In situ tight-seal recordings of taste substance-elicited action currents and voltage-gated Ba currents from single taste bud cells in the peeled epithelium of mouse tongue

Characteristics of A-type voltage-gated K+ currents expressed on sour-sensing type III taste receptor cells in mice

Sour taste is detected by type III taste receptor cells that generate membrane depolarization with action potentials in response to HCl applied to the apical membranes. The shape of action potentials in type III cells exhibits larger afterhyperpolarization due to activation of transient A-type voltage-gated K+ currents. Although action potentials play an important role in neurotransmitter release, the electrophysiological features of A-type K+ currents in taste buds remain unclear. Here, we examined the electrophysiological properties of A-type K+ currents in mouse fungiform taste bud cells using in-situ whole-cell patch clamping. Type III cells were identified with SNAP-25 immunoreactivity and/or electrophysiological features of voltage-gated currents. Type III cells expressed A-type K+ currents which were completely inhibited by 10 mM TEA, whereas IP3R3-immunoreactive type II cells did not. The half-maximal activation and steady-state inactivation of A-type K+ currents were 17.9 ± 4.5 (n = 17) and - 11.0 ± 5.7 (n = 17) mV, respectively, which are similar to the features of Kv3.3 and Kv3.4 channels (transient and high voltage-activated K+ channels). The recovery from inactivation was well fitted with a double exponential equation; the fast and slow time constants were 6.4 ± 0.6 ms and 0.76 ± 0.26 s (n = 6), respectively. RT-PCR experiments suggest that Kv3.3 and Kv3.4 mRNAs were detected at the taste bud level, but not at single-cell levels. As the phosphorylation of Kv3.3 and Kv3.4 channels generally leads to the modulation of cell excitability, neuromodulator-mediated A-type K+ channel phosphorylation likely affects the signal transduction of taste.

Effects of temperature on action potentials and ion conductances in type II taste-bud cells

Temperature strongly influences the intensity of taste, but it remains understudied despite its physiological, hedonic, and commercial implications. The relative roles of the peripheral gustatory and somatosensory systems innervating the oral cavity in mediating thermal effects on taste sensation and perception are poorly understood. Type II taste-bud cells, responsible for sensing sweet, bitter umami, and appetitive NaCl, release neurotransmitters to gustatory neurons by the generation of action potentials, but the effects of temperature on action potentials and the underlying voltage-gated conductances are unknown. Here, we used patch-clamp electrophysiology to explore the effects of temperature on acutely isolated type II taste-bud cell electrical excitability and whole cell conductances. Our data reveal that temperature strongly affects action potential generation, properties, and frequency and suggest that thermal sensitivities of underlying voltage-gated Na+ and K+ channel conductances provide a mechanism for how and whether voltage-gated Na+ and K+ channels in the peripheral gustatory system contribute to the influence of temperature on taste sensitivity and perception.NEW & NOTEWORTHY The temperature of food affects how it tastes. Nevertheless, the mechanisms involved are not well understood, particularly whether the physiology of taste-bud cells in the mouth is involved. Here we show that the electrical activity of type II taste-bud cells that sense sweet, bitter, and umami substances is strongly influenced by temperature. These results suggest a mechanism for the influence of temperature on the intensity of taste perception that resides in taste buds themselves.

Changes of the biophysical properties of voltage-gated Na+ currents during maturation of the sodium-taste cells in rat fungiform papillae

Taste cells are a heterogeneous population of sensory receptors that undergo continuous turnover. Different chemo-sensitive cell lines rely on action potentials to release the neurotransmitter onto nerve endings. The electrical excitability is due to the presence of a tetrodotoxin-sensitive, voltage-gated sodium current (I_Na ) similar to that found in neurons. Since the biophysical properties of neuronal I_Na change during development, we wondered whether the same also occurred in taste cells. Here, we used the patch-clamp recording technique to study I_Na in salt-sensing cells (sodium cells) of rat fungiform papillae. We identified these cells by exploiting the known blocking effect of amiloride on ENaC, the sodium (salt) receptor. Based on the amplitude of I_Na , which is known to increase during development, we subdivided sodium cells into two groups: cells with small sodium current (SSC cells; I_Na  < 1 nA) and cells with large sodium current (LSC cells; I_Na  > 1 nA). We found that: the voltage dependence of activation and inactivation significantly differed between these subsets; a slowly inactivating sodium current was more prominent in LSC cells; membrane capacitance in SSC cells was larger than in LSC cells. mRNA expression analysis of the α-subunits of voltage-gated sodium channels in fungiform taste buds supported the functional data. Lucifer Yellow labelling of recorded cells revealed that our electrophysiological criterion for distinguishing two broad groups of taste cells was in good agreement with morphological observations for cell maturity. Thus, all these findings are consistent with developmental changes in the voltage-dependent properties of sodium-taste cells. KEY POINTS: Taste cells are sensory receptors that undergo continuous turnover while they detect food chemicals and communicate with afferent nerve fibres. The voltage-gated sodium current (I_Na ) is a key ion current for generating action potentials in fully differentiated and chemo-sensitive taste cells, which use electrical signalling to release neurotransmitters. Here we show that, during the maturation of rat taste cells involved in salt detection (sodium cells), the biophysical properties of I_Na , such as voltage dependence of activation and inactivation, change significantly. Our results help reveal how taste cells gain electrical excitability during turnover, a property critical to their operation as chemical detectors that relay sensory information to nerve fibres.

A Taste Bud Organoid-Based Microelectrode Array Biosensor for Taste Sensing

The biological taste system has the unique ability to detect taste substances. Biomaterials originating from a biological taste system have been recognized as ideal candidates to serve as sensitive elements in the development of taste-based biosensors. In this study, we developed a taste bud organoid-based biosensor for the research of taste sensation. Taste bud organoids prepared from newborn mice were cultured and loaded onto the surface of a 64-channel microelectrode array (MEA) chip to explore the electrophysiological changes upon taste; an MEA chip was used to simultaneously record multiple-neuron firing activities from taste bud organoids under different taste stimuli, which helped to reveal the role of taste buds in taste sensing. The obtained results show that taste cells separated from the taste epithelium grew well into spherical structures under 3D culture conditions. These structures were composed of multiple cells with obvious budding structures. Moreover, the multicellular spheres were seeded on a 64-channel microelectrode array and processed with different taste stimuli. It was indicated that the MEA chip could efficiently monitor the electrophysiological signals from taste bud organoids in response to various taste stimuli. This biosensor provides a new method for the study of taste sensations and taste bud functions.

