Acid and salt responses in mouse taste cells

Sodium Homeostasis, a Balance Necessary for Life

Body sodium (Na) levels must be maintained within a narrow range for the correct functioning of the organism (Na homeostasis). Na disorders include not only elevated levels of this solute (hypernatremia), as in diabetes insipidus, but also reduced levels (hyponatremia), as in cerebral salt wasting syndrome. The balance in body Na levels therefore requires a delicate equilibrium to be maintained between the ingestion and excretion of Na. Salt (NaCl) intake is processed by receptors in the tongue and digestive system, which transmit the information to the nucleus of the solitary tract via a neural pathway (chorda tympani/vagus nerves) and to circumventricular organs, including the subfornical organ and area postrema, via a humoral pathway (blood/cerebrospinal fluid). Circuits are formed that stimulate or inhibit homeostatic Na intake involving participation of the parabrachial nucleus, pre-locus coeruleus, medial tuberomammillary nuclei, median eminence, paraventricular and supraoptic nuclei, and other structures with reward properties such as the bed nucleus of the stria terminalis, central amygdala, and ventral tegmental area. Finally, the kidney uses neural signals (e.g., renal sympathetic nerves) and vascular (e.g., renal perfusion pressure) and humoral (e.g., renin-angiotensin-aldosterone system, cardiac natriuretic peptides, antidiuretic hormone, and oxytocin) factors to promote Na excretion or retention and thereby maintain extracellular fluid volume. All these intake and excretion processes are modulated by chemical messengers, many of which (e.g., aldosterone, angiotensin II, and oxytocin) have effects that are coordinated at peripheral and central level to ensure Na homeostasis.

Mechanisms for the Sour Taste

Sour, the taste of acids, provides important sensory information to prevent the ingestion of unripe, spoiled, or fermented foods. In mammals, acids elicit disgust and pain by simultaneously activating taste and somatosensory neurons innervating the oral cavity. Early researchers detected electrical activity in taste nerves upon presenting acids to the tongue, establishing this as the bona fide sour taste. Recent studies have made significant contributions to our understanding of the mechanisms underlying acid sensing in the taste receptor cells at the periphery and the neural circuitry that convey this information to the brain. In this chapter, we discuss the characterization of sour taste receptor cells, the twists and turns eventually leading to the identification of Otopetrin1 (OTOP1) as the sour taste receptor, the pathway of sour taste signaling from the tongue to the brainstem, and other roles sour taste receptor cells play in the taste bud.

Acid-sensing ion channels and transient-receptor potential ion channels in zebrafish taste buds

Sensory information from the environment is required for life and survival, and it is detected by specialized cells which together make up the sensory system. The fish sensory system includes specialized organs that are able to detect mechanical and chemical stimuli. In particular, taste buds are small organs located on the tongue in terrestrial vertebrates that function in the perception of taste. In fish, taste buds occur on the lips, the flanks, and the caudal (tail) fins of some species and on the barbels of others. In fish taste receptor cells, different classes of ion channels have been detected which, like in mammals, presumably participate in the detection and/or transduction of chemical gustatory signals. However, since some of these ion channels are involved in the detection of additional sensory modalities, it can be hypothesized that taste cells sense stimuli other than those specific for taste. This mini-review summarizes current knowledge on the presence of transient-receptor potential (TRP) and acid-sensing (ASIC) ion channels in the taste buds of teleosts, especially adult zebrafish. Up to now ASIC4, TRPC2, TRPA1, TRPV1 and TRPV4 ion channels have been found in the sensory cells, while ASIC2 was detected in the nerves supplying the taste buds.

Amiloride-Insensitive Salt Taste Is Mediated by Two Populations of Type III Taste Cells with Distinct Transduction Mechanisms

