Altered taste sensitivity in obese, prediabetic OLETF rats lacking CCK-1 receptors

Distinct Network Patterns Emerge from Cartesian and XOR Epistasis Models: A Comparative Network Science Analysis

Background

Epistasis, the phenomenon where the effect of one gene (or variant) is masked or modified by one or more other genes, can significantly contribute to the observed phenotypic variance of complex traits. To date, it has been generally assumed that genetic interactions can be detected using a Cartesian, or multiplicative, interaction model commonly utilized in standard regression approaches. However, a recent study investigating epistasis in obesity-related traits in rats and mice has identified potential limitations of the Cartesian model, revealing that it only detects some of the genetic interactions occurring in these systems. By applying an alternative approach, the exclusive-or (XOR) model, the researchers detected a greater number of epistatic interactions and identified more biologically relevant ontological terms associated with the interacting loci. This suggests that the XOR model may provide a more comprehensive understanding of epistasis in these species and phenotypes. To further explore these findings and determine if different interaction models also make up distinct epistatic networks, we leverage network science to provide a more comprehensive view into the genetic interactions underlying BMI in this system.

Results

Our comparative analysis of networks derived from Cartesian and XOR interaction models in rats (Rattus norvegicus) uncovers distinct topological characteristics for each model-derived network. Notably, we discover that networks based on the XOR model exhibit an enhanced sensitivity to epistatic interactions. This sensitivity enables the identification of network communities, revealing novel trait-related biological functions through enrichment analysis. Furthermore, we identify triangle network motifs in the XOR epistatic network, suggestive of higher-order epistasis, based on the topology of lower-order epistasis.

Conclusions

These findings highlight the XOR model's ability to uncover meaningful biological associations as well as higher-order epistasis from lower-order epistatic networks. Additionally, our results demonstrate that network approaches not only enhance epistasis detection capabilities but also provide more nuanced understandings of genetic architectures underlying complex traits. The identification of community structures and motifs within these distinct networks, especially in XOR, points to the potential for network science to aid in the discovery of novel genetic pathways and regulatory networks. Such insights are important for advancing our understanding of phenotype-genotype relationships.

Alteration of Sweet and Bitter Taste Sensitivity with Development of Glucose Intolerance in Non-insulin-Dependent Diabetes Mellitus Model OLETF Rats

Patients with diabetes exhibit altered taste sensitivity, but its details have not been clarified yet. Here, we examined alteration of sweet taste sensitivity with development of glucose intolerance in Otsuka Long-Evans Tokushima Fatty (OLETF) rats as a model of non-insulin-dependent diabetes mellitus. Compared to the cases of Long Evans Tokushima Otsuka (LETO) rats as a control, glucose tolerance of OLETF rats decreased with aging, resulting in development of diabetes at 36-weeks-old. In brief-access tests with a mixture of sucrose and quinine hydrochloride, OLETF rats at 25 or more-weeks-old seemed to exhibit lower sweet taste sensitivity than age-matched LETO ones, but the lick ratios of LETO, but not OLETF, rats for the mixture and quinine hydrochloride solutions decreased and increased, respectively, aging-dependently. Expression of sweet taste receptors, T1R2 and T1R3, in circumvallate papillae (CP) was almost the same in LETO and OLETF rats at 10- and 40-weeks-old, while expression levels of a bitter taste receptor, T2R16, were greater in 40-weeks-old rats than in 10-weeks-old ones in both strains. There was no apparent morphological alteration in taste buds in CP between 10- and 40-weeks-old LETO and OLETF rats. Metagenomic analysis of gut microbiota revealed strain- and aging-dependent alteration of mucus layer-regulatory microbiota. Collectively, we concluded that the apparent higher sweet taste sensitivity in 25 or more-weeks-old OLETF rats than in age-matched LETO rats was due to the aging-dependent increase of bitter taste sensitivity in LETO rats with alteration of the gut microbiota.

Reduced Striatal Dopamine Transporter Availability and Heightened Response to Natural and Pharmacological Stimulation in CCK-1R-Deficient Obese Rats

Alterations in dopamine neurotransmission are associated with obesity and food preferences. Otsuka Long-Evans Tokushima Fatty (OLETF) rats that lack functional cholecystokinin receptor type-1 (CCK-1R), due to a natural mutation, exhibit impaired satiation, are hyperphagic, and become obese. In addition, compared to lean control Long-Evans Tokushima (LETO) rats, OLETF rats have pronounced avidity for over-consuming palatable sweet solutions, have greater dopamine release to psychostimulants, reduced dopamine 2 receptor (D2R) binding, and exhibit increased sensitivity to sucrose reward. This supports altered dopamine function in this strain and its general preference for palatable solutions such as sucrose. In this study, we examined the relationship between OLETF's hyperphagic behavior and striatal dopamine signaling by investigating basal and amphetamine stimulated motor activity in prediabetic OLETF rats before and after access to sucrose solution (0.3 M) compared to non-mutant control LETO rats, as well as availability of dopamine transporter (DAT) using autoradiography. In the sucrose tests, one group of OLETF rats received ad libitum access to sucrose while the other group received an amount of sucrose equal to that consumed by the LETO. OLETFs with ad libitum access consumed significantly more sucrose than LETOs. Sucrose exerted a biphasic effect on basal activity in both strains, i.e., reduced activity for 1 week followed by increased activity in weeks 2 and 3. Basal locomotor activity was reduced (-17%) in OLETFs prior to sucrose, compared to LETOs. Withdrawal of sucrose resulted in increased locomotor activity in both strains. The magnitude of this effect was greater in OLETFs and the activity was increased in restricted compared to ad-libitum-access OLETFs. Sucrose access augmented AMPH-responses in both strains with a greater sensitization to AMPH during week 1, an effect that was a function of the amount of sucrose consumed. One week of sucrose withdrawal sensitized AMPH-induced ambulatory activity in both strains. In OLETF with restricted access to sucrose, withdrawal resulted in no further sensitization to AMPH. DAT availability in the nucleus accumbens shell was significantly reduced in OLETF compared with aged-matched LETO. Together, these findings show that OLETF rats have reduced basal DA transmission and a heightened response to natural and pharmacological stimulation.

Alcohol-induced increase in BMP levels promotes fatty liver disease in male prediabetic stage Otsuka Long-Evans Tokushima Fatty rats

Alcohol consumption exacerbates liver abnormalities in animal models, but whether it increases the severity of liver disease in early diabetic or prediabetic rats is unclear. To investigate the molecular mechanisms underlying alcohol-induced liver steatosis or hepatitis, we used a prediabetic animal model. Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Fatty (LETO) rats were pair-fed with an ethanol-containing liquid diet for 6 weeks. Compared with controls, OLETF and LETO rats displayed more pronounced liver steatosis and higher plasma levels of serum glutamic oxaloacetic transaminase (SGOT) and serum glutamate pyruvate transaminase (SPGT), indicating liver injury. Ethanol-fed LETO (Pd-L-E) rats showed mild liver steatosis and no inflammation compared with ethanol-fed OLETF (Pd-O-E) rats. Although precursor and active SREBP-1 levels in the liver of ethanol-fed OLETF rats significantly increased compared with control diet-fed OLETF rats (Pd-O-C), those of Pd-L-E rats did not. Bone morphogenetic protein (BMP) and TGF-β1 balance in Pd-O-E rats was significantly altered because BMP signaling was upregulated by inducing BMP2, BMP4, BMP7, BMP9, Smad1, and Smad4, whereas TGF-β1, Smad3, and Erk were downregulated. Activation of TGF-β/Smad signaling inhibited BMP2 and BMP9 expression and increased epithelial-mesenchymal transition (EMT) marker levels (Hepcidin, Snail, and Twist) in the liver of LETO rats. Livers of ethanol-fed OLETF rats showed increased levels of vimentin, FSP-1, α-SMA, MMP1, MMP13, and collagen III compared with rats of other groups, whereas EMT marker levels did not change. Thus, BMP exerted anti- and/or pro-fibrotic effects in ethanol-fed rats. Therefore, BMP and TGF-β, two key members of the TGF-β superfamily, play important but diverse roles in liver steatosis in young LETO and OLETF rats.

