| Parkinson's disease (PD) is neurodegenerative condition resulting from a loss of dopaminergic neurons in the substantia nigra. Even though the etiology of PD is unknown, evidence from post-mortem studies indicates that both oxidative stress and energy impairment are involved. In order to decipher the mechanisms underlying dopaminergic cell death in PD, animal models of the disease have been widely utilized. For instance, the parkinsonism-inducing toxin 1-methyl-4-phenyl 1,2,3,6 tetrahydropyridine (MPTP) is known to trigger dopaminergic cell loss in rodents, monkeys, and humans. Its selectivity is ensured by the uptake of its bioactivated product, 1-methyl-4-phenylpyridinium (MPP+), into dopaminergic terminals by the plasma membrane dopamine transporter. Thus, understanding its mechanisms) of MPTP toxicity could provide significant insights into the etiology of PD. Using mouse primary dopaminergic neurons, we show that MPP+ triggers an atypical, non-apoptotic form of neuronal death that can be partially rescued by the potent antioxidant, carboxyfullerene C3. In keeping with this finding, we demonstrate, via the use of redox-sensitive fluorescent probes, that MPP+ rapidly induces the formation of reactive oxygen species (ROS), specifically superoxide anions. These ROS arise from the oxidation of dopamine, which is quickly redistributed from its storage sites by the vesicular uptake of MPP+. MPP + also leads to ATP depletion by blocking electron transport at Complex I, as previously shown in isolated mitochondrial preparations. ROS formation precedes ATP loss by several hours. In addition, MPP+-induced cell death can be completely blocked by the combined addition of dopamine-depleting agents and energy supplements acting downstream of Complex I. Our studies are the first to elucidate the pathways underlying MPP+-induced cell death in primary dopaminergic neurons and provide strong evidence that dopamine exacerbates dopaminergic cell death in response to exogenous toxins and perhaps even in PD. |
