EXPRESSION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTOR GENE IS ALTERED BY GNRH AGONIST DESENSITIZATION IN A MANNER SIMILAR TO THAT OFGONADOTROPIN BETA-SUBUNIT GENES IN NORMAL AND CASTRATED RAT PITUITARY

It was previously established that the administration of a potent GnRH agonist such as triptorelin (n-Trp(6)-GnRH) induced desensitization o f pituitary gonadotropic cells, resulting in decreased expression of g onadotropin beta-subunit genes and the suppression of LH and FSH synth esis and release. Binding of GnRH to the pituitary is also affected by agonist treatment. To examine the desensitizing effects of GnRH agoni st on the expression of the pituitary GnRH receptor (GnRH-R) gene, mal e rats were given triptorelin (long-acting formulation, 300 mu g/kg), and levels of GnRH-R messenger RNA (mRNA) were determined by Northern and dot blot hybridization to a P-32-labeled rat complementary DNA pro be. Abundances of gonadotropin alpha-subunit, LH beta, and FSH beta mR NAs were examined in parallel, using appropriate probes. A rapid time- dependent decrease in the level of GnRH-R mRNA was observed in rats af ter triptorelin administration. A minimum residual level of mRNA, in t he range of 20-25% of the initial value, was attained as early as 5 h after treatment. Levels further stabilized to 25-30% after a small tra nsient increase to 45% on day 5. A single injection was effective for at least 30 days, after which GnRH-R mRNA levels slowly returned to no rmal, suggesting a progressive abolition of agonist effects. A concomi tant acute depletion of mRNA levels was observed for LH beta and FSH b eta (50% decrease in about 48 and 3 h, respectively), whereas the alph a-subunit message increased (rapidly reaching a level 1.8-fold that in control rats after 1-2 days). Castration induced a 3.8-fold elevation in the amounts of GnRH-R mRNA after 3 weeks, whereas alpha, LH beta, and FSH beta mRNAs increased by 6.2-, 7.9-, and 4.2-fold, respectively , compared to corresponding values in intact animals. Administration o f the GnRH agonist readily prevented, for as long as 3 weeks, the stim ulatory effects of castration on the GnRH-R mRNA and mRNAs for the bet a-subunit of gonadotropins, but not for the alpha mRNA, which remained at a high level. When triptorelin was administered 3 weeks postoperat ively, the castration-induced increase in LH beta and FSH beta was tot ally abolished, and no significant effect was noted on alpha-subunit m RNA. In conclusion, these data demonstrate that expression of the GnRH -R gene is subject to regulation and depends on GnRH stimulation, in a manner that indicates susceptibility to desensitizing action by the l ong-acting GnRH analog, triptorelin. This effect of desensitization is observed in normal and castrated rats, suggesting, in addition to a t onic action of endogenous GnRH on the expression of its own receptor g ene, the mediation via GnRH receptors of the effects of gonadectomy. E xpression of GnRH-R and gonadotropin beta-subunit genes respond simila rly to treatments, suggesting concerted regulation, in contrast to tha t of the alpha-subunit gene. The receptor and postreceptor mechanisms by which these genes are similarly or differentially affected by desen sitization remain to be elucidated.