History of exposure to dopaminergic medication does not affect motor cortex plasticity and excitability in Parkinson's disease

Objective: Little is known whether and how chronic exposure to dopaminergic treatment alters physiological mechanisms in Parkinson's disease (PD). Methods: Two clinically similar groups of PD patients, one consisting of drug-naive patients and another of patients already on chronic dopaminergic medication (when off medication), were compared to each other and to a control group. Plasticity and excitability of the hand primary motor cortex of the more affected side were evaluated using transcranial magnetic stimulation (TMS) techniques. Results: There was little difference between two patient groups, and both groups showed similar differences in comparison to controls: decreased facilitatory sensory-motor plasticity (as measured by paired associative stimulation [PAS] protocol), impaired short-interval intracortical inhibition (SICI), and diminished slope of input-output curves at higher TMS intensities. The exception was that 30 min after PAS, intracortical facilitation (ICF) was significantly reduced in drug-naive patients, whereas it changed much less in other two groups. Conclusions: Chronic exposure to dopaminergic drugs does not affect substantially the features of motor cortex excitability and plasticity in PD. There is little interaction between plasticity and excitability features of motor cortex in PD. Significance: Reduced response to facilitatory PAS protocol, reduced SICI, and reduced slope of the input-output curve at higher TMS pulse intensities, seem to be physiological markers for the presence of the pathological disease process in PD. Long term treatment does not seem to change the underlying physiology of the disease