Rapid kinetics of second messenger production in bitter taste

Novel, Fully Characterised Bovine Taste Bud Cells of Fungiform Papillae

Current understanding of functional characteristics and biochemical pathways in taste bud cells have been hindered due the lack of long-term cultured cells. To address this, we developed a holistic approach to fully characterise long term cultured bovine taste bud cells (BTBCs). Initially, cultured BTBCs were characterised using RT-PCR gene expression profiling, immunocytochemistry, flowcytometry and calcium imaging, that confirmed the cells were mature TBCs that express taste receptor genes, taste specific protein markers and capable of responding to taste stimuli, i.e., denatonium (2 mM) and quinine (462.30 μM). Gene expression analysis of forty-two genes implicated in taste transduction pathway (map04742) using custom-made RT-qPCR array revealed high and low expressed genes in BTBCs. Preliminary datamining and bioinformatics demonstrated that the bovine α-gustducin, gustatory G-protein, have higher sequence similarity to the human orthologue compared to rodents. Therefore, results from this work will replace animal experimentation and provide surrogate cell-based throughput system to study human taste transduction.

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.

TRPM4 and TRPM5 are both required for normal signaling in taste receptor cells

Peripheral taste receptor cells use multiple signaling pathways to transduce taste stimuli into output signals that are sent to the brain. Transient receptor potential melastatin 5 (TRPM5), a sodium-selective TRP channel, functions as a common downstream component in sweet, bitter, and umami signaling pathways. In the absence of TRPM5, mice have a reduced, but not abolished, ability to detect stimuli, suggesting that a TRPM5-independent pathway also contributes to these signals. Here, we identify a critical role for the sodium-selective TRP channel TRPM4 in taste transduction. Using live cell imaging and behavioral studies in KO mice, we show that TRPM4 and TRPM5 are both involved in taste-evoked signaling. Loss of either channel significantly impairs taste, and loss of both channels completely abolishes the ability to detect bitter, sweet, or umami stimuli. Thus, both TRPM4 and TRPM5 are required for transduction of taste stimuli.

G-Protein-Dependent and -Independent Pathways in Denatonium Signal Transduction

To clarify the involvement of G protein in denatonium signal transduction, we carried out a whole-cell patch-clamp analysis with isolated taste cells in mice. Two different responses were observed by applying GDP-beta-S, a G-protein inhibitor. One response to denatonium was reduced by GDP-beta-S (G-protein-dependent), whereas the other was not affected (G-protein-independent). These different patterns were also observed by concurrently inhibiting the phospholipase C beta2 and phosphodiesterase pathways via G protein. These data suggest dual, G-protein-dependent and -independent mechanisms for denatonium. Moreover, the denatonium responses were not attenuated by singly inhibiting the phospholipase C beta2 or phosphodiesterase pathway, implying that both pathways were involved in G-protein-dependent transduction. In the G-protein-independent cells, the response was abolished by the depletion of calcium ions within the intracellular store. These results suggest that Ca2+ release from the intracellular store is an important factor. Our data demonstrate multiple transduction pathways for denatonium in mammalian taste cells.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

Electronic tongue: An analytical gustatory tool

Taste is an important organoleptic property governing acceptance of products for administration through mouth. But majority of drugs available are bitter in taste. For patient acceptability and compliance, bitter taste drugs are masked by adding several flavoring agents. Thus, taste assessment is one important quality control parameter for evaluating taste-masked formulations. The primary method for the taste measurement of drug substances and formulations is by human panelists. The use of sensory panelists is very difficult and problematic in industry and this is due to the potential toxicity of drugs and subjectivity of taste panelists, problems in recruiting taste panelists, motivation and panel maintenance are significantly difficult when working with unpleasant products. Furthermore, Food and Drug Administration (FDA)-unapproved molecules cannot be tested. Therefore, analytical taste-sensing multichannel sensory system called as electronic tongue (e-tongue or artificial tongue) which can assess taste have been replacing the sensory panelists. Thus, e-tongue includes benefits like reducing reliance on human panel. The present review focuses on the electrochemical concepts in instrumentation, performance qualification of E-tongue, and applications in various fields.

A conditioned aversion study of sucrose and SC45647 taste in TRPM5 knockout mice

Previously, published studies have reported mixed results regarding the role of the TRPM5 cation channel in signaling sweet taste by taste sensory cells. Some studies have reported a complete loss of sweet taste preference in TRPM5 knockout (KO) mice, whereas others have reported only a partial loss of sweet taste preference. This study reports the results of conditioned aversion studies designed to motivate wild-type (WT) and KO mice to respond to sweet substances. In conditioned taste aversion experiments, WT mice showed nearly complete LiCl-induced response suppression to sucrose and SC45647. In contrast, TRPM5 KO mice showed a much smaller conditioned aversion to either sweet substance, suggesting a compromised, but not absent, ability to detect sweet taste. A subsequent conditioned flavor aversion experiment was conducted to determine if TRPM5 KO mice were impaired in their ability to learn a conditioned aversion. In this experiment, KO and WT mice were conditioned to a mixture of SC45647 and amyl acetate (an odor cue). Although WT mice avoided both components of the stimulus mixture, they avoided SC45647 more than the odor cue. The KO mice also avoided both stimuli, but they avoided the odor component more than SC45647, suggesting that while the KO mice are capable of learning an aversion, to them the odor cue was more salient than the taste cue. Collectively, these findings suggest the TRPM5 KO mice have some residual ability to detect SC45647 and sucrose, and, like bitter, there may be a TRPM5-independent transduction pathway for detecting these substances.

