Impulsive choice in a rodent model of attention-deficit/hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been studied extensively as a purported rodent model of attention-deficit/hyperactivity disorder (ADHD). Because ADHD in humans is partially defined by marked impulsivity, SHRs, if a valid model of ADHD, ought to behave more impulsively than their normotensive parent strain, Wistar Kyoto (WKY). This prediction was evaluated in two experiments that employed an intertemporal choice procedure in which SHRs and WKYs made repeated choices between a single food pellet delivered immediately and three food pellets delivered after a delay. Four or five delays were investigated (1, 3, 6, 12 and 24s); the experiments differed in the manner in which the delays were experienced. In Experiment 1, the delay values changed after each session and were presented in ascending then descending order. SHRs chose more small/immediate reinforcers than WKYs at the longest delays during the ascending series and at nearly all delays during the descending series. In Experiment 2, the delay values remained in effect for several sessions and were presented in random order. Again, the SHRs chose more small/immediate reinforcers than the WKYs at the longest delays. Thus, in the present study, the SHRs were shown to be more impulsive than the WKYs as defined by preference for smaller, immediate reinforcers over larger, delayed ones in an intertemporal choice procedure.