Abstract

To characterize further the molecular mechanism whereby 6-hydroxydopamine (6-OHDA) exerts its cytotoxic effects, a series of 2-and/or 5-methylated derivatives of 6-OHDA was prepared. These analogs exhibit oxidation-reduction potentials similar to that of 6-OHDA, but the corresponding quinones showed different reactivities toward nucleophiles in vitro. The quinone forms of 6-OHDA and 5-methyl-6-OHDA reacted with glutathione, whereas 2-methyl-6-OHDA quinone reacted very slowly, if at all, and 2,5-dimethyl-6-OHDA quinone was unreactive. In vivo both 6-OHDA and 5-methyl-6-OHDA were shown to produce long-term depletion of norepinephrine in mouse brain, whereas 2-methyl-6-OHDA and 2,5-dimethyl-6-OHDA were substantially less active. Only 6-OHDA produced long-term depletion of dopamine. These compounds were also tested for their abilities to inhibit uptake or cause release of [³H]norepinephrine or [³H]dopamine, using chopped rat cerebral cortex or corpus striatum. The observed order of potencies for both release and inhibition of uptake was 6-OHDA > 2-methyl-6-OHDA ≅ 5-methyl-6-OHDA > 2,5-dimethyl-6-OHDA. The differences in potency between 6-OHDA and 5-methyl-6-OHDA in producing long-term depletion of catecholamines can be explained on the basis of differences in affinities for the uptake systems. However, the differences in potency between 5-methyl-6-OHDA and 2-methyl-6-OHDA in producing long-term norepinephrine depletion cannot be rationalized on a similar basis. A more reasonable explanation for this difference is that 5-methyl-6-OHDA quinone can react with cellular nucleophiles, whereas 2-methyl-6-OHDA quinone cannot. Therefore 5-methyl-6-OHDA produces neuronal destruction through a mechanism similar to that of 6-OHDA, that being generation of a reactive electrophilic species, as well as hydrogen peroxide, superoxide anion, and hydroxyl radical. In contrast, the neurotoxicity produced by 2-methyl-6-OHDA probably results only from the effects of hydrogen peroxide, superoxide anion, and hydroxyl radical.