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 response to teat stimulation, 10 brown-Swiss dairy cows were randomized to two 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 naloxone or saline, 10 min before the start of the first or second suckling, respectively. The collected blood samples were assayed for oxytocin. In the first experiment, the plasma levels of beta -endorphin were elevated during and after the unexperienced suckling in three cows, but not in the fourth cow, and the release of oxytocin during suckling was markedly reduced, suggesting 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 comparable with control milking. In emotional stress situations, the release of oxytocin from the pituitary is inhibited with simultaneously elevated beta -endorphin plasma levels. Although there is some evidence for a regulatory role of opioids for the release of oxytocin, other mediators are suggested to be more potent in regulating oxytocin under stress conditions. (C) 2001 Elsevier Science B.V. All rights reserved.