The Neurobiology of Eating Behavior in Obesity: Mechanisms and Therapeutic Targets: A Report from the 23rd Annual Harvard Nutrition Obesity Symposium

Obesity is increasing at an alarming rate. The effectiveness of currently available strategies for the treatment of obesity (including pharmacologic, surgical, and behavioral interventions) is limited. Understanding the neurobiology of appetite and the important drivers of energy intake (EI) can lead to the development of more effective strategies for the prevention and treatment of obesity. Appetite regulation is complex and is influenced by genetic, social, and environmental factors. It is intricately regulated by a complex interplay of endocrine, gastrointestinal, and neural systems. Hormonal and neural signals generated in response to the energy state of the organism and the quality of food eaten are communicated by paracrine, endocrine, and gastrointestinal signals to the nervous system. The central nervous system integrates homeostatic and hedonic signals to regulate appetite. Although there has been an enormous amount of research over many decades regarding the regulation of EI and body weight, research is only now yielding potentially effective treatment strategies for obesity. The purpose of this article is to summarize the key findings presented in June 2022 at the 23rd annual Harvard Nutrition Obesity Symposium entitled "The Neurobiology of Eating Behavior in Obesity: Mechanisms and Therapeutic Targets." Findings presented at the symposium, sponsored by NIH P30 Nutrition Obesity Research Center at Harvard, enhance our current understanding of appetite biology, including innovative techniques used to assess and systematically manipulate critical hedonic processes, which will shape future research and the development of therapeutics for obesity prevention and treatment.

D1 and D2 neurons in the nucleus accumbens enable positive and negative control over sugar intake in mice

Although the consumption of carbohydrates is needed for survival, their potent reinforcing properties drive obesity worldwide. In turn, sugar overconsumption reveals a major role for brain reward systems in regulating sugar intake. However, it remains elusive how different cell types within the reward circuitries control the initiation and termination of sugary meals. Here, we identified the distinct nucleus accumbens cell types that mediate the chemosensory versus postprandial properties of sweet sugars. Specifically, D1 neurons enhance sugar intake via specialized connections to taste ganglia, whereas D2 neurons mediate the termination of sugary meals via anatomical connections to circuits involved in appetite suppression. Consistently, D2, but not D1, neurons partially mediate the satiating effects of glucagon-like peptide 1 (GLP-1) agonists. Thus, these nucleus accumbens cell types function as a behavioral switch, enabling positive versus negative control over sugar intake. Our study contributes to unveiling the cellular and circuit substrates of sugar overconsumption.

An inter-organ neural circuit for appetite suppression

Glucagon-like peptide-1 (GLP-1) is a signal peptide released from enteroendocrine cells of the lower intestine. GLP-1 exerts anorectic and antimotility actions that protect the body against nutrient malabsorption. However, little is known about how intestinal GLP-1 affects distant organs despite rapid enzymatic inactivation. We show that intestinal GLP-1 inhibits gastric emptying and eating via intestinofugal neurons, a subclass of myenteric neurons that project to abdominal sympathetic ganglia. Remarkably, cell-specific ablation of intestinofugal neurons eliminated intestinal GLP-1 effects, and their chemical activation functioned as a GLP-1 mimetic. GLP-1 sensing by intestinofugal neurons then engaged a sympatho-gastro-spinal-reticular-hypothalamic pathway that links abnormal stomach distension to craniofacial programs for food rejection. Within this pathway, cell-specific activation of discrete neuronal populations caused systemic GLP-1-like effects. These molecularly identified, delimited enteric circuits may be targeted to ameliorate the abdominal bloating and loss of appetite typical of gastric motility disorders.

Nausea and the Brain: The Chemoreceptor Trigger Zone Enters the Molecular Age

Area postrema in brainstem has long been known to trigger emesis by detecting blood-borne toxins and pathogens. In this issue, Zhang and colleagues provide a single-cell molecular atlas of this region, opening new possibilities for harnessing its neurons in vivo.

Rethinking Food Reward

The conscious perception of the hedonic sensory properties of caloric foods is commonly believed to guide our dietary choices. Current and traditional models implicate the consciously perceived hedonic qualities of food as driving overeating, whereas subliminal signals arising from the gut would curb our uncontrolled desire for calories. Here we review recent animal and human studies that support a markedly different model for food reward. These findings reveal in particular the existence of subcortical body-to-brain neural pathways linking gastrointestinal nutrient sensors to the brain's reward regions. Unexpectedly, consciously perceptible hedonic qualities appear to play a less relevant, and mostly transient, role in food reinforcement. In this model, gut-brain reward pathways bypass cranial taste and aroma sensory receptors and the cortical networks that give rise to flavor perception. They instead reinforce behaviors independently of the cognitive processes that support overt insights into the nature of our dietary decisions.