Nutritional geometry framework of sucrose taste in Drosophila

Dietary protein (P) and carbohydrate (C) have a major impact on sweet taste sensation. However, it remains unclear whether the balance of P and C influences sweet taste sensitivity. Here, we use the nutritional geometry framework (NGF) to address the interaction of protein and carbohydrates and on sweet taste using Drosophila as a model. Our results reveal that high-protein, low-carbohydrate (HPLC) diets sensitize to sweet taste and low-protein, high-carbohydrate (LPHC) diets desensitize sweet taste in both male and female flies. We further investigate the underlying mechanisms of these two diets' effect on sweet taste using RNA sequencing. When compared to the LPHC diet, the mRNA expression of genes involved in the metabolism of glycine, serine, and threonine is significantly upregulated in the HPLC diet, suggesting these amino acids may mediate sweet taste perception. We further find that sweet sensitization occurs in flies fed with the LPHC diet supplemented with serine and threonine. Our study demonstrates that sucrose taste sensitivity is affected by the balance of dietary protein and carbohydrates possibly through changes in serine and threonine.

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.

Short-term high-fat diet consumption increases body weight and body adiposity and alters brain stem taste information processing in rats

Diet-induced obesity is known to develop whether exposed to a high-energy diet (HED) or a high-fat diet (HFD). However, it is still not clear whether the elevated energy content or the macronutrient imbalance is the key factor in early disease progression. Therefore, this study compared the short-term effects of 2 widely used rodent obesogenic diets, an HFD with 60 kcal% fat content and a carbohydrate-based HED, on the body weight, body fat content, glucose tolerance, and neuronal taste responses in rats. We found that only HFD induced an early significant body weight increase compared with the control normal diet (ND) group, starting on week 4, and resulting in a significantly elevated body adiposity compared with both the ND and HED groups. Oral glucose tolerance test revealed no difference across groups. Subsequently, we also found that HFD resulted in a significant body weight gain even under energy-restricted (isocaloric to ND) conditions. In vivo electrophysiological recordings revealed that only the ad libitum HFD and not the isocaloric-HFD altered the brain stem gustatory neural responses to oral taste stimulation. In conclusion, this study showed that increased fat intake might result in significant body weight gain even under isocaloric and metabolically healthy conditions and demonstrated changes in central taste processing in an early stage of dietary obesity. A better understanding of these initial physiological changes may offer new drug targets for preventing obesity.

OUP accepted manuscript

Administration of cholecystokinin (CCK) or the glucagon-like peptide 1 (GLP-1) receptor agonist Exendin-4 (Ex-4) reduces food intake. Findings in the literature suggest CCK reduces intake primarily as a satiety signal whereas GLP-1 may play a role in both satiety and reward-related feeding signals. Compounds that humans describe as âsweetâ and âfattyâ are palatable yet are signaled via separate transduction pathways. Here, unconditioned lick responses to sucrose and intralipid were measured in a brief-access lick procedure in food-restricted male rats in response to i.p. administration of Ex-4 (3 h before test), CCK (30 min before test), or a combination of both. The current experimental design measures lick responses to water and varying concentrations of both sucrose (0.03, 0.1, and 0.5 M) and intralipid (0.2%, 2%, and 20%) during 10-s trials across a 30-min single test session. This design minimized postingestive influences. Compared with saline-injected controls, CCK (1.0, 3.0, or 6.0 µg/kg) did not change lick responses to sucrose or intralipid. Number of trials initiated and lick responses to both sucrose and intralipid were reduced in rats injected with 3.0 µg/kg, but not 1.0 µg/kg Ex-4. The supplement of CCK did not alter lick responses or trials initiated compared with Ex-4 administration alone. These findings support a role for GLP-1 but not CCK in the oral responsiveness to palatable stimuli. Furthermore, Ex-4-induced reductions were observed for both sucrose and intralipid, compounds representing âsweetâ and âfat,â respectively.

Comparing Taste Detection Thresholds across Individuals Following Vegan, Vegetarian, or Omnivore Diets

Taste perception plays an undisputed role in food choice, preference, and intake. Recent literature suggests that individual diet and taste sensitivity may have a reciprocal relationship, with evidence highlighting that specific diets can alter taste sensitivities. Commensurate with an increase in the prevalence of plant-based diets is the importance of investigating if following a vegetarian or vegan diet is associated with altered taste sensitivities. In this study, the taste detection thresholds for six compounds (i.e., sweet-sucrose, salty-sodium chloride, sour-citric acid, umami-monosodium glutamate, MSG, bitter-caffeine, and metallic-iron II sulphate heptahydrate) were measured for a total of 80 healthy, New Zealand European females aged 18-45 years old, who were categorised as 22 vegans, 23 vegetarians, and 35 omnivores. Each participant's detection thresholds to these compounds were measured across two sessions, using an ascending Method of Limits with two-alternative-forced-choice presentations. The threshold data were analysed using both multivariate (i.e., principal component analysis) and univariate (i.e., ANCOVA) techniques to assess differences across the three types of diet. Multivariate analysis suggested that the omnivore group had distinct taste sensitivity patterns across the six compounds compared to the vegetarian or vegan group, which were characterised by relatively heightened sensitivity to metallic and lowered sensitivity to sweetness. Furthermore, the vegetarian group was shown to have a significantly lower detection threshold to bitterness (i.e., caffeine) relative to the other two groups (p < 0.001). While future study is required to investigate the cause-effect relationship between individual diet and taste sensitivities, the present study provides a systematic evaluation of taste sensitivities of individuals following distinct diets. This information may be valuable to future gustatory research as well as to food manufacturers.

Y1 receptors modulate taste-related behavioral responsiveness in male mice to prototypical gustatory stimuli

Mammalian taste bud cells express receptors for numerous peptides implicated elsewhere in the body in the regulation of metabolism, nutrient assimilation, and satiety. The perturbation of several peptide signaling pathways in the gustatory periphery results in changes in behavioral and/or physiological responsiveness to subsets of taste stimuli. We previously showed that Peptide YY (PYY) - which is present in both saliva and in subsets of taste cells - can affect behavioral taste responsiveness and reduce food intake and body weight. Here, we investigated the contributions of taste bud-localized receptors for PYY and the related Neuropeptide Y (NPY) on behavioral taste responsiveness. Y1R, but not Y2R, null mice show reduced responsiveness to sweet, bitter, and salty taste stimuli in brief-access taste tests; similar results were seen when wildtype mice were exposed to Y receptor antagonists in the taste stimuli. Finally, mice in which the gene encoding the NPY propeptide was deleted also showed reduced taste responsiveness to sweet and bitter taste stimuli. Collectively, these results suggest that Y1R signaling, likely through its interactions with NPY, can modulate peripheral taste responsiveness in mice.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Genetics of mouse behavioral and peripheral neural responses to sucrose

Mice of the C57BL/6ByJ (B6) strain have higher consumption of sucrose, and stronger peripheral neural responses to it, than do mice of the 129P3/J (129) strain. To identify quantitative trait loci (QTLs) responsible for this strain difference and to evaluate the contribution of peripheral taste responsiveness to individual differences in sucrose intake, we produced an intercross (F₂) of 627 mice, measured their sucrose consumption in two-bottle choice tests, recorded the electrophysiological activity of the chorda tympani nerve elicited by sucrose in a subset of F₂ mice, and genotyped the mice with DNA markers distributed in every mouse chromosome. We confirmed a sucrose consumption QTL (Scon2, or Sac) on mouse chromosome (Chr) 4, harboring the Tas1r3 gene, which encodes the sweet taste receptor subunit TAS1R3 and affects both behavioral and neural responses to sucrose. For sucrose consumption, we also detected five new main-effect QTLs, Scon6 (Chr2), Scon7 (Chr5), Scon8 (Chr8), Scon3 (Chr9), and Scon9 (Chr15), and an epistatically interacting QTL pair Scon4 (Chr1) and Scon3 (Chr9). No additional QTLs for the taste nerve responses to sucrose were detected besides Scon2 (Tas1r3) on Chr4. Identification of the causal genes and variants for these sucrose consumption QTLs may point to novel mechanisms beyond peripheral taste sensitivity that could be harnessed to control obesity and diabetes.

