A bitter substance induces a rise in intracellular calcium in a subpopulation of rat taste cells

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

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 Pharmacological Perspective on the Study of Taste

The study of taste has been guided throughout much of its history by the conceptual framework of psychophysics, where the focus was on quantification of the subjective experience of the taste sensations. By the mid-20th century, data from physiologic studies had accumulated sufficiently to assemble a model for the function of receptors that must mediate the initial stimulus of tastant molecules in contact with the tongue. But the study of taste as a receptor-mediated event did not gain momentum until decades later when the actual receptor proteins and attendant signaling mechanisms were identified and localized to the highly specialized taste-responsive cells of the tongue. With those discoveries a new opportunity to examine taste as a function of receptor activity has come into focus. Pharmacology is the science designed specifically for the experimental interrogation and quantitative characterization of receptor function at all levels of inquiry from molecules to behavior. This review covers the history of some of the major concepts that have shaped thinking and experimental approaches to taste, the seminal discoveries that have led to elucidation of receptors for taste, and how applying principles of receptor pharmacology can enhance understanding of the mechanisms of taste physiology and perception.

The pharmacology of bitter taste receptors and their role in human airways

The receptors involved in bitter taste perception (bitter taste receptors--T2Rs) constitute a family of G-protein-coupled receptors, of which around 29 subtypes have been identified in humans. T2R expression was initially thought to be confined to the oral cavity but has recently been described in a range of other tissues (such as the heart, gut, nasal cavity and lungs) and cell types (chemosensory, smooth muscle, endothelial, epithelial and inflammatory cells). Although it is still not clear whether endogenous T2R agonists exist, the T2R receptors recognize many natural and synthetic compounds, such as the acyl-homoserine lactones produced by bacteria, caffeine, chloroquine, and erythromycin. In the upper airways, T2Rs are involved in neurogenic inflammation and bacterial clearance. Their known effects in the lungs are exerted at three different levels. Firstly, T2R agonists increase the beating frequency of cilia on epithelial cells. Secondly, the T2Rs induce bronchial smooth muscle cells to relax. Thirdly, the T2R receptors expressed on immune cells (such as macrophages and mast cells) modulate production of pro-inflammatory mediators. Furthermore, T2R agonists are effective in inhibiting lung inflammation or smooth muscle contraction in ex vivo and asthma animal models, and are known to be involved in bacterial killing in the nasal cavity and enhancing lung function in humans. This review focuses on the pharmacology and physiological functions of T2R receptors in the upper and lower airways. It presents recently acquired knowledge suggesting that T2Rs may become valuable drug targets in the treatment of diseases such as asthma and chronic rhinosinusitis.

Denatonium inhibits growth and induces apoptosis of airway epithelial cells through mitochondrial signaling pathways

Background

Denatonium, a widely used bitter agonist, activates bitter taste receptors on many cell types and plays important roles in chemical release, ciliary beating and smooth muscle relaxation through intracellular Ca²⁺-dependent pathways. However, the effects of denatonium on the proliferation of airway epithelial cells and on the integrity of cellular components such as mitochondria have not been studied. In this study, we hypothesize that denatonium might induce airway epithelial cell injury by damaging mitochondria.

Methods

Bright-field microscopy, cell counting kit-8 (CCK-8) assay and flow cytometry analysis were used to examine cellular morphology, proliferation and cell cycle, respectively. Transmission electron microscopy (TEM) was used to examine mitochondrial integrity. JC-1 dye and western blotting techniques were used to measure mitochondrial membrane potential and protein expression, respectively.

Results

For airway epithelial cells, we observed that denatonium significantly effects cellular morphology, decreases cell proliferation and reduces the number of cells in S phase in a dose-dependent manner. TEM analysis demonstrated that denatonium causes large amplitude swelling of mitochondria, which was confirmed by the loss of mitochondrial membrane potential, the down-regulation of Bcl-2 protein and the subsequent enhancement of the mitochondrial release of cytochrome c and Smac/DIABLO after denatonium treatment.

Conclusions

In this study, we demonstrated for the first time that denatonium damages mitochondria and thus induces apoptosis in airway epithelial cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-015-0183-9) contains supplementary material, which is available to authorized users.

Intragastric gavage with denatonium benzoate acutely induces neuronal activation in the solitary tract nucleus via the vagal afferent pathway

Natural toxic substances have a bitter taste and their ingestion sends signals to the brain leading to aversive oral sensations. In the present study, we investigated chronological changes in c-Fos immunoreactivity in the nucleus tractus solitarius (NTS) to study the bitter taste reaction time of neurons in the NTS. Equal volumes (0.5 mL) of denatonium benzoate (DB), a bitter tastant, or its vehicle (distilled water) were administered to rats intragastrically. The rats were sacrificed at 0, 0.5, 1, 2, 4, 8, or 16 h after treatment. In the vehicle-treated group, the number of c-Fos-positive nuclei started to increase 0.5 h after treatment and peaked 2 h after gavage. In contrast, the number of c-Fos-positive nuclei in the DB-treated group significantly increased 1 h after gavage. Thereafter, the number of c-Fos immunoreactive nuclei decreased over time. The number of c-Fos immunoreactive nuclei in the NTS was also increased in a dose-dependent manner 1 h after gavage. Subdiaphragmatic vagotomy significantly decreased DB-induced neuronal activation in the NTS. These results suggest that intragastric DB increases neuronal c-Fos expression in the NTS 1 h after gavage and this effect is mediated by vagal afferent fibers.

