Transganglionic transport of HRP from the circumvallate papilla of the rat

Abstract

To learn whether horseradish peroxidase (HRP) injections in gustatory papillae on the tongue can be used to study central topographical projections of taste buds and papillae, injections were made into the circumvallate papilla in rats. Labeled central projections after papilla injections were compared to projections after applying HRP to the cut glossopharyngeal nerve. Papilla injections result in HRP transport by afferent and efferent fibers of the glossopharyngeal nerve, and the pattern of central projections is similar to that after labeling the cut nerve. Projections include a separation in the brainstem of afferent, dorsally located fibers and efferent, ventrally located fibers. Afferent fibers project to the solitary nucleus and the trigeminal system. Efferent projections label muscle motorneurons in the nucleus ambiguus and the cells of origin of parasympathetic preganglionic fibers, which form the inferior salivatory nucleus. The parasympathetic neurons labeled after papilla projections are preganglionic fibers to Remak's ganglia in the tongue; postganglionic fibers of these ganglia are the secretomotor supply to the von Ebner's glands. In summary, injections of HRP into gustatory papillae reliably label central projections of the papilla and can be used for studies to discern topography in central projections of the taste system. Injections into the circumvallate papilla also have demonstrated that the parasympathetic neurons innervating von Ebner's glands are located in the inferior salivatory nucleus.