5-Hydroxytryptamine (5-HT or serotonin) is an important cell-to-cell signaling molecule found in all animal phyla. In mammals, substantial concentrations of 5-HT are present in the central and peripheral nervous systems, gastrointestinal tract and cardiovascular system. 5-HT is capable of exerting a wide variety of biological effects by interacting with specific membrane-bound receptors, and at least 13 distinct 5-HT receptor subtypes have been cloned and characterized. With the exception of the 5-HT3 receptor subtype, which is a transmitter-gated ion channel, 5-HT receptors are members of the 7-transmembrane G protein-coupled receptor superfamily. In humans, the serotonergic system is implicated in various physiological processes such as sleep-wake cycles, maintenance of mood, control of food intake and regulation of blood pressure. In accordance with this, drugs that affect 5-HT-containing cells or 5-HT receptors are effective treatments for numerous indications, including depression, anxiety, obesity, nausea, and migraine.