Postnatal development of parvalbumin-immunoreactive cells in the medial entorhinal cortex of the rat : morphology, connectivity and ultrastructure
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Grid cells in layer II of the the medial entorhinal cortex (MEC) generate metric spatial representations. Resent research indicates that the grid-cell firing pattern may emerge through recurrent connections with local parvalbumin-containing (PV+) interneurons. Knowledge about the MEC PV+ cells and their connectivity with grid cells have mainly been obtained from electrophysiological studies in the adult MEC, and little is known about the PV+ cells or their connectivity within the MEC at the ultrastructural level. Although the PV+ cells and their connections must be present before the grid firing pattern can emerge, little is known about the PV+ cells or their connections during development. In this study, maturation of PV+ cells within the rat MEC was examined at postnatal day 10, 15 and 30 by use of immunohistochemistry, light- and electron microscopy. Special attention was paid to PV+ somata within dorsal layer II, and the axonal terminals innervating them. The results indicated that PV was expressed in the MEC at P10, but the expression was especially dense in layer II and appeared to follow a dorso-ventral gradient in layer I. At P10, PV+ somata within dorsal layer II appeared immature, with small nuclear indentations and somatic spines, and were contacted by immature-looking PV+ and immunonegative axon terminals. Maturation of both the PV+ cell bodies and the axonal innervation seemed to occur several weeks into the postnatal period. The average number of PV+ axon terminals appeared to overshoot, followed by a period of pruning. Large variations in the number of PV+ axon terminals within each age group indicated that the interconnectivity between MEC PV+ cells may be more intricate than previously thought.