The nature of the gonadotrophin-releasing hormone (GnRH) stimulus of the pituitary necessary for the oestrogen-induced plasma LH surge was studied in ovariectomized ewes. The sheep were treated with oestradiol benzoate (50 μg i.m.) at 0 h, and the hypothalamic contribution to the LH surge was blocked by pentobarbitone anaesthesia over the time during which the surge was expected (11–31 h). Pituitary responsiveness to exogenous GnRH (100 ng) administered i.v. in a pulsatile mode (once per hour or once per 20 min) over the period 15–30 h was assessed from plasma concentrations of LH. Neither of the GnRH treatments induced patterns of LH secretion similar to those seen in conscious ovariectomized ewes given oestrogen only. Plasma LH secretion in response to hourly GnRH pulses was less (P<0·01) than that associated with oestrogen-induced plasma LH surges in conscious control ewes. With pulses of GnRH administered every 20 min the amount of LH released was greater (P<0·05) than that in oestrogen-treated conscious control ewes. In contrast to the single surge induced by oestradiol in conscious ewes, GnRH pulses given every 20 min elicited phasic patterns of LH secretion consisting of two or three distinct surges. The failure of GnRH treatment to elicit an LH surge similar to an oestrogen-induced surge could reflect inappropriate GnRH treatment regimens, and/or inadequate priming of the pituitary with GnRH after induction of anaesthesia but before GnRH treatment.
J. Endocr. (1988) 116, 143–148