Heterogeneous expression of voltage-dependent Na+ and K+ channels in mammalian retinal bipolar cells.

Retinal bipolar cells show heterogeneous expression of voltage-dependent Na+ and K+ currents. We used whole-cell patch-clamp recordings to investigate the possible roles of these currents in the response properties of bipolar cells in rats. Isolated bipolar cells showed robust spontaneous regenerative activity, but the regenerative potential of rod bipolar cells reached a more depolarized level than that of cone bipolar cells. In both isolated cells and cells in retinal slices, the membrane depolarization evoked by current injection was apparently capped. The evoked membrane potential was again more depolarized in rod bipolar cells than in cone bipolar cells. Application of tetraethylammonium and 4-aminopyridine shifted the spontaneous regenerative potential as well as the evoked potential to a more depolarized level. In addition, a subclass of cone bipolar cells showed a prominent spike in the initial phase of the voltage response when the cells were depolarized from a relatively negative membrane potential. The spike was mediated mainly by tetrodotoxin-sensitive Na+ current. The presence of the spike sped up the response kinetics and enhanced the peak membrane potential. Results of this study raise the possibility that voltage-dependent K+ currents may play a role in defining different membrane operating ranges of rod and cone bipolar cells and that voltage-dependent Na+ currents may enhance the response kinetics and amplitude of certain cone bipolar cells.