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M m-Receptor Definition The term m-opioid (m from morphine) peptide receptor represents the ▶ G-protein-coupled receptor that responds selectively to the majority of clinically useful ▶ opioid drugs. It is usually named the m-receptor or MOR. It is expressed in areas of the nervous system that mediate therapeutic and adverse effects of most opioid drugs. The MOR receptor protein is produced by a single gene. Several mRNA splice variants are known to exist and produce receptor proteins that display different properties when expressed in cells. When activated, the MOR receptor predominantly transduces actions via inhibitory G-proteins. The direct electrophysiological consequences of MOR receptor activation are usually inhibitory. mPET Definition A mPET machine is a relatively high-resolution positron emission tomography imaging device for the noninvasive assessment of small animals, for example, to study receptor occupancy (with specific radioligands) or functional activity (with radiolabeled deoxyglucose) of the brain, to evaluate animal models of psychiatric or neurological disorders, or to develop novel radiotracers for use in man. Cross-References ▶ Deoxyglucose ▶ Positron Emission Tomography mAChR ▶ Muscarinic Receptors Magnesium Pemoline ▶ Pemoline Magnetic Resonance ▶ Nuclear Magnetic Resonance Magnetic Resonance Imaging (Functional) BEN J. HARRISON, CHRISTOS PANTELIS Melbourne Neuropsychiatry Centre, Department of Psychiatry, The University of Melbourne & Melbourne Health, c/o National Neuroscience Facility, Carlton, Melbourne, Australia Synonyms fMRI; Functional magnetic resonance imaging Definition Functional magnetic resonance imaging (fMRI) is a specialized form of MRI and a modern ▶ neuroimaging technique that is typically used for investigating brain activity in animals and humans. Most fMRI experiments measure blood-oxygenation-level dependent (▶ BOLD) contrast – an endogenous hemodynamic signal that reflects blood oxygenation changes linked to neuronal activity. BOLD fMRI is thus an indirect or surrogate measure of neuronal function. Because intracranial recordings of ▶ local field potentials better predict changes in BOLD signal amplitude than ▶ multiunit activity, it is generally considered to reflect synaptic input and local processing in neuronal ensembles as opposed to neuronal spiking activity per se. In conventional applications, BOLD fMRI has a temporal resolution in the order of seconds (1–3 s) and a spatial resolution in the order of millimeters (cubes of tissue 3–5 mm on each side) when covering the whole Ian P. Stolerman (ed.), Encyclopedia of Psychopharmacology, DOI 10.1007/978-3-540-68706-1, # Springer-Verlag Berlin Heidelberg 2010 732 M Magnetic Resonance Imaging (Functional) brain. Largely due to its noninvasive nature and good spatiotemporal resolution, but also through growing understanding of the biophysical basis of the BOLD signal, as well as advances in the acquisition, design, and statistical analysis of brain mapping experiments, BOLD fMRI has become a principal research tool in human ▶ cognitive neuroscience since the mid-1990s. Principles and Role in Psychopharmacology Blood circulation and energy metabolism are closely linked to neuronal synaptic activity in the brain – an observation first suggested by nineteenth century researchers. fMRI, and specifically BOLD contrast fMRI (BOLD fMRI), is a modern neuroimaging technique that exploits the fact that such processes, particularly blood flow and blood oxygenation, are regionally coupled to changes in neuronal activity levels. Background Principles Active neurons consume oxygen that is carried by hemoglobin in capillary red blood cells. Under periods of increased neuronal demand for oxygen (* oxygen utilization), there is an accompanying increase in cerebral blood flow (CBF) to active brain areas, or functional hyperemia. Central to understanding BOLD fMRI is that, during this period of increased blood flow, blood oxygenation increases more than what is required to satisfy the increased neuronal demand for oxygen, leading to local changes in the relative concentration of oxygenated and deoxygenated blood, as well as local cerebral blood volume. Magnetic resonance imaging (MRI), extending the principle of ▶ nuclear magnetic resonance (NMR), has the capacity to measure physiological changes associated with increased (or decreased) neuronal activity, such as measurements of tissue perfusion, blood volume, and blood oxygenation. To understand these measurements, it is necessary to have basic knowledge of the physical, biophysical, and engineering principles of MRI and its functional variants such as BOLD fMRI. For this, several comprehensive introductory texts are available (inc. Buxton 2002; Huettel et al. 2004; Jezzard et al. 2003). Two early discoveries of special relevance to BOLD fMRI are (1) that deoxygenated hemoglobin is ▶ paramagnetic and (2) that there is an oxygenation dependence of the transverse relaxation time of water protons in whole blood at high magnetic field strengths (1.5 or greater). This led Ogawa et al. (1990) to investigate whether altering blood oxygenation levels would influence the visibility of blood vessels on T2*-weighted MR images. When increasing the relative concentration of deoxygenated hemoglobin in blood, they observed reduced T2*-weighted signal intensity in local vasculature on ▶ gradient-echo images (GE). Ogawa and colleagues went on to suggest that their observation of ‘‘BOLD contrast’’ could potentially be used to investigate neuronal activity, albeit indirectly, through changes in blood flow and tissue oxygenation. The first BOLD fMRI studies of the brain in humans were reported in 1992 and involved sensory related activation of the visual and motor cortices (Fig. 1). This work confirmed that MRI could be used to investigate regional changes in brain activity, similar to functional brain mapping studies undertaken at the time with ▶ PET imaging. Since these initial studies, the growth of BOLD fMRI in neuroscience applications has been extraordinary. While BOLD fMRI initially provided a noninvasive and improved brain mapping alternative to PET imaging, it has since taken on its own unique role in cognitive neuroscience research, as well as having a variety of Magnetic Resonance Imaging (Functional). Fig. 1. An early blood-oxygenation-level dependent (BOLD) fMRI study of visual cortex activation in a single human subject. This study was performed in October 1992 at the Magnetic Resonance Centre of Pedralbes in Barcelona, Spain (gradient-echo sequence at 1.5 T, GE Signa), single-slice acquisition, 9664 pixel matrix; round surface coil; TR=7 s. The subject was stimulated with an 8 Hz visual flicker in a blocked-design experiment that compared four blocks of visual stimulation alternating with four blocks of darkness. Eight images were acquired per block. (Image courtesy of J. Pujol.) Magnetic Resonance Imaging (Functional) clinical and commercial applications. According to a recent estimate, over 19,000 peer-reviewed articles are returned from an ISI/Web of Science search with the keyword terms ‘‘fMRI’’ or ‘‘functional MRI’’ or ‘‘functional magnetic resonance imaging,’’ and where the rate of fMRI-related publications has risen from a total of four papers in 1992 to eight papers per day by 2007 (Logothetis 2008). Of the total number of reports, approximately 43% investigated functional localization and/or anatomy associated with specific stimuli or tasks (sensory, cognitive, and emotional); 22% were ‘‘region of interest’’ (ROI) studies examining the physiological properties of distinct brain regions; 8% were related to neuropsychology; 5% were on the properties of the fMRI signal; and the remaining work was related to various topics including plasticity, drug action, experimental design, and analysis methods. For a specific overview of fMRI applications in clinical neuroscience, including ▶ pharmacological fMRI (phfMRI), see Matthews et al. (2006). Spatial and Temporal Resolution Spatial resolution in fMRI experiments is defined by voxel size – 3D rectangular prisms (volume elements) that form the basic unit of measurement in MR images. In whole brain fMRI studies, voxels will typically have a resolution of 3–5 mm (on a side), which is determined by the field of view, matrix size, and slice thickness of the imaged volume. Reducing the size of voxels generally comes at the risk of decreasing signal-to-noise (given the various noise sources in BOLD fMRI; see below) and increasing acquisition times, especially in whole brain studies. Larger voxels, on the other hand, may contain to a larger extent signal contributions from distinct tissue types or regions, known as ‘‘partial volume effects.’’ Determining appropriate voxel size is therefore a trade-off with respect to spatial coverage, resolution, and acquisition time in fMRI studies. Ultimately, the spatial resolution of BOLD fMRI is constrained by the specificity of the brain’s vascular system, which can be said to define the technique’s functional resolution. This refers to the anatomical coincidence between hemodynamic and neuronal activities, which varies in the brain depending on characteristics of the local microvasculature (e.g., density and architecture). BOLD fMRI is sensitive to signal changes in the capillary bed (<10 mm in diameter) and the venous side of the circulation, including venules and larger draining veins (100 mm to mm in diameter), since arteries and arterioles are close to full saturation and contain no deoxygenated blood. In the case of large vessels, signal changes can be displaced up to several millimeters from the activated site and may M obscure the localization of smaller magnitude changes, such as those in the capillary bed supplying active neurons. Higher spatial specificity is therefore advantageous in BOLD fMRI experiments. To this end, advanced acquisition sequences have been developed to emphasize or deemphasize vascular components of the BOLD signal that are distant from neuronal activity, while fMRI experiments can also be designed to better localize hemodynamic responses to specific regions of interest (see example in Fig. 2). The experiments that have compared BOLD fMRI and intracranial microelectrode recordings has shown that the BOLD signal is a robust (linear) predictor of neuronal activity only when considered at the supramillimeter scale (3–6 mm2). Temporal resolution in BOLD fMRI experiments is usually defined by repetition time (TR), or the time taken to acquire one image of the brain or specified volume (number of slices) of interest. In conventional experiments, TR may range from 1 to 3 s, giving such measurements an intermediate level of temporal resolution in-between electrophysiological (ms) and positron emission tomography (PET) imaging (tens of seconds) techniques. Like its spatial resolution, temporal resolution in BOLD fMRI is constrained by technical and physiological factors. In the former case, a major contribution to its current success has been the development of novel acquisition schemes, notably ▶ echo-planar imaging (EPI), that permit rapid functional imaging of T2*weighted images of the whole-brain. Currently, BOLD fMRI with segmented GE-EPI can acquire single slices at a sampling rate of less than 100 ms. Depending on the experimental design, reducing TR (* temporal resolution), will improve the statistical estimation of the BOLD hemodynamic response to a certain extent, although this presents a trade-off between image quality and spatial coverage. Ultimately, temporal resolution in BOLD fMRI is constrained by the slower nature of the hemodynamic response to neuronal activity, whose onset lags behind the timing of actual neuronal events by 4–6 s (see below). Despite these absolute timing differences, well-designed experiments have been able to discriminate the relative timing of BOLD signals between different stimuli or brain regions within a few hundred milliseconds (reviewed in Chapter 8, Huettel et al. 2004). Characteristics and Generation of the BOLD Response As introduced above, BOLD signal is inversely proportional to the concentration of deoxygenated hemoglobin, which is influenced by local changes in three physiological parameters: cerebral blood volume (CBV), CBF, 733 M 734 M Magnetic Resonance Imaging (Functional) Magnetic Resonance Imaging (Functional). Fig. 2. BOLD fMRI of ocular dominance (OD) columns in the human visual cortex: vertical neuronal columns that respond preferentially to visual stimuli presented to one eye rather than to stimuli presented to the other eye. fMRI studies of OD are often presented to showcase the level of spatial specificity that can be achieved with advanced fMRI techniques. In humans, OD columns are separated by approximately 1 mm. Left panel a: The imaging slice from a single subject selected in a study by Yacoub et al. (2007) that permitted a resolution of 0.250.25 mm2 in-plane for a slice thickness of 3 mm. Right panel b: Differential functional OD maps depicting increased activity for left eye stimulation (blue) and right eye stimulation (red) for this subject across distinct sessions (A, B, C & F, G, H) and different filtered averages (D, E and I, J). The upper and lower rows show maps obtained using gradient-echo (GE) and hahn spin-echo (HSE) fMRI, respectively. Both approaches reproduced the expected OD columns, although with increased specificity seen with the HSE method due to its enhanced ability to suppress the influence of large vessels. Pos posterior; RH right hemisphere; IHF inter-hemispheric fissure. (Reproduced with permission from Yacoub et al. 2007 ß 2007 Elsevier Inc.) and the cerebral metabolic rate of oxygen consumption (CMRO2). Signal increases reported in BOLD fMRI experiments are related to the fact that neuronal activity increases regional CBF and glucose utilization (CMRglu) to a larger extent than CMRO2. The net effect of neuronal excitation is therefore to decrease the concentration of deoxygenated hemoglobin (‘‘deoxyhemoglobin washout’’; Brown et al 2007), which in turn increases BOLD signal strength. It is now understood that the characteristic BOLD signal changes observed in fMRI studies reflects the summation of these competing events (CBF, CMRO2, and CBV), resulting in a complex response function that is controlled by several parameters (Buxton 2002). In other words, the BOLD signal does not reflect a single physiological process, but rather represents the combined effects of CBF, CBV, and CMRO2 (Fig. 3). The BOLD response to a short duration event or single stimulus has a canonical hemodynamic waveform shape, which is often described as consisting of (1) a fast response lasting 1–2 s (‘‘initial dip’’) in which there is a small decrease in BOLD signal amplitude, (2) a larger amplitude hyperemia associated with the inflow of oxygenated blood, which peaks at approximately 4–6 s after stimulus presentation, and (3) a refractory period lasting 6–12 s where the signal undershoots the baseline due to the combination of reduced regional CBF and increased CBV (‘‘post-stimulus undershoot’’). The second phase of hyperemia (or hyperoxic phase) is the common focus for detecting increases in brain activity measured by BOLD fMRI. The so-called ‘‘initial dip’’ is suspected to result from early oxygenation changes (oxygen extraction) localized to capillaries, and has been argued as more closely related to neuronal activity than the ensuing hyperemia. However, this observation remains controversial and is not reliably detected in BOLD fMRI studies. Like the BOLD response to a single event, multiple repetitions of the same stimuli in blocks (see below), will see the BOLD response rise to a steady plateau and decline once the block ends, although there are variations to this rule, such as an initial overshoot, slow increasing and decreasing ramps, or an undershoot at the end of the stimulus. Neurovascular Coupling and Neuronal Correlates of BOLD The process by which neural activity influences the hemodynamic properties of the surrounding vasculature (principally arterioles) is referred to as neurovascular coupling, Magnetic Resonance Imaging (Functional) M Magnetic Resonance Imaging (Functional). Fig. 3. Left panel a: Measuring signal changes in fMRI experiments depicted as a complex multistage process, beginning with neuronal activity and ending with BOLD signal measurement as a property of the MRI scanner and pulse sequence. This figure presents a schematic illustration of interactions in the formation of the BOLD signal. Positive/negative arrows indicate positive/negative correlations between the parameters. The right pathway (bold arrows) is the most significant effect in most BOLD fMRI. Right panels b and c: Simplified schematic representation of the BOLD hemodynamic response waveform to a short duration stimulus (b), and to a block of multiple consecutive stimuli (c). (Parts of this figure reproduced with permission from http://www.eecs.umich.edu/~dnoll.) although the mechanism(s) responsible for this is not fully understood. One hypothesis regarding a principal signaling route is that a feedforward pathway involving neuronal-glial interactions after neurotransmitter release stimulates regional CBF. Astrocytes play a crucial role in neurotransmitter recycling, using energy reliant on glycolysis (nonoxidative glucose metabolism) to clear extracellular glutamate and convert it to glutamine after neuronal firing. Increased glycolysis in astrocytes is suspected to trigger intracellular events that couple glutamate cycling rate to the production of vasoactive agents, including nitric oxide and eicosanoids. Therefore, according to this view, neurovascular coupling is mediated by neuronal signaling mechanisms via glial pathways, as opposed to signaling mechanisms of an energy deficit in neurons per se. This view supports, in part, the notion that glycolysis is relevant to the detection of BOLD activity changes and in explaining the apparent mismatch between CBF, CMRglu, and CMRO2 during evoked brain activity (Raichle and Mintun 2006). Detailed biophysical models have also been proposed to explain the complex shape of the hemodynamic response observed in BOLD fMRI studies, accounting for the changes in CBF, CBV, and CMRO2 that accompany increased neuronal activity. Most prominent is the ‘‘balloon model’’ of Buxton and colleagues. According to this work, the apparent discrepancy between CBF and CMRO2 results from how oxygen is supplied to neurons related to its poor diffusion in brain tissue. That is, blood flow must increase more than oxygen consumption to maintain tissue–oxygen gradients supporting oxygen delivery to tissue because its extraction (by passive diffusion) from blood is less efficient at higher flow rates (Buxton 2002). Evidence favoring this model versus the former hypothesis (and vice versa) can be found in expanded form in Buxton et al. (2004) and Raichle and Mintun (2006), respectively. 735 M 736 M Magnetic Resonance Imaging (Functional) Regardless of the precise cause(s) of the physiological changes that give rise to the BOLD signal, evidence has been marshaled in support of a close relationship between evoked hemodynamic and neuronal activity changes. Notably, in the work of Logothetis et al. (2001), which compared BOLD fMRI and intracranial electrophysiological measurements recorded simultaneously in monkeys, BOLD signal was found to be spatially well localized and scaled with neuronal activity. Specifically, these authors reported that the amplitude of the BOLD signal was better correlated with recordings of local field potentials rather than multiunit activity (Logothetis et al. 2001). That is, BOLD signal better reflects the weighted average of synchronized activity of the input signals into a neuronal ensemble than their spiking (action potential) activities. This suggests that BOLD signal changes, primarily reflect input and integrative processes rather than output (communicative) activity. However, there remains some debate about the contributions of different types of neuronal activity to the BOLD signal (local field potentials vs. spiking activity), as the former will be correlated with the latter in many instances (Raichle and Mintun 2006). Experimental Design In a conventional fMRI experiment, ▶ time-series of T2*-weighted images are acquired while subjects are exposed to a specific stimulus or set of stimuli (‘‘task-on’’) that is systematically varied with respect to a ‘‘control-off ’’ condition, typically in the context of a serial or ▶ cognitive subtraction or factorial design. The goal of this approach is to evoke significant changes in blood flow and oxygenation within a given region or network associated with the ‘‘task-on’’ state that will modulate BOLD signal intensity about its mean value. The duration of these stimulus presentations or epochs must be tailored to the dynamics of the hemodynamic response, and will be repeated multiple times to establish sufficient contrast and functional signal to noise ratios for the mapping of ‘‘activation’’ responses. In practice, the magnitude of taskrelated changes in fMRI studies is small (up to5% but usually less) in comparison to the total image intensity and variability across time due to various sources of physical (MR system) and physiological noise. Careful experimental design and the use of post-processing methods for maximizing the detection of activation in the BOLD time series is therefore a critical feature of fMRI studies (Chapter 8–13, Huettel et al. 2004). One common approach is to take advantage of the summed signal as a way of minimizing the influence of noise in fMRI experiments (Fig. 4). The idea here is that the BOLD response summed over several trials will reduce the influence of random noise sources as a result of averaging (Brown et al 2007). Blocked designs that present the same class of stimuli on multiple occasions seek to capitalize on this strategy. In turn, this involves selecting the correct number and timing of stimuli to occur within a block, the duration of the block itself and its number of repetitions, as well as the number of different block types to be included in a single acquisition for later comparison. Overall, block designs are powerful in terms of detecting significant sustained (steady-state) activation in fMRI studies but are generally poor estimators of the time course of the regional hemodynamic response to neuronal events because of their reliance on linear summation of individual responses. Event-related designs are a second common approach in fMRI experiments and involve the presentation of specific stimuli as short duration events in order to detect transient associated changes in neuronal activity. With this approach, each event is separated temporally by an interval ranging from a few to tens of seconds and typically in a random order of predefined range. Investigators typically assume a canonical shape to the hemodynamic response to each stimulus presented and model it as a weighted sum to consecutive stimuli – although this linearity assumption may not hold, especially for the early phase of the hemodynamic response. Compared to blocked designs, eventrelated designs are superior in investigating the shape of regional hemodynamic responses and to compare features such as amplitude or relative timing differences between events. Event-related designs also allow for the investigation of BOLD responses sorted by response types, for instance comparing correct versus incorrect or fast versus slow responses. By comparison, their detection power is relatively poor with respect to blocked designs due to the fewer number of events that can be presented and averaged in a single experimental run. Analysis of BOLD fMRI It was previously stated that the BOLD fMRI time series is influenced by a number of sources of physical and physiological noise. In the former case, this includes system noise that causes fluctuations in MR signal (e.g., signal drift) due to magnetic field inhomogeneities and other factors. In the latter case, this includes gross head motion artifacts, motion related to the cardiac (beat-to-beat) and respiratory (breath-to-breath) cycles, as well as slow variations in respiratory rate and volume, which change the pressure of arterial CO2 – a potent vasodilator. Awareness of these various noise sources in BOLD fMRI studies has led to a range of methods to reduce or mitigate their influence, which continue to be improved upon and refined. Magnetic Resonance Imaging (Functional) M 737 M Magnetic Resonance Imaging (Functional). Fig. 4. Linearity of the hemodynamic response: events leading from the presentation of two stimuli to the generation of a summed BOLD signal. To a certain degree, the BOLD response to successive neural events can be predicted from the summed responses (superposition) to single neural events given an appropriate time delay between them. The figure assumes that stimulus 1 is presented 1 s before stimulus 2 and each stimulus evokes a response that alters CMRO2, cerebral blood flow, and CBV. The net effect of these physiological changes leads to the BOLD response for individual stimuli (a and b). The observed BOLD response (c) is a composite of the unobserved BOLD responses to the single stimuli. (Reproduced with permission from Brown et al. 2007. ß 2007 Springer Netherlands.) Pre-processing of the raw fMRI time-series, prior to statistical analysis, generally has two main goals: firstly, to reduce unwanted or uninteresting variability from data and; secondly, to prepare data for statistical analysis and inference given that many statistical tests applied in fMRI studies make assumptions that are met through such preprocessing steps. In practice, this involves modifying the raw data in a series of steps often including image realignment – to correct and to diagnose head motion artifact in a time series, image normalization – to transform data from different subjects into a common neuroanatomical space, spatial smoothing – to filter (or blur) the data to reduce spatial noise and to improve its normality for statistical parametric tests, and temporal smoothing – to remove low or high frequency noise sources, such as mentioned above. There are a growing number of ways to perform statistical analysis in BOLD fMRI experiments. This has been assisted greatly by the development of publicly available neuroimaging analysis software packages, such as Statistical Parametric Mapping (http://www.fil.ion.ucl.ac.uk/spm/), FMRIB Software Library (http://www.fmrib.ox.ac.uk/fsl/ FSL), Analysis of Functional Neuro Images (http://afni. nimh.nih.gov/). The majority of fMRI studies to date have adopted a conventional voxel-based mapping approach based on extensions of the general linear model for timeseries analysis. The basic premise behind such approaches is that the observed fMRI data can be accounted for by a combination of several experimental (or model) parameters and uncorrelated (or independently distributed) noise. Given the high number of statistical tests performed (voxel by voxel) some correction factor for multiple 738 M Magnetic Resonance Imaging (Functional) Magnetic Resonance Imaging (Functional). Fig. 5. Global functional connectivity of a large-scale and distributed brain network characterized from two distinct task states using independent component analysis (ICA). Top panel a: Correlated fluctuations of the BOLD signal among regions of the so-called ‘‘default-mode network’’ in a group of healthy subjects scanned at rest (NB: time-course plot is of a single subject). Bottom panel b: Correlated fluctuations of the BOLD signal among ‘‘default mode network’’ regions in the same group of subjects performing a moral dilemma task (NB: time course plot is of the group mean). The task consisted of four alternating 30 s control (C) and moral dilemma (D) condition blocks (CDCDCDCD). (Modified from Harrison et al. (2008) Proc Natl Acad Sci USA 105:9781–9789. ß 2008 by the National Academy of Sciences of the USA.) comparisons will generally be applied, leading to the generation of statistically thresholded ‘‘activation’’ maps related to the experiment at hand. This may be performed for the whole brain or specific regions of interest. Other techniques, based on multivariate analysis techniques can also be used to investigate which brain areas are ‘‘activated’’ by a task or a stimulus in fMRI studies. These techniques, as opposed to the general linear model approach, are data driven and therefore do not require the specification of experimental models a priori. Another important distinction between this class of statistical tests and the former is that they are sensitive for testing not only ‘‘where’’ activation occurs in a given experimental context but also how different regions or networks of regions may interact or show interdependence in their activities over time (Fig. 5). Such relationships have been characterized as representing distinct forms of brain-functional connectivity, which has become a topic of specific interest with BOLD fMRI in recent years. Major Strengths of the Method ● Is a safe, noninvasive, highly repeatable and widely available technique for measuring changes in brain activity in vivo. ● Has superior spatial resolution compared to other human neuroimaging techniques. ● Affords high flexibility in experimental design and data modeling. Major Limitations of the Method ● Measures neuronal activity indirectly via changes in blood oxygenation levels. ● Has a temporal resolution in the order of seconds due to the nature of the hemodynamic response. ● Is susceptible to influences of non-neural changes in the body. Cross-References ▶ Cognitive Neuroscience ▶ Cognitive Subtraction ▶ Echo-Planar Imaging ▶ Gradient-Echo Images ▶ Local Field Potentials ▶ Magnetic Resonance Imaging ▶ Multi-Unit Activity ▶ Neuroimaging ▶ Nuclear Magnetic Resonance Magnetic Resonance Imaging ▶ Structural and Imaging ▶ Time-Series Functional Magnetic Resonance References Brown GG, Perthen JE, Liu TT, Buxton RB (2007) A primer on functional magnetic resonance imaging. Neuropsychol Rev 17:107–125 Buxton RB (2002) Introduction to functional magnetic resonance imaging: principles and techniques. Cambridge University Press, Cambridge Buxton RB, Uludag K, Dubowitz DJ, Liu TT (2004) Modeling the hemodynamic response to brain activation. Neuroimage 23:S220–S233 Huettel SA, Song AW, McCarthy G (2004) Functional magnetic resonance imaging. Sinauer Associates, Sunderland Jezzard P, Matthews PM, Smith SM (2003) Functional MRI: an introduction to methods. Oxford University Press, New York Logothetis NK (2008) What we can do and what we cannot do with fMRI. Nature 453:869–879 Logothetis NK, Pauls J, Augath M, Trinath T, Oeltermann A (2001) Neurophysiological investigation of the basis of the fMRI signal. Nature 412:150–157 Matthews PM, Honey GD, Bullmore ET (2006) Application of fMRI in translational medicine and clinical practice. Nat Rev Neurosci 7:732–744 Ogawa S, Lee TM, Nayak AS, Glynn P (1990) Oxygenation-sensitive contrast in magnetic resonance image of rodent brain at high magnetic fields. Magn Reson Med 14:68–78 Raichle ME, Mintun MA (2006) Brain work and brain imaging. Ann Rev Neurosci 29:449–476 Yacoub E, Shmuel A, Logothetis NK, Ugurbil K (2007) Robust detection of ocular dominance columns in humans using Hahn Spin Echo BOLD functional MRI at 7 Telsa. Neuroimage 37:1161–1177 Magnetic Resonance Imaging CHRISTOF BALTES1, THOMAS MUEGGLER1, MARKUS RUDIN1,2 1 Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland 2 Institute of Pharmacology and Toxicology, University of Zurich, Zurich, Switzerland Definition Structural and functional magnetic resonance imaging (structural MRI and fMRI, respectively) are medical imaging techniques widely used in radiology and neuroradiology. While structural MRI allows acquiring three-dimensional images of neuroanatomy with high spatial resolution and excellent soft tissue contrast, fMRI is applied to assess brain activity. fMRI provides information on current status and on neuropharmacological modulation of neuronal activity across the entire brain in a spatially and temporally resolved manner. Structural M 739 MRI and fMRI methods are readily translatable to clinical systems as they are inherently noninvasive and can therefore be applied in human subjects without exposure to radiation such as in nuclear imaging techniques. However, methodological constraints and limitations require careful interpretation of fMRI data. Today, both structural MRI and fMRI have emerged as powerful tools for neuropharmacological research and hold great potential for clinical applications. Principles and Role in Psychopharmacology MRI is a biomedical imaging technique applied in both research laboratories and radiology to visualize the structure and function of the body. MR images represent a weighted distribution of hydrogen atoms (protons) in living tissue, the major contribution being due to water constituting approximately 60–80% of tissue mass (Vlaardingerbroek and den Boer 1999). Hydrogen atoms possess an intrinsic magnetic moment. From a technical point of view, three key components are required to generate the MR signals allowing the formation of an image: the static magnetic field aligning the protons in the direction of the field, the radiofrequency (RF) coil(s) for excitation and reception of the MR signal, and the magnetic gradient field coils for spatial encoding of the MR signal. MRI systems measure the magnetic properties of the protons, which are influenced by electrical and magnetic characteristics of their environment. The local environment varies between tissues or the structures in which the protons are embedded in and are furthermore influenced by physiological processes such as diffusion, perfusion, or blood flow. Consequently, image contrast is governed by a number of MR parameters (Table 1), such as intrinsic relaxation rates (R1, R2, R2*), incoherent motion of water, such as diffusion or perfusion, coherent blood flow in major vessels and water exchange processes between cellular/interstitial fluid and water bound to macromolecules. MRI acquisition parameters can be adapted to emphasize the specific contrast optimal for the structure or the process of interest. In addition, tissue relaxation rates can be altered using exogenous contrast agents based on paramagnetic compounds, such as gadolinium chelates or iron oxide nanoparticles. The strong effect of contrast agents on relaxation rates is due to unpaired electron(s) contained in their electron shell, the magnetic moment of which is approximately 650 times higher than that of protons. Structural MRI Three-dimensional imaging, excellent spatial resolution, and superior soft tissue contrast render MRI the method M 740 M Magnetic Resonance Imaging Magnetic Resonance Imaging. Table 1. Intrinsic MR contrast parameter and biological information derived. Contrast-generating process MRI parameter Information derived Longitudinal relaxation: return to magnetic equilibrium state R1 (=1/T1) Basic structural information: e.g., gray vs. white matter differentiation enhancement following contrast agent administration: e.g., blood–brain barrier integrity or retrograde axonal tracing Transverse relaxation: Loss of phase coherence due to stochastic processes R2 (=1/T2) Basic structural information: sensitive to tissue water content edema formation, inflammation Magnetic susceptibility: Loss of phase coherence due to magnetic field differences R2* (=1/T2*) Basic structural information: gray vs. white matter contrast, hemorrhages Incoherent motion, diffusion: Loss of phase coherence due to molecular diffusion ADC FA CBF and volume (using intravascular contrast agent) BOLD contrast ADC: cellularity (intracelluar vs. extracellular volume fraction) FA: restricted anisotropic diffusion Incoherent motion, perfusion: F Loss of phase coherence due to flow in capillaries Local tissue perfusion Water exchange: MTR Water exchange between two states of different water mobility Macromolecule content: e.g., degree and integrity of myelination ADC apparent diffusion coefficient (mm2/s) BOLD blood oxygen-level dependent (contrast) f tissue perfusion (in ml/s/g tissue or ml/min/100 g tissue) FA fractional anisotropy MTR magnetization transfer ratio is the ratio between the equilibrium magnetization and the steady-state magnetization under saturation conditions Ri relaxation rate (s1) Ti relaxation time (s) of choice for structural neuroimaging. A weak point of the method is its inherent low sensitivity, which has a negative impact on spatial resolution. Correspondingly, identifying small brain structures or assessing subtle/minor pathological alterations, putting high demands on spatial resolution, is challenging. Substantial efforts have been made in recent years to increase MR sensitivity by either moving to higher static magnetic field strengths or by refining RF detection devices, such as the cryogenic detection technology. State-of-the-art MR systems for routine clinical neuroimaging (Konarski et al. 2007) operate at 3 T allowing for spatial resolutions in the order of 1 mm3 (Fig. 1a), while clinical research systems up to 7.0 T have been constructed. In contrast, the high spatial resolution required in rodent brain imaging led to the development of MR systems operating at up to 17.6 T. For example, the combination of high magnetic field strength (9.4 T) with cryogenic RF detection devices enabled the routine recording of high-resolution (52 52 170 m3) mouse brain images (Fig. 1b) in a measurement time of 10 min (Baltes et al. 2009). In structural MRI, usually morphometric comparisons between groups are performed using hypothesisbased selection of regions of interest (ROIs), which is a lengthy process and prone to evaluation errors. More recently, voxel-based morphometry (VBM) has been developed as a fully automated comparison of whole brains on a voxel-by-voxel basis. After spatial normalization of the brains to a stereotactic standard space, brain regions are compared with respect to differences in residual tissue concentrations rather than differences in shape. Clinical application of structural readouts using MRI cover a broad range of disorders from neurodegenerative (i.e., stroke and dementia) to neuropsychiatric diseases (American Psychiatric Association (APA) 2000) (e.g., ▶ schizophrenia, ▶ posttraumatic stress disorder (PTSD), or mood disorders, such as ▶ bipolar affective disorders). For example, VBM studies in schizophrenia generally confirmed and Magnetic Resonance Imaging M Magnetic Resonance Imaging. Fig. 1. Visualization of neuroanatomical structures in the human (a) and in the mouse brain (b) put different demands on spatial resolution. While an in-plane resolution of 0.9 0.9 mm2 in human subjects is sufficient for gray vs. white matter discrimination, a resolution of 52 52 m2 is required in mouse brain to depict cortical structures, such as subfields of the hippocampus. For comparison of the dimensions, the mouse brain image (b) is depicted as inset in the human brain image (a) using the same scale. (Courtesy of R. Luechinger, PhD, University and ETH Zurich, Switzerland.) extended ROI-based studies showing less gray matter concentration in multiple cortical and subcortical regions (Pearlson and Calhoun 2007). In patients suffering, e.g., from bipolar disorders, structural MRI has been applied to assess neuroanatomical abnormalities between healthy subjects and patients. While overall brain volumes appeared to be normal, regional differences have been observed in prefrontal cortex, and subcortical and medial temporal structures involved in the behavioral network, which is known to be affected in bipolar disorders (Strakowski et al. 2005). Similarly, in preclinical MRI, morphometric readouts hold promise to phenotype rodent models of central nervous system (CNS) diseases with applications in mouse models of neurological disorders such as ▶ Alzheimer’s disease (AD) or ▶ Huntington’s disease or in rat models of schizophrenia. Its value for detection of subtle or diffuse morphometric abnormalities, i.e., in neuropsychiatric models that are predictive for disease progression or can serve as surrogate markers for end-stage disease status has to be further validated in carefully planned and analyzed longitudinal studies. While structural MRI provides information on neuroanatomical alterations preceding or accompanying psychiatric diseases, fMRI allows assessing changes in neuronal activation patterns between healthy subjects and patients, which might be more closely related to the disease progression or effects of drug administration. Functional MRI Underlying Biological Processes fMRI is widely applied in clinical and preclinical studies to assess brain function, keeping in mind that the MR method is sensitive to hemodynamic changes prompted by neuronal activity rather than the neuronal activation itself. Local neuronal activity leads to an increased consumption of oxygen and nutrients triggering an increase in local perfusion, i.e., regional cerebral blood flow (CBF) and cerebral blood volume (CBV) (Fig. 2). As the efficiency of oxygen extraction decreases with increasing flow rates, the venous blood contains more oxygenated hemoglobin in the activated state when compared with the resting state. The higher concentration of oxyhemoglobin, and correspondingly the lower