Associative plasticity of the motor cortex in Parkinson's disease

An interventional paired associative stimulation (IPAS) combining the peripheral nerve and the cortical stimulation can induce a long-lasting increase in the excitability of the motor cortex, when two inputs were synchronous at the level of the motor cortex (associative long-term potentiation (LTP)-like effect). The aim of this study is to elucidate whether the dopaminergic system can modulate the associative LTP-like effect in the motor cortex by applying this method to Parkinson's disease (PD). Ten right-handed patients with PD without dopamine replacement therapy for the previous 12 h and five right-handed age-matched healthy volunteers were studied. IPAS consisted of an electrical stimulation of the right median nerve and a following suprathreshold magnetic stimulation over the left primary motor cortex (M1) with the delay of 25 ms. A total of 240 pairs of stimuli were given at 0.2 Hz. Motor evoked potentials (MEPs) were measured from the right abductor pollicis brevis (APB) muscles before (baseline) and immediately after IPAS. IPAS was capable of inducing increased MEP amplitude of the right APB in age-matched healthy volunteers, whereas it did not change MEP amplitude in PD. This result suggests that the dopaminergic system modulates the associative plasticity of the human motor cortex, which may be achieved via basal-ganglia-thalamocortical loops.