Blockade of Rapid Influx of Extracellular Zn2+ into Nigral Dopaminergic Neurons Overcomes Paraquat-Induced Parkinson's Disease in Rats

The herbicide paraquat (PQ) has been reported to enhance the risk of developing Parkinson's disease (PD) from epidemiological studies. PQ-induced reactive oxygen species (ROS) are linked with a selective loss of nigrostriatal dopaminergic neurons. Here, we first report a unique mechanism of nigrostriatal dopaminergic degeneration, in which rapid intracellular Zn2+ dysregulation via PQ-induced ROS production causes PD in rats. When the substantia nigra pars compacta (SNpc) of rats was perfused with PQ, extracellular concentrations of glutamate and Zn2+ were increased and decreased, respectively, in the SNpc. These changes were ameliorated by co-perfusion with Trolox, an antioxidative agent. In in vitro slice experiments, PQ rapidly increased extracellular Zn2+ influx via AMPA receptor activation. Both loss of nigrostriatal dopaminergic neurons and increase in turning behavior in response to apomorphine were markedly reduced by coinjection of PQ and intracellular Zn2+ chelator, i.e., ZnAF-2DA into the SNpc. Furthermore, loss of nigrostriatal dopaminergic neurons induced with a low dose of PQ, which did not induce any behavioral abnormality, was completely blocked by coinjection of ZnAF-2DA. The present study indicates that rapid influx of extracellular Zn2+ into dopaminergic neurons via AMPA receptor activation, which is initially induced by PQ-mediated ROS production in the SNpc, induces nigrostriatal dopaminergic degeneration, resulting in PQ-induced PD in rats. Intracellular Zn2+ dysregulation in dopaminergic neurons is the cause of PQ-induced pathogenesis in the SNpc, and the block of intracellular Zn2+ toxicity leads to defending PQ-induced pathogenesis.