Effects of a single bilateral infusion of R-ketamine in the rat brain regions of a learned helplessness model of depression

Lack of Antidepressant Effects of (2R,6R)-Hydroxynorketamine in a Rat Learned Helplessness Model: Comparison with (R)-Ketamine

BACKGROUND

(R)-Ketamine exhibits rapid and sustained antidepressant effects in animal models of depression. It is stereoselectively metabolized to (R)-norketamine and subsequently to (2R,6R)-hydroxynorketamine in the liver. The metabolism of ketamine to hydroxynorketamine was recently demonstrated to be essential for ketamine's antidepressant actions. However, no study has compared the antidepressant effects of these 3 compounds in animal models of depression.

METHODS

The effects of a single i.p. injection of (R)-ketamine, (R)-norketamine, and (2R,6R)-hydroxynorketamine in a rat learned helplessness model were examined.

RESULTS

A single dose of (R)-ketamine (20 mg/kg) showed an antidepressant effect in the rat learned helplessness model. In contrast, neither (R)-norketamine (20 mg/kg) nor (2R,6R)-hydroxynorketamine (20 and 40 mg/kg) did so.

CONCLUSIONS

Unlike (R)-ketamine, its metabolite (2R,6R)-hydroxynorketamine did not show antidepressant actions in the rat learned helplessness model. Therefore, it is unlikely that the metabolism of ketamine to hydroxynorketamine is essential for ketamine's antidepressant actions.

Rapid Acting Antidepressants in Chronic Stress Models: Molecular and Cellular Mechanisms

Stress-associated disorders, including depression and anxiety, impact nearly 20% of individuals in the United States. The social, health, and economic burden imposed by stress-associated disorders requires in depth research efforts to identify suitable treatment strategies. Traditional medications (e.g., selective serotonin reuptake inhibitors, monoamine oxidase inhibitors) have significant limitations, notably a time lag for therapeutic response that is compounded by low rates of efficacy. Excitement over ketamine, a rapid acting antidepressant effective in treatment resistant patients, is tempered by transient dissociative and psychotomimetic effects, as well as abuse potential. Rodent stress models are commonly used to produce behavioral abnormalities that resemble those observed in stress-associated disorders. Stress models also produce molecular and cellular morphological changes in stress sensitive brain regions, including the prefrontal cortex and hippocampus that resemble alterations observed in depression. Rapid acting antidepressants such as ketamine can rescue stress-associated morphological and behavioral changes in rodent models. Here, we review the literature supporting a role for rapid acting antidepressants in opposing the effects of stress, and summarize research efforts seeking to elucidate the molecular, cellular, and circuit level targets of these agents.

Antidepressant Potential of (R)-Ketamine in Rodent Models: Comparison with (S)-Ketamine

The rapid-acting and long-lasting antidepressant effects of (R,S)-ketamine have recently gained much attention. Although (S)-ketamine has been studied as an active isomer, recent evidence suggests that (R)-ketamine exhibits longer-lasting antidepressant effects than (S)-ketamine in rodents. However, the antidepressant potential of (R)-ketamine has not been fully addressed. In the present study, we compared the antidepressant effects of (R)-ketamine with those of (S)-ketamine in animal models of depression, including a model that is refractory to current medications. Both (R)-ketamine and (S)-ketamine exhibited antidepressant effects at 30 minutes as well as at 24 hours after administration in forced-swimming and tail-suspension tests in mice. At 48 hours after administration, however, (R)-ketamine still exerted a significant antidepressant effect in the tail-suspension test, whereas the effect of (S)-ketamine was no longer observed. Moreover, (R)-ketamine, but not (S)-ketamine, significantly reversed the depressive-like behavior induced by repeated treatments with corticosterone in rats at 24 hours after a single administration. This effect was attenuated by an α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist, suggesting the involvement of AMPA receptor stimulation in the effects. Both (R)-ketamine and (S)-ketamine exhibited practically the same exposure levels in plasma, brain, and cerebrospinal fluid in mice and rats, and both compounds were rapidly eliminated from plasma (<4-8 hours). The present results confirmed the previous findings that (R)-ketamine exerted longer-lasting antidepressant effects than (S)-ketamine in animal models of depression. Moreover, our study is the first to demonstrate that (R)-ketamine exerted a sustained antidepressant effect even in a model that is refractory to currently prescribed antidepressants.

Investigational drugs for treating major depressive disorder

INTRODUCTION

Treatment of patients suffering from major depression could be highly challenging for psychiatrists. Intractability as well as relapse is commonly seen among these patients, leading to functional impairment and poor quality of life. The present review discusses some of the novel investigational drugs that are under pre-clinical or clinical phases in the treatment of major depression. Areas covered: Molecules belonging to different classes such as triple reuptake inhibitors, opioid receptors, ionotropic and metabotropic glutamate receptors, and neurotrophin in the treatment of major depression are covered in this article. Expert opinion: Although the historical discovery of earlier antidepressant molecules (iproniazid and imipramine) is through serendipitous discovery, the present research focuses on discovering novel molecules based on our current pathophysiological knowledge of the disease condition. The fast-acting antidepressant property of N-methyl-d-aspartate (NMDA) receptor molecules, including ketamine is an exciting area of research. Other drug molecules such as amitifadine (triple reuptake inhibitor), ALKS-5461 (kappa receptor antagonist and mu opioidergic receptor agonist), rapastinel (NMDA glutamatergic receptor modulator) are under Phase-III clinical trials and could be approved in the near future for the treatment of major depression.