With an appropriate oral dose of Varenicline (VCN) identified from experiments 1 & 2 of the study (see NCT02933372), the investigators will administer VCN to Parkinson Disease (PD) participants to determine if VCN improves walking speed and measures of balance. PD participants will receive VCN or a placebo (fake drug) for 3 weeks to assess the effects of VCN administration on gait speed and balance. Participants will undergo examinations to assess the intensity of their Parkinsonism and asked questions to assess their mood and thinking.

To demonstrate that α4β2* nAChRs are appropriate therapeutic targets in Parkinson's Disease (PD), it is necessary to study key pharmacokinetic-pharmacodynamic features of α4β2* nAChR in the context of the PD brain with loss of nerve cells that produce the neurotransmitter acetylcholine, a pathologic environment in which they may exhibit unique features. This personalized medicine approach focuses our studies on the subgroup of PD subjects with loss of nerve cells that produce the neurotransmitter acetylcholine identified by Project II and the Clinical Resource Core. The investigators will assess α4β2* nAChR features using PET imaging with the α4β2* nAChR ligand [18-Fluorine]flubatine, subacute administration of the α4β2* nAChR partial agonist Varenicline (VCN), and laboratory measures of gait, balance, and attention. The investigators will use [18-Fluorine] flubatine PET to assess VCN occupancy of brain α4β2* nAChRs (experiments 1 & 2). VCN will be administered to both PD participants (experiment 1) and healthy controls (experiment 2) and both populations will undergo a flubatine PET scan to assess VCN occupancy. Using this PET data to select an appropriate VCN dose, the investigators will perform a pharmacodynamic study (experiment 3) with subacute VCN administration to determine if α4β2* nAChR stimulation improves laboratory measures of gait function, postural control, and attentional function in PD subjects with loss of nerve cells that produce the neurotransmitter acetylcholine.

Inclusion Criteria:

1. PD diagnosis will be based on the United Kingdom Parkinson's Disease Society Brain Bank Research Center (UKPDSBRC) clinical diagnostic criteria. The investigators will enrich the cohort by recruiting subjects at modified Hoehn and Yahr stages 2 or higher, duration of motor disease 5 years or longer, age >65 years, or the Postural Instability and Gait Disorder (PIGD) phenotype. Duration of motor disease will be defined as the time between onset of motor symptoms and time of entry into the study. The PIGD phenotype is defined as described previously. PD subjects with defined cholinergic deficits will be recruited as described in Project II. PD subjects will have cortical cholinergic deficits based on 5th percentile cutoff of the normal controls as defined previously.
2. Stable dopaminergic replacement therapy for 3 months prior to enrollment and expected to maintain stable dopaminergic therapy for duration of study participation.

Exclusion Criteria:

1. Other disorders which may resemble PD with or without dementia, such as vascular dementia, normal pressure hydrocephalus, progressive supranuclear palsy, multiple system atrophy, corticobasal ganglionic degeneration, or toxic causes of parkinsonism. Prototypical cases have distinctive clinical profiles, like vertical supranuclear gaze palsy, early and severe dysautonomia or appendicular apraxia, which may differentiate them from idiopathic PD. The use of the UKPDSBRC clinical diagnostic criteria for PD will mitigate the inclusion of subjects with atypical parkinsonism and all participants will undergo [11-Carbon]dihydrotetrabenazine PET to confirm striatal dopaminergic denervation.
2. Subjects on neuroleptic, anticholinergic (trihexphenidyl, benztropine), or cholinesterase inhibitor drugs.
3. Current or previous (within last 6 months) use of any product or medication containing nicotinic agents,including use of tobacco products such as cigarettes, cigars, pipes, chewing tobacco, etc., electronic cigarettes, over-the-counter nicotine patches, chewing gum containing nicotine, or varenicline.
4. Evidence of a stroke or mass lesion on structural brain imaging (MRI).
5. Participants in whom magnetic resonance imaging (MRI) is contraindicated including, but not limited to, those with a pacemaker, presence of metallic fragments near the eyes or spinal cord, or cochlear implant.
6. Severe claustrophobia precluding MR or PET imaging
7. Subjects limited by participation in research procedures involving ionizing radiation.
8. Pregnancy (test within 48 hours of each PET session) or breastfeeding.
9. Significant risk of cardiovascular event.
10. Active, significant mood disorder.