Serotonin ( ) (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesized in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract of animals including humans. Serotonin is also found in many mushrooms and plants, including fruits and vegetables.

Function

thumb|left|200px|A hydroxy-group at carbon 5 of the carbon skeleton of L-tryptophan without a carboxyl group gives serotonin its descriptive chemical name, 5-hydroxytryptamine.

In the central nervous system, serotonin plays an important role as a neurotransmitter in the modulation of anger, aggression, body temperature, mood, sleep, human sexuality, appetite, and metabolism, as well as stimulating vomiting.

Serotonin has broad activities in the brain, and genetic variation in serotonin receptors and the serotonin transporter, which facilitates reuptake of serotonin into presynapses, have been implicated in neurological diseases. Drugs targeting serotonin-induced pathways are being used in the treatment of many psychiatric disorders, and one focus of clinical research is the influence of genetics on serotonin action and metabolism in psychiatric settings. Such studies have revealed that the variation in the promoter region of the serotonin transporter protein accounts for nearly 10% of total variance in anxiety-related personality, and the effect of this gene on depression was found to interact with the environment.

Levels of serotonin in the brain show association with aggression , and a mutation in the gene which codes for the 5-HT_2A receptor may double the risk of suicide for those with that genotype.

Using the ultimatum game as model, it was shown that people whose serotonin levels have been artificially lowered will reject unfair offers more often than players with normal serotonin levels.

In addition, serotonin is also a peripheral signal mediator. It is found extensively in the human gastrointestinal tract as about 80-90% of the body's total serotonin is found in the enterochromaffin cells in the gut. In the blood, the major storage site is platelets, which collect serotonin for use in mediating post-injury vasoconstriction.

Recent research suggests that serotonin plays an important role in liver regeneration and acts as a mitogen (induces cell division) throughout the body. Recent research also suggests that intestinal serotonin may inhibit bone formation.

Serotonin and SIDS

Defective signalling of serotonin in the brain may be the root cause of sudden infant death syndrome (SIDS). Scientists from the European Molecular Biology Laboratory in Monterotondo, Italy, genetically modified lab mice to produce low levels of the brain signaling protein serotonin. The results showed the mice suffered drops in heart rate and other symptoms of SIDS, and many of the animals died at an early age.