Naloxone cannot abolish the lack of oxytocin release during unexperienced suckling of dairy cows

To evaluate the role of opioids for the regulation of oxytocin release in r esponse to teat stimulation, 10 brown-Swiss dairy cows were randomized to t wo experiments during mid of lactation. In the first experiment, four cows without previous suckling experience were suckled by an alien calf between two normal milkings. Before and during milking or suckling, frequent blood samples were collected via a jugular cannula for determination of oxytocin and B-endorphin. In the second experiment, six cows were treated with nalox one or saline, 10 min before the start of the first or second suckling, res pectively. The collected blood samples were assayed for oxytocin. In the fi rst experiment, the plasma levels of beta -endorphin were elevated during a nd after the unexperienced suckling in three cows, but not in the fourth co w, and the release of oxytocin during suckling was markedly reduced, sugges ting no release of alveolar milk. In the second experiment, the release of oxytocin during suckling was again significantly reduced. Pretreatment with naloxone before suckling did not completely abolish the adverse effect of suckling and the oxytocin plasma level did not increase to levels comparabl e with control milking. In emotional stress situations, the release of oxytocin from the pituitary is inhibited with simultaneously elevated beta -endorphin plasma levels. Al though there is some evidence for a regulatory role of opioids for the rele ase of oxytocin, other mediators are suggested to be more potent in regulat ing oxytocin under stress conditions. (C) 2001 Elsevier Science B.V. All ri ghts reserved.