The role of sodium and potassium transport pathways in taste transduction: an overview

Recent studies have shown that taste sensations are mediatedby a multiplicity of transduction mechanisms. The taste of saltis produced in part by the entry of Na⁺ through channels inthe apical taste cell membrane. Na⁺ transport also mediatessweet perception in some species. The taste of KCI requiresentry of K⁺ through apical potassium channels. The productionof second messengers such as cAMP by taste stimuli or tastemodifiers can depolarize taste cells by inducing an enzymaticcascade that alters K⁺ permeability.     

Intracellular Calcium Increase in Gerbil Taste Cell by Amino Acid Sweeteners

Gustatory transduction mechanisms for sucrose and amino acidsweeteners in gerbil taste cells were studied with Ca²⁺ imagingand whole cell recording techniques. A 100 mM sucrose stimuluswith Ca²⁺ increased the intracellular Ca²⁺ concentration ([Ca²⁺]i)in sweet-sensitive taste cells of the taste bud, but the sucrosestimulus without Ca²⁺ did not change the [Ca²⁺]i. A 10 mM D-phenylalaninesweet stimulus with or without Ca²⁺ similarly increased the[Ca²⁺]i in the taste bud. The addition of 5 µM ionomysin,a Ca²⁺-ionophore, without Ca²⁺ greatly increased the [Ca²⁺]iin the taste bud. The application of 10 mM D-phenylalanine stimuluswithout Ca²⁺ enhanced the outward K⁺ current in isolated tastecells. These results suggest that a sugar sweetener such assucrose induces a depolarization in gerbil taste cells whichactivates voltage-dependent Ca²⁺ channels and that a non-sugarsweetener such as D-phenylalanine releases Ca²⁺ from the internalstores without a depolarization. Chem. Senses 22: 83–91,1997.     

Quantification of fungiform papillae and taste pores in living human subjects

A method developed to quantify taste buds in living human subjectsto study the relationship between taste sensitivity and tastebud distribution was used to count the taste buds in 10 humansubjects; fungiform papillae were mapped in 12 subjects. Tastebuds were identified by staining taste pores with methyleneblue, and images of the papillae and their taste pores wereobtained with videomicroscopy and an image processor. Fungiformpapillae showed a 3.3-fold range in density, from 22.1 to 73.6papillae/cm² with an average of 41.1 ± 16.8/cm² (s.d.,n = 2). There was a 14-fold range in taste pore density, from36 to 511 pores/cm² among subjects, with an average of 193 ±133/cm² (s.d., n = 10). Fungiform papillae contained from 0to 22 taste pores, with an average per subject of 3.75 ±1.4 taste pores/papilla (s.d., n = 10). We hypothesize thatsome differences in human taste sensitivity may be related tothese variations in taste bud density.     

Tastants evoke cAMP signal in taste buds that is independent of calcium signaling

We previously showed that rat taste buds express several adenylyl cyclases (ACs) of which only AC8 is known to be stimulated by Ca²⁺. Here we demonstrate by direct measurements of cAMP levels that AC activity in taste buds is stimulated by treatments that elevate intracellular Ca²⁺. Specifically, 5 µM thapsigargin or 3 µM A-23187 (calcium ionophore), both of which increase intracellular Ca²⁺ concentration ([Ca²⁺]_i), lead to a significant elevation of cAMP levels. This calcium stimulation of AC activity requires extracellular Ca²⁺, suggesting that it is dependent on Ca²⁺ entry rather than release from stores. With immunofluorescence microscopy, we show that the calcium-stimulated AC8 is principally expressed in taste cells that also express phospholipase C₂ (i.e., cells that elevate [Ca²⁺]_i in response to sweet, bitter, or umami stimuli). Taste transduction for sucrose is known to result in an elevation of both cAMP and calcium in taste buds. Thus we tested whether the cAMP increase in response to sucrose is a downstream consequence of calcium elevation. Even under conditions of depletion of stored and extracellular calcium, the cAMP response to sucrose stimulation persists in taste cells. The cAMP signal in response to monosodium glutamate stimulation is similarly unperturbed by calcium depletion. Our results suggest that tastant-evoked cAMP signals are not simply a secondary consequence of calcium modulation. Instead, cAMP and released Ca²⁺ may represent independent second messenger signals downstream of taste receptors. calcium-sensitive adenylyl cyclase; capacitative entry; cross talk; taste transduction     

