Gut fat sensing in the negative feedback control of energy balance--recent advances

Neuropeptidergic Control of Feeding: Focus on the Galanin Family of Peptides

Obesity/overweight are important health problems due to metabolic complications. Dysregulation of peptides exerting orexigenic/anorexigenic effects must be investigated in-depth to understand the mechanisms involved in feeding behaviour. One of the most important and studied orexigenic peptides is galanin (GAL). The aim of this review is to update the mechanisms of action and physiological roles played by the GAL family of peptides (GAL, GAL-like peptide, GAL message-associated peptide, alarin) in the control of food intake and to review the involvement of these peptides in metabolic diseases and food intake disorders in experimental animal models and humans. The interaction between GAL and NPY in feeding and energy metabolism, the relationships between GAL and other substances involved in food intake mechanisms, the potential pharmacological strategies to treat food intake disorders and obesity and the possible clinical applications will be mentioned and discussed. Some research lines are suggested to be developed in the future, such as studies focused on GAL receptor/neuropeptide Y Y₁ receptor interactions in hypothalamic and extra-hypothalamic nuclei and sexual differences regarding the expression of GAL in feeding behaviour. It is also important to study the possible GAL resistance in obese individuals to better understand the molecular mechanisms by which GAL regulates insulin/glucose metabolism. GAL does not exert a pivotal role in weight regulation and food intake, but this role is crucial in fat intake and also exerts an important action by regulating the activity of other key compounds under conditions of stress/altered diet.

Physical and nutrient stimuli differentially modulate gut motility patterns, gut transit rate, and transcriptome in an agastric fish, the ballan wrasse

The effects of nutrient and mechanical sensing on gut motility and intestinal metabolism in lower vertebrates remains largely unknown. Here we present the transcriptome response to luminal stimulation by nutrients and an inert bolus on nutrient response pathways and also the response on gut motility in a stomachless fish with a short digestive tract; the ballan wrasse (Labrus berggylta). Using an in vitro model, we differentiate how signals initiated by physical stretch (cellulose and plastic beads) and nutrients (lipid and protein) modulate the gut evacuation rate, motility patterns and the transcriptome. Intestinal stretch generated by inert cellulose initiated a faster evacuation of digesta out of the anterior intestine compared to digestible protein and lipid. Stretch on the intestine upregulated genes associated with increased muscle activity, whereas nutrients stimulated increased expression of several neuropeptides and receptors which are directly involved in gut motility regulation. Although administration of protein and lipid resulted in similar bulbous evacuation times, differences in intestinal motility, transit between the segments and gene expression between the two were observed. Lipid induced increased frequency of ripples and standing contraction in the middle section of the intestine compared to the protein group. We suggest that this difference in motility was modulated by factors [prepronociceptin (pnoca), prodynorphin (pdyn) and neuromedin U (nmu), opioid neurotransmitters and peptides] that are known to inhibit gastrointestinal motility and were upregulated by protein and not lipid. Our findings show that physical pressure in the intestine initiate contractions propelling the bolus distally, directly towards the exit, whereas the stimuli from nutrients modulates the motility to prolong the residence time of digesta in the digestive tract for optimal digestion.

Morbid obesity and type 2 diabetes alter intestinal fatty acid uptake and blood flow

AIMS

Bariatric surgery is the most effective treatment to tackle morbid obesity and type 2 diabetes, but the mechanisms of action are still unclear. The objective of this study was to investigate the effects of bariatric surgery on intestinal fatty acid (FA) uptake and blood flow.

MATERIALS AND METHODS

We recruited 27 morbidly obese subjects, of whom 10 had type 2 diabetes and 15 were healthy age-matched controls. Intestinal blood flow and fatty acid uptake from circulation were measured during fasting state using positron emission tomography (PET). Obese subjects were re-studied 6 months after bariatric surgery. The mucosal location of intestinal FA retention was verified in insulin resistant mice with autoradiography.

