Effects of a single bilateral infusion of R-ketamine in the rat brain regions of a learned helplessness model of depression

Ketamine's Amelioration of Fear Extinction in Adolescent Male Mice Is Associated with the Activation of the Hippocampal Akt-mTOR-GluA1 Pathway

Fear-related disorders, including post-traumatic stress disorder (PTSD), and anxiety disorders are pervasive psychiatric conditions marked by persistent fear, stemming from its dysregulated acquisition and extinction. The primary treatment for these disorders, exposure therapy (ET), relies heavily on fear extinction (FE) principles. Adolescence, a vulnerable period for developing psychiatric disorders, is characterized by neurobiological changes in the fear circuitry, leading to impaired FE and increased susceptibility to relapse following ET. Ketamine, known for relieving anxiety and reducing PTSD symptoms, influences fear-related learning processes and synaptic plasticity across the fear circuitry. Our study aimed to investigate the effects of ketamine (10 mg/kg) on FE in adolescent male C57 BL/6 mice at the behavioral and molecular levels. We analyzed the protein and gene expression of synaptic plasticity markers in the hippocampus (HPC) and prefrontal cortex (PFC) and sought to identify neural correlates associated with ketamine's effects on adolescent extinction learning. Ketamine ameliorated FE in the adolescent males, likely affecting the consolidation and/or recall of extinction memory. Ketamine also increased the Akt and mTOR activity and the GluA1 and GluN2A levels in the HPC and upregulated BDNF exon IV mRNA expression in the HPC and PFC of the fear-extinguished mice. Furthermore, ketamine increased the c-Fos expression in specific brain regions, including the ventral HPC (vHPC) and the left infralimbic ventromedial PFC (IL vmPFC). Providing a comprehensive exploration of ketamine's mechanisms in adolescent FE, our study suggests that ketamine's effects on FE in adolescent males are associated with the activation of hippocampal Akt-mTOR-GluA1 signaling, with the vHPC and the left IL vmPFC as the proposed neural correlates.

Ketamine and its enantiomers for depression: a bibliometric analysis from 2000 to 2023

Ketamine has demonstrated rapid and sustained antidepressant effects, marking its emergence as an innovative treatment of depression. Despite the growing number of preclinical and clinical studies exploring the antidepressant effects of ketamine and its enantiomers, a comprehensive bibliometric analysis in this field has yet to be conducted. This study employs bibliometric methods and visualization tools to examine the literature and identify key topics related to the antidepressant effects of ketamine and its enantiomers. We sourced publications on the antidepressant effects of ketamine and its enantiomers from the Web of Science Core Collection (WOSCC) database, covering the period from 2000 to 2023. Tools such as VOSviewer, CiteSpace and the R package "bibliometrix" were utilized for visual analysis. The study included 4,274 publications, with a notable increase in publications peaking in 2022. Co-occurrence analysis highlighted two primary research focal points: the efficacy and safety of ketamine and its enantiomers in treating depression, and the mechanisms behind their antidepressant effects. In conclusion, this analysis revealed a significant increase in research on the antidepressant effects of ketamine and its enantiomers over the past two decades, leading to the approval of esketamine nasal spray for treatment-resistant depression. The rapid antidepressant effects of ketamine have spurred further studies into its mechanisms of action and the search for new antidepressants with fewer side effects.

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.

The role of neurotrophic factors in novel, rapid psychiatric treatments

Neurotrophic factors are a family of growth factors that modulate cellular growth, survival, and differentiation. For many decades, it has been generally believed that a lack of neurotrophic support led to the decreased neuronal synaptic plasticity, death, and loss of non-neuronal supportive cells seen in neuropsychiatric disorders. Traditional psychiatric medications that lead to immediate increases in neurotransmitter levels at the synapse have been shown also to elevate synaptic neurotrophic levels over weeks, correlating with the time course of the therapeutic effects of these drugs. Recent advances in psychiatric treatments, such as ketamine and psychedelics, have shown a much faster onset of therapeutic effects (within minutes to hours). They have also been shown to lead to a rapid release of neurotrophins into the synapse. This has spurred a significant shift in understanding the role of neurotrophins and how the receptor tyrosine kinases that bind neurotrophins may work in concert with other signaling systems. In this review, this renewed understanding of synaptic receptor signaling interactions and the clinical implications of this mechanistic insight will be discussed within the larger context of the well-established roles of neurotrophic factors in psychiatric disorders and treatments.

Preliminary evidence that ketamine alters anterior cingulate resting-state functional connectivity in depressed individuals

Activity changes within the anterior cingulate cortex (ACC) are implicated in the antidepressant effects of ketamine, but the ACC is cytoarchitectonically and functionally heterogeneous and ketamine's effects may be subregion specific. In the context of a double-blind randomized placebo-controlled crossover trial investigating the clinical and resting-state fMRI effects of intravenous ketamine vs. placebo in patients with treatment resistant depression (TRD) vs. healthy volunteers (HV), we used seed-based resting-state functional connectivity (rsFC) analyses to determine differential changes in subgenual ACC (sgACC), perigenual ACC (pgACC) and dorsal ACC (dACC) rsFC two days post-infusion. Across cingulate subregions, ketamine differentially modulated rsFC to the right insula and anterior ventromedial prefrontal cortex, compared to placebo, in TRD vs. HV; changes to pgACC-insula connectivity correlated with improvements in depression scores. Post-hoc analysis of each cingulate subregion separately revealed differential modulation of sgACC-hippocampal, sgACC-vmPFC, pgACC-posterior cingulate, and dACC-supramarginal gyrus connectivity. By comparing rsFC changes following ketamine vs. placebo in the TRD group alone, we found that sgACC rsFC was most substantially modulated by ketamine vs. placebo. Changes to sgACC-pgACC, sgACC-ventral striatal, and sgACC-dACC connectivity correlated with improvements in anhedonia symptoms. This preliminary evidence suggests that accurate segmentation of the ACC is needed to understand the precise effects of ketamine's antidepressant and anti-anhedonic action.

Abnormal compositions of gut microbiota and metabolites are associated with susceptibility versus resilience in rats to inescapable electric stress

BACKGROUND

Increasing evidence suggests the role of gut microbiota in resilience versus vulnerability after stress. However, the role of gut microbiota and microbiome-derived metabolites in resilience versus susceptibility in rodents exposed to stress remains unclear.

METHODS

Adult male rats were exposed to inescapable electric stress under the learned helplessness (LH) paradigm. The composition of gut microbiota and metabolites in the brain and blood from control (no stress) rats, LH resilient rats, and LH susceptible rats were examined.

RESULTS

At the genus level, the relative abundances of Asaccharobacter, Eisenbergiella, and Klebsiella in LH susceptible rats were significantly higher than that of LH resilient rats. At the species level, the relative abundances of several microbiome were significantly altered between LH susceptible rats and LH resilient rats. Furthermore, there were several metabolites in the brain and blood altered between LH susceptible rats and LH resilient rats. A network analysis showed correlations between the abundance of several microbiome and metabolites in the brain (or blood).

LIMITATIONS

Detailed roles of microbiome and metabolites are unclear.

CONCLUSIONS

These findings suggest that abnormal compositions of the gut microbiota and metabolites might contribute to susceptibility versus resilience in rats subjected to inescapable electric foot shock.

