Social Reactivation of Fear Engrams Enhances Memory Recall

For group-living species such as humans and rodents, conspecific interactions pervasively shape emotion ([1][1]–[3][2]), attention ([4][3]), and cognitive ability ([5][4]–[8][5]). Higher-order cognitive processes such as memory within a social brain are thus interlaced with social influences. Traditional laboratory rodent cages offer a limited but nonetheless rich multi-modal landscape of communication, including auditory calls ([9][6]–[12][7]), chemical signaling ([13][8], [14][9]), and tactile stimulation ([15][10], [16][11]). The absence of such social encounters in singly housed animals results in cognitive impairments and depression-like phenotypes ([17][12]), likely obscuring how the social brain has evolved to function. It is thus important to understand the relationship between social context and how individuals process memories. As social interaction recruits hippocampal ([18][13]) and amygdalar ([19][14]) circuitry that also serves as hubs for non-social memory traces([20][15]–[24][16]), we hypothesized that pre-existing ensembles in these regions can be modulated by social experiences and lead to changes in memory expression. Here we show that stressful social experiences enhance the recall of previously acquired fear memories in male but not female mice. Activity-dependent tagging of cells in the dentate gyrus (DG) during fear learning revealed that these ensembles were endogenously reactivated during the social experiences in males. These reactivated cells were shown to be functional components of engrams, as optogenetic stimulation of the cells active during the social experience in previously fear conditioned animals was sufficient to drive fear-related behaviors. Our findings suggest that social encounters can reactivate pre-existing DG engrams and thereby strengthen discrete memories. Significance Statement Social environments can bolster and protect cognitive abilities. However, the relationship between social stimuli and individually learned memories remains enigmatic. Our work reveals that exposure to a stressed, naïve non-familiar conspecific or to the ambient olfactory-auditory cues of a recently stressed familiar conspecific induces reactivation of the cellular ensembles associated with a fear memory in the hippocampus. Artificially activating the hippocampal ensemble active during the social experience induces fearful behaviors only in animals that have previously acquired a negative memory, suggesting a fear-driving function of the reactivated ensembles and demonstrating the interaction between individual history and social experience. The neural resurgence of fear-driving ensembles during social experiences leads to a context-specific enhancement of fear recall. Our findings provide evidence that unlike directly physical stressors, ambient social stimuli can reactivate and amplify an individual’s memories. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-3 [3]: #ref-4 [4]: #ref-5 [5]: #ref-8 [6]: #ref-9 [7]: #ref-12 [8]: #ref-13 [9]: #ref-14 [10]: #ref-15 [11]: #ref-16 [12]: #ref-17 [13]: #ref-18 [14]: #ref-19 [15]: #ref-20 [16]: #ref-24