Serotonin

Serotonin, also called 5-hydroxytryptamine (5-HT), is synthesized and/or stored at several sites in the body (Fig. 1). The largest amount by far is found in the intestinal mucosa. Smaller amounts occur in the CNS, where it functions as a neurotransmitter, and in platelets .
Serotonin is synthesized from tryptophan, which is hydroxylated in a BH4-requiring reaction analogous to that catalyzed by phenylalanine hydroxylase. The product, 5-hydroxytryptophan, is decarboxylated to 5-HT. Serotonin has multiple physiologic roles including pain perception and regulation of sleep, appetite, temperature, blood pressure, cognitive functions, and mood (causes a feeling of well-being). [Note: Selective serotonin reuptake inhibitors (SSRI) maintain serotonin levels, thereby functioning as antidepressants.] Serotonin is degraded by MAO to 5-hydroxy-3-indoleacetic acid (5-HIAA).

Figure 1: Synthesis of serotonin. [Note: Serotonin is converted to melatonin, a regulator of circadian rhythm, in the pineal gland.] PLP = pyridoxal phosphate; CO2 = carbon dioxide.