Long-term object recognition memory in aged rats

Rats show spontaneous preference for exploring novel rather than familiar objects. Thus, exploratory activity can be used to evaluate recognition memory. To date, the spontaneous novelty-preference test of object recognition has been used to study long-term recognition memory formation in adult rats (Mumby et al., 2002 Behav Brain Res 132:215-26), but not in aged rats. In the present study we used this behavioral test with 25-27 month-old Wistar rats (n=9), but we found that it was not able to elicit recognition memory. Thus, we developed a new experimental protocol and we tested a second group of rats of the same age (n=18). Each animal received 5 training sessions (1 per day on days 1 and 2, and 1 on day 3) lasting 5 min in a small box (48x26,5x21 cm) containing two identical plastic cubes (8 cm high). We found that the time spent in exploring this pair of objects significantly (P<0.01) decreased on the third session. Twenty-four h after the training 8 rats were tested in the same box, in which one of the two cubes (familiar object) was replaced by a plastic pyramid (8 cm high, novel object). The animals displayed a significantly (P<0.05) longer exploration time of the novel object in comparison with the familiar one. When we applied the new protocol to adult animals (4-6 month-old, n=19), we found that the time spent in exploring the pair of familiar objects significantly (P<0.01) decreased on the second training session, and the time spent in exploring the novel object in comparison with the familiar one was significantly (P<0.05, n=10) longer after 24 h. These findings indicate that the new protocol is able to induce long-term recognition memory formation in aged animals, and allows the evaluation of age-related differences in learning ability.