updated 3:52 p.m. ET Aug. 12, 2008

Variations in a gene that regulates the brain chemical dopamine may help explain why some people are more prone to anxiety than others, a new study suggests.

The gene in question, known as COMT, controls an enzyme that helps break down dopamine in the body. German researchers found that people with a particular, common variant of the gene tend to have an exaggerated "startle" response — a trait that could make them more vulnerable to anxiety disorders.

The findings, reported in the journal Behavioral Neuroscience, mesh with past studies that have found a link between the gene variant and a higher risk of anxiety disorders.

"This single gene variation is potentially only one of many factors influencing such a complex trait as anxiety," lead researcher Dr. Christian Montag, of the University of Bonn, commented in a statement issued by the American Psychological Association.

"Still," he added, "to identify the first candidates for genes associated with an anxiety-prone personality is a step in the right direction."

The researchers hope that by understanding the genetic underpinnings of anxiety and other psychiatric disorders, they can refine treatments.

To examine the relationship between COMT gene variants and the startle reflex, Montag's team recruited 96 young German women who were an average of 22 years old.

There are two major variations of the COMT gene, one known as Val158 and the other Met158. About half of the population carries one copy of each variant; the other half carries two Val copies (about 25 percent) or two Met copies (about 25 percent).

People who carry two Met variants have the lowest activity in the COMT gene, which indicates that they break down dopamine more slowly and therefore have higher levels of the chemical in emotion-regulating centers of the brain.

Genetically programmed 'wariness'
Based on the new findings, these individuals also have a stronger startle response.

To measure the startle reflex, Montag and his colleagues outfitted the women with electrodes that measured activity in their eye muscles as they viewed different computer images; some images were pleasant — babies and animals, for instance — some were emotionally neutral, and some were unpleasant — such as pictures of weapons or injury victims.

As the women viewed the images, a loud white noise would randomly sound, eliciting a startle reflex — here measured as activity in the eye muscles.

Montag's team found that when they showed the women an unpleasant image, those with two Met gene variants had a stronger startle response than those who carried at least one Val variant.

The researchers speculate that higher dopamine levels in the brain may make Met carriers feel more vulnerable to environmental threats or may make them less able to divert their attention from perceived threats.

According to Montag, the Met variant arose relatively recently in evolutionary terms and is seen only in humans, and not in other primates. He speculates that the gene variant gave humans a level of "wariness" to protect them in a threatening world. "It was an advantage to be more anxious in a dangerous environment," he noted.

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