(R)-Ketamine Shows Greater Potency and Longer Lasting Antidepressant Effects Than Its Metabolite (2R,6R)-Hydroxynorketamine

miR-98-5p plays a critical role in depression and antidepressant effect of ketamine

Ketamine has been demonstrated to be a rapid-onset and long-lasting antidepressant, but its underlying molecular mechanisms remain unclear. Recent studies have emerged microRNAs as important modulators for depression treatment. In this study, we report that miR-98-5p is downregulated in the prefrontal cortex and hippocampus of mice subjected to chronic social stress, while overexpressing it by its agonist alleviates depression-like behaviors. More importantly, we demonstrate that miR-98-5p is upregulated by ketamine administration, while inhibition of it by its antagonist blocks the antidepressant effect of ketamine. Our data implicate a novel molecular mechanism underlying the antidepressant effect of ketamine, and that therapeutic strategies targeting miR-98-5p could exert beneficial effects for depression treatment.

Novel rapid-acting glutamatergic modulators: Targeting the synaptic plasticity in depression

Major depressive disorder (MDD) is severely prevalent, and conventional monoaminergic antidepressants gradually exhibit low therapeutic efficiency, especially for patients with treatment-resistant depression. A neuroplasticity hypothesis is an emerging advancement in the mechanism of depression, mainly expressed in the glutamate system, e.g., glutamate receptors and signaling. Dysfunctional glutamatergic neurotransmission is currently considered to be closely associated with the pathophysiology of MDD. Biological function, pharmacological action, and signal attributes in the glutamate system both regulate the neural process. Specific functional subunits could be therapeutic targets to explore the novel glutamatergic modulators, which have fast-acting, and relatively sustained antidepressant effects. Here, the present review summarizes the pathophysiology of MDD found in the glutamate system, exploring the role of glutamate receptors and their downstream effects. These convergent mechanisms have prompted the development of other modulators targeting on glutamate system, including N-methyl-d-aspartate receptor antagonists, selective GluN2B-specific antagonists, glycine binding site agents, and regulators of metabotropic glutamate receptors. Relevant researches underly the putative mechanisms of these drugs, which reverse the damage of depression by regulating glutamatergic neurotransmission. It also provides further insight into the mechanism of depression and exploring potential targets for novel agent development.

Gut microbiota is involved in the antidepressant-like effect of (S)-norketamine in an inflammation model of depression

The non-competitive glutamatergic N-methyl-d-aspartate receptor (NMDAR) antagonist, (R, S)-ketamine (ketamine), is known to exert rapid and long-lasting antidepressant-like effects. However, the widely use of ketamine is restricted owing to severe psychotomimetic side-effects and abuse liability. Very recently, we demonstrated that a major metabolite of ketamine, norketamine, in particular the (S)-enantiomer, had a potent antidepressant-like effect. We here examined the effects of a low-dose of norketamine enantiomers on depression symptoms and detected the changes in the composition of gut microbiota. In the behavioral tests, (S)-norketamine, but not (R)-norketamine, showed antidepressant-like effects in the lipopolysaccharide (LPS)-induced mice. At the genus level, (S)-norketamine, but not (R)-norketamine, significantly attenuated the increase in the levels of Escherichia-Shigella and Adlercreutzia, as well as the reduction in the levels of Harryflintia. At the species level, both (S)-norketamine and (R)-norketamine significantly attenuated the increase in the levels of bacterium ic1379 and Bacteroides sp. Marseille-P3166. Notably, (S)-norketamine was more potent than (R)-norketamine at reducing the levels of bacterium ic1379 and Bacteroides sp. Marseille-P3166. Furthermore, (S)-norketamine, but not (R)-norketamine, significantly attenuated the increased levels of Bacteroides caecigallinarum. In conclusion, this study suggests that the antidepressant-like effects of (S)-norketamine might be associated with the changes in the composition of gut microbiota. Therapeutic strategies improving the gut microbiota might facilitate the benefits for depression treatment.

Intranasal esketamine: From origins to future implications in treatment-resistant depression

The approval of intranasal esketamine for treatment-resistant depression marks the next step in our understanding of and ability to treat treatment-resistant depression. The origin of ketamine is rooted in the search for a phencyclidine analog that could be used as a pre-surgical anesthetic with less emergence delirium. Following its inception, ketamine has been used in a variety of contexts. However, it was the seminal Berman et al., 2000 study, which published positive findings from the first human trial using subanesthetic intravenous ketamine for depression. Since then, a large body of research has investigated ketamine's various proposed antidepressant mechanisms of action, and the role its pharmacokinetic properties and route of administration play in producing its antidepressant effects. The results of this research were the eventual approval of intranasal esketamine for treatment-resistant depression by the U.S. Food and Drug Administration (FDA) in March 2019. By identifying and utilizing predictors of response, we can continue to refine our approach to treating treatment-resistant depression and optimize patient response to intranasal esketamine. In this article, we look at the history, pharmacology, landmark studies, and future implications of intranasal esketamine for treatment-resistant depression.