RESULTS

Compared to lean subjects, morbidly obese subjects had higher duodenal and jejunal FA uptake (P < .001) but similar intestinal blood flow (NS). Within 6 months after bariatric surgery, obese subjects had lost 24% of their weight and 7/10 diabetic subjects were in remission. Jejunal FA uptake was further increased (P < .03). Conversely, bariatric surgery provoked a decrease in jejunal blood flow (P < .05) while duodenal blood flow was preserved. Animal studies showed that FAs were taken up into enterocytes, for the most part, but were also transferred, in part, into the lumen.

CONCLUSIONS

In the obese, the small intestine actively takes up FAs from circulation and FA uptake remains higher than in controls post-operatively. Intestinal blood flow was not enhanced before or after bariatric surgery, suggesting that enhanced intestinal FA metabolism is not driven by intestinal perfusion.

The vagus neurometabolic interface and clinical disease

The nervous system both monitors and modulates body metabolism to maintain homoeostasis. In disease states such as obesity and diabetes, the neurometabolic interface is dysfunctional and contributes to clinical illness. The vagus nerve, in particular, with both sensory and motor fibres, provides an anatomical substrate for this interface. Its sensory fibres contain receptors for important circulating metabolic mediators, including leptin and cholecystokinin, and provide real-time information about these mediators to the central nervous system. In turn, efferent fibres within the vagus nerve participate in a brain-gut axis to regulate metabolism. In this review, we describe these vagus nerve-mediated metabolic pathways and recent clinical trials of vagus nerve stimulation for the management of obesity. These early studies suggest that neuromodulation approaches that employ electricity to tune neurometabolic circuits may represent a new tool in the clinical armamentarium directed against obesity.

Appetite control by the tongue-gut axis and evaluation of the role of CD36/SR-B2

Understanding the mechanisms governing food intake is a public health issue given the dramatic rise of obesity over the world. The overconsumption of tasty energy-dense foods rich in lipids is considered to be one of the nutritional causes of this epidemic. Over the last decade, the identification of fatty acid receptors in strategic places in the body (i.e. oro-intestinal tract and brain) has provided a major progress in the deciphering of regulatory networks involved in the control of dietary intake. Among these lipid sensors, CD36/SR-B2 appears to play a significant role since this membrane protein, known to bind long-chain fatty acid with a high affinity, was specifically found both in enterocytes and in a subset of taste bud cells and entero-endocrine cells. After a short overview on CD36/SR-B2 structure, function and regulation, this mini-review proposes to analyze the key findings about the role of CD36/SR-B2 along of the tongue-gut axis in relation to appetite control. In addition, we discuss whether obesogenic diets might impair lipid sensing mediated by CD36/SR-B2 along this axis.

Targeting the gastrointestinal tract to treat type 2 diabetes

The rising global rates of type 2 diabetes and obesity present a significant economic and social burden, underscoring the importance for effective and safe therapeutic options. The success of glucagon-like-peptide-1 receptor agonists in the treatment of type 2 diabetes, along with the potent glucose-lowering effects of bariatric surgery, highlight the gastrointestinal tract as a potential target for diabetes treatment. Furthermore, recent evidence suggests that the gut plays a prominent role in the ability of metformin to lower glucose levels. As such, the current review highlights some of the current and potential pathways in the gut that could be targeted to improve glucose homeostasis, such as changes in nutrient sensing, gut peptides, gut microbiota and bile acids. A better understanding of these pathways will lay the groundwork for novel gut-targeted antidiabetic therapies, some of which have already shown initial promise.

Central chronic apelin infusion decreases energy expenditure and thermogenesis in mice

Apelin is a bioactive peptide involved in the control of energy metabolism. In the hypothalamus, chronic exposure to high levels of apelin is associated with an increase in hepatic glucose production, and then contributes to the onset of type 2 diabetes. However, the molecular mechanisms behind deleterious effects of chronic apelin in the brain and consequences on energy expenditure and thermogenesis are currently unknown. We aimed to evaluate the effects of chronic intracerebroventricular (icv) infusion of apelin in normal mice on hypothalamic inflammatory gene expression, energy expenditure, thermogenesis and brown adipose tissue functions. We have shown that chronic icv infusion of apelin increases the expression of pro-inflammatory factors in the hypothalamus associated with an increase in plasma interleukin-1 beta. In parallel, mice infused with icv apelin exhibit a significant lower energy expenditure coupled to a decrease in PGC1alpha, PRDM16 and UCP1 expression in brown adipose tissue which could explain the alteration of thermogenesis in these mice. These data provide compelling evidence that central apelin contributes to the development of type 2 diabetes by altering energy expenditure, thermogenesis and fat browning.

