Neurons in the caudal ventrolateral medulla projecting to the paraventricular hypothalamic nucleus receive synaptic inputs from the nucleus of the solitary tract: a light and electron microscopic double-labeling study in the rat.

We used light and electron microscopic techniques to investigate the possibility that neurons in the nucleus of the solitary tract (NST) might send projection fibers to neurons in the caudal ventrolateral medulla (CVLM) projecting to the paraventricular hypothalamic nucleus (PVN) by the anterograde and retrograde double labeling method in the rat. The retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex, was injected into the PVN, and the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was injected into the NST of each rat. Many retrogradely labeled neurons were encountered in the CVLM, including the A1 region. On the other hand, a dense network of anterogradely labeled terminals was found in the CVLM. Electron microscopic examination revealed synaptic contacts between PHA-L-immunoreactive nerve terminals and dendrites of retrogradely labeled neurons in the CVLM. The results indicate that CVLM neurons projecting to the PVN receive axon terminals of NST neurons.