Mu Opioid Receptor Activation Mediates (S)-ketamine Reinforcement in Rats: Implications for Abuse Liability

Beneficial effects of Esketamine on Morphine preference reacquisition in male rats

Addiction is a chronic condition that poses a serious public health challenge, particularly highlighted by the global opioid crisis involving drugs such as morphine (MORPH). One of the major obstacles in effective detoxification is the high relapse rate, with many individuals resuming drug use after withdrawal. Pharmacological treatments developed so far have generally shown limited efficacy in addressing substance use disorder. In this context, esketamine (ESK), the S-ketamine isomer, has been used in cases of treatment-resistant recurrent depression and depression with suicide risk. In our study, rats were treated with two doses of ESK every five days (acute - A-ESK) or daily (sub-chronic - SC-ESK) during MORPH-conditioned place preference (CPP) extinction. After 10 days, the animals were re-exposed to MORPH to assess preference reacquisition in the CPP paradigm. Our findings showed that both acute and sub-chronic ESK (A-ESK and SC-ESK) effectively prevented MORPH-CPP reestablishment. To our knowledge, this is the first experimental study to demonstrate the potential of ESK as a promising treatment for opioid abuse disorder. Clinical studies are needed to confirm its efficacy in human rehabilitation centers.

Ketamine evokes acute behavioral effects via μ-opioid receptor expressing neurons of the central amygdala

BACKGROUND

Ketamine has anesthetic, analgesic, and antidepressant properties which may involve multiple neuromodulatory systems. In humans, the opioid receptor (OR) antagonist naltrexone blocks the antidepressant effect of ketamine. This mechanism may differentiate ketamine from other NMDA receptor antagonists. Animal models that reflect OR-dependent behavioral effects of ketamine may shed light on the brain regions and circuits that contribute to ketamine's antidepressant mechanism in humans.

METHODS

We screened male and female wild-type mice for a behavioral response to ketamine that could be reversed by OR antagonists in several assays, including locomotor activation, analgesia, and the forced swim test. Whole-brain imaging of cFos expression in ketamine-treated mice, pretreated with naltrexone or vehicle, was used to identify brain areas mediating ketamine / OR interactions. Region-specific pharmacological and genetic interference with μOR (MOR) signaling was used to test predictions of whole-brain imaging results in a subset of behavioral assays.

RESULTS

Among a series of behavioral assays, only locomotor-activation was sensitive to ketamine and blocked by an MOR-selective antagonist. Locomotor activation produced by the NMDA receptor antagonist, MK-801, was not OR-dependent. Whole-brain imaging revealed cFos expression in neurons of the central amygdala (CeA) showed the greatest difference between ketamine in the presence versus absence of naltrexone. CeA neurons expressing both MOR and PKCδ were strongly activated by naltrexone, and selectively interrupting MOR function in the CeA either pharmacologically or genetically blocked the locomotor effects of ketamine.

CONCLUSIONS

These data suggest that ketamine acts at MORs expressed in CeA neurons to produce acute hyperlocomotion.

Ketamine-induced static and dynamic functional connectivity changes are modulated by opioid receptors and biological sex in rats

Subanesthetic ketamine is currently used as a rapid-acting treatment for varied neuropsychiatric disorders. However, the mechanistic underpinnings of its therapeutic action remain unclear, and emerging clinical and preclinical evidence highlights a potential involvement of the opioid system. We used pharmacological functional ultrasound imaging data acquired during and after ketamine administration in male and female rats pretreated with naltrexone, an opioid receptor antagonist, or vehicle. We found that ketamine-induced functional connectivity changes are modulated by opioid receptor blockade, and that these responses are dependent on biological sex. Specifically, naltrexone sex-dependently altered the connectivity patterns within the medial prefrontal cortex (mPFC), a key node of the brain's default-mode network, and between the mPFC and other functional nodes. Furthermore, ketamine produced an opioid-dependent shift toward states of increased dysconnectivity and brain entropy in male rats only. Our findings warrant further investigation into the neurophysiological underpinnings of ketamine action and potential sex-specific interactions with opioid receptors.

