Dextran sulfate sodium-induced inflammation and colitis in mice are ameliorated by (R)-ketamine, but not (S)-ketamine: A role of TrkB signaling

Linagliptin mitigates lipopolysaccharide-induced acute kidney injury in mice: Novel renal BDNF/TrkB/NRF2-dependent antioxidant, anti-inflammatory, and antiapoptotic mechanisms

Acute kidney injury (AKI) is a common complication associated with sepsis, yet no effective treatment is currently available. The primary mechanisms involved in lipopolysaccharide (LPS)-induced septic AKI are oxidative stress, inflammation, and apoptosis. This study aimed to investigate the potential renoprotective effects of linagliptin, an antidiabetic dipeptidyl peptidase (DPP)-4 inhibitor, against LPS-induced AKI with special emphasis on renal brain-derived neurotrophic factor (BDNF)/nuclear factor erythroid 2-related factor 2 (NRF2) axis. Mice were divided into control, LPS, LPS + linagliptin, and LPS + linagliptin+ANA-12 (tropomyosin receptor kinase B (TrkB) antagonist) groups. Our results revealed that linagliptin, partially through BDNF augmentation, ameliorated AKI, evidenced by the improved histological structure and function of the kidney where serum creatinine, blood urea nitrogen, cystatin C, and renal kidney injury molecule-1were decreased with increased serum albumin. These improvements result from glucagon-like peptide-1/BDNF/TrkB-mediated NRF2 activation, enhancing antioxidant, anti-inflammatory, and antiapoptotic pathways. Linagliptin, through NRF2 augmentation, suppressed renal myeloperoxidase, malondialdehyde, NLR Family pyrin domain-containing 3 inflammasome, nuclear factor-kappaB, tumor necrosis factor-alpha, monocyte chemoattractant protein-1, B-cell lymphoma 2 (Bcl2)-associated X protein, while boosting the antioxidant glutathione and the antiapoptotic Bcl2 contents. The administration of ANA-12 before linagliptin partially reversed these beneficial effects. Accordingly, our results suggest that linagliptin has therapeutic potential in managing LPS-induced AKI. Furthermore, they provide insights into its underlying mechanisms, highlighting renal BDNF signaling as a potential therapeutic target through downstream NRF2 enhancement and its associated antioxidant, anti-inflammatory, and antiapoptotic effects.

Esketamine mitigates endotoxin-induced hippocampal injury by regulating calcium transient and synaptic plasticity via the NF-α1/CREB pathway

Esketamine (ES) has been shown to confer neuroprotection partly by exerting anti-inflammation, alleviating oxidative stress, enhancing neuronal vitality, and promoting synaptic remodeling. Nonetheless, its precise function in SAE and the associated mechanisms are not understood. In this study, we investigated the neuroprotective potential of ES at behavioral, structural, and functional levels in vivo and in vitro. C57BL/6J mice administered with lipopolysaccharide (LPS) served as the research model and were injected with 10 mg/kg ES intraperitoneally. Fiber photometry was performed to record Ca2+ transients during behavioral assays. The neuronal dendritic architecture and synaptic plasticity were examined using the Golgi staining and transmission electron microscopy. Stereotactic administration of siRNA was performed to suppress the NF-α1 expression and determine the role of the NF-α1/CREB pathway in vitro. The neuroprotective effects of ES were verified in primary neurons and HT22 cells using a conditioned culture. The ES treatment alleviated sepsis symptoms, cognitive impairment, and decreased mortality. It also upregulated the NF-α1 expression in the hippocampal CA1 region and reduced neuroinflammation, oxidative stress, and neuronal loss. Moreover, ES treatment normalized the Ca2+ transients and improved dendritic structure as well as synaptic plasticity. However, NF-α1 knockdown p-CREB downregulation abolished the protective effects of ES. This also reversed the phenotypic characteristics of Ca2+ transients, dendritic structure, and post-synaptic plasticity. ES can abolish the LPS-induced hippocampal neurotoxicity in vitro and in vivo models and modulate neuronal Ca2+ transients and post-synaptic plasticity via the NF-α1/CREB signaling pathway. These findings provide a theoretical basis that will guide the future application of ES to treat hippocampal injury in sepsis.

