Ethanol’s Effects on Operant Responding: Differentiating Reinforcement Efficacy and Motor Performance

Abstract

PETRY, N. M. Ethanol’s effects on operant responding: Differentiating reinforcement efficacy and motor performance. Physiol Behav 64(1) 117–122, 1998. The purpose of this study was to examine the effects of ethanol on operant responding in rats. Drugs may affect response rates by several mechanisms, including altering motor performance or sensitivity to reinforcement. These two processes can be dissociated by applying a quantitative model of reinforced responding the response–strength equation, to response rates obtained during multiple variable-interval (VI) schedules of reinforcement. This equation has two fitted parameters. One ( k) is thought to measure changes in motor performance, and the other ( R e) is thought to measure changes in reinforcement efficacy. In experimental sessions, rats earned sucrose reinforcement on a seven-component multiple VI operant schedule. The average intervals varied from 108 to 3 s and provided reinforcement rates from 30 to 1200/h. Following stable baseline performance, rats were injected intraperitoneally prior to sessions with different doses of ethanol: 0.0, 0.3, 0.6, and 0.9 g/kg. Low to moderate doses of ethanol (0.3 and 0.6 g/kg) significantly decreased response rates on several of the VI schedules but did not alter either of the fitted parameters, suggesting that these doses of ethanol did not affect motor performance or sensitivity to reinforcement. The 0.9 g/kg dose decreased responding maintained by many of the VI schedules and significantly increased the R e parameter, suggesting that the sucrose became relatively less efficacious in ethanol compared to vehicle sessions. This relatively high dose of ethanol, however, did not alter responding maintained by the richest reinforcement schedule, and it did not affect motor performance as defined by the k parameter. These results suggest that, within this paradigm, ethanol affects motivation to respond rather than ability to respond.