A study at the Brookhaven National Laboratory, which appears in the Journal of the American Medical Association, found that ADHD patients lack key proteins which allow them to experience a sense of reward and motivation. Previous research looking at the brains of people with ADHD had uncovered differences in areas controlling attention and hyperactivity but this study suggests ADHD has a profound impact elsewhere in the brain too.

The researchers compared brain scans of 53 adult ADHD patients who had never received treatment with those from 44 people who did not have the condition.

Using a sophisticated form of scan called positron emission tomography (PET), the researchers focused on how the participants' brains handled the chemical dopamine, a key regulator of mood. In particular they measured levels of two proteins - dopamine receptors and transporters - without which dopamine cannot function effectively to influence mood. ADHD patients had lower levels of both proteins in two areas of the brain known as the nucleus accumbens and midbrain. Both form part of the limbic system, responsible for the emotions, and sensations such as motivation and reward. Patients with more pronounced ADHD symptoms had the lowest levels of the proteins in these areas.

The research supports the use of stimulant medications to treat ADHD by raising dopamine levels and also the theory that people with ADHD may be more prone to drug abuse and obesity because they are unconsciously attempting to compensate for a deficient reward system.

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First published in Sptember 2009

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