Turning behavior induced by injections of glutamate receptor antagonists into the substantia nigra of the rat

We have found recently that muscimol microinjections into the subthalamic nucleus produce contralateral turning activity [Murer and Pazo (1993) NeuroReport, 4:1219–1222]. To test the hypothesis that a reduced glutamate action on substantia nigra pars reticulata neurons mediates this turning response, we examined the effect of unilateral intranigral microinjections of the AMPA/kainate receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and the competitive N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). DNQX and AP-5 both produced a dose-dependent contralateral turning response, while vehicle administration did not induce turning activity. Application of glutamate receptor antagonist at adjacent regions of the mesencephalic tegmentum were also ineffective. Coadministration of NMDA or AMPA significantly reduced the turning response induced by AP-5 or DNQX, respectively. Lesions of the nigrostriatal pathway by 6-hydroxydopamine did not modify the response to DNQX or AP-5 administration into the nigra. However, their behavioral effects were significantly reduced by a lesion of the ipsilateral subthalamic nucleus. Our results show that the blockade of a tonic input acting on AMPA/kainate and NMDA receptors located at the substantia nigra produces contralateral turning behavior. The effect seems to involve pars reticulata cells since this area remains unchanged after destruction of dopaminergic neurons. The subthalamic nucleus seems to be the endogenous source of the agonist acting on the nigral glutamate receptors related to turning behavior. © 1996 Wiley-Liss, Inc.