ATP signaling is crucial for communication from taste buds to gustatory nerves

New structure enlivens interest in P2X receptors

P2X receptors are ATP-gated membrane ion channels with multifarious roles, including afferent sensation, autocrine feedback loops, and inflammation. Their molecular operation has been less well elucidated compared with other ligand-gated channels (nicotinic acetylcholine receptors, ionotropic glutamate receptors). This will change with the recent publication of the crystal structure of a closed P2X receptor. Here we re-interpret results from 15 years of experiments using site-directed mutagenesis with a model based on the new structure. Previous predictions of receptor stoichiometry, the extracellular ATP binding site, inter-subunit contacts, and many details of the permeation pathway fall into place in three dimensions. We can therefore quickly understand how the channel operates at the molecular level. This is important not only for ion- channel aficionados, but also those engaged in developing effective antagonists at P2X receptors for potential therapeutic use.

Cell-to-cell communication in intact taste buds through ATP signalling from pannexin 1 gap junction hemichannels

Isolated taste cells, taste buds and strips of lingual tissue from taste papillae secrete ATP upon taste stimulation. Taste bud receptor (Type II) cells have been identified as the source of ATP secretion. Based on studies on isolated taste buds and single taste cells, we have postulated that ATP secreted from receptor cells via pannexin 1 hemichannels acts within the taste bud to excite neighbouring presynaptic (Type III) cells. This hypothesis, however, remains to be tested in intact tissues. In this report we used confocal Ca(2+) imaging and lingual slices containing intact taste buds to test the hypothesis of purinergic signalling between taste cells in a more integral preparation. Incubating lingual slices with apyrase reversibly blocked cell-to-cell communication between receptor cells and presynaptic cells, consistent with ATP being the transmitter. Inhibiting pannexin 1 gap junction hemichannels with CO(2)-saturated buffer or probenecid significantly reduced cell-cell signalling between receptor cells and presynaptic cells. In contrast, anandamide, a blocker of connexin gap junction channels, had no effect of cell-to-cell communication in taste buds. These findings are consistent with the model for peripheral signal processing via ATP and pannexin 1 hemichannels in mammalian taste buds.

The diffuse chemosensory system: exploring the iceberg toward the definition of functional roles

The diffuse chemosensory system (DCS) is an anatomical structure composed of solitary chemosensory cells (SCCs, also called solitary chemoreceptor cells), which have analogies with taste cells but are not aggregated in buds. The concept of DCS has been advanced, after the discovery that cells similar to gustatory elements are present in several organs. The elements forming the DCS share common morphological and biochemical characteristics with the taste cells located in taste buds of the oro-pharyngeal cavity but they are localized in internal organs. In particular, they may express molecules of the chemoreceptorial cascade (e.g. trans-membrane taste receptors, the G-protein alpha-gustducin, PLCbeta2, TRPM5). This article will focus on the mammalian DCS in apparatuses of endodermic origin (i.e. digestive and respiratory systems), which is composed of an enormous number of sensory elements and presents a multiplicity of morphological aspects. Recent research has provided an adequate description of these elements, but the functional role for the DCS in these apparatuses is unknown. The initial findings led to the definition of a DCS structured like an iceberg, with a mysterious "submerged" portion localized in the distal part of endodermic apparatuses. Recent work has focussed on the discovery of this submerged portion, which now appears less puzzling. However, the functional roles of the different cytotypes belonging to the DCS are not well known. Recent studies linked chemosensation of the intraluminal content to local control of absorptive and secretory (exocrine and endocrine) processes. Control of the microbial population and detection of irritants seem to be other possible functions of the DCS. In the light of these new findings, the DCS might be thought to be involved in a wide range of diseases of both the respiratory (e.g. asthma, chronic obstructive pulmonary disease, cystic fibrosis) and digestive apparatuses (absorptive or secretive diseases, dysmicrobism), as well as in systemic diseases (e.g. obesity, diabetes). A description of the functional roles of the DCS might be a first step toward the discovery of therapeutic approaches which target chemosensory mechanisms.

Response latency to lingual taste stimulation distinguishes neuron types within the geniculate ganglion

The purpose of this study was to investigate the role of response latency in discrimination of chemical stimuli by geniculate ganglion neurons in the rat. Accordingly, we recorded single-cell 5-s responses from geniculate ganglion neurons (n = 47) simultaneously with stimulus-evoked summated potentials (electrogustogram; EGG) from the anterior tongue to signal when the stimulus contacted the lingual epithelium. Artificial saliva served as the rinse solution and solvent for all stimuli [(0.5 M sucrose, 0.03-0.5 M NaCl, 0.01 M citric acid, and 0.02 M quinine hydrochloride (QHCl)], 0.1 M KCl as well as for 0.1 M NaCl +1 μM benzamil. Cluster analysis separated neurons into four groups (sucrose specialists, NaCl specialists, NaCl/QHCl generalists and acid generalists). Artificial saliva elevated spontaneous firing rate and response frequency of all neurons. As a rule, geniculate ganglion neurons responded with the highest frequency and shortest latency to their best stimulus with acid generalist the only exception. For specialist neurons and NaCl/QHCl generalists, the average response latency to the best stimulus was two to four times shorter than the latency to secondary stimuli. For NaCl-specialist neurons, response frequency increased and response latency decreased systematically with increasing NaCl concentration; benzamil significantly decreased NaCl response frequency and increased response latency. Acid-generalist neurons had the highest spontaneous firing rate and were the only group that responded consistently to citric acid and KCl. For many acid generalists, a citric-acid-evoked inhibition preceded robust excitation. We conclude that response latency may be an informative coding signal for peripheral chemosensory neurons.

