Increased Potency and Selectivity for Group III Metabotropic Glutamate Receptor Agonists Binding at Dual sites

Novel Inhibitory Site Revealed by Mode of Action on the Metabotropic Glutamate 7 Receptor Venus Flytrap Domain

Metabotropic glutamate (mGlu) receptors play a key role in modulating most synapses in the brain. The mGlu7 receptors inhibit presynaptic neurotransmitter release and offer therapeutic possibilities for post-traumatic stress disorders or epilepsy. Screening campaigns provided mGlu7-specific allosteric modulators as the inhibitor XAP044 (Gee et al. J. Biol. Chem. 2014). In contrast to other mGlu receptor allosteric modulators, XAP044 does not bind in the transmembrane domain but to the extracellular domain of the mGlu7 receptor and not at the orthosteric site. Here, we identified the mode of action of XAP044, combining synthesis of derivatives, modeling and docking experiments, and mutagenesis. We propose a unique mode of action of these inhibitors, preventing the closure of the Venus flytrap agonist binding domain. While acting as a noncompetitive antagonist of L-AP4, XAP044 and derivatives act as apparent competitive antagonists of LSP4-2022. These data revealed more potent XAP044 analogues and new possibilities to target mGluRs.

Metabotropic glutamate receptors (mGluRs) in epileptogenesis: an update on abnormal mGluRs signaling and its therapeutic implications

Epilepsy is a neurological disorder characterized by high morbidity, high recurrence, and drug resistance. Enhanced signaling through the excitatory neurotransmitter glutamate is intricately associated with epilepsy. Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors activated by glutamate and are key regulators of neuronal and synaptic plasticity. Dysregulated mGluR signaling has been associated with various neurological disorders, and numerous studies have shown a close relationship between mGluRs expression/activity and the development of epilepsy. In this review, we first introduce the three groups of mGluRs and their associated signaling pathways. Then, we detail how these receptors influence epilepsy by describing the signaling cascades triggered by their activation and their neuroprotective or detrimental roles in epileptogenesis. In addition, strategies for pharmacological manipulation of these receptors during the treatment of epilepsy in experimental studies is also summarized. We hope that this review will provide a foundation for future studies on the development of mGluR-targeted antiepileptic drugs.

Designing drugs and chemical probes with the dualsteric approach

Traditionally, drugs are monovalent, targeting only one site on the protein surface. This includes orthosteric and allosteric drugs, which bind the protein at orthosteric and allosteric sites, respectively. Orthosteric drugs are good in potency, whereas allosteric drugs have better selectivity and are solutions to classically undruggable targets. However, it would be difficult to simultaneously reach high potency and selectivity when targeting only one site. Also, both kinds of monovalent drugs suffer from mutation-caused drug resistance. To overcome these obstacles, dualsteric modulators have been proposed in the past twenty years. Compared to orthosteric or allosteric drugs, dualsteric modulators are bivalent (or bitopic) with two pharmacophores. Each of the two pharmacophores bind the protein at the orthosteric and an allosteric site, which could bring the modulator with special properties beyond monovalent drugs. In this study, we comprehensively review the current development of dualsteric modulators. Our main effort reason and illustrate the aims to apply the dualsteric approach, including a "double win" of potency and selectivity, overcoming mutation-caused drug resistance, developments of function-biased modulators, and design of partial agonists. Moreover, the strengths of the dualsteric technique also led to its application outside pharmacy, including the design of highly sensitive fluorescent tracers and usage as molecular rulers. Besides, we also introduced drug targets, designing strategies, and validation methods of dualsteric modulators. Finally, we detail the conclusions and perspectives.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

The mGlu7 receptor in schizophrenia - An update and future perspectives

The mGlu₇ receptor belongs to the III group of metabotropic glutamatergic (mGlu) receptors and physiologically serves as an "emergency" receptor that is activated by high, almost pathological, glutamate concentrations. Of all mGlu receptors, this receptor is most highly expressed in the brain. Additionally, relatively intense expression of the receptor was found at the periphery, for example in the bowels or in the reproductive system of male mice, but this review will be focused predominantly on its role in the brain. In the CNS, the receptor is expressed presynaptically, in the center of the synaptic cleft, at the terminals of both excitatory glutamatergic and inhibitory GABAergic neurons. Thus, it may regulate the release of both glutamate and GABA. Schizophrenia is thought to develop as a consequence of a disturbed glutamatergic-GABAergic balance in different parts of the brain. Thus, the mGlu₇ receptor may be involved in the pathophysiology of schizophrenia and consequently constitute the target for antipsychotic drug discovery. In this review, we summarize the available data about mGlu₇ receptor ligands and their activity in animal models of schizophrenia. At present, only a few ligands are available, and negative allosteric modulators (NAMs) appear to exert antipsychotic-like efficacy, indicating that the inhibition of the receptor could constitute a promising target in the search for novel drugs. Additionally, the data concerning the expression of the receptor in the CNS and putative mechanisms by which its inhibition may contribute to the treatment of schizophrenia will be discussed. Finally, the polymorphisms of genes encoding the receptor in schizophrenic patients will also be provided.

