Autonomic nervous system responses associated with primary tastes

The hedonic dimension of the taste sensation plays a crucial role in the control of many taste-mediated responses related to food ingestion or rejection. The purpose of this study was to evaluate the emotional reactivity associated with each primary taste (sweet, salty, sour and bitter) through analysis of the variations of autonomic nervous system (ANS) parameters. Thirty-four healthy non-smoker volunteer subjects (17 males and 17 females, mean age = 28 years) participated in the experiment. Taste stimuli were solutions of 0.3 M sucrose (sweet), 0.15 M NaCl (salty), 0.02 M citric acid (sour) and 0.00015 M quinine sulfate (bitter). Evian mineral water was used as the diluent and control (neutral taste). Throughout the test, five ANS parameters (skin potential and skin resistance, skin blood flow and skin temperature, and instantaneous heart rate) were simultaneously and continuously recorded. Results of the ANOVA evidenced a significant effect of primary taste on skin resistance amplitude (P: < 0.001) and duration (P: < 0.0001), skin temperature amplitude (P: < 0.001), skin blood flow amplitude (vasoconstriction) (P: < 0.0001) and instantaneous heart rate increase (P: < 0.0001). Skin resistance and cardiac responses were the most relevant ANS parameters to distinguish among the taste solutions. The four primary tastes could be associated with significantly different ANS responses in relation to their hedonic valence: the pleasantly connoted and innate-accepted sweet taste induced the weakest ANS responses whereas the unpleasant connoted tastes (salty, sour and bitter) induced stronger ANS responses, the innate-rejected bitter taste inducing the strongest ones. Such a neurovegetative characterization of each primary taste could provide references for the hedonic analysis of the more complex gustative sensation attached to foods.     

Molecular mechanisms underlying the reception and transmission of sour taste information

Taste enables organisms to determine the properties of ingested substances by conveying information regarding the five basic taste modalities: sweet, salty, sour, bitter, and umami. The sweet, salty, and umami taste modalities convey the carbohydrate, electrolyte, and glutamate content of food, indicating its desirability and stimulating appetitive responses. The sour and bitter modalities convey the acidity of food and the presence of potential toxins, respectively, stimulating aversive responses to such tastes. In recent years, the receptors mediating sweet, bitter, and umami tastes have been identified as members of the T1R and T2R G-protein-coupled receptor families; however, the molecular mechanisms underlying sour taste detection have yet to be clearly elucidated. This review covers the molecular mechanisms proposed to mediate the detection and transmission of sour stimuli, focusing on polycystic kidney disease 1-like 3 (Pkd1l3), Pkd2l1, and carbonic anhydrase 4 (Car4).     

Umami evaluation in taste epithelium on microelectrode array by extracellular electrophysiological recording

Umami is one of the basic tastes along with sweet, bitter, sour and salty. It is often elicited by amino acids and can provide a palatable flavor for food. With taste epithelium as the sensing element, microelectrodes can be used to evaluate umami taste by biological responses of the tissue. The electrophysiological activities to umami stimuli are measured with a 60-channel microelectrode array (MEA). Local field potential (LFP) recorded by a MEA system showed different temporal characteristics respectively with l-glutamic acid (l-Glu), l-aspartic acid (l-Asp), l-monosodium glutamate (l-MSG) and l-monosodium aspartate (l-MSA), while remarkable differences were observed between amino acids and their sodium salts. Wealso found that a dose-dependent behavior in the increasing concentrations of umami stimulations and a synergistic enhancement between amino acids and purine nucleotides can be detected. The investigation of this evaluation for umami represents a promising approach for distinguishing and evaluating umami tastants.     

TASTE INTENSITIES OF OIL-IN-WATER EMULSIONS WITH VARYING FAT CONTENT

The objective of this study was to determine the effect fat has on the intensity of sweet, salty, sour, bitter and umami tastes in oil-in-water emulsions. The first experiment used two levels of fat (9% and 17% in oil-in-water emulsions) and two intensities of each taste (high and low). We compared the taste intensities of these emulsions to the intensities of oil-free samples with equal total volume, and to oil free samples of the same aqueous taste compound concentrations. Because of potential confusion between taste intensity and viscosity, we repeated the experiment, having panelists rate both thickness and taste intensity. Diluting with oil, compared to diluting with water, decreased bitterness, but increased the intensity of salty, sweet, sour and umami tastes. When compared to samples with equal aqueous taste compound concentrations, fat suppressed bitterness, but had no effect on the other tastes.     

