Labeling and analysis of chicken taste buds using molecular markers in oral epithelial sheets

Macroscopic, microscopic, and immunofluorescent characterization of the Greek tortoise (Testudo graeca graeca) oropharyngeal floor with concern to its feed adaptation as a herbivorous land reptile

The current investigation focuses on gross anatomy, light, and scanning electron microscopy (SEM) of the Testudo graeca oropharyngeal floor, with particular reference to the immunofluorescence technique to examine its tongue. The T. graeca oropharyngeal floor showed many anatomical structures: the lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. The lower rhamphotheca appeared as a V-shaped jaw line with a highly serrated edge and a median tomium (beak). SEM observations of the lingual apex and the lingual body showed rectangular and conical filiform papillae with porous surfaces and taste pores. Meanwhile, the lingual root had two wings that carried papillae with different shapes: dagger-shaped, conical, bifurcated, and leaf-like papillae, and these papillae lacked taste pores. The laryngeal mound had openings for the laryngeal mucus gland and its secretions. Light microscopy findings showed mucous glands in the propria submucosa and near the mucosal surface of the lingual apex. The lingual root had lingual papillae and two hyaline cartilaginous skeletons between skeletal muscles, and the lingual papillae were elongated filiform, rectangular filiform papillae, and fungiform papillae. The lamina propria constituted the core of the lingual papillae and the mucous gland, they had a positive reaction with the periodic acid schiff (PAS) reagent. The apical surface of the fungiform papillae had taste pores. Under immunofluorescence, the vimentin was detected in taste bud cells, and synaptophysin reacted to the taste buds and nerve bundles. The current study of the Greek tortoise oropharyngeal floor investigated its herbivorous eating habits using its serrated lower rhamphotheca, a large tongue with differently shaped papillae, and numerous mucous glands. RESEARCH HIGHLIGHTS: The Greek tortoise (T. graeca graeca) oropharyngeal floor showed many anatomical structures: lower rhamphotheca, paralingual ridge, lower alveolar ridge, tongue, laryngeal mound, and glottis. SEM and light microscopy observations of the tongue revealed varied types and shapes of lingual papillae with a porous surface on the tongue apex (rectangular or conical filiform papillae), on the tongue body (filiform and fungiform papillae), and on the tongue root (dagger-shaped, conical, bifurcated, and leaf-like papillae). Light microscopy findings: the lamina propria constituted the core of the lingual papillae and had numerous mucous glands that had a slightly magenta-red color with PAS reagent. The apical surface of the fungiform papillae had taste pores. Vimentin and synaptophysin gave a reaction to the taste buds.

Taste Preferences in Broilers: Effect of Age, Delivery Matrix, and Number of Chickens per Pen on Selection and Consumption Behaviour

Due to substantial differences between studies, the understanding of avian taste perception remains incomplete. Also, studies on chicken taste preferences have mainly focused on measuring consumption differences, neglecting consumption behaviour patterns. This study investigated how age, the compound delivery matrix, and the number of birds per pen affect broiler chicken preferences and consumption behaviour, and established their preference values for four taste compounds. Ninety-six one-day-old male broiler chickens (Ross 308) were divided into two age groups (initial: days 7-23; final: days 26-42), with two compound delivery matrices (water or ground wheat) and two numbers of birds (one or two chickens per pen), following a 2 × 2 × 2 factorial design. Four taste compounds (sucrose, monosodium glutamate (MSG), L-lysine, and calcium carbonate) were tested at different concentrations. Preferences were assessed at 2, 4, and 8 h post-test, along with recording various behavioural parameters. Initial-stage birds showed higher (p < 0.001) preference values, time of approach (TA), number of bouts (NB), duration of bouts (DB), and number of pecks (NP) than final-stage birds. Birds exposed to a water matrix also exhibited higher (p < 0.001) preference and NB, while those exposed to a ground wheat matrix showed a higher (p < 0.001) NP. Pens with a pair of birds had a higher (p < 0.003) 2 h preference, TA, NB, DB, and NP, than pens with a single chicken. Chickens showed significant preference values for 100 mM sucrose at 2 h (p = 0.025), 150 mM MSG at 4 h (p = 0.026) and 8 h (p = 0.013), and 300 mM MSG at 2 h (p = 0.013). We concluded that all the variables evaluated influence broilers' taste preferences and consumption behaviour during selection tests. Future studies should prioritize including chickens in the initial stage of the production cycle, testing them in pairs or groups, and delivering compounds via a liquid matrix.

