Serotonin

Serotonin (pronounced /ˌsɛrəˈtoʊnən/) (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesized in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract of animals including humans. Serotonin is also found in many mushrooms and plants, including fruits and vegetables.

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Function

In the central nervous system, serotonin is believed to play an important role as a neurotransmitter, in the regulation of anger, aggression, body temperature, mood, sleep, vomiting, sexuality, and appetite.

In addition, serotonin is also a peripheral signal mediator. For instance, serotonin is found extensively in the human gastrointestinal tract (about 90%),^[1] and the major storage place is platelets in the blood stream.

Neurotransmission

As with all neurotransmitters, the effects of 5-HT on the human mood and state of mind, and its role in consciousness, are very difficult to ascertain.

Gross anatomy

The neurons of the raphe nuclei are the principal source of 5-HT release in the brain.^[2] The raphe nuclei are neurons grouped into about nine pairs and distributed along the entire length of the brainstem, centered around the reticular formation. ^[3]

Axons from the neurons of the raphe nuclei form a neurotransmitter system, reaching large areas of the brain. Axons of neurons in the caudal dorsal raphe nucleus terminate in e.g.:

• deep cerebellar nuclei
• cerebellar cortex
• spinal cord

On the other hand, axons of neurons in the rostral dorsal raphe nucleus terminate in e.g.:

Thus, activation of this serotonin system has effects on large areas of the brain, which explains the effects of therapeutic modulation of it.

Microanatomy

5-HT is thought to be released from serotonergic varicosities into the extra neuronal space, in other words from swellings (varicosities) along the axon, rather than from synaptic terminal buttons (in the manner of classical neurotransmission). From here it is free to diffuse over a relatively large region of space (>20µm) and activate 5-HT receptors located on the dendrites, cell bodies and presynaptic terminals of adjacent neurons.

Receptors

Main article: 5-HT receptor

5-HT receptors are the receptors for serotonin. They 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.

Genetic factors

Genetic variations in alleles which code for serotonin receptors are now known to have a significant impact on the likelihood of the appearance of certain psychological disorders and problems. For instance, a mutation in the allele which codes for the 5-HT_2A receptor appears to double the risk of suicide for those with that genotype. [1]

Termination

Serotonergic action is terminated primarily via uptake of 5-HT from the synapse. This is through the specific monoamine transporter for 5-HT, 5-HT reuptake transporter, on the presynaptic neuron. Various agents can inhibit 5-HT reuptake including MDMA (ecstasy), amphetamine, cocaine, dextromethorphan (an antitussive), tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs).

Other functions

Recent research suggests that serotonin plays an important role in liver regeneration and acts as a mitogen (induces cell division) throughout the body.^[4]

Pathology

Low levels of serotonin may be associated with several disorders, namely increase in aggressive and angry behaviors, clinical depression, obsessive-compulsive disorder (OCD), migraine, irritable bowel syndrome, tinnitus, fibromyalgia, bipolar disorder and anxiety disorders.^{[citation needed]} If neurons of the brainstem that make serotonin — serotonergic neurons — are abnormal in infants, there is a risk of sudden infant death syndrome (SIDS).^[5][6] Low levels of serotonin may also be associated with intense religious experiences.^[7]

Recent research conducted at Rockefeller University shows that in both patients who suffer from depression and in mice that model that disease, levels of the p11 protein are decreased. This protein is related to serotonin transmission within the brain.^[8]

Synthesis

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. There is also evidence that ovarian hormones can affect the expression of TPH in various species, suggesting a possible mechanism for postpartum depression and premenstrual stress syndrome.

Serotonin taken orally does not pass into the serotonergic pathways of the central nervous system because it does not cross the blood-brain barrier. However, tryptophan and its metabolite 5-hydroxytryptophan (5-HTP), from which serotonin is synthesized, can and do cross the blood-brain barrier. These agents are available as dietary supplements and may be effective serotonergic agents.

One product of serotonin breakdown is 5-Hydroxyindoleacetic acid (5 HIAA), which is excreted in the urine. Serotonin and 5 HIAA are sometimes produced in excess amounts by certain tumors or cancers, and levels of these substances may be measured in the urine to test for these tumors.

Psychedelic modulation

There exist many recreational drugs that innately modulate the 5-HT system in such a way to produce alterations in perception, emotional response, and thought process. These include psilocin/psilocybin, DMT, mescaline, LSD, MDMA (ecstasy), MDA, MDEA and ibogaine.

Therapeutic modulation

Various drugs are used to modulate the 5-HT system including some antidepressants, anxiolytics, antiemetics, and triptans.

Many are classified as psychiatric medications, including the monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs), atypical antipsychotics, the selective serotonin reuptake inhibitors (SSRIs), and amphetamines. Research by GW Pharma in the UK has shown that cannabis modulates serotonin levels through G-proteins, also resulting in an antiemetic effect.

