5Hydroxytryptamine

5-hydroxytryptamine, commonly known as serotonin (5-HT), is an organic monoamine neurotransmitter derived from the essential amino acid tryptophan. In humans it exists in two main pools: a central nervous system (CNS) pool within various brain networks, and a peripheral pool largely in the enterochromaffin cells of the gut and in platelets. Serotonin itself cannot cross the blood–brain barrier, so central and peripheral serotonin operate as largely separate systems.

Biosynthesis and storage occur in two enzymatic steps: tryptophan is hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan, which is then decarboxylated to serotonin by aromatic L-amino acid decarboxylase. The neurotransmitter is stored in vesicles and released in a calcium-dependent manner. After release, serotonin acts on a variety of serotonin receptors (5-HT1 through 5-HT7 and their subtypes) to influence numerous physiological processes.

Functions of 5-HT are broad and include modulation of mood, cognition, sleep, appetite, and thermoregulation in the CNS, as well as regulation of gut motility and vascular tone in the periphery. Serotonin is metabolized mainly by monoamine oxidase A to 5-hydroxyindoleacetic acid (5-HIAA), which is excreted in urine. Clinically, abnormalities in serotonin signaling are linked to mood disorders and anxiety; pharmacological agents such as SSRIs increase synaptic 5-HT to treat depression. Peripheral overproduction, as seen in carcinoid tumors, can cause flushing, diarrhea, and other symptoms.