Ketamine, Esketamine, and Arketamine: Their Mechanisms of Action and Applications in the Treatment of Depression and Alleviation of Depressive Symptoms

Research over the past years has compared the enantiomers (S)-ketamine (esketamine) and (R)-ketamine (arketamine) of the previously known racemic mixture called ketamine (R/S-ketamine). Esketamine has been found to be more potent, offering three times stronger analgesic effects and 1.5 times greater anesthetic efficacy than arketamine. It provides smoother anesthesia with fewer side effects and is widely used in clinical settings due to its neuroprotective, bronchodilatory, and antiepileptic properties. Approved by the FDA and EMA in 2019, esketamine is currently used alongside SSRIs or SNRIs for treatment-resistant depression (TRD). On the other hand, arketamine has shown potential for treating neurological disorders such as Alzheimer's, Parkinson's, and multiple sclerosis, offering possible antidepressant effects and anti-inflammatory benefits. While esketamine is already in clinical use, arketamine's future depends on further research to address its safety, efficacy, and optimal dosing. Both enantiomers hold significant clinical value, with esketamine excelling in anesthesia, and arketamine showing promise in neurological and psychiatric treatments.

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.

Exploring the multifaceted potential of (R)-ketamine beyond antidepressant applications

(R, S)- and (S)-ketamine have made significant progress in the treatment of treatment-resistant depression (TRD) and have become a research focus in recent years. However, they both have risks of psychomimetic effects, dissociative effects, and abuse liability, which limit their clinical use. Recent preclinical and clinical studies have shown that (R)-ketamine has a more efficient and lasting antidepressant effect with fewer side effects compared to (R, S)- and (S)-ketamine. However, a recent small-sample randomized controlled trial found that although (R)-ketamine has a lower incidence of adverse reactions in adult TRD treatment, its antidepressant efficacy is not superior to the placebo group, indicating its antidepressant advantage still needs further verification and clarification. Moreover, an increasing body of research suggests that (R)-ketamine might also have significant applications in the prevention and treatment of medical fields or diseases such as cognitive disorders, perioperative anesthesia, ischemic stroke, Parkinson's disease, multiple sclerosis, osteoporosis, substance use disorders, inflammatory diseases, COVID-19, and organophosphate poisoning. This article briefly reviews the mechanism of action and research on antidepressants related to (R)-ketamine, fully revealing its application potential and development prospects, and providing some references and assistance for subsequent expanded research.

Esketamine mitigates cognitive impairment following exposure to LPS by modulating the intestinal flora/subdiaphragmatic vagus nerve/spleen axis

INTRODUCTION

Susceptibility to secondary infection often increases after primary infection. Secondary infections can lead to more severe inflammatory injuries; however, the underlying mechanisms are not yet fully elucidated.

OBJECTIVE

To investigate whether esketamine treatment immediately after primary lipopolysaccharide (LPS) exposure could alleviate cognitive impairment caused by secondary infection.

METHODS

Mice were injected intraperitoneally (IP) with LPS (5 mg/kg) 10 days apart. Esketamine (10, 15, or 30 mg/kg) was administered IP immediately after the primary LPS injection. Splenectomy or subdiaphragmatic vagotomy (SDV) was performed 7 days before secondary LPS exposure or broad-spectrum antibiotic administration.

