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

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.

Neurobehavioral plasticity in the rodent gustatory system induced by regular consumption of a low-calorie sweetener during adolescence

Habitual consumption of low-calorie sweeteners (LCS) during juvenile-adolescence can lead to greater sugar intake later in life. Here, we investigated if exposure to the LCS Acesulfame Potassium (Ace-K) during this critical period of development reprograms the taste system in a way that would alter hedonic responding for common dietary compounds. Results revealed that early-life LCS intake not only enhanced the avidity for a caloric sugar (fructose) when rats were in a state of caloric need, it increased acceptance of a bitterant (quinine) in Ace-K-exposed rats tested when middle-aged. These behavior shifts, which endured months after the end of Ace-K exposure, were accompanied by widespread changes in the peripheral taste system. The anterior tongue had fewer fungiform taste papillae, and the circumvallate papillae had a reduced anterior to posterior span and diminished expression of genes involved in sweet reception, sweet and bitter intracellular signaling, fructose transport, and cellular progeneration in the Ace-K-exposed rats. Ace-K exposure also led to a significant reduction in dopamine-producing cells of the ventral tegmental area. The collective findings reveal that LCS intake early in life alters the taste-brain axis and the behavioral responsiveness to both positive and negative tastants that are important determinants of dietary preferences.

Lactobacillus salivarius and Lactobacillus gasseri supplementation reduces stress-induced sugar craving in mice

OBJECTIVE

Increased intake of sweets or sugar craving may occur in response to chronic stress representing a risk factor for development of eating disorders and obesity. However, no safe treatment of stress-induced sugar craving is available. In this study we analysed effects of two Lactobacillus strains on food and sucrose intake in mice before and during their exposure to a chronic mild stress (CMS).

RESEARCH METHODS & PROCEDURES

C57Bl6 mice were gavaged daily for 27 days with a mix of L. salivarius (LS) LS7892 and L. gasseri (LG) LG6410 strains or with 0.9% NaCl as a control. Following 10 days of gavage, mice were individually placed into the Modular Phenotypic cages, and after 7 days of acclimation were exposed to a CMS model for 10 days. Food, water and 2% sucrose intakes as well as meal pattern were monitored. Anxiety and depressive-like behaviour were analysed by standard tests.

RESULTS

Exposure of mice to CMS was accompanied by increased size of sucrose intake in the control group likely reflecting the stress-induced sugar craving. A consistent, about 20% lower total sucrose intake, was observed in the Lactobacilli-treated group during stress which was mainly due to a reduced number of intakes. Lactobacilli treatment also modified the meal pattern before and during the CMS, showing a decrease of meal number and an increase of meal size with a tendency of reduced total daily food intake. Mild anti-depressive behavioural effects of the Lactobacilli mix were also present.

CONCLUSION

Supplementation of mice with LS LS7892 and LG LG6410 decreases sugar consumption suggesting a potential utility of these strains against stress-induced sugar craving.

Mice Condition Cephalic-Phase Insulin Release to Flavors Associated with Postoral Actions of Concentrated Glucose

Rats can condition cephalic-phase insulin responses (CPIRs) to specific sounds or times of the day that predict food availability. The present study asked whether mice can condition a CPIR to the flavor of sapid solutions that produce postoral glucose stimulation. To this end, we subjected C57BL/6 mice to one of six experimental protocols. We varied both the duration of the five training sessions (i.e., 23 h or 1 h) and the nature of the training solution. In Experiment 1, consumption of a 0.61% saccharin solution was paired with IG co-infusion of a 16% glucose solution. In Experiments 2-6, the mice consumed a training solution containing a mixture of 0.61% saccharin + 16% glucose, 32% sucrose, 32% maltodextrin, flavored 32% maltodextrin, or 16% maltodextrin. We subsequently asked whether consumption of any of these fluids conditioned a CPIR to a test solution that produced a similar flavor, but which did not elicit a CPIR in naïve mice. The mice did condition a CPIR, but only to the solutions containing 32% maltodextrin. We attribute this conditioning to postoral actions of the concentrated maltodextrin solutions.

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.

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.

Protein Extract of a Probiotic Strain of Hafnia alvei and Bacterial ClpB Protein Improve Glucose Tolerance in Mice

A commercial strain of Hafnia alvei (H. alvei) 4597 bacteria was shown to reduce food intake and promote weight loss, effects possibly induced by the bacterial protein ClpB, an antigen-mimetic of the anorexigenic α-melanocyte-stimulating hormone. A decrease in the basal plasma glucose levels was also observed in overweight fasted humans and mice receiving H. alvei. However, it is not known whether H. alvei influences sweet taste preference and whether its protein extract or ClpB are sufficient to increase glucose tolerance; these are the objectives tested in the present study. C57BL/6J male mice were kept under standard diet and were gavaged daily for 17 days with a suspension of H. alvei (4.5 × 10⁷ CFU/animal) or with H. alvei total protein extract (5 μg/animal) or saline as a control. Sweet taste preference was analyzed via a brief-access licking test with sucrose solution. Glucose tolerance tests (GTT) were performed after the intraperitoneal (IP) or intragastric (IG) glucose administration at the 9th and 15th days of gavage, respectively. The expression of regulatory peptides' mRNA levels was assayed in the hypothalamus. In another experiment performed in non-treated C57BL/6J male mice, effects of acute IP administration of recombinant ClpB protein on glucose tolerance were studied by both IP- and IG-GTT. Mice treated with the H. alvei protein extract showed an improved glucose tolerance in IP-GTT but not in IG-GTT. Both groups treated with H. alvei bacteria or protein extract showed a reduction of pancreatic tissue weight but without significant changes to basal plasma insulin. No significant effects of H. alvei bacteria or its total protein extract administration were observed on the sweet taste preference, insulin tolerance and expression of regulatory peptides' mRNA in the hypothalamus. Acute administration of ClpB in non-treated mice increased glucose tolerance during the IP-GTT but not the IG-GTT, and reduced basal plasma glucose levels. We conclude that both the H. alvei protein extract introduced orally and the ClpB protein administered via IP improve glucose tolerance probably by acting at the glucose postabsorptive level. Moreover, H. alvei probiotic does not seem to influence the sweet taste preference. These results justify future testing of both the H. alvei protein extract and ClpB protein in animal models of diabetes.

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.