Pituitary Binding Sites for Gonadotrophin-Releasing Hormone in Booroola Merino Ewes which were Non-Carriers or Homozygotes of the Fecundity Gene F.

Abstract Pituitary binding sites for gonadotrophin-releasing hormone (GnRH) were examined in Booroola ewes on Days 4 and 12 of the oestrous cycle and more than 12 months after ovariectomy. No significant differences were observed in the affinity of the binding sites for [(125) I]D Ala6, des Gly(10) GnRH ethylamide, in non-carriers (++) or homozygotes of the fecundity gene (FF), in intact or ovariectomized animals (overall K(a)= (1.8 +/- 0.1) x 10(9) M(-1); mean +/- SEM, n = 67). In ++ ewes, the number of GnRH binding sites on Day 4 of the oestrous cycle (0.199 +/- 0.025 fmol/mg pituitary, n = 8) was significantly greater (P<0.05) than on Day 12 (0.107 +/- 0.012 fmol/mg pituitary, n = 12). In contrast in FF ewes the number of binding sites on Days 4 and 12 were similar (Day 4: 0.119 +/- 0.030, n = 8; Day 12: 0.092 +/- 0.014, n = 10). On Day 4, but not Day 12, the binding site numbers were significantly higher in ++ compared with FF pituitaries (P<0.05). Likewise on Day 4, but not Day 12, the plasma concentrations of follicle-stimulating hormone were significantly higher (P<0.005) in FF (1.60 +/- 0.15 ng/ml) than ++ ewes (0.96 +/- 0.10 ng/ml). After ovariectomy, no gene-specific differences in binding site numbers were noted despite significantly higher plasma follicle-stimulating hormone concentrations in FF animals. No gene-specific differences in plasma luteinizing hormone concentrations were observed in these animals on Days 4 or 12, or after ovariectomy. Moreover, luteal phase plasma progesterone concentrations were also similar between ++ and FF ewes. We conclude that the gene-specific differences in pituitary responsiveness to GnRH and in pituitary gonadotrophin output are unlikely to be due to differences in GnRH receptor binding affinities.