The good taste of peptides

The taste of peptides is seldom one of the most relevant issues when one considers the many important biological functions of this class of molecules. However, peptides generally do have a taste, covering essentially the entire range of established taste modalities: sweet, bitter, umami, sour and salty. The last two modalities cannot be attributed to peptides as such because they are due to the presence of charged terminals and/or charged side chains, thus reflecting only the zwitterionic nature of these compounds and/or the nature of some side chains but not the electronic and/or conformational features of a specific peptide. The other three tastes, that is, sweet, umami and bitter, are represented by different families of peptides. This review describes the main peptides with a sweet, umami or bitter taste and their relationship with food acceptance or rejection. Particular emphasis will be given to the sweet taste modality, owing to the practical and scientific relevance of aspartame, the well‐known sweetener, and to the theoretical importance of sweet proteins, the most potent peptide sweet molecules. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

灵长类苦味受体基因研究进展

在鲜味、甜味、苦味、咸味和酸味5种味觉形式中,苦味能避免动物摄入有毒有害物质,在动物的生存中发挥着特别重要的作用。苦味味觉的产生依赖于苦味物质与苦味受体的相互作用。苦味受体由苦味受体基因Tas2rs编码,此类基因在不同物种中数量变化较大以适应不同的需求。目前的研究在灵长类中鉴别出了若干苦味受体的配体,并发现有的苦味受体基因所经受的选择压在类群之间、基因之间甚至同一基因不同功能区之间都存在着变化。本文从苦味受体作用的多样性特点,受体与配体的对应关系、受体基因进化模式与食性之间的关系、苦味受体基因的适应性进化方面对灵长类苦味受体基因进行了综述,以期为苦味受体基因在灵长类中的深入研究提供参考。     

Bitter-sweet solution in taste transduction

A contentious issue in taste research might have come to a close. Zhang et al., in this issue of Cell, provide broad support for the notion that the recognition of sweet, umami, and bitter tastes use the same signaling molecules. Moreover, they show that individual taste cells are dedicated to the transduction of only one of these three taste qualities.     

Abnormal taste perception in mice lacking the type 3 inositol 1,4,5-trisphosphate receptor

Inositol 1,4,5-trisphosphate receptor (IP3R) is one of the important calcium channels expressed in the endoplasmic reticulum and has been shown to play crucial roles in various physiological phenomena. Type 3 IP3R is expressed in taste cells, but the physiological relevance of this receptor in taste perception in vivo is still unknown. Here, we show that mice lacking IP3R3 show abnormal behavioral and electrophysiological responses to sweet, umami, and bitter substances that trigger G-protein-coupled receptor activation. In contrast, responses to salty and acid tastes are largely normal in the mutant mice. We conclude that IP3R3 is a principal mediator of sweet, bitter, and umami taste perception and would be a missing molecule linking phospholipase C beta2 to TRPM5 activation.     

Expression of gustducin overlaps with that of type III IP3 receptor in taste buds of the rat soft palate

Type III IP3 receptor (IP3R3) is one of the common critical calcium-signaling molecules for sweet, umami, and bitter signal transduction in taste cells, and the total IP3R3-expressing cell population represents all cells mediating these taste modalities in the taste buds. Although gustducin, a taste cell-specific G-protein, is also involved in sweet, umami, and bitter signal transduction, the expression of gustducin is restricted to different subsets of IP3R3-expressing cells by location in the tongue. Based on the expression patterns of gustducin and taste receptors in the tongue, the function of gustducin has been implicated primarily in bitter taste in the circumvallate (CV) papillae and in sweet taste in the fungiform (FF) papillae. However, in the soft palate (SP), the expression pattern of gustducin remains unclear and little is known about its function. In the present paper, the expression patterns of gustducin and IP3R3 in taste buds of the SP and tongue papillae in the rat were examined by double-color whole-mount immunohistochemistry. Gustducin was expressed in almost all (96.7%) IP3R3-expressing cells in taste buds of the SP, whereas gustducin-positive cells were 42.4% and 60.1% of IP3R3-expressing cells in FF and CV, respectively. Our data suggest that gustducin is involved in signal transduction of all the tastes of sweet, umami, and bitter in the SP, in contrast to its limited function in the tongue.     