Evidence for a role of voltage-sensitive apical K+ channels in sour and salt taste transduction

The taste stimulus KCl is transduced in mudpuppy taste cellsby a voltage-sensitive K⁺ conductance on the apical membrane.Sour taste stimuli produce depolarizing receptor potentialsby blocking this conductance.     

Vasopressin Modulates Membrane Properties of Taste Cells Isolated from Bullfrogs

The effect of arginine vasopressin (AVP) on the membrane propertieswas analyzed in isolated bullfrog taste cells using a perforatedwhole-cell patch-clamp technique. AVP (100 nM) induced threekinds of responses in rod-type taste cells: appearance of inwardcurrent, inhibition of voltage ramp-induced outward currentand enhancement of the outward current. The Ca²⁺-ionophore ionomycin(3 µM) also induced inward current in taste cells. A membrane-permeablecAMP analog, 8-CPT-cAMP (0.3 mM) inhibited voltage ramp-inducedoutward current in some rod cells, but enhanced the currentin other rod cells. The results suggest that AVP may increaseeither intracellular Ca²⁺ level or cAMP level in taste cells,modulating the membrane excitability. Chem. Senses 21: 739–745,1996.     

Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction

Taste receptor cells (TRCs)respond to acid stimulation, initiating perception of sour taste.Paradoxically, the pH of weak acidic stimuli correlates poorly with theperception of their sourness. A fundamental issue surrounding sourtaste reception is the identity of the sour stimulus. We tested thehypothesis that acids induce sour taste perception by penetratingplasma membranes as H⁺ ions or as undissociated moleculesand decreasing the intracellular pH (pH_i) of TRCs. Our datasuggest that taste nerve responses to weak acids (acetic acid andCO₂) are independent of stimulus pH but strongly correlatewith the intracellular acidification of polarized TRCs. Taste nerveresponses to CO₂ were voltage sensitive and were blockedwith MK-417, a specific blocker of carbonic anhydrase. Strong acids(HCl) decrease pH_i in a subset of TRCs that contain apathway for H⁺ entry. Both the apical membrane and theparacellular shunt pathway restrict H⁺ entry such that alarge decrease in apical pH is translated into a relatively smallchange in TRC pH_i within the physiological range. Weconclude that a decrease in TRC pH_i is the proximate stimulus in rat sour taste transduction.

     

The Perception of Saltiness is Eliminated by NaCl Adaptation: Implications for Gustatory Transduction and Coding

The tastes of salts to humans are complex. NaCl is the mostpurely salty of all salts, but even this stimulus tastes sweetat low concentrations and somewhat sour at mid-range intensities.Other salts taste significantly sour or bitter in addition tosalty. Previous studies have shown that the saltiness of simplehalide salts is reduced by adaptation to NaCl, suggesting thata single mechanism might be responsible for the salty tasteof these stimuli. In electrophysiological studies in rodents,the response to NaCl is reduced by application to the tongueof the Na⁺- channel blocker amiloride. Organic Na⁺ salts aremore heavily dependent on this amiloride-sensitive transductioncomponent than NaCl, and are generally less salty and more sour.In order to investigate the relationship between NaCl saltinessand that evoked by other salts, we adapted the tongue to distilledH₂O and to 0.1 M NaCl and obtained direct magnitude estimatesof the taste intensity of 15 organic and inorganic Na⁺, Li⁺,K⁺ and Ca²⁺ salts, matched for total intensity. Subjects dividedthese magnitude estimates among the component taste qualities.Adaptation to NaCl abolished the taste of NaCl and LiCl, andeliminated the saltiness of all other salts. The magnitude estimatesof the bitterness and sourness of many salts increased afterNaCI adaptation. Since recent biophysical data suggest thatadaptation in taste receptors may involve whole-cell mechanisms,we propose that saltiness is reduced by NaCl adaptation becauseit originates in the subset of taste receptors responsive toNaCl. This implies that saltiness is coded within the CNS incells whose receptive fields include the NaCl-sensitive receptorcells and that the degree to which any salt tastes salty isdetermined by its ability to drive these receptors. This modelproposes, for example, that KCl has a salty component becauseit stimulates some of the same receptor cells as NaCl, eventhough the transduction mechanisms for KCl are different thanthose engaged by NaCl. Adaptation to NaCl blocks the saltinessof KCl and other salts because they stimulate NaCl-sensitivereceptor cells. Chem. Senses 20: 545–557, 1995.     

