Positive modulation of NMDA receptors by AGN-241751 exerts rapid antidepressant-like effects via excitatory neurons

Demystifying the Antidepressant Mechanism of Action of Stinels, a Novel Class of Neuroplastogens: Positive Allosteric Modulators of the NMDA Receptor

Plastogens are a class of therapeutics that function by rapidly promoting changes in neuroplasticity. A notable example, ketamine, is receiving great attention due to its combined rapid and long-term antidepressant effects. Ketamine is an N-methyl-D-aspartate receptor (NMDAR) antagonist, and, in addition to its therapeutic activity, it is associated with psychotomimetic and dissociative side effects. Stinels-rapastinel, apimostinel, and zelquistinel-are also plastogens not only with rapid and long-term antidepressant effects but also with improved safety and tolerability profiles compared to ketamine. Previous descriptions of the mechanism by which stinels modulate NMDAR activity have been inconsistent and, at times, contradictory. The purpose of this review is to clarify the mechanism of action and contextualize stinels within a broader class of NMDAR-targeting therapeutics. In this review, we present the rationale behind targeting NMDARs for treatment-resistant depression and other psychiatric conditions, describe the various mechanisms by which NMDAR activity is regulated by different classes of therapeutics, and present evidence for the stinel mechanism. In contrast with previous descriptions of glycine-like NMDAR partial agonists, we define stinels as positive allosteric modulators of NMDAR activity with a novel regulatory binding site.

Phytochemicals in the treatment of patients with depression: a systemic review

Background

Depression is a complex mental disease whose incidence increases every year; 300 million people worldwide currently suffer from it. Women are more likely to suffer from depression, twice the rate as men. It is one of the few illnesses that can lead to suicide, which makes it very dangerous - currently, 700,000 people die from suicide and it is the 4th most common cause of death in people aged 15-29. The treatment strategies for depression is a big challenge for physicians, pharmacists, scientists and classic remedies cause many side effects. Therefore, natural phytotherapy with herbs can prove to be a good solution. Phytotherapy is a popular treatment method used for centuries in Chinese medicine or Ayurveda.

Materials and methods

The study conducted a comprehensive database search PubMed, ClinicalKey and MedNar covered the years 2015 - 2024 to provide the most up-to-date data. 13 randomized controlled trials and 1 meta - analysis were included in the systematic review.

Results

Many plants show anti-inflammatory, antioxidant and cognitive enhancing effects, which are particularly important in depression. In the treatment of depression, plants such as Crocus sativus L. stigma, Lavandula angustifolia, Hypericum perforatum L. and Curcuma longa L. have proven to be effective. They show good effectiveness in human studies and alleviate the symptoms of depression. Herbal products can support classical pharmacotherapy, but this requires further research. Non-commercial clinical trials in the future should provide answers to research questions: at what stage of treatment of patients with MDD will the use of phytochemicals be most appropriate in terms of therapy efficacy and safety for the patient.

Conclusions

Crocus sativus L. stigma, Lavandula angustifolia, Hypericum perforatum L. and Curcuma longa L. in modern medicine can help improve the well-being of patients with depression. The use of herbs as an intervention was associated with a decrease in the concentration of proinflammatory cytokines and an overall improvement in the mood of patients. Further research should be undertaken into combining both therapies in order to improve patients' quality of life and reduce treatment costs.

Research Progress on NMDA Receptor Enhancement Drugs for the Treatment of Depressive Disorder

Major depressive disorder (MDD) is a severe mental illness with a complex etiology. Currently, many medications employed in clinical treatment exhibit limitations such as delayed onset of action and a high incidence of adverse reactions. Therefore, there is a pressing need to develop antidepressants that exhibit enhanced efficacy and safety. The N-methyl-D-aspartate receptor (NMDAR), a distinctive glutamate-gated ion channel receptor, has been implicated in the onset and progression of depressive disorder, as evidenced by both preclinical and clinical research. The NMDAR antagonist, ketamine, exhibits rapid and sustained antidepressant effects, holding promise as a novel therapeutic approach for depressive disorder. However, its psychotomimetic impact and potential for addiction have restricted its widespread clinical application. Notably, over the past decade, studies have suggested that enhancing NMDAR functionality can produce antidepressant effects with improved safety, especially with the emergence of NMDAR-positive allosteric modulators (PAMs). We view this as a potential novel strategy for treating depression, forming the basis for the narrative review that follows.

