Stimulation of gonadotropin-releasing hormone surges by estrogen. II. Roleof cyclic adenosine 3 ',5 '-monophosphate

Release of GnRH surges in female rats is directed by a daily neural signal and occurs only after exposure of the hypothalamus to sustained, elevated e strogen (E-2) levels in serum. We have proposed that preovulatory E-2 coupl es the daily neural signal to the circuitry governing GnRH release by a two -step process, which includes stimulation of neuronal progesterone receptor s (PRs) by E-2 and subsequent activation of PRs by the daily neural signal. In the preceding report we documented that PR activation is obligatory for the stimulation of GnRH surges by E-2. In these studies we assess the vali dity of a second essential feature of this model, that neural signals can a ctivate PRs and thereby prompt the release of GnRH and LH surges. Our effor ts specifically focused on the role of cAMP in mediating neural PR trans-ac tivation leading to GnRH surges. To assess whether cAMP may function as a d aily neural signal, cAMP levels were examined via a competitive binding ass ay in anteroventral periventricular nucleus (AVPV) homogenates obtained at 0900, 1200, 1500, 1800, and 2100 h on all days of the estrous cycle. A sign ificant rise in cAMP concentrations was observed at 1500 h on all estrous c ycle days. A similar rise at the same time was observed in AVPV tissues of ovariectomized (OVX) rats regardless of steroid treatment. No significant i ncrease in cAMP levels was observed at any time point in homogenates of ven tromedial nucleus or cerebral cortex. In a second experiment, female rats w ere OVX on the afternoon of diestrous day 2 and simultaneously administered 30 mu g estradiol benzoate or oil vehicle. On the following day of presump tive proestrus, rats received intracerebroventricular infusions of the cAMP analog, 8-bromo-cAMP, or saline vehicle at 0900 h. Rats treated with 8-bro mo-cAMP exhibited LH surges that were advanced by 3 h compared with those i n saline-treated controls. This advance did not occur in 8-bromo-cAMP-treat ed rats not primed with E-2, or in E-2-treated rats given the antiprogestin RU486. In a third experiment, OVX, estradiol benzoate-primed rats received intracerebroventricular infusions of saline vehicle or the adenylyl cyclas e inhibitor SQ22536; although saline-treated rats exhibited normal LH surge s, no surges were observed in the rats receiving SQ22536. In additional SQ2 2536-treated animals, however, LH surge release was rescued and greatly aug mented by a pharmacological dose of progesterone. These results demonstrate that 1) cAMP levels in the AVPV are significantly elevated at 1500 h on a daily basis; 2) cAMP elevations in the AVPV can prematurely evoke LH surges by a mechanism that requires PR activation; 3) inhibition of adenylyl cycl ase activity in the AVPV blocks LII surges, an action that can be reversed by progesterone; and 4) cAMP generation leads to PR trans-activation in the AVPV. Our observations thus provide support for the hypothesis that an inc rease in intracellular cAMP in the AVPV acts as a component of the daily ne ural signal required to initiate GnRH and subsequent LH surges, and that tr ansmission of this signal is mediated by cAMP-induced PR trans-activation i n the AVPV.