Stimulus-induced currents in isolated taste receptor cells of the larval tiger salamander

Gustatory receptor cells, isolated from the lingual epitheliumof larval tiger salamanders (Ambystoma tigrinum), possess avariety of voltage- and ion-dependent conductances, includinga transient Na⁺ -current (I_Na), a voltage-gated Ca²⁺ -current(I_A). a transient K₊ -current (I_A), a delayed rectifier K⁺ -current(I_K), and a Ca₂₊ -activated K₊ -current (I_K(Ca))- By use ofwhole-cell and excised-patch tight-seal recording techniques,we examined the effects of taste stimuli on the conductancesof taste cells from the tiger salamander. Depolarizing receptorpotentials elicited by NaCl were associated with slow, gradedinward currents which were composed of amiloride-sensitive andtetrodoxin-(TTX)-sensitive components. Potassium chloride producedmaintained inward currents, which usually showed both phasicand tonic components and were only partially blocked by tetraethylammoniumchloride (TEA). Citric and acetic acids elicited slow depolarizationsin taste cells. Under voltage-clamp, acids produced graded inwardcurrents which were composed of two components: one attributableto a transient block of voltage-dependent K₊ -channels and asmaller component which may have resulted from an increasedconductance to cations. Quinine hydrochloride elicited slowdepolarization of taste cells which was associated with a slowlydeveloping maintained inward current under voltage-clamp. Quininesuppressed both voltage-dependent inward and outward currents.In some taste cells, L-arginine elicited small outward currentswhich were attributable to an increase in K₊ conductance. Inother cells, L-arginine produced a decrease in voltage-dependentoutward currents and generated depolarizations associated withinward currents. These results indicate that several independentmechanisms, including amiloride-sensitive Na₊ -channels, andstimulus modulation of voltage-dependent K₊ -channels, are involvedin taste cell responses to chemical stimuli. More than one mechanismis typically present in a single cell. ³Present address: Department of Physiology, Tokyo Medical andDental University, 5-45 Yushima 1-chome, Bunkyo-ku, Tokyo 113,Japan     

Salivary ions and neural taste responses in the hamster

Saliva is a chemically complex fluid that bathes oral surfacesand may affect early events in mammalian gustation. We measuredchorda tympani responses to taste stimuli in hamsters (Mesocricetusauratus) while their tongues were adapted to either water, artificialsaliva or natural saliva. Artificial saliva on the tongue loweredneural responses to taste stimuli that were present in the artificialsaliva and to those stimuli that cross-adated with salivarycomponents. Changing from a water-adapted tongue to one soakedwith pilocarpine-stimulated saliva from donor hamsters led tosignificantly smaller responses to NaCl. Responses to sucrose,NH₄Cl and quinine were unaffected. Chemical analysis of hamstersaliva revealed ‘normal’ mammalian levels of K⁺,Ca²⁺ and Mg²⁺, but unexpectedly low levels of Na⁺ and Cl^–.     

