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

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Abstract
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 Fluoro‐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 rostra1 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).