Granule cells in the mouse hippocampus (shown in blue) that were active during the formation of a discrete memory were selectively targeted to express activity-dependent DREADDs (shown in green; designer receptors exclusively activated by designer drugs). These hippocampal cells were traditionally thought to be more active shortly after memory formation (e.g., one day later) and less so when accessed at more remote time points (e.g. 21 days later).