A doctor investigated whether an insufficient amount of dopamine in the brain causes Parkinson' s disease  The doctor injected dopamine into the bloodstreams of patients with Parkinson' s disease  The progress of the disease was not halted, nor did any of the patients improve
The doctor concluded that Parkinson' s disease must be caused by something other than a lack of dopamine in the brain
Parkinson's disease causes the brain to produce less dopamine than it normally would.