The Parkinsonism-inducing drug 1-methyl-4-phenylpyridinium triggers intracellular dopamine oxidation - A novel mechanism of toxicity

Uptake of the Parkinsonism-inducing toxin, 1-methyl-4-phenylpyridinium (MPP+), into dopaminergic terminals is thought to block Complex I activity leading to ATP loss and overproduction of reactive oxygen species (ROS). The present study indicates that MPP+-induced ROS formation is not mitochondrial in origin but results from intracellular dopamine (DA) oxidation. Although a mean lethal dose of MPP+ led to ROS production in identified dopaminergic neurons, toxic doses of the Complex I inhibitor rotenone did not. Concurrent with ROS formation, MPP+ redistributed vesicular DA to the cytoplasm prior to its extrusion from the cell by reverse transport via the DA transporter. MPP+-induced DA redistribution was also associated with cell death. Depleting cells of newly synthesized and/or stored DA significantly attenuated both superorxide production and cell death, whereas enhancing intracellular DA content exacerbated dopaminergic sensitivity to MPP+. Lastly, depleting cells of DA in the presence of succinate completely abolished MPP+-induced cell death. Thus, MPP+ neurotoxicity is a multi-component process involving both mitochondrial dysfunction and ROS generated by vesicular DA displacement. These results suggest that in the presence of a Complex I defect, misregulation of DA storage could lead to the loss of nigrostriatal neurons in Parkinson's disease.