Ketamine for depression

Over the last two decades, the dissociative anaesthetic agent ketamine, an uncompetitive N-Methyl-D-Aspartate (NMDA) receptor antagonist, has emerged as a novel therapy for treatment-resistant depression (TRD), demonstrating rapid and robust antidepressant effects within hours of administration. Ketamine is a racemic mixture composed of equal amounts of (S)-ketamine and (R)-ketamine. Although ketamine currently remains an off-label treatment for TRD, an (S)-ketamine nasal spray has been approved for use in TRD (in conjunction with an oral antidepressant) in the United States and Europe. Despite the promise of ketamine, key challenges including how to maintain response, concerns regarding short and long-term side-effects and the potential for abuse remain. This review provides an overview of the history of ketamine, its use in psychiatry and its basic pharmacology. The clinical evidence for the use of ketamine in depression and potential adverse effects associated with treatment are summarized. A synopsis of some of the putative neurobiological mechanisms underlying ketamine's rapid-acting antidepressant effects is provided before finally outlining future research directions, including the need to identify biomarkers for predicting response and treatment targets that may be used in the development of next-generation rapid-acting antidepressants that may lack ketamine's side-effects or abuse potential.

Hydroxynorketamines: Pharmacology and Potential Therapeutic Applications

Hydroxynorketamines (HNKs) are formed in vivo after (R,S)-ketamine (ketamine) administration. The 12 HNK stereoisomers are distinguished by the position of cyclohexyl ring hydroxylation (at the 4, 5, or 6 position) and their unique stereochemistry at two stereocenters. Although HNKs were initially classified as inactive metabolites because of their lack of anesthetic effects, more recent studies have begun to reveal their biologic activities. In particular, (2R,6R)- and (2S 6)-HNK exert antidepressant-relevant behavioral and physiologic effects in preclinical models, which led to a rapid increase in studies seeking to clarify the mechanisms by which HNKs exert their pharmacological effects. To date, the majority of HNK research has focused on the actions of (2R,6R)-HNK because of its robust behavioral actions in tests of antidepressant effectiveness and its limited adverse effects. This review describes HNK pharmacokinetics and pharmacodynamics, as well as the putative cellular, molecular, and synaptic mechanisms thought to underlie their behavioral effects, both following their metabolism from ketamine and after direct administration in preclinical studies. Converging preclinical evidence indicates that HNKs modulate glutamatergic neurotransmission and downstream signaling pathways in several brain regions, including the hippocampus and prefrontal cortex. Effects on other neurotransmitter systems, as well as possible effects on neurotrophic and inflammatory processes, and energy metabolism, are also discussed. Additionally, the behavioral effects of HNKs and possible therapeutic applications are described, including the treatment of unipolar and bipolar depression, post-traumatic stress disorder, chronic pain, neuroinflammation, and other anti-inflammatory and analgesic uses. SIGNIFICANCE STATEMENT: Preclinical studies indicate that hydroxynorketamines (HNKs) exert antidepressant-relevant behavioral actions and may also have analgesic, anti-inflammatory, and other physiological effects that are relevant for the treatment of a variety of human diseases. This review details the pharmacokinetics and pharmacodynamics of the HNKs, as well as their behavioral actions, putative mechanisms of action, and potential therapeutic applications.

Esketamine: a glimmer of hope in treatment-resistant depression

The motive of this article is to review the pharmacological and clinical aspects of esketamine (ESK), an NMDA-receptor antagonist approved recently by the FDA for treatment-resistant depression (TRD). PubMed/Medline database was searched using keywords 'esketamine' and 'depression', 'S-ketamine' and 'depression', and 'NMDA antagonist' and 'depression'. Individual trials were searched from ClinicalTrials.gov. We included English-language articles evaluating pharmacokinetics and pharmacodynamics of intranasal (IN) esketamine, along with clinical trial data related to its efficacy and safety in patients diagnosed with TRD. Compared to placebo, IN esketamine causes significant and rapid improvement in depression. Dizziness, vertigo, headache, increase in blood pressure are some of its common adverse effects. With the growing number of patients of TRD, additional effective and safe treatment is the need of the hour. Esketamine appears to be an effective therapy when combined with oral antidepressants in patients with TRD. It is of special value due to the rapid onset of its action. Long-term clinical studies are, however, needed to ascertain its safety profile.

Ketamine-50 years in use: from anesthesia to rapid antidepressant effects and neurobiological mechanisms

Over the past 50 years, ketamine has solidified its position in both human and veterinary medicine as an important anesthetic with many uses. More recently, ketamine has been studied and used for several new indications, ranging from chronic pain to drug addiction and post-traumatic stress disorder. The discovery of the rapid-acting antidepressant effects of ketamine has resulted in a surge of interest towards understanding the precise mechanisms driving its effects. Indeed, ketamine may have had the largest impact for advancements in the research and treatment of psychiatric disorders in the past few decades. While intense research efforts have been aimed towards uncovering the molecular targets underlying ketamine's effects in treating depression, the underlying neurobiological mechanisms remain elusive. These efforts are made more difficult by ketamine's complex dose-dependent effects on molecular mechanisms, multiple pharmacologically active metabolites, and a mechanism of action associated with the facilitation of synaptic plasticity. This review aims to provide a brief overview of the different uses of ketamine, with an emphasis on examining ketamine's rapid antidepressant effects spanning molecular, cellular, and network levels. Another focus of the review is to offer a perspective on studies related to the different doses of ketamine used in antidepressant research. Finally, the review discusses some of the latest hypotheses concerning ketamine's action.

