Translational Concepts of mGluR5 in Synaptic Diseases of the Brain

Nobiletin regulates intracellular Ca2+ levels via IP3R and ameliorates neuroinflammation in Aβ42-induced astrocytes

Astrocytes are the major glial cells in the human brain and provide crucial metabolic and trophic support to neurons. The amyloid-β peptide (Aβ) alter the morphological and functional properties of astrocytes and induce inflammation and calcium dysregulation, contributing to Alzheimer's disease (AD) pathology. Recent studies highlight the role of Toll-like receptor (TLR) 4/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling in inflammation. Reactive oxygen species (ROS) generated due to Aβ, induce apoptosis in the brain cells worsening AD progression. Astrocytic cell surface receptors, such as purinergic receptors (P2Y1 and P2Y2), metabotropic glutamate receptor (mGLUR)5, α7 nicotinic acetylcholine receptor (α7nAChR), and N-methyl-d-aspartate receptors (NMDARs), have been suggested to interact with inositol trisphosphate receptor (IP3R) on the endoplasmic reticulum (ER) to induce Ca2+ movement from ER to cytoplasm, causing Ca2+ dysregulation. We found that the citrus flavonoid nobiletin (NOB) protected primary astrocytes from Aβ42-induced cytotoxicity and inhibited TLR4/NF-κB signaling in Aβ42-induced primary rat astrocytes. NOB was found to regulate Aβ42-induced ROS levels through Keap1-Nrf2 pathway. The receptors P2Y1, P2Y2, mGLUR5, α7nAChR, and NMDARs induced intracellular Ca2+ levels by activating IP3R and NOB regulated them, thereby regulating intracellular Ca2+ levels. Molecular docking analysis revealed a possible interaction between NOB and IP3R in IP3R regulation. Furthermore, RNA sequencing revealed various NOB-mediated biological signaling pathways, such as the AD-presenilin, AD-amyloid secretase, and Wnt signaling pathway, suggesting possible neuroprotective roles of NOB. To conclude, NOB is a promising therapeutic agent for AD and works by modulating AD pathology at various levels in Aβ42-induced primary rat astrocytes.

Presynaptic density determined by SV2A PET is closely associated with postsynaptic metabotropic glutamate receptor 5 availability and independent of amyloid pathology in early cognitive impairment

INTRODUCTION

Metabotropic glutamate receptor 5 (mGluR5) is involved in regulating integrative brain function and synaptic transmission. Aberrant mGluR5 signaling and relevant synaptic failure play a key role in the pathophysiological mechanism of Alzheimer's disease (AD).

METHODS

Ten cognitively impaired (CI) individuals and 10 healthy controls (HCs) underwent [18F]SynVesT-1 and [18F]PSS232 positron emission tomography (PET)/magnetic resonance to assess synaptic density and mGluR5 availability. The associations between mGluR5 availability and synaptic density were examined. A mediation analysis was performed to investigate the possible mediating effects of mGluR5 availability and synaptic loss on the relationship between amyloid deposition and cognition.

RESULTS

CI patients exhibited lower mGluR5 availability and synaptic density in the medial temporal lobe than HCs. Regional synaptic density was closely associated with regional mGluR5 availability. mGluR5 availability and synaptic loss partially mediated the relationship between amyloid deposition and cognition.

CONCLUSIONS

Reductions in mGluR5 availability and synaptic density exhibit similar spatial patterns in AD and are closely linked.

HIGHLIGHTS

Cognitively impaired patients exhibited lower mGluR5 availability and synaptic density in the medial temporal lobe than HCs. Reductions in mGluR5 availability and synaptic density exhibit similar spatial patterns in AD. Regional synaptic density was closely associated with regional mGluR5 availability. mGluR5 availability and synaptic loss partially mediated the relationship between amyloid deposition and global cognition. With further research, modulating mGluR5 availability might be a potential therapeutic strategy for improving synaptic function in AD.

Engineering self-deliverable ribonucleoproteins for genome editing in the brain

The delivery of CRISPR ribonucleoproteins (RNPs) for genome editing in vitro and in vivo has important advantages over other delivery methods, including reduced off-target and immunogenic effects. However, effective delivery of RNPs remains challenging in certain cell types due to low efficiency and cell toxicity. To address these issues, we engineer self-deliverable RNPs that can promote efficient cellular uptake and carry out robust genome editing without the need for helper materials or biomolecules. Screening of cell-penetrating peptides (CPPs) fused to CRISPR-Cas9 protein identifies potent constructs capable of efficient genome editing of neural progenitor cells. Further engineering of these fusion proteins establishes a C-terminal Cas9 fusion with three copies of A22p, a peptide derived from human semaphorin-3a, that exhibits substantially improved editing efficacy compared to other constructs. We find that self-deliverable Cas9 RNPs generate robust genome edits in clinically relevant genes when injected directly into the mouse striatum. Overall, self-deliverable Cas9 proteins provide a facile and effective platform for genome editing in vitro and in vivo.

The neurobehavioral impacts of typical antibiotics toward zebrafish larvae

Due to the widely usage in livestock, aquaculture and clinics, antibiotic residues are existed in aqueous environments and their potential toxicity to aquatic organisms is concerning. Here, we used zebrafish as the model to investigate the neurotoxicity and involved mechanism of seven antibiotics that were frequently detected in surface waters. The results revealed that the short-term exposure to clarithromycin (CLA), chlortetracycline (CTC) and roxithromycin (ROX) induced behavioral effects, with effective concentration of 1 μg/L (CTC and ROX) and 100 μg/L (CLA, CTC and ROX) respectively. A significant decrease in the travel distance and velocity as well as an increase in turn angle was measured. TUNEL assay identified increased cell apoptosis in brain sections of larvae exposed to three neurotoxic antibiotics, which raised the possibility that the behavioral symptoms were associated with neural damage. Transcriptome sequencing showed that the three antibiotics could affect the nervous system of zebrafish including the alteration of synaptogenesis and neurotransmission. Additionally, ROX and CTC affected pathways involved in mitochondrial stress response and endocrine system in zebrafish larvae. Besides, BDNF, ASCL1, and CREBBP are potential upstream regulatory factors that mediated these impacts. These findings indicated that exposure of CTC, ROX and CLA may cause abnormal behavior toward zebrafish larvae under environmental relevant concentration and revealed the potential role of neural cell apoptosis and synaptogenesis signaling in mediating this effect.

Alterations of BDNF, mGluR5, Homer1a, p11 and excitatory/inhibitory balance in corticolimbic brain regions of suicide decedents

BACKGROUND

Developing biological based approaches for preventing suicide has become a priority. In recent years, there has been a surge in studies investigating the role of the glutamatergic system in suicide, although it remains unclear.

METHODS

We evaluated changes in the gene expression of the metabotropic glutamate receptor 5 (mGluR5) and its scaffolding proteins Homer1a and p11 in the dorsolateral prefrontal cortex (DLPFC), amygdala (AMY), and hippocampus (HIP) of 28 suicide decedents (S) (with no clinical psychiatric history or treatment with anxiolytics or antidepressants) and 26 controls (C) by real-time PCR (qPCR). Indeed, we measured BDNF gene expression and VGluT1 and VGAT immunoreactivities in the HIP by qPCR and immunohistochemistry, respectively. Cases and controls matched for age (C: 48.6 ± 11.6 years; S: 46.9 ± 14.5 years) and postmortem interval (PMI; C: 20.1 ± 13h; S: 16.9 ± 5h).

RESULTS

In DLPFC, S had lower p11 gene expression levels, but no differences were found in mGluR5 or Homer1a. In the AMY and HIP, mGluR5 and Homer1a were increased, p11 and BDNF were reduced. In the HIP, there were less VGAT-ir and more VGluT1-ir.

LIMITATIONS

Future studies are necessary to evaluate protein levels, and determine the cell types and potential compensatory mechanisms in a larger sample including S diagnosed with psychiatric disorders, females and different ethnicities.

CONCLUSIONS

This study identified significant alterations in mGluR5, Homer1a, p11, BDNF and excitatory/inhibitory balance in corticolimbic brain areas of S. These results further characterize the biological basis of suicide, contributing to the identification of potential biomarkers for suicide prevention.

