Similar conclusions were quoted in a recent thread:
Widening the scope and the extent of PD therapy

'Qamar et al. write ...:

"Besides dopamine (DA), three further key neurotransmitters have been described to be involved in the pathogenesis of PD; namely noradrenaline (NA), acetylcholine (ACh), and serotonin (5HT)."'

It doesn't seem unreasonable to think of Parkinson's as a neurotransmitter disease, rather than just a lack of dopamine disease. Moreover, the standard dopamine replacement therapy is throwing exogenous levodopa across the whole brain, which must have some effect on other neurotransmitters.

Levodopa does a good job with motor problems, but does little to address non-motor symptoms such as constipation.

John