concentration of paramagnetic deoxyhemoglobin during activation leads to a decrease in R2* relaxation rates, thus an increase in the MR signal. This mechanism, called blood oxygenation level dependent (▶ BOLD) contrast, has found widespread use in the neuroscience community to study brain function under physiological and pathological conditions. It is 741 M 742 M Magnetic Resonance Imaging Magnetic Resonance Imaging. Fig. 2. Schematic representation of the relationship between neuronal activity and the hemodynamic response function. fMRI allows to assess various processes involved such as changes in CBF, CBV, and changes in BOLD contrast. (Adapted from Martin and Sibson 2008.) important to note that an intact neurovascular coupling is essential for the reliability of functional MRI signals. To elucidate the complex mechanism of neurovascular coupling, further fMRI methods have been developed directly assessing the vascular response to neuronal activation such as CBF and CBV changes. Recently, MR arterial spin labeling (ASL) techniques have been described to measure regional CBF. For this purpose, the arterial blood flowing into the brain is magnetically labeled at the level of the common carotid artery. In this way, arterial blood can be used as endogenous contrast agent. Regional CBF values can be estimated from the difference of the MR signal intensities before and during labeling of the inflowing arterial spins taking the finite lifetime of the magnetically labeled state into consideration. ASL is independent of contrast agent administration and allows therefore continuous CBF monitoring. As CBF changes are supposed to be proportional to neuronal activity, ASL presents itself an attractive method for providing a more direct readout of neuronal activity than BOLD and CBV measurements, which are highly nonlinear. Furthermore, CBF measurements are less susceptible to magnetic field variations than BOLD fMRI based on fast-gradient echo sequences. For the assessment of local CBV values, MRI methods have been developed using exogenous contrast agents with a long plasma half-life such as iron oxide nanoparticles leading to an increase in the R2 and R2* relaxation rates. A few minutes after intravenous administration of the contrast agent, a steady-state concentration is reached. The relative change in local relaxation rates R2 and R2* is proportional to the amount of contrast agent in the tissue and thus proportional to the local CBV. Subsequently, neuronal activity prompting local CBV changes can be detected by measuring relative changes in local relaxation rates R2*. As oxygen extraction decreases for increasing flow rates and the ▶ BOLD Contrast decreases at lower magnetic field strength, CBV measurements are more sensitive than fMRI based on the BOLD contrast. Stimulation Paradigms A key component of fMRI experiments is the ▶ Stimulation Paradigm applied to evoke brain activity. A variety of different stimuli ranging from no stimulation (restingstate fMRI) over thermal, sensory, mechanical, visual, or auditory to pharmacological stimuli have been used during fMRI studies. The rationale behind resting state fMRI is to investigate activation differences between a healthy control group and patients suffering, e.g., from schizophrenia (Pearlson and Calhoun 2007) as these experiments do not rely on the ability of the patient to perform certain tasks. Aside from resting state fMRI, which analyzes spontaneous activity/hemodynamic changes in the brain, fMRI studies always rely on at least two measurements comparing state A (e.g., the resting state) with state B (e.g., during stimulusinduced activation). When designing fMRI studies, it is important to consider the dynamics of the hemodynamic response to neuronal activity, which determines the time resolution that can be achieved in the experiment. In ▶ Pharmacological fMRI (▶ phMRI), the functional response to ligand-induced receptor stimulation or Magnetic Resonance Imaging inhibition after drug administration is assessed throughout the brain using the above-described fMRI methods. Consequently, phMRI, like all fMRI methods, relies on intact neurovascular coupling and assesses the functional response induced by drug administration. In humans, phMRI involves BOLD signal acquisition before, during, and after the administration of a drug, while in animals CBF and CBV measurements are also widely established. phMRI studies in the rat measuring BOLD or CBV changes confirmed sufficient sensitivity to detect dosedependent effects of systemically administered receptor ligands. Assessment of alterations in magnitude and spatial extend of neuronal activity induced by pharmacological targeting has been successfully demonstrated for various neurotransmitter systems such as the dopaminergic, ▶ opioid, ▶ GABAergic, glutamatergic, or cannabinoid system. Figure 3 shows an example of phMRI in the mouse. Acute administration of the GABAA receptor antagonist bicuculline led to region-specific CBV changes. The impact and potential of phMRI in the area of psychopharmacology will be discussed in the following for the example of mood and anxiety disorders. Investigations using in vivo neuroimaging techniques within the field of major (clinical) depression and anxiety disorders are largely dominated by measuring serotonergic (5hydroxytryptamine; 5-HT) neurotransmission and receptor level and their response to medication treatment as alterations in serotonin neurotransmitter system have been clearly implicated in the pathophysiology of these two ▶ neuropsychiatric disorders. Based on the availability of radiolabeled 5-HT receptor ligands, distinct ▶ serotonin receptor populations have been imaged using M positron emission tomography (PET) and single positron emission tomography (SPECT). On the other hand, the nature of serotonergic neurocircuitries can be investigated using phMRI by modifying endogenous neurotransmitter levels or manipulating their receptor activity by specific ligands. Most phMRI studies investigated the 5-HT2C receptor system as one of the main targets for novel anxiolytic drugs. Among the different ligands used, meta-chlorophenylpiperazine (m-CPP), a mixed 5HT1B/2C receptor agonist, has been advanced as a useful pharmacological substance for fMRI studies of regional activation in rats and in human beings. Furthermore, the acute and chronic effect of SSRI on region-specific neuronal activation measured by BOLD-fMRI in human beings has been examined using citalopram or in the rats using ▶ fluoxetine. The literature on phMRI investigating specifically the 5-HT1A receptor is rather sparse, but represents an attractive field of research given that 5-HT1A autoreceptor desensitization is one of the suggested mechanisms of action of chronic antidepressants (i.e., ▶ selective serotonin re-uptake inhibitors, SSRIs) and might be responsible for the delayed onset of antidepressants under clinical conditions. Using CBV-fMRI, decreased activity across several brain areas of the rats was mapped with the strongest effects in ▶ hippocampus and septum after acute administration of the 5-HT1A receptor agonist 8-OH-DPAT. Data Analysis and Interpretation The analysis of fMRI data turns out to be especially challenging because of the complex relationship between the physiological processes involved (Fig. 2) and, Magnetic Resonance Imaging. Fig. 3. Example of phMRI in the mouse. CBV changes in the mouse brain during infusion of the GABAA receptor antagonist bicuculline. (a) Percentage DCBV activity maps of brain section +0.74 mm relative to the Bregma showing the highest activity in cortical areas. (b) Temporal profile for three different ROIs (see brain atlas inlet: green=cortex, blue=striatum, orange = control ROI) highlighting the region specificity of the induced CBV changes. 743 M 744 M Magnetic Resonance Imaging furthermore, the small signal changes detected in fMRI, e.g., BOLD signal changes in humans are in the order of 1–5%. For this reason, the fMRI analysis is often simplified by fitting a general linear model (GLM) based on a priori information of the stimulation paradigm convolved with an assumed hemodynamic response function. More sophisticated approaches incorporate the various processes associated with the hemodynamic response to form a biophysically plausible framework such as the balloon model (Buxton et al. 1998). However, phMRI aims at resolving neuronal networks and connectivities throughout the brain. As the functional relationship and the causal dependencies between different brain areas are a priori unknown, hypothesis-driven methods, such as GLM, might lead to false results or to a loss in sensitivity in activated areas. In this case, data-driven or exploratory approaches are assumed to be superior: these include independent component analysis (ICA) or methods detecting temporal or spatial correlations. For example, in waveletbased cluster analysis (WCA), activated pixels are grouped according to their own activity pattern avoiding assumptions derived from the experimental design. As phMRI methods monitor neuronal activity via neurometabolic or neurovascular coupling processes, careful interpretation of phMRI data is indicated taking various processes influencing the phMRI signal into account. Direct systemic effects of the drug administered or influences of the pathology on the vascular tone might lead to changes of the functional response measured by phMRI, which do not necessarily reflect changes in neuronal activity (Mueggler et al. 2002). Furthermore, one needs to keep in mind that phMRI detects brain regions showing hemodynamic changes, which usually exceed local regions of neuronal activity, thus overestimating the region involved, or neurons even project to distant brain regions, which do not reflect the underlying receptor distribution. In small rodents, phMRI is commonly carried out in anesthetized animals, in which the anesthetic might potentially interfere with the ligand–receptor interaction of interest. The remaining topics with respect to fMRI data analysis and interpretation are the understanding of the functional signal changes acquired and their link to neuronal activity even in healthy subjects. The missing clear understanding of the fundamental process makes the interpretation of fMRI under pathological conditions even more complex. Furthermore, fMRI data analysis and interpretation is carried out on groups of healthy or diseased subjects. As described above, normalization of all brains of a group to a standard space reduces within-group difference, thus enhancing between-group difference and accordingly statistical significance of the differences (Konarski et al. 2007). The biological variability and the slightness of the signal changes hamper subject-specific interpretation or diagnosis of fMRI examinations. Advantages and Limitations with Respect to Alternative Neuroimaging Modalities The neuroimaging technique to be applied depends on the biomedical question to be addressed. Assessing structural information commonly requires high spatial resolution and adequate contrast for the structure of interest, while image acquisition time, typically in the order of several minutes, is not an issue because the anatomical structures can be assumed to be static during the data collection period. As alternative imaging modality, computerized tomography (CT) can be applied measuring X-ray attenuation by tissue. Both structural MRI and CT provide three-dimensional information with excellent spatial resolution and the accessibility to deep brain structures allowing to quantitatively document volumetric and morphological changes (Konarski et al. 2007). However, the advantage of structural MRI is completely noninvasiveness, thus enabling repeated measurements in longitudinal studies to monitor disease progression or therapeutic effects and superior soft tissue contrast. In contrast, imaging of dynamic processes, such as brain perfusion, requires not only sufficient sensitivity to detect, but also sufficient temporal resolution to resolve the physiological changes under investigation. The time available for imaging is determined by the physiological process of interest, which has an impact on the fMRI method that can be applied and the spatial resolution that can be achieved. Alternative imaging modalities to fMRI are ▶ SPECT or ▶ PET. Both modalities are using exogenous contrast agents labeled with radionuclides, such as fluorine-18 for PET and iodine-123 for SPECT (Rudin 2005). Although PET is relatively expensive when compared with SPECT, it is superior in terms of sensitivity and spatial resolution, and provides inherently quantitative data. PET provides information on physiological (perfusion) and biochemical processes, such as neuronal glucose metabolism. With respect to SPECT and PET, fMRI is less sensitive, but provides information on functional consequences of drug administration in a spatially highly resolved manner. Due to their noninvasiveness, fMRI and nuclear imaging methods developed in small rodents are readily translatable to clinical applications. The dimensions of small rodents further allow the use of fluorescent molecular tomography (FMT) as alternative neuroimaging method. In this method, a compound labeled with a fluorescent dye is administered. After specific binding of this reporter to the target and after clearance Major and Minor and Mixed Anxiety-Depressive Disorders of the unbound fraction, the fluorophor is excited using laser light in the near-infrared range (Rudin 2005). FMT is superior to MRI in terms of sensitivity, lower detection limits are in the nanomolar range, but provides relatively poor spatial resolution. However, in human subjects deep brain areas are not accessible due to the small penetration depth of light in the near-infrared range. Future Directions Noninvasive neuroimaging has been rapidly developing in the past decade, as various imaging modalities provide an overwhelming amount of information on functional neuroanatomy, neuronal activity, and neuronal networks. However, technical limitations such as low sensitivity in fMRI or poor localization in EEG prevented the assessment of an integrated view of the brain function. These issues have prompted the development of data fusion methods aiming to combine complementary information from different imaging modalities. For example, EEG data have been constrained using fMRI activation maps (Pearlson and Calhoun 2007). Although these methods hold great potential to improve data interpretation, one has to avoid unrealistic assumptions. Alternative approaches under development are trying to combine the strengths of two modalities such as the high sensitivity of molecular imaging with the high spatial resolution and localization of CT and MRI. Substantial efforts have been made to bring forward such hybrid imaging systems combining, e.g., CTPET or MRI-PET for clinical applications. In small animal imaging, CT-FMT or MRI-PET solutions are also pursued. Cross-References ▶ BOLD Contrast ▶ Cerebral Perfusion ▶ Functional MRI ▶ MR Image Analysis ▶ Neuropsychiatric Disorders ▶ Pharmacological fMRI ▶ Stimulation Paradigm ▶ Structural and Functional Imaging ▶ Translational Neuroimaging M Konarski JZ, McIntyre RS, Soczynska JK, Kennedy SH (2007) Neuroimaging approaches in mood disorders: technique and clinical implications. Ann Clin Psychiatry 19:265–277 Martin C, Sibson NR (2008) Pharmacological MRI in animal models: a useful tool for 5-HT research? Neuropharmacology 55:1038–1047 Mueggler T, Sturchler-Pierrat C, Baumann D, Rausch M, Staufenbiel M, Rudin M (2002) Compromised hemodynamic response in amyloid precursor protein transgenic mice. J Neurosci 22:7218–7224 Pearlson GD, Calhoun V (2007) Structural and functional magnetic resonance imaging in psychiatric disorders. Can J Psychiatry 52:158–166 Rudin M (2005) Molecular imaging – basic principles and applications in biomedical research. Imperial College Press, London Strakowski SM, Delbello MP, Adler CM (2005) The functional neuroanatomy of bipolar disorder: a review of neuroimaging findings. Mol Psychiatry 10:105–116 Vlaardingerbroek MT, den Boer JA (1999) Magnetic resonance imaging. Springer-Verlag, Heidelberg Mairungi ▶ Khat Major and Minor and Mixed Anxiety-Depressive Disorders LAWRENCE H. PRICE Department of Psychiatry and Human Behavior, The Warren Alpert Medical School of Brown University, Providence, RI, USA Synonyms Clinical depression; Depression; Unipolar depression Definition Magnetic Resonance References American Psychiatric Association (APA) (2000) Diagnostic and statistical manual of mental disorders DSM-IV-TR (text revision), 4th edn. APA Press, Washington Baltes C, Radzwill N, Bosshard S, Marek D, Rudin M (2009) Micro MR imaging of the mouse brain using a novel 400 MHz cryogenic quadrature RF probe. NMR in biomedicine. Wiley, UK Buxton RB, Wong EC, Frank LR (1998) Dynamics of blood flow and oxygenation changes during brain activation: the balloon model. Magn Reson Med 39:855–864 745 The depressive disorders comprise a spectrum of clinical syndromes characterized by persistent depressed mood or sadness, loss of interest (apathy), or loss of pleasure (anhedonia). These symptoms are to be distinguished from the transient feelings of unhappiness or sadness that constitute normal reactions to the disappointments or losses experienced in everyday life. The symptoms of depressive disorders cause clinically significant distress or impairment in social, occupational, or other important areas of functioning, and are not due to the direct physiological effects of a substance or a general medical condition. Depression is generally differentiated from bereavement, the expectable constellation of depressive symptoms M 746 M Major and Minor and Mixed Anxiety-Depressive Disorders following the loss of a loved one, although depression may be diagnosed if such symptoms are unusually prolonged (e.g., more than 2 months) or are not characteristic of a ‘‘normal’’ grief reaction in the individual’s culture. The depressive disorders are also to be distinguished from depressive episodes of ▶ bipolar disorder, which is diagnosed if the individual has ever had a manic, mixed, or hypomanic episode. Role of Pharmacotherapy Clinical Features, Etiology, and Pathogenesis Recognition of depression as a distinct disease dates back at least as far as the Hippocratic writings of the fifth and fourth centuries B.C. Lifetime prevalence varies among diagnostic categories and according to the specific diagnostic criteria used, with risks for major depression ranging from 10 to 25% in women and 5 to 12% in men. Prevalence rates are unrelated to income, education, marital status, or ethnicity. Depressive disorders may have their onset at any age, but the average is in the mid- 20s. At least 60% of individuals with a single episode of major depression will have subsequent episodes. Major depression is associated with significantly increased mortality; up to 15% of individuals with severe illness die by ▶ suicide. The etiology of depressive disorders is unknown. There is a familial pattern, with major depression 1.5– 3 times more common in individuals with an affected first-degree biological relative; ▶ heritability based on twin studies is 40–50%. However, no single major gene locus has been shown to cause depression. Rather, genetic risk appears to involve multigenic and/or geneenvironment interactions. The short allele of the serotonin transporter-linked polymorphic region (5-HTTLPR), in particular, has been most strongly implicated in geneenvironment interactions leading to depression. Environmental risk factors include early-life abuse and neglect and major life stress. The predominant theories of pathogenesis in depression have focused on monoamine neurotransmission and ▶ hypothalamic-pituitary-adrenal (HPA) axis dysfunction. Early monoamine theories posited simple deficiencies in ▶ serotonin or ▶ norepinephrine function, but such hypotheses have been superseded by complex formulations involving a wide array of intracellular and trans-synaptic signaling pathways. Similarly, while early HPA axis theories emphasized hyperactivity, more recent work suggests that derangements in this neuroendocrine system are more variable, and may be linked to abnormalities in neurotrophins (such as ▶ brain-derived neurotrophic factor (BDNF)) and ▶ neurogenesis. Other theories of pathogenesis have explored the role of reduced neurotransmission in the ▶ dopamine and ▶ GABA systems, altered glutamatergic neurotransmission, impaired ▶ endogenous opioid function, abnormal ▶ circadian rhythms, hypothalamic-pituitary-thyroid (HPT) axis dysfunction, monoamine–acetylcholine imbalances, cytokine-mediated neuroimmune abnormalities, deficient ▶ neurosteroids synthesis, and dysfunction of specific brain structures and circuits. Diagnostic Categories Major Depressive Disorder (Major Depression) The diagnosis of major depression requires the presence of at least five of nine key symptoms during the same 2-week period, with at least one of the symptoms being either (1) depressed mood, or (2) loss of interest or pleasure. In addition to these, the other key symptoms are (3) significantly decreased or increased weight or appetite, (4) insomnia or hypersomnia, (5) psychomotor agitation or retardation, (6) fatigue or loss of energy (anergia), (7) feelings of worthlessness or guilt, (8) diminished concentration or indecisiveness, and (9) recurrent thoughts of death, suicidal ideation, or suicidal behavior. Between 50 and 70% of individuals show clinically significant improvement with a given antidepressant. There is no compelling evidence of differential efficacy between drugs in unselected patients, and drug selection is usually based on a consideration of side-effect profiles. Selective serotonin reuptake inhibitors (▶ SSRIs) are the most frequently used first-line drugs for major depression. This group includes ▶ fluoxetine, ▶ paroxetine, ▶ sertraline, ▶ fluvoxamine, ▶ citalopram, and ▶ escitalopram. Common side effects of these drugs are constipation, diarrhea, dizziness, headache, insomnia, nausea, somnolence, and sexual dysfunction. Other frequently used classes of ▶ antidepressants are the serotonin-norepinephrine reuptake inhibitors (▶ SNRIs) and the norepinephrine reuptake inhibitors (▶ NARIs). The SNRIs include ▶ venlafaxine, desvenlafaxine, ▶ milnacipran, and ▶ duloxetine, which have side-effect profiles generally similar to the SSRIs. The NARIs include ▶ bupropion and ▶ reboxetine, with side effects that include dry mouth, insomnia, headache, nausea, constipation, tremor, and tachycardia; bupropion is less likely to cause sexual dysfunction than the SSRIs or SNRIs, but lowers the seizure threshold at high doses. The ▶ tricyclic antidepressants (TCAs) include ▶ amitriptyline, ▶ imipramine, ▶ clomipramine, ▶ nortriptyline, desipramine, protriptyline, Major and Minor and Mixed Anxiety-Depressive Disorders ▶ trimipramine, amoxapine, and dothiepin. While the TCAs were the first class of drugs used to treat depression, they have largely been supplanted in that role by newer drugs because of their unfavorable side-effect profile (▶ anticholinergic effects, cardiovascular effects, somnolence, and tremor) and their high degree of lethality in overdose. Tetracyclic antidepressants, which include maprotiline, ▶ mirtazapine, and ▶ mianserin, constitute another group of less commonly used agents; maprotiline has side effects similar to the TCAs, whereas mirtazapine and mianserin are notable for causing somnolence, dry mouth, and substantial weight gain. The nonselective ▶ monoamine oxidase inhibitors (MAOIs), which include ▶ phenelzine, ▶ tranylcypromine, and ▶ isocarboxazid, are generally considered third- or later-line drugs, as they can cause life-threatening hypertensive or hyperpyrexic reactions when inadvertently combined with foods or drugs that enhance noradrenergic or serotonergic activity. More selective MAOIs, such as the reversible inhibitor of monoamine oxidase A (RIMA) ▶ moclobemide or the transdermal formulation of the MAO-B selective ▶ selegiline, are far less likely to cause such reactions. The triazolopyridine antidepressants, ▶ trazodone and its congener nefazodone, are now infrequently used as primary treatments for depression; trazodone can cause severe priapism, while nefazodone has been associated with rare fatal hepatic necrosis, and both drugs cause significant somnolence. Major depression of mild to moderate severity may be treated with psychotherapy, with or without medication; ▶ cognitive-behavioral therapy and ▶ interpersonal psychotherapy, in particular, have been supported in controlled clinical trials. Milder cases of depression may also respond to complementary and alternative medical approaches, such as light (phototherapy), exercise, and herbal or dietary supplements (e.g., St. John’s wort (Hypericum perforatum), omega-3 fatty acids, S-adenosine-Lmethionine (SAMe)). Severe and treatment-resistant cases of depression are often managed with combinations of drugs or with ▶ electroconvulsive therapy. Rarely, stereotactic ablative neurosurgery, generally involving lesions in frontolimbic circuitry, is used in intractable cases. The role of newer neuromodulatory approaches, such as ▶ transcranial magnetic stimulation, ▶ vagus nerve stimulation, and ▶ deep brain stimulation, is currently under investigation. Major Depressive Disorder with Psychotic Features (Psychotic Depression) This form of depression is defined by the presence of either ▶ delusions or ▶ hallucinations, and is invariably M 747 severe; inpatient treatment is usually necessary because of profound functional impairment or intense suicidality. Psychotic features whose content reflects the typical depressive themes of personal inadequacy, guilt, disease, death, nihilism, or deserved punishment are considered mood-congruent; mood-incongruent psychotic features generally involve non-depressive persecutory delusions, thought insertion, thought broadcasting, or delusions of control. Pharmacotherapy usually involves the use of an ▶ antipsychotic in combination with an antidepressant. However, ECT is often required. Major Depressive Disorder with Catatonic Features Catatonia is diagnosed when at least two of five key symptoms are present, including (1) motoric immobility, such as catalepsy or stupor, (2) excessive purposeless motor activity (catatonic excitement), (3) extreme negativism (motiveless resistance to instructions or maintenance of a rigid posture against attempts to be moved) or mutism, (4) bizarre posturing, stereotypies, mannerisms, or grimacing, and (5) ▶ echolalia or ▶ echopraxia. Catatonic symptoms often respond acutely to ▶ benzodiazepines. However, since this syndrome generally occurs in the context of psychotic depression, treatment with an antipsychotic/antidepressant combination or ECT is usually necessary for sustained improvement. Major Depressive Disorder with Melancholic Features (Melancholia) The critical feature of this syndrome is profound anhedonia or lack of reactivity (not even transient mood improvement in response to positive events). In addition, at least three of six key symptoms are present, including (1) distinct quality of mood (different from usual feelings of sadness or loss), (2) morning worsening of mood (diurnal variation), (3) early morning awakening, (4) marked psychomotor retardation or agitation, (5) significant anorexia or weight loss, and (6) excessive guilt. Episodes of melancholia are usually severe, and patients with psychotic depression are usually melancholic. As the successor to the historical syndrome of endogenous depression, melancholia was originally defined in an attempt to identify those patients who would have a better response to somatic treatment than other patients. Subsequent research has failed to establish this preferential response, but has shown that melancholic patients are less likely than other patients to respond to placebo. Pharmacotherapy involves standard antidepressant drugs. Some authorities believe that first-generation drugs (i.e., TCAs and MAOIs) are more effective. Severe cases may require ECT. M 748 M Major and Minor and Mixed Anxiety-Depressive Disorders Major Depressive Disorder with Atypical Features (Atypical Depression) This syndrome is defined by the presence of mood reactivity (mood brightening in response to positive events) in conjunction with at least two of four key symptoms, including (1) significant weight gain or appetite increase, (2) hypersomnia, (3) leaden paralysis (heavy feelings in the limbs), and (4) a long-standing pattern of interpersonal rejection sensitivity. The diagnostic criteria for atypical depression reflect early efforts to identify a group of patients who would preferentially respond to MAOIs rather than TCAs. The importance of this distinction has receded as other, safer agents have supplanted the MAOIs, and treatment of atypical depression is now usually initiated with SSRIs or other second-generation antidepressants. However, ongoing research has generally supported the existence of meaningful neurobiological differences between atypical depression and melancholia (e.g., HPA axis hypoactivity in atypical depression vs. hyperactivity in melancholia). Major Depressive Disorder with Postpartum Onset (Postpartum Depression) Episodes of major depression whose onset is within 4 weeks of delivery are considered postpartum (as are similarly timed manic or mixed episodes). Diagnostic criteria are otherwise the same as for other depressive syndromes, although postpartum episodes are usually distinguished by symptom content that is focused on the infant. The mother may express excessive concern for the infant’s well-being, feelings of being overwhelmed, fear of being responsible for the infant, hostility, or apathy. Psychotic symptoms may develop, in which case there may be a risk of infanticide. Postpartum depression must be distinguished from the transient mood lability (‘‘baby blues’’) occurring in the first 10 days postpartum in up to 70% of women, which resolves on its own. Pharmacotherapy for postpartum depression involves standard antidepressant drugs, with antipsychotics if psychotic symptoms are present. All antidepressants are secreted in breast milk, but few specific adverse events have been reported, so benefits and risks of breastfeeding must be addressed. Major Depressive Disorder with Seasonal Pattern (Seasonal Depression, Seasonal Affective Disorder) Episodes of major depression are considered seasonal (as are manic or mixed episodes) if (1) there has been a regular temporal relationship between the onset of the episode and a particular time of year (for depression, usually fall or winter), (2) full remissions also occur at a characteristic time of year (usually spring), (3) in the last 2 years two episodes have occurred with the seasonal pattern, but no nonseasonal episodes have occurred, and (4) seasonal episodes substantially outnumber nonseasonal episodes over the individual’s lifetime. The diagnosis is not made if seasonal psychosocial stressors (e.g., school or work) better account for the seasonal pattern. Seasonal depression is more common at higher latitudes and in younger individuals. Pharmacotherapy usually involves SSRIs or NARIs, although light therapy appears to be equally effective and is often used. Minor Depressive Disorder (Minor Depression) The diagnostic criteria for this syndrome are the same as those for major depression, but fewer symptoms are required. Thus, at least two, but less than five, of the nine key symptoms must be present during the same 2-week period, again with at least one of the symptoms being either depressed mood or loss of interest or pleasure. At present, these diagnostic criteria are considered investigational, and the nosological status of this syndrome is not established. There is only limited support for using pharmacotherapy in the management of minor depression, and while some studies suggest SSRIs may be of benefit, an especially careful assessment of risks and benefits should be undertaken. Mixed Anxiety-Depressive Disorder (Mixed Anxiety-Depression) This syndrome is defined by persistent or dysphoric mood lasting at least 1 month, accompanied by at least four of ten key symptoms lasting at least 1 month, including (1) difficulty in concentrating, (2) sleep disturbance, (3) fatigue, (4) irritability, (5) worry, (6) tearfulness, (7) hypervigilance, (8) anticipating the worst, (9) hopelessness, and (10) low self-esteem. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning, and are not due to the direct physiological effects of a substance or a general medical condition. In addition, criteria have never been met for major depression, ▶ dysthymic disorder, ▶ panic disorder, or ▶ generalized anxiety disorder, and criteria are not currently met for any other anxiety or mood disorder. As in the case of minor depression, mixed anxiety–depression is still considered an investigational diagnosis, and is not recognized in the official nomenclature. However, the greater symptom burden in mixed anxiety–depression supports a more prominent role for pharmacotherapy. SSRIs and SNRIs are preferred, as NARIs may lack efficacy and the side-effect profiles of other antidepressants are often MAO-B Inhibitor poorly tolerated in this population. Benzodiazepines are frequently used adjunctively, especially early in treatment. Cross-References ▶ Antidepressants ▶ Antipsychotic Drugs ▶ Benzodiazepines ▶ Bipolar Disorder ▶ Monoamine Oxidase Inhibitors ▶ NARIs ▶ Neurogenesis ▶ Neurosteroids ▶ SNRIs ▶ SSRI ▶ Suicide References American Psychiatric Association Task Force on DSM-IV (2000) Diagnostic and statistical manual of mental disorders: DSM-IV-TR, 4th edn. American Psychiatric Association, Washington, DC Belmaker RH, Agam G (2008) Major depressive disorder. N Engl J Med 358:55–68 Grunze H, Schüle C, Casey D, Baghai TC (2008) Mood disorders: Depression. In: Tasman A, Maj M, First MB, Kay J, Lieberman JA (eds) Psychiatry, 3rd edn. Wiley, Hoboken, NJ, USA, pp 1283–1332 Mann JJ (2005) The medical management of depression. N Engl J Med 353:1819–1834 Misri S, Kendrick K (2007) Treatment of perinatal mood and anxiety disorders: a review. Can J Psychiatry 52:489–498 Parker G (2005) Beyond major depression. Psychol Med 35:467–474 Boland RJ, Keller MB (2004) Treatment of depression. In: Schatzberg AF, Nemeroff CB (eds) The American Psychiatric publishing textbook of psychopharmacology, 3rd edn. American Psychiatric Publications, Washington, DC, pp 847–864 Stewart JW, McGrath PJ, Quitkin FM, Klein DF (2007) Atypical depression: Current status and relevance to melancholia. Acta Psychiatr Scand (Suppl): 58–71 Tyrka AR, Price LH, Mello MF, Mello AF, Carpenter LL (2006) Psychotic major depression: A benefit-risk assessment of treatment options. Drug Saf 29:491–508 Westrin A, Lam RW (2007) Seasonal affective disorder: A clinical update. Ann Clin Psychiatry 19:239–246 M Cross-References ▶ Antipsychotic Drugs ▶ First-Generation Antipsychotics ▶ Second and Third Generation Antipsychotics MALDI ▶ Matrix-Assisted Laser Desorption Ionization Malleability ▶ Elasticity Malonylurea ▶ Barbiturates Manerix ▶ Moclobemide Mania Definition A state in which the individual experiences euphoria, irritability, lack of sleep, and increased drives, often to the point of poor judgment. Manic-Depressive Illness Major Tranquilizer Synonyms ▶ Bipolar Disorder ▶ Bipolar Disorder in Children Antipsychotic; Neuroleptics Definition A medication used in the treatment of psychotic disorders of any type with a particular emphasis on inducing sedation. The term is less used than its synonyms and is poorly characterized in pharmacological terms. It tends to be used less by psychiatrists than by nonspecialists. 