Anterior striatal projections to the globus pallidus, entopeduncular nucleus and substantia nigra in the rat: the GABA connection.

The projections of the head of the striatum of the rat were studied by autoradiography and horseradish peroxidase (HRP) histochemistry. After injections of [3H]-leucine into the head of the striatum, accumulations of autoradiographic grains were observed over the neuropil of the globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra, pars reticulata (SNR). The striatal projections to GP and EP were confirmed using HRP histochemistry. Injections of HRP into GP resulted in the appearance of reactive perikarya which were localized mainly in the central core region of the striatum. HRP injections into EP also produced labeled cells in the striatum but these were found in different regions of the striatum in different animals. It was not possible to discern topographical relationships. Striatal hemitransections just anterior to GP significantly decreased glutamic acid decarboxylase (GAD) activities in GP and EP, but did not have significant effects on the activity of this enzyme in the whole SN. Identical results were obtained with electrolytic lesions of the head of the striatum when the lesions did not encroach upon GP. Stereotaxic injections of kainic acid into the same region of the striatum significantly decreased GAD activities in GP, EP and SN. However, histological examination of the kainic acid lesions showed involvement of the GP and more caudal striatal regions. This may account for the significantly greater decrease in GAD in the GP and SN after these lesions than after the two other lesioning procedures. These results suggest that the projections from anterior striatum to GP and EP are GABA-containing systems. In contrast, although the same region of the striatum can be shown anatomically to project to the SN, this does not appear to be GABAergic. Rather, these findings are consistent with the view that the major source of the massive GABAergic innervation of the SN originates in the GP or in the caudal striatum immediately anterior, dorsal or lateral to the GP.