DRL performance of spontaneously hypertensive rats: dissociation of timing and inhibition of responses.

In this experiment, we used a differential reinforcement of low rates (DRL) schedule to evaluate the performance of spontaneously hypertensive rats (SHR), Wistar Kyoto (WKY) and Wistar (WIS) rats, with the goal of dissociating the processes of timing and inhibition of responses through the use of two quantitative models: the peak deviation analysis and the temporal regulation model. The subjects were divided in two groups; the first group was exposed to 70 sessions under a DRL 10s schedule. SHR rats showed an apparent temporary deficit in the inhibition of responses process; however, no differences among strains were observed in terms of the timing process. The second group of rats was exposed to 30 sessions in DRL 10s schedule, before receiving three doses (2 mg/kg, 4 mg/kg and 8 mg/kg) of methylphenidate. The results obtained through both models were consistent and indicated that at higher drug doses, the performance of all three strains of rats deteriorated. The impulsivity exhibited by SHR during acquisition supports the idea of these rats as an adequate animal model of ADHD. In contrast, evidence against this relies on the normal temporal processing found and in the worsening effect that methylphenidate produced in the process of inhibition of responses. These mixed results suggest the necessity of exploring timing behavior of other animal models in order to find a reliable animal model of ADHD.