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To elucidate the mechanism of 6-hydroxydopamine (6-OHDA)-induced Zn2+ toxicity, which is involved in neurodegeneration in the substantia nigra pars compacta (SNpc) of rats, we postulated that intracellular hydrogen peroxide (H2O2) produced by 6-OHDA is a trigger for intracellular Zn2+ dysregulation in the SNpc. Intracellular H2O2 level in the SNpc elevated by 6-OHDA was completely inhibited by co-injection of GBR 13069 dihydrochloride (GBR), a dopamine reuptake inhibitor, suggesting that 6-OHDA taken up through dopamine transporters produces H2O2 in the intercellular compartment of dopaminergic neurons. When the SNpc was perfused with H2O2, H2O2 accumulated glutamate in the extracellular compartment and the accumulation was inhibited in the presence of N-(p-amylcinnamoyl)anthranilic acid (ACA), a blocker of the transient receptor potential melastatin 2 (TRPM2) channels. In addition to 6-OHDA, H2O2 also induced intracellular Zn2+ dysregulation via AMPA receptor activation followed by nigral dopaminergic degeneration. Furthermore, 6-OHDA-induced nigral dopaminergic degeneration was completely inhibited by co-injection of HYDROP, an intracellular H2O2 scavenger or GBR into the SNpc. The present study indicates that H2O2 is produced by 6-OHDA taken up through dopamine transporters in the SNpc, is retrogradely transported to presynaptic glutamatergic terminals, activates TRPM2 channels, accumulates glutamate in the extracellular compartment, and induces intracellular Zn2+ dysregulation via AMPA receptor activation, resulting in nigral dopaminergic degeneration. It is likely that intracellular H2O2, but not extracellular H2O2, is a key trigger for nigral dopaminergic degeneration via intracellular Zn2+ dysregulation.

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