RESULTS

Splenomegaly was observed after the primary LPS injection on Days 3 and 10. Splenomegaly was attenuated by treatment with 30 mg/kg esketamine. Esketamine treatment prevented increased plasma proinflammatory cytokines levels and cognitive dysfunction induced by secondary LPS exposure. Mice that underwent splenectomy or SDV had lower proinflammatory cytokines levels, higher hippocampal brain-derived neurotrophic factor (BDNF) levels, and improved cognitive function 1 day after secondary infection, which was not further improved by esketamine. Fecal microbiota transplantation (FMT) from endotoxic mice treated with esketamine attenuated hippocampal BDNF downregulation and cognitive dysfunction only in pseudo germ-free (PGF) mice without splenectomy. FMT with fecal suspensions from esketamine-treated endotoxic mice abrogated splenomegaly only in PGF mice without SDV. Blocking BDNF signaling blocked esketamine's ameliorating effects on secondary LPS exposure-induced cognitive dysfunction.

CONCLUSION

The intestinal flora/subdiaphragmatic vagus nerve/spleen axis-mediated hippocampal BDNF downregulation significantly affected secondary LPS-induced systemic inflammation and cognitive dysfunction. Esketamine preserves cognitive function via this mechanism.

The role of CD38 in inflammation-induced depression-like behavior and the antidepressant effect of (R)-ketamine

CD38 is involved in immune responses, cell proliferation, and has been identified in the brain, where it is implicated in inflammation processes and psychiatric disorders. We hypothesized that dysfunctional CD38 activity in the brain may contribute to the pathogenesis of depression. To investigate the underlying mechanisms, we used a lipopolysaccharide (LPS)-induced depression-like model and conducted behavioral tests, molecular and morphological methods, along with optogenetic techniques. We microinjected adeno-associated virus into the hippocampal CA3 region with stereotaxic instrumentation. Our results showed a marked increase in CD38 expression in both the hippocampus and cortex of LPS-treated mice. Additionally, pharmacological inhibition and genetic knockout of CD38 effectively alleviated neuroinflammation, microglia activation, synaptic defects, and Sirt1/STAT3 signaling, subsequently improving depression-like behaviors. Moreover, optogenetic activation of glutamatergic neurons of hippocampal CA3 reduced the susceptibility of mice to depression-like behaviors, accompanied by reduced CD38 expression. We also found that (R)-ketamine, which displayed antidepressant effects, was linked to its anti-inflammatory properties by suppressing increased CD38 expression and reversing synaptic defects. In conclusion, hippocampal CD38 is closely linked to depression-like behaviors in an inflammation model, highlighting its potential as a therapeutic target for antidepressant development.

Bedside to bench: the outlook for psychedelic research

There has recently been a resurgence of interest in psychedelic compounds based on studies demonstrating their potential therapeutic applications in treating post-traumatic stress disorder, substance abuse disorders, and treatment-resistant depression. Despite promising efficacy observed in some clinical trials, the full range of biological effects and mechanism(s) of action of these compounds have yet to be fully established. Indeed, most studies to date have focused on assessing the psychological mechanisms of psychedelics, often neglecting the non-psychological modes of action. However, it is important to understand that psychedelics may mediate their therapeutic effects through multi-faceted mechanisms, such as the modulation of brain network activity, neuronal plasticity, neuroendocrine function, glial cell regulation, epigenetic processes, and the gut-brain axis. This review provides a framework supporting the implementation of a multi-faceted approach, incorporating in silico, in vitro and in vivo modeling, to aid in the comprehensive understanding of the physiological effects of psychedelics and their potential for clinical application beyond the treatment of psychiatric disorders. We also provide an overview of the literature supporting the potential utility of psychedelics for the treatment of brain injury (e.g., stroke and traumatic brain injury), neurodegenerative diseases (e.g., Parkinson's and Alzheimer's diseases), and gut-brain axis dysfunction associated with psychiatric disorders (e.g., generalized anxiety disorder and major depressive disorder). To move the field forward, we outline advantageous experimental frameworks to explore these and other novel applications for psychedelics.

Mitochondria of intestinal epithelial cells in depression: Are they at a crossroads of gut-brain communication?

