Brain-derived neurotrophic factor protects dopamine neurons against 6-hydroxydopamine and N-methyl-4-phenylpyridinium ion toxicity: involvement of the glutathione system.

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.

Deprenyl suppresses the oxidant stress associated with increased dopamine turnover.

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.

Dopamine turnover and glutathione oxidation: implications for Parkinson disease.

Parkinson disease is characterized by a major loss (approximately 80% or more) of dopaminergic nigrostriatal neurons and by an increased turnover of neurotransmitter by surviving neurons of the 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.

Exposure of striatal [corrected] synaptosomes to L-dopa increases levels of oxidized glutathione.

Incubation of striatal synaptosomes with L-dopa and glucose in either the presence or absence of 10 microM reserpine resulted in a rise in the level of oxidized glutathione (GSSG) within the isolated tissue pellet. The rise in GSSG was concentration dependent in the range of 0.04-1.0 mM L-dopa. With 1.0 mM L-dopa in the presence of reserpine, the GSSG level was elevated by 7.0 +/- 0.7 pmol/mg original striatal tissue, which corresponds to an increase of 38.0 +/- 4.5% compared with control. The rise in GSSG reflects an oxidative stress. The oxidation of dopamine by monoamine oxidase generates H2O2, which is normally detoxified by glutathione peroxidase to yield GSSG. In the presence of clorgyline or pargyline (two monoamine oxidase inhibitors), the rise in GSSG was suppressed by 88-92%, as was the formation of DOPAC. NSD-1055 and carbidopa (two inhibitors of dopa-decarboxylase) also significantly suppressed (50-60%) the rise in GSSG. These data show that the synthesis of dopamine from L-dopa, with subsequent catabolism of dopamine, can evoke a significant rise in the level of GSSG, which reflects the oxidant stress associated with monoamine oxidase activity.

Depletion of brain glutathione in preweanling mice by L-buthionine sulfoximine.

Previous studies indicated that DL-buthionine sulfoximine (DL-BSO), an agent that inhibits the biosynthesis of GSH in liver and other peripheral organs, fails to suppress levels of GSH in the CNS. In the current study, preweanling mice responded to repeated injections of L-BSO with marked declines (79.6-86.5%) of GSH content in brain and spinal cord. In adult mice, the same treatment schedule produced only modest declines (17.8-29.2%) of GSH content in brain and a 55.9% decline in spinal cord. Pretreatment of preweanling mice with L-BSO represents a tool for studying the role of GSH in the CNS.

Reduced and oxidized glutathione in human and monkey brain.

Prior animal studies have indicated that levels of oxidized glutathione (GSSG) in brain and other organs are low, comprising several percent, or less, of the total glutathione. An exception is seen in reports for autopsy and biopsy specimens of human brain, where very high levels of GSSG have been indicated. The latter observations imply an unusual redox state for human brain. In the current study, GSSG and reduced glutathione (GSH) were measured in monkey brain and autopsy specimens of human brain. Levels of GSSG were 1.2% or less, of the total glutathione. These results are similar to those for rodent brain, but disagree with earlier reports concerning human brain.