A sign‐reversing pathway from rods to double and single cones in the retina of the tiger salamander.

Abstract

Signal transmission between rods and cones was studied by passing current into a rod and recording the voltage response in a nearby double or single cone and vice versa. Two types of rod-cone interaction were found. Between immediately adjacent rods and cones, passage of current into either receptor elicited in the other receptor a sustained voltage response of the same sign as the injected current. These signals were still seen in the presence of Co2+, and are probably mediated by the electrical synapses which were seen anatomically between adjacent rods and cones. In addition to this short-range sign-preserving interaction, passing current into a rod elicited a transient sign-inverted signal in cones up to at least 89 .mu.m from the injected rod. No such response was seen in rods for current injection into cones. This signal was greatly reduced by Co2+ ions. Hyperpolarization of the cone to .apprx. -65 mV, with .apprx. 0.1 nA current, reversed this signal, which is presumed to be mediated by a chemical synaptic input to cones. Light flashes suppressed the sign-inverted signal for a period which was longer for brighter flashes. The time of reappearance of the signal was correlated with the return of the rod and horizontal cell potentials to their dark levels. This suppression could also be produced by an annulus of light which produced no light response in the receptors at the center of the annulus, but which did polarize horizontal cells under the center of the annulus. The wave form of the sign-inverted signal was similar to that produced in horizontal cells by current injection into rods, but of opposite sign. If an electrode was left in a cone for some time, the normal hyperpolarizing light response diminished, leaving a depolarizing response produced, presumably, by feedback from horizontal cells. This signal was reversed when the cone was hyperpolarized with .apprx. 0.1 nA current. The sign-inverted response may be mediated by feedback from horizontal cells and, assuming that depolarization increases the rate of release of horizontal cell synaptic transmitter, then the feedback transmitter opens channels in the cone membrane whose currents have a reversal potential .apprx. -65 mV.