Ketamine and its metabolites: Potential as novel treatments for depression

Depression is a well-known serious mental illness, and the onset of treatment using traditional antidepressants is frequently delayed by several weeks. Moreover, numerous patients with depression fail to respond to therapy. One major breakthrough in antidepressant therapy is that subanesthetic ketamine doses can rapidly alleviate depressive symptoms within hours of administering a single dose, even in treatment-resistant patients. However, specific mechanisms through which ketamine exerts its antidepressant effects remain elusive, leading to concerns regarding its rapid and long-lasting antidepressant effects. N-methyl-d-aspartate receptor (NMDAR) antagonists like ketamine are reportedly associated with serious side effects, such as dissociative symptoms, cognitive impairment, and abuse potential, limiting the large-scale clinical use of ketamine as an antidepressant. Herein, we reviewed the pharmacological properties of ketamine and the mechanisms of action underlying the rapid antidepressant efficacy, including the disinhibition hypothesis and synaptogenesis, along with common downstream effector pathways such as enhanced brain-derived neurotrophic factor and tropomyosin-related kinase B signaling, activation of the mechanistic target of rapamycin complex 1 and transforming growth factor β1. We focused on evidence supporting the relevance of these potential mechanisms of ketamine and its metabolites in mediating the clinical efficacy of the drug. Given its reported antidepressant efficacy in preclinical studies and limited undesirable adverse effects, (R)-ketamine may be a safer, more controllable, rapid antidepressant. Overall, understanding the potential mechanisms of action of ketamine and its metabolites in combination with pharmacology may help develop a new generation of rapid antidepressants that maximize antidepressant effects while avoiding unfavorable adverse effects. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

A role of microRNA-149 in the prefrontal cortex for prophylactic actions of (R)-ketamine in inflammation model

MicroRNAs (or miRNAs) are short, regulatory RNAs that act as post-transcriptional repressors of gene expression. Recently, we reported that the nuclear factor of activated T cells 4 (NFATc4) signaling might contribute to sustained prophylactic effects of new antidepressant (R)-ketamine in lipopolysaccharide (LPS)-treated inflammation model of depression. In this study, we examined the role of miRNAs (miR-149 and miR-7688-5p) which can regulate NFATc4 in the prefrontal cortex (PFC) of male mice after administration of LPS (1.0 mg/kg). There was a positive correlation between the expression of Nfatc4 and the expression of miR-149 in the PFC. There was also a negative correlation between gene expression of Nfatc4 and gene expression of miR-7688-5p in the PFC. Gut microbiota analysis showed that pretreatment with (R)-ketamine (10 mg/kg) could restore altered composition of gut microbiota in LPS-treated mice. A network analysis showed that gut microbiota may regulate gene expression of Nfatc4 and miR-149 (or miR-7688-5p) in the PFC. Finally, inhibition of miR-149 by antagomiR-149 blocked LPS-induced depression-like behavior by attenuating LPS-induced expression of NFATc4 in the PFC. These findings suggest that the regulation of NFATc4 signaling by miR-149 might play a role in persistent prophylactic effects of (R)-ketamine, and that gut microbiota may regulate the gene expression of miRNAs in the PFC through gut-microbiota-brain axis.

A key role of miR-132-5p in the prefrontal cortex for persistent prophylactic actions of (R)-ketamine in mice

(R,S)-ketamine is known to elicit persistent prophylactic effects in rodent models of depression. However, the precise molecular mechanisms underlying its action remain elusive. Using RNA-sequencing analysis, we searched for novel molecular target(s) that contribute to the prophylactic effects of (R)-ketamine, a more potent enantiomer of (R,S)-ketamine in chronic restraint stress (CRS) model. Pretreatment with (R)-ketamine (10 mg/kg, 1 day before CRS) significantly ameliorated body weight loss, increased immobility time of forced swimming test, and decreased sucrose preference of sucrose preference test in CRS-exposed mice. RNA-sequencing analysis of prefrontal cortex (PFC) revealed that several miRNAs such as miR-132-5p might contribute to sustained prophylactic effects of (R)-ketamine. Methyl CpG binding protein 2 (MeCP2) is known to regulate brain-derived neurotrophic factor (BDNF) expression. Quantitative RT-PCR confirmed that (R)-ketamine significantly attenuated altered expression of miR-132-5p and its regulated genes (Bdnf, Mecp2, Tgfb1, Tgfbr2) in the PFC of CRS-exposed mice. Furthermore, (R)-ketamine significantly attenuated altered expression of BDNF, MeCP2, TGF-β1 (transforming growth factor β1), and synaptic proteins (PSD-95, and GluA1) in the PFC of CRS-exposed mice. Administration of agomiR-132-5p decreased the expression of Bdnf and Tgfb1 in the PFC, resulting in depression-like behaviors. In contrast, administration of antagomiR-132-5p blocked the increased expression of miR-132-5p and decreased expression of Bdnf in the PFC of CRS-exposed mice, resulting in antidepressant-like effects. In conclusion, our data show a novel role of miR-132-5p in the PFC underlying depression-like phenotypes in CRS model and the sustained prophylactic effects of (R)-ketamine.

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.

Microglial ERK-NRBP1-CREB-BDNF signaling in sustained antidepressant actions of (R)-ketamine

(R,S)-ketamine elicits rapid-acting and sustained antidepressant actions in treatment-resistant patients with depression. (R)-ketamine produces longer-lasting antidepressant effects than (S)-ketamine in rodents; however, the precise molecular mechanisms underlying antidepressant actions of (R)-ketamine remain unknown. Using isobaric Tag for Relative and Absolute Quantification, we identified nuclear receptor-binding protein 1 (NRBP1) that could contribute to different antidepressant-like effects of the two enantiomers in chronic social defeat stress (CSDS) model. NRBP1 was localized in the microglia and neuron, not astrocyte, of mouse medial prefrontal cortex (mPFC). (R)-ketamine increased the expression of NRBP1, brain-derived neurotrophic factor (BDNF), and phosphorylated cAMP response element binding protein (p-CREB)/CREB ratio in primary microglia cultures thorough the extracellular signal-regulated kinase (ERK) activation. Furthermore, (R)-ketamine could activate BDNF transcription through activation of CREB as well as MeCP2 (methyl-CpG binding protein 2) suppression in microglia. Single intracerebroventricular (i.c.v.) injection of CREB-DNA/RNA heteroduplex oligonucleotides (CREB-HDO) or BDNF exon IV-HDO blocked the antidepressant-like effects of (R)-ketamine in CSDS susceptible mice. Moreover, microglial depletion by colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX3397 blocked the antidepressant-like effects of (R)-ketamine in CSDS susceptible mice. In addition, inhibition of microglia by single i.c.v. injection of mannosylated clodronate liposomes (MCLs) significantly blocked the antidepressant-like effects of (R)-ketamine in CSDS susceptible mice. Finally, single i.c.v. injection of CREB-HDO, BDNF exon IV-HDO or MCLs blocked the beneficial effects of (R)-ketamine on the reduced dendritic spine density in the mPFC of CSDS susceptible mice. These data suggest a novel ERK-NRBP1-CREB-BDNF pathways in microglia underlying antidepressant-like effects of (R)-ketamine.

Novel experimental and early investigational drugs for the treatment of bipolar disorder

INTRODUCTION

The quest toward more effective treatments for bipolar disorder (BD) solicits novel drugs and further research on the underpinning neurobiology. The present review aims to critically appraise the existing evidence about the pharmacological treatment of BD toward the development of novel treatment avenues.

AREAS COVERED

The present review appraises animal and human studies concerning both the currently available psychotropic drugs, and the general medicine drugs which may represent a path toward the development of novel drugs for BD. PubMed and Scopus were last accessed on February 20^th, 2021 for records indexed upon inception relevant to the pharmacological treatment of BD. Immune-modulating agents, anti-inflammatory agents, and glutamate antagonists represent the most intriguing potential targets for the development of new drugs for BD, thus receiving critical appraisal in the present text.

EXPERT OPINION

Regardless of the neurobiological pathways worthy of investigation toward the development of experimental drugs for BD, several unmet needs need to be addressed first. In particular, several biomarkers are altered in BD. However, it is the opinion herein expressed by the authors that it remains uncertain what comes first, that is peripheral changes or the disease.

The Resilient Phenotype Induced by Prophylactic Ketamine Exposure During Adolescence Is Mediated by the Ventral Tegmental Area-Nucleus Accumbens Pathway

BACKGROUND

Major depressive disorder is prevalent in children and adolescents and is associated with a high degree of morbidity throughout life, with potentially devastating personal consequences and public health impact. The efficacy of ketamine (KET) as an antidepressant has been demonstrated in adolescent rodents; however, the neurobiological mechanisms underlying these effects are unknown. Recent evidence showed that KET reverses stress-induced (i.e., depressive-like) deficits within major mesocorticolimbic regions, such as the prefrontal cortex, nucleus accumbens (NAc), and hippocampus, in adult rodents. However, little is known about KET's effect in the ventral tegmental area (VTA), which provides the majority of dopaminergic input to these brain regions.

