K. K. and Y. A. contributed equally to this work.Abbreviations used: 5-HT, serotonin; ADHD, attention-deficit/hyperactivity disorder; AUC, area under the curve; DA, dopamine; i.p., intraperitoneally; NA, noradrenaline; PBS, phosphate buffered saline. AbstractAcute administration of atomoxetine and methylphenidate, attention-deficit/hyperactivity disorder (ADHD) drugs, activates catecholaminergic systems in rat brain, but the effects of their chronic administration are not known. This study examined the effects of acute and chronic administration of ADHD drugs on the extracellular levels of noradrenaline (NA), dopamine (DA) and serotonin (5-HT), and the expression of the neuronal activity marker c-Fos in the prefrontal cortex and striatum of mice. Acute ADHD drugs increased NA and DA, but not 5-HT, levels in the prefrontal cortex of mice. Maximal effects of atomoxetine and methylphenidate were observed at 1 mg/kg and 3 mg/kg, respectively. At these doses, both drugs did not affect the spontaneous locomotor activity of mice. Chronic administration of atomoxetine 1 mg/kg and methylphenidate 3 mg/kg for 21 days also increased NA and DA, but not 5-HT, levels in the prefrontal cortex. The increases in NA levels induced by atomoxetine, but not methylphenidate, were reduced by chronic treatment. In contrast, acute and chronic administration of atomoxetine 1 mg/kg and methylphenidate 3 mg/kg did not affect the monoamine levels in the striatum. Acute and chronic atomoxetine 1 mg/kg and methylphenidate 3 mg/kg increased the expression of c-Fos in the prefrontal cortex, but not in the striatum, to a similar extent. These results suggest that acute and chronic administration of the ADHD drugs selectively activate the prefrontal catecholamine systems in mice.

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