Naloxone inhibits arginine vasotocin- (AVT) induced prolactin release in urethane-anesthetized male rats in vivo.

Arginine vasotocin (AVT) (1 microgram) elicited a two-fold increase in serum prolactin (Prl) levels 10 min following its i.v. injection into urethane-anesthetized adult male rats. At 20 min after AVT administration serum Prl concentrations had essentially returned to preinjection levels. Prior treatment of the animals with 0.2 mg/kg body weight of naloxone (Nal), an opiate antagonist, completely blocked the Prl response to a single injection of AVT. Nal itself did not alter the titers of Prl at any time point. The results suggest that the in vivo Prl-releasing effect of AVT is mediated via an interaction with opiate receptors.