Sensitivity to reinforcer delay predicts ethanol's suppressant effects, but itself is unaffected by ethanol

Background: Relative preference for smaller, sooner rewards over larger, later rewards ("delay discounting") is increased by acute ethanol. Additionally, drug-naive levels of delay discounting can predict subsequent ethanol consumption. However, it is unknown whether these phenomena are driven by a difference in sensitivity to the reinforcer delay or a difference in sensitivity to the reinforcer magnitude, because typical delay discounting tasks manipulate both parameters simultaneously. Methods: To disambiguate these factors, two tasks were developed in which animals chose between levers with either different delay contingencies (adjusting delay task) or different magnitude contingencies (adjusting magnitude task). When task performance was stable, rats received ethanol (0, 0.6, and 0.9 g/kg, i.p.). Results: Ethanol did not affect sensitivity to delay or sensitivity to magnitude. However, responding was suppressed at the highest dose of ethanol (0.9 g/kg). Less suppression was found in animals exhibiting high levels of drug-naive sensitivity to delay. Conclusion: Thus, this study suggests that ethanol's effect on standard delay discounting tasks is not due to an alteration in sensitivity to delay or magnitude. Additionally, these data show that animals with high sensitivity to delay are resistant to the behaviorally suppressant effects of ethanol, which suggests that low tolerance for delayed rewards and low sensitivity to the behaviorally suppressant effects of ethanol may partly be driven by the same underlying mechanism. (C) 2013 Elsevier Ireland Ltd. All rights reserved.