Glucose elicits cephalic-phase insulin release in mice by activating KATP channels in taste cells

Selective increases in taste sensitivity to glucose as a function of hunger status

Glucose is critical for normal metabolic function in humans. Accordingly, the ability to sense glucose and glucose-containing saccharides is crucial for maintenance of energy homeostasis. Here, we report the evidence that glucose is perceived relatively stronger compared to fructose or sucralose when subjects are hungry. In the initial experiment, we measured the relative sensitivities between glucose and fructose when subjects were fasted vs. fed. Overnight fasted subjects (n = 22) completed a series of 3-AFC tests comparing one target (glucose from a range of concentrations) and two constants (200 mM fructose) before and after consuming mild-tasting breakfast sandwiches until satiated (738 ± 60 kcal). We found that the relative sensitivity to glucose as compared to fructose was significantly higher when individuals were hungry vs. satiated (p < 0.001). We replicated this finding by comparing the same range of glucose concentrations to a constant sucralose concentration (0.04 mM) (N = 19, p < 0.001). Importantly, when we compared a fixed concentration of sucralose (0.4 mM) to a range of fructose concentrations, we saw no difference in iso-intense concentration before and after eating (N = 19, p > 0.05). These findings support the hypothesis that hunger selectively increases taste sensitivity of glucose compared to other sweeteners.

Role of glutamatergic signaling in the acquisition and expression of learned sugar preferences in C57BL/6 mice

C57BL/6 (B6) mice learn to prefer glucose or sucrose to initially isopreferred or even more preferred nonnutritive sweeteners due to the postoral appetite stimulating (appetition) actions of glucose. Recent evidence indicates that specific duodenal neuropod cells transmit the glucose appetition signal to the brain via glutamatergic synaptic connections with vagal afferents. The present study found that intraperitoneal pretreatment with a glutamatergic receptor antagonist cocktail (kynurenic acid (KA)/D-2-amino-3-phosphonopentanoic acid (AP3)) in B6 mice did not block the expression of their learned preference for 8% glucose solution over an initially-preferred 0.1% sucralose + 0.1% saccharin solution. However, acquisition of the glucose preference was blocked by drug treatment during 1-h training sessions with the two sweeteners. Systemic KA/AP3 injections also did not block the expression of the learned preference for a 10.6% sucrose solution over a 0.6% sucralose solution. Drug effects on the acquisition of the sucrose preference were not determined because sucrose, unlike glucose conditioning, required 24-h training trials. The findings that the 1-h training regimen conditioned 8% glucose, but not 10.6% sucrose, preferences suggest that glucose has more potent appetition actions. This was confirmed by the finding that B6 mice learned to prefer 10.6% glucose to 10.6% sucrose after 1-h or 24-h training despite an initial strong sucrose preference. This action can be explained by 10.6% sucrose's digestion in the gut to glucose and fructose with only glucose activating the gut-brain appetition pathway.

Glucose appetition in C57BL/6J mice: Influence of nonnutritive sweetener experience, food deprivation state and sex differences

In addition to its sweet taste, glucose has potent and rapid postoral actions (appetition) that enhance its reward value. This has been demonstrated by the experience-induced preference for glucose over initially preferred nonnutritive sweetener solutions in 24-h choice tests. However, some sweetener solutions (e.g., 0.8% sucralose) have inhibitory postoral actions that may exaggerate glucose appetition whereas others (e.g., 0.1% sucralose + 0.1% saccharin, S+S) do not. Experiment 1 revealed that food-restricted (FR) male C57BL/6J mice displayed similar rapid glucose appetition effects (stimulation of glucose licking within minutes) and conditioned flavor preferences following 1-h experience with flavored 0.8% sucralose or 0.1% S+S and 8% glucose solutions. Thus, the inhibitory effects of 0.8% sucralose observed in 24-h tests were not apparent in 1-h tests. Experiment 2 evaluated the effects of food deprivation state and sweetener concentration on glucose appetition in female mice. Unlike FR mice tested with 0.1% S+S and 8% glucose, ad libitum (AL) fed mice displayed no stimulation of 8% glucose licking in the 1-h tests. A second ad libitum group (AL) tested with 0.2% S+S and 16% glucose solutions displayed stimulation of 16% glucose licking by the third 1-h test. Both AL groups, like the FR group, developed a preference for the glucose-paired flavor over the S+S paired flavor. Thus, food restriction promotes increased glucose licking but is not required for a conditioned preference. The FR male mice (Exp. 1) and FR female mice (Exp. 2) showed similar appetition responses (licking stimulation and flavor preference) to 8% glucose.

Mechanisms and Functions of Sweet Reception in Oral and Extraoral Organs

The oral detection of sugars relies on two types of receptor systems. The first is the G-protein-coupled receptor TAS1R2/TAS1R3. When activated, this receptor triggers a downstream signaling cascade involving gustducin, phospholipase Cβ2 (PLCβ2), and transient receptor potential channel M5 (TRPM5). The second type of receptor is the glucose transporter. When glucose enters the cell via this transporter, it is metabolized to produce ATP. This ATP inhibits the opening of KATP channels, leading to cell depolarization. Beside these receptor systems, sweet-sensitive taste cells have mechanisms to regulate their sensitivity to sweet substances based on internal and external states of the body. Sweet taste receptors are not limited to the oral cavity; they are also present in extraoral organs such as the gastrointestinal tract, pancreas, and brain. These extraoral sweet receptors are involved in various functions, including glucose absorption, insulin release, sugar preference, and food intake, contributing to the maintenance of energy homeostasis. Additionally, sweet receptors may have unique roles in certain organs like the trachea and bone. This review summarizes past and recent studies on sweet receptor systems, exploring the molecular mechanisms and physiological functions of sweet (sugar) detection in both oral and extraoral organs.

Addition of low sodium does not increase sensitivity to glucose in wild-type mice, or lead to partial glucose taste detection in T1R3 knock-out mice

The sodium glucose cotransporter 1 (SGLT1) has been proposed as a non-T1R glucosensor contributing to glucose taste. Studies have shown that the addition of NaCl at very weak concentrations to a glucose stimulus can enhance signaling in the gustatory nerves of mice and significantly lower glucose detection thresholds in humans. Here, we trained mice with (wild-type; WT) and without (knockout; KO) a functioning T1R3 subunit on a two-response operant detection task to differentially respond to the presence or absence of a taste stimulus immediately after sampling. After extensive training (∼40 sessions), KO mice were unable to reliably discriminate 2 M glucose+0.01 M NaCl from 0.01 M NaCl alone, but all WT mice could. We then tested WT mice on a descending array of glucose concentrations (2.0-0.03 M) with the addition of 0.01 M NaCl vs. 0.01 M NaCl alone. The concentration series was then repeated with glucose alone vs. water. We found no psychophysical evidence of a non-T1R taste transduction pathway involved in the detection of glucose. The addition of NaCl to glucose did not lower taste detection thresholds in WT mice, nor did it render the stimulus detectable to KO mice, even at 2 M. The proposed pathway must contribute to functions other than sensory-discriminative detection, at least when tested under these conditions. Detection thresholds were also derived for fructose and found to be 1/3 log10 lower than for glucose, but highly correlated (r = 0.88) between the two sugars, suggesting that sensitivity to these stimuli in this task was based on a similar neural process.

Activation and inhibition of the sweet taste receptor TAS1R2-TAS1R3 differentially affect glucose tolerance in humans

The sweet taste receptor, TAS1R2-TAS1R3, is expressed in taste bud cells, where it conveys sweetness, and also in intestinal enteroendocrine cells, where it may facilitate glucose absorption and assimilation. In the present study, our objective was to determine whether TAS1R2-TAS1R3 influences glucose metabolism bidirectionally via hyperactivation with 5 mM sucralose (n = 12) and inhibition with 2 mM sodium lactisole (n = 10) in mixture with 75 g glucose loads during oral glucose tolerance tests (OGTTs) in healthy humans. Plasma glucose, insulin, and glucagon were measured before, during, and after OGTTs up to 120 minutes post-prandially. We also assessed individual participants' sweet taste responses to sucralose and their sensitivities to lactisole sweetness inhibition. The addition of sucralose to glucose elevated plasma insulin responses to the OGTT (F(1, 11) = 4.55, p = 0.056). Sucralose sweetness ratings were correlated with early increases in plasma glucose (R2 = 0.41, p<0.05), as well as increases in plasma insulin (R2 = 0.38, p<0.05) when sucralose was added to the OGTT (15 minute AUC). Sensitivity to lactisole sweetness inhibition was correlated with decreased plasma glucose (R2 = 0.84, p<0.01) when lactisole was added to the OGTT over the whole test (120 minute AUC). In summary, stimulation and inhibition of the TAS1R2-TAS1R3 receptor demonstrates that TAS1R2-TAS1R3 helps regulate glucose metabolism in humans and may have translational implications for metabolic disease risk.

Glucose appetition in C57BL/6J mice: Influence of nonnutritive sweetener experience, food deprivation state and sex differences

In addition to its sweet taste, glucose has potent and rapid postoral actions (appetition) that enhance its reward value. This has been demonstrated by the experience-induced preference for glucose over initially preferred nonnutritive sweetener solutions in 24-h choice tests. However, some sweetener solutions (e.g., 0.8% sucralose) have inhibitory postoral actions that may exaggerate glucose appetition whereas others (e.g., 0.1% sucralose + 0.1% saccharin, S+S) do not. Experiment 1 revealed that food-restricted (FR) male C57BL/6J mice displayed similar rapid glucose appetition effects (stimulation of glucose licking within minutes) and conditioned flavor preferences following 1-h experience with flavored 0.8% sucralose or 0.1% S+S and 8% glucose solutions. Thus, the inhibitory effects of 0.8% sucralose observed in 24-h tests were not apparent in 1-h tests. Experiment 2 evaluated the effects of food deprivation state on 1-h glucose appetition. Unlike FR female mice, ad libitum (AL) fed mice displayed no or delayed stimulation of glucose licking depending upon the training solutions used (0.1% S+S vs. 8% glucose, or 0.2% S+S vs. 16% glucose). Both AL groups, like the FR group, developed a preference for the glucose-paired flavor over the S+S paired flavor. Thus, food restriction promotes glucose appetition but is not required for a conditioned preference. Overall, male and female mice showed similar glucose appetition responses although females displayed a more rapid initial glucose response.

Oral stimulation with glucose and fructose, but not sucrose, accelerates gastric emptying in humans

Postprandial regulation of the gastric emptying (GE) rate plays an important role in food intake. Although oral sweetening with glucose may accelerate GE, the effects of different sweetness intensities of glucose (10% and 20%, w/v) and other energy sweeteners (e.g. fructose and sucrose) remain uncertain. The purpose of this study was to determine the effects of different glucose concentrations (Experiment 1) and different sugars with the same sweet taste intensity (Experiment 2) on postprandial GE. In both experiments, after ingesting a 200 kcal carbohydrate solution containing 50 g of maltodextrin, participants repeatedly sipped, but did not swallow, one of three (water, 10% and 20%, w/v glucose) or four (water and equally sweet 20%, w/v glucose, 12%, w/v fructose, and 14%, w/v sucrose) solutions for 1 min every 5 min over a 30 min period. GE was evaluated by measuring the temporal change in the cross-sectional area of the gastric antrum using ultrasound. In Experiment 1, oral stimulation with 20% (w/v) glucose resulted in greater GE than the control stimulus (i.e. water), but the effect of stimulation with 10% (w/v) glucose on GE was not different from that of the control stimulus. In Experiment 2, stimulation with 20% (w/v) glucose or 12% (w/v) fructose resulted in greater GE than the control stimulus. However, the effect of stimulation with 14% (w/v) sucrose on GE did not differ from that of the control stimulus. Consequently, oral stimulation with glucose or fructose solutions of moderate to high sweetness following a meal facilitates postprandial GE.

TRPM4 and PLCβ3 contribute to normal behavioral responses to an array of sweeteners and carbohydrates but PLCβ3 is not needed for taste-driven licking for glucose

The peripheral taste system is more complex than previously thought. The novel taste-signaling proteins TRPM4 and PLCβ3 appear to function in normal taste responding as part of Type II taste cell signaling or as part of a broadly responsive (BR) taste cell that can respond to some or all classes of tastants. This work begins to disentangle the roles of intracellular components found in Type II taste cells (TRPM5, TRPM4, and IP3R3) or the BR taste cells (PLCβ3 and TRPM4) in driving behavioral responses to various saccharides and other sweeteners in brief-access taste tests. We found that TRPM4, TRPM5, TRPM4/5, and IP3R3 knockout (KO) mice show blunted or abolished responding to all stimuli compared with wild-type. IP3R3 KO mice did, however, lick more for glucose than fructose following extensive experience with the 2 sugars. PLCβ3 KO mice were largely unresponsive to all stimuli except they showed normal concentration-dependent responding to glucose. The results show that key intracellular signaling proteins associated with Type II and BR taste cells are mutually required for taste-driven responses to a wide range of sweet and carbohydrate stimuli, except glucose. This confirms and extends a previous finding demonstrating that Type II and BR cells are both necessary for taste-driven licking to sucrose. Glucose appears to engage unique intracellular taste-signaling mechanisms, which remain to be fully elucidated.

Oral glucose sensing in cephalic phase insulin release

Oral stimulation with foods or food components elicits cephalic phase insulin release (CPIR), which limits postprandial hyperglycemia. Despite its physiological importance, the specific gustatory mechanisms that elicit CPIR have not been clearly defined. While most studies point to glucose and glucose-containing saccharides (e.g., sucrose, maltodextrins) as being the most consistent elicitors, it is not apparent whether this is due to the detection of glucose per se, or to the perceived taste cues associated with these stimuli (e.g., sweetness, starchiness). This study investigated potential sensory mechanisms involved with eliciting CPIR in humans, focusing on the role of oral glucose detection and associated taste. Four stimulus conditions possessing different carbohydrate and taste profiles were designed: 1) glucose alone; 2) glucose mixed with lactisole, a sweet taste inhibitor; 3) maltodextrin, which is digested to starchy- and sweet-tasting products during oral processing; and 4) maltodextrin mixed with lactisole and acarbose, an oral digestion inhibitor. Healthy adults (N = 22) attended four sessions where blood samples were drawn before and after oral stimulation with one of the target stimuli. Plasma c-peptide, insulin, and glucose concentrations were then analyzed. Whereas glucose alone elicited CPIR (one-sample t-test, p < 0.05), it did not stimulate the response in the presence of lactisole. Likewise, maltodextrin alone stimulated CPIR (p < 0.05), but maltodextrin with lactisole and acarbose did not. Together, these findings indicate that glucose is an effective CPIR stimulus, but that an associated taste sensation also serves as an important cue for triggering this response in humans.

Sugar signals from oral glucose transporters elicit cephalic-phase insulin release in mice

Cephalic-phase insulin release (CPIR) occurs before blood glucose increases after a meal. Although glucose is the most plausible cue to induce CPIR, peripheral sensory systems involved are not fully elucidated. We therefore examined roles of sweet sensing by a T1R3-dependent taste receptor and sugar sensing by oral glucose transporters in the oropharyngeal region in inducing CPIR. Spontaneous oral ingestion of glucose significantly increased plasma insulin 5 min later in wild-type (C57BL/6) and T1R3-knockout mice, but intragastric infusion did not. Oral treatment of glucose transporter inhibitors phlorizin and phloretin significantly reduced CPIR after spontaneous oral ingestion. In addition, a rapid increase in plasma insulin was significantly smaller in WT mice with spontaneous oral ingestion of nonmetabolizable glucose analog than in WT mice with spontaneous oral ingestion of glucose. Taken together, the T1R3-dependent receptor is not required for CPIR, but oral glucose transporters greatly contribute to induction of CPIR by sugars.

Adrenomedullin Enhances Mouse Gustatory Nerve Responses to Sugars via T1R-Independent Sweet Taste Pathway

On the tongue, the T1R-independent pathway (comprising glucose transporters, including sodium-glucose cotransporter (SGLT1) and the K_ATP channel) detects only sugars, whereas the T1R-dependent (T1R2/T1R3) pathway can broadly sense various sweeteners. Cephalic-phase insulin release, a rapid release of insulin induced by sensory signals in the head after food-related stimuli, reportedly depends on the T1R-independent pathway, and the competitive sweet taste modulators leptin and endocannabinoids may function on these two different sweet taste pathways independently, suggesting independent roles of two oral sugar-detecting pathways in food intake. Here, we examined the effect of adrenomedullin (ADM), a multifunctional regulatory peptide, on sugar sensing in mice since it affects the expression of SGLT1 in rat enterocytes. We found that ADM receptor components were expressed in T1R3-positive taste cells. Analyses of chorda tympani (CT) nerve responses revealed that ADM enhanced responses to sugars but not to artificial sweeteners and other tastants. Moreover, ADM increased the apical uptake of a fluorescent D-glucose derivative into taste cells and SGLT1 mRNA expression in taste buds. These results suggest that the T1R-independent sweet taste pathway in mouse taste cells is a peripheral target of ADM, and the specific enhancement of gustatory nerve responses to sugars by ADM may contribute to caloric sensing and food intake.

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Oral stimulation with maltodextrin: Effect on cephalic phase insulin release

Cephalic phase insulin release (CPIR) occurs following sensory stimulation with food-related stimuli, and has been shown to limit postabsorptive hyperglycemia. While the specific stimuli that elicit CPIR in humans have not been clearly defined, previous research points to sugars as having potential importance. Maltodextrins are a starch-derived food ingredient commonly found in a variety of processed food products. When consumed, salivary α-amylase rapidly cleaves its component saccharides into smaller units, leading to the production of sugars in the mouth. Here, we investigated whether humans elicit CPIR after tasting but not swallowing maltodextrin, and whether the degree of CPIR exhibited is affected by individuals' salivary α-amylase activity. We found that a gelatin-based stimulus containing 22% w/v maltodextrin elicited CPIR in healthy individuals (N = 22) following a modified sham-feeding protocol using both insulin and c-peptide as indices of the response. However, the degree of CPIR measured did not differ across three groupings (low, medium, or high) of effective α-amylase activity by either index. In a follow-up experiment, a subset of participants (N = 14) underwent the same protocol using a gelatin stimulus without maltodextrin, and no observable CPIR ensued. These findings suggest that oral stimulation with maltodextrin elicits CPIR in humans, but that individual differences in effective salivary α-amylase activity may not necessarily be predictive of the degree of CPIR.

Dietary experience with glucose and fructose fosters heightened avidity for glucose-containing sugars independent of TRPM5 taste transduction in mice

OBJECTIVE

Experience with metabolically distinct sugars, glucose and fructose, enhances attraction to the orosensory properties of glucose over fructose. To gain insight into which sensory signals are affected, we investigated how this nutritive learning reshapes behavioral responding to various sugars in brief access taste tests in C57BL6/J (B6) mice and assessed whether sugar-exposed mice lacking the TRPM5 channel involved in G-protein coupled taste transduction could acquire these types of preferences for glucose-containing sugars.

METHODS

B6, TRPM5 knockout (KO), and TRPM5 heterozygous (Het) mice were given extensive access to water (sugar naïve) or 0.316, 0.56, and 1.1 M glucose and fructose (sugar-exposed) and then tested, whilst food deprived, for their relative avidities for glucose, fructose, sucrose, maltose, and/or a non-metabolizable glucose analog in a series of taste tests.

RESULTS

Sugar-exposed B6 mice licked relatively more for glucose than fructose, driven by an increased avidity for glucose, not an avoidance of fructose, and licked more for maltose, compared to their sugar-naïve counterparts. Sugar-exposed B6 mice did not lick with such avidity for a non-metabolizable glucose analog. TRPM5 KO mice took longer to acquire the sugar discrimination than the Het controls, but both groups ultimately licked significantly more for glucose than fructose. Het mice displayed clear preferential licking for sucrose over fructose, while licking comparably high for glucose, sucrose, and maltose. KO mice licked significantly more for maltose than sucrose.

CONCLUSIONS

Collectively, the findings suggest that ingestive experience with glucose and fructose primarily reprograms behavioral responding to a TRPM5-independent orosensory signal generated by glucose-containing sugars.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

A Glucokinase-linked Sensor in the Taste System Contributes to Glucose Appetite

Objectives

Dietary glucose is a robust elicitor of central reward responses and ingestion, but the key peripheral sensors triggering these orexigenic mechanisms are not entirely known. The objective of this study was to determine whether glucokinase, a phosphorylating enzyme with known glucosensory roles, is also expressed in taste bud cells and contributes to the immediate hedonic appeal of glucose-containing substances.

Methods and results

Glucokinase (GCK) gene transcripts were localized in murine taste bud cells with RNAScope®, and GCK mRNA was found to be upregulated in the circumvallate taste papillae in response to fasting and after a period of dietary access to added simple sugars in mice, as determined with real time-qPCR. Pharmacological activation of glucokinase with Compound A increased primary taste nerve and licking responses for glucose but did not impact responsivity to fructose in naïve mice. Virogenetic silencing of glucokinase in the major taste fields attenuated glucose-stimulated licking, especially in mice that also lacked sweet receptors, but did not disrupt consummatory behaviors for fructose or the low-calorie sweetener, sucralose in sugar naïve mice. Knockdown of lingual glucokinase weakened the acquired preference for glucose over fructose in sugar-experienced mice in brief access taste tests.

Conclusions

Collectively, our data establish that glucokinase contributes to glucose appetition at the very first site of nutrient detection, in the oral cavity. The findings expand our understanding of orosensory inputs underlying nutrition, metabolism, and food reward.

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Glucokinase is expressed in the taste bud cells.

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Gustatory glucokinase is upregulated by energy deficit and regular consumption of simple sugars.

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Gustatory glucokinase is required for normal glucose taste detection and contributes to the hedonic appeal of this nutrient.

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.

Cephalic phase insulin release: A review of its mechanistic basis and variability in humans

Cephalic phase insulin release (CPIR) is a transient pulse of insulin that occurs within minutes of stimulation from foods or food-related stimuli. Despite decades of research on CPIR in humans, the body of literature surrounding this phenomenon is controversial due in part to contradictory findings . This has slowed progress towards understanding the sensory and neural basis of CPIR, as well as its overall relevance to health. This review examines up-to-date knowledge in CPIR research and identifies sources of CPIR variability in humans in an effort to guide future research. The review starts by defining CPIR and discussing its presumed functional roles in glucose homeostasis and feeding behavior. Next, the types of stimuli that have been reported to elicit CPIR, as well as the sensory and neural mechanisms underlying the response in rodents and humans are discussed, and areas where knowledge is limited are identified. Finally, factors that may contribute to the observed variability of CPIR in humans are examined, including experimental design, test procedure, and individual characteristics. Overall, oral stimulation appears to be important for eliciting CPIR, especially when combined with other sensory modalities (vision, olfaction, somatosensation). While differences in experimental design and testing procedure likely explain some of the observed inter- and intra-study variability, individual differences also appear to play an important role. Understanding sources of these individual differences in CPIR will be key for establishing its health relevance.

Fructose and the Liver

Chronic diseases represent a major challenge in world health. Metabolic syndrome is a constellation of disturbances affecting several organs, and it has been proposed to be a liver-centered condition. Fructose overconsumption may result in insulin resistance, oxidative stress, inflammation, elevated uric acid levels, increased blood pressure, and increased triglyceride concentrations in both the blood and liver. Non-alcoholic fatty liver disease (NAFLD) is a term widely used to describe excessive fatty infiltration in the liver in the absence of alcohol, autoimmune disorders, or viral hepatitis; it is attributed to obesity, high sugar and fat consumption, and sedentarism. If untreated, NAFLD can progress to nonalcoholic steatohepatitis (NASH), characterized by inflammation and mild fibrosis in addition to fat infiltration and, eventually, advanced scar tissue deposition, cirrhosis, and finally liver cancer, which constitutes the culmination of the disease. Notably, fructose is recognized as a major mediator of NAFLD, as a significant correlation between fructose intake and the degree of inflammation and fibrosis has been found in preclinical and clinical studies. Moreover, fructose is a risk factor for liver cancer development. Interestingly, fructose induces a number of proinflammatory, fibrogenic, and oncogenic signaling pathways that explain its deleterious effects in the body, especially in the liver.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Recent Advances in Understanding Peripheral Taste Decoding I: 2010 to 2020

Taste sensation is the gatekeeper for direct decisions on feeding behavior and evaluating the quality of food. Nutritious and beneficial substances such as sugars and amino acids are represented by sweet and umami tastes, respectively, whereas noxious substances and toxins by bitter or sour tastes. Essential electrolytes including Na+ and other ions are recognized by the salty taste. Gustatory information is initially generated by taste buds in the oral cavity, projected into the central nervous system, and finally processed to provide input signals for food recognition, regulation of metabolism and physiology, and higher-order brain functions such as learning and memory, emotion, and reward. Therefore, understanding the peripheral taste system is fundamental for the development of technologies to regulate the endocrine system and improve whole-body metabolism. In this review article, we introduce previous widely-accepted views on the physiology and genetics of peripheral taste cells and primary gustatory neurons, and discuss key findings from the past decade that have raised novel questions or solved previously raised questions.

Bitter Taste Receptors (T2Rs) are Sentinels that Coordinate Metabolic and Immunological Defense Responses

In addition to being responsible for bitter taste, type 2 taste receptors (T2Rs) regulate endocrine, behavioral, and immunological responses. T2R agonists include indicators of incoming threats to metabolic homeostasis, pathogens, and irritants. This review will provide an overview of T2R-regulated processes throughout the body that function defensively. We propose a broader definition of T2Rs as chemosensory sentinels that monitor toxic, metabolic, and infectious threats and initiate defensive responses.

Food anticipatory hormonal responses: A systematic review of animal and human studies

Food anticipatory hormonal responses (cephalic responses) are proactive physiological processes, that allow animals to prepare for food ingestion by modulating their hormonal levels in response to food cues. This process is important for digesting food, metabolizing nutrients and maintaining glucose levels within homeostasis. In this systematic review, we summarize the evidence from animal and human research on cephalic responses. Thirty-six animal and fifty-three human studies were included. The majority (88 %) of studies demonstrated that hormonal levels are changed in response to cues previously associated with food intake, such as feeding time, smell, and sight of food. Most evidence comes from studies on insulin, ghrelin, pancreatic polypeptide, glucagon, and c-peptide. Moreover, impaired cephalic responses were found in disorders related to metabolism and food intake such as diabetes, pancreatic insufficiency, obesity, and eating disorders, which opens discussions about the etiological mechanisms of these disorders as well as on potential therapeutic opportunities.

Sweetness Perception is not Involved in the Regulation of Blood Glucose after Oral Application of Sucrose and Glucose Solutions in Healthy Male Subjects

SCOPE

This study investigates the effect of the sweetness of a sucrose versus an isocaloric glucose solution in dietary concentrations on blood glucose regulation by adjusting the sweetness level using the sweet taste inhibitor lactisole.

METHODS AND RESULTS

A total of 27 healthy males participated in this randomized, crossover study with four treatments: 10% glucose, 10% sucrose, 10% sucrose + 60 ppm lactisole, and 10% glucose + 60 ppm lactisole. Plasma glucose, insulin, glucagon-like peptide 1, and glucagon levels are measured at baseline and 15, 30, 60, 90, and 120 min after beverage consumption. Test subjects rated the sucrose solution to be sweeter than the isocaloric glucose solution, whereas no difference in sweetness is reported after addition of lactisole to the sucrose solution. Administration of the less sweet glucose solution versus sucrose led to higher blood glucose levels after 30 min, as reflected by a lower ΔAUC for sucrose (1072 ± 136) than for glucose (1567 ± 231). Application of lactisole leads to no differences in glucose, insulin, or glucagon responses induced by sucrose or glucose.

CONCLUSION

The results indicate that the structure of the carbohydrate has a stronger impact on the regulation of blood glucose levels than the perceived sweetness.

Sodium-glucose cotransporter 1 as a sugar taste sensor in mouse tongue

AIM

We investigated potential neuron types that code sugar information and how sodium-glucose cotransporters (SGLTs) and T1Rs are involved.

METHODS

Whole-nerve recordings in the chorda tympani (CT) and the glossopharyngeal (GL) nerves and single-fibre recordings in the CT were performed in T1R3-KO and wild-type (WT) mice. Behavioural response measurements were conducted in T1R3-KO mice using phlorizin (Phl), a competitive inhibitor of SGLTs.

RESULTS

Results indicated that significant enhancement occurred in responses to sucrose and glucose (Glc) by adding 10 mmol/L NaCl but not in responses to KCl, monopotassium glutamate, citric acid, quinine sulphate, SC45647(SC) or polycose in both CT and GL nerves. These enhancements were abolished by lingual application of Phl. In single-fibre recording, fibres showing maximal response to sucrose could be classified according to responses to SC and Glc with or without 10 mmol/L NaCl in the CT of WT mice, namely, Phl-insensitive type, Phl-sensitive Glc-type and Mixed (Glc and SC responding)-type fibres. In T1R3-KO mice, Phl-insensitive-type fibres disappeared. Results from behavioural experiments showed that the number of licks and amount of intake for Glc with or without 10 mmol/L NaCl were significantly suppressed by Phl.

CONCLUSION

We found evidence for the contribution of SGLTs in sugar sensing in taste cells of mouse tongue. Moreover, we found T1R-dependent (Phl-insensitive) type, Glc-type and Mixed (SGLTs and T1Rs)-type fibres. SGLT1 may be involved in the latter two types and may play important roles in the glucose-specific cephalic phase of digestion and palatable food intake.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Residual Glucose Taste in T1R3 Knockout but not TRPM5 Knockout Mice

Knockout (KO) mice missing the sweet taste receptor subunit T1R3 or the signaling protein TRPM5 have greatly attenuated sweetener preferences. Yet both types of KO mice develop preferences for glucose but not fructose in 24-h tests, which has been attributed to the postoral reinforcing actions of glucose. Here we probed for residual sugar taste sensitivity in KO mice. Unlike wildtype (WT) mice, food-restricted T1R3 KO and TRPM5 KO mice displayed little attraction for 8% glucose and 8% fructose in 1-min, two-bottle choice tests. However, in 1-h tests about half of the T1R3 KO mice displayed a significant preference for glucose over fructose (78-84%), while WT mice showed either no or weak preferences (41-56%) for glucose. Following one-bottle training sessions, WT mice display greater glucose preferences although still weaker than those observed in T1R3 KO mice. In contrast, TRPM5 KO mice were indifferent to sugars in 1-h tests but developed a strong preference for glucose over fructose in 24-h tests. T1R3 taste cells contain the sodium glucose cotransporter 1 (SGLT1) and the ATP-gated K+ (K_ATP) metabolic sensor, which may mediate the unlearned glucose preference displayed by T1R3 KO mice. Unlike WT mice, many T1R3 KO mice strongly preferred glucose to a non-metabolizable glucose analog (α-methyl-D-glucopyranoside, MDG) in initial 1-h choice tests. Glucose and MDG are both ligands for SGLT1 which indicates that SGLT1 sensing does not mediate the glucose preference of T1R3 KO mice. Instead, K_ATP sensing and/or other oral sensors are implicated. The MDG findings also argue against postoral sensing as the primary source of the initial glucose preference displayed by T1R3 KO mice. Why only half of the T1R3 KO mice showed this preference in 1-h tests remains to be determined. All T1R3 KO mice preferred glucose to fructose in 24-h tests, which appears to be due to both oral and postoral glucose sensing.

Olfaction contributes to the learned avidity for glucose relative to fructose in mice

When offered glucose and fructose solutions, rodents consume more glucose solution because it produces stronger postoral reinforcement. Intake of these sugars also conditions a higher avidity for glucose relative to fructose. We asked which chemosensory cue mediates the learned avidity for glucose. We subjected mice to 18 days of sugar training, offering them 0.3, 0.6, and 1 M glucose and fructose solutions. Before and after training, we measured avidity for 0.3 and 0.6 M glucose and fructose in brief-access lick tests. First, we replicated prior work in C57BL/6 mice. Before training, the mice licked at a slightly higher rate for 0.6 M fructose; after training, they licked at a higher rate for 0.6 M glucose. Second, we assessed the necessity of the glucose-specific ATP-sensitive K+(KATP) taste pathway for the learned avidity for glucose, using mice with a nonfunctional KATPchannel [regulatory sulfonylurea receptor (SUR1) knockout (KO) mice]. Before training, SUR1 KO and wild-type mice licked at similar rates for 0.6 M glucose and fructose; after training, both strains licked at a higher rate for 0.6 M glucose, indicating that the KATPpathway is not necessary for the learned discrimination. Third, we investigated the necessity of olfaction by comparing sham-treated and anosmic mice. The mice were made anosmic by olfactory bulbectomy or ZnSO4treatment. Before training, sham-treated and anosmic mice licked at similar rates for 0.6 M glucose and fructose; after training, sham-treated mice licked at a higher rate for 0.6 M glucose, whereas anosmic mice licked at similar rates for both sugars. This demonstrates that olfaction contributes significantly to the learned avidity for glucose.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Electrophysiological responses to sugars and amino acids in the nucleus of the solitary tract of type 1 taste receptor double-knockout mice

Strong evidence supports a major role for heterodimers of the type 1 taste receptor (T1R) family in the taste transduction of sugars (T1R2+T1R3) and amino acids (T1R1+T1R3), but there are also neural and behavioral data supporting T1R-independent mechanisms. Most neural evidence for alternate mechanisms comes from whole nerve recordings in mice with deletion of a single T1R family member, limiting conclusions about the functional significance and T1R independence of the remaining responses. To clarify these issues, we recorded single-unit taste responses from the nucleus of the solitary tract in T1R double-knockout (double-KO) mice lacking functional T1R1+T1R3 [KO1+3] or T1R2+T1R3 [KO2+3] receptors and their wild-type background strains [WT; C57BL/6J (B6), 129X1/SvJ (S129)]. In both double-KO strains, responses to sugars and a moderate concentration of an monosodium glutamate + amiloride + inosine 5'-monophosphate cocktail (0.1 M, i.e., umami) were profoundly depressed, whereas a panel of 0.6 M amino acids were mostly unaffected. Strikingly, in contrast to WT mice, no double-KO neurons responded selectively to sugars and umami, precluding segregation of this group of stimuli from those representing other taste qualities in a multidimensional scaling analysis. Nevertheless, residual sugar responses, mainly elicited by monosaccharides, persisted as small "sideband" responses in double-KOs. Thus other receptors may convey limited information about sugars to the central nervous system, but T1Rs appear critical for coding the distinct perceptual features of sugar and umami stimuli. The persistence of amino acid responses supports previous proposals of alternate receptors, but because these stimuli affected multiple neuron types, further investigations are necessary.NEW & NOTEWORTHY The type 1 taste receptor (T1R) family is crucial for transducing sugars and amino acids, but there is evidence for T1R-independent mechanisms. In this study, single-unit recordings from the nucleus of the solitary tract in T1R double-knockout mice lacking T1R1+T1R3 or T1R2+T1R3 receptors revealed greatly reduced umami synergism and sugar responses. Nevertheless, residual sugar responses persisted, mainly elicited by monosaccharides and evident as "sidebands" in neurons activated more vigorously by other qualities.

Recent advances in taste transduction and signaling

In the last few years, single-cell profiling of taste cells and ganglion cells has advanced our understanding of transduction, encoding, and transmission of information from taste buds as relayed to the central nervous system. This review focuses on new knowledge from these molecular approaches and attempts to place this in the context of previous questions and findings in the field. The individual taste cells within a taste bud are molecularly specialized for detection of one of the primary taste qualities: salt, sour, sweet, umami, and bitter. Transduction and transmitter release mechanisms differ substantially for taste cells transducing sour (Type  III cells) compared with those transducing the qualities of sweet, umami, or bitter (Type II cells), although ultimately all transmission of taste relies on activation of purinergic P2X receptors on the afferent nerves. The ganglion cells providing innervation to the taste buds also appear divisible into functional and molecular subtypes, and each ganglion cell is primarily but not exclusively responsive to one taste quality.

Low Calorie Sweeteners Differ in Their Physiological Effects in Humans

Low calorie sweeteners (LCS) are prevalent in the food supply for their primary functional property of providing sweetness with little or no energy. Though tested for safety individually, there has been extremely limited work on the efficacy of each LCS. It is commonly assumed all LCS act similarly in their behavioral and physiological effects. However, each LCS has its own chemical structure that influences its metabolism, making each LCS unique in its potential effects on body weight, energy intake, and appetite. LCS may have different behavioral and physiological effects mediated at the sweet taste receptor, in brain activation, with gut hormones, at the microbiota and on appetitive responses. Further elucidation of the unique effects of the different commercially available LCS may hold important implications for recommendations about their use for different health outcomes.

Low-calorie sweeteners cause only limited metabolic effects in mice

There are widespread concerns that low-calorie sweeteners (LCSs) cause metabolic derangement. These concerns stem in part from prior studies linking LCS consumption to impaired glucose tolerance in humans and rodents. Here, we examined this linkage in mice. In experiment 1, we provided mice with chow, water, and an LCS-sweetened solution (saccharin, sucralose, or acesulfame K) for 28 days and measured glucose tolerance and body weight across the exposure period. Exposure to the LCS solutions did not impair glucose tolerance or alter weight gain. In experiment 2, we provided mice with chow, water, and a solution containing saccharin, glucose, or a mixture of both for 28 days, and tested for metabolic changes. Exposure to the saccharin solution increased the insulinemic response of mice to the glucose challenge, and exposure to the saccharin + glucose solution increased the rate of glucose uptake during the glucose challenge. However, neither of these test solutions altered glucose tolerance, insulin sensitivity, plasma triglycerides, or percent body fat. In contrast, exposure to the glucose solution increased glucose tolerance, early insulin response, insulin sensitivity, and percent body fat. We conclude that whereas the LCS-containing solutions induced a few metabolic changes, they were modest compared with those induced by the glucose solution.

Suppression of sweet sensing with glucose, but not aspartame, delays gastric emptying and glycemic response

Previously, we reported that oral stimulation with Gymnema sylvestre (GS), a plant that selectively suppresses sweet taste sensation in humans, delayed gastric emptying and glycemic response during and after oral glucose ingestion. It is unclear whether these responses are triggered by sweet taste sensing per se. We tested the hypothesis that the effects of sweet taste sensing involving a low-energy sweetener, aspartame, alters gastric emptying, blood glucose, and plasma insulin responses during and after the prandial phase. Nine participants rinsed their mouths with either 25 mL of water (control) or a 2.5% GS solution, and then ingested 200 g (50 g × four times) of either 0.09% aspartame or 15% glucose solution containing 100 mg of ¹³C-sodium acetate. Gastric emptying was measured with a ¹³C breath test. Blood glucose and plasma insulin were measured at baseline as well as during and after ingestion of the sweet solutions. Decreased subjective sweet taste intensity was observed in the GS group for both the aspartame and glucose trials. In the aspartame trial, no measurements showed significant differences between either group. In the glucose trial, gastric emptying was delayed in the GS group compared to controls. In the initial phase, both during and after glucose ingestion in the glucose trial, blood glucose and plasma insulin responses were lower in the GS group than the controls. The presence or absence of sweet taste-sensing involving glucose had a significant effect on gastric emptying and glycemic metabolism, both during and after the prandial phase, as opposed to the effects involving aspartame.

