Perineuronal nets affect parvalbumin expression in GABAergic neurons of the mouse hippocampus

Recent studies have suggested that the perineuronal net (PNN), a specialised extracellular matrix structure, and parvalbumin (PV), an EF-hand calcium-binding protein, are involved in the regulation of plasticity of neural circuits. Here, we aimed to quantitatively estimate the relationship between the two plasticity regulators, PV and PNNs, in the hippocampus of young adult mice. Dual fluorescence staining for PV and Wisteria floribunda agglutinin (a broad PNN marker) showed that a substantial population of PV-expressing (PV+) GABAergic neurons lacked PNNs. Optical disector analysis demonstrated that there were fewer PNN+ neurons than PV+ neurons. The ratio of PNN expression in PV+ neurons was generally lower in the dendritic layers than in the principal cell layers, whereas the ratio of PV expression in PNN+ neurons was effectively 100%. The mean PV fluorescence was significantly higher in PNN+/PV+ neurons than in PNN-/PV+ neurons. Cumulative frequencies for single-cell PV fluorescence indicated that intensely stained PV+ neurons tend to be enwrapped by PNNs, whereas weakly stained PV+ neurons are likely to lack PNNs. We digested the PNNs by a unilateral injection of chondroitinase ABC (chABC) into the dorsal CA1 region. Although the densities of PV+ neurons remained unchanged, the PV fluorescence declined 7days after chABC injection. Quantitative real-time polymerase chain reaction analysis demonstrated a reduction in PV mRNA expression following chABC injection. These findings indicate that the presence or absence of PNNs affects the relative PV expression in GABAergic neurons in the hippocampus.