The gut-brain axis mediates sugar preference

Protein Appetite at the Interface between Nutrient Sensing and Physiological Homeostasis

Feeding behavior is guided by multiple competing physiological needs, as animals must sense their internal nutritional state and then identify and consume foods that meet nutritional needs. Dietary protein intake is necessary to provide essential amino acids and represents a specific, distinct nutritional need. Consistent with this importance, there is a relatively strong body of literature indicating that protein intake is defended, such that animals sense the restriction of protein and adaptively alter feeding behavior to increase protein intake. Here, we argue that this matching of food consumption with physiological need requires at least two concurrent mechanisms: the first being the detection of internal nutritional need (a protein need state) and the second being the discrimination between foods with differing nutritional compositions. In this review, we outline various mechanisms that could mediate the sensing of need state and the discrimination between protein-rich and protein-poor foods. Finally, we briefly describe how the interaction of these mechanisms might allow an animal to self-select between a complex array of foods to meet nutritional needs and adaptively respond to changes in either the external environment or internal physiological state.

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

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

Post-oral sensing of fat increases food intake and attenuates body weight defense

In mammals, changes in weight elicit responses that favor a return to one's previous weight and promote weight stability. It has been hypothesized that palatable sweet and high-fat foods disturb the defense of body weight, leading to weight gain. We find that increasing sweetness or percent calories from fat increases diet palatability but that only increases in nutritive fat content increase caloric intake and body weight. In a mouse model of overfeeding that activates weight defense, high-fat diets, but not sweetened diets, attenuate the defense of body weight, leading to weight gain. The ability of a palatable, high-fat diet to increase food intake does not require tasting or smelling the food. Instead, the direct infusion of a high-fat diet into the stomach increases the ad libitum intake of less palatable, low-fat food. Post-oral sensing of percent calories from fat modulates feeding behavior to alter weight stability.

Learning of food preferences: mechanisms and implications for obesity & metabolic diseases

Omnivores, including rodents and humans, compose their diets from a wide variety of potential foods. Beyond the guidance of a few basic orosensory biases such as attraction to sweet and avoidance of bitter, they have limited innate dietary knowledge and must learn to prefer foods based on their flavors and postoral effects. This review focuses on postoral nutrient sensing and signaling as an essential part of the reward system that shapes preferences for the associated flavors of foods. We discuss the extensive array of sensors in the gastrointestinal system and the vagal pathways conveying information about ingested nutrients to the brain. Earlier studies of vagal contributions were limited by nonselective methods that could not easily distinguish the contributions of subsets of vagal afferents. Recent advances in technique have generated substantial new details on sugar- and fat-responsive signaling pathways. We explain methods for conditioning flavor preferences and their use in evaluating gut-brain communication. The SGLT1 intestinal sugar sensor is important in sugar conditioning; the critical sensors for fat are less certain, though GPR40 and 120 fatty acid sensors have been implicated. Ongoing work points to particular vagal pathways to brain reward areas. An implication for obesity treatment is that bariatric surgery may alter vagal function.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part II

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell's biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells' electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (overviewed in Part I), as well as chemogenetics and thermogenetics (described here, in Part II), which is significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

Sweet taste and obesity

For more than 50 years, there has been evidence for greater consumption of sweet- foods in overweight humans and animals, relative to those that have a normal weight. Furthermore, it has long been suggested that energy deficit resulting from dieting, while moving the individual from a higher weight set point, would result in heightened susceptibility to palatable tastants, namely to sweet tastants. This was the motivation behind the first studies comparing sweet taste perception between individuals with obesity and those of a normal weight. These studies, using direct measures of taste, have been characterized by significant methodological heterogeneity, contributing towards variability in results and conclusions. Nevertheless, some of these findings have been used to support the theory that patients with obesity have decreased taste perception, particularly for sweet tastants. A similar hypothesis has been proposed regarding evidence for reduced brain dopamine receptors in obesity and, in both cases, it is proposed that increased food consumption, and associated weight gain, result from the need to increase sensory and brain stimulation. However, the available literature is not conclusive on the association between obesity and reduced sweet taste perception, with both negative and contradictory findings in comparisons between individuals with obesity and normal weight control subjects, as well as within-subject comparisons before and after bariatric surgery. Nevertheless, following either Roux-en-Y gastric bypass or sleeve gastrectomy, there is evidence of changes in taste perception, particularly for reward-related measures of sweet tastants, that should be further tested and confirmed in large samples, using consensual methodology.

Neural Circuits of Interoception

The present paper considers recent progress in our understanding of the afferent/ascending neural pathways and neural circuits of interoception. Of particular note is the extensive role of rostral neural systems, including cortical systems, in the recognition of internal body states, and the reciprocal role of efferent/descending systems in the regulation of those states. Together these reciprocal interacting networks entail interoceptive circuits that play an important role in a broad range of functions beyond the homeostatic maintenance of physiological steady-states. These include the regulation of behavioral, cognitive, and affective processes across conscious and nonconscious levels of processing. We highlight recent advances and knowledge gaps that are important for accelerating progress in the study of interoception.

Fatigue, Stress, and Functional Status are Associated With Taste Changes in Oncology Patients Receiving Chemotherapy

CONTEXT

A common complaint among oncology patients receiving chemotherapy is altered taste perception.

OBJECTIVE

The purpose of this study was to evaluate for differences in common symptoms and stress levels in patients who reported taste changes.

