Fear conditioning runs in families, VCU study finds

Genetic finding could help researchers as they study why people develop anxiety disorders

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RICHMOND, Va. – An innovative twin study by an international team of researchers suggests that genes influence fear conditioning, or how people respond to dangerous and fear-provoking stimulus.

Researchers say the findings, which are published in the July issue of the Archives of General Psychiatry, have important implications for the study and treatment of anxiety disorders, which affect approximately 19 million American adults and cause irrational fear and apprehension as well as various physical symptoms from chest pain and shortness of breath to heart palpitations and dizziness.

"Historically, researchers have postulated fear conditioning as an experimental model for studying why people develop phobias and anxiety disorders," says Dr. John M. Hettema, assistant professor of psychiatry at Virginia Commonwealth University and the Virginia Institute for Psychiatric and Behavioral Genetics at VCU.  Dr. Hettema also is attending physician at the Anxiety Disorder Specialty Clinic at the VCU Medical Center.

"But nobody really knew if the underlying biological factors involved in the experimentally produced fear conditioning are the same as those that cause phobias and anxiety," he says. "Now we can say that the fear conditioning process in humans is controlled, at least to some extent, by genetic factors, increasing its biological plausibility as a model system to study fear and anxiety. Understanding fear conditioning provides one aspect of how humans develop anxiety syndromes, which could lead to better treatments for these chronic illnesses."

Fear conditioning occurs when fear is associated with a neutral stimulus, such as circles and triangles, after pairing that stimulus with a fear-provoking stimulus, such as an electric shock  Fear conditioning is similar to the Pavlovian concept of training a dog to salivate at the sound of a bell much as it would salivate naturally to the smell of food.

The VCU researchers collaborated with a team from Uppsala University in Sweden to analyze data from an experiment involving 173 pairs of twins enrolled from the Swedish Twin Registry. The twins – about half identical and half fraternal – were shown a series of different images.  Some images, of snakes and spiders, were intended to be naturally fear-related, while others, of circles and triangles, were fear-neutral. The researchers applied a mild electric shock while subjects viewed some but not all of the images, provoking a feeling of discomfort and an association of fear, depending upon which images were paired with the shock.

The researchers found that the identical twins, but not the fraternal twins, responded essentially the same during the fear conditioning, indicating a substantial genetic effect. Researchers frequently use identical and fraternal twins to study the impact of genetic and environmental factors on various health and behavior issues because twins share the same genes. In identical twins, the genes are 100 percent identical. Fraternal twins' genetic makeup mirrors that of typical siblings.

Hettema noted that some types of fear are not only natural but useful to protect people from dangerous situations. When fear or anxiety grows out of proportion to the degree of danger, is chronic or becomes disabling, it then meets criteria for an anxiety disorder, which should be evaluated by a professional for treatment.

The data analysis was supported by grants from the National Institute of Mental Health and the Anxiety Disorders Association of America.