Double P2X2/P2X3 purinergic receptor knockout mice do not taste NaCl or the artificial sweetener SC45647

The P2X ionotropic purinergic receptors, P2X2 and P2X3, are essential for transmission of taste information from taste buds to the gustatory nerves. Mice lacking both P2X2 and P2X3 purinergic receptors (P2X2/P2X3(Dbl-/-)) exhibit no taste-evoked activity in the chorda tympani and glossopharyngeal nerves when stimulated with taste stimuli from any of the 5 classical taste quality groups (salt, sweet, sour, bitter, and umami) nor do the mice show taste preferences for sweet or umami, or avoidance of bitter substances (Finger et al. 2005. ATP signaling is crucial for communication from taste buds to gustatory nerves. Science. 310[5753]:1495-1499). Here, we compare the ability of P2X2/P2X3(Dbl-/-) mice and P2X2/P2X3(Dbl+/+) wild-type (WT) mice to detect NaCl in brief-access tests and conditioned aversion paradigms. Brief-access testing with NaCl revealed that whereas WT mice decrease licking at 300 mM and above, the P2X2/P2X3(Dbl-/-) mice do not show any change in lick rates. In conditioned aversion tests, P2X2/P2X3(Dbl-/-) mice did not develop a learned aversion to NaCl or the artificial sweetener SC45647, both of which are easily avoided by conditioned WT mice. The inability of P2X2/P2X3(Dbl-/-) mice to show avoidance of these taste stimuli was not due to an inability to learn the task because both WT and P2X2/P2X3(Dbl-/-) mice learned to avoid a combination of SC45647 and amyl acetate (an odor cue). These data suggest that P2X2/P2X3(Dbl-/-) mice are unable to respond to NaCl or SC45647 as taste stimuli, mirroring the lack of gustatory nerve responses to these substances.

Residual chemosensory capabilities in double P2X2/P2X3 purinergic receptor null mice: intraoral or postingestive detection?

Mice lacking the purinergic receptors, P2X2 and P2X3 (P2X2/P2X3(Dbl-/-)), exhibit essentially no tastant-evoked activity in the chorda tympani and glossopharyngeal nerves and substantial loss of tastant-evoked behavior as measured in long-term intake experiments. To assess whether the residual chemically driven behaviors in these P2X2/P2X3(Dbl-/-) mice were attributable to postingestive detection or oropharyngeal detection of the compounds, we used brief access lickometer tests to assess the behavioral capabilities of the P2X2/P2X3(Dbl-/-) animals. The P2X2/P2X3(Dbl-/-) mice showed avoidance to high levels (10 mM quinine and 10-30 mM denatonium benzoate) of classical "bitter"-tasting stimuli in 24-h, 2-bottle preference tests but minimal avoidance of these substances in the lickometer tests, suggesting that the strong avoidance in the intake tests was largely mediated by post-oral chemosensors. Similarly, increases in consumption of 1 M sucrose by P2X2/P2X3(Dbl-/-) mice in long-term intake tests were not mirrored by increases in consumption of sucrose in lickometer tests, suggesting that sucrose detection in these mice is mediated by postingestive consequences. In contrast, in brief access tests, P2X2/P2X3(Dbl-/-) mice avoided citric acid and hydrochloric acid at the same concentrations as their wild-type counterparts, indicating that these weak acids activate oropharyngeal chemoreceptors.

Modulation of sweet taste sensitivity by orexigenic and anorexigenic factors

The present study summarized recent findings on roles of leptin and endocannabinoids as modulators of the peripheral components of sweet taste. The positive effect of endocannabinoids on sweet sensitivity was opposed to that of leptin which suppresses sweet sensitivity. Leptin and endocannabinoids, therefore, not only regulate food intake via central nervous systems but also may modulate palatability of foods by altering peripheral sweet taste responses via their cognate receptors. Orexigenic and anorexigenic factors such as endocannnabinoids and leptin may affect energy homeostasis by regulating taste sensitivity.

Reciprocal modulation of sweet taste by leptin and endocannabinoids

Sweet taste perception is important for animals to detect carbohydrate source of calories and has a critical role in the nutritional status of animals. Recent studies demonstrated that sweet taste responses can be modulated by leptin and endocannabinoids [anandamide (N-arachidonoylethanolamine) and 2-arachidonoyl glycerol]. Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor, Ob-Rb. Leptin is shown to selectively suppress sweet taste responses in wild-type mice but not in leptin receptor-deficient db/db mice. In marked contrast, endocannabinoids are orexigenic mediators that act via CB(1) receptors in hypothalamus and limbic forebrain to induce appetite and stimulate food intake. In the peripheral taste system, endocannabinoids also oppose the action of leptin and enhance sweet taste sensitivities in wild-type mice but not in mice genetically lacking CB(1) receptors. These findings indicate that leptin and endocannabinoids not only regulate food intake via central nervous systems but also may modulate palatability of foods by altering peripheral sweet taste responses via their cognate receptors.

Inward rectifier channel, ROMK, is localized to the apical tips of glial-like cells in mouse taste buds

Cells in taste buds are closely packed, with little extracellular space. Tight junctions and other barriers further limit permeability and may result in buildup of extracellular K(+) following action potentials. In many tissues, inwardly rectifying K channels such as the renal outer medullary K (ROMK) channel (also called Kir1.1 and derived from the Kcnj1 gene) help to redistribute K(+). Using reverse-transcription polymerase chain reaction (RT-PCR), we defined ROMK splice variants in mouse kidney and report here the expression of a single one of these, ROMK2, in a subset of mouse taste cells. With quantitative (q)RT-PCR, we show the abundance of ROMK mRNA in taste buds is vallate > foliate > > palate > > fungiform. ROMK protein follows the same pattern of prevalence as mRNA, and is essentially undetectable by immunohistochemistry in fungiform taste buds. ROMK protein is localized to the apical tips of a subset of taste cells. Using tissues from PLCbeta2-GFP and GAD1-GFP transgenic mice, we show that ROMK is not found in PLCbeta2-expressing type II/receptor cells or in GAD1-expressing type III/presynaptic cells. Instead, ROMK is found, by single-cell RT-PCR and immunofluorescence, in most cells that are positive for the taste glial cell marker, Ectonucleotidase2. ROMK is precisely localized to the apical tips of these cells, at and above apical tight junctions. We propose that in taste buds, ROMK in type I/glial-like cells may serve a homeostatic function, excreting excess K(+) through the apical pore, and allowing excitable taste cells to maintain a hyperpolarized resting membrane potential.

