AHI1 and major depressive disorder: To explore the underlying mechanisms of the sex differences, we measured 5-HT levels by HPLC in hippocampal tissues from female Ahi1 KO mice because previous studies have shown that serotonin is involved in regulating sex differences in depression [58], hippocampal 5-HT levels in female Ahi1 KO mice were not different from those in control mice (Fig. 5C), consistent with its normal phenotype.