Identification of cell ensembles responsible for memory retrieval after repetition of learning in the dentate gyrus

Abstract

Reactivation of hippocampal cells activated during contextual fear conditioning (CFC) is required for later memory retrieval and subsequent re-learning. However, whether cells that were active during initial learning are still necessary for memory retrieval after re-learning has not been directly tested. By using FosTRAP mice, I labeled Fos-expressing cells in the dentate gyrus (DG) of hippocampus during contextual fear conditioning with eNpHR3.0-EYFP and examined whether activation of those cells are necessary for memory retrieval. Inhibition of labeled cells impaired specifically for contextual memory retrieval. However, when such labeled cells were inhibited during retrieval after re-learning, mice displayed normal context freezing. I also found that inhibition of labeled Fos-expressing cells during re-conditioning results in light illumination-dependent memory retrieval. Finally, I found that Fos-expressing cells that were active during re-conditioning were required for subsequent memory retrieval. Together, these results suggest formation of distinct neuronal ensembles in the DG that were active during initial learning or re-learning, and identify cell ensembles responsible for memory retrieval after repetition of learning in the dentate gyrus.