An electrode was stereotaxically implanted in the mesencephalic ventromedial tegmentum (VMT) [A:4~4.5,L:2~2.5,H:-5.0~-5.5] of each of ten cats. Subsequently two electrodes were placed in the rostomedial [A:14~15,L:4~5,H:4~5] and anteroventral [A:17~19,L:4~5,H:4~5] parts of the head of the ipsilateral caudate nucleus (CN). Four or five days later the VMT was electrically stimulated in three cats to induce behavioral changes which apparantly resembled human athetoid and choreiform movements. Square pulse current was 2-10 volt, 0.5-1.0 msec in duration and 20-100/sec in frequency. 1) The results of high frequency stimulation of the VMT were as follows. 1 Arrest of all kinds of voluntary movements, and ipsilateral miosis with adduction were observed immediately after the start of stimulation. 2 Turning or circling to the opposite side, elevation or choreo-athetoid movements of contralateral forelimb, and licking or chewing followed two to four seconds later. These movements continued during the electrical stimulation and lasted even after cessation of the stimulation. 3 Vegetative phenomena such as tachycardia or urination were also seen occasionally. 2) Decapitation was performed after irrigation of the brain by 10% formalin. Histological study revealed that the tip of the electrode was stituated in the VMT containing the dorsomedial part of the substantia nigra. 3) High frequency stimulation of the VMT led to numerous firing of neurons of the rostromedial part of the ipsilateral CN, which lasted for several seconds after cessation of the stimulation. Electrical stimulation of the rostromedial part of the CN induced choreo-athetoid movements and turning, which were quite similar to those produced by stimulation of the VMT. 4) Additional CN stimulation altered the involuntary movements induced by stimulation of the VMT. Namely, stimulation of the rostromedial part of the CN increased both choreo-athetoid movements and turning in cats. On the other hand, stimulation of the anteroventral part of the CN decreased them in cats. 5) Finally, turning and circling induced by the VMT stimulation intensified following parenteral administration of L-Dopa, and diminished with haloperidol. These results suggest that the nigrostriatal dopaminergic system plays an important role in the production of circling and choreo-athetoid movements. Especially, the caudate neurons which fired abnormally following repetitive VMT stimulation might be a trigger of these movements.
Ventromedial tegmentum (VMT)
Head of the caudate nucleus (HCN)