Studies on the off-response of the rod photoreceptors in the isolated frog retina.

Transretinal potential changes induced by a 30 sec exposure to 503 nm light were studied in the dark-adapted frog isolated retina. The retina was treated with aspartate and 0.5 mM Ba2+ to suppress the PII and slow PIII components of the electroretinogram, and therefore the response to the light stimulus consisted of the on-response (fast PIII response) and the off-response. The amplitude of the off-response was proportional to that of the on-response when the stimulus intensity was weak. The amplitude ratio of the off-response to the on-response was unaffected by partial bleachings of rhodopsin. In the presence of 700 nm background illumination, the amplitude of the on-response was decreased, whereas that of the off-response was increased. The amplitude of the off-response increased to about four-fold that of the original response at 3 hr after turning on the background illumination, but the effects of 480 nm background light were less remarkable. Both the on- and off-response, however, had a peak spectral sensitivity at about 500 nm, regardless of the presence of background light. From these findings, it was suggested that the red rods contribute to the development of the off-response, but the cones would also contribute through small focal gap junctions between the cones and the red rods.