Genetically-increased taste cell population with G -gustducin-coupled sweet receptors is associated with increase of gurmarin-sensitive taste nerve fibers in mice

Background: The peptide gurmarin is a selective sweet response inhibitor for rodents. In mice, gurmarin sensitivity differs among strains with gurmarin-sensitive C57BL and gurmarin-poorly-sensitive BALB strains. In C57BL mice, sweet-responsive fibers of the chorda tympani (CT) nerve can be divided into two distinct populations, gurmarin-sensitive (GS) and gurmarin-insensitive (GI) types, suggesting the existence of two distinct reception pathways for sweet taste responses. By using the dpa congenic strain (dpa CG) whose genetic background is identical to BALB except that the gene(s) controlling gurmarin sensitivity are derived from C57BL, we previously found that genetically-elevated gurmarin sensitivity in dpa CG mice, confirmed by using behavioral response and whole CT nerve response analyses, was linked to a greater taste cell population co-expressing sweet taste receptors and a G alpha protein, G alpha-gustducin. However, the formation of neural pathways from the increased taste cell population to nerve fibers has not yet been examined. Results: Here, we investigated whether the increased taste cell population with G alpha-gustducin-coupled sweet receptors would be associated with selective increment of GS fiber population or nonselective shift of gurmarin sensitivities of overall sweet-responsive fibers by examining the classification of GS and GI fiber types in dpa CG and BALB mice. The results indicated that dpa CG, like C57BL, possess two distinct populations of GS and GI types of sweet-responsive fibers with almost identical sizes (dpa CG: 13 GS and 16 GI fibers; C57BL: 16 GS and 14 GI fibers). In contrast, BALB has only 3 GS fibers but 18 GI fibers. These data indicate a marked increase of the GS population in dpa CG. Conclusion: These results suggest that the increased cell population expressing T1r2/T1r3/G alpha-gustducin in dpa CG mice may be associated with an increase of their matched GS type fibers, and may form the distinct GS sweet reception pathway in mice. G alpha-gustducin may be involved in the GS sweet reception pathway and may be a key molecule for links between sweet taste receptors and cell type-specific-innervation by their matched fiber class.