Efferent connections of the caudate nucleus, including cortical projections of the striatum and other basal ganglia: an autoradiographic and horseradish peroxidase investigation in the cat.

Tritiated tracer was injected into the head of the caudate nucleus in cats. Following such injections, labeling is present within extensive regions of both the globus pallidus and entopeduncular nucleus, where it presents a mottled or meshlike appearance. These projections are topographically organized in that there is simple correspondence between the mediolateral, dorsoventral, and rostrocaudal origin of the caudate projection and its input to the globus pallidus and entopeduncular nucleus. Transported tracer is also present within the substantia nigra, where it is most abundant within the pars reticularis. However, distinct labeling also overlies cells of the pars compacta, and lesser amounts of labeling are present within the pars lateralis and within the retrorubral area. Following injections of horseradish peroxidase into the caudate nucleus, and subsequent tissue processing by the tetramethylbenzidine (TMB) method of Mesulam ('78), labeled anterograde fibers are present in abundance within the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra, thus confirming the autoradiographic findings. Also, it is especially obvious in this HRP material that, contrary to previous degeneration studies, both the rostromedial and caudolateral parts of the pars lateralis of the substantia nigra contain numerous anterogradely labeled fibers. Retrogradely labeled neurons are also present within the substantia nigra of these same tissue sections, where they are most abundant within the pars compacta, but lesser numbers of labeled neurons are also present within the pars reticularis, pars lateralis, retrorubral area, and ventral tegmental area on the ipsilateral side, and all of these same subdivisions of the substantia nigra on the contralateral side. Also, within the subthalamic nucleus in these experiments, there are anterogradely labeled fibers, as well as retrogradely labeled neurons, which are interpreted to represent a reciprocal connection between the subthalamic nucleus and the striatum. In a separate series of experiments, horseradish peroxidase was injected into the motor cortex-specifically into the anterior sigmoidal gyrus. Following such injections, labeled neurons representing afferents to the motor cortex are found in all subcortical nuclei commonly known as the "basal ganglia," including the caudate nucleus, putamen, globus pallidus, entopeduncular nucleus, substantia innominata, nucleus of the diagonal band of Broca, medial septal nucleus, claustrum, and basolateral amygdaloid nucleus.