People with PD who are depressed should not be lumped in with non PD depressed people.

Here's a study that confirms post-mortem findings:

Monoamine transporter availability in Parkinson’s disease patients with or without depression

"Conclusion Our data indicate that depression in PD is associated with a more pronounced loss of striatal dopamine transporter availability that is most likely secondary to increased dopaminergic degeneration. In addition, depressed PD patients have a lower availability of midbrain/brainstem monoamine transporters than nondepressed PD patients.

These findings provide in vivo evidence in support of the known post-mortem data demonstrating more extensive nerve cell loss in PD with depression and indicate that SPECT imaging can help to identify pathophysiological changes underlying nonmotor symptoms in this common movement disorder."

http://www.springerlink.com/content/v441tk2167663450/