Molecular Functions and Physiological Roles of Gustatory Receptors of the Silkworm Bombyx mori.

Complete elucidation of members of the gustatory receptor (Gr) family in lepidopteran insects began in the silkworm Bombyx mori. Grs of lepidopteran insects were initially classified into four subfamilies based on the results of phylogenetic studies and analyses of a few ligands. However, with further ligand analysis, it has become clear that plant secondary metabolites are important targets not only for Grs in the bitter subfamily but also for the Drosophila melanogaster Gr43a orthologue subfamily and Grs in the sugar subfamily. Gene knockout experiments showed that B. mori Gr6 (BmGr6) and BmGr9 are involved in the recognition of the feeding-promoting compounds chlorogenic acid and isoquercetin in mulberry leaves by the maxillary palps, suggesting that these Grs are responsible for palpation-dependent host recognition without biting. On the other hand, BmGr expression was also confirmed in nonsensory organs. Midgut enteroendocrine cells that produce specific neuropeptides were shown to express specific BmGrs, suggesting that BmGrs are involved in the induction of endocrine secretion in response to changes in the midgut contents. Furthermore, gene knockout experiments indicated that BmGr6 is indeed involved in the secretion of myosuppressin. On the other hand, BmGr9 was shown to induce signal transduction that is not derived from the intracellular signaling cascade mediated by G proteins but from the fructose-regulated cation channel of BmGr9 itself. Cryogenic electron microscopy revealed the mechanism by which the ion channel of the BmGr9 homotetramer opens upon binding of fructose to the ligand-binding pocket. Research on BmGrs has contributed greatly to our understanding of the functions and roles of Grs in insects.

Exploration of the mechanism of temperature influence on bitter taste of theacrine by activating human bitter taste receptor hTAS2R14.

Theacrine, a purine alkaloid derived from Camellia assamica var. kucha, has a distinct bitter taste. Our previous study found the lower recognition threshold of theacrine at 25 °C than 45 °C. This study aims to investigate the bitterness characterizations of theacrine at aforementioned temperatures and its taste perception mechanism. Sensory analysis exhibited higher bitterness intensity for theacrine at 25 °C than 45 °C. Subsequently, flow cytometry was performed to verify the above characterization at the cellular level. It revealed that theacrine could activated the bitter receptor hTAS2R14 and the calcium signal at 25 °C was higher than 45 °C. Ultimately, the interaction mechanism was studied by molecular dynamics simulations, indicating that the conformation of theacrine-hTAS2R14 had a higher binding capacity and better stability at 25 °C. Overall, temperature affected the binding of theacrine to the bitter receptor hTAS2R14, resulting in the stronger bitterness intensity of theacrine at 25 °C than 45 °C.

Taste Sensor Assessment of Bitterness in Medicines: Overview and Recent Topics.

In recent decades, taste sensors have been increasingly utilized to assess the taste of oral medicines, particularly focusing on bitterness, a major obstacle to patient acceptance and adherence. This objective and safe method holds promise for enhancing the development of patient-friendly medicines in pharmaceutical companies. This review article introduces its application in measuring the intensity of bitterness in medicine, confirming the achievement of taste masking, distinguishing taste differences between branded and generic medicines, and identifying substances to suppress bitterness in target medicines. Another application of the sensor is to predict a significant increase in bitterness when medicine is taken with certain foods/beverages or concomitant medication. Additionally, to verify the sensor's predictability, a significant correlation has been demonstrated between the output of a bitter-sensitive sensor designed for drug bitterness (BT0) and the bitterness responses of the human taste receptor hT2R14 from BitterDB (huji.ac.il). As a recent advancement, a novel taste sensor equipped with lipid/polymer membranes modified by 3-Br-2,6-dihydroxybenzoic acid (2,6-DHBA), based on the concept of allostery, is introduced. This sensor successfully predicts the bitterness of non-charged pharmaceuticals with xanthine skeletons, such as caffeine or related compounds. Finally, the future prospects of taste sensors are discussed.

