Immunolocalization of vesicular glutamate transporter 2 and exocytosis-related proteins in afferent nerve endings innervating taste buds in the rat incisive papilla.

The present study aimed to investigate the immunolocalization of vesicular glutamate transporter (VGLUT) 1 and 2, and proteins associated with exocytosis, i.e., core components of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex (synaptosomal-associated protein of 25 kDa, syntaxin 1, and vesicle-associated membrane protein 2) and synaptotagmin-1 (Syt1), in incisive papillary taste buds of rats using double-indirect immunofluorescence. No VGLUT1 immunoreactivity was observed, whereas VGLUT2-immunoreactive punctate products were closely associated with guanine nucleotide-binding protein G(t) subunit α3-immmunoreactive cells in taste buds. VGLUT2 was immunolocalized in P2X3 purinoceptor-expressing afferent nerve endings. Synaptosomal-associated protein of 25 kDa, syntaxin 1, and vesicle-associated membrane protein 2 were immunolocalized in nerve endings containing VGLUT2-immunoreactive products as well as a few cells in taste buds. VGLUT2 was co-immunolocalized in some intragemmal nerve endings immunoreactive for Syt1, a calcium sensor implicated in vesicle membrane fusion. The present results suggest that afferent nerve endings innervating incisive taste buds release glutamate by exocytosis to modulate taste cell function.

Morphology and chemical characteristics of taste buds associated with P2X3-immunoreactive afferent nerve endings in the rat incisive papilla.

The present study investigated the cellular components and afferent innervations of taste buds in the rat incisive papilla by immunohistochemistry using confocal scanning laser microscopy. Taste buds containing guanine nucleotide-binding protein G(t), subunit α3 (GNAT3)-imunoreactive cells were densely distributed in the lateral wall of incisive papilla forming the opening of nasoincisor ducts. GNAT3-immunoreactive cells in the taste buds were slender in shape and the tips of apical processes gathered at one point at the surface of the epithelium. The number of taste buds was 56.8 ± 4.5 in the incisive papilla. The incisive taste buds also contained ectonucleoside triphosphate diphosphohydrolase 2-immunoreactive cells and synaptotagmin-1-immunoreactive cells in addition to GNAT3-immunoreactive cells. Furthermore, GNAT3-immunoreactive cells were immunoreactive to taste transduction molecules such as phospholipase C, ß2-subunit, and inositol 1,4,5-trisphosphate receptor, type 3. P2X3-immunoreactive subepithelial nerve fibers intruded into the taste buds and terminated with hederiform or calix-like nerve endings attached to GNAT3-immunoreactive cells and synaptosomal-associated protein, 25 kDa-immunoreactive cells. Some P2X3-immunoreactive endings were also weakly immunoreactive for P2X2. Furthermore, a retrograde tracing method using fast blue dye indicated that most of the P2X3-immunoreactive nerve endings originated from the geniculate ganglia (GG) of the facial nerve. These results suggest that incisive taste buds are morphologically and cellularly homologous to lingual taste buds and are innervated by P2X3-immunoreactive nerve endings derived from the GG. The incisive papilla may be the palatal taste papilla that transmits chemosensory information in the oral cavity to the GG via P2X3-immunoreactive afferent nerve endings.