Artificial hibernation reveals synaptic engram architecture associated with memory retention

Memories leave lasting physical changes at the synaptic level. Although stable, larger spines are thought to support memory, the high turnover of dendritic spines and the drifting of neuronal representations after memory formation suggest alternative possibilities. To elucidate the structural trace underlying memory retention, we used a mouse model of artificial hibernation. During hibernation, hippocampal neurons exhibited a substantial reduction in their activity and an extensive elimination of dendritic spines and synapses. Despite these changes, their memory and associated hippocampal neuronal representations remained intact. We found that a subset of spines characterized by synaptic contacts with multisynaptic boutons is maintained during hibernation. These findings suggest that synaptic engram architecture, rather than larger spines per se, is resilient to network remodeling and associated with long-term memory retention.