Density counts of cells positive for Arc immunoreactivity in the AC (A), PL (B), or IL (C) subregions of the mPFC of male and female rats exposed to either group or isolation rearing (means . Rats then were subjected to either a 15 minute exposure to a novel same-sex conspecific (Social) or remained in their home cages (No Social). ** p < .01, *** p < .001, Significantly different from same sex, same rearing condition rats not exposed to a novel conspecific. ## p < .01, significantly different from same-sex, isolation reared rats exposed to a novel conspecific. @ p < .05, significantly different from female group-reared rats exposed to a novel conspecific. Values are means ± SEMs.