For what it's worth, I have been quite successful in preventing constipation with a herbal laxative called 'Herb-Lax" manufactured and distributed by the Shaklee Corp. Just Google your local Shaklee distributor. I have used it for the entire 13 years since my Dx. My DW is a distributor.

Regarding the neuro-gastroenterological bit, about three years ago when the above referenced PLOS One paper on finding aggregated alpha synuclein in colon sympathetic neurons appeared, I was already scheduled for a scanning colonoscopy due to abdominal discomfort. The GE's report of my test mentioned that my colon epithelium was pigmented, but that the overall result of my test was normal. I later had an abdominal sonogram which nailed gallstones as the source of my "discomfort". (No cholecystectomy or recurrence of symptoms; yet.)

At that time I wondered if my long-term use of the herbal laxative containing senna as one of its components resulted in the pigmentation of the inside of my colon.
(Lindy, I'm pretty OK with the YUK factor!)
Further, I wonder if my slow motor symptom progression might somehow be related to the long-term use of the senna supplement.?
Robert