A gene found deep in the emotional center of the brain may help explain why some people prefer to skydive while others are content with keeping both feet on the ground.

A new study shows that mice lacking one of a set of genes found in the emotional part of the brain called the amygdala react differently to fear and take more risks than their normal peers.

Researchers found the mice with only one copy of the gene, called neuro2, had an impaired emotional learning and showed abnormal responses to fear. Those without the set of genes did not have normal development and growth of this region of the brain.

"Most of us are familiar with the fact that we can remember things better if those memories are formed at a time when there is a strong emotional impact -- times when we are frightened, angry, or falling in love," says researcher James Olson, MD, PhD, of the Fred Hutchinson Cancer Research Center, in a news release. "That's called emotional-memory formation. The amygdala is the part of the brain that is responsible for formation of emotional memory."

"The contribution we have made is showing that neuroD2 is related to the development of the amygdala. This is the first time that a specific neurodevelopmental gene has been related to these emotional activities in the brain," says Olson.

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Gene Linked to Fear Response

In the study, published in the Proceedings of the National Academy of Sciences, researchers looked at the effects of the neuroD2 gene in the brain development of mice.

First, they found that certain areas of the amygdala did not fully develop in mice bred without the gene. These mice also died within a few weeks after birth.

Second, they conducted a series of experiments to determine what effect having only one copy rather than the normal two copies of the gene had on the mice’s emotional learning and development.

In one experiment, mice were exposed to a tone followed by a mild foot shock. Normal mice responded by crouching down and not moving the next time they heard the tone, which indicates they were expecting a shock. But the mice with only one copy of the gene did not freeze in anticipation of a shock.

Another experiment tested the level of fear in mice with one copy of the neuroD2 gene by putting the mice in a situation that would elicit fear in normal mice. The mice were placed in an elevated maze and given the choice to walk along narrow, unprotected walkways or ones with protective walls.

The results showed that half of the time the neuroD2-deficient mice chose the unprotected paths, while normal mice almost always chose the protected ones.

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Emotional Development May Affect Fear

"Now we're seeing that the neuroD2-deficient mice, when compared to normal littermates, show a profound difference in unconditioned anxiety levels as well as their ability to form emotional memories," says Olson. "All of this matches very well with previous observations that the amygdala is responsible for fear, anxiety, and aggression."

Olson says more research is needed to determine if differences in the dosage of this gene may affect human brain development and eventually shape human behavior, such as making one person more prone to take risks than another.

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By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Lin, C. Proceedings of the National Academy of Sciences, Sept. 26, 2005 early online edition. News release, Fred Hutchinson Cancer Research Center.