Gut microbiota-derived lipopolysaccharide uptake and trafficking to adipose tissue: implications for inflammation and obesity

The composition of the gut microbiota and excessive ingestion of high-fat diets (HFD) are considered to be important factors for development of obesity. In this review we describe a coherent mechanism of action for the development of obesity, which involves the composition of gut microbiota, HFD, low-grade inflammation, expression of fat translocase and scavenger receptor CD36, and the scavenger receptor class B type 1 (SR-BI). SR-BI binds to both lipids and lipopolysaccharide (LPS) from Gram-negative bacteria, which may promote incorporation of LPS in chylomicrons (CMs). These CMs are transported via lymph to the circulation, where LPS is transferred to other lipoproteins by translocases, preferentially to HDL. LPS increases the SR-BI binding, transcytosis of lipoproteins over the endothelial barrier,and endocytosis in adipocytes. Especially large size adipocytes with high metabolic activity absorb LPS-rich lipoproteins. In addition, macrophages in adipose tissue internalize LPS-lipoproteins. This may contribute to the polarization from M2 to M1 phenotype, which is a consequence of increased LPS delivery into the tissue during hypertrophy. In conclusion, evidence suggests that LPS is involved in the development of obesity as a direct targeting molecule for lipid delivery and storage in adipose tissue.

Modeling Energy Dynamics in Mice with Skeletal Muscle Hypertrophy Fed High Calorie Diets

Retrospective and prospective studies show that lean mass or strength is positively associated with metabolic health. Mice deficient in myostatin, a growth factor that negatively regulates skeletal muscle mass, have increased muscle and body weights and are resistant to diet-induced obesity. Their leanness is often attributed to higher energy expenditure in the face of normal food intake. However, even obese animals have an increase in energy expenditure compared to normal weight animals suggesting this is an incomplete explanation. We have previously developed a computational model to estimate energy output, fat oxidation and respiratory quotient from food intake and body composition measurements to more accurately account for changes in body composition in rodents over time. Here we use this approach to understand the dynamic changes in energy output, intake, fat oxidation and respiratory quotient in muscular mice carrying a dominant negative activin receptor IIB expressed specifically in muscle. We found that muscular mice had higher food intake and higher energy output when fed either chow or a high-fat diet for 15 weeks compared to WT mice. Transgenic mice also matched their rate of fat oxidation to the rate of fat consumed better than WT mice. Surprisingly, when given a choice between high-fat diet and Ensure® drink, transgenic mice consumed relatively more calories from Ensure® than from the high-fat diet despite similar caloric intake to WT mice. When switching back and forth between diets, transgenic mice adjusted their intake more rapidly than WT to restore normal caloric intake. Our results show that mice with myostatin inhibition in muscle are better at adjusting energy intake and output on diets of different macronutrient composition than WT mice to maintain energy balance and resist weight gain.

Interventional treatment of obesity and diabetes: An interim report on gastric electrical stimulation

Gastric electrical stimulation has been applied to treat human obesity since 1995. Dilatation of the stomach causes a series of neural reflexes which result in satiation and satiety. In non-obese individuals food ingestion is limited in part by this mechanism. In obese individuals, satiation and satiety are defective and unable to limit energy intake and prevent excessive weight gain. Several gastric electrical stimulatory (GES) devices have been developed, tested in clinical trials and even approved for the treatment of obesity. The design and clinical utility of three devices (Transend®, Maestro® and DIAMOND®) that have been extensively studied are presented as well as that of a new device (abiliti®) which is in early development. The Transcend®, a low energy GES device, showed promising results in open label studies but failed to show a difference from placebo in decreasing weight in obese subjects. The results of the clinical trials in treating obese subjects with the Maestro®, a vagal nerve stimulator, were sufficient to gain approval for marketing the device. The DIAMOND®, a multi-electrode GES device, has been used to treat type 2 diabetes and an associated benefit is to reduce body weight and lower systolic blood pressure.

