serotonin

n. A common neurotransmitter synthesized from the amino acid L-tryptophan. It is important in the raphe nucleus, reticular activating system, and most parts of the central nervous system, as well as in the digestive system and in the smooth muscles of the respiratory and cardiovascular systems. It has been implicated in arousal, emotion, sleep, appetite, pain, and the regulation of reflexes. Lack of serotonin is associated with sleep disorders, and selective serotonin reuptake inhibitors have been found to reduce anxiety and depression. Very high levels of serotonin as occur after ingesting LSD are associated with hallucinations and other psychedelic experiences. Also called 5-hydroxytryptamine.