Rethinking Food Reward

The conscious perception of the hedonic sensory properties of caloric foods is commonly believed to guide our dietary choices. Current and traditional models implicate the consciously perceived hedonic qualities of food as driving overeating, whereas subliminal signals arising from the gut would curb our uncontrolled desire for calories. Here we review recent animal and human studies that support a markedly different model for food reward. These findings reveal in particular the existence of subcortical body-to-brain neural pathways linking gastrointestinal nutrient sensors to the brain's reward regions. Unexpectedly, consciously perceptible hedonic qualities appear to play a less relevant, and mostly transient, role in food reinforcement. In this model, gut-brain reward pathways bypass cranial taste and aroma sensory receptors and the cortical networks that give rise to flavor perception. They instead reinforce behaviors independently of the cognitive processes that support overt insights into the nature of our dietary decisions.

On the roles of the Duodenum and the Vagus nerve in learned nutrient preferences

BACKGROUND AND AIM

In most species, including humans, food preference is primarily controlled by nutrient value. However, the gut-brain pathways involved in preference learning remain elusive. The aim of the present study, performed in C57BL6/J mice, was to characterize the roles in nutrient preference of two critical elements of gut-brain pathways, i.e. the duodenum and vagal gut innervation.

METHODS

Adult wild-type C57BL6/J mice from a normal-weight cohort sustained one of the following three procedures: duodenal-jejunal bypass intestinal rerouting (DJB), total subdiaphragmatic vagotomy (SDV), or sham surgery. Mice were assessed in short-term two-bottle preference tests before and after 24 h s exposures to solutions containing one of glutamate, lipids, sodium, or glucose.

RESULTS

DJB and SDV interfered in preference formation in a nutrient-specific manner: whereas normal preference learning for lipids and glutamate was disrupted by both DJB and SDV, these interventions did not alter the formation of preferences for glucose. Interestingly, sodium preferences were abrogated by DJB but not by SDV.

CONCLUSIONS

Different macronutrients make use of distinct gut-brain pathways to influence food preferences, thereby mirroring nutrient-specific processes of food digestion. Specifically, whereas both vagal innervation and duodenal sensing appear critical for generating responses to fats and protein, glucose preferences recruit post-duodenal, vagal-independent pathways in pair with the control of glucose homeostasis. Overall, our data suggest that the physiological processes involved in digesting and absorbing fats, amino acids, and glucose overlap with those mediating learned preferences for each of these nutrients.

Sugar Metabolism Regulates Flavor Preferences and Portal Glucose Sensing

In most species, including humans, food preference is primarily controlled by nutrient value. In particular, glucose-containing sugars exert exquisitely strong effects on food choice via gut-generated signals. However, the identity of the visceral signals underlying glucose’s rewarding effects remains uncertain. In particular, it is unknown whether sugar metabolism mediates the formation of preferences for glucose-containing sugars. Using the mouse as a model organism, we made use of a combination of conditioning schedules, gastrointestinal nutrient administration, and chromatographic/electrochemical methods to assess the behavioral and neural effects of activating the gut with either metabolizable glucose or a non-metabolizable glucose analog. We show that mice display much superior preferences for flavors associated with intra-gastric infusions of glucose compared to flavors associated with intra-gastric infusions of the non-metabolizable glucose analog α-methyl-D-glucopyranoside (“MDG,” an activator of intestinal sodium/glucose co-transporters). These effects were unaffected by surgical bypassing of the duodenum, suggesting glucose-specific post-absorptive sensing mechanisms. Consistently, intra-portal infusions of glucose, but not of MDG, induced significant rises in dopamine (DA) levels within brain reward circuits. Our data reveal that the unmatched rewarding effects of glucose-containing sugars cannot be accounted for by metabolism-independent activation of sodium/glucose cotransporters; rather, they point to glucose metabolism as the physiological mechanism underlying the potent reward value of sugar-sweetened flavored beverages. In particular, no circulating “gut factors” need to be invoked to explain the reward value of ingested glucose. Thus, instead of circulating gut hormones, portal-mesenteric sensing of glucose emerges as the preferential physiological pathway for sugar reward.

A Neural Circuit for Gut-Induced Reward

The gut is now recognized as a major regulator of motivational and emotional states. However, the relevant gut-brain neuronal circuitry remains unknown. We show that optical activation of gut-innervating vagal sensory neurons recapitulates the hallmark effects of stimulating brain reward neurons. Specifically, right, but not left, vagal sensory ganglion activation sustained self-stimulation behavior, conditioned both flavor and place preferences, and induced dopamine release from Substantia nigra. Cell-specific transneuronal tracing revealed asymmetric ascending pathways of vagal origin throughout the CNS. In particular, transneuronal labeling identified the glutamatergic neurons of the dorsolateral parabrachial region as the obligatory relay linking the right vagal sensory ganglion to dopamine cells in Substantia nigra. Consistently, optical activation of parabrachio-nigral projections replicated the rewarding effects of right vagus excitation. Our findings establish the vagal gut-to-brain axis as an integral component of the neuronal reward pathway. They also suggest novel vagal stimulation approaches to affective disorders.

