NREM consolidation and increased spindle counts improve age-related memory impairments and hippocampal representations

Age-related changes in sleep patterns have been linked to cognitive decline. Specifically, age is associated with increased fragmentation of sleep and wake cycles. Yet it remains unknown if improvements in sleep architecture can ameliorate cellular and cognitive deficits. We evaluated how changes in sleep architecture following sleep restriction affected hippocampal representations and memory in young and old mice. Following training in a hippocampus dependent object/place recognition task, control animals were allowed to sleep normally, while experimental animals underwent 5 hr of sleep restriction (SR). Interestingly, old SR mice exhibited proper object/place memory, similarly to young control mice, whereas young SR and old control mice did not. Successful memory correlated with the presence of two hippocampal cell types: 1) 9Context9 cells, which remained stable throughout training and testing, and 2) 9Object9 cells, which shifted their preferred firing location when objects were introduced to the context and moved during testing. As expected, EEG analysis revealed more fragmented sleep and fewer initial spindles in old controls than young controls during the post-training sleep period. However, following the acute SR session old animals displayed more consolidate NREM and increased spindle count, while young mice did not significantly display changes in sleep architecture. These results indicate that consolidation of NREM sleep and increases in spindle numbers serve to ameliorate age-related memory deficits and allow hippocampal representations to adapt to changing environments.