Different effects of amphetamine on reinforced variations versus repetitions in spontaneously hypertensive rats (SHR)

The spontaneously hypertensive rat (SHR) may serve as an animal model of human attention deficit hyperactivity disorder (ADHD). We compared performances of SHRs and Wistar-Kyoto normotensive controls rats (WKY) in two experiments. When rewarded for varying sequences of responses across two manipulanda, the SHRs were more likely to vary than the WKYs. On the other hand, when rewarded for repetitions of a small number of sequences, the WKYs were more likely to learn to repeat. Both of these results confirm previous findings. Injecting 0.75 mg/kg d-amphetamine facilitated learning by SHRs to repeat the required sequences, with amphetamine-injected SHRs learning as rapidly as saline-injected, control WKYs. On the other hand, amphetamine tended to increase variability in both strains when high levels of variations were required for reward, and to decrease it in both strains when low levels of variability were required. Thus, amphetamine may have different effects on reinforced repetitions vs. reinforced variations.

Spontaneously hypertensive rats (SHR) readily learn to vary but not repeat instrumental responses

When spontaneously hypertensive rats (SHR) and Wystar-Kyoto normotensive control rats (WKY) were rewarded in a 12-arm radial maze (Experiment 1), the SHRs varied their arm choices more, making fewer repetition errors than the WKYs. Similarly when rewards depended on variable sequences of responses on two levers in an operant chamber (Experiment 2), SHRs' sequences were more variable than those of WKYs. A requirement for response variability was then combined with a requirement to repeat selected responses in the radial maze (Experiment 3) and operant chamber (Experiment 4). WKYs learned to repeat more readily than the SHRs, whereas SHRs varied more readily. Thus, when subjects had to repeat responses, SHRs were at a disadvantage, but when variability was adaptive, SHRs excelled. The high variability of SHRs, together with their difficulty in learning to repeat, may have parallels in children diagnosed with attention deficit disorder with hyperactivity (ADDH).