N-Methyl-D-aspartate receptor antagonist d-methadone produces rapid, mTORC1-dependent antidepressant effects

The N-methyl-d-aspartate receptor hypothesis of ketamine's antidepressant action: evidence and controversies

Substantial clinical evidence has unravelled the superior antidepressant efficacy of ketamine: in comparison to traditional antidepressants targeting the monoamine systems, ketamine, as an N-methyl-d-aspartate receptor (NMDAR) antagonist, acts much faster and more potently. Surrounding the antidepressant mechanisms of ketamine, there is ample evidence supporting an NMDAR-antagonism-based hypothesis. However, alternative arguments also exist, mostly derived from the controversial clinical results of other NMDAR inhibitors. In this article, we first summarize the historical development of the NMDAR-centred hypothesis of rapid antidepressants. We then classify different NMDAR inhibitors based on their mechanisms of inhibition and evaluate preclinical as well as clinical evidence of their antidepressant effects. Finally, we critically analyse controversies and arguments surrounding ketamine's NMDAR-dependent and NMDAR-independent antidepressant action. A better understanding of ketamine's molecular targets and antidepressant mechanisms should shed light on the future development of better treatment for depression. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Mouse Model of Parkinson's Disease with Bilateral Dorsal Striatum Lesion with 6-Hydroxydopamine Exhibits Cognitive Apathy-like Behavior

Among the symptoms of Parkinson's disease (PD), apathy comprises a set of behavioral, affective, and cognitive features that can be classified into several subtypes. However, the pathophysiology and brain regions that are involved in these different apathy subtypes are still poorly characterized. We examined which subtype of apathy is elicited in a mouse model of PD with 6-hydroxydopamine (6-OHDA) lesions and the behavioral symptoms that are exhibited. Male C57/BL6J mice were allocated to sham (n = 8) and 6-OHDA (n = 13) groups and locally injected with saline or 4 µg 6-OHDA bilaterally in the dorsal striatum. We then conducted motor performance tests and apathy-related behavioral experiments. We then pathologically evaluated tyrosine hydroxylase (TH) immunostaining. The 6-OHDA group exhibited significant impairments in motor function. In the behavioral tests of apathy, significant differences were observed between the sham and 6-OHDA groups in the hole-board test and novelty-suppressed feeding test. The 6-OHDA group exhibited impairments in inanimate novel object preference, whereas social preference was maintained in the three-chamber test. The number of TH+ pixels in the caudate putamen and substantia nigra compacta was significantly reduced in the 6-OHDA group. The present mouse model of PD predominantly showed dorsal striatum dopaminergic neuronal loss and a decrease in novelty seeking as a symptom that is related to the cognitive apathy component.

A Critical View on New and Future Antidepressants

For the first time after many decades, many new antidepressants have been approved and many more are under various stages of development and will soon be available in the market. The new drugs present a range of new mechanisms of action with benefits in terms of speed of action, tolerability and range of treatable disorders. Neurosteroids have been recently approved and their rapid benefit may extend from postpartum depression to anxious depression and bipolar depression, dextromethorphan and bupropion combination may prove useful in major depression but also in treatment resistant depression, dextromethadone is a possible augmentation in partial antidepressant response, psychedelic drugs have the potential of long lasting benefits after a single administration, though are still experimental treatments. Botulinum has the same advantage of psychedelics of a single administration and its antidepressant effects may last for weeks or more. Further potentially interesting new antidepressant mechanisms include new drug targets, drug repurposing and genetic or epigenetic manipulations. It is therefore important that clinicians are kept up to date with new evidence so that new evidence can be rapidly translated into clinical practice.

Repurposing Ketamine in the Therapy of Depression and Depression-Related Disorders: Recent Advances and Future Potential

Depression represents a prevalent and enduring mental disorder of significant concern within the clinical domain. Extensive research indicates that depression is very complex, with many interconnected pathways involved. Most research related to depression focuses on monoamines, neurotrophic factors, the hypothalamic-pituitary-adrenal axis, tryptophan metabolism, energy metabolism, mitochondrial function, the gut-brain axis, glial cell-mediated inflammation, myelination, homeostasis, and brain neural networks. However, recently, Ketamine, an ionotropic N-methyl-D-aspartate (NMDA) receptor antagonist, has been discovered to have rapid antidepressant effects in patients, leading to novel and successful treatment approaches for mood disorders. This review aims to summarize the latest findings and insights into various signaling pathways and systems observed in depression patients and animal models, providing a more comprehensive view of the neurobiology of anxious-depressive-like behavior. Specifically, it highlights the key mechanisms of ketamine as a rapid-acting antidepressant, aiming to enhance the treatment of neuropsychiatric disorders. Moreover, we discuss the potential of ketamine as a prophylactic or therapeutic intervention for stress-related psychiatric disorders.

Sex-dependent effects of the uncompetitive N-methyl-D-aspartate receptor antagonist REL-1017 in G93A-SOD1 amyotrophic lateral sclerosis mice

Introduction

The pathogenesis of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the demise of motor neurons has been linked to excitotoxicity caused by excessive calcium influx via N-methyl-D-aspartate receptors (NMDARs), suggesting that uncompetitive NMDAR antagonism could be a strategy to attenuate motor neuron degeneration. REL-1017, the dextro-isomer of racemic methadone, is a low-affinity uncompetitive NMDAR antagonist. Importantly, in humans REL-1017 has shown excellent tolerability in clinical trials for major depression.

Methods

Here, we tested if REL-1017 improves the disease phenotypes in the G93A SOD1 mouse, a well-established model of familial ALS, by examining survival and motor functions, as well as the expression of genes and proteins involved in neuroplasticity.

Results

We found a sex-dependent effect of REL-1017 in G93A SOD1 mice. A delay of ALS symptom onset, assessed as 10%-decrease of body weight (p < 0.01 vs. control untreated mice) and an extension of lifespan (p < 0.001 vs. control untreated mice) was observed in male G93A SOD1 mice. Female G93A SOD1 mice treated with REL-1017 showed an improvement of muscle strength (p < 0.01 vs. control untreated mice). Both males and females treated with REL-1017 showed a decrease in hind limb clasping. Sex-dependent effects of REL-1017 were also detected in molecular markers of neuronal plasticity (PSD95 and SYN1) in the spinal cord and in the GluN1 NMDAR subunit in quadricep muscles.

Conclusion

In conclusion, this study provides preclinical in vivo evidence supporting the clinical evaluation of REL-1017 in ALS.

Unique pharmacodynamic properties and low abuse liability of the µ-opioid receptor ligand (S)-methadone

(R,S)-methadone ((R,S)-MTD) is a µ-opioid receptor (MOR) agonist comprised of (R)-MTD and (S)-MTD enantiomers. (S)-MTD is being developed as an antidepressant and is considered an N-methyl-D-aspartate receptor (NMDAR) antagonist. We compared the pharmacology of (R)-MTD and (S)-MTD and found they bind to MORs, but not NMDARs, and induce full analgesia. Unlike (R)-MTD, (S)-MTD was a weak reinforcer that failed to affect extracellular dopamine or induce locomotor stimulation. Furthermore, (S)-MTD antagonized motor and dopamine releasing effects of (R)-MTD. (S)-MTD acted as a partial agonist at MOR, with complete loss of efficacy at the MOR-galanin Gal1 receptor (Gal1R) heteromer, a key mediator of the dopaminergic effects of opioids. In sum, we report novel and unique pharmacodynamic properties of (S)-MTD that are relevant to its potential mechanism of action and therapeutic use. One-sentence summary: (S)-MTD, like (R)-MTD, binds to and activates MORs in vitro, but (S)-MTD antagonizes the MOR-Gal1R heteromer, decreasing its abuse liability.

