Altered pontine taste processing in a rat model of obesity

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.

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.

The neuroscience of sugars in taste, gut-reward, feeding circuits, and obesity

Throughout the animal kingdom sucrose is one of the most palatable and preferred tastants. From an evolutionary perspective, this is not surprising as it is a primary source of energy. However, its overconsumption can result in obesity and an associated cornucopia of maladies, including type 2 diabetes and cardiovascular disease. Here we describe three physiological levels of processing sucrose that are involved in the decision to ingest it: the tongue, gut, and brain. The first section describes the peripheral cellular and molecular mechanisms of sweet taste identification that project to higher brain centers. We argue that stimulation of the tongue with sucrose triggers the formation of three distinct pathways that convey sensory attributes about its quality, palatability, and intensity that results in a perception of sweet taste. We also discuss the coding of sucrose throughout the gustatory pathway. The second section reviews how sucrose, and other palatable foods, interact with the gut-brain axis either through the hepatoportal system and/or vagal pathways in a manner that encodes both the rewarding and of nutritional value of foods. The third section reviews the homeostatic, hedonic, and aversive brain circuits involved in the control of food intake. Finally, we discuss evidence that overconsumption of sugars (or high fat diets) blunts taste perception, the post-ingestive nutritional reward value, and the circuits that control feeding in a manner that can lead to the development of obesity.

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.

SatB2-Expressing Neurons in the Parabrachial Nucleus Encode Sweet Taste

The gustatory system plays an important role in sensing appetitive and aversive tastes for evaluating food quality. In mice, taste signals are relayed by multiple brain regions, including the parabrachial nucleus (PBN) of the pons, before reaching the gustatory cortex via the gustatory thalamus. Recent studies show that taste information at the periphery is encoded in a labeled-line manner, such that each taste modality has its own receptors and neuronal pathway. In contrast, the molecular identity of gustatory neurons in the CNS remains unknown. Here, we show that SatB2-expressing neurons in the PBN play a pivotal role in sweet taste transduction. With cell ablation, in vivo calcium imaging, and optogenetics, we reveal that SatB2^PBN neurons encode positive valance and selectively transmit sweet taste signals to the gustatory thalamus.

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.

Taste Changes after Bariatric Surgery: a Systematic Review

BACKGROUND

Alterations in taste perception and preferences may contribute to dietary changes and subsequent weight loss following bariatric surgery.

METHODS

A systematic search was performed to identify all articles investigating gustation, olfaction, and sensory perception in both animal and human studies following bariatric procedures.

RESULTS

Two hundred fifty-five articles were identified after database searches, bibliography inclusions and deduplication. Sixty-one articles were included. These articles provide evidence supporting changes in taste perception and hedonic taste following bariatric procedures. Taste sensitivity to sweet and fatty stimuli appears to increase post-operatively. Additionally, patients also have a reduced hedonic response to these stimuli.

CONCLUSIONS

Available evidence suggests that there is a change in taste perception following bariatric procedures, which may contribute to long-term maintenance of weight loss following surgery.

Exercise, Obesity and CNS Control of Metabolic Homeostasis: A Review

This review details the manner in which the central nervous system regulates metabolic homeostasis in normal weight and obese rodents and humans. It includes a review of the homeostatic contributions of neurons located in the hypothalamus, the midbrain and limbic structures, the pons and the medullary area postrema, nucleus tractus solitarius, and vagus nucleus, and details how these brain regions respond to circulating levels of orexigenic hormones, such as ghrelin, and anorexigenic hormones, such as glucagon-like peptide 1 and leptin. It provides an insight as to how high intensity exercise may improve homeostatic control in overweight and obese subjects. Finally, it provides suggestions as to how further progress can be made in controlling the current pandemic of obesity and diabetes.

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.

Gustatory neural responses to umami stimuli in the parabrachial nucleus of C57BL/6J mice

Umami is considered to be the fifth basic taste quality and is elicited by glutamate. The mouse is an ideal rodent model for the study of this taste quality because of evidence that suggests that this species, like humans, may sense umami-tasting compounds as unique from other basic taste qualities. We performed single-unit recording of taste responses in the parabrachial nucleus (PbN) of anesthetized C57BL/6J mice to investigate the central representation of umami taste. A total of 52 taste-responsive neurons (22 sucrose-best, 19 NaCl-best, 5 citric acid-best, and 6 quinine-best) were recorded from stimulation period with a large panel of basic and umami-tasting stimuli. No neuron responded best to monopotassium glutamate (MPG) or inosine 5'-monophosphate (IMP), suggesting convergence of input in the central nervous system. Synergism induced by an MPG-IMP mixture was observed in all sucrose-best and some NaCl-best neurons that possessed strong sensitivity to sucrose. In more than half of sucrose-best neurons, the MPG-IMP mixture evoked stronger responses than those elicited by their best stimulus. Furthermore, hierarchical cluster analysis and multidimensional analysis indicated close similarity between sucrose and the MPG-IMP mixture. These results strongly suggest the mixture tastes sweet to mice, a conclusion consistent with previous findings that show bidirectional generalization of conditioned taste aversion between sucrose and umami mixtures, and suppression of taste responses to both sucrose and mixtures by the antisweet polypeptide gurmarin in the chorda tympani nerve. The distribution pattern of reconstructed recording sites of specific neuron types suggested chemotopic organization in the PbN.

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.

It's all a matter of taste: gustatory processing and ingestive decisions

This paper reviews the physiology of taste processing and ingestive decisions.

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.