The herbal extract ALS-L1023 from Melissa officinalis reduces weight gain, elevated glucose levels and β-cell loss in Otsuka Long-Evans Tokushima fatty rats

ETHNOPHARMACOLOGICAL RELEVANCE

Melissa officinalis L. (Labiatae; lemon balm) is a traditional medicinal plant with hypoglycemic and hypolipidemic effects; however, how it imparts its beneficial effects remains unclear. We thus hypothesized that the herbal extract ALS-L1023, isolated from Melissa officinalis, inhibits obesity and diabetes, and tested our hypothesis using Otsuka Long-Evans Tokushima fatty (OLETF) rats, which are an established animal model of type 2 diabetes.

MATERIALS AND METHODS

In this study, 28-week-old OLETF rats were fed a high-fat diet for 4 weeks to induce a marked impairment of the insulin response and were treated with or without ALS-L1023. Subsequently, the variables and determinants of glucose metabolism and pancreatic function were assessed via blood analysis, histology, immunohistochemistry, and real-time polymerase chain reaction.

RESULTS

The administration of ALS-L1023 resulted in a weight reduction without changes in food intake. It also markedly inhibited hyperglycemia and hypoinsulinemia, and restored β-cell mass that was severely impaired in OLETF rats. There was a decrease in lipid accumulation in the liver and skeletal muscle of the obese rats after treatment with ALS-L1023. Concomitantly, there was an increase in the expression levels of fatty acid-oxidizing enzymes (AMPKα2, ACOX, MCAD, and VLCAD) in the liver and skeletal muscle after ALS-L1023 treatment. Furthermore, ALS-L1023 attenuated the pancreatic inflammation including the infiltration of CD68-positive macrophages and mast cells, in addition to attenuating the expression of inflammatory factors (IL-6 and CD68).

CONCLUSIONS

These results suggest that treatment with ALS-L1023 may reduce weight gain, elevated glucose levels, and β-cell loss, by changing the expression of fatty acid-oxidizing enzymes in the liver and skeletal muscle, including inflammatory factors in the pancreas. These findings indicate that ALS-L1023 may be an effective therapeutic strategy to treat human obesity and type 2 diabetes.

Differential Effects of Maternal High Fat Diet During Pregnancy and Lactation on Taste Preferences in Rats

Maternal intake of high fat diet (HFD) increases risk for obesity and metabolic disorders in offspring. Developmental programming of taste preference is a potential mechanism by which this occurs. Whether maternal HFD during pregnancy, lactation, or both, imposes greater risks for altered taste preferences in adult offspring remains a question, and in turn, was investigated in the present study. Four groups of offspring were generated based on maternal HFD access: (1) HFD during pregnancy and lactation (HFD); (2) HFD during pregnancy (HFD-pregnancy); (3) HFD during lactation (HFD-lactation); and (4) normal diet (ND) during pregnancy and lactation (ND). Adult offspring 70 days of age underwent sensory and motivational taste preference testing with various concentrations of sucrose and Intralipid solutions using brief-access automated gustometers (Davis-rigs) and 24 h two-bottle choice tests, respectively. To control for post-gestational diet effects, offspring in all experimental groups were weaned on ND, and did not differ in body weight or glucose tolerance at the time of testing. Offspring exposed to maternal HFD showed increased sensory taste responses for 0.3, 0.6, 1.2 M sucrose solutions in HFD and 0.6 M in HFD-pregnancy groups, compared to animals exposed to ND. Similar effects were noted for lower concentrations of Intralipid in HFD (0.05, 0.10%) and HFD-pregnancy (0.05, 0.10, 0.5%) groups. The HFD-lactation group showed an opposite, diminished responsiveness for sucrose at the highest concentrations (0.9, 1.2, 1.5 M), but not for Intralipid, compared to ND animals. Extended-access two-bottle tests did not reveal major difference across the groups. Our study shows that maternal HFD during pregnancy and lactation has markedly different effects on preferences for palatable sweet and fatty solutions in adult offspring and suggests that such developmental programing may primarily affect gustatory mechanisms. Future studies are warranted for determining the impact of taste changes on development of obesity and metabolic disorders in a “real” food environment with food choices available, as well as to identify specific underlying mechanisms.

Flavor identification inversely correlates with body mass index (BMI)

BACKGROUND AND AIMS

Dietary choices are influenced by several factors including physiological, social, or genetic factors. Among these, flavor is the most important determinant modulating food preferences. The aim of the present study was to assess flavor identification abilities in patients with obesity (Ob) in comparison with matched normal weight (NW) and over-weight (OW) subjects using a specific and validated chemosensory test.

METHODS AND RESULTS

The flavor test was administered to 140 Ob patients recruited in the obesity outpatient Unit at the Federico II University hospital and to the same number of NW and OW subjects matched by sex, age, and smoking habit. Flavor score (FS) inversely correlated with BMI. Median [Q1; Q3] FS was significantly higher in NW (14.5 [12; 16]) than in Ob (13 [10; 15] p < 0.001) and not significantly different from OW (14 [12; 16]) individuals. FS was also higher in OW than in Ob subjects (p < 0.005). When separated according to age quartiles, the BMI-related differences in FS were still significant in younger quartiles, while they were abolished in the older.

CONCLUSIONS

BMI is a critical factor modulating flavor identification, particularly in young subjects. Further investigations are needed to explore the precise mechanism and the causal relationship between body weight and olfactory dysfunctions.

CLINICALTRIAL ID

NCT03506074.

Differential Effects of Diet and Weight on Taste Responses in Diet-Induced Obese Mice

OBJECTIVE

Previous studies have reported that individuals with obesity have reduced taste perception, but the relationship between obesity and taste is poorly understood. Earlier work has demonstrated that diet-induced obesity directly impairs taste. Currently, it is not clear whether these changes to taste are due to obesity or to the high-fat diet exposure. The goal of the current study was to determine whether diet or excess weight is responsible for the taste deficits induced by diet-induced obesity.

METHODS

C57BL/6 mice were placed on either high-fat or standard chow in the presence or absence of captopril. Mice on captopril did not gain weight when exposed to a high-fat diet. Changes in the responses to different taste stimuli were evaluated using live cell imaging, brief-access licking, immunohistochemistry, and real-time polymerase chain reaction.

RESULTS

Diet and weight gain each affected taste responses, but their effects varied by stimulus. Two key signaling proteins, α-gustducin and phospholipase Cβ2, were significantly reduced in the mice on the high-fat diet with and without weight gain, identifying a potential mechanism for the reduced taste responsiveness to some stimuli.

CONCLUSIONS

Our data indicate that, for some stimuli, diet alone can cause taste deficits, even without the onset of obesity.

