Intra-orbitofrontal cortex injection of haloperidol removes the beneficial effect of methylphenidate on reversal learning of spontaneously hypertensive rats in an attentional set-shifting task

Abstract

Numerous studies suggest that attention-deficit/hyperactivity disorder (ADHD) is caused by deficits in catecholaminergic systems. Furthermore, dysfunctions of prefrontal cortex can impair inhibitory controls of ADHD patients, resulting in their impulsive behaviors. Researchers also find that rats with lesions in the orbitofrontal cortex show deficits in the reversal learning of attentional set-shifting task (ASST), a behavioral test frequently used in human studies to asses the inhibition system. However, the role of orbitofrontal dopamine system in the mechanism responsible for the dysfunctions of inhibitory controls in ADHD patients and animal models remains unknown. In the present study, we manipulated orbitofrontal dopamine activities of spontaneously hypertensive rats (SHR), a widely used ADHD animal model, through intra-peritoneal injection of methylphenidate (MPH) and central infusion of haloperidol, and observed their performances in ASST. The results show that juvenile SHRs learned slower than Wistar controls in the first and second reversal learnings of ASST. The deficits could be removed by intra-peritoneal injections of MPH. Furthermore, central infusions of haloperidol in the orbitofrontal cortex blocked the effects of MPH. In conclusions, dopamine activity in orbitofrontal cortex might play a crucial role in the neural mechanism of reversal learning deficits in this animal model of ADHD.