Abnormal topography and altered acquisition of conditioned eyeblink responses in a rodent model of attention-deficit/hyperactivity disorder.

The spontaneously hypertensive rat (SHR) has been suggested as a possible animal model of attention-deficit/hyperactivity disorder (ADHD). Reductions in the volume of the cerebellum and impairments in cerebellar-dependent eyeblink conditioning have been observed in ADHD, prompting investigation into whether SHRs also exhibit eyeblink conditioning impairments. In Experiment 1, SHRs and a control strain, Wistar, were trained on a long-delay eyeblink conditioning task in which a tone conditioned stimulus was paired with a periorbital stimulation unconditioned stimulus (750-ms delay paradigm). SHRs exhibited faster acquisition of eyeblink conditioned responses (CRs) and displayed mistimed (early onset and peak latency) and larger CRs in comparison with Wistar rats. In subsequent extinction training, SHRs were slower to extinguish CRs. The authors conducted Experiment 2 using separate rats to rule out the possibility that the results of Experiment 1 were due to nonassociative responding. SHRs and Wistar rats were presented with explicitly unpaired tone and periorbital stimulation stimuli. There was no evidence of conditioning in either group, nor were there differences between the groups in terms of the number of eyeblink responses elicited by the tone. The current results support the hypothesis of cerebellar abnormalities in this rodent model of ADHD.