Bilateral six-hydroxydopamine administration to PFC prevents the expression of behavioral sensitization to methylphenidate.

Psychostimulants like amphetamine and methylphenidate (MPD) are used to treat attention deficit hyperactivity disorder (ADHD), which is marked by developmentally inappropriate inattention, hyperactivity, and impulsivity. Neuropsychological analyses indicate that ADHD patients are impaired on tasks of behavioral inhibition, reward reversal, and working memory, which are functions of the prefrontal cortex (PFC) and are modulated by the mesocortical dopamine (DA) system. Non-specific electrical lesioning of PFC eliminated the expression of behavioral sensitization elicited by chronic MPD administration. Behavioral sensitization is the progressive augmentation of locomotor activity as a result of repetitive (chronic) exposure to the drug. It is believed that the sensitization to chronic drug treatment is caused due to an increase in DA in the mesocorticolimbic DA system, which includes the PFC. Therefore, this study investigated the role of PFC DA in mediating the behavioral sensitization to repeated administration of MPD in adult male Sprague-Dawley rats. On experimental day (ED) 1, the behavior was recorded post-saline injection. On ED 2, the rats were divided into three groups--control, sham and bilateral 6-OHDA treated group; and the sham and 6-OHDA treated groups underwent respective surgeries. After 5 days of rest following surgery, the post-surgery baseline was recorded on ED 8 following a saline injection. All three groups received 2.5 mg/kg MPD for 6 days (from ED 9 to ED 14), followed by a 3-day washout period (ED 15 to ED 18). On ED 19, a rechallenge injection of 2.5 mg/kg MPD was given and locomotor activity was recorded. It was found that the 6-OHDA lesion group failed to exhibit behavioral sensitization to MPD. The involvement of the dopaminergic afferents of PFC in behavioral sensitization to MPD is discussed.