NORMAL OR INDUCED SECRETORY PATTERNS OF LUTEINIZING-HORMONE AND FOLLICLE-STIMULATING-HORMONE IN ANESTROUS GONADOTROPIN-RELEASING HORMONE-IMMUNIZED AND CYCLIC CONTROL HEIFERS

The objective was to determine the effect of gonadotrophin-releasing h ormone (GnRH), GnRH analogue (GnRH-A) or oestradiol administration on luteinising hormone (LH) and follicle-stimulating hormone (FSH) releas e in GnRH-immunised anoestrous and control cyclic heifers. Thirty-two heifers (477 +/- 7.1 kg) were immunised against either human serum alb umin (HSA; controls; n = 8), or a HSA-GnRH conjugate. On day 70 after primary immunisation, control heifers (n = 4 per treatment; day 3 of c ycle) received either (a) 2.5 mu g GnRH or (b) 2.5 mu g of GnRH-A (Bus erelin(R)) and GnRH-immunised heifers (blocked by GnRH antibody titre; n=6 per treatment) received either (c) saline, (d) 2.5 mu g GnRH, (e) 25 mu g GnRH or (f) 2.5 mu g GnRH-A, intravenously. On day 105, 1 mg oestradiol was injected (intramuscularly) into control (n = 6) and GnR H-immunised anoestrous heifers with either low (13.4 +/- 1.9% binding at 1:640; n = 6) or high GnRH antibody titres (33.4 +/- 4.8% binding; n = 6). Data were analysed by ANOVA. Mean plasma LH and FSH concentrat ions on day 69 were higher (P < 0.05) in control than in GnRH-immunise d heifers (3.1 +/- 0.16 vs. 2.5 +/- 0.12 ng LH ml(-1) and 22.5 +/- 0.7 3 vs. 17.1 +/- 0.64 ng FSH ml(-1), respectively). The number of LH pul ses was higher (P < 0.05) in control than in GnRH-immunised heifers on day 69 (3.4 +/- 0.45 and 1.0 +/- 0.26 pulses per 6 h, respectively). On day 70, 2.5 mu g GnRH increased (P < 0.05) LH concentrations in con trol but not in GnRH-immunised heifers, while both 25 mu g GnRH and 2. 5 mu g GnRH-A increased (P < 0.05) LH concentrations in GnRH-immunised heifers, and 2.5 mu g GnRH-A increased LH in controls. FSH was increa sed (P < 0.05) in GnRH-immunised heifers following 25 mu g GnRH and 2. 5 mu g GnRH-A. Oestradiol challenge increased (P < 0.05) LH concentrat ions during the 13-24 h period after challenge with a greater (P < 0.0 5) increase in control than in GnRH-immunised heifers. FSH concentrati ons were decreased (P < 0.05) for at least 30 h after oestradiol chall enge. In conclusion, GnRH immunisation decreased LH pulsatility and me an LH and FSH concentrations. GnRH antibodies neutralised low doses of GnRH (2.5 mu g), but not high doses of GnRH (25 mu g) and GnRH-A (2.5 mu g). GnRH immunisation decreased the rise in LH concentrations foll owing oestradiol challenge.