Oscillatory activity in the cortico-basal ganglia-thalamic neural circuits in Parkinson's disease

Dopamine is an important neurotransmitter that maintains the balance within the basal ganglia between the direct pathway, which promotes movement, and the indirect pathway, which inhibits movement. Degeneration of dopaminergic neurons in the substantia nigra increases the influence of the indirect pathway, resulting in motor dysfunction in Parkinson's disease (PD). The direct and indirect pathways are composed of basal ganglia and thalamic nuclei, which are interconnected via independent parallel loop circuits with cortical areas and often referred to as cortico-basal ganglia-thalamic (CBT) neural circuits. CBT circuits have been useful in generating hypotheses to describe slowness in PD. Recent work has focused on aberrant neural oscillations within CBT circuits. Although beta (13-30 Hz) oscillations are a common feature of the CBT network, there is growing evidence that abnormally exaggerated beta oscillations, observed after dopamine loss in the CBT circuits, may contribute to motor symptoms of PD. Disruption of abnormal beta oscillations has been associated with the improvement of motor functions during pharmacological treatments, surgical lesions, and electrical stimulation. However, it is not clear how abnormal oscillations originate in the CBT motor network and resonate specifically in the beta band after the loss of dopamine. Most studies have addressed these questions by simultaneous recordings of oscillations in the motor cortex, basal ganglia nuclei, and motor regions of the thalamus in animal models of parkinsonism as well as in PD patients. This review further discusses previous and current studies of the changes in oscillatory activity at the level of CBT neural network in PD.