Brief isolation during infancy enhances the formation of long-term memories in infant rodents

Behavioral tagging, which is well-established in adults, has recently been shown to also occur in infants. Interestingly, while familiarizing the novel experience abolishes behavioral tagging in adults, it appears to be without effect in infants. Familiarization, at least in infants, may act as an experience-dependent switch, closing the hippocampal critical period and thus accelerating the maturation of the hippocampus. In this study, infant (i.e., Postnatal Day 17) rats were placed in a context and shocked. Infants familiarized to an open field arena for 30 min the day before exhibited enhanced retention when tested 1 day later (Experiments 1 and 2). While brief exploration of an open field soon before learning (i.e., the behavioral tagging procedure) resulted in better memory when rats were tested 1, but not 3, days later, familiarized rats had enhanced long-term memory (LTM) at both intervals (Experiment 3). Furthermore, familiarization, but not brief open field exposure, enhanced LTM for a nonhippocampal task (Experiment 4), suggesting that familiarization works through a different mechanism compared with behavioral tagging. Specifically, these results suggest that familiarization results in broader changes to the emotional learning and memory system rather than the hippocampus alone. Further investigation revealed that the effect of familiarization on LTM formation was mediated by isolation, rather than contextual novelty (Experiments 5 and 6), and consistent with this notion, maternal presence during familiarization blocked the enhancements in LTM (Experiment 7). Overall, these findings suggest that isolation during infancy may regulate the maturation of the emotional learning system. (PsycINFO Database Record (c) 2019 APA, all rights reserved).