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Evaluating the effects of presession drug administration on intertemporal choice in nonhumans is a useful approach for identifying compounds that promote impulsive behavior in clinical populations, such as those prescribed the dopamine agonist pramipexole (PPX). Based on the results of previous studies, it is unclear whether PPX increases rats' impulsive choice or attenuates aspects of stimulus control. The present study was designed to experimentally isolate behavioral processes fundamental to intertemporal choice and challenge them pharmacologically with PPX administration. In Experiment 1, the hypothesis that PPX increases impulsive choice as a result of enhanced sensitivity to reinforcer delays was tested and disconfirmed. That is, acute PPX diminished delay sensitivity in a manner consistent with disruption of stimulus control whereas repeated PPX had no effect on delay sensitivity. Experiments 2 and 3 elaborated upon this finding by examining the effects of repeated PPX on rats' discrimination of response-reinforcer contingencies and reinforcer amounts, respectively. Accuracy of both discriminations was reduced by PPX. Collectively these results provide no support for past studies that have suggested PPX increases impulsive choice. Instead, PPX impairs stimulus control over choice behavior. The behavioral approach adopted herein could be profitably integrated with genetic and other biobehavioral models to advance our understanding of impulsive behavior associated with drug administration. [ABSTRACT FROM AUTHOR]

In both humans and nonhumans, prior research demonstrates increased preference for larger-later over smaller-sooner rewards when rewards are bundled together in a series (i.e., when an operant choice produces multiple discrete reward deliveries, as opposed to only a single delivery). These findings can be predicted using a traditional hyperbolic delay-discounting model. The present study was designed to examine the parametric effects of the size of the reward bundle on larger-later reward preference in male rats. During a reward-bundling phase, rats were exposed to bundle-sizes of either 1 (i.e., no bundling), 3, or 9 rewards. Rats in the Bundle-size 9 group showed significantly greater larger-later reward preference across a range of delays (0-17.5 s) than rats in any other group, but no other significant differences in choice were observed between groups. In addition, when choice for unbundled rewards was assessed following the reward-bundling phase, rats in the Bundle-size 9 group showed a significant increase in larger-later reward preference (compared to a pre-test). Obtained data from the reward-bundling phase are compared to model-predicted data, and potential mechanisms of the observed increase in self-control for unbundled rewards are discussed. [ABSTRACT FROM AUTHOR]