The tongue of the red-eared slider (Trachemys scripta elegans): morphological characterization through gross, light, scanning electron, and immunofluorescence microscopic examination

The red-eared slider (Trachemys scripta elegans) is renowned for its remarkable adaptations, yet much of its complex biology remains unknown. In this pioneering study, we utilized a combination of gross anatomy, scanning electron microscopy (SEM), light microscopy, and immunofluorescence techniques to examine the tongue's omnivorous adaptation in this species. This research bridges a critical knowledge gap, enhancing our understanding of this intriguing reptile. Gross examination revealed a unique arrowhead-shaped tongue with a median lingual fissure and puzzle-piece-shaped tongue papillae. SEM unveiled rectangular filiform, conical, and fungiform papillae, with taste pores predominantly on the dorsal surface and mucous cells on the lateral surface of the papillae. Histologically, the tongue's apex featured short rectangular filiform and fungiform papillae, while the body exhibited varying filiform shapes and multiple taste buds on fungiform papillae. The tongue's root contained lymphatic tissue with numerous lymphocytes surrounding the central crypt, alongside lingual skeletal musculature, blood and lymph vessels, and Raffin corpuscles in the submucosa. The lingual striated muscle bundles had different orientations, and the lingual hyaline cartilage displayed a bluish coloration of the ground substance, along with a characteristic isogenous group of chondrocytes. Our research represents the first comprehensive application of immunofluorescence techniques to investigate the cellular intricacies of the red-eared slider's tongue by employing seven distinct antibodies, revealing a wide array of compelling and significant findings. Vimentin revealed the presence of taste bud cells, while synaptophysin provided insights into taste bud and nerve bundle characteristics. CD34 and PDGFRα illuminated lingual stromal cells, and SOX9 and PDGFRα shed light on chondrocytes within the tongue's cartilage. CD20 mapped B-cell lymphocyte distribution in the lingual tonsil, while alpha smooth actin (α-SMA) exposed the intricate myofibroblast and smooth muscle network surrounding the lingual blood vessels and salivary glands. In conclusion, our comprehensive study advances our knowledge of the red-eared slider's tongue anatomy and physiology, addressing a significant research gap. These findings not only contribute to the field of turtle biology but also deepen our appreciation for the species' remarkable adaptations in their specific ecological niches.

Histo-morphological Characterization of the Tongue and Oropharyngeal Cavity of the Shining Sunbird (Cinnyris habessinicus)

Sunbirds, as specialized nectarivores, have developed multiple lingual and oropharyngeal peculiarities imposed by this dietary specialization that particularly extract floral nectar. We have described the functional morphology of the tongues and palates of the shining sunbird, Cinnyris habessinicus, using gross anatomical, histological, and scanning electron microscopic methods. The tongue was bifurcated with fringed lamella and extended posteriorly, forming a broad trough at the lingual body and terminating in two fleshy, alae linguae. The lingual apex and body are nonpapillate and nonglandular, and its root had a muscular pad followed by a conspicuous laryngeal mound bordered by three prominent rows of conical papillae. The lingual root had clusters of mucoid glands with rich acidic mucins, and the laryngeal region had complex papillary distribution at the back margins. Both the lingual body and root had well-developed skeletal elements, musculature, and connective tissues. Furthermore, the palate was membranous and made up of four main ridges with a central choanal slit guarded by choanal papillae. Overall, the presented results showed structural and anatomical features that are the results of the nectarivory dietary niche.

