As the use of amphetamine in the treatment of ADHD has increased, a large body of preclinical data has accrued indicating that amphetamine has the potential to damage brain dopamine-containing neurons in experimental animals. In particular, animals treated with amphetamine develop lasting reductions in striatal dopamine, its major metabolite dihydroxyphenylacetic acid (DOPAC), its rate-limiting enzyme tyrosine hydroxylase, its membrane transporter (DAT), and its vesicular transporter (VMAT2) (Gibb et al., 1994; McCann and Ricaurte, 2004). Anatomic studies indicate that lasting dopaminergic deficits after amphetamine are due to damage of dopaminergic nerve endings in the striatum, with sparing of dopaminergic nerve cell bodies in the substantia nigra.
In particular, the results of the present study indicate that an oral regimen of amphetamine, modeled after dosing regimens used in patients with ADHD, engenders plasma amphetamine concentrations that result in toxicity to brain dopaminergic axon terminals in baboons and squirrel monkeys. These results may have implications for the pathophysiology and treatment of ADHD and raise the question of whether or not plasma monitoring might be indicated in ADHD patients receiving higher, chronic doses of amphetamine.
In early studies, high doses of amphetamine, comparable to amounts used by addicts, were shown to damage dopaminergic pathways. More recent studies, using therapeutic regimens, appear contradictory. One paradigm shows significant decreases in striatal dopamine and transporter density after oral administration of “therapeutic” doses in primates. Another shows morphological evidence of “trophic” dendritic growth in the brains of adult and juvenile rats given systemic injections mimicking “therapeutic” treatment. Imaging studies of ADHD-diagnosed individuals show an increase in striatal dopamine transporter availability that may be reduced by methylphenidate treatment
In contrast to concerns about potential adverse effects of amphetamine on the brain during aging, it is remarkable that the reduction of the heightened risk for substance abuse that is otherwise associated with ADHD by the initiation of stimulant treatment during childhood appears to be accompanied by a congruent reduction in structural brain pathology. Unmedicated children with ADHD had smaller brain white matter volume than medicated children with ADHD (−8.9%, P<.001) or children without ADHD (−10.7%, P<.001), suggesting that early stimulant treatment may normalize brain white matter volume in ADHD 182.
