Hyperexcitability of female serotonin neurons underlies sex-specific anxiety responses
Mood and anxiety disorders display robust sex differences in prevalence, symptom profile, and treatment outcomes, yet the circuit mechanisms underlying this sex bias remain unclear. Here, we identify a serotonergic (5-HT) pathway from the median raphe region (MRR) to the ventral hippocampus (vHP) that drives sex-specific anxiety regulation in mice. Using a multimodal approach combining electrophysiology, fiber photometry, and optogenetics, we show that vHP-projecting 5-HT neurons (5-HTvHP) in the MRR are intrinsically hyperexcitable in females and exhibit delayed adaptation during exposure to aversive environments. At baseline, female mice displayed greater avoidance and reduced risk-assessment behavior. Optogenetic activation of this pathway selectively enhanced anxiety-like behavior and stress-related grooming in females, while leaving locomotion unaffected. Fiber photometry revealed that grooming episodes coincide with transient suppression of 5-HTvHP activity, suggesting an adaptive feedback mechanism to downregulate serotonergic tone under elevated anxiety. Moreover, activation of this pathway disrupted hippocampal theta dynamics during habituation to a novel arena exclusively in females, revealing serotonergic modulation of anxiety and novelty processing. These results were consistent with the identification of sex-specific M-currents, which constrained excitability in male MRR 5-HTvHP neurons, while being largely absent in females. Collectively, our findings uncover a hyperexcitable MRR-vHP serotonergic circuit that drives female-specific anxiety states, providing a mechanistic framework for understanding sex-specific vulnerability to mood and anxiety disorders.