Operant wheel running by rats on fixed ratio schedules of sucrose reinforcement

Abstract

Prior studies have shown that operant wheel running in rats is reinforced by sucrose on fixed ratio (FR) schedules, but wheel running also occurs at a high frequency without experimentally-arranged consequences. Findings that scheduled reinforcement exerts only limited control on operant wheel running served as the basis for the current study. Eight female Long-Evans rats were exposed to a series of FR schedules (FR 5, 15, 40, and 80) for 0.1 ml of 20% sucrose solution and again with 0% sucrose (water). Wheel revolutions at each ratio size produced outcomes (sucrose or water), and wheel running rates were assessed for each scheduled reinforcement rate (FR size). Results showed significantly higher wheel-running rates for sucrose reinforcement than water; however, the difference in rates by outcome was modest and independent of the control exerted by FR size alone. For both outcomes, wheel-running rates decreased, and latency to initiate running increased as FR size increased and the reinforcement rate decreased. The lack of variation in the reinforcing effect of sucrose as the FR size varied differs from findings with operant behaviours such as key pecking and lever pressing. This finding suggests that automatic reinforcement and constraint on the opportunity to run might help explain these differences, but further research is required to substantiate these findings.