TOXICOLOGY AND APPLIED PHARMACOLOGY 138, 285–297 (1996) ARTICLE NO. 0127 Mercury (Hg 2/ ) Enhances the Depressant Effect of Kainate on Ca-Inactivated Potassium Current in Telencephalic Cells Derived from Chick Embryos VLADIMIR A. DYATLOV, 1 ALEXANDER V. PLATOSHIN,* DAVID A. LAWRENCE, AND DAVID O. CARPENTER 2 Wadsworth Center for Laboratories and Research, New York State Department of Health, and School of Public Health, State University of New York at Albany, Albany, New York 12201-0509; and *Bogomolets Institute of Physiology, Kiev 24, Ukraine Received September 25, 1995; accepted February 26, 1996 munocytochemical identification showed that the KA/Hg 2/ -sensi- Mercury (Hg 2/ ) Enhances the Depressant Effect of Kainate on tive Ca-inactivated K / current exists in early differentiating telen- Ca-Inactivated Potassium Current in Telencephalic Cells Derived cephalic neurons. Because depression of the K / current by KA and from Chick Embryos. DYATLOV, V. A., PLATOSHIN, A. V., LAW- Hg 2/ decreases the interspike interval and irreversibly perturbs the RENCE, D. A., AND CARPENTER, D. O. (1996). Toxicol. Appl. Phar- frequency code of information in the nervous system, the expres- macol. 138, 285–297. sion of this current during early neuroembryogenesis may be one of the reasons for the developmental toxicity of inorganic mercury. The effect of HgCl 2 on kainate (KA)-induced depression of a  1996 Academic Press, Inc. voltage-gated potassium (K / ) current in chick embryo telence- phalic cells was studied using conventional and nystatin-perfora- ted whole-cell patch-clamp recordings, fluorescence imaging, and Elemental mercury vapor and inorganic and organic mer- flow cytometry techniques. Hg 2/ (1 mM) alone did not effect the 4- cury compounds are well-known environmental contami- aminopyridine-(4-AP)-sensitive transient K / current in immature nants, and neurotoxicity of organic mercury is documented cells (Embryonic Day 5), but irreversibly potentiated the depres- both in humans (Bakir et al., 1973; McAlpine and Shukuro, sant effect of KA on this K / current. A 50%potentiation of KA- 1958; Svensson et al., 1995) and experimental animals (re- induced depression of the K / current was produced by an applica- viewed in: Abou-Donia and Gupta, 1994; Carpenter, 1994). tion of 0.19 mM Hg 2/ . Application of ionomycin (5 mM) or calcium Observations on both human subjects and animals indicate ionophore A23187 (2 mM) suppressed the K / current. To test the that the developing central nervous system (CNS) is more possibility that the 4-AP-sensitive transient K / current is a Ca- inactivated current, the effect of intracellular Ca 2/ concentration sensitive to damage from organic mercury than the adult ([Ca 2/ ] i ) in the range of 30 nM to 2 mM was determined. The CNS (reviewed in: Clarkson, 1983). Studies on animals amplitude of the K / current was sensitive to [Ca 2/ ] i with half- dosed with methylmercury and inorganic mercury (Hg 2/ ) maximal inactivation at 370 nM at /60 mV. The concentration– suggest similar patterns of mercury distribution in the brain response curve of the K / current inhibition by [Ca 2/ ] i was shifted and the partial demethylation of methylmercury into Hg 2/ to lower [Ca 2/ ] i and the slope of the curve was reduced in the (Mo ¨ller-Madsen, 1990; Charleston et al., 1995). Hg 2/ con- presence of KA. Hg 2/ potentiated these effects of KA. The Ca- tinues to receive attention because there are still a number dependence of the K / current was maximal at the 5th embryonic of open questions regarding its mechanism of action in the day, declined to the 9th embryonic day, and was absent at the developing CNS. 11th embryonic day. Application of Hg 2/ (0.1–1 mM) had no effect At low concentrations (0.3 – 0.5 mM) Hg 2/ greatly potenti- on the basal [Ca 2/ ] i of freshly dissociated cells (10th day in ovo) ates PC12 cell differentiation induced by nerve growth factor and cells in culture (the 4-day cultures from the telencephalon of 5-day-old embryos), but potentiated KA-induced increase of (Rossi et al., 1993). Methylmercury and Hg 2/ affect calcium [Ca 2/ ] i in a Ca-free-EGTA solution in a concentration-dependent homeostasis in eggs of the domestic fowl (Lundholm, 1995), manner. Moreover 1 mM Hg 2/ delayed and reduced the recovery suggesting the possibility of subtle neurotoxic effects of mer- to basal [Ca 2/ ] i after washout of KA. Exposure to 5–30 mM Hg 2/ cury on CNS of embryos. caused an irreversible decline of membrane resistance, an in- We recently studied voltage-gated K / currents in chick creased cell size, and reduced cell granularity and complexity. embryo telencephalic cells (Mike et al., 1995). This study Intracellular recording of spontaneous neuronal activity and im- has shown that the KA-type 3 of glutamate receptor can indi- 1 Permanent address: Bogomolets Institute of Physiology, 4 Bogomolets 3 Abbreviations used: KA, kainate; 4-AP, 4-aminopyridine; TEA, tetra- ethylammonium; TTX, tetrodotoxin; [Ca 2/ ] i , intracellular concentration of Str., Kiev 24, Ukraine. 2 To whom correspondence should be addressed. Fax: 518-473-0752. E- calcium; GFAP, glial fibrillary acidic protein; MAP2, microtubule-associ- ated protein 2; NF 200, neurofilament polypeptide with apparent molecular mail:carpent@wadsworth.org. 285 0041-008X/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.