Hallucinogens in Mental Health: Preclinical and Clinical Studies on LSD, Psilocybin, MDMA, and Ketamine

A revamped interest in the study of hallucinogens has recently emerged, especially with regard to their potential application in the treatment of psychiatric disorders. In the last decade, a plethora of preclinical and clinical studies have confirmed the efficacy of ketamine in the treatment of depression. More recently, emerging evidence has pointed out the potential therapeutic properties of psilocybin and LSD, as well as their ability to modulate functional brain connectivity. Moreover, MDMA, a compound belonging to the family of entactogens, has been demonstrated to be useful to treat post-traumatic stress disorders. In this review, the pharmacology of hallucinogenic compounds is summarized by underscoring the differences between psychedelic and nonpsychedelic hallucinogens as well as entactogens, and their behavioral effects in both animals and humans are described. Together, these data substantiate the potentials of these compounds in treating mental diseases.

Ketamine: A tale of two enantiomers

The discovery of the rapid antidepressant effects of the dissociative anaesthetic ketamine, an uncompetitive N-Methyl-D-Aspartate receptor antagonist, is arguably the most important breakthrough in depression research in the last 50 years. Ketamine remains an off-label treatment for treatment-resistant depression with factors that limit widespread use including its dissociative effects and abuse potential. Ketamine is a racemic mixture, composed of equal amounts of (S)-ketamine and (R)-ketamine. An (S)-ketamine nasal spray has been developed and approved for use in treatment-resistant depression in the United States and Europe; however, some concerns regarding efficacy and side effects remain. Although (R)-ketamine is a less potent N-Methyl-D-Aspartate receptor antagonist than (S)-ketamine, increasing preclinical evidence suggests (R)-ketamine may have more potent and longer lasting antidepressant effects than (S)-ketamine, alongside fewer side effects. Furthermore, a recent pilot trial of (R)-ketamine has demonstrated rapid-acting and sustained antidepressant effects in individuals with treatment-resistant depression. Research is ongoing to determine the specific cellular and molecular mechanisms underlying the antidepressant actions of ketamine and its component enantiomers in an effort to develop future rapid-acting antidepressants that lack undesirable effects. Here, we briefly review findings regarding the antidepressant effects of ketamine and its enantiomers before considering underlying mechanisms including N-Methyl-D-Aspartate receptor antagonism, γ-aminobutyric acid-ergic interneuron inhibition, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor activation, brain-derived neurotrophic factor and tropomyosin kinase B signalling, mammalian target of rapamycin complex 1 and extracellular signal-regulated kinase signalling, inhibition of glycogen synthase kinase-3 and inhibition of lateral habenula bursting, alongside potential roles of the monoaminergic and opioid receptor systems.

Splenic NKG2D confers resilience versus susceptibility in mice after chronic social defeat stress: beneficial effects of (R)-ketamine

The spleen is a large immune organ that plays a key role in the immune system. The precise molecular mechanisms underlying the relationship between the spleen and stress-related psychiatric disorders are unknown. Here we investigated the role of spleen in stress-related psychiatric disorders. FACS analysis was applied to determine the contribution of the spleen to susceptibility and resilience in mice that were subjected to chronic social defeat stress (CSDS). We found a notable increase in splenic volume and weight in CSDS-susceptible mice compared to control (no CSDS) mice and CSDS-resilient mice. The number of granulocytes, but not of T cells and B cells, in the spleen of susceptible mice was higher than in the spleen of both control and resilient mice. Interestingly, NKG2D (natural killer group 2, member D) expression in the spleen of CSDS-susceptible mice was higher than that in control mice and CSDS-resilient mice. In addition, NKG2D expression in the spleen of patients with depression was higher than that in controls. Both increased splenic weight and increased splenic NKG2D expression in CSDS-susceptible mice were ameliorated after a subsequent administration of (R)-ketamine. The present findings indicate a novel role of splenic NKG2D in stress susceptibility versus resilience in mice subjected to CSDS. Furthermore, abnormalities in splenic functions in CSDS-susceptible mice were ameliorated after subsequent injection of (R)-ketamine. Thus, the brain-spleen axis might, at least in part, contribute to the pathogenesis of stress-related psychiatric disorders such as depression.

Time is of the essence: Coupling sleep-wake and circadian neurobiology to the antidepressant effects of ketamine

Several studies have demonstrated the effectiveness of ketamine in rapidly alleviating depression and suicidal ideation. Intense research efforts have been undertaken to expose the precise mechanism underlying the antidepressant action of ketamine; however, the translation of findings into new clinical treatments has been slow. This translational gap is partially explained by a lack of understanding of the function of time and circadian timing in the complex neurobiology around ketamine. Indeed, the acute pharmacological effects of a single ketamine treatment last for only a few hours, whereas the antidepressant effects peak at around 24 hours and are sustained for the following few days. Numerous studies have investigated the acute and long-lasting neurobiological changes induced by ketamine; however, the most dramatic and fundamental change that the brain undergoes each day is rarely taken into consideration. Here, we explore the link between sleep and circadian regulation and rapid-acting antidepressant effects and summarize how diverse phenomena associated with ketamine's antidepressant actions - such as cortical excitation, synaptogenesis, and involved molecular determinants - are intimately connected with the neurobiology of wake, sleep, and circadian rhythms. We review several recently proposed hypotheses about rapid antidepressant actions, which focus on sleep or circadian regulation, and discuss their implications for ongoing research. Considering these aspects may be the last piece of the puzzle necessary to gain a more comprehensive understanding of the effects of rapid-acting antidepressants on the brain.

