Adenylyl cyclase expression and modulation of cAMP in rat taste cells

Adrenomedullin Enhances Mouse Gustatory Nerve Responses to Sugars via T1R-Independent Sweet Taste Pathway

On the tongue, the T1R-independent pathway (comprising glucose transporters, including sodium-glucose cotransporter (SGLT1) and the K_ATP channel) detects only sugars, whereas the T1R-dependent (T1R2/T1R3) pathway can broadly sense various sweeteners. Cephalic-phase insulin release, a rapid release of insulin induced by sensory signals in the head after food-related stimuli, reportedly depends on the T1R-independent pathway, and the competitive sweet taste modulators leptin and endocannabinoids may function on these two different sweet taste pathways independently, suggesting independent roles of two oral sugar-detecting pathways in food intake. Here, we examined the effect of adrenomedullin (ADM), a multifunctional regulatory peptide, on sugar sensing in mice since it affects the expression of SGLT1 in rat enterocytes. We found that ADM receptor components were expressed in T1R3-positive taste cells. Analyses of chorda tympani (CT) nerve responses revealed that ADM enhanced responses to sugars but not to artificial sweeteners and other tastants. Moreover, ADM increased the apical uptake of a fluorescent D-glucose derivative into taste cells and SGLT1 mRNA expression in taste buds. These results suggest that the T1R-independent sweet taste pathway in mouse taste cells is a peripheral target of ADM, and the specific enhancement of gustatory nerve responses to sugars by ADM may contribute to caloric sensing and food intake.

Physiology of the tongue with emphasis on taste transduction

The tongue is a complex multifunctional organ that interacts and senses both interoceptively and exteroceptively. Although it is easily visible to almost all of us, it is relatively understudied and what is in the literature is often contradictory or is not comprehensively reported. The tongue is both a motor and a sensory organ: motor in that it is required for speech and mastication, and sensory in that it receives information to be relayed to the central nervous system pertaining to the safety and quality of the contents of the oral cavity. Additionally, the tongue and its taste apparatus form part of an innate immune surveillance system. For example, loss or alteration in taste perception can be an early indication of infection as became evident during the present global SARS-CoV-2 pandemic. Here, we particularly emphasize the latest updates in the mechanisms of taste perception, taste bud formation and adult taste bud renewal, and the presence and effects of hormones on taste perception, review the understudied lingual immune system with specific reference to SARS-CoV-2, discuss nascent work on tongue microbiome, as well as address the effect of systemic disease on tongue structure and function, especially in relation to taste.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

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

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

G Protein-Coupled Receptors in Taste Physiology and Pharmacology

Heterotrimeric G protein-coupled receptors (GPCRs) comprise the largest receptor family in mammals and are responsible for the regulation of most physiological functions. Besides mediating the sensory modalities of olfaction and vision, GPCRs also transduce signals for three basic taste qualities of sweet, umami (savory taste), and bitter, as well as the flavor sensation kokumi. Taste GPCRs reside in specialised taste receptor cells (TRCs) within taste buds. Type I taste GPCRs (TAS1R) form heterodimeric complexes that function as sweet (TAS1R2/TAS1R3) or umami (TAS1R1/TAS1R3) taste receptors, whereas Type II are monomeric bitter taste receptors or kokumi/calcium-sensing receptors. Sweet, umami and kokumi receptors share structural similarities in containing multiple agonist binding sites with pronounced selectivity while most bitter receptors contain a single binding site that is broadly tuned to a diverse array of bitter ligands in a non-selective manner. Tastant binding to the receptor activates downstream secondary messenger pathways leading to depolarization and increased intracellular calcium in TRCs, that in turn innervate the gustatory cortex in the brain. Despite recent advances in our understanding of the relationship between agonist binding and the conformational changes required for receptor activation, several major challenges and questions remain in taste GPCR biology that are discussed in the present review. In recent years, intensive integrative approaches combining heterologous expression, mutagenesis and homology modeling have together provided insight regarding agonist binding site locations and molecular mechanisms of orthosteric and allosteric modulation. In addition, studies based on transgenic mice, utilizing either global or conditional knock out strategies have provided insights to taste receptor signal transduction mechanisms and their roles in physiology. However, the need for more functional studies in a physiological context is apparent and would be enhanced by a crystallized structure of taste receptors for a more complete picture of their pharmacological mechanisms.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Mammalian Taste Cells Express Functional Olfactory Receptors

The peripheral taste and olfactory systems in mammals are separate and independent sensory systems. In the current model of chemosensation, gustatory, and olfactory receptors are genetically divergent families expressed in anatomically distinct locations that project to disparate downstream targets. Although information from the 2 sensory systems merges to form the perception of flavor, the first cross talk is thought to occur centrally, in the insular cortex. Recent studies have shown that gustatory and olfactory receptors are expressed throughout the body and serve as chemical sensors in multiple tissues. Olfactory receptor cDNA has been detected in the tongue, yet the presence of physiologically functional olfactory receptors in taste cells has not yet been demonstrated. Here we report that olfactory receptors are functionally expressed in taste papillae. We found expression of olfactory receptors in the taste papillae of green fluorescent protein-expressing transgenic mice and, using immunocytochemistry and real-time quantitative polymerase chain reaction experiments, the presence of olfactory signal transduction molecules and olfactory receptors in cultured human fungiform taste papilla (HBO) cells. Both HBO cells and mouse taste papilla cells responded to odorants. Knockdown of adenylyl cyclase mRNA by specific small inhibitory RNA and pharmacological block of adenylyl cyclase eliminated these responses, leading us to hypothesize that the gustatory system may receive olfactory information in the periphery. These results provide the first direct evidence of the presence of functional olfactory receptors in mammalian taste cells. Our results also demonstrate that the initial integration of gustatory and olfactory information may occur as early as the taste receptor cells.

