St. Paul, MN (American Academy of Neurology) -- People with depression are three times more likely to develop Parkinson's disease than people who are not depressed, according to a study published in the May 28 issue of Neurology, the scientific journal of the American Academy of Neurology.

"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, PhD, of Maastricht University in Maastricht, Netherlands.

While depression frequently affects people already diagnosed with Parkinson's disease, this is the first study to show that depression can precede the 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, which amounted to 67,570 people. Both groups were followed for up to 25 years to determine how many people developed Parkinson's over the years. Nineteen of the depressed people developed Parkinson's, compared to 259 of those with no depression.

The researchers say a current theory explaining why depression occurs in Parkinson's patients may also explain why depression precedes Parkinson's. Studies have shown that Parkinson's patients' brains have a lowered level of the neurotransmitter serotonin. Low levels of serotonin play a key role in depression. Serotonin also acts to modulate the release of the neurotransmitter dopamine in the brain. 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.

The study was funded by the Netherlands Organisation for Scientific Research. 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.