ELECTROPHYSIOLOGICAL STUDY OF THE EXCITATORY PARAFASCICULAR PROJECTION TO THE SUBTHALAMIC NUCLEUS AND EVIDENCE FOR IPSILATERAL AND CONTRALATERAL CONTROLS

The activity of subthalamic neurons was recorded extracellularly in an aesthetized rats after stimulation, inhibition or lesioning of the par afascicular nucleus. Electrical stimulation of the parafascicular nucl eus evoked a complex response with two excitatory phases. The first re sponse was correlated with a monosynaptically-driven excitation via a parafascicular input to the subthalamic nucleus. Since the second phas e was observed even when the early excitation was not recorded and was eliminated by lesion of the globus pallidus, we suggest that it is no t generated by a mechanism intrinsic to the subthalamic nucleus and is due to a disinhibitory effect originating from the globus pallidus. M icroinjection of carbachol into the parafascicular nucleus enhanced by 119% the discharge rate of the neurons in the ipsilateral subthalamic nucleus and that of muscimol decreased the discharge rate by 91%. Opp osite changes, a decrease of the discharge rate of 49% after microinje ction of carbachol and an increase of 47% after muscimol, occurred in the contralateral subthalamic nucleus. In contrast to the above result s, the unilateral excitotoxic lesion of the parafascicular nucleus, pe rformed one week before recording, decreased the discharge rate by 69% of the ipsilateral subthalamic nucleus neurons and by 34% that of the contralateral neurons. We suggest that the parafascicular input to th e subthalamic nucleus is an excitatory pathway which can tonically dri ve the neuronal activity in this structure. The opposite changes recor ded in the ipsi- and contralateral subthalamic nucleus during unilater al microinjection of excitatory or inhibitory drugs in the parafascicu lar nucleus emphasize the importance of this thalamic structure in the bilateral regulation of basal ganglia activity via the subthalamic nu cleus.