Taste Responses in the Nucleus of the Solitary Tract of Awake Obese Rats Are Blunted Compared With Those in Lean Rats

Taste perception changes with obesity but the underlying neural changes remain poorly understood. To address this issue, we recorded taste responses from single cells in the nucleus tractus solitarius (NTS, the first synapse in the central gustatory circuit) in awake, diet-induced obese [(DIO; ≥ 8 weeks on a high-energy diet (45%fat, 17% sugar; HED)], and lean rats. Rats were implanted with a bundle of microelectrodes in the NTS and allowed to recover. Water-deprived rats were allowed to freely lick various tastants in an experimental chamber. Taste stimuli included an array of sapid stimuli dissolved in artificial saliva (AS). Each taste trial consisted of five consecutive licks followed by five AS licks presented on a VR5 schedule. Results showed that taste responses (n = 49 for DIO; n = 74 for lean rats) in NTS cells in DIO rats were smaller in magnitude, shorter in duration, and longer in latency that those in lean rats. However, there were proportionately more taste-responsive cells in DIO than in lean rats. Lick coherence in DIO rats was significantly lower than in lean rats, both in taste-responsive, and lick-related cells (n = 172 in lean; n = 65 in DIO). Analyses of temporal coding showed that taste cells in DIO rats conveyed less information about taste quality than cells in lean rats. Collectively, results suggest that a HED produces blunted, but more prevalent, responses to taste in the NTS, and a weakened association of taste responses with ingestive behavior. These neural adaptations may represent both negative effects and compensatory mechanisms of a HED that may underlie deficits in taste-related behavior associated with obesity.

A role for taste receptors in (neuro)endocrinology?

The sense of taste is positioned at the forefront when it comes to the interaction of our body with foodborne chemicals. However, the role of our taste system, and in particular its associated taste receptors, is not limited to driving food preferences leading to ingestion or rejection before other organs take over responsibility for nutrient digestion, absorption and metabolic regulation. Taste sensory elements do much more. On the one hand, extra-oral taste receptors from the brain to the gut continue to sense nutrients and noxious substances after ingestion and, on the other hand, the nutritional state feeds back on the taste system. This intricate regulatory network is orchestrated by endocrine factors that are secreted in response to taste receptor signalling and, in turn regulate the taste receptor cells themselves. The present review summarises current knowledge on the endocrine regulation of the taste perceptual system and the release of hunger/satiety regulating factors by gastrointestinal taste receptors. Furthermore, the regulation of blood glucose levels via the activation of pancreatic sweet taste receptors and subsequent insulin secretion, as well as the influence of bitter compounds on thyroid hormone release, is addressed. Finally, the central effects of tastants are discussed briefly.

Altered salt taste response and increased tongue epithelium Scnna1 expression in adult Engrailed-2 null mice

Sensory impairments are critical for diagnosing and characterizing neurodevelopmental disorders. Taste is a sensory modality often not well characterized. Engrailed-2 (En2) is a transcription factor critical for neural development, and mice lacking En2 (En2^-/-) display signs of impaired social interaction, cognitive processes (e.g., learning and memory, conditioned fear), and neurodevelopmental alterations. As such, En2^-/- mice display the behavioral deficits and neural impairments characteristic of the core symptoms associated with autism spectrum disorder (ASD). The objective of this study was to characterize the taste function in En2^-/- compared with En2^+/+ in adult male mice. Measuring taste responsiveness by an automated gustometer, En2 null mice had decreased lick responses for 1.6 M fructose, whereas they demonstrated an increased taste responsivity (i.e., relative to water) at 0.3 M sodium chloride and 1 M monosodium glutamate. In a separate cohort of mice, En2^-/- mice had an increased preference for sodium chloride over a range of concentrations (0.032-0.3 M) compared with En2^+/+ mice. Regional gene expression of the tongue epithelium demonstrated an increase in Scnn1a, T2R140, T1R3, and Trpm5 and a decrease in Pkd1l3 in En2 null mice. Taken together, such data indicate that deficits in En2 can produce sensory impairments that can have a measurable impact on taste, particularly salt taste.

Toll-like receptor 4 mediates fat, sugar, and umami taste preference and food intake and body weight regulation

OBJECTIVE

Immune and inflammatory pathways play important roles in the pathogenesis of metabolic disorders. This study investigated the role of toll-like receptor 4 (TLR4) in orosensory detection of dietary lipids and sugars.

METHODS

Taste preferences of TLR4 knockout (KO) and wild-type (WT) male mice under a standard and a high-fat, high-sugar diet were assessed with two-bottle tests. Gene expression of taste signaling molecules was analyzed in the tongue epithelium. The role of TLR4 in food intake and weight gain was investigated in TLR4 KO and WT mice fed a high-fat and high-sugar diet for 12 weeks.

RESULTS

Compared to WT mice, TLR4 KO mice showed reduced preference for lipids, sugars, and umami in a two-bottle preference test. The altered taste perception was associated with decreased levels of key taste regulatory molecules in the tongue epithelium. TLR4 KO mice on a high-fat and high-sugar diet consumed less food and drink, resulting in diminished weight gain.

CONCLUSIONS

TLR4 signaling promotes ingestion of sugar and fat by a mechanism involving increased preference for such obesogenic foods.

Improved Gustatory Sensitivity in Morbidly Obese Patients After Laparoscopic Sleeve Gastrectomy

Introduction:

The reduction in the preferences for sweet and fat containing tastes in obese patients who underwent bariatric surgery was relatively well shown; however, there are only limited data on the changes in the sensitivity of other tastes like sour, salty, and bitter.

Methods:

We investigated the changes in gustatory sensitivity of 52 morbidly obese patients (M/F, 22/30; age range, 19-60 years; BMI range, 32.5-63.0 kg/m2) after laparoscopic sleeve gastrectomy. The surgery was performed by the same surgeon using 5 ports technique. Gustatory sensitivity was tested preoperatively and 1 and 3 months after the surgery using standardized Taste Strips test.

Results:

There was a statistically significant improvement in the taste acuity to sweet, sour, salty, and bitter tastants in morbidly obese patients after the laparoscopic sleeve gastrectomy during the follow-up period of 3 months. Median whole test scores of the patients were increased from 11.5 preoperatively to 14 in the first and third months.

Conclusion:

In this study, we were able to show the significant improvement in gustatory sensitivity of morbidly obese patients after laparoscopic sleeve gastrectomy for the first time in literature.

Alterations in sucrose sham-feeding intake as a function of diet-exposure in rats maintained on calorically dense diets

We previously reported that rats increase meal size upon initial presentation of a calorically dense diet. The increase may be attributed to increased orosensory stimulation and/or reduced sensitivity to post-ingestive inhibitory signals. During feeding both types of signals are simultaneously in play; thus here, we compare responses in rats presented a high-energy diet (HE) or 45% high-fat diet (HF) with those of chow-fed controls (CHOW) in a sham-feeding procedure in which post-ingestive feedback is minimized. Measures of sham-feeding to sucrose were taken before diet manipulation (baseline), ~5 days (dynamic phase) and ~6 weeks (static phase) following introduction of the palatable diet, as well as after animals were switched back to standard chow (recovery phase). Some but not all the hypotheses based on our previous findings were confirmed by the outcomes here. Consistent with our hypothesis that enhanced orosensory stimulation during the dynamic phase compared with the static phase would generalize to increased intake of other palatable stimuli, HE rats showed higher sucrose intake during the dynamic phase compared with the static phase. Contrary to what we hypothesized, HE and HF rats did not increase responses to sucrose compared to CHOW rats. In fact, HE rats showed decreased responses compared to CHOW controls. Thus changes in orosensory stimulation do not necessarily generalize to increased intake of other palatable stimuli.

