Serotonin transporter

The serotonin transporter (SERT or 5-HTT) is a type of is_associated_with::monoamine transporter is_associated_with::protein that transports serotonin from the is_associated_with::synaptic cleft to the presynaptic neuron.

This transport of is_associated_with::serotonin by the SERT protein terminates the action of serotonin and recycles it in a sodium-dependent manner. This protein is the target of many antidepressant medications, including those of the is_associated_with::SSRI class. It is a member of the sodium:neurotransmitter symporter family. A repeat length polymorphism in the promoter of this gene has been shown to affect the rate of serotonin uptake and may play a role in is_associated_with::sudden infant death syndrome, aggressive behavior in Alzheimer disease patients, is_associated_with::post-traumatic stress disorder and depression-susceptibility in people experiencing emotional trauma.

Function
The serotonin transporter removes serotonin from the synaptic cleft back into the synaptic boutons. Thus, it terminates the effects of serotonin and simultaneously enables its reuse by the is_associated_with::presynaptic neuron.

Neurons communicate by using chemical messengers like serotonin between cells. The transporter protein, by recycling serotonin, regulates its concentration in a gap, or is_associated_with::synapse, and thus its effects on a receiving neuron’s receptors.

Medical studies have shown that changes in serotonin transporter metabolism appear to be associated with many different phenomena, including is_associated_with::alcoholism, is_associated_with::clinical depression, is_associated_with::obsessive-compulsive disorder (OCD), is_associated_with::romantic love, is_associated_with::hypertension and generalized social phobia.

The serotonin transporter is also present in is_associated_with::platelets; there, serotonin functions as a is_associated_with::vasoconstrictive substance.

Pharmacology
SERT spans the plasma membrane 12 times. It belongs to NE, DA, SERT monoamine transporter family. Transporters are important sites for agents that treat is_associated_with::psychiatric disorders. Drugs that reduce the binding of serotonin to transporters (is_associated_with::selective serotonin reuptake inhibitors, or SSRIs) are used to treat mental disorders. About half of patients with OCD are treated with SSRIs. is_associated_with::Fluoxetine is an example of a is_associated_with::selective serotonin reuptake inhibitor.



Ligands

 * is_associated_with::DASB
 * compound 4b: Ki = 17 pM; 710-fold and 11,100-fold selective over DAT and NET
 * compound (+)-12a: Ki = 180 pM at hSERT; >1000-fold selective over hDAT, hNET, 5-HT1A, and 5-HT6. Isosteres
 * 3-cis-(3-Aminocyclopentyl)indole 8a: Ki = 220 pM

Genetics
The is_associated_with::gene that encodes the serotonin transporter is called solute carrier family 6 (neurotransmitter transporter, serotonin), member 4 (SLC6A4, see is_associated_with::Solute carrier family). In is_associated_with::humans the gene is found on is_associated_with::chromosome 17 on location 17q11.1–q12.

Mutations associated with the gene may result in changes in serotonin transporter function, and experiments with mice have identified more the 50 different phenotypic changes as a result of genetic variation. These phenotypic changes may, e.g., be increased is_associated_with::anxiety and gut dysfunction. Some of the human genetic variations associated with the gene are:
 * Length variation in the serotonin-transporter-gene-linked polymorphic region (is_associated_with::5-HTTLPR)
 * is_associated_with::rs25531 &mdash; a is_associated_with::single nucleotide polymorphism (SNP) in the 5-HTTLPR
 * is_associated_with::rs25532 &mdash; another SNP in the 5-HTTLPR
 * STin2 &mdash; a is_associated_with::variable number of tandem repeats (VNTR) in the functional is_associated_with::intron 2
 * G56A on the second is_associated_with::exon
 * I425V on the ninth exon

Length variation in 5-HTTLPR
The promotor region of the SLC6A4 gene contains a polymorphism with "short" and "long" repeats in a region: 5-HTT-linked polymorphic region (is_associated_with::5-HTTLPR or SERTPR). The short variation has 14 repeats of a sequence while the long variation has 16 repeats. The short variation leads to less transcription for SLC6A4, and it has been found that it can partly account for anxiety-related is_associated_with::personality traits. This polymorphism has been extensively investigated in over 300 scientific studies (as of 2006). The 5-HTTLPR polymorphism may be subdivided further: One study published in 2000 found 14 allelic variants (14-A, 14-B, 14-C, 14-D, 15, 16-A, 16-B, 16-C, 16-D, 16-E, 16-F, 19, 20 and 22) in a group of around 200 Japanese and Caucasian people.