Ketamine metabolite (2R,6R)-hydroxynorketamine reverses behavioral despair produced by adolescent trauma

Early life trauma dramatically increases the risk of developing major depressive disorder (MDD), and is associated with a markedly decreased adult treatment response to antidepressants. Novel treatment approaches are required to treat childhood trauma-associated MDD. Recent studies suggest that the (R,S)-ketamine (ketamine) metabolite, (2R,6R)-hydroxynorketamine (HNK), exerts fast- and long-lasting antidepressant-like effects without ketamine's NMDAR-inhibition-associated adverse side-effect profile. We investigated the therapeutic potential of (2R,6R)-HNK against behavioral despair produced by a novel live-predator stress exposure during adolescence. Male and female C57BL/6J mice were exposed to a live snake or control conditions at post-natal (PND) days 31, 45 and 61. In order to assess the enduring consequences of trauma-exposure, at a minimum of 14 days following the last exposure, mice received inescapable shocks followed by a session with available escape options twenty-four hours later. Mice that manifested enduring escape deficits (helplessness) were treated with vehicle or (2R,6R)-HNK (20 mg/kg, i.p.), 24 h prior to retesting for reversal of escape deficits. We found that a significantly greater number of mice developed the helpless phenotype when they were exposed to the live predator and that the helpless phenotype was reversed in mice treated with (2R,6R)-HNK. There were no sex differences in the response to predator-stress exposure or (2R,6R)-HNK treatment. The live-predator model developed in this study provides an opportunity to further refine our understanding of the neurobiological substrates impacted by adolescent trauma and improve treatment strategies. The demonstrated efficacy of (2R,6R)-HNK in this model suggests a novel therapeutic intervention for a treatment-resistant population.

Ketamine and its metabolite, (2R,6R)-HNK, restore hippocampal LTP and long-term spatial memory in the Wistar-Kyoto rat model of depression

Accumulating evidence implicates dysregulation of hippocampal synaptic plasticity in the pathophysiology of depression. However, the effects of ketamine on synaptic plasticity and their contribution to its mechanism of action as an antidepressant, are still unclear. We investigated ketamine's effects on in vivo dorsal hippocampal (dHPC) synaptic plasticity and their role in mediating aspects of antidepressant activity in the Wistar-Kyoto (WKY) model of depression. dHPC long-term potentiation (LTP) was significantly impaired in WKY rats compared to Wistar controls. Importantly, a single low dose (5 mg/kg, ip) of ketamine or its metabolite, (2R,6R)-HNK, rescued the LTP deficit in WKY rats at 3.5 h but not 30 min following injection, with residual effects at 24 h, indicating a delayed, sustained facilitatory effect on dHPC synaptic plasticity. Consistent with the observed dHPC LTP deficit, WKY rats exhibited impaired hippocampal-dependent long-term spatial memory as measured by the novel object location recognition test (NOLRT), which was effectively restored by pre-treatment with both ketamine or (2R,6R)-HNK. In contrast, in WKYs, which display abnormal stress coping, ketamine, but not (2R,6R)-HNK, had rapid and sustained effects in the forced swim test (FST), a commonly used preclinical screen for antidepressant-like activity. The differential effects of (2R,6R)-HNK observed here reveal a dissociation between drug effects on FST immobility and dHPC synaptic plasticity. Therefore, in the WKY rat model, restoring dHPC LTP was not correlated with ketamine's effects in FST, but importantly, may have contributed to the reversal of hippocampal-dependent cognitive deficits, which are critical features of clinical depression. Our findings support the theory that ketamine may reverse the stress-induced loss of connectivity in key neural circuits by engaging synaptic plasticity processes to "reset the system".

Ketamine's dose related multiple mechanisms of actions: Dissociative anesthetic to rapid antidepressant

Ketamine induces safe and effective anesthesia and displays unusual cataleptic properties that gave rise to the term dissociative anesthesia. Since 1970, clinicians only utilized the drug as an anesthetic or analgesic for decades, but ketamine was found to have rapid acting antidepressant effects in 1990s. Accumulated evidence exhibits NMDAR antagonism may not be the only mechanism of ketamine. The contributions of AMPA receptor, mTor signal pathway, monoaminergic system, sigma-1 receptor, cholinergic, opioid and cannabinoid systems, as well as voltage-gated calcium channels and hyperpolarization cyclic nucleotide gated channels are discussed for the antidepressant effects. Also the effects of ketamine's enantiomers and metabolites are reviewed. Furthermore ketamine's anesthetic and analgesic mechanisms are briefly revisited. Overall, pharmacology of ketamine, its enantiomers and metabolites is very unique. Insight into multiple mechanisms of action will provide further development and desirable clinical effects of ketamine.

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.

Ketamine: The final frontier or another depressing end?

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

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.

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.

Neurobiology of rapid-acting antidepressants: convergent effects on GluA1-synaptic function

Efforts to develop efficacious antidepressant agents with novel mechanisms have been largely unsuccessful since the 1950's until the discovery of ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist that produces rapid and sustained antidepressant actions even in treatment-resistant patients. This finding has ushered in a new era for the development of novel rapid-acting antidepressants that act at the NMDA receptor complex, but without dissociative and psychotomimetic side effects of ketamine. Here, we review the current state of rapid-acting antidepressant drug development, including NMDA channel blockers, glycine site agents, and allosteric modulators, as well as ketamine stereoisomers and metabolites. In addition, we focus on the neurobiological mechanisms underlying the actions of these diverse agents and discuss evidence of convergent mechanisms including increased brain-derived neurotrophic factor signaling, increased synthesis of synaptic proteins, and most notably increased GluR1 and synaptic connectivity in the medial prefrontal cortex. These convergent mechanisms provide insight for potential additional novel targets for drug development (e.g., agents that increase synaptic protein synthesis and plasticity). Importantly, the convergent effects on synapse formation and plasticity also reverse the well-documented neuronal and synaptic deficits associated with stress and depression, and thereby target the underlying pathophysiology of major depressive disorder.

