Mechanisms for Taste Sensation of Carbonation

Carbonation, or the presence of carbon dioxide (CO2) dissolved in solution, is a commonly encountered feature of beverages in the contemporary human diet. While the popularity of carbonation may be attributed to its distinct sensory qualities, the specific orosensory pathways mediating CO2 detection in mammals have not previously been delineated. This dissertation describes the identification of specific cellular and molecular mechanisms that mediate taste sensation of carbonation, using the mouse as a model system. The mammalian gustatory system is sensitive to CO2, and these responses are sensitive to inhibition of carbonic anhydrases, enzymes that catalyze the interconversion of carbon dioxide with carbonic acid. Through gene expression profiling I discovered that the gene carbonic anhydrase IV (Car4), encoding an extracellular enzyme, is specifically expressed in acid sensing taste receptor cells (TRCs). Genetic ablation of the Car4 locus resulted in a major deficit in gustatory CO2 sensation that is stimulus specific, not affecting responses to acid. Ablation or silencing of acid sensing TRCs likewise produced a profound deficit in taste responses to CO2. These studies identified a primary pathway of the gustatory carbonation response, substantiating acid sensing TRC and the Car4 enzyme as key mediators. A smaller gustatory neural response to carbonation remains even in the absence of sour-sensing TRC and/or Car4. To identify additional carbonation sensing pathways, I applied an in vivo calcium-imaging assay to define the ensemble of primary gustatory neurons activated by CO2. These studies revealed that in addition to robust activation of sour sensing neurons, a secondary gustatory pathway for CO2 detection is mediated by subpopulations of bitter and sweet responsive neurons. I identified carbonic anhydrase VII (Car7) as an intracellular carbonic anhydrase specifically expressed by sweet, bitter and umami sensing TRC. Pharmacological and gene expression data support a role for Car7 in transducing the secondary CO2 sensing pathway. These studies suggested that carbonation acts as a complex gustatory stimulus, stimulating sour, sweet and bitter taste qualities simultaneously. The rules governing peripheral encoding of multi-modal taste stimuli are not well understood. To address this issue, I examined the peripheral gustatory response to binary mixtures of taste qualities. I found that most combinations of taste qualities are represented as a superimposition of the component responses. However, neural responses to attractive stimuli, including natural sugars, artificial sweeteners and umami tastants, are selectively suppressed by simultaneous co-stimulation with a sour (acidic) stimulus. Acid-mediated suppression of sweet is cell autonomous, occurring even in the absence of gustatory acid sensing. Remarkably, carbonation stimulates sour signaling without suppressing sweet taste response. These studies suggest that cross-modal interactions at the periphery modulate the sensory response to complex taste stimuli.