METHODS

We characterized behavioral, biochemical, and electrophysiological effects produced by KET treatment in C57BL/6J male mice during adolescence (n = 7-10 per condition) within the VTA and its major projection regions, namely, the NAc and prefrontal cortex. Subsequently, molecular targets within the VTA-NAc projection were identified for viral gene transfer manipulations to recapitulate the effects of stress or KET treatment.

RESULTS

Repeated KET treatment produced a robust proresilient response to chronic social defeat stress. This effect was largely driven by Akt signaling activity within the VTA and NAc, and it could be blocked or recapitulated through direct Akt-viral-mediated manipulation. Additionally, we found that the KET-induced resilient phenotype is dependent on VTA-NAc, but not VTA-prefrontal cortex, pathway activity.

CONCLUSIONS

These findings indicate that KET exposure during adolescence produces a proresilient phenotype mediated by changes in Akt intracellular signaling and altered neuronal activity within the VTA-NAc pathway.

The anterior cingulate cortex as a key locus of ketamine's antidepressant action

The subdivisions of the anterior cingulate cortex (ACC) - including subgenual, perigenual and dorsal zones - are implicated in the etiology, pathogenesis and treatment of major depression. We review an emerging body of evidence which suggests that changes in ACC activity are critically important in mediating the antidepressant effects of ketamine, the prototypical member of an emerging class of rapidly acting antidepressants. Infusions of ketamine induce acute (over minutes) and post-acute (over hours to days) modulations in subgenual and perigenual activity, and importantly, these changes can correlate with antidepressant efficacy. The subgenual and dorsal zones of the ACC have been specifically implicated in ketamine's anti-anhedonic effects. We emphasize the synergistic relationship between neuroimaging studies in humans and brain manipulations in animals to understand the causal relationship between changes in brain activity and therapeutic efficacy. We conclude with circuit-based perspectives on ketamine's action: first, related to ACC function in a central network mediating affective pain, and second, related to its role as the anterior node of the default mode network.

The Sustained Antidepressant Effects of Ketamine Are Independent of the Lateral Habenula

Ketamine is known to have a rapid and lasting antidepressant effect. Recent studies have shown that ketamine exerts it rapid antidepressant effect by blocking burst firing in the lateral habenula (LHb). Whether the sustained antidepressant effect of ketamine occurs through the same mechanism has not been explored. Here, using male rats, we found that local infusion of (R,S)-ketamine into the LHb resulted in a rapid antidepressant-like effect 1 h after infusion, which almost returned to baseline levels after 24 h. Intra-LHb injection of (S)-ketamine also showed a significant antidepressant-like effect 1 h after injection, which recovered at 24 h. No significant antidepressant-like effect was found at 1 or 24 h after the administration of (R)-ketamine into the LHb. Injection of (2R,6R)-hydroxynorketamine, a ketamine metabolite, into the LHb did not result in any obvious antidepressant-like effect 1 or 24 h after injection. Systemic administration of (R,S)-ketamine (intraperitoneally) significantly suppressed LHb bursting activity at 1 h, but the inhibitory effect was reversed 24 h after injection. No significant effect of (R,S)-ketamine on miniature excitatory postsynaptic potentials of LHb neurons was found at 1 or 24 h after systemic application. Our study demonstrated that the sustained antidepressant-like effect of ketamine may not depend on burst firing of LHb neurons.SIGNIFICANCE STATEMENT Ketamine exerts it rapid antidepressant effect by blocking burst firing in the lateral habenula (LHb). However, whether the sustained antidepressant effect of ketamine occurs through the same mechanism has not been explored. In the present study, we demonstrated that the sustained antidepressant effect of ketamine may not depend on the burst firing of LHb neurons. This finding may lead to a novel perspective on LHb in the antidepressant effect of ketamine.

The role of the excitation:inhibition functional balance in the mPFC in the onset of antidepressants

Currently available antidepressants, such as selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs), generally require weeks to months to produce a therapeutic response, but the mechanism of action underlying the delayed onset of antidepressant-like action remains to be elucidated. The balance between excitatory glutamatergic pyramidal neurons and inhibitory γ-aminobutyric acid (GABA) interneurons, i.e., the excitation:inhibition functional (E:I) balance, in the medial prefrontal cortex (mPFC) is critical in regulating several behaviors and might play an important mediating role in the mechanism of rapid antidepressant-like action reported by several studies. In the present study, the multichannel electrophysiological technique was used to record the firing activities of pyramidal neurons and interneurons and investigate the effects of a single dose of fluoxetine and ketamine (both 10 mg/kg, i.p.) on the E:I functional balance in the rat mPFC after 90 min or 24 h, and the forced swimming test (FST) was used to evaluate the antidepressant-like effects of fluoxetine and ketamine. The present study also explored the effects of chronic treatment with fluoxetine (10 mg/kg, i.g.) for 7 d or 21 d on the E:I functional balance in the mPFC. The present results suggested that a single dose of ketamine could both significantly increase the firing activities of pyramidal neurons and significantly decrease the firing activities of interneurons in the mPFC and exerted significant antidepressant-like action on the FST after 90 min and 24 h, but fluoxetine had no such effects under the same conditions. However, chronic treatment with fluoxetine for 21 d (but not 7 d) could significantly affect the firing activities of pyramidal neurons and interneurons in the mPFC. Taken together, the present results indicated that rapid regulation of the E:I functional balance in the mPFC might be an important common mechanism of rapid-acting antidepressants and the delayed onset of SSRIs might be partly attributed to their inability to rapidly regulate the E:I functional balance in the mPFC. The present study provided a new entry point to the development of rapid-acting antidepressants.

Attenuated dopamine signaling after aversive learning is restored by ketamine to rescue escape actions

Escaping aversive stimuli is essential for complex organisms, but prolonged exposure to stress leads to maladaptive learning. Stress alters neuronal activity and neuromodulatory signaling in distributed networks, modifying behavior. Here, we describe changes in dopaminergic neuron activity and signaling following aversive learning in a learned helplessness paradigm in mice. A single dose of ketamine suffices to restore escape behavior after aversive learning. Dopaminergic neuron activity in the ventral tegmental area (VTA) systematically varies across learning, correlating with future sensitivity to ketamine treatment. Ketamine’s effects are blocked by chemogenetic inhibition of dopamine signaling. Rather than directly altering the activity of dopaminergic neurons, ketamine appears to rescue dopamine dynamics through actions in the medial prefrontal cortex (mPFC). Chemogenetic activation of Drd1 receptor positive mPFC neurons mimics ketamine’s effects on behavior. Together, our data link neuromodulatory dynamics in mPFC-VTA circuits, aversive learning, and the effects of ketamine.

Over 264 million people around the world suffer from depression, according to the World Health Organization (WHO). Depression can be debilitating, and while anti-depressant drugs are available, they do not always work. A small molecule drug mainly used for anesthesia called ketamine has recently been shown to ameliorate depressive symptoms within hours, much faster than most anti-depressants. However, the molecular mechanisms behind this effect are still largely unknown.

Most anti-depressant drugs work by restoring the normal balance of dopamine and other chemical messengers in the brain. Dopamine is released by a specialized group of cells called dopaminergic neurons, and helps us make decisions by influencing a wide range of other cells in the brain. In a healthy brain, dopamine directs us to rewarding choices, while avoiding actions with negative outcomes. During depression, these dopamine signals are perturbed, resulting in reduced motivation and pleasure. But it remained unclear whether ketamine’s anti-depressant activity also relied on dopamine.

To investigate this, Wu et al. used a behavioral study called “learned helplessness” which simulates depression by putting mice in unavoidable stressful situations. Over time the mice learn that their actions do not change the outcome and eventually stop trying to escape from unpleasant situations, even if they are avoidable. The experiment showed that dopaminergic neurons in an area of the brain that is an important part of the “reward and aversion” system became less sensitive to unpleasant stimuli following learned helplessness. When the mice received ketamine, these neurons recovered after a few hours.

Individual mice also responded differently to ketamine. The most ‘resilient’, stress-resistant mice, which had distinct patterns of dopamine signaling, also responded most strongly to the drug. Genetic and chemical manipulation of dopaminergic neurons confirmed that ketamine needed intact dopamine signals to work, and revealed that it acted indirectly on dopamine dynamics via another brain region called the medial prefrontal cortex.