Major depressive disorder: hypothesis, mechanism, prevention and treatment

Worldwide, the incidence of major depressive disorder (MDD) is increasing annually, resulting in greater economic and social burdens. Moreover, the pathological mechanisms of MDD and the mechanisms underlying the effects of pharmacological treatments for MDD are complex and unclear, and additional diagnostic and therapeutic strategies for MDD still are needed. The currently widely accepted theories of MDD pathogenesis include the neurotransmitter and receptor hypothesis, hypothalamic-pituitary-adrenal (HPA) axis hypothesis, cytokine hypothesis, neuroplasticity hypothesis and systemic influence hypothesis, but these hypothesis cannot completely explain the pathological mechanism of MDD. Even it is still hard to adopt only one hypothesis to completely reveal the pathogenesis of MDD, thus in recent years, great progress has been made in elucidating the roles of multiple organ interactions in the pathogenesis MDD and identifying novel therapeutic approaches and multitarget modulatory strategies, further revealing the disease features of MDD. Furthermore, some newly discovered potential pharmacological targets and newly studied antidepressants have attracted widespread attention, some reagents have even been approved for clinical treatment and some novel therapeutic methods such as phototherapy and acupuncture have been discovered to have effective improvement for the depressive symptoms. In this work, we comprehensively summarize the latest research on the pathogenesis and diagnosis of MDD, preventive approaches and therapeutic medicines, as well as the related clinical trials.

Ketamine and the neurobiology of depression: Toward next-generation rapid-acting antidepressant treatments

Ketamine has emerged as a transformative and mechanistically novel pharmacotherapy for depression. Its rapid onset of action, efficacy for treatment-resistant symptoms, and protection against relapse distinguish it from prior antidepressants. Its discovery emerged from a reconceptualization of the neurobiology of depression and, in turn, insights from the elaboration of its mechanisms of action inform studies of the pathophysiology of depression and related disorders. It has been 25 y since we first presented our ketamine findings in depression. Thus, it is timely for this review to consider what we have learned from studies of ketamine and to suggest future directions for the optimization of rapid-acting antidepressant treatment.

Courtes A, Jones G, Soares JC, Machado-Vieira R. Pharmacotherapies Targeting GABA-Glutamate Neurotransmission for Treatment-Resistant Depression

Treatment-resistant depression (TRD) is a term used to describe a particular type of major depressive disorder (MDD). There is no consensus about what defines TRD, with various studies describing between 1 and 4 failures of antidepressant therapies, with or without electroconvulsive therapy (ECT). That is why TRD is such a growing concern among clinicians and researchers, and it explains the necessity for investigating novel therapeutic targets beyond conventional monoamine pathways. An imbalance between two primary central nervous system (CNS) neurotransmitters, L-glutamate and γ-aminobutyric acid (GABA), has emerged as having a key role in the pathophysiology of TRD. In this review, we provide an evaluation and comprehensive review of investigational antidepressants targeting these two systems, accessing their levels of available evidence, mechanisms of action, and safety profiles. N-methyl-D-aspartate (NMDA) receptor antagonism has shown the most promise amongst the glutamatergic targets, with ketamine and esketamine (Spravato) robustly generating responses across trials. Two specific NMDA-glycine site modulators, D-cycloserine (DCS) and apimostinel, have also generated promising initial safety and efficacy profiles, warranting further investigation. Combination dextromethorphan-bupropion (AXS-05/Auvelity) displays a unique mechanism of action and demonstrated positive results in particular applicability in subpopulations with cognitive dysfunction. Currently, the most promising GABA modulators appear to be synthetic neurosteroid analogs with positive GABAA receptor modulation (such as brexanolone). Overall, advances in the last decade provide exciting perspectives for those who do not improve with conventional therapies. Of the compounds reviewed here, three are approved by the Food and Drug Administration (FDA): esketamine (Spravato) for TRD, Auvelity (dextromethorphan-bupropion) for major depressive disorder (MDD), and brexanolone (Zulresso) for post-partum depression (PPD). Notably, some concerns have arisen with esketamine and brexanolone, which will be detailed in this study.

A synopsis of multitarget therapeutic effects of anesthetics on depression

Depression is a profound mental disorder that dampens the mood and undermines volition, which exhibited an increased incidence over the years. Although drug-based interventions remain the primary approach for depression treatment, the available medications still can't satisfy the patients. In recent years, the newly discovered therapeutic targets such as N-methyl-D-aspartate (NMDA) receptor, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor, and tyrosine kinase B (TrkB) have brought new breakthroughs in the development of antidepressant drugs. Moreover, it has come to light that certain anesthetics possess pharmacological mechanisms intricately linked to the aforementioned therapeutic targets for depression. At present, numerous preclinical and clinical studies have explored the therapeutic effects of anesthetic drugs such as ketamine, isoflurane, N2O, and propofol, on depression. These investigations suggested that these drugs can swiftly ameliorate patients' depression symptoms and engender long-term effects. In this paper, we provide a comprehensive review of the research progress and potential molecular mechanisms of various anesthetic drugs for depression treatment. By shedding light on this subject, we aim to facilitate the development and clinical implementation of new antidepressant drugs based on anesthetic medications.

Esmethadone-HCl (REL-1017): a promising rapid antidepressant

This review article presents select recent studies that form the basis for the development of esmethadone into a potential new drug. Esmethadone is a promising member of the pharmacological class of uncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists that have shown efficacy for major depressive disorder (MDD) and other diseases and disorders, such as Alzheimer's dementia and pseudobulbar affect. The other drugs in the novel class of NMDAR antagonists with therapeutic uses that are discussed for comparative purposes in this review are esketamine, ketamine, dextromethorphan, and memantine. We present in silico, in vitro, in vivo, and clinical data for esmethadone and other uncompetitive NMDAR antagonists that may advance our understanding of the role of these receptors in neural plasticity in health and disease. The efficacy of NMDAR antagonists as rapid antidepressants may advance our understanding of the neurobiology of MDD and other neuropsychiatric diseases and disorders.

Clinical specificity profile for novel rapid acting antidepressant drugs

Mood disorders are recurrent/chronic diseases with variable clinical remission rates. Available antidepressants are not effective in all patients and often show a relevant response latency, with a range of adverse events, including weight gain and sexual dysfunction. Novel rapid agents were developed with the aim of overcoming at least in part these issues. Novel drugs target glutamate, gamma-aminobutyric acid, orexin, and other receptors, providing a broader range of pharmacodynamic mechanisms, that is, expected to increase the possibility of personalizing treatments on the individual clinical profile. These new drugs were developed with the aim of combining a rapid action, a tolerable profile, and higher effectiveness on specific symptoms, which were relatively poorly targeted by standard antidepressants, such as anhedonia and response to reward, suicidal ideation/behaviours, insomnia, cognitive deficits, and irritability. This review discusses the clinical specificity profile of new antidepressants, namely 4-chlorokynurenine (AV-101), dextromethorphan-bupropion, pregn-4-en-20-yn-3-one (PH-10), pimavanserin, PRAX-114, psilocybin, esmethadone (REL-1017/dextromethadone), seltorexant (JNJ-42847922/MIN-202), and zuranolone (SAGE-217). The main aim is to provide an overview of the efficacy/tolerability of these compounds in patients with mood disorders having different symptom/comorbidity patterns, to help clinicians in the optimization of the risk/benefit ratio when prescribing these drugs.

