mGlu2/3 Receptor Antagonists as Novel Antidepressants

Activation of mGlu 2/3 receptors with the orthosteric agonist LY-404,039 alleviates dyskinesia in experimental parkinsonism

LY-404,039 is an orthosteric agonist at metabotropic glutamate 2 and 3 (mGlu 2/3 ) receptors, with a possible additional agonist effect at dopamine D 2 receptors. LY-404,039 and its pro-drug, LY-2140023, have previously been tested in clinical trials for psychiatric indications and could therefore be repurposed if they were shown to be efficacious in other conditions. We have recently demonstrated that the mGlu 2/3 orthosteric agonist LY-354,740 alleviated L-3,4-dihydroxyphenylalanine (L-DOPA)-induced abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA)-lesioned rat without hampering the anti-parkinsonian action of L-DOPA. Here, we seek to take advantage of a possible additional D 2 -agonist effect of LY-404,039 and see if an anti-parkinsonian benefit might be achieved in addition to the antidyskinetic effect of mGlu 2/3 activation. To this end, we have administered LY-404,039 (vehicle, 0.1, 1 and 10 mg/kg) to 6-OHDA-lesioned rats, after which the severity of axial, limbs and oro-lingual (ALO) AIMs was assessed. The addition of LY-404,039 10 mg/kg to L-DOPA resulted in a significant reduction of ALO AIMs over 60-100 min (54%, P  < 0.05). In addition, LY-404,039 significantly enhanced the antiparkinsonian effect of L-DOPA, assessed through the cylinder test (76%, P  < 0.01). These results provide further evidence that mGlu 2/3 orthosteric stimulation may alleviate dyskinesia in PD and, in the specific case of LY-404,039, a possible D 2 -agonist effect might also make it attractive to address motor fluctuations. Because LY-404,039 and its pro-drug have been administered to humans, they could possibly be advanced to Phase IIa trials rapidly for the treatment of motor complications in PD.

Exploring the activation mechanism of metabotropic glutamate receptor 2

Homomeric dimerization of metabotropic glutamate receptors (mGlus) is essential for the modulation of their functions and represents a promising avenue for the development of novel therapeutic approaches to address central nervous system diseases. Yet, the scarcity of detailed molecular and energetic data on mGlu2 impedes our in-depth comprehension of their activation process. Here, we employ computational simulation methods to elucidate the activation process and key events associated with the mGlu2, including a detailed analysis of its conformational transitions, the binding of agonists, Gi protein coupling, and the guanosine diphosphate (GDP) release. Our results demonstrate that the activation of mGlu2 is a stepwise process and several energy barriers need to be overcome. Moreover, we also identify the rate-determining step of the mGlu2's transition from the agonist-bound state to its active state. From the perspective of free-energy analysis, we find that the conformational dynamics of mGlu2's subunit follow coupled rather than discrete, independent actions. Asymmetric dimerization is critical for receptor activation. Our calculation results are consistent with the observation of cross-linking and fluorescent-labeled blot experiments, thus illustrating the reliability of our calculations. Besides, we also identify potential key residues in the Gi protein binding position on mGlu2, mGlu2 dimer's TM6-TM6 interface, and Gi α5 helix by the change of energy barriers after mutation. The implications of our findings could lead to a more comprehensive grasp of class C G protein-coupled receptor activation.

Group I and group II metabotropic glutamate receptors are upregulated in the synapses of infant rats prenatally exposed to valproic acid

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction and restricted/stereotyped behavior. Prenatal exposure to valproic acid (VPA) is associated with an increased risk of developing ASD in humans and autistic-like behaviors in rodents. Increasing evidence indicates that dysfunctions of glutamate receptors at synapses are associated with ASD. In the VPA rat model, an involvement of glutamate receptors in autism-like phenotypes has been suggested; however, few studies were carried out on metabotropic glutamate (mGlu) receptors.

OBJECTIVES

We examined the protein expression levels of group I (mGlu1 and mGlu5) and group II (mGlu2/3) mGlu receptors in rats prenatally exposed to VPA and evaluated the effect of mGlu receptor modulation on an early autism-like phenotype in these animals.

METHODS

We used western blotting analysis on synaptosomes obtained from forebrain of control and VPA rats at different ages (postnatal day P13, 35, 90) and carried out ultrasonic vocalization (USV) emission test in infant control and VPA rats.

RESULTS

The expression levels of all these receptors were significantly increased in infant VPA rats. No changes were detected in adolescent and adult rats. An acute treatment with the preferential mGlu2/3 antagonist, LY341495, attenuated the impairment in the USV emission in VPA rats. No effect was observed after a treatment with the mGlu5 selective antagonist, MTEP.

CONCLUSIONS

Our findings demonstrate that the expression of group I and group II mGlu receptors is upregulated at synapses of infant VPA rats and suggest that mGlu2/3 receptor modulation may have a therapeutic potential in ASD.

Interaction of hallucinogenic rapid-acting antidepressants with mGlu2/3 receptor ligands as a window for more effective therapies

The desire to find a gold-standard therapy for depression is still ongoing. Developing one universal and effective pharmacotherapy remains troublesome due to the high complexity and variety of symptoms. Over the last decades, the understanding of the mechanism of pathophysiology of depression and its key consequences for brain functioning have undergone significant changes, referring to the monoaminergic theory of the disease. After the breakthrough discovery of ketamine, research began to focus on the modulation of glutamatergic transmission as a new pharmacological target. Glutamate is a crucial player in mechanisms of a novel class of antidepressants, including hallucinogens such as ketamine. The role of glutamatergic transmission is also suggested in the antidepressant (AD) action of scopolamine and psilocybin. Despite fast, robust, and sustained AD action hallucinogens belonging to a group of rapid-acting antidepressants (RAA) exert significant undesired effects, which hamper their use in the clinic. Thus, the synergistic action of more than one substance in lower doses instead of monotherapy may alleviate the likelihood of adverse effects while improving therapeutic outcomes. In this review, we explore AD-like behavioral, synaptic, and molecular action of RAAs such as ketamine, scopolamine, and psilocybin, in combination with mGlu2/3 receptor antagonists.

mGlu2/3 receptor antagonists for depression: overview of underlying mechanisms and clinical development

Triggered by the ground-breaking finding that ketamine exerts robust and rapid-acting antidepressant effects in patients with treatment-resistant depression, glutamatergic systems have attracted attention as targets for the development of novel antidepressants. Among glutamatergic systems, group II metabotropic glutamate (mGlu) receptors, consisting of mGlu2 and mGlu3 receptors, are of interest because of their modulatory roles in glutamatergic transmission. Accumulating evidence has indicated that mGlu2/3 receptor antagonists have antidepressant-like effects in rodent models that mirror those of ketamine and that mGlu2/3 receptor antagonists also share underlying mechanisms with ketamine that are responsible for these antidepressant-like actions. Importantly, contrary to their antidepressant-like profile, preclinical studies have revealed that mGlu2/3 receptor antagonists are devoid of ketamine-like adverse effects, such as psychotomimetic-like behavior, abuse potential and neurotoxicity. Despite some discouraging results for an mGlu2/3 receptor antagonist decoglurant (classified as a negative allosteric modulator [NAM]) in patients with major depressive disorder, clinical trials of two mGlu2/3 receptor antagonists, a phase 2 trial of TS-161 (an orthosteric antagonist) and a phase 1 trial of DSP-3456 (a NAM), are presently on-going. mGlu2/3 receptors still hold promise for the development of safer and more efficacious antidepressants.