Gustatory sensation of l- and d-amino acids in humans

Amino acids are known to elicit complex taste, but most human psychophysical studies on the taste of amino acids have focused on a single basic taste, such as umami (savory) taste, sweetness, or bitterness. In this study, we addressed the potential relationship between the structure and the taste properties of amino acids by measuring the human gustatory intensity and quality in response to aqueous solutions of proteogenic amino acids in comparison to d-enantiomers. Trained subjects tasted aqueous solution of each amino acid and evaluated the intensities of total taste and each basic taste using a category-ratio scale. Each basic taste of amino acids showed the dependency on its hydrophobicity, size, charge, functional groups on the side chain, and chirality of the alpha carbon. In addition, the overall taste of amino acid was found to be the combination of basic tastes according to the partial structure. For example, hydrophilic non-charged middle-sized amino acids elicited sweetness, and l-enantiomeric hydrophilic middle-sized structure was necessary for umami taste. For example, l-serine had mainly sweet and minor umami taste, and d-serine was sweet. We further applied Stevens’ psychophysical function to relate the total-taste intensity and the concentration, and found that the slope values depended on the major quality of taste (e.g., bitter large, sour small).     

The liaison of sweet and savory

The sense of taste provides humans with necessary information about the composition and quality of food. For humans, five basic tastes are readily distinguishable and include sweet, bitter, salty, sour, and savory (or umami). Although each of these qualities has individualized transduction pathways, sweet and umami tastes are believed to share a common receptor element, the T1R3 receptor subunit. The two G-protein-coupled heteromer receptors that comprise an umami stimulus receptor (T1R1-T1R3) and a sweetener receptor (T1R2-T1R3) constitute a potential link between these two qualities of perception. While the role of the individual monomers in each human heteromer has been examined in vitro, very little is known of the implication of this research for human perception, or specifically, how sweet and savory taste perceptions may be connected. Using a psychophysical approach, we demonstrate that lactisole, a potent sweetness inhibitor that binds in vitro to hT1R3, also inhibits a significant portion of the perception of umami taste from monosodium glutamate. Following the molecular logic put forward by Xu et al. (2004, Proc. Natl Acad. Sci. USA, 101, 14258-14263), our psychophysical data support the in vitro hypothesis that the shared T1R3 monomer moderates the activation of both T1R2 and T1R1 in humans and impairs suprathreshold perception, respectively, of sweetness and, to a lesser degree, umaminess in the presence of lactisole.     

Coding of sweet,bitter, and umami tastes: different receptor cells sharing similar signaling pathways

Mammals can taste a wide repertoire of chemosensory stimuli. Two unrelated families of receptors (T1Rs and T2Rs) mediate responses to sweet, amino acids, and bitter compounds. Here, we demonstrate that knockouts of TRPM5, a taste TRP ion channel, or PLCbeta2, a phospholipase C selectively expressed in taste tissue, abolish sweet, amino acid, and bitter taste reception, but do not impact sour or salty tastes. Therefore, despite relying on different receptors, sweet, amino acid, and bitter transduction converge on common signaling molecules. Using PLCbeta2 taste-blind animals, we then examined a fundamental question in taste perception: how taste modalities are encoded at the cellular level. Mice engineered to rescue PLCbeta2 function exclusively in bitter-receptor expressing cells respond normally to bitter tastants but do not taste sweet or amino acid stimuli. Thus, bitter is encoded independently of sweet and amino acids, and taste receptor cells are not broadly tuned across these modalities.     

(+)-(S)-alapyridaine--a general taste enhancer?

N-(1-Carboxyethyl)-6-hydroxymethyl-pyridinium-3-ol inner salt (alapyridaine), recently identified in heated sugar/amino acid mixtures as well as in beef bouillon, has been shown to exhibit general taste-enhancing activities, although tasteless on its own. Differing from other taste enhancers reported so far, racemic (R/S)-alapyridaine and, to an even greater extent (+)-(S)-alapyridaine, the physiologically active enantiomer, are able to enhance more than one basic taste quality. The threshold concentrations for the sweet taste of glucose and sucrose, for the umami taste of monosodium L-glutamate (MSG) and guanosine-5'-monophosphate (GMP), as well as the salty taste of NaCl, were significantly decreased when alapyridaine was present. In contrast, perception of the bitter tastes of caffeine and L-phenylalanine, as well as of sour-tasting citric acid, was unaffected. Furthermore, alapyridaine was shown to intensify known taste synergies such as, for example, the enhancing effect of L-arginine on the salty taste of NaCl, as well as that of GMP on the umami taste of MSG. The activity of (+)-(S)-alapyridaine could be observed not only in solutions of single taste compounds, but also in more complex tastant mixtures; for example, the umami, sweet and salty taste of a solution containing MSG, sucrose, NaCl and caffeine was significantly modulated, thus indicating that alapyridaine is a general taste enhancer.     

