Dark‐adaptation in abnormal (RCS) rats studied electroretinographically.

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Abstract
Electroretinogram (ERG) responses recorded from dark-reared rats with inherited retinal dystrophy (RCS [Royal College of Surgeons]) showed progressive decline in b-wave amplitude and prolongation of the time to the peak of the b-wave with age when compared to dark-reared, normal albino rats. Dark-adaptation was followed in RCS and normal rats by recording the light intensity needed to evoke a criterion ERG response at different time intervals after bleaching about 90% of the rhodopsin. In normal rats, dark-adaptation was governed by 2 mechanisms. The first 25-35 min of recovery was determined by cones. The 2nd branch, determined by the recovery of rods, lasted for about 3 h and proceeded along an exponential time course with time constant of 41.4 .+-. 2.4 min (standard error of mean). In RCS rats the time course of the dark-adaptation after a 90% bleach depended on age. In 25-30 day old rats the recovery curve had at least 3 breaks separating 3 different mechanisms. Rats, 35-40 days old, exhibited double exponential recovery curves; 45-70 day old rats recovery along a single exponential curve similar in time course to the cone branch of dark-adaptation found in normal rats. Action spectra obtained from RCS rats at different time intervals of the recovery curve showed that in young rats, 25-30 days old, small ERG responses recorded before bleaching and at the end of the recovery period were determined by rhodopsin; those recorded during the 1st part of the recovery from 90% bleach were determined by a combination of rods and cones. In RCS rats of advanced age (45-70 days old), rhodopsin was the major contributor to the ERG responses recorded either before bleaching or at the end of the recovery period. The gradual deterioration with age of the ERG in RCS rats could not be explained by either the decrease in quantum catch due to the decrease in rhodopsin content or by the linear relationship between log ERG threshold and pigment concentration. Using estimates of rhodopsin density within surviving rods obtained from retinal densitometry, in RCS rats where more than 30% of normal levels of rhodopsin was located within the functioning rods, the log intensity needed for a criterion ERG response measured at the end of the recovery period from a 90% bleach was linearly related to the fraction of functional rhodopsin. No simple relationship between log ERG threshold and rhodopsin concentration could be found during the course of recovery in the dark from a strong bleaching exposure in RCS rats of all ages.