Type II and III Taste Bud Cells Preferentially Expressed Kainate Glutamate Receptors in Rats

Glutamate-induced cobalt uptake reveals that non-NMDA glutamate receptors (GluRs) are present in rat taste bud cells. Previous studies involving glutamate induced cobalt staining suggest this uptake mainly occurs via kainate type GluRs. It is not known which of the 4 types of taste bud cells express subunits of kainate GluR. Circumvallate and foliate papillae of Sprague-Dawley rats (45~60 days old) were used to search for the mRNAs of subunits of non-NMDA GluRs using RT-PCR with specific primers for GluR1-7, KA1 and KA2. We also performed RT-PCR for GluR5, KA1, PLCbeta2, and NCAM/SNAP 25 in isolated single cells from taste buds. Taste epithelium, including circumvallate or foliate papilla, express mRNAs of GluR5 and KA1. However, non-taste tongue epithelium expresses no subunits of non-NMDA GluRs. Isolated single cell RT-PCR reveals that the mRNAs of GluR5 and KA1 are preferentially expressed in Type II and Type III cells over Type I cells.

The role of transient receptor potential vanilloid-1 on neural responses to acids by the chorda tympani, glossopharyngeal and superior laryngeal nerves in mice

The transient receptor potential vanilloid-1 (TRPV1) receptor acts as a polymodal nociceptor activated by capsaicin, heat, and acid. TRPV1, which is expressed in sensory neurons innervating the oral cavity, is associated with an oral burning sensation in response to spicy food containing capsaicin. However, little is known about the involvement of TRPV1 in responses to acid stimuli in either the gustatory system or the general somatosensory innervation of the oropharynx. To test this possibility, we recorded electrophysiological responses to several acids (acetic acid, citric acid and HCl) and other taste stimuli from the mouse chorda tympani, glossopharyngeal and superior laryngeal nerves, and compared potential effects of iodo-resiniferatoxin (I-RTX), a potent TRPV1 antagonist, on chemical responses of the three nerves. The results indicated that in the chorda tympani nerve, I-RTX (1-100 nM) did not affect responses to acids, sucrose and quinine HCl, but reduced responses to NaCl (I-RTX at concentrations of 10 and 100 nM) and KCl and NH(4)Cl (100 nM). In contrast, in the glossopharyngeal nerve, I-RTX significantly suppressed responses to all acids and salts, but not to sucrose and quinine HCl. Responses to acetic acid were suppressed by I-RTX even at 0.1 nM concentration. The superior laryngeal nerve responded in a concentration-dependent manner to acetic acid, citric acid, HCl, KCl, NH(4)Cl and monosodium l-glutamate. The responses to acetic acid, but not to the other stimuli, were significantly inhibited by I-RTX. These results suggested that TRPV1 may be involved in the mechanism for responses to acids presented to the posterior oral cavity and larynx. This high degree of responsiveness to acetic acid may account for the oral burning sensation, known as a flavor characteristic of vinegar.

Purinergic mechanosensory transduction and visceral pain

In this review, evidence is presented to support the hypothesis that mechanosensory transduction occurs in tubes and sacs and can initiate visceral pain. Experimental evidence for this mechanism in urinary bladder, ureter, gut, lung, uterus, tooth-pulp and tongue is reviewed. Potential therapeutic strategies are considered for the treatment of visceral pain in such conditions as renal colic, interstitial cystitis and inflammatory bowel disease by agents that interfere with mechanosensory transduction in the organs considered, including P2X₃ and P2X_2/3 receptor antagonists that are orally bioavailable and stable in vivo and agents that inhibit or enhance ATP release and breakdown.

Autocrine and paracrine roles for ATP and serotonin in mouse taste buds

Receptor (type II) taste bud cells secrete ATP during taste stimulation. In turn, ATP activates adjacent presynaptic (type III) cells to release serotonin (5-hydroxytryptamine, or 5-HT) and norepinephrine (NE). The roles of these neurotransmitters in taste buds have not been fully elucidated. Here we tested whether ATP or 5-HT exert feedback onto receptor (type II) cells during taste stimulation. Our previous studies showed NE does not appear to act on adjacent taste bud cells, or at least on receptor cells. Our data show that 5-HT released from presynaptic (type III) cells provides negative paracrine feedback onto receptor cells by activating 5-HT(1A) receptors, inhibiting taste-evoked Ca(2+) mobilization in receptor cells, and reducing ATP secretion. The findings also demonstrate that ATP exerts positive autocrine feedback onto receptor (type II) cells by activating P2Y1 receptors and enhancing ATP secretion. These results begin to sort out how purinergic and aminergic transmitters function within the taste bud to modulate gustatory signaling in these peripheral sensory organs.

Eukaryotic vs. prokaryotic chemosensory systems

In the last decades, microbiologists demonstrated that microorganisms possess chemosensory capabilities and communicate with each other via chemical signals. In parallel, it was demonstrated that solitary eukaryotic chemosensory cells are diffusely located on the mucosae of digestive and respiratory apparatuses. It is now evident that on the mucosal surfaces of vertebrates, two chemoreceptorial systems (i.e. eukaryotic and prokaryotic) coexist in a common microenvironment. To date, it is not known if the two chemosensory systems reciprocally interact and compete for detection of chemical cues. This appears to be a fruitful field of study and future researches must consider that the mucosal epithelia possess more chemosensory capabilities than previously supposed.

P2X receptors: dawn of the post-structure era

P2X receptors are non-selective cation channels gated by extracellular ATP. They play key roles in various physiological processes such as nerve transmission, pain sensation and the response to inflammation, making them attractive drug targets for the treatment of inflammatory pain. The recent report of the three-dimensional (3D) crystal structure of zebrafish P2X4.1 represents a step change in our understanding of these membrane ion channels, where previously only low-resolution structural data and inferences from indirect structure–function studies were available. The purpose of this review is to place previous work within the context of the new 3D structure, and to summarize the key questions and challenges which await P2X researchers as we move into the post-structure era.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Modulation and transmission of sweet taste information for energy homeostasis

Perception of sweet taste is important for animals to detect external energy source of calories. In mice, sweet-sensitive cells possess a leptin receptor. Increase of plasma leptin with increasing internal energy storage in the adipose tissue suppresses sweet taste responses via this receptor. Data from our recent studies indicate that leptin may also modulate sweet taste sensation in humans with a diurnal variation in sweet sensitivity. This leptin modulation of sweet taste information to the brain may influence individuals' preference and ingestive behavior, thereby playing important roles in regulation of energy homeostasis.

