Effects of L-DOPA on neuronal activity of the globus pallidus externalis (GPe) and globus pallidus internalis (GPi) in the MPTP-treated monkey.

We studied the effects of L-DOPA on the firing patterns of pallidal neurons in experimental parkinsonism. After a unilateral injection of MPTP, we observed a decrease in the firing rate of GPe neurons, and a slight increase in their bursting activity. In the GPi, there was a considerable augmentation of both neuronal firing frequency and the number of bursting cells. During l-DOPA treatment (10 mg/kg), GPe neurons.pattern is almost unmodified. The firing frequency of GPi neurons, on the contrary, decreased even lower than the control level. A slight reduction was observed in bursting activity. These unexpected results would show that the normalizing effect of L-DOPA on GPi output is limited.