Age related changes of social memory/recognition in male Fischer 344 rats.

Two different habituation-dishabituation test paradigms were used to evaluate differences in social memory/recognition among 3-, 15- and 22-month-old male Fischer 344 rats. For test 1, males received three 2-min exposures to the same stimulus ovariectomized female, followed by three 2-min exposures to a different stimulus female with an inter-trial interval of 6 min. All groups showed a habituation response with investigation times decreasing on trials 2 and 3. Introduction of a different stimulus female on trial 4 (dishabituation) resulted in significant differences with investigation times of the 3-month animals being significantly greater than both the 15- and 22-month animals and those of the 15- being greater than the 22-month animals. Notably, the 22-month-old animals failed to dishabituate on this task. For test 2, all animals received two trials with different stimulus females used in each trial. While investigation times of the 3-month animals remained elevated in trial 2, indicative of an absence of habituation to these different stimuli, those of the 15- and 22-month-old animals decreased significantly, suggesting that habituation had occurred to the task and these animals failed to recognize differences in the stimuli. Taken together, these results demonstrate that the 22-month-old rats show enhanced habituation, but markedly deficient dishabituation responses compared to the 3-month-old animals, while the performance of the 15-month animals was intermediate. These results suggest an age dependent decrement in social memory/recognition processes in the male Fischer 344 rat.