Concomitant behavioral and PFC neuronal activity recorded following dose-response protocol of MPD in adult male rats.

The use of methylphenidate (MPD), a commonly prescribed drug to treat attention-deficit hyperactivity disorder (ADHD), has steadily increased over the past 25 years. This trend has been accompanied by more MPD abuse by ordinary individuals for its cognitive enhancing effects. Therefore, understanding the effects of MPD on the prefrontal cortex (PFC), a brain area involved in higher cortical processing such as executive function, language, planning, and attention regulation, is of particular importance. The goal of this study is to investigate the effects of acute and chronic dose-response characteristics following MPD exposure on both the PFC neuronal population and behavioral activity in freely behaving animals implanted previously with permanent electrodes within the PFC. Four groups of animals were used: saline (control), 0.6, 2.5, and 10.0mg/kg MPD. It was observed that the same dose of either 0.6, 2.5, or 10.0mg/kg repetitive (chronic) MPD exposure elicited behavioral sensitization in some animals and behavioral tolerance in others, and that the majority of PFC units recorded from animals expressing behavioral sensitization to chronic MPD exposure responded to MPD by increasing their neuronal firing rate, whereas the majority of PFC neurons recorded from animals expressing behavioral tolerance in response to chronic MPD responded by decreasing their neuronal firing rate. This data suggests that in animals that display behavioral sensitization, chronic MPD exposure causes an increase in the number of post-synaptic D1 dopamine receptors leading to an increase in behavioral and neuronal firing rate, while in animals that display behavioral tolerance, chronic MPD exposure causes an increase in the number of post-synaptic D2 dopamine receptors leading to a decrease in behavioral and neuronal firing rate. This dichotomy needs to be further investigated.