Brain chemistry that may account for the difficulty some people have concentrating
is the target of research being conducted with single-photon emission computed
tomography (SPECT). Teams at Harvard University and elsewhere have reported
some success in tracking dopamine in the brains of both adult patients and children
with attention-deficit/hyperactivity disorder.
Dopamine is believed to be a central player in the condition, although evidence
is somewhat circumstantial. The belief is based on the effectiveness of psychostimulants
such as Ritalin (methylphenidate) in treating ADHD, said Dr. Darin Dougherty,
an assistant professor of psychiatry at Harvard University.
"We believe ADHD patients are dopamine-deficient, but it's never been proven
in vivo," he said.
Dougherty and colleagues imaged eight adults with ADHD who had at least one
first-degree relative with the condition, as well as age-matched controls. They
used iodine-123 altropane to see the binding potential of dopamine transporters
in the brain and found at least 70% more binding in the brains of ADHD patients
than in controls. Dougherty presented his results at the Society of Nuclear
Medicine meeting in St. Louis in June.
"We're not sure of the ramifications yet," he said. "We'll want to look into
it as a possible diagnostic tool, but if that doesn't work, at least we'll know
more from a pathophysiology standpoint. The jury is out on whether it will be
diagnostically useful."
Researchers at the University of Munich also found a higher number of dopamine
transporter binding sites in ADHD patients, although their research involved
a different ligand, technetium-99m Trodat-1.
The mechanism of Ritalin, the most popular pharmaceutical treatment for ADHD,
may be closer to being unraveled, according to a research group at Brookhaven
National Laboratory in Upton, NY. Ritalin has been hypothesized to act by increasing
the dopamine concentration in the brain. A series of PET scans using 11-raclopride,
a ligand that competes with dopamine for receptors, showed that raclopride levels
decreased substantially in the striatum of the brains of 10 healthy subjects
after they were given a 60-mg oral dose of Ritalin.
Although a single biochemical marker for ADHD may be elusive, Dougherty said,
detecting a genetic component could make diagnosis easier. Patients with a high
presence of the transporter could be tested to find a common blood genotype,
which could one day result in a blood test for the condition.
— by Jane Lowers
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