Antidepressants

The MAOIs prevent the breakdown of monoamine neurotransmitters (including serotonin), 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 this re-uptake of serotonin, again making it stay in the synapse longer. The tricyclic antidepressants (TCAs) inhibit the re-uptake of both serotonin and norepinephrine. The newer selective serotonin re-uptake inhibitors (SSRIs) have fewer (though still numerous) side-effects and fewer interactions with other drugs.

Like many centrally active drugs, prolonged use of SSRIs may not be effective for increasing levels of serotonin as homeostasis may reverse the effects of SSRIs via negative feedback, tolerance or downregulation.

Antiemetics

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 treatment of post-operative nausea and vomiting. Applications to the treatment of depression and other mental and psychological conditions have also been investigated with some positive results.

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.

Chronic diseases resulting from serotonin 5-HT_2B overstimulation

Main article: Cardiac fibrosis

In blood, serotonin stored in platelets is active wherever platelets bind, as a vasoconstictor to stop bleeding, and also as a fibrocyte mitotic, to aid healing. Because of these effects, overdoses of serotonin, or serotonin agonist drugs, may cause acute or chronic pulmonary hypertension from pulmonary vasoconstriction, or else syndromes of retroperitoneal fibrosis or cardiac valve fibrosis (endocardial fibrosis) from overstimulation of serotonic growth receptors on fibrocytes.

Serotonin itself may cause a syndrome of cardiac fibrosis when it is eaten in large quantities in the diet (the Matoki banana of East Africa) or when it is over-secreted by certain mid-gut carcinoid tumors. The valvular fibrosis in such cases is typically on the right side of the heart, since excess serotonin in the serum outside platelets is metabolized in the lungs and does not reach the left circulation.

Serotonergic agonist drugs may cause not only pulmonary hypertension but also 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 caused these syndromes. They are the serotonergic vasoconstrictive anti-migraine drugs (ergotamine and methysergide), the serotonergic appetite suppressant drugs (fenfluramine, chlorphentermine, and aminorex), and certain anti-parkinsonian dopaminergic agonists, which also stimulate serotonergic 5-HT_2B receptors (pergolide and cabergoline, but not the more specific lisuride). A number of these drugs have recently been withdrawn from the market.

Because neither the amino acid L-tryptophan nor the SSRI-class antidepressants raise blood serotonin levels, they are not under suspicion to cause the syndromes described. However, since 5-hydroxytryptophan (5-HTP) does raise blood serotonin levels, it is under some of the same scrutiny as actively serotonergic drugs.

In unicellular organisms

Serotonin is used by a variety of single-cell organisms for various purposes. Selective serotonin re-uptake inhibitors (SSRIs) have been found to be toxic to algae.^[11] The gastrointestinal parasite Entamoeba histolytica secretes serotonin, causing a sustained secretory diarrhea in some patients.^[12][13] Patients infected with Entamoeba histolytica have been found to have highly elevated serum serotonin levels which returned to normal following resolution of the infection.^[14]Entamoeba histolytica also responds to the presence of serotonin by becoming more virulent.^[15]

In plants

Serotonin is found in mushrooms and plants, including fruits and vegetables. The highest values of 25–400 mg/kg have been found in nuts of the walnut (Juglans) and hickory (Carya) genuses. Serotonin concentrations of 3–30 mg/kg have been found in plantain, pineapple, banana, kiwifruit, plums, and tomatoes. Moderate levels from 0.1–3 mg/kg have been found in a wide range of tested vegetables.^[16] Serotonin is one compound of the poison contained in the stinging hairs of the stinging nettle (Urtica dioica). It should be noted that serotonin, unlike its precursors 5-HTP and tryptophan, does not cross the blood–brain barrier. Several plants contain serotonin together with a family of related tryptamines that are methylated at the amino (NH₂) and hydroxy (OH) groups, are N-oxides, or miss the OH group. Examples are plants from the Anadenanthera genus that are used in the hallucinogenic yopo snuff.

In animals

Serotonin as a neurotransmitter is found in all animals, including insects. Several toad venoms, as well as that of the stingray, contain serotonin and related tryptamines.

History

Isolated and named in 1948 by Maurice M. Rapport, Arda Green, and Irvine Page of the Cleveland Clinic,^[17] the name serotonin is something of a misnomer and reflects the circumstances of the compound's discovery. It was initially identified as a vasoconstrictor substance in blood serum – hence serotonin, a serum agent affecting vascular tone. This agent was later chemically identified as 5-hydroxytryptamine (5-HT) by Rapport, and, as the broad range of physiological roles were elucidated, 5-HT became the preferred name in the pharmacological field.

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