TPH1 and colitis: On the other hand, in a dextran sulfate sodium-induced colitis mouse model, a reduction in serotonin via the depletion of the tryptophan hydroxylase 1 (THP1) gene decreases the severity of colitis, which is associated with a decrease in MΦ infiltration and significantly decreased concentrations of proinflammatory cytokines such as TNF29.