A compound, also known as 5-hydroxytryptamine (5-HT), derived from tryptophan, an indole-containing amino acid. It is widely distributed in the animal and vegetable kingdoms. In mammals it is found in gastrointestinal enterochromaffin cells, in blood platelets, and in brain and nerve tissue. Its presence in blood plasma is probably due to disruption of platelets during manipulation of blood in the laboratory. Low levels are found in urine, but metabolic breakdown products, particularly 5-hydroxyindoleacetic acid (5-HIAA), are found. Serotonin is a local vasoconstrictor, plays a role in brain and nerve function and in regulation of gastric secretion and intestinal peristalsis, and has pharmacologic properties.