Disrupting peripheral serotonin synthesis drives sex-specific alterations in sociability and cognition, and reshapes...

Serotonin (5-HT) regulates neurodevelopment and behavior. Whereas central 5-HT is synthesized by tryptophan hydroxylase (TPH)-2, the TPH1 isoform produces 5-HT peripherally. Although these systems are anatomically segregated, TPH1 gene polymorphisms have been linked to neuropsychiatric vulnerability, suggesting that peripheral 5-HT may influence behavior through indirect pathways, potentially involving the gut-immune-brain axis. Here, we examined how lifelong peripheral 5-HT deficiency shapes behavior and cross-system communication in a sex-specific manner using adult TPH1 knockout (TPH1^-/-) rats. Animals underwent behavioral testing combined with neurochemical and gene expression profiling, gut microbiome sequencing, and peripheral leukocyte phenotyping. TPH1^-/- males and females displayed reduced anxiety-like behavior and enhanced attentional performance. TPH1^-/- males also showed improved sociability and cognitive flexibility. These behavioral alterations were accompanied by attenuation of monoaminergic and GABA/glutamatergic signaling, alongside changes in HPA-axis and neuroplasticity-related pathways across cortico-limbic regions. Peripheral 5-HT deficiency further reshaped gut microbiome ecology, selectively reducing α-diversity in males and altering microbial community composition in both sexes. In the colon, TPH1 deficiency downregulated pro-inflammatory cytokines while upregulating adhesion marker expression. Circulating leukocyte populations were also altered, with reductions in γδ T cells, CD127⁺ CD8⁺ T cells, and B cells, and an increase in eosinophils. Integrative analyses showed that individual differences in anxiety-like behavior and cognition were associated with variation spanning neurochemical, microbial, and immune domains. Together, these findings support a role for peripheral 5-HT in behavioral regulation and reveal cross-system associations consistent with sex-specific modulation along the gut-immune-brain axis, while identifying TPH1-dependent signaling as a potential contributor to neuropsychiatric disorders' pathophysiology.