Dynamic regulation of gonadotropin-releasing hormone receptor mRNA levels in the anterior pituitary gland during the rat estrous cycle.

The number of gonadotropin-releasing hormone (GnRH) receptors on pituitary gonadotropes varies substantially during the rat estrous cycle and may modulate pituitary responsiveness to GnRH. The present studies were undertaken to determine to what extent these changes in GnRH receptor number reflect a change in GnRH receptor mRNA expression in the anterior pituitary gland. Using quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), pituitary GnRH receptor mRNA levels were measured at various timepoints throughout the rat estrous cycle. There was a three-fold increase in GnRH receptor mRNA levels on the afternoon of proestrus (PRO) when compared to levels observed on the morning of metestrus (MET). This rise preceded the onset of the LH surge by 6h (1200h). GnRH receptor mRNA levels remained elevated through 2100h PRO, after which they dropped dramatically, and by 2400h PRO were not significantly different from levels observed at 0900h MET. A two-fold increase in GnRH receptor mRNA expression was also observed during the early stages of the estrous cycle (0900h to 1800h MET), and this increase was sustained until 1800h on diestrus, at which time mRNA levels decreased to levels observed at 0900h MET. These results demonstrate that pituitary GnRH receptor mRNAs are dynamically regulated during the rat estrous cycle, with receptor mRNA expression being greatest on the afternoon of PRO, the time of the estrous cycle at which gonadotropes are most sensitive to GnRH stimulation.