Mercury (Hg²⁺) enhances the depressant effect of kainate on Ca-inactivated potassium current in telencephalic cells derived from chick embryos

The effect of HgCl₂ on kainate (KA)-induced depression of a voltage-gated potassium (K⁺) current in chick embryo telencephalic cells was studied using conventional and nystatin-perforated whole-cell patch-clamp recordings, fluorescence imaging, and flow cytometry techniques. Hg²⁺ (1 μM) alone did not effect the 4-aminopyridine-(4-AP)-sensitive transient K⁺ current in immature cells (Embryonic Day 5), but irreversibly potentiated the depressant effect of KA on this K⁺ current. A 50% potentiation of KA-induced depression of the K⁺ current was produced by an application of 0.19 μM Hg²⁺. Application of ionomycin (5 μM) or calcium ionophore A23187 (2 μM) suppressed the K⁺ current. To test the possibility that the 4-AP-sensitive transient K⁺ current is a Ca-inactivated current, the effect of intracellular Ca²⁺ concentration ([Ca²⁺](i)) in the range of 30 nM to 2 μM was determined. The amplitude of the K⁺ current was sensitive to [Ca²⁺](i) with half-maximal inactivation at 370 nM at +60 mV. The concentration-response curve of the K⁺ current inhibition by [Ca²⁺](i) was shifted to lower [Ca²⁺](i) and the slope of the curve was reduced in the presence of KA. Hg²⁺ potentiated these effects of KA. The Ca-dependence of the K⁺ current was maximal at the 5th embryonic day, declined to the 9th embryonic day, and was absent at the 11th embryonic day. Application of Hg²⁺ (0.1-1 μM) had no effect on the basal [Ca²⁺](i) of freshly dissociated cells (10th day in ovo) and cells in culture (the 4-day cultures from the telencephalon of 5-day-old embryos), but potentiated KA-induced increase of [Ca²⁺](i) in a Ca-free-EGTA solution in a concentration-dependent manner. Moreover 1 μM Hg²⁺ delayed and reduced the recovery to basal [Ca²⁺](i) after washout of KA. Exposure to 5-30 μM Hg²⁺ caused an irreversible decline of membrane resistance, an increased cell size, and reduced cell granularity and complexity. Intracellular recording of spontaneous neuronal activity and immunocytochemical identification showed that the KA/Hg²⁺-sensitive Ca-inactivated K⁺ current exists in early differentiating telencephalic neurons. Because depression of the K⁺ current by KA and Hg²⁺ decreases the interspike interval and irreversibly perturbs the frequency code of information in the nervous system, the expression of this current during early neuroembryogenesis may be one of the reasons for the developmental toxicity of inorganic mercury.