Zelquistinel acts at an extracellular binding domain to modulate intracellular calcium inactivation of N-methyl-d-aspartate receptors

Stinels are a novel class of N-methyl-d-aspartate glutamate receptor (NMDAR) positive allosteric modulators. We explored mechanism of action and NR2 subtype specificity of the stinel zelquistinel (ZEL) in HEK 293 cells expressing recombinant NMDARs. ZEL potently enhanced NMDAR current at NR2A (EC50 = 9.9 ± 0.5 nM) and NR2C-containing (EC50 = 9.7 ± 0.6 nM) NMDARs, with a larger ceiling enhancement at NR2B-NMDAR (EC50 = 35.0 ± 0.7 nM), while not affecting NR2D-containing NMDARs. In cells expressing NR2A and NR2C-containing NMDARs, ZEL exhibited an inverted-U dose-response relation, with a low concentration enhancement and high concentration suppression of NMDAR currents. Extracellular application of ZEL potentiated NMDAR receptor activity via prolongation of NMDAR currents. Replacing the slow Ca2+ intracellular chelator EGTA with the fast chelator BAPTA blocked ZEL potentiation of NMDARs, suggesting an action on intracellular Ca2+-calmodulin-dependent inactivation (CDI). Consistent with this mechanism of action, removal of the NR1 intracellular C-terminus, or intracellular infusion of a calmodulin blocking peptide, blocked ZEL potentiation of NMDAR current. In contrast, BAPTA did not prevent high-dose suppression of current, indicating this effect has a different mechanism of action. These data indicate ZEL is a novel positive allosteric modulator that binds extracellularly and acts through a unique long-distance mechanism to reduce NMDAR CDI, eliciting enhancement of NMDAR current. The critical role that NMDARs play in long-term, activity-dependent synaptic plasticity, learning, memory and cognition, suggests dysregulation of CDI may contribute to psychiatric disorders such as depression, schizophrenia and others, and that the stinel class of drugs can restore NMDAR-dependent synaptic plasticity by reducing activity-dependent CDI.

Glutamatergic Modulators for Major Depression from Theory to Clinical Use

Major depressive disorder (MDD) is a chronic, burdensome, highly prevalent disease that is characterized by depressed mood and anhedonia. MDD is especially burdensome as approved monoamine antidepressant treatments have weeks-long delays before clinical benefit and low remission rates. In the past 2 decades, a promising target emerged to improve patient outcomes in depression treatment: glutamatergic signaling. This narrative review provides a high-level overview of glutamate signaling in synaptogenesis and neural plasticity and the implications of glutamate dysregulation in depression. Based on this preclinical evidence implicating glutamate in depression and the rapid improvement of depression with ketamine treatment in a proof-of-concept trial, a range of N-methyl-D-aspartate (NMDA)-targeted therapies have been investigated. While an array of treatments has been investigated in registered phase 2 or 3 clinical trials, the development of most of these agents has been discontinued. Multiple glutamate-targeted antidepressants are actively in development, and two are approved. Nasal administration of esketamine (Spravato®) was approved by the US Food and Drug Administration (FDA) in 2019 to treat adults with treatment-resistant depression and in 2020 for adults with MDD with acute suicidal ideation or behavior. Oral combination dextromethorphan-bupropion (AXS-05, Auvelity® extended-release tablet) was FDA approved in 2022 for the treatment of MDD in adults. These approvals bolster the importance of glutamate in depression and represent an exciting breakthrough in contemporary psychiatry, providing new avenues of treatment for patients as first-line therapy or with either poor response or unacceptable side effects to monoaminergic antidepressants.