Heterozygous c.175C>T variant in PURA gene causes severe developmental delay

Key Clinical Message

This case report presents a child with PURA-related neurodevelopmental disorder, caused by the heterozygous pathogenic variant c.175C>T (p.Gln59*). The clinical symptoms included microcephaly, brachygnathia, central and peripheral hypotonia, and developmental delay (non-verbal), among others. On comparison with published literature, even patients with the same mutation present different clinical symptoms.

Abstract

This case report presents a child with PURA-related neurodevelopmental disorder, caused by the heterozygous pathogenic variant c.175C>T (p.Gln59*), whose symptoms included microcephaly, brachygnathia, the development of a high anterior hairline, hip dysplasia, strabismus, severe hypotonia, developmental delay (non-meaningful verbal), feeding difficulties, and respiratory difficulties. His development ceased with age, such that his development at 10 years corresponded to an infant of 6 months. Moreover, even patients with the same variant can have different clinical symptoms, such as the presence or absence of epilepsy or congenital malformations. Therefore, we should follow his long-term clinical course and provide medical support as necessary.

Altered GABAergic, glutamatergic and endocannabinoid signaling is accompanied by neuroinflammatory response in a zebrafish model of social withdrawal behavior

Introduction

Deficits in social communication are in the core of clinical symptoms characterizing many neuropsychiatric disorders such as schizophrenia and autism spectrum disorder. The occurrence of anxiety-related behavior, a common co-morbid condition in individuals with impairments in social domain, suggests the presence of overlapping neurobiological mechanisms between these two pathologies. Dysregulated excitation/inhibition balance and excessive neuroinflammation, in specific neural circuits, are proposed as common etiological mechanisms implicated in both pathologies.

Methods and Results

In the present study we evaluated changes in glutamatergic/GABAergic neurotransmission as well as the presence of neuroinflammation within the regions of the Social Decision-Making Network (SDMN) using a zebrafish model of NMDA receptor hypofunction, following sub-chronic MK-801 administration. MK-801-treated zebrafish are characterized by impaired social communication together with increased anxiety levels. At the molecular level, the behavioral phenotype was accompanied by increased mGluR5 and GAD67 but decreased PSD-95 protein expression levels in telencephalon and midbrain. In parallel, MK-801-treated zebrafish exhibited altered endocannabinoid signaling as indicated by the upregulation of cannabinoid receptor 1 (CB1R) in the telencephalon. Interestingly, glutamatergic dysfunction was positively correlated with social withdrawal behavior whereas defective GABAergic and endocannabinoid activity were positively associated with anxiety-like behavior. Moreover, neuronal and astrocytic IL-1β expression was increased in regions of the SDMN, supporting the role of neuroinflammatory responses in the manifestation of MK-801 behavioral phenotype. Colocalization of interleukin-1β (IL-1β) with β₂-adrenergic receptors (β₂-ARs) underlies the possible influence of noradrenergic neurotransmission to increased IL-1β expression in comorbidity between social deficits and elevated anxiety comorbidity.

Discussion

Overall, our results indicate the contribution of altered excitatory and inhibitory synaptic transmission as well as excessive neuroinflammatory responses in the manifestation of social deficits and anxiety-like behavior of MK-801-treated fish, identifying possible novel targets for amelioration of these symptoms.

Beyond monoamines: II. Novel applications for PET imaging in psychiatric disorders

Early applications of positron emission tomography (PET) in psychiatry sought to identify derangements of cerebral blood flow and metabolism. The need for more specific neurochemical imaging probes was soon evident, and these probes initially targeted the sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. For nearly 30 years, the centrality of monoamine dysfunction in psychiatric disorders drove the development of an armamentarium of monoaminergic PET radiopharmaceuticals and imaging methodologies. However, continued investments in monoamine-enhancing drug development realized only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely parallelled drug development priorities, resulting in the development of new PET imaging agents for non-monoamine targets. In part two of this review, we survey clinical research studies using the novel targets and radiotracers described in part one across major psychiatric application areas such as substance use disorders, anxiety disorders, eating disorders, personality disorders, mood disorders, and schizophrenia. Important limitations of the studies described are discussed, as well as key methodologic issues, challenges to the field, and the status of clinical trials seeking to exploit these targets for novel therapeutics.

Neuromuscular and Neuromuscular Junction Manifestations of the PURA-NDD: A Systematic Review of the Reported Symptoms and Potential Treatment Options

PURA-related neurodevelopmental disorders (PURA-NDDs) are a rare genetic disease caused by pathogenic autosomal dominant variants in the PURA gene or a deletion encompassing the PURA gene. PURA-NDD is clinically characterized by neurodevelopmental delay, learning disability, neonatal hypotonia, feeding difficulties, abnormal movements, and epilepsy. It is generally considered to be central nervous system disorders, with generalized weakness, associated hypotonia, cognitive and development deficits in early development, and seizures in late stages. Although it is classified predominantly as a central nervous syndrome disorder, some phenotypic features, such as myopathic facies, respiratory insufficiency of muscle origin, and myopathic features on muscle biopsy and electrodiagnostic evaluation, point to a peripheral (neuromuscular) source of weakness. Patients with PURA-NDD have been increasingly identified in exome-sequenced cohorts of patients with neuromuscular- and congenital myasthenic syndrome-like phenotypes. Recently, fluctuating weakness noted in a PURA-NDD patient, accompanied by repetitive nerve stimulation abnormalities, suggested the disease to be a channelopathy and, more specifically, a neuromuscular junction disorder. Treatment with pyridostigmine or salbutamol led to clinical improvement of neuromuscular function in two reported cases. The goal of this systematic retrospective review is to highlight the motor symptoms of PURA-NDD, to further describe the neuromuscular phenotype, and to emphasize the role of potential treatment opportunities of the neuromuscular phenotype in the setting of the potential role of PURA protein in the neuromuscular junction and the muscles.

The metabotropic glutamate receptor 5 as a biomarker for psychiatric disorders

The role of glutamate system in the etiology and pathophysiology of psychiatric disorders has gained considerable attention in the past two decades, including dysregulation of the metabotropic glutamatergic receptor subtype 5 (mGlu5). Thus, mGlu5 may represent a promising therapeutic target for psychiatric conditions, particularly stress-related disorders. Here, we describe mGlu5 findings in mood disorders, anxiety, and trauma disorders, as well as substance use (specifically nicotine, cannabis, and alcohol use). We highlight insights gained from positron emission tomography (PET) studies, where possible, and discuss findings from treatment trials, when available, to explore the role of mGlu5 in these psychiatric disorders. Through the research evidence reviewed in this chapter, we make the argument that, not only is dysregulation of mGlu5 evident in numerous psychiatric disorders, potentially functioning as a disease "biomarker," the normalization of glutamate neurotransmission via changes in mGlu5 expression and/or modulation of mGlu5 signaling may be a needed component in treating some psychiatric disorders or symptoms. Finally, we hope to demonstrate the utility of PET as an important tool for investigating mGlu5 in disease mechanisms and treatment response.

Unravelling Contributions of Astrocytic Connexin 43 to the Functional Activity of Brain Neuron-Glial Networks under Hypoxic State In Vitro

Brain hypoxia remains an Achilles' heel for public health that must be urgently addressed. Hypoxic damage affects both neurons and glial cells, particularly astrocytes, which are in close dynamic bi-directional communication, and are organized in plastic and tightly regulated networks. However, astroglial networks have received limited attention regarding their influence on the adaptive functional rearrangements of neural networks to oxygen deficiency. Herein, against the background of astrocytic Cx43 gap junction blockade by the selective blocker Gap19, we evaluated the features of spontaneous calcium activity and network characteristics of cells in primary cultures of the cerebral cortex, as well as the expression levels of metabotropic glutamate receptors 2 (mGluR2) and 5 (mGluR5) in the early and late periods after simulated hypoxia in vitro. We showed that, under normoxic conditions, blockade of Cx43 leads to an increase in the expression of metabotropic glutamate receptors mGluR2 and mGluR5 and long-term modulation of spontaneous calcium activity in primary cortical cultures, primarily expressed in the restructuring of the functional architectonics of neuron-glial networks through reducing the level of correlation between cells in the network and the percentage of existing correlated connections between cells. Blocking Cx43 during hypoxic injury has a pronounced neuroprotective effect. Together with the increased expression of mGluR5 receptors, a decrease in mGluR2 expression to the physiological level was found, which suggests the triggering of alternative molecular mechanisms of cell adaptation to hypoxia. Importantly, the blockade of Cx43 in hypoxic damage contributed to the maintenance of both the main parameters of the spontaneous calcium activity of primary cortical cultures and the functional architectonics of neuron-glial networks while maintaining the profile of calcium oscillations and calcium signal communications between cells at a highly correlated level. Our results demonstrate the crucial importance of astrocytic networks in functional brain adaptation to hypoxic damage and could be a promising target for the development of rational anti-hypoxic therapy.