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 Induces a Greater Increase in Prefrontal 5-HT Release Than (S)-Ketamine and Ketamine Metabolites via an AMPA Receptor-Independent Mechanism

BACKGROUND

Although recent studies provide insight into the molecular mechanisms of the effects of ketamine, the antidepressant mechanism of ketamine enantiomers and their metabolites is not fully understood. In view of the involvement of mechanisms other than the N-methyl-D-aspartate receptor in ketamine's action, we investigated the effects of (R)-ketamine, (S)-ketamine, (R)-norketamine [(R)-NK], (S)-NK, (2R,6R)-hydroxynorketamine [(2R,6R)-HNK], and (2S,6S)-HNK on monoaminergic neurotransmission in the prefrontal cortex of mice.

METHODS

The extracellular monoamine levels in the prefrontal cortex were measured by in vivo microdialysis.

RESULTS

(R)-Ketamine and (S)-ketamine acutely increased serotonin release in a dose-dependent manner, and the effect of (R)-ketamine was greater than that of (S)-ketamine. In contrast, (S)-ketamine caused a robust increase in dopamine release compared with (R)-ketamine. Both ketamine enantiomers increased noradrenaline release, but these effects did not differ. (2R,6R)-HNK caused a slight but significant increase in serotonin and noradrenaline but not dopamine release. (S)-NK increased dopamine and noradrenaline but not serotonin release. Differential effects between (R)-ketamine and (S)-ketamine were also observed in a lipopolysaccharide-induced model of depression. An α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor antagonist, 2,3-dioxo-6-nitro-1,2,3,4- tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), attenuated (S)-ketamine-induced, but not (R)-ketamine-induced serotonin release, whereas NBQX blocked dopamine release induced by both enantiomers. Local application of (R)-ketamine into the prefrontal cortex caused a greater increase in prefrontal serotonin release than that of (S)-ketamine.

CONCLUSIONS

(R)-Ketamine strongly activates the prefrontal serotonergic system through an AMPA receptor-independent mechanism. (S)-Ketamine-induced serotonin and dopamine release was AMPA receptor-dependent. These findings provide a neurochemical basis for the underlying pharmacological differences between ketamine enantiomers and their metabolites.

(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.

Ketamine metabolite (2R,6R)-hydroxynorketamine enhances aggression via periaqueductal gray glutamatergic transmission

(2R,6R)-hydroxynorketamine (HNK), a metabolite of ketamine, has recently been suggested to be a potent antidepressant for treating animal depression and has rapid-onset and long-lasting action through potentiating glutamatergic transmission. However, its other effects are still unclear. In the present study, we tested the effects of (2R,6R)-HNK on offensive aggression. A resident-intruder (RI) test was used as the main model to test elements of offensive aggression, including threats and bites. Electrophysiological recordings in the ventrolateral periaqueductal gray (vlPAG) were used to measure the functions of glutamatergic synaptic transmission. A single systemic injection of (2R,6R)-HNK, but not (2S,6S)-HNK, increased elements of offensive aggression, including threats and bites, in a dose-dependent manner with long-lasting action. Moreover, (2R,6R)-HNK increased the input-output curve, the AMPA-mediated current, and the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) and decreased the paired-pulse ratio (PPR) in the vlPAG. Furthermore, intra-vlPAG application of (2R,6R)-HNK increased aggressive and biting behaviors, which were abolished by an intra-vlPAG pretreatment with the AMPA receptors antagonist, CNQX. Notably, the intra-vlPAG CNQX pretreatment eliminated systemic (2R,6R)-HNK-enhanced aggressive and biting behaviors. The results of this suggest that (2R,6R)-HNK evokes offensive aggression by increasing vlPAG glutamatergic transmission. Although (2R,6R)-HNK is currently suggested to be effective for treating depression, its side effect of increasing offensive aggression should be a subject of concern in future drug development and therapy.

Abnormalities in Inflammatory Cytokines Confer Susceptible to Chronic Neuropathic Pain-related Anhedonia in a Rat Model of Spared Nerve Injury

Objective

Patients with chronic neuropathic pain (CNP) have a higher incidence to develop depression. However, its pathogenesis has not yet been fully elucidated. Here we aimed to investigate the role of inflammatory cytokines in CNP-related anhedonia, which is a core symptom of depression, and to explore the effects of ketamine and parecoxib on pain and anhedonia.

Methods

A rat model of spared nerve injury (SNI) was constructed to mimic CNP. Hierarchical cluster analysis of sucrose preference test (SPT) was applied to classify the SNI rats into anhedonia susceptible and unsusceptible. Inflammatory cytokines in medial prefrontal cortex (mPFC) of brain, serum and L2–5 spinal cord were measured. Moreover, effects of ketamine or parecoxib on mechanical withdrawal test (MWT) and SPT in anhedonia susceptible rats were detected.

