"This raises the question of whether depression is the first symptom of Parkinson's disease -- that appears before patients have other symptoms and a diagnosis," said study author Agnes Schuurman, Ph.D., of the Netherlands.

Although depression frequently affects people already diagnosed with Parkinson's disease, this is the first study to show that depression can precede the initial clinical symptoms of Parkinson's.

For the study, researchers identified all of the people from a health registry in the southern Netherlands who were diagnosed with depression over a 15-year period. Those 1,358 people were then matched with people in the registry born in the same year but never diagnosed with depression, a group of 67,570 people.

Both groups were followed for up to 25 years to determine how many people developed Parkinson's over the long term. Of the 1,358 persons with depression, 19 developed Parkinson's, compared with 259 of the 67,570 people without depression.

The researchers say a current hypothesis explaining why depression occurs in Parkinson's patients may also explain why depression can precedes symptoms of Parkinson's. Studies have shown that the brains of subjects with Parkinson's disease have a lowered level of serotonin.

Serotonin acts to modulate the release of dopamine. Because the level of dopamine activity is decreased in Parkinson's, researchers believe the amount of serotonin activity is also decreased in compensation. That reduction increases the risk of depression.

"Because the reduced serotonin activity already exists before any motor symptoms begin, the risk of depression is also increased long before any Parkinson's symptoms become apparent," Schuurman said.

Other studies have shown that people with depression are more likely to develop cancer, dementia or coronary heart disease or to later have a stroke.