Microtransplantation of Postmortem Native Synaptic mGluRs Receptors into Xenopus Oocytes for Their Functional Analysis

Metabotropic glutamate receptors (mGluRs) are membrane receptors that play a central role in the modulation of synaptic transmission and neuronal excitability and whose dysregulation is implicated in diverse neurological disorders. Most current understanding about the electrophysiological properties of such receptors has been determined using recombinant proteins. However, recombinant receptors do not necessarily recapitulate the properties of native receptors due to the lack of obligated accessory proteins, some of which are differentially expressed as function of developmental stage and brain region. To overcome this limitation, we sought to microtransplant entire synaptosome membranes from frozen rat cortex into Xenopus oocytes, and directly analyze the responses elicited by native mGluRs. We recorded ion currents elicited by 1 mM glutamate using two electrodes voltage clamp. Glutamate produced a fast ionotropic response (6 ± 0.3 nA) in all microtransplanted oocytes (n = 218 oocytes) and a delayed oscillatory response (52 ± 7 nA) in 73% of them. The participation of Group 1 mGluRs was confirmed by the presence of metabotropic oscillations during the administration of (±)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD; Group 1 mGluR agonist), and the absence of oscillations during co-administration of N-(1-adamantyl)quinoxaline-2-carboxamide (NPS 2390; Group 1 mGluR antagonist). Since both mGluR1 and mGluR5 belong to Group 1 mGluRs, further investigation revealed that mGluR1 antagonism with LY 456236 has little effect on metabotropic oscillations, while mGluR5 antagonism with 100 µM AZD 9272 has significant reduction of metabotropic currents elicited by ACPD and glutamate. We confirmed the expression of mGluR1 and mGluR5 in native synaptosomes by immunoblots, both of which are enhanced when compared to their counterpart proteins in rat cortex tissue lysates. Finally, these results demonstrate the merit of using microtransplantation of native synaptosomes for the study of mGluRs and the contribution of mGluR5 to the metabotropic glutamate signaling, providing a better tool for the understanding of the role of these receptors in neurological disorders.

mGluR5 PAMs rescue cortical and behavioural defects in a mouse model of CDKL5 deficiency disorder

Cyclin-dependent kinase-like 5 (CDKL5) deficiency disorder (CDD) is a devastating rare neurodevelopmental disease without a cure, caused by mutations of the serine/threonine kinase CDKL5 highly expressed in the forebrain. CDD is characterized by early-onset seizures, severe intellectual disabilities, autistic-like traits, sensorimotor and cortical visual impairments (CVI). The lack of an effective therapeutic strategy for CDD urgently demands the identification of novel druggable targets potentially relevant for CDD pathophysiology. To this aim, we studied Class I metabotropic glutamate receptors 5 (mGluR5) because of their important role in the neuropathological signs produced by the lack of CDKL5 in-vivo, such as defective synaptogenesis, dendritic spines formation/maturation, synaptic transmission and plasticity. Importantly, mGluR5 function strictly depends on the correct expression of the postsynaptic protein Homer1bc that we previously found atypical in the cerebral cortex of Cdkl5^-/y mice. In this study, we reveal that CDKL5 loss tampers with (i) the binding strength of Homer1bc-mGluR5 complexes, (ii) the synaptic localization of mGluR5 and (iii) the mGluR5-mediated enhancement of NMDA-induced neuronal responses. Importantly, we showed that the stimulation of mGluR5 activity by administering in mice specific positive-allosteric-modulators (PAMs), i.e., 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) or RO6807794, corrected the synaptic, functional and behavioral defects shown by Cdkl5^-/y mice. Notably, in the visual cortex of 2 CDD patients we found changes in synaptic organization that recapitulate those of mutant CDKL5 mice, including the reduced expression of mGluR5, suggesting that these receptors represent a promising therapeutic target for CDD.

Interaction of JNK and mGluR5 in the regulation of psychomotor behaviours after repeated cocaine administration

Activation of protein kinases after cocaine administration controls psychomotor behaviours by interacting with metabotropic receptors in the brain. This study identified how c-Jun N-terminal kinase (JNK) interacts with metabotropic glutamate receptor 5 (mGluR5) in vitro and in the caudate and putamen (CPu). The potential role of this interaction in the regulation of psychomotor behaviour was also evaluated after administration of cocaine. Active JNK phosphorylates a threonine residue at position 1055 in the carboxyl terminus (CT) of mGluR5 in vitro. The binding of active JNK to the D-motif within CT2 is necessary for that phosphorylation. Interaction of phosphorylated JNK and mGluR5 occurs in the CPu. Unilateral interference of the interaction decreases the repeated cocaine-induced increases in locomotor activity and conditioned place preference. These findings suggest that activation of JNK has the capability to interact with mGluR5 in the CPu. Phosphorylation of mGluR5 following the JNK-mGluR5 interaction may be responsible for the potentiation of behavioural sensitisation and cocaine-wanting behaviour in response to cocaine administration.

Machine Learning algorithm unveils glutamatergic alterations in the post-mortem schizophrenia brain

Schizophrenia is a disorder of synaptic plasticity and aberrant connectivity in which a major dysfunction in glutamate synapse has been suggested. However, a multi-level approach tackling diverse clusters of interacting molecules of the glutamate signaling in schizophrenia is still lacking. We investigated in the post-mortem dorsolateral prefrontal cortex (DLPFC) and hippocampus of schizophrenia patients and non-psychiatric controls, the levels of neuroactive d- and l-amino acids (l-glutamate, d-serine, glycine, l-aspartate, d-aspartate) by HPLC. Moreover, by quantitative RT-PCR and western blotting we analyzed, respectively, the mRNA and protein levels of pre- and post-synaptic key molecules involved in the glutamatergic synapse functioning, including glutamate receptors (NMDA, AMPA, metabotropic), their interacting scaffolding proteins (PSD-95, Homer1b/c), plasma membrane and vesicular glutamate transporters (EAAT1, EAAT2, VGluT1, VGluT2), enzymes involved either in glutamate-dependent GABA neurotransmitter synthesis (GAD65 and 67), or in post-synaptic NMDA receptor-mediated signaling (CAMKIIα) and the pre-synaptic marker Synapsin-1. Univariable analyses revealed that none of the investigated molecules was differently represented in the post-mortem DLPFC and hippocampus of schizophrenia patients, compared with controls. Nonetheless, multivariable hypothesis-driven analyses revealed that the presence of schizophrenia was significantly affected by variations in neuroactive amino acid levels and glutamate-related synaptic elements. Furthermore, a Machine Learning hypothesis-free unveiled other discriminative clusters of molecules, one in the DLPFC and another in the hippocampus. Overall, while confirming a key role of glutamatergic synapse in the molecular pathophysiology of schizophrenia, we reported molecular signatures encompassing elements of the glutamate synapse able to discriminate patients with schizophrenia and normal individuals.

