Uncertain, intermittent access to reward promotes increased reward pursuit

Self-administration procedures have been developed to model the intermittency of cocaine use in humans. These procedures involve intermittent, predictable access to cocaine during daily self-administration sessions. However, human drug use often involves intermittent and unpredictable patterns of drug access. Here, we introduce a new procedure to study the effects of unpredictable, intermittent access (UIntA) to a reinforcer, and we compare this procedure to two existing ones that provide predictable reinforcer availability; continuous (ContA) or intermittent (IntA) access. Three groups of rats self-administered water or a 5% sucrose solution in daily hour-long sessions. UIntA rats had alternating periods of reinforcer ON and OFF of unpredictable duration (1, 5 or 9 min/period). During reinforcer ON periods, reinforcer quantities were also unpredictable (0, 0.1 or 0.2 ml of solution) and were available under a variable ratio 3 schedule of reinforcement (1-6 responses). Both IntA and ContA rats had access to a fixed volume of water or sucrose (0.1 ml), under a fixed ratio 3 schedule of reinforcement. IntA rats had alternating and predictable 5-min reinforcer ON and OFF periods, while ContA rats had 60 minutes of reinforcer access during each session. Following 14 daily self-administration sessions, we found that UIntA rats had the highest rates of responding for water or sucrose reward under progressive ratio and extinction conditions, and the highest levels of cue-induced reinstatement of sucrose seeking. Thus, uncertain, intermittent access to reward promotes increased reward-seeking and -taking behaviours. This has implications for modeling addiction and other disorders of increased reward seeking.