749 MAO-B Inhibitor Definition A drug that blocks the action of monoamine oxidase type B. M 750 M Marijuana Abuse Marijuana Abuse ▶ Cannabis Abuse and Dependence Marijuana Addiction ▶ Cannabis Abuse and Dependence Marijuana Dependence ▶ Cannabis Abuse and Dependence Marplan Mass Spectrometry Imaging ▶ Mass Spectrometry Mass Spectroscopy ▶ Mass Spectrometry Matching Law Definition A behavioral allocation function that relates the rate of operant performance to the rate of reward delivery. The generalized matching law incorporates additional independent variables, such as the strength, amount, imminence, and likelihood of reward. ▶ Isocarboxazid Maternal Deprivation Model Mass Spectrometry Synonyms Mass spectroscopy Definition Mass spectrometry is an analysis technique that measures the molecular mass of an analyte. A mass spectrometer typically consists of three modules: (1) an ion source in which the convert sample is vaporized and ionized to make it analyzable; (2) a mass analyzer, which separates the sample components on the basis of their mass (more precisely their mass/charge ratios); (3) and a detector, which provides data for calculating the abundances of each ion present. Cross-References ▶ Electrospray Ionization (ESI) ▶ Imaging Mass Spectrometry (IMS) ▶ Matrix-Assisted Laser Desorption Ionization (MALDI) ▶ Metabolomics ▶ Neuropeptidomics ▶ Post-Translational Modification ▶ Proteomics ▶ Two-Dimensional Gel Electrophoresis Definition An animal model in which young rats are separated from their mother for a single period of 24 h. The most optimal day for separation is postnatal day 9. These animals develop a large number of schizophrenia-like phenomena in adulthood. Interestingly, most of these phenomena occur after puberty in accordance with the clinical literature on schizophrenia. Cross-References ▶ Schizophrenia: Animal Models ▶ Simulation Model Mating Behavior ▶ Sexual Behavior MATRICS Synonyms Measurement and treatment research to improve cognition in schizophrenia Medazepam Definition This is an initiative of the National Institutes of Health in the USA to enhance the methodology of assessing cognitive impairment in ▶ schizophrenia, using neuropsychological tests, for the purpose of clinical trials. This initiative has also boosted interest in cognitive assessment in experimental animals in order to evaluate putative cognitive enhancing compounds. M 751 Measure of Drug Activity ▶ Potency Measurement and Treatment Research to Improve Cognition in Schizophrenia ▶ MATRICS Matrix-Assisted Laser Desorption Ionization Synonyms Laser desorption ionization; MALDI Definition Formation of gas-phase ions from molecules that are present in a solid or liquid matrix that is irradiated with a pulsed laser. Measurement of Biological Effect Resulting from Interaction with the Receptor ▶ Receptors: Functional Assays Cross-References ▶ Imaging Mass Spectrometry (IMS) ▶ Mass Spectrometry (MS) ▶ Metabolomics ▶ Neuropeptidomics ▶ Proteomics ▶ Two-dimensional Gel Electrophoresis Measurement of Neuronal Activity ▶ Extracellular Recording Measurement of Receptor Signaling MDAS ▶ Receptors: Functional Assays ▶ Memorial Delirium Assessment Scale Mecamylamine MDMA ▶ Methylenedioxymethamphetamine (MDMA) Definition A nicotinic antagonist that is well absorbed from the gastrointestinal tract and crosses the ▶ blood–brain barrier. MEA ▶ Glutamate Microelectrode Arrays ▶ Microelectrode Arrays Measures ▶ Rating Scales and Diagnostic Schemata Medazepam Definition Medazepam is a benzodiazepine that has anxiolytic, sedative, and anticonvulsant properties. It has a very long duration of action, mainly due to a long ▶ elimination half-life (36–200 h) and conversion to active metabolites including the benzodiazepines ▶ diazepam and M 752 M Medial Forebrain Bundle N-desmethyl-medazepam, both of which also are longacting and also have long-acting metabolites. Like most similar compounds, medazepam is subject to ▶ tolerance, ▶ dependence, and ▶ abuse. Drug; Human medicinal product Cross-References Definition ▶ Anxiolytics ▶ Benzodiazepines Any substance or combination of substances that may be used in or administered to human beings either with a view to restoring, correcting, or modifying physiological functions by exerting a pharmacological, immunological, or metabolic action, or to making a medical diagnosis. Medicine Synonyms Medial Forebrain Bundle Definition The MFB is a complex bundle of axons coming from the basal olfactory regions, the periamygdaloid region, and the septal nuclei, and passing to the lateral hypothalamus with some carrying on into the tegmentum. It contains both ascending and descending fibers. It is commonly accepted that the MFB is part of the reward system involved in the integration of reward and pleasure. Electrical stimulation of the MFB is believed to cause sensations of pleasure. Medicines Control ▶ Licensing and Regulation of Medicines Medicines Regulation ▶ Licensing and Regulation of Medicines Megakaryocytes Medial Prefrontal Cortex Synonyms mPFC Definition A brain structure located in ▶ prefrontal cortex that receives dopamine input from the midbrain. It has been implicated in drug reward, behavioral inhibition, and stress reactivity. Definition The megakaryocyte is a nucleated cell originating from the bone marrow and responsible for the production of platelets (thrombocytes), which are necessary for the process of hemostasis. The cytoplasm, exactly as platelets that bud off from it, contains alpha-granules and dense bodies. Megalomania ▶ Delusional Disorder Medical Herbalism ▶ Herbal Remedies Medically Unexplained Symptoms ▶ Somatoform and Body Dysmorphic Disorders Melanin-Concentrating Hormone Definition Lateral hypothalamic peptide involved in the regulation of feeding motivated behaviors including food intake. Like ▶ hypocretin/orexin, cells producing this peptide are located in the lateral hypothalamic region, but melanin-concentrating hormone (MCH)-producing cells do not co-express hypocretin/orexin. Memorial Delirium Assessment Scale a-Melanocyte-Stimulating Hormone Definition Cleaved product of the proopiomelanocortin pre-pro peptide. In the brain, this peptide is critical for the regulation of energy balance and genetic alterations that inactivate this peptide are associated with morbid obesity and insulin resistance. M 753 Definition A cell’s membrane potential is the voltage difference across the plasma membrane that is present in all living cells. Cross-References ▶ Intracellular Recording Membrane Potential Recording ▶ Current Clamp Melatonin Definition Melatonin is a neurotransmitter that participates in the regulation of the sleep/wake cycle. It is produced endogenously and also available over-the-counter as an effective hypnotic for sleep onset. Memantine Definition Memantine is used as an anti-dementia drug with some efficacy in the treatment of moderate to severe ▶ Alzheimer’s disease. It reduces glutamatergic neurotransmission by acting as a low-affinity ▶ NMDA-receptor antagonist and this action is thought to underlie its protective effects against neuronal excitotoxicity. In addition to its antiglutamate action, memantine also acts as a noncompetitive antagonist at serotonin 5-HT3 and nicotinic acetylcholine receptors, and as an agonist at the dopamine D2 receptor; the role of these actions in the anti-dementia properties of the drug is unknown. Adverse side effects include confusion, dizziness, drowsiness, headache, insomnia or sleepiness, agitation, and hallucinations. In addition to Alzheimer’s disease, memantine is currently being tested as a potential treatment for a number of other disorders. Preclinical studies as well as the clinical absence of withdrawal symptoms suggest that this drug has a low abuse potential. Membrane Potential Synonyms Membrane voltage Membrane Voltage ▶ Membrane Potential Memorial Delirium Assessment Scale Synonyms MDAS Definition The memorial delirium assessment scale (MDAS) is a 10-item, 4-point clinician-rated scale (possible range: 0 to 30) designed to quantify the severity of ▶ delirium, validated among hospitalized patients with advanced cancer and AIDS. Items included in the MDAS reflect the diagnostic criteria for delirium in the ▶ DSM-IV, as well as symptoms of delirium from earlier or alternative classification systems (e.g., DSM-III, DSM-III-R, ICD-9). The MDAS is both a good delirium diagnostic screening tool as well as a reliable tool for assessing delirium severity among patients with advanced disease. Scale items assess disturbances in arousal and level of consciousness, as well as in several areas of cognitive functioning (memory, attention, orientation, disturbances in thinking) and psychomotor activity. A cutoff score of 13 is diagnostic of delirium. The MDAS is designed to be administered repeatedly within the same day, in order to allow for objective measurement of changes in delirium severity in response to medical changes or clinical interventions. The MDAS has advantages over other delirium tools in that it is both a diagnostic as well as a severity measure that is ideal for repeated assessments and for use in treatment intervention trials. M 754 M Memory Memory Definition Memory refers to the ability to recover information about past events or knowledge, the process of recovering information about past events or knowledge, as well as cognitive reconstruction. The brain engages in a remarkable reshuffling process in an attempt to extract what is general and what is particular about each passing moment. Memory may be divided into short-term (also known as working or recent memory) and long-term memory. Short-term memory recovers memories of recent events, while long-term memory is concerned with recalling the more distant past. Cross-References ▶ Long-Term Memory ▶ Short-Term and Working Memory in Animals ▶ Short-Term and Working Memory in Humans Memory: Information Storage, Learning and Memory ▶ Long-Term Depression and Memory Memory Persistence ▶ Protein Synthesis and Memory Memory Restabilization ▶ Consolidation and Reconsolidation Memory Stabilization ▶ Consolidation and Reconsolidation Memory Consolidation ▶ Consolidation ▶ Protein Synthesis and Memory Memory Storage ▶ Consolidation ▶ Protein Synthesis and Memory Mental Deficiency ▶ Autism Spectrum Disorders and Mental Retardation Memory-Storage Effect Definition Memory Dysfunction ▶ Dementias and Other Amnestic Disorders Memory-Impairing Drugs ▶ Inhibition of Memory Memory Impairment ▶ Dementias and Other Amnestic Disorders Memory-storage effect refers to the gradual effect of a drug to alter the long-term memory of the estimated duration that is dependent on the translation of the clock reading into memory. In the ▶ Peak Internal (PI) procedure, a memory effect is observed as a gradual horizontal shift in the response function in PI trials following drug administration that is proportional to the estimated duration (see Meck, 1996). Cross-References ▶ Timing Behavior Mental Disorders ▶ Neuropsychiatric Disorders Meta-Analysis Mental Retardation ▶ Autism Spectrum Disorders and Mental Retardation M 755 lower risk of dependence and abuse in comparison to full m-agonists such as ▶ morphine. Meptazinol exhibits not only a short onset of action, but also a shorter duration of action relative to other opioids. Cross-References MEOS ▶ Microsomal Ethanol-Oxidizing System ▶ Addiction ▶ Analgesics ▶ Dependence ▶ Opioids ▶ Pain ▶ Tolerance Meperidine ▶ Opioids ▶ Pethidine Meprobamate Synonyms Miltown Definition Meprobamate is a sedative with a medium duration of action and is generally akin to the barbiturates used in the treatment of ▶ anxiety. It has largely been replaced by the benzodiazepines. Unwanted effects include sedation, headaches, paradoxical excitement, confusion, cognitive and psychomotor impairment, and confusion in the elderly. Interaction with ▶ alcohol can be hazardous. It depresses respiration and is toxic in overdose. Long-term use can induce ▶ dependence with severe withdrawal reactions. Recreational use and abuse can occur: it is a scheduled substance. Cross-References ▶ Barbiturates ▶ Minor Tranquilizers Merital® ▶ Nomifensine Mesolimbic System Definition Brain pathway that is dominated by dopamine projections branching to the subcortical ventral striatum (▶ nucleus accumbens) and to the ▶ prefrontal cortex. Mesotelencephalic Dopamine Reward Systems Synonyms Brain reward systems; Dopamine reward systems Definition The mesotelencephalic dopamine system has three components, the nigrostriatal, mesolimbic, and mesocortical pathways consisting of cell bodies in the substantia nigra and ▶ ventral tegmental area that project to a number of regions including the ▶ nucleus accumbens, ▶ amygdala, striatum, and ▶ prefrontal cortex. These areas of the brain are strongly implicated in reward-related learning. Meptazinol Definition Meptazinol is an opioid analgesic for use in moderate to severe pain. It is most commonly used to treat pain in obstetrics (childbirth). As a partial m-opioid receptor agonist, its mixed agonist/antagonist activity results in a Meta-Analysis Definition A statistical technique for combining data from independent but methodologically similar studies to answer M 756 M Metabolic Encephalopathy related hypotheses and to estimate an overall effect across all the studies. Examples of its use include clinical trials and studies of behavioral and psychiatric genetics. Metabolic Encephalopathy ▶ Delirium Metabolic Toxins ▶ Neurotoxins ▶ Matrix-Assisted Laser Desorption Ionization (MALDI) ▶ Neuropeptidomics ▶ Post-Translational Modification Metabonomics ▶ Metabolomics Metabotropic Glutamate Receptor Synonyms mGluRs Definition Metabolism G-protein-coupled receptor for which glutamate is the endogenous ligand. Synonyms Biotransformation Definition Metabolism is the irreversible transformation of drugs into metabolites. Metabotropic Glutamate Receptors 2 and 3 ▶ Group II Metabotropic Glutamate Receptor Cross-References ▶ Absorption ▶ Distribution ▶ Excretion ▶ Liberation ▶ Pharmacokinetics Meta-Phenylenediamine Synonyms m-PD Definition Metabolomics Synonyms Metabonomics Definition Study of the complete set of small-molecule metabolites, such as metabolic intermediates, hormones, and other signaling molecules, and secondary metabolites, found within a biological sample. All microelectrode arrays sites are electroplated with meta-phenylenediamine (m-PD) by applying a potential of +0.5 V to the Pt sites versus a silver/silver chloride (Ag/AgCl) reference electrode (Bioanalytical Systems, RE-5) in a deoxygenated 0.05 M phosphate buffered saline (PBS, pH 7.1–7.4) with 5.0 mM m-PD. The m-PD forms a size-exclusion layer over the sites, blocking DA, ascorbic acid (AA), DOPAC, and other electroactive compounds. Meta-Plasticity Cross-References Definition ▶ Electrospray Ionization (ESI) ▶ Imaging Mass Spectrometry (IMS) ▶ Mass Spectrometry (MS) Some forms of ▶ neuronal plasticity affect LTP and LTD, which are already forms of plasticity. Therefore, the term ‘‘meta-plasticity’’ was introduced. N-Methyl-D-Aspartate Receptor Metergoline ▶ Methergoline M 757 Methergoline Synonyms Metergoline Methadone Definition Methadone is an opioid racemate drug that acts as an agonist on the m opiate receptor and is the best-studied substance available for opioid maintenance in terms of clinical effectiveness reducing illicit opioid consumption and reducing high-risk behavior such as needle sharing and increasing rates of treatment retention. Definition Methergoline is a synthetic compound that acts as a nonselective ▶ partial agonist at serotonin (5-hydroxytryptamine) receptors. It is potent at some subtypes of both 5-HT1 and 5-HT2 receptors but has little action at 5HT3 receptors. It also acts as a dopamine agonist. Cross-References ▶ Drug Discrimination Cross-References ▶ Opioid Dependence and Its Treatment Methotrimeprazine Methamphetamine ▶ Levomepromazine Synonyms Desoxyephedrine; Methylamphetamine; N-methylamphetamine Definition Methamphetamine is a ▶ psychostimulant and sympathomimetic drug that has a blood ▶ half-life of 9–15 h. The primary metabolite of methamphetamine is ▶ amphetamine, a chemical that itself is a potent psychostimulant. Methamphetamine is clinically available for the treatment of obesity, ▶ narcolepsy, and in some cases ADHD. Methamphetamine is a highly potent drug of abuse, with its illicit use reaching epidemic proportions in several Western countries including North American, Asian, and Pacific regions. Chronic exposure to methamphetamine can lead to schizophrenia-like psychosis and ▶ neurotoxic degeneration of dopaminergic neurons. Methylamphetamine ▶ Methamphetamine N-Methylamphetamine ▶ Methamphetamine N-Methyl-D-Aspartate Receptor Cross-References Synonyms ▶ Addiction ▶ Adolescence and Response to Drugs ▶ Amphetamine ▶ Attention Deficit Hyperactivity Disorder ▶ Dependence ▶ Dopamine ▶ Half-Life ▶ Narcolepsy ▶ Neurotoxicity ▶ Psychomotor Stimulants ▶ Sensitization to Drugs NMDA receptors Definition The N-methyl-D-aspartate receptor (NMDA) receptor is one of several subtypes of glutamate receptor. It is a voltage-sensitive ionotropic receptor (ligand-gated ion channel) that facilitates excitatory transmission of electrical signals between neurons by depolarizing the postsynaptic neuronal membrane. Although it is named after a selective agonist (NMDA), endogenous ligands for the receptor include glutamate or aspartate. Efficient channel M 758 M Methyl Benzene, Toluol opening also requires binding by the co-agonist glycine and a positive change in transmembrane potential. Activation of NMDA receptors results in the opening of an ion channel that is nonselective to cations, allowing the influx of Naþ and Ca2þ and the efflux of Kþ. In addition to facilitating excitatory neurotransmission, NMDA receptors are thought to play a key role in ▶ synaptic plasticity, thereby having an important role in learning and memory. Cross-References ▶ Excitatory Amino Acids and their Antagonists ▶ Glutamate ▶ Glycine Methyl Benzene, Toluol ▶ Toluene Methyl (1R,2R,3S,5S)-3-(Benzoyloxy)-8Methyl-8-Azabicyclo[3.2.1] Octane-2Carboxylate ▶ Cocaine Methylbenzylpropynylamine ▶ Pargyline Methyl Chloroform ▶ Trichloroethane Methylenedioxymethamphetamine (MDMA) IAIN S. MCGREGOR1, MURRAY R. THOMPSON1 PAUL D. CALLAGHAN2 1 School of Psychology, University of Sydney, Sydney, NSW, Australia 2 Radiopharmaceutical Research Institute, Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, NSW, Australia Synonyms E; Eccie; Ecstasy; Hug drug; Love drug; Love hormone; 3,4 Methylenedioxymethamphetamine; XTC Definition MDMA is a popular recreational drug that is renowned for its ability to produce euphoria and unique prosocial effects. It is the best known and most commonly used member of the family of phenethylamines (substitutes for ▶ amphetamines) that are sometimes known as ▶ entactogens, empathogens, or the MDxx class of drugs. MDMA has multiple neurochemical effects, the most prominent of which is to promote the release of serotonin via an action on the ▶ serotonin transporter (SERT). The prosocial effects of MDMA have recently been linked to the release of the neuropeptide ▶ oxytocin. High doses of MDMA can cause long-term depletion of serotonin in the brains of laboratory animals, but whether this also occurs in humans and whether this leads to associated psychopathology such as ▶ depression and ▶ cognitive impairment remains unclear. Pharmacological Properties History MDMA was first synthesized in 1912 by the company E. Merck. Although commonly thought to have been designed as an appetite suppressant, the original patent bears no record of this and simply states that MDMA was deemed to contain primary constituents for therapeutically active compounds. The first reported pharmacological study involving MDMA occurred in 1927 although basic toxicology studies were not undertaken until the 1950s. Further studies at the University of Michigan, supported by the US Army, reported LD50 values for five different species, with the lowest LD50 value found in dogs and the highest in mice. The first systematic use of MDMA was as an adjunct to insight-oriented psychotherapy, with administration of MDMA producing an easily controllable altered state of consciousness with positive emotional and sensual overtones. The colloquial term for MDMA changed from ‘‘Empathy’’ as was used by therapists in the 1970s to ‘‘Ecstasy,’’ emphasizing the drug’s euphoric effects. Heavy media attention in 1985 sensationalized Ecstasy’s euphoric effects and caused a surge in recreational use. In 1986, MDMA became a Schedule 1 drug in the United States, deemed to possess no recognized therapeutic value. By the 1990s, ▶ Ecstasy had become intrinsically linked to the club and rave culture, with its use by groups of young people attending all-night dance parties where Methylenedioxymethamphetamine (MDMA) vigorous dancing occurred to highly repetitive and hypnotic ‘‘techno’’ music. The popularity of MDMA has continued to grow to the point where it is now one of the most widely used illicit drugs in the world. Mechanisms of Action MDMA is a ring-substituted amphetamine, with a methylenedioxy group attached to the aromatic ring of amphetamine. It has multiple, complex pharmacological actions. The most important property is to potently release ▶ serotonin (5-HT) from axon terminals into the synapse and to inhibit 5-HT reuptake. To a lesser extent, MDMA also releases dopamine, noradrenaline, and acetylcholine. MDMA reverses the action of the SERT causing 5-HT stores from the neuron to be pumped into the synapse. An additional related action is to block the reuptake of 5-HT (▶ uptake), which further increases synaptic 5-HT concentrations. Pretreatment with SERT ligands including ▶ SSRIs (e.g., fluoxetine) prevents MDMA-induced 5-HT release in brain slices and in vivo. In addition to these effects on 5-HT efflux, MDMA-mediated inhibition of ▶ monoamine oxidase prevents the breakdown of 5-HT and other ▶ neurotransmitters such as dopamine, further contributing to elevated monoamine levels. A further effect of MDMA is to inhibit tryptophan hydroxylase (the rate-limiting enzyme for 5-HT synthesis). This effect may contribute to depletion of 5-HT stores in the days following MDMA use. MDMA possesses two stereoisomers: ()-MDMA has a higher affinity for postsynaptic 5-HT receptors while (+)-MDMA has a higher affinity for the SERT. The two isomers differ in their behavioral effects in rhesus monkeys and subjective effects in humans. These two isomers also differ in the rate in which they are metabolized across individuals which may result in large interindividual differences in the overall response to MDMA. MDMA also binds to various 5-HT receptors with moderate to high affinity. Receptor-binding studies indicate that MDMA possesses a high affinity for the 5-HT2 family of receptors and a moderate affinity for 5-HT1-type receptors. Activation of ▶ 5-HT1A receptors largely acts to inhibit serotonergic cell firing although the resultant inhibitory effects on 5-HT release are overridden through MDMA-induced effects at the SERT in forebrain regions. MDMA acts to increase synaptic ▶ dopamine levels, but these increases are generally smaller than the increases in 5-HT in any given region. MDMA-induced dopamine release may involve both an indirect ▶ 5-HT2A-receptor mechanism as well as a direct action on the ▶ dopamine transporter (DAT). Dopamine levels are also augmented by an action of MDMA on the vesicular monoamine M 759 transporter (VMAT2) causing dopamine efflux from vesicular stores via carrier-mediated exchange. MDMA also causes a significant release of ▶ norepinephrine via an interaction with the ▶ norepinephrine transporter (NET). ▶ Acetylcholine release also occurs in the prefrontal cortex and dorsal hippocampus following MDMA, and there is also the involvement of GABA, glutamate, nitrergic, and sigma (s1) systems. MDMA causes major endocrine changes (▶ neuroendocrine markers for drug action) including an increase in plasma oxytocin, vasopressin, cortisol, and prolactin. Pharmacokinetics Human ▶ pharmacokinetic studies show that MDMA’s distinctive effects occur at doses of 1 mg/kg or above with peak MDMA serum concentrations observed 2 h post administration, coinciding with peak psychological effects. MDMA has nonlinear pharmacokinetics, with increasing doses resulting in unpredictable blood/body concentrations. Like most other psychoactive drugs, MDMA is primarily metabolized by the liver via the ▶ cytochrome P450 family of enzymes, with the 2D6 isozyme particularly implicated. However, MDMA has a very complex metabolic pathway in comparison to other amphetamine analogs, and this may explain the sometimes complex and unpredictable relationship between Ecstasy tablet intake and acute effects of the drug. The primary metabolites of MDMA in humans, HHMA and HMMA, are readily broken down in the body to orthoquinones, highly reactive compounds that may lead to ▶ free radical-induced brain injury. Neurotoxicity Exposure to relatively high doses of MDMA can cause a long-lasting reduction in brain monoamine levels in a variety of animal species. While rats and primates show a primary reduction in brain 5-HT, mice show primary reductions in brain dopamine. Reductions in SERT density in cortical, limbic, and striatal regions has also been reported in many studies with rats and primates. Given that the SERT protein is primarily located in 5-HT axons, MDMA-induced axotomy has been invoked as the primary reason for this effect and has been confirmed in some histological studies. Abnormal 5-HT axonal immunoreactivity has been seen in primates 7 years post MDMA treatment. However, these pattern of findings do not confirm a neurotoxic effect in the classic sense. Gliosis is not typically observed following MDMA administration; nor is there any damage to serotonergic cell bodies. The widely discussed notion of MDMA-induced ▶ neurotoxicity therefore remains controversial (Baumann et al. 2007). M 760 M Methylenedioxymethamphetamine (MDMA) In addition to global SERT changes, alterations in the density of specific 5-HT receptor subpopulations can be seen following MDMA. Significant reductions in 5-HT2A receptor density in cortical, striatal, thalamic, and hypothalamic regions have been reported in rats months after MDMA treatment, although opposite findings on 5-HT2A receptor density have been reported in some human studies. ▶ 5-HT1B receptor density was reduced in MDMAtreated rats in the globus pallidus, ▶ hippocampus, and medial thalamus but increased in the ▶ nucleus accumbens and lateral septum. High ambient temperatures at the time of dosing, typical of the dance parties where MDMA is often taken, may exacerbate MDMA-induced 5-HT depletion. A ▶ neuroprotective effect of coadministered drugs (e.g., haloperidol, ketanserin, pentobarbitone, and various antioxidants) may result from an induction of hypothermia or by preventing the hyperthermic effects of MDMA. However, some drugs (e.g., ▶ cannabinoids) are protective independently of their body temperature effects. Some human studies have found that Ecstasy users differ from controls on a range of measures related to 5-HT, including a reduction of cerebrospinal 5-HIAA levels and a blunted ▶ neuroendocrine responses to serotonergic ligands. Decreased global and regional SERT density in Ecstasy users have been reported in some ▶ PET imaging and ▶ SPECT imaging studies although these are generally modest effects and may recover with abstinence from the drug. Positive Effects in Humans MDMA induces a positive mood state in humans along with increased energy and euphoria, typical of amphetamine and its derivatives. However, MDMA users also report a unique sense of intimacy and empathy coupled with an increased feeling of closeness to others (▶ entactogen) that is not always typical of amphetamine (▶ social behavior). In addition, MDMA users also report mild ▶ hallucinogen-like enhancement of perceptions and sensations with augmented responses to touch and music. Unlike amphetamines, MDMA appears to have relatively low ▶ abuse potential in humans, perhaps due to rapid ▶ tolerance developing to the positive effects with repeated use. SSRIs attenuate many of the acute psychological effects of MDMA in humans, consistent with a primary action of MDMA on SERT. SSRIs also reduce MDMAinduced heart-rate changes. Other studies showed that MDMA-induced perceptual changes and emotional excitation are partially mediated by post-synaptic 5-HT2A receptors since these effects can be attenuated by ketanserin (Liechti and Vollenweider 2001). The positive acute effects of MDMA in humans may involve other neurochemical systems. Thus, the ▶ antipsychotic drug ▶ haloperidol partially antagonized the positive and mania-like mood states induced by MDMA. In a recent laboratory study, the increased feeling of sociability after MDMA was associated with increased plasma levels of oxytocin in human subjects (Dumont et al. 2009). Effects in Laboratory Animals The acute effects of MDMA have been investigated in a diverse range of laboratory animal species. A key consideration in utilizing animal models is in establishing appropriate species-equivalent dosing levels to model human MDMA use (Green et al. 2009). This issue is still far from resolved with many animal studies using MDMA dose regimes that are in the extreme range. Species-specific ▶ pharmacokinetics also complicate the picture. MDMA has amphetamine-like sympathomimetic effects, increasing blood pressure and heart rate. It exerts a powerful influence on body temperature, with the direction of change (hyperthermia or ▶ hypothermia) dependent upon the ambient temperature of the environment (Green et al. 2003). The hyperthermic response to MDMA appears in part to be reliant upon the mitochondrial uncoupling protein 3 (UCP-3) acting in striated myocytes. MDMA also produces peripheral vasoconstriction, further preventing heat loss. Behaviorally, MDMA causes amphetamine-like ▶ hyperactivity and locomotor ▶ sensitization in rodent species. ▶ Intravenous self-administration of MDMA is seen in mice, rats, and nonhuman primates although rates are significantly less than that of other abused drugs such as the ▶ psychostimulants cocaine and methamphetamine. Self-administration of MDMA in rats is increased at high ambient temperatures, and this may be in part due to augmentation of MDMA-stimulated increases in dopamine and neuronal activation in reward-relevant brain regions. Rats will also show a ▶ conditioned place preference to MDMA, an effect that involves dopamine, ▶ opioid, and ▶ endocannabinoid systems. In line with its characteristic prosocial effects in humans, MDMA reduces aggression and increases social interaction in rodents. In the ▶ social interaction test, rats spend increased times in adjacent contact following acute MDMA treatment, and this effect is also augmented at high ambient temperatures. This prosocial effect of MDMA is Methylenedioxymethamphetamine (MDMA) reduced by oxytocin antagonists and is minimicked by the ▶ 5-HT1A agonist 8-OH-DPAT (McGregor et al. 2008). MDMA-Associated Hazards and Psychopathology Hyperthermia and other components of the ▶ serotonin syndrome are the main acute hazards facing human MDMA users, particularly when the drug is taken in high doses and in hot environments. Despite considerable media attention, lethal effects of MDMA (taken alone) appear comparatively rare. However, combining MDMA with other serotonergic drugs (e.g., ▶ monoamine oxidase inhibitors) can be extremely dangerous due to the possibility of ▶ serotonin syndrome. Other problems for users relate to the fact that Ecstasy tablets do not always contain MDMA, with a wide range of adulterants reported in analytical studies. Acute adverse psychological effects are occasionally reported with MDMA, most commonly ▶ anxiety and paranoia. A greater research focus, however, has been on possible lasting adverse psychological effects of MDMA use, effects that might be associated with serotonin depletion. In various studies, Ecstasy use has been linked to ▶ anxiety, ▶ depression, and ▶ mild cognitive impairment. However, many of these studies have inherent methodological problems. For example, Ecstasy users typically use other substances and coincident heavy cannabis use is a particularly troublesome confound in studies probing cognitive impairment after MDMA. There is also evidence that people with a preexisting childhood tendency toward anxiety and depression are more likely to become Ecstasy users, providing an additional confound. There is therefore a need for prospective longitudinal studies to control for premorbid psychiatric and cognitive problems in assessing MDMA-related harms. An example of this is the recent Netherlands XTC Toxicity (NeXT) study. This has uncovered subtle abnormalities in brain function in a sample of young persons taking MDMA for the first few times (de Win et al. 2008). Preclinical studies are also important in addressing the issue of whether MDMA exposure has lasting adverse consequences. Consistent, lasting adverse effects have been reported in a number of behavioral tests in rodents pretreated with MDMA. These include increased anxiety as assessed in the emergence test and the ▶ elevated plus maze, increased depressive-like symptoms in the Porsolt test (▶ Depression: Animal Models), and impaired ▶ novel object recognition and ▶ spatial memory. The ▶ social interaction test has been found to be particularly sensitive to detecting lasting adverse effects of MDMA in rodents, with decreased social behavior detected even M 761 months after low-dose MDMA exposure. Many of the above long-term effects are seen with low-dose regimes of MDMA that do not deplete brain 5-HT. As yet, unspecified neuroadaptations in nonserotonergic brain systems may therefore underlie these lasting adverse effects (McGregor et al. 2008). Therapeutic Uses Despite concerns relating to the neurotoxicity and possible psychopathology associated with MDMA use, a number of reputable scientists have called for further study of the use of MDMA as a therapeutic for anxiety and depression and relationship issues. This marks something of a return to the original use of MDMA as a tool for assisting ▶ interpersonal psychotherapy in the 1970s and 1980s. The Multidisciplinary Association for Psychedelic Studies (MAPS) (http://www.maps.org/mdma/) is currently sponsoring small clinical trials of MDMA in several countries for the treatment of ▶ traumatic stress disorder, and are also sponsoring a study of MDMA for alleviation of anxiety linked to terminal cancer. Conclusions MDMA is a controversial drug with a unique and complex pharmacology. No other drug, with the possible exception of GHB (▶ sodium oxybate), has the capacity to produce such marked facilitatory effects on ▶ social behavior in humans and other animal species. It is therefore encouraging to see that recent psychopharmacological studies of MDMA have started to focus on the positive prosocial effects of the drug in humans (Bedi et al. 2009; Dumont et al. 2009), and not just on its possible adverse effects and neurotoxicity. Despite a plethora of human and animal studies spanning more than two decades, experts cannot appear to reach a consensus on the relative harms associated with MDMA use: some claim MDMA is largely innocuous (Nutt 2009) while others proclaim its dangers (Parrott 2002). Fortunately, our overall knowledge of MDMA psychopharmacology continues to grow, as research studies involving both human Ecstasy users and laboratory animals given MDMA evolve in their sophistication, scope, and power. Perhaps given another decade of research, a greater consensus will emerge, and we will understand not only how MDMA acts in the brain to produce ‘‘chemical love’’ but also whether this is a good or a bad thing for the health of the individual. Cross-References ▶ Amphetamine ▶ Anxiety: Animal Models M 762 M 3,4 Methylenedioxymethamphetamine ▶ Depression ▶ Dopamine Transporter ▶ Hypothermia ▶ Neurotoxicity ▶ Serotonin Syndrome ▶ Serotonin Transporter ▶ Social Behavior ▶ Transporters ▶ Traumatic Stress Disorder (3R)-N-Methyl-3-(2-Methylphenoxy)-3Phenyl-Propan-1-Amine ▶ Atomoxetine Methylmorphine ▶ Codeine References Baumann MH, Wang X, Rothman RB (2007) 3, 4-Methylenedioxymethamphetamine (MDMA) neurotoxicity in rats: a reappraisal of past and present findings. Psychopharmacology (Berl) 189: 407–424 Bedi G, Phan KL, Angstadt M, de Wit H (2009) Effects of MDMA on sociability and neural response to social threat and social reward. Psychopharmacology (Berl) 207:73–83 de Win MM, Jager G, Booij J, Reneman L, Schilt T, Lavini C, Olabarriaga SD, den Heeten GJ, van den Brink W (2008) Sustained effects of ecstasy on the human brain: a prospective neuroimaging study in novel users. Brain 131:2936–2945 Dumont GJ, Sweep FC, van der Steen R, Hermsen R, Donders AR, Touw DJ, van Gerven JM, Buitelaar JK, Verkes RJ (2009) Increased oxytocin concentrations and prosocial feelings in humans after ecstasy (3, 4-methylenedioxymethamphetamine) administration. Soc Neurosci 4:359–366 Green AR, Mechan AO, Elliott JM, O’Shea E, Colado MI (2003) The pharmacology and clinical pharmacology of 3, 4-methylenedioxymethamphetamine (MDMA, ‘‘ecstasy’’). Pharmacol Rev 55:463–508 Green AR, Gabrielsson J, Marsden CA, Fone KC (2009) MDMA: on the translation from rodent to human dosing. Psychopharmacology (Berl) 204:375–378 Liechti ME, Vollenweider FX (2001) Which neuroreceptors mediate the subjective effects of MDMA in humans? A summary of mechanistic studies. Hum Psychopharmacol 16:589–598 McGregor IS, Callaghan PD, Hunt GE (2008) From ultrasocial to antisocial: a role for oxytocin in the acute reinforcing effects and long-term adverse consequences of drug use? Br J Pharmacol 154:358–368 Nutt DJ (2009) Equasy – an overlooked addiction with implications for the current debate on drug harms. J Psychopharmacol 23:3–5 Parrott AC (2002) Recreational ecstasy/MDMA, the serotonin syndrome, and serotonergic neurotoxicity. Pharmacol Biochem Behav 71:837–844 3,4 Methylenedioxymethamphetamine ▶ Methylenedioxymethamphetamine (MDMA) Methyl-Lorazepam ▶ Lormetazepam Methylphenidate Synonyms Dexmethylphenidate; Ritalin; Vitamin R Definition Methylphenidate (MPH) is the most commonly prescribed psychostimulant for the treatment of attentiondeficit hyperactivity disorder (juvenile and adult forms), although it has also been used in the treatment of narcolepsy and postural orthostatic tachycardia syndrome. It was first synthesized in 1944 and is chemically related to cocaine; it was originally formulated as a mixture of two racemates, 80% ()-erythro and 20% ()-threo, though its efficacy was later realized to derive from the threo isomer. Like most psychomotor stimulants, it acts to enhance ▶ dopamine release and block reuptake with additional effects on ▶ noradrenaline reuptake; it is not thought to affect central serotonin directly, unlike the prototypical psychomotor stimulant ▶ amphetamine, or ▶ cocaine. Its main cognitive effects are to reduce ▶ fatigue, enhance ▶ attention, and reduce ▶ impulsivity. Experimental studies in non-sleep-deprived healthy humans have indicated beneficial effects on ▶ working memory, leading to its current reputation as a ‘‘▶ cognition enhancer.’’ Its mode of action on cognition is probably in fronto-striatal circuits as shown from human imaging and animal studies. Its relatively slow onset of action when taken orally may well contribute to its relative lack of abuse potential in the conventional sense, although it has recently been shown to be a popular ‘‘cognition enhancer’’ in college students where it is sometimes called ‘‘Vitamin R.’’ Adverse effects of methylphenidate include slight growth retardation, appetite suppression, and, occasionally, motor tics. Long-term effects are largely unknown, although it has been suggested to ‘‘protect’’ ADHD juveniles from future drug abuse. Methylphenidate and Related Compounds Cross-References ▶ Attention Deficit and Disruptive Behavior Disorders ▶ Attention Deficit Hyperactivity Disorders: Animal Models ▶ Cognitive Enhancers ▶ Hypersomnia ▶ Impulse Control Disorders ▶ Impulsivity ▶ Methylphenidate and Related Compounds ▶ Pemoline ▶ Psychomotor Performance ▶ Psychomotor Stimulants Methylphenidate and Related Compounds NACHUM DAFNY Department of Neurobiology and Anatomy, University of Texas – Medical School at Houston, Houston, TX, USA Synonyms Ritalin Pharmacological Properties Several newspapers in the US recently reported the contention that healthy people should have the right to boost their brains with psychoactive drugs, drugs that are normally described for disorders such as attention deficit disorder (ADD), ▶ attention deficit/hyperactivity disorder (ADHD), ▶ narcolepsy, and memory-impairment in older individuals (▶ dementia). College students are already taking Ritalin as a ▶ cognitive enhancer before exams to help them study better, and some students contend that ‘‘We should welcome new methods of improving our brain function’’ and that doing it with pills is no more morally objectionable than eating right or getting a good night sleep (Houston Chronicle, December 8, 2008). But what do we know about this psychoactive drug? Psychoactive drug use is a practice that dates back to prehistoric times. A psychoactive drug is a central nervous system (CNS) stimulant that modulates consciousness, perception, mood, and behavior. There is archeological evidence of the use of psychoactive drugs as far back as several thousand years ago. These drugs were used therapeutically as medication, for ritual and spiritual purposes, as well as recreationally to alter one’s mood and to get ‘‘high.’’ Because psychoactive drugs elicit changes in consciousness and mood, the user feels alert, joyful, pleasant, and becomes M euphoric. Many psychoactive drugs are abused despite the risks of negative consequences, that is, ▶ dependence. Methylphenidate belongs to this family of drugs. What is methylphenidate? MPD is one of the most prescribed psychostimulants for the treatment of children and adults with ADHD. ADHD is characterized by a persistent pattern of inattention and/or hyperactivity, with impulsivity more often displayed and more severe than is typical for individuals at a comparable level of development. ADHD is a developmental disorder that affects as much as 5–15% of school-aged children in the US (Kollins et al. 2001). Methylphenidate is a CNS stimulant that is structurally closely related to ▶ amphetamine. The neuropharmacological profile of methylphenidate is also similar to that of ▶ cocaine (Volkow et al. 1999). The drug was first synthesized in 1944 and was used initially as an analeptic for several types of barbiturate-induced coma. It was later used as a drug to improve memory in elderly patients. Since then, its usage has been extended to improve alertness and attention in children and adults with emotional, behavioral, and learning difficulties. Methylphenidate is highly effective in treating ADHD. Methylphenidate may also be useful in providing relief from intractable pain in narcolepsy and chronic fatigue. When methylphenidate is given orally, it is absorbed from the intestinal tract and has a half-life time of about 1 h with equally short duration and efficacy (Solanto 2000). Its peak level following injection (systemic) is faster and is reached in 8–20 min post injection (Kuczenski and Segal 1997), a pattern that is similar to systemic cocaine and amphetamine administration (Volkow et al. 1999). When methylphenidate is given systematically, it binds with similar affinity to the ▶ dopamine transporter (DAT) and has a potency (Ki =200) similar to cocaine (Ki =224) (Kuczenski and Segal 1997). The relationship between drug doses (milligrams of hydrochloride salt/kilogram of body weight) and percentage occupancy of DAT is identical for cocaine and methylphenidate in rodents and humans (Gatley et al. 1999). The dose and route of administration are important because the features of the behavioral and neurochemical responses to the drug are dependent on the speed of the drug to reach the peak level, that is, the rise time of drug concentration (Kuczenski and Segal 1997). Peak levels of methylphenidate following intravenous (i.v.), intraperitoneal (i.p.), and oral administrations were 8–20 min, 15–28 min, and 60–90 min, respectively (Gerasimov et al. 2000). Similar peak levels of i.v., i.p., and oral administrations were obtained following amphetamine and cocaine (Gerasimov et al. 2000). The ability to reach the peak level in a short time (i.v., 8–30 min) is one of the main 763 M 764 M Methylphenidate and Related Compounds factors in eliciting adverse effects such as its euphoria, ▶ tolerance, and ▶ sensitization. Stimulants have been abused for both ‘‘performance enhancement’’ and for recreational purposes. For the former, they suppress appetite, facilitate weight loss, increase wakefulness, and increase focus and attention. Their euphoric effects usually occur when the stimulants are crushed and inhaled through the nose or injected. Methylphenidate is absorbed and metabolized via de-esterification to ritalinic acid and released into urine within 48 h. Brain concentrations of methylphenidate exceed that of plasma, since the psychostimulant is concentrated in catecholamine systems with free passage across the ▶ blood-brain barrier. Methylphenidate has a rapid uptake in the brain similar to amphetamine and cocaine, but differs from amphetamine and cocaine in that the rate of clearance from the brain is much slower. Intravenous (i.v.) or intranasal administration of methylphenidate is associated with a higher mortality rate than cocaine or amphetamine use, and every year more adolescents and young adults use (or misuse) the drug in these routes of administration. The outcome of methylphenidate treatment results primarily in dopamine (DA) release and inhibits the reuptake of DA, norepinephrine (NE), and serotonin (5-HT) (Kuczenski and Segal 1997). Like cocaine, methylphenidate is an indirect catecholamine agonist, since it does not stimulate the catecholamine receptors directly but rather facilitates the action of the catecholamine (Volkow et al. 1999). The therapeutic effects of methylphenidate in the treatment of ADHD has been attributed to its ability to increase the efflux of these neurotransmitters (Askenasy et al. 2007; Dafny and Yang 2006) by binding to their transporters and blocking the reuptake of these neurotransmitters. This causes increases in extracellular DA, NE, and 5-HT levels (Kuczenski and Segal 1997), which has an effect that has been linked to its reinforcing properties (Solanto 2000; Volkow et al. 1999). Methylphenidate has moderate effects on the peripheral circulatory system. In rats, methylphenidate administration in low doses (2.0–5.0 mg/kg) stimulates locomotor activity (Fig. 1) and elicits behavioral sensitization (Fig. 2.). In higher doses (10.0 mg/kg and higher), it stimulates stereotypical behavior and tolerance (Dafny and Yang 2006). The therapeutic effects of stimulants such as methylphenidate and amphetamine are achieved by slow and steady increases of DA, NE, and 5HT which are similar to natural production by the brain. The most commonly prescribed medication for ADHD patients include amphetamine (e.g., Adderall, a mixture of amphetamine salts), methylphenidate (i.e., Ritalin, which is short acting), and formulations such as Concerta that release methylphenidate over a longer period of time. Domestic sales of methylphenidate in the US showed an increase of almost 500% in the last decade. The therapeutic benefits of methylphenidate treatment for ADHD are clear. Moreover, concerns exist that during adolescent years, crucial neurodevelopment occurs with the production and elimination of numerous neuronal synaptic connections, that is, synaptic pruning. Children with ADHD who are going through neurodevelopmental processes are treated with methylphenidate for extended periods of time. Chronic treatment with psychostimulants such as methylphenidate and amphetamine can modulate these neurodevelopmental processes critically, which in turn may alter the body’s homeostasis. Any modulation produced by such psychopharmacological intervention in a still-developing brain should generate significant public health concerns. Additional concerns are raised that psychostimulant therapy given to adolescents and young adults may result in an increased risk for behavioral disorders (Robinson and Berridge 1993), while other reports have shown that psychostimulant treatment in adolescents with ADHD protects them from later substance use. These contradictory reports call for basic in-depth studies to resolve this critical issue. Animal models using behavior and neuronal recordings following acute and chronic methylphenidate treatment can be used to clarify this contradiction. Animal Models for Studies of Methylphenidate Methylphenidate is the drug most often used for treating children and adults with ADHD, for many years. It is expected that these patients would be used to study the prolonged effects of methylphenidate on their physiological and behavioral properties. However, there are ethical, methodological, and economic factors that limit research on children and adults exhibiting ADHD and on the therapeutic effects of methylphenidate. Therefore, research uses animal models for the understanding of the disorders. Researchers deal with a simpler system, which yields data that may be easier to interpret than that of a clinical case. In addition, animal models offer the possibility of understanding neurobiological processes that cannot be readily studied in humans. The question is which animal, and if a rat is selected, which strain? It has been argued that the most adequate model to study the physiological properties of methylphenidate is the one that best mimics a clinical case of ADHD and is able to predict aspects of ADHD behavior. There are differences between different strains of rats in the susceptibility to psychostimulants and their chronic effects such as tolerance or sensitization. Each strain of rats comprises a different gene pool. This results in different susceptibilities to Methylphenidate and Related Compounds M Methylphenidate and Related Compounds. Fig. 1. The figure summarizes the dose-response of acute methylphenidate injection on the horizontal activity and the number of stereotypic activities of female and male WKY, SHR, and SD rats, using the open field assay. Each group consists of N=8 and was given saline on experimental day 1 and methylphenidate on experimental day 2. The 0.6 mg/kg methylphenidate failed to elicit any changes in the horizontal activity of female and male WKY and SHR groups, while this low dose of methylphenidate significantly increased the horizontal activity of male SD rats (Fig. 1 – left column). The 2.5 mg/kg methylphenidate given i.p., increased significantly (*P<0.05) only the horizontal activity of male WKY and SD rats and female SHR and SD rats (Fig. 1 – middle column), as compared to that of the saline control group. Female SHR and SD exhibited significantly (D P<0.05) greater increase in activity compared to their male counterparts. The highest methylphenidate dose (10.0 mg/ kg) induced robust increase in locomotor activity when compared to baseline. * – indicates significant (P<0.05) difference when comparing the animal group to its control day, that is, experimental day 2 to experimental day 1. D indicates significant (P<0.05) difference between the sexes of each rat strain. 