Targeting metaplasticity mechanisms to promote sustained antidepressant actions

The discovery that subanesthetic doses of (R, S)-ketamine (ketamine) and (S)-ketamine (esketamine) rapidly induce antidepressant effects and promote sustained actions following drug clearance in depressed patients who are treatment-resistant to other therapies has resulted in a paradigm shift in the conceptualization of how rapidly and effectively depression can be treated. Consequently, the mechanism(s) that next generation antidepressants may engage to improve pathophysiology and resultant symptomology are being reconceptualized. Impaired excitatory glutamatergic synapses in mood-regulating circuits are likely a substantial contributor to the pathophysiology of depression. Metaplasticity is the process of regulating future capacity for plasticity by priming neurons with a stimulation that alters later neuronal plasticity responses. Accordingly, the development of treatment modalities that specifically modulate the duration, direction, or magnitude of glutamatergic synaptic plasticity events such as long-term potentiation (LTP), defined here as metaplastogens, may be an effective approach to reverse the pathophysiology underlying depression and improve depression symptoms. We review evidence that the initiating mechanisms of pharmacologically diverse rapid-acting antidepressants (i.e., ketamine mimetics) converge on consistent downstream molecular mediators that facilitate the expression/maintenance of increased synaptic strength and resultant persisting antidepressant effects. Specifically, while the initiating mechanisms of these therapies may differ (e.g., cell type-specificity, N-methyl-D-aspartate receptor (NMDAR) subtype-selective inhibition vs activation, metabotropic glutamate receptor 2/3 antagonism, AMPA receptor potentiation, 5-HT receptor-activating psychedelics, etc.), the sustained therapeutic mechanisms of putative rapid-acting antidepressants will be mediated, in part, by metaplastic effects that converge on consistent molecular mediators to enhance excitatory neurotransmission and altered capacity for synaptic plasticity. We conclude that the convergence of these therapeutic mechanisms provides the opportunity for metaplasticity processes to be harnessed as a druggable plasticity mechanism by next-generation therapeutics. Further, targeting metaplastic mechanisms presents therapeutic advantages including decreased dosing frequency and associated diminished adverse responses by eliminating the requirement for the drug to be continuously present.

cAMP-dependent protein kinase signaling is required for (2R,6R)-hydroxynorketamine to potentiate hippocampal glutamatergic transmission

(2R,6R)-Hydroxynorketamine (HNK) is a ketamine metabolite that shows rapid antidepressant-like effects in preclinical studies and lacks the adverse N-methyl-d-aspartate receptor (NMDAR) inhibition-related properties of ketamine. Investigating how (2R,6R)-HNK exerts its antidepressant actions may be informative in the design of novel pharmacotherapies with improved safety and efficacy. We sought to identify the molecular substrates through which (2R,6R)-HNK induces functional changes at excitatory synapses, a prevailing hypothesis for how rapid antidepressant effects are initiated. We recorded excitatory postsynaptic potentials in hippocampal slices from male Wistar Kyoto rats, which have impaired hippocampal plasticity and are resistant to traditional antidepressants. (2R,6R)-HNK (10 µM) led to a rapid potentiation of electrically evoked excitatory postsynaptic potentials at Schaffer collateral CA1 stratum radiatum synapses. This potentiation was associated with a decrease in paired pulse facilitation, suggesting an increase in the probability of glutamate release. The (2R,6R)-HNK-induced potentiation was blocked by inhibiting either cyclic adenosine monophosphate (cAMP) or its downstream target, cAMP-dependent protein kinase (PKA). As cAMP is a potent regulator of brain-derived neurotrophic factor (BDNF) release, we assessed whether (2R,6R)-HNK exerts this acute potentiation through a rapid increase in cAMP-dependent BDNF-TrkB signaling. We found that the cAMP-PKA-dependent potentiation was not dependent on TrkB activation by BDNF, which functionally delimits the acute synaptic effects of (2R,6R)-HNK from its sustained BDNF-dependent actions in vivo. These results suggest that, by potentiating glutamate release via cAMP-PKA signaling, (2R,6R)-HNK initiates acute adaptations in fast excitatory synaptic transmission that promote structural plasticity leading to maintained antidepressant action.NEW & NOTEWORTHY Ketamine is a rapid-acting antidepressant and its preclinical effects are mimicked by its (2R,6R)-(HNK) metabolite. We found that (2R,6R)-HNK initiates acute adaptations in fast excitatory synaptic transmission by potentiating glutamate release via cAMP-PKA signaling at hippocampal Schaffer collateral synapses. This cAMP-PKA-dependent potentiation was not dependent on TrkB activation by BDNF, which functionally delimits the rapid synaptic effects of (2R,6R)-HNK from its sustained BDNF-dependent actions that are thought to maintain antidepressant action in vivo.

Cerebrospinal fluid exploratory proteomics and ketamine metabolite pharmacokinetics in human volunteers after ketamine infusion

Ketamine is a treatment for both refractory depression and chronic pain syndromes. In order to explore ketamine's potential mechanism of action and whether ketamine or its metabolites cross the blood brain barrier, we examined the pharmacokinetics of ketamine and its metabolites-norketamine (NK), dehydronorketamine (DHNK), and hydroxynorketamines (HNKs)-in cerebrospinal fluid (CSF) and plasma, as well as in an exploratory proteomic analysis in the CSF of nine healthy volunteers who received ketamine intravenously (0.5 mg/kg IV). We found that ketamine, NK, and (2R,6R;2S,6S)-HNK readily crossed the blood brain barrier. Additionally, 354 proteins were altered in the CSF in at least two consecutive timepoints (p < 0.01). Proteins in the classes of tyrosine kinases, cellular adhesion molecules, and growth factors, including insulin, were most affected, suggesting an interplay of altered neurotransmission, neuroplasticity, neurogenesis, synaptogenesis, and neural network functions following ketamine administration.

Brain-based correlates of antidepressant response to ketamine: a comprehensive systematic review of neuroimaging studies

Ketamine is an effective antidepressant, but there is substantial variability in patient response and the precise mechanism of action is unclear. Neuroimaging can provide predictive and mechanistic insights, but findings are limited by small sample sizes. This systematic review covers neuroimaging studies investigating baseline (pre-treatment) and longitudinal (post-treatment) biomarkers of responses to ketamine. All modalities were included. We performed searches of five electronic databases (from inception to April 26, 2022). 69 studies were included (with 1751 participants). There was substantial methodological heterogeneity and no well replicated biomarker. However, we found convergence across some significant results, particularly in longitudinal biomarkers. Response to ketamine was associated with post-treatment increases in gamma power in frontoparietal regions in electrophysiological studies, post-treatment increases in functional connectivity within the prefrontal cortex, and post-treatment increases in the functional activation of the striatum. Although a well replicated neuroimaging biomarker of ketamine response was not identified, there are biomarkers that warrant further investigation.

Relative to females, male rats are more willing to forego obtaining sucrose reward in order to prevent harm to their cage mate

RATIONALE

Empathy, or the ability to perceive, share, and act upon the emotions of another, is a crucial social skill and is dysfunctional in autism and schizophrenia. While the complexities of human empathy are difficult to model in rodents, behavioral paradigms utilizing rats which study decision-making in social contexts may provide a translational framework for assessing biological, pharmacotherapeutic, and environmental interventions.

OBJECTIVES

Modify and expand upon the three-session rat harm aversion task, which measures the willingness of rats to cease pressing a lever that earns them sucrose reward but delivers a shock to their cage mate. We sought to test the sustainability of harm aversion across seven sessions in male and female rats.

