Gustatory signaling in the periphery: detection, transmission, and modulation of taste information

A review of taste-active compounds in meat: Identification, influencing factors, and taste transduction mechanism

Poultry and livestock meat are important parts of the human diet. As living standards have improved, food taste has become a major influence on consumer quality assessment and meat purchasing choices. There is increasing research interest in meat taste and meat taste-active compounds, which include free amino acids, flavor nucleotides, taste-active peptides, organic acids, soluble sugars, and inorganic ions. Taste component research is also an important part of sensory science. A deeper understanding of the meat taste perception mechanism and interactions among different taste compounds will promote the development of meat science and sensory evaluation. This article reviews the main taste compounds in meat, factors influencing their concentrations, and the identification and measurement of taste-active compounds, as well as summarizing the mechanisms of taste sensing and perception. Finally, the future of scientific taste component evaluation is discussed. This review provides a theoretical basis for research on meat taste and an important reference for the development of the meat industry.

A comprehensive review of plant-derived salt substitutes: Classification, mechanism, and application

The diseases caused by excessive sodium intake derived from NaCl consumption have attracted widespread attention worldwide, and many researchers are committed to finding suitable ways to reduce sodium intake during the dietary process. Salt substitute is considered an effective way to reduce sodium intake by replacing all/part of NaCl in food without reducing the saltiness while minimizing the impact on the taste and acceptability of the food. Plant-derived natural ingredients are generally considered safe and reliable, and extensive research has shown that certain plant extracts or specific components are effective salt substitutes, which can also give food additional health benefits. However, these plant-derived salt substitutes (PSS) have not been systematically recognized by the public and have not been well adopted in the food industry. Therefore, a comprehensive review of PSS, including its material basis, flavor characteristics, and taste mechanism is helpful for a deeper understanding of PSS, accelerating its research and development, and promoting its application.

Advancements in production, assessment, and food applications of salty and saltiness-enhancing peptides: A review

Salt is important for food flavor, but excessive sodium intake leads to adverse health consequences. Thus, salty and saltiness-enhancing peptides are developed for sodium-reduction products. This review elucidates saltiness perception process and analyses correlation between the peptide structure and saltiness-enhancing ability. These peptides interact with taste receptors to produce saltiness perception, including ENaC, TRPV1, and TMC4. This review also outlines preparation, isolation, purification, characterization, screening, and assessment techniques of these peptides and discusses their potential applications. These peptides are from various sources and produced through enzymatic hydrolysis, microbial fermentation, or Millard reaction and then separated, purified, identified, and screened. Sensory evaluation, electronic tongue, bioelectronic tongue, and cell and animal models are the primary saltiness assessment approaches. These peptides can be used in sodium-reduction food products to produce "clean label" items, and the peptides with biological activity can also serve as functional ingredients, making them very promising for food industry.

Taste-Active Peptides from Triple-Enzymatically Hydrolyzed Straw Mushroom Proteins Enhance Salty Taste: An Elucidation of Their Effect on the T1R1/T1R3 Taste Receptor via Molecular Docking

The objective of our study was to analyze and identify enzymatic peptides from straw mushrooms that can enhance salty taste with the aim of developing saltiness enhancement peptides to reduce salt intake and promote dietary health. We isolated taste-related peptides from the straw mushroom extract using ultrafiltration and identified them using UPLC-Q-TOF-MS/MS. The study found that the ultrafiltration fraction (500-2000 Da) of straw mushroom peptides had a saltiness enhancement effect, as revealed via subsequent E-tongue and sensory analyses. The ultrafiltration fractions (500-2000 Da) were found to contain 220 peptides, which were identified through UPLC-Q-TOF-MS/MS analysis. The interaction of these peptides with the T1R1/T1R3 receptor was also assessed. The investigation highlighted the significant involvement of Asp223, Gln243, Leu232, Asp251, and Pro254 in binding peptides from triple-enzymatically hydrolyzed straw mushrooms to T1R1/T1R3. Based on the binding energy and active site analysis, three peptides were selected for synthesis: DFNALPFK (-9.2 kcal/mol), YNEDNGIVK (-8.8 kcal/mol), and VPGGQEIKDR (-8.9 kcal/mol). Importantly, 3.2 mmol of VPGGQEIKDR increased the saltiness level of a 0.05% NaCl solution to that of a 0.15% NaCl solution. Additionally, the addition of 0.8 mmol of YNEDNGIVK to a 0.05% NaCl solution resulted in the same level of saltiness as a 0.1% NaCl solution.

