Flavor preference conditioning by different sugars in sweet ageusic Trpm5 knockout mice

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.

Conditioned preference and avoidance induced in mice by the rare sugars isomaltulose and allulose

Isomaltulose, a slowly digested isocaloric analog of sucrose, and allulose, a noncaloric fructose analog, are promoted as "healthful" sugar alternatives in human food products. Here we investigated the appetite and preference conditioning actions of these sugar analogs in inbred mouse strains. In brief-access lick tests (Experiment 1), C57BL/6 (B6) mice showed similar concentration dependent increases in licking for allulose and fructose, but less pronounced concentration-dependent increases in licking for isomaltulose than sucrose. In Experiment 2, B6 male were given one-bottle training with a CS+ flavor (e.g., grape) mixed with 8% isomaltulose or allulose and a CS- flavor (e.g., cherry) mixed in water followed by two-bottle CS flavor tests. The isomaltulose mice showed only a weak CS+ flavor preference but a strong preference for the sugar over water. The allulose mice strongly preferred the CS- flavor and water over the sugar. The allulose avoidance may be due to gut discomfort as reported in humans consuming high amounts of the sugar. Experiment 3 found that the preference for 8% sucrose over 8% isomaltulose could be reversed or blocked by adding different concentrations of a noncaloric sweetener mixture (sucralose + saccharin, SS) to the isomaltulose. Experiment 4 revealed that the preference of B6 or FVB/N mice for isomaltulose+0.01%SS or sucrose over 0.1%SS increased after separate experience with the sugars and SS. This indicates that isomaltulose, like sucrose, has postoral appetition effects that enhances the appetite for the sugar. In Experiments 5 and 6, the appetition actions of the two sugars were directly compared by giving mice isomaltulose+0.05%SS vs. sucrose choice tests before and after separate experience with the two sugars. In general, the initial preference the mice displayed for isomaltulose+0.05%SS was reduced or reversed after separate experience with the two sugars although some strain and sex differences were obtained. This indicates that isomaltulose has weaker postoral appetition effects than sucrose.

Effect of irinotecan administration on amiloride-sensitive sodium taste responses in mice

Taste alteration is a frequently reported side effect in patients receiving the chemotherapeutic agent, irinotecan. However, the way in which irinotecan causes taste disturbance and the type of taste impairment that is affected remain elusive. Here, we used the two-bottle preference test to characterize behavioral taste responses and employed immunohistochemical analyses to clarify the types and mechanisms of taste alteration induced, in mice, by irinotecan administration. Irinotecan administration resulted in a reduced intake of sodium taste solution but had no effect on sweet taste responses, as determined in the two-bottle preference test. In the presence of amiloride, which inhibits the function of the epithelial sodium channel (ENaC) in the periphery, the intake of sodium taste solution was comparable between the irinotecan-treated and control groups. Immunohistochemical analyses revealed that α-ENaC immunoreactivity detected in taste bud cells decreased slowly after irinotecan administration, and that administration of irinotecan had little effect on the number of cells expressing the cellular proliferation marker, Ki67, within or around taste buds. Our results imply that irinotecan administration may be responsible for altered behavioral sodium taste responses originating from ENaC function in the periphery, while being accompanied by the reduction of α-ENaC expression at the apical membrane of taste receptor cells without disturbing taste cell renewal.

Fat preference deficits and experience-induced recovery in global taste-deficient Trpm5 and Calhm1 knockout mice

There is much evidence that gustation mediates the preference for dietary fat in rodents. Several studies indicate that mice have fat taste receptors that activate downstream signaling elements, including TRPM5 and CALHM1 ion channels and P2×2/P2×3 purinergic gustatory nerve receptors. Experiment 1 further documented the involvement of TRPM5 in fat appetite by giving Trpm5 knockout (KO) mice, which show global taste deficits, 24-h two-bottle choice tests with ascending concentrations of soybean oil (0.1 - 10% Intralipid) vs. water. Unlike wildtype (WT) mice, naive Trpm5 KO mice were indifferent to 0.5 - 2.5% fat. They preferred 5-10% fat but consumed much less than WT mice. The same KO mice preferred all fat concentrations in a second test series, which is attributed to a postoral fat conditioned attraction to the non-taste flavor qualities of the Intralipid, although they consumed less fat than the WT mice. The fat preference deficits of the Trpm5 KO mice were as great or greater than those observed in Calhm1 KO mice, another KO line with global taste deficits. Experiment 2 examined experience-enhanced fat preferences in Trpm5 KO and Calhm1 KO mice by giving them one-bottle training with 1%, 2.5%, and 5% fat prior to two-bottle fat vs. water tests. The KO mice displayed increased two-bottle preferences for all concentrations, although they still consumed less 1% and 2.5% fat than WT mice. Thus, the postoral actions of fat induce robust preferences for fat in taste-deficient mice, but do not stimulate the high fat intakes observed in WT mice with normal fat taste signaling.

