Received for publication September 16, 2008.
Revised November 24, 2008.
Accepted for publication November 25, 2008.

Cocaine Alters Vesicular Dopamine Sequestration and Potassium-Stimulated Dopamine Release: The Role of D2 Receptor Activation

Abstract

Cocaine is a psychostimulant that inhibits the inward transport of dopamine (DA) via the neuronal DA transporter (DAT), thereby increasing DA concentrations in the synaptic cleft. Cocaine administration also causes a redistribution of striatal vesicular monoamine transporter-2 (VMAT-2)-containing vesicles that co-fractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles) to a non-membrane-associated, cytoplasmic subcellular fraction. Whereas previous studies from our laboratory have focused on the impact of cocaine on cytoplasmic vesicles, the present report describes the pharmacological effects of cocaine on the membrane-associated vesicle population. Results revealed that the redistribution of VMAT-2 and associated vesicles away from synaptosomal membranes is associated with a decrease in total DA transport and DA content in the membrane-associated VMAT-2-containing subcellular fraction. Cocaine also decreases the velocity and magnitude of K⁺-stimulated exocytotic DA release from whole striatal suspensions. The cocaine-induced VMAT-2 redistribution, decrease in DA release, and decrease in total DA transport are mediated by D2 receptors, as these events were prevented by pretreatment with the D2 receptor antagonist, eticlopride. These data suggest that following cocaine administration, D2 receptors are activated due to increased synaptic DA, resulting in a redistribution of DA-containing vesicles away from synaptosomal membranes, thus leading to less DA released after a depolarizing stimulus. These findings provide insight into not only the mechanism of action of cocaine, but also mechanisms underlying the regulation of dopaminergic neurons.

Key words: D2 receptor, cocaine, dopamine, potassium-stimulated release, rotating disk electrode voltammetry, vesicle