Voltage-dependent potassium current is sufficient for the changes in gain and dynamics of isolated retinal bipolar cells of the tiger salamander

Purpose: We previously reported that isolated bipolar cells in the tiger salamander change from low-pass to band-pass behavior with depolarization Depolarization was also associated with a reduction in gain and an apparent decrease in membrane time constant. We investigated the ionic current(s) underlying these changes and examined in detail conditions under which an ionic channel leads to the band-pass behavior. Methods: Pseudo-random binary sequences (M-sequences), fluctuating around different mean currents were injected into isolated bipolar cells in the whole-cell, current-clamp mode. 0th and 1st order kernels were computed. Agents that block potassium and calcium currents were used to investigate ionic mechanisms. Computer simulations were performed to probe ways in which channel parameters affect dynamics Results: 1) When bipolar cells were depolarized, an undershoot appeared in the 1st order kernels (impulse response). The undershoot was eliminated by agents that suppress potassium currents, such as intracellular Cs⁺, Ba⁺⁺ TEA and TPeA, but was resistant to the calcium channel blocker Co⁺⁺, 2) Computer simulations were performed on a model neuron which included a leakage conductance, a voltage-dependent K⁺ channel and a voltage-independent K⁺ channel. The simulations reproduced the changes in gain and dynamics and showed that the voltage-dependent potassium current resulted in a robust undershoot by applying a narrow pulse-like stimulus or the M-sequences to the model neuron. Moreover, the simulation reproduced the quasilinear behavior at each mean injected current, similar to what was observed in experiments. 3)Voltage-dependency by itself is not sufficient to cause an undershoot, but a certain combination of kinetics and ionic constituents gives rise to the band-pass behavior. Conclusions: Voltage-dependent potassium current by itself can and does cause an undershoot in the impulse response of the bipolar cells.