SUMMARY Stressful and traumatic experiences can contribute to mood disorders in some individuals while others are resilient to this process, and pharmacological tools for treating major depression (MDD) are effective on only a subset of patients. Moreover, MDD affects women nearly twice as often as men, but the neurobiological underpinnings of this discrepancy are unclear. Over the past five years of the current funded project, we demonstrated that lower excitability of ventral hippocampal glutamatergic projections to nucleus accumbens (vHPC-NAc) drives increased resilience to stress-induced anhedonia in male mice compared to females, and that this effect is caused directly by testosterone in males. However, the mechanisms by which testosterone regulates excitability of the vHPC-NAc remain unknown. Testosterone produces many of its effects through activation of the androgen receptor (AR) which can alter cell function through signaling cascades in the cytosol or through directly binding to DNA and altering gene expression. We have shown that vHPC-NAc neurons produce AR and respond to AR antagonists, and our preliminary data show that eliminating AR in vHPC-NAc neurons increases excitability. This leads to our overarching hypothesis for years 6-10: that testosterone activation of ARs in vHPC-NAc decreases excitability through changes in gene expression to drive behavioral resilience to stress. This is significant, as identification of AR targets/mechanisms driving resilience may generate novel therapeutic strategies that would be particularly beneficial in females, an unmet need due to the sexual disparity in mood disorders. We will address this using three specific aims: 1) Uncover the mechanisms of testosterone-driven reduction in vHPC-NAc excitability using novel transgenic mice and classical pharmacology; 2) Uncover the mechanisms of testosterone-driven resilience to stress using novel mouse lines and a circuit- specific intersection dual viral vector approach; and 3) Determine circuit-specific gene expression driven by sex, testosterone, and the androgen receptor using translating ribosome affinity purification. Given the established role of vHPC circuitry in emotional regulation, the gene expression studies proposed here will meaningfully impact basic psychiatric research and may elucidate novel treatment strategies for psychiatric disorders that could be particularly beneficial to female patients. Moreover, our understanding of basal sex differences in brain circuitry is lacking, so this work will inform not only stress studies, but will increase understanding of sex as a biological variable in all behaviors that engage vHPC circuitry.