Gastric Bypass Surgery Recruits a Gut PPAR-α-Striatal D1R Pathway to Reduce Fat Appetite in Obese Rats

Bariatric surgery remains the single most effective long-term treatment modality for morbid obesity, achieved mainly by lowering caloric intake through as yet ill-defined mechanisms. Here we show in rats that Roux-en-Y gastric bypass (RYGB)-like rerouting of ingested fat mobilizes lower small intestine production of the fat-satiety molecule oleoylethanolamide (OEA). This was associated with vagus nerve-driven increases in dorsal striatal dopamine release. We also demonstrate that RYGB upregulates striatal dopamine 1 receptor (D1R) expression specifically under high-fat diet feeding conditions. Mechanistically, interfering with local OEA, vagal, and dorsal striatal D1R signaling negated the beneficial effects of RYGB on fat intake and preferences. These findings delineate a molecular/systems pathway through which bariatric surgery improves feeding behavior and may aid in the development of novel weight loss strategies that similarly modify brain reward circuits compromised in obesity.

Integrated Control of Predatory Hunting by the Central Nucleus of the Amygdala

Superior predatory skills led to the evolutionary triumph of jawed vertebrates. However, the mechanisms by which the vertebrate brain controls predation remain largely unknown. Here, we reveal a critical role for the central nucleus of the amygdala in predatory hunting. Both optogenetic and chemogenetic stimulation of central amygdala of mice elicited predatory-like attacks upon both insect and artificial prey. Coordinated control of cervical and mandibular musculatures, which is necessary for accurately positioning lethal bites on prey, was mediated by a central amygdala projection to the reticular formation in the brainstem. In contrast, prey pursuit was mediated by projections to the midbrain periaqueductal gray matter. Targeted lesions to these two pathways separately disrupted biting attacks upon prey versus the initiation of prey pursuit. Our findings delineate a neural network that integrates distinct behavioral modules and suggest that central amygdala neurons instruct predatory hunting across jawed vertebrates.

Striatal Dopamine Links Gastrointestinal Rerouting to Altered Sweet Appetite

Reductions in calorie intake contribute significantly to the positive outcome of bariatric surgeries. However, the physiological mechanisms linking the rerouting of the gastrointestinal tract to reductions in sugar cravings remain uncertain. We show that a duodenal-jejunal bypass (DJB) intervention inhibits maladaptive sweet appetite by acting on dopamine-responsive striatal circuitries. DJB disrupted the ability of recurrent sugar exposure to promote sweet appetite in sated animals, thereby revealing a link between recurrent duodenal sugar influx and maladaptive sweet intake. Unlike ingestion of a low-calorie sweetener, ingestion of sugar was associated with significant dopamine effluxes in the dorsal striatum, with glucose infusions into the duodenum inducing greater striatal dopamine release than equivalent jejunal infusions. Consistently, optogenetic activation of dopamine-excitable cells of the dorsal striatum was sufficient to restore maladaptive sweet appetite in sated DJB mice. Our findings point to a causal link between striatal dopamine signaling and the outcomes of bariatric interventions.

Fatty acid amide supplementation decreases impulsivity in young adult heavy drinkers

Compromised dopamine signaling in the striatum has been associated with the expression of impulsive behaviors in addiction, obesity and alcoholism. In rodents, intragastric infusion of the fatty acid amide oleoylethanolamide increases striatal extracellular dopamine levels via vagal afferent signaling. Here we tested whether supplementation with PhosphoLean™, a dietary supplement that contains the precursor of the fatty acid amide oleoylethanolamide (N-oleyl-phosphatidylethanolamine), would reduce impulsive responding and alcohol use in heavy drinking young adults. Twenty-two individuals were assigned to a three-week supplementation regimen with PhosphoLean™ or placebo. Impulsivity was assessed with self-report questionnaires and behavioral tasks pre- and post-supplementation. Although self-report measures of impulsivity did not change, supplementation with PhosphoLean™, but not placebo, significantly reduced false alarm rate on a Go/No-Go task. In addition, an association was found between improved sensitivity on the Go/No-Go task and reduced alcohol intake. These findings provide preliminary evidence that promoting fatty acid derived gut-brain dopamine communication may have therapeutic potential for reducing impulsivity in heavy drinkers.

Sensing of triacylglycerol in the gut: different mechanisms for fatty acids and 2-monoacylglycerol

