Opioid modulation of the hypothalamo-pituitary-adrenal axis in dairy cows.

Plasma cortisol responses to an intravenous bolus treatment with 250 mg naloxone, 300 mg morphine or a combination, were studied in Holstein-Friesian cows; 4 in early lactation (29-43 d postpartum) and 7 in mid-lactation (90-155 d post-partum). Blood samples were collected every 15 min from 60 min before to 90 min after treatment. Naloxone induced an immediate increase in cortisol concentration, reaching a peak within 30 min. The cortisol response (area under the curve) was positively correlated with pre-naloxone cortisol concentrations (r = 0.7, p < 0.05). The mean increase in cortisol concentration after naloxone appeared to be lower in early lactation (1.8 ng/ml) than in mid-lactation (8.3 ng/ml). In contrast, morphine consistently suppressed mean tonic plasma cortisol concentration by 2.7 ng/ml below baseline for at least 90 min. When given with morphine, naloxone counteracted the suppressive effects; the cortisol response was similar to that after naloxone alone. A cow in mid-lactation, suffering from chronic lameness (joint infection), gave opposite results, i.e., treatment with morphine alone increased cortisol concentration, whereas morphine with naloxone did not result in the expected large increase in plasma cortisol concentration. In conclusion, the hypothalamo-pituitary-adrenal axis of dairy cows appears to be under suppressive opioidergic control. However, the opioidergic system involved in hypothalamo-pituitary-adrenal functions of an animal under chronic stress behaved in an opposite manner.