An integrated multi-level analysis reveals learning-memory deficits and synaptic dysfunction in the rat model exposure to austere environment

Austere environment existing in tank, submarine and vessel has many risk factors including high temperature and humidity, confinement, noise, hypoxia, and high level of carbon dioxide, which may cause depression and cognitive impairment. However, the underlying mechanism is not fully understood yet. We attempt to investigate the effects of austere environment (AE) on emotion and cognitive function in a rodent model. After 21 days of AE stress, the rats exhibit depressive-like behavior and cognitive impairment. Compared with control group, the glucose metabolic level of the hippocampus is significantly decreased using whole-brain positron emission tomography (PET) imaging, and the density of dendritic spines of the hippocampus is remarkably reduced in AE group. Then, we employ a label-free quantitative proteomics strategy to investigate the differentially abundant proteins in rats' hippocampus. It is striking that the differentially abundant proteins annotated by KEGG enrich in oxidative phosphorylation pathway, synaptic vesicle cycle pathway and glutamatergic synapses pathway. The synaptic vesicle transport related proteins (Syntaxin-1A, Synaptogyrin-1 and SV-2) are down-regulated, resulting in the accumulation of intracellular glutamate. Furthermore, the concentration of hydrogen peroxide and malondialdehyde is increased while the activity of superoxide dismutase and complex I and IV of mitochondria is decreased, indicating that oxidative damage to hippocampal synapses is associated with the cognitive decline. The results of this study offer direct evidence, for the first time, that austere environment can substantially cause learning and memory deficits and synaptic dysfunction in a rodent model via behavioral assessments, PET imaging, label-free proteomics, and oxidative stress tests. SIGNIFICANCE: The incidence of depression and cognitive decline in military occupations (for example, tanker and submariner) is significantly higher than that of global population. In the present study, we first established novel model to simulate the coexisting risk factors in the austere environment. The results of this study offer the direct evidences, for the first time, that the austere environment can substantially cause learning and memory deficits by altering plasticity of the synaptic transmission in a rodent model via proteomic strategy, PET imaging, oxidative stress and behavioral assessments. These findings provide valuable information to better understand the mechanisms of cognitive impairment.

Antidepressants in the post-ketamine Era: Pharmacological approaches targeting the glutamatergic system

The efficacy of currently available medications for depression is unsatisfactory, and that has spurred the development of novel antidepressants based on a hypothesis other than the monoamine hypothesis. Recent studies have revealed the importance of the glutamatergic system as a drug target for depression, and the validity of this hypothesis has been underpinned by the discovery of the antidepressant effects of ketamine, leading to the market launch of Spravato® nasal spray which delivers (S)-ketamine (esketamine). However, both ketamine and esketamine have unwanted adverse effects that hinder their routine use in daily practice. Extensive studies have elucidated the mechanisms underlying the antidepressant effects of ketamine, and that has encouraged numerous drug discovery activities to search for agents that retain a ketamine-like antidepressant profile but with lesser adverse effect liabilities. The discovery activities have included attempts to identify 1) the active substance(s) in the circulation after ketamine administration and 2) agents that act on the proposed mechanisms of action of ketamine. Clinical trials of agents discovered in the course of these activities are underway, and in 2022, AUVELITY™ (AXS-05; dextromethorphan with bupropion) was approved by the United States Food and Drug Administration. Drug development of post-ketamine agents should provide novel antidepressants that are safer, but as potent and rapidly acting as ketamine.

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.

Rapid-acting antidepressants targeting modulation of the glutamatergic system: clinical and preclinical evidence and mechanisms

Major depressive disorder (MDD) is a devastating mental illness that affects approximately 20% of the world's population. It is a major disease that leads to disability and suicide, causing a severe burden among communities. Currently available medications for treating MDD target the monoaminergic systems. The most prescribed medications include selective serotonin reuptake inhibitors and selective norepinephrine reuptake inhibitors. However, these medications have serious drawbacks, such as a delayed onset requiring weeks or months to reach efficacy and drug resistance, as one-third of patients are unresponsive to the medications. Therefore, it is imperative to develop novel therapies with rapid action, high efficacy and few adverse effects. The discovery of the rapid antidepressant effect of ketamine has triggered tremendous enthusiasm for studying new antidepressants that target the glutamatergic system in the central nervous system. Many agents that directly or indirectly modulate the glutamatergic system have been shown to provide rapid and lasting antidepressant action. Among these agents, ketamine, an antagonist of metabotropic glutamate 2/3 receptors, and scopolamine, an unspecific muscarinic acetylcholine receptor antagonist, have been extensively studied. In this review, we discuss the clinical and preclinical evidence supporting the antidepressant efficacy of these agents and the current understanding of the underlying mechanisms.

Phosphorylation and regulation of group II metabotropic glutamate receptors (mGlu2/3) in neurons

Group II metabotropic glutamate (mGlu) receptors (mGlu2/3) are Gαi/o-coupled receptors and are primarily located on presynaptic axonal terminals in the central nervous system. Like ionotropic glutamate receptors, group II mGlu receptors are subject to regulation by posttranslational phosphorylation. Pharmacological evidence suggests that several serine/threonine protein kinases possess the ability to regulate mGlu2/3 receptors. Detailed mapping of phosphorylation residues has revealed that protein kinase A (PKA) phosphorylates mGlu2/3 receptors at a specific serine site on their intracellular C-terminal tails in heterologous cells or neurons, which underlies physiological modulation of mGlu2/3 signaling. Casein kinases promote mGlu2 phosphorylation at a specific site. Tyrosine protein kinases also target group II receptors to induce robust phosphorylation. A protein phosphatase was found to specifically bind to mGlu3 receptors and dephosphorylate the receptor at a PKA-sensitive site. This review summarizes recent progress in research on group II receptor phosphorylation and the phosphorylation-dependent regulation of group II receptor functions. We further explore the potential linkage of mGlu2/3 phosphorylation to various neurological and neuropsychiatric disorders, and discuss future research aimed at analyzing novel biochemical and physiological properties of mGlu2/3 phosphorylation.

Where do we go next in antidepressant drug discovery? A new generation of antidepressants: a pivotal role of AMPA receptor potentiation and mGlu2/3 receptor antagonism

INTRODUCTION

Major depressive disorder remains a prevalent world-wide health problem. Currently available antidepressant medications take weeks of dosing, do not produce antidepressant response in all patients, and have undesirable ancillary effects.