Ketamine and Hydroxynorketamine as Novel Pharmacotherapies for the Treatment of Opioid Use Disorders

Opioid use disorder (OUD) has reached epidemic proportions, with many countries facing high levels of opioid use and related fatalities. Although currently prescribed medications for OUD are considered lifesaving, they inadequately address negative affect and cognitive impairment, resulting in high relapse rates to nonmedical opioid use even years after drug cessation (protracted abstinence). Evidence supports the notion that ketamine, an anesthetic and rapid-acting antidepressant drug, holds promise as a candidate for OUD treatment, including the management of acute withdrawal somatic symptoms, negative affect during protracted opioid abstinence, and prevention of retaking nonmedical opioids. In this review, we comprehensively discuss preclinical and clinical research that has evaluated ketamine and its metabolites as potential novel therapeutic strategies for treating OUD. Furthermore, we examine evidence that supports the relevance of the molecular targets of ketamine and its metabolites in relation to their potential effects and therapeutic outcomes in OUD. Overall, existing evidence demonstrates that ketamine and its metabolites can effectively modulate pathophysiological processes affected in OUD, suggesting a promising therapeutic role in the treatment of OUD and the prevention of return to opioid use during abstinence.

Esketamine Treatment for Depression in Adults: A PRISMA Systematic Review and Meta-Analysis

OBJECTIVE

Intranasal esketamine has been approved as an adjunctive therapy for treatment-resistant major depressive disorder with acute suicidal ideation and behavior. The authors conducted a systematic review and meta-analysis of the available data on its efficacy against depression and suicidality as well as its side effects.

METHODS

MEDLINE was searched with the keyword "esketamine" on March 24, 2024, using the PRISMA method. Data processing and statistical analysis were performed with R, version 4.3.3, and the meta-analysis was performed with the METAFOR package.

RESULTS

Of 1,115 articles initially identified, 87 were included for analysis and discussion. At weeks 2-4, randomized controlled trials were mostly negative or failed; however, the meta-analysis returned a weak but significant positive effect for depression (effect size range, 0.15-0.23 at weeks 2-4), similar to augmentation strategies with atypical antipsychotics for treatment-resistant depression. The effect size concerning suicidality was not significant at any time point. The sensitivity analysis produced the same results.

CONCLUSIONS

The study findings suggest that esketamine's efficacy as an add-on to antidepressants is modest in treatment-resistant depression (similar to augmentation strategies with atypical antipsychotics) and is absent against suicidality itself. These findings need to be considered in light of esketamine's abuse potential and the fact that long-term effects are still not fully known. Some alarming signs concerning deaths and emerging suicidality during the testing phase are discussed, along with other regulatory issues.

Redefining Ketamine Pharmacology for Antidepressant Action: Synergistic NMDA and Opioid Receptor Interactions?

Ketamine is a racemic compound and medication comprised of (S)-ketamine and (R)-ketamine enantiomers and its metabolites. It has been used for decades as a dissociative anesthetic, analgesic, and recreational drug. More recently, ketamine, its enantiomers, and its metabolites have been used or are being investigated for the treatment of refractory depression, as well as for comorbid disorders such as anxiety, obsessive-compulsive, and opioid use disorders. Despite its complex pharmacology, ketamine is referred to as an N-methyl-d-aspartate (NMDA) receptor antagonist. In this review, the authors argue that ketamine's pharmacology should be redefined to include opioid receptors and the endogenous opioid system. They also highlight a potential mechanism of action of ketamine for depression that is attributed to bifunctional, synergistic interactions involving NMDA and opioid receptors.

The dopaminergic effects of esketamine are mediated by a dual mechanism involving glutamate and opioid receptors

Esketamine represents a new class of drugs for treating mood disorders. Unlike traditional monoaminergic-based therapies, esketamine primarily targets N-methyl-D-aspartate receptors (NMDAR). However, esketamine is a complex drug with low affinity for NMDAR and can also bind to other targets, such as opioid receptors. Its precise mechanism of action for its antidepressant properties remains debated, as does its potential for misuse. A key component at the intersection of mood and reward processing is the dopaminergic system. In this study, we evaluated the effects of esketamine in locomotion, anxiety tests and operant responding and we used in vivo fiber photometry to explore the neurochemical effects of esketamine in the nucleus accumbens of mice. Our findings demonstrated multifaceted effects of esketamine on neurotransmitter dynamics. In freely behaving mice, esketamine increased locomotion and increased extracellular dopamine tone -by impairing dopamine clearance rather than promoting dopamine release- while decreasing glutamatergic activity. However, it decreased dopamine spontaneous release event frequency and impaired reward-evoked dopamine release, leading to a reduction in operant responding rates. These dopaminergic effects were partially, and conditionally, blocked by the opioid antagonist naloxone and required glutamatergic input. In summary, our study reveals a complex interaction between neurotransmitter systems, suggesting that the neurochemical effects of esketamine are both circuit- and state-dependent.

