Anxiolytic-like activity of the mGLU2/3 receptor agonist LY354740 in the elevated plus maze test is disrupted in metabotropic glutamate receptor 2 and 3 knock-out mice

mGlu2/3 receptor agonist (LY354740) in anxiety

mGlu2/3 Receptors (LY354740) in Anxiety mGlu2/3 receptors when activated decrease glutamate excitation on limbic synapses involved in anxiety. The orally active agonist compound LY354740 (or prodrug LY544344) was active in animal and human models of stress/anxiety. Later clinical studies showed efficacy in generalized anxiety in patients, validating this mechanism clinically. However, the compound was terminated due to rodent seizures in long-term toxicology studies.

Anxiolytic-like effects of an mGluR 5 antagonist and a mGluR 2/3 agonist, and antidepressant-like effects of an mGluR 7 agonist in the chick social separation stress test, a dual-drug screening model of treatment-resistant depression

Modulation of glutamate receptors has demonstrated anxiolytic and/or antidepressant effects in rodent stress models. The chick social-separation stress paradigm exposes socially raised aves to an isolation stressor which elicits distress vocalizations (DVocs) in an attempt to re-establish contact. The model presents a state of panic during the first 5 min followed by a state of behavioral despair during the last 60 to 90 min. Making it useful as a dual anxiolytic/antidepressant screening assay. Further research has identified the Black Australorp strain as a stress-vulnerable, treatment-resistant, and ketamine-sensitive genetic line. Utilizing this genetic line, we sought to evaluate modulation of glutamatergic receptors for potential anxiolytic and/or antidepressant effects. Separate dose-response studies were conducted for the following drugs: the AMPA PAM LY392098, the mGluR 5 antagonist MPEP, the mGluR 2/3 agonist LY404039, the mGluR 2/3 antagonist LY341495, and the mGluR 7 agonist AMN082. The norepinephrine α² agonist clonidine and the NMDA antagonist ketamine were included as comparison for anxiolytic (anti-panic) and antidepressant effects, respectively. As in previous studies, clonidine reduced DVoc rates during the first 5 min (attenuation of panic) and ketamine elevated DVoc rates (attenuation of behavioral despair) during the last 60 min of isolation. The mGluR 2/3 agonist LY404039 and the mGluR 5 antagonist MPEP decreased DVoc rates during the first 5 min of isolation indicative of anxiolytic effects like that of clonidine while the mGluR 7 agonist AMN082 elevated DVoc rates in the later hour of isolation, representative of antidepressant effects like that of ketamine. Collectively, these findings suggest that certain glutamate targets may be clinically useful in treating panic disorder and/or treatment-resistant depression.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Clinical investigations of compounds targeting metabotropic glutamate receptors

Pharmacological modulation of glutamate has long been considered to be of immense therapeutic utility. The metabotropic glutamate receptors (mGluRs) are potential targets for safely altering glutamate-driven excitation. Data support the potential therapeutic use of mGluR modulators in the treatment of anxiety, depression, schizophrenia, and other psychiatric disorders, pain, epilepsy, as well as neurodegenerative and neurodevelopmental disorders. For each of the three mGluR groups, compounds have been constructed that produce either potentiation or functional blockade. PET ligands for mGlu5Rs have been studied in a range of patient populations and several mGlu5R antagonists have been tested for potential efficacy in patients including mavoglurant, diploglurant, basimglurant, GET 73, and ADX10059. Efficacy with mGlu5R antagonists has been reported in trials with patients with gastroesophageal reflux disease; data from patients with Parkinson's disease or Fragile X syndrome have not been as robust as hoped. Fenobam was approved for use as an anxiolytic prior to its recognition as an mGlu5R antagonist. mGlu2/3R agonists (pomaglumated methionil) and mGlu2R agonists (JNJ-40411813, AZD 8529, and LY2979165) have been studied in patients with schizophrenia with promising but mixed results. Antagonists of mGlu2/3Rs (decoglurant and TS-161) have been studied in depression where TS-161 has advanced into a planned Phase 2 study in treatment-resistant depression. The Group III mGluRs are the least developed of the mGluR receptor targets. The mGlu4R potentiator, foliglurax, did not meet its primary endpoint in patients with Parkinson's disease. Ongoing efforts to develop mGluR-targeted compounds continue to promise these glutamate modulators as medicines for psychiatric and neurological disorders.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

Group II metabotropic glutamate receptor (mGlu2 and mGlu3 ) roles in thalamic processing

BACKGROUND AND PURPOSE

As the thalamus underpins almost all aspects of behaviour, it is important to understand how the thalamus operates. Group II metabotropic glutamate (mGlu₂ /mGlu₃ ) receptor activation reduces inhibition in thalamic nuclei originating from the surrounding thalamic reticular nucleus (TRN). Whilst an mGlu₂ component to this effect has been reported, in this study, we demonstrate that it is likely, largely mediated via mGlu₃ .

EXPERIMENTAL APPROACH

The somatosensory ventrobasal thalamus (VB) is an established model for probing fundamental principles of thalamic function. In vitro slices conserving VB-TRN circuitry from wild-type and mGlu₃ knockout mouse brains were used to record IPSPs and mIPSCs. In vivo extracellular recordings were made from VB neurons in anaesthetised rats. A range of selective pharmacological agents were used to probe Group II mGlu receptor function (agonist, LY354740; antagonist, LY341495; mGlu₂ positive allosteric modulator, LY487379 and mixed mGlu₂ agonist/mGlu₃ antagonist LY395756).

KEY RESULTS

The in vitro and in vivo data are complementary and suggest that mGlu₃ receptor activation is largely responsible for potentiating responses to somatosensory stimulation by reducing inhibition from the TRN.

CONCLUSIONS AND IMPLICATIONS

mGlu₃ receptor activation in the VB likely enables important somatosensory information to be discerned from background activity. These mGlu₃ receptors are likely to be endogenously activated via 'glutamate spillover'. In cognitive thalamic nuclei, this mechanism may be of importance in governing attentional processes. Positive allosteric modulation of endogenous mGlu₃ receptor activation may therefore enhance cognitive function in pathophysiological disease states, such as schizophrenia, thus representing a highly specific therapeutic target.

Targeting metabotropic glutamate receptors for the treatment of depression and other stress-related disorders

The discovery of robust antidepressant effects of ketamine in refractory patients has led to increasing focus on agents targeting glutamatergic signaling as potential novel antidepressant strategy. Among the agents targeting the glutamatergic system, compounds acting at metabotropic glutamate (mGlu) receptors are among the most promising agents under studies for depressive disorders. Further, the receptor diversity, distinct distribution in the CNS, and ability to modulate the glutamatergic neurotransmission in the brain areas implicated in mood disorders make them an exciting target for stress-related disorders. In preclinical models, antidepressant and anxiolytic effects of mGlu₅ negative allosteric modulators (NAMs) have been reported. Interestingly, mGlu_2/3 receptor antagonists show fast and sustained antidepressant-like effects similar to that of ketamine in rodents. Excitingly, they can also induce antidepressant effects in the animal models of treatment-resistant depression and are devoid of the side-effects associated with ketamine. Unfortunately, clinical trials of both mGlu₅ and mGlu_2/3 receptor NAMs have been inconclusive, and additional trials using other compounds with suitable preclinical and clinical properties are needed. Although group III mGlu receptors have gained less attention, mGlu₇ receptor ligands have been shown to induce antidepressant-like effects in rodents. Collectively, compounds targeting mGlu receptors provide an alternative approach to fill the outstanding clinical need for safer and more efficacious antidepressants. This article is part of the special Issue on "Glutamate Receptors - mGluRs".

