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

A Single Administration of the Atypical Psychedelic Ibogaine or Its Metabolite Noribogaine Induces an Antidepressant-Like Effect in Rats

Anecdotal reports and open-label case studies in humans indicated that the psychedelic alkaloid ibogaine exerts profound antiaddictive effects. Ample preclinical evidence demonstrated the efficacy of ibogaine, and its main metabolite, noribogaine, in substance-use-disorder rodent models. In contrast to addiction research, depression-relevant effects of ibogaine or noribogaine in rodents have not been previously examined. We have recently reported that the acute ibogaine administration induced a long-term increase of brain-derived neurotrophic factor mRNA levels in the rat prefrontal cortex, which led us to hypothesize that ibogaine may elicit antidepressant-like effects in rats. Accordingly, we characterized behavioral effects (dose- and time-dependence) induced by the acute ibogaine and noribogaine administration in rats using the forced swim test (FST, 20 and 40 mg/kg i.p., single injection for each dose). We also examined the correlation between plasma and brain concentrations of ibogaine and noribogaine and the elicited behavioral response. We found that ibogaine and noribogaine induced a dose- and time-dependent antidepressant-like effect without significant changes of animal locomotor activity. Noribogaine's FST effect was short-lived (30 min) and correlated with high brain concentrations (estimated >8 μM of free drug), while the ibogaine's antidepressant-like effect was significant at 3 h. At this time point, both ibogaine and noribogaine were present in rat brain at concentrations that cannot produce the same behavioral outcome on their own (ibogaine ∼0.5 μM, noribogaine ∼2.5 μM). Our data suggests a polypharmacological mechanism underpinning the antidepressant-like effects of ibogaine and noribogaine.

Ketamine as an antidepressant: overview of its mechanisms of action and potential predictive biomarkers

Ketamine, a drug introduced in the 1960s as an anesthetic agent and still used for that purpose, has garnered marked interest over the past two decades as an emerging treatment for major depressive disorder. With increasing evidence of its efficacy in treatment-resistant depression and its potential anti-suicidal action, a great deal of investigation has been conducted on elucidating ketamine's effects on the brain. Of particular interest and therapeutic potential is the ability of ketamine to exert rapid antidepressant properties as early as several hours after administration. This is in stark contrast to the delayed effects observed with traditional antidepressants, often requiring several weeks of therapy for a clinical response. Furthermore, ketamine appears to have a unique mechanism of action involving glutamate modulation via actions at the N-methyl-D-aspartate (NMDA) and α -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, as well as downstream activation of brain-derived neurotrophic factor (BDNF) and mechanistic target of rapamycin (mTOR) signaling pathways to potentiate synaptic plasticity. This paper provides a brief overview of ketamine with regard to pharmacology/pharmacokinetics, toxicology, the current state of clinical trials on depression, postulated antidepressant mechanisms and potential biomarkers (biochemical, inflammatory, metabolic, neuroimaging sleep-related and cognitive) for predicting response to and/or monitoring of therapeutic outcome with ketamine.

Personality neuroscience and psychopathology: should we start with biology and look for neural-level factors?

"Personality is an abstraction used to explain consistency and coherency in an individual's pattern of affects, cognitions, desires and behaviors [ABCDs]" (Revelle, 2007, p. 37). But personality research currently provides more a taxonomy of patterns than theories of fundamental causes. Psychiatric disorders can be viewed as involving extremes of personality but are diagnosed via symptom patterns not biological causes. Such surface-level taxonomic description is necessary for science, but consistent predictive explanation requires causal theory. Personality constructs, and especially their clinical extremes, should predict variation in ABCD patterns, with parsimony requiring the lowest effective causal level of explanation. But, even biologically inspired personality theories currently use an intuitive language-based approach for scale development that lacks biological anchors. I argue that teleonomic "purpose" explains the organisation and outputs of conserved brain emotion systems, where high activation is adaptive in specific situations but is otherwise maladaptive. Simple modulators of whole-system sensitivity evolved because the requisite adaptive level can vary across people and time. Sensitivity to a modulator is an abstract predictive personality factor that operates at the neural level but provides a causal explanation of both coherence and occasional apparent incoherence in ABCD variation. Neuromodulators impact all levels of the "personality hierarchy" from metatraits to aspects: stability appears altered by serotonergic drugs, neuroticism by ketamine and trait anxiety by simple anxiolytic drugs. Here, the tools of psychiatry transfer to personality research and imply both interaction between levels and oblique factor mappings to ABCD. On this view, much psychopathology reflects extremes of neural-level personality factors, and we can view much pharmacotherapy as temporarily altering personality. So, particularly for personality factors linked to basic emotions and their disorders, I think we should start with evolutionary biology and look directly at conserved neural-level modulators for our explanatory personality constructs and only invoke higher order, emergent, explanations when neural-level explanation fails.

