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Depressed Dopamine Activity Seen in Adult ADHD

Blunted response to methylphenidate suggests the condition is linked to dopamine dysfunction

MONDAY, Aug. 6 (HealthDay News) -- Adults with attention-deficit/hyperactivity disorder (ADHD) have a blunted response to methylphenidate in the left and right caudate, and they have a lower dopamine release than those without the disorder, researchers report in the August issue of the Archives of General Psychiatry.

Nora D. Volkow, M.D., of the National Institute on Drug Abuse in Bethesda, Md., and colleagues measured dopamine activity in 19 adults with ADHD (average age 32) and 24 healthy controls (average age 30) after injecting the subjects with either placebo or methylphenidate. They also assessed the subjects' post-injection ADHD symptoms and asked the subjects if they liked or disliked the injection.

The researchers found that ADHD subjects had fewer dopamine receptors available in the left caudate than controls, and that methylphenidate caused fewer changes to dopamine activity in ADHD subjects than in controls. They also found evidence of reduced binding in the hippocampus and amygdala in ADHD subjects and that the blunted response was associated with symptoms of inattentiveness and increased drug-liking in ADHD subjects.

"The reinforcing responses to methylphenidate were negatively correlated with the dopamine increases, suggesting that decreased dopaminergic activity may also be involved in modulating the magnitude of the reinforcing effects of methylphenidate," the authors write. "This suggests that dopamine dysfunction is involved with symptoms of inattention but may also contribute to substance abuse comorbidity in ADHD."

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