Dietary Lipids Inform the Gut and Brain about Meal Arrival via CD36-Mediated Signal Transduction

Sensing mechanisms for nutrients, in particular dietary fat, operate in the mouth, brain, and gastrointestinal tract and play a key role in regulating feeding behavior and energy balance. Critical to these regulatory mechanisms are the specialized receptors present on taste buds on the tongue, on neurons in specialized centers in the brain, and on epithelial and enteroendocrine cells in the intestinal mucosa. These receptors recognize nutrients and respond by inducing intracellular signals that trigger release of bioactive compounds that influence other organs and help coordinate the response to the meal. Components of dietary fat that are recognized by these receptors are the long-chain fatty acids that act as ligands for 2 G protein-coupled receptors, GPR40 and GPR120, and the fatty acid (FA) translocase/CD36. Recent evidence that emphasizes the important role of CD36 in orosensory, intestinal, and neuronal sensing of FAs under physiologic conditions is highlighted in the review. How this role intersects with that of GPR120 and GPR40 in the regulation of food preference and energy balance is briefly discussed.

Glucoregulatory Relevance of Small Intestinal Nutrient Sensing in Physiology, Bariatric Surgery, and Pharmacology

Emerging evidence suggests the gastrointestinal tract plays an important glucoregulatory role. In this perspective, we first review how the intestine senses ingested nutrients, initiating crucial negative feedback mechanisms through a gut-brain neuronal axis to regulate glycemia, mainly via reduction in hepatic glucose production. We then highlight how intestinal energy sensory mechanisms are responsible for the glucose-lowering effects of bariatric surgery, specifically duodenal-jejunal bypass, and the antidiabetic agents metformin and resveratrol. A better understanding of these pathways lays the groundwork for intestinally targeted drug therapy for the treatment of diabetes.

Intestinal lipid-derived signals that sense dietary fat

Fat is a vital macronutrient, and its intake is closely monitored by an array of molecular sensors distributed throughout the alimentary canal. In the mouth, dietary fat constituents such as mono- and diunsaturated fatty acids give rise to taste signals that stimulate food intake, in part by enhancing the production of lipid-derived endocannabinoid messengers in the gut. As fat-containing chyme enters the small intestine, it causes the formation of anorexic lipid mediators, such as oleoylethanolamide, which promote satiety. These anatomically and functionally distinct responses may contribute to the homeostatic control and, possibly, the pathological dysregulation of food intake.

Fat sensing and metabolic syndrome

Overconsumption of dietary fat contributes to the development of obesity and metabolic syndrome. Recent evidence suggests that high dietary fat may promote these metabolic states not only by providing calories but also by inducing impaired control of energy balance. In normal metabolic states, fat interacts with various organs or receptors to generate signals for the regulation of energy balance. Many of these interactions are impaired by high-fat diets or in obesity, contributing to the development or maintenance of obesity. These impairments may arise largely from fundamental alterations in the hypothalamus where all peripheral signals are integrated to regulate energy balance. This review focuses on various mechanisms by which fat is sensed at different stages of ingestion, circulation, storage, and utilization to regulate food intake, and how these individual mechanisms are altered by high-fat diets or in obesity.

Gut microbiota, nutrient sensing and energy balance

The gastrointestinal (GI) tract is a highly specialized sensory organ that provides crucial negative feedback during a meal, partly via a gut-brain axis. More specifically, enteroendocrine cells located throughout the GI tract are able to sense and respond to specific nutrients, releasing gut peptides that act in a paracrine, autocrine or endocrine fashion to regulate energy balance, thus controlling both food intake and possibly energy expenditure. Furthermore, the gut microbiota has been shown to provide a substantial metabolic and physiological contribution to the host, and metabolic disease such as obesity has been associated with aberrant gut microbiota and microbiome. Interestingly, recent evidence suggests that the gut microbiota can impact the gut-brain axis controlling energy balance, at both the level of intestinal nutrient-sensing mechanisms, as well as potentially at the sites of integration in the central nervous system. A better understanding of the intricate relationship between the gut microbiota and host energy-regulating pathways is crucial for uncovering the mechanisms responsible for the development of metabolic diseases and for possible therapeutic strategies.