Effect of bitter compounds on amylase secretion in murine submandibular glands: Signaling pathway mechanisms

BACKGROUND

Amylase is synthesized in submandibular glands (SMG) and released into the oral cavity to degrade carbohydrates in the mouth. Bitter taste receptors (T2R) belong to the G-protein coupled receptor (GPCR) family and are expressed in the taste cells and also in the digestive tract.

METHODS

The activity of amylase secreted by murine SMG was measured, detecting maltose by Bernfeld's method. Amylase and T2R6 were detected by imunohistochemistry and Western blot. The expression of Ggustducin, Gi, and phospholipase Cβ2 was also studied by Western blot. cAMP levels were measured by radioimmunoassay and inositol monophosphate production was quantified by ELISA.

RESULTS

Theophylline, denatonium and cycloheximide exerted a dose-dependent inhibition on amylase secretion. This effect was reverted by preincubating SMG with an anti-Gαi antibody. cAMP production was increased by the same compounds, an effect that was also abrogated by an anti-Gαi antibody. Bitter compounds reduced inositol monophosphate formation in SMG and H-89, a protein kinase A inhibitor, reverted this action, revealing that this protein kinase down regulates phospholipase C activity.

GENERAL SIGNIFICANCE

We demonstrated that theophylline, denatonium and cycloheximide inhibit salivary amylase secretion, activating an intracellular signaling pathway that involves cAMP and phospholipase C, that cross talks via protein kinase A.

ROS-GC subfamily membrane guanylate cyclase-linked transduction systems: taste, pineal gland and hippocampus

In the continuous efforts to test the validity of the theme that the Ca(2+)-modulated ROS-GC subfamily system is a universal transduction component of the sensory and sensory-linked network of neurons, this article focuses on the presence and variant biochemical forms of this transduction system in the gustatory epithelium, the site of gustatory transduction; in the pineal, a light-sensitive gland; and in the hippocampus neurons, linked with the perception of all SENSES.

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.

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.

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.

Bitterness Suppression of BCAA Solutions by L-Ornithine

The purpose of the present study was to evaluate the bitterness-suppressing effect of L-ornithine (L-Orn) on single or mixed solutions of branched-chain amino acids (BCAAs) using human gustatory sensation tests and an artificial taste sensor. The BCAAs tested (L-isoleucine (L-Ile), L-leucine (L-Leu), and L-valine (L-Val)) are the main components of various enteral nutrients or supplements. The bitterness-suppression effect of L-Orn was also compared with the effect of L-Arg. L-Orn was effective in suppressing the bitterness of single or mixed solutions of BCAAs in human gustatory sensation tests, the effect being similar to or greater than that of L-Arg. The artificial taste sensor was able to predict the bitterness-suppressing effects of L-Orn and L-Arg. The response electric potential patterns of L-Val, L-Leu and L-Ile solutions to which 100 mM L-Arg had been added were quite similar to the sensor response patterns of the 100 mM L-Arg solutions alone. The relative response electric potential patterns of L-Val, L-Leu or L-Ile solutions containing 100 mM L-Orn in channels 5-8 (positively charged) are similar to that of single solution of 100 mM L-Orn.

PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands

Using a brief-access taste assay, we show in the present report that although phospholipase C beta2 knockout (PLCbeta2 KO) mice are unresponsive to low- and midrange concentrations of quinine and denatonium, they do significantly avoid licking higher concentrations of these aversive compounds. PLCbeta2 KO mice displayed no concentration-dependent licking of the prototypical sweetener sucrose but were similar to wild-type mice in their responses to citric acid and NaCl, notwithstanding some interesting exceptions. Although these findings confirm an essential role for PLCbeta2 in taste responsiveness to sucrose and to low- to midrange concentrations of quinine and denatonium in mice as previously reported, they importantly suggest that higher concentrations of the latter two compounds, which are bitter to humans, can engage a PLCbeta2-independent taste transduction pathway.

Mouse taste buds use serotonin as a neurotransmitter

Synapses between gustatory receptor cells and primary sensory afferent fibers transmit the output signal from taste buds to the CNS. Several transmitter candidates have been proposed for these synapses, including serotonin (5-HT), glutamate, acetylcholine, ATP, peptides, and others, but, to date, none has been unambiguously identified. We used Chinese hamster ovary cells stably expressing 5-HT2C receptors as biodetectors to monitor 5-HT release from taste buds. When taste buds were depolarized with KCl or stimulated with bitter, sweet, or sour (acid) tastants, serotonin was released. KCl- and acid-induced 5-HT release, but not release attributable to sweet or bitter stimulation, required Ca2+ influx. In contrast, 5-HT release evoked by sweet and bitter stimulation seemed to be triggered by intracellular Ca2+ release. These experiments strongly implicate serotonin as a taste bud neurotransmitter and reveal unexpected transmitter release mechanisms.

