Serotonin (also known as 5-hydroxytryptamine, or 5-HT), is a monoamine neurotransmitter affecting the serotonin receptors (5-HT1-7). Serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans. It is popularly thought to be a contributor to feelings of well-being and happiness.
Approximately 90% of the human body's total serotonin is located in the digestive system, where it is used to regulate intestinal movements. The remainder is synthesized in serotonergic neurons of the CNS, where it has various functions. These include the regulation of mood, appetite, heart rate and sleep.
Serotonin also has some cognitive functions, including memory and learning. In the blood, it serves as a vasoconstrictor, and could be attributed as the cause of vasoconstriction in most serotonergic drugs.
Serotonin is comprised of a monoamine chain attached to an indole ring at the third carbon. A monoamine chain is made up of an amine group attached to an ethane chain. This monoamine chain can be found in many neurotransmitters, including histamine, dopamine, adrenaline and noradrenaline. It is also found in many drugs, examples being tryptamines and phenethylamines.
Serotonin is synthesized from the α-amino acid tryptophan. Tryptophan is hydroxylated into the metabolic intermediate 5-Hydroxytryptophan (5-HTP) and then decarboxylated into serotonin. This process requires the co-enzyme vitamin B6.
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The serotonin receptor, or 5-HT receptors, are found throughout the central nervous system and the peripheral nervous system.
Drugs targeting the 5-HT system
A serotonin partial agonist is a drug that binds to and activates a serotonin receptor, but only has partial efficacy at the receptor relative to a full agonist.
Most classical psychedelic drugs are partial agonists of the 5-HT2A receptor; among them are LSD, psilocin and mescaline. Many antidepressants, anxiolytics/anti-anxiety drugs, and cluster headache medicines are partial serotonin receptor agonists.
A serotonin inverse agonist activates a serotonin receptor but has the opposite pharmacological effect. Inverse agonists trigger a specific response from a receptor, whereas antagonists inhibit the activity of the receptor.
Some antipsychotics such as pimavanserin are 5-HT2A inverse agonists. 5-HT2A inverse agonists have been researched for the treatment of insomnia with limited success.
A serotonin releasing agent is a drug that induces the release of serotonin from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of serotonin.
A serotonin reuptake inhibitor inhibits the reabsorption of serotonin into the pre-synaptic neuron. Serotonin reuptake inhibitors do this by inhibiting the serotonin transporter, or SERT protein.
Many antidepressants such as venlafaxine (Effexor), citalopram (Celexa) and amitriptyline are serotonin reuptake inhibitors. Many recreational drugs like cocaine and tramadol are also serotonin reuptake inhibitors.
A serotonin receptor antagonist is a type of receptor drug that inhibits action at serotonin receptors.
Many antipsychotics like haloperidol or quetiapine and anti-emetics are serotonin receptor antagonists. One example is galanolactone, a chemical found in ginger, that acts as an anti-emetic via its action as a 5-HT3 antagonist.
Toxicity and harm potential
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Extremely high levels of serotonin can cause a condition known as serotonin syndrome that has toxic and potentially fatal effects. Serotonin syndrome, also known as serotonin toxicity, can be induced via overdose of particular drugs and some drug combinations where both substances have serenonergic actions. Many drug interactions have been found to cause serotonin syndrome, the most notorious of which being the combination of an SSRI antidepressant and an MAOI antidepressant.
Symptoms can start showing within minutes and can include increased heart rate, sweating, anxiety, hyperthermia, shivering, high blood pressure, agitation, seizures, and rarely death. Symptoms usually resolve after 24 hours, but can last up to several months in some cases.
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