Serotonin is a hormone found in the human body that transmits signals between nerve cells and causes blood vessels to narrow. According to the Britannica Online Encyclopedia, serotonin is also known as 5-hydroxytryptamine. It is most concentrated in the midbrain and hypothalamus areas of the brain, but it is also found in intestinal tissue, blood platelets and the mast cells of the immune system.
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In the medical community, serotonin is particularly significant due to its impact on psychiatric health. According to Mayo Clinic's website, abnormalities in neurotransmitter activity may affect behavior or mood and contribute to the development of depression. Serotonin travels between nerve cells via serotonin receptors. During this process, some serotonin is reabsorbed by certain cells in the brain. The reabsorbed serotonin is then no longer available for transmitting signals between nerve cells. In people with clinical depression, it is thought that too much serotonin is reabsorbed, leaving a low level of serotonin available for neurotransmission.
Certain antidepressants, called selective serotonin reuptake inhibitors (SSRIs), block the reabsorption, or reuptake, of serotonin. This leaves more serotonin available in the brain, and this is thought to alleviate the symptoms of depression. SSRIs approved by the FDA for treating depression include citalopram, escitalopram, fluoxetine, paroxetine and sertraline. Side effects include nausea, sexual dysfunction, headache, diarrhea, dry mouth, agitation, rash, nervousness, weight gain, drowsiness and insomnia.
Abnormalities in serotonin levels may affect more than just mood. According to "The Chemistry of the Brain" on the Michigan State University website, migraines may be partially caused by extreme fluctuations in serotonin levels. If you are a migraine sufferer, you may notice that certain triggers, such as food or activity, cause you to develop a migraine. These triggers may cause a spasm in the arteries at the base of your brain and cause platelets to clump together and release serotonin. Before a migraine, the rise in serotonin causes the blood vessels in your head to constrict and lowers your tolerance for pain. The level of serotonin in your brain subsequently drops, causing the constricted blood vessels to suddenly become larger than normal and resulting in a rush of blood to the brain and the throbbing pain characteristic of migraines.
Serotonin is a significant part of human functioning, and having too much or too little serotonin can produce adverse effects. While a lack of serotonin in the brain is thought to be behind the development of clinical depression, too much serotonin has the potential to cause serotonin syndrome. This is a potentially fatal condition marked by agitation, confusion, rapid heart rate, dilated pupils, heavy sweating and loss of muscle coordination. According to Mayo Clinic, serotonin syndrome can occur when you first begin taking or increasing the dosage of SSRI medications as well as with the use of certain dietary supplements and illicit drugs.
A possible correlation has been found between low serotonin levels and heart disease. According to the Duke Health website, psychologist Edward Suarez discovered that when people with low serotonin levels were put under stress, they produced larger amounts of certain immune system proteins known to contribute to heart disease. People with normal or high levels of serotonin did not experience a rise in these proteins under the same stress. This presents the possibility that psychiatric medications formulated to increase serotonin may someday be used to treat heart disease.