>>... It activates glial-cell derived neurotrophic factor...
Doesn't vitamin D also do this?
That asked, there's evidence that GDNF supplied directly to the brains of Parkinson's sufferers can restore the cells (that should be) producing dopamine.
https://www.parkinsons.org.uk/news/gdnf-clinical-trial-offers-hope-restoring-brain-cells-damaged-parkinsons