Maternal stress programs a demasculinization of glutamatergic transmission in stress-related brain regions of aged rats

Brain aging may be programmed by early-life stress. Aging affects males and females differently, but how perinatal stress (PRS) affects brain aging between sexes is unknown. We showed behavioral and neurobiological sex differences in non-stressed control rats that were strongly reduced or inverted in PRS rats. In particular, PRS decreased risk-taking behavior, spatial memory, exploratory behavior, and fine motor behavior in male aged rats. In contrast, female aged PRS rats displayed only increased risk-taking behavior and reduced exploratory behavior. PRS induced large reductions in the expression of glutamate receptors in the ventral and dorsal hippocampus and prefrontal cortex only in male rats. PRS also reduced the expression of synaptic vesicle-associated proteins, glucocorticoid receptors (GR), and mineralocorticoid receptors (MR) in the ventral hippocampus of aged male rats. In contrast, in female aged rats, PRS enhanced the expression of MRs and brain-derived neurotrophic factor (BDNF) in the ventral hippocampus and the expression of glial fibrillary acidic protein (GFAP) and BDNF in the prefrontal cortex. A common PRS effect in both sexes was a reduction in exploratory behavior and metabotropic glutamate (mGlu2/3) receptors in the ventral hippocampus and prefrontal cortex. A multidimensional analysis revealed that PRS induced a demasculinization profile in glutamate-related proteins in the ventral and dorsal hippocampus and prefrontal cortex, as well as a demasculinization profile of stress markers only in the dorsal hippocampus. In contrast, defeminization was observed only in the ventral hippocampus. Measurements of testosterone and 17-β-estradiol in the plasma and aromatase in the dorsal hippocampus were consistent with a demasculinizing action of PRS. These findings confirm that the brains of males and females differentially respond to PRS and aging suggesting that females might be more protected against early stress and age-related inflammation and neurodegeneration. Taken together, these results may contribute to understanding how early environmental factors shape vulnerability to brain aging in both sexes and may lay the groundwork for future studies aimed at identifying new treatment strategies to improve the quality of life of older individuals, which is of particular interest given that there is a high growth of aging in populations around the world.

The Paraventricular Nucleus of the Thalamus as an Integrating and Relay Node in the Brain Anxiety Network

The brain anxiety network is composed of a number of interconnected cortical regions that detect threats and execute appropriate defensive responses via projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and lateral region of the central nucleus of the amygdala (CeL). The paraventricular nucleus of the thalamus (PVT) is anatomically positioned to integrate threat- and arousal-related signals from cortex and hypothalamus and then relay these signals to neural circuits in the NAcSh, BSTDL, and CeL that mediate defensive responses. This review describes the anatomical connections of the PVT that support the view that the PVT may be a critical node in the brain anxiety network. Experimental findings are reviewed showing that the arousal peptides orexins (hypocretins) act at the PVT to promote avoidance of potential threats especially following exposure of rats to a single episode of footshocks. Recent anatomical and experimental findings are discussed which show that neurons in the PVT provide divergent projections to subcortical regions that mediate defensive behaviors and that the projection to the NAcSh is critical for the enhanced social avoidance displayed in rats exposed to footshocks. A theoretical model is proposed for how the PVT integrates cortical and hypothalamic signals to modulate the behavioral responses associated with anxiety and other challenging situations.

Extending the Specificity of DRL 72-s Behavior for Screening Antidepressant-Like Effects of Glutamatergic Clinically Validated Anxiolytic or Antidepressant Drugs in Rats

Both an agonist and its associated prodrug for metabotropic glutamate_2/3 (mGlu_2/3) receptors demonstrated anxiolytic efficacy in large, randomized, multicenter, double-blind, placebo-controlled trials studying patients with generalized anxiety disorder (GAD). These mGlu_2/3 receptor agonists produced robust preclinical anxiolytic-like effects in rodent models. Several different metabotropic glutamate₂ receptor positive allosteric modulators have been found to produce antidepressant-like effects on several preclinical screening paradigms, including differential-reinforcement-of-low-rate 72-second (DRL 72-s) behavior [increased reinforcers, decreased response rate, and cohesive rightward shifts in inter-response time distributions]. Although mGlu_2/3 receptor agonists have not been tested formally for therapeutic effects in treating patients with major depressive disorder, these compounds generally fail to exert antidepressant-like effects in preclinical screening paradigms and did not improve depressive symptoms in GAD trials. Thus, the present studies were designed to test the potential antidepressant-like effects of the mGlu_2/3 receptor agonist 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicarboxylate monohydrate (LY354740) on the DRL 72-s schedule. LY354740 did not test similarly to clinically validated antidepressant drugs when administered alone or when coadministered with the selective serotonin reuptake inhibitor fluoxetine in rats. Another glutamate-based antidepressant drug, the uncompetitive N-methyl-D-aspartate channel blocker racemic ketamine, exerted antidepressant-like effects when administered at subanesthetic doses in rats. The findings further support the specificity of rat DRL 72-s behavior when screening for anxiolytic versus antidepressant drugs and extend testing of compounds with glutamatergic mechanisms of action. SIGNIFICANCE STATEMENT: The metabotropic glutamate_2/3 receptor agonist and clinically validated anxiolytic drug 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicarboxylate monohydrate did not test similar to antidepressant drugs (increased reinforcers, decreased response rate, and cohesive rightward shifts in the inter-response time distribution) when tested on differential-reinforcement-of-low-rate 72-second (DRL 72-s) behavior and also did not enhance the antidepressant-like effects of the serotonin reuptake inhibitor fluoxetine. The uncompetitive N-methyl-D-aspartate receptor antagonist ketamine increased the reinforcement rate, decreased the response rate, and induced a rightward shift in the inter-response time distribution similar to antidepressant drugs; these results confirm the utility of DRL 72-s schedule of reinforcement when testing clinically validated anxiolytic versus antidepressant glutamatergic drugs.

Molecular docking studies of N-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives as potential modulators of glutamate receptors

Introduction: The virtual target-oriented screening is a necessary stage of modern drug-design. In the present study, the affinity of pyridazine derivatives for the most promising antiparkinsonian biotargets – I–III groups of metabotropic and ionotropic NMDA-glutamate receptors – was evaluated.

Materials and methods: Docking of the studied ligands to the active sites of biotargets – mGluR5, mGluR3, mGluR8, NMDA GluN2B – was performed using AutoDockVina. Base of the preparation of ligands and proteins – AutoDockTools-1.5.6. A Discovery Studio Visualizer 2017/R2 was used to visualize the interpretation of the results.

Results and discussion: A high degree of the affinity is predicted for group III of the metabotropic mGlu8 receptors – binding energy from -5.0 to -8.7 kcal/mol, compared to -6.1 kcal/mol of that of the reference drug (L-AP4), as well as for the ionotropic NMDA GluN2B receptors –binding energy from -8.7 to -11.6 kcal/mol, compared to -11.3 kcal/mol of that of ifenprodil.

Conclusion: The prospects of the searching for glutamate receptor modulators in a number of n-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives are proved. Some aspects of the structure-affinity relationship are discussed.

