5-Hydroxytryptamine (5-HT, also commonly known as serotonin) is synthesized in enterochromaffin cells in the intestine and in serotonergic nerve terminals. In the periphery, 5-HT mediates gastrointestinal motility, platelet aggregation, and contraction of blood vessels. Many functions of the central nervous system are influenced by 5-HT, including sleep, motor activity, sensory perception, arousal and appetite. A family of 12 GPCRs and one ion channel mediate the biological effects of 5-HT. 5-HT2A, which couples to Gq/11 to increase intracellular calcium, is widely expressed at central and peripheral sites of 5-HT action, and contributes to many of the physiological effects of 5-HT. The hallucinogenic activity of LSD is mediated in part by its action as an partial to full agonist at 5-HT2A, and the activity of atypical antipsychotics such as clozapine appears to be mediated in part by antagonism of 5-HT2A.