NO EVIDENCE FOR AN OPIOIDERGIC CONTROL OF OXYTOCIN RELEASE IN COWS DURING LATE PREGNANCY, PARTURITION AND EARLY PUERPERIUM

In this study, a possible influence of endogenous opioids on plasma ox ytocin concentrations was investigated in periparturient dairy cows. T he cows were injected before parturition, during calving and 1 day pos t-partum with either the opiate antagonist naloxone or saline. Blood s amples for determination of plasma oxytocin concentrations were collec ted at 5-min intervals from 15.min before to 30 min after naloxone or saline applications. After naloxone injections, in most animals behavi oural changes could be seen but plasma oxytocin concentrations in cows receiving naloxone were not significantly different from correspondin g values for the control group. Therefore, an inhibitory effect of end ogenous opioid systems on oxytocin release could not be demonstrated. Although in cows with undisturbed parturition an opioid influence on o xytocin secretion does not seem to exist, opioid systems could be acti vated when the animal is exposed to an additional, environmental stres s during labour.