Hippocampal nicotinic acetylcholine receptor signaling mediates the anti-allodynic effect of ketamine and morphine on neuropathic pain

The present study investigated the involvement of hippocampal nicotinic acetylcholine receptors (nAChRs) in the anti-allodynic effect of ketamine/morphine on neuropathic pain in adult male Wistar rats. Morphine or ketamine administration decreased the percentage of maximum possible effect (MPE%), indicating an analgesic effect. The most significant decrease occurred with a 5 mg/kg dose of morphine (average MPE% = 98), while a 0.5 mg/kg dose of ketamine resulted in a high response (average MPE% = 91), using decision trees as a machine learning tool. Combining morphine and ketamine improved neuropathic pain (average MPE% = 91). Intra-CA1 microinjection of mecamylamine (2 μg/rat) with morphine (3 mg/kg) reduced neuropathic pain (average MPE% = 94). Co-administration of lower doses of ketamine (0.1 mg/kg, i.p.) and mecamylamine (0.5 or 1 μg/rat) with morphine (3 mg/kg) led to a considerable reduction in pain (average MPE% = 91). Utilizing the generalized least squares (GLS) model enabled the establishment of a continuous relation between drug dose and MPE% as the outcome of interest. There was a 19.60 higher average MPE% for each mg/kg increase in morphine dose. In contrast, there was a 17.05 higher average MPE% for every 0.1 mg/kg increase in ketamine dose. Each 0.1 mg/kg increase in ketamine dose, when combined with morphine (3 mg/kg), led to a 30.85 higher average MPE%. A tenfold impact of increasing mecamylamine dosage on MPE% was observed when paired with morphine. Thus, hippocampal nAChRs play a significant role in mediating the anti-allodynic effect of ketamine and morphine in neuropathic pain.

Insights for the Next Generation of Ketamine for the Treatment of Depressive Disorder

Treatment-resistant depression responds quickly to ketamine. As an N-methyl-d-aspartate receptor (NMDAR) antagonist, ketamine may affect prefrontal cortex (PFC) neurons. Recent investigations reveal that the (R)-enantiomer is the most effective and least abuseable antidepressant. The Food and Drug Administration approves only the (S)-enantiomer for medical usage. (2R,6R)-Hydroxynorketamine (HNK) inhibits mGlu2, linked to a Gi, in presynaptic glutamatergic neurons, increasing brain-derived neurotrophic factor (BDNF) release, which autocrinely activates Tropomyosin receptor kinase B (TrkB) and promotes synaptogenesis. Ketamine, originally an anesthetic, has garnered attention for its many pharmacological effects, including its potential as a rapid-acting antidepressant and recreational use. In this Perspective, we explore the synthesis, pharmacology, metabolism, and effects of ketamine and its metabolites in animal and human studies to explain the difference in the biological activity between the enantiomers.

Off-label Use of Lamotrigine and Naltrexone in the Treatment of Ketamine Use Disorder: A Case Report

BACKGROUND

Ketamine is a dissociative anesthetic increasingly utilized in United States medical settings for the treatment of mental health conditions. Additionally, it is increasingly used in nonmedical settings for its dissociative properties. While nonmedical ketamine use and ketamine use disorder (KUD) have been observed internationally, KUD, and approaches to its treatment, have not been previously described in the US.

CASE PRESENTATION

We present the case of a 32-year-old man with KUD who experienced severe cravings despite receipt of residential and intensive outpatient substance use disorder treatment. He resumed use after an initial period of abstinence and was subsequently started on lamotrigine and naltrexone for treatment of depressive symptoms and cravings. This combination altered his experience while on ketamine, resulting in nausea and decreased hallucinogenic effects. In addition, it substantially decreased his cravings, aiding him in achieving longer-term abstinence in combination with receipt of dialectical behavioral therapy, familial support, and involvement in 12-step programming.

