TPH2 (tryptophan hydroxylase 2) is the rate-limiting enzyme responsible for synthesizing serotonin (5-HT) in the central nervous system, catalyzing the conversion of tryptophan to 5-hydroxytryptophan 1. Unlike TPH1 (expressed in gut enterochromaffin cells), TPH2 is primarily expressed in neurons of the brain stem raphe nuclei and enteric nervous system neurons 2. TPH2 regulates multiple critical functions including mood, sleep, anxiety, and gastrointestinal motility through central serotonergic signaling 2. Notably, TPH2 acts as a master regulator upstream of oxytocin in mammalian social behaviors, with elimination of Tph2 reducing maternal affiliation in mice, rats, and monkeys 3. TPH2 dysfunction is clinically significant in psychiatric and gastrointestinal disorders. Mutations like R441H/R439H cause 60-80% reductions in central serotonin, resulting in depression-like behaviors and gastrointestinal dysmotility 4. Genetic variants (rs4570625, rs4760820, rs11178998) show associations with major depressive disorder and bipolar disorder 56. Therapeutic approaches targeting TPH2 expression, including gut microbiota-mediated upregulation via Roseburia and 5-HTP supplementation, ameliorate both depression and associated gastrointestinal dysfunction 748.