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

The NMDA receptor modulator zelquistinel durably relieves behavioral deficits in three mouse models of autism spectrum disorder

Autism spectrum disorders (ASD) are complex neurodevelopmental disorders characterized by deficient social communication and interaction together with restricted, stereotyped behaviors. Currently approved treatments relieve comorbidities rather than core symptoms. Since excitation/inhibition balance and synaptic plasticity are disrupted in ASD, molecules targeting excitatory synaptic transmission appear as highly promising candidates to treat this pathology. Among glutamatergic receptors, the NMDA receptor has received particular attention through the last decade to develop novel allosteric modulators. Here, we show that positive NMDA receptor modulation by zelquistinel, a spirocyclic β-lactam platform chemical, relieves core symptoms in two genetic and one environmental mouse models of ASD. A single oral dose of zelquistinel rescued, in a dose-response manner, social deficits and stereotypic behavior in Shank3Δex13-16-/- mice while chronic intraperitoneal administration promoted a long-lasting relief of such autistic-like features in these mice. Subchronic oral mid-dose zelquistinel treatment demonstrated durable effects in Shank3Δex13-16-/-, Fmr1-/- and in utero valproate-exposed mice. Carry-over effects were best maintained in the Fmr1 null mouse model, with social parameters being still fully recovered two weeks after treatment withdrawal. Among recently developed NMDA receptor subunit modulators, zelquistinel displays a promising therapeutic potential to relieve core symptoms in ASD patients, with oral bioavailability and long-lasting effects boding well for clinical applications. Efficacy in three mouse models with different etiologies supports high translational value. Further, this compound represents an innovative pharmacological tool to investigate plasticity mechanisms underlying behavioral deficits in animal models of ASD.

Targeting metaplasticity mechanisms to promote sustained antidepressant actions

The discovery that subanesthetic doses of (R, S)-ketamine (ketamine) and (S)-ketamine (esketamine) rapidly induce antidepressant effects and promote sustained actions following drug clearance in depressed patients who are treatment-resistant to other therapies has resulted in a paradigm shift in the conceptualization of how rapidly and effectively depression can be treated. Consequently, the mechanism(s) that next generation antidepressants may engage to improve pathophysiology and resultant symptomology are being reconceptualized. Impaired excitatory glutamatergic synapses in mood-regulating circuits are likely a substantial contributor to the pathophysiology of depression. Metaplasticity is the process of regulating future capacity for plasticity by priming neurons with a stimulation that alters later neuronal plasticity responses. Accordingly, the development of treatment modalities that specifically modulate the duration, direction, or magnitude of glutamatergic synaptic plasticity events such as long-term potentiation (LTP), defined here as metaplastogens, may be an effective approach to reverse the pathophysiology underlying depression and improve depression symptoms. We review evidence that the initiating mechanisms of pharmacologically diverse rapid-acting antidepressants (i.e., ketamine mimetics) converge on consistent downstream molecular mediators that facilitate the expression/maintenance of increased synaptic strength and resultant persisting antidepressant effects. Specifically, while the initiating mechanisms of these therapies may differ (e.g., cell type-specificity, N-methyl-D-aspartate receptor (NMDAR) subtype-selective inhibition vs activation, metabotropic glutamate receptor 2/3 antagonism, AMPA receptor potentiation, 5-HT receptor-activating psychedelics, etc.), the sustained therapeutic mechanisms of putative rapid-acting antidepressants will be mediated, in part, by metaplastic effects that converge on consistent molecular mediators to enhance excitatory neurotransmission and altered capacity for synaptic plasticity. We conclude that the convergence of these therapeutic mechanisms provides the opportunity for metaplasticity processes to be harnessed as a druggable plasticity mechanism by next-generation therapeutics. Further, targeting metaplastic mechanisms presents therapeutic advantages including decreased dosing frequency and associated diminished adverse responses by eliminating the requirement for the drug to be continuously present.

