Role of the G-Protein Subunit α-Gustducin in Taste Cell Responses to Bitter Stimuli

Many bitter stimuli are believed to bind to specific G-protein-coupled membrane receptors on taste cells. Despite the compelling evidence for its pivotal role in bitter taste sensation, a direct involvement of the G-protein subunit α-gustducin in bitter taste transduction in taste cells has not been demonstrated in situ at the cellular level. We recorded activation of taste cells by bitter stimuli using Ca²⁺ imaging in lingual slices and examined α-gustducin immunoreactivity in the same cells. In mice vallate papillae, many, but not all, bitter-responsive cells expressed α-gustducin. In agreement with this correlation, the incidence of cells responding to bitter stimuli was reduced by 70% in mutant mice lacking α-gustducin. Nevertheless, some taste cells lacking α-gustducin responded to bitter stimuli, suggesting that other G-protein α subunits are involved. We found that the G-protein α subunit Gα_i2 is present in most bitter-responsive cells and thus may also play a role in bitter taste transduction. The reduced behavioral sensitivity to bitter stimuli in α-gustducin knock-out mice thus appears to be the consequence of a reduced number of bitter-activated taste cells, as well as reduced sensitivity.