The effects of aging on memory for sequentially presented objects in rats.

The current study investigated memory for sequentially presented objects in young rats 6 months old (n = 12) and aged rats 24 months old (n = 12). Rats were tested on a task involving three exploratory trials and one probe test. During the exploratory trials, the rat explored a set of three sequentially presented object pairs (A-A, B-B, and C-C) for 5 min per pair with a 3-min delay between each pair. Following the exploratory trials, a probe test was conducted where the rat was presented simultaneously with one object from the first exploratory trial (A) and one object from the third exploratory trial (C). Results from the exploratory trials showed no significant age-related differences in exploration, indicating that 24-month-old rats explored the object pairs as much as 6-month-old rats. The probe test demonstrated that 6-month-old rats spent significantly more time exploring object A compared to object C, indicating that young rats show intact temporal order memory for the exploratory trial objects. However, 24-month-old rats showed no preference for object A and spent a relatively equal amount of time exploring objects A and C. The results suggest that temporal order memory declines as a result of age-related changes in the rodent brain. The findings also may reflect differences in attraction to objects with different memory strengths. Since age-related differences were not detected during the exploratory trials, age-related differences on the probe trial were not due solely to decreased exploration, motivation, or locomotion.