CalbindinD28k- and parvalbumin-immunoreactive neurons form complementary sublaminae in the rat superior colliculus

By using light microscopic immunocytochemistry and computer analysis, we have mapped the distributions of two calcium-binding proteins (CaBPs), calbindinD28k (CB) and parvalbumin (PV), in the rat superior colliculus (SC). The patterns of CaBP expression were complementary. A band of heavily labeled, medium-sized CB-immunoreactive cells (CB-cells) was centered in the optic layer (OL), whereas PV-immunoreactive cells (PV-cells) were found predominantly in the intermediate gray layer (IGL), where they were clustered within patches of PV-labeled fibers. The superficial gray layer (SGL) could be divided into two sublaminae. CB-cells were found mostly in the dorsal half of the SGL, whereas PV-cells were scattered throughout the ventral SGL and the dorsal OL. Most of the CaBP-immunoreactive cells in the SGL were small bipolar cells with vertically oriented dendrites; however, there were also some PV-cells with horizontally oriented dendrites. Quantitative analysis of the CaBP distributions reinforced our observations that these cells are distributed in complementary tiers that are not restricted to the traditional laminae. The size and shape of some of these tiers were determined from a three-dimensional reconstruction of serial sections. The complementarity of the CaBP-immunoreactive tiers was also confirmed by fluorescence microscopy of double-labeled sections, in which few if any double-labeled neurons were observed. Complementary tiers of CB-cells and PV-cells have been observed previously in the SC of the cat. The present results demonstrate them in another species and further suggest that there are functional sublaminae in the SC that can be distinguished by CaBP content.