These results shed new light on how a promising new anti-depressant works. In the future, they may also explain why drugs like ketamine work better for some people than others, ultimately helping clinicians select the most effective treatment for individual patients.

An Integrative Approach to Ketamine Therapy May Enhance Multiple Dimensions of Efficacy: Improving Therapeutic Outcomes With Treatment Resistant Depression

Research over the last two decades has established ketamine as a safe, effective, fast-acting, and sustained antidepressant that significantly reduces adverse symptoms associated with depression, even in patients who are treatment resistant. Much of this research has evolved within the framework of several independent branches of scientific inquiry: in addition to the study of ketamine is a non-selective NMDAR antagonist with rapid antidepressant effects, it has also been found effective as a psychoplastogen that stimulates synaptogenesis and increases neuroplasticity, as a powerful anti-inflammatory that may improve inflammation-related depressive symptoms, as a substance that induces beneficial high entropy brain states, and as a subjectively impactful psychedelic agent. Each branch of inquiry has generated independent evidence of ketamine's efficacy but has advanced without substantive coordination or communication with other lines of inquiry. Integrative research that considers these branches of research together may lead toward a better understanding of ketamine's effects and improved treatment protocols and clinical outcomes. Such an overview can inform more comprehensive patient care through: (a) informed patient psychoeducation that encompasses all of ketamine's mechanisms of action; (b) calibration of optimal dosage to ensure induction and maintenance of high entropy brain states during each ketamine session utilizing EEG measurement; (c) Improved management of emergence side effects through proper care for set and setting; (d) inclusion of pre-selected appropriate music to enhance the emotional experience; (e) increased monitoring of ketamine effects on cortical activity, inter-hemispheric imbalance, and inflammation-related levels of cytokines to further improvements in ketamine protocols; and (f) appropriate timing of any adjunctive psychotherapy sessions to coincide with peak neurogenesis at 24-48 h post ketamine treatment.

The alterations of glutamate transporter 1 and glutamine synthetase in the rat brain of a learned helplessness model of depression

BACKGROUND

Although glutamate transmission via astrocytes has been proposed to contribute to the pathophysiology of depression, the precise mechanisms are unknown. Herein, we investigated the levels of glutamate transporter-1 (GLT-1) and glutamine synthetase (GS) of astrocytes in learned helplessness (LH) rats (an animal model of depression) and non-LH rats (an animal model of resilience).

METHODS

We administered inescapable mild electric shock to rats and then discriminated the LH and non-LH rats by a post-shock test. Almost 55% of the rats acquired LH. We then measured the expressions of GLT-1 and GS in several brain regions of LH and non-LH rats by Western blot analysis.

RESULTS

The levels of GLT-1 and GS in the CA-1, CA-3, dentate gyrus (DG), medial prefrontal cortex (mPF), and nucleus accumbens (NAc) of the LH group were significantly higher than those of the control group. The GS levels in the amygdala of the LH rats were significantly decreased compared to the controls. There were significant differences in GLT-1 and GS levels between the non-LH and LH rats in the CA-1 and CA-3.

CONCLUSIONS

These results suggest that the LH rats experienced up-regulations of GLT-1 and GS in the CA-1, CA-3, DG, mPF, and NAc and a down-regulation of GS in the amygdala. It is possible that the effects of the GLT-1 and GS levels on astrocytes in the CA-1 and CA-3 are critical for the differentiation of resilience from vulnerability.

Antidepressant mechanisms of ketamine: Focus on GABAergic inhibition

There has been much recent progress in understanding of the mechanism of ketamine's rapid and enduring antidepressant effects. Here we review recent insights from clinical and preclinical studies, with special emphasis of ketamine-induced changes in GABAergic synaptic transmission that are considered essential for its antidepressant therapeutic effects. Subanesthetic ketamine is now understood to exert its initial action by selectively blocking a subset of NMDA receptors on GABAergic interneurons, which results in disinhibition of glutamatergic target neurons, a surge in extracellular glutamate and correspondingly elevated glutamatergic synaptic transmission. This surge in glutamate appears to be corroborated by the rapid metabolism of ketamine into hydroxynorketamine, which acts at presynaptic sites to disinhibit the release of glutamate. Preclinical studies indicate that glutamate-induced activity triggers the release of BDNF, followed by transient activation of the mTOR pathway and increased expression of synaptic proteins, along with functional strengthening of glutamatergic synapses. This drug-on phase lasts for approximately 2h and is followed by a period of days characterized by structural maturation of newly formed glutamatergic synapses and prominently enhanced GABAergic synaptic inhibition. Evidence from mouse models with constitutive antidepressant-like phenotypes suggests that this phase involves strengthened inhibition of dendrites by somatostatin-positive GABAergic interneurons and correspondingly reduced NMDA receptor-mediated Ca²⁺ entry into dendrites, which activates an intracellular signaling cascade that converges with the mTOR pathway onto increased activity of the eukaryotic elongation factor eEF2 and enhanced translation of dendritic mRNAs. Newly synthesized proteins such as BDNF may be important for the prolonged therapeutic effects of ketamine.

Rapid anti-depressant-like effects of ketamine and other candidates: Molecular and cellular mechanisms

Major depressive disorder takes at least 3 weeks for clinical anti‐depressants, such as serotonin selective reuptake inhibitors, to take effect, and only one‐third of patients remit. Ketamine, a kind of anaesthetic, can alleviate symptoms of major depressive disorder patients in a short time and is reported to be effective to treatment‐resistant depression patients. The rapid and strong anti‐depressant‐like effects of ketamine cause wide concern. In addition to ketamine, caloric restriction and sleep deprivation also elicit similar rapid anti‐depressant‐like effects. However, mechanisms about the rapid anti‐depressant‐like effects remain unclear. Elucidating the mechanisms of rapid anti‐depressant effects is the key to finding new therapeutic targets and developing therapeutic patterns. Therefore, in this review we summarize potential molecular and cellular mechanisms of rapid anti‐depressant‐like effects based on the pre‐clinical and clinical evidence, trying to provide new insight into future therapy.

Lack of dopamine D1 receptors in the antidepressant actions of (R)-ketamine in a chronic social defeat stress model

It is reported that dopamine D₁ receptors in the medial prefrontal cortex play a role in the antidepressant actions of (R,S)-ketamine. However, its role in the antidepressant actions of (R)-ketamine, which is more potent than (S)-ketamine, is unknown. In the locomotion test, tail suspension test, forced swimming test and 1% sucrose preference test, pretreatment with dopamine D₁ receptor antagonist SCH-23390 did not block the antidepressant effects of (R)-ketamine in the susceptible mice after chronic social defeat stress. These findings suggest that dopamine D₁ receptors may not play a major role in the antidepressant actions of (R)-ketamine.

Essential role of microglial transforming growth factor-β1 in antidepressant actions of (R)-ketamine and the novel antidepressant TGF-β1

In rodent models of depression, (R)-ketamine has greater potency and longer-lasting antidepressant effects than (S)-ketamine; however, the precise molecular mechanisms underlying the antidepressant actions of (R)-ketamine remain unknown. Using RNA-sequencing analysis, we identified novel molecular targets that contribute to the different antidepressant effects of the two enantiomers. Either (R)-ketamine (10 mg/kg) or (S)-ketamine (10 mg/kg) was administered to susceptible mice after chronic social defeat stress (CSDS). RNA-sequencing analysis of prefrontal cortex (PFC) and subsequent GSEA (gene set enrichment analysis) revealed that transforming growth factor (TGF)-β signaling might contribute to the different antidepressant effects of the two enantiomers. (R)-ketamine, but not (S)-ketamine, ameliorated the reduced expressions of Tgfb1 and its receptors (Tgfbr1 and Tgfbr2) in the PFC and hippocampus of CSDS susceptible mice. Either pharmacological inhibitors (i.e., RepSox and SB431542) or neutralizing antibody of TGF-β1 blocked the antidepressant effects of (R)-ketamine in CSDS susceptible mice. Moreover, depletion of microglia by the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX3397 blocked the antidepressant effects of (R)-ketamine in CSDS susceptible mice. Similar to (R)-ketamine, the recombinant TGF-β1 elicited rapid and long-lasting antidepressant effects in animal models of depression. Our data implicate a novel microglial TGF-β1-dependent mechanism underlying the antidepressant effects of (R)-ketamine in rodents with depression-like phenotype. Moreover, TGF-β1 and its receptor agonists would likely constitute a novel rapid-acting and sustained antidepressant in humans.

