What is serotonin?
Serotonin is a chemical that works as a neurotransmitter , which is a substance in the body that carries a signal from one nerve cell to another. It is one of several monoamine neurotransmitters, which include adrenaline and dopamine. The human body manufactures serotonin, which many researchers believe plays a role in mood balance. While some serotonin is manufactured and used in the brain, most of the body's supply is in the gastrointestinal tract.
Serotonin was first discovered during the 1930s by Vittorio Erspamer, who isolated the molecules while seeking drugs from natural sources. He was studying cells in the smooth muscles of animals' intestinal tracts when he isolated serotonin, which he named enteramine. Irvine Page, Maurice Rapport, and Arda Green at the Cleveland Clinic were working on a substance they called serotonin in 1948. Page was studying hypertension and trying to understand what caused it. He believed it was something in the blood, but a substance kept showing up in his samples. Wishing to isolate the substance so he could remove it, he sought assistance from Rapport and Green. As a result of this work, in 1952 serotonin and enteramine were found to be one and the same.
Around this time, Betty Mack Twarog was studying neurotransmitters in mussels. She was unable to identify the neurotransmitter that regulated muscle relaxation in the mollusks. After she read the Cleveland Clinic researchers' paper on serotonin, she became intrigued, but the paper only addressed serotonin in mammals. About that time, Erspamer found enteramine in octopus salivary glands, and soon after this discovery, enteramine was identified as serotonin. Twarog obtained a sample of serotonin and soon confirmed her suspicions that the neurotransmitter in the mussels was serotonin. She later worked at the Cleveland Clinic and confirmed her theory that serotonin could be found in the brain. The work of these researchers was instrumental in the development of the field of neuroscience, which is the study of the nervous system.
The chemical name for serotonin is 5-hydroxytryptamine, or 5-HT. It is created by combining an amino acid called tryptophan and a chemical reactor called tryptophan hydroxylase. Between 80 and 90 percent of the body's serotonin is found in the gastrointestinal tract, but it is also found throughout the body in the central nervous system and in platelets.
Serotonin is important because it conveys signals between nerve cells. It cannot pass through the blood-brain barrier, which is a network of cells that restricts access to the brain, therefore all serotonin created in the brain must remain there. For this reason, it is impossible to measure serotonin levels in the brain using blood tests. The chemical is believed to be a necessary component in many functions, including appetite and digestion, memory, mood and social behavior, sexual desire and function, and sleep. Some studies have found a possible connection between disruption in the serotonin network and sudden infant death syndrome (SIDS).
Research has found a relationship between mood and serotonin, and for some time it was believed that serotonin affected mood. Scientists believed low levels of serotonin caused depression; however, studies have not shown this to be the case. Researchers working with mice that cannot manufacture serotonin found no signs of depression in the animals, though the mice did display compulsive and aggressive behavior. It is possible that depression causes a reduction in serotonin production. Other possible reasons for the apparent connection between mood and serotonin include low tryptophan levels, a failure or lack of receptor sites, failure of serotonin to reach receptor sites, and reduced brain cell regeneration.
Medications known as selective serotonin reuptake inhibitors (SSRIs) are prescribed as antidepressants because they have been found to affect serotonin levels. Like other neurotransmitters, serotonin is reabsorbed by the body after it has relayed a neural impulse. SSRIs hinder this absorption process, boosting serotonin levels. Some research suggests that high levels of serotonin improve transmission between brain cells, which improves one's mood.
Too much serotonin may cause health problems, however. Some substances, including dietary supplements such as St. John's wort, medications, and illegal drugs, can overstimulate the central nervous system and some serotonin receptors. Carcinoid tumors, which are often found in the gastrointestinal tract, also may cause high levels of serotonin. Such increases in serotonin may lead to a potentially life-threatening condition known as serotonin syndrome. Symptoms of this include agitation, confusion, diarrhea, headaches, increased heart rate and blood pressure, loss of muscle coordination, muscle rigidity, pupil dilation, shivering, and sweating.
Some research indicates that serotonin levels in the body may be increased without medications. Possible methods include eating foods such as carbohydrates that increase one's level of tryptophan; exercising; light, such as that used in treating seasonal affective disorder; and psychotherapy or other means to affect mood.
Men and women appear to react differently to changes in serotonin levels. A 2007 study in the journal Biological Psychiatry indicated that men with reduced levels of serotonin in the brain became impulsive while women developed a poorer mood and increased caution. Other studies seem to indicate that the interaction of serotonin and hormones may affect women during menopause, the postpartum period, and the premenstrual period—times when hormone levels fluctuate.
Serotonin may also be a factor in aging issues such as dementia. A 2006 study found that the brains of many deceased patients with Alzheimer's disease were deficient in serotonin.
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