The avian taste system

Taste or gustation is the sense evolving from the chemo-sensory system present in the oral cavity of avian species, which evolved to evaluate the nutritional value of foods by detecting relevant compounds including amino acids and peptides, carbohydrates, lipids, calcium, salts, and toxic or anti-nutritional compounds. In birds compared to mammals, due to the relatively low retention time of food in the oral cavity, the lack of taste papillae in the tongue, and an extremely limited secretion of saliva, the relevance of the avian taste system has been historically undermined. However, in recent years, novel data has emerged, facilitated partially by the advent of the genomic era, evidencing that the taste system is as crucial to avian species as is to mammals. Despite many similarities, there are also fundamental differences between avian and mammalian taste systems in terms of anatomy, distribution of taste buds, and the nature and molecular structure of taste receptors. Generally, birds have smaller oral cavities and a lower number of taste buds compared to mammals, and their distribution in the oral cavity appears to follow the swallowing pattern of foods. In addition, differences between bird species in the size, structure and distribution of taste buds seem to be associated with diet type and other ecological adaptations. Birds also seem to have a smaller repertoire of bitter taste receptors (T2Rs) and lack some taste receptors such as the T1R2 involved in sweet taste perception. This has opened new areas of research focusing on taste perception mechanisms independent of GPCR taste receptors and the discovery of evolutionary shifts in the molecular function of taste receptors adapting to ecological niches in birds. For example, recent discoveries have shown that the amino acid taste receptor dimer T1R1-T1R3 have mutated to sense simple sugars in almost half of the living bird species, or SGLT1 has been proposed as a part of a T1R2-independent sweet taste sensing in chicken. The aim of this review is to present the scientific data known to date related to the avian taste system across species and its impact on dietary choices including domestic and wild species.

Oral expressions and functional analyses of the extracellular calcium-sensing receptor (CaSR) in chicken

In vertebrates, the extracellular calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis by sensing slight changes in extracellular Ca²⁺. CaSR is also expressed in mammals including rodent taste cells and is involved in sensing kokumi, a rich, savory quality that enhances the intensities of salty, sweet, and umami tastes. In this study, we focused on chicken CaSR (cCaSR) since calcium is an essential nutrient that is necessary for making eggshell and for the extremely rapid initial growth of bones. First we confirmed that cCaSR is expressed in taste cells. Next we cloned the cCaSR gene from kidney and transiently transfected human embryonic kidney 293 T (HEK293T) cells with the recombinant cCaSR, or empty vector and looked for the agonists and allosteric modulators (including kokumi substances) of cCaSR by Ca²⁺ imaging. We found that cCaSR was activated by extracellular Ca²⁺ and Mg²⁺ in a dose dependent manner. Several L-amino acids and kokumi substances such as glutathione enhanced the response of cCaSR. In addition, NPS2143 as a negative allosteric modulator of human CaSR negatively modulated the response of cCaSR. These results suggest that cCaSR can sense extracellular Ca²⁺ and Mg²⁺ as well as positive and negative allosteric modulators. Taken together, the results imply that CaSR might be a multifunctional receptor for calcium, amino acids, and kokumi substances in chicken. The present finding that functional CaSR is expressed in the chicken oral tissues will allow us to further elucidate the physiological role of CaSR in the chickens' taste sense, and to create new feeds that will contribute to the poultry industry.

Bitter Taste Perception in Chickens

Many behavioral studies and histological analyses of the sense of taste have been conducted in chickens, as it plays an important role in the ingestion of feed. In recent years, various taste receptors have been analyzed, and the functions of fatty acids, umami, and bitter taste receptors in chickens have become clear. In this review, the bitter taste sense in chickens, which is the taste quality by which animals reject poisons, is discussed among a variety of taste qualities. Chickens have taste buds in the palate, the base of the oral cavity, and the root of the tongue. Bitter taste receptors, taste receptor type 2 members 1, 2, and 7 (T2R1, T2R2, and T2R7) are expressed in these tissues. According to functional analyses of bitter taste receptors and behavioral studies, T2R1 and T2R7 are thought to be especially involved in the rejection of bitter compounds in chickens. Furthermore, the antagonists of these two functional bitter taste receptors were also identified, and it is expected that such antagonists will be useful in improving the taste quality of feed materials and poultry drugs that have a bitter taste. Bitter taste receptors are also expressed in extra-oral tissues, and it has been suggested that gastrointestinal bitter taste receptors may be involved in the secretion of gastrointestinal hormones and pathogen defense mechanisms. Thus, bitter taste receptors in chickens are suspected to play major roles in taste sensing and other physiological systems.

