Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat.

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 microm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS.