The role of iron in senescence of dopaminergic neurons in Parkinson's disease.

In Parkinson's disease (PD) an elevation of iron with staging of the disease has been observed in the substantia nigra (SN), especially the zona compacta (ZC). The iron is found to be present in glia, active microglia, macrophages, oligodendrocytes, outside the degenerated dopamine neurons and as a mild halo around Lewy bodies and within melanized dopamine neurons of SNZC. Although in control brains iron is absent in melanized dopamine neurons, in PD it is bound to neuromelanin in a fashion similar to the interaction of iron with synthetic dopamine-melanin. The iron in SNZC is thought to induce oxidative stress and thus be associated with the reported decreases of glutathione peroxidase activity, reduced glutathione (GSH), mitochondrial Complex I activity, calcium binding protein and increase of basal lipid peroxidation. An animal (rat) model of PD has been described in which intranigral iron injection induces a relatively specific lesioning of dopamine neurons resulting in behavioural and biochemical Parkinsonism in rats. Support for the neurotoxicity of iron liberated from an endogenous source has come from the 6-hydroxydopamine model of PD. This neurotoxin is thought to owe its toxicity to the liberation of iron from ferritin, which in turn alters the homeostasis of mitochondrial Ca2+ with the subsequent depletion of tissue GSH, resulting in oxidative stress. Pretreatment of rats with intraventricular injection of a relatively selective prototype iron chelator, desferrioxamine (desferal), attenuates the 6-hydroxydopamine lesion of nigrostriatal dopamine. Thus iron can fulfill the role of a neurotoxin. However it remains to be established whether its role in PD is primary or secondary to some other neurotoxic event.