GONADOTROPE RESPONSIVENESS IN ORCHIECTOMIZED SHEEP .2. EFFECT OF GONADOTROPIN-RELEASING-HORMONE AMPLITUDE SHIFT DURING CONTINUOUS-INFUSION OF ESTRADIOL

Gonadotropin secretion during continuous infusion of estradiol (E2; 5 mug/h, i.v.) and pulsatile delivery of GnRH was assessed in orchidecto mized sheep (wethers). Wethers received 200 ng GnRH per hourly pulse d uring the first 12 h of E2 infusion. During the final 36 h of infusion , animals (n = 6/group) received 0, 200, 400, 800, or 1600 ng GnRH/h p ulse. Amplitude shift to 1600 ng GnRH/h pulse induced preovulatory sur ge-like secretion of LH in animals receiving concurrent E2 infusion. I n the absence of E2, the same change in GnRH amplitude did not signifi cantly affect serum concentrations of LH. Full expression of the GnRH- induced surge of LH in wethers required prior exposure to E2, but was not dependent on pretreatment with low-amplitude GnRH stimulus. In a c ompanion experiment, anterior pituitary tissue was collected from E2-t reated wethers 0, 3, 6, 12, or 24 h after initiation of episodic GnRH delivery (1600 ng/h pulse). Treatment with E2 induced a 5-fold increas e in GnRH receptor concentration. The tissue concentration of GnRH rec eptor was depressed (p < 0.05) within 6 h of initiation of high-amplit ude GnRH stimulation and remained depressed for the duration of GnRH d elivery. Tissue stores of LH were increased (p < 0.05) by E, and reduc ed (p < 0.05) after 3 h of high-amplitude GnRH stimulation. In contras t, tissue stores of FSH were reduced (p < 0.05) by both E2 alone and E 2 with concurrent high-amplitude GnRH stimulation. The concentration o f mRNA for the FSHbeta subunit, but not for the alpha or LHbeta subuni ts, was reduced (p < 0.05) by E2. High-amplitude GnRH stimulation rapi dly depressed (p < 0.05) the tissue concentrations of mRNA for the alp ha and FSHbeta subunits but had no effect (p > 0.05) on the tissue lev el of mRNA for the LHbeta subunit. Tissue levels of mRNA for FSHbeta r emained depressed throughout the period of high-amplitude GnRH stimula tion. In contrast, the GnRH-induced reduction in tissue levels of mRNA for the alpha subunit was transient and tissue levels of mRNA returne d to pretreatment levels within 12 h, despite continued high-amplitude GnRH stimulation. These data indicate that preovulatory surge-like se cretion of LH is evident in wethers pretreated with E2 and receiving h igh-amplitude GnRH stimulus. The GnRH-induced surge of LH in E2-treate d wethers is associated with down-regulation of GnRH receptor, depleti on of gonadotropin reserves, and divergent change in tissue concentrat ions of mRNA for the gonadotropin subunits.