Interaction between parasympathetic and sympathetic nerves in prevertebral ganglia: morphological evidence for vagal efferent innervation of ganglion cells in the rat.

Vagal efferent preganglionic neurons were anterogradely labeled by injecting either DiI or DiA, fluorescent lipophilic carbocyanine dyes, into the dorsal motor nucleus of the vagus of the rat. All neurons of the peripheral nervous system (outside the blood-brain barrier) were then fluorescently counterstained in vivo by injecting Fluorogold (Fluorochrome, Inc., Englewood, CO) intraperitoneally. The upper abdominal prevertebral ganglia, including the numerous microganglia associated with the periarterial plexuses of the celiac and superior mesenteric arteries, were identified and dissected in formalin-fixed tissue under ultraviolet light and stereomicroscopic guidance. In 14 of 15 animals analyzed (93%), labeled vagal efferent fibers were found to penetrate into both the left and right celiac ganglia and the superior mesenteric ganglion, as well as into some of the associated microganglia. These projections formed varicose terminal-like structures, highly suggestive of synaptic contacts surrounding individual ganglion cells. In about half the animals, such vagal innervation was also seen in the left and right suprarenal ganglia. The specificity of vagal efferent labeling was confirmed by control experiments, which included injections in vagotomized animals and direct selective labeling of vagal afferents from the nodose ganglia. It is concluded that vagal efferent preganglionics innervate principal ganglion cells of prevertebral ganglia. These vagal contacts may either directly modulate the postganglionic outflow or else gate some or all of the potential modulatory inputs to these postganglionic neurons, thus allowing the vagal system to exert a more selective influence on sympathetic outflow. Finally, the use of laser scanning confocal microscopy and the in toto Fluorogold staining method for investigations of the peripheral nervous system are discussed.