Parkinson's may progress quietly but insidiously for many years before the neurodegenerative disease is diagnosed, according to a study conducted in mice.
The research, published recently in the journal Nature Communications, sheds new light on the surprising resilience of the brain during the asymptomatic period of Parkinson's.
The researchers from the University of Montreal in Canada demonstrated that movement circuits in the brains of mice are insensitive to an almost total loss of active secretion of dopamine, a chemical messenger recognised for its importance in movement.
In Parkinson's disease, dopamine levels in the brain drop inexorably, they said.
"This observation went against our initial hypothesis, but that's often the way it is in science, and it forced us to re-evaluate our certainties about what dopamine really does in the brain," said Louis-Eric Trudeau, a professor at the University of Montreal.
Using genetic manipulations, the team eliminated the ability of dopamine-producing neurons to release this chemical messenger in response to the normal electrical activity of these cells.
The researchers, including Benoit Delignat-Lavaud, a doctoral student in Trudeau's laboratory, expected to see a loss of motor function in these mice similar to what is seen in individuals with Parkinson's.
Surprisingly, the mice showed a completely normal capacity for movement, they said. Measurements of overall dopamine levels in the brain revealed that extracellular levels of dopamine in the brain of these mice were normal, the researchers said.
These results suggest that the activity of movement circuits in the brain requires only low basal levels of dopamine, they said.
The researchers noted that it is likely that in the early stages of Parkinson's disease, basal dopamine levels in the brain remain sufficiently high for many years, despite the gradual loss of dopamine-producing neurons.
It is only when a minimum threshold is exceeded that motor perturbations appear, they said.
By identifying the mechanisms involved in the secretion of dopamine in the brain, the research could help to identify new approaches to reduce the symptoms of this incurable neurodegenerative disease, the researchers added.