Social Reactivation of Fear Engrams Enhances Recall and Reinstatement

The power of one9s social environment to bidirectionally modulate cognitive abilities is well documented, but the processes by which social experiences impact the cellular substrates of memory remain unknown. Here we show that social interactions and exposure to ambient stimuli emitted by stressed conspecifics, but not individually experienced physical stress, enhance the recall and reinstatement of previously acquired fear memories. Activity-dependent tagging of cells in the dentate gyrus of the hippocampus during fear learning revealed that these ensembles were endogenously reactivated during the social, but not directly stressful, experiences. These reactivated cells were revealed to be functional engrams, as optogenetic stimulation of the cells active during the social experience was sufficient to drive fear-related behaviors only in animals that had previously been fear conditioned. Our findings suggest that social encounters can reactivate pre-existing engrams and thereby strengthen discrete memories.