We had previously identified an axonal projection arising in the caudal magnocellular basal nucleus (cMBN) and terminating within the thalamic reticular nucleus (TRN) in rats. The terminal boutons of this pathway were found to frequently synapse upon both somata and proximal dendrites of TRN neurons, with symmetric membrane specializations. Although we had, by examining labeled neurons following punctate WGA-HRP injections into the TRN, identified some neurons within the cMBN that are both retrogradely labeled and positive for GABA, it was important to confirm the GABAergic disposition of the axons arising within the cMBN itself using anterograde tracers, since the uptake of tracers by fibers of passage was a concern. Using PHA-L in conjunction with MM immunohistochemistry, we find that many, though not all boutons within the TRN arising in the CMBN contain GAD. Collectively these findings indicate that an extrinsic GABAergic input enters into the TRN, and demonstrate that one such input clearly arises within the CMBN. Using dopamine-beta-hydroxylase (DBH) immunohistochemistry, we have proceeded to examine the noradrenergic (NA) input upon the TRN and the cMBN. The highest density of DBH+ boutons occurs in the TRN; the DBH+ axonal boutons are evenly dispersed and give rise exclusively to boutons en passant. In contrast, the DBH+ axons within the cMBN are seen to coalesce into occasional clustered foci which appear to abut upon certain neurons, although many DBH+ axonsals also give rise to dispersed boutons en passant. By combining DBH immunohistochemistry with intracellular injections of the fluorescent dye, Lucifer yellow (LY) into TRN neurons. we examined the relationship of NA axons to the neurons of the TRN. The DBH+ bouton enlargements occurred in relation to dendrites of all diameters, appearing to randomly contact whichever dendrites they happened to intersect. Following fast blue (FB) injections into the TRN, retrogradely labeled cMBN neurons were also injected with LY. Although apparent random contacts of DBH+ axonal boutons with dendritic profiles of injected cMBN neurons were occasionally identified, the focal clusters did not occur in relation to the subset of cMBN neurons which project to the TRN. PHA-L injections into the locus coeruleus (LC) reveal that both populations of DBH+ axons (in the TRN and in the cMBN) arise within the LC, and retrograde double labeling experiments indicate that within the LC, the cell populations projecting to these two sites are largely independent of one another. Thus, the ascending reticular activating system, long held to arise exclusively somewhere within the brainstem reticular formation, may also be substantially influenced by the telencephalon (via the cMBN). One ascending brainstem system, the NA system, may strongly and directly influence the TRN, but does not appear to be poised to have a significant influence upon the signals conveyed by the cMBN-->TRN pathway.
