Most of the discussion of PD is related to the motor symptoms (e.g. tremor, bradykinesia, stiffness and gait). This is in spite of the fact that for many PwP non-motor symptoms (NMS), such as depression, dementia and constipation, have a greater impact on the quality of life.

For me, constipation has more impact on my QOL than any other symptom.

In terms of treatment, most of the discussion is in terms of replacing the dopamine that is lost by PD. But as Qamar et al. write [1]:

"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)."

They go on to describe the possible involvement of these neurotransmitters in about 30 NMS.

Reference

[1] "Presynaptic dopaminergic terminal imaging and non-motor symptoms assessment of Parkinson’s disease: evidence for dopaminergic basis?"
MA Qamar, A Sauerbier, M Politis, H Carr, P Loehrer, and K Ray Chaudhuri
NPJ Parkinsons Dis. 2017; 3: 5.
Presynaptic dopaminergic terminal imaging and non-motor symptoms assessment of Parkinson’s... - Europe PMC Article - Europe PMC

John