The role of gut dysbiosis in depression is well established. However, recent studies have shown that gut microbiota is regulated by intestinal epithelial cell (IEC) mitochondria, which has yet to receive much attention. This review summarizes the recent developments about the critical role of IEC mitochondria in actively maintaining gut microbiota, intestinal metabolism, and immune homeostasis. We propose that IEC mitochondrial dysfunction alters gut microbiota composition, participates in cell fate, mediates oxidative stress, activates the peripheral immune system, causes peripheral inflammation, and transmits peripheral signals through the vagus and enteric nervous systems. These pathological alterations lead to brain inflammation, disruption of the blood-brain barrier, activation of the hypothalamic-pituitary-adrenal axis, activation of microglia and astrocytes, induction of neuronal loss, and ultimately depression. Furthermore, we highlight the prospect of treating depression through the mitochondria of IECs. These new findings suggest that the mitochondria of IECs may be a newly found important factor in the pathogenesis of depression and represent a potential new strategy for treating depression.

Arketamine for cognitive impairment in psychiatric disorders

Cognitive impairment has been observed in patients with various psychiatric disorders, including schizophrenia, major depressive disorder (MDD), and bipolar disorder (BD). Although modern therapeutic drugs can improve certain symptoms (i.e., psychosis, depression) in these patients, these drugs have not been found to improve cognitive impairment. The N-methyl-D-aspartate receptor antagonist (R,S)-ketamine has attracted attention as a rapidly acting antidepressant. In addition to its robust antidepressant effects, (R,S)-ketamine has been suggested to improve cognitive impairment in patients with MDD and BD, despite causing cognitive impairment in healthy control subjects. (R,S)-ketamine is a racemic mixture of equal amounts of (R)-ketamine (or arketamine) and (S)-ketamine (or esketamine). Arketamine has been found to have more potent antidepressant-like actions than esketamine in rodents. Interestingly, arketamine, but not esketamine, has been suggested to improve phencyclidine-induced cognitive deficits in mice. Furthermore, arketamine has been suggested to ameliorate cognitive deficits in rodent offspring after maternal immune activation. In the current article, it is proposed that arketamine has therapeutic potential for treating cognitive impairment in patients with psychiatric disorders. Additionally, the potential role of the gut-microbiome-brain axis in cognitive impairment in psychiatric disorders is discussed.

Activation of σ1-Receptors by R-Ketamine May Enhance the Antidepressant Effect of S-Ketamine

Ketamine is a racemic mixture composed of two enantiomers, S-ketamine and R-ketamine. In preclinical studies, both enantiomers have exhibited antidepressant effects, but these effects are attributed to distinct pharmacological activities. The S-enantiomer acts as an NMDA-channel blocker and as an opioid μ-receptor agonist, whereas the R-enantiomer binds to σ1-receptors and is believed to act as an agonist. As racemate, ketamine potentially triggers four biochemical pathways involving the AGC-kinases, PKA, Akt (PKB), PKC and RSK that ultimately lead to inhibitory phosphorylation of GSK3β in microglia. In patients with major depressive disorder, S-ketamine administered as a nasal spray has shown clear antidepressant activity. However, when compared to intravenously infused racemic ketamine, the response rate, duration of action and anti-suicidal activity of S-ketamine appear to be less pronounced. The σ1-protein interacts with μ-opioid and TrkB-receptors, whereas in preclinical experiments σ1-agonists reduce μ-receptor desensitization and improve TrkB signal transduction. TrkB activation occurs as a response to NMDA blockade. So, the σ1-activity of R-ketamine may not only enhance two pathways via which S-ketamine produces an antidepressant response, but it furthermore provides an antidepressant activity in its own right. These two factors could explain the apparently superior antidepressant effect observed with racemic ketamine compared to S-ketamine alone.

