Summary. Male (N = 8) and female (N = 8) pigs were assigned to receive saline or a potent GnRH antagonist ([Ac-d2Nal1,d4-Cl-Phe2,d-Trp3,d-Arg6, d-Ala10]GnRH*HOAc; 1 mg/kg body weight) at 14 days of age. The GnRH antagonist caused LH to decline (P < 0·01) from 1·7 ng/ml at 0 h to <0·5 ng/ml during 4–32 h in males and females. Concentrations of FSH in gilts declined slowly from 75 ± 8 to 56 ± 5 ng/ml (P < 0·05) at 32 h. In males FSH was low (5·7 ± 0·5 ng/ml) at 0 h and did not change significantly.
To observe the effect of long-term treatment with GnRH antagonist, 10 male and 10 female pigs, 3 days of age, were treated with saline or 1 mg GnRH antagonist per kg body weight every 36 h for 21 days. Concentrations of LH were reduced (P < 0·01) to 0·2–0·4 ng/ml throughout the experimental period in male and female piglets treated with GnRH antagonist. Plasma FSH increased in control females, but remained suppressed (P < 0·001) in females treated with GnRH antagonist. Treatment with the GnRH antagonist suppressed FSH levels in males on Days 8 and 16 (P < 0·05), not on Day 24. Treatment of females with the GnRH antagonist did not influence (P > 0·10) oestradiol-17β concentrations. Administration of GnRH antagonist to males suppressed testosterone and oestradiol-17β values (P < 0·01) and reduced testicular weight (P < 0·01). Concentration of LH/hCG receptors in testes of boars treated with GnRH antagonist was lower (P < 0·10) than in controls, but concentration of FSH receptors was not affected. Basal and potassium-stimulated release of GnRH from the stalk median eminence and medial basal hypothalamus in vitro did not differ between treatment groups. The amount of residual GnRH in hypothalamic tissue was not different in control gilts and in gilts receiving the GnRH antagonist, but it was lower (P < 0·05) in boars treated with GnRH antagonist than in control boars.
Keywords: pig; GnRH antagonist; LH; FSH