Answer: Serotonin is produced in the body by enzymatic reaction of the dietary amino acid tryptophan in the brainstem area called the Raphe nucleus.
Serotonin is a neurotransmitter that can influence brain chemistry and behavior. Serotonin levels in the brain can affect such complex behaviors as reward, motivation, learning, cognition, and anxiety. Serotonin is best known to function in regulation of mood. Serotonin also serves an important function in the gut to regulate motility; about 90% of the body's serotonin is used for this purpose. Chemically, serotonin is also called 5-HT.
Serotonin is synthesized from the amino acid tryptophan. Tryptophan is an essential amino acid, meaning that the body does not produce it endogenously, but can only be obtained from dietary sources. Many foods contain tryptophan, especially egg whites, cheeses, and meats such as pork, chicken, and turkey. It is also sold over the counter as a dietary supplement.
The process of serotonin synthesis is a two step reaction driven by enzyme activity. In the first step, dietary tryptophan is converted into 5-hydroxy-L-tryptophan, or 5-HTP. This is accomplished by the activity of the enzyme tryptophan hydroxylase, or TPH. This enzyme exists in two forms, TPH1 and TPH2, of which TPH2 is found exclusively in neurons.
The second step converts 5-HTP into 5-HT through a decarboxylation process. This chemical reaction is a result of activity of the enzyme aromatic L-amino acid decarboxylase, or AADC. This reaction requires the activity of the cofactor pyridoxal phosphate, the active form of vitamin B6. AADC is expressed in several neurons, and it can act to synthesize other neurotransmitters. For example, AADC converts L-histidine to histamine and L-DOPA to dopamine.
Gut serotonin is produced by enterochromaffin cells, not Raphe cells.