Brainstem afferents to the intralaminar nuclei of the cat thalamus studied by the horseradish peroxidase method.

Brainstem afferents to the intralaminar thalamic nuclei of the cat have been studied by retrograde axonal transport of horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to HRP (WGA-HRP). Injections of HRP or WGA-HRP into the lateral central and paracentral nuclei led to labeling of cells in various diencephalic structures of the injected side. Some cells were labeled in the intralaminar and medial pulvinar nuclei. A large number of labeled cells were detected in the thalamic reticular and ventral lateral geniculate nuclei. The zona incerta and field of Forel contained a few labeled cells. Occasional labeled cells were seen in the dorsomedial, ventromedial, posterior and lateral hypothalamic nuclei. Labeling of cells in the more caudal brainstem structures was found bilaterally, mostly with ipsilateral predominance. Some labeled cells were detected in the pretectal nuclei, the periaductal gray and the pars reticulata of the substantia nigra as well. The nuclei of Darkschewitsch and Cajal contained occasional labeled cells. Numerous cells were labeled in the superior colliculus, mainly in layers 3 and 4. A number of labeled cells were distributed in almost the entire extent of the brainstem reticular formation. It should be mentioned that the gigantocellular reticular nucleus contained labeled cells bilaterally with contralateral predominance. Very few labeled cells were encountered in the ipsilateral dorsal tegmental and parabrachial nuclei. A few cells were labeled in the vestibular nuclei and nucleus prepositus hypoglossi bilaterally. Some labeled cells were detected mainly contralaterally in three cerebellar nuclei; the fastigial, the interposed and the dentate. Injections of WGA-HRP into the medial central nucleus showed a different pattern of labeling. It is noteworthy that no labeled cells were found in the dorsal thalamus, the hypothalamus (except for the lateral mamillary nucleus), the superior colliculus or the cerebellar nuclei.