Ketamine and neuroticism: a double-hit hypothesis of internalizing disorders

Psychiatric disorders can often be viewed as extremes of personality traits. The primary action of drugs that ameliorate these disorders may, thus, be to alter the patient’s position on a relevant trait dimension. Here, we suggest that interactions between such trait dimensions may also be important for disorder. Internalizing disorders show important differences in terms of range of activity and speed of response of medications. Established antidepressant and anxiolytic medications are slow in onset and have differing effects across different internalizing disorders. In contrast, low-dose ketamine is rapidly effective and improves symptom ratings in all internalizing disorders. To account for this, we propose a “double hit” model for internalizing disorders: generation (and maintenance) require two distinct forms of neural dysfunction to coincide. One hit, sensitive to ketamine, is disorder-general: dysfunction of a neural system linked to high levels of the personality trait of neuroticism. The other hit is disorder-specific: dysfunction of one of a set of disorder-specific neural modules, each with its own particular pattern of sensitivity to conventional drugs. Our hypothesis applies only to internalizing disorders. So, we predict that ketamine will be effective in simple phobia and (perhaps partially) in anorexia nervosa, but would make no such prediction about other disorders where neuroticism might also be important secondarily (e.g. attention deficit hyperactivity disorder and schizophrenia).

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.

Ketamine: Leading us into the future for development of antidepressants

Numerous randomized double-blind clinical trials have consistently shown that that a single intravenous administration of a subanesthetic dose of ketamine to treatment-resistant depressed patients significantly improved depressive symptomatology rapidly, within two hours, with the effect lasting up to seven days. Despite its very promising effects, ketamine has long been associated with potential for abuse as it can cause psychotropic side effects, such as hallucinations, false beliefs, and severe impairments in judgment and other cognitive processes. Consequently, within the last two decades preclinical research has been carried out aimed at understanding its mechanisms of action and the brain circuits involved in ketamine's antidepressant effects, both of which are discussed in this review. Furthermore, with the hippocampus being a key target for ketamine's beneficial antidepressant effects, we and others have begun to examine behavioral and neurochemical effects of drugs that act selectively on the hippocampus due to the preferential location of their receptor targets. Such drugs are negative allosteric modulators (NAMs) and positive allosteric modulator (PAM) of the α5-GABA_A receptor. Such compounds are discussed within the framework of how lessons learned with ketamine point to novel classes of drugs, targeting the GABAergic system, that can recapitulate the antidepressant effects of ketamine without its adverse effects.

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.

Dissociating the Clinical Role and Economic Value of Intranasal Esketamine

DISCLOSURES

No funding supported the writing of this commentary. The author has nothing to disclose.

MPTP-induced dopaminergic neurotoxicity in mouse brain is attenuated after subsequent intranasal administration of (R)-ketamine: a role of TrkB signaling

RATIONALE

Parkinson's disease (PD) is characterized as a chronic and progressive neurodegenerative disorder, and PD patients have non-motor features such as depressive symptoms. Although there are several available medications to treat PD symptoms, these medications do not prevent the progression of the disease.

OBJECTIVE

(R)-ketamine has greater and longer-lasting antidepressant effects than (S)-ketamine in animal models of depression. This study was undertaken to investigate whether two enantiomers of ketamine and its metabolite norketamine shows neuroprotective effects in an animal model of PD.

METHODS

Effects of (R)-ketamine, (S)-ketamine, and their metabolites on MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced reduction of dopamine transporter (DAT) and tyrosine hydroxylase (TH) in the mouse striatum and substantia nigra (SNr) were examined.