765 M 766 M Methylphenidate and Related Compounds Methylphenidate and Related Compounds. Fig. 2. This figure demonstrates behavioral sensitization. The embedded histograms in the upper right corner depict the total change from the baseline activity in the initial 2-h following injection at all administration times. The numbers indicate the experimental days. The figure summarizes 15 consecutive recordings (N=8) of four different locomotor activity indices. The locomotor recording of three control days (baseline) was set arbitrarily as 0. Following the control days, six single daily injections of 2.5 mg/kg methylphenidate (i.p.) were given at 07:00, and followed by 5 days of washout. Finally, a re-challenge injection of 2.5 mg/kg methylphenidate (i.p) was administered on experimental day 15. In all the four locomotor indices, the activity on experimental day 15 was significantly elevated as compared to the recording on experimental day 4 (1st day of methylphenidate injection). In the temporal graphs, the filled circles are the recordings after saline injection, while the squares denote the locomotor activity after 2.5 mg/kg methylphenidate. psychostimulants by different strains, and the long-term effects of the drug (sensitization or tolerance) can also be different in each strain. Since no biological marker for ADHD has yet been identified, diagnosis of ADHD is presently based only on behavioral symptoms. Many suggested ▶ animal models of ADHD exist, including rats selected from a general population, rats reared in social isolation, rats exposed to environmental pollutants, rats that have undergone neonatal anoxia, rats that have undergone hippocampal X-irradiation in infancy, rats that have undergone neurotoxic brain lesions, Naples High/Low excitability rats, Methylphenidate and Related Compounds and knockout mice. There are also genetic models, including the spontaneously hypertensive/hyperactive rat (SHR) strain, which was bred from progenitor Wistar Kyoto (WKY) rats (Dafny and Yang 2006). The SHR is a rat strain hyperactive in a variety of behavioral characteristics that are comparable to the behavior of children with ADHD, including motor and cognitive impulsiveness, impaired sustained attention, hyperactivity, and reduced DA function. Therefore, the SHR strain is used most often in ADHD/methylphenidate studies. Most investigators who study the properties of drugs on animals do so in the belief that their work will ultimately be relevant to people. Behavioral Models for Drug Dependence and Abuse The concept of drug ▶ abuse liability is related operationally to the ability of a drug to support ▶ self-administration. There are only two studies that investigated the selfadministration behavior of methylphenidate (Askenasy et al. 2007) and they reported that methylphenidate supported self-administration behavior. They conclude that the reinforcing effects of methylphenidate were similar to those of amphetamine and cocaine. The ▶ conditioned place preference paradigm is an experimental procedure in which, after training, an animal develops a preference for certain environment by virtue of its association with the rewarding state induced by a drug. The procedure consists of a box that is divided into two compartments that are qualitatively different from each other, for example in texture, color, smell, and connected to each other via a chamber that acts as a gateway between the compartments. The studies paired the drug (methylphenidate) with the less preferred compartment and they demonstrated that methylphenidate induced CPP in drug naı̈ve rats (Askenasy et al. 2007). Age differences in methylphenidate effects: The response to psychostimulants was reported to vary with age (Dafny and Yang 2006). During normal development, overproduction of synaptic connections and receptors occurs and follows by their pruning or competitive elimination. The marked overproduction and elimination of synapses and receptors during adolescence may serve as a permissive factor for a number of behavioral/psychiatric disorders, including ADHD (Andersen and Teicher 2000). Between 5 and 15 years of age, in humans, synaptic density in the frontal cortex decreases by approximately 40%. The time course and nature of ADHD parallel the pattern of overproduction and regressive synaptic elimination described earlier. Some adverse consequences of neuronal development in children were reported. Adolescent rats are affected differently by catecholaminergic agonists when compared with adult rats. It was reported that adolescent rats M exhibited an attenuated behavioral response, while adult rats exhibited an increase in behavioral response to psychostimulants (Laviola et al. 1995). Rats exposed to methylphenidate during the period equivalent to human adolescence experienced behavioral changes that endured into adulthood, which suggests that methylphenidate does have a neurobiological effect in adolescents that modulates the ‘‘normal’’ development to adulthood (Dafny and Yang 2006). Studies of cocaine-induced behavioral ▶ sensitization in developing animals have yielded conflicting results, depending upon the age tested, the drug maintenance dose, the intervals between the repetitive drug treatment, and the challenge dose. Adolescent rats showed behavioral sensitization to the locomotor activating effects of cocaine, whereas different locomotor sensitization profiles were found in adult rats (Laviola et al. 1995). However, others have reported that younger animals treated chronically with stimulants rarely exhibited behavioral sensitization and that when sensitization occurred, it persisted for a shorter period of time. When adolescent and adult rats were compared to their responses following chronic stimulant, adolescent rats showed alterations in psychopharmacological sensitivity which apparently did not rely on age-specific decreases in brain drug availability, but rather appeared to be related to alterations in CNS sensitivity (Laviola et al. 1995). It was also reported that adult rats repeatedly exposed to methylphenidate during adolescence were significantly more vulnerable to cocaine, as determined by increased selfinfusion of psychostimulants and increased motor activity. This suggests that adult responses to cocaine are altered following childhood methylphenidate exposure. The ontogeny of brain/behavior relationship during the period between preadolescence, adolescence, and attained sexual maturity needs to receive more attention. Sex Differences in Methylphenidate Effects Biomedical investigation has been conducted almost exclusively on male subjects. The reason for excluding females as subjects in the research is that they have greater biological complexity than males due to their reproductive cycle. It has only recently become evident that the gonadal steroid hormones have multiple functions. Furthermore, sex-related differences are often controversial and not documented. Differences in the response to cocaine and amphetamine are reported to be sexdependent. Observations of ▶ sex differences in response to drug treatment may be due to drug ▶ pharmacokinetics, particularly drug metabolism. The neural systems mediating the behavioral response to psychomotor stimulants are sexually dimorphic and are modulated by genes 767 M 768 M Methylphenoxy-Benzene Propanamine and pituitary and gonadal hormones. For example, estrogen enhances the acute behavioral and neurochemical responses to dopamine (DA), amphetamine, and cocaine in female rats. The effects of gonadal hormones are postulated to have important implications for gender differences in the acute and chronic responses and in the susceptibility of addiction to psychomotor stimulants. There are also remarkable gender differences in the behavioral expression of ADHD patients (Andersen and Teicher 2000). For example, ADHD is more often diagnosed in males than in females and is 2–9-fold more prevalent in males. Females with ADHD may be more severely affected than males, as female ADHD subjects tend to have a higher genetic loading for the disorder. It was hypothesized that there is an extensive overproduction of DA receptors in the male striatum and NAc during prepubertal development, which may help to explain why males are often afflicted with ADHD because dopaminergic activity increases in these regions can produce hyperactivity and stereotypical behavior. Sex differences in ADHD may also be attributed to sex differences in DA receptor density. Striatal D2 receptor density in males increases to 144 26% in between 35 and 40 days, while that in females increases only to 317%. The rise in striatal DA receptors in males parallels early development of ADHD motor symptoms (Dafny and Yang 2006). In general, females were more sensitive than males to methylphenidate, cocaine, and amphetamine. The development of behavioral sensitization to these drugs was a function of sex-specific alterations in the sensitivity to psychostimulants. In addition, accumulating evidence indicates that the antecedents, consequences, and mechanisms of drug abuse and addiction are different in females and males. It was reported that adult female rats were more seriously addicted to psychostimulants and express a more rapid behavioral sensitivity to chronic exposure of these drugs compared to their male counterparts. This sexual dimorphism was only observed in adult rats, suggesting that gonadal hormones secreted in adulthood may modulate the responsiveness to psychostimulants (Dafny and Yang 2006). Dafny N, Yang PB (2006) The role of age, genotype, sex, and route of acute and chronic administration of methylphenidate: a review of its locomotor effects. Brain Res Bull 68:393–405 Gatley SJ, Volkow ND, Gifford AN, Fowler JS, Dewey SL, Ding Y-S, Logan J (1999) Dopamine-transporter occupancy after intravenous doses of cocaine and methylphenidate in mice and human. Psychopharmacol 146:93–100. Gerasimov MD, Franceschi M, Volkow ND, Gifford A, Gatley SJ, Marsteller D, Molina PE, Dewey SL (2000) Comparison between intraperitoneal and oral methylphenidate administration: a microdialysis and locomotor activity study. J Pharmacol Exp Ther 296:51–57 Kollins SH, MacDonald EK, Rush CR (2001) Assessing the abuse potential of methylphenidate in non-human and human subjects: a review. Pharm Biochem Behav 68:611–627 Kuczenski R, Segal DS (1997) Effects of methylphenidate on extracellular dopamine, serotonin, and norepinephrine: comparison with amphetamine. J Neurochem 68:2032–2037 Laviola G, Wood RD, Kuhn C, Francis R, Spear LP (1995) Cocaine sensitization in periadolescent and adult rats. J Pharmacol Exp Ther 275:345–357 Robinson TE, Berridge KC (1993) The neural basis of drug craving: an incentive-sensitization theory of addiction. Brain Res Rev 18:247–291 Solanto MV (2000) Clinical psychopharmacology of AD/HD: implications for animal models. Neurosci Biobehav Rev 24:27–30 Volkow ND, Wang GJ, Fowler JS, Fischman M, Foltin R, Abumrad NN, Gatley SJ, Logan J, Wong C, Gifford A, Ding YS, Hitzemann R, Pappas N (1999) Methylphenidate and cocaine have a similar in vivo potency to block dopamine transporters in the human brain. Life Sci 65:PL7–PL12 Methylphenoxy-Benzene Propanamine ▶ Atomoxetine N-Methyl-N-2-Propynylbenzylamine ▶ Pargyline (–)-1-Methyl-2-(3-Pyridyl)Pyrrolidine Cross-References ▶ ADHD ▶ Psychostimulants References Andersen SL, Teicher MH (2000) Sex differences in dopamine receptors and their relevance to ADHD. Neurosci Biobehav Rev 24:137–141 Askenasy EP, Taber KH, Yang PB, Dafny N (2007) Methylphenidate (ritalin): behavioral studies in the rat. Int J Neurosci 117:1–38 ▶ Nicotine 8 b[(Methylthio)Methyl]-6Propylergoline Monomethanesulfonate ▶ Pergolide Microdialysis Mexazolam Definition Mexazolam is a benzodiazepine derivative that has anxiolytic, anticonvulsant, hypnotic, sedative, amnesic, and muscle-relaxant properties. Cross-References ▶ Anxiolytic ▶ Benzodiazepine mGluRs ▶ Metabotropic Glutamate Receptor Mianserin Definition Mianserin is a tetracyclic second-generation ▶ antidepressant with combined serotonergic–noradrenergic mechanism of action. It increases serotonergic (5HT) and noradrenergic (NA) neurotransmission by acting as an antagonist mainly at 5-HT2 and a2 presynaptic and somatodendritic auto- and hetero-receptors. This drug also has a strong antihistaminic effect but, unlike the ▶ tricyclic antidepressants, it has almost no anticholinergic and cardiotoxic properties. In addition to its antidepressant effects, mianserin also has anxiolytic, sedative-hypnotic, antiemetic, and appetite-enhancing effects. Clinical effects of mianserin usually become noticeable after 1–3 weeks of treatment. Common side effects include dizziness, blurred vision, drowsiness, weight gain, dry mouth, and constipation while more serious adverse reactions may include hypomania, fainting, seizures, and hematological problems. As with other antidepressants, abrupt or rapid discontinuation of mianserin therapy may induce withdrawal effects, such as rebound depression, anxiety, panic attacks, anorexia, and insomnia. Cross-References ▶ SNRI Antidepressants M Microamines ▶ Trace Amines Microdialysis ALBERT ADELL1,2,3, FRANCESC ARTIGAS1,2,3 1 Department of Neurochemistry and Neuropharmacology, Instituto de Investigaciones Biomédicas de Barcelona, Consejo Superior de Investigaciones Cientı́ficas (CSIC), Barcelona, Spain 2 Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain 3 Centro de Investigación Biomédica en Red de Salud Mental (CIBERSAM), Barcelona, Spain Synonyms Brain microdialysis; Intracerebral microdialysis Definition Brain microdialysis is a sampling technique developed to study the concentration of chemical solutes (mainly neurotransmitters) in the extracellular compartment of the brain by means of implanting a tubing of dialysis membrane. During the last decades, the necessity to measure the release of neurotransmitters in vivo in the central nervous system (CNS) has prompted the development of innovative techniques for sampling the extracellular fluid in the brain of experimental animals. Historically, one of the methods that evolved for this purpose was the push–pull perfusion which involved the stereotaxic insertion of a push–pull cannula into a selected area of the brain. Being an open flow system, push–pull perfusion allowed a direct contact of perfusion fluid with brain tissue, which often caused tissue damage, microbial and blood contamination, etc. To circumvent such drawbacks, a semipermeable membrane was attached to the cannula tip and this device was called dialysis bag or dialytrode. This was soon replaced by a more straightforward approach, the intracerebral dialysis, in which the dialysis bag was substituted by a hollow fiber, the dialysis membrane (Ungerstedt 1984). Principles and Role in Psychopharmacology Michael Kohlhaas’ Syndrome ▶ Delusional Disorder 769 The term dialysis refers to the passage of small molecules through a semipermeable membrane, a process driven by a concentration gradient. The endogenous substances diffuse out of the extracellular fluid into the perfusion M 770 M Microdialysis medium. On the other hand, exogenous compounds can be infused locally through the dialysis probe and reach the brain compartment through this concentration gradient. Unlike push–pull perfusion, dialysis is based on a closed flow system. Therefore, only a single perfusion pump is needed. The probe is constantly perfused with a physiological solution at a low flow rate (typically <2 mL/min) and perfusate samples are then collected for further analysis. Due to its relative ease of use, microdialysis has become the technique of choice for the in vivo analysis of neurotransmitters in the extracellular compartment of the brain of experimental animals. Its use has been fundamental in the identification of the mechanism of action of numerous psychoactive drugs. In particular, microdialysis has enabled to clarify neuronal elements (neurotransmitters, receptors) and brain networks affected by two of the most important drug classes in psychiatry: ▶ antidepressants and ▶ antipsychotics (Artigas and Adell 2007). The low concentration of endogenous neurotransmitters in the extracellular brain space has been one of the main difficulties associated with the dialysis technique. The development of highly sensitive ▶ high-performance liquid chromatographic (HPLC) methods has made possible the increasing use of the microdialysis technique for the in vivo analysis of nanomolar concentrations of neurotransmitters and their metabolites (usually at higher concentrations) in brain. Capillary electrophoresis has also been successfully applied to the analysis of amino acids and amines. However, the need to sample for relatively long periods of time (typically >10 min) is one of the main drawbacks of microdialysis, compared with other in vivo techniques assessing brain function, such as electrophysiology and ▶ electrochemical techniques. The dialysis membrane constitutes a real barrier between the perfusion fluid and the interstitial brain space, which usually excludes the transport of large molecules that may interfere with the substances of interest in the analytical procedure. Furthermore, enzymes that could cause a breakdown of the neuroactive compounds are also prevented from being picked up by the dialysate. The implant and functioning of microdialysis probes may cause tissue reactions ranging from an excessive washout of neurotransmitters and metabolites if flow rates are too high, to glial reaction surrounding the probe that may act as an actual barrier for the passage of components from the extracellular brain space to the inner part of the microdialysis probe. These aspects need to be examined in detail while establishing the experimental protocols. In particular, the effect of flow rate and duration of experiments need to be carefully assessed. Methodology Microdialysis probes are implanted stereotaxically in the brain of anesthetized animals. The coordinates for rat or mouse brain are usually taken from the corresponding atlas (Franklin and Paxinos 1997; Paxinos and Watson 2005). This allows a theoretical precision of 0.1 mm in the placement of microdialysis probes, although the actual precision is impaired due to individual variations in the size and shape of brain of experimental animals. Once the dialysis probe has been positioned in the area of the brain to be studied, flushing with artificial cerebrospinal fluid (CSF) is recommended in order to check the integrity of the membrane. Then, the probe is secured to the skull with anchor screws and dental cement. Construction of a Dialysis Probe At present, the type of dialysis probe most commonly used has a concentric structure (Fig. 1). This kind of probe has been used in a wealth of experimental research Microdialysis. Fig. 1. Schematic representation of a concentric microdialysis probe made up of the following components (see text for details): 27 gauge stainless steel tubing (1), 25 gauge stainless steel tubing (2), epoxy resin (3 and 9), dental cement (4), hot-melt adhesive (5), polyethylene tubings (6 and 11), fused silica capillary tubings (7 and 10), and dialysis membrane (8). Small arrows indicate the interchange process through dialysis membrane. Larger arrows indicate the direction of perfusion fluid. Microdialysis because it is well suited for reaching deep structures and/ or small nuclei of the brain. A detailed description of the materials and suppliers can be found in Adell and Artigas (1998). Briefly, the body of the probe is made up of 20-mm long 25 gauge stainless steel tubing. The inflow and outflow tubes threaded through the 25 gauge tubing consist of fused silica capillary tubing of 0.11 mm OD, 0.04 mm ID. The upper exposed end of fused silica tubings are inserted into a 7-mm piece of 27 gauge (0.41 mm OD, 0.20 mm ID) stainless steel tubing. The junction of the 27 and 25 gauge stainless steel tubings is sealed with epoxy glue and covered with dental cement to harden the assembly. Sampling of monoaminergic neurotransmitters usually requires a dialysis membrane consisting of a regenerated cellulose hollow fiber (0.17 mm OD, 0.15 mm ID), with a molecular weight cutoff of 6,000 Da, which is placed over the protruding lower portion of the inlet fused silica tubing and glued with epoxy resin to the inside surface of the 25 gauge stainless steel tubing. The tip of the hollow fiber is sealed also with epoxy glue. The length of the dialysis membrane exposed to the tissue varies according to the brain area to be examined. Finally, the 27 gauge protective steel tubes are friction-fitted with 20-mm lengths of polyethylene tubing of 0.61 mm OD, 0.28 mm ID to facilitate the connection of the probe to the perfusion pump and outflow line. These polyethylene and steel tubes are secured together with hot-melt glue. Perfusion Fluids One of the crucial aspects in microdialysis studies is that the composition of the perfusion medium must be physiological, i.e., isotonic with respect to that of the interstitial space. However, several fluids are being used currently that differ in their electrolytic composition/concentration (reviewed by Benveniste and Hüttemeier 1990). With little variation, the fluids used to perfuse dialysis probes are those derived from Krebs–Ringer solutions or artificial CSF. Typically, the concentration of Ca2+ ions in the perfusion fluid may vary from the physiological 1.2 mM– 3.3 mM. Since Ca2+ ions are essential for the process of exocytosis, some authors have used higher concentrations to stimulate transmitter release. Although the buffering capacity of the extracellular fluid for some cations such as K+ and Ca2+ is high, differences in the ionic composition of the perfusion and interstitial fluids may alter the responsiveness of neurons. Other perfusing solutions also contain glucose to prevent its depletion from the interstitial space produced by the continuous drainage of dialysate. Glucose addition to the perfusion fluid also provides the essential nutrient for neurons to cope with the cell damage and disruption of the M ▶ blood-brain barrier caused by probe implantation. However, the concentration of lactate, pyruvate, aspartate and glutamate in rat cortical dialysates is similar when the perfusion fluid contains 0 or 3 mM glucose, which suggests that the presence of glucose does not play a critical role in neuronal metabolism during microdialysis experiments. In addition, it should be kept in mind that the presence of glucose may favor bacterial growth in the perfusion fluid, thus altering the extraction of neurotransmitters. One common problem inherent to most microdialysis studies is the very low concentration of neurotransmitters in the dialysate caused by efficient mechanisms of removal from interstitial space, such as reuptake or enzymatic degradation. To circumvent this complication, an uptake blocker or an inhibitor of enzymatic breakdown is included in the perfusion medium. In the absence of such agents, the extracellular concentration of a transmitter (but not of its metabolites) reflects the balance between the processes of release and inactivation. However, in the presence of such agents, the release component is amplified and this has to be taken into consideration while interpreting dialysis results. For example, the use of an uptake blocker may allow to detect changes in the extracellular level of transmitters that otherwise may be overlooked. The most common compounds added to the perfusion fluid are uptake inhibitors such as ▶ citalopram for serotonin, ▶ nomifensine for dopamine, desipramine for norepinephrine, and ▶ physostygmine or neostygmine to block the enzymatic degradation of acetylcholine. However, the use of such agents can complicate the interpretation of results, as the higher neurotransmitter concentration they induce may result in the activation of terminal autoreceptors in nerve endings, which usually results in a negative feedback affecting neurotransmitter synthesis and release. On the other hand, the addition of ▶ acetylcholinesterase inhibitors, which increase the concentration of acetylcholine in dialysates, has been shown to influence markedly the interaction between cholinergic and dopaminergic brain systems. Therefore, these modified perfusion fluids must be used with caution, as they may alter the function of drugs whose mechanism of action is examined. The choice of an appropriate flow rate for the perfusion (usually ranging between 0.1 and 2 mL/min) is an important practical point for several reasons. The relative recovery of neurochemical compounds through dialysis membranes declines as the flow rate increases. High flow rates generate a concentration gradient and compounds can be carried away from the extracellular space with the subsequent enrichment of dialysate samples so that the absolute recovery per time unit is greater. However, such 771 M 772 M Microdialysis a washing effect may reduce the tone on terminal autoreceptors and, therefore, alter the dynamics of the release process. For all of the above reasons, low flow rates are preferred in order to approach ideal dialysis conditions and maximize the recovery of transmitter substances from the interstitial space. Flow rates of 0.25–3 mL/min provide a sample volume of 5–60 mL per 20-min fraction, which can be collected in plastic microvials and is easy to handle for further analysis by HPLC procedures. Quantitative Aspects Different attempts have been made to calculate the extracellular concentration of neurotransmitters stemming from the level obtained in dialysate samples. The simplest method is to calibrate the probes for in vitro recovery. To this aim, the probes are immersed in a beaker filled with a known concentration of the substances of interest dissolved in the perfusion fluid. The ratio between the concentration of the substance in the probe effluent and in the medium provides the recovery value of such a substance. The term ‘‘absolute recovery’’ refers to the total amount of a compound that passes into the perfusion fluid per unit of time, whereas ‘‘relative recovery’’ describes the concentration of a compound in the dialysate relative to that in the perfusion medium expressed as a percentage value. The flow rate of the perfusion fluid is inversely related to the relative recovery and the magnitude of absolute recovery is limited by perfusion flow rate. It is important to note herein that the absolute recovery is proportional to the concentration of the substance outside the dialysate, whereas the relative recovery is not. In addition, the calibration in vitro of the probes depends on the temperature and physicochemical stability of the compounds to be analyzed. The validity of these calibration procedures is based on the assumption that the conditions in vitro and in vivo are similar. However, the brain interstitial space is a more complex matrix and the tortuosity of the diffusion created by cell membranes and the drainage of endogenous compounds induced by the continuous perfusion are factors that must be taken into consideration. Several refined mathematical models have been described to determine the actual concentration of transmitters in the extracellular compartment of the brain, yet they are usually too complex to be used routinely in neuropsychopharmacology research. A more practical approach was reported in which the dialysate concentration of a neurotransmitter is measured at different flow rates and extrapolated to a flow rate of zero (Justice 1993). With no net flow, the dialysate is in equilibrium with the extracellular fluid. Therefore, the level found at zero flow should represent the actual in vivo concentration of the transmitter. Finally, it should be considered that, for many applications, the knowledge of the in vivo concentration of a transmitter is not necessary. Instead, the change with respect to baseline level is what actually matters and is to be related to the mechanism of action of drugs. Statistical Analysis of Data The working hypothesis usually tested in microdialysis experiments tests whether a physiological or pharmacological manipulation affects the concentration of a transmitter in dialysate samples. Typically, the experimental approach consists in the collection of several pretreatment samples until stable baseline values are attained. Then, drugs are administered or animals are subjected to certain procedures (e.g., stress, forced motor activity, behavioral manipulation, etc.) and a number of posttreatment samples are collected. These temporal data series are often analyzed by means of analysis of variance (ANOVA) for repeated measures followed by appropriate post hoc tests to compare pre- and posttreatment periods. In more complex experiments (e.g., when assessing the effects of different drug doses or in various areas of the brain), the use of two-way ANOVA for repeated measures is better suited, with dose (or region) as the independent factor and time (or treatment) as the dependent variable. AUCs of the posttreatment periods can be also calculated as an integrated estimate of drug action and compared by means of one- or two-way ANOVA. Neural Origin of Transmitters in Dialysate In order to determine the neural origin of neurotransmitter efflux measured in dialysis experiments, several specific criteria must be fullfilled. First, basal transmitter release from nerve terminals has to be impulse-dependent. This is usually assessed by the addition of the sodium channel blocker tetrodotoxin, which impairs the release of the transmitter. A second requirement for the neuronal origin of a putative transmitter is its disappearance or decrease from dialysate, when Ca2+ is omitted from the perfusion medium. The basis for such an action is that the impulsedependent release of a transmitter by exocytosis is dependent on the availability of extracellular Ca2+ (Augustine et al. 1987). Finally, the ability of elevated concentrations of K+ to depolarize neural structures and stimulate the output of transmitters has been taken as an additional criterion for their neural provenance. Working Practices As detailed in the preceding sections, microdialysis is only a sampling procedure. The combination with appropriate Microdialysis analytical techniques has made it possible to monitor changes in the concentration of small molecules in the interstitial space of the CNS. In addition, researchers have devised a number of different experimental approaches to exploit the capabilities of this technique. Local Administration of Chemicals and Drugs Due to the ability of the microdialysis membrane to allow the passage of small molecules in both directions, microdialysis probes have been used to deliver chemicals in restricted areas of the brain by reverse dialysis. Ions or pharmaceutical agents known to affect neural function can be dissolved in the perfusion fluid and delivered to the brain structures of interest, provided that the molecular weight cut-off of the membrane is appropriate. Changes in the concentration of transmitters can thus be monitored locally or distally, by means of a second dialysis probe implanted in an area anatomically or functionally related to the brain structure in which the first probe is located (see Section 7.3). When appropriate amounts of chemical agents or drugs have to be dissolved in the perfusion fluid, it is important to check that they do not alter pH or osmolarity of the fluid. Usually, once stable baseline values are obtained, the standard dialysis fluid is replaced by one containing the compound(s) of interest. This procedure may be particularly useful when examining the effects of substances with a poor penetration into the brain or when assessing regional differences in the effects of drugs. Quantitative effects of drugs in vivo can be estimated through ▶ ED50 values calculated after local application of drugs. The ED50 values obtained in this manner are by no means comparable to those obtained using cell cultures, membranes, synaptosomes, or other in vitro preparations. Several factors account for these differences, including (1) the recovery of the dialysis membrane, usually much lower than 100%; (2) the diffusion of chemicals within the brain once they have crossed the dialysis membrane and the tortuosity of the neural tissue; (3) the continuous drainage of applied drugs by the CSF; and (4) the unspecific binding to cell membranes, particularly of lipophilic molecules. All these factors contribute to reduce the actual concentration of drugs reaching the active sites in the brain, dramatically. In contrast, in vitro drug affinities for receptors/transporters are calculated under almost ideal conditions, i.e., with enriched preparations and unlimited access of the chemicals to their cellular targets, generally at equilibrium and under nondegrading conditions. Systemic Administration of Drugs Experiments involving the systemic administration of drugs constitute the vast majority of the applications of M 773 microdialysis. In such experiments, however, appropriate controls must be carried out, because the procedure of drug delivery or the vehicle used may change transmitter function due to the associated stress or the sensitivity of some neuronal groups to sensory stimuli. The changes in the concentration of transmitters in an area of the brain after systemic administration of drugs do not necessarily parallel those found after their local application. In general, when drugs are applied locally, larger concentrations are needed to reach effects similar to those obtained after systemic application. This possibly reflects a better distribution of drugs administered systemically through the diffuse network of brain capillaries. Another factor frequently ignored is the fact that the diffusion of a chemical agent delivered by reverse dialysis is limited to a small portion of the brain tissue surrounding the dialysis probe. In contrast, the changes in the extracellular concentration of the transmitter in the same area after systemic administration of a drug results from an integrated response of the whole CNS, i.e., local and transynaptic effects. Dual Probe Models Experiments carried out with two or more probes implanted in the same animal present two distinct advantages. First, such an approach allows to reduce the number of animals used in a single experiment and, second, it is ideal to examine functional interactions between different brain areas. This latter asset was first employed for dopamine and serotonin systems to study how the release in terminal areas is regulated by the activity at the level of cell bodies. This was possible because the cell bodies of those neuronal systems are tightly packed in the midbrain substantia nigra, ventral tegmental area, and raphe nuclei. Therefore, the local application of drugs known to interact with receptors or transporters located on these monoaminergic cell bodies induces changes of the release of the transmitter in projection areas. Dual probe microdialysis studies have been extremely helpful for the study of the functional connections between different brain areas and the transmitter/receptors involved (Adell and Artigas 1991; Santiago et al. 1991). Coupling to Electrical Stimulation Similar to the experiments described in the previous section, electrical stimulation coupled with microdialysis in distal areas has been used to assess the existence of functional connections between brain areas. This is usually achieved by inserting an electrode in an area containing the cell bodies of a certain transmitter system and a dialysis probe in the corresponding projection areas. For instance, the electrical stimulation of the substantia nigra M 774 M Microdialysis or the raphe nuclei results in an enhanced release of dopamine and serotonin, respectively, in projection areas. Similar experimental procedures combining electrical stimulation and microdialysis have been used to study the modulatory role of ▶ prefrontal cortex on dopaminergic and cholinergic activity in subcortical structures such as the dorsal striatum or the ▶ nucleus accumbens. Advantages and Limitations of Microdialysis Since its first applications, microdialysis has become increasingly popular to study brain function. The use of alternative in vivo procedures such as push–pull perfusion or voltammetry has remained constant or even declined during last years. A comparison between microdialysis and voltammetry reveals that microdialysis is applicable to most types of small molecules whereas the use of voltammetry is limited to easily oxidizable compounds such as catecholamines and serotonin. Moreover, microdialysis appears to be simple to use on a routine basis and can easily be applied to study freely moving animals. Table 1 summarizes some of the advantages and limitations of microdialysis. Certainly, microdialysis is by no means a definitive method for the assessment of the active transmitter concentrations in the brain. Yet, it has a number of advantages