AREAS COVERED

The present opinion piece focuses on the major inroads to the creation of new antidepressants. These include N-methyl-D-aspartate (NMDA) receptor antagonists and related compounds like ketamine, psychedelic drugs like psilocybin, and muscarinic receptor antagonists like scopolamine. The preclinical and clinical pharmacological profile of these new-age antidepressant drugs is discussed.

EXPERT OPINION

Preclinical and clinical data have accumulated to predict a next generation of antidepressant medicines. In contrast to the current standard of care antidepressant drugs, these compounds differ in that they demonstrate rapid activity, often after a single dose, and effects that outlive their presence in brain. These compounds also can provide efficacy for treatment-resistant depressed patients. The mechanism of action of these compounds suggests a strong glutamatergic component that involves the facilitation of AMPA receptor function. Antagonism of mGlu2/3 receptors is also relevant to the antidepressant pharmacology of this new class of drugs. Based upon the ongoing efforts to develop these new-age antidepressants, new drug approvals are predicted in the near future.

Advances in translating mGlu2 and mGlu3 receptor selective allosteric modulators as breakthrough treatments for affective disorders and alcohol use disorder

Metabotropic glutamate (mGlu) receptors are promising targets for the treatment of affective disorders and alcohol use disorder (AUD). Nonspecific ligands for Group II (mGlu2 and mGlu3) mGlu receptors have demonstrated consistent therapeutic potential for affective disorders in preclinical models. Disentangling the specific roles of mGlu2 versus mGlu3 receptors in these effects has persisted as a major challenge, in part due to pharmacological limitations. However, the recent development of highly specific allosteric modulators for both mGlu2 and mGlu3 receptors have enabled straightforward and rigorous investigations into the specific function of each receptor. Here, we review recent experiments using these compounds that have demonstrated both similar and distinct receptor functions in behavioral, molecular, and electrophysiological measures associated with basal function and preclinical models of affective disorders. Studies using these selective drugs have demonstrated that mGlu2 is the predominant receptor subclass involved in presynaptic neurotransmitter release in prefrontal cortex. By contrast, the activation of postsynaptic mGlu3 receptors induces a cascade of cellular changes that results in AMPA receptor internalization, producing long-term depression and diminishing excitatory drive. Acute stress decreases the mGlu3 receptor function and dynamically alters transcript expression for both mGlu2 (Grm2) and mGlu3 (Grm3) receptors in brain areas involved in reward and stress. Accordingly, both mGlu2 and mGlu3 negative allosteric modulators show acute antidepressant-like effects and potential prophylactic effects against acute and traumatic stressors. The wide array of effects displayed by these new allosteric modulators of mGlu2 and mGlu3 receptors suggest that these drugs may act through improving endophenotypes of symptoms observed across several neuropsychiatric disorders. Therefore, recently developed allosteric modulators selective for mGlu2 or mGlu3 receptors show promise as potential therapeutics for affective disorders and AUD.

Exploration of group II metabotropic glutamate receptor modulation in mouse models of Rett syndrome and MECP2 Duplication syndrome

Rett syndrome (RTT) and MECP2 Duplication syndrome (MDS) have opposing molecular origins in relation to expression and function of the transcriptional regulator Methyl-CpG-binding protein 2 (MeCP2). Several clinical and preclinical phenotypes, however, are shared between these disorders. Modulation of MeCP2 levels has recently emerged as a potential treatment option for both of these diseases. However, toxicity concerns remain with these approaches. Here, we focus on pharmacologically modulating the group II metabotropic glutamate receptors (mGlu), mGlu₂ and mGlu₃, which are two downstream targets of MeCP2 that are bidirectionally affected in expression in RTT patients and mice (Mecp2^Null/+) versus an MDS mouse model (MECP2^Tg1/o). Mecp2^Null/+ and MECP2^Tg1/o animals also exhibit contrasting phenotypes in trace fear acquisition, a form of temporal associative learning and memory, with trace fear deficiency observed in Mecp2^Null/+ mice and abnormally enhanced trace fear acquisition in MECP2^Tg1/o animals. In Mecp2^Null/+ mice, treatment with the mGlu_2/3 agonist LY379268 reverses the deficit in trace fear acquisition, and mGlu_2/3 antagonism with LY341495 normalizes the abnormal trace fear learning and memory phenotype in MECP2^Tg1/o mice. Altogether, these data highlight the role of group II mGlu receptors in RTT and MDS and demonstrate that both mGlu₂ and mGlu₃ may be potential therapeutic targets for these disorders.

The effects of predator odor (TMT) exposure and mGlu3 NAM pretreatment on behavioral and NMDA receptor adaptations in the brain

A stressor can trigger lasting adaptations that contribute to neuropsychiatric disorders. Predator odor (TMT) exposure is an innate stressor that may activate the metabotropic glutamate receptor 3 (mGlu3) to produce stress adaptations. To evaluate functional involvement, the mGlu3 negative allosteric modulator (NAM, VU6010572; 3 mg/kg, i.p.) was administered before TMT exposure in male, Long Evans rats. Two weeks after, rats underwent context re-exposure, elevated zero maze (ZM), and acoustic startle (ASR) behavioral tests, followed by RT-PCR gene expression in the insular cortex and bed nucleus of the stria terminalis (BNST) to evaluate lasting behavioral and molecular adaptations from the stressor. Rats displayed stress-reactive behaviors in response to TMT exposure that were not affected by VU6010572. Freezing and hyperactivity were observed during the context re-exposure, and mGlu3-NAM pretreatment during stressor prevented the context freezing response. TMT exposure did not affect ZM or ASR measures, but VU6010572 increased time spent in the open arms of the ZM and ASR habituation regardless of stressor treatment. In the insular cortex, TMT exposure increased expression of mGlu (Grm3, Grm5) and NMDA (GriN2A, GriN2B, GriN2C, GriN3A, GriN3B) receptor transcripts, and mGlu3-NAM pretreatment blocked GriN3B upregulation. In the BNST, TMT exposure increased expression of GriN2B and GriN3B in vehicle-treated rats, but decreased expression in the mGlu3-NAM group. Similar to the insular cortex, mGlu3-NAM reversed the stressor-induced upregulation of GriN3B in the BNST. mGlu3-NAM also upregulated GriN2A, GriN2B, GriN3B and Grm2 in the control group, but not the TMT group. Together, these data implicate mGlu3 receptor signaling in some lasting adaptations of predator odor stressor and anxiolytic-like effects.