The endocrinology of taste receptors

Levels of obesity have reached epidemic proportions on a global scale, which has led to considerable increases in health problems and increased risk of several diseases, including cardiovascular and pulmonary diseases, cancer and diabetes mellitus. People with obesity consume more food than is needed to maintain an ideal body weight, despite the discrimination that accompanies being overweight and the wealth of available information that overconsumption is detrimental to health. The relationship between energy expenditure and energy intake throughout an individual's lifetime is far more complicated than previously thought. An improved comprehension of the relationships between taste, palatability, taste receptors and hedonic responses to food might lead to increased understanding of the biological underpinnings of energy acquisition, as well as why humans sometimes eat more than is needed and more than we know is healthy. This Review discusses the role of taste receptors in the tongue, gut, pancreas and brain and their hormonal involvement in taste perception, as well as the relationship between taste perception, overeating and the development of obesity.

Behavioral characterization of the hyperphagia synphilin-1 overexpressing mice

Synphilin-1 is a cytoplasmic protein that has been shown to be involved in the control of energy balance. Previously, we reported on the generation of a human synphilin-1 transgenic mouse model (SP1), in which overexpression of human synphilin-1 resulted in hyperphagia and obesity. Here, behavioral measures in SP1 mice were compared with those of their age-matched controls (NTg) at two time points: when there was not yet a group body weight difference (“pre-obese”) and when SP1 mice were heavier (“obese”). At both time points, meal pattern analyses revealed that SP1 mice displayed higher daily chow intake than non-transgenic control mice. Furthermore, there was an increase in meal size in SP1 mice compared with NTg control mice at the obese stage. In contrast, there was no meal number change between SP1 and NTg control mice. In a brief-access taste procedure, both “pre-obese” and “obese“ SP1 mice displayed concentration-dependent licking across a sucrose concentration range similar to their NTg controls. However, at the pre-obese stage, SP1 mice initiated significantly more trials to sucrose across the testing sessions and licked more vigorously at the highest concentration presented, than the NTg counterparts. These group differences in responsiveness to sucrose were no longer apparent in obese SP1 mice. These results suggest that at the pre-obese stage, the increased trials to sucrose in the SP1 mice reflects increased appetitive behavior to sucrose that may be indicative of the behavioral changes that may contribute to hyperphagia and development of obesity in SP1 mice. These studies provide new insight into synphilin-1 contributions to energy homeostasis.

Diet-induced obesity reduces the responsiveness of the peripheral taste receptor cells

Introduction

Obesity is a growing epidemic that causes many serious health related complications. While the causes of obesity are complex, there is conclusive evidence that overconsumption coupled with a sedentary lifestyle is the primary cause of this medical condition. Dietary consumption is controlled by appetite which is in turn regulated by multiple neuronal systems, including the taste system. However, the relationship between taste and obesity has not been well defined. Growing evidence suggests that taste perception in the brain is altered in obese animals and humans, however no studies have determined if there are altered taste responses in the peripheral taste receptor cells, which is the initiation site for the detection and perception of taste stimuli.

Methodology/Principal Findings

In this study, we used C57Bl/6 mice which readily become obese when placed on a high fat diet. After ten weeks on the high fat diet, we used calcium imaging to measure how taste-evoked calcium signals were affected in the obese mice. We found that significantly fewer taste receptor cells were responsive to some appetitive taste stimuli while the numbers of taste cells that were sensitive to aversive taste stimuli did not change. Properties of the taste-evoked calcium signals were also significantly altered in the obese mice. Behavioral analyses found that mice on the high fat diet had reduced ability to detect some taste stimuli compared to their littermate controls.

Conclusions/Significance

Our findings demonstrate that diet-induced obesity significantly influences peripheral taste receptor cell signals which likely leads to changes in the central taste system and may cause altered taste perception.

Cocaine- and amphetamine-regulated transcript (CART) peptide immunoreactivity in feeding- and reward-related brain areas of young OLETF rats

Cocaine- and amphetamine-regulated transcript (CART) peptide is expressed in brain areas involved in the control of appetite, drug reward and homeostatic regulation and it has an overall anorexigenic effect. Recently, we have shown that CART peptide immunoreactivity was significantly reduced in the rostral part of the nucleus accumbens and in the rostro-medial part of the nucleus of the solitary tract in adult CCK-1 receptor deficient obese diabetic Otsuka Long Evans Tokushima Fatty (OLETF) rats compared to Long Evans Tokushima Otsuka (LETO) lean controls. It is not clear, however, whether altered CART expression is caused primarily by the deficiency in CCK-1 signaling or whether is related to the obese and diabetic phenotype of the OLETF strain which develops at a later age. Therefore, in the present study, CART-immunoreaction in feeding-related areas of the brain was compared in young, age-matched (6-7 weeks old) non-obese, non-diabetic OLETF rats and in LETO controls. We found that, young, non-diabetic OLETF rats revealed unaltered distribution of CART-peptide expressing neurons and axons throughout the brain when compared to age-matched LETO rats. In contrast to previous results observed in the obese diabetic adult rats, intensity of CART immunoreaction did not differ in the areas related to control of food-intake and reward in the young OLETFs compared to young LETO rats. Our findings suggest that factors secondary to obesity and/or diabetes rather than impaired CCK-1 receptor signaling may contribute to altered CART expression in the OLETF strain.

Peptide regulators of peripheral taste function

The peripheral sensory organ of the gustatory system, the taste bud, contains a heterogeneous collection of sensory cells. These taste cells can differ in the stimuli to which they respond and the receptors and other signaling molecules they employ to transduce and encode those stimuli. This molecular diversity extends to the expression of a varied repertoire of bioactive peptides that appear to play important functional roles in signaling taste information between the taste cells and afferent sensory nerves and/or in processing sensory signals within the taste bud itself. Here, we review studies that examine the expression of bioactive peptides in the taste bud and the impact of those peptides on taste functions. Many of these peptides produced in taste buds are known to affect appetite, satiety or metabolism through their actions in the brain, pancreas and other organs, suggesting a functional link between the gustatory system and the neural and endocrine systems that regulate feeding and nutrient utilization.

Feeding and reward: ontogenetic changes in an animal model of obesity

Given that food is a natural reinforcement, deficits in the reward system can lead to disordered eating behavior, inducing or worsening an already existing pre-obese phenotype. In order to evaluate developmental, food-reward-related measures we used the OLETF rat, an animal model of early-onset overeating-induced obesity, and a natural CCK-1 receptor knockout. Dopamine-like-receptor type 1 (D1R) and D2R levels were examined in a reward-related brain area (Nac shell) and sucrose preference was assessed at selected time points from weaning to adulthood (postnatal day [PND]90). In addition, a group of OLETF was pair fed (PF) to the amount of food consumed by same-age LETO controls (from weaning to PND 90) to examine the contribution of overweight to the alteration in DR expression. In addition, we examined food "craving"-like behavior by analyzing microstructural patterns of licking a palatable liquid diet. OLETF rats expressed significantly lower D2R levels than LETO controls only on PND 90. In PF OLETF, weight and D2R levels were normalized. In addition, OLETF presented exaggerated preference for the high sucrose concentration. After 30-day abstinence, OLETF rats presented significant increased initial rate of licking, suggesting food "craving". Thus, adult OLETF rats demonstrated altered D2R signaling similar to drug-induced sensitization, suggesting a link with their avidity for sucrose and their abnormal craving response. However, the current findings of a late deficit appearance and the novel PF results suggest that deficits in the motivation/regulatory systems of the OLETF rat are a developing process (at least from weaning and on) depending on the overeating and obese phenotype of the rats and not only on the CCK mutation.

Could the mechanisms of bariatric surgery hold the key for novel therapies? report from a Pennington Scientific Symposium

Bariatric surgery is the most effective method for promoting dramatic and durable weight loss in morbidly obese subjects. Furthermore, type 2 diabetes is resolved in over 80% of patients. The mechanisms behind the amelioration in metabolic abnormalities are largely unknown but may be due to changes in energy metabolism, gut peptides and food preference. The goal of this meeting was to review the latest research to better understand the mechanisms behind the 'magic' of bariatric surgery. Replication of these effects in a non-surgical manner remains one of the ultimate challenges for the treatment of obesity and diabetes. Promising data on energy metabolism, gastrointestinal physiology, hedonic response and food intake were reviewed and discussed.