DISCUSSION

KUD is a poorly described condition that may become more prevalent as US ketamine use increases. Combining treatment of depressive symptoms and cravings, in this case with lamotrigine and naltrexone, may be a promising pharmacotherapeutic strategy. Lamotrigine, an antiepileptic with glutamate modulating effects, has been utilized to decrease cravings in a variety of substance use disorders. Naltrexone is an opioid antagonist approved for alcohol use disorder and opioid use disorder and is used off-label for stimulant use disorder. This combination offers a possible pharmacotherapeutic option for KUD with more research needed to further evaluate.

Is there a risk of addiction to ketamine during the treatment of depression? A systematic review of available literature

BACKGROUND

Ketamine has demonstrated both rapid and sustained efficacy in treating depression, especially in treatment-resistant cases. However, concerns regarding the addictive potential of ketamine during long-term depression treatment persist among clinicians.

AIM

This review aimed to summarise the evidence on addiction phenomena associated with ketamine treatment of depression.

METHODS

A comprehensive search was conducted in MEDLINE, Embase, PsycInfo and Global Health databases, with additional relevant studies identified through reference lists. Sixteen studies were included, comprising six randomised controlled trials, three single-arm open-label studies, one retrospective study, three case series and three case reports, for a total of 2174 patients.

RESULTS

The studies employed various routes of administration, including intravenous, intramuscular, intranasal, oral and sublingual. Ketamine was administered in the racemic form, except for the studies that utilised intranasal esketamine. Among the included population, four patients were reported to exhibit clear signs of tolerance to the antidepressant effect of ketamine or dependence on the drug, while the majority did not. Cases of addiction phenomena reported in studies that did not meet the inclusion criteria are also discussed.

CONCLUSIONS

Despite the heterogeneity in study designs and outcome assessment methods, the review underscores the relative safety of ketamine treatment for adult patients with depression, emphasising the importance of medically supervised administration, vigilant monitoring and judicious dosing. Future long-term studies employing quantitative scales to assess dependence phenomena could contribute to strengthening the evidence for the safe and effective use of ketamine in the treatment of depression.

N-methyl-D-aspartate Receptors and Depression: Linking Psychopharmacology, Pathology and Physiology in a Unifying Hypothesis for the Epigenetic Code of Neural Plasticity

Uncompetitive NMDAR (N-methyl-D-aspartate receptor) antagonists restore impaired neural plasticity, reverse depressive-like behavior in animal models, and relieve major depressive disorder (MDD) in humans. This review integrates recent findings from in silico, in vitro, in vivo, and human studies of uncompetitive NMDAR antagonists into the extensive body of knowledge on NMDARs and neural plasticity. Uncompetitive NMDAR antagonists are activity-dependent channel blockers that preferentially target hyperactive GluN2D subtypes because these subtypes are most sensitive to activation by low concentrations of extracellular glutamate and are more likely activated by certain pathological agonists and allosteric modulators. Hyperactivity of GluN2D subtypes in specific neural circuits may underlie the pathophysiology of MDD. We hypothesize that neural plasticity is epigenetically regulated by precise Ca2+ quanta entering cells via NMDARs. Stimuli reach receptor cells (specialized cells that detect specific types of stimuli and convert them into electrical signals) and change their membrane potential, regulating glutamate release in the synaptic cleft. Free glutamate binds ionotropic glutamatergic receptors regulating NMDAR-mediated Ca2+ influx. Quanta of Ca2+ via NMDARs activate enzymatic pathways, epigenetically regulating synaptic protein homeostasis and synaptic receptor expression; thereby, Ca2+ quanta via NMDARs control the balance between long-term potentiation and long-term depression. This NMDAR Ca2+ quantal hypothesis for the epigenetic code of neural plasticity integrates recent psychopharmacology findings into established physiological and pathological mechanisms of brain function.

