Zanos et al. reply

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 opioid use and related fatalities. Although currently-prescribed medications for OUD (MOUD) are considered life-saving, they inadequately address negative affect and cognitive impairment, resulting in high relapse rates to non-medical 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 re-taking non-medical opioids. In this review, we comprehensively discuss preclinical and clinical research evaluating ketamine and its metabolites as potential novel therapeutic strategies for treating OUDs. We further examine evidence supporting the relevance of the molecular targets of ketamine and its metabolites in relation to their potential effects and therapeutic outcomes in OUDs. Overall, existing evidence demonstrates that ketamine and its metabolites can effectively modulate pathophysiological processes affected in OUD, suggesting their promising therapeutic role in the treatment of OUD and the prevention of return to opioid use during abstinence.

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.

An integrative view on the cell-type-specific mechanisms of ketamine's antidepressant actions

Over the past six decades, the use of ketamine has evolved from an anesthetic and recreational drug to the first non-monoaminergic antidepressant approved for treatment-resistant major depressive disorder (MDD). Subanesthetic doses of ketamine and its enantiomer (S)-ketamine (esketamine) directly bind to several neurotransmitter receptors [including N-methyl-d-aspartic acid receptor (NMDAR), κ and μ opioid receptor (KOR and MOR)] widely distributed in the brain and across different cell types, implicating several potential molecular mechanisms underlying the action of ketamine as an antidepressant. This review examines preclinical studies investigating cell-type-specific mechanisms underlying the effects of ketamine on behavior and synapses. Cell-type-specific approaches are crucial for disentangling the critical mechanisms involved in the therapeutic effect of ketamine.

Pharmacological Mechanism of Ketamine in Suicidal Behavior Based on Animal Models of Aggressiveness and Impulsivity: A Narrative Review

Around 700,000 people die from suicide each year in the world. Approximately 90% of suicides have a history of mental illness, and more than two-thirds occur during a major depressive episode. Specific therapeutic options to manage the suicidal crisis are limited and measures to prevent acting out also remain limited. Drugs shown to reduce the risk of suicide (antidepressants, lithium, or clozapine) necessitate a long delay of onset. To date, no treatment is indicated for the treatment of suicidality. Ketamine, a glutamate NMDA receptor antagonist, is a fast-acting antidepressant with significant effects on suicidal ideation in the short term, while its effects on suicidal acts still need to be demonstrated. In the present article, we reviewed the literature on preclinical studies in order to identify the potential anti-suicidal pharmacological targets of ketamine. Impulsive-aggressive traits are one of the vulnerability factors common to suicide in patients with unipolar and bipolar depression. Preclinical studies in rodent models with impulsivity, aggressiveness, and anhedonia may help to analyze, at least in part, suicide neurobiology, as well as the beneficial effects of ketamine/esketamine on reducing suicidal ideations and preventing suicidal acts. The present review focuses on disruptions in the serotonergic system (5-HT_B receptor, MAO-A enzyme), neuroinflammation, and/or the HPA axis in rodent models with an impulsive/aggressive phenotype, because these traits are critical risk factors for suicide in humans. Ketamine can modulate these endophenotypes of suicide in human as well as in animal models. The main pharmacological properties of ketamine are then summarized. Finally, numerous questions arose regarding the mechanisms by which ketamine may prevent an impulsive-aggressive phenotype in rodents and suicidal ideations in humans. Animal models of anxiety/depression are important tools to better understand the pathophysiology of depressed patients, and in helping develop novel and fast antidepressant drugs with anti-suicidal properties and clinical utility.

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.