The Caenorhabditis elegans innexin INX-20 regulates nociceptive behavioral sensitivity

Organisms rely on chemical cues in their environment to indicate the presence or absence of food, reproductive partners, predators, or other harmful stimuli. In the nematode Caenorhabditis elegans, the bilaterally symmetric pair of ASH sensory neurons serves as the primary nociceptors. ASH activation by aversive stimuli leads to backward locomotion and stimulus avoidance. We previously reported a role for guanylyl cyclases in dampening nociceptive sensitivity that requires an innexin-based gap junction network to pass cGMP between neurons. Here, we report that animals lacking function of the gap junction component INX-20 are hypersensitive in their behavioral response to both soluble and volatile chemical stimuli that signal through G protein-coupled receptor pathways in ASH. We find that expressing inx-20 in the ADL and AFD sensory neurons is sufficient to dampen ASH sensitivity, which is supported by new expression analysis of endogenous INX-20 tagged with mCherry via the CRISPR-Cas9 system. Although ADL does not form gap junctions directly with ASH, it does so via gap junctions with the interneuron RMG and the sensory neuron ASK. Ablating either ADL or RMG and ASK also resulted in nociceptive hypersensitivity, suggesting an important role for RMG/ASK downstream of ADL in the ASH modulatory circuit. This work adds to our growing understanding of the repertoire of ways by which ASH activity is regulated via its connectivity to other neurons and identifies a previously unknown role for ADL and RMG in the modulation of aversive behavior.

Dietary compounds activate an insect gustatory receptor on enteroendocrine cells to elicit myosuppressin secretion

Sensing of midgut internal contents is important for ensuring appropriate hormonal response and digestion following the ingestion of dietary components. Studies in mammals have demonstrated that taste receptors (TRs), a subgroup of G protein-coupled receptors (GPCRs), are expressed in gut enteroendocrine cells (EECs) to sense dietary compounds and regulate the production and/or secretion of peptide hormones. Although progress has been made in identifying expression patterns of gustatory receptors (GRs) in gut EECs, it is currently unknown whether these receptors, which act as ligand-gated ion channels, serve similar functions as mammalian GPCR TRs to elicit hormone production and/or secretion. A Bombyx mori Gr, BmGr6, has been demonstrated to express in cells by oral sensory organs, midgut and nervous system; and to sense isoquercitrin and chlorogenic acid, which are non-nutritional secondary metabolites of host mulberry. Here, we show that BmGr6 co-expresses with Bommo-myosuppressin (BMS) in midgut EECs, responds to dietary compounds and is involved in regulation of BMS secretion. The presence of dietary compounds in midgut lumen after food intake resulted in an increase of BMS secretions in hemolymph of both wild-type and BmGr9 knockout larvae, but BMS secretions in BmGr6 knockout larvae decreased relative to wild-type. In addition, loss of BmGr6 led to a significant decrease in weight gain, excrement, hemolymph carbohydrates levels and hemolymph lipid levels. Interestingly, although BMS is produced in both midgut EECs and brain neurosecretory cells (NSCs), BMS levels in tissue extracts suggested that the increase in hemolymph BMS during feeding conditions is primarily due to secretion from midgut EECs. Our studies indicate that BmGr6 expressed in midgut EECs responds to the presence of dietary compounds in the lumen by eliciting BMS secretion in B. mori larvae.

Chemosensory Functions After Glossectomy-A Cross-Sectional Pilot Study

OBJECTIVE

To evaluate potential interactions and compensatory mechanisms of subjectively impaired taste function with ortho- and retronasal olfaction after glossectomy.

STUDY DESIGN

In this cross-sectional pilot study, chemosensory functions were assessed in 25 patients with tongue carcinomas after glossectomy. The orthonasal-, retronasal-, and gustatory functions were tested with a mean time of 25 months after surgery with the Sniffin' Sticks odor identification test kit (ISST), the Candy Smell-27 test (CST-27) and the Taste strip test (TST). Visual analog scales (VAS) were additionally used for self-assessment of taste, flavor perception, and odor ranging from 0 (no perception) to 10 (excellent perception) and further correlated with our psychophysical evaluated outcome measures.

