MONDAY, April 17, 2006 (HealthDay News) -- Is the anxiety that often accompanies chronic depression a cause or effect of the illness?
It's a longstanding question among experts, but new research in mice suggests that chronic stress may be a trigger -- not a symptom of -- depression.
The findings could also help improve treatments for the disorder, says a research team from Harvard Medical School and McLean Hospital, in Boston.
It was already known that people with depression tended to have higher levels of the human stress hormone cortisol, but it wasn't clear whether this was a cause or effect of depression. The new mouse study suggests that long-term exposure to cortisol may actually contribute to depression.
For this study, the researchers exposed mice to acute (24 hours) and chronic (17 to 18 days) doses of the rodent stress hormone corticosterone. The hormone was put in the animals' drinking water so that the stress of injections would not cloud the results.
Compared to the mice with short-term exposure, the mice with chronic exposure took much longer to emerge from a small dark compartment into a brightly lit open field. This is a common behavioral test of anxiety in animals. The results suggest that the mice with chronic exposure were more fearful and less willing to explore their new environment.
The researchers also found that mice with chronic exposure showed dulled reactions to being startled, another indication that their nervous system was overwhelmed.
The findings appear in the April issue of the journal Behavioral Neuroscience.
The U.S. National Institute of Mental Health has more about depression.