Cyclic-AMP regulates postnatal development of neural and behavioral responses to NaCl in rats

During postnatal development rats demonstrate an age-dependent increase in NaCl chorda tympani (CT) responses and the number of functional apical amiloride-sensitive epithelial Na+ channels (ENaCs) in salt sensing fungiform (FF) taste receptor cells (TRCs). Currently, the intracellular signals that regulate the postnatal development of salt taste have not been identified. We investigated the effect of cAMP, a downstream signal for arginine vasopressin (AVP) action, on the postnatal development of NaCl responses in 19-23 day old rats. ENaC-dependent NaCl CT responses were monitored after lingual application of 8-chlorophenylthio-cAMP (8-CPT-cAMP) under open-circuit conditions and under ±60 mV lingual voltage clamp. Behavioral responses were tested using 2 bottle/24h NaCl preference tests. The effect of [deamino-Cys1, D-Arg8]-vasopressin (dDAVP, a specific V2R agonist) was investigated on ENaC subunit trafficking in rat FF TRCs and on cAMP generation in cultured adult human FF taste cells (HBO cells). Our results show that in 19-23 day old rats, the ENaC-dependent maximum NaCl CT response was a saturating sigmoidal function of 8-CPT-cAMP concentration. 8-CPT-cAMP increased the voltage-sensitivity of the NaCl CT response and the apical Na+ response conductance. Intravenous injections of dDAVP increased ENaC expression and γ-ENaC trafficking from cytosolic compartment to the apical compartment in rat FF TRCs. In HBO cells dDAVP increased intracellular cAMP and cAMP increased trafficking of γ- and δ-ENaC from cytosolic compartment to the apical compartment 10 min post-cAMP treatment. Control 19-23 day old rats were indifferent to NaCl, but showed clear preference for appetitive NaCl concentrations after 8-CPT-cAMP treatment. Relative to adult rats, 14 day old rats demonstrated significantly less V2R antibody binding in circumvallate TRCs. We conclude that an age-dependent increase in V2R expression produces an AVP-induced incremental increase in cAMP that modulates the postnatal increase in TRC ENaC and the neural and behavioral responses to NaCl.

The Effect of Temperature on Umami Taste

The effect of temperature on umami taste has not been previously studied in humans. Reported here are 3 experiments in which umami taste was measured for monopotassium glutamate (MPG) and monosodium glutamate (MSG) at solution temperatures between 10 and 37 °C. Experiment 1 showed that for subjects sensitive to MPG on the tongue tip, 1) cooling reduced umami intensity whether sampled with the tongue tip or in the whole mouth, but 2) had no effect on the rate of umami adaptation on the tongue tip. Experiment 2 showed that temperature had similar effects on the umami taste of MSG and MPG on the tongue tip but not in the whole mouth, and that contrary to umami taste, cooling to 10 °C increased rather than decreased the salty taste of both stimuli. Experiment 3 was designed to investigate the contribution of the hT1R1-hT1R3 glutamate receptor to the cooling effect on umami taste by using the T1R3 inhibitor lactisole. However, lactisole failed to block the umami taste of MPG at any temperature, which supports prior evidence that lactisole does not block umami taste for all ligands of the hT1R1-hT1R3 receptor. We conclude that temperature can affect sensitivity to the umami and salty tastes of glutamates, but in opposite directions, and that the magnitude of these effects can vary across stimuli and modes of tasting (i.e., whole mouth vs. tongue tip exposures).

Umami the Fifth Basic Taste: History of Studies on Receptor Mechanisms and Role as a Food Flavor

Three umami substances (glutamate, 5'-inosinate, and 5'-guanylate) were found by Japanese scientists, but umami has not been recognized in Europe and America for a long time. In the late 1900s, umami was internationally recognized as the fifth basic taste based on psychophysical, electrophysiological, and biochemical studies. Three umami receptors (T1R1 + T1R3, mGluR4, and mGluR1) were identified. There is a synergism between glutamate and the 5'-nucleotides. Among the above receptors, only T1R1 + T1R3 receptor exhibits the synergism. In rats, the response to a mixture of glutamate and 5'-inosinate is about 1.7 times larger than that to glutamate alone. In human, the response to the mixture is about 8 times larger than that to glutamate alone. Since glutamate and 5'-inosinate are contained in various foods, we taste umami induced by the synergism in daily eating. Hence umami taste induced by the synergism is a main umami taste in human.

