Sex differences in response to chronic mild stress and congenital serotonin deficiency.

Women exhibit a nearly twofold increased risk of developing depression and anxiety disorders when compared to men, a fact that has been hypothesized to result in part from increased stress susceptibility. Here, we used the tryptophan hydroxylase-2 R439H knock-in mouse (Tph2KI) and the chronic unpredictable mild stress (CMS) model to examine sex differences in response to congenital 5-HT deficiency and chronic stress. Our results demonstrate that female mice, but not 5-HT-deficient animals, exhibit significantly increased susceptibility to CMS-induced despair-like behavior in the forced swim test. In addition, female 5-HT-deficient mice exhibit anhedonia-like behavior in the sucrose preference test, whereas male 5-HT-deficient animals do not, suggesting that females exhibit increased sensitivity to at least some of the effects of congenital 5-HT deficiency. Although CMS did not reduce cell proliferation in the hippocampus, low levels of brain 5-HT were associated with increased hippocampal cell proliferation, an effect that was predominantly observed in females. Overall, these results highlight the importance of interactions between psychiatric disease risk factors such as sex, chronic stress and congenital 5-HT deficiency in the development of aberrant emotional behavior.