Transcriptomic signatures of hippocampal active place avoidance memory maintenance

The gene expression changes associated with memory acquisition, consolidation and reconsolidation-all active epochs in memory formation-have been well characterized in the rodent hippocampus. Less is known, however, of the changes in gene expression during the offline maintenance of memory. In this study, we measured the gene expression changes in the dorsal hippocampus of four mice 3 days after consolidation of an active place avoidance memory. We examined gene expression changes in a putative subset of memory-associated neurons by leveraging the immediate early gene <i>in vivo</i> tagging system of the Arc-Cre/flox-eYFP transgenic mouse line. Through spatial transcriptomics we found that memory trained animals exhibited spatially regionalized expression of genes involved in post-synaptic function in CA1, synaptic vesicle transport in CA3, and neuronal differentiation in DG. Surprisingly, these gene expression enrichments were not observed in eYFP mRNA positive spatial spots. To gain granularity into this finding, we carried out single nuclear RNA sequencing, which confirmed enrichment of differentially expressed genes associated with synaptic plasticity and post-synaptic signaling unique to each subregion in trained animals, but not from their eYFP mRNA positive nuclei. Notably, nuclei of hippocampal neurons were largely characterized by their down regulation of genes involved in ATP synthesis and cytoplasmic translation. Our results suggest that two overarching transcriptomic patterns contribute to the functional changes in hippocampal cells during offline memory maintenance: regionally distributed expression of genes linked to synaptic functions (with concomitant sparseness of memory-associated neuronal ensembles) and a reduction of metabolic activity related genes across hippocampal sub-regions.