TRPM5

TRPM5 is a Ca(2+)-activated cation channel that mediates signaling in taste and other chemosensory cells. Within taste cells, TRPM5 is the final element in a signaling cascade that starts with the activation of G protein-coupled receptors by bitter, sweet, or umami taste molecules and that requires the enzyme PLCβ2. PLCβ2 breaks down PIP2 into DAG and IP3, and the ensuing release of Ca(2+) from intracellular stores activates TRPM5. Since its initial discovery in the taste system, TRPM5 has been found to be distributed in sparse chemosensory cells located throughout the digestive track, in the respiratory system, and in the olfactory system. It is also found in pancreatic islets, where it contributes to insulin secretion. This review highlights recent work on the mechanisms of the activation of the TRPM5 channel and its regulation by voltage, phosphoinositides, temperature, and pH. The distribution of the channel in the body and its functional contribution to various sensory and nonsensory processes are discussed.

Relaxant effect of chloroquine in rat ileum: possible involvement of nitric oxide and BKCa

Objectives

Bitter perception has a particularly important role in host defence. However, to date, direct effects of bitter compounds on small intestinal motility have not been shown. This study investigated the effects of bitter compounds on the spontaneous contraction of longitudinal smooth muscle strips of rat ileum.

Methods

Isolated longitudinal smooth muscle strips of rat ileum were used for tension recording in vitro. Immunofluorescence staining was used to identify the localization of TAS2R10 receptors.

Key findings

The spontaneous contraction of rat ileum was decreased after chloroquine administration. Other bitter compounds, such as quinine, denatonium and saccharin, exhibited similar effects. Chloroquine-induced relaxation was not blocked by tetrodotoxin, but was partially reversed by the nitric oxide synthase inhibitor L-NAME or the large conductance Ca2+-activated K+ (BKCa) channel antagonist iberiotoxin. By surgically removing the small intestinal mucosa or bathing in Ca2+-free Krebs solution, the chloroquine-induced relaxation was largely attenuated. The immunofluorescence staining showed that TAS2R10 receptors were expressed in rat ileum.

Conclusions

The results indicate that bitter receptor agonists induce relaxation of longitudinal smooth muscle strips of rat ileum, which is mediated by nitric oxide and BKCa channels.

Changes in taste receptor cell [Ca2+]i modulate chorda tympani responses to bitter, sweet, and umami taste stimuli

The relationship between taste receptor cell (TRC) intracellular Ca(2+) ([Ca(2+)](i)) and rat chorda tympani (CT) nerve responses to bitter (quinine and denatonium), sweet (sucrose, glycine, and erythritol), and umami [monosodium glutamate (MSG) and MSG + inosine 5'-monophosphate (IMP)] taste stimuli was investigated before and after lingual application of ionomycin (Ca(2+) ionophore) + Ca(2+), 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM; Ca(2+) chelator), U73122 (phospholipase C blocker), thapsigargin (Ca(2+)-ATPase blocker), and diC8-PIP(2) (synthetic phosphatidylinositol 4,5-bisphosphate). The phasic CT response to quinine was indifferent to changes in [Ca(2+)](i). However, a decrease in [Ca(2+)](i) inhibited the tonic part of the CT response to quinine. The CT responses to sweet and umami stimuli were indifferent to changes in TRC [Ca(2+)](i). However, a decrease in [Ca(2+)](i) attenuated the synergistic effects of ethanol on the CT response to sweet stimuli and of IMP on the glutamate CT response. U73122 and thapsigargin inhibited the phasic and tonic CT responses to bitter, sweet, and umami stimuli. Although diC8-PIP(2) increased the CT response to bitter and sweet stimuli, it did not alter the CT response to glutamate but did inhibit the synergistic effect of IMP on the glutamate response. The results suggest that bitter, sweet, and umami taste qualities are transduced by [Ca(2+)](i)-dependent and [Ca(2+)](i)-independent mechanisms. Changes in TRC [Ca(2+)](i) in the BAPTA-sensitive cytosolic compartment regulate quality-specific taste receptors and ion channels that are involved in the neural adaptation and mixture interactions. Changes in TRC [Ca(2+)](i) in a separate subcompartment, sensitive to inositol trisphosphate and thapsigargin but inaccessible to BAPTA and ionomycin + Ca(2+), are associated with neurotransmitter release.

Changes in taste receptor cell [Ca2+]i modulate chorda tympani responses to salty and sour taste stimuli

The relationship between taste receptor cell (TRC) Ca(2+) concentration ([Ca(2+)](i)) and rat chorda tympani (CT) nerve responses to salty [NaCl and NaCl+benzamil (Bz)] and sour (HCl, CO(2), and acetic acid) taste stimuli was investigated before and after lingual application of ionomycin+Ca(2+), 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), U73122 (phospholipase C blocker), and thapsigargin (Ca(2+)-ATPase inhibitor) under open-circuit or lingual voltage-clamp conditions. An increase in TRC [Ca(2+)](i) attenuated the tonic Bz-sensitive NaCl CT response and the apical membrane Na(+) conductance. A decrease in TRC [Ca(2+)](i) enhanced the tonic Bz-sensitive and Bz-insensitive NaCl CT responses and apical membrane Na(+) conductance but did not affect CT responses to KCl or NH(4)Cl. An increase in TRC [Ca(2+)](i) did not alter the phasic response but attenuated the tonic CT response to acidic stimuli. A decrease in [Ca(2+)](i) did not alter the phasic response but attenuated the tonic CT response to acidic stimuli. In a subset of TRCs, a positive relationship between [H(+)](i) and [Ca(2+)](i) was obtained using in vitro imaging techniques. U73122 inhibited the tonic CT responses to NaCl, and thapsigargin inhibited the tonic CT responses to salty and sour stimuli. The results suggest that salty and sour taste qualities are transduced by [Ca(2+)](i)-dependent and [Ca(2+)](i)-independent mechanisms. Changes in TRC [Ca(2+)](i) in a BAPTA-sensitive cytosolic compartment regulate ion channels and cotransporters involved in the salty and sour taste transduction mechanisms and in neural adaptation. Changes in TRC [Ca(2+)](i) in a separate subcompartment, sensitive to inositol trisphosphate and thapsigargin but inaccessible to BAPTA, are associated with neurotransmitter release.