Enterocyte-afferent nerve interactions in dietary fat sensing

The central nervous system (CNS) constantly monitors nutrient availability in the body and, in particular, in the gastrointestinal (GI) tract to regulate nutrient and energy homeostasis. Extrinsic parasympathetic and sympathetic nerves are crucial for CNS nutrient sensing in the GI tract. These extrinsic afferent nerves detect the nature and amount of nutrients present in the GI tract and relay the information to the brain, which controls energy intake and expenditure accordingly. Dietary fat and fatty acids are sensed through various direct and indirect mechanisms. These sensing processes involve the binding of fatty acids to specific G protein-coupled receptors expressed either on the afferent nerve fibres or on the surface of enteroendocrine cells that release gut peptides, which themselves can modulate afferent nerve activity through their cognate receptors or have endocrine effects directly on the brain. Further dietary fat sensing mechanisms that are related to enterocyte fat handling and metabolism involve the release of several possible chemical mediators such as fatty acid ethanolamides or apolipoprotein A-IV. We here present evidence for yet another mechanism that may be based on ketone bodies resulting from enterocyte oxidation of dietary fat-derived fatty acids. The presently available evidence suggests that sympathetic rather than vagal afferents are involved, but further experiments are necessary to critically examine this concept.

Role of anorectic N-acylethanolamines in intestinal physiology and satiety control with respect to dietary fat

Anandamide is a well-known agonist for the cannabinoid receptors. Along with endogenous anandamide other non-endocannabinoid N-acylethanolamines are also formed, apparently in higher amounts. These include mainly oleoylethanolamide (OEA), palmitoyelethanolamide (PEA) and linoleoylethanolamide (LEA), and they have biological activity by themselves being anorectic and anti-inflammatory. It appears that the major effect of dietary fat on the level of these molecules is in the gastrointestinal system, where OEA, PEA and LEA in the enterocytes may function as homeostatic signals, which are decreased by prolonged consumption of a high-fat diet. These lipid amides appear to mediate their signaling activity via activation of PPARα in the enterocyte followed by activation of afferent vagal fibers leading to the brain. Through this mechanism OEA, PEA and LEA may both reduce the consumption of a meal as well as increase the reward value of the food. Thus, they may function as homeostatic intestinal signals involving hedonic aspects that contribute to the regulation of the amounts of dietary fat to be ingested.

A fatty gut feeling

The absorptive epithelium of the proximal small intestine converts oleic acid released during fat digestion into oleoylethanolamide (OEA), an endogenous high-affinity agonist of peroxisome proliferator-activated receptor-α (PPAR-α). OEA interacts with this receptor to cause a state of satiety characterized by prolonged inter-meal intervals and reduced feeding frequency. The two main branches of the autonomic nervous system, sympathetic and parasympathetic, contribute to this effect: the former by enabling OEA mobilization in the gut and the latter by relaying the OEA signal to brain structures, such as the hypothalamus, that are involved in feeding regulation. OEA signaling may be a key component of the physiological system devoted to the monitoring of dietary fat intake, and its dysfunction might contribute to overweight and obesity.

Food and symptom generation in functional gastrointestinal disorders: physiological aspects

The response of the gastrointestinal tract (GIT) to ingestion of food is a complex and closely controlled process, which allows optimization of propulsion, digestion, absorption of nutrients, and removal of indigestible remnants. This review summarizes current knowledge on the mechanisms that control the response of the GIT to food intake. During the cephalic phase, triggered by cortical food-related influences, the GIT prepares for receiving nutrients. The gastric phase is dominated by the mechanical effect of the meal volume. Accumulation of food in the stomach activates tension-sensitive mechanoreceptors, which in turn stimulate gastric accommodation and gastric acid secretion through the intrinsic and vago-vagal reflex pathways. After meal ingestion, the tightly controlled process of gastric emptying starts, with arrival of nutrients in the duodenum triggering negative feedback on emptying and stimulating secretion of digestive enzymes through the neural (mainly vago-vagal reflex, but also intrinsic) and endocrine (release of peptides from entero-endocrine cells) pathways. Several types of specialized receptors detect the presence of all main categories of nutrients. In addition, the gastrointestinal mucosa expresses receptors of the T1R and T2R families (taste receptors) and several members of the transient receptor potential channel family, all of which are putatively involved in the detection of specific tastants in the lumen. Activation of nutrient and taste sensors also activates the extrinsic and intrinsic neural, as well as entero-endocrine, pathways. During passage through the small bowel, nutrients are progressively extracted, and electrolyte-rich liquid intestinal content with non-digestible residue is delivered to the colon. The colon provides absorption of the water and electrolytes, storage of non-digestible remnants of food, aboral propulsion of contents, and finally evacuation through defecation.