Missense genetic variants in major bitter taste receptors are associated with diet quality and food intake in a highly admixed underrepresented population.

BACKGROUND AND AIMS: To investigate associations between Single Nucleotide Polymorphisms (SNPs) in the TAS1R and TAS2R taste receptors and diet quality, intake of alcohol, added sugar, and fat, using linear regression and machine learning techniques in a highly admixed population. METHODS: In the ISA-Capital health survey, 901 individuals were interviewed and had socioeconomic, demographic, health characteristics, along with dietary information obtained through two 24-h recalls. Data on 12 components related to food groups, nutrients, and calories was combined into a diet quality score (BHEI-R). BHEI-R, SoFAAs (calories from added sugar, saturated fat, and alcohol) and Alcohol use were tested for associations with 255 TAS2R SNPs and 73 TAS1R SNPs for 637 individuals with regression analysis and Random Forest. Significant SNPs were combined into Genetic taste scores (GTSs). RESULTS: Among 23 SNPs significantly associated either by stepwise linear/logistic regression or random forest with any possible biological functionality, the missense variants rs149217752 in TAS2R40, for SoFAAs, and rs2233997 in TAS2R4, were associated with both BHEI-R (under 4% increase in Mean Squared Error) and SoFAAs. GTSs increased the variance explanation of quantitative phenotypes and there was a moderately high AUC for alcohol use. CONCLUSIONS: The study provides insights into the genetic basis of human taste perception through the identification of missense variants in the TAS2R gene family. These findings may contribute to future strategies in precision nutrition aimed at improving food quality by reducing added sugar, saturated fat, and alcohol intake.

Single-Nucleotide Polymorphisms of TAS2R46 Affect the Receptor Downstream Calcium Regulation in Histamine-Challenged Cells.

Bitter taste receptors (TAS2Rs) expressed in extraoral tissues represent a whole-body sensory system, whose role and mechanisms could be of interest for the identification of new therapeutic targets. It is known that TAS2R46s in pre-contracted airway smooth muscle cells increase mitochondrial calcium uptake, leading to bronchodilation, and that several SNPs have been identified in its gene sequence. There are very few reports on the structure-function analysis of TAS2Rs. Thus, we delved into the subject by using mutagenesis and in silico studies. We generated a cellular model that expresses native TAS2R46 to evaluate the influence of the four most common SNPs on calcium fluxes following the activation of the receptor by its specific ligand absinthin. Then, docking studies were conducted to correlate the calcium flux results to the structural mutation. The analysed SNPs differently modulate the TAS2R46 signal cascade according to the altered protein domain. In particular, the SNP in the sixth transmembrane domain of the receptors did not modulate calcium homeostasis, while the SNPs in the sequence coding for the fourth transmembrane domain completely abolished the mitochondrial calcium uptake. In conclusion, these results indicate the fourth transmembrane domain of TAS2R46 is critical for the intrinsic receptor activity.

Comprehensive Analysis of the Function and Prognostic Value of TAS2Rs Family-Related Genes in Colon Cancer.

In the realm of colon carcinoma, significant genetic and epigenetic diversity is observed, underscoring the necessity for tailored prognostic features that can guide personalized therapeutic strategies. In this study, we explored the association between the type 2 bitter taste receptor (TAS2Rs) family-related genes and colon cancer using RNA-sequencing and clinical datasets from The Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO). Our preliminary analysis identified seven TAS2Rs genes associated with survival using univariate Cox regression analysis, all of which were observed to be overexpressed in colon cancer. Subsequently, based on these seven TAS2Rs prognostic genes, two colon cancer molecular subtypes (Cluster A and Cluster B) were defined. These subtypes exhibited distinct prognostic and immune characteristics, with Cluster A characterized by low immune cell infiltration and less favorable outcomes, while Cluster B was associated with high immune cell infiltration and better prognosis. Finally, we developed a robust scoring system using a gradient boosting machine (GBM) approach, integrated with the gene-pairing method, to predict the prognosis of colon cancer patients. This machine learning model could improve our predictive accuracy for colon cancer outcomes, underscoring its value in the precision oncology framework.