Age-related changes in mouse taste bud morphology, hormone expression, and taste responsivity

Normal aging is a complex process that affects every organ system in the body, including the taste system. Thus, we investigated the effects of the normal aging process on taste bud morphology, function, and taste responsivity in male mice at 2, 10, and 18 months of age. The 18-month-old animals demonstrated a significant reduction in taste bud size and number of taste cells per bud compared with the 2- and 10-month-old animals. The 18-month-old animals exhibited a significant reduction of protein gene product 9.5 and sonic hedgehog immunoreactivity (taste cell markers). The number of taste cells expressing the sweet taste receptor subunit, T1R3, and the sweet taste modulating hormone, glucagon-like peptide-1, were reduced in the 18-month-old mice. Concordant with taste cell alterations, the 18-month-old animals demonstrated reduced sweet taste responsivity compared with the younger animals and the other major taste modalities (salty, sour, and bitter) remained intact.

Expression of calcium binding proteins in mouse type II taste cells

It is well established that calcium is a critical signaling molecule in the transduction of taste stimuli within the peripheral taste system. However, little is known about the regulation and termination of these calcium signals in the taste system. The authors used Western blot, immunocytochemical, and RT-PCR analyses to evaluate the expression of multiple calcium binding proteins in mouse circumvallate taste papillae, including parvalbumin, calbindin D28k, calretinin, neurocalcin, NCS-1 (or frequenin), and CaBP. They found that all of the calcium binding proteins they tested were expressed in mouse circumvallate taste cells with the exception of NCS-1. The authors correlated the expression patterns of these calcium binding proteins with a marker for type II cells and found that neurocalcin was expressed in 80% of type II cells, whereas parvalbumin was found in less than 10% of the type II cells. Calretinin, calbindin, and CaBP were expressed in about half of the type II cells. These data reveal that multiple calcium binding proteins are highly expressed in taste cells and have distinct expression patterns that likely reflect their different roles within taste receptor cells.

Ryanodine receptors selectively contribute to the formation of taste-evoked calcium signals in mouse taste cells

The peripheral taste system uses multiple signaling pathways to transduce a stimulus into an output signal that activates afferent neurons. All of these signaling pathways depend on transient increases in intracellular calcium, but current understanding of these calcium signals is not well developed. Using molecular and physiological techniques, this study establishes that ryanodine receptors (RyRs), specifically isoform 1, are expressed in taste cells and that their physiological function differs among cell types employing different signaling pathways. RyR1 contributes to some taste-evoked signals that rely on calcium release from internal stores but can also supplement the calcium signal that is initiated by opening voltage-gated calcium channels. In taste cells expressing both signaling pathways, RyR1 contributes to the depolarization-induced calcium signal but not to the calcium signal that depends on calcium release from stores. These data suggest that RyR1 is an important regulator of calcium signaling and that its physiological role in taste cells is dictated by the nature of the calcium signaling mechanisms expressed.

Calcium signaling in taste cells: regulation required

Peripheral taste receptor cells depend on distinct calcium signals to generate appropriate cellular responses that relay taste information to the central nervous system. Some taste cells have conventional chemical synapses and rely on calcium influx through voltage-gated calcium channels. Other taste cells lack these synapses and depend on calcium release from stores to formulate an output signal through a hemichannel. Despite the importance of calcium signaling in taste cells, little is known about how these signals are regulated. This review summarizes recent studies that have identified 2 calcium clearance mechanisms expressed in taste cells, including mitochondrial calcium uptake and sodium/calcium exchangers (NCXs). These studies identified a unique constitutive calcium influx that contributes to maintaining appropriate calcium homeostasis in taste cells and the role of the mitochondria and exchangers in this process. The additional role of NCXs in the regulation of evoked calcium responses is also discussed. Clearly, calcium signaling is a dynamic process in taste cells and appears to be more complex than has previously been appreciated.

Sodium/calcium exchangers selectively regulate calcium signaling in mouse taste receptor cells

Taste cells use multiple signaling mechanisms to generate appropriate cellular responses to discrete taste stimuli. Some taste stimuli activate G protein coupled receptors (GPCRs) that cause calcium release from intracellular stores while other stimuli depolarize taste cells to cause calcium influx through voltage-gated calcium channels (VGCCs). While the signaling mechanisms that initiate calcium signals have been described in taste cells, the calcium clearance mechanisms (CCMs) that contribute to the termination of these signals have not been identified. In this study, we used calcium imaging to define the role of sodium-calcium exchangers (NCXs) in the termination of evoked calcium responses. We found that NCXs regulate the calcium signals that rely on calcium influx at the plasma membrane but do not significantly contribute to the calcium signals that depend on calcium release from internal stores. Our data indicate that this selective regulation of calcium signals by NCXs is due primarily to their location in the cell rather than to the differences in cytosolic calcium loads. This is the first report to define the physiological role for any of the CCMs utilized by taste cells to regulate their evoked calcium responses.

