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 dopamine 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 voltammetry. 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 μM. 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 μM and 15.7 ± 2.57 pmol/(s × 10⁶ 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 dopamine to hDAT (3.5 fold to 19.2 × 10⁶ 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.