Screening of Bitterness-Suppressing Agents for Quinine: The Use of Molecularly Imprinted Polymers

The purpose of the present study was to examine the possibility of using molecularly imprinted polymers (MIPs) to screen for bitterness-suppressing agents. Quinine was selected as the bitter substance standard. L-arginine (L-Arg), L-ornithine (L-Orn), L-lysine (L-Lys), and L-citrulline (L-Ctr) were tested as bitterness suppressant candidates. In a high-performance liquid chromatography study using a uniformly sized MIP for cinchonidine, which has a very similar structure to quinine, the retention factor (k) of quinine was significantly shortened by the addition of L-Arg or L-Orn to the mobile phase, whereas slight or no decrease was observed when L-Ctr and L-Lys were added. The abilities of these amino acids to decrease the k of quinine were ranked in the following order: L-Arg = (L-Orn >(L-Ctr >>(L-Lys. A linear relationship between the reciprocal of k and the concentration of the amino acids indicated a single competitive model at a single site. The magnitude of the association constants obtained seemed to be directly related to the inhibitory effect of the test substances on the affinity of quinine for the receptor site. Nuclear magnetic resonance and molecular modeling studies suggested a one-to-two hydrogen-bonding-based complex formation of one quinine molecule with two methacrylic acid molecules (Q-2MAA) in chloroform. In the molecular modeling studies, the N--N distance of the quinine molecule in the assumed Q-2MAA complex was calculated to be 5.12 angstroms, similar to the N - N distances of the two amino acid complexes (L-Arg-2MAA, L-Orn-2MAA), which were 4.84 and 5.30 angstroms, respectively. This suggests that L-Arg and L-Orn may compete with the quinine molecule in the cinchonidine-imprinted space. Finally, the results of human gustatory sensation tests correlated well with the MIP data. The proposed method using MIPs seems to have a potential for screening bitterness-suppressing agents for quinine.

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.

Phospholipase C-beta 2 as a mammalian taste signaling marker is expressed in the multiple gustatory tissues of medaka fish, Oryzias latipes

Phospholipase C-beta 2 (PLC-beta 2) is a key enzyme in mammalian taste signal transduction. To analyze the taste system in fishes at molecular level, we cloned mfplc-beta 2 as a medaka fish homologue of PLC-beta 2. In situ hybridization analysis revealed that mfplc-beta 2 is expressed in the lip and branchial region where chemosensory tissues are distributed. Immunohistochemical detection of nerve fibers near the mfplc-beta 2 positive cells suggests the characteristic of peripheral sensory cells. These results suggest that mfplc-beta 2 is expressed in the gustatory sensory cells of medaka. This may provide a molecular basis for the taste reception at multiple tissues in fish species.

RGS21 is a novel regulator of G protein signalling selectively expressed in subpopulations of taste bud cells

Abstract G-protein-mediated signalling processes are involved in sweet and bitter taste transduction. In particular, the G protein alpha-subunit gustducin has been implicated in these processes. One of the limiting factors for the time-course of cellular responses induced by tastants is therefore the intrinsic GTPase activity of alpha-gustducin, which determines the lifetime of the active G protein complex. In several signalling systems specific 'regulator of G protein signalling' (RGS) proteins accelerate the GTPase activity of G protein alpha-subunits. Using differential screening approaches, we have identified a novel RGS protein termed RGS21, which represents the smallest known member of this protein family. Reverse transcription polymerase chain reaction and in situ hybridization experiments demonstrated that RGS21 is expressed selectively in taste tissue where it is found in a subpopulation of sensory cells. Furthermore, it is coexpressed in individual taste cells with bitter and sweet transduction components including alpha-gustducin, phospholipase Cbeta2, T1R2/T1R3 sweet taste receptors and T2R bitter taste receptors. In vitro binding assays demonstrate that RGS21 binds alpha-gustducin in a conformation-dependent manner and has the potential to interact with the same Galpha subtypes as T1R receptors. These results suggest that RGS21 could play a regulatory role in bitter as well as sweet taste transduction processes.

Some sweet and bitter tastants stimulate inhibitory pathway of adenylyl cyclase via melatonin and alpha 2-adrenergic receptors in Xenopus laevis melanophores

The sweeteners saccharin, D-tryptophan, and neohesperidin dihydrochalcone (NHD) and the bitter tastant cyclo(Leu-Trp) stimulated concentration-dependent pigment aggregation in a Xenopus laevis melanophore cell line similar to melatonin. Like melatonin, these tastants inhibited (by 45-92%) cAMP formation in melanophores; pertussis toxin pretreatment almost completely abolished the tastant-induced cAMP inhibition, suggesting the involvement of the inhibitory pathway (Gi) of adenylyl cyclase. The presence of luzindole (melatonin receptor antagonist) almost completely abolished the inhibition of cAMP formation induced by saccharin, D-tryptophan, and cyclo(Leu-Trp) but only slightly affected the inhibitory effect of NHD. In contrast, the presence of an alpha2-adrenergic receptor antagonist, yohimbine, almost completely abolished the inhibition of cAMP formation induced by NHD but had only a minor effect on that induced by the other tastants. Thus saccharin, D-tryptophan, and cyclo(Leu-Trp) are melatonin receptor agonists whereas NHD is an alpha2-adrenergic receptor agonist, but both pathways lead to the same transduction output and cellular response. Formation of D-myo-inositol 1,4,5-trisphosphate (IP3) in melanophores was reduced (15-58%, no concentration dependence) by saccharin, D-tryptophan, and cyclo(Leu-Trp) stimulation but increased by NHD stimulation. Tastant stimulation did not affect cGMP. Although some of the above tastants were found to be membrane permeant, their direct activation of downstream transduction components in this experimental system is questionable. MT1 and MT2 melatonin receptor mRNAs were identified in rat circumvallate papilla taste buds and nonsensory epithelium, suggesting the occurrence of MT1 and MT2 receptors in these tissues. Melatonin stimulation reduced the cellular content of cAMP in taste cells, which may or may not be related to taste sensation.

