Ontogenesis of prolactin receptor gene expression in the rat olfactory system: potential roles for lactogenic...

The PRL receptor (PRLR) is expressed at very low levels in the olfactory bulb of the adult rat but is detected in abundance in the olfactory epithelium and olfactory bulb of the fetal rat in late gestation. To explore potential roles for the lactogenic hormones in olfactory differentiation and development, we have used in situ hybridization and immunohistochemistry to examine the ontogeny of PRLR gene expression in the rat olfactory system. At midgestation (embryonic day 12.5), messenger RNAs (mRNAs) encoding the long and short isoforms of the rat PRLR were detected in the medial and lateral nasal processes, the epithelial lining of the olfactory pit, and the neuroepithelium lining the cerebral ventricles in the region of the rhinencephalon. PRLR mRNA was also expressed prominently in the frontonasal mesenchyme and in mesenchymal tissue underlying the developing brain and overlying the pontine flexure in the interpeduncular fossa. The distribution of PRLR immunoreactivity was similar to that of PRLR mRNA, indicating that the PRLR gene is translated to that of PRLR mRNA, indicating that the PRLR gene is translated to lactogenic binding protein in the rat embryo in vivo. With advancing gestation, the PRLR was expressed intensely, although discontinuously, in the olfactory epithelium and was detected in the cartilage primordia of the ethmoid, sphenoid, temporal, and mandibular bones. PRLR expression in the vomeronasal organ was confined to the luminal epithelial surface. PRLR mRNA and immunoreactive protein were first detected in the olfactory bulb on embryonic day 18. PRLR expression was most intense initially in the periventricular neuroepithelium; subsequently, robust staining of the mitral and tufted cell neurons became apparent, accompanied by intense PRLR expression in the sensory neuronal cell bodies of the olfactory epithelium. By postnatal day 5, the PRLR was expressed in abundance in mitral and tufted cells of the olfactory bulb and in neuronal cell bodies of the anterior olfactory nucleus and the piriform cortex. PRLR mRNA was also detected in the mitral cells of the olfactory bulb of the lactating rat, although at levels far lower than those in the fetal or neonatal rat. The expression of the PRLR in the olfactory system of the fetal and neonatal rat implicates novel roles for the lactogenic hormones in olfactory differentiation and development and may provide new mechanisms by which the lactogens may regulate neonatal behavior and maternal-infant interactions.