Pharmacological MRI (phMRI) as a tool to differentiate Parkinson's disease-related from age-related changes in basal ganglia function

The prevalence of both parkinsonian signs and Parkinson's disease (PD) per se increases with age. While the pathophysiology of PD has been studied extensively, less is known about the functional changes taking place in the basal ganglia circuitry with age. To specifically address this issue, three groups of rhesus macaques were studied: Normal middle-aged animals (used as controls), middle-aged animals with MPTP-induced parkinsonism, and aged animals (>20 years old) with declines in motor function. All animals underwent the same behavioral and phMRI procedures to measure changes in basal ganglia function in response to dopaminergic drug challenges consisting of apomorphine (APO) administration followed by either a D1 (SCH23390) or D2 (raclopride) receptor antagonist. Significant, functional changes were predominantly seen in the external segment of the globus pallidus (GPe) in aged animals and in the striatum (caudate nucleus and putamen) in MPTP-lesioned animals. Despite significant differences seen in the putamen and GPe between MPTP-lesioned versus aged animals, a similar response profile to dopaminergic stimulations was found between these two groups in the internal segment of the globus pallidus (GPi). In contrast, the pharmacological responses seen in the control animals were much milder compared with the other two groups in all examined areas. Our phMRI findings in MPTP-lesioned parkinsonian and aged animals suggest that changes in basal ganglia function in the elderly may differ from those seen in parkinsonian patients and that phMRI could be used to distinguish PD from other age-associated functional alterations in the brain.