That's because parkisons drugs can upregulate a certain protein that is implicated in all forms of addiction that feature that kind of "seeking" behavior and modification of the reward system!

https://en.wikipedia.org/wiki/FOSB#Role_in_addiction

We found that if you give an addicted rat a chemical which suppresses the expression of this gene, it will improve their addiction situation, reduce seeking behaviors, etc.

We found that in 2015.