PREFERENTIAL EXPRESSION OF LONG FORM PROLACTIN RECEPTOR MESSENGER-RNAIN THE RAT-BRAIN DURING THE ESTROUS-CYCLE, PREGNANCY AND LACTATION - HORMONES INVOLVED IN ITS GENE-EXPRESSION

The mRNA species for prolactin receptor (PRL-R) isoforms, long and sho rt form PRL-Rs, were estimated by the reverse transcription-polymerase chain reaction method in the rat brain (cerebrum) during the oestrous cycle, pregnancy and lactation. The levels of long form PRL-R mRNA in creased at pro-oestrus and oestrus, at the same time as serum prolacti n levels increased, whereas the mRNA level of short form PRL-R was rel atively unchanged. Long form PRL-R mRNA expression was also markedly i ncreased in the brain at mid- and late gestation, and this elevated mR NA level was maintained during the period of lactation. In contrast, b asal levels of short form PRL-R mRNA were also maintained throughout t hese periods of gestation and lactation. Ovariectomy moderately reduce d brain mRNA levels of both long and short form PRL-R from the levels of those in control dioestrous rats, and hypophysectomy further suppre ssed them to the lowest levels. Administration or oestradiol valerate (E(2)V) or 17 alpha-hydroxyprogesterone caproate (17OHPC) to ovariecto mized rats resulted in dramatic increases in long form PRL-R mRNA leve ls in the brain, whereas no significant increase in short form PRL-R m RNA was observed. In rats which were ovariectomized and hypophysectomi zed, the administration of 17OHPC, rat prolactin or rat GH partially r estored the brain level of long form PRL-R mRNA but not short form PRL -R mRNA. E(2)V, on the other hand, had no effect on the expression of brain PRL-R mRNAs in these hypophysectomized rats, suggesting that the stimulatory effect of E(2)V on long form PRL-R mRNA expression in ova riectomized rats was mediated by an enhanced secretion of a pituitary hormone, prolactin. These results suggest that the expression of long form PRL-R mRNA in the rat brain is directly induced by progesterone, prolactin or GH during the oestrous cycle, pregnancy and lactation.