Amiloride and vertebrate gustatory responses to NaCl

Channels as taste receptors in vertebrates

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

Salt-evoked lingual surface potential in humans

Salt sensing in animals involves the epithelial sodium channel (ENaC). If ENaC were involved in human salt sensing, then the lingual surface potential (LSP) would hyperpolarize when exposed to sodium. We developed a chamber to measure the LSP while different solutions superfused the surface of the tongue and a technique to adjust for the junction potentials induced by varying salt concentrations. Changing the superfusion solution from rinse solution (30 mM KCl) to 300 mM NaCl (+30 mM KCl) caused the LSP to hyperpolarize by 10.1 +/- 0.7 mV (n = 13, P < 0.001). With repeated challenge the LSP response was reproducible. Increasing the Na concentration from 100 to 600 mM increased hyperpolarization by 35 +/- 4.8% (n = 9, P < 0.001). To examine whether amiloride affects the LSP, 0.1 mM amiloride was added to 300 mM NaCl; it reduced the hyperpolarization by 18.5 +/- 4.3% (P < 0.005, n = 11). However, the amiloride effect was not uniform: in six volunteers, amiloride inhibited the LSP by as much as 42%, while in five subjects, amiloride inhibited <5% of the LSP. In an amiloride sensitive volunteer, amiloride exerted 50% of its effect at 1 microM. In conclusion, we have demonstrated that the LSP can be measured in humans, that Na hyperpolarizes the LSP, that increasing the Na concentration increases LSP hyperpolarization, and that amiloride inhibits the Na evoked LSP in some humans. While ENaC is involved in sensing salt, its role appears to vary among individuals.

Contribution of Drosophila DEG/ENaC genes to salt taste

The ability to detect salt is critical for the survival of terrestrial animals. Based on amiloride-dependent inhibition, the receptors that detect salt have been postulated to be DEG/ENaC channels. We found the Drosophila DEG/ENaC genes Pickpocket11 (ppk11) and Pickpocket19 (ppk19) expressed in the larval taste-sensing terminal organ and in adults on the taste bristles of the labelum, the legs, and the wing margins. When we disrupted PPK11 or PPK19 function, larvae lost their ability to discriminate low concentrations of Na(+) or K(+) from water, and the electrophysiologic responses to low salt concentrations were attenuated. In both larvae and adults, disrupting PPK11 or PPK19 affected the behavioral response to high salt concentrations. In contrast, the response of larvae to sucrose, pH 3, and several odors remained intact. These results indicate that the DEG/ENaC channels PPK11 and PPK19 play a key role in detecting Na(+) and K(+) salts.

Amiloride blocks salt taste transduction of the glossopharyngeal nerve in metamorphosed salamanders

Studies in the last two decades have shown that amiloride-sensitive Na(+) channels play a role in NaCl transduction in rat taste receptors. However, this role is not readily generalized for salt taste transduction in vertebrates, because functional expression of these channels varies across species and also in development in a species. Glossopharyngeal nerve responses to sodium and potassium salts were recorded in larval and metamorphosed salamanders and compared before and after the oral floor was exposed to amiloride, a blocker of Na(+) channels known to be responsible for epithelial ion transport. Pre-exposure to amiloride (100 microM) did not affect salt taste responses in both axolotls (Ambystoma mexicanum) and larval Ezo salamanders (Hynobius retardatus). In contrast, in metamorphosed Ezo salamanders the nerve responses to NaCl were significantly reduced by amiloride. In amphibians amiloride-sensitive components in salt taste transduction seem to develop during metamorphosis.

Effects of chlorhexidine on human taste perception

Chlorhexidine, a bis-cationic biguanide antiseptic, greatly reduces the perceived intensity of the salty prototype sodium chloride and may prove to be an important probe of mechanisms that underlie the human salty taste quality. Chlorhexidine, which tastes bitter, also reduces quinine hydrochloride taste intensity, but neither sweet sucrose nor sour citric acid is affected. Perceptual intensity rating and quality identification were measured for human subjects before and for 30 min following treatment with 1.34 mM chlorhexidine gluconate. In one experiment, test stimuli were the taste-quality prototypes; in a second experiment, stimuli were series of sodium, halide and sulfate salts. Experiment 1 showed a single 3-min chlorhexidine treatment resulted in reductions in taste intensity that persisted for at least 30 min. Experiment 2 showed a single 2-min chlorhexidine treatment reduced perceptual intensities of halide and sulfate salts except those with divalent cations. Chlorhexidine impaired identification of the salty quality and produced a bitter quality in nonbitter salts and impaired identification of the bitter quality of quinine, but not bitter salts. The specific effect of chlorhexidine on the bitterness of quinine suggests it may bind to the same receptor as quinine. The ability of chlorhexidine to specifically disrupt saltiness of a wide range of salts is consistent with proposed peripheral transduction mechanisms for the salty quality that involve transepithelial ion transport.

