OXYTOCIN RECEPTOR MESSENGER-RIBONUCLEIC-ACID - CHARACTERIZATION, REGULATION, AND CELLULAR-LOCALIZATION IN THE RAT PITUITARY-GLAND

The hypothalamic neuropeptide oxytocin (OT) stimulates the release of several pituitary hormones, including ACTH, LH, and PRL. Although spec ific OT receptors have been identified in anterior pituitary membranes , the structure and cellular localization of these binding sites have not been elucidated. We previously cloned a rat OT receptor (OTR) gene and showed that its expression in rat uterus results in several trans cripts ranging in size from 2.9-6.7 kilobases. In this study we show, by using Northern blot analysis, reverse transcriptase-polymerase chai n reaction, and ultrastructural in situ hybridization that the same OT R gene is also expressed in the pituitary, where it gives rise to a 6. 7- and a 4.8-kilobase messenger RNA. Ultrastructural in situ hybridiza tion combined with immunogold labeling indicated that pituitary OTR ge ne expression is highly cell-specific and restricted to lactotrophs. I n accordance with this finding, only the lactotroph-derived cell line MMQ expressed the OTR gene among several pituitary cell lines tested. Northern blot analysis, reverse transcriptase-polymerase chain reactio n, and in situ hybridization analysis indicated a dramatic increase in pituitary OTR gene expression at the end of gestation and after estro gen treatment. Our results suggest that the OT effect on lactotrophs i s direct, whereas OT actions on gonadotrophs and corticotrophs are eit her indirect or mediated via different receptors. Moreover, our findin gs imply that OT exerts its full potential as a physiological PRL-rele asing factor only towards the end of gestation, and that therefore the role of OT as a hypothalamic PRL-releasing factor may so far have bee n underestimated.