There is increasing evidence that ADHD is a genetic neurobiological disease, and two new studies have added to this by demonstrating a link between ADHD and changes in production of the brain chemical dopamine.

In the first study, researchers found that about one-quarter to one-fifth of children with ADHD have a variant of the dopamine receptor gene (DRD4) which may help cause the behavioural condition by producing thinner tissue in areas of the brain that control attention. Strangely, however, children who have the risk gene also tend to get better in the long term, eventually regaining healthy tissue thickness in the affected brain region - a fact that may explain ADHD's tendency to improve with age.

In a second study, scientists found that ADHD is associated with lowered dopamine production and that the drug Ritalin is able to help some children with ADHD by increasing the brain's production of this chemical. The researchers found that the reduction in dopamine was associated with typical symptoms of inattention, which may explain why many people with ADHD tend to abuse drugs such as nicotine, cocaine and methamphetamine, all of which increase dopamine activity in the brain.

Sources: Archives of General Psychiatry, August 2007; vol 64: pp 921-931and August 2007; vol 64: pp 932-940.

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First published in 2007

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