Ovarian Steroid Regulation of Estrogen and Progesterone Receptor Messenger Ribonucleic Acid in the Anteroventral Periventricular Nucleus of the Rat

The anteroventral periventricular nucleus of the preoptic region (AVPV) represents a key site for hormonal feedback on gonadotropin secretion. It plays a critical role in the neural control of luteinizing hormone secretion and contains high densities of neurons that express receptors for estrogen and progesterone. In this study in situ hybridization was used to examine the expression of mRNAs encoding the estrogen (ER) and progesterone (PR) receptors in the AVPV during the estrous cycle. ER gene expression fluctuated during the cycle with the lowest levels of ER mRNA observed in animals killed on the afternoon of proestrus, and the highest levels present in animals killed during metestrus. This apparent inverse relationship between circulating levels of estradiol (E2) and ER mRNA levels in AVPV neurons was supported by the observation that treatment of ovariectomized rats with E2 suppressed expression of ER mRNA in the AVPV. The influence of progesterone (P4) on ER expression was less pronounced, but a significant increase in ER mRNA in the AVPV was detected 3 h after treatment with P4. In contrast, PR mRNA levels were highest in the AVPV during diestrus and lowest on the morning of proestrus suggesting that PR expression in the AVPV is regulated in a complex manner that may reflect the combined regulatory effects of E2 and P4. E2 treatment caused a dramatic induction of PR mRNA in the AVPV, but P4 did not affect PR mRNA expression acutely, although PR mRNA appears to be attenuated in the AVPV 27 h after P4 treatment. These findings suggest that ovarian steroid hormones regulate ER and PR gene expression in the AVPV during the estrous cycle, which may represent molecular events that contribute to cyclic changes in the responsiveness of AVPV neurons to steroid hormones.