"Domain gauges": A reference system for multivariate profiling of brain fMRI activation patterns induced by psychoactive drugs in rats.

Pharmacological magnetic resonance imaging (phMRI) of the brain has become a widely used tool in both preclinical and clinical drug research. One of its challenges is to condense the observed complex drug-induced brain-activation patterns into semantically meaningful metrics that can then serve as a basis for informed decision making. To aid interpretation of spatially distributed activation patterns, we propose here a set of multivariate metrics termed "domain gauges", which have been calibrated based on different classes of marketed or validated reference drugs. Each class represents a particular "domain" of interest, i.e., a specific therapeutic indication or mode of action. The drug class is empirically characterized by the unique activation pattern it evokes in the brain-the "domain profile". A domain gauge provides, for any tested intervention, a "classifier" as a measure of response strength with respect to the domain in question, and a "differentiator" as a measure of deviation from the domain profile, both along with error ranges. Capitalizing on our in-house database with an unprecedented wealth of standardized perfusion-based phMRI data obtained from rats subjected to various validated treatments, we exemplarily focused on 3 domains based on therapeutic indications: an antipsychotic, an antidepressant and an anxiolytic domain. The domain profiles identified as part of the gauge definition process, as well as the outputs of the gauges when applied to both reference and validation data, were evaluated for their reconcilability with prior biological knowledge and for their performance in drug characterization. The domain profiles provided quantitative activation patterns with high biological plausibility. The antipsychotic profile, for instance, comprised key areas (e.g., cingulate cortex, nucleus accumbens, ventral tegmental area, substantia nigra) which are believed to be strongly involved in mediating an antipsychotic effect, and which are in line with network-level dysfunctions observed in schizophrenia. The domain gauges plausibly positioned the vast majority of the pharmacological and even non-pharmacological treatments. The results also suggest the segregation of sub-domains based on, e.g., the mode of action. Upon judicious selection of domains and careful calibration of the gauges, our approach represents a valuable analytical tool for biological interpretation and decision making in drug discovery.

Pharmacology of basimglurant (RO4917523, RG7090), a unique metabotropic glutamate receptor 5 negative allosteric modulator in clinical development for depression.

Major depressive disorder (MDD) is a serious public health burden and a leading cause of disability. Its pharmacotherapy is currently limited to modulators of monoamine neurotransmitters and second-generation antipsychotics. Recently, glutamatergic approaches for the treatment of MDD have increasingly received attention, and preclinical research suggests that metabotropic glutamate receptor 5 (mGlu5) inhibitors have antidepressant-like properties. Basimglurant (2-chloro-4-[1-(4-fluoro-phenyl)-2,5-dimethyl-1H-imidazol-4-ylethynyl]-pyridine) is a novel mGlu5 negative allosteric modulator currently in phase 2 clinical development for MDD and fragile X syndrome. Here, the comprehensive preclinical pharmacological profile of basimglurant is presented with a focus on its therapeutic potential for MDD and drug-like properties. Basimglurant is a potent, selective, and safe mGlu5 inhibitor with good oral bioavailability and long half-life supportive of once-daily administration, good brain penetration, and high in vivo potency. It has antidepressant properties that are corroborated by its functional magnetic imaging profile as well as anxiolytic-like and antinociceptive features. In electroencephalography recordings, basimglurant shows wake-promoting effects followed by increased delta power during subsequent non-rapid eye movement sleep. In microdialysis studies, basimglurant had no effect on monoamine transmitter levels in the frontal cortex or nucleus accumbens except for a moderate increase of accumbal dopamine, which is in line with its lack of pharmacological activity on monoamine reuptake transporters. These data taken together, basimglurant has favorable drug-like properties, a differentiated molecular mechanism of action, and antidepressant-like features that suggest the possibility of also addressing important comorbidities of MDD including anxiety and pain as well as daytime sleepiness and apathy or lethargy.

A proposal for an inclusive working definition of genetic discrimination to promote a more coherent debate.

Genetic discrimination is an evolving phenomenon that impacts fundamental human rights such as dignity, justice and equity. Although, in the past, various definitions to better conceptualize genetic discrimination have been proposed, these have been unable to capture several key facets of the phenomenon. In this Perspective, we explore definitions of genetic discrimination across disciplines, consider criticisms of such definitions and show how other forms of discrimination and stigmatization can compound genetic discrimination in a way that affects individuals, groups and systems. We propose a nuanced and inclusive definition of genetic discrimination, which reflects its multifaceted impact that should remain relevant in the face of an evolving social context and advancing science. We argue that our definition should be adopted as a guiding academic framework to facilitate scientific and policy discussions about genetic discrimination and support the development of laws and industry policies seeking to address the phenomenon.