Taste Receptor Cells Generate Oscillating Receptor Potentials by Activating G Protein-Coupled Taste Receptors

The receptor potentials of taste receptor cells remain unclear. Here, we demonstrate that taste receptor cells generate oscillating depolarization (n = 7) with action potentials in response to sweet, bitter, umami, and salty taste substances. At a lower concentration of taste substances, taste receptor cells exhibited oscillations in membrane potentials with a low frequency and small magnitude of depolarization. Although the respective waves contained no or 1–2 action potentials, the taste receptor cells generated action potentials continuously in the presence of taste stimuli. Both the frequency and magnitude of oscillations increased when the concentration was increased, to 0.67–1.43 Hz (n = 3) and Δ39–53 mV (n = 3) in magnitude from −64.7 ± 4.2 to −18.7 ± 5.9 mV, which may activate the ATP-permeable ion channels. In contrast, a sour tastant (10-mM HCl) induced membrane depolarization (Δ19.4 ± 9.5 mV, n = 4) with action potentials in type III taste receptor cells. Interestingly, NaCl (1 M) taste stimuli induced oscillation (n = 2) or depolarization (Δ10.5 ± 5.7 mV at the tonic component, n = 9). Our results indicate that the frequency and magnitude of oscillations increased with increasing taste substance concentrations. These parameters may contribute to the expression of taste “thickness.”

Quantitative Analysis of Taste Bud Cell Numbers in the Circumvallate and Foliate Taste Buds of Mice

A mouse single taste bud contains 10-100 taste bud cells (TBCs) in which the elongated TBCs are classified into 3 cell types (types I-III) equipped with different taste receptors. Accordingly, differences in the cell numbers and ratios of respective cell types per taste bud may affect taste-nerve responsiveness. Here, we examined the numbers of each immunoreactive cell for the type II (sweet, bitter, or umami receptor cells) and type III (sour and/or salt receptor cells) markers per taste bud in the circumvallate and foliate papillae and compared these numerical features of TBCs per taste bud to those in fungiform papilla and soft palate, which we previously reported. In circumvallate and foliate taste buds, the numbers of TBCs and immunoreactive cells per taste bud increased as a linear function of the maximal cross-sectional taste bud area. Type II cells made up approximately 25% of TBCs irrespective of the regions from which the TBCs arose. In contrast, type III cells in circumvallate and foliate taste buds made up approximately 11% of TBCs, which represented almost 2 times higher than what was observed in the fungiform and soft palate taste buds. The densities (number of immunoreactive cells per taste bud divided by the maximal cross-sectional area of the taste bud) of types II and III cells per taste bud are significantly higher in the circumvallate papillae than in the other regions. The effects of these region-dependent differences on the taste response of the taste bud are discussed.

Slow recovery from the inactivation of voltage-gated sodium channel Nav1.3 in mouse taste receptor cells

Action potentials play an important role in neurotransmitter release in response to taste. Here, I have investigated voltage-gated Na⁺ channels, a primary component of action potentials, in respective cell types of mouse fungiform taste bud cells (TBCs) with in situ whole-cell clamping and single-cell RT-PCR techniques. The cell types of TBCs electrophysiologically examined were determined immunohistochemically using the type III inositol 1,4,5-triphoshate receptor as a type II cell marker and synaptosomal-associated protein 25 as a type III cell marker. I show that type II cells, type III cells, and TBCs not immunoreactive to these markers (likely type I cells) generate voltage-gated Na⁺ currents. The recovery following inactivation of these currents was well fitted with double exponential curves. The time constants in type III cells (~20 ms and ~ 1 s) were significantly slower than respective time constants in other cell types. RT-PCR analysis indicated the expression of Nav1.3, Nav1.5, Nav1.6, and β1 subunit mRNAs in TBCs. Pharmacological inhibition and single-cell RT-PCR studies demonstrated that type II and type III cells principally express tetrodotoxin (TTX)-sensitive Nav1.3 channels and that ~ 30% of type I cells express TTX-resistant Nav1.5 channels. The auxiliary β1 subunit that modulates gating kinetics was rarely detected in TBCs. As the β1 subunit co-expressed with an α subunit is known to accelerate the recovery from inactivation, it is likely that voltage-gated Na⁺ channels in TBCs may function without β subunits. Slow recovery from inactivation, especially in type III cells, may limit high-frequency firing in response to taste substances.

Age-related electrophysiological changes in mouse taste receptor cells

NEW FINDINGS

What is the central question of this study? Loss of taste or inability to distinguish between different tastes progresses with age. The purpose was to evaluate the age-dependent changes in taste by studying the electrophysiological properties of taste receptor cells. What is the main finding and its importance? Ageing decreased the voltage-gated Na⁺ and K⁺ current densities of type III cells (sour and/or salt receptor cells) but did not affect the current densities in type II cells. At the peripheral levels, the excitability of type III cells was reduced due to ageing, which may affect the signal transduction to taste nerves.

ABSTRACT

The loss of taste due to normal ageing in mammals is assumed to be caused by the ageing of taste receptor cells. We examined the electrophysiological properties of taste receptor cells in the fungiform taste buds of ∼20-month-old mice in situ and subsequently identified their cell types with immunological markers: the inositol 1,4,5-trisphosphate (IP₃ ) receptor (IP₃ R3) for type II cells and a SNARE protein, synaptosomal-associated protein 25 (SNAP-25), for type III cells. Other cells are referred to as non-immunoreactive cells (non-IRCs). Cell types of some cells that could not be identified using cell-type markers were identified based on the electrophysiological feature of the respective cell types. All cell types generated action potentials and a variety of voltage-gated currents. The type II cells mainly expressed tetraethylammonium (TEA)-insensitive and slowly activating outwardly rectifying currents and generated tail currents in repolarization. In contrast, the type III cells expressed TEA-sensitive and faster activating K⁺ currents and did not generate tail currents. These cell type-specific characteristics of voltage-gated currents in ∼20-month-old mice were similar to their respective cell types in ∼2-month-old mice. Also, we showed an age-dependent decrease in Na⁺ and K⁺ current densities in type III cells and an age-dependent increase in outwardly rectifying current density in non-IRCs. Ageing did not affect the voltage-gated current densities in type II cells. The decreased Na⁺ and K⁺ current densities, i.e. the decreased excitability of type III cells, due to ageing may affect the signal transduction to taste nerves.