RESULTS

The TST, ISST, and CST-27 tests revealed that only eight (32%) and 13 (52%) glossectomy patients had normal taste and orthonasal function, e 21 (84%) patients showed normal retronasal function. Importantly, neither extent of resection and reconstruction nor prior radiotherapy affected chemosensory functions. Contrary, 20 (80%) patients rated their taste and flavor perception as acceptable (VAS >5). Results of the TST, ISST, and CST-27 tests did not correlate with the equivalent self-assessments of taste (p = 0.260, r = 0.234), odor (p = 0.588, r = -0.114), and flavor (p = 0.728, r = 0.073) perception.

CONCLUSION

There was a significant discrepancy between self-perception of taste and flavor and assessed gustatory function after glossectomy. A contribution of the intact retronasal olfactory system could be a possible explanation of our results.

LEVEL OF EVIDENCE

4 Laryngoscope, 133:375-382, 2023.

BmGr4 responds to sucrose and glucose and expresses in tachykinin-related peptide-secreting enteroendocrine cells

The regulatory hormones known as tachykinin-related peptides (TRPs) are identified as brain-gut peptides in insects. Dietary components from mulberry leaves, including glucose, induce secretion of TRPs from Bombyx mori midgut. However, the sensory molecules that recognize these compounds are still unknown. Here, we identified the gustatory receptor, BmGr4, as a sucrose and glucose receptor using Ca²⁺ imaging. Immunostaining revealed BmGr4 expression not only in the midgut, but also in the brain. In addition, BmGr4 expression was found to co-localize with TRP-expressing cells in both midgut enteroendocrine cells (EECs) and brain neurosecretory cells (NSCs). Furthermore, dietary nutrients after food intake result in an increase of TRP-level in hemolymph of silkworm larvae. These results provide significant circumstantial evidence for the involvement of the sucrose and glucose receptor, BmGr4, in the elicitation of TRP secretion in midgut EECs and brain NSCs.

Diurnal pattern of salivary alpha-amylase and cortisol under citric acid stimulation in young adults

Background

Saliva composition has diurnal variations. Citric acid stimulation plays a major role in the change of salivary flow rate and salivary composition. However, diurnal variations and sex differences in salivary alpha-amylase (sAA), pH, salivary flow rate (SFR), and salivary cortisol before and after citric acid stimulation remain unclear.

Methods

We recruited 30 healthy volunteers, including 15 women (24.7 ± 1.0 years old) and 15 men (25.3 ± 1.3 years old). At four time points (T1, 7:00; T2, 10:00; T3, 16:00; and T4, 20:00), saliva was collected from healthy volunteers before and after citric acid stimulation; and sAA, pH, SFR and salivary cortisol were measured and compared between men and women.

Results

There were circadian fluctuations in sAA activity, SFR, pH, and cortisol level both before and after citric acid stimulation, and the diurnal fluctuations of these indexes were not affected by citric acid stimulation. There were significant differences in salivary cortisol between men and women before and after acid stimulation in T1. Neither SFR nor pH showed sex-related differences before or after acid stimulation. The variation trend of sAA activity was contrary to that of cortisol, with a significant negative correlation.

Conclusions

Our data suggest that sAA and cortisol showed diurnal fluctuation, and the variation characteristics of male and female under resting state and acid stimulation were basically the same. The variation trend of salivary alpha-amylase activity was opposite to that of cortisol, with significant negative correlation. Our findings may enable the selection of the correct sampling time for research and the selection of appropriate sampling strategies in studies investigating chronic psychosocial conditions.

Novel impacts of saliva with regard to oral health

The maintenance of balanced oral homeostasis depends on saliva. A readily available and molecularly rich source of biological fluid, saliva fulfills many functions in the oral cavity, including lubrication, pH buffering, and tooth mineralization. Saliva composition and flow can be modulated by different factors, including circadian rhythm, diet, age, drugs, and disease. Recent events have revealed that saliva plays a central role in the dissemination and detection of the SARS-CoV-2 coronavirus. A working knowledge of saliva function and physiology is essential for dental health professionals.

Food preferences and thyroid hormones in children and adolescents with obesity

BACKGROUND

Thyroid hormones profoundly affect energy metabolism but their interrelation with food preference, which might contribute to childhood obesity development, are much less understood. In this study, we investigated if thyroid hormone levels are associated with specific modulation of food preference and potentially linked to the level of obesity in children and adolescents.

METHODS

Interrelations between food preference and peripheral thyroid activity were examined in a population of 99 non-obese and 101 obese children and adolescents (12.8 ± 3.6 years of age, 111/89 F/M) randomly selected from the patients of the Obesity and Metabolic Disease Out-patient Research Unit at National Institute for Children's Diseases in Bratislava in a period between December 2017 and March 2020.