METHODS

Same-sex pair-housed rats were assigned as either the observer, which had access to the lever, or the demonstrator, which would receive shocks. After training the observer to press the lever to receive sucrose pellets, the demonstrator was placed into an adjacent chamber at which point lever responses would also deliver a shock. If the observer did not press the lever, no shock and no sucrose was delivered.

RESULTS

A sex difference in harm aversion was observed with female rats having significantly higher response rates and decreased response latencies across the seven test sessions, thus delivering more shocks and obtaining more sucrose, relative to males.

CONCLUSIONS

These data demonstrate that male rats sustain harm aversion to a greater extent relative to females.

Ketamine preservative benzethonium chloride potentiates hippocampal synaptic transmission and binds neurotransmitter receptors and transporters

Benzethonium chloride (BZT) is an excipient used in numerous products including (R,S)-ketamine (ketamine) drug formulations for human and veterinary use. Emerging evidence indicates BZT is pharmacologically active. BZT may therefore contribute to some of the clinical or preclinical effects observed with ketamine. In the present study, we evaluated: (i) the affinity of BZT for neurotransmitter receptors and transporters, (ii) the effects of BZT on hippocampal synaptic transmission in vitro, and (iii) plasma and brain concentrations of BZT following its intraperitoneal administration to male CD1 mice. Radioligand binding assays determined the affinity of BZT for neurotransmitter targets. Effects of BZT on field excitatory postsynaptic potentials (fEPSPs) were established via electrophysiological recordings from slices collected from male C57BL/6J mice. The binding assays revealed that BZT binds to numerous receptors (e.g., σ2 Ki = 7 nM) and transporters (e.g., dopamine transporter Ki = 545 nM). Bath application of BZT potentiated hippocampal fEPSPs in mouse hippocampal slices with an EC50 of 2.03 nM. Following intraperitoneal administration, BZT was detected in the plasma, but not in the brain of mice. These data highlight that studies measuring peripheral endpoints or directly exposing systems, in vitro, intracerebroventricularly, or intracortically, to BZT-containing formulations should account for the direct effects of BZT. Our findings also suggest that earlier data attributing pharmacological effects to ketamine may be confounded by BZT and that additional investigation into the functional impact of BZT is warranted. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

κ-Opioid Receptor Plasma Levels Are Associated With Sex and Diagnosis of Major Depressive Disorder But Not Response to Ketamine

BACKGROUND

Preclinical evidence indicates that the κ-opioid receptor (KOR)/dynorphin pathway is implicated in depressive-like behaviors. Ketamine is believed to partly exert its antidepressant effects by modulating the opioid system. This post hoc study examined the following research questions: (1) at baseline, were there differences in KOR or dynorphin plasma levels between individuals with major depressive disorder (MDD) and healthy volunteers (HVs) or between men and women? (2) in individuals with MDD, did KOR or dynorphin baseline plasma levels moderate ketamine's therapeutic effects or adverse effects? and (3) in individuals with MDD, were KOR or dynorphin plasma levels affected after treatment with ketamine compared with placebo?

METHODS

Thirty-nine unmedicated individuals with MDD (23 women) and 25 HVs (16 women) received intravenous ketamine (0.5 mg/kg) and placebo in a randomized, crossover, double-blind trial. Blood was obtained from all participants at baseline and at 3 postinfusion time points (230 minutes, day 1, day 3). Linear mixed model regressions were used.

RESULTS

At baseline, participants with MDD had lower KOR plasma levels than HVs ( F1,60 = 13.16, P < 0.001), and women (MDD and HVs) had higher KOR plasma levels than men ( F1,60 = 4.98, P = 0.03). Diagnosis and sex had no significant effects on baseline dynorphin levels. Baseline KOR and dynorphin levels did not moderate ketamine's therapeutic or adverse effects. Compared with placebo, ketamine was not associated with postinfusion changes in KOR or dynorphin levels.

CONCLUSIONS

In humans, diagnosis of MDD and biological sex are involved with changes in components of the KOR/dynorphin pathway. Neither KOR nor dynorphin levels consistently moderated ketamine's therapeutic effects or adverse effects, nor were levels altered after ketamine infusion.

TRIAL REGISTRATION

NCT00088699 ( ClinicalTrials.gov ).

Possible psychedelic therapeutic mechanism

Psychedelics act on intracellular serotonin receptors that are not accessible by serotonin alone.

NMDA Receptor Activation-Dependent Antidepressant-Relevant Behavioral and Synaptic Actions of Ketamine

Ketamine is a well-characterized NMDA receptor (NMDAR) antagonist, although the relevance of this pharmacology to its rapid (within hours of administration) antidepressant actions, which depend on mechanisms convergent with strengthening of excitatory synapses, is unclear. Activation of synaptic NMDARs is necessary for the induction of canonical long-term potentiation (LTP) leading to a sustained expression of increased synaptic strength. We tested the hypothesis that induction of rapid antidepressant effects requires NMDAR activation, by using behavioral pharmacology, western blot quantification of hippocampal synaptoneurosomal protein levels, and ex vivo hippocampal slice electrophysiology in male mice. We found that ketamine exerts an inverted U-shaped dose-response in antidepressant-sensitive behavioral tests, suggesting that an excessive NMDAR inhibition can prevent ketamine's antidepressant effects. Ketamine's actions to induce antidepressant-like behavioral effects, up-regulation of hippocampal AMPAR subunits GluA1 and GluA2, as well as metaplasticity measured ex vivo using electrically-stimulated LTP, were abolished by pretreatment with other non-antidepressant NMDAR antagonists, including MK-801 and CPP. Similarly, the antidepressant-like actions of other putative rapid-acting antidepressant drugs (2R,6R)-hydroxynorketamine (ketamine metabolite), MRK-016 (GABAAα5 negative allosteric modulator), and LY341495 (mGlu2/3 receptor antagonist) were blocked by NMDAR inhibition. Ketamine acted synergistically with an NMDAR positive allosteric modulator to exert antidepressant-like behavioral effects and activation of the NMDAR subunit GluN2A was necessary and sufficient for such relevant effects. We conclude rapid-acting antidepressant compounds share a common downstream NMDAR-activation dependent effector mechanism, despite variation in initial pharmacological targets. Promoting NMDAR signaling or other approaches that enhance NMDAR-dependent LTP-like synaptic potentiation may be an effective antidepressant strategy.SIGNIFICANCE STATEMENT The anesthetic and antidepressant drug ketamine is well-characterized as an NMDA receptor (NMDAR) antagonist; though, the relevance and full impact of this pharmacology to its antidepressant actions is unclear. We found that NMDAR activation, which occurs downstream of their initial actions, is necessary for the beneficial effects of ketamine and several other putative antidepressant compounds. As such, promoting NMDAR signaling, or other approaches that enhance NMDAR-dependent long-term potentiation (LTP)-like synaptic potentiation in vivo may be an effective antidepressant strategy directly, or acting synergistically with other drug or interventional treatments.