Results

Tumor necrosis factor (TNF)-α was increased in mPFC, serum and and spinal cord of anhedonia susceptible rats. Furthermore, anhedonia susceptible and unsusceptible rats both increased the interleukin (IL)-1β level in mPFC, serum and spinal cord. IL-6 was altered in serum and spinal cord, but not in mPFC. IL-10 was significantly altered in mPFC and serum, but not in spinal cord. Additionally, ketamine treatment significantly attenuated the decreased results of MWT and SPT in anhedonia susceptible rats, and that parecoxib significantly improved the MWT score, but failed to alter the result of SPT.

Conclusion

These findings suggest that abnormalities in inflammatory cytokines confer susceptible to anhedonia in a rat model of SNI. Ketamine, a fast-acting antidepressant, has pharmacological benefits to alleviate pain and anhedonia symptoms.

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.

Altered Connectivity in Depression: GABA and Glutamate Neurotransmitter Deficits and Reversal by Novel Treatments

The mechanisms underlying the pathophysiology and treatment of depression and stress-related disorders remain unclear, but studies in depressed patients and rodent models are beginning to yield promising insights. These studies demonstrate that depression and chronic stress exposure cause atrophy of neurons in cortical and limbic brain regions implicated in depression, and brain imaging studies demonstrate altered connectivity and network function in the brains of depressed patients. Studies of the neurobiological basis of the these alterations have focused on both the principle, excitatory glutamate neurons, as well as inhibitory GABA interneurons. They demonstrate structural, functional, and neurochemical deficits in both major neuronal types that could lead to degradation of signal integrity in cortical and hippocampal regions. The molecular mechanisms underlying these changes have not been identified but are thought to be related to stress induced excitotoxic effects in combination with elevated adrenal glucocorticoids and inflammatory cytokines as well as other environmental factors. Transcriptomic studies are beginning to demonstrate important sex differences and, together with genomic studies, are starting to reveal mechanistic domains of overlap and uniqueness with regards to risk and pathophysiological mechanisms with schizophrenia and bipolar disorder. These studies also implicate GABA and glutamate dysfunction as well as immunologic mechanisms. While current antidepressants have significant time lag and efficacy limitations, new rapid-acting agents that target the glutamate and GABA systems address these issues and offer superior therapeutic interventions for this widespread and debilitating disorder.

Decoding the Mechanism of Action of Rapid-Acting Antidepressant Treatment Strategies: Does Gender Matter?

Gender differences play a pivotal role in the pathophysiology and treatment of major depressive disorder. This is strongly supported by a mean 2:1 female-male ratio of depression consistently observed throughout studies in developed nations. Considering the urgent need to tailor individualized treatment strategies to fight depression more efficiently, a more precise understanding of gender-specific aspects in the pathophysiology and treatment of depressive disorders is fundamental. However, current treatment guidelines almost entirely neglect gender as a potentially relevant factor. Similarly, the vast majority of animal experiments analysing antidepressant treatment in rodent models exclusively uses male animals and does not consider gender-specific effects. Based on the growing interest in innovative and rapid-acting treatment approaches in depression, such as the administration of ketamine, its metabolites or electroconvulsive therapy, this review article summarizes the evidence supporting the importance of gender in modulating response to rapid acting antidepressant treatment. We provide an overview on the current state of knowledge and propose a framework for rodent experiments to ultimately decode gender-dependent differences in molecular and behavioural mechanisms involved in shaping treatment response.

Antidepressant-relevant concentrations of the ketamine metabolite (2R,6R)-hydroxynorketamine do not block NMDA receptor function

Preclinical studies indicate that (2R,6R)-hydroxynorketamine (HNK) is a putative fast-acting antidepressant candidate. Although inhibition of NMDA-type glutamate receptors (NMDARs) is one mechanism proposed to underlie ketamine's antidepressant and adverse effects, the potency of (2R,6R)-HNK to inhibit NMDARs has not been established. We used a multidisciplinary approach to determine the effects of (2R,6R)-HNK on NMDAR function. Antidepressant-relevant behavioral responses and (2R,6R)-HNK levels in the extracellular compartment of the hippocampus were measured following systemic (2R,6R)-HNK administration in mice. The effects of ketamine, (2R,6R)-HNK, and, in some cases, the (2S,6S)-HNK stereoisomer were evaluated on the following: (i) NMDA-induced lethality in mice, (ii) NMDAR-mediated field excitatory postsynaptic potentials (fEPSPs) in the CA1 field of mouse hippocampal slices, (iii) NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and NMDA-evoked currents in CA1 pyramidal neurons of rat hippocampal slices, and (iv) recombinant NMDARs expressed in Xenopus oocytes. While a single i.p. injection of 10 mg/kg (2R,6R)-HNK exerted antidepressant-related behavioral and cellular responses in mice, the ED₅₀ of (2R,6R)-HNK to prevent NMDA-induced lethality was found to be 228 mg/kg, compared with 6.4 mg/kg for ketamine. The 10 mg/kg (2R,6R)-HNK dose generated maximal hippocampal extracellular concentrations of ∼8 µM, which were well below concentrations required to inhibit synaptic and extrasynaptic NMDARs in vitro. (2S,6S)-HNK was more potent than (2R,6R)-HNK, but less potent than ketamine at inhibiting NMDARs. These data demonstrate the stereoselectivity of NMDAR inhibition by (2R,6R;2S,6S)-HNK and support the conclusion that direct NMDAR inhibition does not contribute to antidepressant-relevant effects of (2R,6R)-HNK.

