Changes in visually guided behavior of Royal College of Surgeons rats as a function of age: a histologic, morphometric, and functional study.

PURPOSE

To compare the changes in visually guided performance as a function of age between Royal College of Surgeons (RCS) dystrophic and congenic rats and to correlate photoreceptor cell number with visually guided performance in age-matched populations of RCS dystrophic rats.

METHODS

The visually guided performances of RCS dystrophic (n=6) and congenic (n=7) rats were studied from 0.75 to 12 months of age using a water escape paradigm that tested their ability to find a submersed, randomly placed platform that used a light source as a clue. The time to find the platform (latency) was recorded. In age-matched dystrophic RCS rats, histopathologic changes were described and the number of photoreceptor cell nuclear profiles per midsagittal retinal section was counted. Changes with age in visually guided behavior and photoreceptor cell populations of RCS dystrophic rats then were compared.

RESULTS

The latency of RCS dystrophic rats increased significantly beyond that of congenic rats after 6 months of age. Photoreceptor cell number in dystrophic rats precipitously decreased through 6 months of age, stabilized at 9 months, and decreased further at 12 months. Two unexpected results were seen in the dystrophic animals: At 6 months of age, as few as 22+/-3 photoreceptor cell nuclei per midsagittal section provided similar latencies as at 2 months when there were as many as 400. Although the number of photoreceptor cells remained stable from 6 to 9 months of age, functional vision significantly deteriorated.

CONCLUSIONS

Two important phenomena were observed. First, the RCS rats performed very well in the water escape test even while their photoreceptor cell population was being decimated. Second, once a low threshold was reached, a dramatic deterioration of visually guided behavior occurred without a further reduction in photoreceptor cell numbers.