Natural product Kaji-ichigoside F1 exhibits rapid antidepression via activating the AMPA-BDNF-mTOR pathway and inhibiting the NMDAR-CaMKIIα pathway

BACKGROUND

Depression is a common and recurrent neuropsychiatric disorder. Recent studies have shown that the N-methyl-d-aspartate (NMDA) receptor (NMDAR) is involved in the pathophysiology of depression. Previous studies have found that Kaji-ichigoside F1 (KF1) has a protective effect against NMDA-induced neurotoxicity. However, the antidepressant mechanism of KF1 has not been confirmed yet.

PURPOSE

In the present study, we aimed to evaluate the rapid antidepressant activity of KF1 and explore the underlying mechanism.

STUDY DESIGN

First, we explored the effect of KF1 on NMDA-induced hippocampal neurons and the underlying mechanism. Second, depression was induced in C57BL/6 mice via chronic unpredictable mild stress (CUMS), and the immediate and persistent depression-like behavior was evaluated using the forced swimming test (FST) after a single administration of KF1. Third, the contributions of NMDA signaling to the antidepressant effect of KF1 were investigated using pharmacological interventions. Fourth, CUMS mice were treated with KF1 for 21 days, and then their depression-like behaviors and the underlying mechanism were further explored.

METHODS

The FST was used to evaluate immediate and persistent depression-like behavior after a single administration of KF1 with or without NMDA pretreatment. The effect of KF1 on depressive-like behavior was investigated in CUMS mice by treating them with KF1 once daily for 21 days through the sucrose preference test, FST, open field test, and tail suspension test. Then, the effects of KF1 on the morphology and molecular and functional phenotypes of primary neuronal cells and hippocampus of mice were investigated by hematoxylin-eosin staining, Nissl staining, propidium iodide staining, TUNEL staining, Ca2+ imaging, JC-1 staining, ELISA, immunofluorescence analysis, RT-PCR, and Western blot.

RESULTS

KF1 could effectively improve cellular viability, reduce apoptosis, inhibit the release of LDH and Ca2+, and increase the mitochondrial membrane potential and the number of dendritic spines numbers in hippocampal neurons. Moreover, behavioral tests showed that KF1 exerted acute and sustained antidepressant-like effects by reducing Glu-levels and ameliorating neuronal damage in the hippocampus. Additionally, in vivo and in vitro experiments revealed that PSD95, Syn1, α-amino-3‑hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and brain-derived neurotrophic factor (BDNF) were upregulated at the protein level, and BDNF and AMPA were upregulated at the mRNA level. NR1 and NR2A showed the opposite trend.

CONCLUSION

These results confirm that KF1 exerts rapid antidepressant effects mainly by activating the AMPA-BDNF-mTOR pathway and inhibiting the NMDAR-CaMKIIα pathway. This study serves as a new reference for discovering rapid antidepressants.

The role of N-methyl-D-aspartate glutamate receptors in Alzheimer's disease: From pathophysiology to therapeutic approaches

N-Methyl-D-aspartate glutamate receptors (NMDARs) are involved in multiple physiopathological processes, including synaptic plasticity, neuronal network activities, excitotoxic events, and cognitive impairment. Abnormalities in NMDARs can initiate a cascade of pathological events, notably in Alzheimer's disease (AD) and even other neuropsychiatric disorders. The subunit composition of NMDARs is plastic, giving rise to a diverse array of receptor subtypes. While they are primarily found in neurons, NMDAR complexes, comprising both traditional and atypical subunits, are also present in non-neuronal cells, influencing the functions of various peripheral tissues. Furthermore, protein-protein interactions within NMDAR complexes has been linked with Aβ accumulation, tau phosphorylation, neuroinflammation, and mitochondrial dysfunction, all of which potentially served as an obligatory relay of cognitive impairment. Nonetheless, the precise mechanistic link remains to be fully elucidated. In this review, we provided an in-depth analysis of the structure and function of NMDAR, investigated their interactions with various pathogenic proteins, discussed the current landscape of NMDAR-based therapeutics, and highlighted the remaining challenges during drug development.

