Dopamine transporter DAT

The principal mechanism for terminating dopamine signaling is reuptake by the presynaptic neuron via the dopamine transporter (DAT). Dopamine that is not taken up is metabolized by the enzymes monoamine oxidase (MAO) and catechol-O-methyl transferase... 

The main target of action of methylphenidate, the most widespread clinically used psychostimulant, is the dopamine transporter (DAT) its inhibition increases intrasynaptic dopamine concentrations. The subcortical dopamine system (mesolimbic and nigrostriatal parts) mediates the unconditioned and conditioned responses toward reinforcement. 

Following the release of dopamine, the primary mode of removal from the synapse is reuptake into the presynaptic neuron via the dopamine transporter (DAT). DAT is dependent upon the energy created by the Na+/K+ pump and is a member of the Na+/Cl -dependent plasma membrane transporter family, as are the norepinephrine and 7-aminobutyric acid (GABA) transporters. Imaging studies utilizing compounds with highly specific affinity for DAT... 

DA TRANSPORTER AND COCAINE DEPENDENCE 5.2.1 Regulation of the Dopamine Transporter (DAT) by Cocaine... 

Many neurotransmitters are inactivated by a combination of enzymic and non-enzymic methods. The monoamines - dopamine, noradrenaline and serotonin (5-HT) - are actively transported back from the synaptic cleft into the cytoplasm of the presynaptic neuron. This process utilises specialised proteins called transporters, or carriers. The monoamine binds to the transporter and is then carried across the plasma membrane it is thus transported back into the cellular cytoplasm. A number of psychotropic drugs selectively or non-selectively inhibit this reuptake process. They compete with the monoamines for the available binding sites on the transporter, so slowing the removal of the neurotransmitter from the synaptic cleft. The overall result is prolonged stimulation of the receptor. The tricyclic antidepressant imipramine inhibits the transport of both noradrenaline and 5-HT. While the selective noradrenaline reuptake inhibitor reboxetine and the selective serotonin reuptake inhibitor fluoxetine block the noradrenaline transporter (NAT) and serotonin transporter (SERT), respectively. Cocaine non-selectively blocks both the NAT and dopamine transporter (DAT) whereas the smoking cessation facilitator and antidepressant bupropion is a more selective DAT inhibitor. 

The neuronal membrane norepinephrine transporter (NET), the dopamine transporter (DAT) and the vesicular membrane transporter (VMAT-2), which is the same in all catecholamine-containing neurons, have similar numbers of predicted transmembrane segments. They have different numbers of amino acids, pharmacological properties and chromosomal localizations. 

Monoamine reuptake inhibitors elevate extracellular levels of serotonin (5-HT), norepinephrine (NE) and/or dopamine (DA) in the brain by binding to one or more of the transporters responsible for reuptake, namely the serotonin transporter (SERT), the norepinephrine transporter (NET) and the dopamine transporter (DAT), thereby blocking the reuptake of the neurotransmitter(s) from the synaptic cleft [1], Monoamine reuptake inhibitors are an established drug class that has proven utility for the treatment of a number of CNS disorders, especially major depressive disorder (MDD). 

N. V. Chemuturi, J. E. Haraldsson, T. Prisinzano, and M. Donovan. Role of dopamine transporter (DAT) in dopamine transport across the nasal mucosa. Life Sci 79 1391-1398 (2006). 

MPTP) and sodium benzoate induce a strong downregulation in the expression of tyrosine hydroxylase mRNA and in the tyrosine hydroxylase-positive cells in the ventral diencephalon. These chemicals also decreased the expression of the dopamine transporter (DAT), a membrane transport protein involved in dopamine reuptake, that is, a specific marker of dopaminergic neurons [9]. 

A prominent radiotracer represented in the first group is 6-[ F]fluorodopamine (6-[ F]FDA). This compound was developed from the knowledge that dopamine binds not only to the dopamine transporter (DAT) but also to NET with high binding affinity [138]. 

Dopamine is removed from the synapse via two mechanisms. First, COMT degrades intrasynaptic DA. Second, the dopamine transporter (DAT) [see (4) in Fig. 2.9], a Na /CD-dependent neurotransmitter transporter, transports DA in either direction, depending on the concentration gradient. The DAT is blocked selectively by drugs such as cocaine, amphetamine, bupropion, and nomifensine. 

Mechanism of action of cocaine and amphetamine on synaptic terminal of dopamine (DA) neurons. Left Cocaine inhibits the dopamine transporter (DAT), decreasing DA clearance from the synaptic cleft and causing an increase in extracellular DA concentration. Right Since amphetamine (Amph) is a substrate of the DAT, it competitively inhibits DA transport. In addition, once in the cell, amphetamine interferes with the vesicular monoamine transporter (VMAT) and impedes the filling of synaptic vesicles. As a consequence, vesicles are depleted and cytoplasmic DA increases. This leads to a reversal of DAT direction, strongly increasing nonvesicular release of DA, and further increasing extracellular DA concentrations. 

Starting in 1993, reports have appeared that in rat striatum and nucleus accum-bens D2-like autoreceptors, when activated, enhanced removal of dopamine from the extracellular space by means of the dopamine transporter (DAT) as studied by voltammetry (Table2). D2 receptor activation also increased [3H]-dopamine uptake in Xenopus oocytes transfected with both the human D2 receptor and the human DAT (Mayfield and Zahniser 2001), as well as the uptake of a fluorescent DAT substrate in human embryonic kidney cells equally transfected with both the human D2 receptor and the human DAT (Bolan et al. 2007). In accord with this autoreceptor-transporter connection, clearence of dopamine from the striatal extracellular space in vivo was reduced in D2 receptor knockout mice (Dickinson et al. 1999). Unfortunately, however, the data in the literature are by no means consistent. No effect of D2 receptor activation on the DAT was observed in another rat striatum study (Prasad and Amara 2001), in synaptosomes from rat and human neocortex (Feuer-stein, unpublished observations), and in PC 12 cells that possess D2 receptors and the DAT (Pothos et al. 1998). In a second study with D2 receptor knockout mice, uptake of dopamine in the striatum was increased rather than decreased (Schmitz et al. 2002). The reason for these discrepancies is not clear. It has been suggested that the clearance measurements by means of voltammetry were not appropriate for assessment of the function of the DAT (Prasad and Amara 2001). 

Methamphetamine (MAP) is a psychostimulant that induces enhanced arousal and euphoria acutely, and psychosis and addiction chronically. MAP enters the terminals/neuron via the monoamine transporters (dopamine transporter DAT, serotonin transporter SERT, or norepinephrine transporter NET), displaces... 

In the rat, the dorsal tier includes cells of the dorsal parts of the VTA and SNc and cells of the RRA innervating the limbic portion of the striatum and limbic cortical fields, as well as the ventral basal forebrain structures, such as the olfactory tubercle and the amygdala. Neurons of the dorsal tier are mostly fusiform, with dendrites oriented horizontally in the mediolateral plane of the SNc. From the neurochemical point of view, neurons of the dorsal tier contain relatively low levels of TH mRNA and dopamine transporter (DAT) mRNA, and the calcium binding protein calbindin is colocalized with DA in most dorsal tier neurons (Gerfen, 1985) (Figs. 4D 13C,D). 

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