Meglumine cyclic adenylate improves neurological function following acute spinal cord injury in rats

Elevation of intracellular cyclic adenosine monophosphate (cAMP) levels facilitates recovery following spinal injury by suppressing secondary pathology and promoting axonal regeneration. However, this treatment strategy is limited by lack of effective and tolerable clinical agents. The present study examined the effects of meglumine cyclic adenylate (MCA) on neurological recovery, cAMP concentration, adenylate cyclase 3 (AC3) activity and phosphodiesterase 4D (PDE4D) activity during early stage acute spinal cord injury (SCI) in rats. A total of 48 Sprague‑Dawley rats were randomly assigned to groups A, B or C, each consisting of 16 animals. SCI was induced by Allen's method using a 7 g x 3 cm extradural weight‑drop impact on spinal cord segment T11. A total of 30 min following SCI, group A received a single 30 mg/kg‑bw i.p. dose of methylprednisolone, group B received 2 mg/kg‑bw i.p. MCA daily for seven days and group C were administered an equal volume of normal saline. Seven days following SCI, the spinal cord samples from eight rats per group were obtained to measure the cAMP concentration, and the activities of AC3 and PDE4D. The remaining eight rats per group were used for behavioral assessments using the inclined plane stability test and Gale scale for up to six weeks post‑SCI. The drug‑treated groups A and B had higher cAMP concentrations and AC3 activities but lower PDE4D activities at the lesion sites, as well as superior behavioral scores post‑SCI compared with the vehicle‑treated group C (P<0.05). Furthermore, cAMP was higher in group B than in group A (P<0.05). It was concluded that MCA may serve as an effective SCI treatment by activating AC3 and suppressing PDE4D.

Angiotensin II modulates salty and sweet taste sensitivities

Understanding the mechanisms underlying gustatory detection of dietary sodium is important for the prevention and treatment of hypertension. Here, we show that Angiotensin II (AngII), a major mediator of body fluid and sodium homeostasis, modulates salty and sweet taste sensitivities, and that this modulation critically influences ingestive behaviors in mice. Gustatory nerve recording demonstrated that AngII suppressed amiloride-sensitive taste responses to NaCl. Surprisingly, AngII also enhanced nerve responses to sweeteners, but had no effect on responses to KCl, sour, bitter, or umami tastants. These effects of AngII on nerve responses were blocked by the angiotensin II type 1 receptor (AT1) antagonist CV11974. In behavioral tests, CV11974 treatment reduced the stimulated high licking rate to NaCl and sweeteners in water-restricted mice with elevated plasma AngII levels. In taste cells AT1 proteins were coexpressed with αENaC (epithelial sodium channel α-subunit, an amiloride-sensitive salt taste receptor) or T1r3 (a sweet taste receptor component). These results suggest that the taste organ is a peripheral target of AngII. The specific reduction of amiloride-sensitive salt taste sensitivity by AngII may contribute to increased sodium intake. Furthermore, AngII may contribute to increased energy intake by enhancing sweet responses. The linkage between salty and sweet preferences via AngII signaling may optimize sodium and calorie intakes.

Taste receptor signalling - from tongues to lungs

Taste buds are the transducing endorgans of gustation. Each taste bud comprises 50-100 elongated cells, which extend from the basal lamina to the surface of the tongue, where their apical microvilli encounter taste stimuli in the oral cavity. Salts and acids utilize apically located ion channels for transduction, while bitter, sweet and umami (glutamate) stimuli utilize G-protein-coupled receptors (GPCRs) and second-messenger signalling mechanisms. This review will focus on GPCR signalling mechanisms. Two classes of taste GPCRs have been identified, the T1Rs for sweet and umami (glutamate) stimuli and the T2Rs for bitter stimuli. These low affinity GPCRs all couple to the same downstream signalling effectors that include Gβγ activation of phospholipase Cβ2, 1,4,5-inositol trisphosphate mediated release of Ca(2+) from intracellular stores and Ca(2+) -dependent activation of the monovalent selective cation channel, TrpM5. These events lead to membrane depolarization, action potentials and release of ATP as a transmitter to activate gustatory afferents. The Gα subunit, α-gustducin, activates a phosphodiesterase to decrease intracellular cAMP levels, although the precise targets of cAMP have not been identified. With the molecular identification of the taste GPCRs, it has become clear that taste signalling is not limited to taste buds, but occurs in many cell types of the airways. These include solitary chemosensory cells, ciliated epithelial cells and smooth muscle cells. Bitter receptors are most abundantly expressed in the airways, where they respond to irritating chemicals and promote protective airway reflexes, utilizing the same downstream signalling effectors as taste cells.

Involvement of NADPH-dependent and cAMP-PKA sensitive H+ channels in the chorda tympani nerve responses to strong acids

To investigate if chorda tympani (CT) taste nerve responses to strong (HCl) and weak (CO(2) and acetic acid) acidic stimuli are dependent upon NADPH oxidase-linked and cAMP-sensitive proton conductances in taste cell membranes, CT responses were monitored in rats, wild-type (WT) mice, and gp91(phox) knockout (KO) mice in the absence and presence of blockers (Zn(2+) and diethyl pyrocarbonate [DEPC]) or activators (8-(4-chlorophenylthio)-cAMP; 8-CPT-cAMP) of proton channels and activators of the NADPH oxidase enzyme (phorbol 12-myristate 13-acetate [PMA], H(2)O(2), and nitrazepam). Zn(2+) and DEPC inhibited and 8-CPT-cAMP, PMA, H(2)O(2), and nitrazepam enhanced the tonic CT responses to HCl without altering responses to CO(2) and acetic acid. In KO mice, the tonic HCl CT response was reduced by 64% relative to WT mice. The residual CT response was insensitive to H(2)O(2) but was blocked by Zn(2+). Its magnitude was further enhanced by 8-CPT-cAMP treatment, and the enhancement was blocked by 8-CPT-adenosine-3'-5'-cyclic monophospho-rothioate, a protein kinase A (PKA) inhibitor. Under voltage-clamp conditions, before cAMP treatment, rat tonic HCl CT responses demonstrated voltage-dependence only at ±90 mV, suggesting the presence of H(+) channels with voltage-dependent conductances. After cAMP treatment, the tonic HCl CT response had a quasi-linear dependence on voltage, suggesting that the cAMP-dependent part of the HCl CT response has a quasi-linear voltage dependence between +60 and -60 mV, only becoming sigmoidal when approaching +90 and -90 mV. The results suggest that CT responses to HCl involve 2 proton entry pathways, an NADPH oxidase-dependent proton channel, and a cAMP-PKA sensitive proton channel.