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. β-Amyloid 42 (Aβ42) oligomers interact with a mGluR5/cellular prion protein (PrP^C) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Neonatal maternal separation affects metabotropic glutamate receptor 5 expression and anxiety-related behavior of adult rats

Exposure to early life stress leads to long-term neurochemical and behavioral alterations. Stress-induced psychiatric disorders, such as depression, have recently been linked to dysregulation of glutamate signaling, mainly via its postsynaptic receptors. The role of metabotropic glutamate receptor 5 (mGluR5) in stress-induced psychopathology has been the target of several studies in humans. In rodents, blockade of mGluR5 produces antidepressant-like actions, whereas mice lacking mGluR5 exhibit altered anxiety-like behaviors and learning. In this study, we used well-known rodent models of early life stress based on mother-infant separation during the first 3 weeks of life in order to examine the effects of neonatal maternal separation on mGluR5 expression and on anxiety-related behavior in adulthood. We observed that brief (15 min) neonatal maternal separation, but not prolonged (3 h), induced increases in mGluR5 mRNA and protein expression levels in medial prefrontal cortex and mGluR5 protein levels in dorsal, but not ventral, hippocampus of adult rat brain. Behavioral testing using the open-field and the elevated-plus maze tasks showed that brief maternal separations resulted in increased exploratory and decreased anxiety-related behavior, whereas prolonged maternal separations resulted in increased anxiety-related behavior in adulthood. The data indicate that the long-lasting effects of neonatal mother-offspring separation on anxiety-like behavior and mGluR5 expression depend on the duration of maternal separation and suggest that the increased mGluR5 receptors in medial prefrontal cortex and hippocampus of adult rats exposed to brief neonatal maternal separations may underlie their heightened ability to cope with stress.

Genetic disruption of Grm5 causes complex alterations in motor activity, anxiety and social behaviors

Autism is a neurodevelopmental disorder characterized by impaired social interactions and restricted and repetitive behaviors. Although group 1 metabotropic glutamate receptors (mGluRs), and in particular mGluR5, have been extensively proposed as potential targets for intervention in autism and other neurodevelopmental disorders, there has not been a comprehensive analysis of the effect of mGluR5 loss on behaviors typically assessed in autism mouse models thought to be correlates of behavioral symptoms of human disorders. Here we present a behavioral characterization of mice with complete or partial loss of mGluR5 (homozygous or heterozygous null mutations in Grm5 gene). We tested several autism related behaviors including social interaction, repetitive grooming, digging and locomotor behaviors. We found that digging and marble burying behaviors were almost completely abolished in mGluR5 ko mice, although self-grooming was not altered. Social interaction was impaired in ko but not in heterozygote (het) mice. In tests of locomotor activity and anxiety related behaviors, mGluR5 ko mice exhibited hyperactivity and reduced anxiety in the open field test but unexpectedly, showed hypoactivity in the elevated zero-maze test. There was no impairment in motor learning in the accelerating rotarod in both ko and het mutant. Together these results provide support for the importance of mGluR5 in motor and social behaviors that are specifically affected in autism disorders.

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC)

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

Excitatory-inhibitory balance within EEG microstates and resting-state fMRI networks: assessed via simultaneous trimodal PET-MR-EEG imaging

The symbiosis of neuronal activities and glucose energy metabolism is reflected in the generation of functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) signals. However, their association with the balance between neuronal excitation and inhibition (E/I-B), which is closely related to the activities of glutamate and γ-aminobutyric acid (GABA) and the receptor availability (RA) of GABAA and mGluR5, remains unexplored. This research investigates these associations during the resting state (RS) condition using simultaneously recorded PET/MR/EEG (trimodal) data. The trimodal data were acquired from three studies using different radio-tracers such as, [11C]ABP688 (ABP) (N = 9), [11C]Flumazenil (FMZ) (N = 10) and 2-[18F]fluoro-2-deoxy-D-glucose (FDG) (N = 10) targeted to study the mGluR5, GABAA receptors and glucose metabolism respectively. Glucose metabolism and neuroreceptor binding availability (non-displaceable binding potential (BPND)) of GABAA and mGluR5 were found to be significantly higher and closely linked within core resting-state networks (RSNs). The neuronal generators of EEG microstates and the fMRI measures were most tightly associated with the BPND of GABAA relative to mGluR5 BPND and the glucose metabolism, emphasising a predominance of inhibitory processes within in the core RSNs at rest. Changes in the neuroreceptors leading to an altered coupling with glucose metabolism may render the RSNs vulnerable to psychiatric conditions. The paradigm employed here will likely help identify the precise neurobiological mechanisms behind these alterations in fMRI functional connectivity and EEG oscillations, potentially benefitting individualised healthcare treatment measures.

Preconditioning by ultra-low dose of tramadol reduces the severity of tramadol-induced seizure: Contribution of glutamate receptors

Tramadol, a weak agonist of mu-opioid receptors, causes seizure via several mechanisms. Preconditioning has been purposed to reduce the epileptic seizures in animal models of epilepsy. The preconditioning effect of tramadol on seizure is not studied yet. This study was designed to evaluate the preconditioning effect of ultra-low dose of tramadol on the seizures induced by tramadol at high dose. Furthermore, regarding the critical role of glutamate signaling in the pathogenesis of epilepsy, the effect of preconditioning on some glutamate signaling elements was also examined. Male Wistar rats received tramadol (2 mg/kg, i.p) or normal saline (1 mL/kg, i.p) in preconditioning and control groups, respectively. After 4 days, the challenging tramadol dose (150 mg/kg) was injected to all rats. Epileptic behaviors were recorded during 50 min. The expression of Norbin (as a regulator of metabotropic glutamate receptor 5), Calponin3 (as a regulator of excitatory synaptic markers), NR1 (NMDA receptor subunit 1) and GluR1 (AMPA receptor subunit 1) was measured in hippocampus, prefrontal cortex (PFC) and amygdala. Preconditioning decreased the number and duration of tremors and tonic-clonic seizures. Norbin, Calponin3, NR1 and GluR1 expression were decreased in hippocampus, and preconditioning had no effect on them. In contrast, it increased Norbin expression in PFC and amygdala, and attenuated NR1 and GluR1 upregulation following tramadol at high dose. These findings indicated that preconditioning by ultra-low dose of tramadol protected the animals against seizures following high dose of tramadol mediated, at least in part, by Norbin up regulation, and NR1 and GluR1 down regulation.

Aβ oligomers induce pathophysiological mGluR5 signaling in Alzheimer's disease model mice in a sex-selective manner

The prevalence, presentation, and progression of Alzheimer's disease (AD) differ between men and women, although β-amyloid (Aβ) deposition is a pathological hallmark of AD in both sexes. Aβ-induced activation of the neuronal glutamate receptor mGluR5 is linked to AD progression. However, we found that mGluR5 exhibits distinct sex-dependent profiles. Specifically, mGluR5 isolated from male mouse cortical and hippocampal tissues bound with high affinity to Aβ oligomers, whereas mGluR5 from female mice exhibited no such affinity. This sex-selective Aβ-mGluR5 interaction did not appear to depend on estrogen, but rather Aβ interaction with cellular prion protein (PrP^C), which was detected only in male mouse brain homogenates. The ternary complex between mGluR5, Aβ oligomers, and PrP^C was essential to elicit mGluR5-dependent pathological suppression of autophagy in primary neuronal cultures. Pharmacological inhibition of mGluR5 reactivated autophagy, mitigated Aβ pathology, and reversed cognitive decline in male APPswe/PS1ΔE9 mice, but not in their female counterparts. Aβ oligomers also bound with high affinity to human mGluR5 isolated from postmortem donor male cortical brain tissue, but not that from female samples, suggesting that this mechanism may be relevant to patients. Our findings indicate that mGluR5 does not contribute to Aβ pathology in females, highlighting the complexity of mGluR5 pharmacology and Aβ signaling that supports the need for sex-specific stratification in clinical trials assessing AD therapeutics.

Rethinking Intellectual Disability from Neuro- to Astro-Pathology

Neurodevelopmental disorders arise from genetic and/or from environmental factors and are characterized by different degrees of intellectual disability. The mechanisms that govern important processes sustaining learning and memory, which are severely affected in intellectual disability, have classically been thought to be exclusively under neuronal control. However, this vision has recently evolved into a more integrative conception in which astroglia, rather than just acting as metabolic supply and structural anchoring for neurons, interact at distinct levels modulating neuronal communication and possibly also cognitive processes. Recently, genetic tools have made it possible to specifically manipulate astrocyte activity unraveling novel functions that involve astrocytes in memory function in the healthy brain. However, astrocyte manipulation has also underscored potential mechanisms by which dysfunctional astrocytes could contribute to memory deficits in several neurodevelopmental disorders revealing new pathogenic mechanisms in intellectual disability. Here, we review the current knowledge about astrocyte dysfunction that might contribute to learning and memory impairment in neurodevelopmental disorders, with special focus on Fragile X syndrome and Down syndrome.