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.

Effect of Quinine Solutions on Intracellular Ca2+ Levels in Neuro-2a Cells-Conventional Physiological Method for the Evaluation of Bitterness-

The purpose of the present study was to examine the effect of quinine on intracellular Ca2+ ([Ca2+]i) levels in cultured neuro-2a cells, and to investigate the possibility of using [Ca2+]i levels to predict the bitterness of quinine solutions. [Ca2+]i levels in neuro-2a cells increased following stimulation by quinine in a concentration-related manner. There was a good linear correlationship between the quinine-induced increase in [Ca2+]i levels increase and the bitterness scores of the quinine solutions as assessed in human gustatory sensation tests (r2=0.918). The quinine-induced increase in [Ca2+]i levels was inhibited by thapsigargin (an inhibitor of the Ca2+ pump into intracellular stores), U73122 (an inhibitor of phospholipase C) and omega-conotoxin (an N-type Ca2+-channel blocker), but not by nifedipine (an L-type Ca2+-channel blocker).

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.

Dominant loss of responsiveness to sweet and bitter compounds caused by a single mutation in alpha -gustducin

Biochemical and genetic studies have implicated alpha-gustducin as a key component in the transduction of both bitter or sweet taste. Yet, alpha-gustducin-null mice are not completely unresponsive to bitter or sweet compounds. To gain insights into how gustducin mediates responses to bitter and sweet compounds, and to elicit the nature of the gustducin-independent pathways, we generated a dominant-negative form of alpha-gustducin and expressed it as a transgene from the alpha-gustducin promoter in both wild-type and alpha-gustducin-null mice. A single mutation, G352P, introduced into the C-terminal region of alpha-gustducin critical for receptor interaction rendered the mutant protein unresponsive to activation by taste receptor, but left its other functions intact. In control experiments, expression of wild-type alpha-gustducin as a transgene in alpha-gustducin-null mice fully restored responsiveness to bitter and sweet compounds, formally proving that the targeted deletion of the alpha-gustducin gene caused the taste deficits of the null mice. In contrast, transgenic expression of the G352P mutant did not restore responsiveness of the null mice to either bitter or sweet compounds. Furthermore, in the wild-type background, the mutant transgene inhibited endogenous alpha-gustducin's interactions with taste receptors, i.e., it acted as a dominant-negative. That the mutant transgene further diminished the residual bitter and sweet taste responsiveness of the alpha-gustducin-null mice suggests that other guanine nucleotide-binding regulatory proteins expressed in the alpha-gustducin lineage of taste cells mediate these responses.

Soa genotype selectively affects mouse gustatory neural responses to sucrose octaacetate

In mice, behavioral acceptance of the bitter compound sucrose octaacetate (SOA) depends on allelic variation of a single gene, Soa. The SW.B6-Soa(b)congenic mouse strain has the genetic background of an "SOA taster" SWR/J strain and an Soa-containing donor chromosome fragment from an "SOA nontaster" C57BL/6J strain. Using microsatellite markers polymorphic between the two parental strains, we determined that the donor fragment spans 5-10 cM of distal chromosome 6. The SWR/J mice avoided SOA in two-bottle tests with water and had strong responses to SOA in two gustatory nerves, the chorda tympani (CT) and glossopharyngeal (GL). In contrast, the SW.B6-Soa(b) mice were indifferent to SOA in two-bottle tests and had very weak responses to SOA in both of these nerves. The SWR/J and SW.B6-Soa(b) mice did not differ in responses of either nerve to sucrose, NaCl, HCl, or the bitter-tasting stimuli quinine, denatonium, strychnine, 6-n-propylthiouracil, phenylthiocarbamide, and MgSO(4). Thus the effect of the Soa genotype on SOA avoidance is mediated by peripheral taste responsiveness to SOA, involving taste receptor cells innervated by both the CT and GL nerves.

Immunocytochemical evidence for co-expression of Type III IP3 receptor with signaling components of bitter taste transduction

BACKGROUND

Taste receptor cells are responsible for transducing chemical stimuli into electrical signals that lead to the sense of taste. An important second messenger in taste transduction is IP3, which is involved in both bitter and sweet transduction pathways. Several components of the bitter transduction pathway have been identified, including the T2R/TRB taste receptors, phospholipase C beta2, and the G protein subunits alpha-gustducin, beta3, and gamma13. However, the identity of the IP3 receptor subtype in this pathway is not known. In the present study we used immunocytochemistry on rodent taste tissue to identify the IP3 receptors expressed in taste cells and to examine taste bud expression patterns for IP3R3.