Responses in the amiloride-insensitive (AI) pathway, one of the two pathways mediating salty taste in mammals, are modulated by the size of the anion of a salt. This "anion effect" has been hypothesized to result from inhibitory transepithelial potentials (TPs) generated across the lingual epithelium as cations permeate through tight junctions and leave their larger and less permeable anions behind (Ye et al., 1991). We tested directly the necessity of TPs for the anion effect by measuring responses to NaCl and Na-gluconate (small and large anion sodium salts, respectively) in isolated taste cells from mouse circumvallate papillae. Using calcium imaging, we identified AI salt-responsive type III taste cells and demonstrated that they compose a subpopulation of acid-responsive taste cells. Even in the absence of TPs, many (66%) AI salt-responsive type III taste cells still exhibited the anion effect, demonstrating that some component of the transduction machinery for salty taste in type III cells is sensitive to anion size. We hypothesized that osmotic responses could explain why a minority of type III cells (34%) had AI salt responses but lacked anion sensitivity. All AI type III cells had osmotic responses to cellobiose, which were significantly modulated by extracellular sodium concentration, suggesting the presence of a sodium-conducting osmotically sensitive ion channel. However, these responses were significantly larger in AI type III cells that did not exhibit the anion effect. These findings indicate that multiple mechanisms could underlie AI salt responses in type III taste cells, one of which may contribute to the anion effect.

SIGNIFICANCE STATEMENT

Understanding the mechanisms underlying salty taste will help inform strategies to combat the health problems associated with NaCl overconsumption by humans. Of the two pathways underlying salty taste in mammals, the amiloride-insensitive (AI) pathway is the least understood. Using calcium imaging of isolated mouse taste cells, we identify two separate populations of AI salt-responsive type III taste cells distinguished by their sensitivity to anion size and show that these cells compose subpopulations of acid-responsive taste cells. We also find evidence that a sodium-conducting osmotically sensitive mechanism contributes to salt responses in type III taste cells. Our data not only provide new insights into the transduction mechanisms of AI salt taste but also have important implications for general theories of taste encoding.

Inflammatory stimuli acutely modulate peripheral taste function

Inflammation-mediated changes in taste perception can affect health outcomes in patients, but little is known about the underlying mechanisms. In the present work, we hypothesized that proinflammatory cytokines directly modulate Na(+) transport in taste buds. To test this, we measured acute changes in Na(+) flux in polarized fungiform taste buds loaded with a Na(+) indicator dye. IL-1β elicited an amiloride-sensitive increase in Na(+) transport in taste buds. In contrast, TNF-α dramatically and reversibly decreased Na(+) flux in polarized taste buds via amiloride-sensitive and amiloride-insensitive Na(+) transport systems. The speed and partial amiloride sensitivity of these changes in Na(+) flux indicate that IL-1β and TNF-α modulate epithelial Na(+) channel (ENaC) function. A portion of the TNF-mediated decrease in Na(+) flux is also blocked by the TRPV1 antagonist capsazepine, although TNF-α further reduced Na(+) transport independently of both amiloride and capsazepine. We also assessed taste function in vivo in a model of infection and inflammation that elevates these and additional cytokines. In rats administered systemic lipopolysaccharide (LPS), CT responses to Na(+) were significantly elevated between 1 and 2 h after LPS treatment. Low, normally preferred concentrations of NaCl and sodium acetate elicited high response magnitudes. Consistent with this outcome, codelivery of IL-1β and TNF-α enhanced Na(+) flux in polarized taste buds. These results demonstrate that inflammation elicits swift changes in Na(+) taste function, which may limit salt consumption during illness.

Attempt to develop taste bud models in three-dimensional culture

Taste buds are the end organs of taste located in the gustatory papillae, which occur on the surface of the oral cavity. The goal of the present study was to establish a culture model mimicking the lingual taste bud of the mouse. To this end, three cell lines were employed: taste bud-derived cell lines (TBD cell lines), a lingual epithelial cell-derived cell line (20A cell line), and a mesenchymal cell-derived cell line (TMD cell line). TBD cells embedded in collagen gel formed three-dimensional clusters, which had an internal cavity equipped with a tight junction-like structure, a microvilluslike structure, and a laminin-positive layer surrounding the cluster. The cells with this epitheliumlike morphology expressed marker proteins of taste cells: gustducin and NCAM. TBD cells formed a monolayer on collagen gel when they were co-cultured with TMD cells. TBD, 20A, and TMD cell lines were maintained in a triple cell co-culture, in which TBD cells were pre-seeded as aggregates or in suspension on the collagen gel containing TMD cells, and 20A cells were laid over the TBD cells. TBD cells in the triple cell co-culture expressed NCAM. This result suggests that co-cultured TBD cells exhibited a characteristic of Type III taste cells. The culture model would be useful to study morphogenesis and functions of the gustatory organ.

