Glutamate-Based Drug Discovery for Novel Antidepressants

4-F-PCP, a Novel PCP Analog Ameliorates the Depressive-Like Behavior of Chronic Social Defeat Stress Mice via NMDA Receptor Antagonism

Major depressive disorder is a leading cause of disability in more than 280 million people worldwide. Monoamine-based antidepressants are currently used to treat depression, but delays in treatment effects and lack of responses are major reasons for the need to develop faster and more efficient antidepressants. Studies show that ketamine (KET), a PCP analog, produces antidepressant effects within a few hours of administration that lasts up to a week. However, the use of KET has raised concerns about side effects, as well as the risk of abuse. 4 -F-PCP analog is a novel PCP analog that is also an NMDA receptor antagonist, structurally similar to KET, and might potentially elicit similar antidepressant effects, however, there has been no study on this subject yet. Herein, we investigate whether 4-F-PCP displays antidepressant effects and explored their potential therapeutic mechanisms. 4-F-PCP at 3 and 10 mg/kg doses showed antidepressant-like effects and repeated treatments maintained its effects. Furthermore, treatment with 4-F-PCP rescued the decreased expression of proteins most likely involved in depression and synaptic plasticity. Changes in the excitatory amino acid transporters (EAAT2, EAAT3, EAAT4) were also seen following drug treatment. Lastly, we assessed the possible side effects of 4-F-PCP after long-term treatment (up to 21 days). Results show that 4-F-PCP at 3 mg/kg dose did not alter the cognitive function of mice. Overall, current findings provide significant implications for future research not only with PCP analogs but also on the next generation of different types of antidepressants.

The Antidepressant-like Activity and Cognitive Enhancing Effects of the Combined Administration of (R)-Ketamine and LY341495 in the CUMS Model of Depression in Mice Are Related to the Modulation of Excitatory Synaptic Transmission and LTP in the PFC

(S)-Ketamine is the first rapid-acting antidepressant drug (RAAD) introduced for the treatment of depression. However, research is still being carried out on the search for further RAADs that will be not only effective but also safe to use. Recent data have indicated that the combined administration of (R)-ketamine and the mGlu2/3 receptor antagonist LY341495 (mixRL) induces rapid and sustained effects in the chronic unpredictable mild stress (CUMS) model of depression in mice, and the use of this drug combination is associated with a low risk of undesirable effects. Considering the possible influence of stress on cortical plasticity and, on the other hand, the role of this plasticity in the mechanism of action of ketamine, we decided to investigate whether mixed RL affects synaptic plasticity in the prefrontal cortex (PFC) in the CUMS model of depression using electrophysiological techniques and explore whether these effects are related to memory impairments. Using behavioral methods, we found that a single administration of mixRL reversed CUMS-induced PFC-dependent memory deficits and alleviated depression-like effects induced by CUMS. In turn, electrophysiological experiments indicated that the amplitude of field potentials as well as paired-pulse responses in CUMS mice were increased, and mixRL was found to reverse these effects. Additionally, the magnitude of long-term potentiation (LTP) was reduced in CUMS mice, and mixRL was shown to restore this parameter. In summary, mixRL appeared to exert its antidepressant effects and cognitive enhancing effects in a mouse model of depression, at least in part, by mechanisms involving modulation of glutamatergic transmission and LTP in the PFC.

Glutamatergic dysregulation in mood disorders: opportunities for the discovery of novel drug targets

INTRODUCTION

Recently, a considerable attention has been paid to glutamatergic conception of mood disorders. The development of new treatment strategies targeted at glutamate provides new opportunities for the treatment of mood disorders. It is expected that these novel therapeutic options will provide a fast and sustained antidepressant effect and will be better tolerated by patients than the currently available antidepressants.

AREAS COVERED

This paper discusses glutamatergic abnormalities in mood disorders and reviews novel glutamate-based drugs developed for the treatment of these disorders. We have searched the PubMed and EMBASE databases, presented the results of relevant clinical studies and also describe novel glutamate-based agents that are under investigation.

EXPERT OPINION

The glutamatergic system plays many important roles in energy metabolism of the brain and neurotransmission; therefore, any attempt to identify novel therapeutic targets within this system seems justified. The effective development of new glutamate-based drugs requires, among others, a more in-depth exploration and understanding of the anatomy, function, and localization of different glutamatergic receptors in the brain. In our opinion, novel glutamate-based antidepressants will find application in the treatment of mood disorders and present an option will be widely used in clinical practice in the future.