Role of group II metabotropic glutamate receptors in ketamine's antidepressant actions

Major Depressive Disorder (MDD) is a serious neuropsychiatric disorder afflicting around 16-17 % of the global population and is accompanied by recurrent episodes of low mood, hopelessness and suicidal thoughts. Current pharmacological interventions take several weeks to even months for an improvement in depressive symptoms to emerge, with a significant percentage of individuals not responding to these medications at all, thus highlighting the need for rapid and effective next-generation treatments for MDD. Pre-clinical studies in animals have demonstrated that antagonists of the metabotropic glutamate receptor subtype 2/3 (mGlu_2/3 receptor) exert rapid antidepressant-like effects, comparable to the actions of ketamine. Therefore, it is possible that mGlu₂ or mGlu₃ receptors to have a regulatory role on the unique antidepressant properties of ketamine, or that convergent intracellular mechanisms exist between mGlu_2/3 receptor signaling and ketamine's effects. Here, we provide a comprehensive and critical evaluation of the literature on these convergent processes underlying the antidepressant action of mGlu_2/3 receptor inhibitors and ketamine. Importantly, combining sub-threshold doses of mGlu_2/3 receptor inhibitors with sub-antidepressant ketamine doses induce synergistic antidepressant-relevant behavioral effects. We review the evidence supporting these combinatorial effects since sub-effective dosages of mGlu_2/3 receptor antagonists and ketamine could reduce the risk for the emergence of significant adverse events compared with taking normal dosages. Overall, deconvolution of ketamine's pharmacological targets will give critical insights to influence the development of next-generation antidepressant treatments with rapid actions.

Translational control by ketamine and its implications for comorbid cognitive deficits in depressive disorders

Ketamine has shown antidepressant effects in patients with major depressive disorder (MDD) resistant to first-line treatments and approved for use in this patient population. Ketamine induces several forms of synaptic plasticity, which are proposed to underlie its antidepressant effects. However, the molecular mechanism of action directly responsible for ketamine's antidepressant effects remains under active investigation. It was recently demonstrated that the effectors of the mammalian target of rapamycin complex 1 (mTORC1) signalling pathway, namely, eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2 (4E-BP1 and 4E-BP2), are central in mediating ketamine-induced synaptic plasticity and behavioural antidepressant-like effect. 4E-BPs are a family of messenger ribonucleic acid (mRNA) translation repressors inactivated by mTORC1. We observed that their expression in inhibitory interneurons mediates ketamine's effects in the forced swim and novelty suppressed feeding tests and the long-lasting inhibition of GABAergic neurotransmission in the hippocampus. In addition, another effector pathway that regulates translation elongation downstream of mTORC1, the eukaryotic elongation factor 2 kinase (eEF2K), has been implicated in ketamine's behavioural effects. We will discuss how ketamine's rapid antidepressant effect depends on the activation of neuronal mRNA translation through 4E-BP1/2 and eEF2K. Furthermore, given that these pathways also regulate cognitive functions, we will discuss the evidence of ketamine's effect on cognitive function in MDD. Overall, the data accrued from pre-clinical research have implicated the mRNA translation pathways in treating mood symptoms of MDD. However, it is yet unclear whether the pro-cognitive potential of subanesthetic ketamine in rodents also engages these pathways and whether such an effect is consistently observed in the treatment-resistant MDD population.

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.

Ketamine: Neuroprotective or Neurotoxic?

Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, has been employed clinically as an intravenous anesthetic since the 1970s. More recently, ketamine has received attention for its rapid antidepressant effects and is actively being explored as a treatment for a wide range of neuropsychiatric syndromes. In model systems, ketamine appears to display a combination of neurotoxic and neuroprotective properties that are context dependent. At anesthetic doses applied during neurodevelopmental windows, ketamine contributes to inflammation, autophagy, apoptosis, and enhances levels of reactive oxygen species. At the same time, subanesthetic dose ketamine is a powerful activator of multiple parallel neurotrophic signaling cascades with neuroprotective actions that are not always NMDAR-dependent. Here, we summarize results from an array of preclinical studies that highlight a complex landscape of intracellular signaling pathways modulated by ketamine and juxtapose the somewhat contrasting neuroprotective and neurotoxic features of this drug.

