Hi, RD42;
Like Lee and Steve, I think the depression is often situational, i.e, simply the emotional effect of the awareness that one has a relentlesly progressive neurodegerative disease. I have taken anti-depressants of various types for about 25 years, as well as 5-6 years of cognitive "talk therapy" with some really good therapists.
Interestingly, once I recieved the PD Dx, I felt confident enough to "graduate" from the theraputic group, even though I still take the Rxs. I think I really still need them to maintain a consistently positive outlook. My wife agrees with this.
Clinical depression and loss of smell sensitivity can precede other PD symptoms by 5-10 years. That was the case for both my older brother and me, at any rate. Fortunately, I can still smell bread baking. MMMMMM!