Processing umami and other tastes in mammalian taste buds

Neuroscientists are now coming to appreciate that a significant degree of information processing occurs in the peripheral sensory organs of taste prior to signals propagating to the brain. Gustatory stimulation causes taste bud cells to secrete neurotransmitters that act on adjacent taste bud cells (paracrine transmitters) as well as on primary sensory afferent fibers (neurocrine transmitters). Paracrine transmission, representing cell-cell communication within the taste bud, has the potential to shape the final signal output that taste buds transmit to the brain. The following paragraphs summarize current thinking about how taste signals generally, and umami taste in particular, are processed in taste buds.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

Umami taste transduction mechanisms

l-Glutamate elicits the umami taste sensation, now recognized as a fifth distinct taste quality. A characteristic feature of umami taste is its potentiation by 5'-ribonucleotides such as guanosine-5'-monophosphate and inosine 5'-monophosphate, which also elicit the umami taste on their own. Recent data suggest that multiple G protein-coupled receptors contribute to umami taste. This review will focus on events downstream of the umami taste receptors. Ligand binding leads to Gbetagamma activation of phospholipase C beta2, which produces the second messengers inositol trisphosphate and diacylglycerol. Inositol trisphosphate binds to the type III inositol trisphosphate receptor, which causes the release of Ca(2+) from intracellular stores and Ca(2+)-dependent activation of a monovalent-selective cation channel, TRPM5. TRPM5 is believed to depolarize taste cells, which leads to the release of ATP, which activates ionotropic purinergic receptors on gustatory afferent nerve fibers. This model is supported by knockout of the relevant signaling effectors as well as physiologic studies of isolated taste cells. Concomitant with the molecular studies, physiologic studies show that l-glutamate elicits increases in intracellular Ca(2+) in isolated taste cells and that the source of the Ca(2+) is release from intracellular stores. Both Galpha gustducin and Galpha transducin are involved in umami signaling, because the knockout of either subunit compromises responses to umami stimuli. Both alpha-gustducin and alpha-transducin activate phosphodiesterases to decrease intracellular cAMP. The target of cAMP in umami transduction is not known, but membrane-permeant analogs of cAMP antagonize electrophysiologic responses to umami stimuli in isolated taste cells, which suggests that cAMP may have a modulatory role in umami signaling.

Multiple receptors underlie glutamate taste responses in mice

l-Glutamate is known to elicit a unique taste, umami, that is distinct from the tastes of sweet, salty, sour, and bitter. Recent molecular studies have identified several candidate receptors for umami in taste cells, such as the heterodimer T1R1/T1R3 and brain-expressed and taste-expressed type 1 and 4 metabotropic glutamate receptors (brain-mGluR1, brain-mGluR4, taste-mGluR1, and taste-mGluR4). However, the relative contributions of these receptors to umami taste reception remain to be elucidated. We critically discuss data from recent studies in which mouse taste cell, nerve fiber, and behavioral responses to umami stimuli were measured to evaluate whether receptors other than T1R1/T1R3 are involved in umami responses. We particularly emphasized studies of umami responses in T1R3 knockout (KO) mice and studies of potential effects of mGluR antagonists on taste responses. The results of these studies indicate the existence of substantial residual responses to umami compounds in the T1R3-KO model and a significant reduction of umami responsiveness after administration of mGluR antagonists. These findings thus provide evidence of the involvement of mGluRs in addition to T1R1/T1R3 in umami detection in mice and suggest that umami responses, at least in mice, may be mediated by multiple receptors.

An evolutionary history of P2X receptors

Adenosine triphosphate (ATP) is an ancient and fundamentally important biological molecule involved in both intracellular and extracellular activities. P2X ionotropic and P2Y metabotropic receptors have been cloned and characterised in mammals. ATP plays a central physiological role as a transmitter molecule in processes including the sensation of pain, taste, breathing and inflammation via the activation of P2X receptors. P2X receptors are structurally distinct from glutamate and Cys-loop/nicotinic receptors and form the third major class of ligand-gated ion channel. Yet, despite the importance of P2X receptors, both as physiological mediators and therapeutic targets, the evolutionary origins and phylogenicity of ATP signalling via P2X receptors remain unclear.

Metabotropic glutamate receptor type 1 in taste tissue

l-Glutamate confers cognitive discrimination for umami taste (delicious or savory) and dietary information to the brain through the activation of G protein-coupled receptors in specialized taste receptor cells of the tongue. The taste heterologous receptor T1R1 plus T1R3 is not sufficient to detect umami taste in mice. The lack of T1R3 diminished but did not abolish nerve and behavioral responses in null mice that still contained umami-sensitive taste receptor cells. The remnant umami responses in T1R3 knockout mice indicate that there are also T1R3 independent receptors. Metabotropic glutamate receptor 1 (mGluR1), which is widely expressed throughout the central nervous system and regulates synaptic signaling, is another l-glutamate receptor candidate. It is found within taste buds, although the amount of l-glutamate in the perisynaptic region is in the order of micromol/L, whereas free dietary l-glutamate is in the mmol/L range. We reexamined the expression of one mGluR1 variant with a lower affinity for l-glutamate that is found in fungiform and circumvallate papillae. This taste mGluR1 receptor responds in vitro to the concentration of l-glutamate usually found in foodstuffs.