Positive AMPA receptor modulation in the treatment of neuropsychiatric disorders: A long and winding road

Glutamatergic transmission is widely implicated in neuropsychiatric disorders, and the discovery that ketamine elicits rapid-acting antidepressant effects by modulating α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) signaling has spurred a resurgence of interest in the field. This review explores agents in various stages of development for neuropsychiatric disorders that positively modulate AMPARs, both directly and indirectly. Despite promising preclinical research, few direct and indirect AMPAR positive modulators have progressed past early clinical development. Challenges such as low potency have created barriers to effective implementation. Nevertheless, the functional complexity of AMPARs sets them apart from other drug targets and allows for specificity in drug discovery. Additional effective treatments for neuropsychiatric disorders that work through positive AMPAR modulation may eventually be developed.

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.

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

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

Ketamine and nitrous oxide: The evolution of NMDA receptor antagonists as antidepressant agents

N-methyl-d-aspartate receptor (NMDAR) antagonists, including ketamine and nitrous oxide, are currently intensely studied as rapid-acting antidepressant agents. Interestingly, both of these compounds are also drugs of abuse. Intravenous ketamine, a dissociative anesthetic that induces complex downstream effects via NMDARs, rapidly reduces depressive and suicidal symptoms in treatment-resistant depression (TRD), as demonstrated by several trials. Recently, the United States Food and Drug Administration (FDA) approved an intranasal version of ketamine (esketamine) for TRD. The United States Drug Enforcement Agency (DEA) lists ketamine as a Class III scheduled drug (moderate-low potential for physical and psychological abuse). The FDA has established a Risk Evaluation and Management Strategy (REMS) program to ensure proper drug storage, handling, dispensing, and monitoring intranasal esketamine to minimize misuse/abuse opportunities. Nitrous Oxide is a colorless, odorless, gas that has been in medical use for over 150 years. The mechanisms of action of nitrous oxide are not fully understood; however, it is known to act as a non-competitive inhibitor of NMDA-type glutamate receptors. Currently, nitrous oxide is used for inhalational general anesthesia and analgesia for short procedures. Inhaled nitrous oxide is also used recreationally, primarily by teens and young adults, but is not believed to have strong addiction potential. In contrast to ketamine, nitrous oxide is not a controlled substance and can be legally purchased without a prescription. A recent double-blind, prospective, cross-over study demonstrated that nitrous oxide reduced depressive symptoms in a group of severely ill TRD patients. Though this is a promising initial study, further investigation is needed.

(2R,6R)-hydroxynorketamine rapidly potentiates hippocampal glutamatergic transmission through a synapse-specific presynaptic mechanism

Preclinical studies indicate that (2R,6R)-hydroxynorketamine (HNK) retains the rapid and sustained antidepressant-like actions of ketamine, but is spared its dissociative-like properties and abuse potential. While (2R,6R)-HNK is thought to exert its antidepressant-like effects by potentiating α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated synaptic transmission, it is unknown how it exerts this effect. The acute synaptic effects of (2R,6R)-HNK were examined by recording field excitatory postsynaptic potentials (fEPSPs) and miniature excitatory postsynaptic currents (mEPSCs) in rat hippocampal slices. (2R,6R)-HNK bath application caused a rapid and persistent potentiation of AMPAR-mediated Schaffer collateral (SC)-CA1 fEPSPs in slices derived from male and female rats. The (2R,6R)-HNK-induced potentiation occurred independent of N-methyl-D-aspartate receptor (NMDAR) activity, was accompanied by a concentration-dependent decrease in paired pulse ratios, and was occluded by raising glutamate release probability. In additon, in the presence of tetrodotoxin, (2R,6R)-HNK increased the frequency, but not amplitude, of mEPSC events, confirming a presynaptic site of action that is independent of glutamatergic network disinhibition. A dual extracellular recording configuration revealed that the presynaptic effects of (2R,6R)-HNK were synapse-selective, occurring in CA1-projecting SC terminals, but not in CA1-projecting temporoammonic terminals. Overall, we found that (2R,6R)-HNK enhances excitatory synaptic transmission in the hippocampus through a concentration-dependent, NMDAR-independent, and synapse-selective increase in glutamate release probability with no direct actions on AMPAR function. These findings provide novel insight regarding (2R,6R)-HNK's acute mechanism of action, and may inform novel antidepressant drug mechanisms that could yield superior efficacy, safety, and tolerability.