RESULTS

Children and adolescents with obesity had a lower preference for food rich in high sucrose and high-complex carbohydrates, while the preference for protein and fat-containing food and that for dietary fibers did not differ between obese and nonobese. In adolescents with obesity, free thyroxine (FT4) correlated positively with the preference for a high protein and high fat-rich diet, irrespective of the fatty acid unsaturation level. Moreover, FT4 correlated negatively with the preference for dietary fibers, which has been also exclusively found in obese adolescents. Individuals with obesity with higher FT4 levels had higher systemic levels of AST and ALT than the population with lower FT4. Multiple regression analysis with age, sex, BMI-SDS, and FT4 as covariates revealed that FT4 and male gender are the major predictors of variability in the preference for a diet high in protein, fat, and monounsaturated fatty acids. FT4 was the sole predictor of the preference for a diet containing saturated and polyunsaturated fatty acids as well as for a diet low in fiber.

CONCLUSION

The link between free thyroxin levels and dietary preference for food rich in fat and protein is present exclusively in individuals with obesity. Higher serum FT4 was linked with elevated AST and ALT in children and adolescents with obesity, and FT4 was the best predictor for preference for food rich in fat and low in fiber. This may indicate that FT4 could contribute to the development of childhood obesity and its complications by modulating food preference.

Taste Perception and Cerebral Activity in the Human Gustatory Cortex Induced by Glucose, Fructose, and Sucrose Solutions

Glucose, fructose, and sucrose are important carbohydrates in Western diets with particular sweetness intensity and metabolisms. No study has compared their cerebral detection and their taste perception. Gustatory evoked potentials (GEPs), taste detection thresholds, intensity perception, and pleasantness were compared in response to glucose, fructose, and sucrose solutions at similar sweetness intensities and at identical molar concentrations. Twenty-three healthy subjects were randomly stimulated with 3 solutions of similar sweetness intensity (0.75 M of glucose, 0.47 M of fructose and 0.29 M of sucrose - sit. A), and with an identical molar concentration (0.29 M - sit. B). GEPs were recorded at gustatory cortex areas. Intensity perception and hedonic values of each solution were evaluated as were gustatory thresholds of the solutions. No significant difference was observed concerning the GEP characteristics of the solutions according to their sweetness intensities (sit. A) or their molar concentration (sit. B). In sit. A, the 3 solutions were perceived to have similar intensities and induced similar hedonic sensations. In sit. B, the glucose solution was perceived to be less intense and pleasant than the fructose and the sucrose solutions (P < 0.001) and the fructose solution was perceived to be less intense and pleasant than the sucrose (P < 0.001). Since GEP recordings were similar for glucose, fructose, and sucrose solutions whatever the concentrations, activation of same taste receptor induces similar cortical activation, even when the solutions were perceived differently. Sweet taste perception seems to be encoded by a complex chemical cerebral neuronal network.

Salivary functions in mastication, taste and textural perception, swallowing and initial digestion

Saliva exerts multiple functions in relation to the initial digestive processes taking place in the upper parts of the gastrointestinal tract. Ingestion of food and beverages, in turn, is a strong stimulus for secretion of saliva with a differential composition depending on the neuronal stimulation pattern. This review paper provides insight into the mechanisms by which saliva acts in relation to taste, mastication, bolus formation, enzymatic digestion and swallowing. Also, the protective functions of saliva including maintenance of dental and mucosal integrity will be discussed as they indirectly influence the digestive process. The final part of this study focuses on the implications of xerostomia and salivary gland dysfunction on gastrointestinal functions.

Genetic Variations in Sweet Taste Receptor Gene Are Related to Chocolate Powder and Dietary Fiber Intake in Obese Children and Adolescents

