Effects of hyperpolarization-activated channel blocker ZD7288 on polar excitations of frog sciatic nerve

Previous studies have demonstrated that Ar(+) laser irradiation shows a more selective blocking effect on the generation of anode-break-excitation (AE) than on cathode-make-excitation (CE), and that the effects of laser irradiation closely resemble those following the application of hyperpolarization-activated current (Ih) blocker, ZD7288. We therefore examined the effects of ZD7288 and tetrodotoxin (TTX) on polar excitations to reveal whether such a selective effect of ZD7288 on AE is specific in frog sciatic nerve. Supramaximal stimuli (10-ms pulse) were applied while for 30 min each channel blocker was applied to the stimulating sites. Analyses of chronological changes in polar excitations were performed using CEs induced by positive stimuli and AEs induced by negative stimuli, because both were generated on the same stimulating grid against the recording grids. TTX application (1 mM) decreased all types of polar excitations at 30 min after initiation of the application. When ZD7288 (1 mM) was applied, the amplitude of AE displayed a significant decrease after 30 min. When TTX or ZD7288 was applied to the middle portion between the stimulating and recording electrode grids, TTX showed the conduction block, but the latter yielded almost no effect. Western blotting analyses demonstrated expressions of the second and the third subunits of hyperpolarization-activated and cyclic-nucleotide-gated nonselective cation channels in frog sciatic nerve. Ih channels thus exist in the frog sciatic nerve, and its specific blocker, ZD7288, has the potential to selectively block the generation of AE.