Linking gustatory neurobiology to behavior in vertebrates

Technological advances in neuroscience in general, and molecular biology in particular, offer tremendous experimental opportunities for researchers studying the vertebrate gustatory system. Ultimately, however, the neurobiological events must be linked to the taste-related behavior of the animal. Although there has been some promising work in this regard, progress has been hampered by an absence of a unified theoretical framework regarding function, unconfirmed assumptions inherent in many experimental designs, and a misguided predilection for researchers to interpret results from a variety of vertebrate models in the context of human psychophysics. This review article offers a heuristic for the organization of taste function and encourages greater coordination between behavioral and neurobiological approaches to the problem of understanding gustatory processes in the nervous system. The potential power of such coordinated efforts is discussed as well as the possible interpretive pitfalls associated with the neural analysis of gustation.

Amiloride increases sodium chloride taste detection threshold in rats

The epithelial sodium-channel blocker amiloride has been shown to inhibit sodium responses in the 7th cranial nerve of the rat. In the signal detection task used in this study, amiloride (100 microM) treatment raised the NaCl threshold by approximately 1 log10 unit. The inhibition constant for amiloride was 1 microM at 0.013 M NaCl. Because the NaCl intake of adult rats has been shown to be related to the level of dietary NaCl exposure early in development, rats were exposed by way of maternal diet to 1 of 3 diets (0.1% NaCl, n = 8; 1.0% NaCl, n = 8; 3.0% NaCl, n = 9) from conception through weaning, to determine whether this treatment affects taste sensitivity. At Postnatal Day 30, rats were placed on 1.0% NaCl chow. This treatment did not affect NaCl detection or amiloride sensitivity in adulthood. The amiloride-induced shifts in NaCl sensitivity functions imply that the transcellular sodium transduction pathway is necessary for normal NaCl detection in the rat.

Modification of behavioral and neural taste responses to NaCl in C57BL/6 mice: effects of NaCl exposure and DOCA treatment

To investigate the possible role of peripheral gustatory responsiveness to changes in NaCl acceptance, we studied NaCl consumption and the chorda tympani nerve responses to lingual application of NaCl in C57BL/6ByJ mice. The mice were treated with 300 mM NaCl (given to drink in 96-h two-bottle tests with water) or with injections of deoxycorticosterone acetate (DOCA; 33 mg/kg daily). Naive mice were neutral to 75 mM NaCl, but mice previously exposed to 300 mM NaCl avoided 75 mM NaCl. The NaCl-exposed (300 mM for 4 days and 75 mM for 2 days) mice had enhanced amiloride-sensitive components of the chorda tympani responses to 10-30 mM NaCl applied at room temperature (24 degrees C). DOCA injections increased acceptance of 300 mM NaCl, but did not change the chorda tympani responses to 100-1000 mM NaCl. However, the DOCA-treated mice had enhanced amiloride-sensitive components of the chorda tympani responses to cold (12 degrees C) 10-30 mM NaCl. These data suggest that peripheral gustatory responsiveness possibly contributes to the NaCl aversion induced by exposure to concentrated NaCl, but not to the DOCA-induced increase of NaCl acceptance.

NaCl-preferring NZB/B1NJ mice and NaCl-avoiding CBA/J mice have similar amiloride inhibition of chorda tympani responses to NaCl

Integrated chorda tympani nerve responses to NaCl were studied in two mouse strains, an NaCl-preferring NZB/B1NJ and an NaCl-avoiding CBA/J. The NaCl responses of both strains had similar magnitude and were suppressed by amiloride to a similar extent. This suggests that peripheral gustatory responsiveness to NaCl is not the only mechanism underlying mouse strain variation in NaCl acceptance.