Parkinson disease is characterized by a major loss (z8O% or more) of dopaminergic nigrostriatal neurons and by an increased turnover of neurotransmitter by surviving neurons ofthe nigrostriatal tract. In theory, increased turnover of dopamine should be associated with an oxidative stress derived from increased production of hydrogen peroxide. The peroxide is formed during the oxidative deamination of dopamine by monoamine oxidase. In experiments with mice, increased presynaptic turnover of dopamine was evoked by injection of reserpine, which interferes with the storage of dopamine in synaptic vesicles. Loss of dopamine and formation of deaminated metabolites were accompanied by a significant rise (87.8%) in the level of oxidized glutathione in brain. This change was observed in the striatum, which is richly innervated by dopamine terminals, but not in the frontal cortex, which receives a much sparser innervation by catecholamine nerve terminals. The rise in oxidized glutathione was seen even though dopamine terminals constitute only 1% or less of the mass of the striatum. Clorgyline, an inhibitor of monoamine oxidase type A, blocked the formation of oxidized glutathione. These observations confirm that a selective increase in neurotransmitter turnover within nigrostriatal nerve terminals can evoke a change in cellular redox status. We suggest that an oxidative stress may play a role in the natural history of Parkinson disease.GSSG reductaseNormally, GSSG is efficiently reduced by glutathione reductase (reduced NADP:oxidized-glutathione oxidoreductase, EC 1.6.4.2) and, as a consequence, levels of GSSG in various tissues, including brain (6), constitute only 1% or less of the total glutathione (GSH + GSSG). Levels of GSSG can rise, however, during exposure of tissues to added peroxides or peroxide-generating cell toxins (7-9). The ratio of oxidized to reduced glutathione reflects, in part, the redox state of the tissue.To evoke increased presynaptic turnover of dopamine, we used the drug reserpine. Reserpine prevents the storage of dopamine in neuronal transmitter vesicles by interfering with the transport of dopamine from the cytosol into the vesicles (10). As a result, the metabolism of dopamine by mitochondrial MAO is accelerated. The net effect is the disappearance of dopamine and the simultaneous formation of acidic metabolites (11,12). This action mimics increased neuronal activity, where increased release and reuptake of transmitter leads to increased presynaptic oxidation of dopamine. We assessed the effect of reserpine on the redox status of dopaminergic nerve terminals in the striatum by measuring changes in the levels of GSSG. Male Swiss-Webster mice (25-30 g; Ace Breeders, Boyertown, PA) received intraperitoneal injections of reserpine (10 mg/kg); control mice received the injection vehicle alone. Where indicated, mice received the MAO inhibitor, clorgyline (2.5 mg/kg, i.p.) 18 hr prior to injection of reserpine. Two hours after reserpine, the mice were rapidly decapitated, the striatum and fro...

Brain-derived neurotrophic factor (BDNF) has recently been shown to enhance the survival of dopamine neurons in cultures derived from the embryonic rat mesencephalon. We now extend this study by demonstrating that, in addition to the effect of sustaining survival of dopaminergic neurons, BDNF also confers protection against the neurotoxic effects of 6-hydroxydopamine (6-OHDA) and N-methyl-4-phenylpyridinium ion (MPP+). Exposure of mesencephalic cultures to either 6-OHDA or MPP+ resulted in a loss of 70-80% of dopaminergic neurons, as determined by tyrosine hydroxylase (TH) immunocytochemistry. In BDNF-treated cultures, loss of TH-positive cells after exposure to either toxin was reduced to only 30%. To facilitate biochemical measurements, we studied SH-SY5Y dopaminergic neuroblastoma cells. BDNF was found to protect these cells from the dopaminergic neurotoxins, 6-OHDA and MPP+. Indicative of oxidative stress, treatment of SH-SY5Y cells with 10 microM 6-OHDA for 24 h caused a fivefold increase in the levels of oxidized glutathione (GSSG). Pretreatment with BDNF for 24 h completely prevented the rise in GSSG. Further examination revealed that BDNF increased the activity of the protective enzyme, glutathione reductase, by 100%. In contrast, BDNF had no effect on the activity of catalase. These results add further impetus to exploring the therapeutic potential of BDNF in animal models of Parkinson's disease.

Tissue glutathione disulfide (GSSG) was studied as an index of changes in redox state in the striatum. When increased turnover of dopamine was provoked in mice by injection of haloperidol (1 mg/kg), the concentration of GSSG in the striatum tripled. Deprenyl (2.5 mg/kg) suppressed the rise in GSSG by 71.9%. These results indicate that deprenyl suppresses an oxidant stress associated with increased dopamine turnover.