Naloxone attenuates voluntary ethanol intake in rats selectively bred for high ethanol preference

The effect of naloxone on voluntary ethanol intake was examined in rats which were selectively bred for oral ethanol preference (High Alcohol Drinking or HAD line). Rats of the HAD line were treated with naloxone in doses of 0.05-18.0 mg/kg b.wt. before access to water alone or to a free-choice between a 10% (v/v) ethanol solution and water. Naloxone suppressed water intake when water was presented as the sole source of fluid. In contrast, naloxone produced a dose-dependent decrease in ethanol consumption, without altering water intake, when rats were given a free-choice between the ethanol solution and water. Selective suppression of ethanol consumption by naloxone was not attributable to changes in blood ethanol concentrations or ethanol elimination rates following naloxone treatment. It appears that although naloxone may attenuate the positively reinforcing properties of both ethanol and water, ethanol drinking is a subset of consummatory behaviors that is particularly sensitive to opioid receptor blockade. The results suggest that activation of the endogenous opioid system may be an important mechanism which serves to maintain continued ethanol drinking.