Mechanisms of ketamine and its metabolites as antidepressants

Treating major depression is a medical need that remains unmet by monoaminergic therapeutic strategies that commonly fail to achieve symptom remission. A breakthrough in the treatment of depression was the discovery that the anesthetic (R,S)-ketamine (ketamine), when administered at sub-anesthetic doses, elicits rapid (sometimes within hours) antidepressant effects in humans that are otherwise resistant to monoaminergic-acting therapies. While this finding was revolutionary and led to the FDA approval of (S)-ketamine (esketamine) for use in adults with treatment-resistant depression and suicidal ideation, the mechanisms underlying how ketamine or esketamine elicit their effects are still under active investigation. An emerging view is that metabolism of ketamine may be a crucial step in its mechanism of action, as several metabolites of ketamine have neuroactive effects of their own and may be leveraged as therapeutics. For example, (2R,6R)-hydroxynorketamine (HNK), is readily observed in humans following ketamine treatment and has shown therapeutic potential in preclinical tests of antidepressant efficacy and synaptic potentiation while being devoid of the negative adverse effects of ketamine, including its dissociative properties and abuse potential. We discuss preclinical and clinical studies pertaining to how ketamine and its metabolites produce antidepressant effects. Specifically, we explore effects on glutamate neurotransmission through N-methyl D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), synaptic structural changes via brain derived neurotrophic factor (BDNF) signaling, interactions with opioid receptors, and the enhancement of serotonin, norepinephrine, and dopamine signaling. Strategic targeting of these mechanisms may result in novel rapid-acting antidepressants with fewer undesirable side effects compared to ketamine.

Rapid-acting antidepressants and the circadian clock

A growing number of epidemiological and experimental studies has established that circadian disruption is strongly associated with psychiatric disorders, including major depressive disorder (MDD). This association is becoming increasingly relevant considering that modern lifestyles, social zeitgebers (time cues) and genetic variants contribute to disrupting circadian rhythms that may lead to psychiatric disorders. Circadian abnormalities associated with MDD include dysregulated rhythms of sleep, temperature, hormonal secretions, and mood which are modulated by the molecular clock. Rapid-acting antidepressants such as subanesthetic ketamine and sleep deprivation therapy can improve symptoms within 24 h in a subset of depressed patients, in striking contrast to conventional treatments, which generally require weeks for a full clinical response. Importantly, animal data show that sleep deprivation and ketamine have overlapping effects on clock gene expression. Furthermore, emerging data implicate the circadian system as a critical component involved in rapid antidepressant responses via several intracellular signaling pathways such as GSK3β, mTOR, MAPK, and NOTCH to initiate synaptic plasticity. Future research on the relationship between depression and the circadian clock may contribute to the development of novel therapeutic strategies for depression-like symptoms. In this review we summarize recent evidence describing: (1) how the circadian clock is implicated in depression, (2) how clock genes may contribute to fast-acting antidepressants, and (3) the mechanistic links between the clock genes driving circadian rhythms and neuroplasticity.

Novel Molecular Targets of Antidepressants

Antidepressants target a variety of proteins in the central nervous system (CNS), the most important belonging to the family of G-protein coupled receptors and the family of neurotransmitter transporters. The increasing number of crystallographic structures of these proteins have significantly contributed to the knowledge of their mechanism of action, as well as to the design of new drugs. Several computational approaches such as molecular docking, molecular dynamics, and virtual screening are useful for elucidating the mechanism of drug action and are important for drug design. This review is a survey of molecular targets for antidepressants in the CNS and computer based strategies to discover novel compounds with antidepressant activity.

Glutamatergic System in Depression and Its Role in Neuromodulatory Techniques Optimization

Depressive disorders are among the most common psychiatric conditions and contribute to significant morbidity. Even though the use of antidepressants revolutionized the management of depression and had a tremendous positive impact on the patient's outcome, a significant proportion of patients with major depressive disorder (MDD) show no or partial or response even with adequate treatment. Given the limitations of the prevailing monoamine hypothesis-based pharmacotherapy, glutamate and glutamatergic related pathways may offer an alternative and a complementary option for designing novel intervention strategies. Over the past few decades, there has been a growing interest in understanding the neurobiological underpinnings of glutamatergic dysfunctions in the pathogenesis of depressive disorders and the development of new pharmacological and non-pharmacological treatment options. There is a growing body of evidence for the efficacy of neuromodulation techniques, including transcranial magnetic stimulation, transcutaneous direct current stimulation, transcranial alternating current stimulation, and photo-biomodulation on improving connectivity and neuroplasticity associated with depression. This review attempts to revisit the role of glutamatergic neurotransmission in the etiopathogenesis of depressive disorders and review the current neuroimaging, neurophysiological and clinical evidence of these neuromodulation techniques in the pathophysiology and treatment of depression.

Antidepressant-like actions of the mGlu2/3 receptor antagonist TP0178894 in the chronic social defeat stress model: Comparison with escitalopram

The metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists are reported to produce ketamine-like rapid-acting and sustained antidepressant-like effects in rodents. In this study, we compared the effects of single administration of the new mGlu2/3 receptor antagonist TP0178894 and the selective serotonin reuptake inhibitor (SSRI) escitalopram in the chronic social defeat stress (CSDS) model of depression, a model which has been shown to be resistant to treatment with a single dose of SSRI. In the tail suspension test and forced swimming test, high dose (3.0 mg/kg) of TP0178894 significantly attenuated the increased immobility time of these tests in CSDS susceptible mice, compared with vehicle-treated mice. In contrast, low doses (0.3 and 1.0 mg/kg) of TP0178894 and escitalopram (10 mg/kg) did not alter the increased immobility time of these two tests. In the sucrose preference test, TP0178894 (3.0 mg/kg) significantly improved the reduced sucrose preference of CSDS susceptible mice, three and seven days after a single dose. In addition, Western blot analyses showed that TP0178894 (3.0 mg/kg), but not low doses of TP0178894 and escitalopram, significantly attenuated the reduced expression of synaptic proteins [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (GluA1) and postsynaptic density protein 95 (PSD-95)] in the prefrontal cortex from CSDS susceptible mice. This study suggests that TP0178894 shows rapid-acting and sustained antidepressant-like effects in CSDS model, as ketamine does.

Frontal cortex genetic ablation of metabotropic glutamate receptor subtype 3 (mGlu3) impairs postsynaptic plasticity and modulates affective behaviors

Clinical and translational studies suggest that prefrontal cortex (PFC) dysregulation is a hallmark feature of several affective disorders. Thus, investigating the mechanisms involved in the regulation of PFC function and synaptic plasticity could aid in developing new medications. In recent years, the mGlu₂ and mGlu₃ subtypes of metabotropic glutamate (mGlu) receptors have emerged as exciting potential targets for the treatment of affective disorders, as mGlu_2/3 antagonists exert antidepressant-like effects across many rodent models. Several recent studies suggest that presynaptic mGlu₂ receptors may contribute to these effects by regulating excitatory transmission at synapses from the thalamus to the PFC. Interestingly, we found that mGlu₃ receptors also inhibit excitatory drive to the PFC but act by inducing long-term depression (LTD) at amygdala-PFC synapses. It remains unclear, however, whether blockade of presynaptic, postsynaptic, or glial mGlu₃ receptors contribute to long-term effects on PFC circuit function and antidepressant-like effects of mGlu_2/3 antagonists. To address these outstanding questions, we leveraged transgenic Grm3^fl/fl mice and viral-mediated gene transfer to genetically ablate mGlu₃ receptors from pyramidal cells in the frontal cortex of adult mice of all sexes. Consistent with a role for mGlu₃ in PFC pyramidal cells, mGlu₃-dependent amygdala-cortical LTD was eliminated following mGlu₃ receptor knockdown. Furthermore, knockdown mice displayed a modest, task-specific anxiolytic phenotype and decreased passive coping behaviors. These studies reveal that postsynaptic mGlu₃ receptors are critical for mGlu₃-dependent LTD and provide convergent genetic evidence suggesting that modulating cortical mGlu₃ receptors may provide a promising new approach for the treatment of mood disorders.

