Methylphenidate (MPH) is one of the most common therapeutics used for the treatment of attention-deficit/hyperactivity disorder (ADHD), which consists of symptoms of inattention, and/or impulsivity and hyperactivity. Acute administration of MPH has been found to decrease impulsive choice in both humans and nonhuman animals, however, little is known about potential long-term changes in impulsive choice due to chronic administration of MPH. In the present experiment, effects of acute and chronic MPH (1.0-10.0mg/kg) were assessed on impulsive choice in the adult male Spontaneously Hypertensive Rat (SHR) to determine the extent of behavioral changes after chronic MPH exposure. Subjects chose between an immediate single food pellet and three food pellets delivered after a delay that increased within session (0 to 16s). At relatively higher doses during acute and chronic administration, choice maintained by the larger reinforcer was disrupted when there was no delay to either outcome, suggesting that MPH may be affecting stimulus control under the current delay-discounting task. When this disruption was not observed, however, MPH effects were selective in that only one intermediate dose (3.0mg/kg) decreased mean impulsive choice at one delay (8s) following acute administration. The same effect was observed following chronic MPH administration except that the dose was higher (5.6 mg/kg) and the delay was shorter (4s). Chronic administration of MPH did not show any negative indicators (e.g., an increase in impulsive choice) when administration was discontinued.
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