Taste responses to divalent cations in the frog glossopharyngeal nerve: competitive inhibition of the Mg2+ response by Ca2+

In the frog glossopharyngeal nerve, single water fibers respondto low CaCl₂ (1–2 mM) and relatively high MgCl₂ (100 mM).In the present study, it was found that stimulation by a mixtureof low CaCl₂ and relatively high MgCl₂ led to a small response.This suggests that the Ca⁺ response is inhibited by the presenceof Mg²⁺ and the Mg²⁺ response is inhibited by the presence ofCa²⁺. Hence, it is suggested that there are different receptorsites for divalent cations in single water fibers of the frogglossopharyngeal nerve, a calcium receptor site (X_Ca) responsiblefor the Ca²⁺ response and a magnesium receptor site (X_Mg) responsiblefor the Mg²⁺ response. It has been reported that Mg²⁺ inhibitsthe Ca²⁺ response by competing with Ca²⁺ for X_Ca (Kitada andShimada, 1980). In the present study, the inhibition of theMg²⁺ response by Ca²⁺ was examined quantitatively under theassumption that the magnitude of the neural response is proportionalto the amount of MgX_Mg complex minus a constant (the thresholdconcentration of the MgX_Mg complex). The results obtained indicatethat Ca²⁺ competes with Mg²⁺ for X^Mg. The apparent dissociationconstants for MgX_Mg complex and CaX_Mg complex, which were obtainedfrom the present study, were 8.0 x 10^–2 M and 7.2 x 10^–4M, respectively. Thus, competition between Ca⁺ and Mg²⁺ forthe distinct receptor sites involved in taste reception wasdemonstrated by the results described in this paper. Since thedivalent cations do not always bring about activation of tastereceptors, the responses to salts in the frog glossopharyngealnerve cannot be explained in terms of changes in the surfacepotential outside the taste cells. The present results suggestthat there exist multiple specific receptor sites for cationsinvolved in salt taste responses, and only the binding of eachseparate cation to its appropriate receptor sites leads to activationof the receptor and the initiation of impulses in sensory nerveendings.     

Effects of pharmacologic reductions in salivary flow on judgments of suprathreshold taste stimuli

The effects of short term salivary flow reductions on suprathresholdjudgments of taste intensity were measured using both a seriesof taste solutions and a comparable series of tastants driedon a filter paper base. Decreases in salivary flow were producedby the oral administration of either Elavil^®, Benadryl^®or atropine. The pharmacologic agents produced a 25 –82% reduction in salivary flow during the period that tastetesting occurred but no measurable effects on perceived tasteintensity were found. The exponents of power functions describingmolar concentration and perceived intensity were unchanged aswas the spread in perceived intensity between aqueous and drystimuli. Significant relationships between individual differencesin salivary flow and suprathreshold taste parameters also failedto emerge.     

ON THE GUSTATORY EFFECTS OF GYMNEMIC ACID AND MIRACULIN IN DOG, PIG AND RABBIT

The effects of gymnemic acid, 1 mg/ml, and miraculin, 1 mg/mlor in a tablet (Miralin^TM) on the taste response to sucrose,citric acid, NaCl and quinine have been studied in dog, pigand rabbit. In the dog gymnemic acid suppressed the responseto sucrose for about 5 min in two out of three animals. It alsoaffected the response to NaCl. Miraculin caused a slight short-lastingincrease in the response to acid. No effects of the taste modifierswere observed in the pig. In the rabbit gymnemic acid did notsuppress the response to sucrose while miraculin enhanced theresponse to citric acid in one out of three animals for a fewminutes.     

