The dopamine transporter (DAT) also (sodium-dependent dopamine transporter) is a membrane-spanning protein coded for in the human by the SLC6A3gene, (also known as DAT1), that pumps the neurotransmitterdopamine out of the synaptic cleft back into cytosol. In the cytosol, other transporters sequester the dopamine into vesicles for storage and later release. Dopamine reuptake via DAT provides the primary mechanism through which dopamine is cleared from synapses, although there may be an exception in the prefrontal cortex, where evidence points to a possibly larger role of the norepinephrine transporter.
DAT is an integral membrane protein that removes dopamine from the synaptic cleft and deposits it into surrounding cells, thus terminating the signal of the neurotransmitter. Dopamine underlies several aspects of cognition, including reward, and DAT facilitates regulation of that signal.
DAT is a symporter that moves dopamine across the cell membrane by coupling the movement to the energetically-favorable movement of sodium ions moving from high to low concentration into the cell. DAT function requires the sequential binding and co-transport of two Na+ions and one Cl− ion with the dopamine substrate. The driving force for DAT-mediated dopamine reuptake is the ion concentration gradient generated by the plasma membrane Na+/K+ ATPase.
In the most widely accepted model for monoamine transporter function, sodium ions must bind to the extracellular domain of the transporter before dopamine can bind. Once dopamine binds, the protein undergoes a conformational change, which allows both sodium and dopamine to unbind on the intracellular side of the membrane.
Studies using electrophysiology and radioactive-labeled dopamine have confirmed that the dopamine transporter is similar to other monoamine transporters in that one molecule of neurotransmitter can be transported across the membrane with one or two sodium ions. Chloride ions are also needed to prevent a buildup of positive charge. These studies have also shown that transport rate and direction is totally dependent on the sodium gradient.
Because of the tight coupling of the membrane potential and the sodium gradient, activity-induced changes in membrane polarity can dramatically influence transport rates. In addition, the transporter may contribute to dopamine release when the neuron depolarizes.
Preliminary evidence suggests that the dopamine transporter couples to L-type voltage-gated calcium channels (particularly Cav1.2 and Cav1.3), which are expressed in virtually all dopamine neurons. As a result of DAT–Cav coupling, DAT substrates that produce depolarizing currents through the transporter are able to open calcium channels that are coupled to the transporter, resulting in a calcium influx in dopamine neurons. This calcium influx is believed to induce CAMKII-mediated phosphorylation of the dopamine transporter as a downstream effect; since DAT phosphorylation by CAMKII results in dopamine efflux in vivo, activation of transporter-coupled calcium channels is a potential mechanism by which certain drugs (e.g., amphetamine) trigger neurotransmitter release.
The initial determination of the membrane topology of DAT was based upon hydrophobic sequence analysis and sequence similarities with the GABA transporter. These methods predicted twelve transmembrane domains (TMD) with a large extracellular loop between the third and fourth TMDs. Further characterization of this protein used proteases, which digest proteins into smaller fragments, and glycosylation, which occurs only on extracellular loops, and largely verified the initial predictions of membrane topology. The exact structure of the Drosophila melanogaster dopamine transporter (dDAT) was elucidated in 2013 by X-ray crystallography.
Location and distribution
Pharmacodynamics of amphetamine in a dopamine neuron
Amphetamine enters the presynaptic neuron across the neuronal membrane or through DAT. Once inside, it binds to TAAR1 or enters synaptic vesicles through VMAT2. When amphetamine enters synaptic vesicles through VMAT2, it collapses the vesicular pH gradient, which in turn causes dopamine to be released into the cytosol (light tan-colored area) through VMAT2. When amphetamine binds to TAAR1, it reduces the firing rate of the dopamine neuron via potassium channels and activates protein kinase A (PKA) and protein kinase C (PKC), which subsequently phosphorylates DAT.PKA-phosphorylation causes DAT to withdraw into the presynaptic neuron (internalize) and cease transport.PKC-phosphorylated DAT may either operate in reverse or, like PKA-phosphorylated DAT, internalize and cease transport. Amphetamine is also known to increase intracellular calcium, an effect which is associated with DAT phosphorylation through a CAMKIIα-dependent pathway, in turn producing dopamine efflux.
Regional distribution of DAT has been found in areas of the brain with established dopaminergic circuitry, including the nigrostriatal, mesolimbic, and mesocortical pathways. The nuclei that make up these pathways have distinct patterns of expression. Gene expression patterns in the adult mouse show high expression in the substantia nigra pars compacta.