Visualization of ATP levels inside single living cells with fluorescence resonance energy transfer-based genetically encoded indicators

Adenosine 5'-triphosphate (ATP) is the major energy currency of cells and is involved in many cellular processes. However, there is no method for real-time monitoring of ATP levels inside individual living cells. To visualize ATP levels, we generated a series of fluorescence resonance energy transfer (FRET)-based indicators for ATP that were composed of the epsilon subunit of the bacterial F(o)F(1)-ATP synthase sandwiched by the cyan- and yellow-fluorescent proteins. The indicators, named ATeams, had apparent dissociation constants for ATP ranging from 7.4 muM to 3.3 mM. By targeting ATeams to different subcellular compartments, we unexpectedly found that ATP levels in the mitochondrial matrix of HeLa cells are significantly lower than those of cytoplasm and nucleus. We also succeeded in measuring changes in the ATP level inside single HeLa cells after treatment with inhibitors of glycolysis and/or oxidative phosphorylation, revealing that glycolysis is the major ATP-generating pathway of the cells grown in glucose-rich medium. This was also confirmed by an experiment using oligomycin A, an inhibitor of F(o)F(1)-ATP synthase. In addition, it was demonstrated that HeLa cells change ATP-generating pathway in response to changes of nutrition in the environment.

Carbon monoxide is a rapid modulator of recombinant and native P2X(2) ligand-gated ion channels

BACKGROUND AND PURPOSE

Carbon monoxide (CO) is a potent modulator of a wide variety of physiological processes, including sensory signal transduction. Many afferent sensory pathways are dependent upon purinergic neurotransmission, but direct modulation of the P2X purinoceptors by this important, endogenously produced gas has never been investigated.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp experiments were used to measure ATP-elicited currents in human embryonic kidney 293 cells heterologously expressing P2X(2), P2X(3), P2X(2/3) and P2X(4) receptors and in rat pheochromocytoma (PC12) cells known to express native P2X(2) receptors. Modulation was investigated using solutions containing CO gas and the CO donor molecule, tricarbonyldichlororuthenium (II) dimer (CORM-2).

KEY RESULTS

CO was a potent and selective modulator of native P2X(2) receptors, and these effects were mimicked by a CO donor (CORM-2). Neither pre-incubation with 8-bromoguanosine-3',5'-cyclomonophosphate nor 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (a potent blocker of soluble guanylyl cyclase) affected the ability of the CO donor to enhance the ATP-evoked P2X(2) currents. The CO donor caused a small, but significant inhibition of currents evoked by P2X(2/3) and P2X(4) receptors, but was without effect on P2X(3) receptors.

CONCLUSIONS AND IMPLICATIONS

These data provided an explanation for how CO might regulate sensory neuronal traffic in physiological reflexes such as systemic oxygen sensing but also showed that CO could be used as a selective pharmacological tool to assess the involvement of homomeric P2X(2) receptors in physiological systems.

TRPV3 in keratinocytes transmits temperature information to sensory neurons via ATP

Transient receptor potential V3 (TRPV3) and TRPV4 are heat-activated cation channels expressed in keratinocytes. It has been proposed that heat-activation of TRPV3 and/or TRPV4 in the skin may release diffusible molecules which would then activate termini of neighboring dorsal root ganglion (DRG) neurons. Here we show that adenosine triphosphate (ATP) is such a candidate molecule released from keratinocytes upon heating in the co-culture systems. Using TRPV1-deficient DRG neurons, we found that increase in cytosolic Ca²⁺-concentration in DRG neurons upon heating was observed only when neurons were co-cultured with keratinocytes, and this increase was blocked by P2 purinoreceptor antagonists, PPADS and suramin. In a co-culture of keratinocytes with HEK293 cells (transfected with P2X₂ cDNA to serve as a bio-sensor), we observed that heat-activated keratinocytes secretes ATP, and that ATP release is compromised in keratinocytes from TRPV3-deficient mice. This study provides evidence that ATP is a messenger molecule for mainly TRPV3-mediated thermotransduction in skin.

Crystal structure of the ATP-gated P2X(4) ion channel in the closed state

P2X receptors are cation-selective ion channels gated by extracellular ATP, and are implicated in diverse physiological processes, from synaptic transmission to inflammation to the sensing of taste and pain. Because P2X receptors are not related to other ion channel proteins of known structure, there is at present no molecular foundation for mechanisms of ligand-gating, allosteric modulation and ion permeation. Here we present crystal structures of the zebrafish P2X(4) receptor in its closed, resting state. The chalice-shaped, trimeric receptor is knit together by subunit-subunit contacts implicated in ion channel gating and receptor assembly. Extracellular domains, rich in beta-strands, have large acidic patches that may attract cations, through fenestrations, to vestibules near the ion channel. In the transmembrane pore, the 'gate' is defined by an approximately 8 A slab of protein. We define the location of three non-canonical, intersubunit ATP-binding sites, and suggest that ATP binding promotes subunit rearrangement and ion channel opening.

Deletion of P2X3 receptors blunts gastro-oesophageal sensation in mice

Prior studies have demonstrated P2X receptor expression in the majority of nodose neurons. Immunoreactivity for P2X receptors has also been seen in putative gastric mechanoreceptors, the intraganglionic laminar endings. We therefore hypothesized that deletion of P2X3 receptors will blunt responses to gastric distension in vagal sensory neurons. Using wildtype and P2X3(-/-) mice, we examined responses to purinergic agonists in retrogradely labelled gastric sensory neurons with patch-clamp techniques. Activation of gastro-oesophageal neurons by fluid distension was studied with intracellular electrodes. Distension-evoked ATP release into the gastric lumen was determined with the luciferase assay and intake and gastric emptying of a solid meal was assessed. ATP triggered inward currents in 80% of gastric nodose neurons. In P2X3(-/-) mice, the peak current density was lower compared to controls. Ten of 14 controls but none of 30 neurons from P2X3(-/-) mice responded to alpha,beta-metATP. Gastro-oesophageal sensory neurons of P2X3(-/-) mice showed a blunted response to fluid distension of oesophagus and stomach. This difference was not explained by differences in distension-evoked ATP release, which did not differ between knockout mice and controls. Food intake during a 3-h period was lower in P2X3(-/-) mice. Gastric emptying of a solid meal was slightly faster in knockout mice after 1.5 h, but did not differ between groups at 3 h. Our data support a role of purinergic signalling in gastric vagal afferents. Considering the role of vagal input in sensations of fullness or nausea, P2X receptors may be interesting treatment targets for dyspeptic symptoms.

