Modulation of inhibitory plasticity in basal ganglia output nuclei of patients with Parkinson's disease.

Deep brain stimulation of certain target structures within the basal ganglia is an effective therapy for the management of the motor symptoms of Parkinson's disease. However, its mechanisms, as well as the pathophysiology of Parkinson's disease, are varied and complex. The classical model of Parkinson's disease states that symptoms may arise as a result of increased neuronal activity in the basal ganglia output nuclei due to downregulated GABAergic striato-nigral/-pallidal projections. We sought to investigate the stimulation and levodopa induced effects on inhibitory synaptic plasticity in these basal ganglia output nuclei, and to determine the clinical relevance of altered plasticity with respect to patients' symptoms. Two closely spaced microelectrodes were advanced into the substantia nigra pars reticulata (potential novel therapeutic target for axial motor symptoms) or globus pallidus internus (conventional therapeutic target) in each of 28 Parkinson's disease patients undergoing subthalamic or pallidal deep brain stimulation surgery. Sets of 1 Hz test-pulses were delivered at different cathodal pulse widths (25, 50, 100, 150, 250 μs) in randomized order, before and after a train of continuous high frequency stimulation at 100 Hz. Increasing the pulse width led to progressive increases in both the amplitudes of extracellular focally evoked inhibitory field potentials and durations of neuronal silent periods. Both of these effects were augmented after a train of continuous high frequency stimulation. Additionally, reductions in the baseline neuronal firing rate persisted beyond 1 min after high frequency stimulation. We found greater enhancements of plasticity in the globus pallidus internus compared to the substantia nigra pars reticulata, and that intraoperative levodopa administration had a potent effect on the enhancement of nigral plasticity. We also found that lower levels of nigral plasticity were associated with higher severity motor symptoms. The findings of this study demonstrate that the efficacy of inhibitory synaptic transmission may be involved in the pathophysiology of Parkinson's disease, and furthermore may have implications for the development of novel stimulation protocols, and advancement of DBS technologies.