Taste, visceral information and exocrine reflexes with glutamate through umami receptors

Chemical substances of foods drive the cognitive recognition of taste with the subsequent regulation of digestion in the gastrointestinal (GI) tract. Tastants like glutamate can bind to taste membrane receptors on the tip of specialized taste cells eliciting umami taste. In chemical-sensing cells diffused through the GI tract, glutamate induces functional changes. Most of the taste-like receptor-expressing cells from the stomach and intestine are neuroendocrine cells. The signaling molecules produced by these neuroendocrine cells either activate afferent nerve endings or release peptide hormones that can regulate neighboring cells in a paracrine fashion or travel through blood to their target receptor. Once afferent sensory fibers transfer the chemical information of the GI content to the central nervous system (CNS) facilitating the gut-brain signaling, the CNS regulates the GI through efferent cholinergic and noradrenergic fibers. Thus, this is a two-way extrinsic communication process. Glutamate within the lumen of the stomach stimulates afferent fibers and increases acid and pepsinogen release; whereas on the duodenum, glutamate increases the production of mucous to protect the mucosa against the incoming gastric acid. The effects of glutamate are believed to be mediated by G protein-coupled receptors expressed at the lumen of GI cells. The specific cell-type and molecular function of each of these receptors are not completely known. Here we will examine some of the glutamate receptors and their already understood role on GI function regulation.

Functional expression of the extracellular-Ca2+-sensing receptor in mouse taste cells

Three types of morphologically and functionally distinct taste cells operate in the mammalian taste bud. We demonstrate here the expression of two G-protein-coupled receptors from the family C, CASR and GPRC6A, in the taste tissue and identify transcripts for both receptors in type I cells, no transcripts in type II cells and only CASR transcripts in type III cells, by using the SMART-PCR RNA amplification method at the level of individual taste cells. Type I taste cells responded to calcimimetic NPS R-568, a stereoselective CASR probe, with Ca(2+) transients, whereas type I and type II cells were not specifically responsive. Consistent with these findings, certain amino acids stimulated PLC-dependent Ca(2+) signaling in type III cells, but not in type I and type II cells, showing the following order of efficacies: Phe~Glu>Arg. Thus, CASR is coupled to Ca(2+) mobilization solely in type III cells. CASR was cloned from the circumvallate papilla into a pIRES2-EGFP plasmid and heterologously expressed in HEK-293 cells. The transfection with CASR enabled HEK-293 cells to generate Ca(2+) transients in response to the amino acids, of which, Phe was most potent. This observation and some other facts favor CASR as the predominant receptor subtype endowing type III cells with the ability to detect amino acids. Altogether, our results indicate that type III cells can serve a novel chemosensory function by expressing the polymodal receptor CASR. A role for CASR and GPRC6A in physiology of taste cells of the type I remains to be unveiled.

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.

Nonsynonymous single nucleotide polymorphisms in human tas1r1, tas1r3, and mGluR1 and individual taste sensitivity to glutamate

Several studies indicate an essential role of the heterodimer Tas1R1-Tas1R3 for monosodium l-glutamate (MSG) detection, although others suggest alternative receptors. Human subjects show different taste sensitivities to MSG, and some are unable to detect the presence of glutamate. Our objective was to study possible relations between phenotype (sensitivity to glutamate) and genotype (polymorphisms in candidate glutamate taste receptors tas1r1, tas1r3, mGluR4, and mGluR1) at the individual level. The sensitivity was measured with a battery of tests to distinguish the effect of sodium ions from the effect of glutamate ions in MSG. A total of 142 genetically unrelated white French subjects were categorized into 27 nontasters (specific ageusia), 21 hypotasters, and 94 tasters. Reverse transcriptase polymerase chain reaction and immunohistochemistry showed expression of tas1r1, tas1r3, and alpha-gustducin in fungiform papillae in all 12 subjects tested, including subjects who presented specific ageusia for glutamate. Amplification and sequencing of cDNA and genomic DNA allowed the identification of 10 nonsynonymous single nucleotide polymorphisms (nsSNPs) in tas1r1 (n = 3), tas1r3 (n = 3), and mGluR1 (n = 4). In our sample of subjects, the frequencies of 2 nsSNPs, C329T in tas1r1 and C2269T in tas1r3, were significantly higher in nontasters than expected, whereas G1114A in tas1r1 was more frequent in tasters. These nsSNPs along with minor variants and other nsSNPs in mGluR1, including T2977C, account for only part of the interindividual variance, which indicates that other factors, possibly including additional receptors, contribute to glutamate sensitivity.

