COMPARISON OF CEREBELLOTHALAMIC AND PALLIDOTHALAMIC PROJECTIONS IN THE MONKEY (MACACA-FUSCATA) - A DOUBLE ANTEROGRADE LABELING STUDY

To address the question of segregated projections from the internal se gment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to t he thalamus in the monkey, we employed a double anterograde labeling s trategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) with biotinylated dextra n amine (BDA) transport. The tissue was processed sequentially for WGA -HRP, and then BDA immunohistochemistry using two different chromogens . Since the two labels were easily distinguishable on the same histolo gical section, the interrelationship between the cerebellar and pallid al projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense ple xuses of labeled fibers and swellings in a patch-like configuration. T he patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense s ingle label in the central portion of the ventral anterior nucleus par s principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) follow ing the cerebellar nuclei injections. Complementary interdigitating pa tches of WGA-HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars ca udalis (VLc), with the density of these projections decreasing along a n anterior to posterior gradient in the thalamus. Occasional patches o f pallidal label were found in VPLo and nucleus X. Conversely, the den sity of cerebellothalamic projections increased along the same gradien t, with the cerebellothalamic territory extending anteriorly beyond th e cell-sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projectio ns primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources. (C) 1996 Wiley-Liss, Inc.