Metabotropic glutamate receptor 5 in bulimia nervosa

Bulimia nervosa (BN) shares central features with substance-related and addictive disorders. The metabotropic glutamate receptor subtype 5 (mGlu5) plays an important role in addiction. Based on similarities between binge eating and substance-related and addictive disorders, we investigated mGlu5 in vivo in 15 female subjects with BN and 15 matched controls. We measured mGlu5 distribution volume ratio (DVR) with positron emission tomography (PET) using [11 C]ABP688. In BN mGlu5 DVR was higher in the anterior cingulate cortex (ACC), subgenual prefrontal cortex, and straight gyrus (p < 0.05). In BN, higher mGlu5 DVR in various brain regions, including ACC, pallidum, putamen, and caudate, positively correlated with "maturity fears" as assessed using the Eating Disorder Inventory-2 (p < 0.05). In BN and controls, smokers had globally decreased mGlu5 DVR. We present the first evidence for increased mGlu5 DVR in BN. Our findings suggest that pharmacological agents inhibiting mGlu5 might have a therapeutic potential in BN.

Acute Sleep Loss Upregulates the Synaptic Scaffolding Protein, Homer1a, in Non-canonical Sleep/Wake Brain Regions, Claustrum, Piriform and Cingulate Cortices

Homer proteins are a component of the post-synaptic density of neurons that are necessary for the maintenance and consolidation of behavioral state. The dominant negative protein homer1a is rapidly increased by neuronal activity and sleep loss. Homer1a knockout mice with globally absent homer1a have reduced ability to sustain wakefulness during the active period. It is not known whether homer1a is required globally or in very specific brain regions or neurons for its role in maintaining wake. In this study, we examined the expression of homer1a, an immediate early gene involved in intracellular signaling cascades, in mice subjected to extended wakefulness. We found that mice displayed increased expression of homer1a in the claustrum, a brain region thought to be involved in consciousness, as well as the cingulate and piriform cortices compared to non-sleep deprived mice. In situ hybridization (ISH) studies also indicate that homer1a is not induced in the known wake promoting regions with sleep deprivation, but is instead upregulated primarily in the claustrum and piriform cortex. Examination of homer1a expression levels with recovery sleep after sleep deprivation indicate that baseline homer1a expression levels were restored. Further, we have identified that homer1a is upregulated in excitatory neurons of the claustrum suggesting that homer1a promotes wakefulness through activating excitatory neurons. This work identifies regions previously unknown to be involved in sleep regulation that respond to acute sleep deprivation or enhanced waking.

Transcriptional corepressor SIN3A regulates hippocampal synaptic plasticity via Homer1/mGluR5 signaling

Long-term memory depends on the control of activity-dependent neuronal gene expression, which is regulated by epigenetic modifications. The epigenetic modification of histones is orchestrated by the opposing activities of 2 classes of regulatory complexes: permissive coactivators and silencing corepressors. Much work has focused on coactivator complexes, but little is known about the corepressor complexes that suppress the expression of plasticity-related genes. Here, we define a critical role for the corepressor SIN3A in memory and synaptic plasticity, showing that postnatal neuronal deletion of Sin3a enhances hippocampal long-term potentiation and long-term contextual fear memory. SIN3A regulates the expression of genes encoding proteins in the postsynaptic density. Loss of SIN3A increases expression of the synaptic scaffold Homer1, alters the metabotropic glutamate receptor 1α (mGluR1α) and mGluR5 dependence of long-term potentiation, and increases activation of ERK in the hippocampus after learning. Our studies define a critical role for corepressors in modulating neural plasticity and memory consolidation and reveal that Homer1/mGluR signaling pathways may be central molecular mechanisms for memory enhancement.

Identification of Potential Dual Negative Allosteric Modulators of Group I mGluR Family: A Shape Based Screening, ADME Prediction, Induced Fit Docking and Molecular Dynamics Approach Against Neurodegenerative Diseases

BACKGROUND

Glutamate is the principal neurotransmitter in the human brain that exerts its effects through ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The mGluRs are a class of C GPCRs that play a vital role in various neurobiological functions, mGluR1 and mGluR5 are the two receptors distributed throughout the brain involved in cognition, learning, memory, and other important neurological processes. Dysfunction of these receptors can cause neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, X-fragile syndrome, anxiety, depression, etc., hence these receptors are high profile targets for pharmaceutical industries.

OBJECTIVE

The objective of our study is to find the novel dual negative allosteric modulators to regulate both mGluR1 and mGluR5.

METHODS

In this study, shape screening protocol was used to find the dual negative allosteric modulators for both mGluR1 and mGluR5 followed by ADME prediction, induced-fit docking (IFD) and molecular dynamics simulations. Further, DFT analysis and MESP studies were carried out for the selected compounds.

RESULTS

Around 247 compounds were obtained from the eMolecules database and clustered through the CANVAS module and filtered with ADME properties. Furthermore, IFD revealed that the top four compounds (16059796, 25004252, 4667236 and 11670690) having good protein-ligand interactions and binding free energies. The molecular electrostatic potential of the top compounds shows interactions in the amine group and the oxygen atom in the negative potential regions. Finally, molecular dynamics simulations were performed with all the selected as well as the reported compound 29 indicates that the screened hits have better stability of protein ligand complex.

CONCLUSION

Hence, from the results, it is evident that top hits 16059796, 25004252, 4667236 and 11670690 could be a novel and potent dual negative allosteric modulators for mGluR1 and mGluR5.

Metabotropic Glutamate Receptors in Alzheimer's Disease Synaptic Dysfunction: Therapeutic Opportunities and Hope for the Future

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the presence of neuritic plaques and neurofibrillary tangles. The impaired synaptic plasticity and dendritic loss at the synaptic level is an early event associated with the AD pathogenesis. The abnormal accumulation of soluble oligomeric amyloid-β (Aβ), the major toxic component in amyloid plaques, is viewed to trigger synaptic dysfunctions through binding to several presynaptic and postsynaptic partners and thus to disrupt synaptic transmission. Over time, the abnormalities in neural transmission will result in cognitive deficits, which are commonly manifested as memory loss in AD patients. Synaptic plasticity is regulated through glutamate transmission, which is mediated by various glutamate receptors. Here we review recent progresses in the study of metabotropic glutamate receptors (mGluRs) in AD cognition. We will discuss the role of mGluRs in synaptic plasticity and their modulation as a possible strategy for AD cognitive improvement.

Rats bred for high anxiety exhibit distinct fear-related coping behavior, hippocampal physiology, and synaptic plasticity-related gene expression

The hippocampus is essential for learning and memory but also regulates emotional behavior. We previously identified the hippocampus as a major brain region that differs in rats bred for emotionality differences. Rats bred for low novelty response (LRs) exhibit high levels of anxiety- and depression-like behavior compared to high novelty responder (HR) rats. Manipulating the hippocampus of high-anxiety LR rats improves their behavior, although no work to date has examined possible HR/LR differences in hippocampal synaptic physiology. Thus, the current study examined hippocampal slice electrophysiology, dendritic spine density, and transcriptome profiling in HR/LR hippocampus, and compared performance on three hippocampus-dependent tasks: The Morris water maze, contextual fear conditioning, and active avoidance. Our physiology experiments revealed increased long-term potentiation (LTP) at CA3-CA1 synapses in HR versus LR hippocampus, and Golgi analysis found an increased number of dendritic spines in basal layer of CA1 pyramidal cells in HR versus LR rats. Transcriptome data revealed glutamate neurotransmission as the top functional pathway differing in the HR/LR hippocampus. Our behavioral experiments showed that HR/LR rats exhibit similar learning and memory capability in the Morris water maze, although the groups differed in fear-related tasks. LR rats displayed greater freezing behavior in the fear-conditioning task, and HR/LR rats adopted distinct behavioral strategies in the active avoidance task. In the active avoidance task, HRs avoided footshock stress by pressing a lever when presented with a warning cue; LR rats, on the other hand, waited until footshocks began before pressing the lever to stop them. Taken together, these findings concur with prior observations of HR rats generally exhibiting active stress coping behavior while LRs exhibit reactive coping. Overall, our current findings coupled with previous work suggest that HR/LR differences in stress reactivity and stress coping may derive, at least in part, from differences in the developing and adult hippocampus.

