“The role of Dopamine in preparatory inhibition : what can we learn from Parkinson’s Disease?”

Recent Transcranial Magnetic Stimulation (TMS) studies have shown that the motor output pathway shows a profound inhibition during the preparation of an action. This phenomenon, called “preparatory inhibition”, remains incompletely understood, as it is still unclear which neural structures support it and what function it exactly serves during action preparation. In the current study, we tested this preparatory inhibition in Parkinson’s Disease Patients (PDP) and matched Healthy Control Subjects (HCS). The aim was to better understand the role of basal ganglia (which are dysfunctional in Parkinson’s Disease [PD]) and the influence of Dopamine Replacement Therapy (DRT) on motor inhibition during action preparation. First, we managed to reproduce preparatory inhibition in HCS, as previously shown in the literature; then, we could demonstrate that this phenomenon was inconsistent in PDP, especially in the hand that was selected for the realization of an upcoming movement. Interestingly, preparatory inhibition seemed to decline throughout the disease. Whereas the presence of DRT did not alter the levels of preparatory inhibition among PDP when taken all together, DRT medication appeared to decrease its strength, especially in the dominant hand, in the patients with the longest duration of PD. This suggests a role of dopamine in the mechanisms at the very basis of preparatory inhibition.