Perikaryal cell labeling in the subthalamic nucleus following the injection of 3H-gamma-aminobutyric acid into the pallidal complex: an autoradiographic study in cat.

Although the subthalamic nucleus is thought to exert a major influence on the corpus striatum output cells, there is little information available on the transmitter or transmitters involved. In a series of autoradiographic experiments in which various different radiolabeled putative transmitter substances were injected separately into the pallidal complex of cats, it was noted that 3H-gamma-aminobutyric acid injection consistently resulted in perikaryal labeling in the subthalamic nucleus. 3H-gamma-aminobutyric acid injection in the lateral part of the external pallidal segment resulted in labeled cells situated laterally in the subthalamic nucleus, while 3H-aminobutyric acid injection in the internal pallidal segment (entopeduncular nucleus) resulted in cell labeling more medially in the subthalamic nucleus. Perikaryal cell labeling was also noted in the lateral putamen following 3H-gamma-aminobutyric acid injection in the external pallidal segment in cats pretreated with systemic amino-oxyacetic acid. No cell groups other than the striatum and subthalamic nucleus could be made to label with 3H-gamma-aminobutyric acid. Furthermore, no perikaryal cell labeling in the subthalamic nucleus was seen to follow injections of 3H-D-aspartate or 3H-serotonin into the pallidal complex. The findings suggest that 3H-gamma-aminobutyric acid, but not aspartate or serotonin, undergoes high affinity uptake and retrograde transport by subthalamo-pallidal neurons. Bearing in mind the many reservations discussed, the observation implies that 3H-gamma-aminobutyric acid may be a transmitter in the subthalamo-pallidal pathway.