Effect of administration of antibody against GnRH on preovulatory LH and FSH surges in the ewe

Summary. An antibody, raised in ewes against synthetic GnRH, was injected intravenously (a) before the expected synchronous surges of LH and FSH or (b) before the second FSH surge, i.e. at 0 or 20 h after the onset of behavioural oestrus. Control ewes were given normal sheep plasma at the same times. One of the 4 ewes given antibody at 0 h showed blockade of the LH surge, whereas in the other 3 ewes there was an average delay of 16 h in the appearance of the LH peak and in 1 of the 3 there was a surge of FSH coincident with the delayed LH surge; no other surges of FSH were detected in any of the ewes between 0 and 48 h after the onset of oestrus. Endoscopic examination at 48–52 h showed follicles of 6–8 mm diameter with a papilla in all the 4 ewes but 2 ewes showed an abnormal luteal phase (32 and 34 days), indicated by plasma progesterone values. The magnitude and the time of appearance of the second FSH surge was unaffected in all the 4 ewes treated with antibody 20 h after the onset of oestrus: 2 of the ewes had 6–8 mm follicles at the time of endoscopy, while the other 2 ewes and the controls had fresh corpora lutea haemorrhagica. Apart from 1 ewe with a long luteal phase all the other 5 treated and control ewes had a fertile oestrus 19·0 ± 0·4 days after the experiment. Administration of antibody 2 h after a 4-h infusion of oestradiol (3 μg/h) to 3 anoestrous ewes completely blocked the LH surge in one and produced a 6 and 3 h delay of an LH surge of decreased magnitude in the other 2 ewes. The FSH surge was blocked in 2 ewes and in the other was asynchronous with the LH surge. The results suggest that GnRH controls the preovulatory synchronous LH and FSH surges, but not the second FSH surge.