Extranuclear projections of rNST neurons expressing gustatory-elicited Fos

Previous studies have demonstrated that gustatory stimulation evokes expression of the immediate-early gene, c-fos in the rostral division of the nucleus of the solitary tract (rNST) (Harrer and Travers [1996] Brain Res. 711:125-137; DiNardo and Travers [1997] J. Neurosci. 17:3826-3839; King et al. [1999] J. Neurosci. 19:3107-3121). The present investigation further defined the phenotype of those neurons by determining their projections, by using immunohistochemistry for the Fos protein and retrograde tracing with Fluoro-Gold. Tracer injections were made into the two major extranuclear targets of rNST, the parabrachial nucleus (PBN) and medullary reticular formation (RF). These structures are thought to play differential roles in higher-order discriminative and homeostatic (PBN) versus reflexive function (RF). After PBN injections, approximately 18% of the Fos-like immunoreactive (FLI) neurons were double-labeled; after RF injections the proportion was 9%. Because only a minority of FLI neurons appear to project to targets outside NST, this suggests that most of these cells have local, intranuclear projections. Comparable proportions of cells were double-labeled after sucrose or quinine, consistent with roles for both tastants in higher-order and reflexive function. On the other hand, regardless of stimulus, twice as many FLI neurons projected to the PBN as to the RF. This could suggest that more FLI neurons contribute to functions mediated by the ascending pathway. However, the results of a recent study prompted a different hypothesis: Because glossopharyngeal nerve section similarly devastates quinine-induced FLI and oral rejection but leaves discriminative function unimpaired, it was proposed that FLI neurons are more important in driving oral motor behavior than discrimination (King et al. [1999] J. Neurosci. 19:3107-3121). A plausible hypothesis for reconciling this apparent discrepancy is that many FLI neurons make local projections in rNST, that in turn give rise to RF connections.