Effects of cocaine on responding for ethanol or sucrose under a progressive ratio schedule

Progressive ratio (PR) schedules have been increasingly used to study motivation for self-administered drugs of abuse, such as psychostimulants and ethanol. In these and other studies, the breaking point (BP) is thought to be a measure of the motivation of the animal to work for a particular reward. Ethanol, a highly abused drug, maintains only low BPs. The present experiment was designed to examine if the low BP achieved by animals working for ethanol could be increased by the administration of a psychostimulant. A group working for a sucrose reinforcer was included for comparison. Rats previously trained to lever press under a PR schedule for 0.1 ml aliquots of 10% ethanol or 5% sucrose reinforcers were dosed once a week with cocaine (0, 5 and 15 mg/kg intraperitoneally) 30 min prior to their daily operant session using a Latin square design. Vehicle and 5 mg/kg cocaine had no effect on BP for any reinforcer, but 15 mg/kg cocaine produced a significantly higher BP (P<0.05) for animals working for either ethanol or sucrose. The same doses of cocaine decreased consumption of, and preference for, a 5% sucrose solution. These results indicate that, although cocaine administration does not increase sucrose preference, it may increase BP values in PR schedules. It is therefore unlikely that the increases in BP reflect cocaine-induced increased motivation, and they may be due to cocaine's stimulant or other properties. These data reinforce opinions that PR schedules may be unsuitable for assessing the effects of experimental manipulations on motivation for drugs with stimulant actions.