Hypothalamic gonadotropin-releasing hormone gene expression during rat estrous cycle

Expression of gonadotropin-releasing hormone (GnRH) gene in the hypothalamus of female rats was studied by the quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. During the estrous cycle, the GnRH mRNA level did not change from diestrus I to II, and then increased with a peak at 1100 h on proestrus. In the afternoon of proestrus, GnRH mRNA decreased rapidly with a nadir at 1600 h, and thereafter increased again and reached a peak at 1100 h on estrus. Since hypothalamic GnRH mRNA was found to be increased by subcutaneous implantation of estradiol-containing silastic tubing in ovariectomized rats, the peak of GnRH gene expression in the proestrous morning might be due to an increase in circulating estrogen in this phase of the estrous cycle. The surge of luteinizing hormone in the proestrous afternoon and the subsequent increase in GnRH mRNA were completely blocked by the injection of MK801, an antagonist for NMDA receptors, suggesting that the excitation of GnRH neurons leads to an increase in GnRH gene expression. This was further supported by the in vitro observation that high K(+)-induced membrane depolarization markedly increased GnRH mRNA in hypothalamic slices. This increase in GnRH mRNA due to neuronal excitation does not seem to require newly synthesized proteins because anisomycin, a protein synthesis inhibitor at the level of translation, did not affect GnRH gene expression. These results suggest that the hypothalamic GnRH mRNA level reached two peaks during the rat estrous cycle, i.e., in proestrous morning and estrous morning, and that estrogen and GnRH neuronal excitation play pivotal roles in regulating GnRH gene expression.