METHODS

Patients were receiving chemotherapy for breast, gastrointestinal, gynecological, or lung cancer. Change in the way food tastes (CFT) was assessed using the Memorial Symptom Assessment Scale before the patients' second or third cycle of chemotherapy. Valid and reliable instruments were used to assess for depressive symptoms, state and trait of anxiety, cognitive impairment, diurnal variations in fatigue and energy, sleep disturbance, and pain. Stress was assessed using the Perceived Stress Scale and the Impact of Events Scale-Revised. Multiple logistic regression was used to evaluate for risk factors associated with CFT.

RESULTS

Of the 1329 patients, 49.4% reported CFT. Patients in the CFT group reported higher levels of depression, anxiety, fatigue, and sleep disturbance as well as higher levels of general and disease specific stress. Factors associated with CFT group included being non-White; receiving an antiemetic regimen that contained a neurokinin-1 receptor antagonist with two other antiemetics; having a lower functional status; higher levels of morning fatigue; and reporting higher scores on the hyperarousal subscale of the Impact of Event Scale-Revised.

CONCLUSIONS

This study provides new evidence on associations between taste changes and common co-occurring symptoms and stress in oncology patients receiving chemotherapy. Clinicians need to evaluate for taste changes in these patients because this symptom can effect patients' nutritional intake and quality of life.

From gut microbiota to host appetite: gut microbiota-derived metabolites as key regulators

Feelings of hunger and satiety are the key determinants for maintaining the life of humans and animals. Disturbed appetite control may disrupt the metabolic health of the host and cause various metabolic disorders. A variety of factors have been implicated in appetite control, including gut microbiota, which develop the intricate interactions to manipulate the metabolic requirements and hedonic feelings. Gut microbial metabolites and components act as appetite-related signaling molecules to regulate appetite-related hormone secretion and the immune system, or act directly on hypothalamic neurons. Herein, we summarize the effects of gut microbiota on host appetite and consider the potential molecular mechanisms. Furthermore, we propose that the manipulation of gut microbiota represents a clinical therapeutic potential for lessening the development and consequence of appetite-related disorders. Video abstract.

Gut-brain communication by distinct sensory neurons differently controls feeding and glucose metabolism

Sensory neurons relay gut-derived signals to the brain, yet the molecular and functional organization of distinct populations remains unclear. Here, we employed intersectional genetic manipulations to probe the feeding and glucoregulatory function of distinct sensory neurons. We reconstruct the gut innervation patterns of numerous molecularly defined vagal and spinal afferents and identify their downstream brain targets. Bidirectional chemogenetic manipulations, coupled with behavioral and circuit mapping analysis, demonstrated that gut-innervating, glucagon-like peptide 1 receptor (GLP1R)-expressing vagal afferents relay anorexigenic signals to parabrachial nucleus neurons that control meal termination. Moreover, GLP1R vagal afferent activation improves glucose tolerance, and their inhibition elevates blood glucose levels independent of food intake. In contrast, gut-innervating, GPR65-expressing vagal afferent stimulation increases hepatic glucose production and activates parabrachial neurons that control normoglycemia, but they are dispensable for feeding regulation. Thus, distinct gut-innervating sensory neurons differentially control feeding and glucoregulatory neurocircuits and may provide specific targets for metabolic control.

Reconsolidation of a post-ingestive nutrient memory requires mTOR in the central amygdala

The central control of feeding behavior and metabolic homeostasis has been proposed to involve a form of post-ingestive nutrient learning independent of the gustatory value of food. However, after such learning, it is unknown which brain regions or circuits are activated to retrieve the stored memory and whether this memory undergoes reconsolidation that depends on protein synthesis after its reactivation through retrieval. In the present study, using a conditioned-flavor-preference paradigm by associating flavors with intra-gastric infusion of glucose to minimize the evaluation of the taste of food, we show that retrieval of the post-ingestive nutrient-conditioned flavor memory stimulates multiple brain regions in mice, including the central nucleus of the amygdala (CeA). Moreover, memory retrieval activated the mammalian target of rapamycin complex 1 (mTORC1) in the CeA, while site-specific or systemic inhibition of mTORC1 immediately after retrieval prevented the subsequent expression of the post-ingestive nutrient-associated flavor memory, leading to a long-lasting suppression of reinstatement. Taken together, our findings suggest that the reconsolidation process of a post-ingestive nutrient memory modulates food preferences.

Reappraising the role of the vagus nerve in GLP-1-mediated regulation of eating

Here, we provide a focused review of the evidence for the roles of the vagus nerve in mediating the regulatory effects of peripherally and centrally produced GLP-1 on eating behaviour and energy balance. We particularly focus on recent studies which have used selective genetic, viral, and transcriptomic approaches to provide important insights into the anatomical and functional organisation of GLP-1-mediated gut-brain signalling pathways. A number of these studies have challenged canonical ideas of how GLP-1 acts in the periphery and the brain to regulate eating behaviour, with important implications for the development of pharmacological treatments for obesity.

