Mercuric(II) chloride modulates single-channel properties of carbachol-activated Cl- channels in cultured neurons of Aplysia californica.

1. The effect of mercuric(II) chloride on kinetic parameters of carbachol-activated single chloride channels were studied in cultured neurons of the marine mollusk, Aplysia californica. 2. Single neurons of Aplysia were cultured in L-15 medium containing 1 mM beta-D-xyloside, which improved the success rate for gigaseal formation by 46%. Carbachol-activated single chloride channels were recorded in the cell-attached patch clamp configuration. Recordings with control solution (1 microM carbachol) and with test solution (1 microM carbachol + 1 microM HgCl2) were performed successively on the same neuron. 3. In both the control and the test solution the open and closed time distributions were fitted with a double-exponential function. However, kinetic analysis revealed that Hg2+ caused a significant reduction of the mean closed time (10.37 +/- 1.08 vs. 3.32 +/- 0.02 msec) and of the second time constant tau 2 of the closed time distribution (2.09 +/- 0.05 vs. 0.66 +/- 0.5 msec). The reduction of tau 2, i.e., fewer events in the longer closed state under the action of Hg2+, may be the physical cause for the reduction of the mean closed time and thus underlies the increased open probability p0 (0.13 +/- 0.01 vs. 0.29 +/- 0.01 msec) of carbachol-activated chloride channels. 4. Inorganic Hg2+ affects the acetylcholine receptor at lower concentrations than previously reported.