| Summary: | Background: Slowness in movement initiation (akinesia) is a cardinal feature of Parkinson's disease (PD), which is still poorly understood. Notably, akinesia is restored by subthalamic nucleus deep brain stimulation (STN-DBS) but not fully reversed by current dopaminergic treatments. It was recently suggested that this disorder is of executive nature (related to inhibitory control of response) and of non-dopaminergic origin (possibly noradrenergic). Objective: To test the double hypothesis that: 1) the ability to control movement initiation is modified by noradrenergic neurotransmission modulation, and 2) this effect is mediated by the regulation of STN activity. Methods: Sixteen STN-DBS PD patients were enrolled in a placebo-controlled study investigating the effects of noradrenergic attenuation by clonidine (∝2-adrenergic receptor agonist). Movement initiation latency was assessed by means of a cue-target reaction time task. Patients, who remained on their chronic dopaminergic medication, were tested on four sessions: two with placebo (ON- or OFF-DBS), and two with a 150 μg oral dose of clonidine (ON- or OFF-DBS). Results: In the OFF stimulation condition, patients were locked into a mode of control maintaining inappropriate response inhibition. This dysfunctional executive setting was overcome by STN-DBS. Clonidine, however, was found to impair specifically the ability to release inhibitory control in the ON-DBS state. Conclusions: Overall our results suggest an important implication of the noradrenergic system in the pathophysiology of akinesia in PD. Reducing the noradrenergic “tonus” may even block the positive action of STN-DBS on akinesia, suggesting, at least by part, a noradrenergic-dependent STN-DBS efficiency.
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