Ketamine: The final frontier or another depressing end?

Two decades ago, the observation of a rapid and sustained antidepressant response after ketamine administration provided an exciting new avenue in the search for more effective therapeutics for the treatment of clinical depression. Research elucidating the mechanism(s) underlying ketamine's antidepressant properties has led to the development of several hypotheses, including that of disinhibition of excitatory glutamate neurons via blockade of N-methyl-d-aspartate (NMDA) receptors. Although the prominent understanding has been that ketamine's mode of action is mediated solely via the NMDA receptor, this view has been challenged by reports implicating other glutamate receptors such as AMPA, and other neurotransmitter systems such as serotonin and opioids in the antidepressant response. The recent approval of esketamine (Spravato™) for the treatment of depression has sparked a resurgence of interest for a deeper understanding of the mechanism(s) underlying ketamine's actions and safe therapeutic use. This review aims to present our current knowledge on both NMDA and non-NMDA mechanisms implicated in ketamine's response, and addresses the controversy surrounding the antidepressant role and potency of its stereoisomers and metabolites. There is much that remains to be known about our understanding of ketamine's antidepressant properties; and although the arrival of esketamine has been received with great enthusiasm, it is now more important than ever that its mechanisms of action be fully delineated, and both the short- and long-term neurobiological/functional consequences of its treatment be thoroughly characterized.

The Effect of Glutamatergic Modulators on Extracellular Glutamate: How Does this Information Contribute to the Discovery of Novel Antidepressants?

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The complexity of glutamatergic signaling challenges glutamate modulator usage.

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Functional biomarkers are needed to understand the MOA of glutamate modulators.

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Evaluating drug effect on EAATs' kinetics may add to antidepressant discovery.

Background

In the search for new antidepressants, clinical researchers have been using drugs that simultaneously modulate multiple targets. During preclinical and clinical trials, the glutamatergic modulators riluzole and ketamine have received particular attention. Glutamatergic agents have a modulatory effect on synaptic transmission, so they can act on both neurons and astrocytes. In addition to influencing the quantity of glutamate released, these modulators can also affect the expression, localization, and functionality of glutamate-binding sites.

Objective

This review discusses the complexity of the glutamatergic system, the ambiguity of data regarding glutamate levels in patients with depression, as well as the mechanisms of action for riluzole and ketamine, which includes their relation to the physiology of glutamatergic transmission. The principal aim is to contribute to the development of novel glutamatergic antidepressant medications whilst emphasizing the need for innovative approaches that evaluate their effects on extracellular glutamate.

Methods

Literature was obtained via PubMed by searching the term depression in combination with each of the following terms: riluzole, ketamine, and glutamate. The search was restricted to full-text articles published in English between 1985 and 2018 relating to both the modulatory mechanisms of glutamatergic-binding proteins and the antidepressant actions of these medicines. Articles about mechanisms associated with synaptic plasticity and antidepressant effects were excluded.

Results

Although experimental data relates glutamatergic signaling to the pathophysiology of major depression and bipolar disorder, the role of glutamate—as well as its extracellular concentration in patients with said disorders—is still unclear. Riluzole's antidepressant action is ascribed to its capacity to reduce glutamate levels in the synaptic cleft, and ketamine's effect has been associated with increased extracellular glutamate levels.

Conclusions

The strategy of using glutamatergic modulators as therapeutic agents requires a better understanding of the role of glutamate in the pathophysiology of depression. Gaining such understanding is a challenge because it entails evaluating different targets as well as the effects of these modulators on the kinetics of glutamate uptake. Essentially, glutamate transport is a dynamic process and, currently, it is still necessary to develop new approaches to assay glutamate in the synaptic cleft. ORCID: 0000-0002-3358-6939.

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.

International Union of Basic and Clinical Pharmacology CIV: The Neurobiology of Treatment-resistant Depression: From Antidepressant Classifications to Novel Pharmacological Targets

Major depressive disorder is one of the most prevalent and life-threatening forms of mental illnesses and a major cause of morbidity worldwide. Currently available antidepressants are effective for most patients, although around 30% are considered treatment resistant (TRD), a condition that is associated with a significant impairment of cognitive function and poor quality of life. In this respect, the identification of the molecular mechanisms contributing to TRD represents an essential step for the design of novel and more efficacious drugs able to modify the clinical course of this disorder and increase remission rates in clinical practice. New insights into the neurobiology of TRD have shed light on the role of a number of different mechanisms, including the glutamatergic system, immune/inflammatory systems, neurotrophin function, and epigenetics. Advances in drug discovery processes in TRD have also influenced the classification of antidepressant drugs and novel classifications are available, such as the neuroscience-based nomenclature that can incorporate such advances in drug development for TRD. This review aims to provide an up-to-date description of key mechanisms in TRD and describe current therapeutic strategies for TRD before examining novel approaches that may ultimately address important neurobiological mechanisms not targeted by currently available antidepressants. All in all, we suggest that drug targeting different neurobiological systems should be able to restore normal function but must also promote resilience to reduce the long-term vulnerability to recurrent depressive episodes.

