Plants of the genus Polygala have been shown to possess protective effects against neuronal death and cognitive impairments in neurodegenerative disorders related to excitotoxicity. Moreover, previous reports from our group have shown the neuroprotective effects of the plant Polygala paniculata against methylmercury (MeHg)-induced neurotoxicity. In this work, we have examined the potential protective effects of three compounds (7-prenyloxy-6-methoxycoumarin, quercetin, and 1,5-dihidroxi-2,3-dimethoxy xanthone) from Polygala species against MeHg- and mercuric chloride (HgCl2)-induced disruption of mitochondrial function under in vitro conditions using mitochondrial-enriched fractions from mouse brain. MeHg and HgCl2 (10-100 microM) significantly decreased mitochondrial viability; this phenomenon was positively correlated to mercurial-induced glutathione oxidation. Among the isolated compounds, only quercetin (100-300 microM) prevented mercurial-induced disruption of mitochondrial viability. Moreover, quercetin, which did not display any chelating effect on MeHg or HgCl2, prevented mercurial-induced glutathione oxidation. The present results suggest that the protective effects of quercetin against mercurial-induced mitochondrial dysfunction is related to the removal of oxidant species generated in the presence of either MeHg or HgCl2. Reinforcing this hypothesis, MeHg and HgCl2 increased the production of hydrogen peroxide in the brain mitochondria, as well as the levels of malondialdehyde. These oxidative phenomena were prevented by co-incubation with quercetin or catalase. These results are the first to show the involvement of hydrogen peroxide as a crucial molecule related to the toxic effects of both organic and inorganic mercurials in brain mitochondria. In addition, the study is the first to show the protective effect of quercetin against mercurial-induced toxicity, pointing to its capability to counteract mercurial-dependent hydrogen peroxide generation as a potential molecular mechanism of protection. Taken together, these data render quercetin a promising molecule for pharmacological studies with respects to mercurials' poisoning.

Methylmercury (MeHg) is a highly neurotoxic pollutant, whose mechanisms of toxicity are related to its pro-oxidative properties. A previous report showed under in vivo conditions the neuroprotective effects of plants of the genus Polygala against MeHg-induced neurotoxicity. Moreover, the flavonoid quercetin, isolated from Polygala sabulosa, displayed beneficial effects against MeHg-induced oxidative damage under in vitro conditions. In this study, we sought for potential beneficial effects of quercetin against the neurotoxicity induced by MeHg in Swiss female mice. Animals were divided into six experimental groups: control, quercetin low dose (5 mg/Kg), quercetin high dose (50 mg/Kg), MeHg (40 mg/L, in tap water), MeHg + quercetin low dose, and MeHg + quercetin high dose. After the treatment (21 days), a significant motor deficit was observed in MeHg + quercetin groups. Biochemical parameters related to oxidative stress showed that the simultaneous treatment with quercetin and MeHg caused a higher cerebellar oxidative damage when compared to the individual exposures. MeHg plus quercetin elicited a higher cerebellar lipid peroxidation than MeHg or quercetin alone. The present results indicate that under in vivo conditions quercetin and MeHg cause additive pro-oxidative effects toward the mice cerebellum and that such phenomenon is associated with the observed motor deficit.

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