The antidepressant actions of ketamine and its enantiomers

Ketamine, an N-methyl-d-aspartate receptor (NMDAR) antagonist first developed as an anesthetic, has shown significant promise as a medication with rapid antidepressant properties in treatment-resistant depression. However, concerns such as adverse side effects and potential misuse liability have limited its widespread use. Racemic ketamine has two enantiomers-(S)- and (R)-ketamine-that appear to have disparate underlying mechanisms. This brief review summarizes some of the most recent preclinical and clinical research regarding the convergent and divergent prophylactic, immediate, and sustained antidepressant effects of (S)- and (R)-ketamine while addressing potential differences in their side effect and misuse liability profiles. Preclinical research suggests divergent mechanisms underlying (S)- and (R)-ketamine, with (S)-ketamine more directly affecting mechanistic target of rapamycin complex 1 (mTORC1) signaling and (R)-ketamine more directly affecting extracellular signal-related kinase (ERK) signaling. Clinical research suggests that (R)-ketamine has a milder side effect profile than (S)-ketamine and decreases depression rating scale scores, but recent randomized, controlled trials found that it had no significant antidepressant efficacy compared to placebo, suggesting that caution is warranted in interpreting its therapeutic potential. Future preclinical and clinical research is needed to maximize the efficacy of each enantiomer, either by optimizing dose, route of administration, or administration paradigm.

Ketamine and serotonergic psychedelics: An update on the mechanisms and biosignatures underlying rapid-acting antidepressant treatment

The discovery of ketamine as a rapid-acting antidepressant spurred significant research to understand its underlying mechanisms of action and to identify other novel compounds that may act similarly. Serotonergic psychedelics (SPs) have shown initial promise in treating depression, though the challenge of conducting randomized controlled trials with SPs and the necessity of long-term clinical observation are important limitations. This review summarizes the similarities and differences between the psychoactive effects associated with both ketamine and SPs and the mechanisms of action of these compounds, with a focus on the monoaminergic, glutamatergic, gamma-aminobutyric acid (GABA)-ergic, opioid, and inflammatory systems. Both molecular and neuroimaging aspects are considered. While their main mechanisms of action differ-SPs increase serotonergic signaling while ketamine is a glutamatergic modulator-evidence suggests that the downstream mechanisms of action of both ketamine and SPs include mechanistic target of rapamycin complex 1 (mTORC1) signaling and downstream GABAA receptor activity. The similarities in downstream mechanisms may explain why ketamine, and potentially SPs, exert rapid-acting antidepressant effects. However, research on SPs is still in its infancy compared to the ongoing research that has been conducted with ketamine. For both therapeutics, issues with regulation and proper controls should be addressed before more widespread implementation. This article is part of the Special Issue on "Ketamine and its Metabolites".

Modulation of Indoleamine 2,3-Dioxygenase 1 During Inflammatory Bowel Disease Activity in Humans and Mice

Background and Aims

Indoleamine 2,3 dioxygenase-1 (IDO1), a key enzyme in tryptophan metabolism, is strongly up-regulated both in human inflammatory bowel disease (IBD) and animal models of colitis, however its role in the pathogenesis is still controversial. In this study, we investigated IDO1 expression and activity in a mouse model of DSS-induced chronic colitis as well as in colon biopsies and sera from IBD patients.

Methods

Chronic colitis was induced in mice through the oral administration of dextran sodium sulfate (DSS), and IDO1 activity was induced by i.p. treatment with N-acetyl serotonin (NAS). IDO1 expression and catalytic activity (measured as Kyn/Trp ratio) was evaluated in sera and tissue samples collected from mice and 93 IBD patients under immunotherapy with Vedolizumab (VDZ) or Ustekinumab (UST).

Results

Strong up-regulation of IDO1 was found in colons of mice with acute colitis, which follows disease activity. Enhanced IDO1 activity by NAS treatment protects the intestinal mucosa during the recovery phase of chronic colitis. In IBD patients, IDO1 expression and activity correlate with the severity of mucosal inflammation with inflamed regions showing higher IDO1 expression compared to non-inflamed regions within the same patient. Endoscopic response to VDZ/UST treatment is associated with decreased expression of IDO1.

