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Challenging the innate immune machinery with the pro-inflammatory agent lipopolysaccharide (LPS) results in the development of a sickness syndrome characterized by numerous depressive-like behavioural and physiological manifestations, most of which overlap with the clinical symptoms of major depression. Although women are known to mount stronger pro-inflammatory responses during infections and being at higher risk to develop depressive disorders compared to men, the vast majority of experimental studies investigating the neurobiological effects of LPS administration have been conducted in males. Herein, we investigated the behavioural effects of LPS administration (0.83mg/kg) in male and female C57BL/6J mice subjected to tests screening for alterations in locomotor activity (open field test), anorexia (food consumption), anhedonia (sucrose preference test), behavioural despair (forced swim test) and grooming behaviour (splash-test). We further mapped the brain's serotonergic and dopaminergic activity in five limbic brain regions implicated in the pathophysiology of major depression (i.e., prefrontal cortex, hippocampus, striatum, amygdala, and hypothalamus) at two critical time-points post-LPS treatment; at 6h when depression of behavioural activity is maximal, and at 24h when depressive-like symptoms develop independently of obvious locomotor performance impairments associated with acute LPS administration. Our findings indicate that the two sexes present with differential behavioural sensitivity to this immune stressor, as impairment of grooming behaviour in the splash test was more persistent in female mice, and anorexia lasted longer in their male counterparts. Notably, LPS affects the brain's serotonergic neurochemistry in a sex-specific manner, as it induced sustained serotonergic hyperactivity in females at 24h post-LPS administration in all the brain regions examined. Moreover, the kinetics of dopaminergic activation appeared to be sex-differentiated upon LPS challenge. Given the higher prevalence of affective disorders in women, a focus of basic science on sex differences that underlie neuroinflammatory processes is imperative in order to elucidate the neuroimmunological substrate of major depression.

One of the most striking discoveries in the treatment of major depression was the finding that infusion of a single sub-anesthetic dose of ketamine induces rapid and sustained antidepressant effects in treatment-resistant depressed patients. However, ketamine's antidepressant-like actions are transient and can only be sustained by repeated drug treatment. Despite the fact that women experience major depression at roughly twice the rate of men, research regarding the neurobiological antidepressant-relevant effects of ketamine has focused almost exclusively on the male sex. Importantly, knowledge regarding the sex-differentiated effects, the frequency and the dose on which repeated ketamine administration stops being beneficial, is limited. In the current study, we investigated the behavioral, neurochemical and synaptic molecular effects of repeated ketamine treatment (10mg/kg; 21days) in male and female C57BL/6J mice. We report that ketamine induced beneficial antidepressant-like effects in male mice, but induced both anxiety-like (i.e., decreased time spent in the center of the open field arena) and depressive-like effects (i.e., enhanced immobility duration in the forced swim test; FST) in their female counterparts. Moreover, repeated ketamine treatment induced sustained sex-differentiated neurochemical and molecular effects, as it enhanced hippocampal synapsin protein levels and serotonin turnover in males, but attenuated glutamate and aspartate levels in female mice. Taken together, our findings indicate that repeated ketamine treatment induces opposite behavioral effects in male and female mice, and thus, present data have far-reaching implications for the sex-oriented use of ketamine in both experimental and clinical research settings.