Metabotropic glutamate receptor orthosteric ligands and their binding sites

Metabotropic glutamate receptors (mGluRs) have been discovered almost four decades ago. Since then, their pharmacology has been largely developed as well as their structural organization. Indeed mGluRs are attractive therapeutic targets for numerous psychiatric and neurological disorders because of their modulating role of synaptic transmission. The more recent drug discovery programs have mostly concentrated on allosteric modulators. However, orthosteric agonists and antagonists have remained unavoidable pharmacological tools as, although not expected, many of them can reach the brain, or can be modified to reach the brain. This review focuses on the most common orthosteric ligands as well as on the few allosteric modulators interacting with the glutamate binding domain. The 3D-structures of these ligands at their binding sites are reported. For most of them, X-Ray structures or docked homology models are available. Because of the high conservation of the binding site, subtype selective agonists were not easy to find. Yet, some were discovered when extending their chemical structures in order to reach selective sites of the receptors.

Allosteric Molecular Switches in Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGlu) are class C G protein-coupled receptors of eight subtypes that are omnipresently expressed in the central nervous system. mGlus have relevance in several psychiatric and neurological disorders, therefore they raise considerable interest as drug targets. Allosteric modulators of mGlus offer advantages over orthosteric ligands owing to their increased potential to achieve subtype selectivity, and this has prompted discovery programs that have produced a large number of reported allosteric mGlu ligands. However, the optimization of allosteric ligands into drug candidates has proved to be challenging owing to induced-fit effects, flat or steep structure-activity relationships and unexpected changes in theirpharmacology. Subtle structural changes identified as molecular switches might modulate the functional activity of allosteric ligands. Here we review these switches discovered in the metabotropic glutamate receptor family..

LSP2-9166, an orthosteric mGlu4 and mGlu7 receptor agonist, reduces cocaine self-administration under a progressive ratio schedule in rats

Cocaine addiction is a serious health issue in Western countries. Despite the regular increase in cocaine consumption across the population, there is no specific treatment for cocaine addiction. Critical roles for glutamate neurotransmission in the rewarding effects of psychostimulants as well as relapse have been suggested and accumulating evidence indicates that targeting mGlu group III receptors could represent a promising strategy to develop therapeutic compounds to treat addiction. In this context, the aim of our study was to examine the effect of LSP2-9166, a mGlu4/mGlu7 receptor orthosteric agonist, on the motivation for cocaine intake. We used an intravenous self-administration paradigm in male Wistar rats as a reliable model of voluntary drug intake. We first evaluated the direct impact of cocaine on Grm4 and Grm7 gene expression. Voluntary cocaine intake under a fixed ratio schedule of injections induced an increase of both mGlu4 and mGlu7 receptor transcripts in nucleus accumbens and hippocampus. We then evaluated the ability of LSP2-9166 to affect cocaine self-administration under a progressive ratio schedule of reinforcement. We found that this compound inhibits the motivation to obtain the drug, although it induced a hypolocomotor effect which could biais motivation index. Our findings demonstrate that mGlu group III receptors represent new targets for decreasing motivation to self-administer cocaine.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Conformational pathway provides unique sensitivity to a synaptic mGluR

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors that operate at synapses. Macroscopic and single molecule FRET to monitor structural rearrangements in the ligand binding domain (LBD) of the mGluR7/7 homodimer revealed it to have an apparent affinity ~4000-fold lower than other mGluRs and a maximal activation of only ~10%, seemingly too low for activation at synapses. However, mGluR7 heterodimerizes, and we find it to associate with mGluR2 in the hippocampus. Strikingly, the mGluR2/7 heterodimer has high affinity and efficacy. mGluR2/7 shows cooperativity in which an unliganded subunit greatly enhances activation by agonist bound to its heteromeric partner, and a unique conformational pathway to activation, in which mGluR2/7 partially activates in the Apo state, even when its LBDs are held open by antagonist. High sensitivity and an unusually broad dynamic range should enable mGluR2/7 to respond to both glutamate transients from nearby release and spillover from distant synapses.