Endocannabinoids measurement in human saliva as potential biomarker of obesity

Background

The discovery of the endocannabinoid system and of its role in the regulation of energy balance has significantly advanced our understanding of the physiopathological mechanisms leading to obesity and type 2 diabetes. New knowledge on the role of this system in humans has been acquired by measuring blood endocannabinoids. Here we explored endocannabinoids and related N-acylethanolamines in saliva and verified their changes in relation to body weight status and in response to a meal or to body weight loss.

Methodology/Principal Findings

Fasting plasma and salivary endocannabinoids and N-acylethanolamines were measured through liquid mass spectrometry in 12 normal weight and 12 obese, insulin-resistant subjects. Salivary endocannabinoids and N-acylethanolamines were evaluated in the same cohort before and after the consumption of a meal. Changes in salivary endocannabinoids and N-acylethanolamines after body weight loss were investigated in a second group of 12 obese subjects following a 12-weeks lifestyle intervention program. The levels of mRNAs coding for enzymes regulating the metabolism of endocannabinoids, N-acylethanolamines and of cannabinoid type 1 (CB₁) receptor, alongside endocannabinoids and N-acylethanolamines content, were assessed in human salivary glands.

The endocannabinoids 2-arachidonoylglycerol (2-AG), N-arachidonoylethanolamide (anandamide, AEA), and the N-acylethanolamines (oleoylethanolamide, OEA and palmitoylethanolamide, PEA) were quantifiable in saliva and their levels were significantly higher in obese than in normal weight subjects. Fasting salivary AEA and OEA directly correlated with BMI, waist circumference and fasting insulin. Salivary endocannabinoids and N-acylethanolamines did not change in response to a meal. CB₁ receptors, ligands and enzymes were expressed in the salivary glands. Finally, a body weight loss of 5.3% obtained after a 12-weeks lifestyle program significantly decreased salivary AEA levels.

Conclusions/Significance

Endocannabinoids and N-acylethanolamines are quantifiable in saliva and their levels correlate with obesity but not with feeding status. Body weight loss significantly decreases salivary AEA, which might represent a useful biomarker in obesity.

Role of the gut in lipid homeostasis

Intestinal lipid transport plays a central role in fat homeostasis. Here we review the pathways regulating intestinal absorption and delivery of dietary and biliary lipid substrates, principally long-chain fatty acid, cholesterol, and other sterols. We discuss the regulation and functions of CD36 in fatty acid absorption, NPC1L1 in cholesterol absorption, as well as other lipid transporters including FATP4 and SRB1. We discuss the pathways of intestinal sterol efflux via ABCG5/G8 and ABCA1 as well as the role of the small intestine in high-density lipoprotein (HDL) biogenesis and reverse cholesterol transport. We review the pathways and genetic regulation of chylomicron assembly, the role of dominant restriction points such as microsomal triglyceride transfer protein and apolipoprotein B, and the role of CD36, l-FABP, and other proteins in formation of the prechylomicron complex. We will summarize current concepts of regulated lipoprotein secretion (including HDL and chylomicron pathways) and include lessons learned from families with genetic mutations in dominant pathways (i.e., abetalipoproteinemia, chylomicron retention disease, and familial hypobetalipoproteinemia). Finally, we will provide an integrative view of intestinal lipid homeostasis through recent findings on the role of lipid flux and fatty acid signaling via diverse receptor pathways in regulating absorption and production of satiety factors.