Ketamine increases vmPFC activity: Effects of (R)- and (S)-stereoisomers and (2R,6R)-hydroxynorketamine metabolite

Ketamine, an NMDA receptor antagonist and fast acting antidepressant, produces a rapid burst of glutamate in the ventral medial prefrontal cortex (mPFC). Preclinical studies have demonstrated that pyramidal cell activity in the vmPFC is necessary for the rapid antidepressant response to ketamine in rodents. We sought to characterize the effects of ketamine and its stereoisomers (R and S), as well as a metabolite, (2R,6R)-hydroxynorketamine (HNK), on vmPFC activity using a genetically encoded calcium indicator (GCaMP6f). Ratiometric fiber photometry was utilized to monitor GCaMP6f fluorescence in pyramidal cells of mouse vmPFC prior to and immediately following administration of compounds. GCaMP6f signal was assessed to determine correspondance of activity between compounds. We observed dose dependent effects with (R,S)-ketamine (3-100 mg/kg), with the greatest effects on GCaMP6f activity at 30 mg/kg and lasting up to 20 min. (S)-ketamine (15 mg/kg), which has high affinity for the NMDA receptor channel produced similar effects to (R,S)-ketamine, but compounds with low NMDA receptor affinity, including (R)-ketamine (15 mg/kg) and (2R,6R)-HNK (30 mg/kg) had little or no effect on GCaMP6f activity. The initial response to administration of (R,S)-ketamine as well as (S)-ketamine is characterized by a brief period of robust GCaMP6f activation, consistent with increased activity of vmPFC pyramidal neurons. Because (2R,6R)-HNK and (R)-ketamine are reported to have antidepressant activity in rodent models the current results indicate that different initiating mechanisms lead to similar brain adaptive consequences that underlie the rapid antidepressant responses.

Rodent ketamine depression-related research: Finding patterns in a literature of variability

Discovering that the anesthetic drug ketamine has rapidly acting antidepressant effects in many individuals with major depression is one of the most important findings in clinical psychopharmacology in recent decades. The initial report of these effects in human subjects was based on a foundation of rodent preclinical studies carried out in the 1990s, and subsequent investigation has included both further studies in individuals with depression, as well as reverse translational experiments in animal models, especially rodents. While there is general agreement in the rodent literature that ketamine has rapidly-acting, and generally sustained, antidepressant-like properties, there are also points of contention across studies, including the precise mechanism of action of this drug. In this review, we briefly summarize prominent yet variable findings regarding the mechanism of action. We also discuss a combination of similarities and variances in the rodent literature in the antidepressant-like effects of ketamine as a function of dose, species and strain, test, stressor, and presumably sex of the experimenter. We then present previously unpublished mouse strain comparison data suggesting that subanesthetic ketamine does not have robust antidepressant-like properties in unstressed animals, and may actually promote depression-like behavior, in contrast to widely reported findings. We conclude that the data best support the notion of ketamine action principally via NMDA receptor antagonism, transiently boosting glutamatergic (and possibly other) signaling in diverse brain circuits. We also suggest that future studies should address in greater detail the extent to which antidepressant-like properties of this drug are stress-sensitive, in an effort to better model major depression present in humans.

Contribution of skeletal muscular glycine to rapid antidepressant effects of ketamine in an inflammation-induced mouse model of depression

RATIONALE

Basic and clinical studies have reported rapid and long-lasting antidepressant effects of ketamine. Although previous studies have proposed several mechanisms underlying the antidepressant effects of ketamine, these mechanisms have not been completely elucidated.

OBJECTIVES

The present study evaluated the effects of systemically administered ketamine treatment in a lipopolysaccharide (LPS)-induced mouse model of depression.

METHODS

Non-targeted metabolomics, western blotting, and behavioral tests (locomotion, tail suspension, and forced swimming tests) were performed.

RESULT

Ketamine significantly attenuated the abnormally increased immobility time in a lipopolysaccharide (LPS)-induced mouse model of depression. Aminomalonic acid, glutaraldehyde, glycine, histidine, N-methyl-L-glutamic acid, and ribose levels in skeletal muscle were altered following ketamine administration. Furthermore, ketamine significantly decreased the LPS-induced increase in glycine receptor A1 (GlyA1) levels. However, the glycine receptor antagonist strychnine did not elicit any pharmacological effects on ketamine-induced alterations in behaviors or muscular GlyA1 levels. Exogenous glycine and L-serine significantly improved depression-like symptoms in LPS-induced mice.

CONCLUSIONS

Our findings suggest that skeletal muscular glycine contributes to the antidepressant effects of ketamine in inflammation. Effective strategies for improving skeletal muscular glycine levels may be a novel approach to depression treatment.

Pharmacological Discrimination of Effects of MK801 on Thalamocortical, Mesothalamic, and Mesocortical Transmissions

N-methyl-d-aspartate/glutamate receptor (NMDAR) is one of the major voltage-sensitive ligand-gated cation channel. Several noncompetitive NMDAR antagonists contribute to pathophysiology of schizophrenia and mood disorders; however, the effects of inhibition of NMDAR on several transmitter system have not been well clarified. Thus, this study determined the selective NMDAR antagonist, MK801 (dizocilpine), on thalamocortical, mesothalamic, and mesocortical transmissions associated with l-glutamate, GABA, serotonin, norepinephrine, and dopamine using multiprobe microdialysis. Perfusion with MK801 into the medial prefrontal cortex (mPFC) increased and decreased respective regional releases of monoamine and GABA without affecting l-glutamate. The mPFC MK801-induced monoamine release is generated by the regional GABAergic disinhibition. Perfusion with MK801 into the reticular thalamic nucleus (RTN) decreased GABA release in the mediodorsal thalamic nucleus (MDTN) but increased releases of l-glutamate and catecholamine without affecting serotonin in the mPFC. The RTN MK801-induced l-glutamate release in the mPFC was generated by GABAergic disinhibition in the MDTN, but RTN MK801-induced catecholamine release in the mPFC was generated by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/glutamate receptor (AMPAR) which received l-glutamate release from thalamocortical glutamatergic terminals in the mPFC. Perfusion with MK801 into the dorsal raphe nucleus (DRN) decreased GABA release in the DRN but selectively increased serotonin release in the MDTN and mPFC. These DRN MK801-induced serotonin releases in the both mPFC and MDTN were also generated by GABAergic disinhibition in the DRN. These results indicate that the GABAergic disinhibition induced by NMDAR inhibition plays important roles in the MK801-induced releases of l-glutamate and monoamine in thalamic nuclei and cortex.

Molecular and cellular mechanisms underlying the antidepressant effects of ketamine enantiomers and its metabolites

Although the robust antidepressant effects of the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine in patients with treatment-resistant depression are beyond doubt, the precise molecular and cellular mechanisms underlying its antidepressant effects remain unknown. NMDAR inhibition and the subsequent α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) activation are suggested to play a role in the antidepressant effects of ketamine. Although (R)-ketamine is a less potent NMDAR antagonist than (S)-ketamine, (R)-ketamine has shown more marked and longer-lasting antidepressant-like effects than (S)-ketamine in several animal models of depression. Furthermore, non-ketamine NMDAR antagonists do not exhibit robust ketamine-like antidepressant effects in patients with depression. These findings suggest that mechanisms other than NMDAR inhibition play a key role in the antidepressant effects of ketamine. Duman's group demonstrated that the activation of mammalian target of rapamycin complex 1 (mTORC1) in the medial prefrontal cortex is reportedly involved in the antidepressant effects of ketamine. However, we reported that mTORC1 serves a role in the antidepressant effects of (S)-ketamine, but not of (R)-ketamine, and that extracellular signal-regulated kinase possibly underlie the antidepressant effects of (R)-ketamine. Several lines of evidence have demonstrated that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are crucial in the antidepressant effects of ketamine and its two enantiomers, (R)-ketamine and (S)-ketamine, in rodents. In addition, (2R,6R)-hydroxynormetamine [a metabolite of (R)-ketamine] and (S)-norketamine [a metabolite of (S)-ketamine] have been shown to exhibit antidepressant-like effects on rodents through the BDNF-TrkB cascade. In this review, we discuss recent findings on the molecular and cellular mechanisms underlying the antidepressant effects of enantiomers of ketamine and its metabolites. It may be time to reconsider the hypothesis of NMDAR inhibition and the subsequent AMPAR activation in the antidepressant effects of ketamine.