Differential fructose and glucose appetition in DBA/2, 129P3 and C57BL/6 × 129P3 hybrid mice revealed by sugar versus non-nutritive sweetener tests

Inbred mouse strains differ in their postoral appetite stimulating response (appetition) to fructose as demonstrated in intragastric (IG) sugar conditioning and oral sugar vs. nonnutritive conditioning experiments. For example, FVB and SWR strains show experience-induced preferences for 8% fructose over a 0.1% sucralose + 0.1% saccharin (S + S) solution, whereas C57BL/6 (B6) and BALB/c strains do not. All strains, however, learn to prefer 8% glucose to S + S after experience, which is attributed to the potent appetition actions of this sugar. The present study extended this analysis to DBA/2 (DBA) and 129P3 (129) inbred mice. In Experiment 1A, ad libitum fed DBA and 129 mice preferred S + S to fructose before and after separate experience with the two sweeteners, indicating an indifference to the postoral nutrient effects of the sugar. When food restricted (Experiment 1B), 129 mice continued to prefer S + S to fructose while DBA mice showed equal preference for the sweeteners after experience, indicating some sensitivity to fructose appetition. In Experiment 1C, both strains acquired significant preferences for glucose over S + S after experience, confirming their sensitivity to postoral glucose appetition. Experiment 2 revealed that C57BL/6 × 129P3 (B6:129) hybrid mice responded like inbred B6 mice and 129 mice in acquiring a preference for glucose but not fructose over S + S. This is of interest because sweet "taste-blind" P2 × 2 / P2 × 3 double-knockout (DKO) mice on a B6:129 genetic background prefer fructose to water in 24 h tests, which is indicative of fructose appetition. Whether differences in the genetic makeup of DKO and B6:129 hybrid mice or other factors explain the fructose appetition of the DKO mice remains to be determined.

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.

Forty-eight hour conditioning produces a robust long lasting flavor preference in rats

Conditioned flavor preference (CFP) learning is a form of associative learning in ingestive behavior. CFP Learning can be rapid and produces preferences of varying strengths that can be exceptionally persistent. We sought to establish a method to produce a robust long-lasting CFP in rats. Rats were given 48-h access (conditioning) to a CS+ flavor (grape or cherry 0.05% Kool-Aid, counterbalanced) mixed with 8% glucose and 0.05% saccharin. In order to determine the strength of conditioning rats were given 14 consecutive days of 24-h access to CS+ and CS- flavors mixed only with 0.05% Kool-Aid and 0.05% saccharin (extinction), then further tested 34 days after the last extinction test (48 days post conditioning) for 2 consecutive days with the CS+ and CS-. We found that not only did the learned CFP fail to extinguish over 14 days of testing, but it also persisted for at least 48 days after conditioning. These data provide a method to produce a robust, long lasting and persistent CFP for use in future ingestive behavior research.

Galactose protects against cell damage in mouse models of acute pancreatitis

Acute pancreatitis (AP), a human disease in which the pancreas digests itself, has substantial mortality with no specific therapy. The major causes of AP are alcohol abuse and gallstone complications, but it also occurs as an important side effect of the standard asparaginase-based therapy for childhood acute lymphoblastic leukemia. Previous investigations into the mechanisms underlying pancreatic acinar cell death induced by alcohol metabolites, bile acids, or asparaginase indicated that loss of intracellular ATP generation is an important factor. We now report that, in isolated mouse pancreatic acinar cells or cell clusters, removal of extracellular glucose had little effect on this ATP loss, suggesting that glucose metabolism was severely inhibited under these conditions. Surprisingly, we show that replacing glucose with galactose prevented or markedly reduced the loss of ATP and any subsequent necrosis. Addition of pyruvate had a similar protective effect. We also studied the effect of galactose in vivo in mouse models of AP induced either by a combination of fatty acids and ethanol or asparaginase. In both cases, galactose markedly reduced acinar necrosis and inflammation. Based on these data, we suggest that galactose feeding may be used to protect against AP.