over its predecessor, the push–pull perfusion, which have led to a more widespread use. The main limitations of microdialysis are the size of the probes and the tissue damage caused by their insertion. For certain applications, size may not be a problem (e.g., to assess the effects of drugs in large brain regions). However, the study of physiologically- or pharmacologically-induced changes of transmitters in small nuclei may pose some constraint because a larger proportion of neurones is damaged. Finally, the low amount of certain neurotransmitters in brain dialysates makes it necessary to collect samples every 20 or 30 min, a time scale which is far from that of neuronal events. This may not be a problem in pharmacological studies because most drugs reach peak levels at a time compatible with the usual periods of sampling of 20 or 30 min. This enables to follow up drug-induced transmitter changes. However, microdialysis may not be suitable for the study of the effects of neuronal stimulation on transmitter release at a physiological time scale. Recent advances in the detection of very low concentration of certain transmitters with capillary electrophoresis have permitted a considerable shortening of the sampling periods. Yet, this is still far from the scale at which neuronal excitation or inhibition is associated to the release of a transmitter. It is hoped that future methodological and technical developments will overcome some of these limitations. Cross-References Microdialysis. Table 1. Advantages and limitations of the microdialysis technique. Advantages Easy to use routinely Easy manufacture of probes No enzymatic degradation of transmitters in samples Coupling to chemical methods of analysis (HPLC, mass spectrometry, capillary electrophoresis, etc.) Possibility of local administration of drugs Possibility of concurrent determination of drugs after systemic administration Dual probe approaches Possibility of concurrent recording of electrical activity Concurrent study of behavior in freely-moving animals Limitations Invasive procedure: causes neuronal death and reactive gliosis Limited spatial resolution Limited temporal resolution Analytical difficulties with some transmitters Low membrane recoveries with high molecular weight compounds ▶ Antidepressants ▶ Antipsychotic Drugs References Adell A, Artigas F (1991) Differential effects of clomipramine given locally or systemically on extracellular 5-hydroxytryptamine in raphe nuclei and frontal cortex. An in vivo brain microdialysis study. Naunyn-Schmiedebergs Arch Pharmacol 343:237–244 Adell A, Artigas F (1998) In vivo Brain microdialysis: principles and applications. In: Boulton AA, Baker GB, Bateson AN (eds) In vivo neuromethods. Humana Press, Totowa, pp 1–33 Artigas F, Adell A (2007) The use of brain microdialysis in antidepressant drug research. In: Westerink BHC, Cremers TIFH (eds) Handbook of microdialysis: methods, applications and clinical aspects. Elsevier/ Academic, Amsterdam, pp 527–543 Augustine GJ, Chanton MP, Smith SJ (1987) Calcium action in synaptic transmitter release. Ann Rev Neurosci 10:633–693 Benveniste H, Hüttemeier PC (1990) Microdialysis-theory and application. Progr Neurobiol 35:195–215 Franklin KBJ, Paxinos G (1997) The mouse brain in stereotaxic coordinates. Academic, San Diego Justice JB Jr (1993) Quantitative microdialysis of neurotransmitters. J Neurosci Meth 48:263–276 Paxinos G, Watson C (2005) The rat brain in stereotaxic coordinates. Elsevier/Academic, Amsterdam Santiago M, Rollema H, De Vries JB, Westerink BHC (1991) Acute effects of intranigral application of MPP+ on nigral and bilateral striatal Microiontophoresis and Related Methods release of dopamine simultaneously recorded by microdialysis. Brain Res 538:226–230 Ungerstedt U (1984) Measurement of neurotransmitter release by intracranial dialysis. In: Marsden CA (ed) Measurement of neurotransmitter release in vivo. Wiley, Chichester, pp 81–105 Microelectrode Arrays Synonyms MEA Definition Ceramic-based multisite microelectrode arrays (MEAs; 15 mm 330 mm or 20 mm 150 mm recording sites) with platinum recording sites and polyimide insulation that have been recently described (Hascup et al., 2007). The triangular design of the microelectrodes yields a microelectrode array ranging in thickness from 37 to 125 mm and an overall length of 8–10 mm. These arrays are wire bonded to printed circuit board holders that adapt MEA’s for measures in brain slices and studies in anesthetized and awake mice, rats, and nonhuman primates. More than 20 varieties of geometries ranging from 4 to 16 Pt recording sites have been designed. Microelectrophoresis ▶ Microiontophoresis and Related Methods Microiontophoresis and Related Methods ROBERTO WILLIAM INVERNIZZI1, ENNIO ESPOSITO2 1 Department of Neuroscience, Mario Negri Institute for Pharmacological Research, Milan, Italy 2 Laboratory of Neurophysiology, Consorzio ‘‘Mario Negri’’ Sud, Santa Maria Imbaro (Chieti), Italy Synonyms Iontophoresis Definition The term microiontophoresis is derived from the ancient Greek term phoretikos, which refers to the production or induction of movement. Microiontophoresis is a technique with which drugs and other ionized particles can M be ejected in very small amounts from solutions contained in glass micropipettes. This ejection is accomplished by applying a voltage across the micropipette and causing the electrode to become polarized. Ionized particles in solution migrate in the applied field and will be ejected from the tip as they carry the current into the tissue. This technique is widely used to determine the effects of various substances on firing parameters of both central and peripheral neurons and muscles. In investigating the phenomenon of synaptic transmission at the neuromuscular junction, during the 1950s, this technique became very popular. A technique appropriate for the study of synaptic pharmacology was first realized by Nastuk (1953) and was later developed by del Castillo and Katz (1955), and it consisted essentially of the microiontophoretic method, i.e., movement of charged particles produced by an electric current, restricted to a micropipette with a tip diameter of the order of 1mm. Thus, solutions of ▶ acetylcholine chloride were used, and by passing a suitable current to this solution, acetylcholine ions could be ejected from the 1mm orifice onto a correspondingly localized area of subsynaptic membrane at the neuromuscular junction. Later, Curtis and his colleagues adopted this technique for studying the mammalian central nervous system (CNS) (Curtis and Eccles 1958b). The experiments of Curtis and coworkers, however, involved an important modification of the original method, in that this group used ▶ multibarrel micropipettes. In the production of these, several lengths of tubing are fused together and then pulled so as to produce a single collective tip, but with each barrel having its own orifice. Multibarrel micropipettes are usually composed of five to seven barrels (Fig. 1). Usually, the central barrel is the recording electrode, whereas the other side barrels contain drug solutions (Fig. 1). As the drug molecules would tend to diffuse from solution in the pipette tip into the extracellular environment, it is necessary to apply a small current to reduce that efflux. This is known as a ‘‘holding’’ or ‘‘retaining’’ current (Fig. 2). It is also a usual practice to include a barrel containing sodium chloride solution, which can be used to control the effects of the current itself. This may be done either by periodically passing through the control barrel the same current used for drug ejection or by passing continuously a current adequate to cancel out the instantaneous sum of ▶ ejecting and ▶ retaining currents passing through the drugcontaining barrels. This is known as ‘‘current balancing.’’ The use of microiontophoresis is suitable for any ionized molecule, but nonionized compounds can be ejected by the closely related variant ‘‘electro-osmosis,’’ which is attributable to the presence of an electrical ‘‘double 775 M 776 M Microiontophoresis and Related Methods layer’’ within the barrel tip. When an aqueous solution is in contact with glass, negative ions are tightly adsorbed on the glass surface, leaving the bulk of solution carrying a net positive charge. The passage of positive (or outward) current then causes the ejection of a small volume of solution containing the compound of interest (Fig. 2). It should be noted, however, that this mechanism has nothing to do with the osmotic pressure of a solution or the establishment of any osmotic gradient. The term electroosmosis derives simply from the fact that the driving force is the movement of the solvent, not the solute, just as the case of osmotic movements across a semipermeable membrane. An alternative method of applying both ionized and nonionized compounds from micropipettes is the use of pressure. A suitable source of pressure, usually a cylinder of compressed gas, is connected to the open end of a micropipette barrel. Pressure usually up to 20 pounds per square inch (p.s.i.) will eject fluid from a 1mm pipette tip. One advantage of micropressure ejection is that it can be applicable to all compounds; however, it is not devoid of problems and artifacts and is unlikely to replace microiontophoresis as a microapplication method. Principles and Role in Psychopharmacology Microiontophoresis and Related Methods. Fig. 1. Examples of different types of multibarrel micropipette assemblies used in microiontophoretic experiments. (a) Standard 7-barrel assembly in most common use, introduced first by Curtis. (b) Twin, or parallel micropipette. (c) Co-axial assembly. (d) Staggered tip multibarrel. (From Hicks, 1984.) General Principles Each barrel of a micropipette assembly to be used for drug ejection is filled with a solution of the ionized compound and the solution is connected to the iontophoresis machine by a suitable lead, which is in contact with the drug solutions. The establishment of a potential difference Microiontophoresis and Related Methods. Fig. 2. Schematic diagram of a micropipette that contains a salt X+Y, showing the direction of current necessary to eject (a) and retain (b) the ion X+. (From Hicks, 1984.) Microiontophoresis and Related Methods between the drug solution and the medium surrounding the barrel tip will then cause the movement of ions through the solution and out of the pipette tip (Fig. 2). A chief advantage of the microiontophoretic method is that it is possible to examine the effects of drugs on single neurons in vivo without affecting the whole nervous system or other physiological responses, such as those that may occur when drugs are administered systemically (Aghajanian 1972). If a voltage is applied to a solution, ions and charged molecules will migrate toward and away from the source of the imposed electrical field depending on the sign of their net charge. This phenomenon is the fundamental principle of microiontophoresis: the desired charged particles are ejected from the mouth of one barrel of a multipipette assembly by appropriately charging the interior of that barrel (Fig. 2). An outward current will cause the ‘‘ejection’’ of positively charged ions, and an inward current flow, the ejection of negatively charged particles. If the pipette assembly is positioned close to a neuron, so that the recordings of its activity can be made through another electrolyte-filled barrel, drugs may be ejected and their pharmacological effects are inferred by the resulting changes in the rate and/or ▶ firing pattern. The Transport Number and the T50 Value An important technical consideration for experiments employing microiontophoresis is the transport number. The transport number is a measure of the amount of drug released from the micropipette by iontophoretic expulsion and it is important, because it helps one to evaluate dose–response relations between different compounds, and it can also provide some indication of the absolute potency of compounds. The transport number varies for individual compounds and is based on the interaction of the following variables: their solubility, the extent of their dissociation in solution, their polarity, and the nature of the external medium into which the drugs are administered. The transport number may be formally described by the following equation: n = RiZFi-1, where n = apparent transport number of the drug ion, Z = valency, F = Faraday’s constant, in Coulombs i = intensity of ▶ ejecting current, in nanoamperes, and Ri = rate of microiontophoretic release (which is equivalent to total release minus the sum of the rate of steady-state spontaneous release and where applicable, the release due to electro-osmosis). During microiontophoresis, the total number of ions transported is related in a direct manner to the amount of current applied to the solution, according to Faraday’s M 777 law. However, only a certain proportion of the charge imposed is carried by the ion species of interest. This value, which is ‘‘n,’’ the transport number, is not constant for a given material but will vary not only from pipette to pipette, but also, to a lesser extent, between different barrels of the same micropipette assembly containing identical solutions. Despite these inconsistencies, it remains valid that under steady-state conditions, drug release from micropipettes conforms to Faraday’s law: the amount of drug released is proportional to the magnitude of current passed (Hicks 1984). Another important parameter to consider when interpreting microiontophoretic data is the T50 value, which is the time taken for a response to reach 50% of its maximum (Fig. 3). The basis for this procedure is the hypothesis that each individual response to an agonist may be considered as a cumulative dose–response relationship M Microiontophoresis and Related Methods. Fig. 3. Time courses of inhibition of neuronal firing following microiontophoretic application of GABA with four different currents, 20 nA (filled circle), 10 nA (o), 5 nA (), and 2 nA (open triangle). Each curve was obtained from the same neuron at a depth of 957 mm in the middle suprasylvian gyrus of the cat cortex. The neuron was driven by continuous microiontophoretic application of L-glutamate (20 nA). Each of the points for 20, 10, and 5 nA applications of GABA is the mean SEM of three values obtained from three separate applications of the same current of GABA. The values of T50 shown are the times taken to achieve 50% inhibition of neuronal firing. (From Hill and Simmonds (1973) Br J Pharmacol 48:1–11.) 778 M Microiontophoresis and Related Methods reflecting the gradual increase in tissue concentration of drug during the ejection period. If a series of such responses are obtained, reaching the same maximum amplitude, they can be readily characterized by the T50 value (Fig. 3). Moreover, T50 value is easier to measure accurately than a response size. Thus, it can be very difficult to obtain reproducible graded response amplitudes to some very potent compounds such as amino acids. Any changes in ▶ firing rate during a response, which ideally should be of the plateau variety, may further complicate any assessment of the response size, whereas in the determination of T50 values, all responses increase to the same maximal level, which may be 100% inhibition or a clear maximal plateau of excitation tending toward overdepolarization. and Watkins 1960). Responses to some of these amino acids, especially ▶ glutamate and aspartate, terminate rapidly when an ejecting iontophoretic current is switched off. It is unclear to what extent this is due to the kinetics of iontophoresis or reflects the presence of rapid and efficient uptake processes. Some authors have reported long-lasting changes of cortical neuronal firing following iontophoresis of glutamate sufficient to at least double the resting firing rate. The development of a series of potent amino acids analogs with very high agonist potency led to the discovery of NMDA and non-NMDA glutamate receptors. This discovery was strengthened by additional findings that phosphonate analogs of amino acids, such as 2-amino-5-phosphonovaleric acid (AP-5) blocked the effects of NMDA, but not of quisqualic and kainic acids. Microiontophoresis in the Central Nervous System It is now more than 50 years since Curtis and Eccles (1958a, b) first employed the technique of microiontophoresis in the CNS. Microiontophoresis has provided so far a great contribution in the identification of the central effects of neurotransmitters, including glutamate, aspartate, g-aminobutyric acid (GABA), noradrenaline, serotonin, dopamine, and a variety of neuropeptides (enkephalins, cholecystokinin, neurotensin, tachykinins). Microiontophoresis also allows the histological confirmation of the sites of electrophysiological recordings, and the neuroanatomical determination of pathways by applying dyes, markers, and materials, which are carried by axonal transport for tracing fiber tracts. Alterations in neuronal sensitivity due to the influence of anesthetic compounds have been monitored when pharmacological agonists have been tested using microiontophoresis. Inhibitory Amino Acids Both glycine and ▶ GABA act as potent inhibitors of neuronal activity in the CNS, usually causing ▶ hyperpolarization associated with increased membrane conductance to chloride. Glycine is selectively antagonized by strychnine, whereas the effects of GABA are blocked by picrotoxin and bicuculline. Microiontophoretic experiments showing potentiation of the inhibitory effects of GABA by ▶ benzodiazepines were among the earlier experimental evidence for the modulatory action of these drugs on ▶ GABAA receptors (Gallagher 1978). Applications The largest number of studies has been concerned with the central nervous system. These studies have yielded information on: (1) the qualitative sensitivity of neurons to putative neurotransmitters and drugs; (2) quantitative estimates of variations and sensitivity in different CNS regions or of different cell types and the following lesions or the administration of drugs; (3) the pharmacology of transmitter receptors; (4) the effects of modifier of putative transmitter effects (antagonistic or enhancing substances) on synaptic transmission; and (5) the mechanisms and ionic conductances underlying transmitters effects. Excitatory Amino Acids Some of the earliest iontophoretic studies demonstrated marked excitatory activity of several simple dicarboxylic acids, including L-glutamic and L-aspartic acids (Curtis Acetylcholine Almost every region of the brain has been examined for its sensitivity to iontophoretically applied cholinergic agents. Most of the earlier work in vivo was concerned primarily with establishing the direction of responses to cholinomimetics and whether the effects involved muscarinic or nicotinic receptors. Many studies examined only cells encountered randomly in a particular brain region, but others have often succeeded in relating the direction of responses to cholinomimetics with some specific function. In the cerebral cortex deep pyramidal tract, cells are excited by ▶ acetylcholine. Several authors have also described an inhibitory action of acetylcholine, largely muscarinic in nature, in more superficial levels of the cortex and an excitatory action, which appears to have a predominant nicotinic pharmacology, in the same superficial layers. Some authors have shown that acetylcholine enhances the stimulus-evoked responses of visually driven cortical units, without affecting the overall excitability of the cell. Thus, orientation and direction specificity of neurons is preserved and increased relative to the nonpreferred responses. This phenomenon is reminiscent of the effects of some amines, which can also increase the Microiontophoresis and Related Methods signal-to-noise ratio by potentiating evoked activity and suppressing background. Noradrenaline Early microiontophoretic studies have shown that ▶ noradrenaline would cause a depression of neuronal firing in the cat cerebral cortex, and a large number of experiments have revealed similar responses in most areas of the CNS. This inhibition often seems to involve a voltagedependent hyperpolarization accompanied by an increased membrane resistance, although a decreased membrane resistance was found on neurons of the locus coeruleus in slice preparation in vitro. The biochemical basis of this hyperpolarization has been the subject of much argument. Although it was originally suggested that they may be mediated by an increase in the intracellular concentration of cyclic AMP, some group failed to reproduce these findings. Overt excitatory effects of noradrenaline have also been observed in many areas of the CNS. Neuronal responses to iontophoretic application of noradrenaline, apparently excitatory as well as inhibitory, can be enhanced by antidepressants. However, this potentiation can occur even after the loss of most amine-containing terminals, and it may be restricted to certain layers of the cortex. The pharmacology of responses to iontophoretically applied noradrenaline has been extensively studied. Some authors have postulated that, in the neocortex, excitatory responses to noradrenaline are mediated by a1-adrenergic receptors, whereas inhibitory responses occur through b-adrenergic receptors. Activation of a2-adrenergic receptors does also elicit inhibitory responses. Dopamine ▶ Dopamine was first tested iontophoretically in the cerebral cortex, where profound suppression of spontaneous cell firing was observed. This action has been confirmed by several authors, although excitatory effects have also been reported. Much attention has been centered on the effects of dopamine in the neostriatum where its action is usually inhibitory in the caudate nucleus. Bunney and Aghajanian (1976) have performed a laminar analysis of amine responses in the rat cerebral cortex. They found that neurons in layers II and III, which receive a dense noradrenergic projection, were more sensitive to noradrenaline than dopamine, whereas the reverse pattern was noted in layers V and VI, which receive a greater dopamine-containing projection. These authors also reported that desipramine, a selective inhibitor of noradrenaline reuptake, would enhance noradrenaline responses in layers II and III, but not in deeper layers, while benztropine enhanced dopamine responses only in M 779 layers V and VI. Dopamine receptors are present not only on innervated cells but also on the dopaminergic neurons themselves: the so-called ▶ autoreceptors. Activation of such receptors by dopamine or apomorphine causes marked inhibition of cell firing, and these effects are blocked by neuroleptic drugs. Microiontophoretic studies of dopamine response pharmacology have mostly proved consistent with behavioral and neurochemical work. Phenothiazines, for example, block dopamine but not noradrenaline responses in the cerebral cortex and the striatum. Iontophoretically applied ▶ a-flupenthixol can also block the effects of dopamine, although intravenously administered a-flupenthixol or ▶ pimozide did not modify neuronal responses to iontophoretic dopamine. Serotonin There is an extensive scientific literature regarding the effects of microiontophoretically applied ▶ serotonin on different areas of the central nervous system. Indeed, the microiontophoretic technique contributed substantially to the elucidation of the physiology and pharmacology of the central serotonergic system. Thus, an important factor controlling the activity of central serotonergic neurons is neuronal feedback inhibition. This is thought to be a homeostatic response, which, under physiological conditions, acts to compensate for increases in synaptic availability of serotonin. Thus, as the concentration of serotonin increases in the brain, the activity of central serotonergic neurons correspondingly decreases. The mechanism underlying this feedback regulation is both local or intrinsic to the raphe region (where serotonergic cell bodies are located) and through a feedback loop from postsynaptic target neurons. Serotonin released in the raphe region from dendrites and possibly from axon terminals appears to inhibit serotonergic neurons by activating somatodendritic autoreceptors, which produces hyperpolarization of the cell membrane via an increase in potassium conductance. Historically, the first drug reported to exert a preferential action on the 5-HT autoreceptor was LSD (lysergic acid diethylamide) applied microiontophoretically on the dorsal raphe nucleus of rats. Subsequently, several other hallucinogenic indoleamines, notably 5MeODMT (5-methoxy-N,N-dimethyltryptamine), were found to share this property with LSD. Since that time, several highly selective 5-HT1A agonist compounds such as 8-OH-DPAT have been synthesized and shown to suppress the firing of serotonergic neurons with potencies comparable with, or even greater than, that of LSD. On the basis of electrophysiological data, the serotonin autoreceptor has been characterized as the 5-HT1A subtype. Microiontophoretic technique also contributed to M 780 M Microiontophoresis and Related Methods characterize the action of serotonin agonists and antagonists and to elucidate the physiological role of serotonin receptor subtypes such as 5-HT1B, 5-HT2A, and 5-HT2C. As regards the 5-HT2C, it was found that this receptor subtype exerts a tonic inhibitory influence on the activity of dopamine-containing neurons in the substantia nigra pars compacta and the ▶ ventral tegmental area. Apparently, this inhibitory effect is mediated through the activation of nondopaminergic (presumably GABA-ergic) neurons in the substantia nigra pars reticulata. Thus, it was recently shown that microiontophoretic application of 5-HT2C receptor agonists stimulates the basal activity of nondopaminergic (presumably GABA-ergic) neurons in the substantia nigra pars reticulata (Invernizzi et al. 2007) (Fig. 4). By using microiontophoresis, it was also found that serotonin exerts a tonic inhibitory influence on the activity of noradrenergic neurons in the locus coeruleus. Opiates and Opioids Microiontophoresis has proved exceedingly valuable for opiate system studies, since it allows the testing of discrete units activated by noxious or nonnoxious stimuli in the same preparation. In most such studies, the applied opiates have depressed noxious stimulus-evoked activity, although usually in parallel with the effects on spontaneous or chemically induced firing. Microiontophoresis has also been proved as a popular means for comparing qualitatively opiate responses in normal and opiate-tolerant animals. Thus, inhibitory responses to ▶ morphine were encountered less frequently in the neocortex of morphine-tolerant rats than in controls. It was shown that iontophoretically applied ▶ naloxone would elicit a large increase of firing in the locus coeruleus noradrenergic neurons in morphine-tolerant rats, presumably as a correlate of the withdrawal phenomenon in such animals. Also, opioid peptides have been tested iontophoretically in many regions of the central nervous system. Opioid peptides were found to excite hippocampal neurons; however, these effects were apparently mediated through an indirect action on transmitter release or to a naloxonesensitive depression of local inhibitory interneurons. Peptides Microiontophoretic or pressure ejection has been used to apply a wide range of endogenous and synthetic peptides to neurons in vivo and in vitro. However, partly because of the lack of selective antagonists, there has been little progress in relating the observed responses to a physiological role, and as a result, attention has been concentrated on the mechanism of the observed responses, and potential interactions with neurotransmitters. ▶ Substance P, for example, appears to interact selectively with acetylcholine. Microiontophoretic substance P has also been found to enhance the response of spinal cord neurons to noxious stimulation but not innocuous ones, in some cases leading to the occurrence of responses in initially unresponsive units. Some excitatory effects of substance P can be mimicked by capsaicin, also applied iontophoretically. It was also reported that the excitatory effect of substance P on noradrenalin-containing neurons in the locus coeruleus is blocked by the selective antagonist [D-Pro2, D-Trp7,9] substance P. Thyrotropin releasing hormone (TRH) has been found to enhance the excitatory effects of acetylcholine on cortical neurons, Microiontophoresis and Related Methods. Fig. 4. Representative rate histogram showing the effect of the selective 5-HT2C receptor agonist RO 60-0175 on the basal activity of a nondopaminergic (presumably GABA-ergic neuron) of the rat substantia nigra pars reticulata. Microiontophoretic application of RO 60175 causes excitation of basal neuronal activity, which is proportional to the amount of the ejecting current applied (numbers above each bar in nA). (From Invernizzi et al. 2007.) Microsomal Ethanol-Oxidizing System M with no effects on resting firing rate. ▶ Somatostatin exerts a potent excitatory effect on hippocampal neurons. ▶ Cholecystokinin (CCK) and ▶ neurotensin are also frequently excitatory while angiotensin has excitant properties, which appear to be restricted to the subfornical organ and related structures. However, it is important to point out that peptides present special problems for microiontophoresis. Larger molecules tend to be adsorbed on to charged surfaces, which include the internal wall of a micropipette tip. Some peptides may also undergo denaturation or degradation during iontophoretic experiments. This problem may be exacerbated if very high currents are applied for long periods of time through high resistance tips, in that any change of local temperature may have a major impact on the stability of a peptide. some as a ‘‘classical’’ neurophysiologic approach to the study of central nervous system, it is likely that it will still contribute substantially to the progress of neuroscience. Advantages and Disadvantages of Microiontophoresis The original microiontophoretic technique was developed for answering questions concerned with synaptic transmission and the neuromuscular junction. Using this preparation, it is a simple matter to microscopically examine the muscle fiber being studied, to determine the distance of the micropipette from the tissue, and to have ready access to known synaptic inputs. These advantages are not valid for the CNS. Nevertheless, with some further precautions and considerations, the technique has been used successfully in the CNS for about 50 years. It is important to consider other potentially confounding technical factors limiting the utility of microiontophoresis, as it is used in central investigations. Of primary concern is the site of drug administration relative to cell soma, where the strongest depolarizing or hyperpolarizing influences are manifested, and the dendritic field, where synaptic influences are normally expressed and where antagonists of transmitters must accumulate to modify trans-synaptic excitations. Another consideration for central investigations also concerns the spatial distribution of drugs in the CNS. Since the CNS is densely packed with cells, microiontophoretically administered compounds cannot affect single neurons in isolation. This must be kept in mind when interpreting the data. References Conclusions Microiontophoresis, an experimental technique introduced more than 50 years ago, prompted a great impetus to the study of the physiology and pharmacology of the central nervous system. By mimicking the synaptic function, it provided a crucial step in establishing the physiological role of most neurotransmitters, including amines, amino acids, and neuropeptides. Although it is now considered by 781 Cross-References ▶ Antidepressants ▶ Excitatory Amino Acids ▶ Extracellular Recording ▶ Hallucinogens ▶ Inhibitory Amino Acids ▶ Intracellular Recording ▶ Neurotensin ▶ Opioids ▶ Somatostatin ▶ Tachykinins Aghajanian GK (1972) LSD and CNS transmission. Annu Rev Pharmacol 12:157–168 Bunney BS, Aghajanian GK (1976) Dopamine and norepinephrine innervated cells in the rat prefrontal cortex: pharmacological differerentiation using microiontophoretic techniques. Life Sci 19:1783–1789 Curtis DR, Eccles RM (1958a) The excitation of Renshaw cells by pharmacological agents applied electrophoretically. J Physiol 141:435–445 Curtis DR, Eccles RM (1958b) The effect of diffusional barriers upon the pharmacology of cells within the central nervous system. J Physiol 141:446–463 Curtis DR, Watkins JC (1960) The excitation and depression of spinal neurones by structurally related amino acids. J Neurochem 6:117–141 del Castillo J, Katz B (1955) On the localization of acetylcholine receptors. J Physiol 128:157–181 Gallager DW (1978) Benzodiazepines: potentiation of a GABA inhibitory response in the dorsal raphe nucleus. Eur J Pharmacol 49:133–143 Hicks TP (1984) The history of development of microiontophoresis in experimental neurobiology. Prog Neurobiol 22:185–240 Invernizzi RW, Pierucci M, Calcagno E, Di Giovanni G, Di Matteo V, Benigno A, Esposito E (2007) Selective activation of 5-HT2C receptors stimulates GABA-ergic function in the rat substantia nigra pars reticulata: a combined in vivo electrophysiological and neurochemical study. Neuroscience 144:1523–1535 Nastuk WL (1953) Membrane potential changes at a single endplate produced by transitory application of acetylcholine with an electrically controlled microjet. Fed Proc 12:102 Microsomal Ethanol-Oxidizing System Synonyms MEOS Definition Secondary pathway of alcohol metabolism in the liver, which plays a pronounced role during heavy and sustained drinking. M 782 M Midazolam Midazolam Definition Midazolam is an ultra short-acting benzodiazepine derivative with potent anxiolytic, amnestic, hypnotic, anticonvulsant, skeletal muscle relaxant, and sedative properties. It is considered an ultra short-acting benzodiazepine, with an ▶ elimination half-life of about 2 h. It is used in some countries for the short-term treatment of insomnia and in many countries as a premedication before surgery. Intravenous midazolam is indicated for procedural sedation (often in combination with an ▶ opioid, such as ▶ fentanyl), for pre-op sedation, for the induction of general anesthesia, and for sedation of ventilated patients in critical care units. Cross-References ▶ Benzodiazepines Mild Cognitive Impairment JOSEF MARKSTEINER, IRENE ADELT Department of Psychiatry and Psychotherapy, LKH Klagenfurt, Klagenfurt, Austria Definition Mild cognitive impairment (MCI) is a relatively recent term. It is used to describe individuals who have ▶ cognitive impairments beyond that expected for their age and education, but that do not interfere significantly with their daily activities (Petersen et al. 1999). The criteria for MCI are shown in the next section. It is considered to be a transitional stage between normal aging and various types of ▶ dementia. Although MCI can present with a variety of symptoms, when memory loss is the predominant symptom it is termed ‘‘amnestic MCI’’ and is frequently seen as a risk factor for Alzheimer’s disease (AD) (Morris et al. 2001). Individuals who have impairments in cognitive domains other than memory are classified as nonamnestic single- or multiple-domain MCI. Subtype of MCI may influence rates of progression to dementia and has a major influence on subsequent type of dementia diagnosis (Yaffe et al. 2006). Studies clearly suggest that MCI patients tend to progress to probable Alzheimer’s disease at a significantly higher rate than healthy individuals of the same age. It is important that people with cognitive impairment are diagnosed as early as possible, so that they can benefit from therapeutic interventions. Criteria for MCI The criteria for MCI are those previously proposed by Petersen et al. (1999) ● Presence of a subjective memory complaint ● Preserved general intellectual functioning ● Demonstration of a memory impairment by cognitive testing ● Intact ability to perform activities of daily living ● Absence of dementia The revised MCI criteria are those proposed by the Stockholm Group consensus. ● Presence of a cognitive complaint from either the subject and/or a family member ● Absence of dementia ● Change from normal functioning ● Decline in any area of cognitive functioning ● Preserved overall general functioning but possibly with increasing difficulty in the performance of activities of daily living Role of Pharmacotherapy Epidemiology and Risk Factors People with MCI are more likely to develop Alzheimer’s or other dementias than are those without cognitive impairment. In fact, about half the people with MCI will progress to ▶ Alzheimer’s disease within 5 years. In the general population, the prevalence of MCI was estimated to be about 3% (Ritchie et al. 2001). In the same population, a prevalence rate of about 18% was reported for ageassociated cognitive decline, which is a similar concept as MCI but is related to normal cognitive aging processes rather than incipient dementia. Most of the studies described an increase in prevalence of MCI with age. A higher prevalence rate of MCI has been found in females. Vascular diseases were identified as risk factors for MCI in some studies. Patients, particularly in general hospitals, represent a high-risk group for MCI, since risk factors like cardiovascular diseases are quite common (Bickel et al. 2006). MCI is also positively associated with stroke and peripheral arterial obstructive disease. Depressive symptoms are highly prevalent among elderly MCI subjects and in cognitively normal elderly individuals. These symptoms are associated with an increased risk of developing MCI (Ravaglia et al. 2008). Mild Cognitive Impairment Depressive symptoms may also increase the conversion rate from MCI to ▶ dementia (Barnes et al. 2006). A synergistic interaction between ▶ apolipoprotein E genotype (epsilon3/epsilon4 or epsilon4/epsilon4) and depression was reported with regard to the incidence of MCI (Geda et al. 2006). Association between MCI and apoE allele 4 status largely depends on MCI subtype. For Alzheimer’s disease, the apoE allele 4 status is probably the most established risk factor. Symptoms How do the memory difficulties in MCI differ from those of normal aging? Numerous studies have examined the cognitive performance of patients with MCI. In general, information on cognitive performance over time is essential for definition of MCI to substantiate a worsening over time. Neuropsychological examinations have demonstrated that, in general, these patients are impaired on tests of memory compared with age-matched healthy individuals. Forgetting things that are usually remembered may be a first symptom of MCI patients. MCI may encompass deficiencies in any or all of the following categories: language, visuospatial ability, for example, placement of things in time and space becomes more difficult; executive function, for example, decision making becomes more challenging; ▶ episodic memory, for example, what happened yesterday. A general recommendation for individuals concerned about their memory would be to discuss these concerns with their physician. MCI patients may report distinct difficulties in other areas of cognition, such as naming objects or people and complex planning tasks. These symptoms are comparable, but less severe, than the neuropsychological deficits found for Alzheimer’s disease, especially for amnestic MCI. A careful interview may reveal that the MCI patient has mild difficulties with daily activities. These problems should be confirmed by a significant other. People with MCI may also experience: ● ● ● ● ● Depression Irritability Anxiety Aggression Apathy Diagnosis In general, a lot of research has focused upon techniques to try to improve ways of identifying people with MCI. The diagnosis of MCI requires considerable clinical judgment, which consists of a comprehensive clinical assessment, M 783 including clinical observation, laboratory examinations, neuropsychological assessment, and neuroimaging. These examinations are also performed to rule out any alternate diagnosis which may lead to cognitive impairment. A similar assessment is usually performed for the diagnosis of Alzheimer’s disease or other types of dementia. As part of physical examination, neurological examination checks for signs of ▶ Parkinson’s disease, strokes, tumors, or other medical conditions that can impair memory as well as physical function. Laboratory Tests Simple blood tests can rule out physical problems that can affect memory, such as vitamin B-12 deficiency or an underactive thyroid gland. Neuroimaging There is emerging evidence that magnetic resonance imaging (MRI) can observe deterioration, including progressive loss of gray matter in the brain, from MCI to Alzheimer disease. MRI-based volumetric measurements of medial temporal lobe structures can discriminate between normal elderly control subjects and patients with Alzheimer’s disease. The extent of medial temporal lobe atrophy distinguishes probable Alzheimer’s disease and amnestic MCI from healthy subjects. Neuroimaging also helps to monitor disease progression from a diagnosis of MCI to different stages of AD. Furthermore, MRI or CT is necessary to rule out the possibility of a tumor, evidence of stroke, or