Sweet-bitter taste interactions in binary mixtures of sweeteners: Relationship between taste receptor activities and sensory perception.

This study investigated the effects of various binary sweetener mixtures on sweetness enhancement and their interactions with sweet or bitter taste receptors, focusing on sensory perception and receptor activity. Acesulfame K or saccharin was mixed with allulose, aspartame, erythritol, fructose, glucose, or sucrose to match a target sucrose sweetness. The effects of the mixtures on sweet and bitter taste receptors (in the human embryonic kidney -293 cells) and sensory taste intensities were evaluated. Sweetness enhancement at the sweet taste receptor level was observed in some cases, with several monosaccharides reducing the acesulfame K- or saccharin-induced bitter taste receptor activity. Combining acesulfame K or saccharin with any of the six sweeteners perceptually enhanced sweetness (60% âˆ¼ 100% in 50:50 ratio), correlating with a reduction in inherent bitterness (-35% âˆ¼ -63% in 50:50 ratio). This finding suggests that sweetness perception likely increased because the monosaccharides mitigate the activation of bitter receptors caused by high-potency sweeteners.

Activation of TAS2R4 signaling attenuates podocyte injury induced by high glucose.

Bitter taste receptors (TAS2Rs) Tas2r108 gene possesses a high abundance in mouse kidney; however, the biological functions of Tas2r108 encoded receptor TAS2Rs member 4 (TAS2R4) are still unknown. In the present study, we found that mouse TAS2R4 (mTAS2R4) signaling was inactivated in chronic high glucose-stimulated mouse podocyte cell line MPC, evidenced by the decreased protein expressions of mTAS2R4 and phospholipase C ß2 (PLCß2), a key downstream molecule of mTAS2R4 signaling. Nonetheless, agonism of mTAS2R4 by quinine recovered mTAS2R4 and PLCß2 levels, and increased podocyte cell viability as well as protein expressions of ZO-1 and nephrin, biomarkers of podocyte slit diaphragm, in high glucose-cultured MPC cells. However, blockage of mTAS2R4 signaling with mTAS2R4 blockers γ-aminobutyric acid and abscisic acid, a Gßγ inhibitor Gallein, or a PLCß2 inhibitor U73122 all abolished the effects of quinine on NLRP3 inflammasome and p-NF-κB p65 as well as the functional podocyte proteins in MPC cells in a high glucose condition. Furthermore, knockdown of mTAS2R4 with lentivirus-carrying Tas2r108 shRNA also ablated the effect of quinine on the key molecules of the above inflammatory signalings and podocyte functions in high glucose-cultured MPC cells. In summary, we demonstrated that activation of TAS2R4 signaling alleviated the podocyte injury caused by chronic high glucose, and inhibition of NF-κB p65 and NLRP3 inflammasome mediated the protective effects of TAS2R4 activation on podocytes. Moreover, activation of TAS2R4 signaling could be an important strategy for prevention and treatment of diabetic kidney disease.

The Growing Complexity of Human Bitter Taste Perception.

Human bitter perception is important for the identification of potentially harmful substances in food. For quite some years, research focused on the identification of activators for ∼25 human bitter taste receptors. The discovery of antagonists as well as increasing knowledge about agonists of different efficacies has substantially added to the intricacy of bitter taste perception. This article seeks to raise awareness for an underestimated new level of complexity when compound mixtures or even whole food items are assessed for their bitter taste.

Sodium oligomannate activates the enteroendocrine-vagal afferent pathways in APP/PS1 mice.