Sensing of dietary triacylglycerol in the proximal small intestine results in physiological, hormonal and behavioural responses. However, the exact physiological pathways linking intestinal fat sensing to food intake and the activation of brain circuits remain to be identified. In this study we examined the role of triacylglycerol digestion for intestinal fat sensing, and compared the effects of the triacylglycerol digestion products, fatty acids and 2-monoacylglycerol, on behavioural, hormonal and dopaminergic responses in behaving mice. Using an operant task in which mice are trained to self-administer lipid emulsions directly into the stomach, we show that inhibiting triacylglycerol digestion disrupts normal behaviour of self-administration in mice, indicating that fat sensing is conditional to digestion. When administered separately, both digestion products, 2-monoacylglycerol and fatty acids, were sensed by the mice, and self-administration patterns of fatty acids were affected by the fatty acid chain length. Peripheral plasma concentrations of the gut hormones GLP-1, GIP, PYY, CCK and insulin did not offer an explanation of the differing behavioural effects produced by 2-monoacylglycerol and fatty acids. However, combined with behavioural responses, striatal dopamine effluxes induced by gut infusions of oleic acid were significantly greater than those produced by equivalent infusions of 2-oleoylglycerol. Our data demonstrate recruitment of different signalling pathways by fatty acids and 2-monoacylglycerol, and suggest that the structural properties of fat rather than total caloric value determine intestinal sensing and the assignment of reward value to lipids.

The neural signature of satiation is associated with ghrelin response and triglyceride metabolism

Eating behavior is guided by a complex interaction between signals conveying information about energy stores, food availability, and palatability. How peripheral signals regulate brain circuits that guide feeding during sensation and consumption of a palatable food is poorly understood. We used fMRI to measure brain response to a palatable food (milkshake) when n=32 participants were fasted and fed with either a fixed-portion or ad libitum meal. We found that larger post-prandial reductions in ghrelin and increases in triglycerides were associated with greater attenuation of response to the milkshake in brain regions regulating reward and feeding including the midbrain, amygdala, pallidum, hippocampus, insula and medial orbitofrontal cortex. Satiation-induced brain responses to milkshake were not related to acute changes in circulating insulin, glucose, or free fatty acids. The impact of a meal on the response to milkshake in the midbrain and dorsolateral prefrontal cortex differed depending upon whether meal termination was fixed or volitional, irrespective of the amount of food consumed. We conclude that satiation-induced changes in brain response to a palatable food are strongly and specifically associated with changes in circulating ghrelin and triglycerides and by volitional meal termination.

Hypothalamic melanin concentrating hormone neurons communicate the nutrient value of sugar

Sugars that contain glucose, such as sucrose, are generally preferred to artificial sweeteners owing to their post-ingestive rewarding effect, which elevates striatal dopamine (DA) release. While the post-ingestive rewarding effect, which artificial sweeteners do not have, signals the nutrient value of sugar and influences food preference, the neural circuitry that mediates the rewarding effect of glucose is unknown. In this study, we show that optogenetic activation of melanin-concentrating hormone (MCH) neurons during intake of the artificial sweetener sucralose increases striatal dopamine levels and inverts the normal preference for sucrose vs sucralose. Conversely, animals with ablation of MCH neurons no longer prefer sucrose to sucralose and show reduced striatal DA release upon sucrose ingestion. We further show that MCH neurons project to reward areas and are required for the post-ingestive rewarding effect of sucrose in sweet-blind Trpm5(-/-) mice. These studies identify an essential component of the neural pathways linking nutrient sensing and food reward. DOI: http://dx.doi.org/10.7554/eLife.01462.001.

Enantiomer-specific selection of amino acids

Dietary intake of L-amino acids impacts on several physiological functions, including the control of gastrointestinal motility, pancreatic secretion, and appetite. However, the biological mechanisms regulating behavioral predilections for certain amino acid types remain poorly understood. We tested the hypothesis that, in mice, the potency with which a given glucogenic amino acid increases glucose utilization reflects its rewarding properties. We have found that: (1) during long-, but not short-, term preference tests, L-alanine and L-serine were preferred over their D-enantiomer counterparts, while no such effect was observed for L-threonine vs. D-threonine; (2) these behavioral patterns were closely associated with the ability of L-amino acids to promote increases in respiratory exchange ratios such that those, and only those, L-amino acids able to promote increases in respiratory exchange ratios were preferred over their D-isomers; (3) these behavioral preferences were independent of gustatory influences, since taste-deficient Trpm5 knockout mice displayed ingestive responses very similar to those of their wild-type counterparts. We conclude that the ability to promote increases in respiratory exchange ratios enhances the reward value of nutritionally relevant amino acids and suggest a mechanistic link between substrate utilization and amino acid preferences.

Glucose utilization rates regulate intake levels of artificial sweeteners

It is well established that animals including humans attribute greater reinforcing value to glucose-containing sugars compared to their non-caloric counterparts, generally termed 'artificial sweeteners'. However, much remains to be determined regarding the physiological signals and brain systems mediating the attribution of greater reinforcing value to sweet solutions that contain glucose. Here we show that disruption of glucose utilization in mice produces an enduring inhibitory effect on artificial sweetener intake, an effect that did not depend on sweetness perception or aversion. Indeed, such an effect was not observed in mice presented with a less palatable, yet caloric, glucose solution. Consistently, hungry mice shifted their preferences away from artificial sweeteners and in favour of glucose after experiencing glucose in a hungry state. Glucose intake was found to produce significantly greater levels of dopamine efflux compared to artificial sweetener in dorsal striatum, whereas disrupting glucose oxidation suppressed dorsal striatum dopamine efflux. Conversely, inhibiting striatal dopamine receptor signalling during glucose intake in sweet-naïve animals resulted in reduced, artificial sweetener-like intake of glucose during subsequent gluco-deprivation. Our results demonstrate that glucose oxidation controls intake levels of sweet tastants by modulating extracellular dopamine levels in dorsal striatum, and suggest that glucose utilization is one critical physiological signal involved in the control of goal-directed sweetener intake.

