SEQUENTIAL EXPOSURE TO ESTROGEN AND TESTOSTERONE (T) AND SUBSEQUENT WITHDRAWAL OF T INCREASES THE LEVEL OF ARGININE-VASOPRESSIN MESSENGER-RIBONUCLEIC-ACID IN THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS OF THE FEMALE RAT

The hypothalamic peptides arginine vasopressin (AVP) and oxytocin (OT) have been implicated as mediators of socio-sexual behaviors in additi on to their roles in osmolar homeostasis (AVP), milk ejection and uter ine contractility (OT), Within 24 h of parturition, OT and AVP messeng er ribonucleic acid (mRNA) levels increase in the hypothalamic paraven tricular, and to a lesser degree, the supraoptic nucleus (PVN and SON) of the rat, We previously reported that the prepartum increase in OT mRNA is related to the spontaneous decline in progesterone levels prio r to parturition. We also reported that increases in PVN and SON OT mR NA can be induced by exposing the ovariectomized rat to a steroid regi men that mimics the steroid milieu of pregnancy, namely sequential est rogen and progesterone and subsequent progesterone withdrawal, Levels of PVN and SON AVP mRNAs were not affected by progesterone withdrawal in late pregnant rats or the steroid regimen that increased OT mRNA in ovariectomized rats, These observations suggest that other factors, p erhaps hormonal, may influence AVP mRNA levels. A decline in testoster one coincident with waning progesterone levels also occurs prepartum, Since peak levels of AVP mRNA prepartum coincide with the prepartum de cline in testosterone, we questioned whether declining testosterone le vels are important for the increase in AVP mRNA levels. To examine a p ossible role for testosterone in the increased level of AVP mRNA in la te pregnancy, we sequentially administered estradiol and testosterone long-term (2 weeks) and removed testosterone 48 h prior to sacrifice. This steroid regimen mimics the estrogen and testosterone pattern of l ate pregnancy in rats, AVP, but not OT, mRNA levels increased signific antly in the PVN of ovariectomized rats receiving this steroid regimen . We also found that implantation of late pregnant rats with testoster one capsules to prevent the spontaneous prepartum decline in testoster one, attenuates the increase in PVN AVP, but not OT, mRNA on day 21 of pregnancy, The data show that sequential estrogen and testosterone an d testosterone withdrawal increase the level of PVN AVP mRNA in the fe male rat.