Scientists may have discovered the earliest warning signs of Parkinson's disease in the brain.
Changes to the serotonin system - which relays messages across the brain - appear to occur years before patients show any visible symptoms, a new study suggests.
The findings, published in journal The Lancet Neurology, could lead to new methods of identifying individuals at a high risk of developing Parkinson's disease.
It has previously been suggested that the neurodegenerative condition, which causes involuntary shaking and slow movement, was linked to changes in the dopamine system.
This study is the first evidence of the important role that brain chemical serotonin plays, the researchers from King's College London said.
Lead investigator Professor Marios Politis said: "Parkinson's disease has traditionally been thought of as occurring due to damage in the dopamine system, but we show that changes to the serotonin system come first, occurring many years before patients begin to show symptoms.
"Our results suggest that early detection of changes in the serotonin system could open doors to the development of new therapies to slow, and ultimately prevent, progression of Parkinson's disease."
The researchers found that the serotonin system starts to malfunction in people with Parkinson's before symptoms affecting their movement, and before changes to the dopamine system.
Heather Wilson, first author of the study, said: "We found that serotonin function was an excellent marker for how advanced Parkinson's disease has become.
"Crucially, we found detectable changes to the serotonin system among patients who were not yet diagnosed.
"Therefore, brain imaging of the serotonin system could become a valuable tool to detect individuals at risk for Parkinson's disease, monitor their progression and help with the development of new treatments."
