Hippocampal CA2 ripples recruit social replay and promote social memory

Abstract

The consolidation of spatial memory depends on the reactivation ( replay) of hippocampal place cells that were active during recent behavior. These reactivations are observed during sharp wave-ripples (SWRs), synchronous oscillatory events that occur during slow-wave sleep1-9 and whose disruption impairs spatial memory consolidation 4,6,7,9. Although the hippocampus encodes a wide range of non-spatial forms of declarative memory, it is not yet known whether SWRs are necessary for non-spatial memory. Moreover, although SWRs can arise from either the hippocampal CA38 or CA210 regions, the relative importance of these sources for memory consolidation is unknown. Here we examined the role of SWRs during the consolidation of social memory, the ability of an animal to recognize and remember a conspecific, focusing on CA2 because of its critical role in social memory11,12,13. We found that ensembles of CA2 pyramidal neurons that were active during social exploration of novel conspecifics were reactivated during SWRs. Importantly, disruption or enhancement of CA2 SWRs suppressed or prolonged social memory, respectively. Thus, SWR reactivation of hippocampal firing related to recent experience appears to be a general mechanism for binding spatial, temporal and sensory information into high-order memory representations.