Identification of the vesicular nucleotide transporter (VNUT) in taste cells

Taste cells are chemosensory epithelial cells that sense distinct taste qualities. It is the type II taste cell that express G-protein coupled receptors to sense either umami, sweet, or bitter compounds. Whereas several reports have suggested involvement of ATP in taste signal transduction, there is a paucity of molecular information about how ATP is stored and being released. The recent discovery of a novel vesicular nucleotide transporter (VNUT) led us to examine whether VNUT exist in the taste tissue where ATP is to be released for taste signal transmission. Here, we report that VNUT is selectively expressed in type II cell but not in type III taste cell. In addition, we show that during taste bud development VNUT expression is always accompanied by the expression of type II taste cell markers. Our results, together with previous studies, strongly suggest that VNUT plays a role in type II taste cell.

P2X(2)- and P2X(3)-positive fibers in fungiform papillae originate from the chorda tympani but not the trigeminal nerve in rats and mice

The subtype 2 and subtype 3 ionotropic purinergic receptors (P2X receptors) are crucial for gustation, but the distribution of these receptors in the geniculate ganglion (GG) and their colocalization in tongue papillae remain unknown. Here we investigated the expression and colocalization of P2X(2) and P2X(3) receptors in the GG and fungiform papillae in rats and mice by using in situ hybridization and immunohistochemistry. In both species, P2X(2) transcripts and immunoreactivity were detected in approximately 50-60% of GG neuronal somata, whereas those of P2X(3) were observed in almost all neurons. In each fungiform papilla, immunoreactivity for both receptors was mostly colocalized and was seen in nerve fibers and their bundles concentrated in the taste buds. Because it is well known that the P2X receptors are involved in not only taste but also nociception, we determined whether the expression originated from the chorda tympani nerve (CT, gustatory) or trigeminal nerve (somatosensory) by cutting the CT in both animals. Most P2X(2) and P2X(3) immunoreactivity in the fungiform papillae was abolished after transection, although the nerve fiber immunoreactivity of transient receptor potential V1 (a marker of somatosensory nerve fibers) remained unchanged, indicating that most fungiform papillae nerve fibers with P2X(2) and P2X(3) receptors were derived from CT. Taken together, these findings suggest that most P2X(2) and P2X(3) receptors in fungiform papillae are used for gustation rather than somatosensation.

Fate mapping of mammalian embryonic taste bud progenitors

Mammalian taste buds have properties of both epithelial and neuronal cells, and are thus developmentally intriguing. Taste buds differentiate at birth within epithelial appendages, termed taste papillae, which arise at mid-gestation as epithelial thickenings or placodes. However, the embryonic relationship between placodes, papillae and adult taste buds has not been defined. Here, using an inducible Cre-lox fate mapping approach with the ShhcreER(T2) mouse line, we demonstrate that Shh-expressing embryonic taste placodes are taste bud progenitors, which give rise to at least two different adult taste cell types, but do not contribute to taste papillae. Strikingly, placodally descendant taste cells disappear early in adult life. As placodally derived taste cells are lost, we used Wnt1Cre mice to show that the neural crest does not supply cells to taste buds, either embryonically or postnatally, thus ruling out a mesenchymal contribution to taste buds. Finally, using Bdnf null mice, which lose neurons that innervate taste buds, we demonstrate that Shh-expressing taste bud progenitors are specified and produce differentiated taste cells normally, in the absence of gustatory nerve contact. This resolution of a direct relationship between embryonic taste placodes with adult taste buds, which is independent of mesenchymal contribution and nerve contact, allows us to better define the early development of this important sensory system. These studies further suggest that mammalian taste bud development is very distinct from that of other epithelial appendages.

Regulation of the benzamil-insensitive salt taste receptor by intracellular Ca2+, protein kinase C, and calcineurin

The regulation of the benzamil (Bz)-insensitive salt taste receptor was investigated by intracellular Ca2+ ([Ca2+]i), protein kinase C (PKC), and the Ca2+-dependent serine-threonine phosphatase, calcineurin (PP2B), by monitoring chorda tympani taste nerve responses to 0.1 M NaCl solutions containing Bz (5x10(-6) M) and resiniferatoxin (RTX; 0-10x10(-6) M) in Sprague-Dawley rats and in wild-type (WT) and transient receptor potential vanilloid-1 knockout (TRPV1 KO) mice. In rats and WT mice, RTX increased the NaCl+Bz chorda tympani responses between 0.25x10(-6) and 1x10(-6) M and inhibited the responses above 1x10(-6) M. Decreasing taste receptor cell (TRC) [Ca2+]i with BAPTA loading, activation of PKC with 4alpha-phorbol-12,13-didecanoate (PMA), or inhibition of PP2B by cyclosporin A or FK-506, enhanced the magnitude of the Bz-insensitive NaCl chorda tympani responses in the presence of RTX and either minimized or completely eliminated the decrease in the chorda tympani response>1x10(-6) M RTX. In contrast, increasing TRC [Ca2+]i with ionomycin inhibited Bz-insensitive NaCl chorda tympani responses in the presence of RTX. No effect of the cited modulators was observed on the chorda tympani responses in WT mice and rats in the presence of TRPV1 blocker SB-366791 (1x10(-6) M) or in TRPV1 KO mice. 32P-labeling demonstrated direct phosphorylation of TRPV1 or TRPV1t in anterior lingual epithelium by PMA, cyclosporin A, or FK-506. PMA also enhanced the RTX-sensitive unilateral apical Na+ flux in polarized fungiform TRC in vitro. We conclude that TRPV1 or its variant TRPV1t is phosphorylated and dephosphorylated by PKC and PP2B, respectively, and either sensitizes or desensitizes the Bz-insensitive NaCl chorda tympani responses to RTX stimulation.

