Cortisol and ACTH release in dairy cows in response to machine milking after pretreatment with morphine and naloxone

The aim of the study was to examine the effect of morphine and naloxone pre treatment on cortisol and ACTH concentrations in response to machine milkin g in dairy cows. In the first part of the experiment, the effects of i.v. m orphine doses (0, 21, 70 and 210 mg, one dose each day) 10 min before morni ng milking were studied in six Brown Swiss dairy cows. In the second part, four cows were treated 1 d after the control milking with 210 mg morphine a t 10 min before milking and the next day with 210 mg naloxone at 15 min bef ore milking followed by 210 mg morphine at 10 min before milking. In additi on, four other cows were treated 1 d after the control milking with 210 mg naloxone at 10 min before milking. Pretreatment with morphine significantly suppressed the machine milking-induced increase of cortisol in blood plasm a as compared with controls. Naloxone pretreatment overcame the inhibitory effect of morphine and elevated milking-induced cortisol concentrations. Na loxone administration alone significantly increased cortisol concentration resulting from milking as compared with controls. However, ACTH concentrati ons did not change in either control or treated animals, suggesting an ACTH -independent release mechanism for cortisol during milking. We conclude tha t the release of cortisol in response to machine milking seems to be modula ted by endogenous opioids at the adrenal level and does not appear to be un der the control of ACTH.