Increases in the reinforcing efficacy of cocaine after particular histories of reinforcement

Abstract

In humans, the progression to cocaine addiction presumably involves increases in the effectiveness of cocaine to function as a reinforcer. Here we use breakpoints assessed using the progressive ratio (PR) schedule as an index of the efficacy of cocaine as a reinforcer. To date, no preclinical studies have demonstrated an increase in breakpoint as a consequence of self-administration history. In the current study, baseline performances on fixed ratio (FR) and PR schedules were determined. Rats were then exposed to different self-administration histories and deprivation periods, and responding under FR and PR schedules was reassessed. Exposure to a discrete-trials procedure (access to cocaine 4 times/hour, 24 hours/day; DT4) for 7 or 10 days, coupled with a deprivation period of 7 days, resulted in increases in breakpoint on a PR schedule, with no change in FR1 schedule responding. Exposure to an FR1 schedule for 72 consecutive hours followed by 7 days of deprivation, failed to change breakpoints, but increased rates of intake assessed with an FR1 schedule. Thus, the type of self-administration history and the length of deprivation experienced contribute to changes in the reinforcing efficacy of cocaine as measured by a PR schedule.