Research Note: Behavioral preference and conditioned taste aversion to oleic acid solution in chickens

Characterization of taste sensitivities to amino acids and sugars by conditioned taste aversion learning in chickens

Taste plays an essential role in regulating the feeding behaviors of animals. The present study aimed to characterize the taste sensory profiles of amino acids and sugars in chickens. To achieve this, we employed a conditioned taste aversion learning method, which is characterized by a specific pairing of gastrointestinal malaise and taste perception. Our findings revealed that chickens were able to learn to avoid L-Val, L-Lys, and L-His through conditioned taste aversion learning, and exhibited a strong aversion to L-Arg. These results suggest that chickens are primarily sensitive to basic amino acids, including L-Lys, which is a crucial limiting amino acid in feeds. Interstingly, this sensitivity to basic amino acids in chickens contrasts with humans, who are mainly sensitive to acidic amino acids as umami taste. Furthermore, despite the absence of a mammalian sweet taste receptor gene in the chicken genome, we demonstrated that chickens learned to avoid glucose, galactose, sucrose, and maltose by conditioned taste aversion learning. Taken together, the present study provides the idea that chickens possess a gustatory perception toward specific amino acids and sugars for the detection of beneficial nutrients in their feeds.

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.

Effect of lithium chloride on food intake, cloacal temperature, voluntary activity, and crop-emptying rate in chicks

Infections frequently accompany with non-specific symptoms such as anorexia and hyperthermia. In addition, there may be unpleasant sensations such as visceral discomfort during infection. Lipopolysaccharide (LPS), a Gram-negative bacteria cell wall component, is known to induce the unpleasant sensation of conditioned taste aversion in mammals. However, the relationship between unpleasant sensations and changes in behavior and physiological conditions has not been investigated extensively in birds. Lithium chloride (LiCl) is a compound that induces unpleasant sensations, including visceral discomfort, although its effects on behavior and physiological conditions have also not been investigated extensively in birds. Thus, the present study was aimed to investigate the effect of an intraperitoneal (IP) injection of LiCl on conditioned visual aversion, food intake, cloacal temperature, voluntary activity, crop-emptying rate, and blood constituents in chicks (Gallus gallus). We also examined the effect of IP injections of LPS and zymosan, a cell wall component of fungus, on conditioned visual aversion formation. First, IP injection of LiCl was confirmed to induce conditioned visual aversion in chicks. An IP injection of LiCl significantly decreased food intake, voluntary activity, and crop-emptying rate but did not affect the temperature. In addition, the injection of LiCl significantly increased plasma corticosterone concentration, indicating that LiCl serves as a stressor in chicks. Finally, IP injections of LPS and zymosan were found to induce conditioned visual aversion in chicks. Collectively, these results suggest that LiCl induces conditioned aversion, anorexia, hypoactivity, and inhibition of crop-emptying in chicks. In addition, LPS and zymosan would induce unpleasant sensations in chicks.

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.

Fatty Acid Taste Receptor GPR120 Activation by Arachidonic Acid, Eicosapentaenoic Acid, and Docosahexaenoic Acid in Chickens

It has been reported that the supplementation of chicken diet with polyunsaturated fatty acids (PUFAs) such as arachidonic acid (AA), eicosapentaenoic acid (EPA), or docosahexaenoic acid (DHA) affects the qualities of eggs and meat. Previous studies have shown that a functional fatty acid taste receptor, G protein-coupled receptor 120 (GPR120), is broadly expressed in chicken oral and gastrointestinal tissues, and chickens have a gustatory perception of oleic acid, which is a chicken GPR120 agonist. The aim of this study was to elucidate the role of chicken GPR120 in response to PUFAs in chicken diets. Ca²⁺ imaging analyses revealed that chicken GPR120 was activated by AA, EPA, and DHA in a concentration-dependent manner. These results suggest that chickens can detect PUFAs via GPR120 in the oral and gastrointestinal tissues, implying that chickens have a gustatory perception of PUFAs.