Discovery of Balovaptan, a Vasopressin 1a Receptor Antagonist for the Treatment of Autism Spectrum Disorder.

We recently reported the discovery of a potent, selective, and brain-penetrant V1a receptor antagonist, which was not suitable for full development. Nevertheless, this compound was found to improve surrogates of social behavior in adults with autism spectrum disorder in an exploratory proof-of-mechanism study. Here we describe scaffold hopping that gave rise to triazolobenzodiazepines with improved pharmacokinetic properties. The key to balancing potency and selectivity while minimizing P-gp mediated efflux was fine-tuning of hydrogen bond acceptor basicity. Ascertaining a V1a antagonist specific brain activity pattern by pharmacological magnetic resonance imaging in the rat played a seminal role in guiding optimization efforts, culminating in the discovery of balovaptan (RG7314, RO5285119) 1. In a 12-week clinical phase 2 study in adults with autism spectrum disorder balovaptan demonstrated improvements in Vineland-II Adaptive Behavior Scales, a secondary end point comprising communication, socialization, and daily living skills. Balovaptan entered phase 3 clinical development in August 2018.

Validation of cerebral blood perfusion imaging as a modality for quantitative pharmacological MRI in rats.

The aim of this study was to validate continuous arterial spin labeling (CASL) as a quantitative imaging modality for pharmacological MRI (phMRI) based on local cerebral blood perfusion. Specifically, the capability of CASL to assess brain-activity signatures of pharmacological interventions in animal models was evaluated with respect to drug discovery in diseases of the central nervous system (CNS). Perfusion as a surrogate for neuronal activity was measured in various brain areas of the rat. The validation approach was threefold. First, perfusion was shown to reliably reflect differential effects of anesthesia on striatal activation. Different baseline levels and different temporal response profiles after amphetamine challenges under isoflurane, propofol, ketamine, and alpha-chloralose anesthesia were consistent with known properties of these anesthetics. Second, remarkable consistency of multi-area baseline perfusion patterns between independent groups of animals confirmed the notion that CASL is highly reproducible and thus particularly suitable for long-term longitudinal studies. Third, administration of the well-characterized psychotomimetic compounds amphetamine and phencyclidine (PCP) elicited dose-dependent activation patterns that were related to the drugs' particular interactions with the dopaminergic and glutamatergic systems, respectively. In conclusion, perfusion-based phMRI is a robust, reliable and valid quantitative technique suitable for evaluating brain-activation patterns in animal models of CNS diseases.

Cortical hypoperfusion in the B6.PS2APP mouse model for Alzheimer's disease: comprehensive phenotyping of vascular and tissular parameters by MRI.

Function and morphology of the cerebral vasculature were studied in the amyloid (Abeta) plaque-containing double-transgenic (TG) B6.PS2APP Alzheimer's disease (AD) mouse model with MRI at an age range of 10 to 17 months. Perfusion, blood volume, and average vessel geometry were assessed in the brain and compared to age-matched controls (wild-type [WT] C57Bl/6). Additionally, the MR relaxation times T(1), T(2), and T(2)* were measured to detect potential pathological changes that might be associated with Abeta plaque depositions. Both decreased perfusion and decreased blood volume were observed in the occipital cortex in B6.PS2APP mice as compared to controls. A significant decrease in T(1) and T(2) was found in the frontal cortex and in the subiculum/parasubiculum. Immunohistochemistry confirmed plaque depositions in the cortex and in the subiculum/parasubiculum. In summary, our data indicate a reduced blood supply of B6.PS2APP mice in the occipital cortex that parallels the findings in cortical regions of patients with AD.

Non-invasive analysis of gallbladder bile composition in cynomolgus monkeys using in vivo 1H magnetic resonance spectroscopy.

