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-HT1A has been implicated in anxiety and depression. Desensitization of 5-HT1A has been correlated with the functional improvements in depression observed 2-3 weeks after initiation of treatment with SSRIs. Decreased 5-HT1A is observed in anxiety disorders, and mice lacking 5-HT1A display increased anxiety. Buspirone, a partial agonist for 5-HT1A , is used clinically for treatment of anxiety disorders.