Serotonin is a hormone, the chemical name is called 5-hydroxytryptamine which is often abbreviated to 5-HT. It is found 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 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. It was first recognised as a powerful vasoconstrictor in blood serum. It was isolated in 1948 by Page and was later found to be associated with the central nervous system.

Serotonin is a monoamine neurotransmitter. Neurotransmitters are chemical messengers within the brain that allow the communication between nerve cells. It plays a part in the regulation of mood, sleep, learning and constriction of blood vessels (vasoconstriction). A low level of serotonin in the space between nerve cells is connected to depression. Serotonin is created in the body from the amino acid tryptophan. Serotonin may also have a role in appetite, vomiting, migraine and anxiety.

Serotonin is naturally produced in the Pineal gland which lies deep at the centre of the human brain.  The average adult human possesses only 5 to 10 mg of serotonin, 90 % of which is in the intestine and the rest in blood platelets and the brain.  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, sexual desire and function, appetite, sleep, memory and learning, temperature regulation, and some social behavior

One role of this 'wonder drug' is as a neurotransmitter, allowing numerous functions in the human body including the control of appetite, sleep, memory and learning, temperature regulation, mood, behaviour, cardiovascular function, muscle contraction, endocrine regulation and depression.  Subsequent to his discovery of Serotonin, Page commented that no physiological substance known possesses such diverse actions in the body as does serotonin.

5-HT is also found in wasp stings and scorpion venom where its function is of an irritant, since intravenous injection of serotonin in humans leads to pain, gasping, coughing, a tingling and prickling sensation, nausea, cramps and other unpleasant symptoms.

The neurons in the brain that release serotonin are found in small dense collections of neurons called Raphe Nuclei.  The Raphe Nuclei are found in the medulla, pons and midbrain which are all located at the top of the spinal cord.  Serotonergic neurons have axons which project to many different parts of the brain, therefore serotonin affects many different behaviors.

Low serotonin levels are believed to be the cause of many cases of mild to severe depression which can lead to symptoms such as anxiety, apathy, fear, feelings of worthlessness, insomnia and fatigue. The most concrete evidence for the connection between serotonin and depression is the decreased concentrations of serotonin metabolites in the cerebrospinal fluid and brain tissues of depressed people.

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, such as Celexa, Lexapro, Prozac, and Paxil -- 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. And while blood levels of serotonin are measurable -- and have been shown to be lower in people who suffer from depression -- what doctors still don't know for certain is whether or not the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.

If depression arises as a result of a serotonin deficiency then pharmaceutical agents that increase the amount of serotonin in the brain should be helpful in treating depressed patients. Anti-depressant medications increase serotonin levels at the synapse by blocking the reuptake of serotonin into the presynaptic cell. Anti-depressants are one of the most highly prescribed medications despite the serious side-effects they can cause.

If depression is mild enough it can sometimes be managed without prescribed medications. The most effective way of raising serotonin levels is with vigorous exercise. Studies have shown that serotonin levels are increased with increased activity and the production of serotonin is increased for some days after the activity. This is the safest way of increasing serotonin levels and many other benefits result from regular exercise.

Antidepressant medications that work on serotonin levels -- medications known as 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.

SSRI antidepressants are generally considered safe; however, a rare side effect of SSRIs is serotonin syndrome. Serotonin syndrome is a condition that occurs when levels of this neurochemical in the brain rise too high. It happens most often when two or more drugs that affect serotonin levels are used simultaneously. For example, if you are taking a category of migraine medicines called triptans, at the same time you are taking an SSRI drug for depression, the end result can be a serotonin overload. The same can occur when you take SSRI supplements, such as St. John's wort.

Serotonin levels can also be controlled through the diet. A diet deficient in omega-3 fatty acids may lower brain levels of serotonin and cause depression. Complex carbohydrates raise the level of tryptophan in the brain resulting in a calming effect. Vitamin C is also required for the conversion of tryptophan into serotonin.

Lysergic Acid Diethylamide, more commonly known as LSD, is a non-toxic, non-addictive molecule which mimics serotonin in the brain. The body 'mistakes' LSD for serotonin and shoots it across the synaptic cleft. LSD has a higher affinity for 5-HT receptors than serotonin, thus the presence of LSD prevents serotonin from sending neural messages in the brain. Once the LSD molecule is bound to the receptor proteins the message is not carried any further. Instead the impulse is redirected to the older parts of the brain, where the bloodstream then takes it to the sense interpretive centres and the motor areas. There are many similarities between the molecules of serotonin and LSD which allows this process to occur, the most obvious being their close structural similarities, particularly to the indole ring.

Recreational drugs such as ecstasy or LSD have also been linked to serotonin syndrome. Symptoms can occur within minutes to hours and generally include restlessness, hallucinations, rapid heartbeat, increased body temperature and sweating, loss of coordination, muscle spasms, nausea, vomiting, diarrhea, and rapid changes in blood pressure . Although not a common occurrence, it can be dangerous and is considered a medical emergency. Treatment consists of drug withdrawal, IV fluids, muscle relaxers, and drugs to block serotonin production.