Roles of dorsomedial hypothalamic cholecystokinin signaling in the controls of meal patterns and glucose homeostasis

A role for dorsomedial hypothalamus (DMH) cholecystokinin (CCK) signaling in feeding control has been proposed. Administration of CCK into the DMH reduces food intake and OLETF rats lacking CCK1 receptors (CCK1R) become hyperphagic and obese. We hypothesized that site specific replenishment of CCK1R in the DMH of OLETF rats would attenuate aspects of their feeding deficits. Recombinant vectors of adeno-associated viral (AAV)-mediated expression of CCK1R (AAVCCK1R) were bilaterally delivered into the DMH of OLETF. OLETF rats with AAVCCK1R injections demonstrated a 65% replenishment of Cck1r mRNA expression in the DMH relative to lean LETO control rats. Although this level of replenishment did not significantly affect overall food intake or body weight through 14 weeks following viral injections, meal patterns were partially normalized in OLETF rats receiving AAVCCK1R with a significant decrease in dark cycle meal size and a small but significant decrease in daily food intake in the meal analysis chambers. Importantly, the elevation in blood glucose level of OLETF rats was attenuated by the AAVCCK1R injections (p=0.03), suggesting a role for DMH CCK signaling in glucose homeostasis. In support of this role, administration of CCK into the DMH of intact rats enhanced glucose tolerance, as this occurred through activation of CCK1R but not CCK2R signaling. In conclusion, partial replenishment of CCK1R in the DMH of OLETF rats, although insufficient for altering overall food intake and body weight, normalizes meal pattern changes and reduces blood glucose levels. Our study also shows a novel role of DMH CCK signaling in glucose homeostasis.

Up-regulation of intestinal type 1 taste receptor 3 and sodium glucose luminal transporter-1 expression and increased sucrose intake in mice lacking gut microbiota

The chemosensory components shared by both lingual and intestinal epithelium play a critical role in food consumption and the regulation of intestinal functions. In addition to nutrient signals, other luminal contents, including micro-organisms, are important in signalling across the gastrointestinal mucosa and initiating changes in digestive functions. A potential role of gut microbiota in influencing food intake, energy homeostasis and weight gain has been suggested. However, whether gut microbiota modulates the expression of nutrient-responsive receptors and transporters, leading to altered food consumption, is unknown. Thus, we examined the preference for nutritive (sucrose) and non-nutritive (saccharin) sweet solutions in germ-free (GF, C57BL/6J) mice compared with conventional (CV, C57BL/6J) control mice using a two-bottle preference test. Then, we quantified mRNA and protein expression of the sweet signalling protein type 1 taste receptor 3 (T1R3) and α-gustducin and Na glucose luminal transporter-1 (SGLT-1) of the intestinal epithelium of both CV and GF mice. Additionally, we measured gene expression of T1R2, T1R3 and α-gustducin in the lingual epithelium. We found that, while the preference for sucrose was similar between the groups, GF mice consumed more of the high concentration (8 %) of sucrose solution than CV mice. There was no difference in either the intake of or the preference for saccharin. GF mice expressed significantly more T1R3 and SGLT-1 mRNA and protein in the intestinal epithelium compared with CV mice; however, lingual taste receptor mRNA expression was similar between the groups. We conclude that the absence of intestinal microbiota alters the expression of sweet taste receptors and GLUT in the proximal small intestine, which is associated with increased consumption of nutritive sweet solutions.

Pontine and thalamic influences on fluid rewards: I. Operant responding for sucrose and corn oil

The reward strength of orosensory sucrose and corn oil was measured using fixed and progressive ratio operant schedules. Because the orosensory effects of the stimuli were of interest, Experiment 1 compared operant responses for sucrose in sham and real feeding rats. The results demonstrated that rats would work for sucrose solutions without the accompanying postingestive effects. Furthermore, the break points for high concentrations of sucrose (1.0 M or 2.0 M) were significantly higher in sham feeding rats than in real feeding controls. Experiment 2 investigated the role of the parabrachial nucleus (PBN) and of the thalamic orosensory area (TOA) in sucrose and corn oil reward. During free access, rats with PBN lesions (PBNx) licked significantly less sucrose solution than their controls, but both groups ingested a similar volume of corn oil emulsion. When an operant was imposed, these same PBNx rats failed to respond for sucrose and continued only modestly for corn oil. In contrast, the TOA lesioned rats (TOAx) showed no impairment in responding for sucrose or corn oil during either the free access or operant sessions. Furthermore, rats with TOA lesions demonstrated significantly higher break points for sucrose than did their controls. Together, the data imply that the PBN but not the TOA is critical for the perception of, or responding to the reward value of sucrose and corn oil.

Interaction between gastric and upper small intestinal hormones in the regulation of hunger and satiety: ghrelin and cholecystokinin take the central stage

Several peptides are produced and released from endocrine cells scattered within the gastric oxyntic and the small intestinal mucosa. These peptide hormones are crucially involved in the regulation of gastrointestinal functions and food intake by conveying their information to central regulatory sites located in the brainstem as well as in the forebrain, such as hypothalamic nuclei. So far, ghrelin is the only known hormone that is peripherally produced in gastric X/A-like cells and centrally acting to stimulate food intake, whereas the suppression of feeding seems to be much more redundantly controlled by a number of gut peptides. Cholecystokinin produced in the duodenum is a well established anorexigenic hormone that interacts with ghrelin to modulate food intake indicating a regulatory network located at the first site of contact with nutrients in the stomach and upper small intestine. In addition, a number of peptides including leptin, urocortin 2, amylin and glucagon-like peptide 1 interact synergistically with CCK to potentiate its satiety signaling effect. New developments have led to the identification of additional peptides in X/A-like cells either derived from the pro-ghrelin gene by alternative splicing and posttranslational processing (obestatin) or a distinct gene (nucleobindin2/nesfatin-1) which have been investigated for their influence on food intake.

Gastric bypass surgery alters behavioral and neural taste functions for sweet taste in obese rats

Roux-en-Y gastric bypass surgery (GBS) is the most effective treatment for morbid obesity. GBS is a restrictive malabsorptive procedure, but many patients also report altered taste preferences. This study investigated the effects of GBS or a sham operation (SH) on body weight, glucose tolerance, and behavioral and neuronal taste functions in the obese Otsuka Long-Evans Tokushima Fatty (OLETF) rats lacking CCK-1 receptors and lean controls (LETO). OLETF-GBS rats lost body weight (-26%) and demonstrated improved glucose tolerance. They also expressed a reduction in 24-h two-bottle preference for sucrose (0.3 and 1.0 M) and decreased 10-s lick responses for sucrose (0.3 through 1.5 M) compared with OLETF-SH or LETO-GBS. A similar effect was noted for other sweet compounds but not for salty, sour, or bitter tastants. In lean rats, GBS did not alter responses to any stimulus tested. Extracellular recordings from 170 taste-responsive neurons of the pontine parabrachial nucleus revealed a rightward shift in concentration responses to oral sucrose in obese compared with lean rats (OLETF-SH vs. LETO-SH): overall increased response magnitudes (above 0.9 M), and maximum responses occurring at higher concentrations (+0.46 M). These effects were reversed by GBS, and neural responses in OLETF-GBS were statistically not different from those in any LETO groups. These findings confirm obesity-related alterations in taste functions and demonstrate the ability of GBS to alleviate these impairments. Furthermore, the beneficial effects of GBS appear to be independent of CCK-1 receptor signaling. An understanding of the underlying mechanisms for reduced preferences for sweet taste could help in developing less invasive treatments for obesity.

