ASCENDING AND DESCENDING PROJECTIONS FROM THE ROSTRAL NUCLEUS OF THE SOLITARY TRACT ORIGINATE FROM SEPARATE NEURONAL POPULATIONS

Anterograde studies have shown that neurons within the rostral (gustat ory) nucleus of the solitary tract project to the parabrachial nucleus , as well as to sites within the medulla including the reticular forma tion and caudal nucleus of the solitary tract. In order to determine t he degree to which the same neurons contribute to both projections, in jections of retrograde tracers were made simultaneously into both the parabrachial nuclei and medullary reticular formation of the rat. Only a small proportion of neurons were double labeled. Consistent with st udies in hamster, labeled neurons projecting to the parabrachial nucle i in rat consisted of both stellate and elongate neurons, concentrated within the central subdivision of the rostral nucleus of the solitary tract. Injections into the medullary reticular formation also labeled both stellate and elongate neurons but these were concentrated in the ventral subdivision of the nucleus. The results of the present study demonstrate that different populations of neurons in the nucleus of th e solitary tract contribute to ascending and descending pathways. This suggests a possbile Functional specialization within the nucleus of t he solitary tract for those neurons whose output eventually reaches th e forebrain compared to those neurons with local connections.