Taste receptors for umami: the case for multiple receptors

Umami taste is elicited by many small molecules, including amino acids (glutamate and aspartate) and nucleotides (monophosphates of inosinate or guanylate, inosine 5'-monophosphate and guanosine-5'-monophosphate). Mammalian taste buds respond to these diverse compounds via membrane receptors that bind the umami tastants. Over the past 15 y, several receptors have been proposed to underlie umami detection in taste buds. These receptors include 2 glutamate-selective G protein-coupled receptors, mGluR4 and mGluR1, and the taste bud-expressed heterodimer T1R1+T1R3. Each of these receptors is expressed in small numbers of cells in anterior and posterior taste buds. The mGluRs are activated by glutamate and certain analogs but are not reported to be sensitive to nucleotides. In contrast, T1R1+T1R3 is activated by a broad range of amino acids and displays a strongly potentiated response in the presence of nucleotides. Mice in which the Grm4 gene is knocked out show a greatly enhanced preference for umami tastants. Loss of the Tas1r1 or Tas1R3 genes is reported to depress but not eliminate neural and behavioral responses to umami. When intact mammalian taste buds are apically stimulated with umami tastants, their functional responses to umami tastants do not fully resemble the responses of a single proposed umami receptor. Furthermore, the responses to umami tastants persist in the taste cells of T1R3-knockout mice. Thus, umami taste detection may involve multiple receptors expressed in different subsets of taste cells. This receptor diversity may underlie the complex perception of umami, with different mixtures of amino acids, peptides, and nucleotides yielding subtly distinct taste qualities.

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.

Interaction between the second messengers cAMP and Ca2+ in mouse presynaptic taste cells

The second messenger, 3',5'-cyclic adenosine monophosphate (cAMP), is known to be modulated in taste buds following exposure to gustatory and other stimuli. Which taste cell type(s) (Type I/glial-like cells, Type II/receptor cells, or Type III/presynaptic cells) undergo taste-evoked changes of cAMP and what the functional consequences of such changes are remain unknown. Using Fura-2 imaging of isolated mouse vallate taste cells, we explored how elevating cAMP alters Ca(2+) levels in identified taste cells. Stimulating taste buds with forskolin (Fsk; 1 microm) + isobutylmethylxanthine (IBMX; 100 microm), which elevates cellular cAMP, triggered Ca(2+) transients in 38% of presynaptic cells (n = 128). We used transgenic GAD-GFP mice to show that cAMP-triggered Ca(2+) responses occur only in the subset of presynaptic cells that lack glutamic acid decarboxylase 67 (GAD). We never observed cAMP-stimulated responses in receptor cells, glial-like cells or GAD-expressing presynaptic cells. The response to cAMP was blocked by the protein kinase A inhibitor H89 and by removing extracellular Ca(2+). Thus, the response to elevated cAMP is a PKA-dependent influx of Ca(2+). This Ca(2+) influx was blocked by nifedipine (an inhibitor of L-type voltage-gated Ca(2+) channels) but was unperturbed by omega-agatoxin IVA and omega-conotoxin GVIA (P/Q-type and N-type channel inhibitors, respectively). Single-cell RT-PCR on functionally identified presynaptic cells from GAD-GFP mice confirmed the pharmacological analyses: Ca(v)1.2 (an L-type subunit) is expressed in cells that display cAMP-triggered Ca(2+) influx, while Ca(v)2.1 (a P/Q subunit) is expressed in all presynaptic cells, and underlies depolarization-triggered Ca(2+) influx. Collectively, these data demonstrate cross-talk between cAMP and Ca(2+) signalling in a subclass of taste cells that form synapses with gustatory fibres and may integrate tastant-evoked signals.

Tonic activity of Galpha-gustducin regulates taste cell responsivity

The taste-selective G protein, alpha-gustducin (alpha-gus) is homologous to alpha-transducin and activates phosphodiesterase (PDE) in vitro. alpha-Gus-knockout mice are compromized to bitter, sweet and umami taste stimuli, suggesting a central role in taste transduction. Here, we suggest a different role for Galpha-gus. In taste buds of alpha-gus-knockout mice, basal (unstimulated) cAMP levels are high compared to those of wild-type mice. Further, H-89, a cAMP-dependent protein kinase inhibitor, dramatically unmasks responses to the bitter tastant denatonium in gus-lineage cells of knockout mice. We propose that an important role of alpha-gus is to maintain cAMP levels tonically low to ensure adequate Ca2+ signaling.

Characterization of umami receptor and coupling G protein in mouse taste cells

Taste receptor cells (TRCs) express multiple umami receptors. We performed physiological investigations to determine whether umami-responding cells in taste buds possess G protein-coupled receptors and to determine what type of G proteins exist if any. To clarify the components that participate in intracellular umami signal transduction in mouse, we recorded the activation of TRCs. TRCs treated with the G protein inhibitor GDP-beta-S lost umami-induced inward currents. Treatment with the Galphai inhibitor, pertussis toxin, did not increase the intracellular Ca2+ level in many TRCs. Immunohistochemical analysis revealed that a subset of TRCs responding to umami stimuli expressed alpha-gustducin. Thus, we demonstrated that umami stimuli were received by G protein-coupled receptors that function together with some of the Galphai family members.

