Gene expression profiling of α-gustducin-expressing taste cells in mouse fungiform and circumvallate papillae

Effects of chronic unpredictable mild stress-induced depression on bitter taste receptor expression in mice

OBJECTIVE

With the rapid increase in the pace of life, people are facing increasing pressures of all kinds, and depression has gradually become a serious psychological disorder in human society, strongly affecting normal social and physiological activities. Depression can disrupt an individual's taste perception and potentially result in taste disorders by affecting and altering taste receptors. This disruption can consequently impact their food preferences and overall eating experiences.

DESIGN

In this study, we used the chronic unpredictable mild stress (CUMS) method to establish a depression model in male C57BL/6 J mice and explored the changes in taste receptor expression in the lingual circumvallate papillae (CP) to elucidate the effects of depression on taste. After 6 weeks of CUMS, behavioral performance evaluations, such as forced swim, open field, and elevated plus maze tests, were conducted in depression model mice. A further two-bottle choice test was subsequently performed to determine the effect of depression on bitter taste, and the expression of bitter taste receptors in the lingual CP was detected via immunofluorescence staining.

RESULTS

In this study, we found for the first time that mice with CUMS-induced depression had decreased bitter taste sensitivity through a two-bottle choice test and demonstrated that the expression of T2r5, a receptor related to bitter taste perception, and the expression of secondary taste signaling proteins in the lingual CP were significantly decreased in mice exposed to CUMS, as determined via qRTPCR and immunofluorescence staining.

CONCLUSIONS

Our study highlights how CUMS influences the perception of bitterness in the peripheral taste system, potentially elucidating stress-induced changes in eating habits.

Taste and its receptors in human physiology: A comprehensive look

Increasing evidence shows that food has significance beyond traditional perception (providing nutrition and energy) in maintaining normal life activities. It is indicated that the sense of taste plays a crucial part in regulating human life activities. Taste is one of the basic physiological sensations in mammals, and it is the fundamental guarantee for them to perceive, select, and ingest nutrients in order to survive. With the advances in electrophysiology, molecular biology, and structural biology, studies on the intracellular and extracellular transduction mechanisms of taste have made great progress and gradually revealed the indispensable role of taste receptors in the regulation and maintenance of normal physiological activities. Up to now, how food regulates life activities through the taste pathway remains unclear. Thus, this review comprehensively and systematically summarizes the current study about the sense of taste, the function of taste receptors, the taste–structure relationship of gustatory molecules, the cross‐talking between distinctive tastes, and the role of the gut–organ axis in the realization of taste. Moreover, we also provide forward‐looking perspectives on taste research to afford a scientific basis for revealing the scientific connotation of taste receptors regulating body health.

The G protein-coupled receptor GPRC5C is a saccharide sensor with a novel 'off' response

GPRC5C is an orphan G protein-coupled receptor (GPCR) that belongs to the class C GPCR family. Although GPRC5C is expressed in various organs, its function and ligand are still undetermined. We found that GPRC5C is expressed in mouse taste cells, enterocytes, and pancreatic α-cells. In functional imaging assays, HEK293 cells heterologously expressing GPRC5C and the chimeric G protein α subunit Gα16-gust44 showed robust intracellular Ca2+ increases in response to monosaccharides, disaccharides, and a sugar alcohol, but not an artificial sweetener or sweet-tasting amino acid. Notably, Ca2+ increases occurred after washout, not during stimulation. Our findings suggest that GPRC5C has receptor properties which lead to novel 'off' responses to saccharide detachment and may work as an internal or external chemosensor specifically tuned to natural sugars.

Anterior and Posterior Tongue Regions and Taste Papillae: Distinct Roles and Regulatory Mechanisms with an Emphasis on Hedgehog Signaling and Antagonism

Sensory receptors across the entire tongue are engaged during eating. However, the tongue has distinctive regions with taste (fungiform and circumvallate) and non-taste (filiform) organs that are composed of specialized epithelia, connective tissues, and innervation. The tissue regions and papillae are adapted in form and function for taste and somatosensation associated with eating. It follows that homeostasis and regeneration of distinctive papillae and taste buds with particular functional roles require tailored molecular pathways. Nonetheless, in the chemosensory field, generalizations are often made between mechanisms that regulate anterior tongue fungiform and posterior circumvallate taste papillae, without a clear distinction that highlights the singular taste cell types and receptors in the papillae. We compare and contrast signaling regulation in the tongue and emphasize the Hedgehog pathway and antagonists as prime examples of signaling differences in anterior and posterior taste and non-taste papillae. Only with more attention to the roles and regulatory signals for different taste cells in distinct tongue regions can optimal treatments for taste dysfunctions be designed. In summary, if tissues are studied from one tongue region only, with associated specialized gustatory and non-gustatory organs, an incomplete and potentially misleading picture will emerge of how lingual sensory systems are involved in eating and altered in disease.

Regional specialization of the tongue revealed by gustatory ganglion imaging

Gustatory information is relayed from the anterior tongue by geniculate ganglion neurons and from the posterior tongue by neurons of the petrosal portion of the jugular/nodose/petrosal ganglion complex. Here, we use in vivo calcium imaging in mice to compare the encoding of taste information in the geniculate and petrosal ganglia, at single-neuron resolution. Our data support an anterior/posterior specialization of taste information coding from the tongue to the ganglia, with petrosal neurons more responsive to umami or bitter and less responsive to sweet or salty stimuli than geniculate neurons. We found that umami (50 mM MPG + 1 mM IMP) promotes salivation when applied to the posterior, but not anterior, tongue. This suggests a functional taste map of the mammalian tongue where the anterior and posterior taste pathways are differentially responsive to specific taste qualities, and differentially regulate downstream physiological functions of taste, such as promoting salivation.

Encoding Taste: From Receptors to Perception

Taste information is encoded in the gustatory nervous system much as in other sensory systems, with notable exceptions. The concept of adequate stimulus is common to all sensory modalities, from somatosensory to auditory, visual, and so forth. That is, sensory cells normally respond only to one particular form of stimulation, the adequate stimulus, such as photons (photoreceptors in the visual system), odors (olfactory sensory neurons in the olfactory system), noxious heat (nociceptors in the somatosensory system), etc. Peripheral sensory receptors transduce the stimulus into membrane potential changes transmitted to the brain in the form of trains of action potentials. How information concerning different aspects of the stimulus such as quality, intensity, and duration are encoded in the trains of action potentials is hotly debated in the field of taste. At one extreme is the notion of labeled line/spatial coding - information for each different taste quality (sweet, salty, sour, etc.) is transmitted along a parallel but separate series of neurons (a "line") that project to focal clusters ("spaces") of neurons in the gustatory cortex. These clusters are distinct for each taste quality. Opposing this are concepts of population/combinatorial coding and temporal coding, where taste information is encrypted by groups of neurons (circuits) and patterns of impulses within these neuronal circuits. Key to population/combinatorial and temporal coding is that impulse activity in an individual neuron does not provide unambiguous information about the taste stimulus. Only populations of neurons and their impulse firing pattern yield that information.