Subclass-specific formation of perineuronal nets around parvalbumin-expressing GABAergic neurons in Ammon's horn of the mouse hippocampus

Perineuronal nets (PNNs) are closely associated with parvalbumin-positive (PV+) neurons, and play a major role in controlling developmental neural plasticity. Considering the recent advances in classification of PV+ neurons, here we aimed to clarify whether PNNs might be associated with specific subclasses of PV+ neurons in the hippocampus. In this study, we labeled PNNs by Wisteria floribunda agglutinin (WFA), and classified PV+ neurons based on the combination of cellular location, molecular expression (neuropeptide Y [NPY], somatostatin [SOM], special AT-rich sequence-binding protein-1 [SATB1]), and retrograde tracing through stereotaxic injection of Fluoro-Gold (FG) into the medial septum. The criteria of each subclass can be summarized as follows: axo-axonic cells, PV+/SATB1-/NPY- cells in the stratum pyramidale; basket cells, PV+/SATB1+/NPY- cells in the stratum pyramidale; bistratified cells, PV+/SATB1+/NPY+ cells in the stratum pyramidale; oriens-lacunosum-moleculare (O-LM) cells, PV+/SOM+/FG- cells in the stratum oriens; hippocampo-septal projection (H-S) cells, and PV+/SOM+/FG+ cells in the stratum oriens. The ratios of formation of WFA+ PNNs around each subclass of PV+ neurons were estimated according to the optical disector principle. The vast majority (over 90%) of putative PV+ basket cells were surrounded by PNNs, while only a minor population (less than 10%) of putative PV+ axo-axonic, O-LM, and H-S cells were enwrapped with PNNs. The ratios of formation of PNNs around putative PV+ bistratified cells were intermediate (25-50%). These findings indicate that PNNs may be specifically associated with PV+ basket cells, and also provide a key to understand the functional significance of PNNs and PV+ neurons in the hippocampus.