The distribution of two calcium binding proteins, calbindin D-28K and parvalbumin, in the entorhinal cortex of the adult mouse.

The immunohistochemical localization of two specific calcium binding proteins, parvalbumin (PV) and calbindin D-28K (CB), were examined in the entorhinal cortex (EC) of the adult mouse. The PV and CB immunoreactivities exhibited a conspicuous regional and laminar distribution in the EC. The overall immunostaining pattern of PV and CB appeared to be complementary in the EC, especially in the medial entorhinal area (MEA). In the dorsal MEA, although layer 2 showed intense PV and CB immunostaining, the PV immunoreactivity was denser in layers 3, 5 and 6a than in layers 4 and 6b, whereas the CB immunoreactivity was denser in layers 4 and 6b than in layers 3, 5 and 6a. Moreover, we recognized the dorsoventral gradation of the PV and CB staining that is, in the dorsal to ventral direction, the intensity of the PV immunostaining in layers 2, 3, 5 and 6a gradually decreased whereas that of the CB immunostaining in those layers gradually increased. In addition, a similar dorsoventral gradation was also observed in the number of PV immunoreactive (PV-IR) and CB-IR neurons in layer 3. In layer 2 of the MEA, the CB-IR neurons were clustered, while displaying a patch-like pattern which could not be recognized in either Nissl staining or PV staining. In contrast, layer 2 of the LEA was separated into two sublayers, the superficial sublayer 2a and the deeper sublayer 2b; both of these sublayers consisted of cell clusters recognized by Nissl staining. These sublayers showed a prominent difference in their CB immunoreactivity; the cells in the layer 2a clusters were CB negative, whereas the cells in the layer 2b clusters were CB-IR. Furthermore, we also recognized a particular region at the most medial part of the MEA, where layer 2 was different from the other portion of the MEA regarding CB immunoreactivity and the cells containing another calcium binding protein, calretinin, were clustered in layer 3. Both the adjacent section technique and the fluorescent double-staining technique clearly revealed that a relatively large number of presumable interneurons contained both PV and CB immunoreactivities. Furthermore, the three neuron groups that were immunoreactive for both PV and CB, immunoreactive for PV alone and immunoreactive for CB alone were heterogeneous in their structural features such as shape and size, and no particular difference was found in their structural features among these three groups.