RESULTS

Antibodies against Type I, II, and III IP3 receptors were tested on sections of rat and mouse circumvallate papillae. Robust cytoplasmic labeling for the Type III IP3 receptor (IP3R3) was found in a large subset of taste cells in both species. In contrast, little or no immunoreactivity was seen with antibodies against the Type I or Type II IP3 receptors. To investigate the potential role of IP3R3 in bitter taste transduction, we used double-label immunocytochemistry to determine whether IP3R3 is expressed in the same subset of cells expressing other bitter signaling components. IP3R3 immunoreactive taste cells were also immunoreactive for PLCbeta2 and gamma13. Alpha-gustducin immunoreactivity was present in a subset of IP3R3, PLCbeta2, and gamma13 positive cells.

CONCLUSIONS

IP3R3 is the dominant form of the IP3 receptor expressed in taste cells and our data suggest it plays an important role in bitter taste transduction.

Bitter taste transduced by PLC-beta(2)-dependent rise in IP(3) and alpha-gustducin-dependent fall in cyclic nucleotides

Current evidence points to the existence of multiple processes for bitter taste transduction. Previous work demonstrated involvement of the polyphosphoinositide system and an alpha-gustducin (Galpha(gust))-mediated stimulation of phosphodiesterase in bitter taste transduction. Additionally, a taste-enriched G protein gamma-subunit, Ggamma(13), colocalizes with Galpha(gust) and mediates the denatonium-stimulated production of inositol 1,4,5-trisphosphate (IP(3)). Using quench-flow techniques, we show here that the bitter stimuli, denatonium and strychnine, induce rapid (50-100 ms) and transient reductions in cAMP and cGMP and increases in IP(3) in murine taste tissue. This decrease of cyclic nucleotides is inhibited by Galpha(gust) antibodies, whereas the increase in IP(3) is not affected by antibodies to Galpha(gust). IP(3) production is inhibited by antibodies specific to phospholipase C-beta(2) (PLC-beta(2)), a PLC isoform known to be activated by Gbetagamma-subunits. Antibodies to PLC-beta(3) or to PLC-beta(4) were without effect. These data suggest a transduction mechanism for bitter taste involving the rapid and transient metabolism of dual second messenger systems, both mediated through a taste cell G protein, likely composed of Galpha(gust)/beta/gamma(13), with both systems being simultaneously activated in the same bitter-sensitive taste receptor cell.

Taste receptor cells that discriminate between bitter stimuli

Recent studies showing that single taste bud cells express multiple bitter taste receptors have reignited a long-standing controversy over whether single gustatory receptor cells respond selectively or broadly to tastants. We examined calcium responses of rat taste receptor cells in situ to a panel of bitter compounds to determine whether individual cells distinguish between bitter stimuli. Most bitter-responsive taste cells were activated by only one out of five compounds tested. In taste cells that responded to multiple stimuli, there were no significant associations between any two stimuli. Bitter sensation does not appear to occur through the activation of a homogeneous population of broadly tuned bitter-sensitive taste cells. Instead, different bitter stimuli may activate different subpopulations of bitter-sensitive taste cells.

Effect of extracellular Ca2+ on the quinine-activated current of bullfrog taste receptor cells

The bitter substance quinine activates a cation current from the frog taste receptor cell. We have analysed the noise associated with this current, and the effect of extracellular Ca2+ on the current, using whole-cell recording on single dissociated cells. Quinine induced an inward current from the taste receptor cell near the resting potential. The response was accompanied by an increase in current fluctuations. From the variance/mean ratio of the quinine-activated current, the single-channel conductance was estimated to be 12 pS in the nominal absence of extracellular Ca2+. In the presence of 1.8 mM Ca2+, this conductance decreased to 5 pS. These values broadly agree with those previously obtained from excised, outside-out membrane patches. The dependence of the current on quinine concentration had a K1/2 of 0.48 mM in the absence of extracellular Ca2+, consistent with measurements from excised patches. The K1/2 value increased to 2.8 mM in 1.8 mM external Ca2+. The maximum current induced by quinine was also reduced by about 20% by Ca2+. The spectral power density distribution of the quinine-activated current could be described by the sum of two Lorentzian functions, with corner frequencies not substantially different in the absence and presence of 1.8 mM external Ca2+. The above results lend further support to the notion that the major component of the response of frog taste receptor cells to quinine comes from an ion channel directly activated by quinine.

The molecular physiology of taste transduction

Taste receptor cells use a variety of mechanisms to transduce chemical information into cellular signals. Seven-transmembrane-helix receptors initiate signaling cascades by coupling to G proteins, effector enzymes, second messengers and ion channels. Apical ion channels pass ions, leading to depolarizing and/or hyperpolarizing responses. New insights into the mechanisms of taste sensation have been gained from molecular cloning of the transduction elements, biochemical elucidation of the transduction pathways, and electrophysiological analysis of the function of taste cell ion channels.

