ADHD Drug Complements Body's Dopamine System

The brain's dopamine system, which has long been linked with reward learning and reward-related behavior, works differently in treated and untreated attention deficit hyperactivity disorder (ADHD) individuals, according to a new study.

"The significant difference we found between treated and untreated ADHD patients provides an important hint on the effect of the most commonly prescribed drug for this disease, which has long baffled and frustrated parents and physicians,” noted co-author of the study, Dr. Felix M Mottaghy, a research fellow at the University of Ulm (Germany). Until this study, there has been no direct evidence pointing to the beneficial effect of methylphenidate (drugs such as Ritalin) on the body's dopamine system, added Dr. Mottaghy. The study was presented by the German researchers at the Society of Nuclear Medicine's (SNM) 53rd annual meeting, held in June 2006 in San Diego (CA, USA).

For years, researchers have hypothesized that methylphenidate relaxes people with ADHD by increasing the release of the neurotransmitter dopamine, improving attention and focus in those who have weak dopamine signals.

The primary features of ADHD are inattention, hyperactivity, and impulsivity, and this disorder can manifest early in a child's life, sometimes as early as preschool. With ADHD, there is an imbalance of several neurotransmitter systems, according to Dr. Mottaghy. "The most affected seemed to be the dopaminergic system. Until now most studies focused on the so-called postsynaptic or receiving part of this system,” he explained. "Our study shows that the beneficial effect of methylphenidate is received via ‘normalization' of the dopamine system. We demonstrated that the brain's dopamine system--including midbrain, the striatum, and the amygdal--is differentially modulated in treated and untreated ADHD patients with respect to healthy normal controls. Methylphenidate leads to a harmonization of the presynaptic dopaminergic neurons that could explain in part the beneficial effects of this central nervous system stimulant.”

The investigators utilized positron emission tomography (PET), a noninvasive brain scanning method, with 18F-DOPA, an imaging agent that is a precursor of dopamine. The researchers also used statistic parametric mapping to obtain the statistical comparison of normalized and reoriented brain images, according to Dr. Mottaghy.



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