Effects of Delay Order and d-Amphetamine on Delay Discounting in Rats

Impulsive choice is defined as choice for a smaller, more immediate reinforcer over a larger, more delayed reinforcer, whereas self-controlled choice is defined as choice for a larger, more delayed reinforcer over a smaller, more immediate reinforcer. Determinants of impulsive and self-controlled choices may be studied by employing delay-discounting procedures. When using a discrete-trials delay-discounting procedure, a difference in delay discounting as a function of order of presentation of delays to larger-reinforcer delivery was reported by one previous study, whereas another study reported no differences. In both studies, subjects had a history of exposure to both ascending and descending order of delay presentations prior to establishing stable delay-discounting functions with one or both order of delay presentations. Experiment 1 used a group design with Sprague-Dawley rats and a discrete-trials, within-session procedure to evaluate effects of order of delay presentation on delay discounting while controlling for history of exposure to different order of delay presentations. Levels of delay discounting (impulsive choice) were not systematically affected by order of delay presentation. Experiment 2 tested the impact of order of delay presentation on effects of d-amphetamine on delay discounting. Relative to saline, acute administration of the two largest doses of d-amphetamine tested resulted in decreases in mean area under the curve, regardless of order of delay presentation