The intensity of the pupillary light reflex does not correlate with the number of retinal photoreceptor cells

The purpose of this study was to determine if the pupillary light reflex (PLR) can serve as an indicator of the number of photoreceptor cells present in the rat retina to a sufficient degree of precision to be useful for testing the functional effects of retinal transplantation. The PLR was measured as percentage constriction of normal Fischer 344 rats (n = 14) and compared to the PLR of light-damaged (1300 luxes/30 days exposure) Fischer rats (n = 13). Additionally, the PLR of RCS-rdy⁺(congenic) rats (n = 8) was compared to the PLR of RCS dystrophic rats (n = 7). Three eyes from each group were randomly chosen for morphometry. The number of photoreceptor nuclear profiles per 60 μm of retinal length was counted at six predetermined loci and averaged. The mean PLR of Light-Damaged F344 group (64%) was significantly different from the mean PLR of the Normal F344 group (75%) (P = 0.003). However, the mean PLR of the RCS Dystrophic group (72%) did not differ from the mean PLR of the RCS Congenic group (71%) (P = 0.82). Morphometry revealed that the mean number of photoreceptor nuclear profiles within each group of animals was vastly different: Normal F344 = 138, Light Damaged F344 = 19, Congenic RCS = 93, and Dystrophic RCS = 1. No correlation was found between intensity of PLR and number of photoreceptors present (r = 0.11, P = 0.78). In summary, our observations of four groups of rats, two with normal retinas and two with vastly diminished photoreceptor cell populations, demonstrate that the PLR intensity is not correlated with the number of photoreceptors. This study suggests that, in the rat, the PLR is not a reliable indicator of the photoreceptor cell population and should not be used to measure the level of functionality of intraretinal transplantation.