Drinking Water with Saccharin Sodium Alters the Microbiota-Gut-Hypothalamus Axis in Guinea Pig

The effects of saccharin, as a type of sweetener additive, on the metabolism and development of mammals are still controversial. Our previous research revealed that saccharin sodium (SS) promoted the feed intake and growth of guinea pigs. In this experiment, we used the guinea pig model to study the physiological effect of SS in the microbiota-gut-hypothalamus axis. Adding 1.5 mM SS to drinking water increased the serum level of glucose, followed by the improvement in the morphology and barrier function of the ileal villus, such as SS supplementation which increased the villus height and villus height/crypt depth ratio. Saccharin sodium (SS) treatment activated the sweet receptor signaling in the ileum and altered GHRP hormone secretion. In the hypothalamus of SS and control (CN) group, RNA-seq identified 1370 differently expressed genes (796 upregulated, 574 downregulated), enriching into the taste signaling transduction, and neuroactive ligand-receptor interaction. LEfSe analysis suggested that Lactobacillaceae-Lactobacillus was the microbe with significantly increased abundance of ileum microorganisms in the SS-treated group, while Brevinema-Andersonii and Erysipelotrichaceae-Ilebacterium were the microbes with significantly increased abundance of the control. Furthermore, SS treatment significantly enhanced the functions of chemoheterotrophy and fermentation of ileal microflora compared to the CN group. Accordingly, SS treatment increased levels of lactic acid and short-chain fatty acids (acetic acid, propionic acid and N-valeric acid) in the ileal digesta. In summary, drinking water with 1.5 mM SS activated sweet receptor signaling in the gut and altered GHRP hormone secretion, followed by the taste signaling transduction in the hypothalamus.

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

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

The specification and function of enteroendocrine cells in Drosophila and mammals: a comparative review

Enteroendocrine cells (EECs) in both invertebrates and vertebrates derive from intestinal stem cells (ISCs) and are scattered along the digestive tract, where they function in sensing various environmental stimuli and subsequently secrete neurotransmitters or neuropeptides to regulate diverse biological and physiological processes. To fulfill these functions, EECs are specified into multiple subtypes that occupy specific gut regions. With advances in single-cell technology, organoid culture experimental systems, and CRISPR/Cas9-mediated genomic editing, rapid progress has been made toward characterization of EEC subtypes in mammals. Additionally, studies of genetic model organisms-especially Drosophila melanogaster-have also provided insights about the molecular processes underlying EEC specification from ISCs and about the establishment of diverse EEC subtypes. In this review, we compare the regulation of EEC specification and function in mammals and Drosophila, with a focus on EEC subtype characterization, on how internal and external regulators mediate EEC subtype specification, and on how EEC-mediated intra- and interorgan communications affect gastrointestinal physiology and pathology.

Glucose metabolic crosstalk and regulation in brain function and diseases

Brain glucose metabolism, including glycolysis, the pentose phosphate pathway, and glycogen turnover, produces ATP for energetic support and provides the precursors for the synthesis of biological macromolecules. Although glucose metabolism in neurons and astrocytes has been extensively studied, the glucose metabolism of microglia and oligodendrocytes, and their interactions with neurons and astrocytes, remain critical to understand brain function. Brain regions with heterogeneous cell composition and cell-type-specific profiles of glucose metabolism suggest that metabolic networks within the brain are complex. Signal transduction proteins including those in the Wnt, GSK-3β, PI3K-AKT, and AMPK pathways are involved in regulating these networks. Additionally, glycolytic enzymes and metabolites, such as hexokinase 2, acetyl-CoA, and enolase 2, are implicated in the modulation of cellular function, microglial activation, glycation, and acetylation of biomolecules. Given these extensive networks, glucose metabolism dysfunction in the whole brain or specific cell types is strongly associated with neurologic pathology including ischemic brain injury and neurodegenerative disorders. This review characterizes the glucose metabolism networks of the brain based on molecular signaling and cellular and regional interactions, and elucidates glucose metabolism-based mechanisms of neurological diseases and therapeutic approaches that may ameliorate metabolic abnormalities in those diseases.

Impact of dietary carbohydrate type and protein-carbohydrate interaction on metabolic health

Reduced protein intake, through dilution with carbohydrate, extends lifespan and improves mid-life metabolic health in animal models. However, with transition to industrialised food systems, reduced dietary protein is associated with poor health outcomes in humans. Here we systematically interrogate the impact of carbohydrate quality in diets with varying carbohydrate and protein content. Studying 700 male mice on 33 isocaloric diets, we find that the type of carbohydrate and its digestibility profoundly shape the behavioural and physiological responses to protein dilution, modulate nutrient processing in the liver and alter the gut microbiota. Low (10%)-protein, high (70%)-carbohydrate diets promote the healthiest metabolic outcomes when carbohydrate comprises resistant starch (RS), yet the worst outcomes were with a 50:50 mixture of monosaccharides fructose and glucose. Our findings could explain the disparity between healthy, high-carbohydrate diets and the obesogenic impact of protein dilution by glucose-fructose mixtures associated with highly processed diets.

Roles for the gut microbiota in regulating neuronal feeding circuits

The gut microbiota has the capacity to affect host appetite via intestinal satiety pathways, as well as complex feeding behaviors. In this Review, we highlight recent evidence that the gut microbiota can modulate food preference across model organisms. We discuss effects of the gut microbiota on the vagus nerve and brain regions including the hypothalamus, mesolimbic system, and prefrontal cortex, which play key roles in regulating feeding behavior. Crosstalk between commensal bacteria and the central and peripheral nervous systems is associated with alterations in signaling of neurotransmitters and neuropeptides such as dopamine, brain-derived neurotrophic factor (BDNF), and glucagon-like peptide-1 (GLP-1). We further consider areas for future research on mechanisms by which gut microbes may influence feeding behavior involving these neural pathways. Understanding roles for the gut microbiota in feeding regulation will be important for informing therapeutic strategies to treat metabolic and eating disorders.

