Effects of naltrexone on the intake of ethanol and flavored solutions in rats

It has been suggested that the endogenous opioid system may mediate the int ake of preferred fluids, perhaps through an attenuation of reinforcement pr operties causing a subsequent shift in palatability. The purpose of the pre sent study was to investigate the effects of the nonspecific opiate antagon ist naltrexone on the intake of 10% ethanol, 0.1% saccharin, 0.0006% quinin e, 0.4% saccharin + 10% ethanol, and 0.4% saccharin + 0.04% quinine solutio ns. Fluid intake was measured in male Long-Evans and Wistar rats under 24-h continuous and 30-min limited-fluid-access drinking paradigms. All rats re ceived injections of naltrexone hydrochloride (10 mg/kg, i.p.) for 5 days a fter baseline intake measures and were monitored for a further 5 days (afte r-treatment phase). Results indicated that naltrexone did not affect intake of any solution when fluids were available over 24 h. However, under limit ed-access conditions, naltrexone caused a decrease in the intake of all flu ids except quinine in both rat strains. On the basis of these findings, it is possible that the effects of this dose of naltrexone were not due to any true conditioning effect on the reinforcement properties of ethanol, but p erhaps to some nonspecific effect of the drug, such as an alteration in pal atability or an attenuation of locomotor activity. As well, due to the inco nsistent results in fluid intake across drinking paradigms, the present fin dings do not provide evidence for an effective role for opiate mediation in ethanol intake as well as any ethanol-sweet fluid intake interactions. (C) 2001 Elsevier Science Inc. All rights reserved.