Enteroendocrine cells (EECs) and vagal afferent neurons constitute functional sensory units of the gut, which have been implicated in bottom-up modulation of brain functions. Sodium oligomannate (GV-971) has been shown to improve cognitive functions in murine models of Alzheimer's disease (AD) and recently approved for the treatment of AD patients in China. In this study, we explored whether activation of the EECs-vagal afferent pathways was involved in the therapeutic effects of GV-971. We found that an enteroendocrine cell line RIN-14B displayed spontaneous calcium oscillations due to TRPA1-mediated calcium entry; perfusion of GV-971 (50, 100 mg/L) concentration-dependently enhanced the calcium oscillations in EECs. In ex vivo murine jejunum preparation, intraluminal infusion of GV-971 (500 mg/L) significantly increased the spontaneous and distension-induced discharge rate of the vagal afferent nerves. In wild-type mice, administration of GV-971 (100 mg· kg-1 ·d-1, i.g. for 7 days) significantly elevated serum serotonin and CCK levels and increased jejunal afferent nerve activity. In 7-month-old APP/PS1 mice, administration of GV-971 for 12 weeks significantly increased jejunal afferent nerve activity and improved the cognitive deficits in behavioral tests. Sweet taste receptor inhibitor Lactisole (0.5 mM) and the TRPA1 channel blocker HC-030031 (10 µM) negated the effects of GV-971 on calcium oscillations in RIN-14B cells as well as on jejunal afferent nerve activity. In APP/PS1 mice, co-administration of Lactisole (30 mg ·kg-1 ·d-1, i.g. for 12 weeks) attenuated the effects of GV-971 on serum serotonin and CCK levels, vagal afferent firing, and cognitive behaviors. We conclude that GV-971 activates sweet taste receptors and TRPA1, either directly or indirectly, to enhance calcium entry in enteroendocrine cells, resulting in increased CCK and 5-HT release and consequent increase of vagal afferent activity. GV-971 might activate the EECs-vagal afferent pathways to modulate cognitive functions.

Membrane-bound chemoreception of bitter bile acids and peptides is mediated by the same subset of bitter taste receptors.

The vertebrate sense of taste allows rapid assessment of the nutritional quality and potential presence of harmful substances prior to ingestion. Among the five basic taste qualities, salty, sour, sweet, umami, and bitter, bitterness is associated with the presence of putative toxic substances and elicits rejection behaviors in a wide range of animals including humans. However, not all bitter substances are harmful, some are thought to be health-beneficial and nutritious. Among those compound classes that elicit a bitter taste although being non-toxic and partly even essential for humans are bitter peptides and L-amino acids. Using functional heterologous expression assays, we observed that the 5 dominant human bitter taste receptors responsive to bitter peptides and amino acids are activated by bile acids, which are notorious for their extreme bitterness. We further demonstrate that the cross-reactivity of bitter taste receptors for these two different compound classes is evolutionary conserved and can be traced back to the amphibian lineage. Moreover, we show that the cross-detection by some receptors relies on "structural mimicry" between the very bitter peptide L-Trp-Trp-Trp and bile acids, whereas other receptors exhibit a phylogenetic conservation of this trait. As some bile acid-sensitive bitter taste receptor genes fulfill dual-roles in gustatory and non-gustatory systems, we suggest that the phylogenetic conservation of the rather surprising cross-detection of the two substance classes could rely on a gene-sharing-like mechanism in which the non-gustatory function accounts for the bitter taste response to amino acids and peptides.

Exendin-4 blockade of T1R2/T1R3 activation improves Pseudomonas aeruginosa-related pneumonia in an animal model of chemically induced diabetes.