Target deconvolution studies of (2R,6R)-hydroxynorketamine: an elusive search

The off-label use of racemic ketamine and the FDA approval of (S)-ketamine are promising developments for the treatment of depression. Nevertheless, racemic ketamine and (S)-ketamine are controlled substances with known abuse potential and their use is associated with undesirable side effects. For these reasons, research efforts have focused on identifying alternatives. One candidate is (2R,6R)-hydroxynorketamine ((2R,6R)-HNK), a ketamine metabolite that in preclinical models lacks the dissociative and abuse properties of ketamine while retaining its antidepressant-like behavioral efficacy. (2R,6R)-HNK's mechanism of action however is unclear. The main goals of this study were to perform an in-depth pharmacological characterization of (2R,6R)-HNK at known ketamine targets, to use target deconvolution approaches to discover novel proteins that bind to (2R,6R)-HNK, and to characterize the biodistribution and behavioral effects of (2R,6R)-HNK across several procedures related to substance use disorder liability. We found that unlike (S)- or (R)-ketamine, (2R,6R)-HNK did not directly bind to any known or proposed ketamine targets. Extensive screening and target deconvolution experiments at thousands of human proteins did not identify any other direct (2R,6R)-HNK-protein interactions. Biodistribution studies using radiolabeled (2R,6R)-HNK revealed non-selective brain regional enrichment, and no specific binding in any organ other than the liver. (2R,6R)-HNK was inactive in conditioned place preference, open-field locomotor activity, and intravenous self-administration procedures. Despite these negative findings, (2R,6R)-HNK produced a reduction in immobility time in the forced swim test and a small but significant increase in metabolic activity across a network of brain regions, and this metabolic signature differed from the brain metabolic profile induced by ketamine enantiomers. In sum, our results indicate that (2R,6R)-HNK does not share pharmacological or behavioral profile similarities with ketamine or its enantiomers. However, it could still be possible that both ketamine and (2R,6R)-HNK exert antidepressant-like efficacy through a common and previously unidentified mechanism. Given its pharmacological profile, we predict that (2R,6R)-HNK will exhibit a favorable safety profile in clinical trials, and we must wait for clinical studies to determine its antidepressant efficacy.

Blood-based biomarkers of antidepressant response to ketamine and esketamine: A systematic review and meta-analysis

(R,S)-ketamine (ketamine) and its enantiomer (S)-ketamine (esketamine) can produce rapid and substantial antidepressant effects. However, individual response to ketamine/esketamine is variable, and there are no well-accepted methods to differentiate persons who are more likely to benefit. Numerous potential peripheral biomarkers have been reported, but their current utility is unclear. We conducted a systematic review/meta-analysis examining the association between baseline levels and longitudinal changes in blood-based biomarkers, and response to ketamine/esketamine. Of the 5611 citations identified, 56 manuscripts were included (N = 2801 participants), and 26 were compatible with meta-analytical calculations. Random-effect models were used, and effect sizes were reported as standardized mean differences (SMD). Our assessments revealed that more than 460 individual biomarkers were examined. Frequently studied groups included neurotrophic factors (n = 15), levels of ketamine and ketamine metabolites (n = 13), and inflammatory markers (n = 12). There were no consistent associations between baseline levels of blood-based biomarkers, and response to ketamine. However, in a longitudinal analysis, ketamine responders had statistically significant increases in brain-derived neurotrophic factor (BDNF) when compared to pre-treatment levels (SMD [95% CI] = 0.26 [0.03, 0.48], p = 0.02), whereas non-responders showed no significant changes in BDNF levels (SMD [95% CI] = 0.05 [-0.19, 0.28], p = 0.70). There was no consistent evidence to support any additional longitudinal biomarkers. Findings were inconclusive for esketamine due to the small number of studies (n = 2). Despite a diverse and substantial literature, there is limited evidence that blood-based biomarkers are associated with response to ketamine, and no current evidence of clinical utility.

Experimenters' sex modulates mouse behaviors and neural responses to ketamine via corticotropin releasing factor

We show that the sex of human experimenters affects mouse behaviors and responses following administration of the rapid-acting antidepressant ketamine and its bioactive metabolite (2R,6R)-hydroxynorketamine. Mice showed aversion to the scent of male experimenters, preference for the scent of female experimenters and increased stress susceptibility when handled by male experimenters. This human-male-scent-induced aversion and stress susceptibility was mediated by the activation of corticotropin-releasing factor (CRF) neurons in the entorhinal cortex that project to hippocampal area CA1. Exposure to the scent of male experimenters before ketamine administration activated CA1-projecting entorhinal cortex CRF neurons, and activation of this CRF pathway modulated in vivo and in vitro antidepressant-like effects of ketamine. A better understanding of the specific and quantitative contributions of the sex of human experimenters to study outcomes in rodents may improve replicability between studies and, as we have shown, reveal biological and pharmacological mechanisms.

(2R,6R)-hydroxynorketamine rapidly potentiates optically-evoked Schaffer collateral synaptic activity

(2R,6R)-hydroxynorketamine (HNK) is a metabolite of ketamine that exerts rapid and sustained antidepressant-like effects in preclinical studies. We hypothesize that the rapid antidepressant actions of (2R,6R)-HNK involve an acute increase in glutamate release at Schaffer collateral synapses. Here, we used an optogenetic approach to assess whether (2R,6R)-HNK promotes glutamate release at CA1-projecting Schaffer collateral terminals in response to select optical excitation of CA3 afferents. The red-shifted channelrhodopsin, ChrimsonR, was expressed in dorsal CA3 neurons of adult male Sprague Dawley rats. Transverse slices were collected four weeks later to determine ChrimsonR expression and to assess the acute synaptic effects of an antidepressant-relevant concentration of (2R,6R)-HNK (10 μM). (2R,6R)-HNK led to a rapid potentiation of CA1 field excitatory postsynaptic potentials evoked by recurrent optical stimulation of ChrimsonR-expressing CA3 afferents. This potentiation is mediated in part by an increase in glutamate release probability, as (2R,6R)-HNK suppressed paired-pulse facilitation at CA3 projections, an effect that correlated with the magnitude of the (2R,6R)-HNK-induced potentiation of CA1 activity. These results demonstrate that (2R,6R)-HNK increases the probability of glutamate release at CA1-projecting Schaffer collateral afferents, which may be involved in the antidepressant-relevant behavioral adaptations conferred by (2R,6R)-HNK in vivo. The current study also establishes proof-of-principle that genetically-encoded light-sensitive proteins can be used to investigate the synaptic plasticity induced by novel antidepressant compounds in neuronal subcircuits.

Negative Allosteric Modulation of Gamma-Aminobutyric Acid A Receptors at α5 Subunit-Containing Benzodiazepine Sites Reverses Stress-Induced Anhedonia and Weakened Synaptic Function in Mice

BACKGROUND

Abnormal reward processing, typically anhedonia, is a hallmark of human depression and is accompanied by altered functional connectivity in reward circuits. Negative allosteric modulators of GABA_A (gamma-aminobutyric acid A) receptors (GABA-NAMs) have rapid antidepressant-like properties in rodents and exert few adverse effects, but molecular targets underlying their behavioral and synaptic effects remain undetermined. We hypothesized that GABA-NAMs act at the benzodiazepine site of GABA_A receptors containing α5 subunits to increase gamma oscillatory activity, strengthen synapses in reward circuits, and reverse anhedonia.

METHODS

Anhedonia was induced by chronic stress in male mice and assayed by preferences for sucrose and female urine (n = 5-7 mice/group). Hippocampal slices were then prepared for electrophysiological recording (n = 1-6 slices/mouse, 4-6 mice/group). Electroencephalography power was quantified in response to GABA-NAM and ketamine administration (n = 7-9 mice/group).

RESULTS

Chronic stress reduced sucrose and female urine preferences and hippocampal temporoammonic-CA1 synaptic strength. A peripheral injection of the GABA-NAM MRK-016 restored hedonic behavior and AMPA-to-NMDA ratios in wild-type mice. These actions were prevented by pretreatment with the benzodiazepine site antagonist flumazenil. MRK-016 administration increased gamma power over the prefrontal cortex in wild-type mice but not α5 knockout mice, whereas ketamine promoted gamma power in both genotypes. Hedonic behavior and AMPA-to-NMDA ratios were only restored by MRK-016 in stressed wild-type mice but not α5 knockout mice.

CONCLUSIONS

α5-Selective GABA-NAMs exert rapid anti-anhedonic actions and restore the strength of synapses in reward regions by acting at the benzodiazepine site of α5-containing GABA_A receptors. These results encourage human studies using GABA-NAMs to treat depression by providing readily translatable measures of target engagement.