Neurotrophin-4 is essential for survival of the majority of vagal afferents to the mucosa of the small intestine, but not the stomach

Vagal afferents form the primary gut-to-brain neural axis, communicating signals that regulate gastrointestinal (GI) function and promote satiation, appetition and reward. Neurotrophin-4 (NT-4) is essential for the survival of vagal smooth muscle afferents of the small intestine, but not the stomach. Here we took advantage of near-complete labeling of GI vagal mucosal afferents in Nav1.8cre-Rosa26tdTomato transgenic mice to determine whether these afferents depend on NT-4 for survival. We quantified the density and distribution of vagal afferent terminals in the stomach and small intestine mucosa and their central terminals in the solitary tract nucleus (NTS) and area postrema in NT-4 knockout (KO) and control mice. NT-4KO mice exhibited a 75% reduction in vagal afferent terminals in proximal duodenal villi and a 55% decrease in the distal ileum, whereas, those in the stomach glands remained intact. Vagal crypt afferents were also reduced in some regions of the small intestine, but to a lesser degree. Surprisingly, NT-4KO mice exhibited an increase in labeled terminals in the medial NTS. These findings, combined with previous results, suggest NT-4 is essential for survival of a large proportion of all classes of vagal afferents that innervate the small intestine, but not those that supply the stomach. Thus, NT-4KO mice could be valuable for distinguishing gastric and intestinal vagal afferent regulation of GI function and feeding. The apparent plasticity of central vagal afferent terminals - an increase in their density - could have compensated for loss of peripheral terminals by maintaining near-normal levels of satiety signaling.

Monitoring In Vivo Neural Activity to Understand Gut-Brain Signaling

Appropriate food intake requires exquisite coordination between the gut and the brain. Indeed, it has long been known that gastrointestinal signals communicate with the brain to promote or inhibit feeding behavior. Recent advances in the ability to monitor and manipulate neural activity in awake, behaving rodents has facilitated important discoveries about how gut signaling influences neural activity and feeding behavior. This review emphasizes recent studies that have advanced our knowledge of gut-brain signaling and food intake control, with a focus on how gut signaling influences in vivo neural activity in animal models. Moving forward, dissecting the complex pathways and circuits that transmit nutritive signals from the gut to the brain will reveal fundamental principles of energy balance, ultimately enabling new treatment strategies for diseases rooted in body weight control.

The gut-brain axis: Identifying new therapeutic approaches for type 2 diabetes, obesity, and related disorders

BACKGROUND

The gut-brain axis, which mediates bidirectional communication between the gastrointestinal system and central nervous system (CNS), plays a fundamental role in multiple areas of physiology including regulating appetite, metabolism, and gastrointestinal function. The biology of the gut-brain axis is central to the efficacy of glucagon-like peptide-1 (GLP-1)-based therapies, which are now leading treatments for type 2 diabetes (T2DM) and obesity. This success and research to suggest a much broader role of gut-brain circuits in physiology and disease has led to increasing interest in targeting such circuits to discover new therapeutics. However, our current knowledge of this biology is limited, largely because the scientific tools have not been available to enable a detailed mechanistic understanding of gut-brain communication.

SCOPE OF REVIEW

In this review, we provide an overview of the current understanding of how sensory information from the gastrointestinal system is communicated to the central nervous system, with an emphasis on circuits involved in regulating feeding and metabolism. We then describe how recent technologies are enabling a better understanding of this system at a molecular level and how this information is leading to novel insights into gut-brain communication. We also discuss current therapeutic approaches that leverage the gut-brain axis to treat diabetes, obesity, and related disorders and describe potential novel approaches that have been enabled by recent advances in the field.

MAJOR CONCLUSIONS

The gut-brain axis is intimately involved in regulating glucose homeostasis and appetite, and this system plays a key role in mediating the efficacy of therapeutics that have had a major impact on treating T2DM and obesity. Research into the gut-brain axis has historically largely focused on studying individual components in this system, but new technologies are now enabling a better understanding of how signals from these components are orchestrated to regulate metabolism. While this work reveals a complexity of signaling even greater than previously appreciated, new insights are already being leveraged to explore fundamentally new approaches to treating metabolic diseases.

Central Neurocircuits Regulating Food Intake in Response to Gut Inputs-Preclinical Evidence

The regulation of energy balance requires the complex integration of homeostatic and hedonic pathways, but sensory inputs from the gastrointestinal (GI) tract are increasingly recognized as playing critical roles. The stomach and small intestine relay sensory information to the central nervous system (CNS) via the sensory afferent vagus nerve. This vast volume of complex sensory information is received by neurons of the nucleus of the tractus solitarius (NTS) and is integrated with responses to circulating factors as well as descending inputs from the brainstem, midbrain, and forebrain nuclei involved in autonomic regulation. The integrated signal is relayed to the adjacent dorsal motor nucleus of the vagus (DMV), which supplies the motor output response via the efferent vagus nerve to regulate and modulate gastric motility, tone, secretion, and emptying, as well as intestinal motility and transit; the precise coordination of these responses is essential for the control of meal size, meal termination, and nutrient absorption. The interconnectivity of the NTS implies that many other CNS areas are capable of modulating vagal efferent output, emphasized by the many CNS disorders associated with dysregulated GI functions including feeding. This review will summarize the role of major CNS centers to gut-related inputs in the regulation of gastric function with specific reference to the regulation of food intake.

