ANALYSIS OF SEVERE PHOTORECEPTOR LOSS AND MORRIS WATER-MAZE PERFORMANCE IN AGED RATS

In a study of aging and memory in 25-27-month-old albino rats, perform ance on a Morris water maze was found to be dependent on the structura l integrity of the retina. Generally, as expected, 'learners' had inta ct retinas, while 'non-learners' had retinas with severe photoreceptor loss and a non-continuous outer nuclear layer, consisting of scattere d cell nuclei. However, contrary to this general correlation between l earning ability and photoreceptor presence, some learners had severely degenerated retinas and occasionally, non-learners had photoreceptor populations that apparently were comparable to those of learners. Rat retinas from these unpredictable, borderline response categories were examined histopathologically and morphometrically with the purpose of determining the minimal number of photoreceptors (PRs) necessary for a nimals to be rated as learners on the Morris water maze. However, amon g these severely damaged retinas of borderline groups, total number of surviving photoreceptors did not vary significantly among the learner , ambiguous or marginal and non-learner groups. The population of surv iving PRs in learners was as low as 0.04% and in non-learners as high as 0.4%, as compared to that of young, adult rats. Therefore, borderli ne learners and non-learners had overlapping surviving PR numbers and the results did not clarify the response difference between these grou ps in the Morris water maze. It is suggested that the pattern of survi ving PRs over the retinal surface, as well as the ratio of surviving r ods to cones and their connectivity with other retinal neurons, may be related to the residual function of degenerated retinas of learner ra ts. Both age-related and light-induced types of photoreceptor degenera tion can confound studies of interactions of age and memory, especiall y if the results depend on evaluation of visually guided behavior in a lbino animals. They, therefore, are essential considerations during de sign of experimental procedures.