Transport stress delays the oestradiol-induced LH surge by a non-opioidergic mechanism in the early postpartum ewe.

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 micrograms 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).(ABSTRACT TRUNCATED AT 250 WORDS)