Combined Glia Inhibition and Opioid Receptor Agonism Afford Highly Potent Analgesics without Tolerance

Commonly prescribed opioid analgesics produce tolerance upon chronic use due in part to induction of hyperalgesia. Given that two reported bivalent ligands (MMG22 and MCC22) produce potent antinociception without tolerance only in inflamed mice, we have investigated the possible cellular and receptor targets of these ligands. The selective microglia inhibitors, minocycline and SB290157, antagonized intrathecal (i.t.) MCC22 antinociception orders of magnitude more potently than MMG22, suggesting that MCC22 selectively targets activated microglia. The astrocyte toxin, l-α-aminoadipic acid antagonized MMG22 antinociception 126-fold without reducing the potency of MCC22, indicating that activated astrocytes are targets of MMG22. MK-801 and Ro25-6981 antagonism of MMG22 antinociception, but not MCC22, is consistent with selective inhibition of activated NMDAR in astrocytes. The antinociception produced by i.t. MMG22 or MCC22 were both antagonized by the selective mu opioid receptor antagonist, β-FNA, implicating interaction of these ligands with MOR in spinal afferent neurons. MCC22 antinociception was potently blocked by kainate or AMPA ion channel antagonists (LY382884; NBQX), in contrast to MMG22. It is concluded that i.t. MMG22 and MCC22 produce exceptional antinociception via potent inhibition of activated spinal glia, thereby leading to desensitization of spinal neurons and enhanced activation of neuronal MOR. Thus, the present study suggests a new approach to treatment of chronic inflammatory pain without tolerance through a single molecular entity that simultaneously inhibits activated glia and stimulates MOR in spinal neurons.

Metabotropic glutamate receptor subtype 5 is altered in LPS-induced murine neuroinflammation model and in the brains of AD and ALS patients

PURPOSE

The aim of the present study was to determine the expression levels of mGluR5 in different mouse strains after induction of neuroinflammation by lipopolysaccharide (LPS) challenge and in the brains of patients with Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS) post mortem to investigate mGluR5 expression in human neurodegenerative diseases.

METHODS

C57BL/6 and CD1 mice were injected intraperitoneally with either 10 mg/kg LPS or saline. mGluR5 and TSPO mRNA levels were measured after 1 and 5 days by qPCR, and mGluR5 protein levels were determined by PET imaging with the mGluR5-specific radiotracer [¹⁸F]PSS232. mGluR5 expression was evaluated in the post-mortem brain slices from AD and ALS patients using in vitro autoradiography.

RESULTS

mGluR5 and TSPO mRNA levels were increased in brains of C57BL/6 and CD1 mice 1 day after LPS treatment and remained significantly increased after 5 days in C57BL/6 mice but not in CD1 mice. Brain PET imaging with [¹⁸F]PSS232 confirmed increased mGluR5 levels in the brains of both mouse strains 1 day after LPS treatment. After 5 days, mGluR5 levels in CD1 mice declined to the levels in vehicle-treated mice but remained high in C57BL/6 mice. Autoradiograms revealed a severalfold higher binding of [¹⁸F]PSS232 in post-mortem brain slices from AD and ALS patients compared with the binding in control brains.

CONCLUSION

LPS-induced neuroinflammation increased mGluR5 levels in mouse brain and is dependent on the mouse strain and time after LPS treatment. mGluR5 levels were also increased in human AD and ALS brains in vitro. PET imaging of mGluR5 levels could potentially be used to diagnose and monitor therapy outcomes in patients with AD and ALS.

Metabotropic Glutamatergic Receptor 5 and Stress Disorders: Knowledge Gained From Receptor Imaging Studies

The metabotropic glutamatergic receptor subtype 5 (mGluR5) may represent a promising therapeutic target for stress-related psychiatric disorders. Here, we describe mGluR5 findings in stress disorders, particularly major depressive disorder (MDD), highlighting insights from positron emission tomography studies. Positron emission tomography studies report either no differences or lower mGluR5 in MDD, potentially reflecting MDD heterogeneity. Unlike the rapidly acting glutamatergic agent ketamine, mGluR5-specific modulation has not yet shown antidepressant efficacy in MDD and bipolar disorder. Although we recently showed that ketamine may work, in part, through significant mGluR5 modulation, the specific role of mGluR5 downregulation in ketamine's antidepressant response is unclear. In contrast to MDD, there has been much less investigation of mGluR5 in bipolar disorder, yet initial studies indicate that mGluR5-specific treatments may aid in both depressed and manic mood states. The direction of modulation needed may be state dependent, however, limiting clinical feasibility. There has been relatively little study of posttraumatic stress disorder or obsessive-compulsive disorder to date, although there is evidence for the upregulation of mGluR5 in these disorders. However, while antagonism of mGluR5 may reduce fear conditioning, it may also reduce fear extinction. Therefore, studies are needed to determine the role mGluR5 modulation might play in the treatment of these conditions. Further challenges in modulating this prevalent neurotransmitter system include potential induction of significant side effects. As such, more research is needed to identify level and type (positive/negative allosteric modulation or full antagonism) of mGluR5 modulation required to translate existing knowledge into improved therapies.

Metabotropic Glutamate Receptor 5 as a Target for the Treatment of Depression and Smoking: Robust Preclinical Data but Inconclusive Clinical Efficacy

The ability of novel pharmacological compounds to improve outcomes in preclinical models is often not translated into clinical efficacy. Psychiatric disorders do not have biological boundaries, and identifying mechanisms to improve the translational bottleneck between preclinical and clinical research domains is an important and challenging task. Glutamate transmission is disrupted in several neuropsychiatric disorders. Metabotropic glutamate (mGlu) receptors represent a diverse class of receptors that contribute to excitatory neurotransmission. Given the wide, yet region-specific manner of expression, developing pharmacological compounds to modulate mGlu receptor activity provides an opportunity to subtly and selectively modulate excitatory neurotransmission. This review focuses on the potential involvement of mGlu5 receptor disruption in major depressive disorder and substance and/or alcohol use disorders. We provide an overview of the justification of targeting mGlu5 receptors in the treatment of these disorders, summarize the preclinical evidence for negatively modulating mGlu5 receptors as a therapeutic target for major depressive disorders and nicotine dependence, and highlight the outcomes of recent clinical trials. While the evidence of mGlu5 receptor negative allosteric modulation has been promising in preclinical investigations, these beneficial effects have not translated into clinical efficacy. In this review, we identify key challenges that may contribute to poor clinical translation and provide suggested approaches moving forward to potentially improve the translation from preclinical to clinical domains. Such approaches may increase the success of clinical trials and may reduce the translational bottleneck that exists in drug discovery for psychiatric disorders.

Atom-based 3D-QSAR, induced fit docking, and molecular dynamics simulations study of thieno[2,3-b]pyridines negative allosteric modulators of mGluR5

Atom-based three dimensional-quantitative structure-activity relationship (3D-QSAR) model was developed on the basis of 5-point pharmacophore hypothesis (AARRR) with two hydrogen bond acceptors (A) and three aromatic rings for the derivatives of thieno[2,3-b]pyridine, which modulates the activity to inhibit the mGluR5 receptor. Generation of a highly predictive 3D-QSAR model was performed using the alignment of predicted pharmacophore hypothesis for the training set (R² = 0.84, SD = 0.26, F = 45.8, N = 29) and test set (Q² = 0.74, RMSE = 0.235, Pearson-R = 0.94, N = 9). The best pharmacophore hypothesis AARRR was selected, and developed three dimensional-quantitative structure activity relationship (3D-QSAR) model also supported the outcome of this study by means of favorable and unfavorable electron withdrawing group and hydrophobic regions of most active compound 42d and least active compound 18b. Following, induced fit docking and binding free energy calculations reveals the reliable binding orientation of the compounds. Finally, molecular dynamics simulations for 100 ns were performed to depict the protein-ligand stability. We anticipate that the resulted outcome could be supportive to discover potent negative allosteric modulators for metabotropic glutamate receptor 5 (mGluR5).

