Hippocampi were bisected, flash-frozen in liquid nitrogen, and stored at -80°C. For ATAC-seq, nuclei were isolated from tissue using iodixanol gradient and DNA was transposed with Tn5 transposase followed by MinElute Reaction Cleanup kit purification. DNA was quantified using the PicoGreen dsDNA Assay Kit. ATAC-seq library preparation was performed in accordance with the ATAC-seq protocol as described by Buenrostro et al. (2015). Libraries were assessed for quality using electrophoresis on an Agilent Bioanalyzer (Agilent) and size-selected for 200~800 bp fragments.