New Clues on Causes of ADHD
Brain Chemical Called Dopamine and a Dopamine Gene May Be Involved, New Studies Show
Aug. 6, 2007 -- Two new studies show that a brain chemical called dopamine may play a role in attention deficit hyperactivity disorder (ADHD).
The studies, published in the August edition of the Archives of General Psychiatry, don't claim to explain exactly what causes ADHD. But the new findings zero in on dopamine's role in the condition.
The first study focuses on the genetics of ADHD -- specifically, on a particular variation of the DRD4 gene, which makes a dopamine receptor in the brain. Dopamine is an important chemical messenger used between nerve cells in the brain. It is linked to many functions including movement and mental health.
The researchers included Philip Shaw, MD, PhD, who works in the child psychiatry branch of the National Institute of Mental Health. Shaw and colleagues studied the DRD4 gene in 105 kids with ADHD and 103 children without ADHD.The children were about 10 years old when the study started. They got their brains scanned using magnetic resonance imaging (MRI); 67 of the kids got follow-up brain scans six years later.
ADHD Gene Clue
The children with ADHD were more likely to have a certain variation of the DRD4 gene than children without ADHD.
But not all kids with ADHD had that particular gene variation. Those who did generally had higher IQ scores than other children with ADHD.
In addition, the gene variation was most common in children whose ADHD improved over time.
"The central finding was that the most important known genetic risk factor for ADHD (the 7-repeat form of the dopamine D4 receptor gene) was associated with having a better clinical outcome and higher intelligence in a large group of children with ADHD," Shaw tells WebMD.
The brain scans may provide a clue about that improvement. Kids with ADHD who had the DRD4 gene variation tended to have a thinner brain cortex in regions related to attention when the study started, but their cortex thickened during the follow-up period.
"The study thus links genes with clinical outcome and brain development," says Shaw. He notes that most of the children took ADHD medications at some point during the study, but that didn't explain the study's findings.
Shaw's team isn't saying that the DRD4 gene is the only gene involved in ADHD. Many genes probably influence ADHD, the researchers note.