Cells of origin of vagal motor neurons projecting to different parts of the stomach in the rat: confocal laser scanning and electron microscopic study

Parasympathetic motor neurons in the dorsal motor nucleus of the vagus (DMV) innervate the stomach by way of the gastric and hepatic branches of the vagus nerve. To investigate whether single neurons of the DMV provide collateral innervations to various parts of the stomach, we injected the retrograde tracer Fluoro-Gold (FG) into the cardia and the retrograde tracer cholera toxin subunit b (CTb) into the antrum or the pylorus of the same animal. Both retrogradely FG-labeled and CTb-labeled neurons were found throughout the DMV. Almost all CTb-labeled neurons (97%) were double-labeled with FG after injection of FG into the cardia and CTb into the antrum, while only a few CTb-labeled neurons (11%) were double-labeled with FG after injection of FG into the cardia and CTb into the pylorus. Thus, the cardia and the antrum received collateral projections, but the pylorus received projections mainly from different neurons in the DMV. These results indicate that different neurons in the DMV activate either the cardia or the pyloric sphincter muscles. We also labeled, retrogradely, the neurons projecting to the cardia and the pylorus in the DMV with cholera toxin-conjugated horseradish peroxidase (CT-HRP) to examine their ultrastructural characteristics. Although the neurons projecting to the cardia (21.6×15.0 µm, 248.0 µm² per section) were significantly smaller than the neurons projecting to the pylorus (27.5×15.9 µm, 323.2 µm² per section), their ultrastructural appearances were similar. Both types of neurons were small-to-medium sized, round or oval in shape, and generally had a small amount of cytoplasm containing a few Nissl bodies and a round nucleus. The average number of axosomatic terminals per section was low in the neurons projecting to the cardia (2.3) and the neurons projecting to the pylorus (3.0). Almost all axon terminals contacting these motor neurons contained round synaptic vesicles and made asymmetric synaptic contacts (Grays type I).