Roles of the monoaminergic system in the antidepressant effects of ketamine and its metabolites

New insights into the involvement of serotonin and BDNF-TrkB signalling in cannabidiol's antidepressant effect

Cannabidiol (CBD) is a phytocannabinoid devoid of psychostimulant properties and is currently under investigation as a potential antidepressant drug. However, the mechanisms underlying CBD's antidepressant effects are not yet well understood. CBD targets include a variety of receptors, enzymes, and transporters, with different binding-affinities. Neurochemical and pharmacological evidence indicates that both serotonin and BDNF-TrkB signalling in the prefrontal cortex are necessary for the antidepressant effects induced by CBD in animal models. Herein, we reviewed the current literature to dissect if these are independent mechanisms or if CBD-induced modulation of the serotonergic neurotransmission could mediate its neuroplastic effects through subsequent regulation of BDNF-TrkB signalling, thus culminating in rapid neuroplastic changes. It is hypothesized that: a) CBD interaction with serotonin receptors on neurons of the dorsal raphe nuclei and the resulting disinhibition of serotonergic neurons would promote rapid serotonin release in the PFC and hence its neuroplastic and antidepressant effects; b) CBD facilitates BDNF-TRKB signalling, especially in the PFC, which rapidly triggers neurochemical and neuroplastic effects. These hypotheses are discussed with perspectives for new drug development and clinical applications.

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, like ketamine, elicit rapid-acting antidepressant activity in the rat chronic mild stress model via cortical mechanisms

BACKGROUND

The highly selective 5-HT1A serotonin receptor "biased" agonists NLX-101 and NLX-204 display, like ketamine, potent and efficacious rapid-acting antidepressant (RAAD) activity in the rat chronic mild stress (CMS) model with systemic (i.p.) administration. They rapidly (within 1 day) reverse anhedonia (i.e., CMS-induced sucrose consumption deficit), attenuate working memory deficit (novel object recognition: NOR), and decrease anxiety behavior in the elevated-plus maze (EPM).

AIMS

Here, we sought to explore the contribution of prefrontal cortex (PFC) 5-HT1A receptor activation in the RAAD activity of NLX compounds.

RESULTS/OUTCOMES

In male Wistar rats, unilateral PFC microinjections of NLX-204 and NLX-101 (16 µg), like ketamine (10 µg), reproduced the effects of their systemic administration: they reversed CMS-induced sucrose consumption deficit, attenuated anxiety (EPM), and reduced working memory deficits (NOR). In addition, unilateral PFC microinjections of the selective 5-HT1A antagonist, WAY-100,635 (2 µg), attenuated the beneficial effects of systemic NLX-204 and NLX-101 (0.16 mg/kg i.p.) in the sucrose intake and NOR models, indicating that these compounds exert their RAAD activity specifically through activation of PFC 5-HT1A receptors.

CONCLUSIONS/INTERPRETATION

These data indicate that 5-HT1A receptor biased agonists share with ketamine a common neuroanatomical site for RAAD activity, which can be obtained not only by targeting glutamatergic/NMDA neurotransmission (ketamine's primary mechanism of action) but also by activating 5-HT1A receptors, as is the case for the NLX compounds. The present observations also reinforce the notion that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve RAAD effects, with additional benefits against cognitive deficits and anxiety in depressed patients, without ketamine's troublesome side effects.

A Case of Stress-Induced Cardiomyopathy After Ketamine Infusion

We report the case of a 64-year-old female with a history of hypothyroidism and isolated parotid sarcoidosis who presented with acute-onset chest pain and dyspnea. Echocardiogram demonstrated transiently reduced ejection fraction with apical hypokinesis, without evidence of obstructive coronary lesions on angiography, compatible with stress-induced cardiomyopathy. She received a ketamine infusion as a mental health treatment shortly before the development of symptoms, suggesting that this medication may have precipitated her cardiomyopathy. In recent years, ketamine has become a popular option for treating mental health disorders, including major depressive disorder and substance use disorders. It should be used with caution in patients with known cardiovascular disease, and its cardiac effects warrant further study.

(R)-ketamine restores anterior insular cortex activity and cognitive deficits in social isolation-reared mice

Chronic social isolation increases the risk of mental health problems, including cognitive impairments and depression. While subanesthetic ketamine is considered effective for cognitive impairments in patients with depression, the neural mechanisms underlying its effects are not well understood. Here we identified unique activation of the anterior insular cortex (aIC) as a characteristic feature in brain-wide regions of mice reared in social isolation and treated with (R)-ketamine, a ketamine enantiomer. Using fiber photometry recording on freely moving mice, we found that social isolation attenuates aIC neuronal activation upon social contact and that (R)-ketamine, but not (S)-ketamine, is able to counteracts this reduction. (R)-ketamine facilitated social cognition in social isolation-reared mice during the social memory test. aIC inactivation offset the effect of (R)-ketamine on social memory. Our results suggest that (R)-ketamine has promising potential as an effective intervention for social cognitive deficits by restoring aIC function.

Unique Effects of (R)-Ketamine Compared to (S)-Ketamine on EEG Theta Power in Rats

Differences in the pharmacological effects of (S)-ketamine and (R)-ketamine are at the focus of research. Clinical data and our rat studies confirmed the antidepressant effect of (S)- but not (R)-ketamine, with similar differences in quantitative electroencephalogram (EEG) and sleep effects. In contrast, studies mainly on mice showed some stronger, preferable effects of (R)-ketamine. EEG theta (5-9 Hz) rhythm originates from the hippocampus, and its power is associated with cognitive functions, attention, and decreased anxiety. To find a brain parameter that is not associated with the antidepressant effect of drugs and may confirm potent in vivo effects of (R)-ketamine in rats, theta EEG power-inducing effects of the two enantiomers were measured and compared for 23 h. EEG-equipped Wistar rats were treated with (R)-ketamine (7.5, 15, 30 mg/kg i.p.), (S)-ketamine (7.5 and 15 mg/kg i.p.), or vehicle at the beginning of the passive phase. Frontoparietal EEG, electromyogram, and motor activity were recorded. (R)-ketamine but not (S)-ketamine dose-dependently increased EEG theta power during wakefulness and rapid eye movement (REM) sleep for 23 h. These results suggest that (R)-ketamine has an effect on a hippocampal function that was not affected by (S)-ketamine and may be associated with neural plasticity and memory encoding.

Long-lasting anti-despair and anti-anhedonia effects of (S)-norketamine in social isolation-reared mice

Alternatives to ketamine without psychotomimetic properties for the treatment of depression have attracted much attention. Here, we examined the anti-despair and anti-anhedonia effects of the ketamine metabolites (S)-norketamine ((S)-NK), (R)-NK, (2S,6S)-hydroxynorketamine, and (2R,6R)-hydroxynorketamine in a mouse model of depression induced by social isolation. All ketamine metabolites examined had acute (30 min after administration) anti-despair-like effects in the forced swim test, but only (S)-NK showed a long-lasting (1 week) effect. Additionally, only (S)-NK improved reduced motivation both 30 min and 24 h after injection in the female encounter test. These results suggest that (S)-NK has potent and long-lasting antidepressant-like effects.

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, display ketamine-like RAAD and anti-TRD activities in rat CMS models

OBJECTIVES

NLX-101 and NLX-204 are highly selective serotonin 5-HT1A 'biased' agonists, displaying potent and efficacious antidepressant-like activity upon acute administration in models such as the forced swim test.

METHODS

we compared the effects of repeated administration of NLX-101, NLX-204 and ketamine in the chronic mild stress (CMS) model of depression, considered to have high translational potential, on sucrose consumption (anhedonia measure), novel object recognition (NOR; working memory measure) and elevated plus maze (EPM; anxiety measure) in male Wistar and Wistar-Kyoto rats (the latter being resistant to classical antidepressants).

RESULTS

in Wistar rats, NLX-204 and NLX-101 (0.08-0.16 mg/kg i.p.), like ketamine (10 mg/kg i.p.) dose-dependently reversed CMS-induced sucrose intake deficit from treatment Day 1, with nearly full reversal observed at the higher dose at Days 8 and 15. These effects persisted for 3 weeks following treatment cessation. In the NOR test, both doses of NLX-101/NLX-204, and ketamine, rescued the deficit in discrimination index caused by CMS on Days 3 and 17; all three compounds increased time spent in open arms (EPM) but only NLX-204 achieved statistical significance on Days 2 and 16. In Wistar-Kyoto rats, all 3 compounds were also active in the sucrose test and, to a lesser extent, in the NOR and EPM. In non-stressed rats (both strains), the three compounds produced no significant effects in all tests.

CONCLUSIONS

these observations further strengthen the hypothesis that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve rapid-acting/sustained antidepressant effects combined with activity against TRD, in addition to providing beneficial effects against memory deficit and anxiety in depressed patients.

The antidepressant actions of ketamine and its enantiomers

Ketamine, an N-methyl-d-aspartate receptor (NMDAR) antagonist first developed as an anesthetic, has shown significant promise as a medication with rapid antidepressant properties in treatment-resistant depression. However, concerns such as adverse side effects and potential misuse liability have limited its widespread use. Racemic ketamine has two enantiomers-(S)- and (R)-ketamine-that appear to have disparate underlying mechanisms. This brief review summarizes some of the most recent preclinical and clinical research regarding the convergent and divergent prophylactic, immediate, and sustained antidepressant effects of (S)- and (R)-ketamine while addressing potential differences in their side effect and misuse liability profiles. Preclinical research suggests divergent mechanisms underlying (S)- and (R)-ketamine, with (S)-ketamine more directly affecting mechanistic target of rapamycin complex 1 (mTORC1) signaling and (R)-ketamine more directly affecting extracellular signal-related kinase (ERK) signaling. Clinical research suggests that (R)-ketamine has a milder side effect profile than (S)-ketamine and decreases depression rating scale scores, but recent randomized, controlled trials found that it had no significant antidepressant efficacy compared to placebo, suggesting that caution is warranted in interpreting its therapeutic potential. Future preclinical and clinical research is needed to maximize the efficacy of each enantiomer, either by optimizing dose, route of administration, or administration paradigm.