Intramedullary connections of the rostral nucleus of the solitary tract in the hamster

The rostral nucleus of the solitary tract (NST) figures prominently in the gustatory system, giving rise to ascending taste pathways that are well documented. Less is known of the local connections of the rostral NST with sites in the medulla. This study defines the intramedullary connections of the rostral NST in the hamster. Small iontophoretic injections of horseradish peroxidase (HRP), confined to the rostral NST, resulted in Golgi-like filling of axons that exited the NST or that interconnected cytoarchitectonic subdivisions within the NST complex. The NST efferent axons terminated sparsely in the trigeminal, facial and hypoglossal motor nuclei, but axons and endings were heavily distributed in the parvicellular reticular formation ventral to the NST. HRP injections centered in this part of the reticular formation resulted in heavy projections to the orofacial motor nuclei. Intranuclear connections, labelled after NST injections, linked NST subdivisions that receive primary afferent taste inputs to subdivisions involved in (1) projections to the preoromotor reticular formation, (2) projections to swallowing motor neurons, (3) activation of preganglionic parasympathetic neurons, and (4) general viscerosensation. In general, the connections defined in the present study provide anatomical details about the substrate for gustatory-motor and gustatory-visceral interactions.