A gut lipid messenger links excess dietary fat to dopamine deficiency

Excessive intake of dietary fats leads to diminished brain dopaminergic function. It has been proposed that dopamine deficiency exacerbates obesity by provoking compensatory overfeeding as one way to restore reward sensitivity. However, the physiological mechanisms linking prolonged high-fat intake to dopamine deficiency remain elusive. We show that administering oleoylethanolamine, a gastrointestinal lipid messenger whose synthesis is suppressed after prolonged high-fat exposure, is sufficient to restore gut-stimulated dopamine release in high-fat-fed mice. Administering oleoylethanolamine to high-fat-fed mice also eliminated motivation deficits during flavorless intragastric feeding and increased oral intake of low-fat emulsions. Our findings suggest that high-fat-induced gastrointestinal dysfunctions play a key role in dopamine deficiency and that restoring gut-generated lipid signaling may increase the reward value of less palatable, yet healthier, foods.

Verbal descriptors influence hypothalamic response to low-calorie drinks

Messages describing foods constitute a pervasive form of reward cueing. Different descriptions may produce particular appeal depending upon the individual. To examine the extent to which verbal descriptors and individual differences interact to influence food preferences, we used functional magnetic resonance imaging to measure brain responses to the same low-calorie drinks preceded by the spoken verbal descriptor "treat" or "healthy" in 27 subjects varying in BMI, eating style and reward sensitivity. Subjects also sampled a prototypical milkshake treat. Despite the fact that the verbal descriptor had no influence on pleasantness ratings, preferential responses to the low-calorie drinks labeled "treat" vs. "healthy" were observed in the midbrain and hypothalamus. These same regions were also preferentially responsive to the prototypical treat. These results reveal a previously undocumented influence of verbal descriptors on brain circuits regulating energy homeostasis.

Metabolic regulation of brain response to food cues

Identification of energy sources depends upon the ability to form associations between food cues and nutritional value. As such, cues previously paired with calories elicit neuronal activation in the nucleus accumbens (NAcc), which reflects the reinforcing value of food. The identity of the physiological signals regulating this response remains elusive. Using fMRI, we examined brain response to noncaloric versions of flavors that had been consumed in previous days with either 0 or 112.5 calories from undetected maltodextrin. We report a small but perceptually meaningful increase in liking for the flavor that had been paired with calories and find that change in liking was associated with changes in insular responses to this beverage. In contrast, NAcc and hypothalamic response to the calorie-paired flavor was unrelated to liking but was strongly associated with the changes in plasma glucose levels produced by ingestion of the beverage when consumed previously with calories. Importantly, because each participant ingested the same caloric dose, the change in plasma glucose depended upon individual differences in glucose metabolism. We conclude that glucose metabolism is a critical signal regulating NAcc and hypothalamic response to food cues, and that this process operates independently from the ability of calories to condition liking.

Flavor-independent maintenance, extinction, and reinstatement of fat self-administration in mice

BACKGROUND

Mounting evidence suggests that overeating may be conceptualized within the same behavioral and neurobiological framework as drug addiction. One potentially important difference between overeating versus drug abuse refers to the sensory stimulation of oral receptors by palatable foods, a feature that may be required for reinforcement during intake. Likewise, postingestive effects and caloric content of food also contribute to reinforcing behavior and might influence the development of compulsive eating behavior. The purpose of the current study was to establish whether intragastric self-administration of fat emulsions, that is, bypassing the oral cavity, recapitulates some of the behavioral and neurobiological hallmarks of psychostimulant self-administration.

METHODS

We used behavioral assays in mice to assess acquisition, maintenance, extinction, and reinstatement of intragastric self-administration of lipid emulsions to determine the extent to which postoral fat self-administration recapitulates psychostimulant self-administration. Striatal dopamine efflux during behavioral tasks was determined by brain microdialysis coupled to chromatographic-electrochemical analyses.

RESULTS

We show that in direct analogy to drug self-administration, 1) decreases in fat dose concentration were met with compensatory increases in response rates aimed at maintaining constant hourly caloric intake; 2) rates of responding markedly increased during both extinction and progressive ratio schedules of reinforcement; and 3) elevations in striatal dopamine levels observed during maintenance were markedly attenuated during extinction sessions, only to be restored on reinstatement.

CONCLUSIONS

Our data thus support the contention that stimulation of oral receptors by caloric foods may not be required for the expression of certain addiction-related neurobehavioral markers.

