Long-term methylphenidate treatment down-regulates c-fos in the striatum of male CD-1 mice.

Methylphenidate (Ritalin) is routinely prescribed to children with attention deficit hyperactivity disorder, but little is known about its long-term consequences on brain development. We treated pre- and peri-pubertal male CD-1 mice with repeated injections of methylphenidate hydrochloride (MPH) and quantified the expression of the immediate early gene c-fos in the striatum. A single injection of MPH (5 or 40 mg/kg) significantly elevated FOS immunoreactivity in the striatum in a dose-dependent manner, compared with saline. Repeated MPH treatment attenuated the effect of a single challenge dose of MPH on striatal c-fos expression. These results replicate those observed with rats and indicate that long-term use of MPH may alter neural activity by down-regulation of gene expression in the striatum.