Antioxidant compounds and Ca 2+ pathway blockers differentially protect against methylmercury and mercuric chloride neurotoxicity

The effects of the environmental contaminants methylmercury (MeHg) and inorganic mercury (HgCl 2 ) on cell viability, intracellular calcium concentration ([Ca 2+ ] i ), and reactive oxygen species (ROS) generation were studied in rat cerebellar granule neuron cultures using fluorescent methods. MeHg exhibited an LC 50 (2.47 μM) tenfold lower than that of HgCl 2 (26.40 μM). To study the involvement of oxidative stress and Ca 2+ homeostasis disruption in mercury‐induced cytotoxicity, we tested the neuroprotective effects of several agents that selectively interfere with these mechanisms. After a 24 hr exposure, the cytotoxic effect of both mercury compounds was reduced by thapsigargin, an inhibitor of endoplasmic reticulum Ca 2+ ‐ATPase; the Ca 2+ channel blocker flunarizine; and the Na + /Ca 2+ exchanger blocker benzamil. All these compounds decreased the mercury‐mediated [Ca 2+ ] i rise. These results indicate that Ca 2+ influx through Ca 2+ channels and the Na + /Ca 2+ exchanger and Ca 2+ mobilization from the endoplasmic reticulum are involved in mercury‐mediated cytotoxicity. The antioxidants probucol and propyl gallate reduced the HgCl 2 toxicity. Probucol and vitamin E partially inhibited the MeHg toxicity after a 24 hr period, whereas propyl gallate completely prevented this effect. Probucol slightly reduced ROS generation in methylmercury‐exposed cultures and decreased mercury‐mediated rise of [Ca 2+ ] i . Propyl gallate abolished ROS generation and partially inhibited the increase of [Ca 2+ ] i induced by both mercury compounds. Propyl gallate also protected human cerebral cortical neuron cultures from the MeHg effect even after 72 hr of MeHg exposure, thus showing a long‐lasting effect. Our data suggest that disruption of redox equilibrium and Ca 2+ homeostasis contribute equally to HgCl 2 ‐mediated toxicity, whereas oxidative stress is the main cause of MeHg neurotoxicity. J. Neurosci. Res. 66:135–145, 2001. © 2001 Wiley‐Liss, Inc.