Evaluation of the Safety, Tolerability, and Pharmacokinetic Profiles of TP0473292 (TS-161), A Prodrug of a Novel Orthosteric mGlu2/3 Receptor Antagonist TP0178894, in Healthy Subjects and Its Antidepressant-Like Effects in Rodents

BACKGROUND

TP0473292 (the active ingredient of TS-161) is a prodrug of a novel metabotropic glutamate (mGlu) 2/3 receptor antagonist being developed for the treatment of patients with depression. This study evaluated the safety, tolerability, and pharmacokinetics of orally administered TS-161 in healthy subjects.

METHODS

This was a first-in-human, phase 1, randomized, double-blind, placebo-controlled, single-ascending dose (15-400 mg TS-161) and 10-day multiple-ascending dose (50-150 mg TS-161) study in healthy subjects, conducted from June 2019 through February 2020. Plasma and urine concentrations of the prodrug and its metabolites, and cerebrospinal fluid (CSF) concentrations of the active metabolite TP0178894 were measured to evaluate the pharmacokinetic profiles after oral administration of TS-161.

RESULTS

Following single and multiple doses, TP0473292 was extensively converted into its active metabolite TP0178894. Plasma concentrations of TP0178894 reached peak (Cmax) within 5 hours post dose and declined with a t1/2 <13 hours. Plasma exposures of TP0178894 increased with increasing dose. TP0178894 penetrated into CSF and reached a Cmax of 9.892 ng/mL at a single dose of 100 mg, which was comparable with IC50 values of antagonist activity at mGlu2/3 receptors. The most frequently observed adverse events that showed exposure-related incidence during the study were nausea, vomiting, and dizziness.

CONCLUSIONS

The mGlu2/3 receptor antagonist prodrug TP0473292 is safe and well-tolerated, is orally bioavailable in humans with extensive conversion into the active metabolite TP0178894 with sufficient CSF penetration to exert the anticipated pharmacological effects, and is a promising candidate for further clinical development in treatment of patients with depression.

Genome-Wide Meta-Analysis Identifies Two Novel Risk Loci for Epilepsy

Epilepsy (affects about 70 million people worldwide) is one of the most prevalent brain disorders and imposes a huge economic burden on society. Epilepsy has a strong genetic component. In this study, we perform the largest genome-wide meta-analysis of epilepsy (N = 8,00,869 subjects) by integrating four large-scale genome-wide association studies (GWASs) of epilepsy. We identified three genome-wide significant (GWS) (p < 5 × 10^–8) risk loci for epilepsy. The risk loci on 7q21.11 [lead single nucleotide polymorphism (SNP) rs11978015, p = 9.26 × 10^–9] and 8p23.1 (lead SNP rs28634186, p = 4.39 × 10^–8) are newly identified in the present study. Of note, rs11978015 resides in upstream of GRM3, which encodes glutamate metabotropic receptor 3. GRM3 has pivotal roles in neurotransmission and is involved in most aspects of normal brain function. In addition, we also identified three genes (TTC21B, RP11-375N15.2, and TNKS) whose cis-regulated expression level are associated with epilepsy, indicating that risk variants may confer epilepsy risk through regulating the expression of these genes. Our study not only provides new insights into genetic architecture of epilepsy but also prioritizes potential molecular targets (including GRM3 and TTC21B) for development of new drugs and therapeutics for epilepsy.

Positive AMPA receptor modulation in the treatment of neuropsychiatric disorders: A long and winding road

Glutamatergic transmission is widely implicated in neuropsychiatric disorders, and the discovery that ketamine elicits rapid-acting antidepressant effects by modulating α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) signaling has spurred a resurgence of interest in the field. This review explores agents in various stages of development for neuropsychiatric disorders that positively modulate AMPARs, both directly and indirectly. Despite promising preclinical research, few direct and indirect AMPAR positive modulators have progressed past early clinical development. Challenges such as low potency have created barriers to effective implementation. Nevertheless, the functional complexity of AMPARs sets them apart from other drug targets and allows for specificity in drug discovery. Additional effective treatments for neuropsychiatric disorders that work through positive AMPAR modulation may eventually be developed.

mGlu2/3 receptor in the prelimbic cortex is implicated in stress resilience and vulnerability in mice

Resilience, referring to "achieving a positive outcome in the face of adversity", is a common phenomenon in daily life. Elucidating the mechanisms of stress resilience is instrumental to developing more effective treatments for stress-related psychiatric disorders such as depression. Metabotropic glutamate receptors (mGlu2/3 and mGlu5) within the medial prefrontal cortex (mPFC) have been recently recognized as promising therapeutic targets for rapid-acting antidepressant treatment. In this study, we assessed the functional roles of the mGlu2/3 and mGlu5 within different subregions of the mPFC in modulating stress resilience and vulnerability by using chronic social defeat stress (CSDS) paradigms in mice. Our results showed that approximately 51.6% of the subjects exhibited depression- or anxiety-like behaviors after exposure to CSDS. When a susceptible mouse was confronted with an attacker, c-Fos expression in the prelimbic cortex (PrL) subregion of the mPFC substantially increased. Compared with the resilient and control groups, the expression of mGlu2/3 was elevated in the PrL of the susceptible group. The expression of mGlu5 showed no significant difference among the three groups in the whole mPFC. Finally, we found that the social avoidance symptoms of the susceptible mice were rapidly relieved by intra-PrL administration of LY341495-an mGluR2/3 antagonists. The above results indicate that mGluR2/3 within the PrL may play an important regulatory role in stress-related psychiatric disorders. Our results are meaningful, as they expand our understanding of stress resilience and vulnerability which may open an avenue to develop novel, personalized approaches to mitigate depression and promote stress resilience.

Targeting the dysfunction of glutamate receptors for the development of novel antidepressants

Increasing evidence indicates that dysfunction of glutamate receptors is involved in the pathophysiology of major depressive disorder (MDD). Although accumulating efforts have been made to elucidate the applications and mechanisms underlying antidepressant-like effects of ketamine, a non-selective antagonist of N-methyl-d-aspartate receptor (NMDAR), the role of specific glutamate receptor subunit in regulating depression is not completely clear. The current review aims to discuss the relationships between glutamate receptor subunits and depressive-like behaviors. Research literatures were searched from inception to July 2020. We summarized the alterations of glutamate receptor subunits in patients with MDD and animal models of depression. Animal behaviors in response to dysfunction of glutamate receptor subunits were also surveyed. To fully understand mechanisms underlying antidepressant-like effects of modulators targeting glutamate receptors, we discussed effects of each glutamate receptor subunit on serotonin system, synaptic plasticity, neurogenesis and neuroinflammation. Finally, we collected most recent clinical applications of glutamate receptor modulators and pointed out the limitations of these candidates in the treatment of MDD.

