A MULTISUBSTRATE KINETIC MECHANISM OF DOPAMINE TRANSPORT IN THE NUCLEUS-ACCUMBENS AND ITS INHIBITION BY COCAINE

Kinetic studies of dopamine transport into suspensions of nucleus accu mbens (NAcc) and effects of Na+ and Cl- as cosubstrates were performed using rotating disk electrode voltammetry, To mimic chemical neurotra nsmission, dopamine was added as a rapid pulse, and transporter-mediat ed clearance of dopamine was evaluated kinetically. This paradigm was shown to approximate a zero trans entry transport experiment, Dopamine was taken up with apparent K-m and V-max values of 1.3 mu M and 375 p mol/s/g wet weight, respectively, Transport exhibited apparent trans a cceleration, Substitution of Na+ with choline or Cl- with isethionate reduced dopamine transport with reaction orders of two and unity, resp ectively, accompanied by reductions in V-max with no changes in K-m. A pparent K-Na and K-Cl values were 70.0 and 92.1 mM, respectively. Dopa mine transport in NAcc was found to follow a partially random, sequent ial mechanism in which dopamine and Na+ bind randomly to the transport er followed by binding of Cl- before transport, Cocaine inhibited dopa mine transport and the influences of the other substrates allosterical ly with an overall K-i of 0.30 mu M. Thus, the general kinetic mechani sm of the transport of dopamine in the NAcc is identical to that previ ously reported by this laboratory for dopamine transport in the striat um. However, the dopamine transporter in the NAcc is more tightly regu lated 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 [subst rate], thereby enhancing dopaminergic signaling more robustly in the N Acc than in the striatum.