Developmental abnormalities in cortical GABAergic system in mice lacking mGlu3 metabotropic glutamate receptors

Polymorphic variants of the gene encoding for metabotropic glutamate receptor 3 (mGlu3) are linked to schizophrenia. Because abnormalities of cortical GABAergic interneurons lie at the core of the pathophysiology of schizophrenia, we examined whether mGlu3 receptors influence the developmental trajectory of cortical GABAergic transmission in the postnatal life. mGlu3^-/- mice showed robust changes in the expression of interneuron-related genes in the prefrontal cortex (PFC), including large reductions in the expression of parvalbumin (PV) and the GluN1 subunit of NMDA receptors. The number of cortical cells enwrapped by perineuronal nets was increased in mGlu3^-/- mice, suggesting that mGlu3 receptors shape the temporal window of plasticity of PV⁺ interneurons. Electrophysiological measurements of GABA_A receptor-mediated responses revealed a more depolarized reversal potential of GABA currents in the somata of PFC pyramidal neurons in mGlu3^-/- mice at postnatal d 9 associated with a reduced expression of the K⁺/Cl^- symporter. Finally, adult mGlu3^-/- mice showed lower power in electroencephalographic rhythms at 1-45 Hz in quiet wakefulness as compared with their wild-type counterparts. These findings suggest that mGlu3 receptors have a strong impact on the development of cortical GABAergic transmission and cortical neural synchronization mechanisms corroborating the concept that genetic variants of mGlu3 receptors may predispose to psychiatric disorders.-Imbriglio, T., Verhaeghe, R., Martinello, K., Pascarelli, M. T., Chece, G., Bucci, D., Notartomaso, S., Quattromani, M., Mascio, G., Scalabrì, F., Simeone, A., Maccari, S., Del Percio, C., Wieloch, T., Fucile, S., Babiloni, C., Battaglia, G., Limatola, C., Nicoletti, F., Cannella, M. Developmental abnormalities in cortical GABAergic system in mice lacking mGlu3 metabotropic glutamate receptors.

Group II metabotropic glutamate receptor blockade promotes stress resilience in mice

Stress is a leading risk factor for the onset and recurrence of major depression. Enhancing stress resilience may be a therapeutic strategy to prevent the development of depression in at-risk populations or its recurrence in depressed patients. Group II metabotropic glutamate receptor (mGlu_2/3) antagonists have been recognized for antidepressant-like actions in preclinical models, but have not been evaluated for prophylactic effects. We assessed the role of mGlu_2/3 in modulating stress resilience using subtype-specific knockout mice lacking mGlu₂ (Grm2^-/-) or mGlu₃ (Grm3^-/-), and pharmacological manipulations of mGlu_2/3 activity during or prior to the induction and reinstatement of stress-induced behavioral deficits. Grm2^-/-, but not Grm3^-/-, mice exhibited reduced forced-swimming test immobility time and were resilient to developing inescapable shock (IES)-induced escape deficits. Grm2^-/- mice were also resilient to developing corticosterone (CORT)-induced escape deficits and chronic social defeat stress-induced anhedonia. Pharmacological blockade of mGlu_2/3 with the antagonist LY341495 during stress prevented the development of IES- and CORT-induced escape deficits, while activation with the agonist LY379268 increased susceptibility to escape deficits. Prophylactic treatment with the LY341495, both systemically and via microinjection into the medial prefrontal cortex (mPFC), up to 7 days before IES, prevented both the induction of escape deficits and their reinstatement by brief re-exposure to IES up to 20 days after treatment. Overall, blockade of mGlu_2/3 enhanced stress resilience and deletion of mGlu₂, but not mGlu₃, conferred a stress-resilient phenotype, indicating that prophylactic treatments reducing mGlu₂ activity may protect against stress-induced changes underlying the development or recurrence of stress-induced disorders, including depression.

The lack of conditioned place preference, but unaltered stimulatory and ataxic effects of alcohol in mGluR3-KO mice

BACKGROUND

Alcohol use associates with environmental cues that can later reinstate drinking patterns without any alcohol exposure. Alcohol-induced reward, when combined with contextual signals of various sensory modalities in the central synapses of mesolimbic reward circuitries, can lead to the formation of conditioned responses.

AIMS

As the activation of glutamatergic synapses is pivotal in such processes, we aimed to investigate whether the metabotropic glutamate receptor subtype 3 plays a role in alcohol-induced behaviours including place preference.

METHODS

The metabotropic glutamate receptor subtype 3 knockout (mGluR3-KO) mouse line was used to study alcohol-induced place preference, locomotor activating and ataxic effects, limited access alcohol drinking, and preference for sucrose and saccharin.

RESULTS

Alcohol-induced horizontal locomotor stimulation and reduced rearing behaviour remained unchanged in the mGluR3-KO mice. However, alcohol-induced place conditioning in an unbiased paradigm setup was lacking in the mGluR3-KO mice, but clearly present in wildtype mice. Locomotor activity was not different between the mGluR3-KO and wildtype mice during the acquisition and expression trials. Alcohol consumption, studied through the 'drinking in the dark' model, remained unchanged in the mGluR3-KO mice, although low consumption in both wildtype and knockout mice hampers interpretation. The mGluR3-KO mice also showed normal sucrose and saccharin preference.

CONCLUSIONS

These studies indicate a role for metabotropic glutamate receptor subtype 3 in the conditioned contextual alcohol responses, but not in stimulatory, and ataxic alcohol effects.

The Role of Metabotropic Glutamate Receptors in Social Behavior in Rodents

The appropriate display of social behavior is critical for the well-being and survival of an individual. In many psychiatric disorders, including social anxiety disorder, autism spectrum disorders, depression and schizophrenia social behavior is severely impaired. Selective targeting of metabotropic glutamate receptors (mGluRs) has emerged as a novel treatment strategy for these disorders. In this review, we describe some of the behavioral paradigms used to assess different types of social behavior, such as social interaction, social memory, aggressive behavior and sexual behavior. We then focus on the effects of pharmacological modulation of mGluR1-8 on these types of social behavior. Indeed, accumulating evidence indicates beneficial effects of selective ligands of specific mGluRs in ameliorating innate or pharmacologically-induced deficits in social interaction and social memory as well as in reducing aggression in rodents. We emphasize the importance of future studies investigating the role of selective mGluR ligands on different types of social behavior to provide a better understanding of the neural mechanisms involved which, in turn, might promote the development of selective mGluR-targeted tools for the improved treatment of psychiatric disorders associated with social deficits.

(2R,6R)-hydroxynorketamine exerts mGlu2 receptor-dependent antidepressant actions

Currently approved antidepressant drugs often take months to take full effect, and ∼30% of depressed patients remain treatment resistant. In contrast, ketamine, when administered as a single subanesthetic dose, exerts rapid and sustained antidepressant actions. Preclinical studies indicate that the ketamine metabolite (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a rapid-acting antidepressant drug candidate with limited dissociation properties and abuse potential. We assessed the role of group II metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) in the antidepressant-relevant actions of (2R,6R)-HNK using behavioral, genetic, and pharmacological approaches as well as cortical quantitative EEG (qEEG) measurements in mice. Both ketamine and (2R,6R)-HNK prevented mGlu_2/3 receptor agonist (LY379268)-induced body temperature increases in mice lacking the Grm3, but not Grm2, gene. This action was not replicated by NMDA receptor antagonists or a chemical variant of ketamine that limits metabolism to (2R,6R)-HNK. The antidepressant-relevant behavioral effects and 30- to 80-Hz qEEG oscillation (gamma-range) increases resultant from (2R,6R)-HNK administration were prevented by pretreatment with an mGlu_2/3 receptor agonist and absent in mice lacking the Grm2, but not Grm3 ^-/-, gene. Combined subeffective doses of the mGlu_2/3 receptor antagonist LY341495 and (2R,6R)-HNK exerted synergistic increases on gamma oscillations and antidepressant-relevant behavioral actions. These findings highlight that (2R,6R)-HNK exerts antidepressant-relevant actions via a mechanism converging with mGlu₂ receptor signaling and suggest enhanced cortical gamma oscillations as a marker of target engagement relevant to antidepressant efficacy. Moreover, these results support the use of (2R,6R)-HNK and inhibitors of mGlu₂ receptor function in clinical trials for treatment-resistant depression either alone or in combination.

