Divergent recruitment of developmentally defined neuronal ensembles supports memory dynamics

Memories are dynamic constructs whose properties change with time and experience. The biological mechanisms underpinning these dynamics remain elusive, particularly concerning how shifts in the composition of memory-encoding neuronal ensembles influence a memory properties' evolution over time. By leveraging a developmental approach to target distinct subpopulations of principal neurons, we show that memory encoding results in the concurrent establishment of multiple memory traces in the mouse hippocampus. Two of these traces are instantiated in subpopulations of early- and late-born neurons and follow distinct reactivation trajectories post-encoding. Notably, the divergent recruitment of these subpopulations underpins memory ensembles’ gradual reorganization, and modulates memory persistence and plasticity across multiple learning episodes. Thus, our findings reveal profound and intricate relationships between ensemble dynamics and memories’ progression over time.