CHICAGO, April 7 (Reuters) - People with depression are more likely to later develop Alzheimer's disease, according to two studies published on Monday, and one team said that chronic stress may damage their brains.
"What we think it suggests is that depression truly is a risk factor for Alzheimer's disease, and not simply a sign that the disease is developing," Dr. Robert Wilson, a neuropsychologist at Rush University Medical Center in Chicago who led one study, said in a telephone interview.
Some researchers have assumed that Alzheimer's causes depression, so Wilson's team tracked 917 retired Catholic priests and nuns, 190 of whom developed Alzheimer's disease. Those with more symptoms of depression at the beginning of the study were more likely to develop Alzheimer's disease.
But Wilson's group did not find a sudden onset or worsening of depression in the few years before symptoms of the brain disease took hold.
"Our thinking is that depression somehow causes damage to part of the brain called the limbic system, and this is the part of the brain that Alzheimer's disease preferentially attacks," 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 Archives of General Psychiatry, were asked about depressive symptoms and not about specific episodes.
"In terms of depressive symptoms, those are fairly consistent from year to year as people have a chronic tendency to be depressed or not be depressed -- it's not just something that randomly varied from year to year," Wilson said.
Wilson cited one theory that chronic stress from depression releases excessive amounts of the brain hormone cortisol that ultimately damages the filigreed connections between brain cells called dendrites.
Depression has not been shown to have a direct relationship to the brain plaques and tangles often found during autopsies on Alzheimer's victims, Wilson said.
"Some people die with lots of plaques and tangles but they don't have dementia. Some people die with few plaques and tangles and they do have dementia. This has long been known that plaques and tangles aren't the whole story. They're other things 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 some physical 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 takes less of a dose because of other changes that are taking place in the brain," such as damage inflicted by depression, he said.
A related theory that depression shrinks the hippocampus and amygdala and paves the way for Alzheimer's was not supported by a Dutch study published on Monday in Neurology, a journal of the American Academy of Neurology.
Magnetic resonance images were taken of the subjects' brains at the beginning of the study, and depression was not associated with brain shrinkage.
But among the 134 of 503 people in the study who reported seeking help for depression, the risk of Alzheimer's was 2.5 times higher than among those who were not depressed.
"We don't know yet whether depression contributes to the development of Alzheimer's disease or whether another unknown factor causes both depression and dementia," said Monique Breteler of Erasmus University in Rotterdam.
(Editing by Maggie Fox and Philip Barbara)