Luteinizing Hormone Secretion in Hypophysial Stalk-Transected Pigs Given Progesterone and Pulsatile Gonadotropin-Releasing Hormone

Abstract

This study was conducted to determine whether progesterone inhibits luteinizing hormone (LH) secretion in female pigs by a direct action on the pituitary gland. Eight ovariectomized, hypophysial stalk-transected gilts were given 1-pg pulses of gonadotropin-releasing hormone iv every 45 min from Day 0 to 12. On Days 5-12, each of four gilts received either progesterone or oil vehicle im at 12-hr intervals. Serum progesterone concentrations in steroid-treated gilts reached 70 2 6.8 ng/ml (mean 2 SE) by Day 8 and remained elevated thereafter, whereas serum progesterone concentrations in oil-treated controls were less than 1 ng/ml for the entire study. Daily serum LH concentrations were not different between gilts treated with progesterone or oil. The 1-pg pulses of gonadotropin-releasing hormone reliably evoked pulses of LH in both treatment groups. The LH pulse frequency and amplitude, assessed from samples collected every 15 min for 6 hr on Day 12, were similar for progesterone-and oil-treated gilts. These results provide evidence that progesterone does not act at the pituitary gland to alter LH secretion in pigs.gonadotropin-releasing hormone (GnRH) from the hypothalamus (5). Progesterone, therefore, may decrease LH secretion in the pig by acting directly on the pituitary gland and/or inhibiting the secretion of GnRH. In an effort to determine the site of the negative feedback of progesterone in pigs, we assessed LH secretion after the administration of progesterone to hypophysial stalktransected (HST) gilts given unvarying pulses of GnRH.

Hypothalamic control of gonadotrophin and prolactin secretion in pigs

“…Secretion of FSH was also abolished after passive immunization against GnRH Esbenshade et al, 1986), but not after hypophysial stalk section . Progesterone (Estienne et al, 1989) and cortisol (unpublished) failed to suppress LH secretion in response to pulsatile administration of GnRH in hypophysial stalk-sectioned gilts, indicating that the well documented negative feedback action of these steroids on pulsatile LH secretion in pigs occurs at the CNS rather than at the pituitary gland.…”

032 Use of Triptorelin Acetate for Inducing Ovulation and Facilitating Fixed Time Artificial Insemination of Sows Weaned on Small-Scale and Niche Market Pig Farms

“…The P4 acts on the hypothalamus to inhibit GnRH secretion and in 4 turn decreases LH production from the anterior pituitary gland. A hypothalamic site for the negative feedback action of the P4 on LH secretion was supported by work from Estienne et al (1989) who demonstrated that P4 did not suppress the secretion of LH in response to pulsatile injections of GnRH in hypophyseal stalk-transected pigs. The inhibition of LH and FSH prevents ovarian follicles from growing to the pre-ovulatory stage and being further selected for ovulation.…”