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What is Serotonin?
Serotonin also referred to as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter. That is, it is used by your body's nervous system to communicate messages throughout the body. The serotonin levels received by groups of receptors serve to regulate a number of bodily functions, such as body temperature, vomiting and appetite.
High levels of serotonin in the intestinal tract, for example, result in a suppressed desire to eat, while lab tests have shown that low levels of serotonin in mice results in increased appetite and obesity. The exact role of serotonin with relation to the human brain and consciousness is still uncertain but a series of studies have shown low serotonin levels to be associated with sleep and mood disorders.
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Understanding Serotonin and Depression
One of the main problems with understanding serotonin depression and investigating the link between the two is that there is no way to measure the level of serotonin in the living brain. While studies have shown that patients suffering from depression, anxiety and a variety of other mood disorders often have decreased levels of serotonin in their blood, it is much harder to explain the mechanism by which serotonin acts to produce these effects. It is not even clear if it is decreased serotonin levels which cause depression or depression which causes decreased serotonin levels.
There are a number of theories regarding the link; Professor Barry Jacobs, for example, believes that serotonin may be implicit in the regeneration of brain cells and that people with low concentrations of serotonin in the brain produce fewer new brain cells and thus suffer from depression. Another theory proposed by evolutionary psychologists is that depression is an adaptation for solving complex problems and that serotonin helps to regulate resources in the brain. There are many other variations of these theories as well; it is still unclear which, if any, are most accurate.
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Treating Depression by Increasing Serotonin Concentration
Despite this uncertainty, many pharmaceutical treatments for depression are believed to work because they trigger increased concentrations of serotonin in the brain. Clinical trials have shown drugs such as Prozac and Celaxa - members of the Selective Serotonin Reuptake Inhibitor (SSRI) class of drugs - relieve depression in up to 60% of patients.
It should be noted however that in many cases these drugs are only moderately more effective than placebos and that there are often a number of side effects in subsets of patients, which only highlights how poorly the mechanism by which the drugs act is really understood. In truth, our understanding of the casual link between serotonin and depression is highly contested and many drugs are prescribed simply because they appear to work and we don't have a better alternative.
Serotonin, then, is a complex neurotransmitter found throughout the body that is known to regulate a variety of bodily functions including appetite and sexual drive. Various studies have shown that mood and sleep disorders are often linked to low levels of serotonin in the body but it is still unclear as to exactly why this should be. SSRI drugs used to treat depression are believed to work by increasing serotonin concentration in the brain but the evidence for the effectiveness is almost entirely consisted of statistical medical trials and as yet no authoritative scientific theory exists to explain the exact mechanism by which they treat depression.
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Carlson, N. (2001) Physiology of behavior (7th edition) Allyn and Bacon
Jacobs, BL (1994) “Serotonin, Motor Activity and Depression-Related Disorders”, American Scientist
Neurotopia, “Depression Post 4: The Serotonin Theory (and why it's probably wrong)” http://scienceblogs.com/neurotopia/2009/03/depression_post_4_the_serotoni.php