MONDAY, Aug. 6, 2007 (HealthDay News) -- Attention-deficit/hyperactivity disorder (ADHD) is a real disease linked to changes in production of the brain chemical dopamine, two new reports suggest.
In the first report, researchers found that a variant of the dopamine receptor gene may help cause the behavioral condition but also improve its long-term outcome.
"If you have a certain variant of this gene, you have a greatly increased risk of having ADHD," said lead researcher Dr. Philip Shaw, a researcher in the Child Psychiatry Branch at the U.S. National Institute of Mental Health. However, "what we found that was surprising was that having this variant was also associated with having a better outcome from ADHD," he said.
"The kids who had the risk gene tended to get better," Shaw said. "They also tended to be a little bit more intelligent."
Not all children with ADHD have this particular gene variant, Shaw said. "About one-quarter to one-fifth of children with ADHD has this gene variant," he said. "That's higher than the general population where about a fifth to a sixth has the variant."
In a second study, scientists found that, in contrast to the common wisdom, ADHD is associated with lowered dopamine production.
Both reports were published in the August issue of the Archives of General Psychiatry.
In the first gene study, Shaw's team compared 105 children with ADHD with 103 healthy children. The youngsters averaged about 10 years of age.
The researchers looked at the children's MRI brain scans as well as their DNA. In addition, 67 of the children with ADHD were evaluated six years later.
Shaw's team found that a variant in the dopamine D4 receptor gene (DRD4) was associated with ADHD. This form of the gene is linked with thinner tissue in areas of the brain that control attention.
Among the children with ADHD seen over six years, those who had this gene variant had better outcomes and had regained healthy tissue thickness in the affected brain region. This may explain ADHD's natural history of improvement with age, the researchers noted.
Further research could help develop treatments that will help children recover from ADHD more quickly, Shaw said.
One expert said the study may be a milestone in understanding ADHD.
"This is a very important study as it adds increasing evidence that ADHD is a heritable disease with genetically determined neurobiological underpinnings and adds further evidence that this is a valid mental disorder, often requiring neurobiological interventions [such as] psychopharmacological treatment," said Dr. Jon A. Shaw, professor and director of child and adolescent psychiatry at the University of Miami Miller School of Medicine.
A second study -- led this time by Dr. Nora D. Volkow, director of the U.S. National Institute on Drug Abuse -- suggests that the ADHD drug Ritalin works by increasing the brain's production of dopamine.
This finding implies that reduced production of dopamine is involved in ADHD and may help explain why many people with the condition also abuse drugs.
"Individuals with ADHD have a decreased function of the brain dopamine system," Volkow said. "ADHD, clearly, is associated with a biochemical dysfunction," she added.
The finding is important, because it belies the myth that ADHD is not a real disease but was just created "to sell medication," Volkow said. Instead, "this finding explains why stimulant medications, such as Ritalin, are beneficial, because they increase dopamine function in the brain," she said.
In the study, Volkow's team performed brain scans of 19 adults with ADHD who had never received Ritalin, as well as 24 healthy individuals. The scans were done after the participants were given shots of Ritalin or placebo.
The team found that the people with ADHD released less dopamine into their blood than those without the disease. However, Ritalin caused less of a decrease in dopamine than usual in these individuals. This reduction in dopamine was associated with typical symptoms of inattention, the researchers found.
Volkow noted that drugs such as nicotine, cocaine and methamphetamine also improve dopamine brain function.
"This may be the reason why individuals that have ADHD are at a much greater risk of abusing substances than the general population, because drugs of abuse increase dopamine brain function, and they will 'feel better,' " Volkow said.
For more on ADHD, visit the U.S. National Institute of Mental Health.