Mechanisms for sweetness

A remarkable amount of information has emerged in the past decade regarding sweet taste physiology. This article reviews these data, with a particular focus on the elucidation of the sweet taste receptor, its location and actions in taste transduction in the mouth, its nontaste functions in the gastrointestinal tract (e.g., in enteroendocrine cells), and the brain circuitry involved in the sensory processing of sweet taste. Complications in the use of rodents to model human sweet taste perception and responses are also considered. In addition, information relating to low-calorie sweeteners (LCS) is discussed in the context of these issues. Particular consideration is given to the known effects of LCS on enteroendocrine cell function.

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.

The insular cortex controls food preferences independently of taste receptor signaling

The insular cortex (IC) contains the primary sensory cortex for oral chemosensation including gustation, and its integrity is required for appropriate control of feeding behavior. However, it remains unknown whether the role of this brain area in food selection relies on the presence of peripheral taste input. Using multielectrode recordings, we found that the responses of populations of neurons in the IC of freely licking, sweet-blind Trpm5^−/− mice are modulated by the rewarding postingestive effects of sucrose. FOS immunoreactivity analyses revealed that these responses are restricted to the dorsal insula. Furthermore, bilateral lesions in this area abolished taste-independent preferences for sucrose that can be conditioned in these Trpm5^−/− animals while preserving their ability to detect sucrose. Overall, these findings demonstrate that, even in the absence of peripheral taste input, IC regulates food choices based on postingestive signals.

Orosensory and Homeostatic Functions of the Insular Taste Cortex

The gustatory aspect of the insular cortex is part of the brain circuit that controls ingestive behaviors based on chemosensory inputs. However, the sensory properties of foods are not restricted to taste and should also include salient features such as odor, texture, temperature, and appearance. Therefore, it is reasonable to hypothesize that specialized circuits within the central taste pathways must be involved in representing several other oral sensory modalities in addition to taste. In this review, we evaluate current evidence indicating that the insular gustatory cortex functions as an integrative circuit, with taste-responsive regions also showing heightened sensitivity to olfactory, somatosensory, and even visual stimulation. We also review evidence for modulation of taste-responsive insular areas by changes in physiological state, with taste-elicited neuronal responses varying according to the nutritional state of the organism. We then examine experimental support for a functional map within the insular cortex that might reflect the various sensory and homeostatic roles associated with this region. Finally, we evaluate the potential role of the taste insular cortex in weight-gain susceptibility. Taken together, the current experimental evidence favors the view that the insular gustatory cortex functions as an orosensory integrative system that not only enables the formation of complex flavor representations but also mediates their modulation by the internal state of the body, playing therefore a central role in food intake regulation.

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.

Sweet taste signaling and the formation of memories of energy sources

The last decade witnessed remarkable advances in our knowledge of the gustatory system. Application of molecular biology techniques not only determined the identity of the membrane receptors and downstream effectors that mediate sweetness, but also uncovered the overall logic of gustatory coding in the periphery. However, while the ability to taste sweet may offer the obvious advantage of eliciting rapid and robust intake of sugars, a number of recent studies demonstrate that sweetness is neither necessary nor sufficient for the formation of long-lasting preferences for stimuli associated with sugar intake. Furthermore, uncoupling sweet taste from ensuing energy utilization may disrupt body weight control. This minireview examines recent experiments performed in both rodents and Drosophila revealing the taste-independent rewarding properties of metabolizable sugars. Taken together, these experiments demonstrate the reinforcing actions of sugars in the absence of sweet taste signaling and point to a critical role played by dopamine systems in translating metabolic sensing into behavioral action. From a mechanistic viewpoint, current evidence favors the concept that gastrointestinal and post-absorptive signals contribute in parallel to sweet-independent sugar acceptance and dopamine release.

Leptin regulates the reward value of nutrient

We developed an assay for quantifying the reward value of nutrient and used it to analyze the effects of metabolic state and leptin. In this assay, mice chose between two sippers, one of which dispensed water and was coupled to optogenetic activation of dopaminergic (DA) neurons and the other of which dispensed natural or artificial sweeteners. This assay measured the reward value of sweeteners relative to lick-induced optogenetic activation of DA neurons. Mice preferred optogenetic stimulation of DA neurons to sucralose, but not to sucrose. However, the mice preferred sucralose plus optogenetic stimulation versus sucrose. We found that food restriction increased the value of sucrose relative to sucralose plus optogenetic stimulation, and that leptin decreased it. Our data suggest that leptin suppresses the ability of sucrose to drive taste-independent DA neuronal activation and provide new insights into the mechanism of leptin's effects on food intake.

