Parkinsonism disrupts the balance between excitatory and inhibitory activity within the primary motor cortex during movement.

Parkinson's disease (PD) has been associated with alterations in neuronal activity in the basal ganglia-thalamocortical (BGTC) network. Previous studies have suggested that cortical disinhibition is a feature of PD, but there has been little direct evidence of the changes in cortical neuronal spiking activity to support this hypothesis. To test the hypothesis that activity in the motor cortex is enhanced in PD, we investigated the effects of parkinsonism on movement-related neuronal activity in the primary motor cortex (M1). Microelectrode arrays were chronically implanted in M1 of two nonhuman primates and populations of cells were collected before and after the induction of parkinsonism using the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. In both animals, we found that during the reaching task in the parkinsonian state, the proportion of M1 neurons that were activated (excited) during movement were increased, while those that were suppressed (inhibited) decreased. These data support the concept that dopaminergic loss in parkinsonism promotes a loss of inhibition in the motor cortex, leading to overactivity in M1 and a disruption in spatial-temporal processing of information within the BGTC circuit that contributes to the motor dysfunction observed in PD.