EFFERENT CONNECTIONS OF THE CAUDAL PART OF THE GLOBUS-PALLIDUS IN THERAT

The efferent connections of the caudal pole of the globus pallidus (GP ) were examined in the rat by employing the anterograde axonal transpo rt of Phaseolus vulgaris leucoagglutinin (PHA-L), and the retrograde t ransport of fluorescent tracers combined with choline acetyltransferas e (ChAT) or parvalbumin (PV) immunofluorescence histochemistry. Labele d fibers from the caudal GP distribute to the caudate-putamen, nucleus of the ansa lenticularis, reuniens, reticular thalamic nucleus (mainl y its posterior extent), and along a thin strip of the zona incerta ad jacent to the cerebral peduncle. The entopeduncular and subthalamic nu clei do not appear to receive input from the caudal GP. Descending fib ers from the caudal GP course in the cerebral peduncle and project to posterior thalamic nuclei (the subparafascicular and suprageniculate n uclei, medial division of the medial geniculate nucleus, and posterior intralaminar nucleus/peripeduncular area) and to extensive brainstem territories, including the pars lateralis of the substantia nigra, lat eral terminal nucleus of the accessory optic system, nucleus of the br achium of the inferior colliculus, nucleus sagulum, external cortical nucleus of the inferior colliculus, cuneiform nucleus, and periaqueduc tal gray. In cases with deposits of PHA-L in the ventral part of the c audal GP, labeled fibers in addition distribute to the lateral amygdal oid nucleus, amygdalostriatal transition area, cerebral cortex (mainly perirhinal, temporal, and somatosensory areas) and rostroventral part of the lateral hypothalamus. Following injections of fluorescent trac er centered in the lateral hypothalamus, posterior intralaminar nucleu s, substantia nigra, pars lateralis, or lateral terminal nucleus, a su bstantial number of retrogradely labeled cells is observed in the caud al GP. None of these cells express ChAT immunoreactivity, but, except for the ones projecting to the lateral hypothalamus, a significant pro portion is immunoreactive to PV. Our results indicate that caudal GP e fferents differ from those of the rostral GP in that they project to e xtensive brainstem territories and appear to be less intimately relate d to intrinsic basal ganglia circuits. Moreover, our data suggest a po ssible participation of the caudal GP in feedback loops involving post erior cortical areas, posterior striatopallidal districts, and posteri or thalamic nuclei. Taken as a whole, the projections of the caudal GP suggest a potential role of this pallidal district in visuomotor and auditory processes. (C) 1996 Wiley-Liss, Inc.