(2R,6R)-hydroxynorketamine exerts mGlu2 receptor-dependent antidepressant actions

Currently approved antidepressant drugs often take months to take full effect, and ∼30% of depressed patients remain treatment resistant. In contrast, ketamine, when administered as a single subanesthetic dose, exerts rapid and sustained antidepressant actions. Preclinical studies indicate that the ketamine metabolite (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a rapid-acting antidepressant drug candidate with limited dissociation properties and abuse potential. We assessed the role of group II metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) in the antidepressant-relevant actions of (2R,6R)-HNK using behavioral, genetic, and pharmacological approaches as well as cortical quantitative EEG (qEEG) measurements in mice. Both ketamine and (2R,6R)-HNK prevented mGlu_2/3 receptor agonist (LY379268)-induced body temperature increases in mice lacking the Grm3, but not Grm2, gene. This action was not replicated by NMDA receptor antagonists or a chemical variant of ketamine that limits metabolism to (2R,6R)-HNK. The antidepressant-relevant behavioral effects and 30- to 80-Hz qEEG oscillation (gamma-range) increases resultant from (2R,6R)-HNK administration were prevented by pretreatment with an mGlu_2/3 receptor agonist and absent in mice lacking the Grm2, but not Grm3 ^-/-, gene. Combined subeffective doses of the mGlu_2/3 receptor antagonist LY341495 and (2R,6R)-HNK exerted synergistic increases on gamma oscillations and antidepressant-relevant behavioral actions. These findings highlight that (2R,6R)-HNK exerts antidepressant-relevant actions via a mechanism converging with mGlu₂ receptor signaling and suggest enhanced cortical gamma oscillations as a marker of target engagement relevant to antidepressant efficacy. Moreover, these results support the use of (2R,6R)-HNK and inhibitors of mGlu₂ receptor function in clinical trials for treatment-resistant depression either alone or in combination.

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

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

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.

International Union of Basic and Clinical Pharmacology CIV: The Neurobiology of Treatment-resistant Depression: From Antidepressant Classifications to Novel Pharmacological Targets

Major depressive disorder is one of the most prevalent and life-threatening forms of mental illnesses and a major cause of morbidity worldwide. Currently available antidepressants are effective for most patients, although around 30% are considered treatment resistant (TRD), a condition that is associated with a significant impairment of cognitive function and poor quality of life. In this respect, the identification of the molecular mechanisms contributing to TRD represents an essential step for the design of novel and more efficacious drugs able to modify the clinical course of this disorder and increase remission rates in clinical practice. New insights into the neurobiology of TRD have shed light on the role of a number of different mechanisms, including the glutamatergic system, immune/inflammatory systems, neurotrophin function, and epigenetics. Advances in drug discovery processes in TRD have also influenced the classification of antidepressant drugs and novel classifications are available, such as the neuroscience-based nomenclature that can incorporate such advances in drug development for TRD. This review aims to provide an up-to-date description of key mechanisms in TRD and describe current therapeutic strategies for TRD before examining novel approaches that may ultimately address important neurobiological mechanisms not targeted by currently available antidepressants. All in all, we suggest that drug targeting different neurobiological systems should be able to restore normal function but must also promote resilience to reduce the long-term vulnerability to recurrent depressive episodes.

