Serotonin is secreted by:

Serotonin is a vasoconstricting hormone, produced in the brain and GI tract. In the GI tract it is produced normally by enterochromaffin cells (argentaffin cells). The principal function is to regulate smooth muscle contraction and peristalsis. Serotonin: also called 5-hydroxytryptamine, is a chemical substance that is derived from the amino acid tryptophan. Principally found stored in three main cell types; (a) Serotonergic neurons in the CNS and in the intestinal myenteric plexus, (b) Enterochromaffin cells in the mucosa of the gastrointestinal tract and (c) Blood platelets Synthesis: Serotonergic neurons and enterochromaffin cells can synthesize serotonin from its precursor amino acid L-tryptophan, whereas platelets rely upon uptake of serotonin for their stores. The biochemical pathway for serotonin synthesis initially involves the conversion of L-tryptophan to 5-hydroxytryptophan by the enzyme L-tryptophan hydroxylase. This enzyme provides the rate limiting step for serotonin synthesis. Metabolism of serotonin is carried out primarily by the enzyme monoamine oxidase (MAO), The action of MAO conves serotonin to 5-hydroxyindole acetaldehyde which in turn is readily metabolized by aldehyde dehydrogenase to produce 5-hydroxyindole acetic acid as the major excreted metabolite of serotonin. Carcinoid syndrome: are neuroendocrine tumours (NETs) of predominantly enterochromaffin cell origin (Kulchitsky cells).Carcinoid syndrome is made up of a constellation of symptoms which are caused by the release of a variety of substances (serotonin, histamine, and substance P, among others) by the carcinoid tumor. It may cause trytophan deficiency and causes decrease in niacin synthesis leading to pellagra. Ref: Principles of medical physiology, By Sabyasachi Sicar, Page 641.