Caffeine increases the motivation to obtain non-drug reinforcers in rats

Background: Caffeine is widely considered to be a reinforcer in humans, but this effect is difficult to measure in non-human animals. We hypothesized that caffeine may have dual reinforcing effects comparable to nicotine - limited primary reinforcing effects, but potent reinforcement enhancing effects. The present studies tested this hypothesis by investigating the effect of caffeine on responding for non-drug rewards. Methods: In two experiments, rats were shaped to respond on a progressive ratio (PR) schedule for sucrose solution (20%, w/v; Experiment 1) or a fixed ratio 2 (FR2) schedule for a moderately reinforcing visual stimulus (VS; Experiment 2). Pretreatment with various doses of caffeine (0-50 mg/kg, intraperitoneal injection) were administered prior to tests over successive week days (M-F). In Experiment 1, acute administration of low-moderate caffeine doses (6.25-25 mg/kg) increased responding for sucrose under the PR schedule. This effect of caffeine declined over the initial 15 test days. In Experiment 2, only acute pretreatment with 12.5 mg/kg caffeine increased responding for the visual stimulus and complete tolerance to this effect of caffeine was observed over the 15 days of testing. In follow up tests we found that abstinence periods of 4 and 8 days resulted in incomplete recovery of the enhancing effects of caffeine. Conclusion: The findings suggest that caffeine enhances the reinforcing effects of non-drug stimuli, but that the pharmacological profile of these effects may differ from other psychomotor stimulants.