Mechanism of neuroprotective effect of mGluR4 agonists

Introduction: This review of literature is to demonstrate a role of group III metabotropic glutamate receptors in maintaining the level of extracellular glutamate in ischemic stroke and neurodegenerative diseases.

Metabotropic glutamate receptors: mGluRs are classified into three groups. It is suggested that the activation of mGluR4 may have a neuroprotective effect.

Role of excitotoxicity in the development and severity of various brain diseases: An increase in the concentration of intracellular Ca2+ is the result of excessive accumulation of glutamate in the extracellular space. And a death of nerve cells occurs after a sequence of biochemical reactions, which was called excitotoxicity. It is followed by an imbalance between glutamatergic excitation and GABA-ergic inhibition. As a result of untimely activation of the inhibitory mechanisms, the accumulation of extracellular glutamate, and consequently the death of neurons, continues, which leads to more severe manifestations of the cerebral ischemia.

Role of modulation of mGluRs activity in neuroprotection: The literature describes a large number of studies proving that inhibition of hyperactive glutamatergic transmission has a neuroprotective effect. The most likely mechanisms of neuroprotection are inhibition of glutamate production in the substantia nigra, which in turn protects against glutamate-mediated excitotoxicity, and the reduction of the inflammatory effects.

Anti-inflammatory effect of mGluR4 agonists in the mechanism of neuroprotective action: The astroglial component may contribute to the protective action of mGluR4 modulators, since astrocytes and microglia have mGluR4.

Conclusion: mGluR4 agonists have the neuroprotective and anti-inflammatory effects.

The mGlu7 receptor provides protective effects against epileptogenesis and epileptic seizures

Finding new targets to control or reduce seizure activity is essential to improve the management of epileptic patients. We hypothesized that activation of the pre-synaptic and inhibitory metabotropic glutamate receptor type 7 (mGlu7) reduces spontaneous seizures. We tested LSP2-9166, a recently developed mGlu7/4 agonist with unprecedented potency on mGlu7 receptors, in two paradigms of epileptogenesis. In a model of chemically induced epileptogenesis (pentylenetetrazole systemic injection), LSP2-9166 induces an anti-epileptogenic effect rarely observed in preclinical studies. In particular, we found a bidirectional modulation of seizure progression by mGlu4 and mGlu7 receptors, the latter preventing kindling. In the intra-hippocampal injection of kainic acid mouse model that mimics the human mesial temporal lobe epilepsy, we found that LSP2-9166 reduces seizure frequency and hippocampal sclerosis. LSP2-9166 also acts as an anti-seizure drug on established seizures in both models tested. Specific modulation of the mGlu7 receptor could represent a novel approach to reduce pathological network remodeling.

Crystallographic and spectroscopic character-ization of 4-nitro-2-(tri-fluoro-meth-yl)benzoic acid and 4-nitro-3-(tri-fluoro-meth-yl)benzoic acid

The title compounds, both C8H4F3NO4, represent two isomers of nitro tri-fluoro-methyl benzoic acid. The compounds each contain a nitro functionality para to the carb-oxy-lic acid group, with the tri-fluoro-methyl substituent ortho to the acid group in the 2-isomer and ortho to the nitro group in the 3-isomer. The regiochemistry with respect to the tri-fluoro-methyl group results in steric inter-actions that rotate the carb-oxy-lic acid group or the nitro group out of the aromatic plane in the 2- and 3-isomer, respectively. Each mol-ecule engages in inter-molecular hydrogen bonding, forming head-to-tail dimers with graph-set notation R 2 2(8) and donor-acceptor hydrogen-bonding distances of 2.7042 (14) Å in the 2-isomer and 2.6337 (16) in the 3-isomer. Recrystallization attempts did not yield untwinned crystals.

Formal [3+2] cycloaddition of vinylcyclopropane azlactones to enals using synergistic catalysis

The present study reports the asymmetric cyclization of enals with vinylcyclopropane azlactones efficiently catalyzed by the combination of achiral Pd(0) complexes and chiral secondary amines. Corresponding spirocyclic azlactones were produced in high yields with moderate diastereoselectivities and excellent enantioselectivities. This protocol provides an efficient and easily-performed route to spirocyclic scaffolds, and densely functionalized cyclopentanes containing quaternary carbon centers.

