Cognitive impairment in old rats: a comparison of object displacement, object recognition and water maze

The behavioral performance of young and aged rats was studied in a repeated-trials test. Young animals reacted to both spatial displacement and novelty, whereas most aged rats lost the ability to react to novelty although maintaining spatial memory. The cluster analysis procedure performed on all the tested subjects enabled the recognition of a consistent group of the aged sample (35%) with a mild degree of spatial and non-spatial memory impairment. Spatial memory impairment of some of the aged animals was also evaluated in the Morris water maze test. On the fifth day of the task, we observed a very low percentage of impaired aged animals, which partially corresponded to the impaired group identified by the object recognition test. In contrast, the subgroup of mildly impaired rats performed similarly to the young animals. We advance that the Morris water maze might represent a stressful experimental condition for aged rats, enhancing the motivational level of animals subjected to this procedure. This condition may alter the cognitive responses. As a consequence, the "mildly impaired" rats, which may be considered an interesting group for investigating memory-enhancing drugs, will infrequently be recognized with the Morris water maze test. Cognitive impairment in aged rats should be studied utilizing a sensitive test in which motivation does not substantially influence the results of the test.