Role of Soluble Epoxide Hydrolase in Metabolism of PUFAs in Psychiatric and Neurological Disorders

Inflammation plays a key role in the pathogenesis of a number of psychiatric and neurological disorders. Soluble epoxide hydrolases (sEH), enzymes present in all living organisms, metabolize epoxy fatty acids (EpFAs) to corresponding 1,2-diols by the addition of a molecule of water. Accumulating evidence suggests that sEH in the metabolism of polyunsaturated fatty acids (PUFAs) plays a key role in inflammation. Preclinical studies demonstrated that protein expression of sEH in the prefrontal cortex, striatum, and hippocampus from mice with depression-like phenotype was higher than control mice. Furthermore, protein expression of sEH in the parietal cortex from patients with major depressive disorder was higher than controls. Interestingly, Ephx2 knock-out (KO) mice exhibit stress resilience after chronic social defeat stress. Furthermore, the sEH inhibitors have antidepressant effects in animal models of depression. In addition, pharmacological inhibition or gene KO of sEH protected against dopaminergic neurotoxicity in the striatum after repeated administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in an animal model of Parkinson’s disease (PD). Protein expression of sEH in the striatum from MPTP-treated mice was higher than control mice. A number of studies using postmortem brain samples showed that the deposition of protein aggregates of α-synuclein, termed Lewy bodies, is evident in multiple brain regions of patients from PD and dementia with Lewy bodies (DLB). Moreover, the expression of the sEH protein in the striatum from patients with DLB was significantly higher compared with controls. Interestingly, there was a positive correlation between sEH expression and the ratio of phosphorylated α-synuclein to α-synuclein in the striatum. In the review, the author discusses the role of sEH in the metabolism of PUFAs in inflammation-related psychiatric and neurological disorders.

Activity-dependent brain-derived neurotrophic factor signaling is required for the antidepressant actions of (2R,6R)-hydroxynorketamine

Ketamine, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, produces rapid and long-lasting antidepressant effects in major depressive disorder (MDD) patients. (2R,6R)-Hydroxynorketamine [(2R,6R)-HNK], a metabolite of ketamine, is reported to produce rapid antidepressant effects in rodent models without the side effects of ketamine. Importantly, (2R,6R)-HNK does not block NMDA receptors like ketamine, and the molecular signaling mechanisms for (2R,6R)-HNK remain unknown. Here, we examined the involvement of BDNF/TrkB/mechanistic target of rapamycin complex 1 (mTORC1) signaling in the antidepressant actions of (2R,6R)-HNK. Intramedial prefrontal cortex (intra-mPFC) infusion or systemic (2R,6R)-HNK administration induces rapid and long-lasting antidepressant effects in behavioral tests, identifying the mPFC as a key region for the actions of (2R,6R)-HNK. The antidepressant actions of (2R,6R)-HNK are blocked in mice with a knockin of the BDNF Val66Met allele (which blocks the processing and activity-dependent release of BDNF) or by intra-mPFC microinjection of an anti-BDNF neutralizing antibody. Blockade of L-type voltage-dependent Ca²⁺ channels (VDCCs), required for activity-dependent BDNF release, also blocks the actions of (2R,6R)-HNK. Intra-mPFC infusion of pharmacological inhibitors of TrkB or mTORC1 signaling, which are downstream of BDNF, also block the actions of (2R,6R)-HNK. Moreover, (2R,6R)-HNK increases synaptic function in the mPFC. These findings indicate that activity-dependent BDNF release and downstream TrkB and mTORC1 signaling, which increase synaptic function in the mPFC, are required for the rapid and long-lasting antidepressant effects of (2R,6R)-HNK, supporting the potential use of this metabolite for the treatment of MDD.

Hyperforin Potentiates Antidepressant-Like Activity of Lanicemine in Mice

N-methyl-D-aspartate receptor (NMDAR) modulators induce rapid and sustained antidepressant like-activity in rodents through a molecular mechanism of action that involves the activation of Ca²⁺ dependent signaling pathways. Moreover, ketamine, a global NMDAR antagonist is a potent, novel, and atypical drug that has been successfully used to treat major depressive disorder (MDD). However, because ketamine evokes unwanted side effects, alternative strategies have been developed for the treatment of depression. The objective of the present study was to determine the antidepressant effects of either a single dose of hyperforin or lanicemine vs. their combined effects in mice. Hyperforin modulates intracellular Ca²⁺ levels by activating Ca²⁺-conducting non-selective canonical transient receptor potential 6 channel (TRPC6) channels. Lanicemine, on the other hand, blocks NMDARs and regulates Ca²⁺ dependent processes. To evaluate the antidepressant-like activity of hyperforin and lanicemine, a set of in vivo (behavioral) and in vitro methods (western blotting, Ca²⁺ imaging studies, electrophysiological, and radioligand binding assays) was employed. Combined administration of hyperforin and lanicemine evoked long-lasting antidepressant-like effects in both naïve and chronic corticosterone-treated mice while also enhancing the expression of the synapsin I, GluA1 subunit, and brain derived neurotrophic factor (BDNF) proteins in the frontal cortex. In Ca²⁺ imaging studies, lanicemine enhanced Ca²⁺ influx induced by hyperforin. Moreover, compound such as MK-2206 (Akt kinase inhibitor) inhibited the antidepressant-like activity of hyperforin in the tail suspension test (TST). Hyperforin reversed disturbances induced by MK-801 in the novel object recognition (NOR) test and had no effects on NMDA currents and binding to NMDAR. Our results suggest that co-administration of hyperforin and lanicemine induces long-lasting antidepressant effects in mice and that both substances may have different molecular targets.