Metabotropic Glutamate2 Receptors Play a Key Role in Modulating Head Twitches Induced by a Serotonergic Hallucinogen in Mice

There is substantial evidence that glutamate can modulate the effects of 5-hydroxytryptamine_2A (5-HT_2A) receptor activation through stimulation of metabotropic glutamate_2/3 (mGlu_2/3) receptors in the prefrontal cortex. Here we show that constitutive deletion of the mGlu₂ gene profoundly attenuates an effect of 5-HT_2A receptor activation using the mouse head twitch response (HTR). MGlu₂ and mGlu₃ receptor knockout (KO) as well as age-matched ICR (CD-1) wild type (WT) mice were administered (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and observed for head twitch activity. DOI failed to produce significant head twitches in mGlu₂ receptor KO mice at a dose 10-fold higher than the peak effective dose in WT or mGlu₃ receptor KO mice. In addition, the mGlu_2/3 receptor agonist LY379268, and the mGlu₂ receptor positive allosteric modulator (PAM) CBiPES, potently blocked the HTR to DOI in WT and mGlu₃ receptor KO mice. Conversely, the mGlu_2/3 receptor antagonist LY341495 (10 mg/kg) increased the HTR produced by DOI (3 mg/kg) in mGlu₃ receptor KO mice. Finally, the mGlu₂ receptor potentiator CBiPES was able to attenuate the increase in the HTR produced by LY341495 in mGlu₃ receptor KO mice. Taken together, all of these results are consistent with the hypothesis that that DOI-induced head twitches are modulated by mGlu₂ receptor activation. These results also are in keeping with a critical autoreceptor function for mGlu₂ receptors in the prefrontal cortex with differential effects of acute vs. chronic perturbation (e.g., constitutive mGlu₂ receptor KO mice). The robust attenuation of DOI-induced head twitches in the mGlu₂ receptor KO mice appears to reflect the critical role of glutamate in ongoing regulation of 5-HT_2A receptors in the prefrontal cortex. Future experiments with inducible knockouts for the mGlu₂ receptor and/or selective mGlu₃ receptor agonists/PAMs/antagonists could provide an important tools in understanding glutamatergic modulation of prefrontal cortical 5-HT_2A receptor function.

Metabotropic glutamate receptors as targets for novel anxiolytics

Anxiety disorders are highly prevalent psychiatric illnesses posing an important social and economic burden. Their current pharmacotherapy shows short term efficacy, though nearly one third of patients do not achieve sustained remission. There is, therefore, a strong medical need for new therapeutic agents acting through novel mechanisms of action. Considerable work has focused on metabotropic glutamate (mGlu) receptors as potential targets for novel anxiolytics. Ligands acting at mGlu receptors showed promising results in preclinical studies, whereas their efficacy was dubious in clinical trials. Recent preclinical and clinical studies have opened new prospects for targeting mGlu receptors to treat anxiety disorders. This review provides an outlook on these progresses.

Co-Activation of Metabotropic Glutamate Receptor 3 and Beta-Adrenergic Receptors Modulates Cyclic-AMP and Long-Term Potentiation, and Disrupts Memory Reconsolidation

Activation of β-adrenergic receptors (βARs) enhances both the induction of long-term potentiation (LTP) in hippocampal CA1 pyramidal cells and hippocampal-dependent cognitive function. Interestingly, previous studies reveal that coincident activation of group II metabotropic glutamate (mGlu) receptors with βARs in the hippocampal astrocytes induces a large increase in cyclic-AMP (cAMP) accumulation and release of adenosine. Adenosine then acts on A₁ adenosine receptors at neighboring excitatory Schaffer collateral terminals, which could counteract effects of activation of neuronal βARs on excitatory transmission. On the basis of this, we postulated that activation of the specific mGlu receptor subtype that mediates this response could inhibit βAR-mediated effects on hippocampal synaptic plasticity and cognitive function. Using novel mGlu receptor subtype-selective allosteric modulators along with knockout mice we now report that the effects of mGlu_2/3 agonists on βAR-mediated increases in cAMP accumulation are exclusively mediated by mGlu₃. Furthermore, mGlu₃ activation inhibits the ability of the βAR agonist isoproterenol to enhance hippocampal LTP, and this effect is absent in slices treated with either a glial toxin or an adenosine A₁ receptor antagonist. Finally, systemic administration of the mGlu_2/3 agonist LY379268 disrupted contextual fear memory in a manner similar to the effect of the βAR antagonist propranolol, and this effect was reversed by the mGlu₃-negative allosteric modulator VU0650786. Taken together, these data suggest that mGlu₃ can influence astrocytic signaling and modulate βAR-mediated effects on hippocampal synaptic plasticity and cognitive function.

Immuno-pharmacological characterization of group II metabotropic glutamate receptors controlling glutamate exocytosis in mouse cortex and spinal cord

BACKGROUND AND PURPOSE

We recently proposed the existence of mGlu3 -preferring autoreceptors in spinal cord terminals and of mGlu2 -preferring autoreceptors in cortical terminals. This study aims to verify our previous conclusions and to extend their pharmacological characterization.

EXPERIMENTAL APPROACH

We studied the effect of LY566332, an mGlu2 receptor positive allosteric modulator (PAM), and of LY2389575, a selective mGlu3 receptor negative allosteric (NAM) modulator, on the mGlu2/3 agonist LY379268-mediated inhibition of glutamate exocytosis [measured as KCl-evoked release of preloaded [3 H]-D-aspartate]. The mGlu2 PAM BINA and the mGlu3 NAM ML337, as well as selective antibodies recognizing the N-terminal of the receptor proteins, were used to confirm the pharmacological characterization of the native receptors.

KEY RESULTS

Cortical synaptosomes possess LY566332-sensitive autoreceptors that are slightly, although significantly, susceptible to LY2389575. In contrast, LY566332-insensitive and LY2389575-sensitive autoreceptors are present in spinal cord terminals. BINA and ML337 mimicked LY566332 and LY2389575, respectively, in controlling LY379268-mediated inhibition of glutamate exocytosis from both cortical and spinal cord synaptosomes. Incubation of cortical synaptosomes with anti-mGlu2 antibody prevented the LY379268-induced inhibition of glutamate exocytosis, and this response was partially reduced by the anti-mGlu3 antibody. Incubation of spinal cord synaptosomes with the anti-mGlu3 antibody abolished LY379268-mediated reduction of glutamate exocytosis from these terminals, while the anti-mGlu2 antibody was inactive. Western blot analysis and confocal microscopy data were largely consistent with these functional observations.

CONCLUSIONS AND IMPLICATIONS

We confirmed that mGlu3 -preferring autoreceptors exist in spinal cord terminals. Differently, cortical glutamatergic terminals possess mGlu2 /mGlu3 heterodimers, whose inhibitory effect is largely mediated by mGlu2 receptors.