A combination of garlic oil and cooked chilli oil could be effective and efficient for pigeon production

This study was conducted to investigate whether the combination of garlic oil and cooked chilli oil is worth using for pigeon production in the context of a total ban on antibiotics in feed additives in China. Two hundred female white king pigeons aged 23 days were randomly divided into five groups with ten replicates (four birds each). In the 47 days trial, the control group was fed with a basal diet, treatment groups were given a basal diet supplemented with 20 mg/kg neomycin sulphate or 2 g/kg corresponding oil (garlic oil or cooked chilli oil or their mixture) respectively. The mixed oil showed obvious antibacterial activity against gram-positive bacterium and its minimal bactericidal concentration against St. aureus, Salmonella and Escherichia coli were all no more than 1.0 mg/ml. In the feeding experiment, pigeons feed with garlic oil with strong bacteriostatic activity had lower FCR and better protein metabolism, and chilli oil showed strong effects of promoting feed intake and weight gain on pigeons but increased serum glucose and lipid content. Compared with the control and the antibiotic group, the mixed oil got increased growth performance, less drip loss of meat, better protein metabolism promoting, and more complete intestinal structure of pigeon. In addition, the breast meat in the mixed oil group had higher total points in the sensory test. The mixed oil combined the strong bacteriostasis of garlic oil with the feeding promotion effect of cooked chilli oil, it improved the comprehensive performance of pigeons and had the feasibility to be popularized as a non-antibiotic strategy in pigeon production.

Overlapping distributions of mammalian types I, II, and III taste cell markers in chicken taste buds

Mammalian taste buds comprise types I, II, and III taste cells, with each type having specific characteristics: glia-like supporting cells (type I), taste receptor cells (type II), and presynaptic cells (type III). In this study, to characterize the peripheral taste-sensing systems in chickens, we analyzed the distributions of the mammalian types I, II, and III taste cell markers in chicken taste buds: glutamate-aspartate transporter (GLAST) for type I; taste receptor type 1 members 1 and 3 (T1R1 and T1R3), taste receptor type 2 member 7 (T2R7), and α-gustducin for type II; and synaptosomal protein 25 (SNAP25) and neural cell adhesion molecule (NCAM) for type III. We found that most GLAST⁺ taste cells expressed α-gustducin and SNAP25 and that high percentages of T1R3⁺ or α-gustducin⁺ taste cells expressed SNAP25 and NCAM. These results demonstrated a unique subset of chicken taste cells expressing multiple taste cell type marker proteins. Taken together, these results provide new insights into the taste-sensing mechanisms in vertebrate taste buds.

Biological Aspects of the Tongue and Oropharyngeal Cavity of the Eurasian Collared Dove (Streptopelia decaocto, Columbiformes, Columbidae): Anatomical, Histochemical, and Ultrastructure Study

We characterized the morphological and anatomical adaptations of the lingual microstructures of the Eurasian collared dove and discussed their implications for its dietary niche. We analyzed tongues of nine S. decaocto using histological, histochemical, stereomicroscopic, and scanning electron microscopic techniques. Our findings showed that the tongue is relatively short with a tapered apex that carries a terminal lingual nail. However, the lingual body has median scales and is bordered laterally by filiform papillae. Further, the tongue body bears a distinctive papillary crest. The tongue root is nonpapillate and infiltered with orifices of the posterior salivary glands. The bulky laryngeal mound has a circular glottic fissure, carrying a single row of papillae at the rear edge. Concurrently, our histological and histochemical findings demonstrate that the tongue has taste buds, anterior and posterior salivary glands, along with an elongated entoglossum that extends from lingual apex to root. Besides, ovoid and globular mucous glands displayed intense alcianophilic reactions. More substantially, the palate is made up of three palatine ridges with a caudal choanal cleft that was bounded by two rows of palatine papillae. Our data indicate multiple and novel structural variations for the lingual and palatal sculptures coopted for their feeding style.

