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29 May 2007 @ 12:12 pm
New Evidence Sheds Light on the Cause of ADHD

Medically Reviewed On: December 05, 2006
Published on: December 05, 2006

(HealthCentersOnline) - Results from a recent brain-imaging study call into question the relationship between dopamine, its associated proteins and the presence of attention deficit hyperactivity disorder (ADHD). Dopamine is a neurotransmitter important in thinking, motivation, short-term memory and some emotions, as well as immune function and motor control.

It was previously believed that unusually high levels of a brain protein responsible for transporting dopamine in some parts of the brain indicated the presence of ADHD. However, researchers found lower levels of these proteins in ADHD patients when measured against the control group.

"These results suggest that dopamine transporter levels alone cannot account for the severity of symptoms of inattention in ADHD," lead study author Nora Volkow said in a recent news release.

ADHD is a set of chronic conditions marked by an inability to pay attention, hyperactivity and impulsive acts. It begins in childhood and can affect all areas of a child's life. Between 3 and 5 percent of school-aged children—or about 2 million children in the United States—have ADHD, according to the National Institute of Mental Health (NIMH). Boys outnumber girls by at least a 3 to 1 ratio, according to National Mental Health Association.

For many years, controversy has surrounded ADHD as some experts have differed over exactly what constitutes the disorder. In recent years, the National Institute of Mental Health has declared definitively that ADHD is a mental health condition.

Dopamine transporter levels were gauged in 20 adults with ADHD who had never used medication, abused drugs (except nicotine) and had no history of mental or neurological disease or were otherwise mentally impaired. Attention levels were also measured using a questionnaire. The same tests were also used in a control group under the same standards.

In order to measure dopamine transporter levels, the participants were each injected with a chemical designed to illuminate dopamine transporters under a positron emission tomography (PET) scanner.

The participants with ADHD showed considerably fewer dopamine transporters than those without the condition in certain regions of the brain. Researchers still believe that dopamine transporters play a key role in the fluctuations in people's attention. However, the findings suggest that at least one other factor in combination with the amount of dopamine transporters leads to the development of ADHD. At every given level of dopamine transporters, patients with ADHD had significantly higher inattention scores than the control group.

"These finding suggest that an additional variable in conjunction with dopamine transporters would be required to account for the severity of the symptoms of inattention in ADHD," Volkow said. "We speculate that this other variable may be lower levels of dopamine release in ADHD subjects."

Lower levels of released dopamine may ultimately cause a drop in the amount of available transporters due to the body's attempt to compensate by reducing the number of transporter proteins. As the number of transporter proteins rises, the amount of dopamine declines, causing a decline in attention. This also accounts for why study participants with higher dopamine levels had better attention capabilities.

The study appears in the current issue of the journal Neuroimage.

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