The present study was designed to investigate whether transport, a mild environmental stressor, could affect the oestradiol-induced LH surge in postpartum ewes and, if so, the mechanism involved. Welsh Mountain ewes, with lambs removed at parturition (day 0) and hand-milked 12 and 48 h later, were given 50 μg oestradiol benzoate intramuscularly at various times postpartum. Blood samples were taken via an indwelling jugular venous catheter every 2 h from 8 to 24 h after oestradiol injection. All results are given as means ± s.d. On day 1 oestradiol was unable to induce an LH surge in any ewe. Transport (10–14 h after oestradiol) delayed the onset of the oestradiol-induced LH surge on day 14 (17·5 ±1·7 vs 14·4±2·0 h, n=5 each; P<0·05), but not on day 28 (14·9 ±2·0 vs 14·0 ±2·4 h, n=5 out of 7). Transport had no effect on the amplitude of the surge on either day. Naloxone treatment (1 mg/kg per 2 h) was unable to prevent the delay caused by transport (18·0±1·1 vs 17·5 ± 1·7 h, n=8 each), and did not affect the amplitude of the surge (28·4±5·3 vs 28·1 ±2·3 ng/ml, n=8 each). The duration of the LH surges were not assessed. On day 7, transport from 16 to 20 h after oestradiol delayed the LH surge (22·8 ±2·0 vs 18·0 ± 2·8 h, n=8 each; P<0·05) and reduced the surge amplitude (19·7 ±1·7 vs 22·8 ±2·8 ng/ml; P<0·05), whilst transport from 10 to 14 h did not. Transport (16 to 20 h) had no effect on surge duration (6·25 ±0·7 vs 6·75 ± 1·0 h). In conclusion, transport inhibited the oestradiol-induced LH release in the early postpartum ewe by a non-opioidergic mechanism, but only if the stressor occurred within 2–3 h of the expected onset of the surge.
Journal of Endocrinology (1994) 142, 447–451
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