The effect of opiate receptor blockade on human CRH-induced PRL secretion was examined in ovariectomized and normal female rhesus monkeys. A bolus iv dose of 100 micrograms CRH acutely stimulated PRL secretion in both normal and ovariectomized animals. The magnitude of the PRL response was greater in ovariectomized monkeys (212% increase; n = 5) than in normal monkeys (56% increase; n = 5). Naloxone pretreatment inhibited CRH-induced PRL release in a dose-dependent fashion, whereas nalmefene did not, in both normal and ovariectomized monkeys. Administration of nalmefene alone significantly stimulated PRL secretion in normal monkeys (57% increase; n = 10), but not in ovariectomized monkeys (10% increase; n = 10). Naloxone alone had no effect. Since nalmefene has a high affinity for both the kappa- and mu-receptors whereas naloxone binds primarily to the mu-receptor, these results suggest that CRH-induced PRL secretion in monkeys is mediated by an endogenous opiate which binds to an opiate receptor other than or in addition to the mu- and kappa-receptors.

The duration of action of nalmefene, a relatively new opiate antagonist, on the hypothalamic-pituitary axis of the rhesus monkey was compared to that exhibited by naloxone. Ovariectomized monkeys (n = 5) were pretreated with 10 mg nalmefene, 10 mg naloxone or an equivalent volume of saline 12, 24, or 48 h prior to the administration of 10 mg morphine. Blood samples were collected by venipuncture 0, 1,2,3 and 4 h after injection of morphine and were assayed for luteinizing hormone (LH) and prolactin (PRL). Duration of action of these two opiate antagonists was estimated from their ability to block morphine inhibition and stimulation of LH and PRL release respectively. Morphine reduced serum LH concentration by 60% and increased PRL levels approximately 4-fold in saline-pretreated animals. Administration of nalmefene either 12 or 24 h, but not 48 h prior to morphine, significantly antagonized the effects of this opiate on LH and PRL release. In contrast, naloxone at all pretreatment intervals failed to block morphine’s effect. We conclude that a single 10 mg bolus injection of nalmefene exerts significant activity at the opiate receptors that mediate the effects of morphine on LH and PRL release for at least 24 h after administration, whereas the same dose of naloxone has a duration of action less than 12 h. Based on this finding it is likely that the effects of endogenous opioid peptides in the rhesus monkey can be chronically antagonized by the daily administration of nalmefene.