Central regulation of stress-evoked peripheral immune responses

Stress-linked psychiatric disorders, including anxiety and major depressive disorder, are associated with systemic inflammation. Recent studies have reported stress-induced alterations in haematopoiesis that result in monocytosis, neutrophilia, lymphocytopenia and, consequently, in the upregulation of pro-inflammatory processes in immunologically relevant peripheral tissues. There is now evidence that this peripheral inflammation contributes to the development of psychiatric symptoms as well as to common co-morbidities of psychiatric disorders such as metabolic syndrome and immunosuppression. Here, we review the specific brain and spinal regions, and the neuronal populations within them, that respond to stress and transmit signals to peripheral tissues via the autonomic nervous system or neuroendocrine pathways to influence immunological function. We comprehensively summarize studies that have employed retrograde tracing to define neurocircuits linking the brain to the bone marrow, spleen, gut, adipose tissue and liver. Moreover, we highlight studies that have used chemogenetic or optogenetic manipulation or intracerebroventricular administration of peptide hormones to control somatic immune responses. Collectively, this growing body of literature illustrates potential mechanisms through which stress signals are conveyed from the CNS to immune cells to regulate stress-relevant behaviours and comorbid pathophysiology.

GluN2B-containing NMDARs in the mammalian brain: pharmacology, physiology, and pathology

Glutamate N-methyl-D-aspartate receptor (NMDAR) is critical for promoting physiological synaptic plasticity and neuronal viability. As a major subpopulation of the NMDAR, the GluN2B subunit-containing NMDARs have distinct pharmacological properties, physiological functions, and pathological relevance to neurological diseases compared with other NMDAR subtypes. In mature neurons, GluN2B-containing NMDARs are likely expressed as both diheteromeric and triheteromeric receptors, though the functional importance of each subpopulation has yet to be disentangled. Moreover, the C-terminal region of the GluN2B subunit forms structural complexes with multiple intracellular signaling proteins. These protein complexes play critical roles in both activity-dependent synaptic plasticity and neuronal survival and death signaling, thus serving as the molecular substrates underlying multiple physiological functions. Accordingly, dysregulation of GluN2B-containing NMDARs and/or their downstream signaling pathways has been implicated in neurological diseases, and various strategies to reverse these deficits have been investigated. In this article, we provide an overview of GluN2B-containing NMDAR pharmacology and its key physiological functions, highlighting the importance of this receptor subtype during both health and disease states.

NMDA receptors as therapeutic targets for depression treatment: Evidence from clinical to basic research

Ketamine, functioning as a channel blocker of the excitatory glutamate-gated N-methyl-d-aspartate (NMDA) receptors, displays compelling fast-acting and sustained antidepressant effects for treatment-resistant depression. Over the past decades, clinical and preclinical studies have implied that the pathology of depression is associated with dysfunction of glutamatergic transmission. In particular, the discovery of antidepressant agents modulating NMDA receptor function has prompted breakthroughs for depression treatment compared with conventional antidepressants targeting the monoaminergic system. In this review, we first summarized the signalling pathway of the ketamine-mediated antidepressant effects, based on the glutamate hypothesis of depression. Second, we reviewed the hypotheses of the synaptic mechanism and network of ketamine antidepressant effects within different brain areas and distinct subcellular localizations, including NMDA receptor antagonism on GABAergic interneurons, extrasynaptic and synaptic NMDA receptor-mediated antagonism, and ketamine blocking bursting activities in the lateral habenula. Third, we reviewed the different roles of NMDA receptor subunits in ketamine-mediated cognitive and psychiatric behaviours in genetically-manipulated rodent models. Finally, we summarized the structural basis of NMDA receptor channel blockers and discussed NMDA receptor modulators that have been reported to exert potential antidepressant effects in animal models or in clinical trials. Integrating the cutting-edge technologies of cryo-EM and artificial intelligence-based drug design (AIDD), we expect that the next generation of first-in-class rapid antidepressants targeting NMDA receptors would be an emerging direction for depression therapeutics. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

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.

