Aging and time-of-day effects on cognition in rats

This study used an animal model to investigate the importance of the time a t which testing occurs for age differences in learning and memory. Groups o f old and young rats were entrained to a 12-hr light-dark schedule and admi nistered tests of delayed alternation and inhibitory avoidance conditioning at the beginning or end of their high-activity cycle. Apart from normal ag e differences in test performance, the behavioral results demonstrated that old but not young rats were affected by the time of testing. In both tasks , old rats tested late in the activity cycle performed significantly worse than did old rats tested early in the cycle, under conditions that challeng ed memory processes that are known to involve the hippocampus. The results indicate that circadian disruption in old age can adversely affect memory a nd related cognitive function, with important implications for inhibitory c ontrol.