Parvalbumin positive basket terminals in layer II of the rat medial entorhinal cortex : postnatal development studied by immuno-electron microscopy
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Layer II (LII) of the adult medial entorhinal cortex (MEC) is highly involved in making up animals’ sense of space, and the intrinsic wiring of this network is thought to underlie the very specific firing patterns of its constituent cells. A key component of this network is the parvalbumin positive (PV+) basket cell, which provides rapid inhibition of local principal cells by targeting their perisomatic region, thus making them intimately involved in regulating the spatial codes of these cells. Also, PV+ basket cells target other interneurons in the local network. Because information about how these connections are refined is scarce, the current study was undertaken to investigate the ultrastructural and cytochemical development of the local output from LII PV+ basket cells. After establishing satisfactory protocols, brains from rats of postnatal day (P) 10, P15 and P30 were immunostained for PV and investigated at the light- and electron microscopic level. The results show that while PV+ cells are already abundant in LII of the MEC at P10, their local connectivity changes considerably over the next couple of weeks. First, the density of PV+ somatic terminals increases significantly between P10 and P15, but then decreases from P15 to P30. Secondly, the boutons themselves change in several respects: their size displays a U shaped trend across age groups; in terms of shape, they get more ellipsoid after P10; and the number of mitochondria increases significantly after P15. The extensive maturation of these features thus mainly occurs between P15 and P30. These results indicate that PV+ boutons in LII of the developing rat MEC undergo periods of overshoot and pruning, with concurrent strengthening of remaining terminals during the latter phase.