Inhibition of suckling-induced prolactin release by μ- and κ-opioid antagonists

Abstract

Evidence suggests that endogenous opioid peptides (EOP) are involved in the hyperprolactinemia and suppression of leuteinizing hormone (LH) release associated with lactation. To address this hypothesis, we investigated the effects of various opioid receptor antagonists on suckling-induced prolactin (PRL) and LH responses in primiparous, lactating rats. All animals were fitted with indwelling jugular catheters to allow serial blood sampling, and some rats received intracerebroventricular (i.c.v.) cannulae for central drug injection. Naloxone (2.0 mg/kg, i.v.) was employed as a broad spectrum opioid antagonist, whereas β-funaltrexamine (β-FNA, 1.0–5.0 μ, i.c.v.), naloxonazine (NAZ, 20 mg/kg, i.v.) and nor-binaltorphimine (nor-BNI, 4.0–16.0 μg, i.c.v.) were used to block μ, μ 1 and κ receptor sites, respectively. In vehicle-treated rats, pup suckling evoked a dramatic increase in plasma PRL and a concurrent decrease in circulating LH. Naloxone caused a modest, though significant, attenuation of the PRL surge during nursing. β-FNA and nor-BNI inhibited suckling-induced PRL release in a dose-related fashion, and at sufficient doses, both antagonists abolished the PRL response. Conversely, the suckling-induced rise in plasma PRL was not affected by NAZ. Naloxone, β-FNA, and NAZ did not alter the profile of circulating LH in suckled rats, but the highest dose nor-BNI (16 μg, i.c.v.) produced a significant elevation in plasma LH. However, even in rats treated with 16.0μg of nor-BNI, plasma LH levels declined in response to the nursing stimulus. Taken together, our results indicate that (1) EOP are important modulators of physiological PRL release in lactating rats, (2) both μ- and κ-opoid receptors are involved in the suckling-induced PRL response, and (3) while a κ-opioid mechanism may inhibit LH secretion during lactation, EOP cannot fully account for suckling-induced LH suppression.