Abstract
Methylmercury (MeHg; 10–100 μM) induced a spontaneous, transient, slow inward current in Purkinje cells in rat cerebellar slices. Insensitivity of this current to tetrodotoxin suggests that its generation is not related to presynaptic firing. The present study was designed to attempt to identify the ionic origin of this current. Neither Gd3+, a nonspecific cation channel blocker, nor tetrakis(2-pyridylmethyl)ethylethylenediamine, which chelates Zn2+, could prevent this current. Following dialysis of cells with a low-[Cl–] pipette solution, the giant currents were inducible only when the cells were held at potentials more positive than 0 mV but not at potentials more negative than –60 mV. In addition, no giant currents were observed when cells were held at 0 mV under symmetrical [Cl–] conditions. Thus, this current seems to be mediated by Cl–. However, it was insensitive to the glycine receptor antagonist strychnine. The anion channel blockers 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) or niflumic acid suppressed GABAA receptor-mediated spontaneous inhibitory postsynaptic currents. Niflumic acid also prevented appearance of this giant current; DIDS was only effective at more positive membrane potentials. Thus, this current seems to be carried by a voltage-dependent Cl– channel. Reducing extracellular Ca2+ concentration and/or intracellular application of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid seemed to be ineffective at preventing appearance of this current. Thus, these data do not seem to support the conclusion that this current is mediated by a Ca2+-activated Cl– channel. The role that this current plays in MeHg-induced neurotoxicity is unknown.
Footnotes
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This work was supported by NIEHS Grants R01-ES03299 and -ES11662. A preliminary report of some of these results was presented at the 2003 Annual Meeting of the Society for Neuroscience (November 7–12, 2003, New Orleans, LA) and was published electronically in abstract form.
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doi:10.1124/jpet.104.080721.
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ABBREVIATIONS: MeHg, methylmercury; ACSF, artificial cerebrospinal fluid; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid; sIPSC, spontaneous inhibitory postsynaptic current; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; ANOVA, analysis of variance; APV, amino-5-phosphonopentanoic acid; mIPSC, miniature inhibitory postsynaptic current; TTX, tetrodotoxin; DIDS, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid; AM, acetoxymethyl ester; TPEN, tetrakis(2-pyridylmethyl)ethylethylenediamine; HP, holding potential.
- Received November 17, 2004.
- Accepted January 31, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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