ReviewTrafficking of dopamine transporters in psychostimulant actions
Introduction
Brain dopamine (DA) has long been known to play a pivotal role in reward [1]. Much evidence supports the importance of DA, as well as several other key neurotransmitters like glutamate, in drug reward, reinforcement and addiction (e.g. [2], [3], [4], [5]). The addiction-related actions of DA are mediated by two major midbrain projection pathways: (i) mesocorticolimbic DA neurons with cell bodies (somatodendritic regions/compartments) in the ventral tegmental area (VTA) and projecting to the medial prefrontal cortex and nucleus accumbens (NAc, or ventral striatum) and (ii) nigrostriatal DA neurons with cell bodies in the substantia nigra pars compacta (SNc) and projecting to the dorsal striatum (dSTR). Mesolimbic DA neurons and elevated DA in NAc are important for motivated drug taking, learned associations and behavioral reinforcement, all of which are essential for initiation of drug taking and development of addiction [2], [6]. Increased mesolimbic DA transmission is necessary and sufficient for cocaine reinforcement, and this system links reward and actions [7], [8]. DA in dSTR also plays a critical, but distinct, role in habit learning; this is involved in cue-induced craving, cocaine-seeking and compulsive behaviors that contribute to long-term drug taking and addiction [9], [10], [11], [12].
DA neurons fire in both tonic and phasic/burst firing patterns, exocytotically releasing DA from vesicles into the synaptic cleft where it diffuses and activates its receptors. DA receptors include postsynaptic D1-like receptors (D1Rs and D5Rs) and D2-like receptors (D2Rs, D3Rs, D4Rs), as well as presynaptic D2-like autoreceptors on the DA neurons themselves. Both D1- and D2-like classes of DA receptors are G protein-coupled receptors (GPCRs) that signal in a relatively slower manner.
The DA transporter (DAT), localized exclusively to DA neurons [13], is the primary mechanism for limiting/terminating DA neurotransmission [14]. DAT is a member of the Solute Carrier 6 gene family (SLC6A3) that encodes several Na+/Cl−-dependent neurotransmitter transporters (neurotransmitter:sodium symporter family) – including transporters for norepinephrine, serotonin, GABA, glycine (see review [15]). DAT, like the other mammalian members of this family of transporters, has intracellular amino- and carboxyl-termini and 12 transmembrane domains (TMs; Fig. 1). These transporters are active when present on the plasma membrane. DAT-mediated DA inward translocation (uptake) uses the neuronal electrochemical gradient to pump DA from the extracellular space back into DA neurons, thereby limiting the temporal and spatial interaction of DA with its receptors. From the neuronal cytoplasm, DA is either taken up into synaptic vesicles by the proton antiporter vesicular monoamine transporter-2 (VMAT-2) or metabolized by monoamine oxidase. Psychomotor stimulant drugs like cocaine and amphetamines produce their activating and addictive effects by interacting with DAT and thereby increasing extracellular DA concentrations [14], [16], [17], [18]. However, their acute mechanisms of action differ: cocaine blocks DAT-mediated uptake of DA whereas amphetamines are substrates for DAT (and VMAT-2) and thereby promote DAT-mediated DA efflux, or Na+-dependent non-vesicular DA release.
DAT activity can be regulated both acutely (minutes to hours) and longer-term (days) (see [18], [19], [20], [21]). As opposed to rapid changes in stimulation-evoked DA release, regulation of DATs occurs more slowly. Nonetheless, it still represents a complementary mechanism by which DA neurotransmission can be fine-tuned. DAT regulation can occur via altered substrate/Na+/Cl− affinity, kinetic activation, trafficking to/from the plasma membrane, and changes in protein turnover. This review will focus on DAT trafficking, which appears to be a common mechanism for acute, dynamic regulation of DAT activity and its potential contribution to psychostimulant drug actions and abuse.
Section snippets
Trafficking of newly-synthesized DAT
Like all transmembrane proteins, DAT is synthesized at the endoplasmic reticulum (ER) membrane, co-translationally translocated through the ER membrane and eventually packaged into COP (coatomer) II vesicles for its anterograde transport to the Golgi apparatus. Since DAT is proposed to be N-glycosylated in the extracellular loop 2 (EL2) (Fig. 1) [22], the transporter must pass through the Golgi to acquire this post-translational modification. From the trans-Golgi network, DAT traffics to the
Regulation of DAT by endocytic trafficking
Transmembrane proteins, that have been delivered through the biosynthetic pathway to the plasma membrane, undergo constitutive and regulated endocytosis. After endocytosis, internalized cargo can be recycled back from endosomes to the plasma membrane, or sorted to lysosomes or other compartments. It is logical to assume that because DAT functions at the cell surface, its constitutive endocytosis is expected to be much slower than, for example, the endocytosis of VMAT-2 that functions in
Regulation of DAT trafficking by intracellular signaling and protein interactions
In addition to PKC-dependent endocytosis, a number of other mechanisms and signaling pathways have been proposed to regulate DAT trafficking. Activity of MAPK/ERK1/2 appears to be important for maximal plasma membrane localization of DAT since inhibition of ERK1/2 activity results in down-regulation of DAT in heterologous expression systems, primary cultures of DA neurons and striatal synaptosomes [47], [49], [50]. Interestingly, MAPK phosphatase MKP-3 appears to have a negative role in
Psychostimulant drugs – cross-talk with DAT trafficking
Most of the evidence that addictive drugs can alter DAT trafficking comes from studies using psychostimulants that interact directly with DAT, viz. amphetamine, methamphetamine and cocaine. Not surprisingly, DAT substrates like amphetamines regulate DAT cell surface number in an opposite manner to DAT inhibitors like cocaine. Thus, acute exposure to amphetamine or methamphetamine reduces surface DATs whereas cocaine increases them (Fig. 3). These studies are discussed below, along with the few
Acknowledgement
We gratefully acknowledge the support by NIDA (R01 DA014204, R37 DA004216 and K05 DA015050).
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