A laser confocal microscopic study of vagal afferent innervation of rat aortic arch: chemoreceptors as well as baroreceptors.

Although the aortic nerves contain vagal afferents that terminate in both the wall of the aortic arch (putative baroreceptors) and its associated glomus tissue (putative chemoreceptors) in most mammalian species, the aortic nerves of the rat have been widely assumed to contain only baro- or pressor afferents. The present study reconsidered this anomaly by characterizing vagal afferent endings and their targets in the aortic arch region of the rat, both qualitatively and quantitatively. Eight Sprague-Dawley rats received intracranial vagal motor rhizotomy unilaterally to eliminate efferents in the nerve and then, two weeks later, injections of the tracer DiI (1,1'-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate) into the ipsilateral nodose ganglion. The aortic arch and its surrounding tissue, with the common carotid and subclavian arteries attached, were examined with both conventional epifluorescence and confocal microscopes. Consistent with earlier observations, vagal afferents formed both flower-spray and end-net terminals rather diffusely within the wall of the aortic arch. More interestingly, vagal afferents also innervated glomus or SIF (i.e., small intensely fluorescent) cell bodies at the junction areas of the common carotid and subclavian arteries. To identify the course of these fibers, six additional animals received DiI injection into the nodose unilaterally after a complete cervical vagotomy caudal to the nodose; in these animals, the aortic nerve had been separated from the vagal trunk and kept intact. There were no marked differences in innervation patterns between the nonvagotomized and the cervically vagotomized animals, indicating that the vagal axons innervating the walls of the blood vessels and the SIF cells in the aortic arch region travel through the aortic nerves. Using a stereological method, we estimated the relative number of chemo- and baroreceptor afferents innervating the aortic arch. About 16.4% (left) and 13.1% (right) of fibers in the aortic nerves innervate SIF cells. These findings challenge the general consensus that the aortic nerves of rats contain exclusively baroreceptor fibers.