Novel antidepressant drugs: Beyond monoamine targets

Treatment of major depressive disorder (MDD) including treatment-resistant depression (TRD) remains a major unmet need. Although there are several classes of dissimilar antidepressant drugs approved for MDD, the current drugs have either limited efficacy or are associated with undesirable side effects and withdrawal symptoms. The efficacy and side effects of antidepressant drugs are mainly attributed to their actions on different monoamine neurotransmitters (serotonin, norepinephrine, and dopamine). Development of new antidepressants with novel targets beyond the monoamine pathways may fill the unmet need in treatment of MDD and TRD. The recent approval of intranasal Esketamine (glutamatergic agent) in conjunction with an oral antidepressant for the treatment of adult TRD patients was the first step toward expanding beyond the monoamine targets. Several other glutamatergic (AXS-05, REL-1017, AV-101, SLS-002, AGN24175, and PCN-101) and GABAergic (brexanolone, zuranolone, and ganaxolone) drugs are currently in different stages of clinical development for MDD, TRD and other indications. The renaissance of psychedelic drugs and the emergence of preliminary positive clinical trial results with psilocybin, Ayahuasca, 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), and lysergic acid diethylamide (LSD) may pave the way towards establishing this class of drugs as effective therapies for MDD, TRD and other neuropsychiatric disorders. Going beyond the monoamine targets appears to be an effective strategy to develop novel antidepressant drugs with superior efficacy, safety, and tolerability for the improved treatment of MDD and TRD.

Emerging drugs for the treatment of major depressive disorder

INTRODUCTION

Major depressive disorder (MDD) continues to be one of the highest contributors to disease burden and years lived with disability in the world. Current existing treatments have been associated with intolerable side effects, long onset of action and suboptimal remission rates. Newer agents are being developed that will be reviewed here, such as glutamate and gamma-aminobutyric acid (GABA) and the reinvigorated testing of psychedelic drugs. This review will summarize the target mechanisms of the newer ADTs currently in development and available on the market.

AREAS COVERED

It briefly covers the existing agents for MDD and treatment-resistant depression (TRD) and the need for new agents with higher efficacy. Therapeutic agents currently in Phase II or later clinical trials are listed and discussed, based on a thorough review of the US National Institutes of Health clinicaltrials.gov index and a search of the Informa Pharmaprojects database. Compounds of interest are grouped into scientific rationale and include atypical antipsychotics, GABA positive allosteric modulators, glutamatergic agents, opioids, orexin 2 receptor antagonists, and psychedelics.

EXPERT OPINION

New therapeutic agents currently in development are promising, with a more rapid onset of action and the ability to augment and treat TRD.

The trilateral interactions between mammalian target of rapamycin (mTOR) signaling, the circadian clock, and psychiatric disorders: an emerging model

Circadian (~24 h) rhythms in physiology and behavior are evolutionarily conserved and found in almost all living organisms. The rhythms are endogenously driven by daily oscillatory activities of so-called "clock genes/proteins", which are widely distributed throughout the mammalian brain. Mammalian (mechanistic) target of rapamycin (mTOR) signaling is a fundamental intracellular signal transduction cascade that controls important neuronal processes including neurodevelopment, synaptic plasticity, metabolism, and aging. Dysregulation of the mTOR pathway is associated with psychiatric disorders including autism spectrum disorders (ASD) and mood disorders (MD), in which patients often exhibit disrupted daily physiological rhythms and abnormal circadian gene expression in the brain. Recent work has found that the activities of mTOR signaling are temporally controlled by the circadian clock and exhibit robust circadian oscillations in multiple systems. In the meantime, mTOR signaling regulates fundamental properties of the central and peripheral circadian clocks, including period length, entrainment, and synchronization. Whereas the underlying mechanisms remain to be fully elucidated, increasing clinical and preclinical evidence support significant crosstalk between mTOR signaling, the circadian clock, and psychiatric disorders. Here, we review recent progress in understanding the trilateral interactions and propose an "interaction triangle" model between mTOR signaling, the circadian clock, and psychiatric disorders (focusing on ASD and MD).

