Intracellular Hydrogen Peroxide Produced by 6-hydroxydopamine is a Trigger for Nigral Dopaminergic Degeneration of Rats Via Rapid Influx of Extracellular Zn

To elucidate the mechanism and significance of 6-hydroxydopamine (6-OHDA)-induced Zn toxicity, which is involved in neurodegeneration in the substantia nigra pars compacta (SNpc) of rats, we postulated that intracellular hydrogen peroxide (HO) produced by 6-OHDA is a trigger for intracellular Zn dysregulation in the SNpc. Intracellular HO level elevated by 6-OHDA in the SNpc 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 HO in the intercellular compartment of dopaminergic neurons. When the SNpc was perfused with HO, glutamate accumulated 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, HO also induced intracellular Zn dysregulation via AMPA receptor activation followed by nigral dopaminergic degeneration. Furthermore, 6-OHDA-induced nigral dopaminergic degeneration was completely inhibited by co-injection of either HYDROP, an intracellular HO scavenger or GBR into the SNpc. The present study indicates that HO 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 Zn dysregulation via AMPA receptor activation, resulting in nigral dopaminergic degeneration prior to movement disorder. It is likely that intracellular HO, but not extracellular HO, is a key trigger for nigral dopaminergic degeneration via intracellular Zn dysregulation.