The umami receptor T1R1-T1R3 heterodimer is rarely formed in chickens

The characterization of molecular mechanisms underlying the taste-sensing system of chickens will add to our understanding of their feeding behaviors in poultry farming. In the mammalian taste system, the heterodimer of taste receptor type 1 members 1/3 (T1R1/T1R3) functions as an umami (amino acid) taste receptor. Here, we analyzed the expression patterns of T1R1 and T1R3 in the taste cells of chickens, labeled by the molecular markers for chicken taste buds (vimentin and α-gustducin). We observed that α-gustducin was expressed in some of the chicken T1R3-positive taste bud cells but rarely expressed in the T1R1-positive and T2R7-positive taste bud cells. These results raise the possibility that there is another second messenger signaling system in chicken taste sensory cells. We also observed that T1R3 and α-gustducin were expressed mostly in the vimentin-positive taste bud cells, whereas T1R1 and bitter taste receptor (i.e., taste receptor type 2 member 7, T2R7) were expressed largely in the vimentin-negative taste bud cells in chickens. In addition, we observed that T1R1 and T1R3 were co-expressed in about 5% of chickens' taste bud cells, which express T1R1 or T1R3. These results suggest that the heterodimer of T1R1 and T1R3 is rarely formed in chickens' taste bud cells, and they provide comparative insights into the expressional regulation of taste receptors in the taste bud cells of vertebrates.

Evolvement of taste sensitivity and taste buds in chickens during selective breeding

Chickens have been reported to have a low taste bud count and thus low taste acuity. However, more recent studies indicate that the earlier reported count of chicken taste buds may have been significantly underestimated. To answer the question of whether the taste sensing system in broiler chickens evolved during the breeding selection over the past decades, we compared the taste sensitivity to bitter and taste buds between a meat-type control strain – the 1955 Athens Canadian Random Bred (ACRB), and a modern high-yielding broiler strain – the 2012 Cobb 500. The behavioral tests showed that the ACRB did not avoid bitter taste solutions of quinine hydrochloride (QHCl) at the examined concentrations (0.5, 1, 2, and 4 mM) (P > 0.05), while the Cobb 500 significantly avoided both the 2 mM and 4 mM QHCl solutions (P < 0.01). The labeling of chicken taste buds using the molecular marker Vimentin revealed that Cobb 500 chickens had a slightly higher number (P < 0.1), but lower density of taste bud clusters in the palate (P < 0.01) and the base of the oral cavity (P < 0.05) compared to the ACRB. We also found that a single amino acid change occurred in the bitter taste receptor T2R7. However, the functional analyses using HEK293T cells transiently expressing T2R7 revealed that the functions of T2R7 were comparable between the two strains. Taken together, our results demonstrated that taste sensitivities could be affected by the selection of the broiler chickens. The modern high-yielding broilers, which have massive feed intake and appetite, had a higher sensitivity to bitter taste stimuli than the meat-type chicken strain which was established decades ago. This evolvement of taste sensitivities may be associated with the alterations of an upper level of taste system, rather than the peripheral taste system, including distribution of taste buds and functions of taste receptors.

Bitter taste receptor T2R7 and umami taste receptor subunit T1R1 are expressed highly in Vimentin-negative taste bud cells in chickens

