Modulation of ethanol-intake by morphine: evidence for a central site of action

Previous studies have shown that subcutaneous administration of low doses of morphine increase, while subcutaneous naloxone decreases, ethanol-intake in rats. However, the site of action of morphine modulation of ethanol-intake remains unclear. In an attempt to elucidate this issue, seven graded doses of morphine were given intracerebroventricularly to rats 15 min prior to an opportunity to consume water and sweetened alcoholic beverage for 2 hr. Two lower doses of intracerebroventricular morphine (1, 3 micrograms) reliably increased ethanol-intake, while higher doses (3-30 micrograms) decreased intake of water. Preference ratios (ethanol-intake divided by total fluid-intake) were reliably increased by morphine doses of 1 microgram and higher. The present data provide support for a central site of morphine modulation of ethanol-intake.