The synergistic mechanism of action of Dajianzhong decoction in conjunction with ketamine in the treatment of depression

Depression is a multifactorial syndrome with a variety of underlying pathological mechanisms. While ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, exhibits a rapid antidepressant action in the central never system (CNS), the potential addiction and psychotomimetic adverse effects of ketamine limit its chronic use in clinical practice. Therefore, it is necessary to discover an additional agent that shows a synergistic antidepressant activity with ketamine to sustain its therapeutic action so as to reduce its use frequency in depression treatment. The present study indicated that Dajianzhong decoction (DJZT), an empirical herbal formula used for the clinical treatment of several inflammation-related intestinal disorders, sustains behavioral and synaptic action of ketamine in depressive mouse models. Additionally, ketamine was also demonstrated to exert a synergistic action with DJZT to alleviate the chronic unpredictable mild stress (CUMS)-induced abnormalities in gut barrier proteins and colonic histology, and subsequently to normalize the diversity and composition of gut microbiota. Furthermore, DJZT was shown to possess an anti-inflammatory activity to prevent activation of NF-κB from releasing proinflammatory cytokines, specifically through inhibiting Th17 cells/IL-17A pathway. Our results uncovered the mechanism of action of DJZT in conjunction with ketamine in depression treatment by which these agents target different pathological factors across biological systems and exert a synergistic activity through a bidirectional communication in the gut-brain axis, and also provided new insights into the systematic treatment of depression.

Zuojinwan ameliorates CUMS-induced depressive-like behavior through inducing ubiquitination of MyD88 via SPOP/MyD88/NF-κB pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Zuojinwan (ZJW) is a traditional Chinese medicine compound, which is often used clinically to treat gastritis and has anti-inflammatory activity. It was found that ZJW is involved in suppressing the expression of inflammatory factors, and neuroinflammation is thought to be associated with the development of depression.

AIM OF THE STUDY

In this study, we investigated whether ZJW could exert antidepressant effects by regulating MyD88 ubiquitination in depressed mice and attempted to elucidate the possible mechanisms.

MATERIALS AND METHODS

Six active compounds of Zuojinwan (ZJW) were identified by HPLC. Then, the effects of ZJW on depression-like behavior in mice were investigated by constructing a chronic unpredictable mild stimulation (CUMS) mouse model. Meanwhile, the effect of ZJW on hippocampal neurons was investigated by Nissl staining. In addition, western blotting, PCR, ELISA, co-immunoprecipitation and immunostaining were used to explore whether ZJW could inhibit neuroinflammation through SPOP/MyD88/NF-κB pathway and thus produce antidepressant effects. Finally, we constructed the AAV-Sh-SPOP virus vector to silence SPOP and verify the mechanism of ZJW's antidepressant action.

RESULTS

ZJW could dramatically ameliorate the depressive behavior induced by CUMS stimulation and alleviate hippocampal neuronal damage. CUMS stimulation resulted in decreased SPOP expression, impaired MyD88 ubiquitination, and activation of downstream NF-κB signaling, which could be reversed by ZJW. In addition, ZJW could significantly ameliorate the abnormal activation of microglia, and the excessive levels of pro-inflammatory factors were inhibited. By blocking the expression of SPOP, we found that ZJW exerted anti-inflammatory and antidepressant effects mainly by promoting the ubiquitination of MyD88 and inhibiting the activation of downstream inflammatory signals.

CONCLUSION

In conclusion, ZJW possesses alleviating effects on depression induced by CUMS stimulation. ZJW can inhibit neuroinflammation and improve neuroinflammation-induced depression-like behaviors through SPOP/MyD88/NF-κB pathway.

M1 acetylcholine receptors in somatostatin interneurons contribute to GABAergic and glutamatergic plasticity in the mPFC and antidepressant-like responses

Alterations in glutamatergic and GABAergic function in the medial prefrontal cortex (mPFC) are prevalent in individuals with major depressive disorder, resulting in impaired synaptic plasticity that compromises the integrity of signal transfer to limbic regions. Scopolamine, a non-selective muscarinic receptor antagonist, produces rapid antidepressant-like effects by targeting M1-type acetylcholine receptors (M1R) on somatostatin (SST) interneurons. So far, these effects have been investigated with relatively short-term manipulations, and long-lasting synaptic mechanisms involved in these responses are still unknown. Here, we generated mice with conditional deletion of M1R (M1f/fSstCre+) only in SST interneurons to determine the role of M1R in modulating long-term GABAergic and glutamatergic plasticity in the mPFC that leads to attenuation of stress-relevant behaviors. We have also investigated whether the molecular and antidepressant-like effects of scopolamine could be mimicked or occluded in male M1f/fSstCre+ mice. M1R deletion in SST-expressing neurons occluded the rapid and sustained antidepressant-like effects of scopolamine, as well as scopolamine-induced increases in c-Fos+/CaMKIIα cells and proteins necessary for glutamatergic and GABAergic function in the mPFC. Importantly, M1R SST deletion resulted in resilience to chronic unpredictable stress in behaviors relevant to coping strategies and motivation, and to a lesser extent, in behaviors relevant to avoidance. Finally, M1R SST deletion also prevented stress-induced impairments in the expression of GABAergic and glutamatergic markers in the mPFC. These findings suggest that the antidepressant-like effects of scopolamine result from modulation of excitatory and inhibitory plasticity via M1R blockade in SST interneurons. This mechanism could represent a promising strategy for antidepressant development.

The novel uncompetitive NMDA receptor antagonist esmethadone (REL-1017) has no meaningful abuse potential in recreational drug users

Esmethadone (REL-1017) is the opioid-inactive dextro-isomer of methadone and a low-affinity, low-potency uncompetitive NMDA receptor antagonist. In a Phase 2, randomized, double-blind, placebo-controlled trial, esmethadone showed rapid, robust, and sustained antidepressant effects. Two studies were conducted to evaluate the abuse potential of esmethadone. Each study utilized a randomized, double-blind, active-, and placebo-controlled crossover design to assess esmethadone compared with oxycodone (Oxycodone Study) or ketamine (Ketamine Study) in healthy recreational drug users. Esmethadone 25 mg (proposed therapeutic daily dose), 75 mg (loading dose), and 150 mg (Maximum Tolerated Dose) were evaluated in each study. Positive controls were oral oxycodone 40 mg and intravenous ketamine 0.5 mg/kg infused over 40 min. The Ketamine study included oral dextromethorphan 300 mg as an exploratory comparator. The primary endpoint was maximum effect (Emax) for Drug Liking, assessed using a bipolar 100-point visual analog scale (VAS). A total of 47 and 51 participants completed the Oxycodone Study and the Ketamine Study, respectively (Completer Population). In both studies, esmethadone doses ranging from therapeutic (25 mg) to 6 times therapeutic (150 mg) had a meaningful and statistically significantly (p < 0.001) lower Drug Liking VAS Emax compared with the positive control. Results were consistent for all secondary endpoints in both studies. In both studies, all doses of esmethadone were statistically equivalent to placebo on Drug Liking VAS Emax (p < 0.05). In the Ketamine Study, Drug Liking VAS Emax scores for esmethadone at all tested doses were significantly lower vs. dextromethorphan (p < 0.05) (exploratory endpoint). These studies indicate no meaningful abuse potential for esmethadone at all tested doses.

Role of Hippocampal miR-132-3p in Modifying the Function of Protein Phosphatase Mg2+/Mn2+-dependent 1 F in Depression

Depression is a common, severe, and debilitating psychiatric disorder of unclear etiology. Our previous study has shown that protein phosphatase Mg2+/Mn2+-dependent 1F (PPM1F) in the hippocampal dentate gyrus (DG) displays significant regulatory effects in depression-related behaviors. miR-132-3p plays a potential role in the etiology of depression. This study explored the effect of miR-132-3p on the onset of depression and the possible underlying mechanism for modulating PPM1F expression during the pathology of depression. We found that miR-132-3p levels in the hippocampus of depressed mice subjected to chronic unpredictable stress (CUS) were dramatically reduced, which were correlated with depression-related behaviors. Knockdown of miR-132-3p in hippocampal DG resulted in depression-related phenotypes and increased susceptibility to stress. miR-132-3p overexpression in hippocampal DG alleviated CUS-induced depression-related performance. We then screened out the potential target genes of miR-132-3p, and we found that the expression profiles of sterol regulatory element-binding transcription factor 1 (Srebf1) and forkhead box protein O3a (FOXO3a) were positively correlated with PPM1F under the condition of miR-132-3p knockdown. Finally, as anticipated, we revealed that the activities of Ca2+/calmodulin-dependent protein kinase II (CAMKII) and adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) were reduced, which underlies the target signaling pathway of PPM1F. In conclusion, our study suggests that miR-132-3p was designed to regulate depression-related behaviors by indirectly regulating PPM1F and targeting Srebf1 and FOXO3a, which have been linked to the pathogenesis and treatment of depression.

