Dual Diagnosis

Methylphenidate (MPD) considered as the gold standard in the treatment of attention defi cit hyperactivity disorder (ADHD),

one of the most common childhood disorders. MPD is also fi nding their way into the hands of healthy adolescent as brain

augmentation to improve cognitive performance and for recreation. Th e possible long term eff ects of MPD exposure in adolescence

are considered controversial. Th e objective of this study is to investigate the acute and chronic dose response characteristics of MPD

on animal behavior concomitant with nucleus accumbens (NAc) neuronal activity recorded in freely behaving adolescent rats. Th e

experiment lasted for 10-16 consecutive days aft er the recovery from implanting 4 permanent semi microelectrodes in the NAc or its

destruction. Four groups of intact animals and eight groups with electrical or chemical NAc lesion were used: saline (control), 0.6,

2.5 and 10 mg/kg MPD groups. Acute MPD was found to elicit a dose response increase in animals' locomotor activity. Re-challenge

with MPD at experimental day 10 (ED10) compared to the eff ect of MPD at ED1 showed no signifi cant diff erences. When the

animals were divided into groups based on their individual responses to chronic MPD exposure, some animals expressed behavioral

tolerance and some expressed behavioral sensitization. NAc destruction prevents the chronic eff ect of MPD to elicit behavioral

sensitization or tolerance. Behavioral sensitization and tolerance is an experimental indicator suggesting that a drug has the potential

to elicit dependent. Electro-physiologically, a dose response characteristic for acute and chronic MPD exposure was observed. With

increasing MPD doses, more NAc units responded by changing their fi ring rate. Moreover, the neuronal responses to chronic MPD

recorded from animals expressing behavioral tolerance were signifi cantly diff erent compared to the neuronal population responses

recorded from animals expressing behavioral sensitization. Th e majority of the NAc units recorded from animals expressing behavioral

tolerance responded to MPD predominately by decreasing their fi ring rates, whereas NAc units recorded from the behaviorally

sensitized animals mainly showed an increase in their fi ring rates.