Differential effects of opioid receptor antagonism on the anti-dyskinetic and anti-parkinsonian effects of sub-anesthetic ketamine treatment in a preclinical model

Sub-anesthetic ketamine treatment has been shown to be an effective therapy for treatment-resistant depression and chronic pain. Our group has previously shown that sub-anesthetic ketamine produces acute anti-parkinsonian, and acute anti-dyskinetic effects in preclinical models of Parkinson's disease (PD). Ketamine is a multifunctional drug and exerts effects through blockade of N-methyl-d-aspartate receptors but also through interaction with the opioid system. In this report, we provide detailed pharmacokinetic rodent data on ketamine and its main metabolites following an intraperitoneal injection, and second, we explore the pharmacodynamic properties of ketamine in a rodent PD model with respect to the opioid system, using naloxone, a pan-opioid receptor antagonist, in unilateral 6-hydroxydopamine-lesioned male rats, treated with 6 mg/kg levodopa (l-DOPA) to establish a model of l-DOPA-induced dyskinesia (LID). As previously reported, we showed that ketamine (20 mg/kg) is highly efficacious in reducing LID and now report that the magnitude of this effect is resistant to naloxone (3 and 5 mg/kg). The higher naloxone dose of 5 mg/kg, however, led to an extension of the time-course of the LID, indicating that opioid receptor activation, while not a prerequisite for the anti-dyskinetic effects of ketamine, still exerts an acute modulatory effect. In contrast to the mild modulatory effect on LID, we found that naloxone added to the anti-parkinsonian activity of ketamine, further reducing the akinetic phenotype. In conclusion, our data show opioid receptor blockade differentially modulates the acute anti-parkinsonian and anti-dyskinetic actions of ketamine, providing novel mechanistic information to support repurposing ketamine for individuals with LID.

What should constitute a control condition in psychedelic drug trials?

Over the past decade there has been a surge in interest in placebo-controlled trials using non-classical 3,4-methylenedioxymethamphetamine (MDMA) and classical psychedelics such as psilocybin, lysergic acid diethylamide (LSD) and N,N-dimethyltryptamine (DMT) to treat neuropsychiatric disorders. However, the success and reliability of these trials depend on the design of the trials, the choice of control conditions, and the ability to blind both participants and researchers. When appropriate control conditions are lacking, it becomes difficult to disentangle placebo and expectation effects from medication effects. Here we explore the neurobiology of placebo and expectation effects, alongside the methodological considerations for selecting suitable control conditions in psychedelic trials. This includes examining the advantages and disadvantages of various control conditions and proposing new directions to enhance the validity of these trials and their regulatory science. By addressing these factors, we aim to improve the reliability of psychedelic research in uncovering the therapeutic benefits of psychedelics beyond placebo and expectation effects.

The association between ketamine and esketamine with alcohol and substance misuse: Reports to the Food and Drug Administration adverse event reporting system (FAERS)

INTRODUCTION

Ketamine and esketamine have been proven to be effective in treating adults with treatment resistant depression (TRD). Preliminary evidence indicates that, when combined with behavioral and psychological interventions, both agents may offer benefits for individuals with substance use disorder (SUD) and alcohol use disorder (AUD). Notwithstanding, concerns have been raised as to whether either or both agents are associated with abuse and/or gateway activity.

METHODS

Herein, we evaluate disproportionate reporting expressed as reporting odds ratios (ROR) for esketamine and ketamine. The outcomes of interest include alcohol problem, alcoholism, alcohol abuse, substance dependence, SUD, substance abuse, drug dependence, drug use disorder and drug abuse as codified by the Medical Dictionary for Regulatory Activities (MedDRA) within the FAERS. The IC025 values were significant for ketamine in cases of alcohol abuse (0.28), substance dependence (1.88), substance use disorder (0.996), substance abuse (0.61), drug dependence (0.56), drug use disorder (1.17) and drug abuse (1.22). Additionally, oxycontin showed significant IC025 values for substance dependence (0.067), substance use disorder (0.094), substance abuse (0.035), and drug dependence (0.27).

RESULTS

We observed significant increases in the reporting odds ratios (RORs) for ketamine with respect to various outcomes: alcohol abuse (ROR 2.84, 95 % CI 1.53-5.28; p = 0.0010), substance dependence (ROR 18.72, 95 % CI 8.49-41.30; p ≤ 0.0001), SUD (ROR 11.40, 95 % CI 4.24-30.65; p ≤ 0.0001), substance abuse (ROR 2.29, 95 % CI 1.73-3.04; p ≤ 0.0001), drug dependence (ROR 1.99, 95 % CI 1.64-2.42; p ≤ 0.0001), drug use disorder (ROR 4.50, 2.94-6.88; p ≤ 0.0001) and drug abuse (ROR 3.72, 3.36-4.12; p ≤ 0.0001). For esketamine, we observed that the ROR was significantly reduced for substance abuse (ROR 0.37, 95 % CI 0.22-0.63; p = 0.0003), drug dependence (ROR 0.13, 95 % CI 0.076-0.23; p ≤ 0.0001) and drug abuse (ROR 0.048, 95 % CI 0.030-0.078; p ≤ 0.0001). To our knowledge, this is the first report of spontaneous adverse events related to these outcomes of interest in the FAERS.