Ketamine administration during a critical period after forced ethanol abstinence inhibits the development of time-dependent affective disturbances

Forced abstinence from chronic two bottle-choice ethanol drinking produces the development of negative affective states in female C57BL/6J mice. We previously reported that this disrupted behavior is acutely reversed by administration of ketamine 30 min-prior to testing. Here we assessed whether ketamine can be used as an inoculant against the development of abstinence- dependent affective disturbances. In parallel, we examined the impact of ketamine administration on long-term potentiation (LTP) in the bed nucleus of the stria terminalis (BNST), a region implicated in affective disturbances. We administered ketamine (3 mg/kg i.p.) to female C57BL/6J mice with a history of chronic ethanol drinking at either the onset, two, or 6 days- post-abstinence and observed its impact on affective behavior in the elevated plus maze (EPM), the Novelty Suppressed Feeding Test (NSFT), and the Forced Swim Test (FST). In addition, we assessed BNST synaptic plasticity with field potential electrophysiology two to 3 weeks into abstinence. We found that early abstinence was associated with disrupted behavior on the EPM. Ketamine administered at the onset of forced abstinence prevented both the deficit in early EPM behavior, and the delayed deficits in NSFT and FST. However, ketamine administered either two or 6 days post-abstinence failed to prevent the abstinence-induced affective disturbances. To begin to explore potential alterations in neural circuit activity that accompanies these actions of ketamine, we assessed the impact of ketamine administration at the onset of forced abstinence and measured LTP induction in the BNST. We find that early ketamine administration persistently increased the capacity for LTP within the BNST. These findings suggest a critical period at the onset of forced abstinence in which ketamine inoculation can prevent the development of affective disturbances, in part by enhancing plasticity within the BNST.

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.

Mechanisms of ketamine action as an antidepressant

Clinical studies have demonstrated that a single sub-anesthetic dose of the dissociative anesthetic ketamine induces rapid and sustained antidepressant actions. Although this finding has been met with enthusiasm, ketamine's widespread use is limited by its abuse potential and dissociative properties. Recent preclinical research has focused on unraveling the molecular mechanisms underlying the antidepressant actions of ketamine in an effort to develop novel pharmacotherapies, which will mimic ketamine's antidepressant actions but lack its undesirable effects. Here we review hypotheses for the mechanism of action of ketamine as an antidepressant, including synaptic or GluN2B-selective extra-synaptic N-methyl-D-aspartate receptor (NMDAR) inhibition, inhibition of NMDARs localized on GABAergic interneurons, inhibition of NMDAR-dependent burst firing of lateral habenula neurons, and the role of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor activation. We also discuss links between ketamine's antidepressant actions and downstream mechanisms regulating synaptic plasticity, including brain-derived neurotrophic factor (BDNF), eukaryotic elongation factor 2 (eEF2), mechanistic target of rapamycin (mTOR) and glycogen synthase kinase-3 (GSK-3). Mechanisms that do not involve direct inhibition of the NMDAR, including a role for ketamine's (R)-ketamine enantiomer and hydroxynorketamine (HNK) metabolites, specifically (2R,6R)-HNK, are also discussed. Proposed mechanisms of ketamine's action are not mutually exclusive and may act in a complementary manner to exert acute changes in synaptic plasticity, leading to sustained strengthening of excitatory synapses, which are necessary for antidepressant behavioral actions. Understanding the molecular mechanisms underpinning ketamine's antidepressant actions will be invaluable for the identification of targets, which will drive the development of novel, effective, next-generation pharmacotherapies for the treatment of depression.

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.

Psychedelics and reconsolidation of traumatic and appetitive maladaptive memories: focus on cannabinoids and ketamine

RATIONALE

Clinical data with 3,4-methylenedioxymethamphetamine (MDMA) in post-traumatic stress disorder (PTSD) patients recently stimulated interest on the potential therapeutic use of psychedelics in disorders characterized by maladaptive memories, including substance use disorders (SUD). The rationale for the use of MDMA in PTSD and SUD is being extended to a broader beneficial "psychedelic effect," which is supporting further clinical investigations, in spite of the lack of mechanistic hypothesis. Considering that the retrieval of emotional memories reactivates specific brain mechanisms vulnerable to inhibition, interference, or strengthening (i.e., the reconsolidation process), it was proposed that the ability to retrieve and change these maladaptive memories might be a novel intervention for PTSD and SUD. The mechanisms underlying MDMA effects indicate memory reconsolidation modulation as a hypothetical process underlying its efficacy.

