Serotonin, or 5-Hyroxytryptamine, is a neurotransmitter primarily found in the central nervous system, gastrointestinal tract and platelets. It is synthesized from the amino acid L-tryptophan by a two-enzyme pathway and contributes to feelings of well-being. Serotonin taken orally does not affect the central nervous system because it cannot cross the blood-brain barrier; however, tryptophan and its metabolite 5-HTP, from which serotonin is synthesized, do cross the blood-brain barrier. Serotonin is mainly metabolized in the liver and excreted by the kidney.
Brain Functions
According to "Basic Neurochemistry: Molecular, Cellular and Medical Aspects, 7th Edition," in the central nervous system, serotonin is involved in the regulation of mood, appetite, sleep, learning and memory, and muscle contraction. Low levels of serotonin are associated with depression. A popular class of anti-depressive drugs are the monoamine oxidase inhibitors, which prevent the breakdown of serotonin and thus increase its concentrations in the brain.
Digestive Tract
The gut contains enterochromaffin cells, which release serotonin when food is ingested. This causes the gut to contract around the food, thus aiding digestion. If harmful substances are ingested, these irritate the entrochromaffin cells, which release more serotonin, and this causes the gut to move faster, i.e. diarrhea occurs. This has the protective function of emptying the gut of the dangerous substances.
Growth
In humans, serotonin controls the release of both insulin and insulin-like growth factor. The former regulates blood sugar while the latter regulates growth. Serotonin can also act directly as a growth factor; in fact, R.B. Matondo and colleagues at Utrecht University, The Netherlands, found that following liver damage, serotonin in the blood stimulates cellular growth, repairing liver damage.
Serotonin Syndrome
While low levels of serotonin are associated with depression and mental disorders, excessively high levels, usually caused by an adverse drug reaction, can be potentially life-threatening. Such a reaction is called serotonin syndrome. It may occur following therapeutic drug use, recreational drug use, or interaction between drugs. The cognitive, autonomic and somatic effects which occur in the serotonin syndrome can range from barely perceptible to fatal. According to "Davidson's Principles and Practice of Medicine, 21st Edition," the symptoms of serotonin syndrome include diarrhea, nausea and vomiting, agitation, increased heart rate and elevated blood pressure, fever, loss of coordination and hallucinations. Symptoms usually set in minutes to hours after the causative drug is taken.
Dietary Effect on Serotonin
Serotonin levels are affected by the diet, mainly by the ratio of tryptophan to phenylalanine and leucine. An increase in this ratio will increase serotonin levels. Foods increasing this ratio include dates, bananas and papaya while foods having the opposite effect include whole wheat and rye bread. A large carbohydrate meal low in protein will generally increase serotonin levels through the action of insulin on amino acids in the blood.
References
- "Basic Neurochemistry: Molecular, Cellular and Medical Aspects, 7th Edition"; George Siegel M.D. et al.; 2005
- "American Journal of Physiology"; Deletion of the serotonin transporter in rats disturbs serotonin homeostasis without impairing liver regeneration; Matondo RB et al; April 2009
- "Davidson's Principles and Practice of Medicine, 21st Edition"; Dr. Nicki R. Colledge, Prof. Brian R. Walker, and Prof. Stuart H. Ralston; 2010