Comparative metabolomic analysis in plasma and cerebrospinal fluid of humans and in plasma and brain of mice following antidepressant-dose ketamine administration

Subanesthetic-dose racemic (R,S)-ketamine (ketamine) produces rapid, robust, and sustained antidepressant effects in major depressive disorder (MDD) and bipolar disorder (BD) and has also been shown to effectively treat neuropathic pain, complex regional pain syndrome, and post-traumatic stress disorder (PTSD). However, to date, its mechanism of action remains unclear. Preclinical studies found that (2 R,6 R;2 S,6 S)-hydroxynorketamine (HNK), a major circulating metabolite of ketamine, elicits antidepressant effects similar to those of ketamine. To help determine how (2 R,6 R)-HNK contributes to ketamine's mechanism of action, an exploratory, targeted, metabolomic analysis was carried out on plasma and CSF of nine healthy volunteers receiving a 40-minute ketamine infusion (0.5 mg/kg). A parallel targeted metabolomic analysis in plasma, hippocampus, and hypothalamus was carried out in mice receiving either 10 mg/kg of ketamine, 10 mg/kg of (2 R,6 R)-HNK, or saline. Ketamine and (2 R,6 R)-HNK both affected multiple pathways associated with inflammatory conditions. In addition, several changes were unique to either the healthy human volunteers and/or the mouse arm of the study, indicating that different pathways may be differentially involved in ketamine's effects in mice and humans. Mechanisms of action found to consistently underlie the effects of ketamine and/or (2 R,6 R)-HNK across both the human metabolome in plasma and CSF and the mouse arm of the study included LAT1, IDO1, NAD+, the nitric oxide (NO) signaling pathway, and sphingolipid rheostat.

Mechanisms of ketamine and its metabolites as antidepressants

Treating major depression is a medical need that remains unmet by monoaminergic therapeutic strategies that commonly fail to achieve symptom remission. A breakthrough in the treatment of depression was the discovery that the anesthetic (R,S)-ketamine (ketamine), when administered at sub-anesthetic doses, elicits rapid (sometimes within hours) antidepressant effects in humans that are otherwise resistant to monoaminergic-acting therapies. While this finding was revolutionary and led to the FDA approval of (S)-ketamine (esketamine) for use in adults with treatment-resistant depression and suicidal ideation, the mechanisms underlying how ketamine or esketamine elicit their effects are still under active investigation. An emerging view is that metabolism of ketamine may be a crucial step in its mechanism of action, as several metabolites of ketamine have neuroactive effects of their own and may be leveraged as therapeutics. For example, (2R,6R)-hydroxynorketamine (HNK), is readily observed in humans following ketamine treatment and has shown therapeutic potential in preclinical tests of antidepressant efficacy and synaptic potentiation while being devoid of the negative adverse effects of ketamine, including its dissociative properties and abuse potential. We discuss preclinical and clinical studies pertaining to how ketamine and its metabolites produce antidepressant effects. Specifically, we explore effects on glutamate neurotransmission through N-methyl D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), synaptic structural changes via brain derived neurotrophic factor (BDNF) signaling, interactions with opioid receptors, and the enhancement of serotonin, norepinephrine, and dopamine signaling. Strategic targeting of these mechanisms may result in novel rapid-acting antidepressants with fewer undesirable side effects compared to ketamine.

Hydroxynorketamine Pharmacokinetics and Antidepressant Behavioral Effects of (2,6)- and (5R)-Methyl-(2R,6R)-hydroxynorketamines

(R,S)-Ketamine is rapidly metabolized to form a range of metabolites in vivo, including 12 unique hydroxynorketamines (HNKs) that are distinguished by a cyclohexyl ring hydroxylation at the 4, 5, or 6 position. While both (2R,6R)- and (2S,6S)-HNK readily penetrate the brain and exert rapid antidepressant-like actions in preclinical tests following peripheral administration, the pharmacokinetic profiles and pharmacodynamic actions of 10 other HNKs have not been examined. We assessed the pharmacokinetic profiles of all 12 HNKs in the plasma and brains of male and female mice and compared the relative potencies of four (2,6)-HNKs to induce antidepressant-relevant behavioral effects in the forced swim test in male mice. While all HNKs were readily brain-penetrable following intraperitoneal injection, there were robust differences in peak plasma and brain concentrations and exposures. Forced swim test immobility rank order of potency, from most to least potent, was (2R,6S)-, (2S,6R)-, (2R,6R)-, and (2S,6S)-HNK. We hypothesized that distinct structure-activity relationships and the resulting potency of each metabolite are linked to unique substitution patterns and resultant conformation of the six-membered cyclohexanone ring system. To explore this, we synthesized (5R)-methyl-(2R,6R)-HNK, which incorporates a methyl substitution on the cyclohexanone ring. (5R)-Methyl-(2R,6R)-HNK exhibited similar antidepressant-like potency to (2R,6S)-HNK. These results suggest that conformation of the cyclohexanone ring system in the (2,6)-HNKs is an important factor underlying potency and that additional engineering of this structural feature may improve the development of a new generation of HNKs. Such HNKs may represent novel drug candidates for the treatment of depression.

Sex-dependent metabolism of ketamine and (2R,6R)-hydroxynorketamine in mice and humans

BACKGROUND

Ketamine is rapidly metabolized to norketamine and hydroxynorketamine (HNK) metabolites. In female mice, when compared to males, higher levels of (2R,6R;2S,6S)-HNK have been observed following ketamine treatment, and higher levels of (2R,6R)-HNK following the direct administration of (2R,6R)-HNK.

AIM

The objective of this study was to evaluate the impact of sex in humans and mice, and gonadal hormones in mice on the metabolism of ketamine to form norketamine and HNKs and in the metabolism/elimination of (2R,6R)-HNK.

METHODS

In CD-1 mice, we utilized gonadectomy to evaluate the role of circulating gonadal hormones in mediating sex-dependent differences in ketamine and (2R,6R)-HNK metabolism. In humans (34 with treatment-resistant depression and 23 healthy controls) receiving an antidepressant dose of ketamine (0.5 mg/kg i.v. infusion over 40 min), we evaluated plasma levels of ketamine, norketamine, and HNKs.

RESULTS

In humans, plasma levels of ketamine and norketamine were higher in males than females, while (2R,6R;2S,6S)-HNK levels were not different. Following ketamine administration to mice (10 mg/kg i.p.), C_max and total plasma concentrations of ketamine and norketamine were higher, and those of (2R,6R;2S,6S)-HNK were lower, in intact males compared to females. Direct (2R,6R)-HNK administration (10 mg/kg i.p.) resulted in higher levels of (2R,6R)-HNK in female mice. Ovariectomy did not alter ketamine metabolism in female mice, whereas orchidectomy recapitulated female pharmacokinetic differences in male mice, which was reversed with testosterone replacement.

CONCLUSION

Sex is an important biological variable that influences the metabolism of ketamine and the HNKs, which may contribute to sex differences in therapeutic antidepressant efficacy or side effects.