Conclusions

This is the first study demonstrating immunomodulatory activity of IDO1 in a chronic mouse model of DSS-induced colitis. As its expression and catalytic activity correlate with the grade of mucosal inflammation and treatment response, IDO1 could represent a promising biomarker for disease severity and treatment monitoring in IBD.

(R)-ketamine as prophylactic and therapeutic drug for neurological disorders: beyond depression

Neurological disorders are the leading cause of disability and the second leading cause of death worldwide. The increasing social and economic burdens of neurological disorders are driven by global population growth and aging. Depression is a common psychiatric symptom in numerous neurological disorders. It is also a risk factor for Alzheimer's disease (AD) and other dementias, Parkinson's disease (PD), and stroke. The rapid-acting and sustained antidepressant actions of (R,S)-ketamine for severe depression was accidentally discovered. Interestingly, (R)-ketamine has greater potency and longer-lasting antidepressant-like effects than (S)-ketamine in rodents. Importantly, its side effects in rodents and humans are lower than those of (R,S)-ketamine and (S)-ketamine. Furthermore, (R)-ketamine could elicit beneficial actions in various rodent models of neurological disorders, including PD, multiple sclerosis (MS), and stroke. In this article, we review the potential of (R)-ketamine as a prophylactic or therapeutic drug for neurological disorders including AD and other dementias, PD, MS, and stroke.

Repeated intermittent administration of (R)-ketamine during juvenile and adolescent stages prevents schizophrenia-relevant phenotypes in adult offspring after maternal immune activation: a role of TrkB signaling

Maternal immune activation (MIA) plays a role in the etiology of schizophrenia. MIA by prenatal exposure of polyinosinic:polycytidylic acid [poly(I:C)] in rodents caused behavioral and neurobiological changes relevant to schizophrenia in adult offspring. We investigated whether the novel antidepressant (R)-ketamine could prevent the development of psychosis-like phenotypes in adult offspring after MIA. We examined the effects of (R)-ketamine (10 mg/kg/day, twice weekly for 4 weeks) during juvenile and adolescent stages (P28-P56) on the development of cognitive deficits, loss of parvalbumin (PV)-immunoreactivity in the medial prefrontal cortex (mPFC), and decreased dendritic spine density in the mPFC and hippocampus from adult offspring after prenatal poly(I:C) exposure. Furthermore, we examined the role of TrkB in the prophylactic effects of (R)-ketamine. Repeated intermittent administration of (R)-ketamine during juvenile and adolescent stages significantly blocked the development of cognitive deficits, reduced PV-immunoreactivity in the prelimbic (PrL) of mPFC, and decreased dendritic spine density in the PrL of mPFC, CA3 and dentate gyrus of the hippocampus from adult offspring after prenatal poly(I:C) exposure. Furthermore, pretreatment with ANA-12 (TrkB antagonist: twice weekly for 4 weeks) significantly blocked the beneficial effects of (R)-ketamine on cognitive deficits of adult offspring after prenatal poly(I:C) exposure. These data suggest that repeated intermittent administration of (R)-ketamine during juvenile and adolescent stages could prevent the development of psychosis in adult offspring after MIA. Therefore, (R)-ketamine would be a potential prophylactic drug for young subjects with high-risk for psychosis.

Nuclear factor of activated T cells 4 in the prefrontal cortex is required for prophylactic actions of (R)-ketamine