Deficits in gastrointestinal responses controlling food intake and body weight

The gastrointestinal tract serves as a portal sensing incoming nutrients and relays mechanical and chemosensory signals of a meal to higher brain centers. Prolonged consumption of dietary fat causes adaptive changes within the alimentary, metabolic, and humoral systems that promote a more efficient process for energy metabolism from this rich source, leading to storage of energy in the form of adipose tissue. Furthermore, prolonged ingestion of dietary fats exerts profound effects on responses to signals involved in termination of a meal. This article reviews the effects of ingested fat on gastrointestinal motility, hormone release, and neuronal substrates. It focuses on changes in sensitivity to satiation signals resulting from chronic ingestion of high-fat diet, which may lead to disordered appetite and dysregulation of body weight.

Reduced sensitivity to cholecystokinin in male rats fed a high-fat diet is reversible

Adult rats chronically fed a high-fat (HF) diet maintain reduced sensitivity to cholecystokinin (CCK). We hypothesized that, similar to adult rats, pups fed a HF diet would also exhibit reduced sensitivity to CCK. To test this, male pups fed low-fat (LF) and HF isoenergetic (16.2 kJ/g) diets were administered CCK intraperitoneally (0.125-1 microg/kg) 1 wk following dietary adaptation. After receiving 0.5 microg/kg CCK, pups fed the HF diet suppressed food intake less (8.9 +/- 5.0%) than pups fed the LF diet (28.9 +/- 4.7%; P < 0.05) relative to intakes after saline administration. We then assessed the development and extinction of changes in CCK sensitivity by switching the diets between the groups. The HF-fed group, when switched to the LF diet, regained sensitivity by wk 4 and suppressed food intake following administration of 0.25 microg/kg CCK (33.1 +/- 5.7%; P < 0.05). The LF-fed group, when switched to the HF diet, lost sensitivity by wk 2 and did not suppress food intake after administrations of CCK compared with saline. Finally, we examined if HF-fed rats have an increased sensitivity to corn oil during brief access tests using a multibottle gustometer. At oil concentrations of 25, 75, and 100%, rats fed the HF diet sampled more oil than LF-fed rats (P < 0.05). These findings demonstrate that male rat pups fed a HF diet exhibit reduced sensitivity to CCK, the development of this reduced sensitivity is quicker than its extinction, and rats consuming a HF diet have increased oral sensitivity to oils.

Altered orosensory sensitivity to oils in CCK-1 receptor deficient rats

CCK-1 receptor deficient Otsuka Long Evans Tokushima Fatty (OLETF) rats are hyperphagic, which leads to subsequent obesity and diabetes. Additionally, they have increased sham intake and enhanced preference for sucrose solutions relative to control, Long Evans Tokushima Otsuka (LETO) rats. To determine the effects of oil on ingestion, we first measured real feeding of various concentrations of oil emulsions (12.5, 25, 50, 75, and 100%) in rats that were fed ad libitum. Secondly, to isolate the orosensory compontent of oils from post-ingestive consequences, as well as determine the contribution of energy status, we measured sham feeding in OLETF and LETO rats using one-bottle acceptance tests while non-deprived and overnight food deprived. Finally, to assess the orosensory effects of nutritive and non-nutritive oils, we used two-bottle preference tests in sham fed OLETF and LETO rats. We found that real feeding resulted in increased intake of high oil concentrations for OLETF rats relative to LETO rats. Similarly, OLETF rats consumed significantly more of higher concentration corn oils than LETO while non-deprived sham feeding. Conversely, OLETF rats overconsumed low concentration corn oil compared to LETO during overnight deprived sham-feeding tests. In two-bottle sham-feeding preference tests, both non-deprived OLETF and LETO rats preferred corn to mineral oil. Collectively, these results show that increased oil intake in OLETF rats is driven by both peripheral deficits to satiation and altered orosensory sensitivity.

Nutritional status alters saccharin intake and sweet receptor mRNA expression in rat taste buds

Sweet taste usually signifies the presence of caloric food. It is commonly accepted that a close association exists among sweet taste perception, preference, and nutritional status. However, the mechanisms involved remain unknown. To investigate whether nutritional status affects the preference for palatable solutions and alters sweet taste receptor gene expression in rats, we measured saccharin intake and preference using a two-bottle preference test, and changes in body weight, plasma leptin levels, and gene expression for the sweet taste receptor in taste buds in high-fat diet-induced obese rats and chronically diet-restricted rats. We found that the consumption and preference ratios for 0.01 and 0.04 M saccharin were significantly lower in the high-fat diet-induced obese rats than in the normal diet rats, while the serum leptin levels were markedly increased in obese rats. Consistent with the changes in saccharin intake, the gene expression level of the sweet taste receptor T1R3 was significantly decreased in the high-fat diet-induced obese rats compared with the control rats. By contrast, the chronically diet-restricted rats showed remarkably enhanced consumption and preference for 0.04 M saccharin. The serum leptin concentration was decreased, and the gene expression of the leptin receptor was markedly increased in the taste buds. In conclusion, our results suggest that nutritional status alters saccharin preference and the expression of T1R3 in taste buds. These processes may be involved in the mechanisms underlying the modulation of peripheral sweet taste sensitivity, in which leptin plays a role.

Parabrachial coding of sapid sucrose: relevance to reward and obesity

Cumulative evidence in rats suggests that the pontine parabrachial nuclei (PBN) are necessary for assigning hedonic value to taste stimuli. In a series of studies, our laboratory has investigated the parabrachial coding of sapid sucrose in normal and obese rats. First, using chronic microdialysis, we demonstrated that sucrose intake increases dopamine release in the nucleus accumbens, an effect that is dependent on oral stimulation and on concentration. The dopamine response was independent of the thalamocortical gustatory system but was blunted substantially by lesions of the PBN. Similar lesions of the PBN but not the thalamic taste relay diminished cFos activation in the nucleus accumbens caused by sucrose ingestion. Recent single-neuron recording studies have demonstrated that processing of sucrose-evoked activity in the PBN is altered in Otsuka Long Evans Tokushima Fatty (OLETF) rats, which develop obesity due to chronic overeating and express increased avidity to sweet. Compared with lean controls, taste neurons in OLETF rats had reduced overall sensitivity to sucrose and altered concentration responses, with decreased responses to lower concentrations and augmented responses to higher concentrations. The decreased sensitivity to sucrose was specific to NaCl-best neurons that also responded to sucrose, but the concentration effects were carried by the sucrose-specific neurons. Collectively, these findings support the hypothesis that the PBN enables taste stimuli to engage the reward system and, in doing so, influences food intake and body weight regulation. Obesity, in turn, may further alter the gustatory code via forebrain connections to the taste relays or hormonal changes consequent to weight gain.

The Korean traditional medicine Gyeongshingangjeehwan inhibits obesity through the regulation of leptin and PPARalpha action in OLETF rats

Gyeongshingangjeehwan (GGEx), which comprises Liriope platyphylla F.T. Wang & T. Tang (Liliaceae), Platycodongrandiflorum A. DC. (Campanulaceae), Schisandrachinensis K. Koch (Magnoliaceae), and Ephedra sinica Stapf (Ephedraceae), has traditionally been used as an anti-obesity drug in Korean local clinics, although there is no evidence concerning the scientific analyses of its effects and mechanism(s) of action. Thus, we investigated the effects of GGEx on obesity, as well as the mechanism by which GGEx functions, in Otsuka Long-Evans Tokushima Fatty (OLETF) male rats. Compared with obese OLETF control rats, administration of GGEx for 8 weeks significantly decreased food intake and plasma leptin levels as well as body weight gain and abdominal fat in OLETF rats. GGEx treatment not only decreased circulating triglycerides, but also inhibited lipid accumulation in the liver. GGEx increased the hepatic mRNA levels of PPARalpha target genes responsible for fatty acid beta-oxidation. Consistent with the in vivo data, GGEx elevated PPARalpha reporter gene expression in NMu2Li liver cells. These results suggest that GGEx may effectively prevent obesity and hypertriglyceridemia in part through the inhibition of feeding and the activation of hepatic PPARalpha.

