Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter and hormone that has many important physiological functions throughout the body. In the gastro-intestinal tract, 5-HT regulates electrolyte and fluid homeostasis, visceral sensation, motility and modulates immune responses. After its release by enterochromaffin cells, 5-HT is transported intracellularly via the Na+/Cl- dependent serotonin transporter (SLC6A4, SERT). We have previously shown that 5-HT activates the aryl hydrocarbon receptor (AhR) in intestinal epithelial cells (IECs) via a SERT-dependent mechanism.