Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parturient rats.

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were given naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximately doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected by delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before and during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increases and pituitary stores of this peptide are decreased by day 20 of gestation, when labor has not yet begun.(ABSTRACT TRUNCATED AT 250 WORDS)