Baicalin attenuates chronic unpredictable mild stress-induced hippocampal neuronal apoptosis through regulating SIRT1/PARP1 signaling pathway

Baicalin (BA), a flavonoid glycoside extracts from Scutellaria baicalensis Georgi, has been reported to exert antidepressant effects. Emerging evidence indicates that neuronal apoptosis plays a crucial role in the pathogenesis of depression. Poly (ADP-ribose) polymerase-1 (PARP1) is established as a key regulator of the cellular apoptosis. In the present study, we explored whether BA exerts antidepressant effects by regulating PARP1 signaling pathway and elucidated the underlying mechanisms. We found that administration of BA (30 mg/kg, 60 mg/kg) alleviated chronic unpredictable mild stress (CUMS)-induced depressive-like behaviors by increasing sucrose consumption in sucrose preference test (SPT), improving activity status in open field test (OFT) and reducing rest time in tail suspension test (TST). Hematoxylin and eosin (HE) staining and Nissl staining showed that BA ameliorated CUMS-induced neuronal damage in the hippocampus. Moreover, BA significantly upregulated anti-apoptotic protein Bcl-2, downregulated pro-apoptotic protein Bax and cleaved-caspase-3 after CUMS in hippocampal of mice. Intriguingly, western blot and immunohistochemistry (IHC) results showed that the protein level of PARP1 was significantly increased in hippocampal tissue after CUMS, which was reversed by BA treatment. In primary hippocampal neurons (PHNs), BA abrogated the neuronal apoptosis caused by PARP1 overexpression. Meanwhile, BA significantly increased the protein level of SIRT1, SIRT1 inhibitor (EX-527) treatment reversed the effect of BA on reducing the protein level of PARP1 and neuronal apoptosis in CUMS-induced mice. Overall, our results indicated that BA attenuated the CUMS-induced hippocampal neuronal apoptosis through regulating the SIRT1/PARP1 signaling pathway.

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'.

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.

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.

The effect of a methadone-initiated memory reconsolidation updating procedure in opioid use disorder: A translational study

BACKGROUND

Opioid use disorder (OUD) is a chronic relapsing psychiatric disorder. An unconditioned stimulus (US)-triggers a memory reconsolidation updating procedure (MRUP) that has been developed and demonstrated its effectiveness in decreasing relapse to cocaine and heroin in preclinical models. However, utilizations of abused drugs as the US to initiate MRUP can be problematic. We therefore designed a translational rat study and human study to evaluate the efficacy of a novel methadone-initiated MRUP.

METHODS

In the rodent study, male rats underwent heroin self-administration training for 10 consecutive days, and were randomly assigned to receive saline or methadone at 10 min, 1 h or 6 h before extinction training after 28-day withdrawal. The primary outcome was operant heroin seeking after reinstatement. In the human experimental study, male OUD patients were randomly assigned to get MRUP at 10 min or 6 h after methadone or methadone alone. The primary outcomes included experimental cue-induced heroin craving change, sustained abstinence and retention in the study at post intervention and the 5 monthly follow-up assessments. The secondary outcomes were changes in physiological responses including experimental cue-induced blood pressure and heart rate.

FINDINGS

Methadone exposure but not saline exposure at 10 min or 1 h before extinction decreased heroin-induced reinstatement of heroin seeking after 28-day of withdrawal in rats (F (8,80) = 8.26, p < 0.001). In the human study, when the MRUP was performed 10 min, but not 6 h after methadone dosing, the MRUP promoted sustained abstinence from heroin throughout 5 monthly follow-up assessments compared to giving methadone alone without MRUP (Hazard Ratio [95%CI] of 0.43 [0.22, 0.83], p = 0.01). The MRUP at 10 min, but not at 6 h after dosing also decreased experimental cue-induced heroin craving and blood pressure increases during the 6-month study duration (group × months × cue types, F (12, 63·3) = 2.41, p = 0.01).

INTERPRETATION

The approach of MRUP within about 1 to 6 h after a methadone dose potently improved several key outcomes of OUD patients during methadone maintenance treatment, and could be a potentially novel treatment to prevent opioid relapse.

FUNDING

National Natural Science Foundation of China (NO. U1802283, 81761128036, 82001400, 82001404 and 31671143) and Chinese National Programs for Brain Science and Brain-like Intelligence Technology (NO. 2021ZD0200800).

Alteration in NMDAR subunits in different brain regions of chronic unpredictable mild stress (CUMS) rat model

N-Methyl-d-aspartate receptor (NMDAR) signaling pathway has been implicated in the pathogenesis and treatment of depression. However, the role of NMDAR subunits in depression is still unclear. In this study, alteration in all seven NMDAR subunits in several brain areas of rats exposed to chronic unpredictable mild stress (CUMS), an animal model of depression, was detected. Our findings demonstrated that: (1) CUMS could induce a reduction in sucrose preference, an indicator of typical depression-like behaviors; (2) CUMS significantly reduced the NMDAR subunits of GluN2B and GluN3 in the medial prefrontal cortex (mPFC), but not altered all seven NMDAR subunits in hippocampus and corpus callosum of rats; (3) subunit composition of NMDARs in corpus callosum was different from that in mPFC, PFC and hippocampus; and (4) the mRNA expressions of GluN2B, GluN3A and GluN3B in mPFC as well as mRNA expression of GluN2C in corpus callosum were correlated to sucrose preference in rats. These findings suggested that GluN2B and GluN3 in mPFC may contribute to the pathophysiology of depression.

Esmethadone (REL-1017) and Other Uncompetitive NMDAR Channel Blockers May Improve Mood Disorders via Modulation of Synaptic Kinase-Mediated Signaling

This article presents a mechanism of action hypothesis to explain the rapid antidepressant effects of esmethadone (REL-1017) and other uncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists and presents a corresponding mechanism of disease hypothesis for major depressive disorder (MDD). Esmethadone and other uncompetitive NMDAR antagonists may restore physiological neural plasticity in animal models of depressive-like behavior and in patients with MDD via preferential tonic block of pathologically hyperactive GluN2D subtypes. Tonic Ca²⁺ currents via GluN2D subtypes regulate the homeostatic availability of synaptic proteins. MDD and depressive behaviors may be determined by reduced homeostatic availability of synaptic proteins, due to upregulated tonic Ca²⁺ currents through GluN2D subtypes. The preferential activity of low-potency NMDAR antagonists for GluN2D subtypes may explain their rapid antidepressant effects in the absence of dissociative side effects.

Targeting NMDA Receptors at the Neurovascular Unit: Past and Future Treatments for Central Nervous System Diseases

The excitatory neurotransmission of the central nervous system (CNS) mainly involves glutamate and its receptors, especially N-methyl-D-Aspartate receptors (NMDARs). These receptors have been extensively described on neurons and, more recently, also on other cell types. Nowadays, the study of their differential expression and function is taking a growing place in preclinical and clinical research. The diversity of NMDAR subtypes and their signaling pathways give rise to pleiotropic functions such as brain development, neuronal plasticity, maturation along with excitotoxicity, blood-brain barrier integrity, and inflammation. NMDARs have thus emerged as key targets for the treatment of neurological disorders. By their large extracellular regions and complex intracellular structures, NMDARs are modulated by a variety of endogenous and pharmacological compounds. Here, we will present an overview of NMDAR functions on neurons and other important cell types involved in the pathophysiology of neurodegenerative, neurovascular, mental, autoimmune, and neurodevelopmental diseases. We will then discuss past and future development of NMDAR targeting drugs, including innovative and promising new approaches.