OBJECTIVE

Mechanistic and clinical studies with other two classes of psychedelic substances, namely cannabinoids and ketamine, are providing data in support of a potential use in PTSD and SUD based on the modulation of traumatic and appetitive memory reconsolidation, respectively. Here, we review preclinical and clinical data on cannabinoids and ketamine effects on biobehavioral processes related to the reconsolidation of maladaptive memories.

RESULTS

We report the findings supporting (or not) the working hypothesis linking the potential therapeutic effect of these substances to the underlying reconsolidation process. We also proposed possible approaches for testing the use of these two classes of drugs within the current paradigm of reconsolidation memory inhibition.

CONCLUSIONS

Metaplasticity may be the process in common between cannabinoids and ketamine/ketamine-like substance effects on the mediation and potential manipulation of maladaptive memories.

Synthesis, absolute configuration and in vitro cytotoxicity of deschloroketamine enantiomers: rediscovered and abused dissociative anaesthetic

Deschloroketamine has been synthesized, the absolute configuration of enantiomers elucidated and the in vitro cytotoxicity of the enantiomers determined using nine different cell lines.

Psychedelics and reconsolidation of traumatic and appetitive maladaptive memories: focus on cannabinoids and ketamine

RATIONALE

Clinical data with 3,4-methylenedioxymethamphetamine (MDMA) in post-traumatic stress disorder (PTSD) patients recently stimulated interest on the potential therapeutic use of psychedelics in disorders characterized by maladaptive memories, including substance use disorders (SUD). The rationale for the use of MDMA in PTSD and SUD is being extended to a broader beneficial "psychedelic effect," which is supporting further clinical investigations, in spite of the lack of mechanistic hypothesis. Considering that the retrieval of emotional memories reactivates specific brain mechanisms vulnerable to inhibition, interference, or strengthening (i.e., the reconsolidation process), it was proposed that the ability to retrieve and change these maladaptive memories might be a novel intervention for PTSD and SUD. The mechanisms underlying MDMA effects indicate memory reconsolidation modulation as a hypothetical process underlying its efficacy.

OBJECTIVE

Mechanistic and clinical studies with other two classes of psychedelic substances, namely cannabinoids and ketamine, are providing data in support of a potential use in PTSD and SUD based on the modulation of traumatic and appetitive memory reconsolidation, respectively. Here, we review preclinical and clinical data on cannabinoids and ketamine effects on biobehavioral processes related to the reconsolidation of maladaptive memories.

RESULTS

We report the findings supporting (or not) the working hypothesis linking the potential therapeutic effect of these substances to the underlying reconsolidation process. We also proposed possible approaches for testing the use of these two classes of drugs within the current paradigm of reconsolidation memory inhibition.

CONCLUSIONS

Metaplasticity may be the process in common between cannabinoids and ketamine/ketamine-like substance effects on the mediation and potential manipulation of maladaptive memories.

BDNF at the synapse: why location matters

Neurotrophic factors, a family of secreted proteins that support the growth, survival and differentiation of neurons, have been intensively studied for decades due to the powerful and diverse effects on neuronal physiology, as well as their therapeutic potential. Such efforts have led to a detailed understanding on the molecular mechanisms of neurotrophic factor signaling. One member, brain-derived neurotrophic factor (BDNF) has drawn much attention due to its pleiotropic roles in the central nervous system and implications in various brain disorders. In addition, recent advances linking the rapid-acting antidepressant, ketamine, to BDNF translation and BDNF-dependent signaling, has re-emphasized the importance of understanding the precise details of BDNF biology at the synapse. Although substantial knowledge related to the genetic, epigenetic, cell biological and biochemical aspects of BDNF biology has now been established, certain aspects related to the precise localization and release of BDNF at the synapse have remained obscure. A recent series of genetic and cell biological studies have shed light on the question-the site of BDNF release at the synapse. In this Perspectives article, these new insights will be placed in the context of previously unresolved issues related to BDNF biology, as well as how BDNF may function as a downstream mediator of newer pharmacological agents currently under investigation for treating psychiatric disorders.

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.