Uptake of 1-methyl-4-phenylpyridinium ion (MPP+) and ATP content in synaptosomes

Symptoms such as those in Parkinson's disease are known to be induced by the neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). We tried to quantitatively measure synaptosomal MPP+ uptake using an MPP+ selective electrode to study the correlation between MPP+ uptake and respiratory inhibition. Synaptosomal MPP+ uptake was low but could be increased by the addition of glucose as an energy substrate, or increased with an increase in the concentration of MPP+. The rate of uptake was 0.2 nmol/mg protein/min at 50 microM MPP+. Tetraphenylboron (TPB+), which enhances cation permeability, increased MPP+ uptake, and the increase was proportional to the TPB+ concentration. When external MPP+ concentration was increased above 200 microM, ATP was depleted and the uptake of MPP+ decreased, which resulted in the release of intrasynaptosomal MPP+. MPP+ uptake was also decreased by depolarization of the membrane potential in synaptosomes. MPP+ was presumed to be distributed across both the synaptosomal and inner mitochondrial membranes, and to be affected by membrane potential as a lipophilic cation. When respiration of the inner mitochondria was inhibited by increasing the intrasynaptosomal MPP+ concentration, the concentration of MPP+ in cytosol was presumed to increase by the release of MPP+ from the mitochondria, and synaptosomal MPP+ uptake would then be decreased.