Oral cocaine as a reinforcer: acquisition conditions and importance of stimulus control

Schedule-induced polydipsia in rats was established in daily, 3h sessions with a fixed-interval 1min food schedule, and continued with two fluids available on concurrent fixed-ratio 6 schedules. A 2% ethanol solution was preferred to water, and succeeding drug solutions (0.16mg/ml cocaine, 0.1mg/ml caffeine, 0.01mg/ml nicotine, 0.11mg/ml lidocaine) also were preferred. Except for lidocaine, these drugs are known to function as reinforcers. Drug solution position was alternated daily, with location indicated by a discriminative stimulus (S(D)) light. Subsequent S(D) manipulations indicated that lidocaine and cocaine preferences were attributable to the prior association of the S(D) with ethanol rather than to the pharmacological effects of the currently accessible drug. Furthermore, when concurrent water choices were instituted, animals continued to choose the water source indicated by the S(D). The effectiveness and durability of the S(D) in determining polydipsic choice attests to the importance of the associative history of environmental S(D)s in triggering and maintaining drug seeking and drug taking. A model is outlined which suggests that drug abuse is a special case of a more general set of excessive behaviors induced by current environmental conditions, with choice of behavior remaining under S(D) control determined, in part, by the associative history of the S(D).