Gustatory hedonic value: potential function for forebrain control of brainstem taste processing

Among well-nourished populations, eating beyond homeostatic needs when presented with caloric-dense palatable food evidences the assertion that an increasing proportion of consumption is driven by pleasure, not just by the need for calories. This presents a major health crisis because the affective component of foods constitutes a behavioral risk factor that promotes over consumption [Sorensen, L.B., Moller, P., Flint, A., Martens, M., Raben, A., 2003. Effect of sensory perception of foods on appetite and food intake: a review of studies on humans. Int. J. Obes. Relat. Metab. Disord. 27, 1152-1166; Yeomans, M.R., Blundell, J.E., Leshem, M., 2004. Palatability: response to nutritional need or need-free stimulation of appetite? Br. J. Nutr. 92 (Suppl. 1), S3-S14]. Overweight or obese individuals have an increased risk of developing hypertension, stroke, heart disease, chronic musculoskeletal problems, type-2 diabetes, and certain types of cancers [Hill, J.O., Catenacci, V., Wyatt, H.R., 2005. Obesity: overview of an epidemic. Psychiatr. Clin. N. Am. 28, 1-23, vii]. The etiology of obesity is complex involving genetic, metabolic, and behavioral factors, but ultimately results from long-term energy imbalance. Evidence indicates that learned and some forms of unlearned control of ingestive behavior driven by palatability (i.e. hedonic value) are critically dependent on reciprocal interactions between brainstem gustatory nuclei and the ventral forebrain. This review discusses the current understanding of centrifugal control of taste processing in subcortical gustatory nuclei and the potential role of such modulation in hedonic responding.

Sucrose taste but not Polycose taste conditions flavor preferences in rats

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This "poly" taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS-) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS- when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS- flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.

Altered pontine taste processing in a rat model of obesity

The increased palatability of modern diet contributes to eating beyond homeostatic need and in turn to the growing prevalence of obesity. How palatability is coded in taste-evoked neural activity and whether this activity differs between obese and lean remains unknown. To investigate this, we used extracellular single-unit recording in the second central gustatory relay, the pontine parabrachial nucleus while stimulating the tongue with various concentrations of sucrose (0.01-1.5 M) in Otsuka Long Evans Tokushima Fatty (OLETF) rats, lacking CCK-1R. The analyses included a total of 179 taste-responsive neurons in age-matched prediabetic, obese OLETF and lean Long Evans Tokushima Otsuka (LETO) controls. Compared with LETO, we found more NaCl-, and fewer sucrose-responsive neurons (67 vs. 47% and 14 vs. 32%), and an overall reduced response magnitude to sucrose in the OLETF rats. Further, in the obese rats there was a rightward shift in sucrose concentration-response functions relative to lean controls with a higher response-threshold (0.37+/-0.05 vs. 0.23+/-0.2 M, P<0.05) and maximal neural response to higher sucrose concentrations (0.96+/-0.07 vs. 0.56+/-0.5 M, P<0.001). These findings demonstrate altered central gustatory processing for sucrose in obese OLETF rat and further support the notion that palatability is encoded in the across neuron pattern.

Unraveling the obesity of OLETF rats

Cholecystokinin (CCK) is a brain gut peptide that plays an important role in satiety. CCK inhibits food intake by reducing meal size. CCK's satiety actions are mediating through its interaction with CCK1 receptors. Otsuka Long Evans Tokushima Fatty (OLETF) rats are a CCK1 receptor knockout model that allows the study of multiple CCK functions. OLETF rats are hyperphagic with the hyperphagia expressed as a significant increase in the size of meals. OLETF rat obesity is secondary to the hyperphagia and has been proposed to derive from two regulatory deficits. One is secondary to the loss of a feedback satiety signal. The other results from increased dorsomedial hypothalamic NPY expression. Recent studies have examined developmental aspects of altered feeding, body weight and orexigenic signaling in OLETF rats. OLETF rats demonstrate increases in meal size in independent ingestion tests as early as two days of age. OLETF pups are also more efficient in suckling situations. Consistent with such developmental differences, examinations of patterns of hypothalamic gene expression in OLETF pups indicate significant increases in DMH NPY expression as early as postnatal day 15. Access to a running wheel and the resulting exercise have age dependent effects on OLETF food intake and obesity. With running wheel access shortly after weaning, food intake decreases to the levels of LETO controls. When running wheel access is discontinued, food intake temporarily increases resulting in an intermediate phenotype and the absence of diabetes. Together these data demonstrate roles for peripheral CCK and CCK in feeding and body weight control and support the use of the OLETF rat as a model for examining obesity development and for investigating how interventions at critical developmental time points can alter genetic influences on food intake and body weight.

Obese OLETF rats exhibit increased operant performance for palatable sucrose solutions and differential sensitivity to D2 receptor antagonism

CCK-1-receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and exhibit a greater preference for sucrose compared with lean controls [Long-Evans Tokushima Otsuka (LETO)]. To directly assess motivation to work for sucrose reward in this model of obesity and type 2 diabetes, we examined the operant performance of OLETF rats at nondiabetic and prediabetic stages (14 and 24 wk of age, respectively) on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement. To evaluate the involvement of dopamine systems, the effects of the D1 receptor antagonist SCH23390 (100 and 200 nmol/kg ip) and the D2 receptor antagonist raclopride (200 and 400 nmol/kg ip), were also tested on PR responding for sucrose. Compared with age-matched LETO rats, 14-wk-old OLETF rats emitted more licks on the "active" empty spout operant on the FR-10 schedule of reinforcement to obtain 0.01 M and 0.3 M sucrose and completed higher ratio requirements on the PR schedule to gain access to 0.3 M and 1.0 M sucrose. At 24 wk, this effect was limited to 1.0 M sucrose. Both antagonists were potent in reducing operant responding to 0.3 M sucrose in both strains at both ages, and there was no strain effect to SCH23390 at either age. OLETF rats, on the other hand, showed an increased sensitivity to the higher dose of raclopride, resulting in reduced responding to sucrose reinforcement at 24 wk. Taken together, these findings provide the first direct evidence for an increased motivation for sucrose reward in the OLETF rats and suggest altered D2 receptor regulation with the progression of obesity and prediabetes.

Altered dopamine D2 receptor function and binding in obese OLETF rat

A decrease in D2-like receptor (D2R) binding in the striatum has been reported in obese individuals and drug addicts. Although natural and drug rewards share neural substrates, it is not clear whether such effects also contribute to overeating on palatable meals as an antecedent of dietary obesity. Therefore, we investigated receptor density and the effect of the D2R agonist quinpirole (0.05, 0.5 mg/kg, S.C.) on locomotor activity and sucrose intake in a rat model of diet-induced obesity, the CCK-1 receptor-deficient Otsuka Long Evans Tokushima Fatty (OLETF) rat. Compared to age-matched lean controls (LETO), OLETF rats expressed significantly lower [125I]-iodosulpride binding in the accumbens shell (-16%, p<0.02). Whereas the high dose of quinpirole increased motor activity in both strains equally, the low dose reduced activity more in OLETF. Both doses significantly reduced sucrose intake in OLETF but not LETO rats. These findings demonstrate an altered D2R signaling in obese OLETF rats similar to drug-induced sensitization and suggest a link between this effect and avidity for sucrose in this model.