FAILURE OF GONADOTROPIN-RELEASING-HORMONE (GNRH) PULSES TO INCREASE LUTEINIZING HORMONE-BETA MESSENGER-RIBONUCLEIC-ACID IN GNRH-DEFICIENT FEMALE RATS

Gonadotropin subunit gene transcription and messenger RNA (mRNA) level s are differentially regulated by GnRH pulse frequency and amplitude i n the male rat. The rapid changes of subunit mRNA levels and LH and FS H secretion during the estrous cycle, particularly the rapid rise in L H-beta subunit mRNA on proestrus afternoon, suggest that physiological changes in the pattern of GnRH action may also be important in female rats. However, in the absence of a GnRH-deficient female model the ro le of varying GnRH stimulation remains to be determined.We have charac terized a GnRH-deficient model by administering the alpha-adrenergic a ntagonist phenoxybenzamine (PBZ) to ovariectomized (OVX) rats. Initial experiments showed that PBZ given 24 h earlier abolished the afternoo n LH surge in OVX estradiol (E2) replaced rats whereas LH responses to exogenous GnRH were preserved. A PBZ regimen of 15 mg/kg ip at OVX fo llowed by 10 mg/kg at 24 h and 5 mg/kg at 48 h prevented the increase in alpha, LH-beta, and FSH-beta mRNAs and LH and FSH secretion for 72 h post-OVX. LH and FSH responses to GnRH pulses were preserved suggest ing that PBZ blocked the post-OVX increase in hypothalamic GnRH secret ion. The suppressive effect of PBZ appeared to be specific to the hypo thalamic-pituitary-ovarian axis as plasma PRL, TSH, and corticosterone were not decreased compared to controls. We have used this GnRH-defic ient OVX female model to investigate the effects of exogenous GnRH pul ses on subunit mRNA expression. GnRH pulses (5-250 ng/30 min for 12-24 h) were administered via an intraatrial catheter beginning 24 h after OVX and the first PBZ injection (OVX + PBZ + saline pulses to control s). Expression of a and FSH-beta mRNAs and LH and FSH secretion were i ncreased by GnRH pulse doses of 5-25 ng to values similar to or greate r than those in OVX controls though the higher doses of GnRH/pulse did not increase FSH-beta mRNA or plasma FSH. However, LH-beta mRNA level s were not increased by GnRH pulses. GnRH pulses were also given to ra ts replaced with proestrus concentrations of estradiol alone or in com bination with progesterone (P). Again, no demonstrable increases in LH -beta mRNA expression were observed. Alpha-mRNA concentrations were fu rther increased in the presence of E2 alone, and P in combination with E2, produced an augmented response of FSH-beta subunit mRNA. These da ta suggest that ovarian steroid hormones act directly on the gonadotro pe to augment alpha and FSH-beta mRNA responses to GnRH. We conclude t hat the alpha-antagonist PBZ prevents the increase in GnRH secretion i n the E2-treated OVX rat and suppresses GnRH release after OVX (assess ed by plasma LH, FSH, and subunit mRNA levels). Administration of puls atile GnRH to OVX-PBZ treated animals increases alpha and FSH-beta mRN A expression to OVX levels. The absence of an LH-beta mRNA response to GnRH, despite the presence of ovarian steroid hormones, suggests that other factors are required to produce the rapid physiological rise in LH-beta mRNA expression in female rats on proestrus.