CONCLUSION

Mixed RORs were observed across aspects of SUD and AUD for both ketamine and esketamine. Due to limitations in the FAERS, establishing causal links between new onset alcohol and substance misuse with either agent remains inconclusive. Possible beneficial effects on measures of SUD and AUD were observed. It is currently unclear, but possible, whether both agents have differential ameliorative effects across dimensions of SUD and AUD, which is a focus of ongoing research.

Endogenous opiates and behavior: 2023

This paper is the forty-sixth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2023 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug and alcohol abuse (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Esketamine in depression: putative biomarkers from clinical research

The discovery of racemic (R, S)-ketamine as a rapid-acting antidepressant and the subsequent FDA approval of its (S)-enantiomer, esketamine, for treatment-resistant depression (TRD) are significant advances in the development of novel neuropsychiatric therapeutics. Esketamine is now recognized as a powerful tool for addressing persistent symptoms of TRD compared to traditional oral antidepressants. However, research on biomarkers associated with antidepressant response to esketamine has remained sparse and, to date, has been largely extrapolated from racemic ketamine studies. Genetic, proteomic, and metabolomic profiles suggest that inflammation and mitochondrial function may play a role in esketamine's antidepressant effects, though these preliminary results require verification. In addition, neuroimaging research has consistently implicated the prefrontal cortex, striatum, and anterior cingulate cortex in esketamine's effects. Esketamine also shows promise in perioperative settings for reducing depression and anxiety, and these effects appear to correlate with increased peripheral biomarkers such as brain-derived neurotrophic factor and serotonin. Further indications are likely to be identified with the continued repurposing of racemic ketamine, providing further opportunity for biomarker study and mechanistic understanding of therapeutic effects. Novel methodologies and well-designed biomarker-focused clinical research trials are needed to more clearly elucidate esketamine's therapeutic actions as well as biologically identify those most likely to benefit from this agent, allowing for the improved personalization of antidepressant treatment.

The role of NMDARs in the anesthetic and antidepressant effects of ketamine

BACKGROUND

As a phencyclidine (PCP) analog, ketamine can generate rapid-onset and substantial anesthetic effects. Contrary to traditional anesthetics, ketamine is a dissociative anesthetic and can induce loss of consciousness in patients. Recently, the subanaesthetic dose of ketamine was found to produce rapid-onset and lasting antidepressant effects.

AIM

However, how different concentrations of ketamine can induce diverse actions remains unclear. Furthermore, the molecular mechanisms underlying the NMDAR-mediated anesthetic and antidepressant effects of ketamine are not fully understood.

METHOD

In this review, we have introduced ketamine and its metabolism, summarized recent advances in the molecular mechanisms underlying NMDAR inhibition in the anesthetic and antidepressant effects of ketamine, explored the possible functions of NMDAR subunits in the effects of ketamine, and discussed the future directions of ketamine-based anesthetic and antidepressant drugs.

RESULT

Both the anesthetic and antidepressant effects of ketamine were thought to be mediated by N-methyl-D-aspartate receptor (NMDAR) inhibition.

CONCLUSION

The roles of NMDARs have been extensively studied in the anaesthetic effects of ketamine. However, the roles of NMDARs in antidepressant effects of ketamine are complicated and controversial.

Opioid receptor expressing neurons of the central amygdala gate behavioral effects of ketamine in mice

Ketamine has anesthetic, analgesic, and antidepressant properties which may involve multiple neuromodulatory systems. In humans, the opioid receptor (OR) antagonist naltrexone blocks the antidepressant effect of ketamine. It is unclear whether naltrexone blocks a direct effect of ketamine at ORs, or whether normal functioning of the OR system is required to realize the full antidepressant effects of treatment. In mice, the effect of ketamine on locomotion, but not analgesia or the forced swim test, was sensitive to naltrexone and was therefore used as a behavioral readout to localize the effect of naltrexone in the brain. We performed whole-brain imaging of cFos expression in ketamine-treated mice, pretreated with naltrexone or vehicle, and identified the central amygdala (CeA) as the area with greatest difference in cFos intensity. CeA neurons expressing both μOR (MOR) and PKCμ were strongly activated by naltrexone but not ketamine, and selectively interrupting MOR function in the CeA either pharmacologically or genetically blocked the locomotor effects of ketamine. These data suggest that MORs expressed in CeA neurons gate behavioral effects of ketamine but are not direct targets of ketamine.

