Low doses of naltrexone reduce palatability and consumption of ethanol in outbred rats.

The opioid antagonist naltrexone, at doses of 1 and 3 mg/kg, has been shown to decrease the palatability and consumption of 10% ethanol in rats. However, a dose of 0.5 mg/kg of naltrexone has produced equivocal results. The purpose of the present study was to clarify the effects of low doses of naltrexone (0.0 [control], 0.25, 0.50, 0.75, and 1.0 mg/kg) on palatability and consumption of 10% ethanol. Sixty-four, male, Long-Evans hooded rats were divided into five groups matched for ethanol consumption. Each rat was injected over four consecutive days with one of five doses of naltrexone exactly 30 min before taste reactivity testing with 10% ethanol. When reactivity testing was completed, rats were acclimated to drink during a period of restricted access to fluid under conditions of mild fluid deprivation. Then, on four consecutive test days, rats were injected with naltrexone 10 min before 10% ethanol was made available for 30 min. Although each dose of naltrexone decreased ingestive responding to 10% ethanol over four days, this effect was not statistically reliable. However, all doses of naltrexone produced significant increases in aversive responding to the ethanol solution. Naltrexone, at the three highest doses, produced significant decreases in consumption of 10% ethanol. These results were consistent with the interpretation that naltrexone, even at low doses, reliably reduces palatability and consumption of 10% ethanol.