Vitamin D-Dependent Calcium-Binding Protein in Rat Brain: Biochemical and Immunocytochemical Characterization*

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Abstract
Immunoreactive calcium-binding protein (CaBP) has been characterized in rat brain both biochemically and immunocytochemically. In this study antiserum to chick CaBP was used to characterize this protein and to describe its distribution in neurons and fibers of the rat fore- and midbrain. Immunostaining in neuronal elements was judged specific for this protein by the absence of staining in tissue sections after adsorption of the antiserum with either chick intestinal CaBP or the 28,000-dalton fraction from rat brain, but not with other molecular weight fractions with calcium-binding activity. Immunoreactive CaBP was found to have a widespread distribution throughout the central nervous system, and was present in most but not all major neuronal cell groups and fiber tracts. The protein was limited primarily to neuronal elements and some ependymal cells, and was absent in glia and blood vessels. The proportion of immunoreactivity in neuronal perikarya and fibers varied among nuclei and within a given structure at different rostral-caudal levels. Immunoreactivity was prominent in neocortex, hippocampal formation (primarily in CA1 and granular cells of the dentate gyrus), hypothalamus, and amygdala. These areas are responsible for the regulation of a variety of pituitary hormones, and several bind steroids. Immunoreactive CaBP was also a major constituent of nonlimbic system pathways. The widespread distribution of immunoreactive CaBP in the central nervous system suggests that CaBP and the vitamin D endocrine system may play a significant role in the regulation of mammalian brain function.