Serotonin is a neurotransmitter. Its chemical name is 5-Hydroxytryptamine or 5-HT. It is derived from tryptophan. Serotonin is found in all vertebrates, mainly in the gastrointestinal tract, blood platelets and central nervous system. Its name is derived from its effect on blood pressure: serotonin is a part of the "serum" which regulates the "tonus" of blood vessels.