Chronic Intake of Commercial Sweeteners Induces Changes in Feeding Behavior and Signaling Pathways Related to the Control of Appetite in BALB/c Mice

Nonnutritive sweetener use is a common practice worldwide. Although considered safe for human consumption, accumulating evidence suggests these compounds may affect metabolic homeostasis; however, there is no consensus on the role of frequent sweetener intake in appetite and weight loss. We sought to determine whether frequent intake of commercial sweeteners induces changes in the JAK2/STAT3 signaling pathway in the brain of mice, as it is involved in the regulation of appetite and body composition. We supplemented adult BALB/c mice with sucrose, steviol glycosides (SG), or sucralose, daily, for 6 weeks. After supplementation, we evaluated body composition and expression of total and phosphorylated JAK2, STAT3, and Akt, as well as SOCS3 and ObRb, in brain tissue. Our results show that frequent intake of commercial SG decreases energy intake, adiposity, and weight gain in male animals, while increasing the expression of pJAK2 and pSTAT3 in the brain, whereas sucralose increases weight gain and pJAK2 expression in females. Our results suggest that chronic intake of commercial sweeteners elicits changes in signaling pathways that have been related to the control of appetite and energy balance in vivo, which may have relevant consequences for the nutritional state and long term health of the organism.

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.

Behavioral Evidence for More than One Taste Signaling Pathway for Sugars in Rats

By conventional behavioral measures, rodents respond to natural sugars, such as glucose and fructose, as though they elicit an identical perceptual taste quality. Beyond that, the metabolic and sensory effects of these two sugars are quite different. Considering the capacity to immediately respond to the more metabolically expedient sugar, glucose, would seem advantageous for energy intake, the present experiment assessed whether experience consuming these two sugars would modify taste-guided ingestive responses to their yet unknown distinguishing orosensory properties. One group (GvF) had randomized access to three concentrations of glucose and fructose (0.316, 0.56, 1.1 m) in separate 30-min single access training sessions, whereas control groups received equivalent exposure to the three glucose or fructose concentrations only, or remained sugar naive. Comparison of the microstructural licking patterns for the two sugars revealed that GvF responded more positively to glucose (increased total intake, increased burst size, decreased number of pauses), relative to fructose, across training. As training progressed, GvF rats began to respond more positively to glucose in the first minute of the session when intake is principally taste-driven. During post-training brief-access taste tests, GvF rats licked more for glucose than for fructose, whereas the other training groups did not respond differentially to the two sugars. Additional brief access testing showed that this did not generalize to Na-saccharin or galactose. Thus, in addition to eliciting a common taste signal, glucose and fructose produce distinct signals that are apparently rendered behaviorally relevant and hedonically distinct through experience. The taste pathway(s) underlying this remain to be identified.

SIGNIFICANCE STATEMENT

The T1R2+T1R3 heterodimer is thought by many to be the only taste receptor for sugars. Although most sugars have been conventionally shown to correspondingly produce a unitary taste percept (sweet), there is reason to question this model. Here, we demonstrate that rats that repeatedly consumed two metabolically distinct sugars (glucose and fructose), and thus have had the opportunity to associate the tastes of these sugars with their differential postoral consequences, initially respond identically to the orosensory properties of the two sugars but eventually respond more positively to glucose. Thus, in addition to the previously identified common taste pathway, glucose and fructose must engage distinct orosensory pathways, the underlying molecular and neural mechanisms of which now await discovery.

Ghrelin signaling is not essential for sugar or fat conditioned flavor preferences in mice

The oral and post-oral actions of sugar and fat stimulate intake and condition flavor preferences in rodents through a process referred to as appetition. Ghrelin is implicated in food reward processing, and this study investigated its involvement in nutrient conditioning in mice. In Exp. 1 ghrelin receptor-null (GHSR-null) and C57BL/6 wildtype (WT) mice learned to prefer a flavor (CS+) mixed into 8% glucose over another flavor (CS-) mixed into a "sweeter" but non-nutritive 0.1% sucralose+saccharin (S+S) solution. In Exp. 2 treating WT mice with a ghrelin receptor antagonist [(D-Lys3)-GHRP-6] during flavor training did not prevent them from learning to prefer the CS+ glucose over the CS-S+S flavor. GHSR-null and WT mice were trained in Exp. 3 to drink a CS+ paired with intragastric (IG) infusion of 16% glucose and a CS- paired with IG water. Both groups drank more CS+ than CS- in training and preferred the CS+ to CS- in a choice test. The same (Exp. 4) and new (Exp. 5) GHSR-null and WT mice learned to prefer a CS+ flavor paired with IG fat (Intralipid) over a CS- flavor paired with IG water. GHSR-null and WT mice also learned to prefer a CS+ flavor added to 8% fructose over a CS- added to water. Together, these results indicate that ghrelin receptor signaling is not required for flavor preferences conditioned by the oral or post-oral actions of sugar and fat. This contrasts with other findings implicating ghrelin signaling in food reward processing and food-conditioned place preferences.

Food for the brain

Diet is a major issue facing humanity. To combat malnourishment and diseases associated with overnutrition, both research and technological breakthroughs are needed.