Classic symptoms of attention deficit hyperactivity disorder, including inattention and the lack of impulse control, might be caused by a disruption of a chemical in the brain that helps cells communicate, a U.S. study suggests.

Studies have already indicated that dopamine, a neurotransmitter needed for normal functioning of the central nervous system, is disrupted in some pathways of the brain in people with ADHD.

Now a study has pinpointed sites in the brain where this seems to occur, a researcher says in Tuesday's issue of the Journal of the American Medical Association.

Dopamine is considered crucial to our ability to perceive rewards and be motivated, said Dr. Nora Volkow of the U.S. National Institute on Drug Abuse. Volkow and her colleagues suspect that faulty dopamine transmission may be to blame for the difficulty people with ADHD experience trying to finish tasks that have no immediate payoff — the difference between doing homework, for instance, and playing a video game.

Reward pathway probed

ADHD, a childhood psychiatric disorder, affects an estimated three per cent to five per cent of adults. For the recent study, researchers used PET scans to compare the dopamine reward pathways in the brains of 53 adults who had ADHD and 44 adults who did not.

The scans in ADHD patients showed lower dopamine availability in two regions of the brain that are important to reward and motivation.

"The lower than normal availability in the accumbens and midbrain regions supports the hypothesis of an impairment of the dopamine reward pathway in ADHD," the researchers wrote.

The findings reflect a shift in thinking about ADHD away from a lack of attention and towards reward pathways, Volkow said.

The findings help explain why people with ADHD have a hard time focusing on tasks they don't find interesting. The research also reinforces the idea that creating ways to make school and work tasks seem more rewarding might improve performance.

The research was carried out at Brookhaven National Laboratory in Upton, N.Y., and received support from the Intramural Research Program of the National Institutes of Health, the National Institute of Mental Health and the U.S. Department of Energy.

Some authors of the study reported receiving support from several pharmceutical makers.