Discovery of VU2957 (Valiglurax): An mGlu4 Positive Allosteric Modulator Evaluated as a Preclinical Candidate for the Treatment of Parkinson's Disease

Herein, we report the discovery of a novel potent, selective, CNS penetrant, and orally bioavailable mGlu₄ PAM, VU0652957 (VU2957, Valiglurax). VU2957 possessed attractive in vitro and in vivo pharmacological and DMPK properties across species. To advance toward the clinic, a spray-dried dispersion (SDD) formulation of VU2957 was developed to support IND-enabling toxicology studies. Based on its overall profile, VU2957 was evaluated as a preclinical development candidate for the treatment of Parkinson's disease.

A Light-Controlled Allosteric Modulator Unveils a Role for mGlu4 Receptors During Early Stages of Ischemia in the Rodent Cerebellar Cortex

Metabotropic glutamate receptors (mGlus) are G Protein coupled-receptors that modulate synaptic transmission and plasticity in the central nervous system. Some act as autoreceptors to control neurotransmitter release at excitatory synapses and have become attractive targets for drug therapy to treat certain neurological disorders. However, the high degree of sequence conservation around the glutamate binding site makes the development of subtype-specific orthosteric ligands difficult to achieve. This problem can be circumvented by designing molecules that target specific less well conserved allosteric sites. One such allosteric drug, the photo-switchable compound OptoGluNAM4.1, has been recently employed to reversibly inhibit the activity of metabotropic glutamate 4 (mGlu₄) receptors in cell cultures and in vivo. We studied OptoGluNAM4.1 as a negative modulator of neurotransmission in rodent cerebellar slices at the parallel fiber – Purkinje cell synapse. Our data show that OptoGluNAM4.1 antagonizes pharmacological activation of mGlu₄ receptors in a fully reversible and photo-controllable manner. In addition, for the first time, this new allosteric modulator allowed us to demonstrate that, in brain slices from the rodent cerebellar cortex, mGlu₄ receptors are endogenously activated in excitotoxic conditions, such as the early phases of simulated cerebellar ischemia, which is associated with elevated levels of extracellular glutamate. These findings support OptoGluNAM4.1 as a promising new tool for unraveling the role of mGlu₄ receptors in the central nervous system in physio-pathological conditions.

Structural Basis for ( S)-3,4-Dicarboxyphenylglycine (DCPG) As a Potent and Subtype Selective Agonist of the mGlu8 Receptor

( S)-3,4-Dicarboxyphenylglycine (DCPG) was first reported in 2001 as a potent orthosteric agonist with high subtype selectivity for the mGlu₈ receptor, but the structural basis for its high selectivity is not well understood. We have solved a cocrystal structure of recombinant human mGlu₈ amino terminal domain (ATD) protein bound to ( S)-DCPG, which possesses the largest lobe opening angle observed to date among known agonist-bound mGlu ATD crystal structures. The binding conformation of ( S)-DCPG observed in the crystal structure is significantly different from that in the homology model built from an l-glutamate-bound rat mGlu₁ ATD crystal structure, which has a smaller lobe opening angle. This highlights the importance of considering various lobe opening angles when modeling mGlu ATD-ligand complex. New homology models of other mGlu receptors based on the ( S)-DCPG-bound mGlu₈ ATD crystal structure were explored to rationalize ( S)-DCPG's high mGlu₈ receptor subtype selectivity.

Metabotropic Glutamate Receptor 7: A New Therapeutic Target in Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) are characterized by a wide range of symptoms including delayed speech, intellectual disability, motor dysfunction, social deficits, breathing problems, structural abnormalities, and epilepsy. Unfortunately, current treatment strategies are limited and innovative new approaches are sorely needed to address these complex diseases. The metabotropic glutamate receptors are a class of G protein-coupled receptors that act to modulate neurotransmission across many brain structures. They have shown great promise as drug targets for numerous neurological and psychiatric diseases. Moreover, the development of subtype-selective allosteric modulators has allowed detailed studies of each receptor subtype. Here, we focus on the metabotropic glutamate receptor 7 (mGlu₇) as a potential therapeutic target for NDDs. mGlu₇ is expressed widely throughout the brain in regions that correspond to the symptom domains listed above and has established roles in synaptic physiology and behavior. Single nucleotide polymorphisms and mutations in the GRM7 gene have been associated with idiopathic autism and other NDDs in patients. In rodent models, existing literature suggests that decreased mGlu₇ expression and/or function may lead to symptoms that overlap with those of NDDs. Furthermore, potentiation of mGlu₇ activity has shown efficacy in a mouse model of Rett syndrome. In this review, we summarize current findings that provide rationale for the continued development of mGlu₇ modulators as potential therapeutics.