In addition to altering the expression of SERT protein and concentrations of extracellular serotonin in the brain, the 5-HTTLPR variation is associated with changes in brain structure. One study found less is_associated_with::grey matter in perigenual is_associated_with::anterior cingulate cortex and is_associated_with::amygdala for short allele carriers of the is_associated_with::5-HTTLPR polymorphism compared to subjects with the long/long genotype.

In contrast, a 2008 meta-analysis found no significant overall association between the 5-HTTLPR polymorphism and autism. A hypothesized is_associated_with::gene-environment interaction between the short/short allele of the 5-HTTLPR and life stress as predictor for major depression has suffered a similar fate: after an influential initial report there were mixed results in replication, and a 2009 meta-analysis was negative. See is_associated_with::5-HTTLPR for more information.

rs25532
rs25532 is a SNP (C>T) close to the site of 5-HTTLPR. It has been examined in connection with is_associated_with::obsessive compulsive disorder (OCD).

I425V
I425V is a rare mutation on the ninth exon. Researchers have found this genetic variation in unrelated families with is_associated_with::OCD, and that it leads to faulty transporter function and regulation. A second variant in the same gene of some patients with this mutation suggests a genetic "double hit", resulting in greater biochemical effects and more severe symptoms.

VNTR in STin2
Another noncoding polymorphism is a is_associated_with::VNTR in the second is_associated_with::intron (is_associated_with::STin2). It is found with three is_associated_with::alleles: 9, 10 and 12 repeats. A is_associated_with::meta-analysis has found that the 12 repeat allele of the STin2 VNTR polymorphism had some minor (with is_associated_with::odds ratio 1.24) but statistically significant association with is_associated_with::schizophrenia. A 2008 meta-analysis found no significant overall association between the STin2 VNTR polymorphism and is_associated_with::autism. Furthermore, a 2003 meta-analysis of is_associated_with::affective disorders, is_associated_with::major depressive disorder and is_associated_with::bipolar disorder, found a little association to the intron 2 VNTR polymorphism, but the results of the meta-analysis depended on a large effect from one individual study.

The polymorphism has also been related to is_associated_with::personality traits with a Russian study from 2008 finding individuals with the STin2.10 allele having lower is_associated_with::neuroticism score as measured with the is_associated_with::Eysenck Personality Inventory.

Neuroimaging
The distribution of the serotonin transporter in the is_associated_with::brain may be imaged with is_associated_with::positron emission tomography using is_associated_with::radioligands called is_associated_with::DASB and DAPP, and the first studies on the human brain were reported in 2000. DASB and DAPP are not the only radioligands for the serotonin transporter. There are numerous others, with the most popular probably being the is_associated_with::β-CIT radioligand with an iodine-123 is_associated_with::isotope that is used for brain scanning with is_associated_with::single photon emission computed tomography (SPECT). The radioligands have been used to examine whether variables such as age, gender or is_associated_with::genotype are associated with differential serotonin transporter binding. Healthy subjects that have a high score of is_associated_with::neuroticism &mdash; a personality trait in the is_associated_with::Revised NEO Personality Inventory &mdash; have been found to have more serotonin transporter binding in the is_associated_with::thalamus.

Neuroimaging and genetics
Studies on the serotonin transporter have combined neuroimaging and genetics methods, e.g., a is_associated_with::voxel-based morphometry study found less is_associated_with::grey matter in perigenual is_associated_with::anterior cingulate cortex and is_associated_with::amygdala for short allele carriers of the is_associated_with::5-HTTLPR polymorphism compared to subjects with the long/long genotype.