Effects of 6-hydroxydopamine-induced severe or partial lesion of the nigrostriatal pathway on the neuronal activity of pallido-subthalamic network in the rat.

The origin of changes in the neuronal activity of the globus pallidus (GP) and the subthalamic nucleus (STN) in animal models of Parkinson's disease (PD) is still controversial. The aim of the study was to investigate the neuronal activity of STN and GP neurons under urethane anesthesia in an early and in an advanced stage PD rat model. 6-Hydroxydopamine (6-OHDA) injection into the striatum induced a partial lesion of dopamine cells in the substantia nigra pars compacta (SNc) and fibers in the striatum. The GP firing rate decreased significantly with no significant change of the pattern. 6-OHDA injection into the SNc induced a total or subtotal lesion without any change in the firing rate and patterns of GP neurons. Concerning the STN, after partial lesion, the firing rate remained unchanged but the firing pattern significantly changed towards a more irregular and bursty pattern. In rats with total or subtotal lesion of the SNc the firing rate increased significantly and the relative amount of tonic neurons significantly decreased. Our results demonstrate that neuronal reactivity in the basal ganglia network considerably differs in the early versus late stage model of PD. We showed that the pathological activity of STN neurons after severe lesion is not mediated by the GP. Moreover, the unchanged activity of GP neurons is likely to be a consequence of the STN hyperactivity. These data suggest that in the GP-STN-GP network, the excitatory influence of the STN-GP pathway overrides that of the GABAergic GP-STN pathway, questioning the classical model of basal ganglia organization.