Nigrostriatal lesion induces D2‐modulated phase‐locked activity in the basal ganglia of rats

There is a debate as to what modifications of neuronal activity underlie the clinical manifestations of Parkinson's disease and the efficacy of antiparkinsonian pharmacotherapy. Previous studies suggest that release of GABAergic striatopallidal neurons from D2 receptor‐mediated inhibition allows spreading of cortical rhythms to the globus pallidus (GP) in rats with 6‐hydroxydopamine‐induced nigrostriatal lesions. Here this abnormal spreading was thoroughly investigated. In control urethane‐anaesthetized rats most GP neurons were excited during the active part of cortical slow waves (‘direct‐phase’ neurons). Two neuronal populations having opposite phase relationships with cortical and striatal activity coexisted in the GP of 6‐hydroxydopamine‐lesioned rats. ‘Inverse‐phase’ GP units exhibited reduced firing coupled to striatal activation during slow waves, suggesting that this GP oscillation was driven by striatopallidal hyperactivity. Half of the pallidonigral neurons identified by antidromic stimulation exhibited inverse‐phase activity. Therefore, spreading of inverse‐phase oscillations through pallidonigral axons might contribute to the abnormal direct‐phase cortical entrainment of basal ganglia output described previously. Systemic administration of the D2 agonist quinpirole to 6‐hydroxydopamine‐lesioned rats reduced GP inverse‐phase coupling with slow waves, and this effect was reversed by the D2 antagonist eticlopride. Because striatopallidal hyperactivity was only slightly reduced by quinpirole, other mechanisms might have contributed to the effect of quinpirole on GP oscillations. These results suggest that antiparkinsonian efficacy may rely on other actions of D2 agonists on basal ganglia activity. However, abnormal slow rhythms may promote enduring changes in functional connectivity along the striatopallidal axis, contributing to D2 agonist‐resistant clinical signs of parkinsonism.