Several classes of drugs target the 5-HT system, including some antidepressants, antipsychotics, anxiolytics, antiemetics, and antimigraine drugs, as well as, the psychedelic drugs and empathogens. Mechanism of action At rest,<unk>  is stored within the vesicles of presynaptic neurons. When stimulated by nerve impulses,<unk>  is released as a neurotransmitter into the synapse, reversibly binding to the postsynaptic receptor to induce a nerve impulse on the postsynaptic neuron.<unk>  can also bind to auto-receptors on the presynaptic neuron to regulate the synthesis and release of<unk> . Normally<unk>  is taken back into the presynaptic neuron to stop its action, then reused or broken down by monoamine oxidase. Psychedelic drugs The serotonergic psychedelic drugs psilocin/psilocybin, DMT, mescaline, psychedelic mushroom and LSD are agonists, primarily at<unk> 2A/2C receptors. The empathogen-entactogen MDMA releases<unk>  from synaptic vesicles of neurons. Antidepressants Drugs that alter<unk>  levels are used in treating depression, generalized anxiety disorder, and social phobia. Monoamine oxidase inhibitors (MAOIs) prevent the breakdown of monoamine neurotransmitters (including<unk> , and therefore increase concentrations of the neurotransmitter in the brain. MAOI therapy is associated with many adverse drug reactions, and patients are at risk of hypertensive emergency triggered by foods with high tyramine content, and certain drugs. Some drugs inhibit the re-uptake of<unk> , making it stay in the synaptic cleft longer. The tricyclic antidepressants (TCAs) inhibit the reuptake of both<unk>  and norepinephrine. The newer selective<unk>  reuptake inhibitors (SSRIs) have fewer side-effects and fewer interactions with other drugs. Certain SSRI medications have been shown to lower<unk>  levels below the baseline after chronic use, despite initial increases. The 5-HTTLPR gene codes for the number of<unk>  transporters in the brain, with more<unk>  transporters causing decreased duration and magnitude of serotonergic signaling. The 5-HTTLPR polymorphism (l/l) causing more<unk>  transporters to be formed is also found to be more resilient against depression and anxiety.<unk>  syndrome Extremely high levels of<unk>  can cause a condition known as<unk>  syndrome, with toxic and potentially fatal effects. In practice, such toxic levels are essentially impossible to reach through an overdose of a single antidepressant drug, but require a combination of serotonergic agents, such as an SSRI with a MAOI, which may occur in therapeutic doses. The intensity of the symptoms of<unk>  syndrome vary over a wide spectrum, and the milder forms are seen even at nontoxic levels. It is estimated that 14% of patients experiencing<unk>  syndrome overdose on SSRIs; meanwhile the fatality rate is between 2% to 12%. Antiemetics Some 5-HT3 antagonists, such as ondansetron, granisetron, and tropisetron, are important antiemetic agents. They are particularly important in treating the nausea and vomiting that occur during anticancer chemotherapy using cytotoxic drugs. Another application is in the treatment of postoperative nausea and vomiting. Other Some serotonergic agonist drugs cause fibrosis anywhere in the body, particularly the syndrome of retroperitoneal fibrosis, as well as cardiac valve fibrosis. In the past, three groups of serotonergic drugs have been epidemiologically linked with these syndromes. These are the serotonergic vasoconstrictive antimigraine drugs (ergotamine and methysergide), the serotonergic appetite suppressant drugs (fenfluramine, chlorphentermine, and aminorex), and certain anti-Parkinsonian dopaminergic agonists, which also stimulate serotonergic 5-HT2B receptors. These include pergolide and cabergoline, but not the more dopamine-specific lisuride. As with fenfluramine, some of these drugs have been withdrawn from the market after groups taking them showed a statistical increase of one or more of the side effects described. An example is pergolide. The drug was declining in use since it was reported in 2003 to be associated with cardiac fibrosis. Two independent studies published in The New England Journal of Medicine in January 2007 implicated pergolide, along with cabergoline, in causing valvular heart disease. As a result of this, the FDA removed pergolide from the United States market in March 2007. (Since cabergoline is not approved in the United States for Parkinson's Disease, but for hyperprolactinemia, the drug remains on the market. Treatment for hyperprolactinemia requires lower doses than that for Parkinson's Disease, diminishing the risk of valvular heart disease). Methyl-tryptamines and hallucinogens Several plants contain<unk>  together with a family of related tryptamines that are methylated at the amino (NH2) and (OH) groups, are N-oxides, or miss the OH group. These compounds do reach the brain, although some portion of them are metabolized by monoamine oxidase enzymes (mainly MAO-A) in the liver. Examples are plants from the genus Anadenanthera that are used in the hallucinogenic yopo snuff. These compounds are widely present in the leaves of many plants, and may serve as deterrents for animal ingestion.<unk>  occurs in several mushrooms of the genus Panaeolus.
Serotonin
N-Acetylserotonin
Acetylcholine
Endorphins
Opioid peptide
Dopamine
Dopamine 
Beta-Endorphin