Chemogenetics Reveal an Anterior Cingulate-Thalamic Pathway for Attending to Task-Relevant Information

In a changing environment, organisms need to decide when to select items that resemble previously rewarded stimuli and when it is best to switch to other stimulus types. Here, we used chemogenetic techniques to provide causal evidence that activity in the rodent anterior cingulate cortex and its efferents to the anterior thalamic nuclei modulate the ability to attend to reliable predictors of important outcomes. Rats completed an attentional set-shifting paradigm that first measures the ability to master serial discriminations involving a constant stimulus dimension that reliably predicts reinforcement (intradimensional-shift), followed by the ability to shift attention to a previously irrelevant class of stimuli when reinforcement contingencies change (extradimensional-shift). Chemogenetic disruption of the anterior cingulate cortex (Experiment 1) as well as selective disruption of anterior cingulate efferents to the anterior thalamic nuclei (Experiment 2) impaired intradimensional learning but facilitated 2 sets of extradimensional-shifts. This pattern of results signals the loss of a corticothalamic system for cognitive control that preferentially processes stimuli resembling those previously associated with reward. Previous studies highlight a separate medial prefrontal system that promotes the converse pattern, that is, switching to hitherto inconsistent predictors of reward when contingencies change. Competition between these 2 systems regulates cognitive flexibility and choice.

The nerve not taken

Nutrients entering the gut influence our brains through uncharacterized circuits. In this issue of Cell Metabolism, Goldstein et al. (2021) show hypothalamic neurons responding, via distinct neural paths, to nutrients infused in different intestinal segments.

Hypothalamic detection of macronutrients via multiple gut-brain pathways

Food intake is tightly regulated by complex and coordinated gut-brain interactions. Nutrients rapidly modulate activity in key populations of hypothalamic neurons that regulate food intake, including hunger-sensitive agouti-related protein (AgRP)-expressing neurons. Because individual macronutrients engage specific receptors in the gut to communicate with the brain, we reasoned that macronutrients may utilize different pathways to reduce activity in AgRP neurons. Here, we revealed that AgRP neuron activity in hungry mice is inhibited by site-specific intestinal detection of different macronutrients. We showed that vagal gut-brain signaling is required for AgRP neuron inhibition by fat. In contrast, spinal gut-brain signaling relays the presence of intestinal glucose. Further, we identified glucose sensors in the intestine and hepatic portal vein that mediate glucose-dependent AgRP neuron inhibition. Therefore, distinct pathways are activated by individual macronutrients to inhibit AgRP neuron activity.

Appetite self-regulation declines across childhood while general self-regulation improves: A narrative review of the origins and development of appetite self-regulation

This narrative review discusses the origins and development of appetite self-regulation (ASR) in childhood (from infancy to age 6 or 7 years). The origins, or foundations, are the biological infrastructure associated with appetite regulation and appetite self-regulation. Homeostatic regulation in infancy is examined and then evidence about developmental change in components of ASR. The main ASR-related components covered are: delay-of-gratification, caloric compensation, eating in the absence of hunger, food responsiveness/hedonics and fussy eating. The research included behavioral measures, parent-reports of appetitive traits and fMRI studies. There were two main trends in the evidence: a decline across childhood in the components of ASR associated with food approach (and therefore an increase in disinhibited eating), and wide individual differences. The decline in ASR contrasts with general self-regulation (GSR) where the evidence is of an improvement across childhood. For many children, bottom-up automatic reactive processes via food reward/hedonics or food avoidance as in fussy eating, appear not to be matched by improvements in top-down regulatory capacities. The prominence of bottom-up processes in ASR could be the main factor in possible differences in developmental paths for GSR and ASR. GSR research is situated in developmental science with its focus on developmental processes, theory and methodology. In contrast, the development of ASR at present does not have a strong developmental tradition to access and there is no unifying model of ASR and its development. We concluded (1) outside of mean-level or normative changes in the components of ASR, individual differences are prominent, and (2) there is a need to formulate models of developmental change in ASR together with appropriate measurement, research designs and data analysis strategies.

Citrus Polyphenols in Brain Health and Disease: Current Perspectives

In addition to essential micronutrients such as vitamin C, citrus fruits represent a considerably rich source of non-essential bioactive compounds, in particular flavanones which form a sub-set of the flavonoid group. Preclinical studies have demonstrated the neuroprotective potential of citrus flavonoids and have highlighted both the well-established (anti-inflammatory and anti-oxidative properties), and newly emerging (influence upon blood-brain barrier function/integrity) mechanistic actions by which these neurological effects are mediated. Encouragingly, results from human studies, although limited in number, appear to support this preclinical basis, with improvements in cognitive performance and disease risk observed across healthy and disease states. Therefore, citrus fruits - both as whole fruit and 100% juices - should be encouraged within the diet for their potential neurological benefit. In addition, there should be further exploration of citrus polyphenols to establish therapeutic efficacy, particularly in the context of well-designed human interventions.

Pharmacology of TAS1R2/TAS1R3 Receptors and Sweet Taste

The detection of energy-rich sweet food items has been important for our survival during evolution, however, in light of the changing lifestyles in industrialized and developing countries our natural sweet preference is causing considerable problems. Hence, it is even more important to understand how our sense of sweetness works, and perhaps even, how we may deceive it for our own benefit. This chapter summarizes current knowledge about sweet tastants and sweet taste modulators on the compound side as well as insights into the structure and function of the sweet taste receptor and the transduction of sweet signals. Moreover, methods to assess the activity of sweet substances in vivo and in vitro are compared and discussed.

