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
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
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.
Adults With ADHD
The second study included 19 adults with ADHD who had never taken ADHD
medications, as well as 24 adults without ADHD. Participants were in their
early 30s, on average.
The researchers included Nora Volkow, MD, of the National Institute on Drug
Volkow's team studied participants' dopamine activity in a series of tests
that included intravenous injections of the active ingredient of the ADHD drug
Ritalin. The tests showed that adults with ADHD had a sluggish dopamine
"By documenting that subjects with ADHD have decreased brain dopamine
activity, it gives us an understanding of why medications such as Ritalin and
Adderall, which increase dopamine in the brain, would be beneficial to ADHD
subjects by strengthening the weak dopamine signals in their brains,"
Volkow tells WebMD.
"It also provides evidence that ADHD is a disease that connotes specific
disruption of the dopamine system," Volkow notes.
She adds that "since drugs of abuse like nicotine and cocaine also
temporarily increase brain dopamine activity, these findings also help us
understand why subjects with ADHD are at much greater risk of abusing
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