title: 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 Ca(2+) ([Ca(2+)]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 Ca(2+) 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 [Ca(2+)]o, pretreatment with caffeine, cyclopiazonic acid (CPA), thapsigargin or ruthenium red (RR) to deplete ryanodine-sensitive and insensitive intracellular Ca(2+) stores or mitochondria, or a combination of lowering [Ca(2+)]o and increased BAPTA buffering. Lowering [Ca(2+)]o significantly reduced sIPSC or mIPSC frequency and amplitudes, but failed to completely prevent MeHg-induced 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 Ca(2+) transport inhibitor RR, or a combination of lowering [Ca(2+)]o and BAPTA buffering reduced but did not prevent MeHg-induced changes in mIPSC frequency. Consistently, confocal Ca(2+) imaging under low [Ca(2+)]o conditions or in the presence of caffeine or CPA exhibited a marked reduction of MeHg-induced increases in [Ca(2+)]i in both molecular and granule layers. Thus, these results verify that a combination of extracellular Ca(2+) influx and Ca(2+) release from different intracellular Ca(2+) pools all contribute to MeHg-induced increase in [Ca(2+)]i and spontaneous GABA release, although extracellular Ca(2+) appears to be the primary contributor.
authors.affiliation: Department of Pharmacology/Toxicology, Michigan State University, B-331 Life Sciences Building, 1355 Bogue Street, East Lansing, Michigan 48824-1317 atchiso1@msu.edu.
authors.forename: Yukun
authors.initials: Y
authors.lastname: Yuan
authors.affiliation: Department of Pharmacology/Toxicology, Michigan State University, B-331 Life Sciences Building, 1355 Bogue Street, East Lansing, Michigan 48824-1317 atchiso1@msu.edu.
authors.forename: William D
authors.initials: WD
authors.lastname: Atchison
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002531:Cerebellum / Q000187:drug effects* / Q000378:metabolism; D004785:Environmental Pollutants / Q000633:toxicity*; D005260:Female; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D053444:Inhibitory Postsynaptic Potentials / Q000187:drug effects*; D008297:Male; D008767:Methylmercury Compounds / Q000633:toxicity*; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D018408:Patch-Clamp Techniques; D011689:Purkinje Cells / Q000187:drug effects / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D004785:Environmental Pollutants; D007211:Indoles; D008767:Methylmercury Compounds; D002110:Caffeine; D019284:Thapsigargin; C004925:methylmercuric chloride; D002118:Calcium; C000543:cyclopiazonic acid
keywords: cerebellar slice; intracellular Ca2+; methylmercury; synaptic transmission