The dark adaptation of single units in the isolated frog retina following partial bleaching of rhodopsin

Abstract

The dark adaptation of single units following partial bleaching of rhodopsin has been studied in isolated perfused retinac of frogs ( Rana esculenta). This dark adaptation occurs without regeneration of rhodopsin, as absence of regeneration is a feature of the perfused retina. The shape of the dark adaptation curves depends on the amount of pigment bleached. If less than 35–40% of rhodopsin have been bleached, the curves show two phases separated by a distinct kink. Bleaching more than 35–40% of the pigment is followed by a recovery process that exhibits one phase only. If a second phase initiated by a kink appears, the spectral sensitivity curve is finally found to be scotopic. In the case of single branched curves, spectral sensitivity distributions show up eventually that are similar to Granit's narrow shaped modulator curves. The analysis of scotopic thresholds finally reached shows that, within a certain range, the scotopic sensitivity is proportional to the fraction of light absorbed. This result suggests the dominance of photochemical processes over nervous ones in setting the sensitivity level.