Cells of origin of subcortical afferents to the caudate nucleus: a horseradish peroxidase study in the cat.

The horseradish peroxidase tracing method was used to investigate the sources of subcortical input to the caudate nucleus in the cat. Of considerable interest was the finding, not previously described, that labeled neurons were present outside of the intralaminar complex, in the ventral anterior and mediodorsal thalamic nuclei. Another new finding was that retrogradely labeled cells were present within the basolateral amygdaloid nucleus, following the caudate injections. In addition, neurons containing enzyme granules were found within all of the intralaminar thalamic nuclei, most prominently in the central lateral and centromedian nuclei. Also, HRP-positive cells were strikingly apparent within the ipsilateral pars compacta of the substantia nigra, and a few labeled cells were found in the ipsilateral pars reticularis, and the contralateral pars compacta of the substantia nigra. Cells containing peroxidase granules were also found in the ventral tegmental area and the retrorubral area. The most caudal projection to the caudate nucleus in the cat was demonstrated by labeled neurons found bilaterally in the dorsal nucleus of the mesencephalic raphe.