The influence of morphine and naloxone on plasma oxytocin concentration in the rat.

Subcutaneous injections of morphine decrease the plasma oxytocin level significantly in both normohydrated rats and rats dehydrated for 24 hrs. This effect of morphine is naloxone-reversible. Injections of naloxone increase the plasma oxytocin level in a time dependent manner, when the oxytocin system is stimulated by dehydration, but have no effect on basal oxytocin level. This observation indicates liberation of an endogenous opioid, during stimulation of the neurohypophysis, leading to inhibition of the oxytocin secretion. A discrepancy in the time course between the plasma oxytocin enhancing effect and the morphine reversing effect of naloxone is observed. On the basis of this observation two different opioid receptors are suggested to be involved in the effect of naloxone on the neurohypophysis.