Molecular evolution of candidate sour taste receptor gene PKD1L3 in mammals

The PKD1L3 gene encodes an ion channel protein that can interact with the PKD2L1 protein to form a candidate sour taste receptor. In the present study, we have analyzed the evolutionary patterns of PKD1L3 genes from 10 mammalian species. The results showed that PKD1L3 genes have evolved under a dominant purifying selection force. However, for some branches and sites, PKD1L3 genes were detected to have been operated by positive selection. Moreover, some of these positive evolutionary sites are likely to participate in acid stimulus recognition. In rodents, PKD1L3 genes evolved more rapidly than other mammalian lineages. Combined with other functional research reports, our results suggest that rodents may not be the most appropriate model for functional research on the PKD1L3 gene.

Genetic variation in taste and its influence on food selection

Abstract Taste perception plays a key role in determining individual food preferences and dietary habits. Individual differences in bitter, sweet, umami, sour, or salty taste perception may influence dietary habits, affecting nutritional status and nutrition-related chronic disease risk. In addition to these traditional taste modalities there is growing evidence that "fat taste" may represent a sixth modality. Several taste receptors have been identified within taste cell membranes on the surface of the tongue, and they include the T2R family of bitter taste receptors, the T1R receptors associated with sweet and umami taste perception, the ion channels PKD1L3 and PKD2L1 linked to sour taste, and the integral membrane protein CD36, which is a putative "fat taste" receptor. Additionally, epithelial sodium channels and a vanilloid receptor, TRPV1, may account for salty taste perception. Common polymorphisms in genes involved in taste perception may account for some of the interindividual differences in food preferences and dietary habits within and between populations. This variability could affect food choices and dietary habits, which may influence nutritional and health status and the risk of chronic disease. This review will summarize the present state of knowledge of the genetic variation in taste, and how such variation might influence food intake behaviors.

