Turnover of Fear Engram Cells by Repeated Experience

A sparse population of neurons active during a learning event has been identified as memory engram cells. However, cells that are recruited to support memory when experience is repeated have been scarcely explored. Evidence from previous studies provides contradictory views. To address these questions, we employed learning-dependent cell labeling in the lateral amygdala (LA) and applied electrophysiological recording, spine imaging, and optogenetic tools to the labeled neurons with or without retraining. We found that engram cells established from original fear learning became dispensable for memory retrieval specifically with relearning, and this correlated with a reduction of synaptic transmission and loss of dendritic spines in these neurons. Despite such decreased connectivity, direct activation of these neurons resulted in fear-memory recall. We further identified that repeated memory was encoded in neurons active during relearning. These results suggest a shift in neuronal ensembles encoding fear memory in the LA by relearning through disconnection of the existing engram neurons established from original experience.