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Serotonin or 5-Hydroxytryptamine (5-HT) is a neurotransmitter synthesized from the amino acid tryptophan in the central nervous system and the gastrointestinal tract. Serotonin is thought to play an important role in a number of physiological and psychological functions, including apetite, thermoregulation, sleep, alertness, and mood.
Most of the body's serotonin is found within the gastrointestinal tract (nearly 80%). It is synthesized when an organism is exposed to light, promoting apetite. Excessive amounts of serotonin are collected by blood platelets found in veins near the gut. Serotonin deficiencies can be caused by excessive or undereating, as well as a lack of sunlight or if too much is collected by the platelets.
The source of serotonin within the CNS is the Raphe Nuclei in the brainstem.
Deficiencies in serotonin are vulnerable to disorders such as clinical depression, Obsessive-Compulsive Disorder, anxiety disorders, bipolar disorder, irritable bowel syndrome, ear infections, and migraines. Also, an organism's activity cycle is affected, changing the organism's eating and sleeping habits. Consequently, medications such as antidepressants increase serotonin levels in the brain, or stabilise them, and are often used to treat these disorders in mammals.
1. Saladin's Anatomy & Physiology 5th Edition