The organization of vagal innervation of rat pancreas using cholera toxin—horseradish peroxidase conjugate

Abstract

The present study was initiated to address the current controversy concerning the parasympathetic innervation of the pancreas, using a more sensitive tracer. The location of retrogradely labeled neurons within the dorsal motor nucleus of the vagus (DMV) was examined 48 h following injections of cholera toxin - horseradish peroxidase (CT-HRP) into designated areas of the rat pancreas. The brainstem and spinal cord were searched for any additional labeled neurons located outside of the DMV. Separate groups of animals were used for control injections into the adipose tissue of the greater omentum, the spleen, abdominal musculature, and the diaphragm. In addition, CT-HRP was dripped over the surfaces of the abdominal viscera in another group of animals. These control cases were designed to indicate whether diffusion of the neural tracer away from injection sites had occurred and had resulted in labeling of neurons which did not innervate the injected areas. Following injection of CT-HRP into the right lobe of the pancreas, labeled neurons were found primarily within the medial region of the left DMV. Injection of CT-HRP into the left lobe of the pancreas resulted in retrogradely labeled neurons predominantly within the medial region of the right DMV. Following injections into the entire pancreas, neural labeling was seen bilaterally within the DMV and was concentrated within the medial regions, with a slightly higher degree of labeling within the right DMV. No labeled neurons were seen within the nucleus ambiguus or other areas of the brainstem or spinal cord following pancreatic injections. Furthermore, no afferent labeling within the nucleus of the solitary tract (NTS) was observed, although a very small number of neurons within the nodose ganglia were labeled. The dendrites of backfilled DMV neurons could be seen extending across the midline to the contralateral DMV as well as dorsally into certain subnuclei of the NTS, and to the borders of the area postrema and the fourth ventricle. These results indicate that both the motor and sensory innervation of the rat pancreas are more restricted than has been previously suggested.