CHANGES IN VISUALLY GUIDED BEHAVIOR OF ROYAL-COLLEGE OF SURGEONS RATSAS A FUNCTION OF AGE - A HISTOLOGIC, MORPHOMETRIC, AND FUNCTIONAL-STUDY

PURPOSE. TO compare the changes in visually guided performance as a fu nction of age between Royal College of Surgeons (RCS) dystrophic and c ongenic 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) an d congenic (n = 7) rats were studied from 0.75 to 12 months of age usi ng a water escape paradigm that tested their ability to find a submers ed, randomly placed platform that used a light source as a clue. The t ime to find the platform (latency) was recorded. In age-matched dystro phic RCS rats, histopathologic changes were described and the number o f photoreceptor cell nuclear profiles per midsagittal retinal section was counted. Changes with age in visually guided behavior and photorec eptor cell populations of RCS dystrophic rats then were compared. RESU LTS. The latency of RCS dystrophic rats increased significantly beyond that of congenic rats after 6 months of age. Photoreceptor cell numbe r in dystrophic rats precipitously decreased through 6 months of age, stabilized at 9 months, and decreased further at 12 months. Two Unexpe cted results n;ere seen in ther 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. CON CLUSIONS. Two important phenomena were observed. First, the RCS rats p erformed very well in the water escape test even while their photorece ptor cell population was being decimated. Second, once a low threshold was reached, a dramatic deterioration of visually guided behavior occ urred without a further reduction in photoreceptor cell numbers.