Labeling afferent nerves in the tongue by peripheral transganglionic transport of HRP and WGA–HRP in the rat

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Abstract
Peripheral transganglionic transport of horseradish pcroxidase (HRP) and wheat germ agglutinin–horseradish peroxidase conjugate (WGA–HRP) was used to label afferent fibers in the taste buds and lingual epithelium of the rat. Microinjections of the tracer were made in the brain stem central projection area of the afferent nerves to the tongue. Optimal labeling of nerve endings in the tongue was obtained when 2 μl of 20% HRP was injected into the brain stem and postinjection survival times of 24–36 h were used. The distribution of single nerves was studied by using this tracing procedure in combination with strategic transections of the various afferent nerves supplying the tongue. Labeled nerve fibers from the combined chorda tympani–lingual nerve were found in the epithelium and in taste buds in the fungiform and anterior foliate papillae of the anterior 3/4 of the tongue. Labeled nerve fibers in the epithelium of the anterior 2/3 of the tongue but none in taste buds were found when the lingual nerve alone was studied, although numerous perigeminal fibers were found. The glossopharyngeal nerve was found to innervate die posterior 1/4 of the tongue epithelium including the taste buds of the circumvallate papillae. The glossopharyngeal nerve on one side was found to innervate the taste buds on both sides of the midline. The results show that this tracing procedure can be a useful supplement to other methods for studying afferent nerves in the tongue.