Group II Metabotropic Glutamate Receptors Modulate Sound Evoked and Spontaneous Activity in the Mouse Inferior Colliculus

Little is known about the functions of Group II metabotropic glutamate receptors (mGluRs2/3) in the inferior colliculus (IC), a midbrain structure that is a major integration region of the central auditory system. We investigated how these receptors modulate sound-evoked and spontaneous firing in the mouse IC in vivo. We first performed immunostaining and tested hearing thresholds to validate vesicular GABA transporter (VGAT)-ChR2 transgenic mice on a mixed CBA/CaJ x C57BL/6J genetic background. Transgenic animals allowed for optogenetic cell-type identification. Extracellular single neuron recordings were obtained before and after pharmacological mGluR2/3 activation. We observed increased sound-evoked firing, as assessed by the rate-level functions (RLFs), in a subset of both GABAergic and non-GABAergic IC neurons following mGluR2/3 pharmacological activation. These neurons also displayed elevated spontaneous excitability and were distributed throughout the IC area tested, suggesting a widespread mGluR2/3 distribution in the mouse IC.

Maternal cafeteria diet exposure primes depression-like behavior in the offspring evoking lower brain volume related to changes in synaptic terminals and gliosis

Maternal nutritional programming by caloric exposure during pregnancy and lactation results in long-term behavioral modification in the offspring. Here, we characterized the effect of maternal caloric exposure on synaptic and brain morphological organization and its effects on depression-like behavior susceptibility in rats' offspring. Female Wistar rats were exposed to chow or cafeteria (CAF) diet for 9 weeks (pre-pregnancy, pregnancy, and lactation) and then switched to chow diet after weaning. By postnatal day 60, the male Wistar rat offspring were tested for depressive-like behavior using operational conditioning, novelty suppressed feeding, sucrose preference, and open-field test. Brain macro and microstructural morphology were analyzed using magnetic resonance imaging deformation-based morphometry (DBM) and western blot, immunohistochemistry for NMDA and AMPA receptor, synaptophysin and myelin, respectively. We found that the offspring of mothers exposed to CAF diet displayed deficient motivation showing decrease in the operant conditioning, sucrose preference, and suppressed feeding test. Macrostructural DBM analysis showed reduction in the frontomesocorticolimbic circuit volume including the nucleus accumbens (NAc), hippocampus, and prefrontal cortex. Microstructural analysis revealed reduced synaptic terminals in hippocampus and NAc, whereas increased glial fibrillary acidic protein in hippocampus and lateral hypothalamus, as well as a decrease in the hippocampal cell number and myelin reduction in the dentate gyrus and hilus, respectively. Also, offspring exhibited increase of the GluR1 and GLUR2 subunits of AMPA receptor, whereas a decrease in the mGluR2 expression in hippocampus. Our findings reveal that maternal programming might prime depression-like behavior in the offspring by modulating macro and micro brain organization of the frontomesocorticolimbic circuit.

Exposure to the predator odor TMT induces early and late differential gene expression related to stress and excitatory synaptic function throughout the brain in male rats

Persistent changes in brain stress and glutamatergic function are associated with post-traumatic stress disorder (PTSD). Rodent exposure to the predator odor trimethylthiazoline (TMT) is an innate stressor that produces lasting behavioral consequences relevant to PTSD. As such, the goal of the present study was to assess early (6 hours and 2 days-Experiment 1) and late (4 weeks-Experiment 2) changes to gene expression (RT-PCR) related to stress and excitatory function following TMT exposure in male, Long-Evans rats. During TMT exposure, rats engaged in stress reactive behaviors, including digging and immobility. Further, the TMT group displayed enhanced exploration and mobility in the TMT-paired context 1 week after exposure, suggesting a lasting contextual reactivity. Gene expression analyses revealed upregulated FKBP5 6 hours post-TMT in the hypothalamus and dorsal hippocampus. Two days after TMT, GRM3 was downregulated in the prelimbic cortex and dorsal hippocampus, but upregulated in the nucleus accumbens. This may reflect an early stress response (FKBP5) that resulted in later glutamatergic adaptation (GRM3). Finally, another experiment 4 weeks after TMT exposure showed several differentially expressed genes known to mediate excitatory tripartite synaptic function in the prelimbic cortex (GRM5, DLG4 and SLC1A3 upregulated), infralimbic cortex (GRM2 downregulated, Homer1 upregulated), nucleus accumbens (GRM7 and SLC1A3 downregulated), dorsal hippocampus (FKBP5 and NR3C2 upregulated, SHANK3 downregulated) and ventral hippocampus (CNR1, GRM7, GRM5, SHANK3 and Homer1 downregulated). These data show that TMT exposure induces stress and excitatory molecular adaptations, which could help us understand the persistent glutamatergic dysfunction observed in PTSD.

The effect of mGlu2/3 receptors on synaptic activities to different types of GABAergic interneurons in the anterior cingulate cortex

Antagonists of the group II metabotropic glutamate (mGlu) 2/3 receptors have been shown to have a rapid antidepressant effect. GABAergic interneurons play a crucial role in major depressive disorder (MDD) and possibly mediate the rapid antidepressant effect. However, how mGlu2/3 receptors regulate synaptic activities to GABAergic interneurons is not fully understood. In the present work, we studied the effect of mGlu2/3 receptors on excitatory and inhibitory synaptic activities to somatostatin (SST)- and parvalbumin (PV)-expressing interneurons, two major types of GABAergic interneurons, in the anterior cingulate cortex (ACC) that is strongly indicated in MDD. We found that activation of mGlu2/3 receptors by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine (DCG-IV), an agonist of mGlu2/3 receptors, remarkably reduced the frequency, but not the amplitude, of spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) and the amplitude of evoked EPSCs in both types. The reduction in the frequency of sEPSCs and the amplitude of evoked EPSCs was more pronounced in SST interneurons. DCG-IV, however, did not affect spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) and evoked IPSCs in both types. LY341495, an antagonist of mGlu2/3 receptors, enhanced the amplitude of evoked EPSCs without affecting sEPSCs and mEPSCs in both types. It also did not affect sIPSCs and evoked IPSCs except slightly increasing the frequency of mIPSCs in SST interneurons. Our results indicate that mGlu2/3 receptors primarily regulate excitatory synaptic activities to the two types of GABAergic interneurons in the ACC.

mGlu2/3 receptor as a novel target for rapid acting antidepressants

Given that ketamine, a noncompetitive N-methyl-d-aspartate receptor antagonist that exerts rapid antidepressant effects in patients with treatment-resistant depression, also has undesirable adverse effects, agents that can be used as alternatives to ketamine have been actively pursued. Group II metabotropic glutamate (mGlu) receptors, consisting of mGlu2 and mGlu3 receptors, have emerged as one of the most promising targets in the development of ketamine-like antidepressants. Indeed, mGlu2/3 receptor antagonists have been demonstrated to exert rapid antidepressant effects in animal models and to be efficacious in animal models refractory to conventional antidepressants. Moreover, there are striking similarities between mGlu2/3 receptor antagonists and ketamine in terms of not only their antidepressant profiles, but also the underlying mechanisms of their antidepressant effects. Nonetheless, studies in rodents have shown that mGlu2/3 receptor antagonists do not cause ketamine-like adverse events, such as psychotomimetic-like behavior, abuse potential or neurotoxicity, supporting the usefulness of mGlu2/3 receptor antagonists as alternatives to ketamine. In this chapter, the past and recent research on the antidepressant effects of mGlu2/3 receptor antagonists will be reviewed. In particular, the potential of mGlu2/3 receptor antagonists as novel ketamine-like antidepressants will be emphasized.

