Functional Changes in Rod and Cone Pathways After Photoreceptor Loss in Light-Damaged Rats

Abstract

Purpose: To determine the functional changes in the rod and cone pathways after photoreceptor loss by continuous light exposure. Methods: Fifty-four male Sprague-Dawley rats were exposed to diffuse fluorescent light of 2000 lux for 24 or 48 hr. Two weeks after the light exposure, full-field scotopic and photopic electroretinograms (ERGs) were elicited by different stimulus intensities with a maximum luminance of 0.84 log cd-s/m2. The amplitudes of the a- and b-waves of the scotopic ERGs and the b-wave of the photopic ERGs were measured. The animals were sacrificed after the ERG recordings, and the number of surviving rod and cone nuclei in the outer nuclear layer was counted. Results: The logarithm (log) of the amplitudes of the maximum rod a-wave (rod Vamax) and b-wave (rod Vbmax) was reduced monotonically with a decrease in the rod nucleus counts (p < 0.0001). The regression line for the rod Vamax decrease was significantly steeper than that for the rod Vbmax (p < 0.005). The maximum b-wave amplitudes of the photopic ERGs (cone Vbmax) were significantly correlated with the number of cone nuclei in a log-linear fashion. The slopes of the regression lines for the rod Vbmax and cone Vbmax were 0.0067 and 0.0140, respectively, which indicates that the amplitude of the cone b-wave was more severely affected than that of the rod b-waves by light-induced photoreceptor degeneration (p < 0.005). Conclusions: The amplitudes of the rod and cone ERGs were correlated with rod and cone nuclei counts in a log-linear fashion in light-damaged rats. The functional loss from the photoreceptor death had a greater effect on the cone pathway than on the rod pathway when the retinal function was assessed by the b-wave.