NaCl responsive taste cells in the mouse fungiform taste buds

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

Neurogenetics of courtship and mating in Drosophila

The reproductive biology of Drosophila melanogaster is described and critically discussed, primarily with regard to genetic studies of sex-specific behavior and its neural underpinnings. The investigatory history of this system includes, in addition to a host of recent neurobiological analyses of reproductive phenotypes, studies of mating as well as the behaviors leading up to that event. Courtship and mating have been delved into mostly with regard to male-specific behavior and biology, although a small number of studies has also pointed to the neural substrates of female reproduction. Sensory influences on interactions between courting flies have long been studied, partly by application of mutants and partly by surgical experiments. More recently, molecular-genetic approaches to sensations passing between flies in reproductive contexts have aimed to "dissect" further the meaning of separate sensory modalities. Notable among these are olfactory and contact-chemosensory stimuli, which perhaps have received an inordinate amount of attention in terms of the possibility that they could comprise the key cues involved in triggering and sustaining courtship actions. But visual and auditory stimuli are heavily involved as well--appreciated mainly from older experiments, but analyzable further using elementary approaches (single-gene mutations mutants and surgeries), as well as by applying the molecularly defined factors alluded to above. Regarding regulation of reproductive behavior by components of Drosophila's central nervous system (CNS), once again significant invigoration of the relevant inquiries has been stimulated and propelled by identification and application of molecular-genetic materials. A distinct plurality of the tools applied involves transposons inserted in the fly's chromosomes, defining "enhancer-trap" strains that can be used to label various portions of the nervous system and, in parallel, disrupt their structure and function by "driving" companion transgenes predesigned for these experimental purposes. Thus, certain components of interneuronal routes, functioning along pathways whose starting points are sensory reception by the peripheral nervous system (PNS), have been manipulated to enhance appreciation of sexually important sensory modalities, as well as to promote understanding of where such inputs end up within the CNS: Where are reproductively related stimuli processed, such that different kinds of sensation would putatively be integrated to mediate sex-specific behavioral readouts? In line with generic sensory studies that have tended to concentrate on chemical stimuli, PNS-to-CNS pathways focused upon in reproductive experiments relying on genic enhancers have mostly involved smell and taste. Enhancer traps have also been applied to disrupt various regions within the CNS to ask about the various ganglia, and portions thereof, that contribute to male- or female-specific behavior. These manipulations have encompassed structural or functional disruptions of such regions as well as application of molecular-genetic tricks to feminize or masculinize a given component of the CNS. Results of such experiments have, indeed, identified certain discrete subsets of centrally located ganglia that, on the one hand, lead to courtship defects when disrupted or, on the other, must apparently maintain sex-specific identity if the requisite courtship actions are to be performed. As just implied, perturbations of certain neural tissues not based on manipulating "sex factors" might lead to reproductive behavioral abnormalities, even though changing the sexual identity of such structures would not necessarily have analogous consequences. It has been valuable to uncover these sexually significant subsets of the Drosophila nervous system, although it must be said that not all of the transgenically based dissection outcomes are in agreement. Thus, the good news is that not all of the CNS is devoted to courtship control, whereby any and all locales disrupted might have led to sex-specific deficits; but the bad news is that the enhancer-trap approach to these matters has not led to definitive homing-in on some tractable number of mutually agreed-upon "courtship centers" within the brain or within the ventral nerve cord (VNC). The latter neural region, which comprises about half of the fly's CNS, is underanalyzed as to its sex-specific significance: How, for example, are various kinds of sensory inputs to posteriorly located PNS structures processed, such that they eventually end up modulating brain functions underlying courtship? And how are sex-specific motor outputs mediated by discrete collections of neurons within VNC ganglia--so that, for instance, male-specific whole-animal motor actions and appendage usages are evoked? These behaviors can be thought of as fixed action patterns. But it is increasingly appreciated that elements of the fly's reproductive behavior can be modulated by previous experience. In this regard, the neural substrates of conditioned courtship are being more and more analyzed, principally by further usages of various transgenic types. Additionally, a set of molecular neurogenetic experiments devoted to experience-dependent courtship was based on manipulations of a salient "sex gene" in D. melanogaster. This well-defined factor is called fruitless (fru). The gene, its encoded products, along with their behavioral and neurobiological significance, have become objects of frenetic attention in recent years. How normal, mutated, and molecularly manipulated forms of fru seem to be generating a good deal of knowledge and insight about male-specific courtship and mating is worthy of much attention. This previews the fact that fruitless matters are woven throughout this chapter as well as having a conspicuous section allocated to them. Finally, an acknowledgment that the reader is being subjected to lengthy preview of an article about this subject is given. This matter is mentioned because--in conjunction with the contemporary broadening and deepening of this investigatory area--brief summaries of its findings are appearing with increasing frequency. This chapter will, from time to time, present our opinion that a fair fraction of the recent minireviews are replete with too many catch phrases about what is really known. This is one reason why the treatment that follows not only attempts to describe the pertinent primary reports in detail but also pauses often to discuss our views about current understandings of sex-specific behavior in Drosophila and its underlying biology.

The chemistry and physiology of sour taste--a review

Sour taste is the key element in the flavor profile of food acidulants. Understanding the chemistry and physiology of sour taste is critical for efficient control of flavor in the formulation of acid and acidified foods. After a brief introduction to the main applications of food acidulants, several chemical parameters associated with sour taste are discussed. Special emphasis is given to hydrogen ions, protonated (undissociated) acid species, titratable acidity, anions, molar concentration, and physical and chemical properties of organic acids. This article also presents an overview of the physiology of sour taste and proposed theories for the transduction mechanisms for sour taste. The physiology of sour taste perception remains controversial and significant diversity exists among species with regard to cellular schemes used for detection of stimuli. The variety of mechanisms proposed, even within individual species, highlights the complexity of elucidating sour taste transduction. However, recent evidence suggests that at least one specific sour taste receptor protein has been identified.

Claudin-based permeability barriers in taste buds

Tight junctions operate as semipermeable barriers in epithelial tissue, separating the apical from the basolateral sides of the cells. Membrane proteins of the claudin family represent the major tight junction constituents, and some reinforce permeability barriers, whereas others create pores based on solute size and ion selectivity. To outline paracellular permeability pathways in gustatory tissue, all claudins expressed in mouse taste buds and in human fungiform papillae have been characterized. Twelve claudins are expressed in murine taste-papillae-enriched tissue, and five of those are expressed in human fungiform papillae. A subset of the claudins expressed in mouse papillae is uniquely found in taste buds. By immunohistochemistry, claudin 4 has been found in mouse taste epithelium, with high abundance around the taste pore. Claudin 6 is explicitly detected inside the pore, claudin 7 was found at the basolateral side of taste cells, and claudin 8 was found around the pore. With the ion permeability features of the different claudins, a highly specific permeability pattern for paracellular diffusion is apparent, which indicates a peripheral mechanism for taste coding.

