A) Example image of WT naïve auditory cortex, with cropped image used for analysis (B). The far right panels identify examples of PNN surrounding non-PV cells (C; arrow), PV cells without PNN (E; arrow) and co-localized PV/PNN cells (D; arrow). F) Fear conditioning caused a decrease in PV density in both superficial (K) and deep layers, specifically in PV cells that were not surrounded by PNN (I; N). There was no genotype difference in PV cell density. L) There were overall fewer PNN cells in Fmr1 KO auditory cortex (deep layers), which were surrounding non-PV cells (O), but no difference was seen in PNNs which surround PV cells (H; M). In superficial layers WT mice up-regulate PNNs after naïve conditioning, and down-regulate them after fear conditioning, while Fmr1 KO mice show no change (G). Conditioning effect #, ##, ###; paired comparison *, **, *** (p = 0.05, 0.01, 0.001). N per group: WT Nv = 5; WT FrC = 5; WT C = 3, Fmr1 KO Nv = 6, Fmr1 KO FrC = 6, Fmr1 KO C = 5. Image # per group: WT Nv = 19, WT FrC = 18, WT C = 15, Fmr1 KO Nv = 18, Fmr1 KO FrC = 18, Fmr1 KO C = 18.