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Chronic peri-adolescent stress in humans increases risk to develop a substance use disorder during adulthood. Rats reared in social isolation during peri-adolescence (aSI; 1 rat/cage) period show greater ethanol and cocaine intake compared to group housed (aGH; 4 rats/cage) rats. In addition, aSI rats have a heightened dopamine response in the nucleus accumbens (NAc) to rewarding and aversive stimuli. Furthermore, single pulse electrical stimulation in slices containing NAc core elicits greater dopamine release in aSI rats. Here, we further investigated dopamine release kinetics and machinery following aSI. Dopamine release, across a wide range of stimulation intensities and frequencies, was significantly greater in aSI rats. Interestingly, subthreshold intensity stimulations also resulted in measurable dopamine release in accumbal slices from aSI but not aGH rats. Extracellular [Ca 2+ ] manipulations revealed augmented calcium sensitivity of dopamine release in aSI rats. The readily releasable pools of dopamine, examined by bath application of Ro-04-1284/000, a vesicular monoamine transporter 2 (VMAT2) inhibitor, were depleted faster in aGH rats. Western blot analysis of release machinery proteins (VMAT2, Synaptogyrin-3, Syntaxin-1, and Munc13-3) showed no difference between the two groups. Tyrosine hydroxylase (TH) protein expression levels, however, were elevated in aSI rats. The greater dopamine release could potentially be explained by higher levels of TH, the rate-limiting step for dopamine synthesis. This augmented responsivity of the dopamine system and heightened dopamine availability post-aSI may lead to an increased risk of addiction vulnerability.

The reinforcing efficacy of cocaine is thought to stem from inhibition of the dopamine transporter (DAT) and subsequent increases in extracellular dopamine concentrations in the brain. In humans, this hypothesis has generally been supported by positron emission tomography imaging studies where the percent of DATs occupied by cocaine is used as a measure of cocaine activity in the brain. Interpretation of these studies, however, often relies on the assumption that measures of DAT occupancy directly correspond with functional DAT blockade. In the current studies, we used in vivo and in vitro fast scan cyclic voltammetry in mice to measure dopamine uptake inhibition following varying doses of cocaine as well as two high affinity DAT inhibitors. We then compared dopamine clearance rates following these drug treatments to dopamine clearance obtained from DAT knockout mice as a proxy for complete DAT blockade. We found that administration of abused doses of cocaine resulted in approximately 2% of maximal DAT blockade. Overall, our data indicate that abused doses of cocaine produce a relatively modest degree of DA uptake inhibition, and suggest that the relationship between DAT occupancy and functional blockade of the DAT is more complex than originally posited.

Dopamine transporter (DAT) levels vary across brain regions and individuals, and are altered by drug history and disease states; however, the impact of altered DAT expression on psychostimulant effects in brain has not been systematically explored. Using fast scan cyclic voltammetry, we measured the effects of elevated DAT levels on presynaptic dopamine parameters as well as the uptake inhibition potency of the blockers cocaine and methylphenidate (MPH) and the releaser amphetamine (AMPH) in the nucleus accumbens core. Here we found that increases in DAT levels, resulting from either genetic overexpression or MPH self-administration, caused markedly increased maximal rates of uptake (Vmax) that were positively correlated with the uptake inhibition potency of AMPH and MPH, but not cocaine. AMPH and MPH were particularly sensitive to DAT changes, with a 100% increase in Vmax resulting in a 200% increase in potency. The relationship between Vmax and MPH potency was the same as that for AMPH, but was different from that for cocaine, indicating that MPH more closely resembles a releaser with regard to uptake inhibition. Conversely, the effects of MPH on stimulated dopamine release were similar to those of cocaine, with inverted U-shaped increases in release over a concentration-response curve. This was strikingly different from the release profile of AMPH, which showed only reductions at high concentrations, indicating that MPH is not a pure releaser. These data indicate that although MPH is a DAT blocker, its uptake-inhibitory actions are affected by DAT changes in a similar manner to releasers. Together, these data show that fluctuations in DAT levels alter the potency of releasers and MPH but not blockers and suggest an integral role of the DAT in the addictive potential of AMPH and related compounds.

A great deal of research has focused on investigating neurobiological alterations induced by chronic psychostimulant use in an effort to describe, understand, and treat the pathology of psychostimulant addiction. It has been known for several decades that dopamine neurotransmission in the nucleus accumbens is integrally involved in the selection and execution of motivated and goal-directed behaviors, and that psychostimulants act on this system to exert many of their effects. As such, a large body of work has focused on defining the consequences of psychostimulant use on dopamine signaling in the striatum as it relates to addictive behaviors. Here, we review presynaptic dopamine terminal alterations observed following self-administration of cocaine and amphetamine, as well as possible mechanisms by which these alterations occur and their impact on the progression of addiction.

Fast scan cyclic voltammetry in brain slices (slice voltammetry) has been used over the last several decades to increase substantially our understanding of the complex local regulation of dopamine release and uptake in the striatum. This technique is routinely used for the study of changes that occur in the dopamine system associated with various disease states and pharmacological treatments, and to study mechanisms of local circuitry regulation of dopamine terminal function. In the context of this Review, we compare the relative advantages of voltammetry using striatal slice preparations versus in vivo preparations, and highlight recent advances in our understanding of dopamine release and uptake in the striatum specifically from studies that use slice voltammetry in drug-naïve animals and animals with a history of psychostimulant self-administration.