Bovine neurophysin-II stimulates prolactin release without involvement of dopaminergic prolactin-release inhibiting factor receptor in the estradiol-primed male rat

Neurophysins have been considered to be physiologically inert carrier proteins for the neurohypophysial hormones, oxytocin and vasopressin. We have observed that bovine neurophysin-II indirectly stimulates prolactin release in estradiol-primed male rats. The release of prolactin is regulated by a dual hypothalamic control system, the prolactin-release-inhibiting factor and the prolactin-releasing factor. We have tried to clarify whether neurophysin-II is acting through stimulation of prolactin-releasing factor by eliminating the possibility of dopaminergic prolactin release-inhibiting factor release. Male rats were primed with estradiol and functional dopaminergic prolactin release-inhibiting factor receptors were completely blocked by pretreatment with a large dose of pimozide (3 mg/kg), a dopaminergic receptor blocking agent. The neurophysin-II stimulated prolactin release in the rats which did not have any functional dopaminergic prolactin release-inhibiting factor receptors suggesting that neurophysin-II likely initiates a chain of events which eventually stimulates prolactin-releasing factor release since the possibility of involvement of the dopaminergic prolactin release-inhibiting factor system is eliminated. Opioids are known to be one of a chain of events which transmit external stress into a stimulation of prolactin release. Naloxone, a mu-receptor antagonist, was injected 20 min before neurophysin-II administration into rats which were primed with estradiol and pretreated with pimozide (3 mg/kg), but the naloxone administration did not block the prolactin release stimulated by neurophysin-II injection. This result indicates that opioids are not one of the chain of events between initiation of stimulation by neurophysin-II and prolactin release.