Compensation by pulsatile GnRH infusions for incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and castrated male pigs

The aim of this study was to investigate incompetence for oestradiol-induce d LH surges in long-term ovariectomized gilts and male pigs. Gilts (250 day s old; n = 36), which had been ovariectomized 30 (OVX 30) or 100 days (OVX 100) before the start of treatment, were challenged i.m. with oestradiol be nzoate and were either given no further treatment, fed methallibure to inhi bit endogenous GnRH release or fed methallibure and given i.v. pulses of 10 0 or 200 ng GnRH agonist at 1 h intervals during the LH surge (48-96 h afte r oestradiol benzoate). The same treatments were applied to long-term orchi dectomized male pigs (ORC, n = 23). In addition, one ORC group was not inje cted with oestradiol benzoate but was fed methallibure and given pulses of 200 ng GnRH agonist. Oestradiol benzoate alone induced an LH surge in the O VX 30 group only (5/6 gilts), methallibure suppressed (P < 0.05) oestradiol benzoate-induced LH secretion, while pulses of 100 ng GnRH agonist in anim als fed methallibure produced LH surges in four of six OVX 30 and four of s ix OVX 100 gilts. The induced LH surges were similar to those produced by o estradiol benzoate alone in OVX 30 gilts. Pulses of 200 ng GnRH: agonist pr oduced LH surges in OVX 30 (6/6) and OVX 100 (6/6) gilts and increased the magnitude of the induced LH surge in OVX 100 gilts (P < 0.05 compared with 100 ng GnRH agonist or OVX 30 control). Pulses of 200 ng GnRH agonist also induced LH surge release in ORC. male pigs (5/6), but were unable to increa se LH concentrations in a surge-like manner in ORC animals that had not bee n given oestradiol benzoate, indicating that oestradiol increases pituitary responsiveness to GnRH. These results support the hypothesis that oestradi ol must inhibit secretion of LH before an LH surge can occur. It is conclud ed that incompetence for oestradiol-induced LH surges in long-term ovarian secretion-deprived gilts and in male pigs is due to the failure of oestradi ol to promote a sufficient increase in the release of GnRH.