μ1-opioid antagonist naloxonazine alters ethanol discrimination and consumption

The endogenous opioid system is implicated in excessive ethanol-drinking behavior. However, the role of individual opioid receptor subtypes in the mechanism underlying excessive ethanol-drinking behavior is not yet well understood. Therefore, we investigated the ability of a selective μ1-opioid antagonist, naloxonazine, to modulate ethanol-drinking behavior and ethanol discrimination in a rat model with the use of ethanol self-administration and drug discrimination paradigms. The effects of naloxonazine (0.001–10 mg/kg) on ethanol intake were examined in Sprague–Dawley rats under conditions of limited access to 10% (wt./vol.) ethanol and ad libitum access to food and water. Pretreatment with high doses of naloxonazine (1–10 mg/kg) significantly reduced ethanol consumption. When the effects of naloxonazine on food intake in free-feeding male rats were examined, naloxonazine (1.8–10 mg/kg) significantly suppressed 24-h food intake. Another group of rats was trained to discriminate ethanol (1.25 g/kg, i.p.) from saline on a fixed-ratio schedule (FR 10), and ethanol dose-response tests were conducted once rats had acquired ethanol–saline discrimination. Injections were given 15 min before ethanol dose-response tests were conducted, and after characterization of the ethanol dose-response curve, the effects of naloxonazine on ethanol discrimination were assessed by administering naloxonazine (0.001–10 mg/kg, i.p.) 15 min before ethanol administration. Treatment with naloxonazine (0.001–1.8 mg/kg, i.p.) before the ED100 dose of ethanol partially antagonized the discriminative stimulus of ethanol without having any effect on the response rate. The results support the suggestion of involvement of μ1-opioid receptors in the discriminative effects of ethanol and ethanol-drinking behavior.