n.
Chemical (5-hydroxytryptamine, or 5-HT) derived from the amino acid tryptophan .
It occurs in brain and intestinal tissue, platelets, and certain connective-tissue cells and is a component of many animal venoms (e.g., wasp, toad). A strong stimulator of blood-vessel constriction and a neurotransmitter , serotonin concentrates in certain brain areas, especially the midbrain and hypothalamus . Some cases of depression are apparently caused by reduced amounts or activity of serotonin in the brain; many antidepressants counteract that condition. Excessive brain serotonin activity may cause migraine headaches and nausea . LSD may act by inhibiting the action of serotonin.