Functional partnership between mGlu3 and mGlu5 metabotropic glutamate receptors in the central nervous system

mGlu5 receptors are involved in mechanisms of activity-dependent synaptic plasticity, and are targeted by drugs developed for the treatment of CNS disorders. We report that mGlu3 receptors, which are traditionally linked to the control of neurotransmitter release, support mGlu5 receptor signaling in neurons and largely contribute to the robust mGlu5 receptor-mediated polyphosphoinositide hydrolysis in the early postnatal life. In cortical pyramidal neurons, mGlu3 receptor activation potentiated mGlu5 receptor-mediated somatic Ca²⁺ mobilization, and mGlu3 receptor-mediated long-term depression in the prefrontal cortex required the endogenous activation of mGlu5 receptors. The interaction between mGlu3 and mGlu5 receptors was also relevant to mechanisms of neuronal toxicity, with mGlu3 receptors shaping the influence of mGlu5 receptors on excitotoxic neuronal death. These findings shed new light into the complex role played by mGlu receptors in physiology and pathology, and suggest reconsideration of some of the current dogmas in the mGlu receptor field.

Group II metabotropic glutamate receptor interactions with NHERF scaffold proteins: Implications for receptor localization in brain

The group II metabotropic glutamate receptors mGluR2 and mGluR3 are key modulators of glutamatergic neurotransmission. In order to identify novel Group II metabotropic glutamate receptor (mGluR)-interacting partners, we screened the C-termini of mGluR2 and mGluR3 for interactions with an array of PDZ domains. These screens identified the Na+/H+ exchanger regulatory factors 1 and 2 (NHERF-1 & -2) as candidate interacting partners. Follow-up co-immunoprecipitation studies demonstrated that both mGluR2 and mGluR3 can associate with NHERF-1 and NHERF-2 in a cellular context. Functional studies revealed that disruption of PDZ interactions with mGluR2 enhanced receptor signaling to Akt. However, further studies of mGluR2 and mGluR3 signaling in astrocytes in which NHERF expression was reduced by gene knockout (KO) and/or siRNA knockdown techniques revealed that the observed differences in signaling between WT and mutant mGluR2 were likely not due to disruption of interactions with the NHERF proteins. Electron microscopic analyses revealed that Group II mGluRs were primarily expressed in glia and unmyelinated axons in WT, NHERF-1 and NHERF-2 KO mice, but the relative proportion of labeled axons over glial processes was higher in NHERF-2 KO mice than in controls and NHERF-1 KO mice. Interestingly, our anatomical studies also revealed that loss of either NHERF protein results in ventriculomegaly, which may be related to the high incidence of hydrocephaly that has previously been observed in NHERF-1 KO mice. Together, these studies support a role for NHERF-1 and NHERF-2 in regulating the distribution of Group II mGluRs in the murine brain, while conversely the effects of the mGluR2/3 PDZ-binding motifs on receptor signaling are likely mediated by interactions with other PDZ scaffold proteins beyond the NHERF proteins.

Comparative Effects of LY3020371, a Potent and Selective Metabotropic Glutamate (mGlu) 2/3 Receptor Antagonist, and Ketamine, a Noncompetitive N-Methyl-d-Aspartate Receptor Antagonist in Rodents: Evidence Supporting the Use of mGlu2/3 Antagonists, for the Treatment of Depression

The ability of the N-methyl-d-aspartate receptor antagonist ketamine to alleviate symptoms in patients suffering from treatment-resistant depression (TRD) is well documented. In this paper, we directly compare in vivo biologic responses in rodents elicited by a recently discovered metabotropic glutamate (mGlu) 2/3 receptor antagonist 2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY3020371) with those produced by ketamine. Both LY3020371 and ketamine increased the number of spontaneously active dopamine cells in the ventral tegmental area of anesthetized rats, increased O₂ in the anterior cingulate cortex, promoted wakefulness, enhanced the efflux of biogenic amines in the prefrontal cortex, and produced antidepressant-related behavioral effects in rodent models. The ability of LY3020371 to produce antidepressant-like effects in the forced-swim assay in rats was associated with cerebrospinal fluid (CSF) drug levels that matched concentrations required for functional antagonist activity in native rat brain tissue preparations. Metabolomic pathway analyses from analytes recovered from rat CSF and hippocampus demonstrated that both LY3020371 and ketamine activated common pathways involving GRIA2 and ADORA1. A diester analog of LY3020371 [bis(((isopropoxycarbonyl)oxy)-methyl) (1S,2R,3S,4S,5R,6R)-2-amino-3-(((3,4-difluorophenyl)thio)methyl)-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylate (LY3027788)] was an effective oral prodrug; when given orally, it recapitulated effects of intravenous doses of LY3020371 in the forced-swim and wake-promotion assays, and augmented the antidepressant-like effects of fluoxetine or citalopram without altering plasma or brain levels of these compounds. The broad overlap of biologic responses produced by LY3020371 and ketamine supports the hypothesis that mGlu2/3 receptor blockade might be a novel therapeutic approach for the treatment of TRD patients. LY3020371 and LY3027788 represent molecules that are ready for clinical tests of this hypothesis.

Developmental expression of mGlu2 and mGlu3 in the mouse brain

The glutamatergic system directs central nervous system (CNS) neuronal activity and may underlie various neuropsychiatric disorders. Glutamate transmits its effects through multiple receptor classes. Class II metabotropic glutamate receptors, mGlu2 and mGlu3, play an important role in regulating synaptic release of different neurotransmitter systems and consequently modulate signaling across several neuronal subtypes. Drugs targeting mGlu2 and mGlu3 are seen as potential therapeutics for various psychiatric and neurological disorders, and defining their expression through development can aid in understanding their distinct function. Here, non-radioactive in situ hybridization was used to detect mGlu2 and mGlu3 mRNA in the CNS of 129SvEv mice at PN1, PN8, PN25, PN40, and PN100. At PN1, mGlu2 and mGlu3 are strongly expressed cortically, most notably in layer III and V. Subcortically, mGlu2 is detected in thalamic nuclei; mGlu3 is highly expressed in the striatum. By PN8, the most notable changes are in hippocampus and cortex, with mGlu2 densely expressed in the dentate gyrus, and showing increased cortical levels especially in medial cortex. At PN8, mGlu3 is observed in cortex and striatum, with highest levels detected in reticular thalamic nucleus. At PN25 patterns of expression approximated those observed across adulthood (PN40 & PN100): mGlu2 expression was high in cortex and dentate gyrus while mGlu3 showed expression in the reticular thalamic nucleus, cortex, and striatum. These studies provide a foundation for future research seeking to parse out the roles of mGlu2 from mGlu3, paving the way for better understanding of how these receptors regulate activity in the brain.

The Emerging Role of Metabotropic Glutamate Receptors in the Pathophysiology of Chronic Stress-Related Disorders

Chronic stress-related psychiatric conditions such as anxiety, depression, and alcohol abuse are an enormous public health concern. The etiology of these pathologies is complex, with psychosocial stressors being among the most frequently discussed risk factors. The brain glutamatergic neurotransmitter system has often been found involved in behaviors and pathophysiologies resulting from acute stress and fear. Despite this, relatively little is known about the role of glutamatergic system components in chronic psychosocial stress, neither in rodents nor in humans. Recently, drug discovery efforts at the metabotropic receptor subtypes of the glutamatergic system (mGlu1-8 receptors) led to the identification of pharmacological tools with emerging potential in psychiatric conditions. But again, the contribution of individual mGlu subtypes to the manifestation of physiological, molecular, and behavioral consequences of chronic psychosocial stress remains still largely unaddressed. The current review will describe animal models typically used to analyze acute and particularly chronic stress conditions, including models of psychosocial stress, and there we will discuss the emerging roles for mGlu receptor subtypes. Indeed, accumulating evidence indicates relevance and potential therapeutic usefulness of mGlu2/3 ligands and mGlu5 receptor antagonists in chronic stress-related disorders. In addition, a role for further mechanisms, e.g. mGlu7-selective compounds, is beginning to emerge. These mechanisms are important to be analyzed in chronic psychosocial stress paradigms, e.g. in the chronic subordinate colony housing (CSC) model. We summarize the early results and discuss necessary future investigations, especially for mGlu5 and mGlu7 receptor blockers, which might serve to suggest improved therapeutic strategies to treat stress-related disorders.

