Old mice fail to integrate a memory update into an existing hippocampal engram.

Existing memories can be updated by the presentation of new information during memory retrieval. Memory updating is impaired with age, and recent reports indicate this process is more susceptible to age-related impairments than the formation of new memories. However, the neuronal mechanisms underlying age-related updating impairments are unknown. Here, we investigated how memory engrams within the dorsal hippocampus encode a memory update in the young and old brain. We found that old mice tended to re-engage a smaller proportion of the original memory engram during the update session and chemogenetically increasing the activation of this engram alleviated age-related updating deficits. A range of therapeutically relevant behavioral and pharmacological approaches promoting re-engagement of the training engram also improved memory updating in old mice. Together, these results identify a novel mechanism by which memory updating is impaired with age and expand our understanding of how the brain organizes related information.