Rapid-acting antidepressant ketamine, its metabolites and other candidates: A historical overview and future perspective

Major depressive disorder (MDD) is one of the most disabling psychiatric disorders. Approximately one-third of the patients with MDD are treatment resistant to the current antidepressants. There is also a significant therapeutic time lag of weeks to months. Furthermore, depression in patients with bipolar disorder (BD) is typically poorly responsive to antidepressants. Therefore, there exists an unmet medical need for rapidly acting antidepressants with beneficial effects in treatment-resistant patients with MDD or BD. Accumulating evidence suggests that the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with MDD or BD. Ketamine is a racemic mixture comprising equal parts of (R)-ketamine (or arketamine) and (S)-ketamine (or esketamine). Because (S)-ketamine has higher affinity for NMDAR than (R)-ketamine, esketamine was developed as an antidepressant. On 5 March 2019, esketamine nasal spray was approved by the US Food and Drug Administration. However, preclinical data suggest that (R)-ketamine exerts greater potency and longer-lasting antidepressant effects than (S)-ketamine in animal models of depression and that (R)-ketamine has less detrimental side-effects than (R,S)-ketamine or (S)-ketamine. In this article, the author reviews the historical overview of the antidepressant actions of enantiomers of ketamine and its major metabolites norketamine and hydroxynorketamine. Furthermore, the author discusses the other potential rapid-acting antidepressant candidates (i.e., NMDAR antagonists and modulators, low-voltage-sensitive T-type calcium channel inhibitor, potassium channel Kir4.1 inhibitor, negative modulators of γ-aminobutyric acid, and type A [GABA_A ] receptors) to compare them with ketamine. Moreover, the molecular and cellular mechanisms of ketamine's antidepressant effects are discussed.

Abnormal composition of gut microbiota is associated with resilience versus susceptibility to inescapable electric stress

Increasing evidence indicates that abnormalities in the composition of gut microbiota might play a role in stress-related disorders. In the learned helplessness (LH) paradigm, ~60-70% rats are susceptible to LH in the face of inescapable electric stress. The role of gut microbiota in susceptibility in the LH paradigm is unknown. In this study, male rats were exposed to inescapable electric stress under the LH paradigm. The compositions of gut microbiota and short-chain fatty acids were assessed in fecal samples from control rats, non-LH (resilient) rats, and LH (susceptible) rats. Members of the order Lactobacillales were present at significantly higher levels in the susceptible rats than in control and resilient rats. At the family level, the number of Lactobacillaceae in the susceptible rats was significantly higher than in control and resilient rats. At the genus level, the numbers of Lactobacillus, Clostridium cluster III, and Anaerofustis in susceptible rats were significantly higher than in control and resilient rats. Levels of acetic acid and propionic acid in the feces of susceptible rats were lower than in those of control and resilient rats; however, the levels of lactic acid in the susceptible rats were higher than those of control and resilient rats. There was a positive correlation between lactic acid and Lactobacillus levels among these three groups. These findings suggest that abnormal composition of the gut microbiota, including organisms such as Lactobacillus, contributes to susceptibility versus resilience to LH in rats subjected to inescapable electric foot shock. Therefore, it appears likely that brain-gut axis plays a role in stress susceptibility in the LH paradigm.

(R)-Ketamine exerts antidepressant actions partly via conversion to (2R,6R)-hydroxynorketamine, while causing adverse effects at sub-anaesthetic doses

BACKGROUND AND PURPOSE

(R)-Ketamine (arketamine) may have utility as a rapidly acting antidepressant. While (R)-ketamine has lower potency than (R,S)-ketamine to inhibit NMDA receptors in vitro, the extent to which (R)-ketamine shares the NMDA receptor-mediated adverse effects of (R,S)-ketamine in vivo has not been fully characterised. Furthermore, (R)-ketamine is metabolised to (2R,6R)-hydroxynorketamine (HNK), which may contribute to its antidepressant-relevant actions.

EXPERIMENTAL APPROACH

Using mice, we compared (R)-ketamine with a deuterated form of the drug (6,6-dideutero-(R)-ketamine, (R)-d2 -ketamine), which hinders its metabolism to (2R,6R)-HNK, in behavioural tests predicting antidepressant responses. We also examined the actions of intracerebroventricularly infused (2R,6R)-HNK. Further, we quantified putative NMDA receptor inhibition-mediated adverse effects of (R)-ketamine.

KEY RESULTS

(R)-d2 -Ketamine was identical to (R)-ketamine in binding to and functionally inhibiting NMDA receptors but hindered (R)-ketamine's metabolism to (2R,6R)-HNK. (R)-Ketamine exerted greater potency than (R)-d2 -ketamine in several antidepressant-sensitive behavioural measures, consistent with a role of (2R,6R)-HNK in the actions of (R)-ketamine. There were dose-dependent sustained antidepressant-relevant actions of (2R,6R)-HNK following intracerebroventricular administration. (R)-Ketamine exerted NMDA receptor inhibition-mediated behaviours similar to (R,S)-ketamine, including locomotor stimulation, conditioned-place preference, prepulse inhibition deficits, and motor incoordination, with approximately half the potency of the racemic drug.

CONCLUSIONS AND IMPLICATIONS

Metabolism of (R)-ketamine to (2R,6R)-HNK increases the potency of (R)-ketamine to exert antidepressant-relevant actions in mice. Adverse effects of (R)-ketamine require higher doses than those necessary for antidepressant-sensitive behavioural changes in mice. However, our data revealing that (R)-ketamine's adverse effects are elicited at sub-anaesthetic doses indicate a potential risk for sensory dissociation and abuse liability.

Lateral Habenular Burst Firing as a Target of the Rapid Antidepressant Effects of Ketamine

The revolutionary discovery of the rapid antidepressant ketamine has been a milestone in psychiatry field in the last half century. Unlike conventional antidepressants that often take weeks to months to show efficacy, ketamine causes rapid antidepressant effects, emerging as early as within 1h after administration. However, how ketamine improves mood symptoms so quickly has remained elusive. Here, we first introduce the historical background of ketamine as a rapid antidepressant. We then discuss current hypotheses underlying ketamine's rapid antidepressant effects, with a focus on our latest discovery that ketamine silences NMDAR-dependent burst firing in the 'anti-reward center', the lateral habenula. While ketamine may act on many brain regions, we argue that its rapid antidepressant effects are critically dependent on ketamine's action in the lateral habenula, with this brain region acting as a primary site of action (or one among a few primary nodes). This molecular-, cellular-, and circuit-based mechanism advances our understanding of the etiology of depression and suggests a new conceptual framework for the rapid antidepressant effects of ketamine.

Beneficial effects of (R)-ketamine, but not its metabolite (2R,6R)-hydroxynorketamine, in the depression-like phenotype, inflammatory bone markers, and bone mineral density in a chronic social defeat stress model

Inflammatory bone markers may play a role in the antidepressant actions of (R)-ketamine in susceptible mice after chronic social defeat stress (CSDS). In this study, we compared the effects of (R)-ketamine and its final metabolite (2R,6R)-hydroxynorketamine (HNK) in depression-like phenotypes, inflammatory bone markers and bone mineral density (BMD) in CSDS susceptible mice. We measured plasma levels of inflammatory bone markers, which included osteoprotegerin (OPG), receptor activator of nuclear factor κB ligand (RANKL), and osteopontin after behavioral tests. (R)-ketamine, but not (2R,6R)-HNK, elicited rapid and sustained antidepressant effects in CSDS susceptible mice. Furthermore, (R)-ketamine, but not (2R,6R)-HNK, significantly improved the increased plasma levels of RANKL and decreased OPG/RANKL ratio in CSDS susceptible mice. Moreover, (R)-ketamine, but not (2R,6R)-HNK, significantly attenuated the decreased BMD in CSDS susceptible mice. These findings demonstrate that (R)-ketamine may have beneficial effects in depression-like phenotype and abnormalities in bone functions of CSDS susceptible mice. It is, therefore, likely that (R)-ketamine would be a potential therapeutic drug for abnormalities in bone metabolism in depressed patients.

