A hormone, also called 5-hydroxytryptamine, in the pineal gland, blood platelets, the digestive tract, and the brain.

Serotonin acts both as a chemical messenger that transmits nerve signals between nerve cells and that causes blood vessels to narrow. Changes in the serotonin levels in the brain can alter the mood. For example, medications that affect the action of serotonin are used to treat depression.

Serotonin is also defined as a vasoconstrictor, liberated by blood platelets, that inhibits gastric secretion and stimulates smooth muscle; present in relatively high concentrations in some areas of the central nervous system (hypothalamus, basal ganglia), and occurring in many peripheral tissues and cells and in carcinoid tumors.