Serotonin

A substance found in many tissues, particularly blood platelets, the digestive tract lining, and the brain. Serotonin is released from platelets at the site of bleeding, where it constricts small blood vessels, reducing blood loss. In the digestive tract, it inhibits gastric secretion and stimulates smooth muscle of the intestine. In the brain, where it acts as a neurotransmitter, levels are reduced in people who are depressed; certain antidepressants raise the level. Serotonin agonists are used to block its effects in acute migraine attacks.