Transient receptor potential (TRP) channels and taste sensation

Humans have 5 basic taste sensations: sweet, bitter, sour, salty, and umami (taste of 1-amino acids). Among 33 genes related to transient receptor potential (TRP) channels, 3--including TRP-melastatin 5 (TRPM5), polycystic kidney disease-1-like 3 (PKD1L3), and polycystic kidney disease-2-like 1 (PKD2L1)--are specifically and abundantly expressed in taste receptor cells. TRP-melastatin 5 is co-expressed with taste receptors T1Rs and T2Rs, and functions as a common downstream component in sweet, bitter, and umami taste signal transduction. In contrast, polycystic kidney disease-1-like 3 and polycystic kidney disease-2-like 1 are co-expressed in distinct subsets of taste receptor cells not expressing TRP-melastatin 5. In the heterologous expression system, cells expressing both polycystic kidney disease-1-like 3 and polycystic kidney disease-2-like 1 responded to sour stimuli, showing a unique "off-response" property. Genetic ablation of poly-cystic kidney disease-2-like 1-expressing cells resulted in elimination of gustatory nerve response to sour stimuli, indicating that cells expressing polycystic kidney disease-2-like 1 function as sour taste detectors. These results suggest that polycystic kidney disease-1-like 3/polycystic kidney disease-2-like 1 may play a significant role, possibly as taste receptors, in sour taste sensation.

Parallel processing in mammalian taste buds?

ROPER, S.D. Parallel processing in mammalian taste buds? Physiol Behav XXX(Y) 000-000, 2009. There is emerging evidence that two parallel lines of gustatory information are generated in taste buds. One pathway leads to higher cortical centers and is involved in discriminating basic taste qualities (sweet, bitter, sour, salty, umami) and perceiving flavors. The other pathway may conduct information involved in physiological reflexes such as swallowing, salivation, and cephalic phase digestion. If this notion is true, the existence of two populations of taste bud cells that have different functional characteristics may lie at the origins of the two pathways. This speculative concept is explored in this review of taste signal processing in mammalian taste buds.

Purinergic signalling in autonomic control

Intercellular purinergic signalling, which utilizes ATP as a transmitter, is fundamental for the operation of the autonomic nervous system. ATP is released together with 'classical' transmitters from sympathetic and parasympathetic nerves supplying various peripheral targets, modulates neurotransmission in autonomic ganglia, has an important role in local enteric neural control and coordination of intestinal secretion and motility, and acts as a common mediator for several distinct sensory modalities. Recently, the role of ATP-mediated signalling in the central nervous control of autonomic function has been addressed. Emerging data demonstrate that in the brain ATP is involved in the operation of several key cardiorespiratory reflexes, contributes to central processing of viscerosensory information, mediates central CO(2) chemosensory transduction and triggers adaptive changes in breathing, and modulates the activities of the brainstem vagal preganglionic, presympathetic and respiratory neural networks.

The neuropeptides CCK and NPY and the changing view of cell-to-cell communication in the taste bud

The evolving view of the taste bud increasingly suggests that it operates as a complex signal processing unit. A number of neurotransmitters and neuropeptides and their corresponding receptors are now known to be expressed in subsets of taste receptor cells in the mammalian bud. These expression patterns set up hard-wired cell-to-cell communication pathways whose exact physiological roles still remain obscure. As occurs in other cellular systems, it is likely that neuropeptides are co-expressed with neurotransmitters and function as neuromodulators. Several neuropeptides have been identified in taste receptor cells including cholecystokinin (CCK), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), and glucagon-like peptide 1 (GLP-1). Of these, CCK and NPY are the best studied. These two peptides are co-expressed in the same presynaptic cells; however, their postsynaptic actions are both divergent and antagonistic. CCK and its receptor, the CCK-1 subtype, are expressed in the same subset of taste receptor cells and the autocrine activation of these cells produces a number of excitatory physiological actions. Further, most of these cells are responsive to bitter stimuli. On the other hand, NPY and its receptor, the NPY-1 subtype, are expressed in different cells. NPY, acting in a paracrine fashion on NPY-1 receptors, results in inhibitory actions on the cell. Preliminary evidence suggests the NPY-1 receptor expressing cell co-expresses T1R3, a member of the T1R family of G-protein coupled receptors thought to be important in detection of sweet and umami stimuli. Thus the neuropeptide expressing cells co-express CCK, NPY, and CCK-1 receptor. Neuropeptides released from these cells during bitter stimulation may work in concert to both modulate the excitation of bitter-sensitive taste receptor cells while concurrently inhibiting sweet-sensitive cells. This modulatory process is similar to the phenomenon of lateral inhibition that occurs in other sensory systems.

ATP release from non-excitable cells

All cells release nucleotides and are in one way or another involved in local autocrine and paracrine regulation of organ function via stimulation of purinergic receptors. Significant technical advances have been made in recent years to quantify more precisely resting and stimulated adenosine triphosphate (ATP) concentrations in close proximity to the plasma membrane. These technical advances are reviewed here. However, the mechanisms by which cells release ATP continue to be enigmatic. The current state of knowledge on different suggested mechanisms is also reviewed. Current evidence suggests that two separate regulated modes of ATP release co-exist in non-excitable cells: (1) a conductive pore which in several systems has been found to be the channel pannexin 1 and (2) vesicular release. Modes of stimulation of ATP release are reviewed and indicate that both subtle mechanical stimulation and agonist-triggered release play pivotal roles. The mechano-sensor for ATP release is not yet defined.