Comparison of l-monosodium glutamate and l-amino acid taste in rats

T1R2/T1R3 heterodimers are selectively responsive to sweet substances whereas T1R1/T1R3 receptors are selective for umami substances, represented by monosodium glutamate (MSG), and for L-amino acids. If a single receptor is responsible for detection of umami and L-amino acids, then it would be predicted that MSG and L-amino acids elicit similar tastes in rats. The present study compared the taste profile of MSG with four amino acids (glycine, L-proline, L-serine and L-arginine) using conditioned taste aversion, detection threshold, and taste discrimination methods. These experiments were designed to either reduce or neutralize the taste of sodium associated with MSG and the other amino acids. Detection threshold studies showed that rats were most sensitive to L-arginine and least sensitive to L-proline. Glycine and L-serine thresholds were similar to those previously reported for MSG. Like MSG, a conditioned taste aversion to each of the four amino acids generalized to sucrose in the presence of amiloride, a sodium channel blocker. Rats showed moderate generalization of aversion between MSG and L-arginine, suggesting that these two amino acids taste only moderately alike. However, the taste aversion experiments indicated that glycine, L-serine, and L-proline elicit taste sensations similar to MSG when amiloride is present. Discrimination experiments further compared the tastes of these three amino acids with MSG. When the sodium taste associated with MSG was reduced or neutralized, glycine and L-proline elicited tastes very similar but not identical to the taste of MSG. Low (but not higher) concentrations of L-serine were also difficult for rats to discriminate from MSG. While there are taste qualities common to all of these amino acids, the perceptual differences found in this study, combined with previous reports, suggest either multiple taste receptors and/or multiple signaling pathways may be involved in umami and amino acid taste perception in rats.

The G-protein coupling properties of the human sweet and amino acid taste receptors

The human T1R taste receptors are family C G-protein-coupled receptors (GPCRs) that act as heterodimers to mediate sweet (hT1R2 + hT1R3) and umami (hT1R1 + hT1R3) taste modalities. Each T1R has a large extracellular ligand-binding domain linked to a seven transmembrane-spanning core domain (7TMD). We demonstrate that the 7TMDs of hT1R1 and hT1R2 display robust ligand-independent constitutive activity, efficiently catalyzing the exchange of GDP for GTP on Galpha subunits. In contrast, relative to the 7TMDs of hT1R1 and hT1R2, the 7TMD of hT1R3 couples poorly to G-proteins, suggesting that in vivo signaling may proceed primarily through hT1R1 and hT1R2. In addition, we provide direct evidence that the hT1Rs selectively signal through Galpha(i/o) pathways, coupling to multiple Galpha(i/o) subunits as well as the taste cell specific Gbeta(1)gamma(13) dimer.

Expression of group II metabotropic glutamate receptors in rat gustatory papillae

Glutamate is one candidate for the neurotransmitters and/or neuromodulators involved in taste signaling in taste buds. Group II metabotropic glutamate receptors (mGluRs: mGluR2 and mGluR3) are known to function as presynaptic receptors that regulate the release of glutamate and/or other neurotransmitters in the central nervous system. Group II mGluRs are negatively linked to adenylyl cyclase through Galphai subunits and thereby reduce the turnover of cAMP. In rat taste tissues, a subset of adenylyl-cyclase-8-expressing taste cells coexpress the Galphai subunits gustducin and Galphai2. However, the expression patterns of group II mGluRs in rat taste tissues have not yet been elucidated. We have therefore examined the expression patterns of mGluR2, mGluR3, and gustducin in rat gustatory tissues. Reverse transcription/polymerase chain reaction assays have revealed that mGluR2 and mGluR3 mRNAs are expressed in the circumvallate papillae. In situ hybridization analyses have detected positive signals for mGluR2 and mGluR3 mRNAs only in the circumvallate taste buds. Among the fungiform, foliate, and circumvallate papillae, an antibody against mGluR2/3 labels a subset of taste bud cells and nerve fibers immediately beneath the taste lingual epithelium. Double-labeling experiments have demonstrated that mGluR2/3-positive cells coexpress gustducin. These results indicate that mGluR2 and mGluR3 are coupled to Galphai subunits and play roles in glutamate-mediated signaling in taste transductions.

Taste-signaling proteins are coexpressed in solitary intestinal epithelial cells

The taste system, made up of taste receptor cells clustered in taste buds at the surface of the tongue and the soft palate, plays a key role in the decision to ingest or reject food and thereby is essential in protecting organisms against harmful toxins and in selecting the most appropriate nutrients. To determine if a similar chemosensory system exists in the gastrointestinal tract, we used immunohistochemistry and real-time polymerase chain reaction (PCR) to investigate which taste-signaling molecules are expressed in the intestinal mucosa. The PCR data showed that T1r1, T1r2, T1r3, alpha-gustducin, phospholipase Cbeta2 (PLCbeta2), and Trpm5 are expressed in the stomach, small intestine, and colon of mice and humans, with the exception of T1r2, which was not detected in the mouse and human stomach or in the mouse colon. Using transgenic mice expressing enhanced green fluorescent protein under the control of the Trpm5 promoter, we found colocalization of Trpm5 and alpha-gustducin in tufted cells at the surface epithelium of the colon, but these cells did not express T1r3 or PLCbeta2. In the duodenal glands, 43%, 33%, and 38% of Trpm5-expressing cells also express PLCbeta2, T1r3, or alpha-gustducin, respectively. The duodenal gland cells that coexpress PLCbeta2 and Trpm5 morphologically resemble enteroendocrine cells. We found a large degree of colocalization of Trpm5, alpha-gustducin, T1r1, and T1r3 in tufted cells of the duodenal villi, but these cells rarely expressed PLCbeta2. The data suggest that these duodenal cells are possibly involved in sensing amino acids.

