Enhanced reactivity and vulnerability to cocaine following methylphenidatetreatment in adolescent rats

Treatment of attention deficit hyperactivity disorder with the psychostimul ant drug methylphenidate (MP) has increased dramatically among schoolchildr en. We tested whether repeated exposure to moderate doses of MP (5 and 10 m g/kg IP for 5 or 7 days) in adolescent rats increased reactivity to cocaine measured by motor responses (ambulations and rearing) to a cocaine challen ge in adulthood. We later tested whether repeated exposure to a low dose of MP (2 mg/kg IP for 7 days) enhanced the psychomotor effects of cocaine, me asured by different challenge doses (0-30 mg/kg) as well as to the reinforc ing effects of cocaine, measured by self-administration of low-dose infusio n (75 mug/kg, IV). We found that exposure to moderate doses of MP enhanced psychomotor responses to cocaine but exposure to a low dose only increased cocaine self-administration. These results suggest that adolescent exposure to low doses of MP in rats may increase the incentive value of low reinfor cers, thereby rendering adult rats more susceptible to cocaine self-adminis tration. (C) 2001 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.