Cell-type-independent expression of inwardly rectifying potassium currents in mouse fungiform taste bud cells

Inwardly rectifying potassium (Kir) channels play key roles in functions, including maintaining the resting membrane potential and regulating the action potential duration in excitable cells. Using in situ whole-cell recordings, we investigated Kir currents in mouse fungiform taste bud cells (TBCs) and immunologically identified the cell types (type I-III) expressing these currents. We demonstrated that Kir currents occur in a cell-type-independent manner. The activation potentials we measured were -80 to -90 mV, and the magnitude of the currents increased as the membrane potentials decreased, irrespective of the cell types. The maximum current densities at -120 mV showed no significant differences among cell types (p>0.05, one-way ANOVA). The density of Kir currents was not correlated with the density of either transient inward currents or outwardly rectifying currents, although there was significant correlation between transient inward and outwardly rectifying current densities (p<0.05, test for no correlation). RT-PCR studies employing total RNA extracted from peeled lingual epithelia detected mRNAs for Kir1, Kir2, Kir4, Kir6, and Kir7 families. These findings indicate that TBCs express several types of Kir channels functionally, which may contribute to regulation of the resting membrane potential and signal transduction of taste.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

A subset of taste receptor cells express biocytin-permeable channels activated by reducing extracellular Ca2+ concentration

Taste receptor cells (type II cells) transmit taste information to taste nerve fibres via ATP-permeable channels, including calcium homeostasis modulator (CALHM), connexin and/or pannexin1 channels, via the paracrine release of adenosine triphosphate (ATP) as a predominant transmitter. In the present study, we demonstrate that extracellular Ca²⁺ -dependent biocytin-permeable channels are present in a subset of type II cells in mouse fungiform taste buds using biocytin uptake, immunohistochemistry and in situ whole-cell recordings. Type II cells were labelled with biocytin in an extracellular Ca²⁺ concentration ([Ca²⁺ ]_out )-sensitive manner. We found that the ratio of biocytin-labelled type II cells to type II cells per taste bud was approximately 20% in 2 mM Ca²⁺ saline, and this ratio increased to approximately 50% in nominally Ca²⁺ -free saline. The addition of 300 µM GdCl₃ , which inhibits various channels including CALHM1 channels, significantly inhibited biocytin labelling in nominally Ca²⁺ -free saline, whereas the addition of 20 µM ruthenium red did not. Moreover, Cs⁺ -insensitive currents increased in nominally Ca²⁺ -free saline in approximately 40% of type II cells. These increased currents appeared at a potential of above -35 mV, reversed at approximately +10 mV and increased with depolarization. These results suggest that biocytin labels type II cells via ion channels activated by [Ca²⁺ ]_out reduction, probably "CALHM-like" channels, on the basolateral membrane and that taste receptor cells can be categorized into two groups based on differences in the expression levels of [Ca²⁺ ]_out -dependent biocytin-permeable channels. These data indicate electrophysiological and pharmacologically relevant properties of biocytin-permeable channels and suggest their contributions to taste signal transduction.

Action potentials and ion conductances in wild-type and CALHM1-knockout type II taste cells

Taste bud type II cells fire action potentials in response to tastants, triggering nonvesicular ATP release to gustatory neurons via voltage-gated CALHM1-associated ion channels. Whereas CALHM1 regulates mouse cortical neuron excitability, its roles in regulating type II cell excitability are unknown. In this study, we compared membrane conductances and action potentials in single identified TRPM5-GFP-expressing circumvallate papillae type II cells acutely isolated from wild-type (WT) and Calhm1 knockout (KO) mice. The activation kinetics of large voltage-gated outward currents were accelerated in cells from Calhm1 KO mice, and their associated nonselective tail currents, previously shown to be highly correlated with ATP release, were completely absent in Calhm1 KO cells, suggesting that CALHM1 contributes to all of these currents. Calhm1 deletion did not significantly alter resting membrane potential or input resistance, the amplitudes and kinetics of Na⁺ currents either estimated from action potentials or recorded from steady-state voltage pulses, or action potential threshold, overshoot peak, afterhyperpolarization, and firing frequency. However, Calhm1 deletion reduced the half-widths of action potentials and accelerated the deactivation kinetics of transient outward currents, suggesting that the CALHM1-associated conductance becomes activated during the repolarization phase of action potentials.NEW & NOTEWORTHY CALHM1 is an essential ion channel component of the ATP neurotransmitter release mechanism in type II taste bud cells. Its contribution to type II cell resting membrane properties and excitability is unknown. Nonselective voltage-gated currents, previously associated with ATP release, were absent in cells lacking CALHM1. Calhm1 deletion was without effects on resting membrane properties or voltage-gated Na⁺ and K⁺ channels but contributed modestly to the kinetics of action potentials.

Cell-type-dependent action potentials and voltage-gated currents in mouse fungiform taste buds

Taste receptor cells fire action potentials in response to taste substances to trigger non-exocytotic neurotransmitter release in type II cells and exocytotic release in type III cells. We investigated possible differences between these action potentials fired by mouse taste receptor cells using in situ whole-cell recordings, and subsequently we identified their cell types immunologically with cell-type markers, an IP3 receptor (IP3 R3) for type II cells and a SNARE protein (SNAP-25) for type III cells. Cells not immunoreactive to these antibodies were examined as non-IRCs. Here, we show that type II cells and type III cells fire action potentials using different ionic mechanisms, and that non-IRCs also fire action potentials with either of the ionic mechanisms. The width of action potentials was significantly narrower and their afterhyperpolarization was deeper in type III cells than in type II cells. Na(+) current density was similar in type II cells and type III cells, but it was significantly smaller in non-IRCs than in the others. Although outwardly rectifying current density was similar between type II cells and type III cells, tetraethylammonium (TEA) preferentially suppressed the density in type III cells and the majority of non-IRCs. Our mathematical model revealed that the shape of action potentials depended on the ratio of TEA-sensitive current density and TEA-insensitive current one. The action potentials of type II cells and type III cells under physiological conditions are discussed.

Subtype-dependent postnatal development of taste receptor cells in mouse fungiform taste buds

Taste buds contain two types of taste receptor cells, inositol 1,4,5-triphosphate receptor type 3-immunoreactive cells (type II cells) and synaptosomal-associating protein-25-immunoreactive cells (type III cells). We investigated their postnatal development in mouse fungiform taste buds immunohistochemically and electrophysiologically. The cell density, i.e. the number of cells per taste bud divided by the maximal area of the horizontal cross-section of the taste bud, of type II cells increased by postnatal day (PD)49, where as that of type III cells was unchanged throughout the postnatal observation period and was equal to that of the adult cells at PD1. The immunoreactivity of taste bud cell subtypes was the same as that of their respective subtypes in adult mice throughout the postnatal observation period. Almost all type II cells were immunoreactive to gustducin at PD1, and then the ratio of gustducin-immunoreactive type II cells to all type II cells decreased to a saturation level, ∼60% of all type II cells, by PD15. Type II and III cells generated voltage-gated currents similar to their respective adult cells even at PD3. These results show that infant taste receptor cells are as excitable as those of adults and propagate in a subtype-dependent manner. The relationship between the ratio of each taste receptor cell subtype to all cells and taste nerve responses are discussed.

Hypertonicity augments bullfrog taste nerve responses to inorganic salts

The tonicity of taste stimulating solutions has been usually ignored, though taste substances themselves yielded the tonicity. We investigated the effect of hypertonicity on bullfrog taste nerve responses to inorganic salts by adding nonelectrolytes such as urea and sucrose that elicited no taste nerve responses. Here, we show that hypertonicity alters bullfrog taste nerve-response magnitude and firing pattern. The addition of urea or sucrose enhances the taste nerve-response magnitude to NaCl and shifts the concentration-response curve to the left. The effect of hypertonicity on responses to CaCl(2) is bimodal; hypertonicity suppresses CaCl(2) responses at concentrations less than ~30 mM and enhances them at concentrations greater than ~30 mM. The hypertonicity also enhances response magnitude to other monovalent salts. The extent of the enhancing effects depends on the difference between the mobility of the cation and anion in the salt. We quantitatively suggest that both the enhancing and suppressing effects result from the magnitude and direction of local circuit currents generated by diffusion potentials across tight junctions surrounding taste receptor cells.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.

