Evaluating the role of inhibiting the biosynthesis of estrogens in the sex-specific antidepressant-like effects of ketamine in rats

Ketamine has been recently approved to treat resistant depression; however preclinical studies showed sex differences in its efficacy. Sex steroids, such as estrogen 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 the role of inhibiting estrogen biosynthesis with letrozole, an aromatase inhibitor that catalyzes the conversion of androgen into estrogen, in the differential antidepressant-like-response induced by ketamine with sex. 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). 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 estrogen production with letrozole induced on itself an antidepressant-like response in female rats, while also improved ketamines response in male rats (i.e., quicker response, only after 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. The present results demonstrated a sex-specific role for inhibiting estrogen biosynthesis in the antidepressant-like response induced by ketamine in male rats. Moreover, letrozole presented itself as a potential antidepressant for females with persistent effects over time. Clearly, estrogen production is key in modulating, in a sex-specific manner, affective-like responses and thus deserve further studies.