Kinetic studies of dopamine transport into suspensions of nucleus accumbens (NAcc) and effects of Na + and Cl -as cosubstrates were performed using rotating disk electrode voltammetry. To mimic chemical neurotransmission, dopamine was added as a rapid pulse, and transporter-mediated clearance of dopamine was evaluated kinetically. This paradigm was shown to approximate a zero trans entry transport experiment. Dopamine was taken up with apparent Km and Vmax values of 1.3 1iM and 375 pmol/s/g wet weight, respectively.Transport exhibited apparent trans acceleration. Substitution of Nawith choline or CL with isethionate reduced dopamine transport with reaction orders of two and unity, respectively, accompanied by reductions in Vmax with no changes in Km. Apparent KNa and K~1values were 70.0 and 92.1 mM, respectively. Dopamine transport in NAcc was found to follow a partially random, sequential mechanism in which dopamine and Na~bind randomly to the transporter followed by binding of Cl -before transport. Cocaine inhibited dopamine transport and the influences of the other substrates allosterically with an overall Kõ f 0.30~sM.Thus, the general kinetic mechanism of the transport of dopamine in the NAcc is identical to that previously reported by this laboratoryfor dopamine transport in the striatum. However, the dopamine transporter in the NAcc is more tightly regulated by Na~, possesses a higher kinetic turnover rate, is four times more sensitive to cocaine than the striatal transporter, and exhibits cocaine inhibition independent of [substrate]. These findings suggest that cocaine modulates chemical signaling in NAcc differently than in striatum, providing down-regulation of function irrespective of [substrate],thereby enhancing dopaminergic signaling more robustly in the NAcc than in the striatum. Key Words: Cocaine-Dopamine-GBR 1 2909-RTI-1 20-Inhibition of transportNucleus accumbens-Rotating disk voltammetry-Kinetic mechanism of transport. J. Neurochem. 69, 1093Neurochem. 69, -1105Neurochem. 69, (1997.The effect of cocaine to produce self-administration behaviors in rodents has been correlated with its binding to the neuronal transporter for dopamine (Ritz et al., 1987). Thus, it has been hypothesized that inhibition of the reuptake of dopamine, resulting in tonically increased synaptic levels of dopamine, mediates addictive behaviors. The anatomical locus for mediating addiction could include individual or combinations of dopaminergic areas of the brain and at pre-and postsynaptic loci. However, recent evidence suggests that the behavioral influences of cocaine, via its primary effects on the dopaminergic transporter, may reside in the mesolimbic areas of the brain, most notably the nucleus accumbens (NAcc) (Koob and Bloom, 1988;Wise and Hoffman, 1992;Chang et al., 1994;Hitri et al., 1994). Although increased pre-and postsynaptic receptor occupation by dopamine in time and/or duration is the gross result of cocaine inhibition of transporter function, it has not been established whether differences ...