Dye-permeable, voltage-gated channel on mouse fungiform taste bud cells

We show here the expression, permeability and pharmacology of a voltage-gated channel in certain taste bud cells (TBCs) which is known to be permeable to Lucifer Yellow CH (LY) and known to release ATP as a neurotransmitter in response to taste substances. LY dissolved in a 200 mM K(+)-containing solution label TBCs immunoreactive to PLCβ2, a phospholipase subtype, but not the TBC subtype immunoreactive to SNAP-25, a SNARE protein. In addition to these subtypes, LY also labelled a few of the non-immunoreactive TBCs. Monovalent and divalent anion probes with molar mass less than 1200 also label PLCβ2-immunoreactive TBCs and a few non-immunoreactive TBCs, whereas a cation probe, rhodamine B, labels the cell membrane of TBCs nonselectively and K(+) independently. The number of LY-labelled TBCs is decreased by 5 μM DIDS (4,4'-diisothiocyanostilbene-2-2'disulfonate), 1mM octanol and 10(-5)M H(+), but not by 10 μM carbenoxolone, 2mM probenecid, 10mM TEA, or 30 μM flufenamic acid. PLCβ2-immunoreactive TBCs and a few non-immunoreactive TBCs generate a TEA-insensitive outwardly rectifying current. DIDS decreases this current in magnitude with IC(50) of ~0.4 μM in a voltage-independent manner. Also 10(-5)M H(+) and 1mM octanol decreases the current magnitude, but 10 μM carbenoxolone and 2mM probenecid do not. These results show that the LY-permeable channel preferably permeates anions and occurs not only on PLCβ2-immunoreactive TBCs but also on certain non-immunoreactive TBCs. Also the results show that the pharmacology of the LY-permeable channel is different from hemichannels reported. The discussion focuses on the pharmacology and the role of the LY-permeable channel.

Quantitative analysis of taste bud cell numbers in fungiform and soft palate taste buds of mice

Mammalian taste bud cells (TBCs) consist of several cell types equipped with different taste receptor molecules, and hence the ratio of cell types in a taste bud constitutes the taste responses of the taste bud. Here we show that the population of immunohistochemically identified cell types per taste bud is proportional to the number of total TBCs in the taste bud or the area of the taste bud in fungiform papillae, and that the proportions differ among cell types. This result is applicable to soft palate taste buds. However, the density of almost all cell types, the population of cell types divided by the area of the respective taste buds, is significantly higher in soft palates. These results suggest that the turnover of TBCs is regulated to keep the ratio of each cell type constant, and that taste responsiveness is different between fungiform and soft palate taste buds.

Voltage-gated sodium channels in taste bud cells

Background

Taste bud cells transmit information regarding the contents of food from taste receptors embedded in apical microvilli to gustatory nerve fibers innervating basolateral membranes. In particular, taste cells depolarize, activate voltage-gated sodium channels, and fire action potentials in response to tastants. Initial cell depolarization is attributable to sodium influx through TRPM5 in sweet, bitter, and umami cells and an undetermined cation influx through an ion channel in sour cells expressing PKD2L1, a candidate sour taste receptor. The molecular identity of the voltage-gated sodium channels that sense depolarizing signals and subsequently initiate action potentials coding taste information to gustatory nerve fibers is unknown.

Results

We describe the molecular and histological expression profiles of cation channels involved in electrical signal transmission from apical to basolateral membrane domains. TRPM5 was positioned immediately beneath tight junctions to receive calcium signals originating from sweet, bitter, and umami receptor activation, while PKD2L1 was positioned at the taste pore. Using mouse taste bud and lingual epithelial cells collected by laser capture microdissection, SCN2A, SCN3A, and SCN9A voltage-gated sodium channel transcripts were expressed in taste tissue. SCN2A, SCN3A, and SCN9A were expressed beneath tight junctions in subsets of taste cells. SCN3A and SCN9A were expressed in TRPM5 cells, while SCN2A was expressed in TRPM5 and PKD2L1 cells. HCN4, a gene previously implicated in sour taste, was expressed in PKD2L1 cells and localized to cell processes beneath the taste pore.

Conclusion

SCN2A, SCN3A and SCN9A voltage-gated sodium channels are positioned to sense initial depolarizing signals stemming from taste receptor activation and initiate taste cell action potentials. SCN2A, SCN3A and SCN9A gene products likely account for the tetrodotoxin-sensitive sodium currents in taste receptor cells.

Voltage-gated sodium channels in taste bud cells

BACKGROUND

Taste bud cells transmit information regarding the contents of food from taste receptors embedded in apical microvilli to gustatory nerve fibers innervating basolateral membranes. In particular, taste cells depolarize, activate voltage-gated sodium channels, and fire action potentials in response to tastants. Initial cell depolarization is attributable to sodium influx through TRPM5 in sweet, bitter, and umami cells and an undetermined cation influx through an ion channel in sour cells expressing PKD2L1, a candidate sour taste receptor. The molecular identity of the voltage-gated sodium channels that sense depolarizing signals and subsequently initiate action potentials coding taste information to gustatory nerve fibers is unknown.

RESULTS

We describe the molecular and histological expression profiles of cation channels involved in electrical signal transmission from apical to basolateral membrane domains. TRPM5 was positioned immediately beneath tight junctions to receive calcium signals originating from sweet, bitter, and umami receptor activation, while PKD2L1 was positioned at the taste pore. Using mouse taste bud and lingual epithelial cells collected by laser capture microdissection, SCN2A, SCN3A, and SCN9A voltage-gated sodium channel transcripts were expressed in taste tissue. SCN2A, SCN3A, and SCN9A were expressed beneath tight junctions in subsets of taste cells. SCN3A and SCN9A were expressed in TRPM5 cells, while SCN2A was expressed in TRPM5 and PKD2L1 cells. HCN4, a gene previously implicated in sour taste, was expressed in PKD2L1 cells and localized to cell processes beneath the taste pore.

CONCLUSION

SCN2A, SCN3A and SCN9A voltage-gated sodium channels are positioned to sense initial depolarizing signals stemming from taste receptor activation and initiate taste cell action potentials. SCN2A, SCN3A and SCN9A gene products likely account for the tetrodotoxin-sensitive sodium currents in taste receptor cells.