Discovery, Structure-Activity Relationship, and Biological Characterization of a Novel Series of 6-((1 H-Pyrazolo[4,3- b]pyridin-3-yl)amino)-benzo[ d]isothiazole-3-carboxamides as Positive Allosteric Modulators of the Metabotropic Glutamate Receptor 4 (mGlu4)

This work describes the discovery and characterization of novel 6-(1 H-pyrazolo[4,3- b]pyridin-3-yl)amino-benzo[ d]isothiazole-3-carboxamides as mGlu₄ PAMs. This scaffold provides improved metabolic clearance and CYP1A2 profiles compared to previously discovered mGlu₄ PAMs. From this work, 27o (VU6001376) was identified as a potent (EC₅₀ = 50.1 nM, 50.5% GluMax) and selective mGlu₄ PAM with an excellent rat DMPK profile ( in vivo rat CL_p = 3.1 mL/min/kg, t_1/2 = 445 min, CYP1A2 IC₅₀ > 30 μM). Compound 27o was also active in reversing haloperidol induced catalepsy in a rodent preclinical model of Parkinson's disease.

Profiling of orthosteric and allosteric group-III metabotropic glutamate receptor ligands on various G protein-coupled receptors with Tag-lite® assays

Group-III metabotropic glutamate (mGlu) receptors are important synaptic regulators and are potential druggable targets for Parkinson disease, autism and pain. Potential drugs include orthosteric agonists in the glutamate binding extracellular domain and positive allosteric modulators interacting with seven-pass transmembrane domains. Orthosteric agonists are rarely completely specific for an individual group-III mGlu subtype. Furthermore they often fail to pass the blood-brain barrier and they constitutively activate their target receptor. These properties limit the potential therapeutic use of orthosteric agonists. Allosteric modulators are more specific and maintain the biological activity of the targeted receptor. However, they bind in a hydrophobic pocket and this limits their bio-availability and increases possible off-target action. It is therefore important to characterize the action of potential drug targets with a multifaceted and deeply informative assay. Here we aimed at multifaceted deep profiling of the effect of seven different agonists, and seven positive allosteric modulators on 34 different G protein-coupled receptors by a Tag-lite^® assay. Our results did not reveal off-target activity of mGlu orthosteric agonists. However, five allosteric modulators had either positive or negative effects on non-cognate G protein-coupled receptors. In conclusion, we demonstrate the power of the Tag-lite^® assay for potential drug ligand profiling on G protein-coupled receptors and its potential to identify positive allosteric compounds.

Discovery and characterization of N-(1,3-dialkyl-1H-indazol-6-yl)-1H-pyrazolo[4,3-b]pyridin-3-amine scaffold as mGlu4 positive allosteric modulators that mitigate CYP1A2 induction liability

Previous reports from our laboratory disclosed the structure and activity of a novel 1H-pyrazolo[4,3-b]pyridine-3-amine scaffold (VU8506) which showed excellent potency, selectivity and in vivo efficacy in preclinical rodent models of Parkinson's disease. Unfortunately, this compound suffered from significant CYP1A2 induction as measured through upstream AhR activation (125-fold) and thus was precluded from further advancement in chronic studies. Herein, we report a new scaffold developed recently which was systematically studied in order to mitigate the CYP1A2 liabilities presented in the earlier scaffolds. We have identified a novel structure that maintains the potency and selectivity of other mGlu₄ PAMs, leading to 9i (hmGlu₄ EC₅₀ = 43 nM; AhR activation = 2.3-fold).

Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

Due to the essential roles of glutamate, detection and response to a large range of extracellular concentrations of this excitatory amino acid are necessary for the fine-tuning of brain functions. Metabotropic glutamate receptors (mGluRs) are implicated in shaping the activity of many synapses in the central nervous system. Among the eight mGluR subtypes, there is increasing interest in studying the mGlu₃ receptor which has recently been linked to various diseases, including psychiatric disorders. This receptor displays striking functional properties, with a high and, often, full basal activity, making its study elusive in heterologous systems. Here, we demonstrate that Cl^- ions exert strong positive allosteric modulation of glutamate on the mGlu₃ receptor. We have also identified the molecular and structural determinants lying behind this allostery: a unique interactive "chloride-lock" network. Indeed, Cl^- ions dramatically stabilize the glutamate-induced active state of the extracellular domain of the mGlu₃ receptor. Thus, the mGlu₃ receptors' large basal activity does not correspond to a constitutive activity in absence of agonist. Instead, it results mostly from a Cl^-mediated amplified response to low ambient glutamate concentrations, such as those measured in cell media. This strong interaction between glutamate and Cl^- ions allows the mGlu₃ receptor to sense and efficiently react to sub-micromolar concentrations of glutamate, making it the most sensitive member of mGluR family.