Evidence for two populations of bitter responsive taste cells in mice

Taste receptor cells use multiple signaling mechanisms to detect different taste stimuli in the oral cavity. Ionic stimuli (sour, salty) interact directly with ion channels to elicit responses, whereas bitter, sweet, and umami tastants activate G protein-coupled receptors to initiate phospholipase C (PLC)-dependent release of calcium from intracellular stores. However, the precise role for PLC in taste responses remains unclear. One study reported that bitter, sweet, and umami detection is abolished in PLCbeta2 knock-out animals, indicating that the perception of these stimuli depends solely on PLCbeta2. In contrast, another study found that PLCbeta2 knock-out mice have a reduced, but not abolished, capacity to detect these taste qualities, suggesting a PLCbeta2-independent signaling pathway may be involved in the detection of taste stimuli. Since PLCbeta2-expressing taste cells do not have conventional synapses or express voltage-gated calcium channels (VGCCs), we sought to determine if any taste cells responding to bitter express VGCCs. We characterized calcium responses generated by bitter stimuli to activate the PLC pathway and 50 mM KCl to activate VGCCs. Comparisons of evoked calcium responses found that these two stimuli generated significantly different responses. Surprisingly, although most responsive taste cells responded to bitter or 50 mM KCl, some taste cells responded to both. Analysis of dual responsive cells found that bitter responses were inhibited by the PLC inhibitor U73122. Immunocytochemical analysis detected PLCbeta3 and IP(3)R1, indicating the presence of multiple PLC signaling pathways in taste cells.

Effects of quinine on the intracellular calcium level and membrane potential of PC 12 cultures

The mechanism for the perception of bitterness appears to be quite complicated, even for quinine, which is a model bitter substance, and thus has yet to be completely elucidated. To investigate the possibility of being able to predict the bitterness of quinine solutions, we examined the effects of quinine on intracellular calcium ion concentration ([Ca(2+)]i) and membrane potentials in PC 12 cultures. [Ca(2+)]i and membrane potentials were analysed by fluorescence confocal microscopic imaging using the Ca(2+)-sensitive probe Calcium Green 1/AM and the membrane potential-sensitive probe bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC(4)(3)). Quinine elicited an increase in the membrane potential along with a concentration-dependent increase in [Ca(2+)]i. These increases were inhibited by extracellular Ca(2+)-free conditions, thapsigargin, which is a Ca(2+)-pump inhibitor, and U73122, which is a phospholipase C inhibitor. The quinine-induced increase in [Ca(2+)]i levels was inhibited by nifedipine, an L-type Ca(2+)-channel blocker, omega-conotoxin, a T-type Ca(2+)-channel blocker, and BMI-40, which is a bitterness-masking substance. These results suggest that responses in PC 12 cultures may be used as a simple model of bitterness perception.

Intragastric infusion of denatonium conditions flavor aversions and delays gastric emptying in rodents

Because most naturally occurring toxins taste bitter to humans, any mechanism that reduces the rate at which bitter substances are ingested and digested should be adaptive. Based on the recent discovery of T2R bitter taste receptors in the gastrointestinal tract of rodents, we asked whether intragastric (IG) infusion of denatonium (a ligand for T2R receptors) would condition a flavor aversion and/or delay gastric emptying. Four experiments tested for post-oral responses to denatonium in rodents. First, Sprague-Dawley rats were trained to associate intake of a flavored solution (the CS+) with IG denatonium infusions, and intake of a different-flavored solution (the CS-) with IG water infusions during 30 min/day sessions. The rats acquired an aversion to the CS+ flavor when it was paired with IG infusions of 10 mM (but not 2.5 mM) denatonium. Intragastric infusions of 10 mM denatonium also delayed gastric emptying of food in the same rats. Second, we asked how long it took for rats to suppress their drinking while being infused IG with 10 mM denatonium. Rats drinking a palatable solution paired with IG infusions of 10 mM denatonium suppressed their licking within 6 min, as compared to rats infused IG with water. Third, we trained C57BL/6J (B6) mice 24 h/day to associate a CS+ flavor paired with IG infusions of 12 mM denatonium (diluted to 6 mM by orally consumed CS+). Like rats, the mice acquired a robust aversion to the CS+ flavor when it was paired with IG infusions of denatonium. A final experiment assessed the potential toxicity of denatonium. To this end, we gave B6 mice a 6 mM denatonium solution as their only source of water for 3 weeks. The mice grew normally and did not display any clinical signs of denatonium toxicosis. This study provides the first evidence that rodents respond to the presence of "bitter" substances in their gastrointestinal tract by generating both behavioral and physiological responses.

Gustducin and transducin: a tale of two G proteins

In the vertebrate taste cell, heterotrimeric guanine nucleotide-binding proteins (G proteins) are involved in the transduction of both bitter and sweet taste stimulants. The bitter compound denatonium raises the intracellular Ca2+ concentration in rat taste cells, apparently via G protein-mediated increases in inositol trisphosphate. Sucrose causes a G protein-dependent generation of cAMP in rat taste bud membranes; elevation of cAMP levels leads to taste cell depolarization. To identify and characterize those proteins involved in the taste transduction process, we have cloned G protein alpha subunit (G alpha) cDNAs from rat taste cells. Using degenerate primers corresponding to conserved regions of G proteins, we used the polymerase chain reaction to amplify and clone taste cell G alpha cDNAs. Eight distinct G alpha cDNAs were isolated, cloned and sequenced from a taste cell library. Among these clones was alpha gustducin, a novel taste G alpha closely related to the transducins. In addition to alpha gustducin, we cloned rod and cone transducins from taste cells. This is the first identification of transducin expression outside photoreceptor cells. The primary sequence of alpha gustducin shows similarities to the transducins in the receptor interaction domain and the phosphodiesterase activation site. These sequence similarities suggest that gustducin and transducin regulate taste cell phosphodiesterase, probably in bitter taste transduction.

Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.

The transduction channel TRPM5 is gated by intracellular calcium in taste cells

Bitter, sweet, and umami tastants are detected by G-protein-coupled receptors that signal through a common second-messenger cascade involving gustducin, phospholipase C beta2, and the transient receptor potential M5 (TRPM5) ion channel. The mechanism by which phosphoinositide signaling activates TRPM5 has been studied in heterologous cell types with contradictory results. To resolve this issue and understand the role of TRPM5 in taste signaling, we took advantage of mice in which the TRPM5 promoter drives expression of green fluorescent protein and mice that carry a targeted deletion of the TRPM5 gene to unequivocally identify TRPM5-dependent currents in taste receptor cells. Our results show that brief elevation of intracellular inositol trisphosphate or Ca2+ is sufficient to gate TRPM5-dependent currents in intact taste cells, but only intracellular Ca2+ is able to activate TRPM5-dependent currents in excised patches. Detailed study in excised patches showed that TRPM5 forms a nonselective cation channel that is half-activated by 8 microM Ca2+ and that desensitizes in response to prolonged exposure to intracellular Ca2+. In addition to channels encoded by the TRPM5 gene, we found that taste cells have a second type of Ca2+-activated nonselective cation channel that is less sensitive to intracellular Ca2+. These data constrain proposed models for taste transduction and suggest a link between receptor signaling and membrane potential in taste cells.

Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.

TRPM5 and taste transduction

TRPM5 is a cation channel that it is essential for transduction of bitter, sweet and umami tastes. Signaling of these tastes involves the activation of G protein-coupled receptors that stimulate phospholipase C (PLC) beta2, leading to the breakdown of phosphatidylinositol bisphosphate (PIP2) into diacylglycerol (DAG) and inositol trisphosphate (IP3), and release of Ca2+ from intracellular stores. TRPM5 forms a nonselective cation channel that is directly activated by Ca2+ and it is likely to be the downstream target of this signaling cascade. Therefore, study of TRPM5 promises to provide insight into fundamental mechanisms of taste transduction. This review highlights recent work on the mechanisms of activation of the TRPM5 channel. The mouse TRPM5 gene encodes a protein of 1,158 amino acids that is proposed to have six transmembrane domains and to function as a tetramer. TRPM5 is structurally most closely related to the Ca(2+)-activated channel TRPM4 and it is more distantly related to the cold-activated channel TRPM8. In patch clamp recordings, TRPM5 channels are activated by micromolar concentrations of Ca2+ and are permeable to monovalent but not divalent cations. TRPM5 channel activity is strongly regulated by voltage, phosphoinositides and temperature, and is blocked by acid pH. Study of TRPM4 and TRPM8, which show similar modes of regulation, has yielded insights into possible structural domains of TRPM5. Understanding the structural basis for TRPM5 function will ultimately allow the design of pharmaceuticals to enhance or interfere with taste sensations.

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.

Separate populations of receptor cells and presynaptic cells in mouse taste buds

Taste buds are aggregates of 50-100 cells, only a fraction of which express genes for taste receptors and intracellular signaling proteins. We combined functional calcium imaging with single-cell molecular profiling to demonstrate the existence of two distinct cell types in mouse taste buds. Calcium imaging revealed that isolated taste cells responded with a transient elevation of cytoplasmic Ca2+ to either tastants or depolarization with KCl, but never both. Using single-cell reverse transcription (RT)-PCR, we show that individual taste cells express either phospholipase C beta2 (PLCbeta2) (an essential taste transduction effector) or synaptosomal-associated protein 25 (SNAP25) (a key component of calcium-triggered transmitter exocytosis). The two functional classes revealed by calcium imaging mapped onto the two gene expression classes determined by single-cell RT-PCR. Specifically, cells responding to tastants expressed PLCbeta2, whereas cells responding to KCl depolarization expressed SNAP25. We demonstrate this by two methods: first, through sequential calcium imaging and single-cell RT-PCR; second, by performing calcium imaging on taste buds in slices from transgenic mice in which PLCbeta2-expressing taste cells are labeled with green fluorescent protein. To evaluate the significance of the SNAP25-expressing cells, we used RNA amplification from single cells, followed by RT-PCR. We show that SNAP25-positive cells also express typical presynaptic proteins, including a voltage-gated calcium channel (alpha1A), neural cell adhesion molecule, synapsin-II, and the neurotransmitter-synthesizing enzymes glutamic acid decarboxylase and aromatic amino acid decarboxylase. No synaptic markers were detected in PLCbeta2 cells by either amplified RNA profiling or by immunocytochemistry. These data demonstrate the existence of at least two molecularly distinct functional classes of taste cells: receptor cells and synapse-forming cells.

