Glutamate modulators and beyond: A neuroscience revolution in the making

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.

Oral ketamine may offer a solution to the ketamine conundrum

Ketamine has received considerable attention for its rapid and robust antidepressant response over the past decade. Current evidence, in clinical populations, predominantly relates to parenterally administered ketamine, which is reported to produce significant undesirable side effects, with additional concerns regarding long-term safety and abuse potential. Attempts to produce a similar drug to ketamine, without the psychotomimetic side effects, have proved elusive. Orally administered ketamine has a different pharmacological profile to parentally administered ketamine, suggesting it may be a viable alternative. Emerging evidence regarding the efficacy and tolerability of oral ketamine suggests that it may be a favourable route of administration, as it appears to obtain similarly beneficial treatment effects, but without the cost and medical resources required in parenteral dosing. The pharmacological effects may be due to the active metabolite norketamine, which has been found to be at substantially higher levels via oral dosing, most likely due to first-pass clearance. Despite bioavailability and peak plasma concentrations both being lower than when administered parenterally, evidence suggests that low-dose oral ketamine is clinically effective in treating pain. This may also be due to the actions of norketamine and therefore, its relevance to the mental health context is explored in this narrative review.

Rethinking ketamine and esketamine action: Are they antidepressants with mood-stabilizing properties?

Ketamine and esketamine, the S-enantiomer of the racemic mixture, have recently generated considerable interest as potential therapeutic agents for Treatment-Resistant Depression (TRD), a complex disorder that includes various psychopathological dimensions and distinct clinical profiles (e.g., comorbid personality disorder, bipolar spectrum, dysthymic disorder). This perspective article provides a comprehensive overview of the action of ketamine/esketamine from a dimensional point of view, taking into account the high prevalence of bipolarity in TRD and the evidence of the efficacy of these substances on mixed features, anxiety, dysphoric mood, and, generally, bipolar traits. Additionally, the article underscores the complexity of the pharmacodynamic mechanisms of action of ketamine/esketamine, which goes beyond the non-competitive antagonism of NMDA-R. The need for further research and evidence is highlighted, mainly to evaluate the efficacy of esketamine nasal spray in bipolar depression, the presence of bipolar elements as a predictor of response, and the potential role of these substances as mood stabilizers. The article implies that, in the future, ketamine/esketamine could be used with fewer limitations, not only as antidepressants for the most severe form of depression but also as valuable tools to stabilize subjects with mixed symptoms or bipolar spectrum.

Stress, mental disorder and ketamine as a novel, rapid acting treatment

The experience of stress is often utilised in models of emerging mental illness and neurobiological systems are implicated as the intermediary link between the experience of psychological stress and the development of a mental disorder. Chronic stress and prolonged glucocorticoid exposure have potent effects on neuronal architecture particularly in regions that modulate the hypothalamic-pituitary-adrenal (HPA) axis and are commonly associated with psychiatric disorders. This review provides an overview of stress modulating neurobiological and neurochemical systems which underpin stress-related structural and functional brain changes. These changes are thought to contribute not only to the development of disorders, but also to the treatment resistance and chronicity seen in some of our most challenging mental disorders. Reports to date suggest that stress-related psychopathology is the aetiological mechanism of these disorders and thus we review the rapid acting antidepressant ketamine as an effective emerging treatment. Ketamine, an N-methyl D-aspartate (NMDA) receptor antagonist, is shown to induce a robust treatment effect in mental disorders via enhanced synaptic strength and connectivity in key brain regions. Whilst ketamine's glutamatergic effect has been previously examined, we further consider ketamine's capacity to modulate the HPA axis and associated pathways.

New investigational agents for the treatment of major depressive disorder

INTRODUCTION

Pharmacotherapy of depression is characterized by the delayed onset of action, chronic treatment requirements, and insufficient effectiveness. Ketamine, with its rapid action and long-lasting effects, represents a breakthrough in the modern pharmacotherapy of depression.

AREAS COVERED

: The current review summarizes the latest findings on the mechanism of the antidepressant action of ketamine and its enantiomers and metabolites. Furthermore, the antidepressant potential of psychedelics, non-hallucinogenic serotonergic modulators and metabotropic glutamate receptor ligands was discussed.

EXPERT OPINION

Recent data indicated that to achieve fast and long-acting antidepressant-like effects, compounds must induce durable effects on the architecture and density of dendritic spines in brain regions engaged in mood regulation. Such mechanisms underlie the actions of ketamine and psychedelics. These compounds trigger hallucinations; however, it is thought that these effects might be essential for their antidepressant action. Behavioral studies with serotonergic modulators affecting 5-HT1A (biased agonists), 5-HT4 (agonists), and 5-HT-7 (antagonists) receptors exert rapid antidepressant-like activity, but they seem to be devoid of this effects. Another way to avoid psychomimetic effects and achieve the desired rapid antidepressant-like effects is combined therapy. In this respect, ligands of metabotropic receptors show some potential.