Afferent projections to the thalamic mediodorsal nucleus in the cat studied by retrograde and anterograde axonal transport of horseradish peroxidase.

The afferent pathways to the thalamic mediodorsal nucleus (MD) in the cat were studied using the methods of anterograde and retrograde axonal transport of horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP (WGA-HRP). The MD receives fibers from the prefrontal cortex in a topically organized manner in accordance with the thalamocortical projections. The medial or ventral portion of the MD receives afferents from the islands of Calleja of the olfactory tubercle, the nucleus of the diagonal band, the amygdala and the claustrum. The lateral hypothalamic nucleus sends a moderate number of fibers to the medial MD, but other hypothalamic nuclei send only a few fibers to the MD. The lateral or dorsal portion of the MD receives fibers from the nucleus of the diagonal band, the ventral pallidum and the entopeduncular nucleus, but only few from the olfactory tubercle and the amygdala. The thalamic reticular nucleus sends many fibers to the MD without showing any topography. The MD, particularly its lateral part, receives afferents from brainstem structures, such as the substantia nigra, superior colliculus, reticular formation, raphe nuclei and nucleus loci coerulei. Only the interpeduncular nucleus sends fibers mainly to the medial part of the MD. The cerebellar nuclei send only a few fibers to the lateral part of the MD at posterior levels.