Expression of galanin and the galanin receptor in rat taste buds

Galanin, a 29-amino-acid neuropeptide, was initially isolated from porcine intestine. It has a wide spread distribution in the central nervous system and is also present in the primary sensory neuron. Galanin has been suggested to be involved in numerous neuronal and endocrine functions as a neurotransmitter and neuromodulator. We examined the expression of galanin and galanin receptors by using a reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and in situ hybridization. RT-PCR analysis showed that mRNA of galanin and GalR2 were detected in the taste bud-containing epithelium of the circumvallate papilla of rats. Immunohistochemical analyses detected galanin was detected in a subset of taste bud cells of the circumvallate papillae. Double-label studies showed that galanin colocalized with alpha-gustducin, NCAM, and PLCbeta2. Our results of double staining with galanin and taste cell markers indicate that galanin-expressing taste cells are type II and type III cells. Taken together with previous studies, these findings show that galanin may function as a taste bud neurotransmitter. Furthermore, GalR2 mRNA was expressed in some taste bud cells. This suggests that, galanin release may not only excite the peripheral afferent nerve fiber but also may act on neighboring taste receptor cells via the activation of GalR2.

Human taste genetics

Humans show substantial differences in taste sensitivity to many different substances. Some of this variation is known to be genetic in origin, and many other inter-individual differences are likely to be partially or wholly determined by genetic mechanisms. Recent advances in the understanding of taste at the molecular level have provided candidate genes that can be evaluated for contributions to phenotypic differences in taste abilities. This approach has provided an understanding of variation in the ability to taste phenylthiocarbamide (PTC), and has resolved long-standing controversies about the genetics of this classic human genetic trait. Significant coding sequence variation exists in taste receptor genes, which suggests that PTC tasting may indicate more general taste sensory variation. However, many aspects of taste perception remain poorly characterized. Better understanding of the molecular components of salty and sour tastes is still needed, as is a more complete picture of second messenger and downstream signaling mechanisms for all taste modalities. More general studies of linkage and association between genetic markers and taste phenotypes may reveal genes encoding proteins that were previously unsuspected to be involved in this sensory process.

Calretinin immunoreactivity in taste buds and afferent fibers of the grey mullet Chelon labrosus

The presence of the calcium-binding protein calretinin in taste buds of a teleost, the thick-lipped grey mullet, was investigated using immunohistochemical techniques. Taste bud sensory cells had calretinin immunoreactivity. The nerve fiber plexus innervating taste buds, the ganglia and the viscerosensory roots projecting to the vagal lobe, also showed calretinin immunoreactivity. These results demonstrate for the first time the occurrence of calretinin in the taste buds and the taste afferent system of a teleost.

Elucidation of mammalian bitter taste

A family of approximately 30 TAS2R bitter taste receptors has been identified in mammals. Their genes evolved through adaptive diversification and are linked to chromosomal loci known to influence bitter taste in mice and humans. The agonists for various TAS2Rs have been identified and all of them were established as bitter tastants. TAS2Rs are broadly tuned to detect multiple bitter substances, explaining, in part, how mammals can recognize numerous bitter compounds with a limited set of receptors. The TAS2Rs are expressed in a subset of taste receptor cells, which are distinct from those mediating responses to other taste qualities. However, cells devoted to the detection of sweet, umami, and bitter stimuli share common signal transduction components. Transgenic expression of a human TAS2R in sweet or bitter taste receptor-expressing cells of mice induced either strong attraction or aversion to the receptor's cognate bitter tastant. Thus, dedicated taste receptor cells appear to function as broadly tuned detectors for bitter substances and are wired to elicit aversive behavior.

Extracellular proton sensing of the rat gustatory cyclic nucleotide-gated channel

Elevations of the intracellular levels of cyclic nucleotides appear to cause the cation influx through gustatory cyclic nucleotide-gated (CNGgust) channels expressed in taste cells. Although changes in the oral pH may directly regulate the activity of the CNGgust channel, the mechanism of pH-dependent control of the channel is not understood. In the present study, we combined the whole-cell patch-clamp recording and the site-directed mutagenesis to investigate the effect of extracellular pH on the ion permeation through CNGgust channels expressed in HEK293 cells. Extracellular acidification strongly inhibited ion permeation through open CNGgust channels. Mutation of Glu(289) remarkably attenuated the pH-dependence of the channel, suggesting that Glu(289) in the pore-forming region is a major proton acceptor site. However, the mutant E289A-CNGgust channel possesses the other residual protonation/deprotonation site. The channel activity, tightly regulated by pH(o) and [cNMP](i), suggests the involvement of its pH(o)-dependent ion permeation in taste signal transduction events.

Intracellular Ca2+ and the phospholipid PIP2 regulate the taste transduction ion channel TRPM5

The transduction of taste is a fundamental process that allows animals to discriminate nutritious from noxious substances. Three taste modalities, bitter, sweet, and amino acid, are mediated by G protein-coupled receptors that signal through a common transduction cascade: activation of phospholipase C beta2, leading to a breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate, which causes release of Ca2+ from intracellular stores. The ion channel, TRPM5, is an essential component of this cascade; however, the mechanism by which it is activated is not known. Here we show that heterologously expressed TRPM5 forms a cation channel that is directly activated by micromolar concentrations of intracellular Ca2+ (K1/2 = 21 microM). Sustained exposure to Ca2+ desensitizes TRPM5 channels, but PIP2 reverses desensitization, partially restoring channel activity. Whole-cell TRPM5 currents can be activated by intracellular Ca2+ and show strong outward rectification because of voltage-sensitive gating of the channels. TRPM5 channels are nonselective among monovalent cations and not detectably permeable to divalent cations. We propose that the regulation of TRPM5 by Ca2+ mediates sensory activation in the taste system.

