Effects of d-Amphetamine on Delay Discounting with Different Response Requirements in Rats.

Impulsive choice is often examined using a delay-discounting procedure where choice is between two reinforcers that differ in amount and delay. Impulsive choice can be influenced by many factors, including drug administration or the addition of a constant delay to the smaller, more immediate and larger, more delayed reinforcer. Little research has examined whether increasing the number of responses (fixed-ratio) or amount of time (fixed-interval) required to earn the smaller, more immediate and larger, more delayed reinforcers would decrease impulsive choice. The current experiments used a modified Evenden and Ryan (1996) procedure, and choice was examined under conditions where the response requirement for both outcomes was a small, intermediate, and large fixed ratio (FR) (Experiment 1) and when the response requirement for both outcomes was a short, intermediate, and long fixed interval (FI) (Experiment 2). Steeper discounting functions (more impulsive choices) were obtained when the small FR and short FI were in effect, shallower functions (more self-controlled choices) were obtained when the large FR and long FI were in effect, and intermediate functions were obtained when the intermediate FR and FI were in effect. Manipulating the response requirement within this procedure can generate differences in impulsive choice within subject. Effects of stimulant drugs on impulsive choice may depend on the baseline rate of delay discounting, and this has been demonstrated using between-subjects designs. Acute effects of d-amphetamine on different rates of delay discounting obtained within subject were examined under different FR and FI requirements. Drug effects generally were not systematic across subjects, and the largest dose of d-amphetamine tested disrupted control by reinforcer amount