Gα-gustducin Is Extensively Coexpressed with Sweet and Bitter Taste Receptors in both the Soft Palate and Fungiform Papillae but Has a Different Functional Significance

To clarify the regional differences in the expression and functional significance of G alpha-gustducin in soft palate (SP) and fungiform (FF) taste buds, we examined the coexpression of G alpha-gustducin with taste receptors and the impact of G alpha-gustducin knockout (gKO) on neural responses to several sweet and bitter compounds. Sweet responses from both the greater superficial petrosal (GSP) and chorda tympani (CT) nerves in gKO mice were markedly depleted, reflecting overlapping expression of G alpha-gustducin and Tas1r2. However, although G alpha-gustducin was expressed in 87% and 88% of Tas2rs cells in the SP and FF, respectively, there were no statistically significant differences in the CT responses to quinine-HCl (QHCl) and denatonium (Den) between gKO and wild-type (WT) mice. In contrast, GSP responses to these compounds were markedly reduced in gKO mice with an apparent elevation of thresholds (> 10-fold). These results suggest that 1) G alpha-gustducin plays a critical role in sweet transduction in both the SP and the FF, 2) other G alpha subunits coexpressed with G alpha-gustducin in the FF are sufficient for responses to QHCl and Den, and 3) robust GSP responses to QHCl and Den occur in the SP by a G alpha-gustducin-dependent mechanism, which is absent in the FF.