The purpose of the present study was (i) to establish a modality for non-invasively probing bile composition in cynomolgus monkeys and (ii) to ascertain the variability in biliary metabolism by repeatedly assessing gallbladder bile in situ. Localised in vivo (1)H magnetic resonance spectroscopy (MRS) provided high-resolution spectra of gallbladder bile that allowed for the first time different species of bile acids, their taurine and glycine conjugates, and phospholipids to be identified and quantified in situ. A combined cross-sectional and longitudinal study of bile composition was conducted over 4 weeks in monkeys kept under standardised nutritional conditions. All biles were composed of the same major constituents. Bile acids contributed 267+/-47 micromol/ml whereof cholate, deoxycholate and chenodeoxycholate were the most abundant primary bile acids. Bile acid conjugation reached an extent of 100%. However, the actual quantitative contributions of different bile constituents varied distinctly. Correlation analysis revealed that intra-individual variability (r=0.77+/-0.03) was significantly (p<0.01) smaller than inter-individual variability (r=0.68+/-0.01), thus purporting the notion that bile composition is a hallmark of individual metabolism. Extension of quantitative bile analysis by in vivo (1)H-MRS to pathological states will provide a rapid and non-invasive modality for monitoring an important, yet elusive compartment of cholesterol and lipid metabolism.

Identification of a Corticohabenular Circuit Regulating Socially Directed Behavior.

BACKGROUND: The prefrontal cortex (PFC) has been implicated in the pathophysiology of social dysfunction, but the specific circuit partners mediating PFC function in health and disease are unclear. METHODS: The excitatory designer receptor exclusively activated by designer drugs (DREADD) hM3Dq was used to induce PFC activation during social behavior measured in the three-chamber sociability assay (rats/mice). Functional magnetic resonance imaging was combined with hM3Dq-mediated PFC activation to identify novel nodes in the "social brain" in a hypothesis-free manner. In multiplexed DREADD experiments, hM3Dq and the inhibitory KORDi were used to bidirectionally modulate PFC activity and measure social behavior and global functional magnetic resonance imaging signature. To characterize the functional role of specific nodes identified in this functional magnetic resonance imaging screen, we used anterograde and retrograde tracers, optogenetic and DREADD-assisted circuit mapping, and circuit behavioral experiments. RESULTS: PFC activation suppressed social behavior and modulated activity in a number of regions involved in emotional behavior. Bidirectional modulation of PFC activity further refined this subset of brain regions and identified the habenula as a node robustly correlated with PFC activity. Furthermore, we showed that the lateral habenula (LHb) receives direct synaptic input from the PFC and that activation of LHb neurons or the PFC inputs to the LHb suppresses social preference. Finally, we demonstrated that LHb inhibition can prevent the social deficits induced by PFC activation. CONCLUSIONS: The LHb is thought to provide reward-related contextual information to the mesolimbic reward system known to be involved in social behavior. Thus, PFC projections to the LHb may represent an important part of descending PFC pathways that control social behavior.

Glutamate and GABA in autism spectrum disorder-a translational magnetic resonance spectroscopy study in man and rodent models.

Autism spectrum disorder (ASD) is a pervasive neurodevelopmental syndrome with a high human and economic burden. The pathophysiology of ASD is largely unclear, thus hampering development of pharmacological treatments for the core symptoms of the disorder. Abnormalities in glutamate and GABA signaling have been hypothesized to underlie ASD symptoms, and may form a therapeutic target, but it is not known whether these abnormalities are recapitulated in humans with ASD, as well as in rodent models of the disorder. We used translational proton magnetic resonance spectroscopy ([1H]MRS) to compare glutamate and GABA levels in adult humans with ASD and in a panel of six diverse rodent ASD models, encompassing genetic and environmental etiologies. [1H]MRS was performed in the striatum and the medial prefrontal cortex, of the humans, mice, and rats in order to allow for direct cross-species comparisons in specific cortical and subcortical brain regions implicated in ASD. In humans with ASD, glutamate concentration was reduced in the striatum and this was correlated with the severity of social symptoms. GABA levels were not altered in either brain region. The reduction in striatal glutamate was recapitulated in mice prenatally exposed to valproate, and in mice and rats carrying Nlgn3 mutations, but not in rodent ASD models with other etiologies. Our findings suggest that glutamate/GABA abnormalities in the corticostriatal circuitry may be a key pathological mechanism in ASD; and may be linked to alterations in the neuroligin-neurexin signaling complex.

Synaptic Regulation of a Thalamocortical Circuit Controls Depression-Related Behavior.

