People with depression are three times more likely to develop Parkinson's Disease than people who are not depressed, scientists have suggested.

People who have already developed Parkinson's - a incurable degenerative disease of the nervous system - often suffer from depression.

But the study from Maastricht University in the Netherlands is the first to suggest depression can precede symptoms of Parkinson's becoming evident.

Parkinson's slowly renders sufferers increasingly immobile and eventually leads to death.

We certainly need more research in this area to understand what is taking place

Robert Meadowcroft, Parkinson's Disease Society

People with the disease develop tremors and facial tics. As the disease progresses, sufferers develop trembling of the arms and legs, stiffness and rigidity of the muscles and slowness of movement.

It tends to affect people over 40, though 10% of sufferers are younger.

Brain chemical levels

Researchers looked at health records for all 1,358 people diagnosed with depression in the southern Netherlands over a 15 year period.

They compared them with 67,570 people of the same age who had never been diagnosed with depression.

Both groups were followed for up to 25 years to see how many developed Parkinson's.

Nineteen of the depressed people developed Parkinson's, compared to 259 of those with no depression.

The research team say low levels of a particular brain chemical in both depression and Parkinson's could explain why depression occurs in Parkinson's patients and perhaps also why depression precedes Parkinson's.

People with Parkinson's have been shown to have low serotonin levels, something also seen in depression.

Serotonin also acts to regulate another chemical, dopamine, in the brain.

As the level of dopamine activity falls in Parkinson's, the researchers suggest the amount of serotonin activity also falls.

That reduction increases the risk of depression.

First direct link

Dr Agnes Schuurman who led the research, said: "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.

"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."

Robert Meadowcroft, director of policy and research at the Parkinson's Disease Society, told BBC News Online: "We think it's the first time that a direct link has been made between a history of depression and the later development of Parkinson's.

"However, it must be emphasised that the majority of people in the study who suffered depression did not go on to develop Parkinson's."

He said previous research has focussed on the role of dopamine in Parkinson's, but this research suggested other changes, such as reduced serotonin levels could also play a part.

"These findings are interesting, and we certainly need more research in this area to understand what is taking place."

The study is published in Neurology, the scientific journal of the American Academy of Neurology.