Studies of delay discounting in humans have revealed that drug users have an exaggerated preference for small, immediate over large, delayed rewards, even when the immediate reward is substantially smaller than the delayed reward. These studies have been interpreted to mean that drug users are more impulsive than nonusers and, possibly, that drug use has altered the processes by which individuals evaluate the benefits and costs associated with rewards. These processes are believed to determine the efficacy of drug and nondrug reinforcers to control behavior. However there are costs other than delay which can cause the value of a reinforcer to be discounted. Two such costs are uncertainty and effort. Five studies are proposed to evaluate the impact of all three costs on reinforcer efficacy (delay to the receipt of a reward, uncertainty about the receipt of the reward, and the effort required to obtain the reward) using a discounting procedure that was developed by the Principal Investigator and her colleague for use with rats. The basic hypotheses are that 1) the rats that discount delayed rewards more will also discount uncertain and effortful rewards more and 2) that discounting for the three different types of cost will be similarly effected by a series of behavioral and pharmacological manipulations. Experiment 1 determines whether rats discount large, expensive rewards to similar degrees regardless of the cost type by assessing the correlation between delay discounting, uncertainty discounting and effort discounting in the same animals, and examines the impact of chronic injections of nicotine on discounting. Experiment 2 examines how the magnitude of reward affects delay, uncertainty and effort discounting. Experiment 3 determines whether preference switches from the small, cheaper reward to the large, expensive reward when a fixed cost is added to both alternatives. Experiment 4 examines whether exposure to high-reinforcer-cost conditions leads to decreases in delay, uncertainty and effort discounting. Experiment 5 investigates the role of dopamine in modulating delay, uncertainty and effort discounting. These studies will provide information about the variables taken into account when the brain evaluates reinforcer efficacy and procedures that can be used to alter reinforcer efficacy, both of which will have implications for drug abuse treatment. Further, the data collected in these studies will provide a basis for future studies, which will examine whether differences in how individuals evaluate the value of reinforcers are causally related to the initiation, progression and cessation of drug use and abuse.