(R,S)-ketamine and (2R,6R)-hydroxynorketamine differentially affect memory as a function of dosing frequency

A single subanesthetic infusion of ketamine can rapidly alleviate symptoms of treatment-resistant major depression. Since repeated administration is required to sustain symptom remission, it is important to characterize the potential untoward effects of prolonged ketamine exposure. While studies suggest that ketamine can alter cognitive function, it is unclear to what extent these effects are modulated by the frequency or chronicity of treatment. To test this, male and female adolescent (postnatal day [PD] 35) and adult (PD 60) BALB/c mice were treated for four consecutive weeks, either daily or thrice-weekly, with (R,S)-ketamine (30 mg/kg, intraperitoneal) or its biologically active metabolite, (2R,6R)-hydroxynorketamine (HNK; 30 mg/kg, intraperitoneal). Following drug cessation, memory performance was assessed in three operationally distinct tasks: (1) novel object recognition to assess explicit memory, (2) Y-maze to assess working memory, and (3) passive avoidance to assess implicit memory. While drug exposure did not influence working memory performance, thrice-weekly ketamine and daily (2R,6R)-HNK led to explicit memory impairment in novel object recognition independent of sex or age of exposure. Daily (2R,6R)-HNK impaired implicit memory in the passive-avoidance task whereas thrice-weekly (2R,6R)-HNK tended to improve it. These differential effects on explicit and implicit memory possibly reflect the unique mechanisms by which ketamine and (2R,6R)-HNK alter the functional integrity of neural circuits that subserve these distinct cognitive domains, a topic of clinical and mechanistic relevance to their antidepressant actions. Our findings also provide additional support for the importance of dosing frequency in establishing the cognitive effects of repeated ketamine exposure.

A comparison of the pharmacokinetics and NMDAR antagonism-associated neurotoxicity of ketamine, (2R,6R)-hydroxynorketamine and MK-801

With the increasing use of ketamine as an off-label treatment for depression and the recent FDA approval of (S)-ketamine for treatment-resistant depression, there is an increased need to understand the long-term safety profile of chronic ketamine administration. Of particular concern is the neurotoxicity previously observed in rat models following acute exposure to high doses of ketamine, broadly referred to as 'Olney's lesions'. This type of toxicity presents as abnormal neuronal cellular vacuolization, followed by neuronal death and has been associated with ketamine's inhibition of the N-methyl-d-aspartate receptor (NMDAR). In this study, a pharmacological and neuropathological analysis of ketamine, the potent NMDAR antagonist MK-801, and the ketamine metabolite (2R,6R)-hydroxynorketamine [(2R,6R)-HNK)] in rats is described following both single dose and repeat dose drug exposures. Ketamine dosing was studied up to 20 mg/kg intravenously for the single-dose neuropathology study and up to 60 mg/kg intraperitoneally for the multiple-dose neuropathology study. MK-801 dosing was studied up to 0.8 mg/kg subcutaneously for both the single and multiple-dose neuropathology studies, while (2R,6R)-HNK dosing was studied up to 160 mg/kg intravenously in both studies. These studies confirm dose-dependent induction of 'Olney's lesions' following both single dose and repeat dosing of MK-801. Ketamine exposure, while showing common behavioral effects, did not induce wide-spread Olney's lesions. Treatment with (2R,6R)-HNK did not produce behavioral effects, toxicity or any evidence of Olney's lesion formation. Based on these results, future NMDAR-antagonist neurotoxicity studies should strongly consider taking pharmacokinetics more thoroughly into account.

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.

Ketamine and the Future of Rapid-Acting Antidepressants

The therapeutic onset of traditional antidepressants is delayed by several weeks and many depressed patients fail to respond to treatment altogether. In contrast, subanesthetic ketamine can rapidly alleviate symptoms of depression within hours of a single administration, even in patients who are considered treatment-resistant. Ketamine is thought to exert these effects by restoring the integrity of neural circuits that are compromised in depression. This hypothesis stems in part from preclinical observations that ketamine can strengthen synaptic connections by increasing glutamate-mediated neurotransmission and promoting rapid neurotrophic factor release. An improved understanding of how ketamine, and other novel rapid-acting antidepressants, give rise to these processes will help foster future therapeutic innovation. Here, we review the history of antidepressant treatment advances that preceded the ketamine discovery, critically examine mechanistic hypotheses for how ketamine may exert its antidepressant effects, and discuss the impact this knowledge has had on ongoing drug discovery efforts.

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.

Alpha2B-Adrenergic Receptor Overexpression in the Brain Potentiate Air Pollution-induced Behavior and Blood Pressure Changes

Fine ambient particulate matter (PM2.5) is able to induce sympathetic activation and inflammation in the brain. However, direct evidence demonstrating an essential role of sympathetic activation in PM2.5-associated disease progression is lacking. We assess the contribution of α2B-adrenergic receptor (Adra2b) in air pollution-associated hypertension and behavioral changes in this study. Wild-type mice and Adra2b-transgenic mice overexpressing Adra2b in the brain (Adra2bTg) were exposed to concentrated PM2.5 or filtered air for 3 months via a versatile aerosol concentrator exposure system. Mice were fed with a high salt diet (4.0% NaCl) for 1 week at week 11 of exposure to induce blood pressure elevation. Intra-arterial blood pressure was monitored by radio-telemetry and behavior changes were assessed by open field, light-dark, and prepulse inhibition tests. PM2.5 exposure increased Adra2b in the brain of wild-type mice. Adra2b overexpression enhanced the anxiety-like behavior and high salt diet-induced blood pressure elevation in response to air pollution but not filtered air exposure. Adra2b overexpression induced upregulation of inflammatory genes such as TLR2, TLR4, and IL-6 in the brain exposed to PM2.5. In addition, there were increased frequencies of activated effector T cells and increased expression of oxidative stress-related genes, such as SOD1, NQO1, Nrf2, and Gclm in Adra2bTg mice compared with wild-type mice. Our results provide new evidence of distinct behavioral changes consistent with anxiety and blood pressure elevation in response to high salt intake and air pollution exposure, highlighting the importance of centrally expressed Adra2b in the vulnerability to air pollution exposure.

A Randomized Trial of the N-Methyl-d-Aspartate Receptor Glycine Site Antagonist Prodrug 4-Chlorokynurenine in Treatment-Resistant Depression

BACKGROUND

Ketamine has rapid-acting antidepressant effects but is associated with psychotomimetic and other adverse effects. A 7-chlorokynurenic acid is a potent and specific glycine site N-methyl-d-aspartate receptor antagonist but crosses the blood-brain barrier inefficiently. Its prodrug, L-4-chlorokynurenine (4-Cl-KYN), exerts acute and sustained antidepressant-like effects in rodents and has no reported psychotomimetic effects in either rodents or healthy volunteers. This study examined whether 4-Cl-KYN has rapid antidepressant effects in individuals with treatment-resistant depression.

METHODS

After a 2-week drug-free period, 19 participants with treatment-resistant depression were randomized to receive daily oral doses of 4-Cl-KYN monotherapy (1080 mg/d for 7 days, then 1440 mg/d for 7 days) or placebo for 14 days in a randomized, placebo-controlled, double-blind, crossover manner. The primary outcome measure was the Hamilton Depression Rating Scale score, assessed at several time points over a 2-week period; secondary outcome measures included additional rating scale scores. Pharmacokinetic measures of 7-chlorokynurenic acid and 4-Cl-KYN and pharmacodynamic assessments were obtained longitudinally and included 1H-magnetic resonance spectroscopy brain glutamate levels, resting-state functional magnetic resonance imaging, and plasma and cerebrospinal fluid measures of kynurenine metabolites and neurotrophic factors.

RESULTS

Linear mixed models detected no treatment effects, as assessed by primary and secondary outcome measures. No difference was observed for any of the peripheral or central biological indices or for adverse effects at any time between groups. A 4-Cl-KYN was safe and well-tolerated, with generally minimal associated adverse events.

CONCLUSIONS

In this small crossover trial, 4-Cl-KYN monotherapy exerted no antidepressant effects at the doses and treatment duration studied.ClinicalTrials.gov identifier: NCT02484456.

