COLLATERALS OF RECURRENT LARYNGEAL NERVE-FIBERS INNERVATE THE THYMUS - A FLUORESCENT TRACER AND HRP INVESTIGATION OF EFFERENT VAGAL NEURONSIN THE RAT BRAIN-STEM

The origin and course of efferent vagal fibers, which innervate the ra t thymus, were investigated by a fluorescent retrograde double labelin g method, using Fast blue (FB) and Diamidino yellow dihydrochloride (D Y) as tracers. In the same animal, one tracer was injected into the cr anial portion of the right lobe of the thymus and the other dye was de posited around the cut end of the right recurrent laryngeal nerve. The neuronal population giving origin to the recurrent nerve was mapped b y using retrograde labeling with HRP applied to the central stump of t he nerve. The HRP retrograde axonal transport showed that most efferen t vagal fibers of the recurrent nerve have their perikarya in the nucl eus retroambigualis (NRA), nucleus ambiguus (NA), and to a lesser exte nt in the nucleus retrofacialis (NRF). In fluorescent retrograde doubl e labeling of thymus and recurrent laryngeal nerve both single and dou ble labeled cells were found. The cells labeled by the injections into the thymus were colocalized with the neurons labeled by the tracer de posited in the recurrent laryngeal nerve to the NRA, NA, and NRF. More over along the rostrocaudal extent of the NRF and NA double labeled ce lls were present, showing that some of the thymic efferents are collat erals of the recurrent nerve fibers. Our experiments shown that some t hymic vagal fibres originate from neurons of nucleus dorsalis nervi va gi (NDV) as demonstrated both by HRP and FB injected thymuses. The pos sible role of these efferents in thymic function is briefly discussed. (C) 1998 Elsevier Science B.V. All rights reserved.