Uptake sites of horseradish peroxidase after injection into peritoneal structures: defining some pitfalls

Following injection of horseradish peroxidase (HRP) either into the jejunal wall or the peritoneal cavity, neurons in the dorsal motor nucleus of the vagus nerve, celiac, nodose and spinal ganglia, and ventral and lateral horns of the spinal cord from the mid-thoracic to lumbar segments were labeled. When HRP was injected into the wall of the exteriorized gut, neurons of the spinal cord were not labeled. Furthermore, there was a significant decrease in the number of labeled neurons in the dorsal motor nucleus and ganglia examined. These results indicate that HRP injected into the intestinal wall could leak into the peritoneal cavity and be taken up and transported to neuronal cell bodies by nerve fibers not terminating in the injection area. The leakage of HRP to nearby abdominal structures and its subsequent uptake by nerve fibers is attributed to the lack of a diffusion barrier across the surfaces of the intestinal wall and the abdominal structures. It is suggested that in applying the HRP techniques for the study of neuronal connections in the peripheral nervous system, it is essential that carefully pla-ned control experiments be undertaken which can overcome the problem of mislabeling due to diffusion from the injection site.