Antidepressant Effects and Mechanisms of Group II mGlu Receptor-Specific Negative Allosteric Modulators

In this issue of Neuron, Joffe et al. (2020) assess the antidepressant-relevant effects and underlying neural mechanisms of negative allosteric modulators selective for either metabotropic glutamate receptors 2 (mGlu₂) or 3 (mGlu₃). Negative modulation of both receptors enhanced excitatory glutamatergic input to mouse prefrontal cortex pyramidal cells, leading to antidepressant-relevant actions.

Classical conditioning of antidepressant placebo effects in mice

INTRODUCTION

Placebo effects in human clinical trials for depression treatment are robust and often comparable to drug effects. Placebo effects are traditionally difficult to study in rodents due to the slow-onset action of classical antidepressant drugs. We hypothesized that the rapid antidepressant actions of ketamine would allow modeling antidepressant placebo effects in rodents.

METHODS

Male and female CD-1 mice received either ketamine or saline injections with concomitant exposure to specific environmental conditioning stimuli, for a total of three drug/conditioning sessions each 2 weeks apart. Two weeks later, during an evocation phase, mice were exposed to the drug-paired conditioning stimuli or no conditioned stimuli followed by testing for motor stimulatory actions and antidepressant-like effects using the forced swim test. Negative (no ketamine administration at any time) and positive (acute ketamine administration prior to evocation testing) control groups were included as comparators.

RESULTS

Both male and female mice exhibited increased locomotor activity following ketamine administration during the conditioning phase, which was not observed following exposure to the conditioning stimuli. Exposure to the conditioning stimuli previously paired with ketamine, similar to an acute ketamine administration, reduced immobility time in the forced swim test both 1 and 24 h after administration in male, but not female, mice.

CONCLUSIONS

These results represent the first evidence of antidepressant-like placebo-conditioned effects in an animal model. The developed approach can be used as a model to explore the neurobiological mechanisms of placebo effects, their possible sexually dimorphic effects, and relevance to mechanisms underlying antidepressant action.

Group II metabotropic glutamate receptor blockade promotes stress resilience in mice

Stress is a leading risk factor for the onset and recurrence of major depression. Enhancing stress resilience may be a therapeutic strategy to prevent the development of depression in at-risk populations or its recurrence in depressed patients. Group II metabotropic glutamate receptor (mGlu_2/3) antagonists have been recognized for antidepressant-like actions in preclinical models, but have not been evaluated for prophylactic effects. We assessed the role of mGlu_2/3 in modulating stress resilience using subtype-specific knockout mice lacking mGlu₂ (Grm2^-/-) or mGlu₃ (Grm3^-/-), and pharmacological manipulations of mGlu_2/3 activity during or prior to the induction and reinstatement of stress-induced behavioral deficits. Grm2^-/-, but not Grm3^-/-, mice exhibited reduced forced-swimming test immobility time and were resilient to developing inescapable shock (IES)-induced escape deficits. Grm2^-/- mice were also resilient to developing corticosterone (CORT)-induced escape deficits and chronic social defeat stress-induced anhedonia. Pharmacological blockade of mGlu_2/3 with the antagonist LY341495 during stress prevented the development of IES- and CORT-induced escape deficits, while activation with the agonist LY379268 increased susceptibility to escape deficits. Prophylactic treatment with the LY341495, both systemically and via microinjection into the medial prefrontal cortex (mPFC), up to 7 days before IES, prevented both the induction of escape deficits and their reinstatement by brief re-exposure to IES up to 20 days after treatment. Overall, blockade of mGlu_2/3 enhanced stress resilience and deletion of mGlu₂, but not mGlu₃, conferred a stress-resilient phenotype, indicating that prophylactic treatments reducing mGlu₂ activity may protect against stress-induced changes underlying the development or recurrence of stress-induced disorders, including depression.

(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.

Sex-Specific Involvement of Estrogen Receptors in Behavioral Responses to Stress and Psychomotor Activation

Fluctuating hormone levels, such as estradiol might underlie the difference in the prevalence of psychiatric disorders observed in women vs. men. Estradiol exert its effects primarily through binding on the two classical estrogen receptor subtypes, alpha (ERα) and beta (ERβ). Both receptors have been suggested to a have role in the development of psychiatric disorders, however, most of the current literature is limited to their role in females. We investigated the role of estrogen receptors on cognition (novel-object recognition), anxiety (open-field test, elevated-plus maze, and light/dark box), stress-responsive behaviors (forced-swim test, learned helplessness following inescapable shock, and sucrose preference), pre-pulse inhibition (PPI) and amphetamine-induced hyperlocomotion in both male and female mice either lacking the ERα or ERβ receptor. We found that female Esr1 ^-/- mice have attenuated pre-pulse inhibition, whereas female Esr2 ^-/- mice manifested enhanced pre-pulse inhibition. No pre-pulse inhibition difference was observed in male Esr1 ^-/- and Esr2 ^-/- mice. Moreover, amphetamine-induced hyperlocomotion was decreased in male Esr1 ^-/-, but not Esr2 ^-/- mice, while female Esr1 ^-/- and Esr2 ^-/- mice showed an enhanced response. Genetic absence of ERα did not alter the escape capability or sucrose preference following inescapable shock in both male and female mice. In contrast, female, but not male Esr2 ^-/- mice, manifested decreased escape failures compared with controls. Lack of Esr2 gene in male mice was associated with decreased sucrose preference following inescapable shock, suggesting susceptibility for development of anhedonia following stress. No sucrose preference differences were found in female Esr2 ^-/- mice following inescapable shock stress. Lastly, we demonstrated that lack of Esr1 or Esr2 genes had no effect on memory and anxiety-like behaviors in both male and female mice. Our findings indicate a differential sex-specific involvement of estrogen receptors in the development of stress-mediated maladaptive behaviors as well as psychomotor activation responses suggesting that these receptors might act as potential treatment targets in a sex-specific manner.

Mouse, rat, and dog bioavailability and mouse oral antidepressant efficacy of (2R,6R)-hydroxynorketamine

BACKGROUND

(R,S)-ketamine has gained attention for its rapid-acting antidepressant actions in patients with treatment-resistant depression. However, widespread use of ketamine is limited by its side effects, abuse potential, and poor oral bioavailability. The ketamine metabolite, (2R,6R)-hydroxynorketamine, exerts rapid antidepressant effects, without ketamine's adverse effects and abuse potential, in rodents.

METHODS

We evaluated the oral bioavailability of (2R,6R)-hydroxynorketamine in three species (mice, rats, and dogs) and also evaluated five candidate prodrug modifications for their capacity to enhance the oral bioavailability of (2R,6R)-hydroxynorketamine in mice. Oral administration of (2R,6R)-hydroxynorketamine was assessed for adverse behavioral effects and for antidepressant efficacy in the mouse forced-swim and learned helplessness tests.

RESULTS

(2R,6R)-hydroxynorketamine had absolute bioavailability between 46-52% in mice, 42% in rats, and 58% in dogs. Compared to intraperitoneal injection in mice, the relative oral bioavailability of (2R,6R)-hydroxynorketamine was 62%, which was not improved by any of the candidate prodrugs tested. Following oral administration, (2R,6R)-hydroxynorketamine readily penetrated the brain, with brain to plasma ratios between 0.67-1.2 in mice and rats. Oral administration of (2R,6R)-hydroxynorketamine to mice did not alter locomotor activity or precipitate behaviors associated with discomfort, sickness, or stereotypy up to a dose of 450 mg/kg. Oral (2R,6R)-hydroxynorketamine reduced forced-swim test immobility time (15-150 mg/kg) and reversed learned helplessness (50-150 mg/kg) in mice.

CONCLUSIONS

These results demonstrate that (2R,6R)-hydroxynorketamine has favorable oral bioavailability in three species and exhibits antidepressant efficacy following oral administration in mice.

