ARC and Alzheimer disease: Our findings of unchanged Arc transcription in the visual cortex contrasted with a previous study from our laboratory showing decreased Arc expression in the CA1 pyramidal layer of the hippocampus in the rTg4510 model mice after exploration of a novel environment [21] and with the earlier mRNA profiling of the human AD brains which found lower amounts of the Arc mRNA in tangle-bearing CA1 neurons compared to tangle-free neurons [32].