Autonomic brainstem projections to the pancreas: a retrograde transneuronal viral tracing study in the rat

The present study describes brainstem nuclei that participate in the autonomic innervation of the pancreas, using a retrograde viral transneuronal tracing technique. It aimed at identifying the neuronal architecture of the parasympathetic, gustatory-induced insulin release by the endocrine pancreas (preabsorptive insulin response, PIR). Autonomic pathways organized for reflex adjustments of the end organ, as it happens in the PIR, involve relatively simple circuits. This implies a short brainstem circuit from the rostral gustatory nucleus of the solitary tract to the dorsal motor nucleus of the vagus. The present findings confirm projections to the pancreas, originating from preganglionic neurons in the dorsal motor nucleus of the vagus. Transneuronal labeling was detected in the medial, and to a lesser extent in the lateral nucleus of the solitary tract mainly at caudal and intermediate levels. Furthermore, infected neurons were seen in the brainstem in the dorsal and ventral part of the medullary reticular formation, in the area postrema and in the raphe nuclei. Sparse labeling was found in the gustatory zone of the nucleus tractus solitarius. These results indicate that a direct connection between the rostral nucleus tractus solitarius and the medial dorsal motor nucleus of the vagus is very unlikely, so that one or more intermediate stations may be involved. Candidates to complete this pathway are the intermediate or caudal nucleus tractus solitarius, the medullary reticular formation or the parabrachial nucleus.