Issues of gustatory neural coding: where they stand today

The basic issues of gustatory neural coding are revisited. Questions addressed and conclusions drawn are: (1) what is the physical dimension across which gustatory neurons are sensitive, and upon which taste perceptions are based? The dimension that unites the various taste qualities is not physical, but physiological: a dimension of well-being, bounded by toxins at one extreme and nutrients at the other. (2) How broadly tuned are taste cells across the dimension? There are instances of specificity, but most mammalian taste cells respond to a range of qualities. (3) Are there basic taste qualities? Sweet, salty, sour, and bitter are widely accepted as basic tastes. Umami and starch tastes are considered basic by some. (4) Is taste topographically organized? There is some degree of physical separation among neurons most responsive to different taste qualities, but this does not appear to be sufficient precision to act as a meaningful coding mechanism. (5) Are there gustatory neuron types? Neurons, separated into categories according to their response profiles, respond as members of their category to the challenges of conditioned aversions and preferences, sodium deprivation, hyperglycemia, and receptor blockade, while cells from other categories react differently. This indicates the existence of functionally distinct types of taste cells. (6) Is the quality signal coded within the activity of the single most appropriate category of neurons, or is it carried by the pattern of response across neuronal categories? Both the breadth of responsiveness and the logical ambiguity of the signal in any one category of neurons argue that the taste message is carried by a pattern of activity across gustatory neuron types.

Rapid entry of bitter and sweet tastants into liposomes and taste cells: implications for signal transduction

Some amphipathic bitter tastants and non-sugar sweeteners are direct activators of G proteins and stimulate transduction pathways in cells not related to taste. We demonstrate that the amphipathic bitter tastants quinine and cyclo(Leu-Trp) and the non-sugar sweetener saccharin translocate rapidly through multilamellar liposomes. Furthermore, when rat circumvallate (CV) taste buds were incubated with the above tastants for 30 s, their intracellular concentrations increased by 3.5- to 7-fold relative to their extracellular concentrations. The time course of this dramatic accumulation was also monitored in situ in rat single CV taste buds under a confocal laser-scanning microscope. Tastants were clearly localized to the taste cell cytosol. It is proposed that, due to their rapid permeation into taste cells, these amphipathic tastants may be available for activation of signal transduction components (e. g., G proteins) directly within the time course of taste sensation. Such activation may occur in addition to the action of these tastants on putative G protein-coupled receptors. This phenomenon may be related to the slow taste onset and lingering aftertaste typically produced by many bitter tastants and non-sugar sweeteners.

IP(3)-Independent release of Ca(2+) from intracellular stores: A novel mechanism for transduction of bitter stimuli

A variety of substances with different chemical structures elicits a bitter taste. Several different transduction mechanisms underlie detection of bitter tastants; however, these have been described in detail for only a few compounds. In addition, most studies have focused on mammalian taste cells, of which only a small subset is responsive to any particular bitter compound. In contrast, approximately 80% of the taste cells in the mudpuppy, Necturus maculosus, are bitter-responsive. In this study, we used Ca(2+) imaging and giga-seal whole cell recording to compare the transduction of dextromethorphan (DEX), a bitter antitussive, with transduction of the well-studied bitter compound denatonium. Bath perfusion of DEX (2.5 mM) increased the intracellular Ca(2+) level in most taste cells. The DEX-induced Ca(2+) increase was inhibited by thapsigargin, an inhibitor of Ca(2+) transport into intracellular stores, but not by U73122, an inhibitor of phospholipase C, or by ryanodine, an inhibitor of ryanodine-sensitive Ca(2+) stores. Increasing intracellular cAMP levels with a cell-permeant cAMP analogue and a phosphodiesterase inhibitor enhanced the DEX-induced Ca(2+) increase, which was inhibited partially by H89, a protein kinase A inhibitor. Electrophysiological measurements showed that DEX depolarized the membrane potential and inhibited voltage-gated Na(+) and K(+) currents in the presence of GDP-beta-S, a blocker of G-protein activation. DEX also inhibited voltage-gated Ca(2+) channels. We suggest that DEX, like quinine, depolarizes taste cells by block of voltage-gated K channels, which are localized to the apical membrane in mudpuppy. In addition, DEX causes release of Ca(2+) from intracellular stores by a phospholipase C-independent mechanism. We speculate that the membrane-permeant DEX may enter taste cells and interact directly with Ca(2+) stores. Comparing transduction of DEX with that of denatonium, both compounds release Ca(2+) from intracellular stores. However, denatonium requires activation of phospholipase C, and the mechanism results in a hyperpolarization rather than a depolarization of the membrane potential. These data support the hypothesis that single taste receptor cells can use multiple mechanisms for transducing the same bitter compound.

Activation by bitter substances of a cationic channel in membrane patches excised from the bullfrog taste receptor cell

