<unk>  () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction. Biochemically, the indoleamine molecule derives from the amino acid tryptophan, via the (rate-limiting) hydroxylation of the 5 position on the ring (forming the intermediate 5-hydroxytryptophan), and then decarboxylation to produce<unk> .<unk>  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, Merkel cells located in the skin, pulmonary neuroendocrine cells and taste receptor cells in the tongue. Additionally,<unk>  is stored in blood platelets and is released during agitation and vasoconstriction, where it then acts as an agonist to other platelets. Approximately 90% of the human body's total<unk>  is located in the enterochromaffin cells in the GI tract, where it regulates intestinal movements. About 8% is found in platelets and 1–2% in the CNS. The<unk>  is secreted luminally and basolaterally, which leads to increased<unk>  uptake by circulating platelets and activation after stimulation, which gives increased stimulation of myenteric neurons and gastrointestinal motility. The remainder is synthesized in<unk> otonergic neurons of the CNS, where it has various functions. These include the regulation of mood, appetite, and sleep.<unk>  also has some cognitive functions, including memory and learning. Several classes of antidepressants, such as the SSRIs and the SNRIs among others, interfere with the normal reabsorption of<unk>  after it is done with the transmission of the signal, therefore augmenting the neurotransmitter levels in the synapses.<unk>  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<unk> , where it can<unk> ve as a vasoconstrictor or a vasodilator while regulating hemostasis and blood clotting. In high concentrations,<unk>  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<unk>  mediated release of nitric oxide from endothelial cells. Additionally, it inhibits the release of norepinephrine from adrenergic nerves.<unk>  is also a growth factor for some types of cells, which may give it a role in wound healing. There are various<unk>  receptors.<unk>  is metabolized mainly to 5-HIAA, chiefly by the liver. Metabolism involves first oxidation by monoamine oxidase to the corresponding aldehyde. The rate-limiting step is hydride transfer from<unk>  to the flavin cofactor. There follows oxidation by aldehyde dehydrogenase to 5-HIAA, the indole acetic-acid derivative. The latter is then excreted by the kidneys. Besides mammals,<unk>  is found in all bilateral animals including worms and insects, as well as in fungi and in plants.<unk> 's presence in insect venoms and plant spines<unk> ves to cause pain, which is a side-effect of<unk>  injection.<unk>  is produced by pathogenic amoebae, and its effect in the human gut is diarrhea. Its widespread presence in many seeds and fruits may<unk> ve to stimulate the digestive tract into expelling the seeds.
Serotonin
N-Acetylserotonin
Acetylcholine
Endorphins
Opioid peptide
Dopamine
Dopamine 
Beta-Endorphin