Intravenous infusion of xenon-containing liposomes generates rapid antidepressant-like effects

AIM

Similar to ketamine, xenon gas acts as a glutamatergic N-methyl-d-aspartate receptor antagonist, but devoid of propensity to cause untoward effects. Herein, we loaded xenon gas into a liposomal carrier called xenon-containing liposomes (Xe-liposome) for systemic delivery, and investigated its effect as an antidepressant and also analyzed synaptic biomarkers including brain-derived neurotrophic factor (BDNF), protein kinase B (AKT), mammalian target of rapamycin (mTOR), protein kinase C (PKC) and extracellular signal-regulated kinase-1/2 (ERK1/2) in blood and brain.

METHODS

Xe-liposomes (15 μl/mg) were prepared by a pressurized freeze-thaw method, and injected via the lateral tail vein (0.6 mL/rat) in male Wistar rats. The uncaging of xenon gas from circulating Xe-liposome was facilitated by continuous ultrasound application externally on the neck over the internal common carotid artery. One-hour after Xe-liposome infusion, animals were assessed for depression-like behaviors using a forced swimming test (FST), and spontaneous locomotor activity. Blood, as well as frontal cortex and hippocampal samples were obtained for immunoblotting and/or enzyme-linked immune sorbent assays.

RESULTS

Acute intravenous infusion of Xe-liposome, at 6 mg/kg, showed an increase in swimming time in the FST (p < 0.006), indicating antidepressant-like phenotypes. Higher doses of Xe-liposomes (9 mg/kg) failed to improve swimming duration. This behavioral discrepancy was not associated with locomotion aberrations, as gross activity of rats remained similar for both doses. In biochemical analyses of frontal cortex, protein levels of BDNF increased by 64%, and enhanced phosphorylation of AKT (43%) and mTOR (93%) was observed at the 6 mg/kg dose level of Xe-liposomes, while these biomarkers and phosphorylated PKC and ERK1/2 levels remained unchanged at the higher dose. Moreover, Xe-liposomal treatment did not change the plasma and protein levels of BDNF, and phosphorylated AKT, mTOR, PKC and ERK1/2 hippocampal expressions.

CONCLUSION

Xe-liposomes mediate a rapid antidepressant-like effect through activation of AKT/mTOR/BDNF signaling pathway.

Depression and schizophrenia viewed from the perspective of amino acidergic neurotransmission: Antipodes of psychiatric disorders

Depression and schizophrenia are burdensome, costly serious and disabling mental disorders. Moreover the existing treatments are not satisfactory. As amino-acidergic (AA) neurotransmitters built a vast majority of brain neurons, in this article we plan to focus on drugs influencing AA neurotransmission in both diseases: we will discuss several facts concerning glutamatergic and GABA-ergic neurotransmission in these diseases, based mainly on preclinical experiments that used stimulators and/or blockers of both neurotransmitter systems. In general a picture emerges showing, that treatments that increase excitatory effects (with either antagonists or agonists) tend to evoke antidepressant effects, while treatments that increase inhibitory effects tend to display antipsychotic properties. Moreover, it seems that the antidepressant activity of a given compound excludes it as a potential antipsychotic and vice versa.