The NMDA receptor (NMDAR) antagonist ketamine elicits a long-lasting antidepressant response in patients with treatment-resistant depression. Understanding how antagonism of NMDARs alters synapse and circuit function is pivotal to developing circuit-based therapies for depression. Using virally induced gene deletion, ex vivo optogenetic-assisted circuit analysis, and in vivo chemogenetics and fMRI, we assessed the role of NMDARs in the medial prefrontal cortex (mPFC) in controlling depression-related behavior in mice. We demonstrate that post-developmental genetic deletion of the NMDAR subunit GluN2B from pyramidal neurons in the mPFC enhances connectivity between the mPFC and limbic thalamus, but not the ventral hippocampus, and reduces depression-like behavior. Using intersectional chemogenetics, we show that activation of this thalamocortical circuit is sufficient to elicit a decrease in despair-like behavior. Our findings reveal that GluN2B exerts input-specific control of pyramidal neuron innervation and identify a medial dorsal thalamus (MDT)→mPFC circuit that controls depression-like behavior.

Altered fronto-striatal functions in the Gdi1-null mouse model of X-linked Intellectual Disability.

RAB-GDP dissociation inhibitor 1 (GDI1) loss-of-function mutations are responsible for a form of non-specific X-linked Intellectual Disability (XLID) where the only clinical feature is cognitive impairment. GDI1 patients are impaired in specific aspects of executive functions and conditioned response, which are controlled by fronto-striatal circuitries. Previous molecular and behavioral characterization of the Gdi1-null mouse revealed alterations in the total number/distribution of hippocampal and cortical synaptic vesicles as well as hippocampal short-term synaptic plasticity, and memory deficits. In this study, we employed cognitive protocols with high translational validity to human condition that target the functionality of cortico-striatal circuitry such as attention and stimulus selection ability with progressive degree of complexity. We previously showed that Gdi1-null mice are impaired in some hippocampus-dependent forms of associative learning assessed by aversive procedures. Here, using appetitive-conditioning procedures we further investigated associative learning deficits sustained by the fronto-striatal system. We report that Gdi1-null mice are impaired in attention and associative learning processes, which are a key part of the cognitive impairment observed in XLID patients.

A novel anesthesia regime enables neurofunctional studies and imaging genetics across mouse strains.

Functional magnetic resonance imaging (fMRI) has revolutionized neuroscience by opening a unique window that allows neurocircuitry function and pathological alterations to be probed non-invasively across brain disorders. Here we report a novel sustainable anesthesia procedure for small animal neuroimaging that overcomes shortcomings of anesthetics commonly used in rodent fMRI. The significantly improved preservation of cerebrovascular dynamics enhances sensitivity to neural activity changes for which it serves as a proxy in fMRI readouts. Excellent cross-species/strain applicability provides coherence among preclinical findings and is expected to improve translation to clinical fMRI investigations. The novel anesthesia procedure based on the GABAergic anesthetic etomidate was extensively validated in fMRI studies conducted in a range of genetically engineered rodent models of autism and strains commonly used for transgenic manipulations. Etomidate proved effective, yielded long-term stable physiology with basal cerebral blood flow of ~0.5 ml/g/min and full recovery. Cerebrovascular responsiveness of up to 180% was maintained as demonstrated with perfusion- and BOLD-based fMRI upon hypercapnic, pharmacological and sensory stimulation. Hence, etomidate lends itself as an anesthetic-of-choice for translational neuroimaging studies across rodent models of brain disorders.

Temporal profile of brain response to alprazolam in patients with generalized anxiety disorder.

This study investigated the temporal pattern of brain response to emotional stimuli during 28 days of alprazolam treatment among patients with generalized anxiety disorder (GAD) randomized 2:1 to drug or placebo in a double-blind design. Functional magnetic resonance imaging scans obtained during an emotion face matching task (EFMT) and an affective stimulus expectancy task (STIMEX) were performed at baseline, one hour after initial drug administration and 28 days later. Alprazolam significantly reduced scores on the Hamilton Anxiety Scale and the Penn State Worry Questionnaire after one week and 28 days of treatment. Brain activation in the amygdala during the EFMT and in the insula during the STIMEX was reduced one hour after alprazolam administration but returned to baseline levels at Day 28. Exploratory analyses revealed significant treatment differences in brain activity during the STIMEX on Day 28 in frontal lobe, caudate nucleus, middle temporal gyrus, secondary visual cortex, and supramarginal gyrus. These results are consistent with the notion that the neural mechanisms supporting sustained treatment effects of benzodiazepines in GAD differ from those underlying their acute effects.

