Unilateral innervation of guinea pig vallate taste buds as determined by glossopharyngeal neurectomy and HRP neural tracing.

The innervation pattern by primary afferent nerve fibres and the neurotrophic effect on taste cells were investigated in the guinea pig vallate taste bud by means of glossopharyngeal neurectomy and horseradish peroxidase (HRP) or wheat germ agglutinin-horseradish peroxidase (WGA-HRP) tracing. In the glossopharyngeal neurectomy study, taste buds in the vallate papillae of adult guinea pigs were denervated by unilateral resection of the right glossopharyngeal nerve. Denervated animals were killed on days 1, 3 and 5 and weeks 1-9, 12 and 24 postneurectomy. The results showed that, on the denervated side, the taste buds decreased significantly in number during the 1st 2 wk, and disappeared completely by wk 3; no mature taste buds were present even 24 wk after neurectomy. This suggests that the vallate taste buds disappear in the absence of the glossopharyngeal nerve. In the neural tracing study, HRP or WGA-HRP was injected into the proximal end of the right glossopharyngeal nerve, near the jugular foramen. After a survival time of 24 h, the vallate papillae were sectioned and examined by light and electron microscopy. Light microscopy revealed that the HRP or WGA-HRP-labelled fibres innervated the vallate taste buds of the injected side. Most of the taste cells in the buds were labelled with HRP or WGA-HRP reaction products from the basal to the apical region. At the ultrastructural level, the reaction products were confined to the cytoplasm of the labelled cells, which were identified as type I, II and III cells, but not basal cells. Labelled intragemmal nerve profiles were seen among the taste cells. No synapse formation was seen with nerve profiles abutting on type I and II cells, whereas on certain type III cells, typical synapses were formed. We conclude that both the right and left vallate papilla in the guinea pig are unilaterally innervated by the glossopharyngeal nerve without cross-innervation.