Sustained Activity of Metabotropic Glutamate Receptor: Homer, Arrestin, and Beyond

When activated, metabotropic glutamate receptors (mGlus) exert long-lasting changes within the glutamatergic synapses. One mechanism is a tonic effect of downstream signal transduction pathways via sustained activation of mGlu itself. Like many other G protein-coupled receptors (GPCRs), mGlu can exist in a constitutively active state, which persists agonist independently. In this paper, we review the current knowledge of the mechanisms underlying the constitutive activity of group I mGlus. The issues concerning Homer1a mechanism in the constitutive activity of group I mGlus and recent findings regarding the significant role of β-arrestin in sustained GPCR activity are also discussed. We propose that once in a state of sustained activation, the mGlu persistently activates downstream signaling pathways, including various adaptor proteins and kinases, such as β-arrestin and mitogen-activated protein kinases. In turn, these effector molecules bind to or phosphorylate the mGlu C-terminal binding domains and consequently regulate the activation state of the mGlu.

Group 1 metabotropic glutamate receptors 1 and 5 form a protein complex in mouse hippocampus and cortex

The group 1 metabotropic glutamate receptors 1 and 5 (mGluR1/5) have been implicated in mechanisms of synaptic plasticity and may serve as potential therapeutic targets in autism spectrum disorders. The interactome of group 1 mGluRs has remained largely unresolved. Using a knockout‐controlled interaction proteomics strategy we examined the mGluR5 protein complex in two brain regions, hippocampus and cortex, and identified mGluR1 as its major interactor in addition to the well described Homer proteins. We confirmed the presence of mGluR1/5 complex by (i) reverse immunoprecipitation using an mGluR1 antibody to pulldown mGluR5 from hippocampal tissue, (ii) coexpression in HEK293 cells followed by coimmunoprecipitation to reveal the direct interaction of mGluR1 and 5, and (iii) superresolution microscopy imaging of hippocampal primary neurons to show colocalization of the mGluR1/5 in the synapse.

Pu-erh Tea Protects the Nervous System by Inhibiting the Expression of Metabotropic Glutamate Receptor 5

Glutamate is one of the major excitatory neurotransmitters of the CNS and is essential for numerous key neuronal functions. However, excess glutamate causes massive neuronal death and brain damage owing to excitotoxicity via the glutamate receptors. Metabotropic glutamate receptor 5 (mGluR5) is one of the glutamate receptors and represents a promising target for studying neuroprotective agents of potential application in neurodegenerative diseases. Pu-erh tea, a fermented tea, mainly produced in Yunnan province, China, has beneficial effects, including the accommodation of the CNS. In this study, pu-erh tea markedly decreased the transcription and translation of mGluR5 compared to those by black and green teas. Pu-erh tea also inhibited the expression of Homer, one of the synaptic scaffolding proteins binding to mGluR5. Pu-erh tea protected neural cells from necrosis via blocked Ca²⁺ influx and inhibited protein kinase C (PKC) activation induced by excess glutamate. Pu-erh tea relieved rat epilepsy induced by LiCl-pilocarpine in behavioural and physiological assays. Pu-erh tea also decreased the expression of mGluR5 in the hippocampus. These results show that the inhibition of mGluR5 plays a role in protecting neural cells from glutamate. The results also indicate that pu-erh tea contains biological compounds binding transcription factors and inhibiting the expression of mGluR5 and identify pu-erh tea as a novel natural neuroprotective agent.

Circadian variation of metabotropic glutamate receptor 5 availability in the rat brain

The metabotrophic subtype 5 glutamate receptor (mGluR5) plays a critical role in synaptic plasticity besides its involvement in numerous neurological disorders, such as depression. As mGluR5 availability in humans is altered in sleep deprivation, we hypothesized that mGluR5 availability underlies a circadian variation. To investigate whether mGluR5 underlies potential circadian changes we measured its density in a randomized fashion at six different daytimes in 11 adult Sprague-Dawley rats. mGluR5 density was quantified by positron emission tomography (PET) using the radioactive ligand [¹¹ C]ABP688. [¹¹ C]ABP688 uptake was quantified in nine regions of interest with a reference tissue model. Significant differences in the binding potential (BP_ND ) and therefore mGluR5 availability between the different circadian times were found in cortex, cingulate cortex, amygdala, caudate putamen and nucleus accumbens. Further post-hoc statistical analysis (Tukey-Kramer test) of the different time-points revealed significant changes in BP_ND between 07:00 hours (start of light-on phase) and 15:00 hours (last time-point of the light-on phase) in the caudate putamen. This study shows that mGluR5 availability is increased during the light-on, or sleep phase, of rodents by approximately 10%. Given that altered mGluR5 densities play a role in psychiatric disorders, further investigation is warranted to evaluate their circadian involvement in mood changes in humans.

Layer-specific potentiation of network GABAergic inhibition in the CA1 area of the hippocampus

One of the most important functions of GABAergic inhibition in cortical regions is the tight control of spatiotemporal activity of principal neuronal ensembles. However, electrophysiological recordings do not provide sufficient spatial information to determine the spatiotemporal properties of inhibitory plasticity. Using Voltage Sensitive Dye Imaging (VSDI) in mouse hippocampal slices, we demonstrate that GABA_A-mediated field inhibitory postsynaptic potentials undergo layer-specific potentiation upon activation of metabotropic glutamate receptors (mGlu). VSDI recordings allowed detection of pharmacologically isolated GABA_A-dependent hyperpolarization signals. Bath-application of the selective group-I mGlu receptor agonist, (S)-3,5-Dihydroxyphenylglycine (DHPG), induces an enhancement of the GABAergic VSDI-recorded signal, which is more or less pronounced in different hippocampal layers. This potentiation is mediated by mGlu₅ and downstream activation of IP₃ receptors. Our results depict network GABAergic activity in the hippocampal CA1 region and its sub-layers, showing also a novel form of inhibitory synaptic plasticity tightly coupled to glutamatergic activity.

mGlu5 positive allosteric modulation normalizes synaptic plasticity defects and motor phenotypes in a mouse model of Rett syndrome

Rett syndrome (RS) is a neurodevelopmental disorder that shares many symptomatic and pathological commonalities with idiopathic autism. Alterations in protein synthesis-dependent synaptic plasticity (PSDSP) are a hallmark of a number of syndromic forms of autism; in the present work, we explore the consequences of disruption and rescue of PSDSP in a mouse model of RS. We report that expression of a key regulator of synaptic protein synthesis, the metabotropic glutamate receptor 5 (mGlu5) protein, is significantly reduced in both the brains of RS model mice and in the motor cortex of human RS autopsy samples. Furthermore, we demonstrate that reduced mGlu5 expression correlates with attenuated DHPG-induced long-term depression in the hippocampus of RS model mice, and that administration of a novel mGlu5 positive allosteric modulator (PAM), termed VU0462807, can rescue synaptic plasticity defects. Additionally, treatment of Mecp2-deficient mice with VU0462807 improves motor performance (open-field behavior and gait dynamics), corrects repetitive clasping behavior, as well as normalizes cued fear-conditioning defects. Importantly, due to the rationale drug discovery approach used in its development, our novel mGlu5 PAM improves RS phenotypes and synaptic plasticity defects without evoking the overt adverse effects commonly associated with potentiation of mGlu5 signaling (i.e. seizures), or affecting cardiorespiratory defects in RS model mice. These findings provide strong support for the continued development of mGlu5 PAMs as potential therapeutic agents for use in RS, and, more broadly, for utility in idiopathic autism.

mGluR5 Positive and Negative Allosteric Modulators Differentially Affect Dendritic Spine Density and Morphology in the Prefrontal Cortex