1. The response to bitter-tasting substances was recorded in outside-out membrane patches excised from the taste receptor cell of the bullfrog fungiform papilla. 2. Application of a bitter-tasting substance, quinine or denatonium, induced channel openings under conditions in which none of the second messenger candidates or their precursors (e.g. cyclic nucleotide, inositol 1,4,5-trisphosphate, Ca2+, ATP and GTP) were present on either side of the membrane. The response could be recorded > 10 min after excision of the patch membrane. These data suggest that the channel was directly gated by the bitter-tasting substances. 3. No change in response was detected upon addition to the cytoplasmic side of either GDPbetaS (1 mM) or GTPgammaS (1 mM), suggesting that the G protein cascade has no direct relation to response generation. 4. The quinine-induced current was dose dependent. The lowest effective concentration was approximately 0.1 mM, and the saturating concentration was near 1 mM. The dose-response curve was fitted by the Hill equation with a K of 0.52 mM and a Hill coefficient of 3.8. 5. The single channel conductance measured in 120 mM NaCl solution was 10 pS. The channel was cation selective, and the ratio of the permeabilities for Na+, K+ and Cs+ (PNa : PK : PCs) was 1 : 0.48 : 0.39. The unitary conductance was dependent on the extracellular Ca2+ concentration ([Ca2+]o); 9.2 pS in a nominally Ca2+-free solution, and 4.5 pS in 1. 8 mM [Ca2+]o. 6. The dose dependence, the ion selectivity and the dependence of the unitary conductance on [Ca2+]o were almost identical to those of the quinine-induced whole-cell current reported previously, indicating that the channel activity observed in the excised membrane is the basis of the whole-cell current. 7. The present observations suggest the new possibility that the cationic channel directly gated by bitter substances is involved in the bitter taste transduction mechanism.

Blocking taste receptor activation of gustducin inhibits gustatory responses to bitter compounds

Gustducin, a transducin-like guanine nucleotide-binding regulatory protein (G protein), and transducin are expressed in taste receptor cells where they are thought to mediate taste transduction. Gustducin and transducin are activated in the presence of bovine taste membranes by several compounds that humans perceive to be bitter. We have monitored this activation with an in vitro assay to identify compounds that inhibited taste receptor activation of transducin by bitter tastants: AMP and chemically related compounds inhibited in vitro responses to several bitter compounds (e.g., denatonium, quinine, strychnine, and atropine). AMP also inhibited behavioral and electrophysiological responses of mice to bitter tastants, but not to NaCl, HCl, or sucrose. GMP, although chemically similar to AMP, inhibited neither the bitter-responsive taste receptor activation of transducin nor the gustatory responses of mice to bitter compounds. AMP and certain related compounds may bind to bitter-responsive taste receptors or interfere with receptor-G protein coupling to serve as naturally occurring taste modifiers.

Possible novel mechanism for bitter taste mediated through cGMP

Taste is the least understood among sensory systems, and bitter taste mechanisms pose a special challenge because they are elicited by a large variety of compounds. We studied bitter taste signal transduction with the quench-flow method and monitored the rapid kinetics of the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) production and degradation in mouse taste tissue. In response to the bitter stimulants, caffeine and theophylline but not strychnine or denatonium cGMP levels demonstrated a rapid and transient increase that peaked at 50 ms and gradually declined throughout the following 4.5 s. The theophylline- and caffeine-induced effect was rapid, transient, concentration dependent and gustatory tissue-specific. The effect could be partially suppressed in the presence of the soluble guanylyl cyclase (GC) inhibitor 10 microM ODQ and 30 microM methylene blue but not 50 microM LY 83583 and boosted by nitric oxide donors 25 microM NOR-3 or 100 microM sodium nitroprusside. The proposed mechanism for this novel cGMP-mediated bitter taste signal transduction is cGMP production partially by the soluble GC and caffeine-induced inhibition of one or several phosphodiesterases.

Cellular mechanisms of taste transduction

Taste receptor cells respond to gustatory stimuli using a complex arrangement of receptor molecules, signaling cascades, and ion channels. When stimulated, these cells produce action potentials that result in the release of neurotransmitter onto an afferent nerve fiber that in turn relays the identity and intensity of the gustatory stimuli to the brain. A variety of mechanisms are used in transducing the four primary tastes. Direct interaction of the stimuli with ion channels appears to be of particular importance in transducing stimuli reported as salty or sour, whereas the second messenger systems cyclic AMP and inositol trisphosphate are important in transducing bitter and sweet stimuli. In addition to the four basic tastes, specific mechanisms exist for the amino acid glutamate, which is sometimes termed the fifth primary taste, and for fatty acids, a so-called nonconventional taste stimulus. The emerging picture is that not only do individual taste qualities use more than one mechanism, but multiple pathways are available for individual tastants as well.

Amino acid-activated channels in the catfish taste system

Membrane vesicles derived from external taste epithelia of channel catfish (Ictalurus punctatus) were incorporated into lipid bilayers on the tips of patch pipettes. Consistent with previous experiments (Teeter, J. H., J. G. Brand, and T. Kumazawa. 1990. Biophys. J. 58:253-259), micromolar (0.5-200 microM) concentrations of L-arginine (L-Arg), a potent taste stimulus for catfish, activated a nonselective cation conductance in some bilayers, which was antagonized by D-Arg. Two classes of L-Arg-gated receptor/channels were observed in reconstituted taste epithelial membranes: one with a unitary conductance of 40-60 pS, and the other with a conductance of 75-100 pS. A separate class of nonselective cation channels, with a conductance of 50-65 pS, was activated by high concentrations of L-proline (L-Pro) (0.1-3 mM), which is the range necessary to elicit neural responses in catfish taste fibers. The L-Pro-activated channels were not affected by either L- or D-Arg, but were blocked by millimolar concentrations of D-Pro. Conversely, neither L- nor D-Pro altered the activity of either class of L-Arg-activated channels, which were blocked by micromolar concentrations of D-Arg. These results are consistent with biochemical, neurophysiological, and behavioral studies indicating that taste responses of channel catfish to L-Arg are mediated by high-affinity receptors that are part of or closely coupled to nonselective cation channels directly gated by low concentrations of L-Arg, while responses to L-Pro are mediated by distinct, low-affinity receptors also associated with nonselective cation channels.