Tastants evoke cAMP signal in taste buds that is independent of calcium signaling

We previously showed that rat taste buds express several adenylyl cyclases (ACs) of which only AC8 is known to be stimulated by Ca2+. Here we demonstrate by direct measurements of cAMP levels that AC activity in taste buds is stimulated by treatments that elevate intracellular Ca2+. Specifically, 5 microM thapsigargin or 3 microM A-23187 (calcium ionophore), both of which increase intracellular Ca2+ concentration ([Ca2+]i), lead to a significant elevation of cAMP levels. This calcium stimulation of AC activity requires extracellular Ca2+, suggesting that it is dependent on Ca2+ entry rather than release from stores. With immunofluorescence microscopy, we show that the calcium-stimulated AC8 is principally expressed in taste cells that also express phospholipase Cbeta2 (i.e., cells that elevate [Ca2+]i in response to sweet, bitter, or umami stimuli). Taste transduction for sucrose is known to result in an elevation of both cAMP and calcium in taste buds. Thus we tested whether the cAMP increase in response to sucrose is a downstream consequence of calcium elevation. Even under conditions of depletion of stored and extracellular calcium, the cAMP response to sucrose stimulation persists in taste cells. The cAMP signal in response to monosodium glutamate stimulation is similarly unperturbed by calcium depletion. Our results suggest that tastant-evoked cAMP signals are not simply a secondary consequence of calcium modulation. Instead, cAMP and released Ca2+ may represent independent second messenger signals downstream of taste receptors.

Umami responses in mouse taste cells indicate more than one receptor

A number of gustatory receptors have been proposed to underlie umami, the taste of L-glutamate, and certain other amino acids and nucleotides. However, the response profiles of these cloned receptors have not been validated against responses recorded from taste receptor cells that are the native detectors of umami taste. We investigated umami taste responses in mouse circumvallate taste buds in an intact slice preparation, using confocal calcium imaging. Approximately 5% of taste cells selectively responded to L-glutamate when it was focally applied to the apical chemosensitive tips of receptor cells. The concentration-response range for L-glutamate fell approximately within the physiologically relevant range for taste behavior in mice, namely 10 mm and above. Inosine monophosphate enhanced taste cell responses to L-glutamate, a characteristic feature of umami taste. Using pharmacological agents, ion substitution, and immunostaining, we showed that intracellular pathways downstream of receptor activation involve phospholipase C beta2. Each of the above features matches those predicted by studies of cloned and expressed receptors. However, the ligand specificity of each of the proposed umami receptors [taste metabotropic glutamate receptor 4, truncated metabotropic glutamate receptor 1, or taste receptor 1 (T1R1) and T1R3 dimers], taken alone, did not appear to explain the taste responses observed in mouse taste cells. Furthermore, umami responses were still observed in mutant mice lacking T1R3. A full explanation of umami taste transduction may involve novel combinations of the proposed receptors and/or as-yet-undiscovered taste receptors.

Functional expression of metabotropic GABAB receptors in primary cultures of astrocytes from rat cerebral cortex

GABA(B) receptor subunits are widely expressed on neurons throughout the central nervous system (CNS), at both pre- and postsynaptic sites, where they mediate the late and slow component of the inhibitory response to the major inhibitory neurotransmitter GABA. Recently, GABA(B) receptors have been reported to be expressed in astrocytes and microglia in the rat CNS by immunocytochemistry. However, there are few reports available for the functional characterization of GABA(B) receptors on astrocytes. In the present study, we therefore investigated the functional expression and characteristics of GABA(B) receptors in primary cultures of astrocytes from rat cerebral cortex. In the presence of 10 microM GTP, forskolin concentration-dependently increased adenylylcyclase (AC) activity in membranes prepared from rat astrocytes. The selective GABA(B) agonist (R)-baclofen concentration-dependently reduced forskolin-stimulated AC activity in the presence of 10 microM GTP. This effect was reversed by the selective GABA(B) antagonists, CGP-55845 and CGP-54626, and was completely abolished by treatment of astrocytic membranes with pertussis toxin. In addition, RT-PCR, Western blotting, and immunocytochemistry clearly showed that metabotropic GABA(B) receptor isoforms (GABA(B)R1 and GABA(B)R2) are expressed in rat cerebrocortical astrocytes. Taken collectively, these results demonstrate that functionally active metabotropic GABA(B) receptors are expressed in rat cerebrocortical astrocytes.

Effect of metabotropic glutamate receptor agonists and signal transduction modulators on feeding by a caterpillar

Feeding in codling moth caterpillars was induced by the general glutamate receptor activator monosodium glutamate (MSG) and by three different mGluR agonists known to specifically stimulate different classes of vertebrate metabotropic glutamate receptors, including: (1S,3R)-ACPD, which stimulates group I mGluRs (2R,4R)-APDC, which stimulates group II mGluRs and L-AP4, which stimulates some group III mGluRs. Experiments exposing larvae to combinations of specific mGluR agonists and specific signal transduction modulators suggest that each tested mGluR uses a different signaling pathway. First, feeding stimulatory effects of (1S,3R)-ACPD were abolished by phospholipase C inhibitor, U 73122, but remained unaffected by adenylate cyclase activator, NKH 477, or phosphodiesterase inhibitor, Rolipram. Second, (2R,4R)-APDC induced feeding in presence of U 73122 or Rolipram, but lost its feeding stimulatory effects in presence of NKH 477. Finally, L-AP4 did not induce feeding in presence of Rolipram, but maintained its feeding stimulatory effects in presence of U 73122 or NKH 477. The activity of the general glutamate receptor activator MSG was abolished by NKH 477, and Rolipram. U 73122 did not affect MSG-stimulated feeding. These results suggest that transduction of MSG taste in the codling moth caterpillar relies mostly on cAMP-dependent signaling pathways.

