The effect of light on the spread of signals through the rod network of the salamander retina.

Adjacent rods in the amphibian retina are electrically coupled to each other by gap junctions. By injecting current pulses into one rod and recording the voltage change produced in nearby rods, we have studied the extent to which signals spread between rods in the presence and absence of illumination. Light has little effect on the steady potentials produced in nearby rods by the injection of a hyperpolarizing current, but does affect the propagation of transient signals through the rod network. The responses to injection of depolarizing current are increased by light. These effects of light were mimicked by hyperpolarizing the rod network (non-uniformly) by injecting continuous current (on top of which current pulses were superimposed to monitor signal spread). This suggests that the effects of light are due solely to the rod hyperpolarization produced by light. The effects of light are not completely predicted from computer simulations based on a previous characterization of the properties of isolated rods; these experiments thus reveal an inadequacy in the description of the rod membrane currents in that model. Light-induced hyperpolarization of cones has no effect on signal spread between rods. The functional significance of these results is discussed.