NaCl responsive taste cells in the mouse fungiform taste buds

Previous studies have demonstrated that rodents' chorda tympani (CT) nerve fibers responding to NaCl can be classified according to their sensitivities to the epithelial sodium channel (ENaC) blocker amiloride into two groups: amiloride-sensitive (AS) and -insensitive (AI). The AS fibers were shown to respond specifically to NaCl, whereas AI fibers broadly respond to various electrolytes, including NaCl. These data suggest that salt taste transduction in taste cells may be composed of at least two different systems; AS and AI ones. To further address this issue, we investigated the responses to NaCl, KCl and HCl and the amiloride sensitivity of mouse fungiform papilla taste bud cells which are innervated by the CT nerve. Comparable with the CT data, the results indicated that 56 NaCl-responsive cells tested were classified into two groups; 25 cells ( approximately 44%) narrowly responded to NaCl and their NaCl response were inhibited by amiloride (AS cells), whereas the remaining 31 cells ( approximately 56%) responded not only to NaCl, but to KCl and/or HCl and showed no amiloride inhibition of NaCl responses (AI cells). Amiloride applied to the basolateral side of taste cells had no effect on NaCl responses in the AS and AI cells. Single cell reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated that ENaC subunit mRNA was expressed in a subset of AS cells. These findings suggest that the mouse fungiform taste bud is composed of AS and AI cells that can transmit taste information differently to their corresponding types of CT fibers, and apical ENaCs may be involved in the NaCl responses of AS cells.

Functional expression of M3, a muscarinic acetylcholine receptor subtype, in taste bud cells of mouse fungiform papillae

Taste bud cells (TBCs) express various neurotransmitter receptors assumed to facilitate or modify taste information processing within taste buds. We investigated the functional expression of muscarinic acetylcholine receptor (mAChR) subtypes, M1-M5, in mouse fungiform TBCs. ACh applied to the basolateral membrane of TBCs elevates the intracellular Ca(2+) level in a concentration-dependent manner with the 50% effective concentration (EC(50)) of 0.6 microM. The Ca(2+) responses occur in the absence of extracellular Ca(2+) and are inhibited by atropine, a selective antagonist against mAChRs. The order of 50% inhibitory concentration (IC(50)) examined with a series of antagonists selective to mAChR subtypes shows the expression of M3 on TBCs. Perforated whole-cell voltage clamp studies show that 1 microM ACh blocks an outwardly rectifying current and that 100 nM atropine reverses the block. Reverse transcriptase-mediated polymerase chain reaction studies suggest the expression of M3 but not the other mAChR subtypes. Immunohistochemical studies show that phospholipase Cbeta-immunoreactive TBCs and synaptosome-associated protein of 25 kDa-immunoreactive nerve endings are immunoreactive to a transporter that packs ACh molecules into synaptic vesicles (vesicular acetylcholine transporter). These results show that M3 occurs on a few fungiform TBCs and suggest that a few nerve endings, and probably a few TBCs, release ACh by exocytosis. The role of ACh in taste responses is discussed.

Localization of amiloride-sensitive sodium current and voltage-gated calcium currents in rat fungiform taste cells

Recent studies have shown that taste cells transducing bitter, sweet, and umami stimuli do not possess high-threshold voltage-gated calcium channels required for synaptic transmission at conventional synapses, suggesting some sort of signal processing inside taste buds prior to the activation of nerve endings. To evaluate whether this is a general paradigm for the physiology of taste reception, we studied the transduction pathway underlying the detection of sodium ions (salty stimulus). In laboratory rodents, Na(+) is thought to be transduced, at least in part, through amiloride-sensitive sodium channels (ASSCs). Therefore we used the patch-clamp techniques to analyze the occurrence pattern of amiloride-sensitive sodium currents and voltage-gated calcium currents (both low-voltage-activated T-type current and high-voltage-activated L-type current) among taste cells in rat fungiform papillae. Because taste cells turnover, we focused our attention on cells possessing large voltage-gated sodium currents, a sign of "mature" cells. We found that cells expressing functional ASSCs either did not possess any calcium currents or exhibited only T-type calcium currents, which is believed to play a role in repetitive firing. On the contrary, cells lacking functional ASSCs were endowed with L-type calcium currents, which are thought to mediate calcium influx required for neurotransmitter exocytosis. Therefore our data suggest that sodium-detecting cells are unlikely to use conventional synaptic communication to transfer taste information to nerve endings. Our findings on sodium taste detection support the recent model of taste transduction, involving separate groups of taste cells: chemosensitive cells and cells forming conventional synapses.

Coding channels for taste perception: information transmission from taste cells to gustatory nerve fibers

Taste signals are first detected by the taste receptor cells, which are located in taste buds existing in the tongue, soft palate, larynx and epiglottis. Taste receptor cells contact with the chemical compounds in oral cavity through the apical processes which protrude into the taste pore. Interaction between chemical compounds and the taste receptor produces activation of taste receptor cells directly or indirectly. Then the signals are transmitted to gustatory nerve fibers and higher order neurons. A recent study demonstrated many similarities between response properties of taste receptor cells with action potentials and those of the gustatory nerve fibers innervating them, suggesting information derived from receptor cells generating action potentials may form a major component of taste information that is transmitted to gustatory nerve fibers. These findings may also indicate that there is no major modification of taste information sampled by taste receptor cells in synaptic transmission from taste cells to nerve fibers although there is indirect evidence. In the peripheral taste system, gustatory nerve fibers may selectively contact with taste receptor cells that have similar response properties and convey constant taste information to the higher order neurons.

Taste Responsiveness of Fungiform Taste Cells With Action Potentials

It is known that a subset of taste cells generate action potentials in response to taste stimuli. However, responsiveness of these cells to particular tastants remains unknown. In the present study, by using a newly developed extracellular recording technique, we recorded action potentials from the basolateral membrane of single receptor cells in response to taste stimuli applied apically to taste buds isolated from mouse fungiform papillae. By this method, we examined taste-cell responses to stimuli representing the four basic taste qualities (NaCl, Na saccharin, HCl, and quinine-HCl). Of 72 cells responding to taste stimuli, 48 (67%) responded to one, 22 (30%) to two, and 2 (3%) to three of four taste stimuli. The entropy value presenting the breadth of responsiveness was 0.158 ± 0.234 (mean ± SD), which was close to that for the nerve fibers (0.183 ± 0.262). In addition, the proportion of taste cells predominantly sensitive to each of the four taste stimuli, and the grouping of taste cells based on hierarchical cluster analysis, were comparable with those of chorda tympani (CT) fibers. The occurrence of each class of taste cells with different taste responsiveness to the four taste stimuli was not significantly different from that of CT fibers except for classes with broad taste responsiveness. These results suggest that information derived from taste cells generating action potentials may provide the major component of taste information that is transmitted to gustatory nerve fibers.