Sucralose and Cardiometabolic Health: Current Understanding from Receptors to Clinical Investigations

The excess consumption of added sugar is consistently found to be associated with weight gain, and a higher risk of type 2 diabetes mellitus, coronary heart disease, and stroke. In an effort to reduce the risk of cardiometabolic disease, sugar is frequently replaced by low- and null-calorie sweeteners (LCSs). Alarmingly, though, emerging evidence indicates that the consumption of LCSs is associated with an increase in cardiovascular mortality risk that is amplified in those who are overweight or obese. Sucralose, a null-caloric high-intensity sweetener, is the most commonly used LCS worldwide, which is regularly consumed by healthy individuals and patients with metabolic disease. To explore a potential causal role for sucralose in increased cardiovascular risk, this present review summarizes the preclinical and clinical data from current research detailing the effects of sucralose on systems controlling food intake, glucose homeostasis, and gut microbiota.

Viscera affectum anno: the gut beyond eating behaviours

One of the most pleasurable, yet dangerous, activities of our daily life is eating. But once food has been swallowed, all we can do is to trust our gut. Several remarkable studies published in 2020 have expanded our knowledge on how the gut is intertwined with essential behaviours beyond food.

Gut-to-brain signals in feeding control

Interoceptive signals from gut and adipose tissue and sensory cues from the environment are integrated by hubs in the brain to regulate feeding behavior and maintain homeostatic control of body weight. In vivo neural recordings have revealed that these signals control the activity of multiple layers of hunger neurons and eating is not only the result of feedback correction to a set point, but can also be under the influence of anticipatory regulations. A series of recent technical developments have revealed how peripheral and sensory signals, in particular, from the gut are conveyed to the brain to integrate neural circuits. Here, we describe the mechanisms involved in gastrointestinal stimulation by nutrients and how these signals act on the hindbrain to generate motivated behaviors. We also consider the organization of multidirectional intra- and extrahypothalamic circuits and how this has created a framework for understanding neural control of feeding.

Food allergy as a biological food quality control system

Food is simultaneously a source of essential nutrients and a potential source of lethal toxins and pathogens. Consequently, multiple sensory mechanisms evolved to monitor the quality of food based on the presence and relative abundance of beneficial and harmful food substances. These include the olfactory, gustatory, and gut chemosensory systems. Here we argue that, in addition to these systems, allergic immunity plays a role in food quality control by mounting allergic defenses against food antigens associated with noxious substances. Exaggeration of these defenses can result in pathological food allergy.

What Does the Taste System Tell Us About the Nutritional Composition and Toxicity of Foods?

One of the distinctive features of the human taste system is that it categorizes food into a few taste qualities - e.g., sweet, salty, sour, bitter, and umami. Here, I examined the functional significance of these taste qualities by asking what they tell us about the nutritional composition and toxicity of foods. I collected published data on the composition of raw and unprocessed foods - i.e., fruits, endosperm tissues, starchy foods, mushrooms, and meats. Sweet taste is thought to help identify foods with a high caloric or micronutrient density. However, the sweetest foods (fruits) had a relatively modest caloric density and low micronutrient density, whereas the blandest foods (endosperm tissues and meats) had a relatively high caloric and high micronutrient density. Salty taste is thought to be a proxy for foods high in sodium. Sodium levels were higher in meats than in most plant materials, but raw meats lack a salient salty taste. Sour taste (a measure of acidity) is thought to signify dangerous or spoiled foods. While this may be the case, it is notable that most ripe fruits are acidic. Umami taste is thought to reflect the protein content of food. I found that free L-glutamate (the prototypical umami tastant) concentration varies independently of protein content in foods. Bitter taste is thought to help identify poisonous foods, but many nutritious plant materials taste bitter. Fat taste is thought to help identify triglyceride-rich foods, but the role of taste versus mouthfeel in the attraction to fatty foods is unresolved. These findings indicate that the taste system provides incomplete or, in some cases, misleading information about the nutritional content and toxicity of foods. This may explain why inputs from the taste system are merged with inputs from the other cephalic senses and intestinal nutrient-sensing systems. By doing so, we create a more complete sensory representation and nutritional evaluation of foods.

Mental Disorders Linked to Crosstalk between The Gut Microbiome and The Brain

Often called the second brain, the gut communicates extensively with the brain and vice versa. The conversation between these two organs affects a variety of physiological mechanisms that are associated with our mental health. Over the past decade, a growing body of evidence has suggested that the gut microbiome builds a unique ecosystem inside the gastrointestinal tract to maintain the homeostasis and that compositional changes in the gut microbiome are highly correlated with several mental disorders. There are ongoing efforts to treat or prevent mental disorders by regulating the gut microbiome using probiotics. These attempts are based on the seminal findings that probiotics can control the gut microbiome and affect mental conditions. However, some issues have yet to be conclusively addressed, especially the causality between the gut microbiome and mental disorders. In this review, we focus on the mechanisms by which the gut microbiome affects mental health and diseases. Furthermore, we discuss the potential use of probiotics as therapeutic agents for psychiatric disorders.

