Pattern of gonadotropin-releasing hormone (GnRH)-like stimuli sufficient to induce follicular growth and ovulation in ewes passively immunized against GnRH

Abstract

The pattern of GnRH-like stimuli capable of inducing follicular growth, ovulation, and luteal function was evaluated in ewes passively immunized against GnRH. The estrous cycles of 30 regularly cyclic sheep were synchronized using vaginal pessaries impregnated with a synthetic progestogen. Animals were passively immunized against GnRH (groups 2-5, n = 6) or the carrier protein, keyhole limpet hemocyanin (KLH; group 1, n = 6), at the time of pessary removal (PR). Circhoral delivery of saline (groups 1, 2, and 5) or low amplitude GnRH agonist (des-Gly10 GnRH ethylamide [100 ng/hourly pulse]; groups 3 and 4) was initiated at PR and continued for 3 (groups 4 and 5) or 12 days (groups 1-3). In groups 4 and 5, the amplitude of the GnRH-like stimulus was increased to 800 ng/hourly pulse (stimulus-shift) during the 24-h period beginning 72 h after PR. The amplitude of the hourly stimulus was adjusted to 100 ng/pulse 96 h after PR and continued at that level to Day 12. The endocrine changes associated with follicle growth and maturation (serum concentrations of estradiol [E2] above 10 pg/ml), ovulation (surge-like secretion of LH and FSH), and normal luteal function (serum concentrations of progesterone [P] above 2 ng/ml) were evident in ewes passively immunized against KLH (group 1). In this group, the preovulatory surge of gonadotropins was noted 48.7 ± 1.2 h after PR. These endocrine events were blocked by passive immunization against GnRH (group 2). Immunoneutralization of GnRH and concurrent episodic delivery of the GnRH agonist in an unvarying pattern (100 ng/hourly pulse; group 3) increased serum concentrations of E2, but did not lead to surge-like secretion of gonadotropins or luteinization. Similarly, the stimulus-shift paradigm in the absence of pre-exposure to low amplitude GnRH-like stimuli (group 5) failed to induce follicle growth or luteinization. Only animals receiving tonic GnRH-like stimuli before stimulus-shift (group 4) showed follicle growth leading to ovulation and normal luteal function in a pattern comparable to that of the control group (group 1). The magnitude of the natural and stimulus-shift-induced LH surges was comparable in groups 1 and 4, but attenuated (p < 0.05) in group 5. These data indicate that follicular growth, ovulation, and normal luteal function are blocked by passive immunization against GnRH. Follicle growth, but not ovulation, is reinstated by episodic delivery of GnRH-like stimuli of unvarying amplitude. However, the anovulatory effect of passive immunization can be reversed by pulsatile delivery of a GnRH agonist utilizing a stimulus-shift paradigm.