Parkinson’s is much more than dopamine
Scientific American article postulates role of Serotonin in PD

http://blogs.scientificamerican.com/...than-dopamine/

"But what this tells us is that it is no longer enough to target the dopamine related symptoms of Parkinson’s alone. It’s a start, but it’s not the best treatment. The authors propose doing multiple grafts of dopamine and serotonin cells, and I think for less severe symptoms there might be reason to combine L-DOPA with an SSRI or something else to increase serotonin function while possible. Clearly, more studies need to be done on how to help these nonmotor symptoms of Parkinson’s. Because restored motor activity is wonderful and incredibly important, but it doesn’t necessary mean a cure."