Late-Life Depression \[LLD\]), is prevalent, disabling, and associated with high rates of completed suicide. Among the LLD patients at the highest risk of these adverse outcomes are those who manifest decreased processing speed and/or decreased gait speed. To develop urgently needed novel therapeutics for LLD, a reasonable approach is to target systems underlying the development and persistence of psychomotor slowing. One such approach has been to augment dopaminergic signaling since post-mortem experiments and in vivo neuroimaging studies have implicated age-related dopaminergic decline in the development of slowing. L-DOPA is the immediate precursor of dopamine, is converted to dopamine in presynaptic dopaminergic nerve terminals, and enhances dopaminergic transmission in multiple brain regions. As opposed to other dopaminergic interventions (i.e., dopamine receptor agonists and stimulants), a large literature shows beneficial effects of L-DOPA on cognitive performance and gait in patients with Parkinson's disease, all while being a safe and well-tolerated medication that is difficult to differentiate from placebo in terms of side effects.

A second therapeutic strategy that has been tested for LLD and is relevant to psychomotor slowing is aerobic exercise training. A number of reports and meta-analytic reviews suggest that exercise is an effective non-pharmacologic treatment for depression, including depression in older adults. The largest recent study found that progressive aerobic exercise conducted three times weekly for 30min over 24 weeks was effective for depression and was tolerated extremely well (14.3% drop-out rate, 70% intervention adherence). Exercise training may be effective for LLD by counteracting deleterious age-related changes related to its development and maintenance, such as by reducing pro-inflammatory cytokines, normalizing hypothalamic-pituitary-adrenal axis hyperactivity, and decreasing physical disability and social isolation. Exercise also appears to facilitate adaptive neuroplastic changes in the hippocampus, prefrontal cortex (PFC), and anterior cingulate cortex (ACC) as well as increased white matter connectivity.

While both dopaminergic augmentation and exercise are promising interventions, neither treatment alone may be sufficient to address the serious adverse medical and psychiatric outcomes associated with LLD and psychomotor slowing. In our preliminary study (NYSPI IRB# 7270), L-DOPA was associated with significant improvements in gait speed, but the effect size of this improvement was only moderate (d=0.4). L-DOPA failed to increase average gait speed in this study above the 1m/s threshold associated with functional disability and increased mortality risk in epidemiologic samples. While exercise has not been studied specifically in this patient population, meta-analyses of exercise interventions in older adults suggest overall effects on gait speed are modest (d=0.3) and perhaps not clinically significant. Thus, one goal of this study is to combine these interventions having complementary mechanisms of action to realize a greater therapeutic benefit.

This study includes task-based functional MRI that will allow us to probe the differential therapeutic mechanisms of L-DOPA and exercise and further elucidate the nature of effort-based decision making and reward deficits in LLD. Decision making about voluntary behavior requires weighing the benefit of potential rewards against the effort cost required to achieve them. This calculation is performed by separable populations of dopaminergic midbrain neurons whose signals for value and effort are integrated with the ventral striatum (VS). Anterior VS (AVS) consistently has been shown to encode subjective value, increasing with the probability of reward and decreasing with effort discounting, while recent work suggests dorsomedial VS (dmVS) activates during the initiation of effortful action. We hypothesize that older adults are biased toward inactivity (and thereby at risk for depression) on the basis of dopaminergic decline that diminishes subjective value estimates and increases the effort cost of action (i.e., by the development of slowing). Among PD patients, L-DOPA increases willingness to work independently of facilitating movement by increasing subjective value estimates. By increasing fitness and helping individuals learn about their increasing capacities, exercise may facilitate effort initiation. Below, we evaluate whether complementary effects on effortful behavior may be achievable via L-DOPA increasing subjective value and Exercise reducing effort cost.