Figure 2.

Metabolism of tryptophan through different pathways: the kynurenine pathway that leads to the production of kynurenine and its derivatives in reliance on indoleamine 2,3-dioxygenase (IDO); the serotonin pathway, including the enterochromaffin cells (ECCs) in which tryptophan is transformed into serotonin (5-HT) and its derivatives and is dependent on short-chain fatty acids (SCFAs) which are biosynthesized by spore-forming Clostridiales. ECCs can be activated by the autonomic nervous system (ANS) to secrete 5-HT into the gut lumen, where it can communicate with the intestinal microbiota. Figure was created using the Vectornator application.