Dynamics of hippocampal reactivation for temporal association memory in mice

Reactivation refers to the re-emergence of activity in neuronal ensembles that were active during information encoding. Hippocampal CA1 neuronal ensembles generate firing activities that encode the temporal association among time-separated events. However, whether and how temporal association memory-related CA1 neuronal ensembles reactivate during sleep and their role in temporal association memory consolidation remain unclear. We utilized multiple unit recordings to monitor CA1 neuronal activity in mice learning a trace eyeblink conditioning (tEBC) task, in which presentation of the conditioned stimulus (CS, a light flash) was paired with presentation of the unconditioned stimulus (US, corneal puff) by a time-separated interval. We found that the CS-US paired training mice exhibited few conditioned eyeblink responses (CRs) at the initial-learning stage (ILS) and an asymptotic level of CRs at the well-learning stage (WLS). More than one third of CA1 pyramidal cells (PYR) in the CS-US paired training mice manifested a CS-evoked firing activity that was sustained from the CS to time-separated interval. The CS-evoked PYR firing activity was required for the tEBC acquisition and was greater when the CRs occurred. Intriguingly, the CS-evoked firing PYR ensembles reactivated, which coincided with increased hippocampal ripples during post-training sleep. The reactivation of CS-evoked firing PYR ensembles diminished across learning stages, with greater strength in the ILS. Disrupting the ripple-associated PYR activity impaired both the reactivation of CS-evoked firing PYR ensembles and tEBC consolidation. Our findings highlight the features of hippocampal CA1 neuronal ensemble reactivation during sleep, which support the consolidation of temporal association memory.