Frequent reward eliminates differences in activity between hyperkinetic rats and controls.

The spontaneously hypertensive rat (SHR) is a strain that is a potential animal model of attention-deficit hyperactivity disorder (ADHD)/childhood hyperkinesis/disturbance of activity and attention, as SHR exhibit both hyperactivity and attention problems. Altered reinforcement (reward) processes have been suggested as the mechanism for the development of hyperactivity in SHR and ADHD. The purpose of the present study was to analyze basic reinforcement and response processes in the hyperactive SHR and in the progenitor Wistar-Kyoto (WKY) control strain. The results show that differences between the strains emerge in response rates maintained by infrequent reinforcers rather than in asymptotic rates of responding maintained by higher rates of reinforcement. The SHR strain required fewer reinforcers per minute to maintain high rates of responding, but when reinforcer rates were high enough to maintain asymptotic response rates in both strains, the rates were similar. Thus, the increased response rates for hyperactive rats at low reinforcement rates may reflect not only barpresses for water, but possibly also barpresses for other stimuli the animal produces by pressing the lever switch. Extrapolating to hyperactive children (ADHD), the results suggest that ADHD behavior might be made more similar to non-ADHD if reinforcers were scheduled more frequently.