The Pavlovian power of palatable food: lessons for weight-loss adherence from a new rodent model of cue-induced overeating

Binge eating, overeating and food addiction: Approaches for examining food overconsumption in laboratory rodents

Overeating ranges in severity from casual overindulgence to an overwhelming drive to consume certain foods. At its most extreme, overeating can manifest as clinical diagnoses such as binge eating disorder or bulimia nervosa, yet subclinical forms of overeating such as emotional eating or uncontrolled eating can still have a profoundly negative impact on health and wellbeing. Although rodent models cannot possibly capture the full spectrum of disordered overeating, studies in laboratory rodents have substantially progressed our understanding of the neurobiology of overconsumption. These experimental approaches range from simple food-exposure protocols that promote binge-like eating and the development of obesity, to more complex operant procedures designed to examine distinct 'addiction-like' endophenotypes for food. This review provides an overview of these experimental approaches, with the view to providing a comprehensive resource for preclinical investigators seeking to utilize behavioural models for studying the neural systems involved in food overconsumption.

Food addiction, hormones and blood biomarkers in humans: A systematic literature review

BACKGROUND

Food addiction may play a role in rising obesity rates in connection with obesogenic environments and processed food availability, however the concept of food addiction remains controversial. While animal studies show evidence for addictive processes in relation to processed foods, most human studies are psychologically focussed and there is a need to better understand evidence for biological mechanisms of food addiction in humans. Several key hormones are implicated in models of food addiction, due to their key roles in feeding, energy metabolism, stress and addictive behaviours. This systematic literature review examines evidence for relationships between food addiction, hormones and other blood biomarkers.

METHODS

A series of literature searches was performed in Scopus, PsychInfo, MedLine, ProQuest, CINAHL and Web of Science. A total of 3111 articles were found, of which 1045 were duplicates. Articles were included if they contained a psychometric measurement of food addiction, such as the Yale Food Addiction Scale, as well as addressed the association between FA and hormones or blood biomarkers in humans. Articles were assessed for eligibility by two independent reviewers.

RESULTS

Sixteen studies were identified that examined relationships between food addiction and blood biomarkers, published between 2015 and 2021. Significant findings were reported for leptin, ghrelin, cortisol, insulin and glucose, oxytocin, cholesterol, plasma dopamine, thyroid stimulating hormone (TSH), haemoglobin A1c (HbA1c), triglyceride (TG), amylin, tumour necrosis factor alpha (TNF- α) and cholecystokinin (CCK). Methodological issues included small sample sizes and variation in obesity status, sex and mental health-related comorbidities. Due to methodological limitations, definite connections between FA, hormones and other blood biomarkers cannot yet be determined.

CONCLUSION

This systematic review identified preliminary evidence linking FA symptoms to hormones and other blood biomarkers related to feeding, addiction, and stress. However, due to the small number of studies and methodological limitations, further research is needed to evaluate biopsychosocial models of FA and to resolve controversies.

Food cue reactivity: Neurobiological and behavioral underpinnings

The modern obesogenic environment contains an abundance of food cues (e.g., sight, smell of food) as well cues that are associated with food through learning and memory processes. Food cue exposure can lead to food seeking and excessive consumption in otherwise food-sated individuals, and a high level of food cue responsivity is a risk factor for overweight and obesity. Similar food cue responses are observed in experimental rodent models, and these models are therefore useful for mechanistically identifying the neural circuits mediating food cue responsivity. This review draws from both experimental rodent models and human data to characterize the behavioral and biological processes through which food-associated stimuli contribute to overeating and weight gain. Two rodent models are emphasized - cue-potentiated feeding and Pavlovian-instrumental transfer - that provide insight in the neural circuits and peptide systems underlying food cue responsivity. Data from humans are highlighted that reveal physiological, psychological, and neural mechanisms that connect food cue responsivity with overeating and weight gain. The collective literature identifies connections between heightened food cue responsivity and obesity in both rodents and humans, and identifies underlying brain regions (nucleus accumbens, amygdala, orbitofrontal cortex, hippocampus) and endocrine systems (ghrelin) that regulate food cue responsivity in both species. These species similarities are encouraging for the possibility of mechanistic rodent model research and further human research leading to novel treatments for excessive food cue responsivity in humans.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

The Orexigenic Force of Olfactory Palatable Food Cues in Rats

Environmental cues recalling palatable foods motivate eating beyond metabolic need, yet the timing of this response and whether it can develop towards a less palatable but readily available food remain elusive. Increasing evidence indicates that external stimuli in the olfactory modality communicate with the major hub in the feeding neurocircuitry, namely the hypothalamic arcuate nucleus (Arc), but the neural substrates involved have been only partially uncovered. By means of a home-cage hidden palatable food paradigm, aiming to mimic ubiquitous exposure to olfactory food cues in Western societies, we investigated whether the latter could drive the overeating of plain chow in non-food-deprived male rats and explored the neural mechanisms involved, including the possible engagement of the orexigenic ghrelin system. The olfactory detection of a familiar, palatable food impacted upon meal patterns, by increasing meal frequency, to cause the persistent overconsumption of chow. In line with the orexigenic response observed, sensing the palatable food in the environment stimulated food-seeking and risk-taking behavior, which are intrinsic components of food acquisition, and caused active ghrelin release. Our results suggest that olfactory food cues recruited intermingled populations of cells embedded within the feeding circuitry within the Arc, including, notably, those containing the ghrelin receptor. These data demonstrate the leverage of ubiquitous food cues, not only for palatable food searching, but also to powerfully drive food consumption in ways that resonate with heightened hunger, for which the orexigenic ghrelin system is implicated.