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.

Pharmacological Comparative Characterization of REL-1017 (Esmethadone-HCl) and Other NMDAR Channel Blockers in Human Heterodimeric N-Methyl-D-Aspartate Receptors

Excessive Ca²⁺ currents via N-methyl-D-aspartate receptors (NMDARs) have been implicated in many disorders. Uncompetitive NMDAR channel blockers are an emerging class of drugs in clinical use for major depressive disorder (MDD) and other neuropsychiatric diseases. The pharmacological characterization of uncompetitive NMDAR blockers in clinical use may improve our understanding of NMDAR function in physiology and pathology. REL-1017 (esmethadone-HCl), a novel uncompetitive NMDAR channel blocker in Phase 3 trials for the treatment of MDD, was characterized together with dextromethorphan, memantine, (±)-ketamine, and MK-801 in cell lines over-expressing NMDAR subtypes using fluorometric imaging plate reader (FLIPR), automated patch-clamp, and manual patch-clamp electrophysiology. In the absence of Mg²⁺, NMDAR subtypes NR1-2D were most sensitive to low, sub-μM glutamate concentrations in FLIPR experiments. FLIPR Ca²⁺ determination demonstrated low μM affinity of REL-1017 at NMDARs with minimal subtype preference. In automated and manual patch-clamp electrophysiological experiments, REL-1017 exhibited preference for the NR1-2D NMDAR subtype in the presence of 1 mM Mg²⁺ and 1 μM L-glutamate. Tau off and trapping characteristics were similar for (±)-ketamine and REL-1017. Results of radioligand binding assays in rat cortical neurons correlated with the estimated affinities obtained in FLIPR assays and in automated and manual patch-clamp assays. In silico studies of NMDARs in closed and open conformation indicate that REL-1017 has a higher preference for docking and undocking the open-channel conformation compared to ketamine. In conclusion, the pharmacological characteristics of REL-1017 at NMDARs, including relatively low affinity at the NMDAR, NR1-2D subtype preference in the presence of 1 mM Mg²⁺, tau off and degree of trapping similar to (±)-ketamine, and preferential docking and undocking of the open NMDAR, could all be important variables for understanding the rapid-onset antidepressant effects of REL-1017 without psychotomimetic side effects.

The N-Methyl-D-Aspartate Receptor Blocker REL-1017 (Esmethadone) Reduces Calcium Influx Induced by Glutamate, Quinolinic Acid, and Gentamicin

REL-1017 (esmethadone) is a novel N-methyl-D-aspartate receptor (NMDAR) antagonist and promising rapid antidepressant candidate. Using fluorometric imaging plate reader (FLIPR) assays, we studied the effects of quinolinic acid (QA) and gentamicin, with or without L-glutamate and REL-1017, on intracellular calcium ([Ca²⁺]_in) in recombinant cell lines expressing human GluN1-GluN2A, GluN1-GluN2B, GluN1-GluN2C, and GluN1-GluN2D NMDAR subtypes. There were no effects of QA on [Ca²⁺]_in in cells expressing GluN1-GluN2C subtypes. QA acted as a low-potency, subtype-selective, NMDAR partial agonist in GluN1-GluN2A, GluN1-GluN2B, and GluN1-GluN2D subtypes. REL-1017 reduced [Ca²⁺]_in induced by QA. In cells expressing the GluN1-GluN2D subtype, QA acted as an agonist in the presence of 0.04 μM L-glutamate and as an antagonist in the presence of 0.2 μM L-glutamate. REL-1017 reduced [Ca²⁺]_in induced by L-glutamate alone and with QA in all cell lines. In the absence of L-glutamate, gentamicin had no effect. Gentamicin was a positive modulator for GluN1-GluN2B subtypes at 10 μM L-glutamate, for GluN1-GluN2A at 0.2 μM L-glutamate, and for GluN1-GluN2A, GluN1-GluN2B, and GluN1-GluN2D at 0.04 μM L-glutamate. No significant changes were observed with GluN1-GluN2C NMDARs. REL-1017 reduced [Ca²⁺]_in induced by the addition of L-glutamate in all NMDAR cell lines in the presence or absence of gentamicin. In conclusion, REL-1017 reduced [Ca²⁺]_in induced by L-glutamate alone and when increased by QA and gentamicin. REL-1017 may protect cells from excessive calcium entry via NMDARs hyperactivated by endogenous and exogenous molecules.

REL-1017 (esmethadone; D-methadone) does not cause reinforcing effect, physical dependence and withdrawal signs in Sprague Dawley rats

REL-1017 (esmethadone, D-methadone) is the opioid-inactive d-isomer of racemic D,L-methadone. REL-1017 may exert antidepressant effects via uncompetitive N-methyl-D-aspartate receptor (NMDAR) channel block. As REL-1017 is expected to exert central nervous system activity, full characterization of its abuse potential is warranted. We evaluated lack of reinforcing effect, physical dependence, and withdrawal of REL-1017 in Sprague Dawley rats. (1) Self-administration Study Rats were trained to self-administer oxycodone intravenously (IV) and then were subjected to 3-day substitution tests where saline, oxycodone, and REL-1017 were self-delivered IV by a fixed number of lever presses; (2) Drug Discontinuation Study Rats were treated for 30 days by oral gavage with vehicle, REL-1017, ketamine or morphine and evaluated for withdrawal with functional observational batteries (FOBs). In the self-administration study, rats treated with saline, vehicle, and all REL-1017 doses showed the typical "extinction burst" pattern of response, characterized by an initial rapid increase of lever-pressing followed by a rapid decrease over 3 days. Rats treated with oxycodone maintained stable self-injection, as expected for reinforcing stimuli. In the withdrawal study, REL-1017 did not engender either morphine or ketamine withdrawal signs over 9 days following abrupt discontinuation of drug exposure. REL-1017 showed no evidence of abuse potential and did not engender withdrawal symptomatology.

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.

Investigational Drugs for the Treatment of Depression (Part 1): Monoaminergic, Orexinergic, GABA-Ergic, and Anti-Inflammatory Agents

Therapeutic management of depression has currently important limitations, and its low efficacy is reflected in high rates of non-response even after multiple trials of antidepressants. Almost two-thirds of the patients diagnosed with major depression who received a 4–6 weeks trial of antidepressant could not reach remission, and more than 30% of these patients are considered treatment-resistant. In bipolar depression, the situation is also discouraging if we analyze the high suicide rate, the risk for the treatment-emergent affective switch when antidepressants are added, the high rate of treatment resistance (up to 25%), and the severe functional impairments associated with these episodes. Therefore, new therapeutic agents are needed, as well as new pathogenetic models for depression. The vast majority of the currently approved antidepressants are based on the monoamine hypothesis, although new drugs exploiting different neurotransmitter pathways have been recently approved by FDA. Brexanolone, an allopregnanolone analog, is an example of such new antidepressants, and its approval for post-partum depression inspired the search for a new generation of neurosteroids and GABA-ergic modulators, with an easier way of administration and superior tolerability profile. Orexin receptors antagonists are also extensively studied for different psychiatric disorders, depression included, in phase II trials. Antiinflammatory drugs, both cyclo-oxygenase 2 inhibitors and biological therapy, are investigated in patients with depressive disorders based on the proven correlation between inflammation and mood disorders in preclinical and clinical studies. Also, a new generation of monoamine-based investigational drugs is explored, ranging from triple reuptake inhibitors to atypical antipsychotics, in patients with major depression. In conclusion, there is hope for new treatments in uni- and bipolar depression, as it became clear, after almost seven decades, that new pathogenetic pathways should be targeted to increase these patients’ response rate.