OBJECTIVE: Poorly controlled diabetes frequently exacerbates lung infection, thereby complicating treatment strategies. Recent studies have shown that exendin-4 exhibits not only hypoglycemic but also anti-inflammatory properties. This study aimed to explore the role of exendin-4 in lung infection with diabetes, as well as its association with NOD1/NF-κB and the T1R2/T1R3 sweet taste receptor. METHODS: 16HBE human bronchial epithelial cells cultured with 20 mM glucose were stimulated with lipopolysaccharide (LPS) isolated from Pseudomonas aeruginosa (PA). Furthermore, Sprague‒Dawley rats were fed a high-fat diet, followed by intraperitoneal injection of streptozotocin and intratracheal instillation of PA. The levels of TNF-α, IL-1ß and IL-6 were evaluated using ELISAs and RT‒qPCR. The expression of T1R2, T1R3, NOD1 and NF-κB p65 was assayed using western blotting and immunofluorescence staining. Pathological changes in the lungs of the rats were observed using hematoxylin and eosin (H&E) staining. RESULTS: At the same dose of LPS, the 20 mM glucose group produced more proinflammatory cytokines (TNF-α, IL-1ß and IL-6) and had higher levels of T1R2, T1R3, NOD1 and NF-κB p65 than the normal control group (with 5.6 mM glucose). However, preintervention with exendin-4 significantly reduced the levels of the aforementioned proinflammatory cytokines and signaling molecules. Similarly, diabetic rats infected with PA exhibited increased levels of proinflammatory cytokines in their lungs and increased expression of T1R2, T1R3, NOD1 and NF-κB p65, and these effects were reversed by exendin-4. CONCLUSIONS: Diabetic hyperglycemia can exacerbate inflammation during lung infection, promote the increase in NOD1/NF-κB, and promote T1R2/T1R3. Exendin-4 can ameliorate PA-related pneumonia with diabetes and overexpression of NOD1/NF-κB. Additionally, exendin-4 suppresses T1R2/T1R3, potentially through its hypoglycemic effect or through a direct mechanism. The correlation between heightened expression of T1R2/T1R3 and an intensified inflammatory response in lung infection with diabetes requires further investigation.

Discovery of bitter masking compounds from Allspice (Pimenta dioica) using sensory guided isolation.

Bitter substances in functional foods and beverages can act as nutraceuticals, offering potential health benefits. However, their unpleasant sensory impact reduces the consumption of these foods. Consequently, the discovery of bitter masking compounds is crucial for enhancing the intake of bioactive compounds in functional foods and beverages. Bitter taste is mediated by TAS2Rs, a sub-family of G-protein-coupled receptors. TAS2R14 is especially pivotal in the perception of bitterness, as it is one of the most broadly tuned bitter receptors. In this study, allspice was extracted and purified to yield five single compounds based on sensory guided fractionation. The structures of each compound were determined based on nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HR-MS). In a sensory evaluation, compound 1 exhibited bitter masking activity against quinine. Molecular docking analysis revealed that compound 1 could act as an antagonist of the TAS2R14 bitter receptor.

Synchronized Expansion and Contraction of Olfactory, Vomeronasal, and Taste Receptor Gene Families in Hystricomorph Rodents.

Chemical senses, including olfaction, pheromones, and taste, are crucial for the survival of most animals. There has long been a debate about whether different types of senses might influence each other. For instance, primates with a strong sense of vision are thought to have weakened olfactory abilities, although the oversimplified trade-off theory is now being questioned. It is uncertain whether such interactions between different chemical senses occur during evolution. To address this question, we examined four receptor gene families related to olfaction, pheromones, and taste: olfactory receptor (OR), vomeronasal receptor type 1 and type 2 (V1R and V2R), and bitter taste receptor (T2R) genes in Hystricomorpha, which is morphologically and ecologically the most diverse group of rodents. We also sequenced and assembled the genome of the grasscutter, Thryonomys swinderianus. By examining 16 available genome assemblies alongside the grasscutter genome, we identified orthologous gene groups among hystricomorph rodents for these gene families to separate the gene gain and loss events in each phylogenetic branch of the Hystricomorpha evolutionary tree. Our analysis revealed that the expansion or contraction of the four gene families occurred synchronously, indicating that when one chemical sense develops or deteriorates, the others follow suit. The results also showed that V1R/V2R genes underwent the fastest evolution, followed by OR genes, and T2R genes were the most evolutionarily stable. This variation likely reflects the difference in ligands of V1R/V2Rs, ORs, and T2Rs: species-specific pheromones, environment-based scents, and toxic substances common to many animals, respectively.