Astroglial correlates of neuropsychiatric disease: From astrocytopathy to astrogliosis

Complex roles for astrocytes in health and disease continue to emerge, highlighting this class of cells as integral to function and dysfunction of the nervous system. In particular, escalating evidence strongly implicates a range of changes in astrocyte structure and function associated with neuropsychiatric diseases including major depressive disorder, schizophrenia, and addiction. These changes can range from astrocytopathy, degeneration, and loss of function, to astrogliosis and hypertrophy, and can be either adaptive or maladaptive. Evidence from the literature indicates a myriad of changes observed in astrocytes from both human postmortem studies as well as preclinical animal models, including changes in expression of glial fibrillary protein, as well as changes in astrocyte morphology and astrocyte-mediated regulation of synaptic function. In this review, we seek to provide a comprehensive assessment of these findings and consequently evidence for common themes regarding adaptations in astrocytes associated with neuropsychiatric disease. While results are mixed across conditions and models, general findings indicate decreased astrocyte cellular features and gene expression in depression, chronic stress and anxiety, but increased inflammation in schizophrenia. Changes also vary widely in response to different drugs of abuse, with evidence reflective of features of astrocytopathy to astrogliosis, varying across drug classes, route of administration and length of withdrawal.

An update on ketamine and its two enantiomers as rapid-acting antidepressants

Introduction: Depression is one of the most disabling diseases worldwide. Approximately one-third of depressed patients are treatment-resistant to the currently available antidepressants and there is a significant therapeutic time lag of weeks to months. There is a clear unmet need for rapid-acting and more efficacious treatments. (R,S)-ketamine, an old anesthetic drug, appears now to be going through a renaissance. Areas covered: This paper reviews recent literature describing the antidepressant effects of ketamine and its enantiomer (S)-ketamine in patients with major depressive disorder (MDD) and bipolar disorder (BD). Furthermore, the authors discuss the therapeutic potential of (R)-ketamine, another enantiomer of (R,S)-ketamine, and (S)-norketamine. Expert commentary: A number of clinical studies have demonstrated that (R,S)-ketamine has rapid-acting and sustained antidepressant activity in treatment-resistant patients with MDD, BD, and other psychiatric disorders. Off-label use of ketamine for mood disorders is proving popular in the United States. Meanwhile, preclinical data suggests that (R)-ketamine can exert longer-lasting antidepressant effects than (S)-ketamine in animal models of depression, and (R)-ketamine may have less detrimental side effects than (R,S)-ketamine and (S)-ketamine. Additionally, (S)-norketamine exhibits rapid and sustained antidepressant effects, with a potency similar to that of (S)-ketamine. Unlike (S)-ketamine, (S)-norketamine does not cause behavioral and biochemical abnormalities and could be a safer than (S)-ketamine too.

Lack of deuterium isotope effects in the antidepressant effects of (R)-ketamine in a chronic social defeat stress model

RATIONALE

(R,S)-ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, exhibits rapid and long-lasting antidepressant effects and anti-suicidal ideation in treatment-resistant patients with depression. However, the precise mechanisms underlying the antidepressant actions of (R,S)-ketamine are unknown. Although the previous report demonstrated the deuterium isotope effects in the antidepressant actions of (R,S)-ketamine, the deuterium isotope effects in the antidepressant actions of (R)-ketamine, which is more potent than (S)-ketamine, are unknown.

METHODS

We examined whether deuterium substitution at the C6 position could affect antidepressant effects of (R)-ketamine in a chronic social defeat stress (CSDS) model.

RESULTS

Pharmacokinetic studies showed that levels of (2R,6R)-d₁-hydroxynorketamine [(2R,6R)-d₁-HNK], a final metabolite of (R)-d₂-ketamine, in the plasma and brain after administration of (R)-d₂-ketamine (10 mg/kg) were lower than those of (2R,6R)-HNK from (R)-ketamine (10 mg/kg), indicating deuterium isotope effects in the production of (2R,6R)-HNK. In contrast, levels of (R)-ketamine and its metabolite (R)-norketamine in the plasma and brain were the same for both compounds. In a CSDS model, both (R)-ketamine (10 mg/kg) and (R)-d₂-ketamine (10 mg/kg) showed rapid and long-lasting (7 days) antidepressant effects, indicating no deuterium isotope effect in the antidepressant effects of (R)-ketamine.

CONCLUSIONS

The present study suggests that deuterium substitution of hydrogen at the C6 position slows the metabolism from (R)-ketamine to (2R,6R)-HNK in mice. In contrast, we did not find the deuterium isotope effects in terms of the rapid and long-lasting antidepressant effects of (R)-ketamine in a CSDS model. Therefore, it is unlikely that (2R,6R)-HNK is essential for antidepressant effects of (R)-ketamine.

Lack of Antidepressant Effects of Low-Voltage-Sensitive T-Type Calcium Channel Blocker Ethosuximide in a Chronic Social Defeat Stress Model: Comparison with (R)-Ketamine

BACKGROUND

A recent study demonstrated that low-voltage-sensitive T-type calcium channel blocker ethosuximide shows rapid antidepressant actions. This study was conducted to compare the antidepressant actions of ethosuximide and (R)-ketamine in a chronic social defeat stress model.

METHODS

Ethosuximide (100, 200, or 400 mg/kg), (R)-ketamine (10 mg/kg), or saline was administered i.p. to chronic social defeat stress-susceptible mice. Subsequently, locomotion test, tail suspension test, forced swimming test, and 1% sucrose preference test were performed.

RESULTS

(R)-ketamine showed rapid and long-lasting antidepressant actions in chronic social defeat stress-susceptible mice. In contrast, ethosuximide did not attenuate the increased immobility time of tail suspension test and forced swimming test in chronic social defeat stress-susceptible mice. In the sucrose preference test, ethosuximide did not improve decreased sucrose preference in chronic social defeat stress-susceptible mice.

CONCLUSIONS

Unlike (R)-ketamine, ethosuximide did not show rapid and sustained antidepressant effects in a chronic social defeat stress model. Therefore, it is unlikely that low-voltage-sensitive T-type calcium channel inhibitors may have ketamine-like robust antidepressant actions.

(2R,6R)-Hydroxynorketamine is not essential for the antidepressant actions of (R)-ketamine in mice

(R,S)-Ketamine has rapid and sustained antidepressant effects in depressed patients. Although the metabolism of (R,S)-ketamine to (2 R,6 R)-hydroxynorketamine (HNK), a metabolite of (R)-ketamine, has been reported to be essential for its antidepressant effects, recent evidence suggests otherwise. The present study investigated the role of the metabolism of (R)-ketamine to (2 R,6 R)-HNK in the antidepressant actions of (R)-ketamine. Antidepressant effects were evaluated using the forced swimming test in the lipopolysaccharide (LPS)-induced inflammation model of mice and the tail suspension test in naive mice. To prevent the metabolism of (R)-ketamine to (2 R,6 R)-HNK, mice were pretreated with cytochrome P450 (CYP) inhibitors. The concentrations of (R)-ketamine, (R)-norketamine, and (2 R,6 R)-HNK in plasma, brain, and cerebrospinal fluid (CSF) samples were determined using enantioselective liquid chromatography-tandem mass spectrometry. The concentrations of (R)-norketamine and (2 R,6 R)-HNK in plasma, brain, and CSF samples after administration of (R)-norketamine (10 mg/kg) and (2 R,6 R)-HNK (10 mg/kg), respectively, were higher than those generated after administration of (R)-ketamine (10 mg/kg). Nonetheless, while (R)-ketamine attenuated, neither (R)-norketamine nor (2 R,6 R)-HNK significantly altered immobility times of LPS-treated mice. Treatment with CYP inhibitors prior to administration of (R)-ketamine increased the plasma levels of (R)-ketamine, while generation of (2 R,6 R)-HNK was almost completely blocked. (R)-Ketamine exerted the antidepressant effects at a lower dose in the presence of CYP inhibitors than in their absence, which is consistent with exposure levels of (R)-ketamine but not (2 R,6 R)-HNK. These results indicate that metabolism to (2 R,6 R)-HNK is not necessary for the antidepressant effects of (R)-ketamine and that unmetabolized (R)-ketamine itself may be responsible for its antidepressant actions.

