ARC and Mental deterioration: In the present study, we identified two hippocampal genes, the brain‐specific activity‐dependent transcription factor Npas4 (neuronal Per‐Arnt‐Sim domain protein 4) and the immediate early gene Arc (activity‐regulated cytoskeleton‐associated protein), as being differentially expressed with ageing, cognitive decline and 11β‐HSD1 deficiency.