Central distribution of subdiaphragmatic vagal branches in the rat

In the rat, the subdiaphragmatic vagus nerves (SDX) have five major branches--the right gastric, the left gastric, the coeliac, the accessory coeliac, and the hepatic. Although these branches innervate more than the organs after which they are named, some mediate specific behavioral functions. In addition to the SDX trunk, the central stump of each of these branches was incubated in horseradish peroxidase (HRP) for 6 hours in anesthetized rats. After processing the vagal ganglia, pons, medulla, and upper cervical spinal cord of each preparation, the sections were examined for both retrogradely and anterogradely transported HRP reaction product. When only one nerve had been incubated, retrogradely labeled neurons were confined primarily to the ipsilateral ganglion, medulla, and spinal cord. Within the brain, a few labeled neurons occurred within the nucleus ambiguus (NA) and the reticular formation caudal to the NA, but the vast majority appeared in the dorsal motor nucleus of the vagus (DMX). The axons of most labeled neurons in the NA distributed in the gastric branches; those from cells caudal to the NA, probably distributed in the coeliac branch. Most labeled DMX cells also distributed with the gastric branches. Those on the lateral tip of the right DMX, however, had axons in the coeliac branch; those on the left DMX tip, in the accessory coeliac. After incubation of the SDX trunk, anterograde HRP reaction product occurred in the caudomedial nucleus of the solitary tract (NST) just rostral and subjacent to the area postrema (AP). Unlike the retrograde label, anterograde reaction product was bilateral, but always weaker contralaterally. Within the SDX distribution, the afferent axons from the gastric branches exhibited one pattern of termination; those from the coeliac, accessory coeliac, and hepatic branches, another. The gastric branch distributions began dorsolaterally in the SDX termination zone and continued caudally beneath the AP. Immediately subjacent to the AP, gastric branch terminals were never dense and the entire distribution faded at the level of the obex. The coeliac and accessory coeliac distributions began dorsomedially within the SDX termination zone and intensified caudally in a thin band immediately subjacent to the AP. The densest label was associated with the caudal half of the AP, but the distribution thinned rapidly caudal to the obex. The hepatic distribution was similar to that of the coeliac branches but never achieved similar density. Physiological and behavioral data correlate with the anatomical picture in that the efferent functions appear to be more densely localized than the afferent functions.