Emerging Frontier of Peripheral Nerve and Organ Interfaces

The development of new tools to interface with the nervous system, empowered by advances in electronics and materials science, has transformed neuroscience and is informing therapies for neurological and mental conditions. Although the vast majority of neural engineering research has focused on advancing tools to study the brain, understanding the peripheral nervous system and other organs can similarly benefit from these technologies. To realize this vision, the neural interface technologies need to address the biophysical, mechanical, and chemical challenges posed by the peripheral nerves and organs. In this Perspective, we discuss design considerations and recent technological advances to modulate electrical signaling outside the central nervous system. The innovations in bioelectronics borne out of interdisciplinary collaborations between biologists and physical scientists may not only advance fundamental study of peripheral (neuro)physiology but also empower clinical interventions for conditions including neurological, gastrointestinal, and immune dysfunction.

tVNS Increases Liking of Orally Sampled Low-Fat Foods: A Pilot Study

Recently a role for the vagus nerve in conditioning food preferences was established in rodents. In a prospective controlled clinical trial in humans, invasive vagus nerve stimulation shifted food choice toward lower fat content. Here we explored whether hedonic aspects of an orally sampled food stimulus can be modulated by non-invasive transcutaneous vagus nerve stimulation (tVNS) in humans. In healthy participants (n = 10, five women, 20-32 years old, no obesity) we tested liking and wanting ratings of food samples with varying fat or sugar content with or without tVNS in a sham-controlled within-participants design. To determine effects of tVNS on food intake, we also measured voluntary consumption of milkshake. Spontaneous eye blink rate was measured as a proxy for dopamine tone. Liking of low-fat, but not high-fat puddings, was higher for tVNS relative to sham stimulation. Other outcomes showed no differences. These findings support a role for the vagus nerve promoting post-ingestive reward signals. Our results suggest that tVNS may be used to increase liking of low-calorie foods, which may support healthier food choices.

Western Diet: Implications for Brain Function and Behavior

The Western diet (WD) pattern characterized by high daily intake of saturated fats and refined carbohydrates often leads to obesity and overweight, and it has been linked to cognitive impairment and emotional disorders in both animal models and humans. This dietary pattern alters the composition of gut microbiota, influencing brain function by different mechanisms involving the gut-brain axis. In addition, long-term exposure to highly palatable foods typical of WD could induce addictive-like eating behaviors and hypothalamic-pituitary-adrenal (HPA) axis dysregulation associated with chronic stress, anxiety, and depression. In turn, chronic stress modulates eating behavior, and it could have detrimental effects on different brain regions such as the hippocampus, hypothalamus, amygdala, and several cortical regions. Moreover, obesity and overweight induce neuroinflammation, causing neuronal dysfunction. In this review, we summarize the current scientific evidence about the mechanisms and factors relating WD consumption with altered brain function and behavior. Possible therapeutic interventions and limitations are also discussed, aiming to tackle and prevent this current pandemic.

Enteric neurons increase maternal food intake during reproduction

Reproduction induces increased food intake across females of many animal species1-4, providing a physiologically relevant paradigm for the exploration of appetite regulation. Here, by examining the diversity of enteric neurons in Drosophila melanogaster, we identify a key role for gut-innervating neurons with sex- and reproductive state-specific activity in sustaining the increased food intake of mothers during reproduction. Steroid and enteroendocrine hormones functionally remodel these neurons, which leads to the release of their neuropeptide onto the muscles of the crop-a stomach-like organ-after mating. Neuropeptide release changes the dynamics of crop enlargement, resulting in increased food intake, and preventing the post-mating remodelling of enteric neurons reduces both reproductive hyperphagia and reproductive fitness. The plasticity of enteric neurons is therefore key to reproductive success. Our findings provide a mechanism to attain the positive energy balance that sustains gestation, dysregulation of which could contribute to infertility or weight gain.

The gut microbiota-brain axis in behaviour and brain disorders

In a striking display of trans-kingdom symbiosis, gut bacteria cooperate with their animal hosts to regulate the development and function of the immune, metabolic and nervous systems through dynamic bidirectional communication along the 'gut-brain axis'. These processes may affect human health, as certain animal behaviours appear to correlate with the composition of gut bacteria, and disruptions in microbial communities have been implicated in several neurological disorders. Most insights about host-microbiota interactions come from animal models, which represent crucial tools for studying the various pathways linking the gut and the brain. However, there are complexities and manifest limitations inherent in translating complex human disease to reductionist animal models. In this Review, we discuss emerging and exciting evidence of intricate and crucial connections between the gut microbiota and the brain involving multiple biological systems, and possible contributions by the gut microbiota to neurological disorders. Continued advances from this frontier of biomedicine may lead to tangible impacts on human health.

Effect of short-term intake of high- and low-concentrations of sucrose solution on the neurochemistry of male and female mice

The effect of short-term intake of high- and low-concentrations of sucrose solution on the neurochemistry of male and female mice was studied. The body weight, feed intake, sucrose solution consumption and brain monoamine neurotransmitters were determined after 34 days' intake of 1% and 8% sucrose solutions. The gene expression and protein levels related to dopamine and opioids were also determined. The results showed that the intake of 1% and 8% sucrose solution for 34 days did not cause significant changes in the weight development of both male and female mice. The preference for sucrose varies with sex. Both males and females had greater preference for the high concentration sucrose solution than the low concentration sucrose solution. The continuous intake of sucrose stimulated the release of monoamine neurotransmitters (DA, 5-HT, NE) in the brains of mice, and the reward effect of 8% sucrose solution is significantly higher than that of 1% sucrose solution. The sex of mice did not affect the release of neurotransmitters. The gene expressions of D1 and D2 were up-regulated in the 1% sucrose group of male mice, while the OPRM1 gene expression was down-regulated. The expression of these three genes in the 8% sucrose group of male mice was all down-regulated, while the gene expressions of D1 and D2 in the 1% and 8% sucrose group (p < 0.05) of female mice were both up-regulated.