RESULTS

MPTP-induced reduction of DAT in the striatum was attenuated by subsequent repeated intranasal administration of both enantiomers of ketamine although (R)-ketamine was more potent than (S)-ketamine. MPTP-induced reduction of TH in the striatum and SNr was attenuated by administration of (R)-ketamine, but not (S)-ketamine. Interestingly, MPTP-induced reduction of DAT in the striatum was also attenuated by a single intranasal administration of (R)-ketamine. In contrast, MPTP-induced reduction of DAT in the striatum was not attenuated by repeated intranasal administration of two enantiomers of norketamine. Furthermore, the pretreatment with TrkB antagonist ANA-12 significantly blocked the neuroprotective effects of (R)-ketamine in the MPTP-induced reduction of DAT in the striatum.

CONCLUSIONS

These findings suggest that (R)-ketamine can protect against MPTP-induced neurotoxicity in the mouse brain via TrkB activation. Therefore, (R)-ketamine could represent a therapeutic drug for neurodegenerative disorders such as PD.

Phencyclidine-induced cognitive deficits in mice are ameliorated by subsequent repeated intermittent administration of (R)-ketamine, but not (S)-ketamine: Role of BDNF-TrkB signaling

The N-methyl-d-aspartate receptor (NMDAR) antagonists including phencyclidine (PCP) and ketamine produce cognitive deficits in rodents and humans. We previously reported that (R)-ketamine produced the beneficial effects compared to (S)-ketamine in several animal models including depression. Here we compared the effects of two enantiomers of ketamine on cognitive deficits in mice after repeated administration of PCP. PCP (10 mg/kg/day for 10 days)-induced cognitive deficits were ameliorated by subsequent repeated intermittent administration of (R)-ketamine (10 mg/kg/day, twice weekly for 2-weeks), but not (S)-ketamine. Western blot analysis showed decreased levels of brain-derived neurotrophic factor (BDNF) and decreased ratio of phosphorylated-TrkB (p-TrkB) to TrkB in the prefrontal cortex (PFC) and hippocampus of PCP-treated mice. Furthermore, PCP-induced reduction of BDNF and p-TrkB/TrkB ratio in the PFC and hippocampus of PCP-treated mice was ameliorated by subsequent intermittent administration of (R)-ketamine. Interestingly, the beneficial effects of (R)-ketamine were blocked by pretreatment with TrkB inhibitor ANA-12. These findings suggest that (R)-ketamine could ameliorate PCP-induced cognitive deficits via activation of BDNF-TrkB signaling in the brain. Therefore, (R)-ketamine could be a potential therapeutic drug for cognitive impairment in patients with schizophrenia.

Esketamine for treatment resistant depression: a trick of smoke and mirrors?

In March 2019, the US Food and Drug Administration (FDA) approved a nasal spray formulation of esketamine for the treatment of resistant depression in adults. Esketamine is the S-enantiomer of ketamine, an FDA-approved anaesthetic, known to cause dissociation and, occasionally, hallucinations. While ketamine has not been approved for depression in the USA or in any other country, it has been used off-label in cases of severe depression. This commentary critically reviewed the evidence on esketamine submitted to the FDA, aiming to draw implications for clinical practice, research and regulatory science.

Beneficial effects of anti-RANKL antibody in depression-like phenotype, inflammatory bone markers, and bone mineral density in male susceptible mice after chronic social defeat stress

Multiple lines of evidence suggest a link between depression and osteoporosis in elderly people. Receptor activator of nuclear factor-κB ligand (RANKL) plays a role in the pathology of osteoporosis, and anti-RANKL antibody has been used in the treatment of osteoporosis. In this study, we investigated whether anti-mouse RANKL antibody could attenuate depression-like phenotypes, inflammatory bone markers and bone mineral density (BMD) in male susceptible mice after chronic social defeat stress (CSDS). We measured plasma levels of inflammatory bone markers, including osteoprotegerin (OPG), RANKL, and osteopontin. A single intravenous injection of anti-RANKL (2 mg/kg) elicited rapid antidepressant effects in CSDS susceptible mice. Furthermore, anti-RANKL significantly improved the increased plasma levels of RANKL and decreased OPG/RANKL ratio in CSDS susceptible mice. Moreover, anti-RANKL significantly attenuated the decreased BMD in CSDS susceptible mice. Interestingly, there is a positive correlation between anhedonia-like behavior and OPG/RANKL ratio in mice. These findings demonstrate that anti-RANKL may have beneficial effects in depression-like phenotype and abnormalities in bone functions of CSDS susceptible mice. It is, therefore, likely that anti-human RANKL antibody (i.e., Denosumab) would be a potential therapeutic drug for depression and osteoporosis.