In the mammalian taste system, the taste receptor type 2 (T2R) family mediates bitter taste, and the taste receptor type 1 (T1R) family mediates sweet and umami tastes (the heterodimer of T1R2/T1R3 forms the sweet taste receptor, and the heterodimer of T1R1/T1R3 forms the umami taste receptor). In the chicken genome, bitter (T2R1, T2R2, and T2R7) and umami (T1R1 and T1R3) taste receptor genes have been found. However, the localization of these taste receptors in the taste buds of chickens has not been elucidated. In the present study, we demonstrated that the bitter taste receptor T2R7 and the umami taste receptor subunit T1R1 were expressed specifically in the taste buds of chickens labeled by Vimentin, a molecular marker for chicken taste buds. We analyzed the distributions of T2R7 and T1R1 on the oral epithelial sheets of chickens and among 3 different oral tissues of chickens: the palate, the base of the oral cavity, and the posterior tongue. We found that the distribution patterns and numbers were similar between taste bud clusters expressing these receptors and those expressing Vimentin. These results indicated broad distributions of T2R7 and T1R1 in the gustatory tissues of the chicken oral cavity. In addition, 3D-reconstructed images clearly revealed that high levels of T2R7 and T1R1 were expressed in Vimentin-negative taste bud cells. Taken together, the present results indicated the presence of bitter and umami sensing systems in the taste buds of chickens, and broad distribution of T2R7 and T1R1 in the chicken oral cavity.

Abundant proliferating cells within early chicken taste buds indicate a potentially "built-in" progenitor system for taste bud growth during maturation in hatchlings

Like other epithelial cells, taste bud cells have a short life span and undergo continuous turnover. An active stem or progenitor cell niche is essential for taste bud formation and maintenance. Early taste bud cells have a life span of ~4 days on average in chicken hatchlings when taste buds grow rapidly and undergo maturation. The average life span is shorter than that of mature taste bud cells of rodents (~10-12 days on average). To better understand the mechanism underlying taste bud growth and homeostasis in chickens, we analyzed the distribution of proliferating cells in different tissue compartments, including taste buds, the surrounding epithelium and the underlying connective tissue in P1-3 hatchlings and P45 chickens. Unlike rodents, which lack proliferating cells within both early and mature taste buds, chickens possessed abundant proliferating cells within early taste buds. Further, at post-hatch day 45, when taste buds are mature and undergo continuous cell renewal, taste buds also contained proliferating cells, though to a lesser extent. These proliferating cells in early taste buds, indicated by PCNA⁺ and BrdU⁺ cells, primarily localized to the basal region of taste buds and were largely unlabeled by the two known molecular markers for chicken taste bud cells (Vimentin and α-Gustducin), suggesting their undifferentiated status. Our data indicate that early chicken taste buds have "built-in" progenitors in order to grow to and maintain their large size and rapid cell turnover in hatchlings.

Nutrient sensing, taste and feed intake in avian species

The anatomical structure and function of beaks, bills and tongue together with the mechanics of deglutition in birds have contributed to the development of a taste system denuded of macrostructures visible to the human naked eye. Studies in chickens and other birds have revealed that the avian taste system consists of taste buds not clustered in papillae and located mainly (60 %) in the upper palate hidden in the crevasses of the salivary ducts. That explains the long delay in the understanding of the avian taste system. However, recent studies reported 767 taste buds in the oral cavity of the chicken. Chickens appear to have an acute sense of taste allowing for the discrimination of dietary amino acids, fatty acids, sugars, quinine, Ca and salt among others. However, chickens and other birds have small repertoires of bitter taste receptors (T2R) and are missing the T1R2 (related to sweet taste in mammals). Thus, T1R2-independent mechanisms of glucose sensing might be particularly relevant in chickens. The chicken umami receptor (T1R1/T1R3) responds to amino acids such as alanine and serine (known to stimulate the umami receptor in rodents and fish). Recently, the avian nutrient chemosensory system has been found in the gastrointestinal tract and hypothalamus related to the enteroendocrine system which mediates the gut-brain dialogue relevant to the control of feed intake. Overall, the understanding of the avian taste system provides novel and robust tools to improve avian nutrition.

