Ensembles and engrams in mouse cortical and sub-thalamic brain regions supporting context and memory recall

Associative learning supports learning about outcomes associated with contexts and cues. During learning, cellular ensembles that become active can be incorporated into a memory engram and later reactivated to support memory recall. Studies exploring engram formation and reactivation have primarily used contextual conditioning in mice and made little distinction between engrams encoding contextual information versus cue-associated learning and recall. Furthermore, often missing in such analyses is exploration of sex differences in engram profiles. Using auditory fear conditioning and activity-dependent tagging in mice, we set out to disaggregate context-associated engrams from those associated with learning and recall while also profiling potential sex differences. Specifically, we quantified cellular activity during context exposure, fear recall, extinction training, and extinction recall in cortical and subthalamic brain regions supporting learning and memory. We found that male mice had larger ensembles of cells active in the infralimbic prefrontal cortex (IL-PFC) during context exposure while female mice recalling a fear memory had a significantly greater proportion of cells that were active in the IL-PFC independent of context. Across sexes, we found greater reactivation of extinction engrams in the IL-PFC compared to contextual engrams. While we found ensembles and engrams in the prelimbic prefrontal cortex (PL-PFC) and zona incerta (ZI), no sex differences were noted in these regions. These results not only emphasize that there is a distinction to be made between ensembles and engrams encoding contextual information from those encoding cue-associated learning and recall, but also highlight sex differences in ensemble and engram allocation.