bleeding which could also cause some of the symptoms seen in MCI patients. Biomarker Currently, a lot of research is done with biomarkers in Alzheimer’s disease and in MCI patients. A promising area of research for biochemical diagnosis of AD and mixed forms of dementia is the analysis of cerebrospinal fluid (CSF) Biological markers can serve as early diagnostic indicators, and as markers of preclinical pathological change. They are likely to take on an increasingly important role in the diagnosis and monitoring of AD, and probably also in MCI. Some biomarkers are currently in the process of implementation as outcome variables. In a large multicenter study, the CSF biomarkers Ab42, T-tau, and P-tau could predict with good accuracy which MCI patients will develop AD, which is a finding that was previously found in smaller studies (Mattsson et al. 2009). The sensitivity and specificity and ease of use are the most important factors that ultimately define the usefulness of a biomarker for diagnosis. M 784 M Mild Cognitive Impairment Neuropathology MCI often causes the same types of brain changes seen in Alzheimer’s disease or other forms of dementia. The difference between MCI and other types of dementia lies in the extent of these changes. The available data suggests that MCI is associated with the early stages of the neuropathological changes that are found in the lesions of AD; including the accumulation of neuritic ▶ plaques, neurofibrillary tangles, synaptic and neurotransmitter associated deficits, and significant neuronal cell death. There is evidence suggesting that while amnestic, MCI patients may not meet the neuropathologic criteria for Alzheimer’s disease, and these patients may be in a transitional stage of evolving Alzheimer’s disease. who took ▶ donepezil (Petersen et al. 2005). However, that difference disappeared by the end of the study. A recent study showed that depression is predictive of progression from amnestic MCI to Alzheimer’s disease, and treatment with donepezil delayed progression to AD among depressed subjects with amnestic MCI (Lu et al. 2009). Also ▶ galantamine has not changed the conversion rate from MCI to Alzheimer’s disease, but may increase the risk of sudden death from heart attacks and strokes when used in people who have MCI (Winblad et al. 2008). However, several clinical trials are in progress to determine if any medications will prevent or delay the rate of progression from MCI to dementia. High Blood Pressure Drugs Treatments and Drugs In general, there is no cure for MCI. There are several medications as well as many nonmedication approaches that can potentially improve MCI-related symptoms. Treatment of coexisting conditions, such as high blood pressure or depression, may help reduce cognitive problems. As MCI may represent a prodromal state to clinical Alzheimer’s disease, treatments proposed for Alzheimer’s disease, such as antioxidants and ▶ cholinesterase inhibitors have also been tested for MCI. Patients with MCI are frequently being treated with ‘‘off label’’ cholinesterase inhibitors and ▶ memantine, as well as other possible cognition-enhancing drugs. Acetylcholinesterase-inhibitors (AChEIs) and Memantine Randomized, placebo-controlled trials examining the therapeutic value of ▶ acetylcholinesterase-inhibitors (AChEIs) have been performed during the last years. All of them have had a negative outcome with regard of the primary outcome parameter which was to prevent the conversion from MCI to real Alzheimer’s disease. Negative results have also been obtained with steroidal or anti-inflammatory compounds or antioxidants. As these compounds are thought to target the early steps in the pathophysiological cascade of dementia, the negative results are disappointing. So far, no disease modifying drugs with a proven efficacy are available for the treatment of MCI patients. As a consequence of these largely negative studies, there is no proven treatment or therapy for MCI. For example, ▶ rivastigmine failed to stop or slow progression to Alzheimer’s disease or on cognitive function for individuals with MCI (Feldman et al. 2007). Donepezil showed only minor, short-term benefits. During the first year of a 3-year study, the rate of progression from MCI to Alzheimer’s was significantly lower in the people People who have MCI are also more likely to have problems with the blood vessels inside their brains. High blood pressure can worsen these problems and cause memory difficulties. Therefore, antihypertensive drugs are under investigation of whether they can reduce the conversion rate from MCI to dementia. But also because of other medical complications, it is essential to keep blood pressure at normal levels. ▶ Antidepressants Depression is common in people who have MCI, and depression, itself, can cause memory problems. Treating ▶ depression may help to improve memory. However, the studies to date were of short duration, and it is not clear whether there is a longer-term benefit associated with antidepressant treatment. Further studies are needed to investigate the role of antidepressants in depressed MCI patients, especially whether they can reduce the conversion rate from MCI to dementia. Antioxidants The antioxidant vitamin E may help to protect brain cells from the oxidative stress that appears to play a role in dementia, but it works no better than placebo in relieving the symptoms or delaying the progression of MCI. Ginkgo appears to improve memory and concentration in older adults with no major memory problems, but it is still uncertain if ginkgo can reduce the memory problems associated with MCI. Physical Activity Observational studies have shown that physical activity reduces the risk of cognitive decline. In a ▶ randomized controlled trial of a 24-week physical activity intervention, participants in the intervention group improved suggesting that a 6-month program of physical activity Minimal Cerebral Dysfunction provided a modest improvement in cognition over an 18-month follow-up period (Lautenschlager et al. 2008). In general, study results are to a certain extent controversial whether physical activities can prevent or reverse MCI. Nevertheless, physical activity can be part of a healthy lifestyle for older people with or without MCI. Conclusion The concept of MCI is currently of high interest. A number of studies have been dealing with different aspects of MCI with regard to diagnosis and therapeutic strategies. Upto now, no drug is licensed for the indication of MCI. However, there is hope that new compounds may be efficacious in the treatment of MCI to reduce the conversion rate from MCI to dementia. References Barnes DE, Alexopoulos GS, Lopez OL, Williamson JD, Yaffe K (2006) Depressive symptoms, vascular disease, and mild cognitive impairment: findings from the Cardiovascular Health Study. Arch Gen Psychiatry 63:273–279 Bickel H, Mosch E, Seigerschmidt E, Siemen M, Forstl H (2006) Prevalence and persistence of mild cognitive impairment among elderly patients in general hospitals. Dement Geriatr Cogn Disord 21:242–250 Feldman HH, Ferris S, Winblad B et al (2007) Effect of rivastigmine on delay to diagnosis of Alzheimer’s disease from mild cognitive impairment: the InDDEx study. Lancet Neurol 6:501–512 Geda YE, Knopman DS, Mrazek DA, Jicha GA, Smith GE, Negash S, Boeve BF, Ivnik RJ, Petersen RC, Pankratz VS, Rocca WA (2006) Depression, apolipoprotein E genotype, and the incidence of mild cognitive impairment: a prospective cohort study. Arch Neurol 63:435–440 Lautenschlager NT, Cox KL, Flicker L, Foster JK, van Bockxmeer FM, Xiao J, Greenop KR, Almeida OP (2008) Effect of physical activity on cognitive function in older adults at risk for Alzheimer disease: a randomized trial. JAMA 300:1027–1037 Lu PH, Edland SD, Teng E, Tingus K, Petersen RC, Cummings JL (2009) Donepezil delays progression to AD in MCI subjects with depressive symptoms. Neurology 72:2115–2121 Mattsson N, Zetterberg H, Hansson O, Andreasen N, Parnetti L, Jonsson M, Herukka SK, van der Flier WM, Blankenstein MA, Ewers M, Rich K, Kaiser E, Verbeek M, Tsolaki M, Mulugeta E, Rosen E, Aarsland D, Visser PJ, Schroder J, Marcusson J, de Leon M, Hampel H, Scheltens P, Pirttila T, Wallin A, Jonhagen ME, Minthon L, Winblad B, Blennow K (2009) CSF biomarkers and incipient Alzheimer disease in patients with mild cognitive impairment. JAMA 302:385–393 Morris JC, Storandt M, Miller JP, McKeel DW, Price JL, Rubin EH, Berg L (2001) Mild cognitive impairment represents early-stage Alzheimer disease. Arch Neurol 58:397–405 Petersen RC, Smith GE, Waring SC, Ivnik RJ, Tangalos EG, Kokmen E (1999) Mild cognitive impairment: clinical characterization and outcome. Arch Neurol 56:303–308 Petersen RC, Thomas RG, Grundman M, Bennett D, Doody R, Ferris S, Galasko D, Jin S, Kaye J, Levey A, Pfeiffer E, Sano M, van Dyck CH, Thal LJ (2005) Vitamin E and donepezil for the treatment of mild cognitive impairment. N Engl J Med 352:2379–2388 M 785 Ravaglia G, Forti P, Lucicesare A, Rietti E, Pisacane N, Mariani E, Dalmonte E (2008) Prevalent depressive symptoms as a risk factor for conversion to mild cognitive impairment in an elderly Italian cohort. Am J Geriatr Psychiatry 16:834–843 Ritchie K, Artero S, Touchon J (2001) Classification criteria for mild cognitive impairment: a population-based validation study. Neurology 56:37–42 Winblad B, Gauthier S, Scinto L, Feldman H, Wilcock GK, Truyen L, Mayorga AJ, Wang D, Brashear HR, Nye JS (2008) Safety and efficacy of galantamine in subjects with mild cognitive impairment. Neurology 70:2024–2035 Yaffe K, Petersen RC, Lindquist K, Kramer J, Miller B (2006) Subtype of mild cognitive impairment and progression to dementia and death. Dement Geriatr Cogn Disord 22:312–319 Milnacipran Synonyms Milnacipran hydrochloride Definition Milnacipran, under the brand name Ixel, became available in the late 1990s in Europe and several other countries for the treatment of Major Depressive Disorder. It was approved in 2009 for the treatment of fibromyalgia in the USA. Milnacipran Hydrochloride ▶ Milnacipran Miltown ▶ Meprobamate Minimal Brain Damage ▶ Attention Deficit and Disruptive Behavior Disorders Minimal Cerebral Dysfunction ▶ Attention Deficit and Disruptive Behavior Disorders M 786 M Minipress® has beneficial effects on symptoms of anxiety and sleep disturbances. Its most common side effects are increased appetite and subsequent weight gain, drowsiness, and dizziness. Minipress® ▶ Prazosin Cross-References ▶ Antidepressants Minor Cerebral Dysfunction ▶ Attention Deficit and Disruptive Behavior Disorders Mismatch Field ▶ Mismatch Negativity Minor Tranquilizer Synonyms Antianxiety Tranquilizer medication; Anxiolytic; Sedative; Mismatch Negativity Definition Synonyms A medication used in the treatment of anxiety disorders and also as a nonspecific sedative. It is a more precise term for the drugs used to lessen anxiety than its synonyms. The distinction from sedative is not a clear one. Minor tranquilizers lie on a continuum from mild antianxiety effects through more pronounced effects and induction of sleep and relaxation to anesthesia of increasing depth. Mismatch field; MMN Cross-References ▶ Anxiolytics ▶ Benzodiazepines ▶ Generalized Anxiety Disorder Miraa ▶ Khat Mirtazapine Definition Mismatch Negativity (MMN) is similar to the P300 in that it is an event-related potential component seen when subjects are presented with a rare or ‘‘deviant’’ stimulus in a train of frequent or regular stimuli. It can be found using stimuli in a variety of sensory modalities but has been most researched using auditory or visual stimuli. The deviant stimuli can vary from the frequent ones in any way, for example, tone, pitch, loudness. Unlike the P300, subjects do not need to be paying conscious attention to the stimuli. MMN is a negative potential observed over fronto-central scalp with a typical latency of 150–250 ms after the onset of the deviant stimulus. Cross-References ▶ Event-Related Potentials Misoprostol Synonyms Synonyms Remeron; Zispin Cytotec Definition Definition Mirtazapine is the first atypical antidepressant with noradrenergic and specific serotonergic receptor antagonism (NaSSa), introduced in 1994, with fewer serotonergic, anticholinergic, and antiadrenergic side effects, and comparable effectiveness to ▶ tricyclic antidepressants. It also Misoprostol is an FDA-approved drug for the prevention of nonsteroidal anti-inflammatory drug-induced gastric ulcers. Misoprostol is also widely used by obstetricians and gynecologists for the induction of labor, therapeutic abortion, and the early termination of pregnancy. Modafinil Chemically, misoprostol is a synthetic prostaglandin E1 analogue. The most commonly reported adverse effects of misoprostol are: diarrhea, abdominal pain, nausea, and headache. Cross-References M 787 or pass through to the detector, where they make irregular baseline noise, and rapid spikes. Cross-References ▶ High Pressure Liquid Chromatography ▶ Autism: Animal Models Moclobemide Mitochondrial Complex Chain Synonyms Aurorix; Manerix Definition Mitochondria provide the energy source of cells by synthesis of adenosine triphosphate (ATP). A series of enzymes and cofactors (complexes I – V) in the inner membrane of the mitochondrion oxidize carbohydrates to release electrons that drive the proton pumps necessary to provide the energy for ATP synthesis. A range of drugs and toxins act on these enzyme complexes to disrupt or enhance energy production via the mitochondrial electron transport chain, affecting cell metabolism and ultimately cell survival. Definition Moclobemide is an antidepressant that inhibits reversibly and preferentially MAO-A, introduced in 1977, but not approved by the FDA in the USA. It is mainly used in the treatment of ▶ major depression and ▶ social anxiety, and it is claimed to have a favorable side-effect profile, but with equal effectiveness to ▶ tricyclic antidepressants and ▶ SSRIs within 1 week of treatment. It is rapidly absorbed and has a relatively short ▶ half-life, but the CNS effects persist for many hours, and it is considered safe. Cross-References MK486 ▶ Antidepressants ▶ Monoamine Oxidase Inhibitors ▶ Carbidopa Modafinil MMN ▶ Mismatch Negativity Mobile phase MICHAEL MINZENBERG University of California, Davis School of Medicine, Imaging Research Center, Sacramento, CA, USA Synonyms 2-[(diphenylmethyl)sulfinyl]acetamide Definition The mobile phase is the buffer that is pumped through the chromatographic column. All chemicals used to prepare the buffer should be of at least HPLC grade, and they should be made up using ultrapure, HPLC-grade, water. In addition, the buffer should be thoroughly filtered which is best achieved by vacuum filtration. This has the benefit that it also removes dissolved gas in the mobile phase that otherwise can come out of solution causing minute air bubbles to form. These get lodged in the column, with resultant adverse effects on the separation, Definition Modafinil is a non-amphetamine ▶ psychostimulant currently FDA-approved for the treatment of sleepiness in ▶ Narcolepsy and shift-work sleep disorder. Pharmacological Properties Modafinil (brand name Provigil) is a racemate, with the two enantiomers being approximately equipotent in behavioral effects, but different in ▶ pharmacokinetic profile. The R-enantiomer (armodafinil) reaches higher M 788 M Modafinil plasma concentrations than the racemic form between 6 and 14 h after administration, with a longer duration of wake-promoting activity in healthy adults. Modafinil is readily absorbed after single or multiple oral doses, reaching peak plasma concentrations 2–4 h after administration. The presence of food in the gastrointestinal tract can slow the rate of absorption but does not affect the total extent of absorption. Steady-state plasma concentrations are achieved between 2 and 4 days with repeated dosing. It is highly lipophilic, and approximately 60% bound to plasma proteins, primarily albumin. Major pharmacokinetic parameters are independent of doses in the range of 200–600 mg/day. The major circulating metabolites, modafinil acid and modafinil sulfone, do not exert any significant activity in the brain or the periphery. The ▶ elimination half-life is approximately 12–15 h, and single daily dosing is adequate and common in clinical practice. Elimination occurs primarily in the liver, via amide hydrolysis and, to a lesser extent, by ▶ cytochrome P450-mediated oxidation. Excretion occurs in the urine, with less than 10% of the oral dose excreted as the unchanged drug. Elimination is slowin the elderly or in individuals with hepatic or renal impairment. Some drug-drug interactions are apparent with modafinil. In vitro, modafinil exerts a reversible inhibition of CYP2C19, a smaller but concentration-dependent induction of CYP 1A2, 2B6 and 3A4, and a suppression of 2C9 activity. There are significant interactions of modafinil with ethinylestradiol and ▶ triazolam, though not with ▶ methylphenidate, dextroamphetamine, or warfarin. Neurochemical Effects of Modafinil Modafinil Effects on Catecholamine Systems Modafinil is structurally unrelated to ▶ amphetamine, with a differing profile of pharmacological and behavioral effects. While the in vitro potency of modafinil in binding either the ▶ dopamine transporter (▶ DAT) and ▶ norepinephrine transporter (▶ NET) is low relative to methylphenidate, ▶ bupropion, or benztropine, modafinil shows DAT occupancy comparable to ▶ methylphenidate at clinically relevant doses (Madras et al. 2006). Modafinil has a complex profile of effects on central dopamine (▶ DA) and norepinephrine (NE) systems, lacking many neurochemical and behavioral effects observed with amphetamine administration. For instance, in contrast to amphetamine, modafinil does not significantly affect DA release or turnover; it shows negligible effects on cerebral cortical blood flow and shows different patterns of metabolic activation compared to amphetamine; it does not produce behavioral stereotypies or rebound hypersomnia; and in healthy humans, modafinil has effects on the resting EEG that are distinct from amphetamine. Nevertheless, a parenteral administration of modafinil does raise extracellular DA levels in the rat ▶ prefrontal cortex (de Saint Hilaire et al. 2001), and in the caudate nucleus of narcoleptic dogs, though only minimally in the rat ▶ hypothalamus (de Saint Hilaire et al. 2001). It causes a modest increase in DA in the accumbens after intraperitoneal doses up to 300 mg/kg. In rat brain slices, modafinil inhibits the activity of ▶ ventral tegmental area DA neurons, an effect that appears to be mediated by D2 receptors. This suggests that modafinil inhibits DA reuptake, leading to DA cell body ▶ autoreceptor activation, to diminish DA cell firing. Modafinil effects on wakefulness are abolished in DAT knockout mice. In a rodent ▶ drug discrimination paradigm, modafinil partially generalizes to a cocaine-like stimulus; in addition, modafinil effects on activity levels in mice are modestly attenuated by D1 receptor antagonism, though not by D2 antagonism. Modafinil reduces blood prolactin levels in humans, without effects on blood growth hormone or thyroid stimulating hormone. Overall, these findings suggest that modafinil effects on arousal and behavioral activity are at least partly mediated by synaptic DA, but in a manner differing from that of amphetamine, and possibly favoring corticostriatal over subcortical limbic circuits. Modafinil also elevates extracellular NE levels in ▶ prefrontal cortex (along with DA) and hypothalamus. Pretreatment with a-adrenergic receptor antagonists diminishes modafinil-induced increases in arousal and activity in rats and monkeys. However, modafinil does not reduce cataplexy in dogs or humans with narcolepsy, a feature that is similar to other DAT inhibitors, and in contrast to a1B receptor agonists and NET inhibitors. In addition, pretreatment with low doses of the a2 antagonist ▶ yohimbine potentiates modafinil-induced wakefulness and activity, whereas higher doses attenuate the activity increases. This biphasic response to yohimbine suggests that low doses may preferentially block the inhibitory terminal a2 autoreceptor to enhance NE release and thus augment postsynaptic adrenergic receptor activation by modafinil, whereas higher doses also block postsynaptic a2 receptors, attenuating modafinil effects. These findings make it likely that postsynaptic a2 receptors mediate some of the behavioral effects of modafinil. Importantly, modafinil also augments pupillary dilation parameters in a manner consistent with phasic activity of locus coeruleus neurons. This effect may also be mediated through a2 receptor activation; in this case, those receptors (autoreceptors) are located on locus coeruleus cell Modafinil bodies. Modest attenuation of modafinil-induced arousal and activity has also been observed after pretreatment with the b-blocker ▶ propranolol, suggesting that postsynaptic b receptors also mediate these modafinil effects. Taken together, these varied findings suggest that modafinil may potentiate both DA and NE neurotransmission. It appears likely that the elevations in extracellular NE observed after modafinil are responsible for the majority of the adrenergic receptor-mediated effects, which may involve a2, a1, and b receptors. D1 and D2 receptors probably also mediate modafinil effects on cognition and behavior. In addition, however, Wisor and Eriksson (2005) have proposed that the elevated synaptic DA resulting from DAT inhibition may lead to DA activation of adrenergic receptors. There remains the possibility that enhanced DA in the ▶ prefrontal cortex results from competition with increased NE levels for binding to the NET, which plays an important role in terminating DA action in the prefrontal cortex. DA has an affinity for cloned mouse a1B receptors that is on the same order of magnitude as NE and DA can activate adrenergic receptors in various brain regions. These observations suggest a mechanism whereby the modafinil inhibition of DAT inhibition may be related to adrenergic receptor-mediated behavioral effects. Modafinil Effects on GABA, Glutamate, and Serotonin Systems Modafinil also has consistent effects on central ▶ glutamate and gamma amino-butyric acid (▶ GABA) neurotransmitter systems. The regional effects on extracellular glutamate occur at ascending doses in this order: thalamus=hypothalamus<striatum=hippocampus. Glutamate levels in the globus pallidus and substantia nigra are unchanged after the highest doses administered. These effects on glutamate may interact with adrenergic mechanisms. Modafinil also causes a dose-dependent decrease in extracellular GABA. These regional GABA effects occur at ascending doses in this order: cortex<striatum/pallidum = hypothalamus <thalamus = hippocampus =substantia nigra=nucleus accumbens. The effects on extracellular GABA may be mediated by modafinil effects on other neurotransmitter systems Cortical GABA effects require intact catecholamine neurons. In addition, modafinil elevates extracellular serotonin (5HT) in the frontal cortex, central nucleus of the amygdala, and dorsal raphe nucleus, but minimally in the hypothalamus. Modafinil and the 5HT reuptake inhibitors ▶ fluoxetine, ▶ paroxetine, and ▶ imipramine mutually enhance the effects of each other on elevations in M 789 cortical 5HT. Taken together, this literature suggests that modafinil effects on GABA are at least partly mediated by 5HT. Ultimately, modafinil effects on GABA may be mediated by adrenergic effects on 5HT activity. Modafinil Effects on Orexin and Histamine Systems The clinical efficacy of modafinil in narcolepsy, a condition characterized by deficient orexin (▶ hypocretin) in the brain, suggests that modafinil may have clinically relevant effects on this neurochemical system. Modafinil does activate orexin cells in the perifornical area of mice and rats. However, modafinil induces wakefulness more potently in orexin knockout mice than in wild-type mice, with similar patterns of Fos-immunoreactivity. In addition, modafinil does not bind to the orexin 1 receptor and retains effects on both extracellular striatal DA and wakepromoting activity in orexin 2 receptor-deficient narcoleptic dogs. Therefore, modafinil effects on arousal do not appear to be mediated through the orexin system, and the precise role of orexin in the cognitive and clinical effects of modafinil remains unknown. Modafinil also elevates extracellular histamine (HA) in the anterior hypothalamus. However, a direct injection of modafinil into the tuberomamillary nucleus (the site of HA cell bodies) does not affect HA release. Given the multiple effects on catecholamines, 5HT and GABA described earlier for modafinil, it appears likely that modafinil effects on HA are mediated by one or more of these other neurotransmitter systems. Effects of Modafinil on Cognition Studies in rodents indicate that modafinil can improve ▶ working memory performance in a dose- and delaydependent manner, and that the processing of contextual cues is also enhanced with modafinil. These effects may be augmented with sustained dosing regimens. In healthy humans (with or without undergoing sleep deprivation), working memory, recognition memory, ▶ sustained attention, and other tasks dependent on cognitive control (and on function of the prefrontal cortex) are enhanced with modafinil. Some evidence suggests that the magnitude of modafinil effects in healthy adults may depend on underlying cognitive abilities. Those with high general intellectual abilities, or high performance in specific cognitive domains, appear to exhibit less improvement after modafinil, suggesting that these individuals already experience optimal levels/patterns of catechalamine activity in the modulation of cognition. Among psychiatric populations, there is now consistent evidence that modafinil (in well-tolerated dosing regimens) improves attention and response inhibition in children and adolescents with attention-deficit/hyperactivity disorder (▶ ADHD). These M 790 M Model Organisms of Hyperkinetic Syndrome improvements in cognition may form the basis for clinical efficacy in ADHD. Among adult psychiatric patients, modafinil improves several cognitive functions dependent on the prefrontal cortex in ▶ schizophrenia, ▶ major depression and adult ADHD, with some null findings reported in schizophrenia. However, these studies have significant limitations evident in their design. The range of clinical samples and cognitive functions that are subject to modafinil treatment study is expected to expand in the future. isolated cases. Modafinil also appears to have a relatively low potential for abuse, which may be a function of its pharmacodynamic profile and/or its physical properties, being insoluble in water and unstable at high temperatures, which minimizes its bioavailability upon smoking or intravenous use. Nonetheless, careful clinical judgment should be exercised in the decision to initiate therapy with modafinil, with particular attention to both its side-effect profile and its potential for drug–drug interactions. Clinical Effects of Modafinil Modafinil has consistently shown efficacy in measures of alertness in narcolepsy and shift-work sleep disorder, in ▶ randomized, double-blind placebo-controlled studies. In these studies, modafinil has shown efficacy in openlabel extension phases extending for as long as 136 weeks, and it has been well tolerated, with no evidence of significant adverse events or abuse. Modafinil has also been evaluated for the treatment of ▶ fatigue and sedation in a number of other neurological and medical conditions, including multiple sclerosis, idiopathic ▶ Parkinson’s disease, chronic fatigue syndrome, polio, HIV infection, ▶ dementias, obstructive sleep apnea, post-anesthetic sedation, and fibromyalgia, with generally favorable but somewhat mixed results (Ballon and Feifel 2006). Among studies of adult psychiatric patients, two studies of patients with ▶ major depression have found significant improvements in mood symptoms on modafinil compared to placebo, and modafinil has been associated with greater rates of abstinence in cocaine-dependent adults. It also shows clinical efficacy in adults with ADHD. In contrast, adjunct modafinil has shown modest and inconsistent efficacy for symptoms of schizophrenia, though these studies have been plagued by small sample sizes and other methodological limitations. In childhood/adolescent ADHD, modafinil improves parent, teacher, and clinician ratings of ADHD symptoms in several short-term (4–9 weeks), randomized, double blind, placebo-controlled trials, at mean doses ranging from 195 to 368 mg daily. Throughout these clinical trials, modafinil has been well tolerated. However, case reports have appeared describing significant adverse events in routine clinical use of modafinil, including the exacerbation of psychosis, acute mania, clozapine toxicity, premature ventricular contractions, and irritability and verbal aggression. Nonetheless, these events have not been observed at a significant rate in modafinil-treated patients compared to placebotreated patients in clinical trials and no serious (e.g., lifethreatening) sequelae have ensued in these reported ▶ Attention Deficit and Disruptive Behavior Disorders ▶ Drug Interactions Cross-References References Ballon JS, Feifel D (2006) A systematic review of modafinil: Potential clinical uses and mechanisms of action. J Clin Psychiatry 67:554–566 de Saint Hilaire Z, Orosco M, Rouch C, Blanc G, Nicolaidis S (2001) Variations in extracellular monoamines in the prefrontal cortex and medial hypothalamus after modafinil administration: a microdialysis study in rats. Neuroreport 12:3533–3537 Fava M, Thase ME, DeBattista C (2005) A multicenter, placebocontrolled study of modafinil augmentation in partial responders to selective serotonin reuptake inhibitors with persistent fatigue and sleepiness. J Clin Psychiatry 66:85–93 Greenhill LL, Biederman J, Boellner SW, Rugino TA, Sangal RB, Earl CQ et al (2006) A randomized, double-blind, placebo-controlled study of modafinil film-coated tablets in children and adolescents with attention-deficit/hyperactivity disorder. J Am Acad Child Adol Psychiatry 45:503–511 Madras BK, Xie Z, Lin Z, Jassen A, Panas H, Lynch L et al (2006) Modafinil occupies dopamine and norepinephrine transporters in vivo and modulates the transporters and trace amine activity in vitro. JPET 319:561–569 Minzenberg MJ, Carter CS (2008) Modafinil: A review of neurochemical actions and effects on cognition. Neuropsychopharmacology 33(7): 1477–1502 Minzenberg MJ, Watrous AJ, Yoon JH, Ursu S, Carter CS (2008) Modafinil Shifts human locus coeruleus to low-tonic, high-phasic activity during functional MRI. Science 322(5908):1700–1702 Robertson P Jr, Hellriegel ET (2003) Clinical pharmacokinetic profile of modafinil. Clin Pharmacokinetics 42:123–137 Turner DC, Clark L, Pomarol-Clotet E, McKenna P, Robbins TW, Sahakian BJ (2004) Modafinil improves cognition and attentional set shifting in patients with chronic schizophrenia. Neuropsychopharmacology 29:1363–1373 Wisor JP, Eriksson KS (2005) Dopaminergic-adrenergic interactions in the wake promoting mechanism of modafinil. Neuroscience 132:1027–1034 Model Organisms of Hyperkinetic Syndrome ▶ Attention Deficit Hyperactivity Disorders: Animal Models Monoamine Oxidase Inhibitors Mogadon ▶ Nitrazepam M 791 Definition Monoamine oxidase inhibitors (MAOIs) inhibit the enzyme MAO, of which there are two major isoforms, MAO-A and MAO-B. Pharmacological Properties Molecular Imaging ▶ Positron Emission Tomography (PET) Imaging Molindone Definition Molindone, a primarily dopamine D2 blocking dehydroindolone ▶ antipsychotic with an ▶ elimination halflife of 6.5 h, is mainly metabolized by 2D6 CYP450 isoenzymes. It is normally regarded as a first-generation antipsychotic. Cross-References ▶ First-Generation Antipsychotics Monoamine Depletion ▶ Amine Depletors Monoamine Hypotheses ▶ Aminergic Hypotheses for Depression ▶ Aminergic Hypotheses for Schizophrenia Monoamine Oxidase Inhibitors ANDREW HOLT1, DARRELL D. MOUSSEAU2, GLEN B. BAKER3 1 Department of Pharmacology, University of Alberta, Edmonton, AB, Canada 2 Cell Signalling Laboratory, Department of Psychiatry, University of Saskatchewan, Saskatoon, SK, Canada 3 Neurochemical Research Unit and Bebensee Schizophrenia Research Unit, Department of Psychiatry, University of Alberta, Edmonton, AB, Canada History MAOIs were originally developed as ▶ antidepressants because of their ability to restore brain levels of the biogenic amines ▶ noradrenaline and 5-hydroxytrytamine (or ▶ serotonin), both of which are thought to be functionally deficient in depression. Yet, several of these drugs have also proven useful in the treatment of ▶ panic disorder, ▶ social anxiety disorder, ▶ eating disorders, pain syndromes, ▶ Parkinson’s disease, and ▶ Alzheimer’s disease. MAOIs are no longer first-line drugs for most depressive or anxiety disorders. However, they do remain important agents when first- or second-line drugs prove to be ineffective or intolerable, and are especially effective in ▶ atypical depression and depression associated with anxiety, panic, or phobias (Kennedy 1994; Kennedy et al. 2005). The MAOIs may be classified according to selectivity (for either the MAO-A or MAO-B isoform) and reversibility (reversible or irreversible) (see Kennedy et al. 2005; Stahl and Felker 2008 and Youdim et al. 2006). Noradrenaline and serotonin are preferred substrates for MAO-A, while b-phenylethylamine and benzylamine are selective for MAO-B. Selective inhibitors of MAO-A and MAO-B include clorgyline and l-deprenyl, respectively. Figure 1 and Table 1 show several MAOIs, some of which are currently available clinically, and others that are used only pre-clinically or are at various stages of development. Side Effects While the inhibition of MAO-A is required for antidepressant activity, clinically used irreversible inhibitors of MAO prevent the inactivation of dietary sympathomimetic amines, such as tyramine, by MAO-A in the gut wall and by MAO-A and MAO-B in the liver. The resulting increase in systemic tyramine stimulates the release of noradrenaline from sympathetic varicosities in the vascular wall. This also occurs with clorgyline, despite its lack of effect upon hepatic MAO-B. As a consequence of MAO-A inhibition, noradrenaline accumulates and produces a series of symptoms, usually starting with headaches and, if not dealt with appropriately, potentially culminating in a hypertensive crisis. This adverse effect is called the ‘‘cheese effect,’’ so-named because it was originally observed in patients taking MAOIs who had ingested foods M 792 M Monoamine Oxidase Inhibitors Monoamine Oxidase Inhibitors. Fig. 1. Some monoamine oxidase inhibitors and their structures. Monoamine Oxidase Inhibitors Monoamine Oxidase Inhibitors. Table 1. Some monoamine oxidase inhibitors and their actions. Generic Reversible/irreversible selectivity Iproniazid Irreversible, nonselective Isoniazid Irreversible, nonselective Phenelzine Irreversible, nonselective Isocarboxazid Irreversible, nonselective Tranylcypromine Irreversible, nonselective Clorgyline Irreversible, MAO-A selective Pargyline Irreversible, MAO-B selective Selegiline Irreversible, MAO-B selective Moclobemide Reversible, MAO-A selective Brofaromine Reversible, MAO-A selective Lazabemide Very slowly reversible, MAO-B selective Mofegiline Irreversible, MAO-B selective Milacemide Partially reversible, nonselective Toloxatone Reversible, MAO-A selective Befloxatone Reversible, MAO-A selective Pirlindole Reversible, MAO-A selective Rasagiline Irreversible, MAO-B selective Ladostigil Irreversible, MAO-B selective PF 9601N Irreversible, MAO-B selective Aliphatic N-propargylamines Irreversible, MAO-B selective Table modified from Kennedy et al. 2005 such as aged cheeses that have a high tyramine content. This food–drug interaction can occur with some of the MAOIs used currently in the clinic, including ▶ tranylcypromine, ▶ phenelzine, and ▶ isocarboxazid (all of which act as irreversible inhibitors of both MAO-A and MAOB). Thus, when patients receive a prescription for such MAOIs, they should be informed by their physician and pharmacist of the foods that should be avoided while on these medications (Stahl and Felker 2008). Concern about the food–drug interaction mentioned above led to the development of selective MAO-B inhibitors such as l-deprenyl (▶ selegiline) as putative antidepressants. However, this selectivity for MAO-B meant that l-deprenyl did not exhibit antidepressant effects until selectivity was lost at higher doses that also caused irreversible MAO-A inhibition. Nonetheless, l-deprenyl is used in the treatment of Parkinson’s disease and has been demonstrated to have ▶ neuroprotective properties M in a large number of neurotoxicity tests in vivo and in vitro (Tatton and Chalmers-Redman 1996; Youdim et al. 2006). In fact, the neuroprotective actions of this drug have stimulated considerable research into related drugs as potential neuroprotective agents (▶ rasagiline is an example of productive research in this area). Transdermal selegiline has recently been reported to be effective as an antidepressant (Culpepper and Kovalick 2008; Stahl and Felker 2008); under these conditions, the drug is delivered directly into the systemic circulation, thereby avoiding extensive first-pass metabolism and reaching the brain in sufficiently high concentrations to inhibit both MAOA and MAO-B. In a further effort to develop antidepressants that would not precipitate the cheese effect, reversible inhibitors of MAO-A (RIMAs) were developed. ▶ Moclobemide, a competitive RIMA available clinically for several years, retains antidepressant properties, but the risk of a hypertensive crisis is reduced since dietary tyramine can still compete with moclobemide for the active site on MAO-A. RIMAs have a further advantage over irreversible inhibitors in that after cessation of treatment with the latter drugs, a period of approximately 2 weeks is required before MAO fully recovers (following the synthesis of new enzyme). In the case of RIMAs, the recovery of enzyme occurs as inhibitors are cleared from the body and is usually complete in 2–5 days. This is an important consideration when shifting a patient to a drug regimen that would otherwise be contraindicated because it would result in elevated levels of serotonin (e.g., a ▶ selective serotonin reuptake inhibitor [SSRI]). Although a hypertensive crisis is a feared adverse effect associated with irreversible MAOIs, this is usually the result of a food–drug or drug–drug interaction. Paradoxically, orthostatic hypotension is a more common adverse cardiovascular effect with MAOIs. ▶ Serotonin syndrome may result if an MAOI is given with another drug that also increases the availability of serotonin (see Table 1). ▶ Insomnia can be a problem with tranylcypromine (an MAOI with a structure similar to that of amphetamine), and the irreversible MAOIs can produce weight gain, peripheral edema, and ▶ sexual dysfunction (Kennedy et al. 2005). A discontinuation syndrome (arousal, mood disturbances, and somatic symptoms) may occur with phenelzine or tranylcypromine if they are discontinued abruptly. Drug–Drug Interactions In addition to the food–drug interaction mentioned earlier, drug–drug interactions must be taken into consideration when prescribing MAOIs; some of these may be life 793 M 794 M Monoamine Oxidase Inhibitors Monoamine Oxidase Inhibitors. Table 2. Potential drug–drug interactions involving monoamine oxidase inhibitors. Interacting drugs Examples Possible result Drugs that stimulate the release of or inhibit the reuptake of noradrenaline at sympathetic neurons; decongestants Amphetamines, methylphenidate, ephedrine, phenylephrine, phenylpropanolamine, pseudoephedrine, oxymetazoline, some antidepressants, tramadol, sibutramine, phentermine Hypertension Drugs metabolised by monoamine oxidase Phenylephrine (oral), sumatriptan, citalopram Hypertension, increased serum levels of sumatriptan, citalopram Drugs that inhibit serotonin reuptake SSRIs, clomipramine, imipramine, meperidine, dextromethorphan, propoxyphene, venlafaxine, chlorpheniramine, brompheniramine, tramadol Serotonin syndrome, confusion, agitation, hypomania, sweating, myoclonus, fever, coma, possible fatality Serotonin agonists Sumatriptan Serotonin syndrome b-Blockers Increased hypotension, bradycardia Oral hypoglycemics Increase hypoglycemic affects Table modified from Kennedy et al. 2005 and Stahl and Felker 2008 threatening. These potential drug–drug interactions are summarized in Table 2. Metabolism Most of the MAOIs are metabolized extensively, and various ▶ cytochrome P450 (CYP) enzymes are involved in this metabolism (see Kennedy et al. 2005). Beyond Inhibition of MAO The MAOIs are multifaceted drugs and have been reported to bind to a wide variety of other enzymes, receptor systems, and uptake pumps that may contribute to their therapeutic and/or adverse effects. Depending on the MAOI involved, interactions with the following have been reported: other amine oxidases; various transaminases, decarboxylases, dehydrogenases, cytochromes P450 (CYPs); biogenic amine receptors and transporters; imidazoline binding sites; and sigma receptors (Holt et al. 2004). Several MAOIs also cause marked increases in brain levels of ▶ trace amines such as b-phenylethylamine and ▶ tryptamine, both of which can affect the normal function of classical neurotransmitter amines such as noradrenaline, dopamine, and serotonin. There has been extensive interest in recent years in the possible neuroprotective effects of MAOIs and their potential for the treatment of neurodegenerative disorders. For example, the MAO-B inhibitors l-deprenyl and rasagiline are neuroprotective and are used as anti-Parkinsonian drugs and, to a lesser extent, as therapeutic agents in Alzheimer’s disease (Youdim et al. 2006). In many cases, these neuroprotective actions appear to be independent of the inhibition of MAO. l-Deprenyl and rasagiline have been reported to prevent the initiation of apoptotic cascades by up-regulating the anti-apoptotic protein Bcl-2 and down-regulating pro-apoptotic proteins such as Bad and Bax, and to prevent the activation and nuclear localization of glyceraldehyde-3-phosphate dehydrogenase (Youdim et al. 2006). Preliminary reports indicate that l-deprenyl might be useful for the treatment of negative symptoms in ▶ schizophrenia. Phenelzine has been shown to provide neuroprotection in an animal model (global ischemia) of stroke, and its contribution to neuroprotection might rely on actions as diverse as the inhibition of ▶ GABA transaminase (GABA-T) and the elevation of brain GABA, the elevation of brain ornithine, and the sequestration of toxic aldehydes such as 3-aminopropanal and acrolein (Baker et al. 2007; Sowa et al. 2004). Although unrelated to MAO, semicarbazide-sensitive amine oxidase (SSAO) is inhibited by some MAOIs (e.g., phenelzine). SSAO has been the subject of extensive research in the areas of Mood Disorders inflammation, neuropsychiatry, and cardiovascular complications of diabetes in recent years, and might play a role in Alzheimer’s disease-related pathology (Jiang et al. 2008). The RIMA moclobemide has been reported to have anti-Parkinsonian activity and neuroprotective effects in a model of cerebral ischemia. Both tranylcypromine and phenelzine have been shown to increase the expression of ▶ brain-derived neurotrophic factor (BDNF) in rat brain after chronic administration. Clorgyline (an irreversible inhibitor of MAO-A), like l-deprenyl and rasagiline, contains an N-propargyl moiety, and has been reported to be neuroprotective in vitro and in vivo (Baker et al. 2007), albeit at doses lower than are required to inhibit MAO. Increased activity and expression of MAO have been reported in Alzheimer’s disease, suggesting that MAOIs should be investigated more thoroughly as adjunctive drugs in this neuropsychiatric disorder. Ladostigil, a drug that combines the properties of rasagiline with anticholinesterase activity, is currently in phase II clinical trials for treatment of Alzheimer’s disease (Youdim et al. 2006), while related drugs that also possess iron-chelating properties or inhibitory potency versus ▶ glutamate release are in preclinical development. In summary, although MAOIs are not currently used extensively in mood and anxiety disorders because of clinically relevant food–drug and drug–drug interactions, they continue to have an important place in the treatment of psychiatric and neurological disorders and are exciting keys to the development of future drugs with potential neuroprotective activity. Acknowledgments The authors are grateful to CIHR, the Davey Endowment, the CRC/CFI programs and the Abraham and Freda Berger Memorial Fund for funding and to Sara Tomlinson for secretarial support when preparing this review. Cross-References ▶ Antidepressants ▶ Brain-Derived Neurotrophic Factor ▶ Depression ▶ Neuroprotection ▶ Panic Disorder ▶ Social Anxiety Disorder ▶ Trace Amines References Baker GB, Sowa B, Todd KG (2007) Amine oxidases and their inhibitors: what can they tell us about neuroprotection and the development of drugs for neuropsychiatric disorders? J Psychiatry Neurosci 32:313–315 M Culpepper L, Kovalick LJ (2008) A review of the literature on the selegiline transdermal system: An effective and well-tolerated monoamine oxidase inhibitor for the treatment of depression. Prim Care Companion J Clin Psychiatry 10:25–30 Holt A, Berry MD, Boulton AA (2004) On the binding of monoamine oxidase inhibitors to some sites distinct from the MAO active site, and effects thereby elicited. Neurotoxicology 25 (1–2):251–266 Jiang ZJ, Richardson JS, Yu PH (2008) The contribution of cerebral vascular semicarbazide-sensitive amine oxidase to cerebral amyloid angiopathy in Alzheimer’s disease. Neuropathol Appl Neurobiol 34:194–204 Kennedy SH (ed) (1994) Clinical advances in monoamine oxidase inhibitor therapies. American Psychiatric Press, Washington, DC Kennedy SH, Holt A, Baker GB (2005) Monoamine oxidase inhibitors. In: Sadock BJ, Sadock VA (eds) Comprehensive textbook of psychiatry, vol 2, 8th edn. Lippincott Williams & Wilkins, New York, pp 2854–2863 Sowa BN, Todd KG, Tanay VAMI, Holt A, Baker GB (2004) Amine oxidase inhibitors and development of neuroprotective drugs. Curr Neuropsychopharmacol 2:153–168 Stahl SM, Felker A (2008) Monoamine oxidase inhibitors: A modern guide to an unrequited class of antidepressants. Trends Psychopharmacol 13(10):855–870 Tatton WG, Chalmers-Redman RM (1996) Modulation of gene expression rather than monoamine oxidase inhibition: (-)-deprenylrelated compounds in controlling neurodegeneration. Neurology 47:S171–S183 Youdim MBH, Edmondson D, Tipton KF (2006) The therapeutic potential of monoamine oxidase inhibitors. Nature Rev Neurosci 7:295–309 Monoamines Definition Monoamines (so-called because they have one organic substituent attached to the nitrogen atom) include ▶ serotonin, ▶ norepinephrine, and ▶ dopamine, all of which are neurotransmitters that are important in the pathophysiology and treatment of psychiatric disorders. Monoamines are subdivided into ▶ catecholamines and ▶ indoleamines. Mood Disorders Synonyms Affective disorders Definition Depressive, manic, and hypomanic disorders like ▶ major depression, ▶ mania, hypomania, ▶ bipolar disorder, ▶ dysthymia, or cyclothymia. 