Amiloride Effects on Taste Quality: Comparison of Single and Multiple Response Category Procedures

Although there is compelling evidence that amiloride reducesthe intensity of Na⁺ and Li⁺ salts in humans, its effects onsaltiness are conflicting. Many salts elicit not only a saltytaste but also one or more side tastes (sweetness, sournessor bitterness). Some studies have shown a suppression of saltinessby amiloride; others show no effect on saltiness but a significantreduction in sourness. In the experiments demonstrating a reductionof saltiness, subjects estimated only saltiness; in those showingan amiloride effect on sourness and not saltiness, subjectsestimated all qualities on each trial. The present study examinesthe role of the psychophysical method in these conflicting results.We have investigated the effects of amiloride on taste qualityby modifying only the instructions to the subjects, keepingall other variables constant. One group of subjects (intensity-only)gave magnitude estimates of the overall intensity of a LiCIconcentration series. A second group (salty-only) was instructedto estimate the saltiness of the stimuli, and a third group(sour-only) estimated their sourness. Finally, a fourth group(profile) rated all of the taste qualities on each stimuluspresentation, using a modified taste profile method. The ratingsof all groups were made comparable by the use of 0.1 mM quinine-HCIas a modulus. When subjects used only one response category,amiloride reduced their estimates (of intensity, saltiness orsourness), but if subjects attended to all four qualities, amiloridespecifically reduced the sourness of LJCI and had no significanteffect on its saltiness. Comparison of the saltiness estimatesof the salty-only group to the sum of the salty and sour estimatesof the profile group demonstrated that subjects combined thesesensations when presented with only one response alternative.To reveal the effect of amiloride on a specific quality of asalt, the psychophysical method must allow subjects to attendto all qualities on each trial. These data and previous resultssuggest that apical Na⁺ channels on the taste receptor cellmembrane mediate the sourness but not the saltiness of Na⁺ andLi⁺ salts. Chem. Senses 22: 267–275, 1997.     

An intensity/time study of the taste of amino acids

An intensity/time study of the taste of selected amino acidswas carried out. Intensity, persistence and total gustatoryresponse were assessed at five concentrations. Ten amino acidswere assessed for sweetness and eleven amino acids were assessedfor bitterness, four amino acids being assessed for both sweetnessand bitterness. Both a linear function and a power function,I = Kcⁿ (where I is taste intensity, c is concentration, K isa constant and n is the exponent of taste intensity), were fittedto the data. The accession efficiencies for taste recognitionand taste detection were found. Kinetic equations were usedto find K_m, the affinity of the receptor site for the sapidmolecule. Limited relationships between chemical structure ofthe amino acids and their temporal properties were found.     

Taste quality changes as a function of salt concentration in single human taste papillae

A series of experiments were performed to determine whetherconcentration-dependent taste quality changes occur in simplesalt solutions when presentation of these solutions is restrictedto single fungiform taste papillae. Preliminary experiments,using small area, dorsal tongue stimulation, revealed the presenceof a sour-salty confusion in response to NaCl and HCl stimulation.This confusion was found to be greater at higher concentrationsand was affected by a pre-rinse. Taste quality changes as afunction of solution concentration for NaCl, KCl, and LiCl stimulationof single papillae were found to parallel those found previouslywith whole-mouth stimulation, although the sour component wasgreater at high concentrations, reflecting the effect of thepreviously identified sour-salty confusion. The data are discussedwithin the context of Dzendolet's (1968) physicochemical theoryof taste quality changes in salts. ^*This research was conducted as part of a thesis submitted inpartial fulfilment of the requirements for the degree of Doctorof Philosophy at the University of Massachusetts/Amherst. ^{* *}Current address: Behavioral Sciences Division, Food SciencesLaboratory, US Army Natick Research and Development Command,Natick, MA 01760, USA     

Genetic Sensitivity to 6-n-Propylthiouracil (PROP) and Hedonic Responses to Bitter and Sweet Tastes

