(June 6, 2006) -- Commonly prescribed drugs to treat attention deficit hyperactivity disorder (ADHD) seem to have an effect on dopamine released in the brain of people with the disorder.

New German research shows that different amounts of dopamine, which is related to positive reinforcement-based learning and behavior, are released in the brains of people treated for ADHD, compared with those untreated for their disorder.

"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," Felix M. Mottaghy, a research fellow at the University of Ulm, said in a prepared statement. Mottaghy added that this is the first study to identify the positive effect that ADHD drugs have on the brain's dopamine system -- although that's long been a theory.

"This is a very preliminary, basic science study... however, future studies of the dopamine system could aid differential diagnosis in hyperactive children," said Mottaghy.

Researchers tested individuals with ADHD, both treated and untreated, with a PET scan of the brain and 18F-DOPA, a drug related to dopamine. Then, the brain images that resulted were statistically mapped.

"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," said Mottaghy. "Our study shows that the beneficial effect of methylphenidate is received via 'normalization' of the dopamine system."

Results of the study were to be presented at this week's annual meeting of the Society of Nuclear Medicine, in San Diego.

"We demonstrated that the brain's dopamine system -- including midbrain, the striatum and the amygdala -- is differentially modulated in treated and untreated ADHD patients with respect to healthy normal controls," Mottaghy said.

Last updated: 06/06

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