Role of gut nutrient sensing in stimulating appetite and conditioning food preferences

The discovery of taste and nutrient receptors (chemosensors) in the gut has led to intensive research on their functions. Whereas oral sugar, fat, and umami taste receptors stimulate nutrient appetite, these and other chemosensors in the gut have been linked to digestive, metabolic, and satiating effects that influence nutrient utilization and inhibit appetite. Gut chemosensors may have an additional function as well: to provide positive feedback signals that condition food preferences and stimulate appetite. The postoral stimulatory actions of nutrients are documented by flavor preference conditioning and appetite stimulation produced by gastric and intestinal infusions of carbohydrate, fat, and protein. Recent findings suggest an upper intestinal site of action, although postabsorptive nutrient actions may contribute to flavor preference learning. The gut chemosensors that generate nutrient conditioning signals remain to be identified; some have been excluded, including sweet (T1R3) and fatty acid (CD36) sensors. The gut-brain signaling pathways (neural, hormonal) are incompletely understood, although vagal afferents are implicated in glutamate conditioning but not carbohydrate or fat conditioning. Brain dopamine reward systems are involved in postoral carbohydrate and fat conditioning but less is known about the reward systems mediating protein/glutamate conditioning. Continued research on the postoral stimulatory actions of nutrients may enhance our understanding of human food preference learning.

The gut-brain dopamine axis: a regulatory system for caloric intake

Post-ingestive factors are known to strongly modulate feeding behavior by providing feedback signals to the central nervous system on the current physiological state of the organism. Of particular interest is the identification of the physiological pathways that permit the brain to sense post-ingestive signals. We will review recent evidence supporting the concept that direct stimulation of the gastrointestinal tract with nutrients induces release of the catecholamine neurotransmitter dopamine. In addition, changes in dopamine efflux produced by direct stimulation of the gastrointestinal tract were found to reflect the caloric load of the infusates, suggesting that dopamine signaling may function as a central caloric sensor that mediates adjustments in intake according to the caloric density of a meal. Consistent with the above, blockade of dopamine signaling disrupts flavor-nutrient associations and impairs the regulatory capacity to maintain constant caloric intake during intra-gastric feeding. Future research must determine the exact pathways linking gut nutrient administration to dopamine efflux. Current evidence points to parallel contributions by pre- and post-absorptive pathways, indicating that dopamine systems constitute a site of convergence through which distinct physiological signals can exert control over ingestive behaviors.

Regulation of fat intake in the absence of flavour signalling

Animals, including humans, can achieve precise regulation of caloric intake by adjusting consumption in response to covert changes in energy density. It remains unknown, however, whether the presence of flavour cues are required for the ability to maintain constant caloric intake. Also unknown are the brain circuits that may function as the central calorie monitors that control adaptive adjustments in energy intake. Here we show that mice trained to lick a dry spout in order to receive intra-gastric infusions of a fat emulsion maintained constant hourly caloric intake by adjusting the number of dry licks in response to changes in caloric density. Animals also increased dry licking according to hunger levels, and developed conditioned preferences for dry sippers associated with high calorie infusions. Importantly, striatal dopamine levels were closely associated with the amount of calories ingested, rather than with the number of dry licks produced. Dopamine levels in dorsal and ventral striatum also reflected caloric density in mice passively receiving intra-gastric infusions of fat emulsions. Consistent with the above, systemic administration of the dopamine receptor blocker haloperidol markedly increased the production of dry licks needed to obtain high-calorie infusions, as if the caloric density of the infusions had been diluted. Conversely, haloperidol markedly decreased the production of dry licks needed to obtain low-calorie infusions. Taken together, our results support the proposition that brain dopamine circuits function as one central sensor of calorie ingestion, since (1) extracellular striatal dopamine levels fluctuate in proportion to the caloric density of nutrients infused in the gut; and (2) inhibiting dopamine receptor signalling disrupts the animals' ability to maintain constant caloric intake across experimental sessions.