Effects of ad libitum ingestion of monosodium glutamate on weight gain in C57BL6/J mice

Although the umami compound monosodium glutamate (MSG) is a widely used flavor enhancer, controversy still persists regarding the effects of MSG intake on body weight. It has been claimed, in particular, that chronic MSG intake may result in excessive body weight gain and obesity. In this study we assessed the effects of chronic (16 weeks) ad libitum MSG on body weight and metabolism of C57BL6/J mice. Adult male mice were divided in four experimental groups and fed with either a low-fat (LF) or high-fat (HF) diet and with either two bottles of plain water or one bottle containing 1% MSG and another one containing water according to a factorial design. Mice were monitored weekly for body weight and food/fluid intake for 15 weeks. At the end of the experiments, the circulating levels of leptin, insulin, total protein, total cholesterol, triglyceride, blood urea nitrogen, and non-esterified fatty acids were also analyzed. Our results show that MSG intake did not influence body weight in either LF or HF groups. Interestingly, although animals overall displayed strong preferences for MSG against water, preferences were relatively higher in LF compared to HF group. Consistent with the body weight data, while significant differences in leptin, insulin, total cholesterol, and non-esterified fatty acids were found between HF and LF groups, such an effect was not influenced by MSG intake. Finally, indirect calorimetry measurements revealed similar energy expenditure levels between animals being presented water only and MSG only. In summary, our data does not support the notion that ad libitum MSG intake should trigger the development of obesity or other metabolic abnormalities.

Metabolic sensing in brain dopamine systems

The gustatory system allows the brain to monitor the presence of chemicals in the oral cavity and initiate appropriate responses of acceptance or rejection. Among such chemicals are the nutrients that must be rapidly recognized and ingested for immediate oxidation or storage. In the periphery, the gustatory system consists of a highly efficient sensing mechanism, where distinct cell types express receptors that bind specifically to chemicals associated with one particular taste quality. These specialized receptors connect to the brain via dedicated pathways, the stimulation of which triggers stereotypic behavioral responses as well as neurotransmitter release in brain reward dopamine systems. However, evidence also exists in favor of the concept that the critical regulators of long-term nutrient choice are physiological processes taking place after ingestion and independently of gustation. We will appraise the hypothesis that organisms can develop preferences for nutrients independently of oral taste stimulation. Of particular interest are recent findings indicating that disrupting nutrient utilization interferes with activity in brain dopamine pathways. These findings establish the metabolic fate of nutrients as previously unanticipated reward signals that regulate the reinforcing value of foods. In particular, it suggests a role for brain dopamine reward systems as metabolic sensors, allowing for signals generated by the metabolic utilization of nutrients to regulate neurotransmitter release and food reinforcement.

Gustatory and homeostatic functions of the rodent parabrachial nucleus

Previous studies demonstrate that lesions to the rodent parabrachial nucleus (PBN) disrupt the formation of gustatory-postingestive associations, while preserving gustatory and viscerosensory functions. This suggests that the rodent PBN functions essentially as an integrative circuit, supporting the conditioning of tastants to postingestive factors. In the case of primates, however, anatomical studies have failed to demonstrate gustatory projections from medullary nuclei to PBN. It should therefore be inferred that the primate PBN lacks the associative functions assigned to its rodent counterpart. Moreover, the ability of rodent midbrain dopaminergic systems to respond to the activation of palatable tastants depends on the integrity of the gustatory PBN. However, recent studies demonstrate that caloric palatable compounds do not require taste signaling to produce elevated brain dopamine levels. This raises the possibility that, in rodents, PBN neurons are important for the detection of postingestive effects of nutrients that occur independently of gustatory input. If confirmed, such function would assign non-associative roles to the rodent PBN, approximating its functional organization to its primate counterpart. We are currently testing this possibility by monitoring the behavioral responses to caloric glucose solutions in sweet-blind mice having sustained bilateral lesions to the PBN. Preliminary results indicate that the rodent PBN regulates nutrient intake even when no gustatory inputs are involved. This favors the assignment of non-gustatory, homeostatic functions to the rodent PBN during feeding, a concept that brings an additional perspective on the rodent versus primate functional discrepancy associated with the anatomy of this pontine nucleus.

Sweet taste signaling functions as a hypothalamic glucose sensor

Brain glucosensing is essential for normal body glucose homeostasis and neuronal function. However, the exact signaling mechanisms involved in the neuronal sensing of extracellular glucose levels remain poorly understood. Of particular interest is the identification of candidate membrane molecular sensors that would allow neurons to change firing rates independently of intracellular glucose metabolism. Here we describe for the first time the expression of the taste receptor genes Tas1r1, Tas1r2 and Tas1r3, and their associated G-protein genes, in the mammalian brain. Neuronal expression of taste genes was detected in different nutrient-sensing forebrain regions, including the paraventricular and arcuate nuclei of the hypothalamus, the CA fields and dentate gyrus of the hippocampus, the habenula, and cortex. Expression was also observed in the intra-ventricular epithelial cells of the choroid plexus. These same regions were found to express the corresponding gene products that form the heterodimeric T1R2/T1R3 and T1R1/T1R3 sweet and l-amino acid taste G-protein coupled receptors, respectively, along with the taste G-protein α-gustducin. Moreover, in vivo studies in mice demonstrated that the hypothalamic expression of taste-related genes is regulated by the nutritional state of the animal, with food deprivation significantly increasing expression levels of Tas1r1 and Tas1r2 in hypothalamus, but not in cortex. Furthermore, exposing mouse hypothalamic cells to a low-glucose medium, while maintaining normal l-amino acid concentrations, specifically resulted in higher expression levels of the sweet-associated gene Tas1r2. This latter effect was reversed by adding the non-metabolizable artificial sweetener sucralose to the low-glucose medium, indicating that taste-like signaling in hypothalamic neurons does not require intracellular glucose oxidation. Taken together, our findings suggest that the heterodimeric G-protein coupled sweet receptor T1R2/T1R3 is a candidate membrane-bound brain glucosensor.

