Expression of proenkephalin mRNA in developing cerebellar cortex of the rat: expression levels coincide with maturational gradients in Purkinje cells

Abstract

The cellular localization of proenkephalin (PE) mRNA expression was systematically examined in midsagittal (vermal) sections of the developing rat cerebellar cortex by in situ hybridization. PE mRNA was initially detected in Golgi cells of postnatal day 7 (PND 7) rats and in each group thereafter. Moreover, PND 7 rats also displayed an intense layer of PE mRNA hybridization signal over the Purkinje mell layer. By PND 14,mdistinct cellular labeling was observed in a subpopulation of Purkinje cells in all lobules of the vermis except lobule III. At PND 7 and 14, the area and level of intensity of Purkinje cell associated PE mRNA hybridization signal followed a gradient that was most intense caudally but then decreased rostrally. At PND 21, the proportion of labeled Purkinje cells and the intensity of PE hybridization signal was evenly dispersed between the anterior and posterior lobules of the cerebellar vermis. PE hybridization signal was not detected in the developing neural cells of the external granular layer or the interneurons of the molecular layer in the vermis. These results indicate that the ontogeny of PE mRNA expression in Purkinje cells is developmentally regulated since levels of expression closely follow the chronological order of settling and maturation of these neurons. Based on prior evidence that endogenous opioids inhibit the growth of Purkinje cell dendrites and dendritic spines, PE expression is likely to be important for Purkinje cell maturation.