Childhood obesity is a major public health problem. It has a direct impact on the quality of life of children and adolescents, as well as on their future risk of developing chronic diseases. Dietary patterns rich in fats and sugars and lacking dietary fibers, vitamins, and minerals, as well as lack of physical exercise have been associated with the rise of obesity prevalence. However, factors that contribute to the preference for foods rich in these nutrients are not well established. Taste is recognized as an important predictor of food choices, and polymorphisms in taste-related genes may explain the variability of taste preference and food intake. The aim of this research is to evaluate the influence of polymorphisms of the sweet taste receptor gene TAS1R2 on diet and metabolic profile in obese children and adolescents. A cross-sectional study with 513 obese children and adolescents and 135 normal-weight children was carried out. A molecular study was performed for the single nucleotide polymorphisms (SNPs) rs9701796 and rs35874116 of TAS1R2, and dietary intake, anthropometric parameters (weight, height, waist circumference, waist-to-height ratio (WHtR)), and metabolic profile (including fasting glucose, insulin, triglyceride, high-density lipoprotein (HDL)-cholesterol, and leptin levels) were analyzed. The variant rs9701796 was associated with increased waist-height ratio, as well as with a higher chocolate powder intake in obese children. The variant rs35874116 was associated with a lower dietary fiber intake. In conclusion, there was no relationship between genotypes and risk of obesity. Obese adolescents carrying the serine allele of SNP rs9701796 in TAS1R2 showed higher waist-to-height ratio and chocolate powder intake, whereas those carrying the valine allele of SNP rs35874116 in TAS1R2 were characterized by lower dietary fiber intake.

Gustatory Salivation Is Associated with Body Mass Index, Daytime Sleepiness, and Snoring in Healthy Young Adults

The taste detection system for oral fatty acid may be related to obesity. In addition, sleep is intrinsically and closely related to food intake and metabolism. However, the association of gustatory salivation with body mass index (BMI), daytime sleepiness, or sleep habits is largely unknown. Therefore, we evaluated the relationship between gustatory salivation and BMI, Epworth sleepiness scale (ESS, a daytime sleepiness scale) or sleep habits among 26 healthy young individuals (20 males and 6 females; mean age: 26.0 ± 4.3 years). We also measured the saliva flow rate (SFR) that was induced by gum-chewing or each of three prototypical tastants (acetic acid, sucrose, and NaCl). Further, the SFR was induced by fatty acid, provided as oleic acid (OA) homogenized in non-fat milk. All participants showed normal rates of salivation during resting and gum-chewing states. The increase in the SFR induced by OA, but not by each of the three tastants, was associated with BMI. Moreover, both daytime sleepiness level and frequency of snoring were associated with the increase in the SFR induced by NaCl. These results suggest that BMI is associated with salivation induced by oral fatty acid exposure. Additionally, the regulatory mechanism for the NaCl-induced salivation reflex may have a relationship with impairments of the respiratory control system that are related to snoring during sleep and lead to daytime sleepiness because of insufficient sleep. Thus, measurement of gustatory salivation might contribute to the evaluation and prevention of obesity and sleep-related breathing disorders.

Association of Oral Fat Sensitivity with Body Mass Index, Taste Preference, and Eating Habits in Healthy Japanese Young Adults

Oral fat sensitivity (OFS, the ability to detect fat) may be related to overeating-induced obesity. However, it is largely unknown whether OFS affects taste preference and eating habits. Therefore, we aimed to evaluate (1) the association between body mass index (BMI) and OFS and (2) the relationship of OFS with four types of taste preference (sweet, sour, salty, and bitter) and eating habits using serial concentrations of oleic acid (OA) homogenized in non-fat milk and a self-reported questionnaire. Participants were 25 healthy Japanese individuals (mean age: 27.0 ± 5.6 years), among whom the OA detection threshold was significantly associated with BMI. Participants were divided into two subgroups based on oral sensitivity to 2.8 mM OA: hypersensitive (able to detect 2.8 mM OA, n = 16) and hyposensitive (unable to detect 2.8 mM OA, n = 9). The degree of sweet taste preference of the hypersensitive group was significantly higher than that of the hyposensitive group. Furthermore, there was significantly higher degree of preference for high-fat sweet foods than low-fat sweet foods in the hypersensitive group. There was also a significant inverse correlation between the OA detection threshold and the degree of both spare eating and postprandial satiety. Thus, OFS is associated not only with BMI, but also with the preference for high-fat sweet foods and eating habits. The present study provides novel insights that measuring OFS may be useful for assessing the risk of obesity associated with overeating in countries, including Japan, where BMI is increasing in the population.

Aversive Behavior in the Nematode C. elegans Is Modulated by cGMP and a Neuronal Gap Junction Network

All animals rely on their ability to sense and respond to their environment to survive. However, the suitability of a behavioral response is context-dependent, and must reflect both an animal's life history and its present internal state. Based on the integration of these variables, an animal's needs can be prioritized to optimize survival strategies. Nociceptive sensory systems detect harmful stimuli and allow for the initiation of protective behavioral responses. The polymodal ASH sensory neurons are the primary nociceptors in C. elegans. We show here that the guanylyl cyclase ODR-1 functions non-cell-autonomously to downregulate ASH-mediated aversive behaviors and that ectopic cGMP generation in ASH is sufficient to dampen ASH sensitivity. We define a gap junction neural network that regulates nociception and propose that decentralized regulation of ASH signaling can allow for rapid correlation between an animal's internal state and its behavioral output, lending modulatory flexibility to this hard-wired nociceptive neural circuit.

