A developmental brain-wide screen identifies retrosplenial cortex as a key player in the emergence of persistent memory.

Memories formed early in life are short-lived while those formed later persist. Recent work revealed that infant memories are stored in a latent state. But why they fail to be retrieved is poorly understood. Here we investigated brain-wide circuit mechanisms underlying infantile amnesia. We performed a screen that combined contextual fear conditioning, activity-dependent neuronal tagging at different postnatal ages, tissue clearing and light sheet microscopy. We observed striking developmental changes in regional activity patterns between infant, juvenile, and adult mice, including changes in the retrosplenial cortex (RSP) that aligned with the emergence of persistent memory. We then performed a series of targeted investigations of RSP structure and function across development. Chronic chemogenetic reactivation of tagged RSP ensembles during the week after learning enhanced memory in adults and juveniles, but not in infants. However, after 33 days, reactivating infant-tagged RSP ensembles recovered forgotten memories. Changes in the developmental functions of RSP memory ensembles were accompanied by changes in dendritic spine density and the likelihood that those ensembles could be reactivated by contextual cues. These studies show that RSP ensembles store latent infant memories, reveal the time course of RSP functional maturation, and suggest that immature RSP functional networks contribute to infantile amnesia.