Ionic mechanisms underlying the responses of off-center bipolar cells in the carp retina. I. Studies on responses evoked by light.

Off-center bipolar cells show hyperpolarizing responses to spot illumination in the receptive field center and depolarization responses to an annulus in the surround. To understand the ionic mechanisms underlying these responses, we examined the current-voltage relationship of these bipolar cells, input resistance changes during their light-evoked responses, and the reversal potentials of these responses. Off-center bipolar cells generally showed inward rectification when they were hyperpolarized and outward rectification when they were strongly depolarized. The membrane potential at which the I-V relationship deviated from linearity varied in individual cells. Hyperpolarizing center responses were generally accompanied by a resistance increase, irrespective of signal inputs either from red-sensitive cones or from rods, and the response polarities reversed at greater than +50 mV. Depolarizing surround responses were accompanied by a resistance decrease with a reversal potential at about +28 mV (one case). From the above observations, it is suggested that the center responses are generated by a decrease in sodium conductance (gNa) and the surround response is generated by an increase in gNa.