Reactivation of hippocampal ensemble activity patterns in the aging rat: Insights into memory consolidation within the aged brain

Abstract

In young rats, the pattern of neuronal ensemble activity correlations expressed among hippocampal pyramidal cells during behavior, persists during subsequent off-line periods, such as quiet wakefulness and slow-wave sleep. This process may facilitate the consolidation of memories. The present study explored the hypothesis that age-related changes in this process might contribute to age-related memory impairments. Neuronal activity was recorded from CAI pyramidal cells in young and old rats during track running and off-line periods before and after track running. Two methods were used to quantify memory reactivation, one which measured similarities in activity patterns without considering the temporal order of neuronal activity (EV) and one that quantified the off-line preservation of temporal asymmetries formed between neurons during behavior (TB). A consistent similarity between the resting epoch activity patterns and those from the preceding behavior epoch was observed in both age groups, using the EV method. This similarity was strongest during sharp wave events. With the EV method, no age differences in the reactivation process were found in experiments using a familiar environment. In addition, the aged group exhibited greater reactivation versus the young group in the novel track experiments. In light of the observed age-related plasticity deficits these results suggest that memory reactivation, measured by EV, is not dependent on such mechanisms. In contrast, the TB method revealed a significant age-related deficit in the off-line preservation of temporal asymmetries. Thus, it appears that newly formed activity sequences are not preserved during off-line reactivation in aged animals, possibly because the new storage of activity sequences requires intact synaptic plasticity.