Each year, panic attacks strike about 2.4 million American adults "out of the blue," with feelings of intense fear and physical symptoms sometimes confused with a heart attack. Unchecked, the disorder often sets in motion a debilitating psychological sequel syndrome of agoraphobia, avoiding public places. Panic disorder runs in families and researchers have long suspected that it has a genetic component. The new finding, combined with evidence from recent animal studies, suggests that genes might increase risk for the disorder by coding for decreased expression of the receptors, say the researchers.
NIMH grantee Dr. Rene Hen, Columbia University, and colleagues, reported in 2002 that a strain of gene "knockout" mice, engineered to lack the receptor during a critical period in early development, exhibit anxiety traits in adulthood, such as a reluctance to begin eating in an unfamiliar environment. More recent experiments with the knockout mice show that a popular SSRI (serotonin selective reuptake inhibitor) drug produces its anti-anxiety effects by stimulating the formation of new neurons in the hippocampus via the serotonin 5-HT1A receptor.
In the current study, Neumeister and Drevets used PET scans (positron emission tomography) to visualize 5-HT1A receptors in brain areas of interest in 16 panic disorder patients - seven of whom also suffered from major depression - and 15 matched healthy controls. A new radioactive tracer (FCWAY), developed by NIH Clinical Center PET scan scientists, binds to the receptors, revealing their locations and a numerical count by brain region. Subjects also underwent structural MRI (magnetic resonance imaging) scans, which were overlaid with their PET scan data to precisely match it with brain structures.
In the panic disorder patients, including those who also had depression, receptors were reduced by an average of nearly a third in the anterior cingulate in the front middle part of the brain, the posterior cingulate, in the rear middle part of the brain, and in the raphe, in the midbrain (See images below.). Previous functional brain imaging studies have implicated both the anterior and posterior cingulate in the regulation of anxiety. Stimulation of 5-HT1A receptors in the raphe regulates serotonin synthesis and release. In an earlier PET study of depressed patients, using a different tracer, Drevets and colleagues found less dramatic reductions of the receptor in the anterior and posterior cingulate, but a 41 percent reduction in the raphe. These findings add to evidence for overlap between depression and anxiety disorders.
Although animal experiments have shown that cortisol secretion triggered by repeated stress reduces expression of the gene that codes for the 5-HT1A receptor, such stress hormone elevations are usually not found in panic disorder. Noting the recent discovery of a variant of the 5-HT1A receptor gene linked to major depression and suicide, the researchers suggest that reduced expression of the receptor "may be a source of vulnerability in humans, and that abnormal function of these receptors appears to specifically impact the cortical circuitry involved in the regulation of anxiety."
Other researchers who participated in the study are Drs. Earle Bain, Allison Nugent, Omer Bonne, David Luckenbaugh, Dennis Charney, NIMH; Richard Carson, William Eckelman, Peter Herscovitch, Warren G. Magnuson Clinical Center.
The National Institute of Mental Health (NIMH) and the Warren G. Magnuson Clinical Center are parts of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research. NIH is a component of the U.S. Department of Health and Human Services.