Identification and characterization of human taste receptor genes belonging to the TAS2R family

The sense of taste is a chemosensory system responsible for basic food appraisal. Humans distinguish between five primary tastes: bitter, sweet, sour, salty and umami. The molecular events in the perception of bitter taste are believed to start with the binding of specific water-soluble molecules to G-protein-coupled receptors encoded by the TAS2R/T2R family of taste receptor genes. TAS2R receptors are expressed at the surface of taste receptor cells and are coupled to G proteins and second messenger pathways. We have identified, cloned and characterized 11 new bitter taste receptor genes and four new pseudogenes that belong to the human TAS2R family. Their encoded proteins have between 298 and 333 amino acids and share between 23 and 86% identity with other human TAS2R proteins. Screening of a mono-chromosomal somatic cell hybrid panel to assign the identified bitter taste receptor genes to human chromosomes demonstrated that they are located in chromosomes 7 and 12. Including the 15 sequences identified, the human TAS2R family is composed of 28 full-length genes and 16 pseudogenes. Phylogenetic analyses suggest a classification of the TAS2R genes in five groups that may reflect a specialization in the detection of specific types of bitter chemicals.     

Trpm5 null mice respond to bitter, sweet, and umami compounds

Trpm5 is a calcium-activated cation channel expressed selectively in taste receptor cells. A previous study reported that mice with an internal deletion of Trpm5, lacking exons 15-19 encoding transmembrane segments 1-5, showed no taste-mediated responses to bitter, sweet, and umami compounds. We independently generated knockout mice null for Trpm5 protein expression due to deletion of Trpm5's promoter region and exons 1-4 (including the translation start site). We examined the taste-mediated responses of Trpm5 null mice and wild-type (WT) mice using three procedures: gustatory nerve recording [chorda tympani (CT) and glossopharyngeal (NG) nerves], initial lick responses, and 24-h two-bottle preference tests. With bitter compounds, the Trpm5 null mice showed reduced, but not abolished, avoidance (as indicated by licking responses and preference ratios higher than those of WT), a normal CT response, and a greatly diminished NG response. With sweet compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, and absent or greatly reduced nerve responses. With umami compounds, Trpm5 null mice showed no licking response, a diminished preference ratio, a normal NG response, and a greatly diminished CT response. Our results demonstrate that the consequences of eliminating Trmp5 expression vary depending upon the taste quality and the lingual taste field examined. Thus, while Trpm5 is an important factor in many taste responses, its absence does not eliminate all taste responses. We conclude that Trpm5-dependent and Trpm5-independent pathways underlie bitter, sweet, and umami tastes.     

Functional diversification of taste cells in vertebrates

Tastes are senses resulting from the activation of taste cells distributed in oral epithelia. Sweet, umami, bitter, sour, and salty tastes are called the five “basic” tastes, but why five, and why these five? In this review, we dissect the peripheral gustatory system in vertebrates from molecular and cellular perspectives. Recent behavioral and molecular genetic studies have revealed the nature of functional taste receptors and cells and show that different taste qualities are accounted for by the activation of different subsets of taste cells. Based on this concept, the diversity of basic tastes should be defined by the diversity of taste cells in taste buds, which varies among species.     

Responses of Primate Taste Cortex Neurons to the Astringent Tastant Tannic Acid

In order to advance knowledge of the neural control of feeding,we investigated the cortical representation of the taste oftannic acid, which produces the taste of astringency. It isa dietary component of biological importance particularly toarboreal primates. Recordings were made from 74 taste responsiveneurons in the orbitofrontal cortex. Single neurons were foundthat were tuned to respond to 0.001 M tannic acid, and representeda subpopulation of neurons that was distinct from neurons responsiveto the tastes of glucose (sweet), NaCl (salty), HCI (sour),quinine (bitter) and monosodium glutamate (umami). In addition,across the population of 74 neurons, tannic acid was as wellrepresented as the tastes of NaCI, HCI quinine or monosodiumglutamate. Multidimensional scaling analysis of the neuronalresponses to the tastants indicates that tannic acid lies outsidethe boundaries of the four conventional taste qualities (sweet,sour, bitter and salty). Taken together these data indicatethat the astringent taste of tannic acid should be consideredas a distinct taste quality, which receives a separate representationfrom sweet, salt, bitter and sour in the primate cortical tasteareas. Chem. Senses 21: 135–145, 1996.     