Diet, Microbiota and Brain Health: Unraveling the Network Intersecting Metabolism and Neurodegeneration

Increasing evidence gives support for the idea that extra-neuronal factors may affect brain physiology and its predisposition to neurodegenerative diseases. Epidemiological and experimental studies show that nutrition and metabolic disorders such as obesity and type 2 diabetes increase the risk of Alzheimer's and Parkinson's diseases after midlife, while the relationship with amyotrophic lateral sclerosis is uncertain, but suggests a protective effect of features of metabolic syndrome. The microbiota has recently emerged as a novel factor engaging strong interactions with neurons and glia, deeply affecting their function and behavior in these diseases. In particular, recent evidence suggested that gut microbes are involved in the seeding of prion-like proteins and their spreading to the central nervous system. Here, we present a comprehensive review of the impact of metabolism, diet and microbiota in neurodegeneration, by affecting simultaneously several aspects of health regarding energy metabolism, immune system and neuronal function. Advancing technologies may allow researchers in the future to improve investigations in these fields, allowing the buildup of population-based preventive interventions and development of targeted therapeutics to halt progressive neurologic disability.

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

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

Taste transduction and channel synapses in taste buds

The variety of taste sensations, including sweet, umami, bitter, sour, and salty, arises from diverse taste cells, each of which expresses specific taste sensor molecules and associated components for downstream signal transduction cascades. Recent years have witnessed major advances in our understanding of the molecular mechanisms underlying transduction of basic tastes in taste buds, including the identification of the bona fide sour sensor H⁺ channel OTOP1, and elucidation of transduction of the amiloride-sensitive component of salty taste (the taste of sodium) and the TAS1R-independent component of sweet taste (the taste of sugar). Studies have also discovered an unconventional chemical synapse termed "channel synapse" which employs an action potential-activated CALHM1/3 ion channel instead of exocytosis of synaptic vesicles as the conduit for neurotransmitter release that links taste cells to afferent neurons. New images of the channel synapse and determinations of the structures of CALHM channels have provided structural and functional insights into this unique synapse. In this review, we discuss the current view of taste transduction and neurotransmission with emphasis on recent advances in the field.

FGF21 Signals to Glutamatergic Neurons in the Ventromedial Hypothalamus to Suppress Carbohydrate Intake

Fibroblast growth factor 21 (FGF21) is an endocrine hormone produced by the liver that regulates nutrient and metabolic homeostasis. FGF21 production is increased in response to macronutrient imbalance and signals to the brain to suppress sugar intake and sweet-taste preference. However, the central targets mediating these effects have been unclear. Here, we identify FGF21 target cells in the hypothalamus and reveal that FGF21 signaling to glutamatergic neurons is both necessary and sufficient to mediate FGF21-induced sugar suppression and sweet-taste preference. Moreover, we show that FGF21 acts directly in the ventromedial hypothalamus (VMH) to specifically regulate sucrose intake, but not non-nutritive sweet-taste preference, body weight, or energy expenditure. Finally, our data demonstrate that FGF21 affects neuronal activity by increasing activation and excitability of neurons in the VMH. Thus, FGF21 signaling to glutamatergic neurons in the VMH is an important component of the neurocircuitry that functions to regulate sucrose intake.

Deconstructing the Direct Reciprocal Hippocampal-Anterior Thalamic Pathways for Spatial Learning

The hippocampus is essential for normal memory but does not act in isolation. The anterior thalamic nuclei may represent one vital partner. Using DREADDs, the behavioral consequences of transiently disrupting anterior thalamic function were examined, followed by inactivation of the dorsal subiculum. Next, the anterograde transport of an adeno-associated virus expressing DREADDs was paired with localized intracerebral infusions of a ligand to target specific input pathways. In this way, the direct projections from the anterior thalamic nuclei to the dorsal hippocampal formation were inhibited, followed by separate inhibition of the dorsal subiculum projections to the anterior thalamic nuclei. To assay spatial working memory, all animals performed a reinforced T-maze alternation task, then a more challenging version that nullifies intramaze cues. Across all four experiments, deficits emerged on the spatial alternation task that precluded the use of intramaze cues. Inhibiting dorsal subiculum projections to the anterior thalamic nuclei produced the severest spatial working memory deficit. This deficit revealed the key contribution of dorsal subiculum projections to the anteromedial and anteroventral thalamic nuclei for the processing of allocentric information, projections not associated with head-direction information. The overall pattern of results provides consistent causal evidence of the two-way functional significance of direct hippocampal-anterior thalamic interactions for spatial processing. At the same time, these findings are consistent with hypotheses that these same, reciprocal interactions underlie the common core symptoms of temporal lobe and diencephalic anterograde amnesia.

SIGNIFICANCE STATEMENT It has long been conjectured that the anterior thalamic nuclei might be key partners with the hippocampal formation and that, respectively, they are principally responsible for diencephalic and temporal lobe amnesia. However, direct causal evidence for this functional relationship is lacking. Here, we examined the behavioral consequences of transiently silencing the direct reciprocal interconnections between these two brain regions on tests of spatial learning. Disrupting information flow from the hippocampal formation to the anterior thalamic nuclei and vice versa impaired performance on tests of spatial learning. By revealing the conjoint importance of hippocampal-anterior thalamic pathways, these findings help explain why pathology in either the medial diencephalon or the medial temporal lobes can result in profound anterograde amnesic syndromes.