Region-Specific Enhancement of c-fos Expression by Combined Treatment With NMDA Receptor Agonists and Antagonists With Antidepressant Potential

Rapastinel, formerly Glyx-13, is a novel positive allosteric modulator of the N-methyl-D-aspartate-receptor (NMDAR) that counteracts psychotomimetic actions of NMDAR antagonists. We set out to evaluate the effect of rapastinel alone or in combination with the global and GluN2B subunit-specific NMDAR antagonists MK-801 and Ro25-6981, respectively, on neuronal activation in relevant regions using c-fos brain mapping. Whereas rapastinel alone did not trigger significant c-fos expression beyond the prelimbic cortex, it strongly increased the c-fos expression induced by MK-801 in hippocampal, cingulate, and retrosplenial areas. Similar results were obtained when rapastinel was replaced by D-cycloserine. Our results reveal new interactions at network level between NMDAR modulators with possible implications regarding their therapeutic effects.

Zelquistinel Is an Orally Bioavailable Novel NMDA Receptor Allosteric Modulator That Exhibits Rapid and Sustained Antidepressant-Like Effects

BACKGROUND

The role of glutamatergic receptors in major depressive disorder continues to be of great interest for therapeutic development. Recent studies suggest that both negative and positive modulation of N-methyl-D-aspartate receptors (NMDAR) can produce rapid antidepressant effects. Here we report that zelquistinel, a novel NMDAR allosteric modulator, exhibits high oral bioavailability and dose-proportional exposures in plasma and the central nervous system and produces rapid and sustained antidepressant-like effects in rodents by enhancing activity-dependent, long-term synaptic plasticity.

METHODS

NMDAR-mediated functional activity was measured in cultured rat brain cortical neurons (calcium imaging), hNR2A or B subtype-expressing HEK cells, and synaptic plasticity in rat hippocampal and medial prefrontal cortex slices in vitro. Pharmacokinetics were evaluated in rats following oral administration. Antidepressant-like effects were assessed in the rat forced swim test and the chronic social deficit mouse model. Target engagement and the safety/tolerability profile was assessed using phencyclidine-induced hyperlocomotion and rotarod rodent models.

RESULTS

Following a single oral dose, zelquistinel (0.1-100 µg/kg) produced rapid and sustained antidepressant-like effects in the rodent depression models. Brain/ cerebrospinal fluid concentrations associated with zelquistinel antidepressant-like activity also increased NMDAR function and rapidly and persistently enhanced activity-dependent synaptic plasticity (long-term potentiation), suggesting that zelquistinel produces antidepressant-like effects by enhancing NMDAR function and synaptic plasticity. Furthermore, Zelquistinel inhibited phencyclidine (an NMDAR antagonist)-induced hyperlocomotion and did not impact rotarod performance.

CONCLUSIONS

Zelquistinel produces rapid and sustained antidepressant effects by positively modulating the NMDARs, thereby enhancing long-term potentiation of synaptic transmission.

Investigational Drugs for the Treatment of Depression (Part 2): Glutamatergic, Cholinergic, Sestrin Modulators, and Other Agents

Many investigational drugs with antidepressant activity are currently explored in different phases of clinical research, with indications such as major depressive disorder, treatment-resistant major depression, bipolar depression, post-partum depression, and late-life depression. Although the vast majority of the antidepressants in clinical use are based on the monoaminergic hypothesis of depression, recent data supported the launching on the market of two new, non-monoamine-modulating drugs. Esketamine for treatment-resistant major depression and brexanolone for post-partum depression are two exceptions from the monoaminergic model, although their use is still limited by high costs, unique way of administration (only intravenously for brexanolone), physicians’ reluctance to prescribe new drugs, and patients’ reticence to use them. Glutamatergic neurotransmission is explored based on the positive results obtained by intranasal esketamine, with subanesthetic intravenous doses of ketamine, and D-cycloserine, traxoprodil, MK-0657, AXS-05, AVP-786, combinations of cycloserine and lurasidone, or dextromethorphan and quinidine, explored as therapeutic options for mono- or bipolar depression. Sestrin modulators, cholinergic receptor modulators, or onabotulinumtoxinA have also been investigated for potential antidepressant activity. In conclusion, there is hope for new treatments in uni- and bipolar depression, as it became clear, after almost 7 decades of monoamine-modulating antidepressants, that new pathogenetic pathways should be targeted to increase the response rate in this population.