Three sweet receptor genes are clustered in human chromosome 1

A search of the human genome database led us to identify three human candidate taste receptors, hT1R1, hT1R2, and hT1R3, which contain seven transmembrane domains. All three genes map to a small region of Chromosome (Chr) 1. This region is syntenous to the distal end of Chr 4 in mouse, which contains the Sac (saccharin preference) locus that is involved in detecting sweet tastants. A genetic marker, DVL1, which is linked to the Sac locus, is within 1700 bp of human T1R3. Recently, the murine T1Rs and its human ortholog have been independently identified in combination as sweet and umami receptors near the Sac locus. All three hT1Rs genes are expressed selectively in human taste receptor cells in the fungiform papillae, consistent with their role in taste perception.

Individual mouse taste cells respond to multiple chemical stimuli

Sensory organs are specialized to detect and decode stimuli in terms of intensity and quality. In the gustatory system, the process of identifying and distinguishing taste qualities (e.g. bitter versus sweet) begins in taste buds. A central question in gustatory research is how information about taste quality is extracted by taste receptor cells. For instance, whether and how individual taste cells respond to multiple chemical stimuli is still a matter for debate. A recent study showed that taste cells expressing bitter-responsive taste receptors do not also express sweet-responsive taste receptors and vice versa. These results suggest that the gustatory system may use separate cellular pathways to process bitter and sweet signals independently. Results from electrophysiological studies, however, reveal that individual taste receptor cells respond to stimuli representing multiple taste qualities. Here we used non-invasive Ca(2+) imaging in slices of lingual tissue containing taste buds to address the issue of quality detection in murine taste receptor cells. We recorded calcium transients elicited by chemical stimuli representing different taste qualities (sweet, salty, sour and bitter). Many receptor cells (38 %) responded to multiple taste qualities, with some taste cells responding to both appetitive ("sweet") and aversive ("bitter") stimuli. Thus, there appears to be no strict and separate detection of taste qualities by distinct subpopulations of taste cells in peripheral gustatory sensory organs in mice.

Dual actions of caffeine on voltage-dependent currents and intracellular calcium in taste receptor cells

Although the numerous stimuli representing the taste quality of bitterness are known to be transduced through multiple mechanisms, recent studies have suggested an unpredicted complexity of the transduction pathways for individual bitter stimuli. To investigate this notion more thoroughly, a single prototypic bitter stimulus, caffeine, was studied by using patch-clamp and ratiometric imaging techniques on dissociated rat taste receptor cells. At behaviorally relevant concentrations, caffeine produced strong inhibition of outwardly and inwardly rectifying potassium currents. Caffeine additionally inhibited calcium current, produced a weaker inhibition of sodium current, and was without effect on chloride current. Consistent with its effects on voltage-dependent currents, caffeine caused a broadening of the action potential and an increase of the input resistance. Caffeine was an effective stimulus for elevation of intracellular calcium. This elevation was concentration dependent, independent of extracellular calcium or ryanodine, and dependent on intracellular stores as evidenced by thapsigargin treatment. These dual actions on voltage-activated ionic currents and intracellular calcium levels suggest that a single taste stimulus, caffeine, utilizes multiple transduction mechanisms.

Acetylcholine increases intracellular Ca2+ in taste cells via activation of muscarinic receptors

Previous studies suggest that acetylcholine (ACh) is a transmitter released from taste cells as well as a transmitter in cholinergic efferent neurons innervating taste buds. However, the physiological effects on taste cells have not been established. I examined effects of ACh on taste-receptor cells by monitoring [Ca2+]i. ACh increased [Ca2+]i in both rat and mudpuppy taste cells. Atropine blocked the ACh response, but D-tubocurarine did not. U73122, a phospholipase C inhibitor, and thapsigargin, a Ca2+-ATPase inhibitor that depletes intracellular Ca2+ stores, blocked the ACh response. These results suggest that ACh binds to M1/M3/M5-like subtypes of muscarinic ACh receptors, causing an increase in inositol 1,4,5-trisphosphate and subsequent release of Ca2+ from the intracellular stores. A long incubation with ACh induced a transient response followed by a sustained phase of [Ca2+]i increase. In Ca2+-free solution, the sustained phases disappeared, suggesting that Ca2+ influx is involved in the sustained phase. Depletion of Ca2+ stores by thapsigargin alone induced Ca2+ influx. These findings suggest that Ca2+ store-operated channels may be present in taste cells and that they may participate in the sustained phase of [Ca2+]i increase. Immunocytochemical experiments indicated that the M1 subtype of muscarinic receptors is present in both rat and mudpuppy taste cells.

Taste receptor cell responses to the bitter stimulus denatonium involve Ca2+ influx via store-operated channels

Previous studies in rat and mouse have shown that brief exposure to the bitter stimulus denatonium induces an increase in [Ca2+]i due to Ca2+ release from intracellular Ca2+ stores, rather than Ca2+ influx. We report here that prolonged exposure to denatonium induces sustained increases in [Ca2+]i that are dependent on Ca2+ influx. Similar results were obtained from taste cells of the mudpuppy, Necturus maculosus, as well as green fluorescent protein (GFP) tagged gustducin-expressing taste cells of transgenic mice. In a subset of mudpuppy taste cells, prolonged exposure to denatonium induced oscillatory Ca2+ responses. Depletion of Ca2+ stores by thapsigargin also induced Ca2+ influx, suggesting that Ca2+ store-operated channels (SOCs) are present in both mudpuppy taste cells and gustducin-expressing taste cells of mouse. Further, treatment with thapsigargin prevented subsequent responses to denatonium, suggesting that the SOCs were the source of the Ca2+ influx. These data suggest that SOCs may contribute to bitter taste transduction and to regulation of Ca2+ homeostasis in taste cells.

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.