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.

Antidepressant Effects and Mechanisms of Group II mGlu Receptor-Specific Negative Allosteric Modulators

In this issue of Neuron, Joffe et al. (2020) assess the antidepressant-relevant effects and underlying neural mechanisms of negative allosteric modulators selective for either metabotropic glutamate receptors 2 (mGlu₂) or 3 (mGlu₃). Negative modulation of both receptors enhanced excitatory glutamatergic input to mouse prefrontal cortex pyramidal cells, leading to antidepressant-relevant actions.

The binding mode of vilazodone in the human serotonin transporter elucidated by ligand docking and molecular dynamics simulations

Vilazodone is a novel antidepressant used for the treatment of major depressive disorder (MDD) with a primary action mechanism of inhibiting the human serotonin reuptake transporter (hSERT) and acting as a 5-HT_1A receptor partial agonist. The interaction between vilazodone and the 5-HT_1A receptor has been reported, however, the binding mode of vilazodone in the hSERT remains elusive. In the current study, to elucidate the molecular mechanism of vilazodone binding in the hSERT, the drug and its five analogs were docked into the hSERT crystal structure as initial conformations and were sampled by 400 ns molecular dynamics (MD) simulations. Through the analysis of the profiles of protein-ligand binding free energies, interaction fingerprints, and conformational rearrangements, the binding mode of vilazodone in the hSERT was revealed. As a result, unlike the classical antidepressants located in the S1 site of the hSERT, vilazodone adopted a linear pose in the binding pocket. Its arylpiperazine fragment occupies the central site (S1) and interacts with Y95, D98, I172, Y176, F335, F341, S438, and T439, while the indole fragment extends to the allosteric site (S2) via interacting with the ionic switch (R104/E403) between the two sites. The new insights obtained are not only helpful in understanding the binding mode of vilazodone in the hSERT, but also provide valuable guidance to the discovery of novel antidepressant drugs.

AMPA receptor-mTORC1 signaling activation is required for neuroplastic effects of LY341495 in rat hippocampal neurons

The group II metabotropic glutamate 2/3 (mGlu_2/3) receptor antagonist LY341495 produces antidepressant-like effects by acting on mammalian target of rapamycin complex 1 (mTORC1) signaling and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors in rodent. We investigated whether LY341495 affects neuroplasticity via these mechanisms in rat primary hippocampal cultures under conditions of dexamethasone (DEX)-induced neurotoxicity. Ketamine was used for comparison. Hippocampal cultures were treated with LY341495 under conditions of DEX-induced toxicity. Changes in mTORC1-mediated proteins were determined by Western blotting analyses. Changes in dendritic outgrowth and spine density were evaluated via immunostaining. LY341495 significantly prevented DEX-induced decreases in the levels of mTORC1, 4E-BP1, and p70S6K phosphorylation as well as the levels of the synaptic proteins. These effects were blocked by pretreatment with the AMPA receptor inhibitor 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo(f)quinoxaline (NBQX) and the mTORC1 inhibitor rapamycin. LY341495 significantly attenuated DEX-induced decreases in dendritic outgrowth and spine density. Pretreatment with rapamycin and NBQX blocked these effects of LY341495. Further analyses indicted that induction of BDNF expression produced by LY341495 was blocked by pretreatment with NBQX and rapamycin. LY341495 has neuroplastic effects by acting on AMPA receptor-mTORC1 signaling under neurotoxic conditions. Therefore, activation of AMPA receptor and mTORC1 signaling, which enhance neuroplasticity, may be novel targets for new antidepressants.

Role of BDNF in the pathophysiology and treatment of depression: Activity-dependent effects distinguish rapid-acting antidepressants

The pathophysiology and treatment of depression have been the focus of intense research and while there is much that remains unknown, modern neurobiological approaches are making progress. This work demonstrates that stress and depression are associated with atrophy of neurons and reduced synaptic connectivity in brain regions such as the hippocampus and prefrontal cortex that contribute to depressive behaviors, and conversely that antidepressant treatment can reverse these deficits. The role of neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF), has been of particular interest as these factors play a key role in activity-dependent regulation of synaptic plasticity. Here, we review the literature demonstrating that exposure to stress and depression decreases BDNF expression in the hippocampus and PFC and conversely that antidepressant treatment can up-regulate BDNF in the adult brain and reverse the effects of stress. We then focus on rapid-acting antidepressants, particularly the NMDA receptor antagonist ketamine, which produces rapid synaptic and antidepressant behavioral actions that are dependent on activity-dependent release of BDNF. This rapid release of BDNF differs from typical monoaminergic agents that require chronic administration to produce a slow induction of BDNF expression, consistent with the time lag for the therapeutic action of these agents. We review evidence that other classes of rapid-acting agents also require BDNF release, demonstrating that this is a common, convergent downstream mechanism. Finally, we discuss evidence that the actions of ketamine are also dependent on another growth factor, vascular endothelial growth factor (VEGF) and its complex interplay with BDNF.

mGlu2 and mGlu3 Negative Allosteric Modulators Divergently Enhance Thalamocortical Transmission and Exert Rapid Antidepressant-like Effects

Non-selective antagonists of metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) exert rapid antidepressant-like effects by enhancing prefrontal cortex (PFC) glutamate transmission; however, the receptor subtype contributions and underlying mechanisms remain unclear. Here, we leveraged newly developed negative allosteric modulators (NAMs), transgenic mice, and viral-assisted optogenetics to test the hypothesis that selective inhibition of mGlu₂ or mGlu₃ potentiates PFC excitatory transmission and confers antidepressant efficacy in preclinical models. We found that systemic treatment with an mGlu₂ or mGlu₃ NAM rapidly activated biophysically unique PFC pyramidal cell ensembles. Mechanistic studies revealed that mGlu₂ and mGlu₃ NAMs enhance thalamocortical transmission and inhibit long-term depression by mechanistically distinct presynaptic and postsynaptic actions. Consistent with these actions, systemic treatment with either NAM decreased passive coping and reversed anhedonia in two independent chronic stress models, suggesting that both mGlu₂ and mGlu₃ NAMs induce antidepressant-like effects through related but divergent mechanisms of action.

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.