795 M 796 M Mood Stabilizers Mood Stabilizers GUY M. GOODWIN University Department of Psychiatry, Warneford Hospital, Headington, Oxford, UK Synonyms Long-term treatments for bipolar disorder Definition Mood stabilizers are pragmatically defined by their clinical efficacy in ▶ bipolar disorder. Bipolar disorder is a complex condition as it is expressed as episodic periods of contrasting mood disturbance – ▶ mania and ▶ depression – and its long-term or maintenance treatment must prevent new episodes of both. Any medicine that achieves this can be said to be a mood stabilizer. Pharmacological Properties History The first mood stabilizer was ▶ lithium. It was discovered over 60 years ago by guided serendipity. Lithium salts of urea were found by an Australian doctor John Cade to be sedative in animals. He had reasoned that urea itself was an active component, but realized that, in fact, lithium was unexpectedly tranquilizing. Immediate trials in patients with ▶ mania suggested acute efficacy and subsequent experience showed that lithium could markedly modify the course of bipolar disorder (then called manicdepression) in the long term. The effects were anecdotally so dramatic for some extremely disabled patients that a group of psychiatrists and scientists, led most notably by Mogens Schou, quickly became self-proclaimed lithium enthusiasts and by the 1960s, had influenced practice in many parts of the world. The history of lithium’s acceptance was interrupted by highly vocal criticism of the methodology of its early adopters. While the critics were right about the methodology, they were wrong about lithium (which they proclaimed on equally little evidence to be a dangerous yet inactive placebo). Subsequent trials have repeatedly shown that lithium is an effective medicine for the prevention of relapse (especially to the manic pole) in bipolar disorder (Geddes et al. 2004). Where the illness starts with and tends to relapse to the manic pole, lithium can produce remarkable mood stability. However, such success is seen in only about 30% of patients with the severe form of the disorder. Therefore, for many patients, other mood stabilizers are required to meet their unmet needs. ▶ Valproate and ▶ carbamazepine are also often described as mood stabilizers, although the evidence is weaker than for lithium. Both also tend to be most active against mania. There is also good evidence for ▶ antipsychotics in mood stabilization (Mahli et al. 2005): ▶ quetiapine acts against both poles of the illness. It also turns out that other agents can be effective in long-term treatment against, for example, the manic pole of the illness, without having an important impact on depression (e.g., ▶ aripiprazole, ▶ olanzapine, and ▶ risperidone), and vice versa (e.g., ▶ lamotrigine). Some authors have argued for extending the term mood stabilizer to include medicines with all these effects. Even more loosely, there was at one time a tendency to extend the term mood stabilizer to include ▶ anticonvulsants, effectively by extrapolation from the examples of ▶ valproate and ▶ carbamazepine. In the case of ▶ topiramate and ▶ gabapentin, subsequent clinical trials proved to be negative. There seems little advantage to a more liberal definition except for marketing the drugs. Mechanism of Action It follows from the definition that there are no pharmacological properties that define mood stabilizers as a class. Moreover, our understanding of the neurobiology of mania and depression remains highly provisional. However, the fact of clinical efficacy has often preceded an understanding of mechanism and has been a major driver to further research. Thus, the individual medicines effective in stabilizing mood have a variety of effects defined for the most part in animal experiments. Lithium inhibits the phosphoinositide secondmessenger system in the brain and peripheral tissues. This may be the basis for its therapeutic (and nontheraputic) effects, although it has not been investigated as fully as might be expected. There are also some effects on the ▶ monoamine function in the brain, which have weak parallels with better-defined psychotropic drugs like ▶ antidepressants and antipsychotics. In recent years, interest has been directed to downstream cellular changes in transcription factors and other molecular pathways that may mediate ▶ neuroprotection. Lithium has some actions in common with valproate in these cellular models. The net actions of the anticonvulsants are variously pro-GABAergic and antiglutamatergic. The efficacy of lamotrigine in bipolar depression is of particular interest Mood Stabilizers given its lack of neurotoxicity at effective doses and selective effects on membrane polarization and glutamate release. ▶ Glutamate is increasingly implicated in the neuronal circuits that are believed to regulate mood, and there is an evolving pharmacology targeted on the glutamate system. Whether this next generation of molecules will be effective is still an open question. The antipsychotics that can prevent long-term relapse in bipolar disorder have a common action in blocking dopamine receptors. However, most have additional actions on serotonergic function in the brain and; in the specific case of quetiapine, an active metabolite that appears to block noradrenergic reuptake, a property shared with a number of antidepressants. Whether these effects, in addition to dopamine blockade, are important remains an open question. ▶ Pharmacokinetics Lithium provides an unusual example for psychopharmacology of a drug whose plasma levels are routinely monitored in clinical practice. It is necessary because the drug has a narrow therapeutic ratio: in other words, the difference between a blood level that is just effective and that which can poison the patient is relatively small. For efficacy, 0.5 mmol/L is regarded as the minimum effective level in plasma taken 18 h after the last dose of lithium. Levels over 1.5 mmol/L are potentially toxic and definitely not recommended. The commonest well-tolerated level is around 0.7 mmol/L, although slightly higher levels (0.8–1.0 mmol/L) are often recommended. Choice of level should always be informed by any symptoms the patient describes. A variety of effects can limit doses to those achieving well below 1.0 mmol/L. Although not routinely used to guide treatment, levels of valproate and carbamazepine are often available because of their use in epilepsy. A valproate level between 50 and 125g/mL has been associated with acute response in mania. Importantly, because combined treatments are so often necessary in bipolar patients, carbamazepine promotes enzyme induction and can lower the levels of a range of other agents (including the contraceptive pill); hence, higher doses of comedication may often be necessary. Conversely, valproate approximately doubles the availability of lamotrigine, thus halving the dose required for efficacy. In the elderly, it is common for the required dosage to be substantially less than those used in younger people. Side effects are an important guide, as ever, to what the dosage should be. The highest well-tolerated dose (whatever the actual figure) is usually the best choice in bipolar disorder. M Tolerability and Safety In general, mood-stabilizing medicines have to be reasonably well tolerated because patients are expected to take them week in and week out for many years – often indefinitely. There is a tendency for doctors to underestimate the adverse impact of medicines on their patients. The most important adverse subjective effects of psychotropic drugs that are used to treat bipolar disorder tend to be tiredness, sedation, and weight gain. In addition, lithium can produce tremor, increased urine volumes, and thyroid dysfunction. Attention to minimizing these problems (by optimizing doses) is essential for good adherence to prescribed medicines. Weight gain was formerly seen as a largely cosmetic problem. It is now realized to be a much more important because obesity combined with lack of exercise and smoking is associated with the so-called metabolic syndrome. This is a composite term for biochemical, blood pressure, and weight indices associated with older age and higher body mass index. It is the prelude to diabetes, coronary heart disease, and stroke. Several antipsychotics used to treat bipolar disorder, including ▶ clozapine, ▶ olanzapine, and ▶ quetiapine are particularly associated with increased weight gain and the risk of dyslipidaemia, hypercholesterolaemia, and elevated glucose. In an increasingly obese population, this is a growing concern and requires active prevention wherever possible and treatment of risk factors. In pregnancy, there is a risk of teratogenicity from several of the medicines used as mood stabilizers. Thus, especially in the first 3 months of fetal development, drugs may interfere with the formation of the most complex organs. The neural tube and heart are especially vulnerable. Lowest risks appear to be associated with the antipsychotics. Higher ▶ teratogenic risks are associated with lithium and especially the anticonvulsants (valproate > carbamazepine > lamotrigine). Decisions about the use of medication in pregnancy by women with bipolar disorder are always difficult. Sudden discontinuation (Goodwin, 1993) or switching medicines risks destabilizing mood and precipitating relapse of the bipolar disorder. Treatment of an acute episode in a pregnant woman may be both highly stressful and require much higher doses of psychotropic drugs than would be required for prophylaxis. Treatment issues also arise postpartum. Childbirth greatly increases the risk of relapse in patients with bipolar disorder in the weeks and months after delivery. Bipolar women with a previous history of a severe postpartum episode (puerperal psychosis) and bipolar women with a 797 M 798 M Moperone family history of puerperal psychosis will have a >50% risk of severe relapse. Medication given to prevent this outcome will often appear in breast milk and have potential consequencies for the neonate. Conclusion The term mood stabilizer is in common use, even though it imprecisely describes what such medicines do and certainly does not define a meaningful pharmacological action or even set of actions. The strictest definition, proposed by Bauer and Mitchner (2004) is prophylaxis and the prevention of recurrence and evidence of short-term efficacy for both poles of the illness. Lithium may just meet the criterion despite its relative weakness against depression; quetiapine certainly does but clinical experience is less. Therefore, for any agent to be called a mood stabilizer, we probably need to know how it performs in comparison to lithium in the long term. We also need to be able to gauge its relative effects against the manic and depressive poles of the illness. Medicines that reduce the risk of relapse to mania and/or depression in bipolar patients can contribute to mood stability, whatever we call them. Some contemporary uses of the term are overinclusive. However, mood stability is a goal that patients, their families, and attending doctors share. If a medicine helps, then doctors are more likely to prescribe it and patients are more likely to take it. In other words, the term mood stabilizer is chosen to be comforting and persuading. This does not detract from the scientific challenge to understand the mechanisms underlying mood stabilization and how we improve treatment. British Association for Psychopharmacology. J Psychopharmacol 23:346–388 Harwood AJ, Agam G (2003) Search for a common mechanism of mood stabilisers. Biochem Pharmacol 66:179–189 Malhi GS, Berk M, Bourin M, Ivanovski B, Dodd S, Lagopoulos J, Mitchell PB (2005) Atypical mood stabilisers: a ‘typical’ role for atypical antispcyhotics. Acta Psychtr Scand 111(Suppl 426):29–38 Moperone Definition Moperone is a first-generation (typical) antipsychotic drug that belongs to the ▶ butyrophenone type approved in Japan for the treatment of ▶ schizophrenia. It has higher antagonist affinity for D2- than 5-HT2A-receptors. It also has high binding affinity for sigma receptors. It can induce ▶ extrapyramidal motor side effects, insomnia, and thirst, but it displays generally low toxicity. Cross-References ▶ Butyrophenones ▶ Extrapyramidal Motor Side Effects ▶ First-Generation Antipsychotics Morals ▶ Ethical Issues in Animal Psychopharmacology ▶ Ethical Issues in Human Psychopharmacology Cross-References ▶ Anticonvulsants ▶ Antidepressants ▶ Bipolar Disorder ▶ Classification of Psychoactive Drugs ▶ Glutamate ▶ Monoamines References Bauer MS, Mitchner L (2004) What is a ‘‘mood stabilizer’’? An evidencebased response. Am J Psychiatry 161(1):3–18 Geddes JR, Burgess S, Hawton K, Jamison K, Goodwin GM (2004) Longterm lithium therapy for bipolar disorder: systematic review and meta-analysis of randomised controlled trials. Am J Psychiatry 161:217–222 Goodwin GM (1994) The recurrence of mania after lithium withdrawal: implications for the use of lithium in the treatment of bipolar affective disorder. Br J Psychiatry 164:149–152 Goodwin GM (2009) Evidence-based guidelines for treating bipolar disorder: revised second edition – recommendations from the Morphine Definition Morphine is a highly potent opiate analgesic drug. It is the principal active ingredient in opium that is derived from the opium poppy, Papaver somniferum. It is considered to be the prototypical ▶ m-opioid agonist. It acts directly on the m-opioid receptors to relieve pain. Morphine has a high potential for addiction; tolerance and both physical and psychological dependence develop rapidly. Cross-References ▶ Addiction ▶ Analgesics ▶ Dependence ▶ Diamorphine Motor Activity and Stereotypy Morphine-Like Compounds ▶ Mu-Opioid Agonists Morphogenesis ▶ Ontogeny M 799 Mosaic ▶ Chimera Mosapramine Synonyms Y-516 Definition Morris Water Maze Synonyms Morris water navigation task Definition The Morris water maze was developed by Richard Morris (Morris 1984) and is used to assess ▶ spatial learning in rats and mice. The apparatus comprises a pool of varying diameter, typically 1.2–2m (smaller for mice) and depth of approximately 60 cm which is filled with opaque water and contains a hidden platform. The animal is placed in the pool at different starting points and swims to the hidden platform (there are many variations of this to assess, e.g., ▶ working memory, daily learning, ▶ delayed match to sample). There are no intramaze cues and the dominant strategy in relocating the hidden platform is thought to be truly spatial. The main measure for the water maze is the latency to find the platform. In order to control for search strategies, other measures can be taken, including time spent in each quadrant during the main trials and probe trials (where the platform is removed), and analyses of the path length. The advantages of the Morris water maze are the rapid acquisition of the task, the ability to assess learning and performance, markers of motivation, and motor ability (e.g., swim speed), and the innate motivation of rats to want to find the platform without being distressed. Mosapramine is a first-generation (typical) antipsychotic drug that belongs to the iminodibenzyl class approved in Japan for the treatment of schizophrenia. It is a potent dopamine antagonist with high affinity for D2, D3, and D4 receptors, but lower affinity for 5-HT2A-receptors. It can induce extrapyramidal motor side effects and drowsiness, but it displays generally low toxicity. Cross-References ▶ Extrapyramidal Motor Side Effects ▶ First-Generation Antipsychotics ▶ Schizophrenia Motivational Valence ▶ Taste Reactivity Test Motor Activity and Stereotypy STEPHEN C. FOWLER Department of Pharmacology and Toxicology 5036 Malott Hall, Schieffelbusch Institute for Life Span Studies, University of Kansas, Lawrence, KS, USA Cross-References ▶ Spatial Learning References Morris R (1984) Developments of a water-maze procedure for studying spatial learning in the rat. J Neurosci Methods 11(1):47–60 Synonyms Exploratory behavior; Locomotor activity; Motor activity: Repetitious behavior; Repetitive behavior; Spontaneous activity; Stereotypy Definition Morris Water Navigation Task ▶ Morris Water Maze Motor activity generally refers to a laboratory animal’s horizontal movements within an enclosure that permits the use of a variety of methods for quantifying such M 800 M Motor Activity and Stereotypy movements. The term is rarely used in the context of laboratory animals exercising in running wheels or on treadmills. Stereotypy refers to abnormally repetitive behavior that reflects dysfunction of nervous system induced by drugs (especially those that elevate brain ▶ dopamine levels), brain lesions, genetic mutations, or environmental circumstances (e.g., wild animals in captivity). Depending on the context in which the term is used, stereotypy or stereotyped behavior may include specific features of locomotor activity (e.g., moving predominantly in the same direction along the inside perimeter of an enclosure) or may be restricted to repetitive behaviors that primarily occur in the absence of locomotion (e.g., rhythmic head movements, known as focused stereotypy; gnawing on the wire-mesh floor of a cage, known as oral stereotypy). Impact of Psychoactive Drugs The locomotor activity assay is one of the most frequently used procedures in the initial exploration of a drug’s putative psychoactive effects. Typically, a mouse or rat is administered a fixed dose of a drug and is then placed in an enclosure with a flat floor large enough to enable free movement of several body lengths in any direction in the horizontal plane. A human observer or an instrument is used to quantify the distance an animal moves. Other species-typical rodent behaviors, such as grooming or rearing, may also be recorded depending on the objective of the research, the recording method, and other experimental conditions. Comparisons of a variety of techniques for measuring locomotor activity can be found in Fowler et al. (2001). The success of locomotor activity as a drug assay depends largely on the fact that most types of mice and rats spontaneously engage in ‘‘exploratory’’ behaviors, when placed in environments new to them. As it moves from place to place in the new environment, a mouse or rat uses its sensory organs and information processing endowments to learn the available speciesrelevant attributes of the environment. Importantly, the study of drug effects on spontaneously expressed behavior is economical because the experimenter does not need to impose any preconditions on the animals (e.g., neither food restriction nor explicit training in a specific task, etc.) in order to assure that easily measurable amounts of species-typical behavior will occur in placebo-treated animals (‘‘controls’’). The behavioral output of controls then affords a baseline condition with which the effects of drug treatments can be compared. Depending on type of drug, drug dose, amount of experience in the environment, amount of experience with the drug, duration of the observation period, route of drug administration, lighting conditions, time of day, and many additional variables, the drug treatment may increase, decrease, or have no effect on measures of locomotor activity compared to control performance. When interpreting the results of these kinds of experiments, one should keep in mind the fact that a priori suppositions about a drug’s ‘‘stimulant’’ or ‘‘depressant’’ activity may not be confirmed in a straightforward manner. For example, the drug, ▶ pentobarbital, which in clinical terms is described as a ‘‘sedative-hypnotic,’’ increases locomotor activity in rodents at low doses and decreases or abolishes locomotor activity at higher doses as an anesthestic dose level is neared. The ▶ psychomotor stimulant drug, d-amphetamine sulfate, also increases locomotor activity in rats at doses around 1.0 mg/kg, but at doses around 5.0 mg/kg of ▶ amphetamine produces a focused stereotypy syndrome characterized by the absence of locomotion and the concurrent expression of rapid rhythmic head movements. In research situations where a priori information about a drug is lacking, a relatively wide range of doses should be used in locomotor activity assays to detect lower-dose increases and higher-dose decreases in locomotor activity. Amphetamine-Induced Locomotor Activity: Illustrative Data Figure 1 provides data illustrating increased locomotor activity induced by amphetamine. The measure of locomotor activity was the ▶ distance traveled during six 10-min time blocks in a 1-h recording session. Distance traveled was calculated from the rats’ center of force x–y coordinates as the rats moved on the 28 cm28 cm load plate of a ▶ force-plate actometer (Fowler et al 2001). The saline group exhibited rapid ▶ habituation during the first 40 min, and the distance traveled remained low for the remainder of the hour. Compared with the saline group, the amphetamine-treated rats showed significantly more locomotor activity at all but during the first 10-min interval, the standard errors of the mean (sem) overlapped. The curvature in the amphetamine plot between block 2 and block 6 is likely the result of changes in the brain level of amphetamine. Brain ▶ microdialysis studies have shown that intraperitoneally administered d-amphetamine sulfate reaches a peak level at about 22–26 min after injection and is eliminated with a halflife of about 40–45 min. During the sixth time block (see Fig. 1), the amphetamine group exhibited more distance traveled than the control group in every time block except the first, suggesting that amphetamine continued to induce abnormally high levels of locomotion 1 h after treatment. Given that the time-related diminution of locomotion in the control group is a representative example Motor Activity and Stereotypy of habituation, one can hypothesize that amphetamine, at least in part, produced elevated levels of locomotor activity by interfering with the habituation process. A well designed experiment with ▶ cocaine, a psychomotor stimulant with many pharmacological effects similar to those of amphetamine, supports this hypothesis (Carey et al. 2003). Thus, the increased locomotor activity induced by a variety of CNS-active drugs is not simply a ‘‘motor effect,’’ because the multiple sites of action of such drugs reside in multiple anatomical loci that perform information processing, associative, and motivational functions, as well as motor functions. Amphetamine-Induced Focused Stereotypy In male Sprague Dawley rats, the first experience with a 2.5 mg/kg, ip, dose of amphetamine will induce an increase in locomotor activity in a large majority of the subjects compared to the untreated controls (see Fig. 2). Motor Activity and Stereotypy. Fig. 1. Effect of d-amphetamine sulfate on locomotor activity in two separate groups (n=8) of male Sprague Dawley rats. The amphetamine was injected intraperitoneally a few seconds before the rats were individually placed in a dark force-plate actometer for a 1-h recording session. Data are based on the rats’ first exposures to the apparatus. The saline group exhibited habituation during the first 40 min, and distance traveled remained low for the remainder of the hour. Compared to the saline group, the amphetamine-treated rats showed hyperactivity throughout the hour. Over the entire hour the saline group mean distance traveled was 59.71 m (s.e.m. 7.23 m), and the amphetamine-treated group traveled 133.51 m (s.e.m. 6.59 m). M However, when given repeatedly, the same dose of amphetamine predominantly evokes, a syndrome characterized by an absence of locomotion accompanied by rapid head movements that have been described as ‘‘sniffing’’ and/or ‘‘head bobbing’’. This syndrome is focused stereotypy or the ‘‘stationary phase’’ of the amphetamine response, and it has been observed in observation arenas as large as 3.0 m3.5 m (Schiorring 1971). The focused stereotypy score in Fig. 2 was calculated by combining a quantitative measure of spatial confinement with the variance of vertical force variation within the boundaries of the space used (see Fowler et al. 2007a for details). This score is low for a sleeping animal (high spatial confinement but low force variance because of a lack of ‘‘in-place’’ movements). The focused stereotypy score is also low when spatial confinement is low (movements are dispersed across the floor), despite the presence of high force variance associated with ambulation. When spatial confinement is pronounced and force variance is also high, a high focused stereotypy score is obtained. In Fig. 2, the switch from first-dose locomotor stimulation to tenth-dose substantial focused stereotypy (Fig. 2, panel b) can be appreciated by comparing the distance traveled data of Injection 1 (panel a, triangles) with the same measure for Injection 10 (panel a, squares), during the second 30 min of the session. Correspondingly, during the same time period, the focused stereotypy scores after Injection 1 were near zero, but averaged higher than 1 after Injection 10. This change in response topography from locomotion to stationarity and focused stereotypy is the result of a ▶ sensitization process, whereby rats become more sensitive to amphetamine or other psychomotor stimulants as repeated dosing ensues. After injection 10, distance traveled (see Fig. 2, panel a, squares) did not drop to no-drug levels because three rats did not make a complete switch to focused stereotypy and continued to display substantial locomotor activation. However, all but one of the eight rats after the tenth injection showed a decrease in distance traveled compared to Injection 1. The aberrant rat actually exhibited evidence of sensitization of locomotor response, as suggested by its 67.9% increase in distance traveled between injection 1 and 10. Two important points can be made from these observations: (1) Individual differences in response to drugs are to be expected, especially when genetically heterogenous outbred strains of rats are used, and (2) when amphetamine or similar drugs are under study, at doses near the threshold for expression of focused stereotypy, the observed behavioral effect may be in opposite directions, with a subset of rats showing increased locomotor activity and another subset exhibiting locomotor suppression. 801 M 802 M Motor Activity and Stereotypy Motor Activity and Stereotypy. Fig. 2. Measures from a force-plate actometer showing how the behavioral effects of d-amphetamine sulfate at 2.5 mg/kg depends on time after treatment, amount of experience with the drug, and the route of administration. After experiencing a 4-h habituation session (black circles, panels a and b), in subsequent sessions eight male Sprague Dawley rats received injections (Inj) of amphetamine eleven times, 3–4 days apart. The first ten injections were given ip, and the 11th injection (green stars) was administered sc in the same volume and dose as the previous ip injections. Panel (a) shows distance traveled for successive 3-min periods in the 4-h recording session. In panel (b) are plotted, also in 3-min intervals, group mean focused stereotypy scores for the indicated treatment conditions. Panel (c) shows the group mean power spectra of the vertical force variations over 15-min periods for the same treatments. The distinctive spectral peaks near 10-hz in the red and green power spectra reflect the rhythm of the head movements of focused stereotypy. Another important point is that a univariate (i.e., singledependent-variable method such as distance traveled) approach to behavioral pharmacology with ▶ indirectacting dopamine agonists can easily lead to erroneous interpretations of a drug’s effects. For example, if one did not have a stereotypy score (Figure 2, panel b) and relied only on the distance traveled information (Figure 2, panel a), one may erroneously conclude that, between Injection 1 and Injection 10, ▶ tolerance (decrease in distance traveled) had occurred instead of sensitization. Effect of Route of Administration on Focused Stereotypy An 11th injection of 2.5 mg/kg amphetamine was given, but this injection was by the subcutaneous route (sc). Figure 2 (panels a and b, stars) shows that changing the route of administration from ip to sc intensified and lengthened the expression of focused stereotypy. Gentry et al. (2004) also found more pronounced focused stereotypy for 3.0 mg/kg sc ▶ methamphetamine compared to the same dose given ip. The group mean increase in Motor Activity and Stereotypy focused stereotypy after the sc injection resulted from (1) a recruitment of the three previously low-stereotypy rats to full stereotypy and (2) all five rats that were already expressing focused stereotypy had higher scores after sc amphetamine treatment compared to the previous ip treatment with the same dose. ▶ Pharmacokinetic studies (Gentry et al. 2004) have shown that sc compared with ip amphetamine in male Sprague Dawley rats reaches a lower peak blood concentration, takes longer to reach its peak, and is eliminated more slowly (i.e., longer duration of action). Thus, the pharmacokinetic data indicating a longer duration of action for sc compared to ip administration of amphetamine are consistent with the behavioral data shown in Fig. 2 (in panels a and b, compare behavioral measures for injections 10 and 11). However, neither the reported pharmacokinetic time-topeak nor peak level achieved is in accord with the behavioral data. In Fig. 2 (panel b), focused stereotypy scores began to rise earlier in the session for the sc injection than for the ip injection (opposite the pharmacokinetic later rise in concentration), and the sc-related focused stereotypy scores reached a substantially higher level than those seen for the ip injection (again, opposite the lower pharmacokinetic peak for sc dosing). While no explanation is yet available for this difference in the stereotypyevoking efficacy of the ip and sc injection methods, the empirical data show that route of administration can substantially influence the focused-stereotypy-inducing effects of amphetamine. Rhythmicity of Head Movements During Focused Stereotypy During the expression of amphetamine-induced focused stereotypy, not only are the head movements observably repetitive they are also strongly rhythmic (Fowler et al. 2001; Fowler et al. 2007a). Application of signal processing techniques, such as ▶ power spectral analysis, to a rat’s variations in vertical force recorded with a force-plate actometer shows that the head movements of focused stereotypy have a tightly regulated rhythm near 10 Hz (Hz=cycles/s). Panel c in Fig. 2 presents group mean power spectra that were calculated for each 15 min interval in the 4-h session. Once the expression of focused stereotypy begins in male Sprague Dawley rats, the near 10-Hz rhythm can be continuously sustained for an hour or more (see Fig. 2, panel c, top row of functions, frames 2, 3, 4, and 5 from left to right). The near 10-Hz rhythm has been recorded in rats separately treated with four different indirect-acting dopamine agonists: amphetamine, ▶ nomifensine, ▶ methylphenidate, and cocaine. The precision of rhythm regulation in rats expressing focused M 803 stereotypy rivals or exceeds that of consummatory licking (7.0 Hz), hindlimb scratching behind the ear (8.0 Hz), within-bout vibrissal whisking (e.g., 7.0 Hz), or grooming (forepaws/face/head: 7.2 Hz; flank-licking: 3.6 Hz). The existence of these species-typical rhythmic reflexes invite the conjecture that the head-movement rhythm of amphetamine focused stereotypy is also reflexive. Both the atypical ▶ antipsychotic drug, ▶ clozapine, and the a-1 noradrenergic antagonist, ▶ prazosin, have been shown to slow the head-movement rhythm of amphetamine-induced focused stereotypy without abolishing the overall syndrome (Fowler et al. 2007a). The frank ▶ rhythmicity of amphetamine-induced head movements adds a new measurable dimension to the focused stereotypy response, and experimental analyzes of rhythm production and modulation may provide insights into the neurotransmitter systems and neuroanatomical loci that mediate druginduced stereotypy. Locomotor Activity and Stereotypy in ▶ Drug Self-Administration Experiments Measurements of locomotor activity and stereotypy in rodents have been used in drug-self-administration research in at least two different ways. One way exemplified by the work of Piazza et al. (1989), used the locomotor activity response to a novel environment (i.e., a 170-cm perimeter, 10-cm wide circular track with photobeams to detect the rats’ locomotion) to analyze individual differences in rats’ susceptibility to acquiring amphetamine self-administration later in another environment. Piazza et al. (1989) found that rats with a relatively low level of locomotor response to novelty did not acquire the amphetamine self-administration nose-poke response, while those rats exhibiting higher amount of locomotor activity acquired the response. This rodent-based experiment was one of the first to suggest that behavioral response to novelty and susceptibility to drug-abuse-like behaviors are correlated. A second way that locomotor activity and stereotypy have been combined with self-administration research is illustrated by an experiment that used a forceplate actometer as the floor of a self-administration chamber where five lever-presses by rats resulted in an intravenous cocaine infusion in a 24-h ‘‘binge’’ session (Fowler et al. 2007b). The work provided measures of rotational behavior (‘‘circling in the same direction’’), distance traveled, power spectra of the vertical force of movements (as in panel c of Fig. 2), and focused stereotypy in order to determine whether or not self-administered cocaine produced the same kind of unconditioned behaviors that are elicited by noncontingently administered bolus doses of cocaine. The results showed that all six M 804 M Motor Activity and Stereotypy rats displayed behaviors typical of noncontingently administered ▶ bolus doses. The average duration of active cocaine intake was 14.7 h (1.8 h). During this time, rats rotated in the same direction an average of 1206.5 (372.7) times, traveled an average of 914.6 m (65.9 m), and exhibited a power spectral peak near 10 Hz during 32.0% (8.4%) of the active binge duration. These data show that locomotor activity and focused stereotypy are major behavioral manifestations of self-administered cocaine and raise the possibility that unconditioned behaviors evoked by indirect-acting dopamine agonists contribute to the reinforcing effects of psychomotor stimulant drugs. Differences in Expression of Stimulant-Induced Behaviors in Rats and Mice Although it is difficult to offer accurate generalizations that are valid for all strains of rats or mice, mice tend to move more than rats, and mice tend to have shallower locomotor activity habituation functions than rats. Response topographies of stimulant-induced stereotypies of rats and mice have similarities and differences. Generally, amphetamine and other indirect-acting dopamine agonists, in sufficient doses, evoke some degree of spatial confinement in both rats and mice. Also, after very high doses of these drugs, most types of laboratory rats and mice exhibit self-injury (usually biting the dorsal wrist area sufficiently to cause bleeding