Comparison of the immunocytochemical distribution of the phosphoprotein B-50 in the cerebellum and hippocampus of immature and adult rat brain.

In this study we compare the distribution of the phosphoprotein B-50 in two regions of immature and adult rat brain using affinity-purified antibodies to B-50. In the cerebellum of the 8-day-old rat we observed distinct patterns of distribution of B-50 immunoreactivity (BIR) in the premigratory zone and the developing molecular layer, likely associated with outgrowing parallel and climbing fibers contacting Purkinje cells in the internal granular layer and in axons coursing through the cerebellar medulla. In contrast, in adult cerebellum, a sparcer distribution of BIR as punctuate deposits is observed in the molecular layer, outlining dendritic trees and the perikarya of neurons. At relatively lower density BIR is found dispersed between the cells of the granular layer and along fibers in the white matter. In the immature hippocampal formation, fibers penetrating between unstained cells of the stratum pyramidale and the subiculum, and neuropil areas are immunostained. In the adult rat a graded immunostaining pattern corresponding to the laminar structure of the hippocampal formation is found with high density of BIR in the strata oriens, radiatum, parts of stratum lacunosum molecular and in the stratum molecular adjoining the field of the proximal apical dendrites of the granule cells. BIR appears to be absent from the proximal part of the mossy fiber pathway. In neuropil areas of adult hippocampus and cerebellum BIR is fairly restricted to dot-like deposits indicating a synaptic localization. This is in correspondence with our previous ultrastructural findings. The present observations in developing brain of B-50-like components in fibers, as well, suggest that B-50 (and/or B-50-like precursors) are involved in neurite outgrowth.