THE HYPOTHALAMIC-PITUITARY-ADRENAL AXIS IN POSTPARTUM EWES

The aim of the study was to investigate whether following the painful and stressful process of parturition, basal and stimulated cortisol secretion differed with time post-partum, and if opioids have any role in the control of basal cortisol secretion in the early post-partum ewe. In Experiment 1, spring-lambing Welsh Mountain ewes had their lambs removed at parturition (Day 0). On Days 1, 7,14 and 28, blood samples were taken from the ewes every 2 h via an indwelling jugular venous catheter from 08:00 to 22:00 h. Half of the group (n = 5 ) were transported in an enclosed truck from 10:00 to 14:00 h. Basal cortisol concentrations were significantly (P<0.05) lower on Day 1 (2.52 +/- 1.36 nmol l-1) than on Days 7, 14 and 28 (9.68 +/- 2.27 nmol l-1, 8.24 +/- 3.64 nmol l-1, 9.59 +/- 0.6 nmol l-1, respectively). There was a significant positive correlation (r=0.69, P<0.05) between the cortisol response to stress, measured as area under the cortisol curve during transport, and day post-partum, until after Day 14. There was no statistical difference in the response between Days 14 and 28 (57.95 +/- 15.6 nmol l-1 vs. 58.75 +/- 8.41 nmol l-1). In Experiment 2, eight suckled ewes were divided into two groups on Days 1-4 post-partum. One group (n=4) was injected intravenously with 1 mg kg-1 of naloxone every hour for 5 h; the control group received saline. All animals were bled every 15 min, via an indwelling jugular venous catheter, from 1 h before to 1 h after treatment. Naloxone had no effect on basal cortisol secretion (7.21 +/- 0.78 nmol l-1 vs. 5.85 +/- 1.92 nmol l-1) in the early post-partum ewe. In conclusion, basal cortisol concentrations were reduced immediately after parturition, by non-opioidergic mechanisms, and there was a reduced cortisol response to transport until after Day 14. This indicates that parturition temporarily disrupts the hypothalamic-pituitary-adrenal axis in sheep.