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.

Novel Pharmacological Approaches to the Treatment of Depression

Major depressive disorder is one of the most prevalent mental health disorders. Monoamine-based antidepressants were the first drugs developed to treat major depressive disorder. More recently, ketamine and other analogues were introduced as fast-acting antidepressants. Unfortunately, currently available therapeutics are inadequate; lack of efficacy, adverse effects, and risks leave patients with limited treatment options. Efforts are now focused on understanding the etiology of depression and identifying novel targets for pharmacological treatment. In this review, we discuss promising novel pharmacological targets for the treatment of major depressive disorder. Targeting receptors including N-methyl-D-aspartate receptors, peroxisome proliferator-activated receptors, G-protein-coupled receptor 39, metabotropic glutamate receptors, galanin and opioid receptors has potential antidepressant effects. Compounds targeting biological processes: inflammation, the hypothalamic-pituitary-adrenal axis, the cholesterol biosynthesis pathway, and gut microbiota have also shown therapeutic potential. Additionally, natural products including plants, herbs, and fatty acids improved depressive symptoms and behaviors. In this review, a brief history of clinically available antidepressants will be provided, with a primary focus on novel pharmaceutical approaches with promising antidepressant effects in preclinical and clinical studies.

Novel Glutamatergic Modulators for the Treatment of Mood Disorders: Current Status

The efficacy of standard antidepressants is limited for many patients with mood disorders such as major depressive disorder (MDD) and bipolar depression, underscoring the urgent need to develop novel therapeutics. Both clinical and preclinical studies have implicated glutamatergic system dysfunction in the pathophysiology of mood disorders. In particular, rapid reductions in depressive symptoms have been observed in response to subanesthetic doses of the glutamatergic modulator racemic (R,S)-ketamine in individuals with mood disorders. These results have prompted investigation into other glutamatergic modulators for depression, both as monotherapy and adjunctively. Several glutamate receptor-modulating agents have been tested in proof-of-concept studies for mood disorders. This manuscript gives a brief overview of the glutamate system and its relevance to rapid antidepressant response and discusses the existing clinical evidence for glutamate receptor-modulating agents, including (1) broad glutamatergic modulators ((R,S)-ketamine, esketamine, (R)-ketamine, (2R,6R)-hydroxynorketamine [HNK], dextromethorphan, Nuedexta [a combination of dextromethorphan and quinidine], deudextromethorphan [AVP-786], axsome [AXS-05], dextromethadone [REL-1017], nitrous oxide, AZD6765, CLE100, AGN-241751); (2) glycine site modulators (D-cycloserine [DCS], NRX-101, rapastinel [GLYX-13], apimostinel [NRX-1074], sarcosine, 4-chlorokynurenine [4-Cl-KYN/AV-101]); (3) subunit (NR2B)-specific N-methyl-D-aspartate (NMDA) receptor antagonists (eliprodil [EVT-101], traxoprodil [CP-101,606], rislenemdaz [MK-0657/CERC-301]); (4) metabotropic glutamate receptor (mGluR) modulators (basimglurant, AZD2066, RG1578, TS-161); and (5) mammalian target of rapamycin complex 1 (mTORC1) activators (NV-5138). Many of these agents are still in the preliminary stages of development. Furthermore, to date, most have demonstrated relatively modest effects compared with (R,S)-ketamine and esketamine, though some have shown more favorable characteristics. Of these novel agents, the most promising, and the ones for which the most evidence exists, appear to be those targeting ionotropic glutamate receptors.