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of type 5 metabotropic glutamate receptors (mGluR5) are currently being investigated as novel treatments for neuropsychiatric diseases including drug addiction, schizophrenia, and Fragile X syndrome. However, only a handful of studies have examined the effects of mGluR5 PAMs or NAMs on the structural plasticity of dendritic spines in otherwise naïve animals, particularly in brain regions mediating executive function. In the present study, we assessed dendritic spine density and morphology in pyramidal cells of the medial prefrontal cortex (mPFC) after repeated administration of either the prototypical mGluR5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5- yl)benzamide (CDPPB, 20 mg/kg), the clinically utilized mGluR5 NAM 1-(3-chlorophenyl)-3-(3-methyl-5-oxo-4Himidazol- 2-yl)urea (fenobam, 20 mg/kg), or vehicle in male Sprague-Dawley rats. Following once daily treatment for 10 consecutive days, coronal brain sections containing the mPFC underwent diolistic labeling and 3D image analysis of dendritic spines. Compared to vehicle treated animals, rats administered fenobam exhibited significant increases in dendritic spine density and the overall frequency of spines with small (<0.2 μm) head diameters, decreases in frequency of spines with medium (0.2-0.4 μm) head diameters, and had no changes in frequency of spines with large head diameters (>0.4 μm). Administration of CDPPB had no discernable effects on dendritic spine density or morphology, and neither CDPPB nor fenobam had any effect on spine length or volume. We conclude that mGluR5 PAMs and NAMs differentially affect mPFC dendritic spine structural plasticity in otherwise naïve animals, and additional studies assessing their effects in combination with cognitive or behavioral tasks are needed.

Alteration by p11 of mGluR5 localization regulates depression-like behaviors

Mood disorders and antidepressant therapy involve alterations of monoaminergic and glutamatergic transmission. The protein S100A10 (p11) was identified as a regulator of serotonin receptors, and it has been implicated in the etiology of depression and in mediating the antidepressant actions of selective serotonin reuptake inhibitors. Here we report that p11 can also regulate depression-like behaviors via regulation of a glutamatergic receptor in mice. p11 directly binds to the cytoplasmic tail of metabotropic glutamate receptor 5 (mGluR5). p11 and mGluR5 mutually facilitate their accumulation at the plasma membrane, and p11 increases cell surface availability of the receptor. Whereas p11 overexpression potentiates mGluR5 agonist-induced calcium responses, overexpression of mGluR5 mutant, which does not interact with p11, diminishes the calcium responses in cultured cells. Knockout of mGluR5 or p11 specifically in glutamatergic neurons in mice causes depression-like behaviors. Conversely, knockout of mGluR5 or p11 in GABAergic neurons causes antidepressant-like behaviors. Inhibition of mGluR5 with an antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), induces antidepressant-like behaviors in a p11-dependent manner. Notably, the antidepressant-like action of MPEP is mediated by parvalbumin-positive GABAergic interneurons, resulting in a decrease of inhibitory neuronal firing with a resultant increase of excitatory neuronal firing. These results identify a molecular and cellular basis by which mGluR5 antagonism achieves its antidepressant-like activity.

Loss of Metabotropic Glutamate Receptor 5 Function on Peripheral Benzodiazepine Receptor in Mice Prenatally Exposed to LPS

Parental microglial induced neuroinflammation, triggered by bacterial- or viral infections, can induce neuropsychiatric disorders like schizophrenia and autism to offspring in animal models. Recent investigations suggest that microglia, the resident immune cells of the brain, provides a link between neurotransmission, immune cell activation, brain inflammation and neuronal dysfunction seen with the offspring. Relatively little is known about how reduction of brain inflammation and restoration of glial function are associated with diminution of brain degeneration and behavioral deficits in offspring. Increased mGluR5 expression and the long-lasting excitotoxic effects of the neurotoxin during brain development are associated with the glial dysfunctions. We investigated the relationship of mGluR5 and PBR and how they regulate glial function and inflammatory processes in mice prenatally exposed to LPS (120μg/kg, between gestational days 15 and 17), an inflammatory model of a psychiatric disorder. Using PET imaging, we showed that pharmacological activation of mGluR5 during 5 weeks reduced expression of classic inflammation marker PBR in many brain areas and that this molecular association was not present in LPS-exposed offspring. The post-mortem analysis revealed that the down regulation of PBR was mediated through activation of mGluR5 in astrocytes. In addition, we demonstrated that this interaction is defective in a mouse model of the psychiatric deficit offering a novel insight of mGluR5 involvement to brain related disorders and PBR related imaging studies. In conclusion, mGluR5 driven glutamatergic activity regulates astrocytic functions associated with PBR (cholesterol transport, neurosteroidogenesis, glial phenotype) during maturation and could be associated with neuropsychiatric disorders in offspring.

Decreased expression of mGluR5 within the dorsolateral prefrontal cortex in autism and increased microglial number in mGluR5 knockout mice: Pathophysiological and neurobehavioral implications

Metabotropic glutamate receptor 5 (mGluR5) and microglial abnormalities have been implicated in autism spectrum disorder (ASD). However, controversy exists as to whether the receptor is down or upregulated in functioning in ASD. In addition, whilst activation of mGluR5 has been shown to attenuate microglial activation, its role in maintaining microglial homeostasis during development has not been investigated. We utilised published microarray data from the dorsolateral prefrontal cortex (DLPFC) of control (n=30) and ASD (n=27) individuals to carry out regression analysis to assess gene expression of mGluR5 downstream signalling elements. We then conducted a post-mortem brain stereological investigation of the DLPFC, to estimate the proportion of mGluR5-positive neurons and glia. Finally, we carried out stereological investigation into numbers of microglia in mGluR5 knockout mice, relative to wildtype littermates, together with assessment of changes in microglial somal size, as an indicator of activation status. We found that gene expression of mGluR5 was significantly decreased in ASD versus controls (p=0.018) as well as downstream elements SHANK3 (p=0.005) and PLCB1 (p=0.009) but that the pro-inflammatory marker NOS2 was increased (p=0.047). Intensity of staining of mGluR5-positive neurons was also significantly decreased in ASD versus controls (p=0.016). Microglial density was significantly increased in mGluR5 knockout animals versus wildtype controls (p=0.011). Our findings provide evidence for decreased expression of mGluR5 and its signalling components representing a key pathophysiological hallmark in ASD with implications for the regulation of microglial number and activation during development. This is important in the context of microglia being considered to play key roles in synaptic pruning during development, with preservation of appropriate connectivity relevant for normal brain functioning.

Researching glutamate - induced cytotoxicity in different cell lines: a comparative/collective analysis/study

Although glutamate is one of the most important excitatory neurotransmitters of the central nervous system, its excessive extracellular concentration leads to uncontrolled continuous depolarization of neurons, a toxic process called, excitotoxicity. In excitotoxicity glutamate triggers the rise of intracellular Ca²⁺ levels, followed by up regulation of nNOS, dysfunction of mitochondria, ROS production, ER stress, and release of lysosomal enzymes. Excessive calcium concentration is the key mediator of glutamate toxicity through over activation of ionotropic and metabotropic receptors. In addition, glutamate accumulation can also inhibit cystine (CySS) uptake by reversing the action of the CySS/glutamate antiporter. Reversal of the antiporter action reinforces the aforementioned events by depleting neurons of cysteine and eventually glutathione’s reducing potential. Various cell lines have been employed in the pursuit to understand the mechanism(s) by which excitotoxicity affects the cells leading them ultimately to their demise. In some cell lines glutamate toxicity is exerted mainly through over activation of NMDA, AMPA, or kainate receptors whereas in other cell lines lacking such receptors, the toxicity is due to glutamate induced oxidative stress. However, in the greatest majority of the cell lines ionotropic glutamate receptors are present, co-existing to CySS/glutamate antiporters and metabotropic glutamate receptors, supporting the assumption that excitotoxicity effect in these cells is accumulative. Different cell lines differ in their responses when exposed to glutamate. In this review article the responses of PC12, SH-SY5Y, HT-22, NT-2, OLCs, C6, primary rat cortical neurons, RGC-5, and SCN2.2 cell systems are systematically collected and analyzed.