Sorry for the delayed response - I only just spotted your message.
It's a very good question and your 'constipation' analogy is pretty close to the mark - why didn't I think of that?
Basically, before this new study, researchers believed that dopamine release only occurred when the dopamine-producing nerve cells are sufficently 'excited'.
This depends on the electrical activity within the nerve cell reaching the threshold in order to trigger an action potential to cause dopamine release.
These diagrams may help explain this a bit better: http://en.wikipedia.org/wiki/Threshold_potential
We know that the dopamine-producing nerve cells are not working properly in Parkinson's - and they are struggling to produce and release enough dopamine to get the messages through to coordinate movement.
This new research suggests there may be a way to help these nerve cells out. Stimulating cholinergic interneurons seems to trigger dopamine release without the need for electrical activity within the dopamine nerve cells.
This finding could have exciting implications for developing new drugs that, by targeting these cholinergic interneurons, can help the dopamine nerve cells release dopamine better.
These types of drugs could help dopamine-boosting drugs (like levodopa) work better for people with Parkinson's, and mean they need to take less of them - ultimately this could improve symptoms and reduce side effects.
I don't like to bandy the 'breakthrough' word around too often, but this study certainly comes close.
I hope this helps explain a bit more - thanks again for your questions!