The fatty acid translocase gene CD36 and lingual lipase influence oral sensitivity to fat in obese subjects

The precise orosensory inputs engaged for dietary lipids detection in humans are unknown. We evaluated whether a common single nucleotide polymorphism (rs1761667) in the CD36 gene that reduces CD36 expression and the addition of orlistat, a lipase inhibitor, to reduce FA release from triacylglycerols (TGs), the main component of dietary fats, would attenuate fat orosensory sensitivity in humans. Twenty-one obese subjects with different rs1761667 genotypes (6 AA, 7 AG, and 8 GG) were studied on two occasions in which oleic acid and triolein orosensory detection thresholds were measured using emulsions prepared with and without orlistat. Subjects homozygous for the G-allele had 8-fold lower oral detection thresholds for oleic acid and triolein than subjects homozygous for the A allele, which associates with lower CD36 expression (P = 0.03). Thresholds for heterozygous subjects were intermediate. The addition of orlistat increased detection thresholds for triolein (log threshold = -0.3 ± 0.2 vs. 0.3 ± 0.1; P < 0.001) but not oleic acid (log threshold = -1.0 ± 0.2 vs. -0.8 ± 0.2; P > 0.2). In conclusion, this is the first experimental evidence for a role of CD36 in fat gustatory perception in humans. The data also support involvement of lingual lipase and are consistent with the concept that FA and not TG is the sensed stimulus.

CD36 genetics and the metabolic complications of obesity

PURPOSE OF REVIEW

The review summarizes our current understanding of the function of the fatty acid translocase, CD36, in lipid metabolism with an emphasis on the influence of CD36 genetic variants and their potential contribution to obesity-related complications.

RECENT FINDINGS

Studies in rodents implicate CD36 in a number of metabolic pathways with relevance to obesity and its associated complications. These include pathways related to fat utilization such as taste perception, intake, intestinal processing, and storage in adipose tissue. Dysfunction in these pathways, coupled with the ability of CD36 to transduce intracellular signals that initiate inflammation in response to excess fat supply, promotes metabolic pathology. In the last few years, the relevance of discoveries in rodents to humans has been highlighted by genetic studies, which identified common CD36 variants that influence circulating lipid levels and cardiometabolic phenotypes.

SUMMARY

Recent genetic studies suggest that CD36 plays an important role in lipid metabolism in humans and may be involved in obesity-related complications. These findings may accelerate the translation of CD36 metabolic functions determined in rodents to humans. Importantly, these studies highlight the potential utility of assessing CD36 expression and common single-nucleotide polymorphism genotypes.

Rapid post-oral stimulation of intake and flavor conditioning by glucose and fat in the mouse

Although widely assumed to have only satiating actions, nutrients in the gut can also condition increases in intake in some cases. Here we studied the time course of post-oral nutrient stimulation of ingestion in food-restricted C57BL/6J mice. In experiment 1, mice adapted to drink a 0.8% sucralose solution 1 h/day, rapidly increased their rate of licking (within 4-6 min) when first tested with an 8% glucose solution and even more so in tests 2 and 3. Other mice decreased their licking rate when switched from sucralose to 8% fructose, a sugar that is sweet like glucose but lacks positive post-oral effects in mice. The glucose-stimulated drinking is due to the sugar's post-oral rather than taste properties, because sucralose is highly preferred to glucose and fructose in brief choice tests. A second experiment showed that the glucose-stimulated ingestion is associated with a conditioned flavor preference in both intact and capsaicin-treated mice. This indicates that the post-oral stimulatory action of glucose is not mediated by capsaicin-sensitive visceral afferents. In experiment 3, mice consumed flavored saccharin solutions as they self-infused water or glucose via an intragastric (IG) catheter. The glucose self-infusion stimulated ingestion within 13-15 min in test 1 and produced a conditioned increase in licking that was apparent in the initial minute of tests 2 and 3. Experiment 4 revealed that IG self-infusions of a fat emulsion also resulted in post-oral stimulation of licking in test 1 and conditioned increases in tests 2 and 3. These findings indicate that glucose and fat can generate stimulatory post-oral signals early in a feeding session that increase ongoing ingestion and condition increases in flavor acceptance and preference revealed in subsequent feeding sessions. The test procedures developed here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients.