How the vital nutrients of salt and water are sensed is not fully understood. This project will investigate these two questions in humans based upon preliminary data that suggest sodium (Na+) in saliva is an important factor in the mechanisms for sensing both nutrients. This possibility first came to light via a chance finding that exposing the tongue tip to water prior to tasting NaCl interfered with perception of saltiness and increased perception of NaCl's sweet and sour side-tastes. A subsequent experiment indicated that these effects could be counteracted by adding NaCl to water in concentrations similar to Na+ in saliva. These results provide the first evidence that salivary Na+ may be important for encoding the quality of salt taste in humans and raises the possibility that rapid dilution and rinsing of salivary Na+ from the mouth serves as a gustatory signal for water. A preliminary fMRI study has supported this hypothesis by showing that adding salivary concentrations of Na+ in the form of NaCl markedly reduces the normal brain response to water, particularly in gustatory cortex. Together these data suggest that Na+ in saliva provides a steady-state gustatory signal from which positive deviations are consistent with the arrival of salt, and negative deviations signal the arrival of pure water. We will build upon the preliminary data to test this overarching hypothesis via 3 specific aims:
Aim 1 will determine if salivary Na+ facilitates encoding of NaCl saltiness rather than its side-sides tastes by measuring the taste quality and intensity of suprathreshold NaCl after exposure to pure water compared to after exposure to water containing NaCl and/or NaHCO3. KCl and KHCO3 will also be tested to rule-out osmolarity as a factor, and Na+ in unstimulated saliva will be measured (here and in Aims 2 & 3) to determine if salivary Na+ contributes to individual differences in perception of the NaCl side-tastes.
Aim 2 will investigate the peripheral mechanisms and perceptual properties of the sweet and sour side-tastes using lactisole to block activity in the sweet taste receptor TAS1R2/TAS1R3 and amiloride to block the sodium channel ENaC. Finally, Aim 3 will use fMRI to test the hypothesis that the gustatory brain response to pure water depends on rinsing salivary Na+ from the tongue by measuring the response to water with and without a wide range of NaCl concentrations that encompass salivary levels of Na+. KCl will again be tested to rule-out osmolarity as a factor, and amiloride will also be used to block ENaC to determine if the gustatory brain response to water depends upon Na+ signaling via this channel.
Understanding how humans perceive the essential nutrients of salt and water taste is necessary both for developing more palatable salt substitutes to combat the adverse health effects of hypertension, and for developing new strategies to reduce the life-threatening risks of dehydration through a better understanding of the sensory signals for water and thirst.
