Differential effects of Zn2+ on the kinetics and cocaine inhibition of dopamine transport by the human and rat dopamine transporters

Zn2+ may play a major role in the modulation of neurotransmission because it modulates membrane receptors and channels. Recent literature has shown Zn2+ inhibits doparnine transport by the dopamine transporter (DAT), the main target of cocaine and some other drugs of abuse. Cocaine inhibits DAT and modulation of the DAT by Zn2+ may alter effects of cocaine on dopamine neurotransmission. This study investigates how Zn2+ changes DAT kinetics and its inhibition by cocaine. Steady-state and pre-steady-state kinetics of DAT activity were investigated using rotating disk electrode voltarnmetry. Values of K-M and V-max in hDAT and effects of cocaine match those in the literature. Zn2+ allosterically inhibited transport in the human DAT (hDAT) with a K-I=7.9 +/- 0.42 mu M. Removal of endogenous Zn2+ with penicillamine in hDAT increased transport values. In contrast, Zn2+ did not alter transport by rat DAT (rDAT),with K-M and V-max values of 1.2 0.49 mu M and 15.7 +/- 2.57 pmol/(s x 10(6) cells), respectively, and removal of Zn2+ did not increase dopamine transport values. Zn2+ allosterically reduced the inhibition by cocaine in hDAT. Results of pre-steady-state studies demonstrated that Zn2+ increases the second order binding rate constant for doparnine to hDAT (3.5 fold to 19.2 x 10(6) M-1 s(-1) for hDAT). In rat striatal homogenates Zn2+ increased initial dopamine transport velocity and decreased cocaine inhibition providing evidence for differences in sensitivity to Zn2+ between the three different preparations. Modulation of the DAT by Zn2+ needs to be assessed further in development of cocaine antagonists. (c) 2007 Elsevier B.V. All rights reserved.