Is the History Repeated? Can (2R,6R)-Hydroxynorketamine be Another Antidepressant?

Historically, identification of active metabolites has contributed to drug discovery for psychiatric disorders. It has led to the identification of new medications such as desipramine (a metabolite of imipramine) and paliperidone (a metabolite of risperidone). (R,S)-Ketamine, which has been regarded as the greatest breakthrough in depression research, is rapidly and stereoselectively metabolized into a variety of metabolites. Therefore, identification of an active substance after administration of (R,S)-ketamine is a critical issue, not only to delineate the underlying mechanisms but also to pave the way to develop a new antidepressant. Recently, one of the metabolites of (R,S)-ketamine, namely, (2R,6R)-hydroxynorketamine (HNK) was proposed as an active metabolite formed after administration of (R,S)-ketamine, and even as being essential for (R,S)-ketamine to exert its antidepressant effects. However, this is still controversial. Indeed, we demonstrated that the antidepressant effect of (2R,6R)-HNK is not as potent as that of its parent compounds ((R)-ketamine and (R,S)-ketamine), and that (2R,6R)-HNK is not essential for (R)-ketamine to exert its antidepressant effects. From the historical point of view, however, there is potential to discover new medications by further investigations of (2R,6R)-HNK. Therefore, more careful and thorough investigation of (2R,6R)-HNK is needed for the discovery of more efficacious and safer antidepressants.

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.

Comparison of rapid and long-lasting antidepressant effects of negative modulators of α5-containing GABAA receptors and (R)‑ketamine in a chronic social defeat stress model

The negative allosteric modulators (NAMs: L-655,708 and MRK-016) of α5 subunit-containing GABA_A receptors are reported to show rapid-acting antidepressant effects in rodents. However, there are no reports comparing these NAMs and (R)‑ketamine, (R)-enantiomer of the rapid-acting antidepressant ketamine, in a chronic social defeat stress (CSDS) model. Here we measured expression of α5 GABA_A receptor in the brain regions from CSDS susceptible mice and postmortem brain samples from depressed patients. Expression of α5 GABA_A receptor in the prefrontal cortex and hippocampus from CSDS susceptible mice was significantly higher than that of control mice. Furthermore, expression of α5 GABA_A receptor in the parietal cortex from depressed patients was also higher than that of control subjects. In the tail suspension and forced swimming tests, (R)‑ketamine and MRK-016 significantly attenuated the increased immobility time in the susceptible mice, compared with the vehicle-treated group. In the sucrose preference test, (R)‑ketamine and MRK-016 significantly enhanced the reduced preference in CSDS susceptible mice two days after a single injection. Unlike (R)‑ketamine, MRK-016 did not attenuate the reduced sucrose preference in susceptible mice 7 days after a single injection. In contrast, L-655,708 did not show antidepressant effects in the same model. In conclusion, this study shows that increased levels of α5 GABA_A receptors in the PFC and hippocampus may play a role in depression-like phenotype after CSDS. It is unlikely that MRK-016 has long-lasting antidepressant effects although it elicits rapid-acting antidepressant effects.

AMPA Receptor Activation-Independent Antidepressant Actions of Ketamine Metabolite (S)-Norketamine

BACKGROUND

Ketamine, an N-methyl-D-aspartate receptor antagonist, exerts robust antidepressant effects in patients with treatment-resistant depression. The precise mechanisms underlying ketamine's antidepressant actions remain unclear, although previous research suggests that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) activation plays a role. We investigated whether (S)-norketamine and (R)-norketamine, the two main metabolites of (R,S)-ketamine, also play a significant role in ketamine's antidepressant effects and whether the effects are mediated by AMPAR.

METHODS

Cellular mechanisms of antidepressant action of norketamine enantiomers were examined in mice.

RESULTS

(S)-Norketamine had more potent antidepressant effects than (R)-norketamine in inflammation and chronic social defeat stress models. Furthermore, (S)-norketamine induced more beneficial effects on decreased dendritic spine density and synaptogenesis in the prefrontal cortex and hippocampus compared with (R)-norketamine. Unexpectedly, AMPAR antagonists did not block the antidepressant effects of (S)-norketamine. The electrophysiological data showed that, although (S)-norketamine inhibited N-methyl-D-aspartate receptor-mediated synaptic currents, (S)-norketamine did not enhance AMPAR-mediated neurotransmission in hippocampal neurons. Furthermore, (S)-norketamine improved reductions in brain-derived neurotrophic factor-tropomyosin receptor kinase B signaling in the prefrontal cortex of mice susceptible to chronic social defeat stress, whereas the tropomyosin receptor kinase B antagonist and a mechanistic target of rapamycin inhibitor blocked the antidepressant effects of (S)-norketamine. In contrast to (S)-ketamine, (S)-norketamine did not cause behavioral abnormalities, such as prepulse inhibition deficits, reward effects, loss of parvalbumin immunoreactivity in the medial prefrontal cortex, or baseline gamma-band oscillation increase.

CONCLUSIONS

Our data identified a novel AMPAR activation-independent mechanism underlying the antidepressant effects of (S)-norketamine. (S)-Norketamine and its prodrugs could be novel antidepressants without the detrimental side effects of (S)-ketamine.