TPH1 and colitis: For instance, IDO1–/– mice are more susceptible to 2,4,6-trinitrobenzene sulfate-induced colitis (Takamatsu et al., 2013) whereas TpH1–/– mice show enhanced protection in response to DSS- or dinitrobenzene sulfonic acid-induced colitis (Ghia et al., 2009), suggesting that while kynurenine is protective in the gut, 5-HT might be deleterious.