The potential antidepressant action and adverse effects profile of scopolamine co-administered with the mGlu7 receptor allosteric agonist AMN082 in mice

Scopolamine, a muscarinic cholinergic receptor antagonist, exerts fast and prolonged antidepressant effects in the clinic. In contrast, the current treatments for major depressive disorder (MDD) require long-term drug administration. On the other hand, the sole use of scopolamine might be related to the high risk of adverse effects. Therefore, it may be preferable to reduce its therapeutic dose. A new approach might include the co-administration of low-dose scopolamine with selected ligands of metabotropic glutamate (mGlu) receptors, which are known to possess antidepressant-like activity in several rodent tests and models of depression. The aim of the present study was to evaluate the potential antidepressant activity of low-dose scopolamine combined with an allosteric agonist of mGlu7 receptors, AMN082 in C57BL/6 mice. It was found that the combination of scopolamine (0.1 mg/kg) and AMN082 (1 mg/kg) exerted significant antidepressant-like effects in the tail suspension test (TST), but these effects were not observed in the mGlu7^-/- mice. Furthermore, low-dose AMN082 co-administered with low-doses scopolamine (0.03 and 0.1 mg/kg) induced antidepressant-like activity in the forced swim test (FST) in mice. The tested compounds did not affect locomotor activity and did not impair spatial memory in the Morris water maze (MWM) test or motor coordination in the rotarod test. The results strongly indicated that there is an enhanced antidepressant-like action of scopolamine by AMN082. Co-administration of scopolamine with AMN082 might be a new strategy with better efficacy and a lower risk of adverse effects compared with the sole use of scopolamine or AMN082.

The role of glutamatergic modulation in the mechanism of action of ketamine, a prototype rapid-acting antidepressant drug

Over the past decade, ketamine has been one of the most commonly studied potential antidepressants. This is because it produces a spectacularly rapid and persistent therapeutic effect in people suffering from severe treatment-resistant depression (TRD), for which classical drugs are ineffective. Similar efficacy was demonstrated for scopolamine, a drug belonging to a completely different pharmacological group. This interesting coincidence piqued the interest of psychopharmacologists and prompted them to search for a possible common mechanism of these rapid acting antidepressant drugs (RAADs). A thorough explanation of this mechanism is also important because each of these substances induces serious side effects. Knowing the mechanism responsible for the therapeutic efficacy of RAADs could lead to minimizing, or even avoiding certain undesirable effects. This review provides an overview of the mechanism of action of a prototype RAAD, ketamine, in animal models, with a particular focus on the roles of NMDA receptors, AMPA receptors, synaptogenesis, and modulation of glutamate transmission by other modulators of this system, such as mGlu receptor ligands. Recently studied roles for ketamine enantiomers and metabolites in its rapid antidepressant effect are also considered. Finally, the results of multiple clinical trials are reported and discussed in relation to basic research. This review concludes that success in introducing novel therapeutic RAADs will depend on better cooperation and integration of neuroscience research and clinical practice.

Role of mGlu7 receptor in morphine rewarding effects is uncovered by a novel orthosteric agonist

Opiate dependence is a major health issue and despite the existence of opioid substitution treatment, relapse frequently occurs. Group III metabotropic glutamate (mGlu) receptors has received much attention as a putative target in ethanol and cocaine addiction, but no data on opiate addiction exist. So we investigated the role of group III mGlu receptors in morphine rewarding effects through the expression and the reinstatement of conditioned place preference (CPP) using a newly synthesized mGlu4/mGlu7 receptor orthosteric agonist, LSP2-9166. We found that LSP2-9166 blocked morphine CPP expression and reinstatement after extinction. Blockade of CPP expression with LSP2-9166 was abolished when using XAP044, a mGlu7 antagonist. We also found that LSP2-9166 at the dose active for blocking morphine reward was devoid of any effect on locomotion, hedonic state, spatial memory, anxiety or depression. Altogether our data demonstrated that group III mGlu receptors, and more specifically mGlu7, might be a valuable target in opiate addiction.

New drugs in psychiatry: focus on new pharmacological targets

The approval of psychotropic drugs with novel mechanisms of action has been rare in recent years. To address this issue, further analysis of the pathophysiology of neuropsychiatric disorders is essential for identifying new pharmacological targets for psychotropic medications. In this report, we detail drug candidates being examined as treatments for psychiatric disorders. Particular emphasis is placed on agents with novel mechanisms of action that are being tested as therapies for depression, schizophrenia, or Alzheimer’s disease. All of the compounds considered were recently approved for human use or are in advanced clinical trials. Drugs included here are new antipsychotic medications endowed with a preferential affinity at dopamine D3 receptor (cariprazine) or at glutamatergic or cannabinoid receptors, as well as vortioxetine, a drug approved for managing the cognitive deficits associated with major depression. New mechanistic approaches for the treatment of depression include intravenous ketamine or esketamine or intranasal esketamine. As for Alzheimer’s disease, the possible value of passive immunotherapy with agents such as aducanumab is considered to be a potential disease-modifying approach that could slow or halt the progressive decline associated with this devastating disorder.