Transient receptor potential family members PKD1L3 and PKD2L1 form a candidate sour taste receptor

Animals use their gustatory systems to evaluate the nutritious value, toxicity, sodium content, and acidity of food. Although characterization of molecular identities that receive taste chemicals is essential, molecular receptors underlying sour taste sensation remain unclear. Here, we show that two transient receptor potential (TRP) channel members, PKD1L3 and PKD2L1, are coexpressed in a subset of taste receptor cells in specific taste areas. Cells expressing these molecules are distinct from taste cells having receptors for bitter, sweet, or umami tastants. The PKD2L1 proteins are accumulated at the taste pore region, where taste chemicals are detected. PKD1L3 and PKD2L1 proteins can interact with each other, and coexpression of the PKD1L3 and PKD2L1 is necessary for their functional cell surface expression. Finally, PKD1L3 and PKD2L1 are activated by various acids when coexpressed in heterologous cells but not by other classes of tastants. These results suggest that PKD1L3 and PKD2L1 heteromers may function as sour taste receptors.

Acid-sensing ion channels in taste buds

Taste receptor cells detect gustatory stimuli using a complex arrangement of ion channels, G protein-coupled receptors, and signaling cascades. Sour and salty tastes are detected by ion channels in the rat. Using a combination of homology screening and functional expression approaches, we screened a rat circumvallate papilla cDNA library and identified acid-sensing ion channel-2a (ASIC2a) and ASIC2b as candidates for the rat sour-sensing channels. In situ hybridization and reverse transcription-polymerase chain reaction experiments revealed that ASIC2a and ASIC2b transcripts were localized in taste bud cells. Immunohistochemistry and immunoprecipitation also revealed that both subunits were expressed in a subset of taste cells and that some of the cells expressed ASIC2a/ASIC2b heteromeric assemblies. Electrophysiological studies demonstrated that stimulation of acetic acid produced larger ASIC2 currents than did hydrochloric acid at the same pH. ASIC2a/ ASIC2b channels generated maximal inward currents at pH <or= 2.0, which agrees well with the in vivo pH-sensitivity of rat taste cells. The amiloride-sensitivity of ASIC2a/ ASIC2b heteromer lessened with decreasing pH and almost completely disappeared at pH 2.0. These data suggest that ASIC2a and ASIC2b may play roles in sour taste transduction.

Acid-sensing ion channel-2 is not necessary for sour taste in mice

The acid-sensitive cation channel acid-sensing ion channel-2 (ASIC2) is widely believed to be a receptor for acid (sour) taste in mammals on the basis of its physiological properties and expression in rat taste bud cells. Using reverse transcriptase-PCR, we detected expression of ASIC1 and ASIC3, but not ASIC4, in mouse and rat taste buds and nonsensory lingual epithelium. Surprisingly, we did not detect mRNA for ASIC2 in mouse taste buds, although we readily observed its expression in rat taste buds. Furthermore, in Ca2+ imaging experiments, ASIC2 knock-out mice exhibited normal physiological responses (increases in intracellular Ca2+ concentrations) to acid taste stimuli. Our results indicate that ASIC2 is not required for acid taste in mice, and that if a universal mammalian acid taste transduction mechanism exists, it likely uses other acid-sensitive receptors or ion channels.

Taste receptor cells express pH-sensitive leak K+ channels

Two-pore domain K+ channels encoded by genes KCNK1-17 (K2p1-17) play important roles in regulating cell excitability. We report here that rat taste receptor cells (TRCs) highly express TASK-2 (KCNK5; K2p5.1), and to a much lesser extent TALK-1 (KCNK16; K2p16.1) and TASK-1 (KCNK3; K2p3.1), and suggest potentially important roles for these channels in setting resting membrane potentials and in sour taste transduction. Whole cell recordings of isolated TRCs show that a leak K+ (Kleak) current in a subset of TRCs exhibited high sensitivity to acidic extracellular pH similar to reported properties of TASK-2 and TALK-1 channels. A drop in bath pH from 7.4 to 6 suppressed 90% of the current, resulting in membrane depolarization. K+ channel blockers, BaCl2, but not tetraethylammonium (TEA), inhibited the current. Interestingly, resting potentials of these TRCs averaged -70 mV, which closely correlated with the amplitude of the pH-sensitive Kleak, suggesting a dominant role of this conductance in setting resting potentials. RT-PCR assays followed by sequencing of PCR products showed that TASK-1, TASK-2, and a functionally similar channel, TALK-1, were expressed in all three types of lingual taste buds. To verify expression of TASK channels, we labeled taste tissue with antibodies against TASK-1, TASK-2, and TASK-3. Strong labeling was seen in some TRCs with antibody against TASK-2 but not TASK-1 and TASK-3. Consistent with the immunocytochemical staining, quantitative real-time PCR assays showed that the message for TASK-2 was expressed at significantly higher levels (10-100 times greater) than was TASK-1, TALK-1, or TASK-3. Thus several K2P channels, and in particular TASK-2, are expressed in rat TRCs, where they may contribute to the establishment of resting potentials and sour reception.

