NEW YORK, Jun 14 (Reuters Health) - People who are prone to panic may be more likely to have a specific variation in a gene that controls serotonin, a neurotransmitter that plays a key role in regulating moods and emotions, researchers report.

      In experiments testing fear responses in psychologically healthy study participants, investigators found that those with the "long" variant of the serotonin transporter gene, known as 5-HTT, showed a tendency to become more anxious under stress.

      The 5-HTT gene regulates the brain chemical serotonin, controlling its return to brain cells after it has been released. The long variant of 5-HTT causes brain cells to take up serotonin more quickly after its release than a short variant of the gene.

      Lead study author Norman B. Schmidt told Reuters Health that serotonin clearly plays a role in anxiety, but exactly what role is not clear. Low serotonin levels may contribute to some psychological disorders, although such disorders are complex and are probably caused by a number of different factors. Past studies have suggested that low levels of serotonin can lead to depression, anxiety, aggression and sleep problems.

      Schmidt, an associate professor of psychology at Ohio State University in Columbus, and his colleagues report their findings in the June 14th issue of the Journal of Abnormal Psychology.

      In the study, 72 men and women were separated into groups based on whether their two copies of the 5-HTT gene--one from each parent--were long or short. To measure their fear responses, Schmidt's team had each participant take a breath of air that was a mix of carbon dioxide and oxygen, which was designed to make the subjects feel momentarily breathless.

      Those with two copies of the long 5-HTT variant reported greater anxiety during their bout with breathlessness compared with people with the short gene variant, the report indicates.

      These findings do not show, however, that people with a long 5-HTT are prone to anxiety disorders. This gene "accounts for only a small piece of the pie if the pie is anxiety," Schmidt said. Another 10 to 15 genes probably contribute to such emotional responses, he added.