"These deficits in the brain's reward system may help explain clinical symptoms of ADHD, including inattention and reduced motivation, as well as the propensity for complications such as drug abuse and obesity among ADHD patients," said lead author Nora Volkow, Director of the National Institute on Drug Abuse and a long-time collaborator on neuroimaging research at Brookhaven Lab.
"Finding ways to address the underlying reward-system deficit could improve the direct clinical outcome of ADHD, and potentially reduce the likelihood of other negative consequences of this condition," said study co-author Gene-Jack Wang, chair of Brookhaven's medical department.
During the study, the researchers recruited 53 adult ADHD patients who had never received treatment and 44 healthy control subjects - all of whom had been carefully screened to eliminate potentially confounding variables.
The scientists used positron emission tomography (PET) to measure two markers of the dopamine system - dopamine receptors, to which the chemical messenger binds to propagate the "reward" signal, and dopamine transporters, which take up and recycle excess dopamine after the signal is sent.
The results showed that the ADHD patients had lower levels of dopamine receptors and transporters in the accumbens and midbrain,
These two key regions of the brain directly involved in processing motivation and reward.
In addition, the measurements of dopamine markers correlated with measures of behaviour and clinical observations of ADHD symptoms, such as reduced levels of attention as measured by standard psychological tests.
"Our findings imply that these deficits in the dopamine reward pathway play a role in the symptoms of inattention in ADHD and could underlie these patients' abnormal responses to reward," said Volkow.
"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," Volkow added.
The study appears in Journal of the American Medical Association.