Fig. 2
From: The role of serotonin and diet in the prevalence of irritable bowel syndrome: a systematic review
Serotonin Synthesis, activation, and degradation: 5-HT is synthesized in the intestinal epithelium by enterochromaffin cells from Tryptophan (Trp) by tryptophan hydroxylase (TPH1). Indigenous bacteria (including Streptococcus, Enterococcus, Escherichia, and Clostridium) in human microbiota promote 5-HT biosynthesis from colonic enterochromaffin cells (ECs), which supply 5-HT to the mucosa, lumen, and circulating platelets. Luminal distention results in the basal release of 5-HT into the interstitial space of the mucosa, leading to the activation of 5-HT receptors on intrinsic primary afferent neurons (IPANs) in both submucosal and myenteric plexuses. 5-HT is inactivated through the action of serotonin reuptake transporter (SERT), which is expressed by intestinal epithelial cells. Once intracellular, 5-HT is degraded by monoamine oxidase (MAO) into 5-hydroxyindoleacetic acid (5-HIAA). Platelets also express SERT. Gut Bacterial species involved in Tryptophan metabolism include Streptococcus, Enterococcus, and Escherichia, and Clostridium