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Evidence for a change in oestradiol negative feedback and LH pulse frequency in post-partum ewes

Summary. Plasma LH levels and the effects of ovariectomy and of chronic and acute administration of oestradiol-17β were compared in post-partum (anoestrous) and cyclic Merino ewes during the breeding season. Plasma LH and LH pulses were detected in more post-partum (anoestrous) than cyclic (dioestrous) ewes (10/10 and 4/10 respectively). The post-partum ewes had a greater pulse frequency (1·8 ± 0·42 compared with 0·4 ± 0·16 pulses/6 h) and higher plasma LH levels (1·16 ± 0·32 compared with 0·17 ± 0·05 ng/ml). Ovariectomized post-partum ewes demonstrated fewer LH pulses (3·8 ± 0·36 compared with 5·0 ± 0·15 pulses/6 h) than did ovariectomized cyclic ewes. The depression of plasma LH following acute i.v. injection of 3 µg oestradiol-17β was similar (0·05 < P < 0·1) for the two groups (80·5 ± 3·6 and 66·3 ± 4·1%). At 7 days after ovariectomy and an oestradiol implant more cyclic ewes than post-partum ewes showed detectable plasma LH and LH pulses (6/10 compared with 1/9 ewes). By 66 h after implant removal pulse frequency was less in the post-partum ewes (3·2 ± 0·55 compared with 5·5 ± 0·56 pulses/6 h).

These findings indicate that in post-partum ewes there is an increased inhibitory (negative-feedback) effect of oestradiol on LH release and a lower intrinsic frequency of pulsatile release of LH. These factors may be causally related to ovarian acyclicity.