Staining in the striatum and nucleus accumbens of the mesolimbic pathway was dense and heterogeneous. In the striatum, DAT is localized in the plasma membrane of axon terminals. Double immunocytochemistry demonstrated DAT colocalization with two other markers of nigrostriatal terminals, tyrosine hydroxylase and D2 dopamine receptors. The latter was thus demonstrated to be an autoreceptor on cells that release dopamine. TAAR1 is a presynaptic intracellular receptor that is also colocalized with DAT and which has the opposite effect of the D2 autoreceptor when activated; i.e., it internalizes dopamine transporters and induces efflux through reversed transporter function via PKA and PKC signaling.
Surprisingly, DAT was not identified within any synaptic active zones. These results suggest that striatal dopamine reuptake may occur outside of synaptic specializations once dopamine diffuses from the synaptic cleft.
Within the perikarya of pars compacta neurons, DAT was localized primarily to rough and smooth endoplasmic reticulum, Golgi complex, and multivesicular bodies, identifying probable sites of synthesis, modification, transport, and degradation.
Genetics and regulation
The gene for DAT, known as DAT1, is located on chromosome 5p15.
The protein encoding region of the gene is over 64 kb long and comprises 15 coding segments or exons.
This gene has a variable number tandem repeat (VNTR) at the 3’ end (rs28363170) and another in the intron 8 region. Differences in the VNTR have been shown to affect the basal level of expression of the transporter; consequently, researchers have looked for associations with dopamine-related disorders.
The rate at which DAT removes dopamine from the synapse can have a profound effect on the amount of dopamine in the cell. This is best evidenced by the severe cognitive deficits, motor abnormalities, and hyperactivity of mice with no dopamine transporters. These characteristics have striking similarities to the symptoms of ADHD.
Differences in the functional VNTR have been identified as risk factors for bipolar disorder and ADHD. Data has emerged that suggests there is also an association with stronger withdrawal symptoms from alcoholism, although this is a point of controversy. An allele of the DAT gene with normal protein levels is associated with non-smoking behavior and ease of quitting. Additionally, male adolescents particularly those in high-risk families (ones marked by a disengaged mother and absence of maternal affection) who carry the 10-allele VNTR repeat show a statistically significant affinity for antisocial peers.
Cocaine blocks DAT by binding directly to the transporter and reducing the rate of transport. In contrast, amphetamine enters the presynaptic neuron directly through the neuronal membrane or through DAT, competing for reuptake with dopamine. Once inside, it binds to TAAR1 or enters synaptic vesicles through VMAT2. When amphetamine binds to TAAR1, it reduces the firing rate of the postsynaptic neuron and triggers protein kinase A and protein kinase C signaling, resulting in DAT phosphorylation. Phosphorylated DAT then either operates in reverse or withdraws into the presynaptic neuron and ceases transport. When amphetamine enters the synaptic vesicles through VMAT2, dopamine is released into the cytosol. Amphetamine also produces dopamine efflux through a second TAAR1-independent mechanism involving CAMKIIα-mediated phosphorylation of the transporter, which putatively arises from the activation of DAT-coupled L-type calcium channels by amphetamine.
The dopaminergic mechanisms of each drug are believed to underlie the pleasurable feelings elicited by these substances.
Dopamine transporter has been shown to interact with:
Apart from these innate protein-protein interactions, recent studies demonstrated that viral proteins such as HIV-1Tat protein interacts with the DAT and this binding may alter the dopamine homeostasis in HIV positive individuals which is a contributing factor for the HIV-associated neurocognitive disorders.
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^ abVandenbergh DJ, Persico AM, Hawkins AL, Griffin CA, Li X, Jabs EW, Uhl GR (December 1992). "Human dopamine transporter gene (DAT1) maps to chromosome 5p15.3 and displays a VNTR". Genomics. 14 (4): 1104–6. doi:10.1016/S0888-7543(05)80138-7. PMID1478653.
^ abWheeler DD, Edwards AM, Chapman BM, Ondo JG (August 1993). "A model of the sodium dependence of dopamine uptake in rat striatal synaptosomes". Neurochemical Research. 18 (8): 927–36. doi:10.1007/BF00998279. PMID8371835. S2CID42196576.
