Apr. 7, 2008— -- CHICAGO, April 7 (Reuters) - People with depression aremore likely to later develop Alzheimer's disease, according totwo studies published on Monday, and one team said that chronicstress may damage their brains.
"What we think it suggests is that depression truly is arisk factor for Alzheimer's disease, and not simply a sign thatthe disease is developing," Dr. Robert Wilson, aneuropsychologist at Rush University Medical Center in Chicagowho led one study, said in a telephone interview.
Some researchers have assumed that Alzheimer's causesdepression, so Wilson's team tracked 917 retired Catholicpriests and nuns, 190 of whom developed Alzheimer's disease.Those with more symptoms of depression at the beginning of thestudy were more likely to develop Alzheimer's disease.
But Wilson's group did not find a sudden onset or worseningof depression in the few years before symptoms of the braindisease took hold.
"Our thinking is that depression somehow causes damage topart of the brain called the limbic system, and this is thepart of the brain that Alzheimer's disease preferentiallyattacks," Wilson said.
The limbic system includes the hippocampus and amygdala,which play key roles in emotions and memory.
The subjects in the study, which appeared in the Archivesof General Psychiatry, were asked about depressive symptoms andnot about specific episodes.
"In terms of depressive symptoms, those are fairlyconsistent from year to year as people have a chronic tendencyto be depressed or not be depressed -- it's not just somethingthat randomly varied from year to year," Wilson said.
Wilson cited one theory that chronic stress from depressionreleases excessive amounts of the brain hormone cortisol thatultimately damages the filigreed connections between braincells called dendrites.
Depression has not been shown to have a direct relationshipto the brain plaques and tangles often found during autopsieson Alzheimer's victims, Wilson said.
"Some people die with lots of plaques and tangles but theydon't have dementia. Some people die with few plaques andtangles and they do have dementia. This has long been knownthat plaques and tangles aren't the whole story. They're otherthings going on that are causing loss of cognition and memory... we think depression is involved in one of those pathways,"he said.
Everyone reaching their 70s and 80s likely has somephysical manifestations of Alzheimer's in their aging brains,Wilson said, but not all develop the mind-robbing symptoms.
"We think that some people are more vulnerable or it takesless of a dose because of other changes that are taking placein the brain," such as damage inflicted by depression, hesaid.
A related theory that depression shrinks the hippocampusand amygdala and paves the way for Alzheimer's was notsupported by a Dutch study published on Monday in Neurology, ajournal of the American Academy of Neurology.
Magnetic resonance images were taken of the subjects'brains at the beginning of the study, and depression was notassociated with brain shrinkage.
But among the 134 of 503 people in the study who reportedseeking help for depression, the risk of Alzheimer's was 2.5times higher than among those who were not depressed.
"We don't know yet whether depression contributes to thedevelopment of Alzheimer's disease or whether another unknownfactor causes both depression and dementia," said MoniqueBreteler of Erasmus University in Rotterdam.
(Editing by Maggie Fox and Philip Barbara)