An Update on the Sense of Taste in Chickens: A Better Developed System than Previously Appreciated

Taste is important in guiding nutritive choices and motivating food intake. The sensory organs for taste are the taste buds, that transduce gustatory stimuli into neural signals. It has been reported that chickens have a low taste bud number and thus low taste acuity. However, more recent studies indicate that chickens have a well-developed taste system and the reported number and distribution of taste buds may have been significantly underestimated. Chickens, as a well-established animal model for research, are also the major species of animals in the poultry industry. Thus, a clear understanding of taste organ formation and the effects of taste sensation on nutrition and feeding practices is important for improving livestock production strategies. In this review, we provide an update on recent findings in chicken taste buds and taste sensation indicating that the chicken taste organ is better developed than previously thought and can serve as an ideal system for multidisciplinary studies including organogenesis, regenerative medicine, feeding and nutritional choices.

Short-term perception of and conditioned taste aversion to umami taste, and oral expression patterns of umami taste receptors in chickens

Umami taste is one of the five basic tastes (sweet, umami, bitter, sour, and salty), and is elicited by l-glutamate salts and 5'-ribonucleotides. In chickens, the elucidation of the umami taste sense is an important step in the production of new feedstuff for the animal industry. Although previous studies found that chickens show a preference for umami compounds in long-term behavioral tests, there are limitations to our understanding of the role of the umami taste sense in chicken oral tissues because the long-term tests partly reflected post-ingestive effects. Here, we performed a short-term test and observed agonists of chicken umami taste receptor, l-alanine and l-serine, affected the solution intakes of chickens. Using this method, we found that chickens could respond to umami solutions containing monosodium l-glutamate (MSG) + inosine 5'-monophosphate (IMP) within 5 min. We also demonstrated that chickens were successfully conditioned to avoid umami solution by the conditioned taste aversion test. It is noted that conditioning to umami solution was generalized to salty and sweet solutions. Thus, chickens may perceive umami taste as a salty- and sweet-like taste. In addition, we found that umami taste receptor candidates were differentially expressed in different regions of the chicken oral tissues. Taken together, the present results strongly suggest that chickens have a sense of umami taste and have umami taste receptors in their oral tissue.

Bitter Taste Sensitivity and the Expression of Bitter Taste Receptors at Different Growth Stages of Chicks

Bitterness is one of the five basic tastes, and sensitivity to bitterness is important in that it enables animals to avoid harmful and toxic substances. In humans, taste sensitivity decreases with age, although the extent of loss varies depending on the taste quality. In chickens (Gallus gallus domesticus), baby chicks have been found to be more sensitive to salt and sour taste qualities than adults. In this study, therefore, we investigated the growth-associated changes in bitter taste sensitivity in chicks. We examined the behavioral perceptions toward the bitter compounds chloramphenicol and andrographolide in chicks at three different growth stages. Then, we measured the relative expression of the functional bitter taste receptors in the chick palate. In behavioral drinking tests, the 0–1-week-old chicks consumed a significantly lower amount of bitter solutions than water, whereas the 8–9-week-old chicks showed lower avoidance of the bitter solutions than the 0–1-week-old and 4–5-week-old chicks. Real-time PCR assay showed that the 0–1-week-old chicks had significantly higher expression of one of the functional bitter taste receptors in the palate than that in the older chicks. These results suggest that baby chicks are more sensitive to bitterness than older chicks. These findings may be useful in the production of new feedstuff for chicks according to their growth stages.

Oral lipase activities and fat-taste receptors for fat-taste sensing in chickens

It has been reported that a functional fat-taste receptor, GPR120, is present in chicken oral tissues, and that chickens can detect fat taste in a behavioral test. However, although triglycerides need to be digested to free fatty acids to be recognized by fat-taste receptors such as GPR120, it remains unknown whether lipase activities exist in chicken oral tissues. To examine this question, we first cloned another fat-taste receptor candidate gene, CD36, from the chicken palate. Then, using RT-PCR, we determined that GPR120 and CD36 were broadly expressed in chicken oral and gastrointestinal tissues. Also by RT-PCR, we confirmed that several lipase genes were expressed in both oral and gastrointestinal tissues. Finally, we analyzed the lipase activities of oral tissues by using a fluorogenic triglyceride analog as a lipase substrate. We found there are functional lipases in oral tissues as well as in the stomach and pancreas. These results suggested that chickens have a basic fat-taste reception system that incorporates a triglycerides/oral-lipases/free fatty acids/GPR120 axis and CD36 axis.

