The organization of subcortical projections from neurons within the nucleus of the diagonal band of broca
Description
The present study investigated the organization of subcortical projections from the nucleus of the diagonal band (nDB) in the rat. Data were collected from several types of anterograde tracing experiments to determine the distribution of efferent projections from the nDB. These data indicate that the nDB gives rise an ascending, a dorsal descending and a ventral descending system The dorsal descending pathway, the focus of the present investigation, contained fibers from the nDB which coursed within either the stria medullaris or the inferior thalamic peduncle and terminated within the paratenial nucleus, the mediodorsal nucleus or the lateral habenula. Retrograde tracers also were employed to determine the location and types of neurons within the nDB which give rise to the efferent pathways. A difference in the location of neurons within the nDB which projected to the cerebral cortex, the dorsal thalamus or the epithalamus was apparent. For example, neurons which projected to the neocortex were located throughout the horizontal limb of the nDB. Likewise, epithalamic projecting neurons occupied the medial and ventral portions of the horizontal limb and were distributed among some of the cortically projecting neurons. In contrast, neurons which projected to the dorsal thalamus were located along the dorsolateral edge of the nDB. Thus, the neurons projecting to the paratenial nucleus and the mediodorsal nucleus were segregated from the neurons within the nDB which projected to the habenula. The dorsolateral region of the nDB, therefore, may actually be a subnucleus that is distinct from the rest of the diagonal band To determine if the thalamic projecting neurons within the nDB were cholinergic, tracers were placed within the thalamus and the tissue was reacted for the presence of choline acetyltransferase (ChAT) activity as well as for the tracer. Following injections placed within either the dorsal thalamus or the epithalamus, the vast majority of retrogradely labeled neurons within the nDB were not double labeled with ChAT immunoreactivity. These data indicate that the neurons within the nDB which project to the paratenial nucleus, the mediodorsal nucleus and the lateral habenula are not likely to be cholinergic