Behavioral tagging in infant rats

Abstract

Recent studies have shown that exposure to a novel environment may stabilize the persistence of weak memories, a phenomenon often attributed to a process referred to as “behavioral tagging.” While this phenomenon has been repeatedly demonstrated in adult animals, no studies to date have examined whether it occurs in infant animals, which is surprising given that infants exhibit an impaired ability to form long-term memories (LTMs). In the present study, infant (i.e., postnatal day (P) 17) rats were placed in a context and repeatedly shocked. Infant rats given brief open field exposure 1 h, but not 2 h, prior to conditioning exhibited enhanced retention when tested 1 d later (Experiments 1 and 2), but comparable retention when tested shortly after training (Experiment 2). Thus, exploration of an open field facilitates subsequent context fear memories by enhancing the persistence of the memory rather than strengthening the context-shock association at encoding. While exploration of an open field did not lead to better memory when animals were tested 3 d later (Experiment 3), a brief pretest shock led to a more pronounced reinstatement effect in rats exposed to the open field 1 h before conditioning (Experiment 4). Finally, unlike what has been reported in adults, Experiments 5 and 6 suggest that familiarization of the open field before subsequent exposure does not abolish the behavioral tagging effect in infants. Overall, while these findings suggest that similar behavioral tagging mechanisms to those reported in adults might be involved in the formation of LTMs in infant rats, they also suggest that there may be developmental differences in the retention of familiarization experiences.