Interactions between Salt and Acid Stimuli

If for no other reason than the fact that eating and drinking gives us pleasure, it is of interest to understand the physiology of gustation. In this issue, Lyall et al. (2002a) have uncovered the key cellular mechanisms that occur in taste receptor cells (TRCs) when certain mixtures of chemicals (tastants) are applied to the anterior tongue. In much the same way as music produced by a quartet differs from that produced by each instrument, compared with the sensations produced by individual tastants, in mixtures they can produce an entirely different taste sensation, increase the intensity of one of the tastants, or, as shown in the work of Lyall et al. (2002a), suppress the response to one of the tastants. One question frequently asked in gustatory circles is where the interaction between the different tastants first takes place. Is it at the periphery, in taste receptor cells and/or in higher CNS centers? Lyall and colleagues have shown that, in agreement with psycho-physical studies, the neural responses to NaCl are reduced in the presence of acidic stimuli. Although previous nerve recordings have shown that neural responses to NaCl are inhibited at low pHs (Biedler, 1954; Ogawa, 1969), until this present study, the mechanisms underlying this effect have not been delineated. Lyall et al. (2002a) showed that these responses could be understood at the level of TRCs, where a decrease in intracel-lular pH reduces Na ϩ influx through amiloride-sensitive epithelial sodium channels (ENaCs). This reduction in Na ϩ influx results in a decreased neural response and ultimately in a decreased sensation to NaCl. One can appreciate the practicality of this result if one accidentally pours too much salt on a steak. The solution to reduce the salty taste is to add acid to the steak (I am not saying it will be palatable, just less salty). In this commentary, I will review some of the basic anatomy and physiology of the peripheral gustatory system and then show how it relates to measurements of the epithelial properties of the tongue, and simultaneous recordings from primary gustatory neurons. Fig. 1 shows a taste bud embedded in a stratified epi-thelium. Taste buds are comprised of ‫ف‬ 50 –100 neu-roepithelial cells, the taste receptor cells that extend from the taste pore, which is in direct contact with the tastants in the mouth, to the basement membrane that separates the epithelium from the papillary layer. Tight junctions …