Relation between potassium-channel kinetics and the intrinsic dynamics in isolated retinal bipolar cells

Characterization of the intrinsic dynamics of isolated retinal bipolar cells by a whole-cell patch-clamp technique combined with estimation of effective impulse responses across a range of mean injected currents reveals strikingly adaptive behavior. At resting potential, bipolar cells' effective impulse response is slow, high gain, and low pass. Depolarization speeds up response, decreases gain, and, in most cells, induces bandpass behavior.This adaptive behavior involves two K⁺ currents. The delayed-rectifier accounts for the observed gain reduction, speed increase, and bandpass behavior. The A-channel further shortens the impulse responses but suppresses bandpass features. Computer simulations of model neurons with a delayed-rectifier and varying A-channel conductances reveal that impulse responses largely reflect the flux of electrical charge through the two K⁺ channels. The A-channel broadens the frequency response and preempts the action of the delayed-rectifier, thereby reducing the associated bandpass features. Admixtures of the two K⁺ channels produce the observed variety of dynamics of retinal bipolar cells.