Neurobiological Drivers of Mobility Resilience: The Dopaminergic System - Placebo-Controlled Arm

Walking with age becomes both slower and less 'automated', requiring more attention and prefrontal resources. As a result older adults have a greater risk of adverse mobility outcomes and falls. Walking disturbances in the elderly have been linked to changes in both cerebral, in particular small vessel disease (cSVD), and peripheral systems. There is an urgent need to identify factors that can help compensate for these harmful factors and reduce walking impairments, as there are currently no effective treatments available. Although effective mobility is the end result of the functional capacity of both central and peripheral systems, the brain's unique modulatory and adaptive capacity may provide clues for novel interventions. For example, investigators have recently discovered that \~20% of older adults maintain fast walking speed even in the presence of age related cSVD and peripheral system impairments, thus appearing resilient to these harmful factors. The investigators work suggests that the nigrostriatal dopamine (DA) system may be a source of this resilience. As investigators recent findings suggest, DA neurotransmission positively predicts walking speed; it also attenuates the negative effects of age related cSVD and peripheral system impairments on walking speed. These findings are consistent with post-mortem evidence that a combination of loss of nigral DA neurons and cSVD best predict age-related walking impairment. The nigrostriatal DA system plays a critical role in motor control; nigrostriatal. DA neurotransmission regulates the automated execution of overlearned motor tasks via its connections with sensorimotor cortical and subcortical areas. The investigators hypothesize that higher nigrostriatal DA neurotransmission drives resilience to cSVD and peripheral system impairments, via higher connectivity of sensorimotor networks, thus increasing automaticity of walking and reducing prefrontal engagement while walking. Unlike cSVD and brain structural impairments, DA neurotransmission is potentially modifiable, thereby offering novel approaches to treat non-resilient elderly in a targeted fashion. This study is an arm of a previously completed translational pilot biomechanistic target engagement study in older adults with slow walking and/or parkinsonian signs (NCT04325503). This sub-study will further investigate this biomechanistic target engagement using a double-blind, placebo-controlled study design. The study will include elderly men and women age 60 or older with evidence of mild parkinsonian signs (MPS, or slow gait (\< 1m/s)).