Identification of a phospholipase C beta subtype in rat taste cells

From rat circumvallate papillae a novel phospholipase C (PLC) subtype has been cloned and identified as most closely related to human PLC beta2. The corresponding mRNA was only detected in sensory lingual tissue but not in non-taste lingual tissue or any other tissues examined by Northern blot analysis. In situ hybridization revealed that a subset of taste receptor cells of circumvallate papillae was specifically labeled. A functional involvement of this PLC beta subtype in taste signal transduction emerged from biochemical analysis monitoring the second messenger response in circumvallate preparations induced by denatonium benzoate. This bitter agent elicited a rapid and transient increase of the inositol 1,4,5-trisphosphate level; this response was blocked by U73122, a potent inhibitor of PLC, and by PLC beta2-specific antibodies. These data indicate that a phospholipase C beta2 isoform mediates a denatonium benzoate-induced second messenger response of taste sensory cells in the circumvallate papillae.

Gustducin and its role in taste

The mechanisms responsible for taste signal transductions are very complex. A key molecule, alpha-gustducin, a primarily taste-specific G protein alpha-subunit, was discovered in 1992 and was later found to be involved in both bitter and sweet taste transduction. A proposed mechanism for alpha-gustducin involves coupling specific cell-surface receptors with a cyclic nucleotide phosphodiesterase which would open a cyclic nucleotide-suppressible cation channel leading to influx of calcium, and ultimately leading to release of neurotransmitter. Although "knock-out" animals deficient in the alpha-gustducin gene clearly demonstrate that gustducin is an essential molecule for tasting certain bitter and sweet compounds, the precise role of alpha-gustducin in bitter and sweet taste is presently unclear. Indeed, there are several other signaling mechanisms in sweet and bitter taste, apparently unrelated to alpha-gustducin, that increase cyclic AMP or inositol 1,4,5 trisphosphate. Thus, proposed models for alpha-gustducin and those found by other laboratories may be parallel and interdependent.

Inositol 1,4,5-trisphosphate activates non-selective cation conductance via intracellular Ca2+ increase in isolated frog taste cells

The effect of intracellular Ca2+ increase was analysed in isolated frog taste cells under the whole-cell patch clamp. External application of a Ca2+-ionophore, ionomycin (3 microM) induced the sustained inward current of -200+/-17 pA (mean +/- SE, n = 23) at -50 mV in taste cells. The ionomycin-induced response was observed in most of the cells exposed in the drug, but not when 10 mM BAPTA (1,2-bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) was included in the pipette (eight cells). Steady-state I-V relationships of ionomycin-induced currents were almost linear and reversed at -8+/-1 mV (n = 23). The simultaneous removal of Na+ and Ca2+ from the external solution eliminated the response completely (three cells). Intracellular dialysis with 1 mM Ca2+ or 50 microM inositol 1,4,5-trisphosphate (IP3) in K+-internal solution also induced an inward current in the taste cells. The Ca2+-induced and IP3-induced responses were observed in 82% and 36% of the cells dialysed with the drugs, respectively. The Ca2+-induced and IP3-induced currents were inhibited by external Cd2+ (1-2 mM). The reversal potentials of the inward currents were -15+/-3 mV (n = 9) in Ca2+ dialysis and -11+/-3 mV (n = 13) in IP3 dialysis. The half-maximal Ca2+ concentration in the pipette to induce the inward current was approximately 170 microM. The results suggest that IP3 can depolarize the taste cell with mediation by intracellular Ca2+.

Behavioral specificity of the bitter taste gene Soa

In mice, aversion to the bitter acetylated sugar sucrose octaacetate (SOA) is determined by a single genetic locus with three alleles. SWR/J (SW) inbred mice are SOA tasters: They avoid many compounds characterized as bitter-tasting by humans, at concentrations to which C3HeB/FeJ (C3:SOA demitasters) mice are less sensitive. C3.SW-Soa(a) congenic taster mice contain the taster allele transposed to a 99% C3 bitter-insensitive genetic background. SW, C3, C3.SW-Soa(a) congenic taster, and C3.SW demitaster mice were behaviorally tested with a series of 48-h two-bottle preference tests to determine the influence of the Soa(a) taster allele on sensitivity to a variety of bitter-tasting compounds. Soa allelic variation had a major effect on sensitivity to 0.003-1.0 mM SOA and several concentrations of the bitter-tasting alkaloids brucine, strychnine, and quinine. Effects were also found for 0.1 mM denatonium and 1 mM propylthiouracil. For caffeine, cycloheximide, thiamine, and two nonbitter compounds (NaCl and calcium hydroxide), the SW mice avoided lower concentrations than the other strains, but this avoidance was not due to the Soa(a) allele because both the C3 inbred and C3.SW-Soa(a) congenics were less sensitive. These results suggest the Soa gene product influences sensitivity to a subset of bitter-tasting compounds.