By Jenny Hope
Last updated at 8:07 AM on 09th September 2009
Hyperactive children have low levels of a brain chemical vital for causing them to pay attention in return for rewards, claim researchers.
It is the first objective evidence that deficiencies in the brain's system for reward and motivation are linked to the disorder affecting thousands of children.
Brain imaging tests run by US researchers show a lack of dopamine - a chemical messenger - in children with attention deficit hyperactivity disorder (ADHD).
The chemical is essential for helping trigger behaviour involving motivation and reward, which results in reduced levels of attention and behavioural problems.
Up to 100,000 British children with ADHD are given the drug Ritalin, which appears to act by releasing dopamine to improve concentration.
Around three to seven per cent of children are believed to have ADHD, with many continuing to experience problems as adults.
Lead researcher Nora Volkow, of the Brookhaven Laboratory, New York, said it was the first definitive evidence that patients suffering from ADHD have lower-than-normal levels of proteins essential for activating the brain's reward system.
She said 'These deficits in the brain's reward system may help explain the clinical symptoms of ADHD, including inattention and reduced motivation, as well as the propensity for complications such as dug abuse and obesity among ADHD patients.'
Results from previous studies had been less conclusive because in many cases patients had undergone treatment or underlying conditions that may have affected the functioning of the dopamine system, she said.
The latest study looked at 53 adults with ADHD who had never received treatment and 44 healthy people.
The researchers used a PET (positron emission tomography) scanner to measure two markers of the dopamine system.
Lying in a PET scanner, each patient was injected with a tiny amount of a 'radiotracer' compound designed to bind to dopamine receptors and transporters in a bid to track their activity levels.
The results 'clearly showed' that ADHD had lower levels of dopamine active in the areas of brain important for processing motivation and reward, compared with healthy people.
The measurements of dopamine markers correlated with clinical symptoms such as reduced levels of attention, said a report in the Journal of the American Medical Association.
Funding for the study came from US National Institutes on Mental Health, Alcohol and Drug Abuse.
Dr Volkow said the results meant current use of medication such as Ritalin was justified because it would be compensating for poor dopamine levels in ADHD students.
She said 'Our findings imply that these deficits in the dopamine reward pathway play a role in symptoms of inattention in ADHD and could underlie these patients' abnormal responses to reward.
'This pathway plays a key role in reinforcement, motivation, and in learning how to associate various stimuli with rewards.
'Its involvement in ADHD supports the use of interventions to enhance the appeal and relevance of school and work tasks to improve performance.
'Our results also support the continued use of stimulant medications - the most common pharmacological treatment for ADHD - which have been shown to increase attention to cognitive tasks by elevating brain dopamine.'
Co-author Gene-Jack Wang, chairman of Brookhaven's medical department, said: 'Other studies from our group suggest that patients who abuse drugs or overate may be unconsciously attempting to compensate for a deficient reward system by boosting their dopamine levels.
'Finding ways to address the underlying reward-system deficit could improve the direct clinical outcome of ADHD,' he added.
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