Ultrastructural demonstration of a gastric monosynaptic vagal circuit in the nucleus of the solitary tract in rat.

Previous light microscopic work in the rat has demonstrated that many dendrites of vagal gastric motoneurons extend beyond the cytoarchitectural boundaries of the dorsal motor nucleus of the vagus (DMV) into the nucleus of the solitary tract (NST), where they overlap with the central terminal field of vagal gastric primary sensory neurons (Shapiro and Miselis, 1985a). In the present ultrastructural study, the synaptic relationship of vagal afferent terminals and vagal motor neurons and dendrites was examined. Following injection of cholera toxin-HRP conjugate (CT-HRP) into the dorsal and ventral musculature of the stomach wall, centrally transported tracer was visualized using tetramethylbenzidine or diaminobenzidine histochemistry. Light microscopy confirmed the presence of retrogradely labeled gastric DMV neurons whose dendrites extended into the overlying NST. The subnucleus gelatinosus of the dorsomedial NST at a level just rostral to the obex contained the highest density of vagal motoneuronal dendrites coextensive with transganglionically labeled vagal sensory afferents. Ultrastructural analysis of several NST subfields at this rostrocaudal level revealed many asymmetric synaptic contacts between labeled sensory afferents and labeled motoneuronal dendrites in the subnucleus gelatinosus but not elsewhere. Following CT-HRP injection into the cervical vagus nerve in separate animals, vagovagal synaptic contacts were again observed only in the subnucleus gelatinosus, despite extensive afferent labeling in other NST subnuclei. The demonstration of a monosynaptic gastric vagovagal circuit provides the anatomical substrate for the relay of gastric interoceptive information directly to gastric vagal motoneurons, which may play a unique role in the central regulation of gastric motility and secretion.