Roles of hormones in taste signaling

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.

Physiology of the tongue with emphasis on taste transduction

The tongue is a complex multifunctional organ that interacts and senses both interoceptively and exteroceptively. Although it is easily visible to almost all of us, it is relatively understudied and what is in the literature is often contradictory or is not comprehensively reported. The tongue is both a motor and a sensory organ: motor in that it is required for speech and mastication, and sensory in that it receives information to be relayed to the central nervous system pertaining to the safety and quality of the contents of the oral cavity. Additionally, the tongue and its taste apparatus form part of an innate immune surveillance system. For example, loss or alteration in taste perception can be an early indication of infection as became evident during the present global SARS-CoV-2 pandemic. Here, we particularly emphasize the latest updates in the mechanisms of taste perception, taste bud formation and adult taste bud renewal, and the presence and effects of hormones on taste perception, review the understudied lingual immune system with specific reference to SARS-CoV-2, discuss nascent work on tongue microbiome, as well as address the effect of systemic disease on tongue structure and function, especially in relation to taste.

Decreased taste sensitivity to sucrose in dopamine D3 receptor mutant mice

Dopamine plays a key role in food rewards and sweet-taste stimulation. We examined the basis for behavioral responses to sweet taste in dopamine D3 receptor-deficient (D3-/-) mice by determining whether the absence of D3 receptors affects the sensitivity to dilute sucrose solutions. In experiment 1, we measured the intensity generalization threshold of conditioned taste aversion (CTA) to a 0.2 M sucrose solution. Results showed that the generalization thresholds were 0.025-0.05 M in D3-/- mice and 0.0025-0.005 M in wild-type (WT) mice. In experiment 2, we found that D3-/- and WT mice had similar capabilities to form and extinguish CTAs. Since the intensity generalization threshold is mainly due to a combination of sweet-taste sensitivity and the robust nature of CTA formation, the results showed that taste sensitivity to sucrose in D3-/- mice was lower than that in WT mice. In experiment 3, to test whether the peripheral sensory signaling may also be affected by the disruption of the dopamine D3 receptors, the mRNA expression levels of sweet-taste-related proteins in taste buds of D3-/- mice were determined. The T1R1 and BDNF mRNA expression levels in D3-/- mice were higher than the controls, whereas T1R2, T1R3, α-gustducin, and TRPM5 mRNA were similar. These findings suggest that disruption of dopamine D3 receptor-mediated signaling decreases the sweet-taste sensitivity and alters the mRNA expression levels of some taste-related molecules.

Glucagon-like peptide-1, a matter of taste?

Understanding of gustatory coding helps to predict, and perhaps even modulate the ingestive decision circuitry, especially when eating behaviour becomes dysfunctional. Preclinical research demonstrated that glucagon like peptide 1 (GLP-1) is locally synthesized in taste bud cells in the tongue and that GLP-1 receptor exists on the gustatory nerves in close proximity to GLP-1 containing taste bud cells. In humans, the tongue has not yet been addressed as clinically relevant target for GLP-1 based therapies. The primary aim of the current review was to elaborate on the role of GLP- 1 in mammalian gustatory system, in particular in the perception of sweet. Secondly, we aimed to explore what modulates gustatory coding and whether the GLP-1 based therapies might be involved in regulation of taste perception. We performed a series of PubMed, Medline and Embase databases systemic searches. The Population-Intervention-Comparison-Outcome (PICO) framework was used to identify interventional studies. Based on the available data, GLP-1 is specifically involved in the perception of sweet. Aging, diabetes and obesity are characterized by diminished taste and sweet perception. Calorie restriction and bariatric surgery are associated with a diminished appreciation of sweet food. GLP-1 receptor agonists (RAs) modulate food preference, yet its modulatory potential in gustatory coding is currently unknown. Future studies should explore whether GLP-1 RAs modulate taste perception to the extent that changes of food preference and consumption ensue.

Sex differences in fat taste responsiveness are modulated by estradiol

Sex as a biological variable has been the focus of increasing interest. Relatively few studies have focused, however, on differences in peripheral taste function between males and females. Nonetheless, there are reports of sex-dependent differences in chemosensitivity in the gustatory system. The involvement of endogenous changes in ovarian hormones has been suggested to account for taste discrepancies. Additionally, whether sex differences exist in taste receptor expression, activation, and subsequent signaling pathways that may contribute to different taste responsiveness is not well understood. In this study, we show the presence of both the nuclear and plasma membrane forms of estrogen receptor (ER) mRNA and protein in mouse taste cells. Furthermore, we provide evidence that estrogen increases taste cell activation during the application of fatty acids, the chemical cue for fat taste, in taste receptor cells. We found that genes important for the transduction pathway of fatty acids vary between males and females and that these differences also exist across the various taste papillae. In vivo support for the effect of estrogens in taste cells was provided by comparing the fatty acid responsiveness in male, intact female, and ovariectomized (OVX) female mice with and without hormone replacement. In general, females detected fatty acids at lower concentrations, and the presence of circulating estrogens increased this apparent fat taste sensitivity. Taken together, these data indicate that increased circulating estrogens in the taste system may play a significant role in physiology and chemosensory cellular activation and, in turn, may alter taste-driven behavior.NEW & NOTEWORTHY Using molecular, cellular, and behavioral analyses, this study shows that sex differences occur in fat taste in a mouse model. Female mice are more responsive to fatty acids, leading to an overall decrease in intake and fatty acid preference. These differences are linked to sex hormones, as estradiol enhances taste cell responsiveness to fatty acids during periods of low circulating estrogen following ovariectomy and in males. Estradiol is ineffective in altering fatty acid signaling during a high-estrogen period and in ovariectomized mice on hormone replacement. Thus, taste receptor cells are a direct target for actions of estrogen, and there are multiple receptors with differing patterns of expression in taste cells.

Glucagon-Like Peptide 1 and Taste Perception: From Molecular Mechanisms to Potential Clinical Implications

Preclinical studies provided some important insights into the action of glucagon-like peptide 1 (GLP-1) in taste perception. This review examines the literature to uncover some molecular mechanisms and connections between GLP-1 and the gustatory coding. Local GLP-1 production in the taste bud cells, the expression of GLP-1 receptor on the adjacent nerves, a functional continuum in the perception of sweet chemicals from the gut to the tongue and an identification of GLP-1 induced signaling pathways in peripheral and central gustatory coding all strongly suggest that GLP-1 is involved in the taste perception, especially sweet. However, the impact of GLP-1 based therapies on gustatory coding in humans remains largely unaddressed. Based on the molecular background we encourage further exploration of the tongue as a new treatment target for GLP-1 receptor agonists in clinical studies. Given that pharmacological manipulation of gustatory coding may represent a new potential strategy against obesity and diabetes, the topic is of utmost clinical relevance.

Glucagon-like peptide-1, a matter of taste?

Understanding of gustatory coding helps to predict, and perhaps even modulate the ingestive decision circuitry, especially when eating behaviour becomes dysfunctional. Preclinical research demonstrated that glucagon like peptide 1 (GLP-1) is locally synthesized in taste bud cells in the tongue and that GLP-1 receptor exists on the gustatory nerves in close proximity to GLP-1 containing taste bud cells. In humans, the tongue has not yet been addressed as clinically relevant target for GLP-1 based therapies. The primary aim of the current review was to elaborate on the role of GLP- 1 in mammalian gustatory system, in particular in the perception of sweet. Secondly, we aimed to explore what modulates gustatory coding and whether the GLP-1 based therapies might be involved in regulation of taste perception. We performed a series of PubMed, Medline and Embase databases systemic searches. The Population-Intervention-Comparison-Outcome (PICO) framework was used to identify interventional studies. Based on the available data, GLP-1 is specifically involved in the perception of sweet. Aging, diabetes and obesity are characterized by diminished taste and sweet perception. Calorie restriction and bariatric surgery are associated with a diminished appreciation of sweet food. GLP-1 receptor agonists (RAs) modulate food preference, yet its modulatory potential in gustatory coding is currently unknown. Future studies should explore whether GLP-1 RAs modulate taste perception to the extent that changes of food preference and consumption ensue.

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.

Sweet taste receptor signaling network: possible implication for cognitive functioning

Sweet taste receptors are transmembrane protein network specialized in the transmission of information from special "sweet" molecules into the intracellular domain. These receptors can sense the taste of a range of molecules and transmit the information downstream to several acceptors, modulate cell specific functions and metabolism, and mediate cell-to-cell coupling through paracrine mechanism. Recent reports indicate that sweet taste receptors are widely distributed in the body and serves specific function relative to their localization. Due to their pleiotropic signaling properties and multisubstrate ligand affinity, sweet taste receptors are able to cooperatively bind multiple substances and mediate signaling by other receptors. Based on increasing evidence about the role of these receptors in the initiation and control of absorption and metabolism, and the pivotal role of metabolic (glucose) regulation in the central nervous system functioning, we propose a possible implication of sweet taste receptor signaling in modulating cognitive functioning.

Vismodegib, an antagonist of hedgehog signaling, directly alters taste molecular signaling in taste buds

Vismodegib, a highly selective inhibitor of hedgehog (Hh) pathway, is an approved treatment for basal-cell carcinoma. Patients on treatment with vismodegib often report profound alterations in taste sensation. The cellular mechanisms underlying the alterations have not been studied. Sonic Hh (Shh) signaling is required for cell growth and differentiation. In taste buds, Shh is exclusively expressed in type IV taste cells, which are undifferentiated basal cells and the precursors of the three types of taste sensing cells. Thus, we investigated if vismodegib has an inhibitory effect on taste cell turnover because of its known effects on Hh signaling. We gavaged C57BL/6J male mice daily with either vehicle or 30 mg/kg vismodegib for 15 weeks. The gustatory behavior and immunohistochemical profile of taste cells were examined. Vismodegib-treated mice showed decreased growth rate and behavioral responsivity to sweet and bitter stimuli, compared to vehicle-treated mice. We found that vismodegib-treated mice had significant reductions in taste bud size and numbers of taste cells per taste bud. Additionally, vismodegib treatment resulted in decreased numbers of Ki67- and Shh-expressing cells in taste buds. The numbers of phospholipase Cβ2- and α-gustducin-expressing cells, which contain biochemical machinery for sweet and bitter sensing, were reduced in vismodegib-treated mice. Furthermore, vismodegib treatment resulted in reduction in numbers of T1R3, glucagon-like peptide-1, and glucagon-expressing cells, which are known to modulate sweet taste sensitivity. These results suggest that inhibition of Shh signaling by vismodegib treatment directly results in alteration of taste due to local effects in taste buds.

The gut-brain axis in obesity

Currently the only effective treatment for morbid obesity with a proven mortality benefit is surgical intervention. The underlying mechanisms of these surgical techniques are unclear, but alterations in circulating gut hormone levels have been demonstrated to be at least one contributing factor. Gut hormones seem to communicate information from the gastrointestinal tract to the regulatory appetite centres within the central nervous system (CNS) via the so-called 'Gut-Brain-Axis'. Such information may be transferred to the CNS either via vagal or non-vagal afferent nerve signalling or directly via blood circulation. Complex neural networks, distributed throughout the forebrain and brainstem, are in control of feeding and energy homoeostasis. This article aims to review how appetite is potentially regulated by these gastrointestinal hormones. Identification of the underlying mechanisms of appetite and weight control may pave the way to develop better surgical techniques and new therapies in the future.

Neural circuits and motivational processes for hunger

How does an organism's internal state direct its actions? At one moment an animal forages for food with acrobatic feats such as tree climbing and jumping between branches. At another time, it travels along the ground to find water or a mate, exposing itself to predators along the way. These behaviors are costly in terms of energy or physical risk, and the likelihood of performing one set of actions relative to another is strongly modulated by internal state. For example, an animal in energy deficit searches for food and a dehydrated animal looks for water. The crosstalk between physiological state and motivational processes influences behavioral intensity and intent, but the underlying neural circuits are poorly understood. Molecular genetics along with optogenetic and pharmacogenetic tools for perturbing neuron function have enabled cell type-selective dissection of circuits that mediate behavioral responses to physiological state changes. Here, we review recent progress into neural circuit analysis of hunger in the mouse by focusing on a starvation-sensitive neuron population in the hypothalamus that is sufficient to promote voracious eating. We also consider research into the motivational processes that are thought to underlie hunger in order to outline considerations for bridging the gap between homeostatic and motivational neural circuits.

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.

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.