No Sex-Specific Differences in the Acute Antidepressant Actions of (R)-Ketamine in an Inflammation Model

BACKGROUND

Although previous reports suggest sex-specific differences in the antidepressant actions of (R,S)-ketamine, these differences in the antidepressant actions of (R)-ketamine, which is more potent than (S)-ketamine, are unknown.

METHODS

Saline or (R)-ketamine was administered 23 hours post lipopolysaccharide administration to adult male or female mice. Subsequently, antidepressant effects were assessed using a forced swimming test. Furthermore, the concentration of (R)-ketamine and its 2 major metabolites, (R)-norketamine and (2R,6R)-hydroxynorketamine, was measured in the plasma and brain after the administration of (R)-ketamine in the mice.

RESULTS

(R)-ketamine (10 mg/kg) significantly attenuated the increased immobility time of forced swimming test in the lipopolysaccharide-treated mice. There were no sex-specific differences in the concentrations of (R)-ketamine and its 2 metabolites in the plasma and brain.

CONCLUSIONS

These findings showed no sex-specific differences in terms of the acute antidepressant effects and pharmacokinetic profile of (R)-ketamine.

Beyond Ketamine: New Approaches to the Development of Safer Antidepressants

Background:

Ketamine has been reported to exert rapid and sustained antidepressant effects in patients with depression, including patients with treatment-resistant depression. However, ketamine has several drawbacks such as psychotomimetic/dissociative symptoms, abuse potential and neurotoxicity, all of which prevent its routine use in daily clinical practice.

Methods:

Therefore, development of novel agents with fewer safety and usage concerns for the treatment of depression has been actively investigated. From this standpoint, searching for active substances (stereoisomers and metabolites) and agents acting on the N-methyl-D-aspartate (NMDA) receptor have recently gained much attention.

Results:

The first approach includes stereoisomers of ketamine, (R)-ketamine and (S)-ketamine. Although (S)-ketamine has been considered as the active stereoisomer of racemic ketamine, recently, (R)-ketamine has been demonstrated to exert even more prolonged antidepressant effects in animal models than (S)-ketamine. Moreover, ketamine is rapidly metabolized into several metabolites, and some metabolites are speculated as being active substances exerting antidepressant effects. Of such metabolites, one in particular, namely, (2R,6R)-hydroxynorketamine, has been reported to be responsible for the antidepressant effects of ketamine. The second approach includes agents acting on the NMDA receptor, such as glycine site modulators and GluN2B subunit-selective antagonists. These agents have been tested in patients with treatment-resistant depression, and have been found to exhibit rapid antidepressant effects like ketamine.

Conclusion:

The above approaches may be useful to overcome the drawbacks of ketamine. Elucidation of the mechanisms of action of ketamine may pave the way for the development of antidepressant that are safer, but as potent and rapidly acting as ketamine.

Lack of metabolism in (R)-ketamine's antidepressant actions in a chronic social defeat stress model

Since the metabolism of (R,S)-ketamine to (2R,6R)-hydroxynorketamine (HNK) is reported to be essential for ketamine's antidepressant effects, there is an increasing debate about antidepressant effects of (2R,6R)-HNK. Using pharmacokinetic and behavioral techniques, we investigated whether intracerebroventricular (i.c.v.) infusion of (R)-ketamine or (2R,6R)-HNK show antidepressant effects in a chronic social defeat stress (CSDS) model of depression. Low levels of (2R,6R)-HNK in the brain after i.c.v. infusion of (R)-ketamine were detected, although brain levels of (2R,6R)-HNK were markedly lower than those after i.c.v. infusion of (2R,6R)-HNK. Furthermore, high levels of (2R,6R)-HNK in the blood and liver after i.c.v. infusion of (R)-ketamine or (2R,6R)-HNK were detected. A single i.c.v. infusion of (R)-ketamine showed rapid and long-lasting (7 days) antidepressant effects in a CSDS model. In contrast, i.c.v. infusion of (2R,6R)-HNK did not show any antidepressant effect in the same model, although brain concentration of (2R,6R)-HNK was higher than after i.c.v. infusion of (R)-ketamine. This study suggest that (R)-ketamine in the periphery after washout from the brain is metabolized to (2R,6R)-HNK in the liver, and subsequently, (2R,6R)-HNK enters into brain tissues. Furthermore, it is unlikely that (2R,6R)-HNK is essential for the antidepressant actions of (R)-ketamine in a CSDS model.

Mechanistic Target of Rapamycin-Independent Antidepressant Effects of (R)-Ketamine in a Social Defeat Stress Model

BACKGROUND

The role of the mechanistic target of rapamycin (mTOR) signaling in the antidepressant effects of ketamine is controversial. In addition to mTOR, extracellular signal-regulated kinase (ERK) is a key signaling molecule in prominent pathways that regulate protein synthesis. (R)-Ketamine has a greater potency and longer-lasting antidepressant effects than (S)-ketamine. Here we investigated whether mTOR signaling and ERK signaling play a role in the antidepressant effects of two enantiomers.

METHODS

The effects of mTOR inhibitors (rapamycin and AZD8055) and an ERK inhibitor (SL327) on the antidepressant effects of ketamine enantiomers in the chronic social defeat stress (CSDS) model (n = 7 or 8) and on those of ketamine enantiomers in these signaling pathways in mouse brain regions were examined.

RESULTS

The intracerebroventricular infusion of rapamycin or AZD8055 blocked the antidepressant effects of (S)-ketamine, but not (R)-ketamine, in the CSDS model. Furthermore, (S)-ketamine, but not (R)-ketamine, significantly attenuated the decreased phosphorylation of mTOR and its downstream effector, ribosomal protein S6 kinase, in the prefrontal cortex of susceptible mice after CSDS. Pretreatment with SL327 blocked the antidepressant effects of (R)-ketamine but not (S)-ketamine. Moreover, (R)-ketamine, but not (S)-ketamine, significantly attenuated the decreased phosphorylation of ERK and its upstream effector, mitogen-activated protein kinase/ERK kinase, in the prefrontal cortex and hippocampal dentate gyrus of susceptible mice after CSDS.

CONCLUSIONS

This study suggests that mTOR plays a role in the antidepressant effects of (S)-ketamine, but not (R)-ketamine, and that ERK plays a role in (R)-ketamine's antidepressant effects. Thus, it is unlikely that the activation of mTOR signaling is necessary for antidepressant actions of (R)-ketamine.

Hydroxynorketamine: Implications for the NMDA Receptor Hypothesis of Ketamine's Antidepressant Action

The prevailing hypothesis of ketamine’s unique antidepressant effects implicates N-methyl-d-aspartate receptor (NMDAR) inhibition-dependent enhancement of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated transmission, activation of intracellular signalling pathways and increased synaptogenesis. Recently, however, a seminal study by Zanos et al. directly challenged the NMDAR hypothesis of ketamine with the claim that an active ketamine metabolite, (2R,6R)-hydroxynorketamine, devoid of NMDAR binding properties or key side effects of its parent compound, is both necessary and sufficient for ketamine’s antidepressant effects in rodents. However, following these encouraging initial findings, one preclinical study failed to replicate the antidepressant effects of (2R,6R)-hydroxynorketamine (HNK), while others have questioned the metabolite’s contribution to ketamine’s therapeutic effects or argued against rejecting the NMDAR hypothesis of ketamine action. In light of these potentially paradigm-shifting, but highly controversial, findings, this review will summarise and critically evaluate the evidence for and against the NMDA receptor hypothesis of ketamine action, with a particular focus on (2R,6R)-HNK and the implications of its discovery for understanding ketamine’s mechanism of action in depression. Ultimately, uncovering the molecular mechanisms underlying the therapeutic effects of ketamine and possibly (2R,6R)-HNK, will aid the development of novel and more efficacious antidepressant agents so urgently needed to address a major public health concern, and could hold potential for the treatment of other stress-related psychopathologies, including bipolar disorder, post-traumatic stress disorder and suicidality.