Assessing expression of TAS2R16 receptor on the tongue of elderly persons

In conducted study, we assessed expression of TAS2R16 receptor gene on the tongue of elderly persons. The TAS2R16 receptor belongs to family of G-protein coupled bitter taste receptors and is expressed in type 2 taste cells, which are a part of taste buds. The taste buds are distributed across the tongue's surface on the specialised structures called papillae. The TAS2R16 receptor mediates bitter taste in response to β-glucopyranosides such as salicin. The purpose of conducted study was to examine, whether the ageing process influence gene expression and hence the perception of taste at the molecular level. Ageing process is often related to either decreased or total lost perception of taste qualities. It is due to physiological changes in the oral cavity. The changes in taste cell membranes involve altered function of ion channels and receptors, which ultimately lead to decreased tasting ability of elderly people. In addition, various causes, such as oral and systemic diseases, drug administration, lifestyle (i.e. smoking) and some oral conditions (wearing dentures, dental caries and coated tongue), may extracerbate this issue. Loss of taste may become a large factor in reduction of appetite, which may lead to malnutrition. To accomplish the objective of this study, we recruited ten elderly persons. One 25-year old human was used as calibrator. We used non-invasive scrapping method for collecting taste cells from fungiform papillae of each subject. A multiplex TaqMan real-time PCR was performed to amplify cDNA of TAS2R16 and PGK1 genes, whereas the last one served as housekeeping gene. The TAS2R16 gene expression for elderly persons relative to that of young one was calculated according to the 2-ΔCT formula. Results pointed out to increased expression of TAS2R16 gene by 2-fold in 5th and 8th seniors. It is assumed that they perceive more intense bitterness from salicin at the molecular level than 25-year old person. The 2nd, 3rd, 7th and 10th elderly persons have had decreased expression level about 70%, whereas in case of 6th one that was even about 90%. It is supposed that these subjects, in particular last one, respond to salicin very weakly. This data may show evidence of almost total loss of taste. The causes and consequences are discussed in more detail.

Molecular mechanisms of taste recognition: considerations about the role of saliva

The gustatory system plays a critical role in determining food preferences and food intake, in addition to nutritive, energy and electrolyte balance. Fine tuning of the gustatory system is also crucial in this respect. The exact mechanisms that fine tune taste sensitivity are as of yet poorly defined, but it is clear that various effects of saliva on taste recognition are also involved. Specifically those metabolic polypeptides present in the saliva that were classically considered to be gut and appetite hormones (i.e., leptin, ghrelin, insulin, neuropeptide Y, peptide YY) were considered to play a pivotal role. Besides these, data clearly indicate the major role of several other salivary proteins, such as salivary carbonic anhydrase (gustin), proline-rich proteins, cystatins, alpha-amylases, histatins, salivary albumin and mucins. Other proteins like glucagon-like peptide-1, salivary immunoglobulin-A, zinc-α-2-glycoprotein, salivary lactoperoxidase, salivary prolactin-inducible protein and salivary molecular chaperone HSP70/HSPAs were also expected to play an important role. Furthermore, factors including salivary flow rate, buffer capacity and ionic composition of saliva should also be considered. In this paper, the current state of research related to the above and the overall emerging field of taste-related salivary research alongside basic principles of taste perception is reviewed.

Mouth rinsing with a bitter solution without ingestion does not improve sprint cycling performance

PURPOSE

Recently, we have shown that combining mouth rinsing with the ingestion of a 2 mM quinine solution immediately before a 30-s cycling sprint significantly improves performance. However, the strong bitterness of such a solution produces an unpleasant taste and evokes nausea at higher concentrations. Given the possibility that mouth rinsing with quinine without ingesting it may not produce nausea, a mouth rinse only protocol may be a more practical approach to administer quinine for improving exercise performance. The purpose of the present study was to determine whether mouth rinsing with quinine without ingesting it improves 30-s sprint cycling performance.

METHODS

Twelve competitive male cyclists performed a 30-s maximal cycling sprint immediately after rinsing their mouth for 10 s with either a 10 mM bitter quinine solution (QUI), plain water (WAT), a 7.1 % w/v sweet glucose solution (GLU), or no solution at all (control; CON). Sprint performance was assessed, and heart rate, ratings of perceived exertion and blood variables were measured pre- and post-exercise.

