Aromatase inhibition and ketamine in rats: sex-differences in antidepressant-like efficacy

Ketamine, a fast-acting antidepressant recently approved for treatment-resistant depression, shows sex differences in efficacy. In adult Sprague-Dawley rats, ketamine (5 mg/kg, 7 days) produced antidepressant-like effects in males but not in females. Inhibiting estrogen synthesis with letrozole (1 mg/kg, 8 days) induced an antidepressant-like response in females and accelerated ketamine's effect in males (after a single dose). Both ketamine's effect in males and letrozole's effect in females persisted up to 65 days post-treatment, suggesting long-term, sex-specific benefits.

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Abstract

Ketamine is a novel fast-acting antidepressant recently approved for treatment-resistant depression. Since preclinical studies showed sex differences in its efficacy, probably driven by sex hormones (estrogens and testosterone), we evaluated the antidepressant-like effects of ketamine in male and female rats when the biosynthesis of estrogens was inhibited. To do so, we utilized letrozole, an inhibitor of the aromatase enzyme responsible for the conversion of testosterone into estrogens. The results showed, in line with the prior literature, sex-differences in the antidepressant-like response of ketamine; with efficacy in male rats and a lack of response for females. Aromatase inhibition with letrozole induced a faster response for ketamine in male rats, while did not change the lack of response for females. However, aromatase inhibition on itself was capable of inducing an antidepressant-like response in female rats. Interestingly, both ketamine's and letrozole's antidepressant-like effects in male and female rats respectively showed long-term beneficial effects, up to 65 days post-treatment. Background Ketamine has been recently approved to treat resistant depression; however preclinical studies showed sex differences in its efficacy. Sex steroids, such as estrogens and testosterone, both in the periphery and locally in the brain, are regarded as important modulators of these sex differences. Therefore, the present study evaluated how inhibiting the biosynthesis of estrogens with letrozole (an aromatase inhibitor) could affect the observed sex differences in ketamine's antidepressant-like-response. Methods We performed several consecutive studies in adult Sprague–Dawley rats to evaluate potential sex differences in the antidepressant-like effects of ketamine (5 mg/kg, 7 days, i.p.), letrozole (1 mg/kg, 8 days, i.p.) and their combination (letrozole pre-treatment 3 h before ketamine). Acute and repeated antidepressant-like responses were ascertained in a series of behavioral tests (forced-swim, novelty-suppressed feeding, two-bottle choice for sucrose preference). Results The main results proved clear sex differences in the antidepressant-like response induced by ketamine, which was observed following a repeated paradigm in adult male rats, but rendered inefficacious in female rats. Moreover, decreasing estrogens production with letrozole induced on itself an antidepressant-like response in female rats, while also increased ketamine's response in male rats (i.e., quicker response observed after only a single dose). Interestingly, both the antidepressant-like effects induced by ketamine in male rats or letrozole in female rats persisted over time up to 65 days post-treatment, suggesting long-term sex-directed benefits for these drugs. Conclusions The present results demonstrated a sex-specific role for aromatase inhibition with letrozole in the antidepressant-like response induced by ketamine in male rats. Moreover, letrozole itself presented as a potential antidepressant for females with persistent effects over time. Clearly, the production of estrogens is key in modulating, in a sex-specific manner, affective-like responses and thus deserve further studies. There are clear sex differences in the antidepressant-like effects of ketamine. Ketamine showed efficacy in adult male rats while rendered ineffective in females. Aromatase inhibition with letrozole improved the effects observed in male rats. Aromatase inhibition with letrozole induced an antidepressant-like effect in females. The antidepressant-like effects of ketamine or letrozole persisted up to 2 months.