Glycine: an alternative transmitter candidate of the pallidosubthalamic projection neurons in the rat

Autoradiographic retrograde tracing techniques with radioactive transmitters were used to analyse the identity of a putative transmitter in the rat pallidosubthalamic (GP-STN) pathway. One to 2 hours after the stereotaxic injection of 3H-glycine restricted to the STN, a large number of neuronal somata were radiolabeled in the GP. No comparable labeling was observed following the injection of 3H-gamma-aminobutyric acid (3H-GABA) into the same nucleus even with survival times as long as 6 hours. Specifically, no significant somatic labeling was detected either in the GP or in the caudoputamen (CPU). Only when 3H-GABA was injected into the substantia nigra did CPU and GP neurons become labeled. On the contrary, STN neuronal somata were invariably labeled 6 hours after the intrapallidal injection of 3H-GABA, whereas no perikaryal labeling was observed in the STN after 3H-glycine injection into the GP. The perikaryal labeling was prevented in all cases by intraventricular administration of colchicine 1 day before the isotope injections. The observations suggest that 3H-glycine was preferentially transported retrogradely through the GP-STN pathway, and 3H-GABA through the STN-GP projection. In view of the recent controversy on the role of GABA as a putative transmitter of the GP-STN projection, we now propose glycine as an alternative transmitter candidate of these critically situated neurons in the basal ganglia.