Food reward in the absence of taste receptor signaling

Food palatability and hedonic value play central roles in nutrient intake. However, postingestive effects can influence food preferences independently of palatability, although the neurobiological bases of such mechanisms remain poorly understood. Of central interest is whether the same brain reward circuitry that is responsive to palatable rewards also encodes metabolic value independently of taste signaling. Here we show that trpm5-/- mice, which lack the cellular machinery required for sweet taste transduction, can develop a robust preference for sucrose solutions based solely on caloric content. Sucrose intake induced dopamine release in the ventral striatum of these sweet-blind mice, a pattern usually associated with receipt of palatable rewards. Furthermore, single neurons in this same ventral striatal region showed increased sensitivity to caloric intake even in the absence of gustatory inputs. Our findings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction.

The neural mechanisms of gustation: a distributed processing code

Whenever food is placed in the mouth, taste receptors are stimulated. Simultaneously, different types of sensory fibre that monitor several food attributes such as texture, temperature and odour are activated. Here, we evaluate taste and oral somatosensory peripheral transduction mechanisms as well as the multi-sensory integrative functions of the central pathways that support the complex sensations that we usually associate with gustation. On the basis of recent experimental data, we argue that these brain circuits make use of distributed ensemble codes that represent the sensory and post-ingestive properties of tastants.

Neural Ensemble Coding of Satiety States

The motivation to start or terminate a meal involves the continual updating of information on current body status by central gustatory and reward systems. Previous electrophysiological and neuroimaging investigations revealed region-specific decreases in activity as the subject's state transitions from hunger to satiety. By implanting bundles of microelectrodes in the lateral hypothalamus, orbitofrontal cortex, insular cortex, and amygdala of hungry rats that voluntarily eat to satiety, we have measured the behavior of neuronal populations through the different phases of a complete feeding cycle (hunger-satiety-hunger). Our data show that while most satiety-sensitive units preferentially responded to a unique hunger phase within a cycle, neuronal populations integrated single-unit information in order to reflect the animal's motivational state across the entire cycle, with higher activity levels during the hunger phases. This distributed population code might constitute a neural mechanism underlying meal initiation under different metabolic states.

Cognitive Modulation of Olfactory Processing

We showed how cognitive, semantic information modulates olfactory representations in the brain by providing a visual word descriptor, "cheddar cheese" or "body odor," during the delivery of a test odor (isovaleric acid with cheddar cheese flavor) and also during the delivery of clean air. Clean air labeled "air" was used as a control. Subjects rated the affective value of the test odor as significantly more unpleasant when labeled "body odor" than when labeled "cheddar cheese." In an event-related fMRI design, we showed that the rostral anterior cingulate cortex (ACC)/medial orbitofrontal cortex (OFC) was significantly more activated by the test stimulus and by clean air when labeled "cheddar cheese" than when labeled "body odor," and the activations were correlated with the pleasantness ratings. This cognitive modulation was also found for the test odor (but not for the clean air) in the amygdala bilaterally.

Methamphetamine activates reward circuitry in drug naïve human subjects

Amphetamines are highly addictive drugs that have pronounced effects on emotional and cognitive behavior in humans. These effects are mediated through their potent dopaminergic agonistic properties. Dopamine has also been implicated in the modulation of responses of the 'reward circuit' in animal and human studies. In this study we use functional magnetic resonance imaging (fMRI) to identify the brain circuitry involved in the psychostimulant effect of methamphetamine in psychostimulant-naïve human subjects. Seven healthy volunteers were scanned in a 3T MR imaging system. They received single-blind intravenous infusions of methamphetamine (0.15 mg/kg), and rated their experience of 'mind-racing' on a button press throughout the experiment. Data were analyzed with statistical parametric mapping methods. Amphetamine administration activated the medial orbitofrontal cortex, the rostral part of the anterior cingulate cortex, and the ventral striatum. Ratings of 'mind-racing' after methamphetamine infusion correlated with activations in the rostral part of the anterior cingulate cortex and in the ventral striatum. In addition, activations in the medial orbitofrontal cortex were independent of motor and related responses involved in making the ratings. These findings indicate that the first administration of a psychostimulant to human subjects activates classical reward circuitry. Our data also support recent hypotheses suggesting a central role for the orbitofrontal cortex in drug reinforcement and the development of addiction.