Bitter substances suppress afferent responses to an appetitive mixture: evidence for peripheral integration of chemosensory stimuli

The processes that lead from detection of chemicals, transduction, and coding with the appropriate message to initiate ingestion of a palatable meal or to reject a potentially noxious substance are poorly understood in vertebrates owing to the complex organization of the taste system. As a first step in elucidating the cellular basis of the behavioral differences elicited by appetitive stimuli and bitter compounds, we recorded from the afferent nerves conveying peripheral chemosensory information to the CNS in the head of the leech, Hirudo medicinalis. Superfusion of the chemosensory region of the lip of Hirudo with a mixture of NaCl (150 mM) and arginine (1 mM), an appetitive solution that elicits ingestion, increased the neuronal activity in the afferent cephalic nerves, for example (Zhang X, Wilson RJ, Li Y, Kleinhaus AL. 2000. Chemical and thermal stimuli have short-lived effects on the Retzius cell in the medicinal leech. J Neurobiol 43:304-311.). In the present paper we show that superfusing the lip with quinine or denatonium reduced the basal neural activity in the afferents. Furthermore, these bitter substances in the appetitive solution counteracted the increased activity the appetitive solution evoked in the cephalic nerves. Thus, the neural activity evoked by the application of appetitive and aversive stimuli to the chemosensory area of the lip paralleled the opposite behavioral responses to the same chemicals. The results suggest that individual leech taste cells possess receptors for both types of stimuli. Therefore, the leech may be a good model system in which to study peripheral taste events in cells that may possess multiple receptors and transduction mechanisms that interact to integrate information.

Differentiation of the lingual and palatal gustatory epithelium of the rat as revealed by immunohistochemistry of alpha-gustducin

We used alpha-gustducin, a taste-cell-specific G protein to investigate the onset of taste transduction and its relation to the development of the palatal and lingual taste buds. Frozen cryostat and paraffin sections were prepared from the palatal and lingual gustatory epithelium of the rat from birth till postnatal day 21 (PN 21d). At PN 1-7d, alpha-gustducin-immunoreactive solitary ovoid or bipolar cells were scattered among the oral epithelium either horizontally along the oral surface or vertically oriented between the basal lamina and oral surface. In the circumvallate and foliate papillae, these cells became wrapped in alpha-gustducin-immunonegative cells surrounded by an extracellular space forming a bud-like structure. Simultaneously, different stages of typical taste buds were recognized, but alpha-gustducin was only expressed in some neonatally developed pored buds. At PN 1d, alpha-gustducin was expressed in pored taste buds with a relatively higher frequency recorded in the soft palate as compared with the nasoincisor, circumvallate, and foliate papillae. The immunoreactive cells were spindle shaped with elongated processes extending from the base to the pore of the taste buds. During the second week, the solitary cells could no longer be recognized while the total counts of immunoreactive cells within the taste buds gradually increased. We argue that taste transduction is essentially required from the time of birth and can be fulfilled by both of the solitary chemosensory cells, which are immunoreactive for alpha-gustducin and scattered in the oral epithelium, and the taste cells within the mature taste buds. Moreover, the onset of taste transduction accomplished by the palatal taste buds developed earlier than that achieved by taste buds in the circumvallate and foliate papillae.

Receptors and transduction in taste

Taste is the sensory system devoted primarily to a quality check of food to be ingested. Although aided by smell and visual inspection, the final recognition and selection relies on chemoreceptive events in the mouth. Emotional states of acute pleasure or displeasure guide the selection and contribute much to our quality of life. Membrane proteins that serve as receptors for the transduction of taste have for a long time remained elusive. But screening the mass of genome sequence data that have recently become available has provided a new means to identify key receptors for bitter and sweet taste. Molecular biology has also identified receptors for salty, sour and umami taste.

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.

Electrophysiological characteristics of rat gustatory cyclic nucleotide--gated channel expressed in Xenopus oocytes

The complementary DNA encoding gustatory cyclic nucleotide--gated ion channel (or gustCNG channel) cloned from rat tongue epithelial tissue was expressed in Xenopus oocytes, and its electrophysiological characteristics were investigated using tight-seal patch-clamp recordings of single and macroscopic channel currents. Both cGMP and cAMP directly activated gustCNG channels but with markedly different affinities. No desensitization or inactivation of gustCNG channel currents was observed even in the prolonged application of the cyclic nucleotides. Single-channel conductance of gustCNG channel was estimated as 28 pS in 130 mM of symmetric Na(+). Single-channel current recordings revealed fast open-close transitions and longer lasting closure states. The distribution of both open and closed events could be well fitted with two exponential components and intracellular cGMP increased the open probability (P(o)) of gustCNG channels mainly by increasing the slower opening rate. Under bi-ionic conditions, the selectivity order of gustCNG channel among divalent cations was determined as Na(+) approximately K(+) > Rb(+) > Li(+) > Cs(+) with the permeability ratio of 1:0.95:0.74:0.63:0.49. Magnesium ion blocked Na(+) currents through gustCNG channels from both intracellular and extracellular sides in voltage-dependent manners. The inhibition constants (K(i)s) of intracellular Mg(2+) were determined as 360 +/- 40 microM at 70 mV and 8.2 +/- 1.5 mM at -70 mV with z delta value of 1.04, while K(i)s of extracellular Mg(2+) were as 1.1 +/- 0.3 mM at 70 mV and 20.0 +/- 0.1 microM at -70 mV with z delta of 0.94. Although 100 microM l-cis-diltiazem blocked significant portions of outward Na(+) currents through both bovine rod and rat olfactory CNG channels, the gustCNG channel currents were minimally affected by the same concentration of the drug.