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 rece ptor (PRLR) in the late gestational fetal rat were examined by in situ hybridization, immunohistochemistry, and radioligand binding. Antisen se and sense strand RNA probes encoding the long and short isoforms of the rat PRLR were hybridized to tissue sections under stringent condi tions. Messenger RNA (mRNA) encoding the two isoforms of the receptor was expressed widely in tissues derived from all three germ layers; th ese included various tissues not known previously to contain lactogeni c receptors, such as the olfactory neuronal epithelium and olfactory b ulb, 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, cho roid plexus, thymus, liver, pancreas, and epidermis. Immunohistochemic al studies using monoclonal anti-PRLR antibodies demonstrated that the distribution of PRLR immunoreactivity was similar to that of PRLR mRN A, 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 placenta l lactogen II-binding sites in fetal adrenal cortex, renal tubules, sm all intestinal villi, pancreatic ductules and islets, hepatic parenchy mal 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 feta l and neonatal development.