Acid-sensitive two-pore domain potassium (K2P) channels in mouse taste buds

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Identification of sour-taste receptor genes

Taste cells located in taste buds respond to gustatory stimuli using a complex arrangement of ion channels, receptor molecules and signaling cascades. Previous electrophysiological experiments have shown that sour taste (essentially a taste of protons) is mediated, at least in part, by apically located amiloride-sensitive channels in the rat. Here, the molecular cloning of sour-taste receptor genes is described. Using a combination of homology screening and functional expression approaches, we screened a rat circumvallate papilla cDNA library and identified acid-sensing ion channel-2a (ASIC2a) and ASIC2b, amiloride-sensitive proton-activated cation channels. In situ hybridization and reverse transcription-polymerase chain reaction experiments showed that ASIC2a and ASIC2b transcripts were localized in taste cells. Immunohistochemical and immunoprecipitation studies revealed that both channels were expressed in a subset of taste cells and that some of the cells expressed ASIC2a/ASIC2b heteromeric assemblies. Immunoelectron microscopic analyses demonstrated that some of the ASIC2a-immunopositive cells had the characteristics of type III cells, which make synaptic contacts with gustatory afferent neurons. According to our electrophysiological studies, stimulation by acetic acid generated larger inward currents in ASIC2a- or ASIC2a/ASIC2b-expressing oocytes than those induced by hydrochloric acid at the same proton concentration and these findings are in good agreement with the well-known taste phenomenon that acetic acid is more sour than hydrochloric acid at equal pH. Taken together, the present results strongly suggest that mucosal protons dissociated from sour-taste substances induce taste cell depolarization through the activation of the ASIC2a and ASIC2a/ASIC2b channels, which leads to transmitter release onto gustatory neurons.

Channels as taste receptors in vertebrates

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

Amiloride-insensitive currents of the acid-sensing ion channel-2a (ASIC2a)/ASIC2b heteromeric sour-taste receptor channel

Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH of < or =2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.

Serotonin differentially modulates the electrical properties of different subsets of taste receptor cells in bullfrog

Serotonin (5-hydroxytryptamin, 5-HT) is localized in taste bud cells of vertebrates. Effects of the external application of 5-HT on the membrane currents of frog taste receptor cells (TRCs) were investigated using patch-clamp technique in whole-cell configuration. The 5-HT (0.1-1 micro m) and 5-HT1A receptor agonist (+/-)-8-OH-2-(D1-n-propyl-amino)tetralin (8-OH-DPAT) (1-20 micro m) inhibited both voltage-gated sodium current (INa) and voltage-gated potassium current (IK) in 50% of TRCs, but potentiated IK without any significant effect on INa in another subset of 18% of TRCs. Voltage-gated currents in the residual TRCs were not affected by 5-HT or 8-OH-DPAT. External application of 10 micro m forskolin and 300 micro m 8-cpt cAMP [8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate] mimicked the inhibitory effect of 5-HT and 8-OH-DPAT on IK and INa while internal dialysis with 50 micro m protein kinase A inhibitor prevented the 5-HT-mediated inhibitory effects on IK and INa in TRCs. Internal dialysis of TRCs with high Ca2+-pipette solution (1 micro m) increased the IK in 58% of TRCs. The 5-HT reversibly increased the [Ca2+]i in 17% of TRCs when measured by Ca2+-imaging using a Ca2+-sensitive dye (fura-2 AM). These results suggest that 5-HT differentially modulates the voltage-gated membrane currents in different subsets of TRCs.

Electrophysiology of Necturus taste cells

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

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.