Can Slot-Machine Reward Schedules Induce Gambling Addiction in Rats?

Similar to drugs of abuse, random-ratio reward schedules are highly motivating and, in humans, are thought to foster gambling addiction. Animal gambling models, however, have not yet demonstrated the compulsivity so characteristic of drug addiction. Three criteria have been used to evaluate addiction-like behavior in drug models: (1) response inhibition when reward is not available, (2) persistence under a progressive ratio schedule, in which the response-to-reward ratio is stretched, and (3) persistence in spite of punishment. We tested whether prolonged exposure (6 weeks) to a gambling-like reward schedule would induce addiction-like symptoms in rats. In two studies, separate groups were trained to respond to either random- or fixed-ratio schedules for food reward. We found that rats trained on random-ratio schedules showed higher response rates and dramatically shorter pauses after rewards. Tests of addiction-like behavior, however, were largely negative. Response rates were not different during cued no-reward periods nor when reward was coupled with punishment. We also found no group differences when food was devalued nor in reinstatement of reward-seeking after a 1-week delay. The sole exception to this pattern was that rats in the second experiment showed greater persistence on a progressive ratio test. After experiment two, subjects were also orally administered pramipexole, which caused increased perseveration during progressive ratio testing, especially in the random ratio group. While, it is possible that longer training or more appetitive rewards might have led to addiction-like behavior, our results, on the surface, suggest that random-ratio schedules are motivating but not addictive.