The projections of the entopeduncular nucleus and globus pallidus in rat as demonstrated by autoradiography and horseradish peroxidase histochemistry

Abstract

The efferent projections of the globus pallidus (GP) and entopeduncular nucleus (EP) in the rat were studied using autoradiographic and horseradish peroxidase (HRP) histochemical techniques. Following stereotaxic injections of 3 H‐leucine into EP, transported label was observed in the lateral part of the ventromedial nucleus (VM) of the thalamus, and more laterally, in the ventrolateral aspect of the ventral lateral‐ventral anterior thalamic complex (VALv). Selective accumulation of autoradiographic grains was also found in the parafascicular nucleus (Pf) and in the lateral habenular nucleus. HRP injections into the thalamus confirmed the entopeduncular projections to these nuclei. Comparison of grain distribution in the thalamus after 3 H‐leucine injections into EP and the zona reticulata of the substantia nigra (Clavier et al., '76) indicates that there is considerable overlap between nigral and EP projections to both VM and Pf. The nigrothalamic and entopedunculothalamic projections to VM and VALv are discussed with reference to the possibility that in the rat these nuclei are homologous to specific nuclei within the ventral lateral‐ventral anterior complex in primates. Injections of 3 H‐leucine into GP resulted in major accumulations of autoradiographic grains in the medial part of the subthalamic nucleus and the substantia nigra. Light and diffuse labelling was also observed in the dorsal thalamus. However, a consistent feature of the GP injections was that they appeared to result in diffusion of isotope to the reticular nucleus of the thalamus, and in view of the widespread thalamic projections of this nucleus, the label in dorsal thalamus could not be positively attributed to pallidal efferents. HRP injections throughout the thalamus failed to produce reactive cells in the GP. These experiments provided no firm support therefore for a recent suggestion that there are extensive projections from the globus pallidus to the thalamus in rat.