Vulnerability to depressive behavior induced by overexpression of striatal Shati/Nat8l via the serotonergic neuronal pathway in mice

The number of patients with depressive disorders is increasing. However, the mechanism of depression onsets has not been completely revealed. We previously identified Shati/Nat8l, an N-acetyltransferase, in the brain using an animal model of psychosis. In this study, we revealed the involvement of Shati/Nat8l in the vulnerability to major depression. Shati/Nat8l mRNA was increased only in the striatum of mice, which were exposed to chronic social defeat stress. Shati/Nat8l-overexpressed mice showed impairment in social interaction and sucrose preference after the subthreshold social defeat (microdefeat) stress. These depression-like behaviors were restored by fluvoxamine and LY341495 injection prior to these tests. Furthermore, the intracerebral administration of only fluvoxamine, but not of LY341495, to the dorsal striatum and direct infusion of LY341495 to the dorsal raphe also rescued. Taken together, Shati/Nat8l in the striatum has an important role in the vulnerability to depression onsets by regulating the origin of serotonergic neuronal system via GABAergic projection neuron in the dorsal raphe from the dorsal striatum.

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.

Glutamatergic Signaling in the Central Nervous System: Ionotropic and Metabotropic Receptors in Concert

Glutamate serves as both the mammalian brain's primary excitatory neurotransmitter and as a key neuromodulator to control synapse and circuit function over a wide range of spatial and temporal scales. This functional diversity is decoded by two receptor families: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The challenges posed by the complexity and physiological importance of each of these subtypes has limited our appreciation and understanding of how these receptors work in concert. In this review, by comparing both receptor families with a focus on their crosstalk, we argue for a more holistic understanding of neural glutamate signaling.

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu₂ and mGlu₃) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu₃ induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu₃-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu₃-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu₃ activation in vivo prevented the stress-induced maladaptive changes to amydalo-cortical physiology and motivated behavior. These data demonstrate that mGlu₃ mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu₃ modulation as a treatment for stress-related psychiatric disorders.

Comparison of antidepressant and side effects in mice after intranasal administration of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine

The N-methyl-d-aspartate receptor (NMDAR) antagonist (R,S)-ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with depression although intranasal use of (R,S)-ketamine in ketamine abusers is popular. In March 5, 2019, nasal spray of (S)-ketamine for treatment-resistant depression was approved as a new antidepressant by the US Food Drug Administration. Clinical study of (R)-ketamine is underway. In a chronic social defeat stress (CSDS) model, we compared the antidepressant effects of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine after a single intranasal administration. Furthermore, we also compared the side effects (i.e., locomotion, prepulse inhibition (PPI), abuse liability) of these three compounds in mice. The order of potency of antidepressant effects after a single intranasal administration was (R)-ketamine > (R,S)-ketamine > (S)-ketamine. In contrast, the order of locomotor activity and prepulse inhibition (PPI) deficits after a single intranasal administration was (S)-ketamine > (R,S)-ketamine > (R)-ketamine. In the conditioned place preference (CPP) test, both (S)-ketamine and (R,S)-ketamine increased CPP scores in mice after repeated intranasal administration, in a dose dependent manner. In contrast, (R)-ketamine did not increase CPP scores in mice. These findings suggest that intranasal administration of (R)-ketamine would be a safer antidepressant than (R,S)-ketamine and (S)-ketamine.

mGlu2/3 receptor antagonists

Abnormalities of glutamatergic transmission are implicated in neuropsychiatric disorders. Among the glutamate receptors, metabotropic (mGlu) 2/3 receptors have recently gained much attention as molecular targets for the treatment of several neuropsychiatric disorders including depression and anxiety. Both orthosteric and allosteric antagonists of mGlu2/3 receptors have been synthesized, and their therapeutic potential has been examined. These research activities have demonstrated the promise of mGlu2/3 receptor antagonists as potential treatment agents for the above-mentioned neuropsychiatric disorders. In particular, it has been considered that the antidepressant effects of mGlu2/3 receptor antagonists are worthy of pursuing, since the antidepressant profiles as well as synaptic/neural mechanisms involved in the actions of mGlu2/3 receptor antagonists are similar to those of ketamine, which has been demonstrated to show potent, rapid and sustained efficacy in patients with depression, even those resistant to the conventionally prescribed antidepressants. In this chapter, the general pharmacology of mGlu2/3 receptor antagonists and their therapeutic potential are reviewed. In particular, I focus on the usefulness of mGlu2/3 receptor antagonists as novel antidepressants, in comparison with ketamine.

Targeting of Metabotropic Glutamate Receptors for the Development of Novel Antidepressants

Since discovering that ketamine has robust antidepressant effects, the glutamatergic system has been proposed as an attractive target for the development of novel antidepressants. Among the glutamatergic system, metabotropic glutamate (mGlu) receptors are of interest because mGlu receptors play modulatory roles in glutamatergic transmission, consequently, agents acting on mGlu receptors might not exert the adverse effects associated with ketamine. mGlu receptors have eight subtypes that are classified into three groups, and the roles of each mGlu receptor subtype in depression are being investigated. To date, the potential use of mGlu5 receptor antagonists and mGlu2/3 receptor antagonists as antidepressants has been actively investigated, and the mechanisms underlying these antidepressant effects are being delineated. Although the outcomes of clinical trials using an mGlu5 receptor negative allosteric modulator and an mGlu2/3 receptor negative allosteric modulator have not been encouraging, these trials have been inconclusive, and additional trials using other compounds with more appropriate profiles are needed. In contrast, the roles of group III mGlu receptors have not yet been fully elucidated because of a lack of suitable pharmacological tools. Nonetheless, investigations of the use of mGlu4 and mGlu7 receptors as drug targets for the development of antidepressants have been ongoing, and some interesting evidence has been obtained.

(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.

Role of Serotonergic System in the Antidepressant Actions of mGlu2/3 Receptor Antagonists: Similarity to Ketamine

Numerous studies have demonstrated the antidepressant effects of group II metabotropic glutamate (mGlu2/3) receptor antagonists in various rodent models. Importantly, it has been shown that the antidepressant effects of mGlu2/3 receptor antagonists in rodent models are similar to those of ketamine, which exerts rapid and long-lasting antidepressant effects in patients with major depressive disorders, including patients with treatment-resistant depression. In addition, the synaptic mechanisms underlying the effects of mGlu2/3 receptor antagonists are reported to be similar to those underlying the effects of ketamine. The roles of the serotonergic system in the antidepressant effects of mGlu2/3 receptor antagonists have recently been demonstrated. Moreover, it was investigated how mGlu2/3 receptor antagonists interact with the serotonergic system to exert antidepressant effects. Notably, the same neural mechanisms as those underlying the effects of ketamine may be involved in the antidepressant actions of the mGlu2/3 receptor antagonists. In this review, we shall summarize the antidepressant potential of mGlu2/3 receptor antagonists and their mechanisms of action in comparison with those of ketamine. In particular, we shall focus on the roles of the serotonergic system in the antidepressant actions of mGlu2/3 receptor antagonists.