GABAergic projection from the ventral pallidum and globus pallidus to the subthalamic nucleus

There exists a topographically organized projection from the globus pallidus and ventral pallidum to the subthalamic nucleus and adjacent lateral hypothalamus. The participation of GABA as a neurotransmitter in this projection was evaluated by retrograde labeling of cells in the pallidal area from an iontophoretic deposit of Fluor-Gold in the subthalamus combined with in situ hybridization for mRNA of the GABA synthetic enzyme, glutamate decarboxylase (GAD). A rostrocaudal gradient in the contribution of GABA to the projection was demonstrated with a relatively small percentage of retrogradely labeled cells in the rostral ventral pallidum containing GAD mRNA (7%) compared to the caudal globus pallidus which had over 70% of the Fluoro-Gold containing cells double-labeled for GAD mRNA. Overall the ventral pallidum contribution to the subthalamic nucleus was less GABAergic than the portion arising from the globus pallidus (35% vs. 61%, respectively).