Molecular Pharmacology and Neurobiology of Rapid-Acting Antidepressants

For decades, symptoms of depression have been treated primarily with medications that directly target the monoaminergic brain systems, which typically take weeks to exert measurable effects and months to exert remission of symptoms. Low, subanesthetic doses of ( R,S)-ketamine (ketamine) result in the rapid improvement of core depressive symptoms, including mood, anhedonia, and suicidal ideation, occurring within hours following a single administration, with relief from symptoms typically lasting up to a week. The discovery of these actions of ketamine has resulted in a reconceptualization of how depression could be more effectively treated in the future. In this review, we discuss clinical data pertaining to ketamine and other rapid-acting antidepressant drugs, as well as the current state of pharmacological knowledge regarding their mechanism of action. Additionally, we discuss the neurobiological circuits that are engaged by this drug class and that may be targeted by a future generation of medications, for example, hydroxynorketamine; metabotropic glutamate receptor 2/3 antagonists; and N-methyl-d-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and γ-aminobutyric acid receptor modulators.

Plasma metabolomic profiling of a ketamine and placebo crossover trial of major depressive disorder and healthy control subjects

(R,S)-Ketamine produces rapid, robust, and sustained antidepressant effects in major depressive disorder. Specifically, its pharmacological efficacy in treatment refractory depression is considered a major breakthrough in the field. However, the mechanism of action of ketamine's rapid effect remains to be determined. In order to identify pathways that are responsible for ketamine's effect, a targeted metabolomic approach was carried out using a double-blind, placebo-controlled crossover design, with infusion order randomized with medication-free patients with treatment-resistant major depressive disorder (29 subjects) and healthy controls (25 subjects). The metabolomic profile of these subjects was characterized at multiple time points, and a comprehensive analysis was investigated between the following: MDD and healthy controls, treatment and placebo in both groups and the corresponding response to ketamine treatment. Ketamine treatment resulted in a general increase in circulating sphingomyelins, levels which were not correlated with response. Ketamine response resulted in more pronounced effects in the kynurenine pathway and the arginine pathway at 4 h post-infusion, where a larger decrease in circulating kynurenine levels and a larger increase in the bioavailability of arginine were observed in responders to ketamine treatment, suggesting possible mechanisms for response to ketamine treatment.

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine's pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

7B2 chaperone knockout in APP model mice results in reduced plaque burden

Impairment of neuronal proteostasis is a hallmark of Alzheimer's and other neurodegenerative diseases. However, the underlying molecular mechanisms leading to pathogenic protein aggregation, and the role of secretory chaperone proteins in this process, are poorly understood. We have previously shown that the neural-and endocrine-specific secretory chaperone 7B2 potently blocks in vitro fibrillation of Aβ42. To determine whether 7B2 can function as a chaperone in vivo, we measured plaque formation and performed behavioral assays in 7B2-deficient mice in an hAPPswe/PS1dE9 Alzheimer's model mouse background. Surprisingly, immunocytochemical analysis of cortical levels of thioflavin S- and Aβ-reactive plaques showed that APP mice with a partial or complete lack of 7B2 expression exhibited a significantly lower number and burden of thioflavin S-reactive, as well as Aβ-immunoreactive, plaques. However, 7B2 knockout did not affect total brain levels of either soluble or insoluble Aβ. While hAPP model mice performed poorly in the Morris water maze, their brain 7B2 levels did not impact performance. Since 7B2 loss reduced amyloid plaque burden, we conclude that brain 7B2 can impact Aβ disposition in a manner that facilitates plaque formation. These results are reminiscent of prior findings in hAPP model mice lacking the ubiquitous secretory chaperone clusterin.

Cigarette smoke and nicotine effects on brain proinflammatory responses and behavioral and motor function in HIV-1 transgenic rats

Cognitive impairment in HIV-1 infection is associated with the induction of chronic proinflammatory responses in the brains of infected individuals. The risk of HIV-related cognitive impairment is increased by cigarette smoking, which induces brain inflammation in rodent models. To better understand the role of smoking and the associated immune response on behavioral and motor function in HIV infection, wild-type F344 and HIV-1 transgenic (HIV1Tg) rats were exposed to either smoke from nicotine-containing (regular) cigarettes, smoke from nicotine-free cigarettes, or to nicotine alone. The animals were then tested using the rotarod test (RRT), the novel object recognition test (NORT), and the open field test (OFT). Subsequently, brain frontal cortex from the rats was analyzed for levels of TNF-α, IL-1, and IL-6. On the RRT, impairment was noted for F344 rats exposed to either nicotine-free cigarette smoke or nicotine alone and for F344 and HIV1Tg rats exposed to regular cigarette smoke. Effects from the exposures on the OFT were seen only for HIV1Tg rats, for which function was worse following exposure to regular cigarette smoke as compared to exposure to nicotine alone. Expression levels for all three cytokines were overall higher for HIV1Tg than for F344 rats. For HIV1Tg rats, TNF-α, IL-1, and IL-6 gene expression levels for all exposure groups were higher than for control rats. All F344 rat exposure groups also showed significantly increased TNF-α expression levels. However, for F344 rats, IL-1 expression levels were higher only for rats exposed to nicotine-free and nicotine-containing CS, and no increase in IL-6 gene expression was noted with any of the exposures as compared to controls. These studies, therefore, demonstrate that F344 and HIV1Tg rats show differential behavioral and immune effects from these exposures. These effects may potentially reflect differences in the responsiveness of the various brain regions in the two animal species as well as the result of direct toxicity mediated by the proinflammatory cytokines that are produced by HIV proteins and by other factors that are present in regular cigarette smoke.

Isoflurane but Not Halothane Prevents and Reverses Helpless Behavior: A Role for EEG Burst Suppression?

BACKGROUND

The volatile anesthetic isoflurane may exert a rapid and long-lasting antidepressant effect in patients with medication-resistant depression. The mechanism underlying the putative therapeutic actions of the anesthetic have been attributed to its ability to elicit cortical burst suppression, a distinct EEG pattern with features resembling the characteristic changes that occur following electroconvulsive therapy. It is currently unknown whether the antidepressant actions of isoflurane are shared by anesthetics that do not elicit cortical burst suppression.

METHODS

In vivo electrophysiological techniques were used to determine the effects of isoflurane and halothane, 2 structurally unrelated volatile anesthetics, on cortical EEG. The effects of anesthesia with either halothane or isoflurane were also compared on stress-induced learned helplessness behavior in rats and mice.

RESULTS

Isoflurane, but not halothane, anesthesia elicited a dose-dependent cortical burst suppression EEG in rats and mice. Two hours of isoflurane, but not halothane, anesthesia reduced the incidence of learned helplessness in rats evaluated 2 weeks following exposure. In mice exhibiting a learned helplessness phenotype, a 1-hour exposure to isoflurane, but not halothane, reversed escape failures 24 hours following burst suppression anesthesia.

CONCLUSIONS

These results are consistent with a role for cortical burst suppression in mediating the antidepressant effects of isoflurane. They provide rationale for additional mechanistic studies in relevant animal models as well as a properly controlled clinical evaluation of the therapeutic benefits associated with isoflurane anesthesia in major depressive disorder.