RESULTS

Mean power output during the 30-s sprint (QUI 888 ± 38; CON 873 ± 39; WAT 885 ± 37; GLU 873 ± 42 W; p = 0.431) as well as peak power (QUI 1230 ± 61; CON 1,208 ± 65; WAT 1,220 ± 70; GLU 1,202 ± 59 W; p = 0.690) were similar between the four conditions. There were no significant differences in any other performance measures, heart rate, subjective ratings or blood variables between conditions.

CONCLUSIONS

Mouth rinsing with a bitter tasting quinine solution without ingestion does not improve 30-s sprint cycling performance.

Occurrence of gustducin-immunoreactive cells in von Ebner's glands of guinea pigs

An immunohistochemical examination of guinea-pig taste buds in vallate papillae revealed gustducin-immunoreactive cells in the area of von Ebner’s glands, minor salivary glands. Since there have been no reports describing those cells in these locations for other species, we investigated these glands in order both to localize the cells and compare their immunoreactive characteristics with corresponding cells in the vallate taste buds. The gustducin-immunoreactive cells coincided with cells containing no secretory granules in the end portion of the glands, which was supported by the electron-microscopic immunocytochemistry. Double immunofluorescence microscopy confirmed these cells to be entirely immunopositive to type III inositol 1,4,5-triphosphate receptor (IP₃R-3), phospholipase Cβ2 (PLCβ2), and villin and also partly immunopositive to neuron-specific enolase (NSE) and calbindin D-28K. The gustducin-immunoreactive cells in the vallate taste buds exhibited completely the same immunoreactivities for these five molecules. Accordingly, the present results give credence to a consideration that the gustducin-immunnoreactive cells in both locations are identical in function(s) e.g., chemo-reception.

Taste responses in mice lacking taste receptor subunit T1R1

The T1R1 receptor subunit acts as an umami taste receptor in combination with its partner, T1R3. In addition, metabotropic glutamate receptors (brain and taste variants of mGluR1 and mGluR4) are thought to function as umami taste receptors. To elucidate the function of T1R1 and the contribution of mGluRs to umami taste detection in vivo, we used newly developed knock-out (T1R1(-/-)) mice, which lack the entire coding region of the Tas1r1 gene and express mCherry in T1R1-expressing cells. Gustatory nerve recordings demonstrated that T1R1(-/-) mice exhibited a serious deficit in inosine monophosphate-elicited synergy but substantial residual responses to glutamate alone in both chorda tympani and glossopharyngeal nerves. Interestingly, chorda tympani nerve responses to sweeteners were smaller in T1R1(-/-) mice. Taste cell recordings demonstrated that many mCherry-expressing taste cells in T1R1(+/-) mice responded to sweet and umami compounds, whereas those in T1R1(-/-) mice responded to sweet stimuli. The proportion of sweet-responsive cells was smaller in T1R1(-/-) than in T1R1(+/-) mice. Single-cell RT-PCR demonstrated that some single mCherry-expressing cells expressed all three T1R subunits. Chorda tympani and glossopharyngeal nerve responses to glutamate were significantly inhibited by addition of mGluR antagonists in both T1R1(-/-) and T1R1(+/-) mice. Conditioned taste aversion tests demonstrated that both T1R1(-/-) and T1R1(+/-) mice were equally capable of discriminating glutamate from other basic taste stimuli. Avoidance conditioned to glutamate was significantly reduced by addition of mGluR antagonists. These results suggest that T1R1-expressing cells mainly contribute to umami taste synergism and partly to sweet sensitivity and that mGluRs are involved in the detection of umami compounds.

