Water maze performance of aged Sprague-Dawley rats in relation to retinal morphologic measures

The spatial learning ability of aged male and female Sprague-Dawley rats was assessed using the Morris water maze. To determine the influence of age-related visual deficits on performance levels, retinal morphologic measures were correlated with water maze performance for each rat. Rats were first trained on the water maze task at 21 months of age and were retrained 3 or 4 times at 6-week intervals. After the last training session the rats were killed and their eyes were removed for histopathologic and morphometric evaluation of the retinas. There was a large degree of retinal degeneration in all of the aged Sprague-Dawley rats with an average decrease in the thickness of the retinal outer nuclear layer (photoreceptor nuclei containing layer) of 85% in old males and 95% in old females. Some rats, however, had less degeneration of the retinas than others, and the degree of retinal degeneration was strongly related to performance levels on the water maze task. Among the aged rats in this study with the least retinal degeneration, there was little evidence for a subset of rats that were unable, with extensive training, to learn a platform position. Of the 41 rats with the least retinal degeneration (out of a total of 81), only one was a clear non-learner on the water maze task, whereas, of the 27 rats with the most retinal degeneration, 20 were non-learners. These results illustrate the potentially serious confounding effects of deteriorating visual ability on attempts to assess cognitive functioning of aged albino rats on tasks requiring utilization of visual cues.