Mechanism of Accumulation of the 1-methyl-4-Phenylpyridinium Species into Mouse Brain Synaptosomes

The mechanism of accumulation of 1-methyl-4-phenylpyridinium ion (MPP+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, into neuronal terminals was studied using mouse brain synaptosomes as an in vitro model. Addition of MPP+ to synaptosomal preparations, essentially devoid of contamination by extrasynaptosomal mitochondria, resulted in its time- and concentration-dependent accumulation. Intrasynaptosomal concentrations of 79 and 106 microM were reached 10 and 30 min, respectively, after addition of 50 microM MPP+. The accumulation of 50 microM MPP+ into synaptosomes was only slightly affected by the catecholamine uptake blockers mazindol and nomifensine; in contrast, it was markedly enhanced by tetraphenylborate, a lipophilic anion that increases the rate of accumulation of permeant cations via a Nernstian concentration gradient, MPP+ accumulation was significantly increased or decreased as a consequence of hyperpolarization or depolarization, respectively, of the plasma membrane of synaptosomes. This effect was evident after incubation for 10 min. Changes in mitochondrial membrane potential also affected MPP+ accumulation, although only after 30 min of incubation. Data indicate that polarization of neuronal membranes may significantly contribute to the accumulation of MPP+ into nerve terminals.