T1R2+T1R3-independent chemosensory inputs contributing to behavioral discrimination of sugars in mice

Simple sugars are thought to elicit a unitary sensation, principally via the "sweet" taste receptor type 1 taste receptor (T1R)2+T1R3, yet we previously found that rats with experience consuming two metabolically distinct sugars, glucose and fructose, subsequently licked more for glucose than fructose, even when postingestive influences were abated. The results pointed to the existence of an orosensory receptor that binds one sugar but not the other and whose signal is channeled into neural circuits that motivate ingestion. Here we sought to determine the chemosensory nature of this signal. First, we assessed whether T1R2 and/or T1R3 are necessary to acquire this behavioral discrimination, replicating our rat study in T1R2+T1R3 double-knockout (KO) mice and their wild-type counterparts as well as in two common mouse strains that vary in their sensitivity to sweeteners [C57BL/6 (B6) and 129X1/SvJ (129)]. These studies showed that extensive exposure to multiple concentrations of glucose and fructose in daily one-bottle 30-min sessions enhanced lick responses for glucose over fructose in brief-access tests. This was true even for KO mice that lacked the canonical "sweet" taste receptor. Surgical disconnection of olfactory inputs to the forebrain (bulbotomy) in B6 mice severely disrupted the ability to express this experience-dependent sugar discrimination. Importantly, these bulbotomized B6 mice exhibited severely blunted responsiveness to both sugars relative to water in brief-access lick tests, despite the fact that they have intact T1R2+T1R3 receptors. The results highlight the importance of other sources of chemosensory and postingestive inputs in shaping and maintaining "hardwired" responses to sugar.

Neurons with diverse phenotypes project from the caudal to the rostral nucleus of the solitary tract

The nucleus of the solitary tract is a potential site for taste-visceral interactions. Connections from the caudal, visceral area of the nucleus (cNST) to the rostral, gustatory zone (rNST) have been described, but the phenotype of cells giving rise to the projection(s) and their distribution among rNST subdivisions are unknown. To determine these characteristics of the intrasolitary pathway, we injected pan-neuronal and floxed AAV viruses into the cNST of mice expressing cre in glutamatergic, GABAergic, or catecholaminergic neurons. Particular attention was paid to the terminal field distribution in rNST subdivisions by simultaneously visualizing P2X2 localized to gustatory afferent terminals. All three phenotypically identified pathways terminated in rNST, with the density greatest for glutamatergic and sparsest for catecholaminergic projections, observations supported by retrograde tracing. Interestingly, cNST neurons had more prominent projections to rNST regions medial and ventral to P2X2 staining, i.e., the medial and ventral subdivisions. In addition, GABAergic neurons projected robustly to the lateral subdivision and adjacent parts of the reticular formation and spinal trigeminal nucleus. Although cNST neurons also projected to the P2X2-rich central subdivision, such projections were sparser. These findings suggest that cNST visceral signals exert stronger excitatory and inhibitory influences on local autonomic and reflex pathways associated with the medial and ventral subdivisions compared to weaker modulation of ascending pathways arising from the central subdivision and ultimately destined for the forebrain.

Oral Post-Oral Actions of Low-Calorie Sweeteners: A Tale of Contradictions and Controversies

OBJECTIVE

Many scientists and laypeople alike have concerns about low-calorie sweeteners (LCSs). These concerns stem from both a dissatisfaction with the taste of LCSs and reports that they cause metabolic disruptions (e.g., weight gain, glucose intolerance).

METHODS

This article provides a critical review of the literature on LCSs from the standpoint of their taste, gastrointestinal, and metabolic effects; biological fate in the body; and impact on ingestion and glucose homeostasis.

RESULTS AND CONCLUSIONS

Mammals can readily discriminate between LCSs and sugars because both types of sweetener activate distinct oral and post-oral sensory pathways. LCSs differ in their ability to access post-oral tissues, but few studies have incorporated this observation into their design. It is difficult to extrapolate results between mice, rats, and humans because of interspecies differences in the taste and post-oral actions of LCSs and the fact that investigators often use different response measures in rodents and humans. There is confounding in the experimental design of some of the most widely cited studies of LCS-induced metabolic disruptions. The uncritical acceptance of these studies has generated considerable controversy. More work is needed to obtain a clearer understanding of the metabolic effects of LCSs.

From appetite setpoint to appetition: 50years of ingestive behavior research

I review the main themes of my 50-year research career in ingestive behavior as a graduate student at the University of Chicago and a professor at the City University of New York. A seminar course with my Ph.D. mentor, S. P. Grossman, sparked my interest in the hypothalamic obesity syndrome. I developed a wire knife to dissect the neuropathways and the functional disorder responsible for the syndrome. An elevated appetite setpoint that permitted the overconsumption of palatable foods appeared central to the hypothalamic syndrome. In brain-intact rats, providing an assortment of highly palatable foods (the cafeteria diet) stimulated diet-induced obesity that mimicked elements of hypothalamic obesity. Studies of the determinants of food palatability led to the discovery of a "new" carbohydrate taste (maltodextrin taste) and the confirmation of a fatty taste. In addition to oral taste receptors, gut nutrient sensors stimulated the intake/preference for carbohydrate- and fat-rich foods via an appetition process that stimulates brain reward systems. My research career greatly benefited from many diligent and creative students, collaborators and technicians and research support from my university and the National Institutes of Health.

Evolution of complex, discreet nutrient sensing pathways

PURPOSE OF REVIEW

The current review summarizes and discusses current research on differences elicited between sugars and nonnutritive sweeteners via sugar-sensing pathways.

RECENT FINDINGS

Sugars, sweeteners, and sweetening agents are all perceived as sweet tasting because of their ability to bind to the type 1 taste receptor family of sweet taste receptors in the oral cavity. The ability of a wide variety of chemical ligands to activate the sweet taste receptor highlights the importance of sweet-tasting foods during human evolution. The sweet taste receptor has been located in the gut, and differences between oral and gut sugar-sensing pathways are discussed.

SUMMARY

Differences in the sweetness transduction cascade, and neuronal signalling may result in incretin hormone release upon activation of the sweet taste receptor from some sweeteners, but not others.

CD36 Modulates Fasting and Preabsorptive Hormone and Bile Acid Levels

CONTEXT

Abnormal fatty acid (FA) metabolism contributes to diabetes and cardiovascular disease. The FA receptor CD36 has been linked to risk of metabolic syndrome. In rodents CD36 regulates various aspects of fat metabolism, but whether it has similar actions in humans is unknown. We examined the impact of a coding single-nucleotide polymorphism in CD36 on postprandial hormone and bile acid (BA) responses.

OBJECTIVE

To examine whether the minor allele (G) of coding CD36 variant rs3211938 (G/T), which reduces CD36 level by ∼50%, influences hormonal responses to a high-fat meal (HFM).

DESIGN

Obese African American (AA) women carriers of the G allele of rs3211938 (G/T) and weight-matched noncarriers (T/T) were studied before and after a HFM.

SETTING

Two-center study.

PARTICIPANTS

Obese AA women.

INTERVENTION

HFM.

MAIN OUTCOME MEASURES

Early preabsorptive responses (10 minutes) and extended excursions in plasma hormones [C-peptide, insulin, incretins, ghrelin fibroblast growth factor (FGF)19, FGF21], BAs, and serum lipoproteins (chylomicrons, very-low-density lipoprotein) were determined.

RESULTS

At fasting, G-allele carriers had significantly reduced cholesterol and glycodeoxycholic acid and consistent but nonsignificant reductions of serum lipoproteins. Levels of GLP-1 and pancreatic polypeptide (PP) were reduced 60% to 70% and those of total BAs were 1.8-fold higher. After the meal, G-allele carriers displayed attenuated early (-10 to 10 minute) responses in insulin, C-peptide, GLP-1, gastric inhibitory peptide, and PP. BAs exhibited divergent trends in G allele carriers vs noncarriers concomitant with differential FGF19 responses.

CONCLUSIONS

CD36 plays an important role in the preabsorptive hormone and BA responses that coordinate brain and gut regulation of energy metabolism.

Preparation and characterization of isolated low degree of polymerization food-grade maltooligosaccharides

Research involving human responses to the consumption of starch and its hydrolysis products would benefit from convenient sources of well defined, low cost, food-grade maltooligosaccharides (MOS). This report addresses such need by presenting an approach to obtain aforementioned MOS. A chromatography-ready MOS sample containing proportionately high amounts of low degree of polymerization (DP) MOS is initially prepared from commercially-available maltodextrins (MD) by taking advantage of the DP-dependent differential solubility of MOS in aqueous-ethanol solutions. The low DP-enriched MOS preparation is subsequently fractionated via preparative column chromatography using cellulose-based stationary phases and step-gradient aqueous-ethanol mobile phases. The resulting fractions yielded isolated food-grade MOS ranging in DP from 3 to 7. NMR spectra of isolated MOS indicated minimal amounts of branched saccharides. Typical yields from a single fractionation protocol (2 g MD starting material), including solvent partitioning through preparative chromatography, ranged from ∼40 mg for DP 4, 5, and 7 to ∼100 mg for DP 3 and 6.

Human taste detection of glucose oligomers with low degree of polymerization

Studies have reported that some animals, including humans, can taste mixtures of glucose oligomers (i.e., maltooligosaccharides, MOS) and that their detection is independent of the known T1R2/T1R3 sweet taste receptor. In an effort to understand potential mechanisms underlying the taste perception of glucose oligomers in humans, this study was designed to investigate: 1) the variability of taste sensitivity to MOS with low degree-of-polymerization (DP), and 2) the potential role of hT1R2/T1R3 in the MOS taste detection. To address these objectives, a series of food grade, narrow-DP-range MOS were first prepared (DP 3, 3–4, 5–6, and 6–7) by fractionating disperse saccharide mixtures. Subjects were then asked to discriminate these MOS stimuli as well as glucose (DP 1) and maltose (DP 2) from blanks after the stimuli were swabbed on the tongue. All stimuli were presented at 75 mM with and without a sweet taste inhibitor (lactisole). An α-glucosidase inhibitor (acarbose) was added to all test stimuli to prevent oral digestion of glucose oligomers. Results showed that all six stimuli were detected with similar discriminability in normal tasting conditions. When the sweet receptor was inhibited, DP 1, 2, and 3 were not discriminated from blanks. In contrast, three higher-DP paired MOS stimuli (DP 3–4, 5–6, and 6–7) were discriminated from blanks at a similar degree. Overall, these results support the presence of a sweet-independent taste perception mechanism that is stimulated by MOS greater than three units.

Taste buds: cells, signals and synapses

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Flavor preferences conditioned by nutritive and non-nutritive sweeteners in mice

Recent studies suggest that preferences are conditioned by nutritive (sucrose) but not by non-nutritive (sucralose) sweeteners in mice. Here we compared the effectiveness of nutritive and non-nutritive sweeteners to condition flavor preferences in three mouse strains. Isopreferred sucrose and sucralose solutions both conditioned flavor preferences in C57BL/6J (B6) mice but sucrose was more effective, consistent with its post-oral appetition action. Subsequent experiments compared flavor conditioning by fructose, which has no post-oral appetition effect in B6 mice, and a sucralose+saccharin mixture (SS) which is highly preferred to fructose in 24-h choice tests. Both sweeteners conditioned flavor preferences but fructose induced stronger preferences than SS. Training B6 mice to drink a flavored SS solution paired with intragastric fructose infusions did not enhance the SS-conditioned preference. Thus, the post-oral nutritive actions of fructose do not explain the sugar's stronger preference conditioning effect. Training B6 mice to drink a flavored fructose solution containing SS did not reduce the sugar-conditioned preference, indicating that SS does not have an off-taste that attenuates conditioning. Although B6 mice strongly preferred flavored SS to flavored fructose in a direct choice test, they preferred the fructose-paired flavor to the SS-paired flavor when these were presented in water. Fructose conditioned a stronger flavor preference than an isopreferred saccharin solution, indicating that sucralose is not responsible for the limited SS conditioning actions. SS is highly preferred by FVB/NJ and CAST/EiJ inbred mice, yet conditioned only weak flavor preferences. It is unclear why highly or equally preferred non-nutritive sweeteners condition weaker preferences than fructose, when all stimulate the same T1r2/T1r3 sweet receptor. Recent findings support the existence of non-T1r2/T1r3 glucose taste sensors; however, there is no evidence for receptors that respond to fructose but not to non-nutritive sweeteners.