Three Mutations Convert the Selectivity of a Protein Sensor from Nicotinic Agonists to S-Methadone for Use in Cells, Organelles, and Biofluids

We report a reagentless, intensity-based S-methadone fluorescent sensor, iS-methadoneSnFR, consisting of a circularly permuted GFP inserted within the sequence of a mutated bacterial periplasmic binding protein (PBP). We evolved a previously reported nicotine-binding PBP to become a selective S-methadone-binding sensor, via three mutations in the PBP's second shell and hinge regions. iS-methadoneSnFR displays the necessary sensitivity, kinetics, and selectivity─notably enantioselectivity against R-methadone─for biological applications. Robust iS-methadoneSnFR responses in human sweat and saliva and mouse serum enable diagnostic uses. Expression and imaging in mammalian cells demonstrate that S-methadone enters at least two organelles and undergoes acid trapping in the Golgi apparatus, where opioid receptors can signal. This work shows a straightforward strategy in adapting existing PBPs to serve real-time applications ranging from subcellular to personal pharmacokinetics.

REL-1017 (Esmethadone), A Novel NMDAR Blocker for the Treatment of MDD is Not Neurotoxic in Sprague-Dawley Rats

REL-1017 (esmethadone; dextromethadone; (S)-methadone) is the opioid-inactive dextro-isomer of the racemic mixture, (R, S)-methadone. REL-1017 acts as a low affinity, low potency N-methyl-D-aspartate receptor (NMDAR) channel blocker with rapid, robust, and sustained therapeutic effects in patients with major depressive disorder (MDD). Systemic administration of NMDAR blockers may cause transient and reversible pathomorphological alterations in brain cortical neurons characterized by cytoplasmic vacuolization, which are called Olney’s lesions, and may also lead to irreversible neuronal necrosis. We determined whether REL-1017 administration via oral gavage for 1–4 days to Sprague-Dawley rats could produce Olney’s lesions and cortical neuronal death and microgliosis as compared with MK-801, a known neurotoxic potent NMDAR blocker. As previously reported, MK-801 produced Olney’s lesions, neuronal necrosis and cortical microgliosis, and impaired behavior and activity. In contrast, administration of REL-1017 at low (20–31.25 mg/kg in females and males), medium (40–62.5 mg/kg) or high (80–110 mg/kg) doses did not cause pathomorphological changes in brain neurons and did not cause impaired behavior and activity. In conclusion, REL-1017 did not produce initial or cumulative neurotoxic effects or other evidence of damage to cortical neurons, further encouraging the development of REL-1017 as a potentially safe novel candidate for rapid treatment of MDD.

A new paradigm shift in antidepressant therapy? From dual-action to multitarget-directed ligands

Major Depressive Disorder is a chronic, recurring, and potentially fatal disease affecting up to 20% of the global population. Since the monoamine hypothesis was proposed more than 60 years ago, only a few relevant advances have been achieved, with very little disease course changing, from a pharmacological perspective. Moreover, since negative efficacy studies with novel molecules are frequent, many pharmaceutical companies have put new studies on hold. Fortunately, relevant clinical studies are currently being performed, and extensive striving is being developed by universities, research centers, and other public and private institutions. Depression is no longer considered a simple disease but a multifactorial one. New research fields are emerging in what could be a paradigm shift: the multitarget approach beyond monoamines. In this review, we summarize the present and the past of antidepressant drug discovery, with the aim to shed some light on the current state of the art in clinical and preclinical advances to face this increasingly devastating disease.

REL-1017 (Esmethadone) as Adjunctive Treatment in Patients With Major Depressive Disorder: A Phase 2a Randomized Double-Blind Trial

Objective: The purpose of this study was to examine the effects of REL-1017 (esmethadone), a novel N-methyl-d-aspartate receptor (NMDAR) channel blocker, in patients with major depressive disorder who failed to benefit from one to three standard antidepressant treatments in their current major depressive episode. Methods: A 7-day phase 2 multicenter randomized double-blind placebo-controlled trial, comprising three arms, was conducted to assess the safety, tolerability, pharmacokinetics, and efficacy of two dosages of REL-1017 (25 mg or 50 mg orally once a day). Patients were randomly assigned in a 1:1:1 ratio to placebo (N=22), REL-1017 25 mg/day (N=19), or REL-1017 50 mg/day (N=21). Safety scales included the 4-item Positive Symptom Rating Scale for psychotomimetic symptoms, the Clinician-Administered Dissociative States Scale for dissociative symptoms, the Clinical Opiate Withdrawal Scale for withdrawal signs and symptoms, and the Columbia-Suicide Severity Rating Scale for suicidality. The primary efficacy endpoint was the Montgomery-Åsberg Depression Scale (MADRS) score. All 62 randomly assigned patients were included in the full analysis set population analysis. Results: Patients experienced mild or moderate transient adverse events and no evidence of dissociative or psychotomimetic effects, opioid effects, or withdrawal signs and symptoms. The improvement in MADRS score shown on day 4 in both of the REL-1017 dosage groups was sustained through day 7 (last dose) and day 14 (7 days after the last dose), with effect sizes from 0.7 to 1.0. Conclusions: This trial showed favorable safety, tolerability, and pharmacokinetic profiles and suggests that REL-1017 may have rapid and sustained antidepressant effects compared with placebo in patients with inadequate responses to antidepressant treatments. These results will need confirmation in larger and longer trials.

Novel drug developmental strategies for treatment-resistant depression

Major depressive disorder is a leading cause of disability worldwide. Because conventional therapies are ineffective in many patients, novel strategies are needed to overcome treatment‐resistant depression (TRD). Limiting factors of successful drug development in the last decades were the lack of (1) knowledge of pathophysiology, (2) translational animal models and (3) objective diagnostic biomarkers. Here, we review novel drug targets and drug candidates currently investigated in Phase I–III clinical trials. The most promising approaches are inhibition of glutamatergic neurotransmission by NMDA and mGlu₅ receptor antagonists, modulation of the opioidergic system by κ receptor antagonists, and hallucinogenic tryptamine derivates. The only registered drug for TRD is the NMDA receptor antagonist, S‐ketamine, but add‐on therapies with second‐generation antipsychotics, certain nutritive, anti‐inflammatory and neuroprotective agents seem to be effective. Currently, there is an intense research focus on large‐scale, high‐throughput omics and neuroimaging studies. These results might provide new insights into molecular mechanisms and potential novel therapeutic strategies.

MiR-139-5p has an antidepressant-like effect by targeting phosphodiesterase 4D to activate the cAMP/PKA/CREB signaling pathway

Background

Phosphodiesterase 4D (PDE4D) inhibitor is commonly used to treat depression, but side effects seriously decrease its efficacy. PDE4D was a downstream target mRNA of miR-139-5p. Therefore, we examined the effects of hippocampal miR-139-5p gain- and loss-of-function on depression-like behaviors, the expression level of PDE4D, and hippocampus neurogenesis.

Methods

Bioinformatic analyses were carried out to to screen differential genes. Quantitative real-time polymerase chain reaction (qRT-PCR) and luciferase reporter assay were used to confirm the relationship between miR-139-5p and PDE4D. MiR-139-5p mimics, miR-139-5p inhibitor, or miR-NC were used to explore the function of miR-139-5p in HT-22 cells. We further explored the role of miR-139-5p in vivo using AAV-injection. Elisa, western blotting, and fluorescence in situ hybridization (FISH) were used to detect the expression of miR-139-5p and PDE4D in CRC tissues.

