Multiple Sources of Ca2+ Contribute to Methylmercury-Induced Increased Frequency of Spontaneous Inhibitory Synaptic Responses in Cerebellar Slices of Rat

Abstract

We previously showed that elevated intracellular Ca2+ ([Ca2+]i) in the molecular layer and granule cells in cerebellar slices is responsible for the initial increases in frequency of spontaneous or miniature inhibitory postsynaptic currents (sIPSCs or mIPSCs) of Purkinje cells following methylmercury (MeHg) treatment. To identify the contribution of different Ca2+ sources to MeHg-induced stimulation of spontaneous GABA release, we examined sIPSC or mIPSC frequency of Purkinje cells in acutely prepared cerebellar slices using whole-cell patch-clamp recording techniques under conditions of lowered [Ca2+]o, pretreatment with caffeine, cyclopiazonic acid (CPA), thapsigargin or rutheniumred (RR) to deplete ryanodine-sensitive and insensitive intracellular Ca2+ stores or mitochondria, or a combination of lowering [Ca2+]o and increased BAPTA buffering. Lowering [Ca2+]o significantly reduced sIPSC or mIPSC frequency and amplitudes, but failed to completely prevent MeHginduced increase in these events frequency. Caffeine, CPA, or thapisgargin also minimized MeHg-induced increase in sIPSC frequency, yet none of them completely blocked MeHg-induced increase in sIPSC frequency. Similarly, the mitochondrial Ca2+ transport inhibitor RR, or a combination of lowering [Ca2+]o and BAPTA buffering reduced but did not prevent MeHginduced changes in mIPSC frequency. Consistently, confocal Ca2+ imaging under low [Ca2+]o conditions or in the presence of caffeine or CPA exhibited a marked reduction of MeHg-induced increases in [Ca2+]i in both molecular and granule layers. Thus, these results verify that a combination of extracellular Ca2+ influx and Ca2+ release from different intracellular Ca2+ pools all contribute to MeHg-induced increase in appears to be the primary contributor.