DIFFERENTIAL REGULATION OF OXYTOCIN AND VASOPRESSIN MESSENGER-RIBONUCLEIC-ACID LEVELS BY GONADAL-STEROIDS IN POSTPARTUM RATS

We previously reported that sequential estradiol and progesterone expo sure followed by progesterone withdrawal increases oxytocin (OT), but not arginine vasopressin (AVP), messenger ribonucleic acid (mRNA) in t he hypothalamic paraventricular nucleus (PVN) of the rat. Substitution of testosterone for progesterone and subsequent testosterone withdraw al in the estrogen-primed rat increases PVN AVP mRNA levels. At the en d of pregnancy (day 21), rats are exposed to high estrogen and declini ng progesterone and testosterone concentrations. Coincident with these changes in circulating gonadal steroid hormones are increases in OT a nd AVP mRNAs. If progesterone levels are sustained at term, OT levels are attenuated and if testosterone is sustained, AVP mRNA levels are a ttenuated. Immediately postpartum, however, OT and AVP mRNA levels dec line compared to term levels. To further determine the role of estroge n in the regulation of OT and AVP mRNAs, we performed two experiments. In the first experiment, we administered estrogen during the peripart um period to determine if estrogen supplementation prevents the relati ve attenuation of OT and AVP mRNAs that is seen after parturition. Day 18 pregnant rats were given estradiol-filled or empty capsules and sa crificed on day 2 of lactation. By Northern analysis, significant diff erences in PVN AVP, but not OT, mRNA were found between the estrogen- and sham-treated lactational animals, P < 0.02. In the second experime nt, we determined if sustaining estrogen after progesterone is removed in steroid-treated ovariectomized rats is essential for the increase in OT mRNA. Ovariectomized rats were given either empty capsules or se quential estradiol- and progesterone-filled capsules and both were sus tained for 12 days. When progesterone-filled capsules were removed, es tradiol-filled capsules were either removed or left in place, and the animals were sacrificed 48 h later. PVN OT mRNA was analyzed by Northe rn blot hybridization. OT mRNA was increased in both of the steroid-tr eated groups to the same degree, compared to sham-treated animals, P = 0.04. In summary, estrogen supplementation during early lactation pre vents the attenuation of PVN AVP, but not OT, mRNA after parturition. In the estrogen-primed ovariectomized rat, it is not necessary to sust ain estrogen to see the effects of progesterone withdrawal upon PVN OT mRNA.