Characterization of Bitter Taste Receptor-Dependent Autophagy in Oral Epithelial Cells.

Microbial dysbiosis is an important trigger in the development of oral diseases. Oral keratinocytes or gingival epithelial cells (GECs) offer protection against various microbial insults. Recent studies suggest that GECs expressed higher level of bitter taste receptor 14 (T2R14) compared to other taste receptors and toll-like receptors and act as innate immune sentinels. Macroautophagy or autophagy is a cellular conserved process involved in the regulation of host innate immune responses against microbial infection. Here, we describe a robust method for evaluation of T2R14-dependent autophagy flux in GECs. Autophagy flux was detected using Western blot analysis in GECs and further was confirmed using Acridine Orange-dependent flow cytometry analysis.

Recombinant expression and tryptophan-assisted analysis of human sweet taste receptor T1R3's extracellular domain in sweetener interaction studies.

The human palate can discern multiple tastes; however, it predominantly perceives five fundamental flavors: sweetness, saltiness, sourness, bitterness, and umami. Sweetness is primarily mediated through the sweet taste receptor, a membrane-bound heterodimeric structure comprising T1R2-T1R3. However, unraveling the structural and mechanistic intricacies of the sweet taste receptor has proven challenging. This study aimed to address this knowledge gap by expressing an extracellular N-terminal domain encompassing the cysteine-rich domain of human hT1R3 (hT1R3-TMD) in Escherichia coli. The expressed protein was obtained as inclusion bodies, purified by metal affinity chromatography, and refolded using the dilution-refolding method. Through rigorous analysis, we confirmed the successful refolding of hT1R3-TMD and elucidated its structural characteristics using circular dichroism spectroscopy. Notably, the refolded protein was found to exist as either a monomer or a dimer, depending on its concentration. A tryptophan fluorescence quenching assay revealed that the dissociation constants for sucrose, sucralose, and brazzein were >9500 µM, 2380 µM and 14.3 µM, respectively. Our findings highlight the utility of this E. coli expression system for producing functional hT1R3-TMD for investigations and demonstrate the efficacy of the tryptophan fluorescence quenching assay in revealing complex interactions between sweet taste receptors and various sweeteners.

Tastant-receptor interactions: insights from the fruit fly.

Across species, taste provides important chemical information about potential food sources and the surrounding environment. As details about the chemicals and receptors responsible for gustation are discovered, a complex view of the taste system is emerging with significant contributions from research using the fruit fly, Drosophila melanogaster, as a model organism. In this brief review, we summarize recent advances in Drosophila gustation and their relevance to taste research more broadly. Our goal is to highlight the molecular mechanisms underlying the first step of gustatory circuits: ligand-receptor interactions in primary taste cells. After an introduction to the Drosophila taste system and how it encodes the canonical taste modalities sweet, bitter, and salty, we describe recent insights into the complex nature of carboxylic acid and amino acid detection in the context of sour and umami taste, respectively. Our analysis extends to non-canonical taste modalities including metals, fatty acids, and bacterial components, and highlights unexpected receptors and signaling pathways that have recently been identified in Drosophila taste cells. Comparing the intricate molecular and cellular underpinnings of how ligands are detected in vivo in fruit flies reveals both specific and promiscuous receptor selectivity for taste encoding. Throughout this review, we compare and contextualize these Drosophila findings with mammalian research to not only emphasize the conservation of these chemosensory systems, but to demonstrate the power of this model organism in elucidating the neurobiology of taste and feeding.

