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. Using this PET data to select an appropriate VCN dose, the investigators will perform a pharmacodynamic study 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.