Synthesis and Pharmacological Characterization of C4-(Thiotriazolyl)-substituted-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylates. Identification of (1R,2S,4R,5R,6R)-2-Amino-4-(1H-1,2,4-triazol-3-ylsulfanyl)bicyclo[3.1.0]hexane-2,6-dicarboxylic Acid (LY2812223), a Highly Potent, Functionally Selective mGlu2 Receptor Agonist

Identification of orthosteric mGlu(2/3) receptor agonists capable of discriminating between individual mGlu2 and mGlu3 subtypes has been highly challenging owing to the glutamate-site sequence homology between these proteins. Herein we detail the preparation and characterization of a series of molecules related to (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate 1 (LY354740) bearing C4-thiotriazole substituents. On the basis of second messenger responses in cells expressing other recombinant human mGlu2/3 subtypes, a number of high potency and efficacy mGlu2 receptor agonists exhibiting low potency mGlu3 partial agonist/antagonist activity were identified. From this, (1R,2S,4R,5R,6R)-2-amino-4-(1H-1,2,4-triazol-3-ylsulfanyl)bicyclo[3.1.0]hexane-2,6-dicarboxylic acid 14a (LY2812223) was further characterized. Cocrystallization of 14a with the amino terminal domains of hmGlu2 and hmGlu3 combined with site-directed mutation studies has clarified the underlying molecular basis of this unique pharmacology. Evaluation of 14a in a rat model responsive to mGlu2 receptor activation coupled with a measure of central drug disposition provides evidence that this molecule engages and activates central mGlu2 receptors in vivo.

Regulation of neuronal communication by G protein-coupled receptors

Neuronal communication plays an essential role in the propagation of information in the brain and requires a precisely orchestrated connectivity between neurons. Synaptic transmission is the mechanism through which neurons communicate with each other. It is a strictly regulated process which involves membrane depolarization, the cellular exocytosis machinery, neurotransmitter release from synaptic vesicles into the synaptic cleft, and the interaction between ion channels, G protein-coupled receptors (GPCRs), and downstream effector molecules. The focus of this review is to explore the role of GPCRs and G protein-signaling in neurotransmission, to highlight the function of GPCRs, which are localized in both presynaptic and postsynaptic membrane terminals, in regulation of intrasynaptic and intersynaptic communication, and to discuss the involvement of astrocytic GPCRs in the regulation of neuronal communication.

The role of group II metabotropic glutamate receptors in cognition and anxiety: comparative studies in GRM2(-/-), GRM3(-/-) and GRM2/3(-/-) knockout mice

Group II metabotropic glutamate receptors (mGlu2 and mGlu3, encoded by GRM2 and GRM3) have been implicated in both cognitive and emotional processes, although their precise role remains to be established. Studies with knockout (KO) mice provide an important approach for investigating the role of specific receptor genes in behaviour. In the present series of experiments we extended our prior characterisation of GRM2/3^−/− double KO mice and, in complementary experiments, investigated the behavioural phenotype of single GRM2^−/− and GRM3^−/− mice. We found no consistent effect on anxiety in either the double or single KO mice. The lack of an anxiety phenotype in any of the lines contrasts with the clear anxiolytic effects of mGlu2/3 ligands. Motor co-ordination was impaired in GRM2/3^−/− mice, but spared in single GRM2^−/− and GRM3^−/− mice. Spatial working memory (rewarded alternation) testing on the elevated T-maze revealed a deficit in GRM2^−/− mice throughout testing, whereas GRM3^−/− mice exhibited a biphasic effect (initially impaired, but performing better than controls by the end of training). A biphasic effect on activity levels was seen for the GRM2^−/− mice. Overall, the phenotype in both GRM2^−/− and GRM3^−/− mice was less pronounced – if present at all – compared to GRM2/3^−/− mice, across the range of task domains. This is consistent with possible redundancy of function and/or compensation in the single KO lines. Results are discussed with reference to a possible role for group II metabotropic glutamate receptors at the interface between arousal and behavioural performance, according to an inverted U-shaped function.

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GRM2^−/− mice exhibited impaired spatial short-term memory (rewarded alternation).

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GRM3^−/− mice displayed bi-directional effects on this spatial short-term memory task.

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GRM2^−/− mice displayed bi-directional effects on activity levels.

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There was no consistent anxiety effect in either double or single knockout mice.

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Behavioural phenotypes were weaker (or absent) in single than in double knockout mice.

Levels of the Rab GDP dissociation inhibitor (GDI) are altered in the prenatal restrain stress mouse model of schizophrenia and are differentially regulated by the mGlu2/3 receptor agonists, LY379268 and LY354740

LY379268 and LY354740, two agonists of mGlu2/3 metabotropic glutamate receptors, display different potencies in mouse models of schizophrenia. This differential effect of the two drugs remains unexplained. We performed a proteomic analysis in cultured cortical neurons challenged with either LY379268 or LY354740. Among the few proteins that were differentially influenced by the two drugs, Rab GDP dissociation inhibitor-β (Rab GDIβ) was down-regulated by LY379268 and showed a trend to an up-regulation in response to LY354740. In cultured hippocampal neurons, LY379268 selectively down-regulated the α isoform of Rab GDI. Rab GDI inhibits the activity of the synaptic vesicle-associated protein, Rab3A, and is reduced in the brain of schizophrenic patients. We examined the expression of Rab GDI in mice exposed to prenatal stress ("PRS mice"), which have been described as a putative model of schizophrenia. Rab GDIα protein levels were increased in the hippocampus of PRS mice at postnatal days (PND)1 and 21, but not at PND60. At PND21, PRS mice also showed a reduced depolarization-evoked [(3)H]d-aspartate release in hippocampal synaptosomes. The increase in Rab GDIα levels in the hippocampus of PRS mice was reversed by a 7-days treatment with LY379268 (1 or 10 mg/kg, i.p.), but not by treatment with equal doses of LY354740. These data strengthen the validity of PRS mice as a model of schizophrenia, and show for the first time a pharmacodynamic difference between LY379268 and LY354740 which might be taken into account in an attempt to explain the differential effect of the two drugs across mouse models.

Loss of metabotropic glutamate receptor 2 escalates alcohol consumption

Identification of genes influencing complex traits is hampered by genetic heterogeneity, the modest effect size of many alleles, and the likely involvement of rare and uncommon alleles. Etiologic complexity can be simplified in model organisms. By genomic sequencing, linkage analysis, and functional validation, we identified that genetic variation of Grm2, which encodes metabotropic glutamate receptor 2 (mGluR2), alters alcohol preference in animal models. Selectively bred alcohol-preferring (P) rats are homozygous for a Grm2 stop codon (Grm2 *407) that leads to largely uncompensated loss of mGluR2. mGluR2 receptor expression was absent, synaptic glutamate transmission was impaired, and expression of genes involved in synaptic function was altered. Grm2 *407 was linked to increased alcohol consumption and preference in F2 rats generated by intercrossing inbred P and nonpreferring rats. Pharmacologic blockade of mGluR2 escalated alcohol self-administration in Wistar rats, the parental strain of P and nonpreferring rats. The causal role of mGluR2 in altered alcohol preference was further supported by elevated alcohol consumption in Grm2 (-/-) mice. Together, these data point to mGluR2 as an origin of alcohol preference and a potential therapeutic target.

