My "PD" is modulated by:
Norepinehrine loss in CNS. Amantadine helps.
Dysautonomia (ANS pathologies). Eg, elevated norepinephrine in periphery.
Midface hypoplasia (eg, severe malocclusion, high arched palate). Also, I've come to believe: distorted development of my insular cortices and interconnected neural tissues.
For related discussion, see:
Dysautonomia subgroup in PD - HU PD.
Serotonin (affected by levodopa) is a chemical precursor to melatonin and helps maintaining sleep while dopamine is a precursor to norepinephrine which is involved in producing and releasing melatonin in the pineal gland. Thus a double whammy combined NT deficits with l-dopa induced altered DNA methylation and PD progression could lead to sundowning syndrome, where melatonin replacement and consistent outdoor physical exercise could be helpful.
List of serotonin deficiency symptoms -
Is vision adversely affected by PD?
Cardiovascular symptoms caused by PD 
My PD
Pain and Weakest on side affected by PD
Mouth affected by PD?
