Fig. 3.

Arc expression in the granule cells of the dentate gyrus differentially correlates with age in rats. (A) Individual examples of behaviorally induced Arc RNA staining, a correlate of spike activity, in hippocampal granule cells of a young and an old rat. Red is the nuclear counterstain propidium iodide, and green is fluorescent-tagged (CY3) Arc RNA. (Scale bar, 20 μm.) (B) The average percent of Arc-positive neurons measured from the three different age groups (9, 15, and 24 months) are shown for individual hippocampal subregions. Note that there are significantly fewer Arc-positive granule cells in old rats compared with stability in the numbers of Arc-positive pyramidal cells. On average, older rats express Arc RNA in fewer cells than did young rats, and this significant age-related decline in Arc expression was observed only in the dentate gyrus.