Food drugs as drivers of therapeutic knowledge and the role of chemosensory qualities.

ETHNOPHARMACOLOGICAL RELEVANCE: Chemosensory qualities of botanical drugs are important cues for anticipating physiologic consequences. Whether a botanical drug is used for both, food and medicine, or only as medicine depends on taste preferences, nutritional content, cultural background, and the individual and overall epidemiological context. MATERIAL AND METHODS: We subjected 540 botanical drugs described in De Materia Medica having at least one oral medical application to a tasting panel. The 540 drugs were grouped into those only used for medicine (388) and those also used for food (152). The associations with chemosensory qualities and therapeutic indications were compared across the two groups. We considered 22 experimentally assessed chemosensory qualities and 39 categories of therapeutic use groups. We wanted to know, 1): which chemosensory qualities increase the probability of an orally applied botanical drug to be also used for food ? 2): which chemosensory qualities augment the probability of an orally applied botanical drug to be only used for medicine? and 3): whether there are differences in therapeutic indications between orally applied botanical drugs also used for food (food drugs) and botanical drugs applied exclusively for medicinal purposes (non-food drugs) and, if yes, how the differences can be explained. RESULTS: Chemosensory qualities augmenting the probability of an orally applied botanical drug to be also used for food were sweet, starchy, salty, burning/hot, fruity, nutty, and cooling. Therapeutics used for diarrhoea, as libido modulators, purgatives, laxatives, for expelling parasites, breast and lactation and increasing diuresis, were preferentially sourced from food drugs while drugs used for liver and jaundice, vaginal discharge and humoral management showed significant negative associations with food dugs in ancient Greek-Roman materia medica. CONCLUSION: Therapeutics used for ailments of body organs involved in the digestion of food and the excretion of waste products showed a tendency to be sourced from food drugs. Arguably, the daily consumption of food offered the possibility for observing post-prandial physiologic and pharmacologic effects which led to a high therapeutic versatility of food drugs and the possibility to understand benefits of taste and flavour qualities. The difference in chemosensory qualities between food drugs and non-food drugs is demarcating the organoleptic requirements of food rather than that of medicine.

Alteration of Sweet and Bitter Taste Sensitivity with Development of Glucose Intolerance in Non-insulin-Dependent Diabetes Mellitus Model OLETF Rats.

Patients with diabetes exhibit altered taste sensitivity, but its details have not been clarified yet. Here, we examined alteration of sweet taste sensitivity with development of glucose intolerance in Otsuka Long-Evans Tokushima Fatty (OLETF) rats as a model of non-insulin-dependent diabetes mellitus. Compared to the cases of Long Evans Tokushima Otsuka (LETO) rats as a control, glucose tolerance of OLETF rats decreased with aging, resulting in development of diabetes at 36-weeks-old. In brief-access tests with a mixture of sucrose and quinine hydrochloride, OLETF rats at 25 or more-weeks-old seemed to exhibit lower sweet taste sensitivity than age-matched LETO ones, but the lick ratios of LETO, but not OLETF, rats for the mixture and quinine hydrochloride solutions decreased and increased, respectively, aging-dependently. Expression of sweet taste receptors, T1R2 and T1R3, in circumvallate papillae (CP) was almost the same in LETO and OLETF rats at 10- and 40-weeks-old, while expression levels of a bitter taste receptor, T2R16, were greater in 40-weeks-old rats than in 10-weeks-old ones in both strains. There was no apparent morphological alteration in taste buds in CP between 10- and 40-weeks-old LETO and OLETF rats. Metagenomic analysis of gut microbiota revealed strain- and aging-dependent alteration of mucus layer-regulatory microbiota. Collectively, we concluded that the apparent higher sweet taste sensitivity in 25 or more-weeks-old OLETF rats than in age-matched LETO rats was due to the aging-dependent increase of bitter taste sensitivity in LETO rats with alteration of the gut microbiota.