Electrophysiologically identified subpopulations of taste bud cells

The heterogeneous population of mammalian taste cells includes several cellular subtypes specializing in distinct physiological functions. They are poorly understood at the single cell level because the available physiological data have generally been obtained from unidentified taste cells. We recorded them from individual taste cells isolated from circumvallate, foliate, and fungiform papilla of the mouse, employing the patch clamp technique, and tried to elucidate whether universal electrophysiological criteria may be established for the identification of functionally different cellular subpopulations. It was found that irrespective of the papillae type, most ( approximately 96%) of robust taste cells could be categorized into three distinct subgroups on the basis of families of whole-cell (WC) currents exhibited in response to membrane polarization. The validity of this quite simple criterion was further confirmed by using different voltage clamp protocols, ion substitutions, and channel blockers to record different ionic currents, including voltage-gated (VG) Ca(2+), inward-rectifying K(+), and hyperpolarization-activated currents. Given that our findings are based on the statistically significant number of recordings, we believe that the electrophysiological identification of taste cells presented here may be effective for further studies on single taste cell physiology, including taste transduction.

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.

Voltage-gated channels involved in taste responses and characterizing taste bud cells in mouse soft palates

Taste bud cells (TBCs) on soft palates differ from those on tongues in innervation and chemosensitivity. We investigated voltage-gated channels involved in the taste responses of TBCs on mouse soft palates under in-situ tight-seal voltage/current-clamp conditions. Under the cell-attached mode, TBCs spontaneously fired action currents, which were blocked by application of 1 microM TTX to TBC basolateral membranes. Firing frequencies increased in response to taste substances applied to TBC receptor membranes. Under the whole-cell clamp mode, as expected, TBCs produced various voltage-gated currents such as TTX-sensitive Na+ currents (INa), outward currents (Iout) including TEA-sensitive and insensitive currents, inward rectifier K+ currents (Iir), and Ca2+ currents including T-type, P/Q-type, and L-type Ca2+ currents. We classified TBCs into three types based on the magnitude of their voltage-gated Na+ currents and membrane capacitance. HEX type (60% of TBCs examined) was significantly larger in Na+ current magnitude and smaller in membrane capacitance than LEX type (23%). NEX type (17%) had no Na+ currents. HEX type was equally distributed within single taste buds, while LEX type was centrally distributed, and NEX type was peripherally distributed. There were correlations between these electrophysiological cell types and morphological cell types determined by three-dimensional reconstruction. The present results show that soft palate taste buds contain TBCs with different electrophysiological properties, and suggest that their co-operation is required in taste transduction.

Lucifer Yellow slows voltage-gated Na+ current inactivation in a light-dependent manner in mice

Lucifer Yellow CH (LY), a membrane-impermeant fluorescent dye, has been used in electro-physiological studies to visualize cell morphology, with little concern about its pharmacological effects. We investigated its effects on TTX-sensitive voltage-gated Na+ channels in mouse taste bud cells and hippocampal neurons under voltage-damp conditions. LY applied inside cells irreversibly slowed the inactivation of Na+ currents upon exposure to light of usual intensities. The inactivation time constant of Na+ currents elicited by a depolarization to -15 mV was increased by fourfold after a 5 min exposure to halogen light of 3200 Ix at source (3200 Ix light), and sevenfold after a 1-min exposure to 12,000 Ix light. A fraction of the Na+ current became non-inactivating following the exposure. The non-inactivating current was approximately 20 % of the peak total Na+ current after a 5 min exposure to 3200 Ix light, and approximately 30 % after a 1 min exposure to 12,000 Ix light. Light-exposed LY shifted slightly the current-voltage relationship of the peak Na+ current and of the steady-state inactivation curve, in the depolarizing direction. A similar light-dependent decrease in kinetics occurred in whole-cell Na+ currents of cultured mouse hippocampal neurones. Single-channel recordings showed that exposure to 6500 Ix light for 3 min increased the mean open time of Na+channels from 1.4 ms to 2.4 ms without changing the elementary conductance. The pre-incubation of taste bud cells with 1 mM dithiothreitoL a scavenger of radical species, blocked these LY effects. These results suggest that light-exposed LY yields radical species that modify Na+ channels.

Electrophysiology of Necturus taste cells

Taste buds are sensory end organs that detect chemical substances occurring in foodstuffs and relay the relative information to the brain. The mechanisms by which the chemical stimuli are converted into biological signals represent a central issue in taste research. Our understanding of how taste buds accomplish this operation relies on the detailed knowledge of the biological properties of taste bud cells-the taste cells-and of the functional processes occurring in these cells during chemostimulation. The amphibian Necturus maculosus (mudpuppy) has proven to be a very useful model for studying basic cellular processes of vertebrate taste reception, some of which are still awaiting to be explored in mammals. The main advantages offered by Necturus are the large size of its taste cells and the relative accessibility of its taste buds, which can therefore be handled easily for experimental manipulations. In this review, I summarize the functional properties of Necturus taste cells studied with electrophysiological techniques (intracellular recordings and patch-clamp recordings). My focus is on ion channels in taste cells and on their role in signal transduction, as well as on the functional relationships among the cells inside Necturus taste buds. This information has revealed to be well suited to outline some of the general physiological processes occurring during taste reception in vertebrates, including mammals, and may represent a useful framework for understanding how taste buds work.

Postnatal development of membrane excitability in taste cells of the mouse vallate papilla

The mammalian peripheral taste system undergoes functional changes during postnatal development. These changes could reflect age-dependent alterations in the membrane properties of taste cells, which use a vast array of ion channels for transduction mechanisms. Yet, scarce information is available on the membrane events in developing taste cells. We have addressed this issue by studying voltage-dependent Na+, K+, and Cl- currents (I(Na), I(K), and I(Cl), respectively) in a subset of taste cells (the so-called "Na/OUT" cells, which are electrically excitable and thought to be sensory) from mouse vallate papilla. Voltage-dependent currents play a key role during taste transduction, especially in the generation of action potentials. Patch-clamp recordings revealed that I(Na), I(K), and I(Cl) were expressed early in postnatal development. However, only I(K) and I(Cl) densities increased significantly in developing Na/OUT cells. Consistent with the rise of I(K) density, we found that action potential waveform changed markedly, with an increased speed of repolarization that was accompanied by an enhanced capability of repetitive firing. In addition to membrane excitability changes in putative sensory cells, we observed a concomitant increase in the occurrence of glia-like taste cells (the so called "leaky" cells) among patched cells. Leaky cells are likely involved in dissipating the increase of extracellular K+ during action potential discharge in chemosensory cells. Thus, developing taste cells of the mouse vallate papilla undergo a significant electrophysiological maturation and diversification. These functional changes may have a profound impact on the transduction capabilities of taste buds during development.

