Gonadotrophin-releasing hormone secretion (GnRH) in anoestrous ewes and the induction of GnRH surges by oestrogen

Anoestrous ewes were studied to determine the pattern of secretion of gonadotrophin-releasing hormone (GnRH) in the resting state and following a single i.m. injection of 50 μg oestradiol benzoate. In three out of four untreated ewes, two or three GnRH pulses were observed over a 6-h sampling period. In the fourth sheep the GnRH pulse frequency was higher (six pulses/6 h), but GnRH pulse amplitudes were lower. Following oestrogen treatment, GnRH pulses continued until the occurrence of an LH surge 12 h later. In five out of six sheep sampled during the oestrogen-induced LH surge a marked rise in GnRH secretion was seen. In the sixth ewe a large pulse of GnRH was seen at the start of the LH surge followed by increased GnRH secretion. It is concluded that GnRH pulse frequency is lower, generally, during anoestrus than during the mating season, and that oestrogen treatment of anoestrous ewes causes a surge in GnRH secretion unlike that seen in similarly treated ovariectomized ewes or the natural cyclic preovulatory changes in GnRH secretion.