It was previously established that the administration of a potent GnRH agonist such as triptorelin (D-Trp6-GnRH) induced desensitization of pituitary gonadotropic cells, resulting in decreased expression of gonadotropin β-subunit genes and the suppression of LH and FSH synthesis and release. Binding of GnRH to the pituitary is also affected by agonist treatment. To examine the desensitizing effects of GnRH agonist on the expression of the pituitary GnRH receptor (GnRH-R) gene, male rats were given triptorelin (long-acting formulation, 300 μg/kg), and levels of GnRH-R messenger RNA (mRNA) were determined by Northern and dot blot hybridization to a 32P-labeled rat complementary DNA probe. Abundances of gonadotropin α-subunit, LH β, and FSH β mRNAs were examined in parallel, using appropriate probes. A rapid time-dependent decrease in the level of GnRH-R mRNA was observed in rats after triptorelin administration. A minimum residual level of mRNA, in the 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 transient 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 normal, suggesting a progressive abolition of agonist effects. A concomitant acute depletion of mRNA levels was observed for LHβ and FSHβ (50% decrease in about 48 and 3 h, respectively), whereas the α-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 α, LHβ, and FSHβ mRNAs increased by 6.2-, 7.9-, and 4.2-fold, respectively, compared to corresponding values in intact animals. Administration of the GnRH agonist readily prevented, for as long as 3 weeks, the stimulatory effects of castration on the GnRH-R mRNA and mRNAs for theβ-subunit of gonadotropins, but not for the α mRNA, which remained at a high level. When triptorelin was administered 3 weeks postoperatively, the castration-induced increase in LHβ and FSHβ was totally abolished, and no significant effect was noted on α-subunit mRNA. 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 long-acting GnRH analog, triptorelin.This effect of desensitization is observed in normal and castrated rats, suggesting, in addition to a tonic action of endogenous GnRH on the expression of its own receptor gene, the mediation via GnRH receptors of the effects of gonadectomy. Expression of GnRH-R and gonadotropin P-subunit genes respond similarly to treatments, suggesting concerted regulation, in contrast to that of the a-subunit gene. The receptor and postreceptor mechanisms by which these genes are similarly or differentially affected by desensitization remain to be elucidated. © 1995 by The Endocrine Society.
