Area of expression of PNNs across the auditory cortex was highest at 4 hours after fear conditioning, but not different from controls by 24 hours after fear conditioning (A). In a separate group of animals sacrificed at 4 hours after tone experience (B), area of expression of PNNs was highest for the paired group as compared to home cage controls, and groups receiving only tone presentation or unpaired stimulation. Numbers of cells surrounded by PNNs are higher at 4 hours after fear conditioning than home cage group and the group sacrificed 24 hours after fear conditioning (C). In the same group of animals as in (B), the number of cells surrounded by PNNs was highest in the paired group 4 hours after auditory fear conditioning as compared to the home cage controls, tone alone and unpaired group 4 hours after tone experience (D). *P < 0.05 vs. vehicle. All values are means ±SEM.