Fourier-, Hilbert- and wavelet-based signal analysis: are they really different approaches?

Spectral signal analysis constitutes one of the most important and most commonly used analytical tools for the evaluation of neurophysiological signals. It is not only the spectral parameters per se (amplitude and phase) which are of interest, but there is also a variety of measures derived from them, including important coupling measures like coherence or phase synchrony. After reviewing some of these measures in order to underline the widespread relevance of spectral analysis, this report compares the three classical spectral analysis approaches: Fourier, Hilbert and wavelet transform. Recently, there seems to be increasing acceptance of the notion that Hilbert- or wavelet-based analyses be in some way superior to Fourier-based analyses. The present article counters such views by demonstrating that the three techniques are in fact formally (i.e. mathematically) equivalent when using the class of wavelets that is typically applied in spectral analyses. Moreover, spectral amplitude serves as an example to show that Fourier, Hilbert and wavelet analysis also yield equivalent results in practical applications to neuronal signals.

Task-related coupling from high- to low-frequency signals among visual cortical areas in human subdural recordings.

Cortical cooperativity during cognitive demands includes high- and low-frequency activities, which raises the question whether there are interdependencies between fast and slow processes and how they are reflected in electrical brain signals. We had the opportunity to record signals intracranially from occipital visual areas in an epileptic patient and quantified inter-areal signal coupling while the patient performed a visual delayed-match-to-sample task. We computed coherence, phase consistency and amplitude envelope correlation and we also determined inter-frequency coupling through correlation between low-frequency signal components and amplitude envelopes of high-frequency components. There was a pronounced task-related increase of correlation between gamma-band (28-70 Hz) signal envelopes from a superior (occipital) and low-frequency (0-3.5 Hz) signals from an inferior (occipital) visual area, lasting for approximately 1 s and possibly reflecting a short-term memory encoding process. The correlational delay between envelopes and low-frequency components was 40 ms. In contrast, coherence, phase consistency and envelope correlation showed event-, but no task-related changes of intra-areal and no changes of inter-areal coupling. Our data suggest a specific effect of gamma-activity in the superior onto low-frequency activity in the inferior area. We argue that temporal dispersion of conduction delays might prevent coherent transmission of high-frequency signals and thus account for the absence of gamma-coherence. As such dispersion is a general property of long-range projections, envelope-to-signal correlation possibly reflects a general neuronal mechanism. Hence, our method provides a powerful tool for detecting such inter-areal interactions not visible with conventional linear coupling measures.

Different types of signal coupling in the visual cortex related to neural mechanisms of associative processing and perception.

The hypothesis of object representation by synchronization in the visual cortex has been supported by our recent experiments in monkeys. They demonstrated local synchrony among gamma activities (30-90 Hz) and their perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few mm, challenging the synchronization hypothesis for larger cortical object representations. The restriction is due to randomly changing phase relations among locally synchronized patches which, however, form continuous waves of gamma-activity, traveling across object representations. The phase continuity of these waves may support coding of object continuity. Interactions across still larger distances, measured among cortical areas in human data, involve amplitude envelopes of gamma signals. Based on models with spiking neurons we discuss potentially underlying mechanisms. Most important for gamma synchronization are local facilitatory connections with distance-dependent delays. They also explain the occurrence of gamma waves and the restriction of gamma-synchrony. Fast local feedback inhibition generates gamma oscillations and supports local synchrony, while slow shunting inhibitory feedback supports figure-ground segregation. Finally, dispersion in inter-areal far projections destroys coherence of gamma signals, but preserves their amplitude modulations. In conclusion, we propose that the hypothesis of associative processing by gamma synchronization be extended to more general forms of signal coupling.

Neural mechanisms of visual associative processing.

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans--related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma-waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma-activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma-waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma-synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.

Chronic metabotropic glutamate receptor 5 inhibition corrects local alterations of brain activity and improves cognitive performance in fragile X mice.

