<Tr> <Td> </Td> <Td> </Td> </Tr> <P> Serotonin (/ ˌsɛrəˈtoʊnɪn, ˌsɪərə - /) or 5 - hydroxytryptamine (5 - HT) is a monoamine neurotransmitter, and is popularly thought to be a contributor to feelings of well - being and happiness . Biochemically, the indoleamine molecule is derived from the amino acid tryptophan . Serotonin is primarily found in the enteric nervous system located in the gastrointestinal tract (GI tract). However, it is also produced in the central nervous system (CNS), specifically in the Raphe nuclei located in the brainstem . Additionally, serotonin is stored in blood platelets and is released during agitation and vasoconstriction, where it then acts as an agonist to other platelets . </P> <P> Approximately 90% of the human body's total serotonin is located in the enterochromaffin cells in the GI tract, where it is used to regulate intestinal movements . The serotonin is secreted luminally and basolaterally which leads to increased serotonin uptake by circulating platelets and activation after stimulation, which gives increased stimulation of myenteric neurons and gastrointestinal motility . The remainder is synthesized in serotonergic neurons of the CNS, where it has various functions . These include the regulation of mood, appetite, and sleep . Serotonin also has some cognitive functions, including memory and learning . Modulation of serotonin at synapses is thought to be a major action of several classes of pharmacological antidepressants . </P> <P> Serotonin secreted from the enterochromaffin cells eventually finds its way out of tissues into the blood . There, it is actively taken up by blood platelets, which store it . When the platelets bind to a clot, they release serotonin, where it can serve as a vasoconstrictor or a vasodilator while regulating hemostasis and blood clotting . In high concentrations, serotonin acts as a vasoconstrictor by contracting endothelial smooth muscle directly or by potentiating the effects of other vasoconstrictors (e.g. angiotensin II, norepinephrine). The vasoconstrictive property is mostly seen in pathologic states affecting the endothelium such as atherosclerosis or chronic hypertension . In physiologic states, vasodilation occurs through the serotonin mediated release of nitric oxide from endothelial cells . Additionally, it inhibits the release of norepinephrine from adrenergic nerves . Serotonin is also a growth factor for some types of cells, which may give it a role in wound healing . There are various serotonin receptors . </P>

Where is the largest cache of serotonin located within the body