Methylphenidate-induced increases in vesicular dopamine sequestration and dopamine release in the striatum: the role of muscarinic and dopamine D2 receptors.

Methylphenidate (MPD) administration alters the subcellular distribution of vesicular monoamine transporter-2 (VMAT-2)-containing vesicles in rat striatum. This report reveals previously undescribed pharmacological features of MPD by elucidating its receptor-mediated effects on VMAT-2-containing vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles) and on striatal dopamine (DA) release. MPD administration increased DA transport into, and decreased the VMAT-2 immunoreactivity of, the membrane-associated vesicle subcellular fraction. These effects were mimicked by the D2 receptor agonist quinpirole and blocked by the D2 receptor antagonist eticlopride. Both MPD and quinpirole increased vesicular DA content. However, MPD increased, whereas quinpirole decreased, K(+)-stimulated DA release from striatal suspensions. Like MPD, the muscarinic receptor agonist, oxotremorine, increased K(+)-stimulated DA release. Both eticlopride and the muscarinic receptor antagonist scopolamine blocked MPD-induced increases in K(+)-stimulated DA release, whereas the N-methyl-d-aspartate receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) was without effect. This suggests that D2 receptors mediate both the MPD-induced redistribution of vesicles away from synaptosomal membranes and the MPD-induced up-regulation of vesicles remaining at the membrane. This results in a redistribution of DA within the striatum from the cytoplasm into vesicles, leading to increased DA release. However, D2 receptor activation alone is not sufficient to mediate the MPD-induced increases in striatal DA release because muscarinic receptor activation is also required. These novel findings provide insight into the mechanism of action of MPD, regulation of DA sequestration/release, and treatment of disorders affecting DA disposition, including attention-deficit hyperactivity disorder, substance abuse, and Parkinson's disease.