The study, published in Behavioral Neuroscience, lays some of the blame on stress hormones. Those stress hormones — such as cortisol and corticotropin-releasing hormone — can help respond to an immediate threat.
But if stress stays high instead of easing up, those hormones can boost anxiety and lead to mood disorders. That's the theory explored in the new study.
The researchers included Paul Ardayfio, BSC, a graduate student in molecular neurobiology at Harvard Medical School and its affiliate, Boston's McLean Hospital. They studied female mice, not people, but the findings may hold clues about how chronic stress affects mood disorders.
Brief or Constant Stress
Ardayfio and colleagues spiked the mice's drinking water with corticosterone, a stress hormone. That way, they avoided stressing the mice out with injections.
Some mice got the spiked water for 17 or 18 days, mimicking long-term exposure to the stress hormone. For comparison, the other mice only got the spiked water for one day.
The mice got two tests, without any training to prepare for those tests. In one test, mice in a dark part of a cage got the chance to explore a bright, open part of a cage.
The mice that drank the spiked water day after day were more hesitant to enter the exposed space. The researchers interpreted that hesitancy as anxiety.
What's That Sound?
In the other test, the researchers exposed the mice to a high-frequency sound.
You might expect that mice under constant corticosterone exposure would have an exaggerated reaction to that sound. But that's not what happened.
The mice under long-term exposure to corticosterone had a dulled reaction to that sound the first 10 times they heard it, the study shows.
Constant exposure to the stress hormone may have depressed those mice, dimming immediate reactions, the researchers write. In other words, the study suggests that long-term stress may have left those mice less prepared to handle a stressful event instead of improving their stress reactions.
SOURCES: Ardayfio, P. Behavioral Neuroscience, April 2006; Vol. 120. News release, American Psychological Association.
By Miranda Hitti
Reviewed by Louise Chang, M.D.
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