BACKGROUND: Fragile X syndrome (FXS) is the most common genetic cause for intellectual disability. Fmr1 knockout (KO) mice are an established model of FXS. Chronic pharmacological inhibition of metabotropic glutamate receptor 5 (mGlu5) in these mice corrects multiple molecular, physiological, and behavioral phenotypes related to patients' symptoms. To better understand the pathophysiology of FXS and the effect of treatment, brain activity was analyzed using functional magnetic resonance imaging in relation to learning and memory performance. METHODS: Wild-type (WT) and Fmr1 KO animals receiving chronic treatment with the mGlu5 inhibitor CTEP or vehicle were evaluated consecutively for 1) learning and memory performance in the inhibitory avoidance and extinction test, and 2) for the levels of brain activity using continuous arterial spin labeling based functional magnetic resonance imaging. Neural activity patterns were correlated with cognitive performance using a multivariate regression analysis. Furthermore, mGlu5 receptor expression in brains of untreated mice was analyzed by autoradiography and saturation analysis using [(3)H]-ABP688. RESULTS: Chronic CTEP treatment corrected the learning deficit observed in Fmr1 KO mice in the inhibitory avoidance and extinction test and prevented memory extinction in WT and Fmr1 KO animals. Chronic CTEP treatment normalized perfusion in the amygdala and the lateral hypothalamus in Fmr1 KO mice and furthermore decreased perfusion in the hippocampus and increased perfusion in primary sensorimotor cortical areas. No significant differences in mGlu5 receptor expression levels between Fmr1 WT and KO mice were detected. CONCLUSIONS: Chronic mGlu5 inhibition corrected the learning deficits and partially normalized the altered brain activity pattern in Fmr1 KO mice.

Preserved modular network organization in the sedated rat brain.

Translation of resting-state functional connectivity (FC) magnetic resonance imaging (rs-fMRI) applications from human to rodents has experienced growing interest, and bears a great potential in pre-clinical imaging as it enables assessing non-invasively the topological organization of complex FC networks (FCNs) in rodent models under normal and various pathophysiological conditions. However, to date, little is known about the organizational architecture of FCNs in rodents in a mentally healthy state, although an understanding of the same is of paramount importance before investigating networks under compromised states. In this study, we characterized the properties of resting-state FCN in an extensive number of Sprague-Dawley rats (n = 40) under medetomidine sedation by evaluating its modular organization and centrality of brain regions and tested for reproducibility. Fully-connected large-scale complex networks of positively and negatively weighted connections were constructed based on Pearson partial correlation analysis between the time courses of 36 brain regions encompassing almost the entire brain. Applying recently proposed complex network analysis measures, we show that the rat FCN exhibits a modular architecture, comprising six modules with a high between subject reproducibility. In addition, we identified network hubs with strong connections to diverse brain regions. Overall our results obtained under a straight medetomidine protocol show for the first time that the community structure of the rat brain is preserved under pharmacologically induced sedation with a network modularity contrasting from the one reported for deep anesthesia but closely resembles the organization described for the rat in conscious state.

Neuropharmacological and neurobiological relevance of in vivo ¹H-MRS of GABA and glutamate for preclinical drug discovery in mental disorders.

Proton magnetic resonance spectroscopy ((1)H-magnetic resonance spectroscopy (MRS)) is a translational modality with great appeal for neuroscience since the two major excitatory and inhibitory neurotransmitters, glutamate, and GABA, can be noninvasively quantified in vivo and have served to explore disease state and effects of drug treatment. Yet, if (1)H-MRS shall serve for decision making in preclinical pharmaceutical drug discovery, it has to meet stringent requirements. In particular, (1)H-MRS needs to reliably report neurobiologically relevant but rather small changes in neurometabolite levels upon pharmacological interventions and to faithfully appraise target engagement in the associated molecular pathways at pharmacologically relevant doses. Here, we thoroughly addressed these matters with a three-pronged approach. Firstly, we determined the sensitivity and reproducibility of (1)H-MRS in rat at 9.4 Tesla for detecting changes in GABA and glutamate levels in the striatum and the prefrontal cortex, respectively. Secondly, we evaluated the neuropharmacological and neurobiological relevance of the MRS readouts by pharmacological interventions with five well-characterized drugs (vigabatrin, 3-mercaptopropionate, tiagabine, methionine sulfoximine, and riluzole), which target key nodes in GABAergic and glutamatergic neurotransmission. Finally, we corroborated the MRS findings with ex vivo biochemical analyses of drug exposure and neurometabolite concentrations. For all five interventions tested, (1)H-MRS provided distinct drug dose-effect relationships in GABA and glutamate over preclinically relevant dose ranges and changes as low as 6% in glutamate and 12% in GABA were reliably detected from 16 mm(3) volumes-of-interest. Taken together, these findings demonstrate the value and limitation of quantitative (1)H-MRS of glutamate and GABA for preclinical pharmaceutical research in mental disorders.