serotonin
n. a common monoamine neurotransmitter in the brain, particularly the raphe nucleus, and in other parts of the central nervous system; it also is found in the gastrointestinal tract, in smooth muscles of the cardiovascular and bronchial systems, and in blood platelets. It is synthesized from the dietary amino acid L-tryptophan (see tryptophan hydroxylase), and in the pineal gland it is converted to melatonin. It is primarily degraded by monoamine oxidase, which yields its principal metabolic product, 5-hydroxyindoleacetic acid (5-HIAA). Serotonin has roles in emotional processing, mood, appetite, sexual desire and performance, sleep, pain processing, hallucinations, and reflex regulation. For example, levels of serotonin correlate negatively with aggression, and release of serotonin may promote sleep. It is implicated in many psychological conditions, including depressive disorders, anxiety disorders, sleep
disorders, aggression, and psychosis; many common psychotropic drugs affect neurotransmission mediated by serotonin. Also called 5-hydroxytryptamine (5-HT).