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

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

Efficacy of esketamine for the treatment of postpartum depression and pain control following cesarean section: a randomized, double-blind, controlled clinical trial

BACKGROUND

Postpartum depression (PPD) following a cesarean delivery is a frequently seen complication. Despite the prophylactic effects of ketamine, the impact of esketamine on PPD in women undergoing cesarean section remains uncertain. This study aimed to assess the effectiveness of esketamine as an adjunct to patient-controlled intravenous analgesia (PCIA) in preventing PPD in women undergoing caesarean section.

METHODS

A total of 275 parturients undergoing caesarean section and subsequent patient-controlled intravenous analgesia (PCIA) were randomly assigned to receive either the control treatment (sufentanil 2 µg/kg + tropisetron 10 mg) or the experimental treatment with additional esketamine (1.5 mg/kg). The primary outcome measured was the incidence of postpartum depression (PPD), classified by Edinburgh Postnatal Depression Scale (EPDS) scores equal to or greater than 13 indicating PPD. Secondary outcomes included cumulative sufentanil consumption during specific time periods (0-24 h, 24-48 h, and 0-48 h) after the surgical procedure and numerical rating scale (NRS) scores at rest and during movements.

RESULTS

The final analysis included a total of 246 postpartum women who had undergone caesarean delivery. On postoperative day 42, the incidence of depression among the control group was 17.6%, which was significantly higher compared to the esketamine group with a rate of 8.2% (P = 0.02). The EPDS scores also showed a significant difference between the two groups, with a mean score of 9.02 ± 2.21 in the control group and 6.87 ± 2.14 in the esketamine group (p < 0.0001). In terms of pain management, the esketamine group showed lower sufentanil consumption in the 0-24 h (42.5 ± 4.58 µg vs. 50.15 ± 5.47 µg, P = 0.04) and 0-48 h (87.40 ± 9.51 µg vs. 95.10 ± 9.36 µg, P = 0.04) postoperative periods compared to the control group. Differences in movement were also observed between the two groups at 24 and 48 h after the cesarean Sect. (3.39 ± 1.57 vs. 4.50 ± 0.80, P = 0.02; 2.43 ± 0.87 vs. 3.56 ± 0.76, P = 0.02). It is worth noting that the frequency of side effects observed in both groups was comparable.

CONCLUSIONS

Esketamine at a dose of 1.5 mg/kg, when used as a supplement in PCIA, has been shown to significantly reduce the occurrence of PPD within 42 days. Additionally, it has been found to decrease cumulative consumption of sufentanil over a 48-hour period following cesarean operation, all without increasing the rate of adverse effects.

TRIAL REGISTRATION

Registered in the Chinese Clinical Trial Registry (ChiCTR2200067054) on December 26, 2022.

Sex dependence of opioid-mediated responses to subanesthetic ketamine in rats

Subanesthetic ketamine is increasingly used for the treatment of varied psychiatric conditions, both on- and off-label. While it is commonly classified as an N-methyl D-aspartate receptor (NMDAR) antagonist, our picture of ketamine's mechanistic underpinnings is incomplete. Recent clinical evidence has indicated, controversially, that a component of the efficacy of subanesthetic ketamine may be opioid dependent. Using pharmacological functional ultrasound imaging in rats, we found that blocking opioid receptors suppressed neurophysiologic changes evoked by ketamine, but not by a more selective NMDAR antagonist, in limbic regions implicated in the pathophysiology of depression and in reward processing. Importantly, this opioid-dependent response was strongly sex-dependent, as it was not evident in female subjects and was fully reversed by surgical removal of the male gonads. We observed similar sex-dependent effects of opioid blockade affecting ketamine-evoked postsynaptic density and behavioral sensitization, as well as in opioid blockade-induced changes in opioid receptor density. Together, these results underscore the potential for ketamine to induce its affective responses via opioid signaling, and indicate that this opioid dependence may be strongly influenced by subject sex. These factors should be more directly assessed in future clinical trials.