Peripheral gustatory processing of sweet stimuli by golden hamsters

Behaviors and taste-nerve responses to bitter stimuli are linked to compounds that bind T2 receptors expressed in one subset of taste-bud receptor cells (TRCs); and behavioral and neural responses to sweet stimuli are linked to chemical compounds that bind a T1 receptor expressed in a different TRC subset. Neural and behavioral responses to bitter-sweet mixtures, however, complicate the ostensible bitter and sweet labeled lines. In the golden hamster, Mesocricetus auratus, quinine hydrochloride, the bitter prototype, suppresses chorda tympani (CT) nerve responses to the sweet prototype: sucrose. This bitter-sweet inhibition was tested with concentration series of sucrose and dulcin, a hydrophobic synthetic sweetener that hamsters behaviorally cross-generalize with sucrose. Dulcin, sucrose and other sweeteners activate one subset of CT fibers: S neurons; whereas, quinine activates a separate subset of CT fibers: E neurons. Whole-nerve and S-neuron CT responses to a sweetener concentration series, mixed with 0, 1, 3 and 10 mM quinine, were measured for 0-2.5 s transient and/or 2.6-10 s steady-state response periods. Ten-sec total single-fiber records, aligned at response onset, were averaged for 100 ms bins to identify response oscillations. Quinine inhibition of dulcin and sucrose responses was identical. Each log molar increment in quinine resulted in equivalent declines in response to either sweetener. Furthermore, sucrose response decrements paralleled response increments in quinine-sensitive CT neurons to the same quinine increases. A 1.43 Hz bursting rhythm to the sweeteners was unchanged by quinine inhibition or decreases in sweetener concentration. Taste-bud processing, possibly between-cell inhibition and within-cell negative feedback, must modify signals initiated by T1 receptors before they are transmitted to the brain.

Receptors for bitter, sweet and umami taste couple to inhibitory G protein signaling pathways

Taste receptors are thought to couple to the G protein Galpha-gustducin to initiate signal transduction cascades leading to taste perception. To further characterize the G protein-coupling selectivity of these receptors, we expressed them in HEK293 cells and monitored the modulation of different signaling pathways upon stimulation. We found that the bitter compound cycloheximide induces phosphorylation of extracellular signal-regulated kinases1 and 2 (ERK 1/2) and inhibits cAMP accumulation in HEK293 cells expressing the mouse bitter T2R(5) receptor. These effects are totally abolished upon treatment with pertussis toxin. On the other hand, sweeteners and monosodium glutamate induce phosphorylation of ERK1/2 and inhibit cAMP accumulation in HEK293 cells expressing the human sweet T1R(2)/T1R(3) receptor and the human umami T1R(1)/T1R(3) receptor, respectively. The effects of these taste modalities are also prevented by treatment with pertussis toxin. Collectively, our results show that taste receptors can functionally couple to Galpha(i/o) proteins to transmit intracellular signals.

Umami taste responses are mediated by alpha-transducin and alpha-gustducin

The sense of taste comprises at least five distinct qualities: sweet, bitter, sour, salty, and umami, the taste of glutamate. For bitter, sweet, and umami compounds, taste signaling is initiated by binding of tastants to G-protein-coupled receptors in specialized epithelial cells located in the taste buds, leading to the activation of signal transduction cascades. Alpha-gustducin, a taste cell-expressed G-protein alpha subunit closely related to the alpha-transducins, is a key mediator of sweet and bitter tastes. Alpha-gustducin knock-out (KO) mice have greatly diminished, but not entirely abolished, responses to many bitter and sweet compounds. We set out to determine whether alpha-gustducin also mediates umami taste and whether rod alpha-transducin (alpha(t-rod)), which is also expressed in taste receptor cells, plays a role in any of the taste responses that remain in alpha-gustducin KO mice. Behavioral tests and taste nerve recordings of single and double KO mice lacking alpha-gustducin and/or alpha(t-rod) confirmed the involvement of alpha-gustducin in bitter (quinine and denatonium) and sweet (sucrose and SC45647) taste and demonstrated the involvement of alpha-gustducin in umami [monosodium glutamate (MSG), monopotassium glutamate (MPG), and inosine monophosphate (IMP)] taste as well. We found that alpha(t-rod) played no role in taste responses to the salty, bitter, and sweet compounds tested or to IMP but was involved in the umami taste of MSG and MPG. Umami detection involving alpha-gustducin and alpha(t-rod) occurs in anteriorly placed taste buds, however taste cells at the back of the tongue respond to umami compounds independently of these two G-protein subunits.

The receptors for mammalian sweet and umami taste

Sweet and umami (the taste of monosodium glutamate) are the main attractive taste modalities in humans. T1Rs are candidate mammalian taste receptors that combine to assemble two heteromeric G-protein-coupled receptor complexes: T1R1+3, an umami sensor, and T1R2+3, a sweet receptor. We now report the behavioral and physiological characterization of T1R1, T1R2, and T1R3 knockout mice. We demonstrate that sweet and umami taste are strictly dependent on T1R-receptors, and show that selective elimination of T1R-subunits differentially abolishes detection and perception of these two taste modalities. To examine the basis of sweet tastant recognition and coding, we engineered animals expressing either the human T1R2-receptor (hT1R2), or a modified opioid-receptor (RASSL) in sweet cells. Expression of hT1R2 in mice generates animals with humanized sweet taste preferences, while expression of RASSL drives strong attraction to a synthetic opiate, demonstrating that sweet cells trigger dedicated behavioral outputs, but their tastant selectivity is determined by the nature of the receptors.