The regulation of prolactin receptor messenger ribonucleic acid levels in the sheep liver before birth: Relative roles of the fetal hypothalamus, cortisol, and the external photoperiod

We have investigated the separate actions of hypothalamo-pituitary disconne ction (HPD), with or without cortisol administration, and changes in the ex ternal photoperiod on the regulation of the levels of messenger RNA (mRNA) encoding long (PRLR1) and short (PRLR2) forms of PRL receptor in the liver of the fetal Iamb. In pregnant Merino ewes (n = 20), the hypothalamus and p ituitary were surgically disconnected in 13 fetuses (HPD group), and fetal vascular catheters were implanted in the HPD group and in an additional 7 f etuses (intact + saline group) between 104-120 days gestation (d). Fetal sh eep in the HPD group were infused with either cortisol (3.5 mg/4.8 mi salin e/24 h; HPD + F; n = 5) or saline for 5 days between 134-141 d, and saline was also infused in the intact group within the same gestational age range. A second group of pregnant ewes (n = 12) was kept in a 12-h Light, 12-h da rk cycle from 70 d until implantation of fetal vascular catheters between 1 06-120 d, after which ewes were allocated to either a long photoperiod (16 h of Light, 8 h of darkness; LL group; n = 6) or a short photoperiod (8 h o f light, 16 h of darkness, SL group; n = 6) regimen. Circulating cortisol c oncentrations were higher (P < 0.05) in the intact fetal sheep (18.7 +/- 3. 8 nmol/liter) than in the HPD + saline group (1.5 +/- 0.6 nmol/liter), and were further increased (P < 0.05) in the KPD + cortisol group (97.4 +/- 23. 7 nmol/liter). Fetal PRL concentrations were lower (P < 0.05) in the HPD saline (10.6 +/- 4.3 ng/ml) and HPD I. cortisol (5.6 +/- 2 ng/ml) groups co mpared with those in the intact group (38.9 +/- 6,8 ng/ml). The levels of h epatic PRLR mRNA were higher (P < 0.05) in the intact (PRLR1, 27.4 +/- 6.1; PRLR2, 17.7 +/- 2.5) and HPD + cortisol(PRLR1, 23.4 +/- 0.41 PRLR2 15.3 +/ - 3.0) groups than in the HPD + saline group (PRLR1, 10.6 +/- 1.8; PRLR2, 8 .9 +/- 1.8) at 140/141 d. The mean plasma PRL concentration in the LL group (70 +/- 9 ng/ml) was higher (P < 0.05) than that in the SL group (34 +/- 1 5 ng/ml), whereas the levels of hepatic PRLR1 mRNA IU group, 4.6 +/- 0.9; ( SL group, 4.3 +/- 0.8) and PRLR2 mRNA (LL group. 3.4 +/- 0.4; SL group, 3.0 +/- 0.5) at 140-141 d were not different. These data indicate that cortiso l acts directly or indirectly to maintain hepatic PRLR mRNA levels in the s heep fetus during late pregnancy. In contrast, changes in the external phot operiod and circulating PRL concentrations in the sheep fetus do not direct ly after PRLR expression in the fetal liver. These studies provide further insight into the role that the PRL axis may play in the transduction of sig nals about the external environment to the fetus as it prepares far the tra nsition to extrauterine life.