Acid detection by taste receptor cells

Sourness is a primary taste quality that evokes an innate rejection response in humans and many other animals. Acidic stimuli are the unique sources of sour taste so a rejection response may serve to discourage ingestion of foods spoiled by acid producing microorganisms. The investigation of mechanisms by which acids excite taste receptor cells (TRCs) is complicated by wide species variability and within a species, apparently different mechanisms for strong and weak acids. The problem is further complicated by the fact that the receptor cells are polarized epithelial cells with different apical and basolateral membrane properties. The cellular mechanisms proposed for acid sensing in taste cells include, the direct blockage of apical K(+) channels by protons, an H(+)-gated Ca(2+) channel, proton conduction through apical amiloride-blockable Na(+) channels, a Cl(-) conductance blocked by NPPB, the activation of the proton-gated channel, BNC-1, a member of the Na(+) channel/degenerin super family, and by stimulus-evoked changes in intracellular pH. Acid-induced intracellular pH changes appear to be similar to those reported in other mammalian acid-sensing cells, such as type-I cells of the carotid body, and neurons found in the ventrolateral medulla, nucleus of the solitary tract, the medullary raphe, and the locus coceuleus. Like type-I carotid body cells and brainstem neurons, isolated TRCs demonstrate a linear relationship between intracellular pH (pH(i)) and extracellular pH (pH(o)) with slope, DeltapH(i)/DeltapH(o) near unity. Acid-sensing cells also appear to regulate pH(i) when intracellular pH changes occur under iso-extracellular pH conditions, but fail to regulate their pH when pH(i) changes are induced by decreasing extracellular pH. We shall discuss the current status of proposed acid-sensing taste mechanisms, emphasizing pH-tracking in receptor cells.

A novel pharmacological probe links the amiloride-insensitive NaCl, KCl, and NH(4)Cl chorda tympani taste responses

Chorda tympani taste nerve responses to NaCl can be dissected pharmacologically into amiloride-sensitive and -insensitive components. It is now established that the amiloride-sensitive, epithelial sodium channel acts as a sodium-specific ion detector in taste receptor cells (TRCs). Much less is known regarding the cellular origin of the amiloride-insensitive component, but its anion dependence indicates an important role for paracellular shunts in the determination of its magnitude. However, this has not precluded the possibility that undetected apical membrane ion pathways in TRCs may also contribute to its origin. Progress toward making such a determination has suffered from lack of a pharmacological probe for an apical amiloride-insensitive taste pathway. We present data here showing that, depending on the concentration used, cetylpyridinium chloride (CPC) can either enhance or inhibit the amiloride-insensitive response to NaCl. The CPC concentration giving maximal enhancement was 250 microM. At 2 mM, CPC inhibited the entire amiloride-insensitive part of the NaCl response. The NaCl response is, therefore, composed entirely of amiloride- and CPC-sensitive components. The magnitude of the maximally enhanced CPC-sensitive component varied with the NaCl concentration and was half-maximal at [NaCl] = 62 +/- 11 (SE) mM. This was significantly less than the corresponding parameter for the amiloride-sensitive component (268 +/- 71 mM). CPC had similar effects on KCl and NH(4)Cl responses except that in these cases, after inhibition with 2 mM CPC, a significant CPC-insensitive response remained. CPC (2 mM) inhibited intracellular acidification of TRCs due to apically presented NH(4)Cl, suggesting that CPC acts on an apical membrane nonselective cation pathway.

Distribution of gustatory sensitivities in rat taste cells: whole-cell responses to apical chemical stimulation

Several taste transduction mechanisms have been demonstrated in mammals, but little is known about their distribution within and across receptor cells. We recorded whole-cell responses of 120 taste cells of the rat fungiform papillae and soft palate maintained within the intact epithelium in a modified Ussing chamber, which allowed us to flow tastants across the apical membrane while monitoring the activity of the cell with a patch pipette. Taste stimuli were: 0.1 m sucrose, KCl, and NH(4)Cl, 0.032 m NaCl, and 3.2 mm HCl and quinine hydrochloride (QHCl). When cells were held at their resting potentials, taste stimulation resulted in conductance changes; reversible currents >5 pA were considered reliable responses. Sucrose and QHCl produced a decrease in outward current and membrane conductance, whereas NaCl, KCl, NH(4)Cl, and HCl elicited inward currents accompanied by increased conductance. Combinations of responses to pairs of the four basic stimuli (sucrose, NaCl, HCl, and QHCl) across the 71-84 cells tested with each pair were predictable from the probabilities of responses to individual stimuli, indicating an independent distribution of sensitivities. Of 62 cells tested with all four basic stimuli, 59 responded to at least one of the stimuli; 16 of these (27.1%) responded to only one, 20 (33.9%) to two, 15 (25.4%) to three, and 8 (13.6%) to all of the basic stimuli. Cells with both inward (Na(+)) and outward (K(+)) voltage-activated currents were significantly more broadly tuned to gustatory stimuli than those with only inward currents.

Mouse taste cells with glialike membrane properties

Taste buds are sensory structures made up by tightly packed, specialized epithelial cells called taste cells. Taste cells are functionally heterogeneous, and a large proportion of them fire action potentials during chemotransduction. In view of the narrow intercellular spaces within the taste bud, it is expected that the ionic composition of the extracellular fluid surrounding taste cells may be altered significantly by activity. This consideration has led to postulate the existence of glialike cells that could control the microenvironment in taste buds. However, the functional identification of such cells has been so far elusive. By using the patch-clamp technique in voltage-clamp conditions, I identified a new type of cells in the taste buds of the mouse vallate papilla. These cells represented about 30% of cells patched in taste buds and were characterized by a large leakage current. Accordingly, I named them "Leaky" cells. The leakage current was carried by K(+), and was blocked by Ba(2+) but not by tetraethylammonium (TEA). Other taste cells, such as those possessing voltage-gated Na(+) currents and thought to be chemosensory in function, did not express any sizeable leakage current. Consistent with the presence of a leakage conductance, Leaky cells had a low input resistance (approximately 0.25 G Omega). In addition, their zero-current ("resting") potential was close to the equilibrium potential for potassium ions. The electrophysiological analysis of the membrane currents remaining after pharmacological block by Ba(2+) revealed that Leaky cells also possessed a Cl(-) conductance. However, in resting conditions the membrane of these cells was about 60 times more permeable to K(+) than to Cl(-). The resting potassium conductance in Leaky cells could be involved in dissipating rapidly the increase in extracellular K(+) during action potential discharge in chemosensory cells. Thus Leaky cells might represent glialike elements in taste buds. These findings support a model in which specific cells control the chemical composition of intercellular fluid in taste buds.

