OVARIAN-STEROID REGULATION OF ESTROGEN AND PROGESTERONE-RECEPTOR MESSENGER-RIBONUCLEIC-ACID IN THE ANTEROVENTRAL PERIVENTRICULAR NUCLEUS OFTHE RAT

The anteroventral periventricular nucleus of the preoptic region (AVPV ) represents a key site for hormonal feedback on gonadotropin secretio n. It plays a critical role in the neural control of luteinizing hormo ne secretion and contains high densities of neurons that express recep tors for estrogen and progesterone. In this study in situ hybridizatio n was used to examine the expression of mRNAs encoding the estrogen (E R) and progesterone (PR) receptors in the AVPV during the estrous cycl e. ER gene expression fluctuated during the cycle with the lowest leve ls of ER mRNA observed in animals killed on the afternoon of proestrus , and the highest levels present in animals killed during metestrus. T his apparent inverse relationship between circulating levels of estrad iol (E2) and ER mRNA levels in AVPV neurons was supported by the obser vation that treatment of ovariectomized rats with E2 suppressed expres sion 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 reg ulated in a complex manner that may reflect the combined regulatory ef fects of E2 and P4. E2 treatment caused a dramatic induction of PR mRN A in the AVPV, but P4 did not affect PR mRNA expression acutely, altho ugh PR mRNA appears to be attenuated in the AVPV 27 h after P4 treatme nt. These findings suggest that ovarian steroid hormones regulate ER a nd PR gene expression in the AVPV during the estrous cycle, which may represent molecular events that contribute to cyclic changes in the re sponsiveness of AVPV neurons to steroid hormones.