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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‐γ‐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.