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Opioids and suckling in relation to inhibition of oestradiol-induced LH secretion in postpartum ewes

The effects of time postpartum and suckling status on the oestradiol-induced LH surge in ewes and some of the possible mechanisms involved were investigated. Lambs were removed from nonsuckled ewes at parturition (day 0) and the ewes hand-milked 12 and 48 h later to prevent discomfort. Nonsuckled ewes on day 1, 7, 14 or 28, and suckled ewes on day 7 or 28 postpartum were injected i.m. with 50 µg oestradiol benzoate (hour 0). A further eight suckled ewes were given oestradiol on day 1 (as above), and four were also given naloxone (1 mg kg^–1 h^–1) from 10 until 14 h after administration of oestradiol benzoate. There was no LH surge in response to oestradiol on day 1. In nonsuckled ewes the onset of the LH surge was significantly (P < 0.05) delayed when oestradiol was administered on day 7 (17.4 ± 1.5 h), compared with when it was injected on days 14 and 28 postpartum (14.4 ± 1.7 and 14.0 ± 1.4 h). The LH surge was also delayed (P < 0.05) when oestradiol was administered on day 7 (18.0 ± 2.8 h) in suckled ewes. There was no difference in the duration or amplitude of the LH surge with time postpartum in nonsuckled ewes. Suckling significantly (P<0.05) reduced the amplitude of the LH surge on both day 7 (21.0 ± 2.4 versus 26.4 ± 2.3 ng ml^–1) and day 28 (23.6 ± 3.2 versus 28.8 ± 2.9 ng ml^–1), but had no effect on the onset or duration of the surge. There was no difference in the timing or the amplitude of the oestradiol peak following the oestradiol benzoate challenge with time postpartum. Naloxone had no effect on the mean LH concentration (control, 2.3 ± 1.6; naloxone-treated, 2.2 ± 1.5 ng ml^–1) or on the pattern of LH secretion on day 1. In conclusion, time postpartum influenced the timing of the onset, while suckling reduced the amplitude, of the oestradiol-induced LH surge. The lack of an oestradiol-induced LH surge on day 1 postpartum in the ewe is due to a nonopioidergic mechanism.