Sensory attributes of complex tasting divalent salts are mediated by TRPM5 and TRPV1 channels

Complex tasting divalent salts (CTDS) are present in our daily diet, contributing to multiple poorly understood taste sensations. CTDS evoking metallic, bitter, salty, and astringent sensations include the divalent salts of iron, zinc, copper, and magnesium. To identify pathways involved with the complex perception of the above salts, taste preference tests (two bottles, brief access) were performed in wild-type (WT) mice and in mice lacking (1) the T1R3 receptor, (2) TRPV1, the capsaicin receptor, or (3) the TRPM5 channel, the latter being necessary for the perception of sweet, bitter, and umami tasting stimuli. At low concentrations, FeSO(4) and ZnSO(4) were perceived as pleasant stimuli by WT mice, and this effect was fully reversed in TRPM5 knock-out mice. In contrast, MgSO(4) and CuSO(4) were aversive to WT mice, but for MgSO(4) the aversion was abolished in TRPM5 knock-out animals, and for CuSO(4), aversion decreased in both TRPV1- and TRPM5-deficient animals. Behavioral tests revealed that the T1R3 subunit of the sweet and umami receptors is implicated in the hedonically positive perception of FeSO(4) and ZnSO(4). For high concentrations of CTDS, the omission of TRPV1 reduced aversion. Imaging studies on heterologously expressed TRPM5 and TRPV1 channels are consistent with the behavioral experiments. Together, these results rationalize the complexity of metallic taste by showing that at low concentrations, compounds such as FeSO(4) and ZnSO(4) stimulate the gustatory system through the hedonically positive T1R3-TRPM5 pathway, and at higher concentrations, their aversion is mediated, in part, by the activation of TRPV1.

The sweet taste quality is linked to a cluster of taste fibers in primates: lactisole diminishes preference and responses to sweet in S fibers (sweet best) chorda tympani fibers of M. fascicularis monkey

Background

Psychophysically, sweet and bitter have long been considered separate taste qualities, evident already to the newborn human. The identification of different receptors for sweet and bitter located on separate cells of the taste buds substantiated this separation. However, this finding leads to the next question: is bitter and sweet also kept separated in the next link from the taste buds, the fibers of the taste nerves? Previous studies in non-human primates, P. troglodytes, C. aethiops, M. mulatta, M. fascicularis and C. jacchus, suggest that the sweet and bitter taste qualities are linked to specific groups of fibers called S and Q fibers. In this study we apply a new sweet taste modifier, lactisole, commercially available as a suppressor of the sweetness of sugars on the human tongue, to test our hypothesis that sweet taste is conveyed in S fibers.

Results

We first ascertained that lactisole exerted similar suppression of sweetness in M. fascicularis, as reported in humans, by recording their preference of sweeteners and non- sweeteners with and without lactisole in two-bottle tests. The addition of lactisole significantly diminished the preference for all sweeteners but had no effect on the intake of non-sweet compounds or the intake of water. We then recorded the response to the same taste stimuli in 40 single chorda tympani nerve fibers. Comparison between single fiber nerve responses to stimuli with and without lactisole showed that lactisole only suppressed the responses to sweeteners in S fibers. It had no effect on the responses to any other stimuli in all other taste fibers.

Conclusion

In M. fascicularis, lactisole diminishes the attractiveness of compounds, which taste sweet to humans. This behavior is linked to activity of fibers in the S-cluster. Assuming that lactisole blocks the T1R3 monomer of the sweet taste receptor T1R2/R3, these results present further support for the hypothesis that S fibers convey taste from T1R2/R3 receptors, while the impulse activity in non-S fibers originates from other kinds of receptors. The absence of the effect of lactisole on the faint responses in some S fibers to other stimuli as well as the responses to sweet and non-sweet stimuli in non-S fibers suggest that these responses originate from other taste receptors.

GABA expression in the mammalian taste bud functions as a route of inhibitory cell-to-cell communication

Recent advances have underscored cell-to-cell communication as an important component of the operation of taste buds with individual taste receptor cells (TRCs) communicating with one another by means of a number of neurotransmitters and neuropeptides, although functional roles are not yet understood. Here, we characterize the presence, distribution pattern, phenotype, and functional consequences of a previously undescribed inhibitory route within the taste bud mediated by the classic neurotransmitter GABA and its receptors. By using immunocytochemistry, subsets of TRCs within rat taste buds were identified as expressing GABA, and its synthetic enzyme glutamate decarboxylase (GAD). GAD expression was verified with Western blotting. Immunofluorescent studies revealed complex coexpression patterns of GAD with the TRC protein markers gustducin, neural cell adhesion molecule, protein gene product 9.5, and synaptosomal-associated protein of 25 kDa that collectively outline hardwired signaling pathways of GABAergic TRCs. RT-PCR and immunocytochemistry demonstrated that both GABA(A) and GABA(B) receptors are expressed in the taste bud. The later was observed in a subset TRCs paracrine to GAD-expressing TRCs. Physiological effects of GABA were examined by patch clamp recordings. GABA and the GABA(A) agonists muscimol and isoguvacine enhanced isolated chloride currents in a dose-dependent manner. Also, GABA and the GABA(B) agonist baclofen both elicited increases of the inwardly rectifying potassium currents that could be blocked by the GABA(B) receptor antagonist CGP 35348 and the G protein blocker GDP-betaS. Collectively, these data suggest that GABAergic TRCs are able to shape the final chemosensory output of the bud by means of processes of cell-to-cell modulation.

Voltage dependence of ATP secretion in mammalian taste cells

Mammalian type II taste cells release the afferent neurotransmitter adenosine triphosphate (ATP) through ATP-permeable ion channels, most likely to be connexin (Cx) and/or pannexin hemichannels. Here, we show that ion channels responsible for voltage-gated (VG) outward currents in type II cells are ATP permeable and demonstrate a strong correlation between the magnitude of the VG current and the intensity of ATP release. These findings suggest that slowly deactivating ion channels transporting the VG outward currents can also mediate ATP secretion in type II cells. In line with this inference, we studied a dependence of ATP secretion on membrane voltage with a cellular ATP sensor using different pulse protocols. These were designed on the basis of predictions of a model of voltage-dependent transient ATP efflux. Consistently with curves that were simulated for ATP release mediated by ATP-permeable channels deactivating slowly, the bell-like and Langmuir isotherm–like potential dependencies were characteristic of ATP secretion obtained for prolonged and short electrical stimulations of taste cells, respectively. These observations strongly support the idea that ATP is primarily released via slowly deactivating channels. Depolarizing voltage pulses produced negligible Ca²⁺ transients in the cytoplasm of cells releasing ATP, suggesting that ATP secretion is mainly governed by membrane voltage under our recording conditions. With the proviso that natural connexons and pannexons are kinetically similar to exogenously expressed hemichannels, our findings suggest that VG ATP release in type II cells is primarily mediated by Cx hemichannels.