Preference Reversals and Effects of d-Amphetamine on Within-Session Delay Discounting in Rats

Impulsive choice is correlated with behavioral problems such as attention-deficit/hyperactivity-disorder and can be assessed using delay-discounting procedures in which subjects choose between a smaller, more immediate reinforcer (impulsive choice) and a larger, more delayed reinforcer (self-controlled choice). A within-session delay-discounting procedure in which choice was between one food pellet delivered immediately and three food pellets delivered after increasing delays was used to examine effects of adding or subtracting delays common to both reinforcers on impulsive choice in male Sprague-Dawley rats (N = 8). For all subjects, delay discounting was observed regardless of whether delays common to both reinforcers were added or subtracted. Using area under the curve (AUC) to quantify impulsive choice, adding delays common to both reinforcers decreased impulsive choice whereas subtracting delays common to both reinforcers increased impulsive choice. Prior to d-amphetamine administration, subjects were rank ordered into High-impulsive or Low-impulsive groups using AUC at the final delay condition. Subjects in the High-impulsive group made more impulsive choices than subjects in the Low-impulsive group and effects of d-amphetamine (0.03, 0.1, 0.18, 0.3, 0.56, 1.0, 1.8 mg/kg) generally depended on these differences. Select doses of d-amphetamine decreased impulsive choice for subjects in the High-impulsive group but not for subjects in the Low-impulsive group. These results show that levels of impulsive choice can be altered by changing the delay common to both reinforcers and suggest that effects of d-amphetamine on impulsive choice are determined, in part, by baseline levels of impulsive choice