An opioid antagonist stimulates myometrial activity in early postpartum cows

The influence of the opioid antagonist naltrexone on myometrial activity of cows was tested during the early postpartum period, when uterine contractions are a highly sensitive indicator of oxytocin actions. Registering intrauterine pressure oscillations in five first calf heifers revealed that myometrial activity was enhanced after naltrexone was administered (0.02 mg/kg BW). However, plasma concentrations of oxytocin were not consistently elevated. In addition, naltrexone did not reliably potentiate the suckling-induced peak of oxytocin release in plasma. In all cases, naltrexone injections caused increases in the cows' respiratory rates and distinct behavioral changes during the first days postpartum, but not later. We conclude that: 1) In the early postpartum period, an inhibitory opioidergic control of myometrial activity may exist in cattle. 2) This control does not seem to be caused by a change in circulating oxytocin levels. 3) Opioids may exert an inhibitory influence on respiratory functions in cattle.