A serotonin-sensitive sensor for investigation of taste cell-to-cell communication

Taste receptor cells are the taste sensation elements for sour, salty, sweet, bitter and umami sensations. It was demonstrated that there are cell-to-cell communications between type II (sour) and type III (sweet, bitter and umami) taste cells. Serotonin (5-HT) is released from type III cells, which is the only type of taste cells that has synaptic process with sensory afferent fibers. Then, taste information is transmitted via fibers to the brain. During this process, 5-HT plays important roles in taste information transmission. In order to explore a sensor to detect 5-HT released from taste cell or taste cell networks, we develop a 5-HT sensitive sensor based on LAPS chip. This sensor performs with a detection limit of 3.3 × 10(-13)M and a sensitivity of 19.1 mV per concentration decade. Upon the stimuli of sour and mix (bitter, sweet and umami) tastants, 5-HT released from taste cells could be detected flexibly, benefit from the addressability of LAPS chip. The experimental results show that the local concentration of 5-HT is around several nM, which is consistent with those from other methods. In addition, immunofluorescent imaging technique is utilized to confirm the functional existence of both type II and III cells in a cluster of isolated taste cells. Different types of taste cells are labeled with corresponding specific antibody. This 5-HT sensitive LAPS chip provides a potential and promising way to detect 5-HT and to investigate the taste coding and information communication mechanisms.     

Taste development from health education among schoolchildren: a two-year intervention study

It is necessary to develop a system of nutritional education which can be understood among schoolchildren who have not yet received a basic education. In the present study, we conducted an educational program for lower-grade schoolchildren, which contained dish selection, an agricultural experience, a cooking experience, and a lecture on digestive absorption. We evaluated the effect of this program on development by measuring taste sensitivity regarding sweet, sour, salty and bitter tastes. For the baseline period, there was no significant difference between the intervention school and the control school in each variable. At follow-up periods, both the intervention and the control schools showed an increasing sense of taste. In the intervention school, development of sensitivity to the sweet, the sour, and the bitter taste was significant. In the control school, development of sensitivity to the sweet and the bitter taste was significant. The increases in the sense of the sour and the bitter tastes and the sum of the four tastes for the intervention subjects were significantly larger than comparable values for the control subjects. These results suggest that the development of taste sensitivity is affected by nutritional education for lower-grade elementary schoolchildren.     

The taste, odor and hedonic quality of polyglycerols

Twenty subjects judged the taste and odor intensity and thetaste and odor pleasantness/unpleasantness of five concentrationsof sucrose, glycerol, a commercial triglycerol, a syntheticlinear diglycerol and a synthetic linear triglycerol. Judgmentsof intensity were made using the method of magnitude estimation;judgments of pleasantness/unpleasantness were made using a graphicline scale. Only the two linear polyglycerols had appreciableodor intensity. Both were described as having an ‘acrid’or ‘burnt caramel’ quality. The odor exponent forthe linear triglycerol was extremely high (1.44) and may beattributed to its intensely unpleasant quality. Sucrose wascharacterized solely by sweet taste, glycerol and the commercialtriglycerol by sweet and bitter tastes, the linear diglycerolby sweet, bitter and sour tastes, and the linear triglycerolby bitter and sour tastes. The relationships between perceivedtaste intensity and concentration were well described by powerfunctions, although the slope of the psychophysical functionfor the linear triglycerol was markedly lower than that forthe other compounds. The relative order of taste intensitieswas: linear triglycerol > sucrose > glycerol = lineardiglycerol > commercial triglycerol. Judgments of taste (andodor) pleasantness/unpleasantness showed only sucrose and glycerolto have positive hedonic qualities. All the polyglycerols werejudged unpleasant at all concentrations. Differences in thetaste and odor characteristics of the commercial and synthetictriglycerols were attributed to the commercial product beinga mixture of over 20 compounds. Although the synthetic lineardi- and triglycerols are effective in lowering water activity,these data suggest that more purified crystalline forms mustbe synthesized before they can be used effectively as humectantsfor intermediate moisture foods.