Decreased striatal dopamine in group II metabotropic glutamate receptor (mGlu2/mGlu3) double knockout mice

BACKGROUND

Group II metabotropic glutamate receptors (mGlu2 and mGlu3, encoded by Grm2 and Grm3) have been the focus of attention as treatment targets for a number of psychiatric conditions. Double knockout mice lacking mGlu2 and mGlu3 (mGlu2/3-/-) show a subtle behavioural phenotype, being hypoactive under basal conditions and in response to amphetamine, and with a spatial memory deficit that depends on the arousal properties of the task. The neurochemical correlates of this profile are unknown. Here, we measured tissue levels of dopamine, 5-HT, noradrenaline and their metabolites in the striatum and frontal cortex of mGlu2/3-/- double knockout mice, using high performance liquid chromatography. We also measured the same parameters in mGlu2-/- and mGlu3-/- single knockout mice.

RESULTS

mGlu2/3-/-mice had reduced dopamine levels in the striatum but not in frontal cortex, compared to wild-types. In a separate cohort we replicated this deficit and, using tissue punches, found it was more prominent in the nucleus accumbens than in dorsolateral striatum. Noradrenaline, 5-HT and their metabolites were not altered in the striatum of mGlu2/3-/- mice, although the noradrenaline metabolite MHPG was increased in the cortex. In mGlu2-/- and mGlu3-/- single knockout mice we found no difference in any monoamine or metabolite, in either brain region, compared to their wild-type littermates.

CONCLUSIONS

Group II metabotropic glutamate receptors impact upon striatal dopamine. The effect may contribute to the behavioural phenotype of mGlu2/3-/- mice. The lack of dopaminergic alterations in mGlu2-/- and mGlu3-/- single knockout mice reveals a degree of redundancy between the two receptors. The findings support the possibility that interactions between mGlu2/3 and dopamine may be relevant to the pathophysiology and therapy of schizophrenia and other disorders.

Reversal of novelty-induced hyperlocomotion and hippocampal c-Fos expression in GluA1 knockout male mice by the mGluR2/3 agonist LY354740

Dysfunctional glutamatergic neurotransmission has been implicated in schizophrenia and mood disorders. As a putative model for these disorders, a mouse line lacking the GluA1 subunit (GluA1-KO) of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor displays a robust novelty-induced hyperlocomotion associated with excessive neuronal activation in the hippocampus. Agonists of metabotropic glutamate 2/3 receptors (mGluR2/3) inhibit glutamate release in various brain regions and they have been shown to inhibit neuronal activation in the hippocampus. Here, we tested a hypothesis that novelty-induced hyperlocomotion in the GluA1-KO mice is mediated via excessive hippocampal neuronal activation by analyzing whether an mGluR2/3 agonist inhibits this phenotypic feature. GluA1-KO mice and littermate wildtype (WT) controls were administered with (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740) (15 mg/kg, i.p.) 30 min before a 2-h exposure to novel arenas after which c-Fos immunopositive cells were analyzed in the hippocampus. LY354740 (15 mg/kg) decreased hyperactivity in male GluA1-KO mice, with only a minimal effect in WT controls. This was observed in two cohorts of animals, one naïve to handling and injections, another pre-handled and accustomed to injections. LY354740 (15 mg/kg) also reduced the excessive c-Fos expression in the dorsal hippocampal CA1 pyramidal cell layer in maleGluA1-KO mice, while not affecting c-Fos levels in WT mice. In female mice, no significant effect for LY354740 (15 mg/kg) on hyperactive behavior or hippocampal c-Fos was observed in either genotype or treatment cohort. A higher dose of LY354740 (30 mg/kg) alleviated hyperlocomotion of GluA1-KO males, but not that of GluA1-KO females. In conclusion, the excessive behavioral hyperactivity of GluA1-KO mice can be partly prevented by reducing neuronal excitability in the hippocampus with the mGluR2/3 agonist suggesting that the hippocampal reactivity is strongly involved in the behavioral phenotype of GluA1-KO mice.

Pharmacological characterization of JNJ-40068782, a new potent, selective, and systemically active positive allosteric modulator of the mGlu2 receptor and its radioligand [3H]JNJ-40068782

Modulation of the metabotropic glutamate type 2 (mGlu2) receptor is considered a promising target for the treatment of central nervous system diseases such as schizophrenia. Here, we describe the pharmacological properties of the novel mGlu2 receptor positive allosteric modulator (PAM) 3-cyano-1-cyclopropylmethyl-4-(4-phenyl-piperidin-1-yl)-pyridine-2(1H)-one (JNJ-40068782) and its radioligand [(3)H]JNJ-40068782. In guanosine 5'-O-(3-[(35)S]thio)triphosphate binding, JNJ-40068782 produced a leftward and upward shift in the glutamate concentration-effect curve at human recombinant mGlu2 receptors. The EC50 of JNJ-40068782 for potentiation of an EC20-equivalent concentration of glutamate was 143 nM. Although JNJ-40068782 did not affect binding of the orthosteric antagonist [(3)H]2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid (LY-341495), it did potentiate the binding of the agonist [(3)H](2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine (DCG-IV), demonstrating that it can allosterically affect binding at the agonist recognition site. The binding of [(3)H]JNJ-40068782 to human recombinant mGlu2 receptors in Chinese hamster ovary cells and rat brain receptors was saturable with a KD of ∼10 nM. In rat brain, the anatomic distribution of [(3)H]JNJ-40068782 was consistent with mGlu2 expression previously described and was most abundant in cortex and hippocampus. The ability of structurally unrelated PAMs to displace [(3)H]JNJ-40068782 suggests that PAMs may bind to common determinants within the same site. It is noteworthy that agonists also increased the binding affinity of [(3)H]JNJ-40068782. JNJ-40068782 influenced rat sleep-wake organization by decreasing rapid eye movement sleep with a lowest active dose of 3 mg/kg PO. In mice, JNJ-40068782 reversed phencyclidine-induced hyperlocomotion with an ED50 of 5.7 mg/kg s.c. Collectively, the present data demonstrate that JNJ-40068782 has utility in investigating the potential of mGlu2 modulation for the treatment of diseases characterized by disturbed glutamatergic signaling and highlight the value of [(3)H]JNJ-40068782 in exploring allosteric binding.