Convergent Mechanisms Underlying Rapid Antidepressant Action

Traditional pharmacological treatments for depression have a delayed therapeutic onset, ranging from several weeks to months, and there is a high percentage of individuals who never respond to treatment. In contrast, ketamine produces rapid-onset antidepressant, anti-suicidal, and anti-anhedonic actions following a single administration to patients with depression. Proposed mechanisms of the antidepressant action of ketamine include N-methyl-D-aspartate receptor (NMDAR) modulation, gamma aminobutyric acid (GABA)-ergic interneuron disinhibition, and direct actions of its hydroxynorketamine (HNK) metabolites. Downstream actions include activation of the mechanistic target of rapamycin (mTOR), deactivation of glycogen synthase kinase-3 and eukaryotic elongation factor 2 (eEF2), enhanced brain-derived neurotrophic factor (BDNF) signaling, and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs). These putative mechanisms of ketamine action are not mutually exclusive and may complement each other to induce potentiation of excitatory synapses in affective-regulating brain circuits, which results in amelioration of depression symptoms. We review these proposed mechanisms of ketamine action in the context of how such mechanisms are informing the development of novel putative rapid-acting antidepressant drugs. Such drugs that have undergone pre-clinical, and in some cases clinical, testing include the muscarinic acetylcholine receptor antagonist scopolamine, GluN2B-NMDAR antagonists (i.e., CP-101,606, MK-0657), (2R,6R)-HNK, NMDAR glycine site modulators (i.e., 4-chlorokynurenine, pro-drug of the glycineB NMDAR antagonist 7-chlorokynurenic acid), NMDAR agonists [i.e., GLYX-13 (rapastinel)], metabotropic glutamate receptor 2/3 (mGluR2/3) antagonists, GABAA receptor modulators, and drugs acting on various serotonin receptor subtypes. These ongoing studies suggest that the future acute treatment of depression will typically occur within hours, rather than months, of treatment initiation.

Dopamine and Stress System Modulation of Sex Differences in Decision Making

Maladaptive decision making is associated with several neuropsychiatric disorders, including problem gambling and suicidal behavior. The prevalence of these disorders is higher in men vs women, suggesting gender-dependent regulation of their pathophysiology underpinnings. We assessed sex differences in decision making using the rat version of the Iowa gambling task. Female rats identified the most optimal choice from session 1, whereas male rats from session 5. Male, but not female rats, progressively improved their advantageous option responding and surpassed females. Estrus cycle phase did not affect decision making. To test whether pharmacological manipulations targeting the dopaminergic and stress systems affect decision making in a sex-dependent manner, male and female rats received injections of a dopamine D₂ receptor (D₂R) antagonist (eticlopride), D₂R agonist (quinpirole), corticotropin-releasing factor 1 (CRF₁) antagonist (antalarmin), and α₂-adrenergic receptor antagonist (yohimbine; used as a pharmacological stressor). Alterations in mRNA levels of D₂R and CRF₁ were also assessed. Eticlopride decreased advantageous responding in male, but not female rats, whereas quinpirole decreased advantageous responding specifically in females. Yohimbine dose-dependently decreased advantageous responding in female rats, whereas decreased advantageous responding was only observed at higher doses in males. Antalarmin increased optimal choice responding only in female rats. Higher Drd2 and Crhr1 expression in the amygdala were observed in female vs male rats. Higher amygdalar Crhr1 expression was negatively correlated with advantageous responding specifically in females. This study demonstrates the relevance of dopaminergic- and stress-dependent sex differences to maladaptive decision making.

Synthesis and N-Methyl-d-aspartate (NMDA) Receptor Activity of Ketamine Metabolites

Ketamine is rapidly metabolized in the human body to a variety of metabolites, including the hydroxynorketamines. At least two hydroxynorketamines have significant antidepressant action in rodent models, with limited action against the N-methyl-d-aspartate (NMDA) receptor. The synthesis of 12 hydroxynorketamines and their binding affinity to the NMDA receptor is presented here.

Animal models to improve our understanding and treatment of suicidal behavior

Worldwide, suicide is a leading cause of death. Although a sizable proportion of deaths by suicide may be preventable, it is well documented that despite major governmental and international investments in research, education and clinical practice suicide rates have not diminished and are even increasing among several at-risk populations. Although nonhuman animals do not engage in suicidal behavior amenable to translational studies, we argue that animal model systems are necessary to investigate candidate endophenotypes of suicidal behavior and the neurobiology underlying these endophenotypes. Animal models are similarly a critical resource to help delineate treatment targets and pharmacological means to improve our ability to manage the risk of suicide. In particular, certain pathophysiological pathways to suicidal behavior, including stress and hypothalamic-pituitary-adrenal axis dysfunction, neurotransmitter system abnormalities, endocrine and neuroimmune changes, aggression, impulsivity and decision-making deficits, as well as the role of critical interactions between genetic and epigenetic factors, development and environmental risk factors can be modeled in laboratory animals. We broadly describe human biological findings, as well as protective effects of medications such as lithium, clozapine, and ketamine associated with modifying risk of engaging in suicidal behavior that are readily translatable to animal models. Endophenotypes of suicidal behavior, studied in animal models, are further useful for moving observed associations with harmful environmental factors (for example, childhood adversity, mechanical trauma aeroallergens, pathogens, inflammation triggers) from association to causation, and developing preventative strategies. Further study in animals will contribute to a more informed, comprehensive, accelerated and ultimately impactful suicide research portfolio.

Reduced levels of Cacna1c attenuate mesolimbic dopamine system function

Genetic variation in CACNA1C, which codes for the L-type calcium channel (LTCC) Ca_v 1.2, is associated with clinical diagnoses of bipolar disorder, depression and schizophrenia. Dysregulation of the mesolimbic-dopamine (ML-DA) system is linked to these syndromes and LTCCs are required for normal DAergic neurotransmission between the ventral tegmental area (VTA) and nucleus accumbens (NAc). It is unclear, however, how variations in CACNA1C genotype, and potential subsequent changes in expression levels in these regions, modify risk. Using constitutive and conditional knockout mice, and treatment with the LTCC antagonist nimodipine, we examined the role of Cacna1c in DA-mediated behaviors elicited by psychomotor stimulants. Using fast-scan cyclic voltammetry, DA release and reuptake in the NAc were measured. We find that subsecond DA release in Cacna1c haploinsufficient mice lacks normal sensitivity to inhibition of the DA transporter (DAT). Constitutive haploinsufficiency of Cacna1c led to attenuation of hyperlocomotion following acute administration of stimulants specific to DAT, and locomotor sensitization of these mice to the DAT antagonist GBR12909 did not reach the same level as wild-type mice. The maintenance of sensitization to GBR12909 was attenuated by administration of nimodipine. Sensitization to GBR12909 was attenuated in mice with reduced Cacna1c selectively in the VTA but not in the NAc. Our findings show that Cacna1c is crucial for normal behavioral responses to DA stimulants and that its activity in the VTA is required for behavioral sensitization. Cacna1c likely exerts these effects through modifications to presynaptic ML-DA system function.

Decreased Nucleus Accumbens Expression of Psychiatric Disorder Risk Gene Cacna1c Promotes Susceptibility to Social Stress

BACKGROUND

Polymorphisms in the CACNA1C gene are associated with human mood disorders. The rodent social defeat model of stress/mood-disorder susceptibility results in maladaptive consequences mediated by altered function of mesolimbic circuits.

METHODS

mRNA levels of Cacna1c in the nucleus accumbens of mice exposed to social defeat were assessed. Cacna1c was selectively deleted in the nucleus accumbens of floxed Cacna1c mice using viral Cre-recombinase to examine Cacna1c in social defeat susceptibility.

RESULTS

Reduced expression of Cacan1c in the nucleus accumbens is associated with increased susceptibility to social defeat stress, and a knockdown of Cacna1c in the nucleus accumbens significantly increases susceptibility measured by social interaction and female urine preference.

CONCLUSIONS

Cacna1c reduction causally predisposes to the maladaptive outcomes of social stress. Normal Cacna1c function in the nucleus accumbens is crucial for resiliency to social stressors. Variations in expression of CACNA1C in the nucleus accumbens may mediate human risk for developing mood disorders and be a target for therapeutic intervention.