(R, S)-ketamine has prophylactic antidepressant-like effects in rodents; however, the precise molecular mechanisms underlying its action remain unknown. Using RNA-sequencing analysis, we searched novel molecular target(s) that contribute to the prophylactic effects of (R)-ketamine, a more potent enantiomer of (R, S)-ketamine. Pretreatment with (R)-ketamine (10 mg/kg, 6 days before) significantly ameliorated body weight loss, splenomegaly, and increased immobility time of forced swimming test in lipopolysaccharide (LPS: 1.0 mg/kg)-treated mice. RNA-sequencing analysis of prefrontal cortex (PFC) and subsequent IPA (Ingenuity Pathway Analysis) revealed that the nuclear factor of activated T cells 4 (NFATc4) signaling might contribute to sustained prophylactic effects of (R)-ketamine. Quantitative RT-PCR confirmed that (R)-ketamine significantly attenuated the increased gene expression of NFATc4 signaling (Nfatc4, Cd4, Cd79b, H2-ab1, H2-aa) in the PFC of LPS-treated mice. Furthermore, pretreatment with NFAT inhibitors (i.e., NFAT inhibitor and cyclosporin A) showed prophylactic effects in the LPS-treated mice. Similar to (R)-ketamine, gene knockdown of Nfatc4 gene by bilateral injection of adeno-associated virus (AAV) into the mPFC could elicit prophylactic effects in the LPS-treated mice. In conclusion, our data implicate a novel NFATc4 signaling pathway in the PFC underlying the prophylactic effects of (R)-ketamine for inflammation-related depression.

Molecular mechanisms underlying the antidepressant actions of arketamine: beyond the NMDA receptor

The discovery of robust antidepressant actions exerted by the N-methyl-D-aspartate receptor (NMDAR) antagonist (R,S)-ketamine has been a crucial breakthrough in mood disorder research. (R,S)-ketamine is a racemic mixture of equal amounts of (R)-ketamine (arketamine) and (S)-ketamine (esketamine). In 2019, an esketamine nasal spray from Johnson & Johnson was approved in the United States of America and Europe for treatment-resistant depression. However, an increasing number of preclinical studies show that arketamine has greater potency and longer-lasting antidepressant-like effects than esketamine in rodents, despite the lower binding affinity of arketamine for the NMDAR. In clinical trials, non-ketamine NMDAR-related compounds did not exhibit ketamine-like robust antidepressant actions in patients with depression, despite these compounds showing antidepressant-like effects in rodents. Thus, the rodent data do not necessarily translate to humans due to the complexity of human psychiatric disorders. Collectively, the available studies indicate that it is unlikely that NMDAR plays a major role in the antidepressant action of (R,S)-ketamine and its enantiomers, although the precise molecular mechanisms underlying antidepressant actions of (R,S)-ketamine and its enantiomers remain unclear. In this paper, we review recent findings on the molecular mechanisms underlying the antidepressant actions of (R,S)-ketamine and its potent enantiomer arketamine. Furthermore, we discuss the possible role of the brain-gut-microbiota axis and brain-spleen axis in stress-related psychiatric disorders and in the antidepressant-like action of arketamine. Finally, we discuss the potential of arketamine as a treatment for cognitive impairment in psychiatric disorders, Parkinson's disease, osteoporosis, inflammatory bowel diseases, and stroke.

(R)-Ketamine ameliorates lethal inflammatory responses and multi-organ injury in mice induced by cecum ligation and puncture

AIMS

Sepsis is a life-threatening organ dysfunction syndrome arising from infection-induced uncontrolled systemic inflammatory responses. Patients surviving severe sepsis also exhibit increased mortality due to enhanced vulnerability to infections. In this study, we examined whether (R)-ketamine could prevent against lethal sepsis-induced systemic inflammation and inflammatory organ injury.

MAIN METHODS

Septic model was induced by cecal ligation and puncture (CLP) surgery on adult mice. (R)-ketamine (10 or 15 mg/kg) was administrated intraperitoneally (i.p.) 24 h before and/or immediately after CLP.

