Serotonin
Serotonin, or 5-hydroxytryptamine, is found in large quantities in the enterochromaffin cells of the gastrointestinal mucosa, in platelets and in serotoninergic neurons of the central nervous system. It is a potent vasoconstrictor and neurotransmitter and has multiple other complex effects. Its measurement is useful in the investigation of abnormal proliferation of enterochromaffin cells.