and exposure of subcutaneous structures). Despite these similarities, the topography of the focused stereotypies in rats and mice can be very different in appearance. For example, while rats exhibit a complete and enduring arrest of locomotion during focused stereotypy, mice tend to retain a substantial locomotor component in their stereotypy response. Typically, mice observed in force-plate actometers do not display the narrow-band head-movement rhythm, characteristic of the rat response to a 5.0 mg/kg dose of amphetamine. Different stereotypy topographies in mice and rats make it difficult if not impossible to devise a rating scale that can be used to quantify stereotypies equally well in both species. Comparing stereotypies in rats and mice is further hindered by the heterogeneity among inbred strains of mice in their responses to amphetamine and other psychomotor stimulants. For example, BALB/cJ mice show a low-dose decrease in their locomotor activity response to 1.0 mg/kg amphetamine, and at a higher dose of 10.0 mg/kg, BALB/cJ mice exhibit vertical leaping (‘‘popping’’) behavior. C57BL/ 6J, DBA/2J, 129SvJ, and C3H/HeJ mice exhibit neither lowdose suppression of locomotor activity nor higher-dose vertical leaping behavior. Neurobehavioral Interpretations and Clinical Importance of Stereotypy Stereotyped behaviors are generally thought to reflect the dysfunction of the central nervous system (Robbins et al. 1990; Teitelbaum et al. 1990). Persons with mental retardation, autism, schizophrenia, Tourette’s syndrome, Huntington’s disease, or obsessive compulsive disorder (OCD) often exhibit excessively repetitious and apparently purposeless behaviors as do persons who have used excessive amounts of amphetamine over multi-day periods. Substantial clinical and experimental evidence points to ▶ dopamine as the brain neurotransmitter with a major role in the expression of stereotyped behavior in the aforementioned clinical abnormalities. In rodent studies of the behavioral effects of psychomotor stimulants, links have been established between dopaminergically innervated subcortical structures and expression of increased locomotor activation and stereotypy induction. Dopamine agonists are thought to induce locomotor activity by acting on the ▶ nucleus accumbens, while actions of the drugs on the caudate/putamen are believed to be critical for the expression of focused stereotypy. In the clinic, dopamine receptor-blocking drugs (e.g., the antipsychotic drug ▶ haloperidol) can be effective in suppressing the spontaneously occurring stereotypies, such as hand flapping, in persons with autism. In the laboratory, most, but not all, antipsychotic drugs can block the expression of amphetamine-induced focused stereotypy. Although a massive amount of empirical evidence implicates a role for dopamine in the evocation of locomotor activity and the expression of stereotyped behavior, much remains to be discovered about dopamine’s interaction with a multiplicity of other neurotransmitters (e.g., glutamate, GABA, acetylcholine, serotonin, neuropeptides) and their receptors in the nucleus accumbens, caudate/ putamen, and elsewhere in the brain. Given the large number of drug targets located in these brain regions, the measurement of pharmacologically induced locomotor activity and stereotypy is likely to continue to be important in the conduct of psychopharmacology research for many decades to come. Acknowledgment This work was supported by MH043429 and HD002528. Cross-References ▶ Aminergic Hypotheses for Schizophrenia ▶ Antipsychotic Drugs ▶ Cocaine ▶ Habituation Movement Disorders Induced by Medications ▶ Methylphenidate and Related Compounds ▶ Open Field Test ▶ Phenotyping of Behavioral Characteristics ▶ Psychomotor Stimulants ▶ Self-Administration of Drugs ▶ Sensitization to Drugs M 805 Motor Learning ▶ Verbal and Non-Verbal Learning in Humans Motor Memory References Carey RJ, DePalma G, Damianopoulos E (2003) Cocaine-conditioned behavioral effects: a role for habituation processes. Pharmacol Biochem Behav 74:701–712 Fowler SC, Birkestrand BR, Chen R, Moss SJ, Vorontsova E, Wang G, Zarcone TJ (2001) A force-plate actometer for quantitating rodent behaviors: illustrative data on locomotion, rotation, spatial patterning, stereotypies and tremor. J Neurosci Methods 107: 107–124 Fowler SC, Pinkston JW, Vorontsova E (2007a) Clozapine and prazosin slow the rhythm of head movements during focused stereotypy induced by d-amphetamine in rats. Psychopharmacology 192: 219–230 Fowler SC, Covington HE, Miczek K (2007b) Stereotyped and complex motor routines expressed during cocaine self-administration: results from a twenty-four hour binge of unlimited cocaine access in rats. Psychopharmacology 192:465–478 Gentry WB, Ghafoor AU, Wessinger WD, Laurenzana EM, Hendrickson HP, Owens SM (2004) (+)-Methamphetamine-induced spontaneous behavior in rats depends on route of (+)METH administration. Pharmacol Biochem Behav 79:751–760 Piazza PV, Deminière JM, Le Moal M, Simon H (1989) Factors that predict individual vulnerability to amphetamine self-administration. Science 245:1511–1513 Robbins TW, Mittleman G, O’Brien J, Winn P (1990) The neuropsychological significance of stereotypy induced by stimulant drugs. In: Cooper SJ, Dourish CT (eds) Neurobiology of stereotyped behaviour. Clarendon Press, Oxford, pp 25–63 Schiorring E (1971) Amphetamine induced selective stimulation of certain behaviour items with concurrent inhibition of others in an open-field test with rats. Behaviour 39:1–17 Teitelbaum P, Pellis SM, DeVietti TL (1990) Disintegration into stereotypy induced by drugs or brain damage: a microdescriptive behavioural analysis. In: Cooper SJ, Dourish CT (eds) Neurobiology of stereotyped behaviour. Clarendon Press, Oxford, pp 169–199 Motor Activity: Repetitious Behavior ▶ Motor Activity and Stereotypy Motor Inhibition ▶ Behavioral Inhibition Definition A motor memory develops by repetition of movements that the individual is already able to perform. Repetition is supposed to improve motor skills and to enhance some degree of automation. Presumably, long term potentiation is involved in the induction of a motor memory. Movement Disorder ▶ Tic Disorders with Childhood Onset ▶ Tics Movement Disorders Induced by Medications THOMAS R. E. BARNES Department of Psychological Medicine, Imperial College, London, UK Synonyms Antipsychotic-induced movement disorders; drug-induced motor syndromes; EPS; Extrapyramidal side effects Definition Discussion of medication-induced movement disorder generally refers to the side effects of ▶ antipsychotic medication which affect motor behavior. However, other drugs, such as lithium, valproate, antidepressants, and psychostimulants like amphetamine, can cause movement problems, principally tremor. Patients developing antipsychotic drug-induced movement disorders exhibit a range of neurological phenomena: ▶ dyskinesia, ▶ dystonia, parkinsonian features including bradykinesia, tremor and rigidity, and restless movements as part of antipsychotic-induced akathisia. While some motor syndromes such as parkinsonism, acute dystonia, and acute akathisia are more common during acute drug treatment, others M 806 M Movement Disorders Induced by Medications such as tardive dyskinesia, tardive dystonia, and chronic akathisia are more common with long-term treatment. The motor phenomena exhibited by those developing these disorders can cause functional impairment and be socially stigmatizing; and parkinsonism, akathisia, and dystonia also have mental manifestations that can be unpleasant and distressing (Owens 1999). Further, the disorders can confound clinical assessment of the psychotic illness. For example, features of parkinsonism such as bradykinesia overlap phenomenologically with symptoms of depression and negative symptoms, while akathisia may be misdiagnosed as anxiety or an exacerbation of psychotic symptoms. Role of Pharmacotherapy The four main diagnostic categories of ▶ extrapyramidal side effects (EPS) associated with antipsychotic medication are ▶ parkinsonism, ▶ akathisia, ▶ dystonia, and ▶ tardive dyskinesia. The pathophysiology of these movement disorders involves the dopamine D2 receptor-blocking properties of antipsychotic drugs. However, it should be noted that both parkinsonism and dyskinesia are observed in antipsychotic-naive patients with first-episode psychoses, suggesting that such movements reflect a neuro-dysfunction intrinsic to the pathophysiology of schizophrenia (Pappa and Dazzan 2009). While EPS were relatively common with the ▶ first-generation antipsychotics (FGAs: such as chlorpromazine, fluphenazine, haloperidol, and trifluoperazine), one of the main claims for ▶ second-generation antipsychotics (SGAs: such as aripiprazole, clozapine, olanzapine, quetiapine, and risperidone) has been a lower risk of developing such disorders, although the individual SGAs vary in their liability for EPS burden. Clinical trials and ▶ meta-analyses (Leucht et al. 2003) suggest that while SGAs have a lower liability for acute EPS and tardive dyskinesia when compared with haloperidol, even at low dosage, the evidence that this is the case in relation to other FGAs in moderate dosage is less convincing. Further, the common use of high dose or combined SGAs, or combined SGAs and FGAs, in clinical practice may compromise any such advantage. Parkinsonism and akathisia remain major problems despite the widespread use of SGAs. With regard to tardive dyskinesia, the evidence suggests a lower risk with SGAs (Correll and Schenk 2008), but prospective, longitudinal studies of SGAs as monotherapy are required to quantify the risks of tardive dyskinesia with particular drugs. Parkinsonism Antipsychotic-induced parkinsonism (sometimes called ‘‘pseudoparkinsonism’’) usually occurs within days of beginning antipsychotic treatment or after a dosage increase. It comprises a triad of bradykinesia, rigidity, and tremor. Bradykinesia is probably the core feature, and manifests itself as difficulty with the initiation of movements and slowness and interruption of the normal flow of movement. When tested by passive movement of the limbs, muscle rigidity may be revealed as being of the lead-pipe (i.e., stiffness that is uniform throughout the range of movement) or cogwheel (i.e., a ratchet-like resistance) type. Other clinical signs include a mask-like expression, lack of spontaneous gesture, and a reduction in the normal arm swing when walking. Subjectively, patients experience slowed thinking (‘‘bradyphrenia’’), fatigue, weakness and stiffness, and sometimes apathy, and diminished interest and initiative. There is some evidence that the development of parkinsonism may indicate an increased risk of developing tardive dyskinesia later. While drug-induced parkinsonism mimics idiopathic ▶ Parkinson’s disease, it is rather more an akinetic rigid syndrome, with the classical resting tremor being relatively uncommon. Nevertheless, the coincidental onset of idiopathic Parkinson’s disease should be borne in mind as a differential diagnosis, and if this is suspected because of the nature of the clinical presentation, or because the signs and symptoms prove to be persistent after antipsychotic discontinuation, a neurology referral may be indicated. Treatment options include reduction of the dose of the causal, or suspected, antipsychotic, or switching to another with evidence for a lower risk of parkinsonism. Prescription of an ▶ antimuscarinic agent (also called ▶ anticholinergic or antiparkinsonian) may be useful (Barnes and McPhillips 1996). The anticholinergic agents most commonly used are as follows: ▶ Benzatropine (benztropine) mesylate –0.5–6 mg/ day by mouth in one to two divided doses; can be sedative so if one dose is greater, administered at bedtime. Tablets are not available in the United Kingdom. ▶ Trihexyphenidyl hydrochloride (benzhexol hydrochloride) –1–2 mg daily, increased gradually; usual maintenance dose 5–15 mg daily in three to four divided doses, to a maximum of 20 mg daily. ▶ Orphenadrine hydrochloride –150 mg a day by mouth in divided doses initially, titrated slowly upwards if necessary. Usual dose range is 150–300 mg daily in divided doses. ▶ Procyclidine hydrochloride –2.5–5 mg by mouth up to three times a day initially, titrated slowly upwards if necessary. Usual maximum is 30 mg a day in divided doses. Biperiden –2 mg by mouth, one to two times a day. Anticholinergic drug prescription should be regularly reviewed, partly because parkinsonism can wane Movement Disorders Induced by Medications spontaneously, and after 3 months or so there may no longer be a need for such treatment, and partly because antimuscarinic drugs are associated with their own unwanted effects such as blurred vision, headaches, dry mouth, increased heart rate, difficulty in urinating and constipation, as well as confusion and disorientation, inability to concentrate, and memory impairment. The elderly are at greater risk of anticholinergic side effects. Akathisia ▶ Akathisia is characterized by a subjective feeling of inner restlessness and objectively by increased restless movement (Barnes 1992). These movements are not dyskinetic (involuntary, repetitive movements) but rather resemble normal patterns of restless movement. Most typically, they involve the legs, for example, walking on the spot, pacing around, or shuffling and tramping of the legs when sitting. When required to sit or stand still, patients can experience a mounting sense of tension and a compulsive desire to move in an attempt to gain some respite. The clinical significance of akathisia relates not only to the subjective dysphoria and unease experienced, but also its adverse influence on medication adherence, and its status, like parkinsonism, as a possible risk factor for tardive dyskinesia. Treatment options include reduction of the dose of the causal antipsychotic, or switching to another with evidence for a lower risk of akathisia. Low-dose ▶ propranolol (initially 20 mg bd) or other lipophilic beta-blockers, an anticholinergic drug (e.g., procyclidine, see above), or a ▶ benzodiazepine (e.g., diazepam 2–4 mg tds) may be helpful. The evidence base for such interventions is limited, though strongest for propranolol or other lipophilic beta-blockers (Miller and Fleischhacker 2000). Note that the beta-blockers are contraindicated in those with asthma or peripheral vascular disease. Taylor et al. (2007) also cite evidence for a possible reduction in symptoms with low-dose ▶ clonazepam, diphenhydramine (an antihistamine), and 5HT2 antagonists such as cyproheptadine, ▶ mirtazapine, ▶ trazodone, and ▶ mianserin. Dystonia ▶ Dystonia is defined as sustained muscle contractions causing twisting and repetitive movements or abnormal postures, which can be painful. Acute dystonia usually occurs early in drug treatment and is short-lived, although it can be distressing and frightening. It can also occur as an antipsychotic drug-withdrawal phenomenon. Tardive dystonia occurs late in the treatment, tends to be persistent, and in severe cases can be disabling and disfiguring. The neck, and jaw and tongue are the most M common sites to be affected by the muscle spasms, but trunk and limbs can also be involved. With regard to treatment options for acute dystonia, the intervention of choice is an anticholinergic drug (Barnes and McPhillips 1996), for example, procyclidine (see above for oral administration if symptoms are mild. Also by intramuscular or intravenous injection as an emergency treatment for acute drug-induced dystonia, 5–10 mg), benzatropine (see above for oral administration if symptoms are mild, but this drug may also be given intramuscularly or even intravenously as an emergency treatment for acute drug-induced dystonia: 1–2 mg repeated if symptoms reappear, to a maximum of 6 mg daily), orphenadrine (see above for oral administration if symptoms are mild), or trihexyphenidyl hydrochloride (see above for oral administration). Patients may be given an oral antimuscarinic drug to take prn (‘‘as required’’) should there be any signs of recurrence within the next few days. Other drug treatments advocated include antihistamines, such as diphenhydramine and benzodiazepines, such as ▶ diazepam (Owens 1999). With regard to treatment options for tardive dystonia, clinicians should first consider the differential diagnosis, including ▶ idiopathic torsion dystonia or secondary dystonia associated with conditions such as ▶ Huntington’s disease or Wilson’s disease. Further, there is some overlap with the features of tardive dyskinesia, with which tardive dystonia may coexist. The most common phenomena are sustained, forced, involuntary closing of the eyelids (blepharospasm), twisting of the neck to one side (torticollis) or drawing the head back (retrocollis), and involvement of the laryngeal and pharyngeal muscles affecting speech and swallowing. The condition can be hard to treat. Withdrawal of antipsychotic medication is not usually a realistic clinical option for people with an established psychotic illness. Switching to an antipsychotic with a lower liability for EPS may be helpful in a proportion of cases over time, and the best evidence is for clozapine. There are also several specific drug treatments that may be beneficial: Dopamine-depleting agents (▶ tetrabenazine, ▶ reserpine) are used in low dose, and have a reasonable evidence base to support their efficacy. They have a range of potentially unpleasant side effects including drowsiness, parkinsonism, depression, and orthostatic hypotension, although tetrabenazine has a lower side-effect burden than reserpine. There is evidence for benefit with anticholinergic agents, sometimes in high dosage, although the relevant studies have yielded mixed results, and use of these drugs runs the risk of exacerbating any co-existing tardive dyskinesia. 807 M 808 M m-PD Benzodiazepines (such as clonazepam), which have muscle relaxant properties, are also commonly used. For focal tardive dystonia which has not responded to standard measures, local injection of botulinum A toxin into the affected muscle by a neurologist with experience of the technique may be effective. Tardive Dyskinesia ▶ Tardive dyskinesia is a syndrome of abnormal involuntary movements, repetitive and stereotypic in nature and commonly referred to as choreiform (i.e., rapid, jerky). The condition usually appears late in the course of treatment, and is partly related to advancing age and possibly the psychotic illness for which the antipsychotic medication is prescribed. Tardive dyskinesia most commonly affects the oro-facial muscles; characteristically a combination of movements is observed, including chewing, tongue twisting and protrusion, and lip smacking and puckering. But virtually all parts of the body can be involved including the trunk and limbs, and respiratory muscles. The condition is most pronounced when patients are aroused and tends to ease during states of relaxation. The abnormal involuntary movements of tardive dyskinesia can result in considerable social and physical disability, although patients are usually unaware of them. Treatment options include reducing the dosage of the antipsychotic or switching to an SGA with evidence for a low liability for tardive dyskinesia, particularly if the patient has been receiving an FGA. Limited data from small studies do not provide convincing evidence of the value of these approaches (Soares-Weiser and Rathbone 2005), but clozapine is probably the antipsychotic most likely to diminish dyskinetic movements in patients with existing tardive dyskinesia. Discontinuing antimuscarinic agents may also be a worthwhile therapeutic option, given the evidence that such drugs can worsen tardive dyskinesia, but the available evidence does not allow for any confident statement on the likely effects of such a strategy. A range of other potential anti-dyskinetics have been tested (Owens 1999; Taylor et al. 2007), including dopamine depleters (e.g., tetrabenazine, reserpine, oxypertine), cholinomimetic agents (e.g., choline, lecithin, deanol), GABA agonists (e.g., sodium ▶ valproate, gamma-vinyl GABA), calcium channel blockers (e.g., diltiazem, verapamil), and vitamin E (alpha-tocopherol). But none has the strength of evidence for efficacy or data on adverse effects that would allow for a clinical recommendation as a treatment for tardive dyskinesia (Soares and McGrath 1999). Cross-References ▶ Anticholinergic/Antimuscarinic Agents ▶ Antipsychotic Drugs ▶ Extrapyramidal side effects ▶ First Generation Antipsychotics References Barnes TRE (1992) Neuromuscular effects of neuroleptics: akathisia. In: Lieberman J, Kane JM (eds) Adverse effects of psychotropic drugs. Guilford Publications, New York, pp 201–217 Barnes TRE, McPhillips MA (1996) Antipsychotic-induced extrapyramidal symptoms: role of anticholinergic drugs in treatment. CNS Drugs 6:315–330 Correll CU, Schenk EM (2008) Tardive dyskinesia and new antipsychotics. Curr Opin Psychiatry 21:151–156 Leucht S, Wahlbeck K, Hamann J, Kissling W (2003) New generation antipsychotics versus low-potency conventional antipsychotics: a systematic review and meta-analysis. Lancet 361:1581–1589 Miller CH, Fleischhacker WW (2000) Managing antipsychotic-induced acute and chronic akathisia. Drug Saf 22:73–81 Owens DGC (1999) A guide to the extrapyramidal side effects of antipsychotic drugs. Cambridge University Press, Cambridge Pappa S, Dazzan P (2009). Spontaneous movement disorders in antipsychotic-naı̈ve patients with first-episode psychoses: a systematic review. Psychol Med 39:1065–1076 Soares KV, McGrath JJ (1999) The treatment of tardive dyskinesia – a systematic review and meta-analysis. Schizophr Res 39:1–16 Soares-Weiser K, Rathbone J (2005) Neuroleptic reduction and/or cessation and neuroleptics as specific treatments for tardive dyskinesia. Cochrane Database Syst Rev, Issue 3. Art. No.: CD000459, doi: 10.1002/14651858.CD000459.pub2 Taylor D, Paton C, Kerwin R (2007) The Maudsley prescribing guidelines, 9th edn. Informa Healthcare, London m-PD ▶ Meta-Phenylenediamine mPFC ▶ Medial Prefrontal Cortex MR Image Analysis Definition Unlike, for example, X-ray or computer tomography (CT), MR images usually do not provide absolute values of MR contrast parameters, as such measurements are MUS accompanied with long acquisition times that are not acceptable in clinical applications. For this reason, MR quantification is mostly based on relative measurements in comparison to a defined reference such as baseline data acquired under normal or resting conditions, or data derived from a healthy control group. In functional MRI, small and relative signal changes caused by the hemodynamic response to changes in neuronal activity are assessed. Sophisticated analysis methods are essential to detect these small changes ranging from approaches that incorporate prior knowledge from the stimulation paradigm applied to solely data-driven or exploratory methods. Cross-References ▶ Functional MRI ▶ Stimulation Paradigm mRNA Splice Variants M 809 Multiple-Unit Spiking Activity ▶ Multiunit Activity Multiunit Activity Synonyms Multiple-unit spiking activity Definition The electrophysiologically recorded multiunit activity (MUA) is thought to represent the average spiking of small neuronal populations close to the vicinity of the placed microelectrode. It is obtained by band-pass filtering the recorded signal in a frequency range of 400 to a few thousand Hz. Cross-References ▶ Magnetic Resonance Imaging (Functional) ▶ Alternative Splicing M MSI ▶ Imaging Mass Spectrometry Mu-Opioid Agonists Synonyms Morphine-like compounds; Opioid analgesics Definition Multibarrel Micropipette Definition An assembly of glass micropipettes usually fused together and terminating in a common tip; used for the concomitant recording of neuronal activity and the application of transmitters, drugs or other compounds of interest. Multi-Infarct Dementia ▶ Vascular Dementia Multimeric Protein Complex Definition A complex formed by several proteins. These are drugs acting selectively on the mu receptors of the endogenous ▶ opioid system. Examples are ▶ morphine, ▶ fentanyl, and sufentanil. Cross-References ▶ Endogenous Opioids ▶ Opioids Murungu ▶ Khat MUS ▶ Somatoform and Body Dysmorphic Disorders 810 M Muscarine Muscarine Definition A poisonous substance that is found in certain types of mushrooms. The substance mimics the actions of ▶ acetylcholine at muscarinic acetylcholine receptors. Muscarinic Agonists ▶ Muscarinic Antagonists Cholinergic Receptor Agonists and Agonists and Muscarinic Antagonists ▶ Muscarinic Cholinergic Antagonists ▶ Scopolamine Receptor Muscarinic Cholinergic Receptor Agonists and Antagonists MICHAEL E. RAGOZZINO, HOLDEN D. BROWN Department of Psychology, University of Illinois at Chicago, Chicago, IL, USA Synonyms Muscarinic agonists; Muscarinic antagonists Definition Muscarinic acetylcholine receptors represent one of the two classes of receptors that mediate the actions of acetylcholine in the nervous system and certain body parts. Muscarinic acetylcholine receptors were so named owing to their greater sensitivity to ▶ muscarine over ▶ nicotine. Muscarinic agonists activate and antagonists block, muscarinic acetylcholine receptors at an orthosteric or ▶ allosteric site. Pharmacological Properties Muscarinic cholinergic receptors are ▶ G-protein coupled receptors that are ubiquitously expressed in the central nervous system. There are different muscarinic receptor subtypes referred to as M1–M5, when a receptor subtype is described based on pharmacology, and m1–m5, when based on their molecular properties. The M1, M3, and M5 muscarinic receptor subtypes are coupled with Gq proteins resulting in the mobilization of intracellular calcium. The M2 and M4 muscarinic receptor subtypes are linked to Go proteins with the activation of these receptors producing a decrease of intracellular cyclic AMP. Antibodies specific to the muscarinic acetylcholine receptor proteins indicate that the m1, m2, and m4 receptors are most abundant in the brain (Levey et al. 1991). The m1 receptor is most concentrated in the neocortex, ▶ hippocampus, ▶ striatum, and ▶ amygdala. These receptors are found to be postsynaptic. The m2 receptor is most abundant in the basal forebrain, thalamus, neocortex, and striatum. These receptors are located on cholinergic terminals and are also located postsynaptically. The m4 receptor has its highest density in the striatum and hippocampus. These receptors are found postsynaptically, as well as presynaptically, on cholinergic neurons. The m3 and m5 receptors are sparser in the brain, with the m3 receptor being the most common in the neocortex, thalamus, and hippocampus, while the highest density of m5 receptors is located in the substantia nigra. It appears that these receptors are present postsynaptically. The identification of different muscarinic receptor subtypes has led to interest in developing muscarinic agonists and antagonists that target a specific type of muscarinic receptor. The development of selective receptor subtype compounds arises in part to understand the function(s) of muscarinic acetylcholine receptor subtypes. This work has been carried out predominantly in animal models. Interest in the development of drugs that target specific muscarinic receptors has also emerged because various neurodegenerative and psychiatric disorders have shown alterations in specific muscarinic acetylcholine receptor subtypes. The effects of muscarinic agonists and antagonists that prefer one type of muscarinic acetylcholine receptor to the other subtypes are described in the following text. The focus is on M1, M2, and M4-preferring drugs because most compounds developed preferentially act at these muscarinic receptor subtypes (see Fig. 1). M1 Muscarinic Receptor-Preferring Drugs There has been considerable effort in the development of pharmacological agents that selectively act at the M1 muscarinic acetylcholine receptor, in particular M1 muscarinic agonists. This is, in large part, because of studies indicating that M1 muscarinic acetylcholine receptors are altered in ▶ Alzheimer’s disease and ▶ schizophrenia (Langmead et al. 2008). Both of these conditions are marked by cognitive deficits and thus there has been an Muscarinic Cholinergic Receptor Agonists and Antagonists M 811 Muscarinic Cholinergic Receptor Agonists and Antagonists. Fig. 1. Chemical structures of muscarinic M1, M2 and M4 agonists and antagonists. M interest in developing selective M1 muscarinic agonists to alleviate the cognitive deficits in these conditions. A recent double-blind placebo-control study using xanomeline, a muscarinic M1-preferring agonist, in schizophrenic patients showed significant improvement in overall positive and negative symptom ratings, along with enhanced verbal learning with only mild side effects (Shekhar et al. 2008). Several M1 muscarinic-preferring agonists have been developed, which have shown learning and memory benefits in animal models, but overall, have not fared as well in clinical trials (Langmead et al. 2008). This is likely due to the lack of high selectivity for the M1 muscarinic acetylcholine receptor leading to the activation of other muscarinic receptor subtypes and unwanted side effects, e.g., nausea, diarrhea, sweating, and salivation. Some of the strongest evidence suggesting that M1 muscarinic receptors support learning and memory comes from experiments examining the effects of M1 muscarinic-preferring antagonists in animal models. For example, systemic administration of M1-preferring receptor antagonist, dicyclomine impairs both learning and memory in a variety of behavioral paradigms. Infusions of the M1-preferring antagonist pirenzepine into specific brain regions of the rodent impair learning or memory (Tzavos et al. 2004). Taken together, several experiments indicate that the blockade of M1 muscarinic acetylcholine receptors in various brain areas impairs learning and memory indicating that M1 muscarinic acetylcholine receptors may support several forms of learning and memory. A main limitation of compounds such as pirenzepine and dicylomine is that they do not exhibit a strong selectivity for M1 muscarinic receptors compared to the other muscarinic receptor subtypes. Another approach to studying muscarinic receptor activity is through the use of snake toxins that bind to specific muscarinic receptor subtypes. Muscarinic-toxin 7 (MT-7) is one such compound that exhibits greater selectivity for the M1 muscarinic receptor over other subtypes. Because MT-7 acts as a more selective M1 muscarinic receptor antagonist compared to that of pirenzepine and dicyclomine, it has been used to study the role of M1 muscarinic receptors in learning. A recent experiment demonstrated that injections of MT-7 into the rodent dorsomedial striatum does not affect the initial learning of a spatial discrimination, but specifically impairs spatial reversal learning (McCool et al. 2008). Thus, the results from the blockade of M1 muscarinic receptors indicate that this muscarinic receptor subtype is important for learning, memory, and behavioral flexibility. 812 M Muscarinic Cholinergic Receptor Agonists and Antagonists Consistent with numerous studies demonstrating that compounds which preferentially block M1 muscarinic receptors impair learning and memory, treatment with M1 muscarinic-preferring agonists have shown to facilitate learning and memory. The drug, McN-A-343, is often considered to be a M1 muscarinic-preferring agonist. Although there is limited muscarinic receptor subtype selectivity for this drug, there is evidence that the drug enhances spatial working memory. There are several M1 muscarinic agonists that have been reported to have cognitive benefits in preclinical tests. Furthermore, the activation of M1 muscarinic acetylcholine receptors may have neuroprotective effects by reducing the amyloid-beta peptide and tau protein associated with plaques and neurofibrillary tangles, respectively. However, M1-preferring agonists have failed or led to significant side effects in clinical trials. Xanolemine, talsaclidine, and WAY 132983 are all examples of compounds that have shown to have pro-cognitive benefits but led to unwanted side effects. The limitations of these drugs that act at the ▶ orthosteric site of the M1 muscarinic acetylcholine receptor, likely results because these agents do not display a strong selectivity for the M1 muscarinic acetylcholine receptor compared to the other muscarinic receptor subtypes. A new generation of M1 muscarinic agonists are being developed that act at the allosteric site of the M1 muscarinic acetylcholine receptor (Langmead et al. 2008). These newer compounds hold further promise for clinical effectiveness because the allosteric binding site for agonists does not seem conserved among the other muscarinic acetylcholine receptor subytpes. Thus, the development of selective allosteric agonists at the M1 receptor site may be effective in reducing cognitive deficits while minimizing unwanted side effects. M2 Muscarinic Receptor-Preferring Drugs Another pharmacological approach to modify brain cholinergic activity has been through M2 muscarinic acetylcholine receptors. Some M2 muscarinic acetylcholine receptors are found to be heteroreceptors in different brain regions. However, many M2 receptors act as autoreceptors providing negative feedback at cholinergic terminals. The localization of M2 muscarinic acetylcholine receptors on cholinergic neuron terminals has led to the examination of M2-preferring antagonists on brain acetylcholine activity and cognitive function. In particular, M2-preferring antagonists enhance brain acetylcholine efflux as measured by in vivo ▶ microdialysis (Ragozzino et al. 2009). Furthermore, the administration of M2-preferring antagonists such as SCH 72788, BIBN99, AF-DX 116, or methoctramine given either systemically or centrally improve memory ▶ consolidation, as well as ▶ working memory on a variety of tasks (Lazaris et al. 2003). To provide more direct evidence that changes in acetylcholine output are related to learning, a study demonstrated that the infusion of the M2-preferring agonist, oxotremorine sesquifumarate, into the dorsomedial striatum simultaneously blocked a behaviorally induced increase in striatal acetylcholine output and impaired reversal learning. These effects were reversed by the M2preferring antagonist, AF-DX 116. Thus, the blockade of M2 muscarinic acetylcholine receptors can provide a mechanism for modulating brain acetylcholine release and cognitive functioning (Ragozzino et al. 2009). The clinical use of M2 muscarinic acetylcholine receptor antagonists is suggested in diseases involving cholinergic degeneration and cognitive impairment, such as ▶ Alzheimer’s disease (Langmead et al. 2008). A number of compounds have been developed that exhibit significant selectivity for the M2 muscarinic acetylcholine receptor. One of the serious drawbacks about using a M2 muscarinic antagonist as a drug therapy is that M2 muscarinic receptors are located in the heart where they slow heart rate. Thus, treatment with a M2 muscarinic antagonist can lead to tachycardia. M4 Muscarinic Receptor-Preferring Drugs The M4 muscarinic acetylcholine receptor is another muscarinic acetylcholine receptor subtype in which there is significant interest in developing selective agents to generate novel treatments for both ▶ schizophrenia and ▶ Parkinson’s disease. Despite interest in the M4 muscarinic acetylcholine receptor, there are a relative lack of compounds that are highly selective for this muscarinic receptor subtype. Comparable to developing new targets for the M1 muscarinic receptor, the development of positive allosteric modulators for the M4 muscarinic receptor holds promise in generating novel treatments for various disorders and diseases. Interest in the M4 muscarinic acetylcholine receptor related to schizophrenia has evolved from findings indicating hyperactivity of the neurotransmitter dopamine in the striatum is observed in schizophrenia (Langmead et al. 2008). Moreover, the activation of M4 muscarinic acetylcholine receptors may inhibit striatal dopamine efflux. PTAC and BuTAC are two compounds that are M4preferring agonists. These drugs have shown to reduce apomorphine-induced ▶ prepulse inhibition (Jones et al. 2005), a paradigm commonly used to screen the effectiveness of antipsychotic drugs. As with many other muscarinic agonists, these drugs do not display a strong selectivity for the M4 muscarinic acetylcholine receptor (Langmead et al. Mutant Animal 2008). Therefore, there is a real possibility of producing unwanted side effects with such treatments. Interest in the M4 muscarinic acetylcholine receptor related to ▶ Parkinson’s disease is also related to dopaminergic–cholinergic interactions in the striatum. Owing to the reduced striatal dopamine activity in Parkinson’s disease, treatment with a M4 muscarinic acetylcholine receptor antagonist may have benefits in enhancing dopaminergic transmission and reducing symptoms in the disease. In support of that idea, tropicamide, a muscarinic acetylcholine receptor antagonist with moderate binding selectivity for the M4 muscarinic acetylcholine receptor subtype, suppresses tremulous jaw movements in rats, a model of Parkinson’s disease, without significant impairment on memory tasks (Betz et al. 2007). Several other compounds that have a high affinity for the M4 muscarinic acetylcholine receptor and show selectivity over other muscarinic acetylcholine receptor subtypes may prove beneficial in the treatment of Parkinson’s disease (Böhme et al. 2002). Conclusions Muscarinic acetylcholine receptors are found throughout the central nervous system. There are various neurodegenerative disorders, as well as psychiatric disorders that exhibit abnormalities in muscarinic acetylcholine receptor function. These various diseases and disorders often exhibit altered muscarinic acetylcholine receptor function for specific muscarinic receptor subtypes. Thus, the development of compounds that are selective for the different muscarinic receptor subtypes provides an opportunity for novel treatments that can reduce severe impairments in cognitive or motor functioning. Cross-References ▶ Allosteric Site ▶ Alzheimer’s Disease ▶ Long-Term Potentiation ▶ Muscarine ▶ Orthosteric Site ▶ Parkinson’s Disease ▶ Pre-pulse Inhibition ▶ Schizophrenia selective antagonists at M(4) muscarinic receptors. J Med Chem 45:3094–3102 Jones CK, Eberle EL, Shaw DB, McKinzie DL, Shannon HE (2005) Pharmacological interactions between the muscarinic cholinergic and dopaminergic systems in the modulation of prepulse inhibition in rats. J Pharmacol Exp Ther 312:1055–1063 Langmead CJ, Watson J, Reavill C (2008) Muscarinic acetylcholine receptors as CNS drug targets. Pharmacol Ther 117:232–243 Lazaris A, Cassel S, Stemmelin J, Cassel JC, Kelche C (2003) Intrastriatal infusions of methoctramine improve memory in cognitively impaired aged rats. Neurobiol Aging 24:599–606 Levey AI, Kitt CA, Simonds WF, Price DL, Brann MR (1991) Identification and localization of muscarinic acetylcholine receptor proteins with subtype-specific antibodies. J Neurosci 11:3218–3226 McCool MF, Patel S, Talati R, Ragozzino ME (2008) Differential involvement of M1-type and M4-type muscarinic cholinergic receptors in the dorsomedial striatum in task switching. Neurobiol Learn Mem 89:114–124 Ragozzino ME, Mohler EG, Prior M, Palencia CA, Rozman S (2009) Acetylcholine activity in selective striatal regions supports behavioral flexibility. Neurobiol Learn Mem 91:13–22 Shekhar A, Potter WZ, Lightfoot J, Lienemann J, Dubé S, Mallinckrodt C, Bymaster FP, McKinzie DL, Felder CC (2008) Selective muscarinic receptor agonist xanomeline as a novel treatment approach for schizophrenia. Am J Psychiatry 165:1033–1039 Tzavos A, Jih J, Ragozzino ME (2004) Differential effects of M1 uscarinic receptor blockade and nicotinic receptor blockade in the dorsomedial striatum on response reversal learning. Behav Brain Res 154:245–253 Muscarinic Receptors Synonyms mAChR Definition ▶ G-protein-coupled acetylcholine receptors found in the plasma membranes of certain neurons and other cells. They play several roles, including acting as the main end receptor stimulated by acetylcholine released from postganglionic fibers in the parasympathetic nervous system. Mutant ▶ Transgenic Organism References Betz AJ, McLaughlin PJ, Burgos M, Weber SM, Salamone JD (2007) The muscarinic receptor antagonist tropicamide suppresses tremulous jaw movements in a rodent model of parkinsonian tremor: possible role of M4 receptors. Psychopharmacology 194:347–359 Böhme TM, Augelli-Szafran CE, Hallak H, Pugsley T, Serpa K, Schwarz RD (2002) Synthesis and pharmacology of benzoxazines as highly M Mutant Animal ▶ Genetically Modified Animals 813 M 814 M Myelination Myelination Definition A process by which axonal trees of major projection neurons (i.e., neurons that project to distant brain regions) are sheathed in myelin, a fatty membrane produced by surrounding neuroglial cells. This myelin sheathing insulates the propagation of action-potentials, the electrical signals that convey information from the cell body to the axonal synaptic terminals, from nonspecific dissipation in the neuropil. Myelin sheathing reduces the energy requirements while increasing the speed of long-range information relays in the brain.