Further studies of opioids and intake of sweetened alcoholic beverage.

Rats were placed on a daily regimen of water deprivation followed by a limited opportunity to take either water or a sweetened alcoholic beverage. Across days of opportunity, they took less water and more alcohol until they were taking considerable amounts of alcohol. Naloxone, the classic antagonist at opioid receptors, was given prior to opportunity to drink and it reduced intakes of alcoholic beverage. Small doses of morphine, the classic agonist, increased intakes of alcoholic beverage at doses as low as 0.41 mg/kg. The effects of two other antagonists at opioid receptors (LY117413 and MR2266) were also tested. Both reduced intakes of alcohol. The data showing that more than one or two opioid antagonists reduce intakes of alcoholic beverages, even palatable beverages usually taken in large amounts, and that morphine increases intakes at very low doses, strengthen the idea that endogenous opioid systems are involved in modulating intake of alcohol.