Ionic basis of high-pass filtering of small signals by the network of retinal rods in the toad

Abstract

The electrical spread of excitation in the network of rods was studied by intracellular recording in the isolated perfused retina of the toad Bufo marinus . It was observed, as in the retina of the snapping turtle (Detwiler et al . 1980), that the peak of a response elicited by a dim, bar-shaped flash of light occurred earlier when the stimulus was laterally displaced from the impaled rod. Therefore the rod network behaved as a high-pass filter, an effect that could be produced by a time-varying voltage-dependent conductance in the rod membrane. Concentrations of Cs + up to 10 mm, which blocked the relaxation from peak to plateau of the rod’s response to bright light, had no effect on the high pass filtering of small signals. Raising external K + concentration from 2.6 to 10 mm abolished the high-pass filtering property of the rod network. It is concluded that at least two time-varying voltage-dependent conductances shape the rod photoresponse. One, which is blocked by Cs + , is responsible for the relaxation from peak to plateau of the rod’s response to bright light. The other is a time-varying voltage-dependent K + conductance responsible for the high-pass filtering of small signals.