Antagonists reversibly reverse chemical LTD induced by group I, group II and group III metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors are implicated in many neurological and psychiatric diseases and are the targets of therapeutic agents currently in clinical development. Their activation has diverse effects in the central nervous system (CNS) that includes an involvement in synaptic plasticity. We previously reported that the brief exposure of hippocampal slices to dihydroxyphenylglycine (DHPG) can result in a long-term depression (LTD) of excitatory synaptic transmission. Surprisingly, this LTD could be fully reversed by mGlu receptor antagonists in a manner that was itself fully reversible upon washout of the antagonist. Here, 15 years after the discovery of DHPG-LTD and its reversible reversibility, we summarise these initial findings. We then present new data on DHPG-LTD, which demonstrates that evoked epileptiform activity triggered by activation of group I mGlu receptors can also be reversibly reversed by mGlu receptor antagonists. Furthermore, we show that the phenomenon of reversible reversibility is not specific to group I mGlu receptors. We report that activation of group II mGlu receptors in the temporo-ammonic pathway (TAP) and mossy fibre pathway within the hippocampus and in the cortical input to neurons of the lateral amygdala induces an LTD that is reversed by LY341495, a group II mGlu receptor antagonist. We also show that activation of group III mGlu8 receptors induces an LTD at lateral perforant path inputs to the dentate gyrus and that this LTD is reversed by MDCPG, an mGlu8 receptor antagonist. In conclusion, we have shown that activation of representative members of each of the three groups of mGlu receptors can induce forms of LTD than can be reversed by antagonists, and that in each case washout of the antagonist is associated with the re-establishment of the LTD. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Characterisation of an mGlu8 receptor-selective agonist and antagonist in the lateral and medial perforant path inputs to the dentate gyrus

Since its characterisation in 2001, the mGlu8-selective agonist DCPG has been widely used to explore the potential functional role of this group III mGlu receptor within the central nervous system. This research has implicated mGlu8 receptors in a number of disease states and conditions such as epilepsy and anxiety, suggesting that mGlu8-selective ligands may hold important therapeutic potential. However, there is evidence that DCPG exerts off-target effects at higher concentrations, limiting its use as an mGlu8-selective agonist. Here, we have used field recordings in rat hippocampal slices to investigate the effects of DCPG in the lateral perforant path (LPP), a pathway known to express high levels of mGlu8. We show that DCPG does inhibit excitatory transmission in this pathway, but produces a biphasic concentration-response curve suggesting activation of two distinct receptor types. The putative mGlu8-selective antagonist MDCPG antagonises the high, but not the low, potency component of this concentration-response curve. In addition, higher concentrations of DCPG also depress excitatory transmission in the medial perforant path (MPP), a pathway expressing very low levels of mGlu8 receptors. Experiments in slices from mice lacking mGlu8 receptors indicate that concentrations of DCPG >1 μM produce large non-selective effects in both the LPP and MPP. Further experiments in slices from mGlu2, 4 and 7 knock-out mice, as well as in an mGlu2-deficient substrain of Wistar rat, reveal that these non-selective effects are mediated primarily by mGlu2 receptors. Taken together, our results confirm the mGlu8-selectivity of DCPG at submicromolar concentrations, but suggest that care must be taken when employing higher concentrations of the agonist, which may additionally activate mGlu2 receptors, especially at synapses where their expression is high. MDCPG may be a useful tool in determining whether observable DCPG effects are attributable to mGlu8, versus mGlu2, receptor activation.

Pharmacological profiling of native group II metabotropic glutamate receptors in primary cortical neuronal cultures using a FLIPR

The group II metabotropic glutamate (mGlu) receptors comprised of the mGlu2 and mGlu3 receptor subtypes have gained recognition in recent years as potential targets for psychiatric disorders, including anxiety and schizophrenia. In addition to studies already indicating which subtype mediates the anxiolytic and anti-psychotic effects observed in disease models, studies to help further define the preferred properties of selective group II mGlu receptor ligands will be essential. Comparison of the in vitro properties of these ligands to their in vivo efficacy and tolerance profiles may help provide these additional insights. We have developed a relatively high-throughput native group II mGlu receptor functional assay to aid this characterisation. We have utilised dissociated primary cortical neuronal cultures, which after 7 days in vitro have formed functional synaptic connections and display periodic and spontaneous synchronised calcium (Ca(2+)) oscillations in response to intrinsic action potential bursts. We herein demonstrate that in addition to non-selective group II mGlu receptor agonists, (2R,4R)-APDC, LY379268 and DCG-IV, a selective mGlu2 agonist, LY541850, and mGlu2 positive allosteric modulators, BINA and CBiPES, inhibit the frequency of synchronised Ca(2+) oscillations in primary cultures of rat and mouse cortical neurons. Use of cultures from wild-type, mGlu2(-/-), mGlu3(-/-) and mGlu2/3(-/-) mice allowed us to further probe the contribution of mGlu2 and mGlu3, and revealed LY541850 to be a partial mGlu2 agonist and a full mGlu3 antagonist. Overnight pre-treatment of cultures with these ligands revealed a preferred desensitisation profile after treatment with a positive allosteric modulator. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Selective activation of either mGlu2 or mGlu3 receptors can induce LTD in the amygdala

Group II metabotropic glutamate (mGlu) receptors are known to induce a long-term depression (LTD) of synaptic transmission in many brain regions including the amygdala. However the roles of the individual receptor subtypes, mGlu2 and mGlu3, in LTD are not well understood. In particular, it is unclear whether activation of mGlu3 receptors is sufficient to induce LTD at synapses in the CNS. In the present study, advantage was taken of a Wistar rat strain not expressing mGlu2 receptors (Ceolin et al., 2011) to investigate the function of mGlu3 receptors in the amygdala. In this preparation, the group II agonist, DCG-IV induced an LTD of the cortical, but not the intra-nuclear, synaptic input to the lateral amygdala. This LTD was concentration dependent and was blocked by the group II mGlu receptor antagonist, LY341495. To investigate further the role of mGlu3 receptors, we used LY395756 (an mGlu2 agonist and mGlu3 antagonist), which acts as a pure mGlu3 receptor antagonist in this rat strain. This compound alone had no effect on basal synaptic transmission, but blocked the LTD induced by DCG-IV. Furthermore, we found that DCG-IV also induces LTD in mGlu2 receptor knock-out (KO) mice to a similar extent as in wild-type mice. This confirms that the activation of mGlu3 receptors alone is sufficient to induce LTD at this amygdala synapse. To address whether mGlu2 activation alone is also sufficient to induce LTD at this synapse we used LY541850 (the active enantiomer of LY395756) in wild-type mice. LY541850 induced a substantial LTD showing that either receptor alone is capable of inducing LTD in this pathway. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Differentiating the roles of mGlu2 and mGlu3 receptors using LY541850, an mGlu2 agonist/mGlu3 antagonist

Despite the potential therapeutic relevance of group II metabotropic glutamate (mGlu) receptors, there has been a lack of pharmacological tools for separating the roles of mGlu2 and mGlu3 receptor subtypes. LY541850 was claimed from human mGlu receptors expressed in non-neuronal cells to be a selective orthosteric mGlu2 agonist and mGlu3 antagonist. We have verified this pharmacological profile of LY541850 in hippocampal slices. Field excitatory post-synaptic potentials (fEPSPs) evoked by stimulation of the temporo-ammonic path (TAP) input to CA1 stratum lacunosum moleculare (SLM) were inhibited by LY541850 in mGlu3-/- mice (EC(50) 38 nM) and wild-type littermates (EC(50) 42 nM) to a similar extent but were not significantly affected in mGlu2-/- mice. The group II agonist, DCG-IV, inhibited the fEPSP in all three genotypes. Co-application of DCG-IV and LY541850 in mGlu3-/- and wild-type littermates resulted in an additive effect, whereas in mGlu2-/- mice, LY541850 reversed the inhibitory action of DCG-IV. These results confirm the selective mGlu2 agonist and mGlu3 antagonist actions of LY541850. A similar profile of activity was seen in medial perforant path synapse to the dentate gyrus. Systemic administration of LY541850 to wild-type mice, reduced the increase in locomotor activity following both phencyclidine and amphetamine administration. These data support the hypothesis that mGlu2 receptors mediate the antipsychotic effects of mixed group II agonists. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.