Re-evaluation of an animal model for ADHD using a free-operant choice task

Previous research using free-operant procedures have reported that the Spontaneously Hypertensive Rat (SHR) is more impulsive and inattentive than the Wistar-Kyoto (WKY) rat. Recently these behavioural differences have been suggested to be a consequence of differences in the overall activity of these strains. This study compared SHRs to WKYs on locomotor activity and delay sensitivity using a delayed reinforcement (DR) and extinction (EXT) task. SHRs maintained higher locomotor activity than WKYs, however no significant group differences were found on the total lever presses in the DR or EXT tasks. During the DR task, SHRs shifted to selecting the immediate small reinforcer significantly faster than WKYs as the delay increased. WKYs predominantly selected the lever previously associated with the delayed large reinforcer throughout the EXT task, while the SHRs showed no such preference. The significant group differences found on lever selection during the DR and EXT tasks suggests that SHRs are more sensitive to delays, therefore providing further support for the face validity of the SHR as an animal model of ADHD.