RNA-Seq analysis on chicken taste sensory organs: An ideal system to study organogenesis

RNA-Seq is a powerful tool in transcriptomic profiling of cells and tissues. We recently identified many more taste buds than previously appreciated in chickens using molecular markers to stain oral epithelial sheets of the palate, base of oral cavity, and posterior tongue. In this study, RNA-Seq was performed to understand the transcriptomic architecture of chicken gustatory tissues. Interestingly, taste sensation related genes and many more differentially expressed genes (DEGs) were found between the epithelium and mesenchyme in the base of oral cavity as compared to the palate and posterior tongue. Further RNA-Seq using specifically defined tissues of the base of oral cavity demonstrated that DEGs between gustatory (GE) and non-gustatory epithelium (NGE), and between GE and the underlying mesenchyme (GM) were enriched in multiple GO terms and KEGG pathways, including many biological processes. Well-known genes for taste sensation were highly expressed in the GE. Moreover, genes of signaling components important in organogenesis (Wnt, TGFβ/ BMP, FGF, Notch, SHH, Erbb) were differentially expressed between GE and GM. Combined with other features of chicken taste buds, e.g., uniquely patterned array and short turnover cycle, our data suggest that chicken gustatory tissue provides an ideal system for multidisciplinary studies, including organogenesis and regenerative medicine.

From Cell to Beak: In-Vitro and In-Vivo Characterization of Chicken Bitter Taste Thresholds

Bitter taste elicits an aversive reaction, and is believed to protect against consuming poisons. Bitter molecules are detected by the Tas2r family of G-protein-coupled receptors, with a species-dependent number of subtypes. Chickens demonstrate bitter taste sensitivity despite having only three bitter taste receptors—ggTas2r1, ggTas2r2 and ggTas2r7. This minimalistic bitter taste system in chickens was used to determine relationships between in-vitro (measured in heterologous systems) and in-vivo (behavioral) detection thresholds. ggTas2r-selective ligands, nicotine (ggTas2r1), caffeine (ggTas2r2), erythromycin and (+)-catechin (ggTas2r7), and the Tas2r-promiscuous ligand quinine (all three ggTas2rs) were studied. Ligands of the same receptor had different in-vivo:in-vitro ratios, and the ggTas2r-promiscuous ligand did not exhibit lower in-vivo:in-vitro ratios than ggTas2r-selective ligands. In-vivo thresholds were similar or up to two orders of magnitude higher than the in-vitro ones.

Identification of functional bitter taste receptors and their antagonist in chickens

Elucidation of the taste sense of chickens is important not only for the development of chicken feedstuffs for the chicken industry but also to help clarify the evolution of the taste sense among animals. There are three putative chicken bitter taste receptors, chicken T2R1 (cT2R1), cT2R2 and cT2R7, which were identified using genome information and cell-based assays. Previously, we have shown that cT2R1 is a functional bitter taste receptor through both cell-based assays and behavioral tests. In this study, therefore, we focused on the sensitivities of the other two bitter receptors, cT2R2 and cT2R7, by using their agonists in behavioral tests. We tested three agonists of cT2R2 and three agonists of cT2R7. In a 10-min drinking study, the intakes of cT2R2 agonist solutions were not different from that of water. On the other hand, the intakes of cT2R7 agonist solutions were significantly lower compared to water. In addition, we constructed cT2R1-and cT2R7-expressing cells in order to search for an antagonist for these functional bitter taste receptors. By using Ca²⁺ imaging methods, we found that 6-methoxyflavanone (6-meth) can inhibit the activities of both cT2R1 and cT2R7. Moreover, 6-meth also inhibited the reduction of the intake of bitter solutions containing cT2R1 or cT2R7 agonists in behavioral tests. Taken together, these results suggested that cT2R7 is a functional bitter taste receptor like cT2R1, but that cT2R2 is not, and that 6-meth is an antagonist for these two functional chicken bitter taste receptors. This is the first identification of an antagonist of chicken bitter receptors.