Optical recordings of taste responses from fungiform papillae of mouse in situ

Single taste buds in mouse fungiform papillae consist of approximately 50 elongated cells (TBCs), where fewer than three TBCs have synaptic contacts with taste nerves. We investigated whether the non-innervated TBCs were chemosensitive using a voltage-sensitive dye, tetramethylrhodamine methyl ester (TMRM), under in situ optical recording conditions. Prior to the optical recordings, we investigated the magnitude and polarity of receptor potentials under in situ whole-cell clamp conditions. In response to 10 mM HCl, several TBCs were depolarized by approximately 25 mV and elicited action potentials, while other TBCs were hyperpolarized by approximately 12 mV. The TBCs eliciting hyperpolarizing receptor potentials also generated action potentials on electrical stimulation. A mixture of 100 mM NaCl, 10 mM HCl and 500 mM sucrose depolarized six TBCs and hyperpolarized another three TBCs out of 13 identified TBCs in a taste bud viewed by optical section. In an optical section of another taste bud, 1 M NaCl depolarized five TBCs and hyperpolarized another two TBCs out of 11 identified TBCs. The number of chemosensitive TBCs was much larger than the number of innervated TBCs in a taste bud, indicating the existence of chemosensitivity in non-innervated TBCs. There was a tendency for TBCs eliciting the same polarity of receptor potential to occur together in taste buds. We discuss the role of non-innervated TBCs in taste information processing.

Acid and salt responses in mouse taste cells

Acid and salt responses of taste cells induced by natural stimulation have not been investigated with exception of early studies with conventional microelectrode method, due to the toxicity of high concentration of salt or low pH of acid stimuli applied to isolated taste cells. This indicates that the application of rapid and localized stimulation to the apical membrane of taste cells is necessary for recording of natural responses to salt or acid stimuli using patch clamp technique. Recently we have developed a procedure to accomplish the quasi-natural condition including rapid, localized stimuli to the apical receptive membrane and the maintenance of taste bud polarity. In this review, we present our recent results obtained under quasi-natural condition using patch clamp techniques, comparing with the previously proposed hypothesis. One of our major finding is the fact that the acid-induced responses of taste cells in the mouse fungiform papillae are never suppressed by amiloride but an apical proton-gated conductance and a basolateral Cl(-) conductance possibly contribute to sour transduction. On the other hand, salt-induced responses are suppressed by amiloride, although the salt-induced responses recorded from a single cell involve both amiloride-sensitive and -insensitive components. Furthermore, the amiloride-insensitive component of salt responses possibly consists of multiple subcomponents including an apical sodium-gated nonselective cation conductance and a basolateral Cl(-) conductance. Recent reports also support the hypothesis that both acid and salt responses require specific receptor mechanisms of inorganic cations such as H(+) and Na(+) at the apical receptive membrane.

Adenosine triphosphate mobilizes cytosolic calcium and modulates ionic currents in mouse taste receptor cells

By using photometry and the patch clamp technique, we identified P(2Y)-like receptors in mouse taste receptor cells (TRCs) and found them to be coupled to Ca(2+) mobilization and ionic current modulation. Particularly, adenosine triphosphate (ATP) and the P(2Y) agonist 2-methylthio-ATP increased intracellular Ca(2+) by stimulating the phosphoinositide pathway, whereas beta, gamma-methylene-D-ATP, a P(2X) agonist, was ineffective. In a distinctive TRC subpopulation, ATP closed Ca(2+) channels. This regulation may underlie the negative feedback tuning neurotransmitter release. By mobilizing intracellular Ca(2+), ATP activated Ca(2+)-dependent Cl(-) channels, the intracellular event that may universally occur upon taste stimulation triggering IP(3) formation and Ca(2+) release in the TRC cytoplasm.

Putative mammalian taste receptors: a class of taste-specific GPCRs with distinct topographic selectivity

Taste represents a major form of sensory input in the animal kingdom. In mammals, taste perception begins with the recognition of tastant molecules by unknown membrane receptors localized on the apical surface of receptor cells of the tongue and palate epithelium. We report the cloning and characterization of two novel seven-transmembrane domain proteins expressed in topographically distinct subpopulations of taste receptor cells and taste buds. These proteins are specifically localized to the taste pore and are members of a new group of G protein-coupled receptors distantly related to putative mammalian pheromone receptors. We propose that these genes encode taste receptors.

Sour transduction involves activation of NPPB-sensitive conductance in mouse taste cells

We examined the sour taste transduction mechanism in the mouse by applying whole cell patch-clamp technique to nondissociated taste cells from the fungiform papillae. Localized stimulation with 0.5 M NaCl and 25 mM citric acid (pH 3.0) of the apical membrane enabled us to obtain responses from single taste cells under a quasi-natural condition. Of 28 taste cells examined, 11 cells (39%) responded to 0. 5 M NaCl alone and 2 cells (7%) responded to 25 mM citric acid alone, indicating the presence of salty- and sour-specific taste cells. Ten cells (36%) responded to both NaCl and citric acid and 5 cells (18%) responded to neither salt nor citric acid. Amiloride reversibly suppressed NaCl-induced responses in mouse taste cells but not citric acid-induced responses. On the other hand, a Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), reversibly suppressed all the citric-acid-induced responses. Most of the NaCl-induced current responses displayed an inwardly rectifying property, whereas all the citric-acid-induced responses displayed an outwardly rectifying property. The reversal potential for NPPB-sensitive component in citric-acid-induced current responses was -2 +/- 7 mV (mean +/- SE, n = 4), which was close to the equilibrium potential of Cl- (ECl), whereas the reversal potential for NPPB-insensitive component was 34 +/- 8 mV (n = 4). The reversal potential of citric-acid-induced current responses (19 +/- 8 mV, n = 4) was mostly present at the middle point between reversal potentials of NPPB-sensitive and -insensitive current components. In some taste cells, an inorganic cation channel blocker, Cd2+, suppressed citric-acid-induced responses, but an inorganic stretch-activated cation channel blocker, Gd3+, did not affect these responses. These results suggest that salt- and acid-induced responses were mediated by differential transduction mechanisms in mouse taste cells and that NPPB-sensitive Cl- channels play a more important role to sour taste transduction rather than amiloride-sensitive Na+ channels. However, the fact that the reversal potentials of citric-acid-induced responses had more positive than ECl suggests that Ca2+ or H+ permeable and poorly selective cation channels, which should be amiloride insensitive, may be activated by citric acid.

A method for in-situ tight-seal recordings from single taste bud cells of mice

In order to investigate taste transduction mechanisms, we developed a method to irrigate the receptor and basolateral membranes of mouse taste bud cells with different solutions under tight-seal recording conditions. A peeled tongue epithelium with taste bud cells was mounted on a recording platform designed to separate irrigating solutions for each membrane. The mucosal surface (receptor membrane side) of the peeled epithelium facing an inner chamber was always irrigated with deionized water or stimulating solutions, and the serosal surface (basolateral membrane side) was irrigated with a physiological saline solution. A recording electrode was placed on the basolateral membrane of a taste bud cell under an upright-microscope with a x 40-water-immersed objective. Investigated taste bud cells generated action potentials, and 1 M glucose, 200 mM NaCl, and 10 mM quinine elicited inward current. Irrigation with deionized water for more than 1 h had no effect. The resistance of the peeled tongue epithelium was 1570 +/- 343 omega cm2. These results show that the peeled tongue epithelium protects basolateral membranes from deionized water or stimulating solutions as the tongue epithelium does in situ and that this method is suitable to investigate the role of each membrane in taste transduction.