Genetically mediated sensitivity to the bitter taste of 6-n-propylthiouracil(PROP) has been associated with greater acuity for bitter andfor some sweet tastes. Thus far, few studies have explored therelationship between PROP taste sensitivity and hedonic responsesto bitter and sweet. In this study, 87 normal-weight young womenwere divided into PROP non-tasters (n = 18), regular tasters(n = 49), and supertasters (n = 20), based on their PROP detectionthresholds and the scaling of five suprathreshold solutionsof PROP and NaCl. Non-tasters had thresholds >1.8 x 10^–4mol/l PROP. Supertasters had thresholds <3.2 x 10^–5mol/l PROP and PROP/NaCl ratios >1.70. As expected, dislikeof the bitter taste of PROP was determined by its perceivedintensity, which was greater among supertasters than among regulartasters or non-tasters. Significant correlations were observedbetween PROP taste thresholds and the sum of intensity ratings(r = –0.61) and between summed intensity and summed hedonicratings (r = –0.80). PROP taste sensitivity was weaklylinked to enhanced perception of sweet taste, but did not predicthedonic responses to sucrose or to saccharin solutions. Giventhat the dislike of PROP solutions is determined by their perceivedintensity, hedonic responses to PROP solutions may provide arapid way of screening for PROP taster status. Chem. Senses22: 27–37, 1997.     

Bimodal (Taste/Tactile) Fibers Innervate the Maxillary Barbel in the Channel Catfish

Analysis of single fibers isolated from a branch of the facial/trigeminalcomplex innervating the maxillary barbel of the channel catfish,Ictalurus punctatus, indicated the existence of bimodal (taste/tactile)fibers. Of the 60 single fibers recorded, 14 (23%) respondedto both taste (amino acid) and tactile stimulation, 43 (72%)were responsive to only tactile stimulation and three (5%) respondedonly to taste stimulation. Quinine hydrochloride at a concentrationof 1.0 mM suppressed the mechanosensory activity of the bimodalfibers, but had no effect on the tactile-only fibers. Chem.Senses 22: 477–482, 1997. ¹Current address: Department of Otolaryngology, Kagoshima UniversityMedical School, 8-35-1 Sakuragaoka, Kagoshima 890, Japan ²Current address: Department of Oral Physiology, Ohu UniversitySchool of Detistry, 31-1 Misumido, Tomita, Koriyama, Fukushima963, Japan     

Changes in Outward K+ Currents in Response to Two Types of Sweeteners in Sweet Taste Transduction of Gerbil Taste Cells

Using the whole cell patch clamp technique, we measured changesin outward K⁺ currents of gerbil taste cells in response todifferent kinds of sweeteners. Outward K⁺ currents of the tastecell induced by depolarizing pulses were suppressed by sweetstimuli such as 10 mM Na-saccharin. The membrane-permeable analogof cAMP, cpt-cAMP, also decreased outward K⁺ currents. On theother hand, the K+ currents were enhanced by amino acid sweetenerssuch as 10 mM D-tryptophan. The outward K⁺ current was enhancedby external application of Ca²⁺-transporting ionophore, 5 µMionomycin, and intracellular application of 5 µM inositol-1,4,5-trisphosphate(IP₃). The outward K⁺ currents were no longer suppressed by10 mM Na-saccharin containing 20 µM gurmarin, but werestill enhanced by 10 mM D-tryptophan containing 20 µMgurmarin. These results suggest that sweet taste transductionfor one group of sweeteners such as Na-saccharin in gerbilsis concerned with an increase of the intracellular cAMP level,and that the transduction for the other group of sweetenerssuch as D-tryptophan is concerned with an increase of the intracellularIP₃ level which releases Ca²⁺ from the internal stores. Chem.Senses 22: 163–169, 1997.     

INFLUENCE OF COLOR ON TASTE THRESHOLDS

Increasing molar concentrations of sweet, sour, bitter and saltywere evaluated in colorless and colored (red, green, yellow)water solutions by 28 untrained students. Green color statisticallyincreased sweet taste threshold sensitivity while yellow colordecreased taste sensitivity. Red color did not affect the tastesensitivity of sweet. In the case of sour, both yellow and greencolors decreased sensitivity with red having no affect. Redcolor decreased bitter taste sensitivity with yellow and greencolor having no effect. No significant differences due to coloraffected salty taste sensitivity. Thus, psychological colorassociation can alter reports of certain basic taste sensations. ^*Scientific Series Paper Number 1764 of the Colorado State UniversityExperiment Station.