September 10, 2009
ADHD And Deficits In Brain's Reward System
A new brain-imaging study published in JAMA this week suggests that patients suffering from attention deficit hyperactivity disorder (ADHD) have lower-than-normal levels of certain proteins essential for experiencing reward and motivation.
According to lead author Nora Volkow, Director of the National Institute on Drug Abuse, "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."
It was believed that individuals with ADHD had abnormalities in the brain's dopamine-mediated motivation/reward system but this study provides the first definitive evidence. Prior to this study, it was not clear whether people with ADHD had abnormalities in the brain's dopamine-mediated motivation/reward system. Previous studies were relatively small and may have been complicated by the fact that some ADHD patients had undergone treatments, or had a history of drug abuse or other conditions that can affect the dopamine system.
The study used positron emission tomography (PET) to measure dopamine synaptic markers (transporters and D2/D3 receptors) in 53 nonmedicated adults with ADHD and 44 healthy controls between 2001-2009 at Brookhaven National Laboratory. While in the PET scanner, each patient was injected with a minute amount of a "radiotracer" compound. Different tracers were used for each target, and patients were scanned for each at separate times. By detecting the signal from the radiotracers, the PET machine can measure the receptor and transporter locations and concentrations in various parts of the brain.
The results clearly showed that, relative to the healthy control subjects, the ADHD patients had lower levels of dopamine receptors and transporters in the accumbens and midbrain - two key regions of the brain directly involved in processing motivation and reward. In addition, the measurements of dopamine markers correlated with measures of behavior 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. 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," said Volkow.
JAMA Article
Vol. 302 No. 10, September 9, 2009