Spices: the savory and beneficial science of pungency

Spicy food does not only provide an important hedonic input in daily life, but has also been anedoctically associated to beneficial effects on our health. In this context, the discovery of chemesthetic trigeminal receptors and their spicy ligands has provided the mechanistic basis and the pharmacological means to investigate this enticing possibility. This review discusses in molecular terms the connection between the neurophysiology of pungent spices and the "systemic" effects associated to their trigeminality. It commences with a cultural and historical overview on the Western fascination for spices, and, after analysing in detail the mechanisms underlying the trigeminality of food, the main dietary players from the transient receptor potential (TRP) family of cation channels are introduced, also discussing the "alien" distribution of taste receptors outside the oro-pharingeal cavity. The modulation of TRPV1 and TRPA1 by spices is next described, discussing how spicy sensations can be turned into hedonic pungency, and analyzing the mechanistic bases for the health benefits that have been associated to the consumption of spices. These include, in addition to a beneficial modulation of gastro-intestinal and cardio-vascular function, slimming, the optimization of skeletal muscle performance, the reduction of chronic inflammation, and the prevention of metabolic syndrome and diabetes. We conclude by reviewing the role of electrophilic spice constituents on cancer prevention in the light of their action on pro-inflammatory and pro-cancerogenic nuclear factors like NFκB, and on their interaction with the electrophile sensor protein Keap1 and the ensuing Nrf2-mediated transcriptional activity. Spicy compounds have a complex polypharmacology, and just like any other bioactive agent, show a balance of beneficial and bad actions. However, at least for moderate consumption, the balance seems definitely in favour of the positive side, suggesting that a spicy diet, a caveman-era technology, could be seriously considered in addition to caloric control and exercise as a measurement to prevent and control many chronic diseases associate to malnutrition from a Western diet.

The belly rules the nose: feeding state-dependent modulation of peripheral chemosensory responses

Feeding history and the presence of food dramatically alter chemosensory behaviors. Recent results indicate that internal nutritional state can gate peripheral gustatory and olfactory sensory responses to affect behavior. Focusing primarily on recent work in C. elegans and Drosophila, I describe the neuromodulatory mechanisms that translate feeding state information into changes in chemosensory neuron response properties and behavioral output.

Modulation of sweet responses of taste receptor cells

Taste receptor cells play a major role in detection of chemical compounds in the oral cavity. Information derived from taste receptor cells, such as sweet, bitter, salty, sour and umami is important for evaluating the quality of food components. Among five basic taste qualities, sweet taste is very attractive for animals and influences food intake. Recent studies have demonstrated that sweet taste sensitivity in taste receptor cells would be affected by leptin and endocannabinoids. Leptin is an anorexigenic mediator that reduces food intake by acting on leptin receptor Ob-Rb in the hypothalamus. Endocannabinoids such as anandamide [N-arachidonoylethanolamine (AEA)] and 2-arachidonoyl glycerol (2-AG) are known as orexigenic mediators that act via cannabinoid receptor 1 (CB1) in the hypothalamus and limbic forebrain to induce appetite and stimulate food intake. At the peripheral gustatory organs, leptin selectively suppresses and endocannabinoids selectively enhance sweet taste sensitivity via Ob-Rb and CB1 expressed in sweet sensitive taste cells. Thus leptin and endocannabinoids not only regulate food intake via central nervous systems but also modulate palatability of foods by altering peripheral sweet taste responses. Such reciprocal modulation of leptin and endocannabinoids on peripheral sweet sensitivity may play an important role in regulating energy homeostasis.

Obesity and appetite control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.

Obesity and appetite control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.

Bitter taster status predicts susceptibility to vection-induced motion sickness and nausea

BACKGROUND

Sensitivity to bitter taste and susceptibility to nausea are both protective mechanisms that guard against toxin ingestion, and both these traits vary within and between populations. Thus, we postulated that they may have co-evolved, such that they are associated.

METHODS

Bitter taster status was determined in 40 subjects (13 men, 27 women) by measuring the differential perceived taste intensity between salt and n-propylthiouracil using a labeled magnitude scale; susceptibility to vection-induced motion sickness and nausea was assessed using an optokinetic drum, a validated multi-symptom scoring scale, and electrogastrography.

KEY RESULTS

Taster status distribution was 25% non-tasters (NT), 40% tasters (T), and 35% supertasters (ST). Gender had no impact on this distribution, but females had a higher mean maximum symptom score than males (12.4 ± 1.4 vs 7.3 ± 2.0). Non-tasters displayed a faster and larger increase in mean symptom scores, had a higher percentage of subjects with high maximum symptom scores, and had a higher mean maximum score than T or ST, (14.8 ± 2.6 vs 7.1 ± 1.8, vs 9.8 ± 2.0). Taster status did not affect the gastric myoelectric frequency response to vection.

CONCLUSIONS & INFERENCES

Non-tasters are more susceptible to vection-induced motion sickness and nausea than T or ST, suggesting these two traits may have co-evolved in a reciprocal manner: in environments where the NT trait conferred an evolutionary advantage by enabling intake of fruits and vegetables containing bitter, yet beneficial, phytonutrients, increased nausea susceptibility may have arisen to maintain protection against ingested toxins.