KEY FINDINGS

Combined prophylactic and therapeutic use of (R)-ketamine (10 mg/kg), as well as either prophylactic or therapeutic use of (R)-ketamine at a single dose of 15 mg/kg did not reduce 14-day mortality after CLP. However, combined prophylactic and therapeutic use of (R)-ketamine (15 mg/kg) significantly increased 14-day survival rate, attenuated sepsis-induced marked drop in the rectal temperature and increase in the plasma levels of inflammatory cytokines [i.e., interleukin (IL)-6, IL-17A, tumor necrosis factor (TNF)-α, IL-1β, and IL-10] 12 h after CLP. Furthermore, (R)-ketamine alleviated sepsis-induced increase in the organ injury markers such as alanine aminotransferase (ALT), aspartate aminotransferase (AST), myocardial kinase (CK-MB), and creatinine 24 h after CLP. Moreover, the increased lung wet/dry weight ratio, pulmonary morphological injury and the pulmonary levels of inflammatory cytokines were also attenuated by (R)-ketamine.

SIGNIFICANCE

Combined prophylactic and therapeutic use of (R)-ketamine could attenuate systemic inflammation and inflammatory multi-organ injury in mice after CLP-induced lethal sepsis. Therefore, (R)-ketamine would be a potential prophylactic and therapeutic drug for patients prone to sepsis.

(R)-ketamine ameliorates the progression of experimental autoimmune encephalomyelitis in mice

Multiple sclerosis (MS) is an immune-mediated neurological disease that attacks the central nervous system, including spinal cord and brain. Experimental autoimmune encephalomyelitis (EAE) is the most commonly used model for MS. Depression is the most prevalent comorbidity in MS patients. We previously demonstrated that (R)-ketamine would be a novel antidepressant without side effects of ketamine. This study was undertaken to investigate whether (R)-ketamine could attenuate disease progression in EAE mouse model. (R)-ketamine (10 mg/kg/day for 15 days) significantly attenuated the reduction of body weight in EAE model mice compared to saline-treated mice. Furthermore, (R)-ketamine ameliorated the clinical EAE scores compared to saline-treated mice. Moreover, (R)-ketamine significantly attenuated the marked increases in the pathological scores, microglial activation, and blood-brain barrier integrity in the spinal cord compared to saline-treated mice. In conclusion, the current study suggests that (R)-ketamine could ameliorate EAE clinical scores and pathological changes in the spinal cord of EAE mice. Therefore, it is likely that (R)-ketamine would be a new potential prophylactic drug for MS.

(R)-Ketamine attenuates LPS-induced endotoxin-derived delirium through inhibition of neuroinflammation

RATIONALE

(R)-Ketamine produced beneficial effects in a variety of models of inflammatory diseases, including low dose of bacterial lipopolysaccharide (LPS) (0.5-1.0 mg/kg)-induced endotoxemia. LPS-treated mice have been used as animal model of delirium.

OBJECTIVES

We investigated the effects of (R)-ketamine in neuroinflammation and cognitive impairment in rodents after administration of high dose of LPS.

METHODS

LPS (5 mg/kg) or saline was administered intraperitoneally (i.p.) to mice. (R)-Ketamine (10 mg/kg) was administrated i.p. 24 h before and/or 10 min after LPS injection.

RESULTS

LPS (5.0 mg/kg) caused a remarkable splenomegaly and increased plasma levels of pro-inflammatory cytokines [i.e., interleukin (IL-6), IL-17A, and interferon (IFN)-γ]. There were positive correlations between spleen weight and plasma cytokines levels. Furthermore, LPS led to increased levels of pro-inflammatory cytokines in the prefrontal cortex (PFC) and hippocampus. Moreover, LPS impaired the natural and learned behaviors, as demonstrated by a decrease in the number of mice's entries and duration in the novel arm in the Y maze test and an increase in the latency of mice to eat the food in the buried food test. Interestingly, the treatment with (R)-ketamine (twice 24 h before and 10 min after LPS injection) significantly attenuated LPS-induced splenomegaly, central and systemic inflammation, and cognitive impairment.

CONCLUSION

Our results highlighted the importance of combined prophylactic and therapeutic use of (R)-ketamine in the attenuation of LPS-induced systemic inflammation, neuroinflammation, and cognitive impairment in mice. It is likely that (R)-ketamine could be a prophylactic drug for delirium.