The why, when, where, how, and so what of so-called rapidly acting antidepressants

Developing antidepressants that are not only more effective but are rapidly acting is the Holy Grail for psychiatry. We review multiple issues that arise in determining rapid responses in antidepressant trials. The current status of purportedly rapid acting agents is first reviewed. Then, a number of key questions/issues are addressed: Is there a unifying definition for rapid response across studies? Should rapid response criteria be based on required measurable effects on overall improvement? On specific symptoms such as psychomotor retardation, depressed mood, or anhedonia? In associated symptoms such as anxiety or insomnia? When should onset be considered rapid-by Day 3? Day7? Day 14? If there is a rapid response, for how long should the effects be maintained? Is maintenance of effect dependent on continuing the medication? Is rapid response associated with specific mechanisms of action? Do the mechanisms of action suggest possible risk for drug abuse? How important is rapid response really in an often chronic or recurrent depressive disorder? In which types of patients could rapid response be particularly important? What are the study design issues that need to be considered for assessing rapid response, including: selection of specific types of depressed patients, multiple doses of drug studied, designation of primary and secondary outcome measures, specific time points at which to determine efficacy, requirements for demonstrating durability, etc. A framework for approaching this complex area is developed for both researchers and clinicians.

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Effects of perioperative use of esketamine on postpartum depression risk in patients undergoing cesarean section: A randomized controlled trial

BACKGROUND

Postpartum depression (PPD) is a prevalent public health issue. Although ketamine has prophylactic effects on PPD in women undergoing cesarean section, the effects of esketamine on PPD remain unclear. This trial aimed to evaluate the efficacy of perioperative esketamine infusion on PPD risk by assessing Edinburgh Postnatal Depression Scale (EPDS) scores and blood biomarkers.

METHODS

A total of 150 participants undergoing elective cesarean section were randomly allocated to receive either esketamine or normal saline. Since 27 participants were excluded due to consent withdrawal or loss to follow-up, 123 patients were included. The primary outcome was the prevalence of PPD risk. Secondary outcomes included the prevalence of postpartum anxiety (PPA) risk, levels of biomarkers, postoperative pain intensity, and cumulative sufentanil consumption.

RESULTS

The prevalence of PPD and PPA risk at 3 days, 42 days, 3 months, and 6 months postpartum did not differ between the two groups. Furthermore, EPDS scores, pain intensity at rest, and during coughing on postoperative days (POD) 1 and 2 did not differ between the two groups. Sufentanil consumption during 0-12 h, 12-24 h, 0-24 h, and 0-48 h postoperatively were significantly lower in the esketamine group compared to the control group. Blood biomarkers did not differ between the two groups on POD 3.

LIMITATIONS

The sample size was small. PPD risk was simply screened, not diagnosed.

CONCLUSIONS

Perioperative administration of esketamine did not decrease the incidence of PPD risk in women after elective cesarean section. However, esketamine reduced opioid consumption.

Converging circuits between pain and depression: the ventral tegmental area as a therapeutic hub

Chronic pain and depression are highly prevalent pathologies and cause a major socioeconomic burden to society. Chronic pain affects the emotional state of the individuals suffering from it, while depression worsens the prognosis of chronic pain patients and may diminish the effectiveness of pain treatments. There is a high comorbidity rate between both pathologies, which might share overlapping mechanisms. This review explores the evidence pinpointing a role for the ventral tegmental area (VTA) as a hub where both pain and emotional processing might converge. In addition, the feasibility of using the VTA as a possible therapeutic target is discussed. The role of the VTA, and the dopaminergic system in general, is highly studied in mood disorders, especially in deficits in reward-processing and motivation. Conversely, the VTA is less regarded where it concerns the study of central mechanisms of pain and its mood-associated consequences. Here, we first outline the brain circuits involving central processing of pain and mood disorders, focusing on the often-understudied role of the dopaminergic system and the VTA. Next, we highlight the state-of-the-art findings supporting the emergence of the VTA as a link where both pathways converge. Thus, we envision a promising part for the VTA as a putative target for innovative therapeutic approaches to treat chronic pain and its effects on mood. Finally, we emphasize the urge to develop and use animal models where both pain and depression-like symptoms are considered in conjunction.