Assessment of sensitivity loss of middle and inner retina following light damage in rats using the ERG B-wave and STR

Purpose. The decrease in retinal sensitivity measured by the ERG b-wave in several rat models of photoreceptor degeneration, including retinal light damage (Rapp and Williams, 1979) and RCS inherited retinal dystrophy (Perlman, 1978), exceeds that predicted by loss of photon catch alone. However, we recently reported that the threshold in RCS rats for the STR (which reflects activity in amacrine cells, and is recorded 1-2 log units below b-wave threshold) was elevated much less than the b-wave threshold. We wanted to see whether a similar effect on ERG thresholds occurred from retinal light damage. Methods. RCS dystrophic (RCS p⁺ rdy⁺)(n=3) and congenic control rats (RCS p⁺) (n=8) were reared in 50 lux cyclic light. Controls were exposed to 1200 lux white fluorescent light for 96 hr at 20 weeks of age. Pupils were kept fully dilated during constant light exposure with 1% atropine. Dark adapted ERGs were recorded one week prior and two weeks after light damage. Measurements of outer nuclear layer (ONL) thickness along the vertical meridian, including the region most affected by damaging light, were made in retinas from light damaged and 60 day old RCS dystrophic rats. Results. Light damage elevated the STR threshold by 1.0 ± 0.6 log units, whereas b-wave threshold was elevated 2.5 ± 0.7 log units. This difference is statistically significant (p<0.0002). Similar threshold elevations were found previously for the STR (1.1 ± 0.5) and b-wave (27 ± 0.7) of 60 old RCS dystrophic rats. Although the distribution of photoreceptor cell loss was markedly different in the two types of degeneration, the over all decrease was approximately 80% in both. Conclusions. Retinal sensitivity at the bipolar cell level, as reflected by the b-wave threshold, is affected more than sensitivity at the amacrine cell level, reflected in the STR threshold, in both inherited and light induced photoreceptor degeneration in RCS rats. Though the reason for this difference is not understood, these results suggest the possibility that the STR threshold may correlate more closely with loss of quantal catch than the b-wave threshold in these degenerations.