Serotonin (5-HT)
Serotonin is synthesized extensively in the human gastrointestinal tract (about 90%),[1] and the major storage place is platelets in the blood stream.
In the body, serotonin is synthesized from the amino acid tryptophan by a short metabolic pathway consisting of two enzymes – tryptophan hydroxylase (TPH) and amino acid decarboxylase (DDC). The TPH mediated reaction is the rate limiting step in the pathway. TPH has been shown to exist in two forms; TPH1, found in several tissues and TPH2, which is a brain specific isoform. There is evidence that genetic polymorphisms in both these subtypes influence susceptibility to anxiety and depression (Nash et al 2005; Zhang et al 2005). There is also evidence that ovarian hormones can effect the expression of TPH in various species, suggesting a possible mechanism for postpartum depression and premenstrual stress syndrome (Hiroi et al 2006). The gut secretes over 95% of the body's serotonin.
Serotonin (5-HT) receptors are also used by other psychoactive drugs, including LSD, DMT, and psilocybin, the active ingredient in psychedelic mushrooms.
In the field of neurochemistry, 5-HT receptors are receptors for the neurotransmitter and peripheral signal mediator serotonin (5-HT). 5-HT receptors are located on the cell membrane of nerve cells and other cell types in animals and mediate the effects of serotonin as the endogenous ligand and of a broad range of pharmaceutical and hallucinogenic drugs. With the exception of the 5-HT3 receptor, a ligand gated ion channel, all other 5-HT receptors are G protein coupled seven transmembrane (or heptahelical) receptors that activate an intracellular second messenger cascade.
5-HT1 receptors are Gi/Go coupled, mediating cellular effects through decreasing cellular levels of cyclic adenosine monophosphate (cAMP).
5-HT2 receptors are Gq/G11 coupled, mediating cellular effects through increasing cellular levels of inositol trisphosphate (IP3) and diacylglycerol (DAG). Three subtypes exist, namely 5-HT2A, 5-HT2B, and 5-HT2C (formerly called 5-HT1C).
The 5-HT3 receptor is a ligand-gated Na+ and K+ cation channel, resulting in a direct plasma membrane depolarization.
The 5-HT4 receptor is Gs coupled, mediating cellular effects through increasing cellular levels of cAMP.
The 5-HT5A receptor is G protein coupled; the primary coupling appears to be through Gi/o, inhibiting adenylate cyclase activity.[7] The 5-HT5B subtype exists, but has not been detected in humans.
The 5-HT7 receptor is Gs coupled, mediating cellular effects through increasing cellular levels of cAMP.
Serotonin syndrome
Extremely high levels of serotonin can have toxic and potentially fatal effects, causing a condition known as serotonin syndrome. In practice, such toxic levels are essentially impossible to reach through an overdose of a single anti-depressant drug, but require a combination of serotonergic agents, such as an SSRI with an MAOI.[9] The intensity of the symptoms of serotonin syndrome vary over a wide spectrum, and the milder forms are seen even at non-toxic levels.[10] For example, recreational doses of MDMA (ecstasy) will generally cause such symptoms but only rarely lead to true toxicity.
http://en.wikipedia.org/wiki/Serotonin