Differential regulation of oxytocin and vasopressin messenger ribonucleic acid levels by gonadal steroids in postpartum rats

Abstract

We previously reported that sequential estradiol and progesterone exposure followed by progesterone withdrawal increases oxytocin (OT), but not arginine vasopressin (AVP), messenger ribonucleic acid (mRNA) in the hypothalamic paraventricular nucleus (PVN) of the rat. Substitution of testosterone for progesterone and subsequent testosterone withdrawal in the estrogen-primed rat increases PVN AVP mRNA levels. At the end of pregnancy (day 21), rats are exposed to high estrogen and declining progesterone and testosterone concentrations. Coincident with these changes in circulating gonadal steroid hormones are increases in OT and AVP mRNAs. If progesterone levels are sustained at term, OT levels are attenuated and if testosterone is sustained, AVP mRNA levels are attenuated. Immediately postpartum, however, OT and AVP mRNA levels decline compared to term levels. To further determine the role of estrogen in the regulation of OT and AVP mRNAs, we performed two experiments. In the first experiment, we administered estrogen during the peripartum period to determine if estrogen supplementation prevents the relative attenuation of OT and AVP mRNAs that is seen after parturition. Day 18 pregnant rats were given estradiol-filled or empty capsules and sacrificed on day 2 of lactation. By Northern analysis, significant differences in PVN AVP, but not OT, mRNA were found between the estrogen- and sham-treated lactational animals, P < 0.02. In the second experiment, 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 sequential estradiol- and progesterone-filled capsules and both were sustained for 12 days. When progesterone-filled capsules were removed, estradiol-filled capsules were either removed or left in place, and the animals were sacrificed 48 h later. PVN OT mRNA was analyzed by Northern blot hybridization. OT mRNA was increased in both of the steroid-treated groups to the same degree, compared to sham-treated animals, P = 0.04. In summary, estrogen supplementation during early lactation prevents the attenuation of PVN AVP, but not OT, mRNA after parturition. In the estrogen-primed ovariectomized rat, it is not necessary to sustain estrogen to see the effects of progesterone withdrawal upon PVN OT mRNA.

Introduction

During the 48 h preceding and following parturition, dramatic changes in oxytocin (OT) and arginine vasopressin (AVP) mRNA levels occur in the hypothalamic paraventricular and supraoptic nuclei (PVN and SON) of the rat [3]. OT and AVP mRNA levels are at their peak at term (day 21 of gestation) 7, 11, 12and precipitously decline after delivery [3]. We recently reported that the changing pattern of gonadal steroid hormones that occurs during pregnancy and lactation accounts for these changes in OT and AVP mRNA levels 4, 9. Specifically, rising estrogen levels during pregnancy combined with declining progesterone and testosterone levels at term account for the increases in OT and AVP mRNAs, respectively 4, 9. If progesterone levels are sustained on day 21 of pregnancy, OT, but not AVP, mRNA levels are attenuated [4]. If testosterone levels are sustained at term, AVP mRNA levels are attenuated [9]. Thus, progesterone and testosterone are inhibitory to OT and AVP expression, respectively. In the setting of prior estrogen priming, the declines in progesterone and testosterone are stimulatory to OT and AVP mRNAs, respectively.

The peak levels of OT and AVP mRNA levels at term are not sustained following delivery [3], possibly due to the changing gonadal steroid milieu postpartum. During week one postpartum, estrogen levels are low [6], except for the brief postpartum surge of estradiol that occurs with the postpartum estrus [5], progesterone levels rise [6]and both OT and AVP mRNA levels are attenuated compared to term levels [3]. Thus, even if estrogen alone were sustained postpartum, OT mRNA levels might not increase because progesterone, which is inhibitory to OT expression [4], rises postpartum. On the other hand, the increase in AVP mRNA at term pregnancy might persist postpartum if estrogen levels were sustained because there is no postpartum rise in testosterone [8], which is inhibitory to AVP mRNA [9]. To test this hypothesis, we determined the effects of sustaining estrogen during early lactation upon PVN OT and AVP mRNA levels. We report that estrogen supplementation sustains AVP, but not OT, mRNA levels in early lactation.

We also used a second experimental paradigm in the ovariectomized rat to determine whether the ‘upregulatory’ effects upon OT of progesterone withdrawal in the estrogen-primed rat is dependent upon sustaining estrogen after progesterone levels decline. In a prior report [4], sequential estrogen and progesterone administration to, and subsequent progesterone removal from, ovariectomized rats increased OT mRNA. In that study, estrogen was maintained at the time of progesterone withdrawal. In the present study, we maintained estrogen at the time of progesterone withdrawal and compared this effect to simultaneously withdrawing both. We now report that progesterone withdrawal is critical to the increase in OT mRNA, irrespective of whether estrogen is maintained or simultaneously withdrawn.