DOWN-REGULATION OF THE GONADOTROPIN-RELEASING-HORMONE RECEPTOR MESSENGER-RIBONUCLEIC-ACID BY ACTIVATION OF ADENYLYL-CYCLASE IN ALPHA-T3-1 PITUITARY GONADOTROPE CELLS

Pituitary regulation of reproductive processes depends on the sensitiv ity of gonadotrope cells to both positive and negative regulators. Hyp othalamic GnRH is the primary stimulus for gonadotropin synthesis and secretion. Therefore, the ability of the gonadotrope to respond to GnR H and the status of GnRH receptors (GnRH-R) are critical in the contro l of reproduction. In the present study, we address the role of GnRH a nd two second messenger activators, a phorbol ester (12-O-tetradecanoy lphorbol-13-acetate) and forskolin, in the regulation of GnRH-R gene e xpression in the alpha T3-1 gonadotrope cell line. Using Northern blot analysis to monitor endogenous GnRH-R steady state messenger RNA (mRN A) levels, we found that although GnRH and 12-O-tetradecanoylphorbol-1 3-acetate do not change GnRH-R mRNA levels, forskolin causes a time-de pendent decline. All three treatments stimulate glycoprotein cu-subuni t gene expression. To dissect the molecular mechanism of forskolin act ion on GnRH-R gene expression, de novo RNA synthesis was inhibited wit h the transcription inhibitor, actinomycin-D (act-D). Act-D alone does not change GnRH-R message levels. However, in the presence of both ac t-D and forskolin, GnRH-R mRNA levels decline dramatically. These data demonstrate that forskolin alters GnRH-R posttranscriptionally by des tabilizing its mRNA. Our data do not, however, exclude possible direct transcriptional effects. This study suggests that activators of the p rotein kinase-A pathway may alter gonadotrope sensitivity to GnRH by d ecreasing GnRH-R gene expression and, therefore, indirectly affect rep roductive status.