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.

(R)-Ketamine Rapidly Ameliorates the Decreased Spine Density in the Medial Prefrontal Cortex and Hippocampus of Susceptible Mice After Chronic Social Defeat Stress

BACKGROUND

A recent study demonstrated that spine formation rates by ketamine in the prefrontal cortex (PFC) were not altered at 3-6 h following a single injection, but were markedly altered at 12-24 h. Here, we investigated the acute (3 h post-treatment) effects of (R)-ketamine in the decreased spine density in the medial PFC (mPFC) and hippocampus in susceptible mice after chronic social defeat stress (CSDS).

METHODS

(R)-ketamine (10 mg/kg) or saline was administered intraperitoneally to CSDS-susceptible mice. Dendritic spine density in the mPFC and hippocampus was measured 3 h after a single injection.

RESULTS

(R)-ketamine significantly ameliorated the decreased spine density in the prelimbic area of mPFC, Cornu Ammonis3, and dentate gyrus of the hippocampus of CSDS-susceptible mice.

CONCLUSIONS

This study suggests that (R)-ketamine rapidly ameliorates the decreased spine density in the mPFC and hippocampus of CSDS-susceptible mice, resulting in its rapid-acting antidepressant effects.

Esketamine for treatment resistant depression

Introduction: Treatment Resistant Depression (TRD) is a common and burdensome condition with poor outcomes and few treatment options. Esketamine is the S-enantiomer of ketamine and has recently been FDA approved in the United States for treating depression that has failed to respond to trials of two or more antidepressants. Areas covered: This review will briefly discuss current treatment options for TRD, then review esketamine. Relevant literature was identified through online database searches, and clinical trial data were provided by Janssen Pharmaceuticals. Pharmacology, including kinetics and dynamics, is discussed, then clinical data regarding efficacy and safety for esketamine from Phase 2-3 trials are reviewed. Expert opinion: In the expert opinion, the authors discuss multiple factors including patient, physician, and social factors that will influence the use of esketamine. While the efficacy of esketamine compared to off-label use of racemic ketamine remains unclear, both esketamine's approval for use in TRD and longer-term safety data may position it preferentially above racemic ketamine, although factors such as cost and monitoring requirements may limit its use. While questions remain regarding duration and frequency of treatment, as well as addictive potential, esketamine is a novel treatment option offering new hope for TRD.

Antidepressants and nose-to-brain delivery: drivers, restraints, opportunities and challenges

Why is nose-to-brain delivery considered to be a strategy that directly allows the access of antidepressants to the brain? In which circumstances can the intranasal pathway be applicable? Are there any requirements to follow? What triggers the antidepressant market? Which constraints are imposed during discovery programs? What opportunities can arise and what is their current status of development? Are they already translated into clinical practice? Which challenges are expected from recent development strategies? This review aims at providing a critical appraisal of nose-to-brain delivery of antidepressants, framed within a comprehensive analysis of drivers, restraints, opportunities and challenges.

Comparison of antidepressant and side effects in mice after intranasal administration of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine

The N-methyl-d-aspartate receptor (NMDAR) antagonist (R,S)-ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with depression although intranasal use of (R,S)-ketamine in ketamine abusers is popular. In March 5, 2019, nasal spray of (S)-ketamine for treatment-resistant depression was approved as a new antidepressant by the US Food Drug Administration. Clinical study of (R)-ketamine is underway. In a chronic social defeat stress (CSDS) model, we compared the antidepressant effects of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine after a single intranasal administration. Furthermore, we also compared the side effects (i.e., locomotion, prepulse inhibition (PPI), abuse liability) of these three compounds in mice. The order of potency of antidepressant effects after a single intranasal administration was (R)-ketamine > (R,S)-ketamine > (S)-ketamine. In contrast, the order of locomotor activity and prepulse inhibition (PPI) deficits after a single intranasal administration was (S)-ketamine > (R,S)-ketamine > (R)-ketamine. In the conditioned place preference (CPP) test, both (S)-ketamine and (R,S)-ketamine increased CPP scores in mice after repeated intranasal administration, in a dose dependent manner. In contrast, (R)-ketamine did not increase CPP scores in mice. These findings suggest that intranasal administration of (R)-ketamine would be a safer antidepressant than (R,S)-ketamine and (S)-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.