Results

Here, we showed that PDE4D messenger RNA (mRNA) was a direct target of microRNA (miR)-139-5p, which was downregulated in a chronic ultra-mild stress (CUMS)-induced depression mouse model. Moreover, in experiments in vitro, miR-139-5p mimic repressed PDE4D expression in HT-22 cells, but promoted phosphorylated cyclic-AMP response element-binding protein (p-CREB) and brain-derived neurotrophic factor (BDNF) expression. Interestingly, adeno-associated virus (AAV)-miR-139-5p downregulated susceptibility to stress-induced depression-like behaviors in mice. AAV-miR-139-5p suppressed PDE4D in mouse hippocampal cells, increasing expression level of cyclic adenosine monophosphate (cAMP), p-CREB, and BDNF, and stimulating mouse hippocampal neurogenesis.

Conclusions

Our findings suggested that miR-139-5p acted like an antidepressant by targeting PDE4D, thereby regulating the cAMP/protein kinase A (PKA)/CREB/BDNF pathway to improve depression.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Neurobiology of the Rapid-Acting Antidepressant Effects of Ketamine: Impact and Opportunities

The discovery of the rapid-acting antidepressant effects of ketamine has 1) led to a paradigm shift in our perception of what is possible in treating severe depression; 2) spurred a wave of basic, translation, and clinical research; and 3) provided an unprecedented investigational tool to conduct longitudinal mechanistic studies that may capture behavioral changes as complex as clinical remission and relapse within hours and days of treatment. Unfortunately, these advances did not yet translate into clinical biomarkers or novel treatments, beyond ketamine. In contrast to slow-acting antidepressants, in which targeting monoaminergic receptors identified several efficacious drugs with comparable mechanisms, the focus on the receptor targets of ketamine has failed in several clinical trials over the past decade. Thus, it is becoming increasingly crucial that we concentrate our effort on the downstream molecular mechanisms of ketamine and their effects on the brain circuitry and networks. Honoring the legacy of our mentor, friend, and colleague Ron Duman, we provide a historical note on the discovery of ketamine and its putative mechanisms. We then detail the molecular and circuits effect of ketamine based on preclinical findings, followed by a summary of the impact of this work on our understanding of chronic stress pathology across psychiatric disorders, with particular emphasis on the role of synaptic connectivity and its brain network effects in the pathology and treatment of clinical depression.

Inhibition of GABA interneurons in the mPFC is sufficient and necessary for rapid antidepressant responses

Major depressive disorder (MDD) is associated with alterations of GABAergic interneurons, notably somatostatin (Sst) as well as parvalbumin (Pvalb), in cortical brain areas. In addition, the antidepressant effects of rapid-acting drugs are thought to occur via inhibition of GABA interneurons. However, the impact of these interneuron subtypes in affective behaviors as well as in the effects of rapid-acting antidepressants remains to be determined. Here, we used a Cre-dependent DREADD-chemogenetic approach to determine if inhibition of GABA interneurons in the mPFC of male mice is sufficient to produce antidepressant actions, and conversely if activation of these interneurons blocks the rapid and sustained antidepressant effects of scopolamine, a nonselective acetylcholine muscarinic receptor antagonist. Chemogenetic inhibition of all GABA interneurons (Gad1+), as well as Sst+ and Pvalb+ subtypes in the mPFC produced dose and time-dependent antidepressant effects in the forced swim and novelty suppressed feeding tests, and increased synaptic plasticity. In contrast, stimulation of Gad1, Sst, or Pvalb interneurons in mPFC abolished the effects of scopolamine and prevented scopolamine induction of synaptic plasticity. The results demonstrate that transient inhibition of GABA interneurons promotes synaptic plasticity that underlies rapid antidepressant responses.

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.

REL-1017 (Esmethadone) Increases Circulating BDNF Levels in Healthy Subjects of a Phase 1 Clinical Study

Brain-derived neurotrophic factor (BDNF), a neurotrophin widely expressed in the central nervous system, exhibits important effects on neural plasticity. BDNF has been implicated in the mechanism of action of ketamine, a N-methyl-d-aspartic acid receptor (NMDAR) antagonist with rapid anti-depressant effects in humans. REL-1017 (esmethadone), the d-optical isomer of the racemic mixture d-l-methadone, is devoid of clinically relevant opioid activity at doses expected to exert therapeutic NMDAR antagonistic activity in humans. The present study was conducted to ascertain the effects of oral administration of 25 mg of REL-1017 for 10 days on plasma BDNF in healthy subjects confined to an inpatient unit for a phase 1 clinical trial. We observed an increase in post-treatment BDNF plasma levels compared to pre-treatment levels. Post-treatment, Day 10 BDNF plasma levels ranged from 2 to 17 times pre-treatment levels in the 25 mg REL-1017 treatment group, whereas in the placebo group, BDNF plasma levels remained unchanged (p = 0.028). Diastolic blood pressure decreased significantly in subjects treated with REL-1017, while no effect could be observed in the placebo group. In conclusion, the administration of 25 mg REL-1017 significantly increased BDNF plasma levels and significantly decreased diastolic blood pressure in healthy subjects confined to an inpatient unit for a phase 1 clinical trial.

A single administration of ascorbic acid rapidly reverses depressive-like behavior and hippocampal synaptic dysfunction induced by corticosterone in mice

Ketamine is the prototype for glutamate-based fast-acting antidepressants. The establishment of ketamine-like drugs is still a challenge and ascorbic acid has emerged as a candidate. This study investigated the ascorbic acid's ability to induce a fast antidepressant-like response and to improve hippocampal synaptic markers in mice subjected to chronic corticosterone (CORT) administration. CORT was administered for 21 days, followed by a single administration of ascorbic acid (1 mg ∕Kg, p.o.), ketamine (1 mg ∕Kg, i.p.) or fluoxetine (10 mg ∕Kg, p.o.) in mice. Depressive-like behavior, hippocampal synaptic proteins immunocontent, dendrite spines density in the dentate gyrus (DG) were analyzed 24 h following treatments. The administration of ascorbic acid or ketamine, but not fluoxetine, counteracted CORT-induced depressive-like behavior in the tail suspension test (TST). CORT administration reduced PSD-95, GluA1, and synapsin (synaptic markers) immunocontent, and these alterations were reversed by ascorbic acid or ketamine, but only ketamine reversed the CORT-induced reduction on GluA1 immunocontent. In the ventral and dorsal DG, CORT decreased filopodia-, thin- and stubby-shaped spines, while ascorbic acid and ketamine abolished this alteration only in filopodia spines. Ascorbic acid and ketamine increased mushroom-shaped spines density in ventral and dorsal DG. Therefore, the results show that a single administration of ascorbic acid, in a way similar to ketamine, rapidly elicits an antidepressant-like response and reverses hippocampal synaptic deficits caused by CORT, an effect associated with increased levels of synaptic proteins and dendritic remodeling.

Sedative and analgesic effects of two subanaesthetic doses of ketamine in combination with methadone and a low dose of dexmedetomidine in healthy dogs

OBJECTIVE

To evaluate the sedative, analgesic and recovery characteristics of two subanaesthetic ketamine doses in combination with dexmedetomidine and methadone for intramuscular sedation in healthy Beagles.

STUDY DESIGN

Randomized, blinded, crossover, experimental study.

ANIMALS

Six healthy adult Beagles.

