The prolactin receptor in the fetal rat: cellular localization of messenger ribonucleic acid, immunoreactive protein, and ligand-binding activity and induction of expression in late gestation.

The cellular distribution and developmental expression of the PRL receptor (PRLR) in the late gestational fetal rat were examined by in situ hybridization, immunohistochemistry, and radioligand binding. Antisense and sense strand RNA probes encoding the long and short isoforms of the rat PRLR were hybridized to tissue sections under stringent conditions. Messenger RNA (mRNA) encoding the two isoforms of the receptor was expressed widely in tissues derived from all three germ layers; these included various tissues not known previously to contain lactogenic receptors, such as the olfactory neuronal epithelium and olfactory bulb, trigeminal and dorsal root ganglia, cochlear duct, brown adipose tissue, submandibular glands, whisker follicles, tooth primordia, and proliferative and maturing chondrocytes of developing bones. Prominent expression of PRLR mRNA was also detected in the fetal adrenal cortex, gastrointestinal and bronchial mucosae, renal tubular epithelia, choroid plexus, thymus, liver, pancreas, and epidermis. Immunohistochemical studies using monoclonal anti-PRLR antibodies demonstrated that the distribution of PRLR immunoreactivity was similar to that of PRLR mRNA, suggesting that the PRLR mRNA is translated to receptor protein in the fetus in vivo. The encoding of functional PRL receptor proteins by fetal PRLR mRNA was revealed by the presence of specific rat placental lactogen II-binding sites in fetal adrenal cortex, renal tubules, small intestinal villi, pancreatic ductules and islets, hepatic parenchymal cells, choroid plexus ependymal cells, and microsomal fractions of fetal lung and thymus. Levels of expression of PRLR mRNA and protein increased between days 17.5 and 20.5 of gestation in a number of fetal tissues, including the adrenal, pancreas, small intestine, pituitary, thymus, liver, and submandibular gland. The widespread expression of the PRLR in the fetal rat and the induction of receptor expression in late gestation suggest novel roles for the lactogenic hormones in fetal and neonatal development.