A hormone, also called 5-hydroxytryptamine, in the pineal gland, blood platelets, the digestive tract, and the brain. Serotonin acts both as a chemical messenger that transmits nerve signals between nerve cells and that causes blood vessels to narrow.
Changes in the serotonin levels in the brain can alter the mood. For example, medications that affect the action of serotonin are used to treat depression.
As a neurotransmitter, serotonin helps to relay messages from one area of the brain to another. Because of the widespread distribution of its cells, it is believed to influence a variety of psychological and other body functions. Of the approximately 40 million brain cells, most are influenced either directly or indirectly by serotonin. This includes brain cells related to mood and function, appetite, sleep, memory and learning, temperature regulation, and some social behavior.
In terms of our body function, serotonin can also affect the functioning of our cardiovascular system, muscles, and various elements in the endocrine system. Researchers have also found evidence that serotonin may play a role in regulating milk production in the breast, and that a defect within the serotonin network may be one underlying cause of SIDS (sudden infant death syndrome).
There are many researchers who believe that an imbalance in serotonin levels may influence mood in a way that leads to depression. Possible problems include low brain cell production of serotonin, a lack of receptor sites able to receive the serotonin that is made, inability of serotonin to reach the receptor sites, or a shortage in tryptophan, the chemical from which serotonin is made. If any of these biochemical glitches occur, researchers believe it can lead to depression, as well as obsessive-compulsive disorder, anxiety, panic, and even excess anger.
One theory about how depression develops centers on the regeneration of brain cells -- a process that some believe is mediated by serotonin, and ongoing throughout our lives. According to Princeton neuroscientist Barry Jacobs, PhD, depression may occur when there is a suppression of new brain cells and that stress is the most important precipitator of depression. He believes that common antidepressant medications known as SSRIs, such as Celexa, Lexapro, Prozac, Paxil and Zoloft, which are designed to boost serotonin levels, help kick off the production of new brain cells, which in turn allows the depression to lift.
Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain. Therefore, there have not been any studies proving that brain levels of this or any neurotransmitter are in short supply when depression or any mental illness develops. Blood levels of serotonin are measurable -- and have been shown to be lower in people who suffer from depression – but researchers don't know if blood levels reflect the brain's level of serotonin.
Also, researchers don't know whether the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.
Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain. The Antidepressant medications that work on serotonin levels -- SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin and norepinephrine reuptake inhibitors) -- are believed to reduce symptoms of depression, but exactly how they work is not yet fully understood.
Serotonin is one of the key neurotransmitters responsible for regulating mood. An article published by McGill University's Department of Psychiatry and published in the April 2002 issue of the scientific journal "Pharmacology Biochemistry and Behavior" says individuals with high serotonin levels are less likely to experience clinical depression or suffer from impulse control disorders than individuals with low serotonin levels.
Another health benefit of serotonin is decreased risk of death from cardiovascular problems such as heart attacks and strokes. High serotonin levels decrease feelings of overall hostility. Less hostility, reports Young, has been shown by 45 different scientific studies to reduce risk of developing cardiovascular disease, but the benefits don't stop there. Young also reports low hostility levels also increase your chances of surviving a heart attack if you do have one.
Doctors have found that certain antidepressants work on serotonin to improve moods for people with anxiety disorders. Selective serotonin reuptake inhibitors, or SSRIs, block the absorption of serotonin by the brain. Absorption normally takes place after serotonin performs a function. By blocking the process, more serotonin becomes available for patients to relieve anxiety, according to the Anxiety Disorders Association of America. The serotonin-altering medications work effectively for generalized anxiety disorder, panic disorder, phobias and post-traumatic stress disorder. People with obsessive-compulsive disorder benefit when using higher doses of the medication. Other antidepressants called serotonin-norepinephrine reuptake inhibitors, known as SNRIs, increase levels of serotonin and norepinephrine, an adrenal hormone.
Antidepressants, of course, also benefit people with depression. Drugs involved with increasing serotonin levels help to improve moods and emotional responses in patients. SSRIs are the most popular types of antidepressants for depression, according to the National Institute of Mental Health. Drugs include sertraline, fluoxetine, citalopram, escitalopram and paroxetine. Doctors also prescribe SNRIs, such as duloxetine and venlafaxine. These drugs are also prescribed for anxiety disorders. SSRIs and SNRIs are among the newer class of drugs that have fewer side effects overall than older drugs.
Low levels of serotonin may result in disturbed sleep patterns, according to CBC News. Serotonin levels, measured through blood tests, were found to be lower among 437 day workers when compared to 246 workers on rotating shifts by researchers at the University of Buenos Aires. The shift workers experienced a poor quality of sleep, sleeping one to four hours less than average, the researchers concluded. Poor sleep quality may affect work performance, risking accidents or injury. Medications that affect serotonin levels help to improve sleep for people with sleeping disorders. They can be especially helpful for patients with depression and disturbed sleep. Doctors may prescribe trazodone, which affects serotonin levels, for sleep disorders.
Side effects from too much serotonin include nervousness, headache, drowsiness, insomnia, agitation and fatigue. Although all serotonin medications have the same mechanism of action, they all have different pharmacological characteristics. In addition, not every patient will experience every neurological side effect with every medication, as an individual might have a side effect with one type of serotonin medication but not with another.
Increased levels of serotonin can include weight loss, changes in appetite and nausea, explains University Of Michigan Health System. This is because serotonin affects the hypothalamus, which is the gland that controls appetite.
Too much serotonin in the brain can cause symptoms of serotonin syndrome. This sometimes life-threatening condition can occur when the patient taking serotonin medications is also taking medication to treat other medical or neurological conditions, explains the American Headache Society. Patients who take certain migraine headache medications together with SSRIs can develop this syndrome which can cause elevated blood pressure, agitation, hallucinations, tremors, diarrhea, vomiting and elevation in body temperature.