METHODS

Dogs were randomly given three treatments: dexmedetomidine (3 μg kg^-1) and methadone (0.3 mg kg^-1) combined with ketamine at 1 and 2 mg kg^-1 (K1 and K2, respectively) or saline (K0), intramuscularly. Sedation score, response to tail clamping and rectal temperature were recorded at baseline, 5, 15, 25, 35, and 45 minutes posttreatment. Pulse rate (PR), respiratory rate, oxygen haemoglobin saturation and noninvasive blood pressure were also recorded at baseline and every 5 minutes until 45 minutes posttreatment. Onset and duration of recumbency, response to venous catheterization and recovery quality were also assessed. Sedation and physiological variables were compared between treatments and within treatments compared to baseline (analysis of variance). Nonparametric data were analysed with the Friedman and Cochran's Q tests; p < 0.050.

RESULTS

Increased sedation was found at 15 (K0 and K1), 25 (all treatments) and 35 (K1) minutes compared with baseline. Sedation score, onset (3-12 minutes) and duration of recumbency (29-51 minutes) were similar between treatments. Recovery quality was considered acceptable in all cases. Response to tail clamping was inconsistent within treatments with no differences between them. None of the dogs responded to venous catheterization. There were no differences between treatments in physiological variables, except for PR which was higher in K2 than in K0. Oxygen supplementation was required in five and three dogs administered saline and ketamine, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

The addition of 1 or 2 mg kg^-1 of ketamine to methadone and dexmedetomidine combination did not enhance sedation or antinociception in healthy dogs. Recovery quality was unaffected.

MTORC1 signaling as a biomarker in major depressive disorder and its pharmacological modulation by novel rapid-acting antidepressants

Major depressive disorder (MDD) is a multifactorial psychiatric disorder with obscure pathophysiology. A biomarker-based approach in combination with standardized interview-based instruments is needed to identify MDD subtypes and novel therapeutic targets. Recent findings support the impairment of the mammalian target of rapamycin complex 1 (mTORC1) in MDD. No well-established biomarkers of mTORC1 disease- and treatment-modulated activity are currently available for use in early phase antidepressant drug (AD) development. This review aims to summarize biomarkers of mTORC1 activity in MDD and to suggest how these could be implemented in future early clinical trials on mTORC1 modulating ADs. Therefore, a PubMed-based narrative literature review of the mTORC1 involvement in MDD was performed. We have summarized recent pre-clinical and clinical findings linking the MDD to the impaired activity of several key biomarkers related to mTORC1. Also, cases of restoration of these impairments by classical ADs and novel fast-acting investigational ADs are summarized. The presented biomarkers may be used to monitor pharmacological effects by novel rapid-acting mTORC1-targeting ADs. Based on findings in the peripheral blood mononuclear cells, we argue that those may serve as an ex vivo model for evaluation of mTORC1 activity and propose the use of the summarized biomarkers for this purpose. This could both facilitate the selection of a pharmacodynamically active dose and guide future early clinical efficacy studies in MDD. In conclusion, this review provides a blueprint for the rational development of rapid-acting mTORC1-targeting ADs.

The physiology of regulated BDNF release

The neurotrophic factor BDNF is an important regulator for the development of brain circuits, for synaptic and neuronal network plasticity, as well as for neuroregeneration and neuroprotection. Up- and downregulations of BDNF levels in human blood and tissue are associated with, e.g., neurodegenerative, neurological, or even cardiovascular diseases. The changes in BDNF concentration are caused by altered dynamics in BDNF expression and release. To understand the relevance of major variations of BDNF levels, detailed knowledge regarding physiological and pathophysiological stimuli affecting intra- and extracellular BDNF concentration is important. Most work addressing the molecular and cellular regulation of BDNF expression and release have been performed in neuronal preparations. Therefore, this review will summarize the stimuli inducing release of BDNF, as well as molecular mechanisms regulating the efficacy of BDNF release, with a focus on cells originating from the brain. Further, we will discuss the current knowledge about the distinct stimuli eliciting regulated release of BDNF under physiological conditions.

Rislenemdaz treatment in the lateral habenula improves despair-like behavior in mice

The specific GluN2B antagonist rislenemdaz (Ris; a.k.a. MK-0657 and CERC-301) is in phase II clinical trial as an antidepressive drug, but the working mechanism for its antidepressant effects is not clearly understood. Given the important role of the lateral habenula (LHb) in the pathogenesis of depression and the fact that GluN2B-containing N-methyl-D-aspartate receptors and brain-derived neurotrophic factor (BDNF) are expressed in the LHb, we conducted a study to examine whether the LHb mediates Ris' antidepressant effects in a chronic restraint stress (CRS)-induced depressive-like mouse model. In this study, Ris was administered systemically or locally into the LHb. Short hairpin RNAs were used to knockdown BDNF in the LHb. Depressive-like behaviors were assessed with the open field test, forced swimming test, tail suspension test, and sucrose preference test. Expression of GluN2B, BDNF, and c-Fos in the LHb were analyzed with western blotting and immunohistochemistry under condition with Ris administered systemically or with BDNF knockdown in the LHb. We found that both systemic and intra-LHb administration of Ris alleviated CRS-induced despair-like behavior and that systemic Ris reduced LHb expression of GluN2B, BDNF, and c-Fos (a neuronal activity marker). Specific knockdown of BDNF in the LHb prevented CRS-induced despair-like behavior, while preventing CRS-induced increases in BDNF and c-Fos expression in the LHb. Together these results suggest that Ris may exert its antidepressant effects through affecting the LHb such as downregulating BDNF expression in the LHb.

Effects of ketamine and other rapidly acting antidepressants on hippocampal excitatory and inhibitory transmission

A single sub-anesthetic intravascular dose of the use-dependent NMDAR antagonist, ketamine, improves mood in patients with treatment resistant depression within hours that can last for days, creating an entirely new treatment strategy for the most seriously ill patients. However, the psychomimetic effects and abuse potential of ketamine require that new therapies be developed that maintain the rapid antidepressant effects of ketamine without the unwanted side effects. This necessitates a detailed understanding of what cellular and synaptic mechanisms are immediately activated once ketamine reaches the brain that triggers the needed changes to elicit the improved behavior. Intense research has centered on the effects of ketamine, and the other rapidly acting antidepressants, on excitatory and inhibitory circuits in hippocampus and medial prefrontal cortex to determine common mechanisms, including key modifications in synaptic transmission and the precise location of the NMDARs that mediate the rapid and sustained antidepressant response. We review data comparing the effects of ketamine with other NMDAR receptor modulators and the muscarinic M1 acetylcholine receptor antagonist, scopolamine, together with evidence supporting the disinhibition hypothesis and the direct inhibition hypothesis of ketamine's mechanism of action on synaptic circuits using preclinical models.

Traditional Chinese Medicine in Depression Treatment: From Molecules to Systems

Depression is a multigenetic or multifactorial syndrome. The central neuron system (CNS)-orientated, single target, and conventional antidepressants are insufficient and far from ideal. Traditional Chinese Medicine (TCM) has historically been used to treat depression up till today, particularly in Asia. Its holistic, multidrug, multitarget nature fits well with the therapeutic idea of systems medicine in depression treatment. Over the past two decades, although efforts have been made to understand TCM herbal antidepressants at the molecular level, many fundamental questions regarding their mechanisms of action remain to be addressed at the systems level in order to better understand the complicated herbal formulations in depression treatment. In this Mini Review, we review and discuss the mechanisms of action of herbal antidepressants and their acting targets in the pathological systems in the brain, such as monoamine neurotransmissions, hypothalamic–pituitary–adrenal (HPA) axis, neurotropic factor brain-derived neurotrophic factor (BDNF) cascade, and glutamate transmission. Some herbal molecules, constituents, and formulas are highlighted as examples to discuss their mechanisms of action and future directions for comprehensive researches at the systems level. Furthermore, we discuss pharmacological approaches to integrate the mechanism of action from the molecular level into the systems level for understanding of systems pharmacology of TCM formulations. Integration of the studies at the molecular level into the systems level not only represents a trend in TCM study but also promotes our understanding of the system-wide mechanism of action of herbal antidepressant formulations.