^ abcdeCameron KN, Solis E, Ruchala I, De Felice LJ, Eltit JM (November 2015). "Amphetamine activates calcium channels through dopamine transporter-mediated depolarization". Cell Calcium. 58 (5): 457–66. doi:10.1016/j.ceca.2015.06.013. PMC4631700. PMID26162812. One example of interest is CaMKII, which has been well characterized as an effector of Ca2+ currents downstream of L-type Ca2+ channels [21,22]. Interestingly, DAT is a CaMKII substrate and phosphorylated DAT favors the reverse transport of dopamine [48,49], constituting a possible mechanism by which electrical activity and L-type Ca2+ channels may modulate DAT states and dopamine release. ... In summary, our results suggest that pharmacologically, S(+)AMPH is more potent than DA at activating hDAT-mediated depolarizing currents, leading to L-type Ca2+ channel activation, and the S(+)AMPH-induced current is more tightly coupled than DA to open L-type Ca2+ channels.
^Sulzer D, Cragg SJ, Rice ME (August 2016). "Striatal dopamine neurotransmission: regulation of release and uptake". Basal Ganglia. 6 (3): 123–148. doi:10.1016/j.baga.2016.02.001. PMC4850498. PMID27141430. Despite the challenges in determining synaptic vesicle pH, the proton gradient across the vesicle membrane is of fundamental importance for its function. Exposure of isolated catecholamine vesicles to protonophores collapses the pH gradient and rapidly redistributes transmitter from inside to outside the vesicle. ... Amphetamine and its derivatives like methamphetamine are weak base compounds that are the only widely used class of drugs known to elicit transmitter release by a non-exocytic mechanism. As substrates for both DAT and VMAT, amphetamines can be taken up to the cytosol and then sequestered in vesicles, where they act to collapse the vesicular pH gradient.
^"TAAR1". GenAtlas. University of Paris. 28 January 2012. Retrieved 29 May 2014. • tonically activates inwardly rectifying K(+) channels, which reduces the basal firing frequency of dopamine (DA) neurons of the ventral tegmental area (VTA)
^Krause J (April 2008). "SPECT and PET of the dopamine transporter in attention-deficit/hyperactivity disorder". Expert Review of Neurotherapeutics. 8 (4): 611–25. doi:10.1586/1473718.104.22.1681. PMID18416663. S2CID24589993. Zinc binds at ... extracellular sites of the DAT , serving as a DAT inhibitor. In this context, controlled double-blind studies in children are of interest, which showed positive effects of zinc [supplementation] on symptoms of ADHD [105,106]. It should be stated that at this time [supplementation] with zinc is not integrated in any ADHD treatment algorithm.
^ abScholze P, Nørregaard L, Singer EA, Freissmuth M, Gether U, Sitte HH (June 2002). "The role of zinc ions in reverse transport mediated by monoamine transporters". The Journal of Biological Chemistry. 277 (24): 21505–13. doi:10.1074/jbc.M112265200. PMID11940571. The human dopamine transporter (hDAT) contains an endogenous high affinity Zn2+ binding site with three coordinating residues on its extracellular face (His193, His375, and Glu396). ... Although Zn2+ inhibited uptake, Zn2+ facilitated [3H]MPP+ release induced by amphetamine, MPP+, or K+-induced depolarization specifically at hDAT but not at the human serotonin and the norepinephrine transporter (hNET).
^Scassellati C, Bonvicini C, Faraone SV, Gennarelli M (October 2012). "Biomarkers and attention-deficit/hyperactivity disorder: a systematic review and meta-analyses". Journal of the American Academy of Child and Adolescent Psychiatry. 51 (10): 1003–1019.e20. doi:10.1016/j.jaac.2012.08.015. PMID23021477. With regard to zinc supplementation, a placebo controlled trial reported that doses up to 30 mg/day of zinc were safe for at least 8 weeks, but the clinical effect was equivocal except for the finding of a 37% reduction in amphetamine optimal dose with 30 mg per day of zinc.110
^Yang B, Chan RC, Jing J, Li T, Sham P, Chen RY (June 2007). "A meta-analysis of association studies between the 10-repeat allele of a VNTR polymorphism in the 3'-UTR of dopamine transporter gene and attention deficit hyperactivity disorder". American Journal of Medical Genetics. Part B, Neuropsychiatric Genetics. 144B (4): 541–50. doi:10.1002/ajmg.b.30453. PMID17440978. S2CID22881996.
^Sander T, Harms H, Podschus J, Finckh U, Nickel B, Rolfs A, Rommelspacher H, Schmidt LG (February 1997). "Allelic association of a dopamine transporter gene polymorphism in alcohol dependence with withdrawal seizures or delirium". Biological Psychiatry. 41 (3): 299–304. doi:10.1016/S0006-3223(96)00044-3. PMID9024952. S2CID42947314.