ABSTRACT Memory engrams are both necessary and sufficient to mediate behavioral outputs. Defensive behaviors such as freezing and avoidance are commonly examined during hippocampal-mediated fear engram reactivation, yet how reactivation of these cellular populations across different contexts engages the brain to produce a variety of defensive behaviors is relatively unclear. To address this, we first optogenetically reactivated a tagged fear engram in the dentate gyrus (DG) subregion of the hippocampus across three distinct contexts. We found that there were differential amounts of light-induced freezing depending on the size of the context in which reactivation occurred: mice demonstrated robust light-induced freezing in the most spatially restricted of the three contexts but not in the largest. We then utilized graph theoretical analyses to identify brain-wide alterations in cFos co-activation during engram reactivation across the smallest and largest contexts. Our manipulations conferred greater positive cFos correlations and recruited regions spanning putative fear and defense systems as hubs in the respective networks. Moreover, reactivating DG-mediated engrams generated network topologies across experimental conditions, emphasizing both shared and distinct features. By identifying and manipulating the circuits supporting memory function, as well as their corresponding brain-wide activity patterns, it is thereby possible to resolve systems-level biological mechanisms mediating memory’s capacity to modulate behavioral states. SIGNIFICANCE STATEMENT Implementing appropriate defensive behaviors across disparate environments is essential for survival. Memories can be used to select these responses. Recent work identified and artificially manipulated cellular ensembles within the hippocampus that mediate fear memory recall, yet how these populations engage brain-wide pathways that mediate defensive behaviors under environmental contingencies is unclear. We demonstrated here that reactivation across environments of various sizes elicits different behavioral responses and corresponding brain-wide network dynamics. These findings establish the flexibility of memory-bearing ensembles in generating brain and behavior states.

Hippocampal engrams generate flexible behavioral responses and brain-wide network states