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Compensatory mechanisms in Parkinson's disease Palmer, Samantha Jane


Parkinson’s disease (PD) is a common movement disorder, affecting 1% of the population over the age of 65. Pathologically, PD results from degeneration of nigral dopaminergic neurons, however symptoms do not appear until an estimated 50% of these cells are lost, suggesting compensatory mechanisms exist which mask disease onset, and may later delay progression of the disease. Compensation may take place over various spatial and temporal scales, from changes in synaptic dopamine release and synthesis that take place over a period of minutes, to recruitment of novel, widespread networks of brain regions for a specific task, which may require formation of new connections over an extended period of time. Neuroimaging techniques have recently allowed the investigation of regional and network changes in activation related to motor performance in PD, however the question of whether such changes represent a downstream effect of basal ganglia degeneration, or a compensatory change, remains difficult to determine. Here, we applied an approach from research into Alzheimer’s Disease, where abnormal activation patterns are studied in the context of tasks of increasing difficulty, such that inferences regarding their compensatory nature can be made. We show that individuals with PD are able to increase the recruitment of normal networks for a motor task (motor reserve) as a form of compensation, in addition to compensatory recruitment of novel networks to accomplish the same task as healthy controls. In particular, we observe a switch from striato-thalamo-cortical (STC) motor loops to cerebello-thalamo-cortical (CTC) loops as a compensatory strategy. This compensatory recruitment involves changes in the amplitude, spatial extent, and connectivity of regions within the CTC pathway. However, this compensation does not come without a price, since we show that compensatory CTC recruitment involving disconnection between the STC and CTC loops occurs in subjects with tremor-dominant PD, but not akinetic-rigidity-dominant PD, supporting a growing body of evidence that suggests the cerebellum plays an important role in the generation of PD tremor. Together, this body of research has implications for treatments that target the symptom of tremor in PD, as therapies which minimize tremor might also reduce beneficial aspects of compensation.

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