Light potentiation of horizontal cells in the isolated rabbit retina

When the retinas of some fishes and amphibians are dark-adapted the hyperpolarising response of horizontal cells (HCs) to a light stimulus is suppressed (= dark suppression) but is restored to that of light adaptation when a light stimulus is rapidly repeated (= light potentiation (LP). This phenomenon, which had not been previously demonstrated in a mammal, has been recorded from isolated rabbit retinas. The steady amount of LP of HCs after several light stimuli was expressed as the percentage of induced hyperpolarisation when that of the dark-adapted retina is taken as 100%. The LP was small (113±13%, n=14), when the HCs had stable and large responses (>15 mV), but varied greatly between HCs. Those HCs with slight LP (106±3% n=8) designated as x-HCs, mostly exhibited larger overshoots than did those HCs with stronger LP (123±14, n=6) and designated y-HCs. These mostly had a smaller overshoot. Other HCs (z-HCs) were unstable and were only slightly hyperpolarised by the first light stimulus after dark adaptation but showed the strongest LP (193±48%, n=7). These results could indicate a variable degree of light potentiation in normally-functioning HCs, which can be classified accordingly. It is possible, however, that the degree of LP is small in all normally-functioning HCs, but as the HCs in isolated mammalian retinal preparations deteriorate, the phenomenon of LP is progressively exaggerated. LP is not peculiar to HCs and can also occur in cells which depolarise in response to a light stimulus, and can be evident in the PIII component of the ERG.