Breaking It Down: Investigation of Binge Eating Components in Animal Models to Enhance Translation

Binge eating (BE) is a core eating disorder behavior that is present across nearly all eating disorder diagnoses (e. g., bulimia nervosa, binge eating disorder, anorexia nervosa binge/purge subtype), and is also widely present in the general population. Despite the prevalence of BE, limited treatment options exist and there are often high rates of relapse after treatment. There is evidence showing that genetic factors contribute to the heritability of BE and support for biological contributions to BE. However, more work is needed to fully understand neurobiological mechanisms underlying BE. One approach to target this problem is to separate BE into its distinct clinical components that can be more easily modeled using pre-clinical approaches. To date, a variety of animal models for BE have been used in pre-clinical studies; but there have been challenges translating this work to human BE. Here, we review these pre-clinical approaches by breaking them down into three clinically-significant component parts (1) consumption of a large amount of food; (2) food consumption within a short period of time; and (3) loss of control over eating. We propose that this rubric identifies the most frequently used and effective ways to model components of BE behavior using pre-clinical approaches with the strongest clinical relevance. Finally, we discuss how current pre-clinical models have been integrated with techniques using targeted neurobiological approaches and propose ways to improve translation of pre-clinical work to human investigations of BE that could enhance our understanding of BE behavior.

Hedonic Eating: Sex Differences and Characterization of Orexin Activation and Signaling

Palatable taste can stimulate appetite in the absence of hunger, and individual differences in hedonic eating may be critical to overeating. Women are more prone to obesity and binge eating than men, which warrants comparisons of hedonic versus physiological consumption and the underlying neural substrates in both sexes. The current study examined palatable (high-sugar) food consumption in male and female rats under physiological hunger and satiety, and the role of the neuropeptide orexin/hypocretin (ORX). Across multiple tests, females consistently consumed similar amounts of palatable food regardless of whether they were hungry or sated prior to testing. In contrast, males typically adjusted their consumption according to their hunger/satiety state. This difference was specific to palatable food consumption, as both sexes ate standard chow according to their hunger state. ORX is important in food motivation and reward behaviors. Thus, to begin to determine the neuronal mechanisms of hedonic eating, we examined activation and signaling of ORX neurons. We systematically characterized Fos induction patterns of ORX neurons across the entire rostrocaudal extent of the lateral hypothalamus and found that they were activated by food and by fasting in both sexes. Then, we showed that systemic blockade of ORX receptor 1 signaling with SB-334867 decreased palatable food consumption in hungry and sated rats of both sexes. These results demonstrate sex differences in hedonic eating; increased susceptibility in females to overeat palatable food regardless of hunger state, and that ORX is a critical neuropeptide mechanism of hedonic eating in both sexes.

The orexinergic neural pathway from the lateral hypothalamus to the nucleus accumbens and its regulation of palatable food intake

OBJECTIVE

To explore the orexinergic pathway from the lateral hypothalamus (LHA) to the nucleus accumbens (NAc) and its regulation on the palatable food intake.

METHODS

Fluorescent gold retrograde tracing combined with fluoro-immunohistochemical staining were used to observe the projection of orexinergic neurons from LHA to NAc. The orexin-A expression in LHA and c-Fos in NAc were studied after electrical stimulation of LHA. The firing rates of neurons were monitored by single-unit extracellular electric discharge recording and the palatable food intake were measured after orexin microinjection in NAc or electrical stimulation of LHA.

RESULTS

(1) Fluorescent gold retrograde tracing combined with fluoro-immunohistochemical staining showed some orexinergic neural projection from the LHA to the NAc shell. (2) Electrical stimulation of LHA significantly enhanced the expression of orexin-A in LHA and the expression of c-Fos in NAc (P < .05). (3) The results of single-unit extracellular discharge recording showed that the microinjection of orexin in NAc or electrical stimulation of LHA significantly increased the discharge activity of gastric distension responsive neurons in NAc, and the effect could be partly blocked by pretreatment of orexin-A receptor inhibitor SB334867 in NAc (P < .05). (4) Microinjection orexin-A in NAc or electrical stimulation of LHA significantly increased the palatable food intake in rats, and the effect also was partly inhibited by pretreatment of SB334867 in NAc (P < .05).

CONCLUSION

There is an orexinergic pathway from LHA to NAc, which may have potential regulatory effects on food reward and obesity.

Learned Overeating: Applying Principles of Pavlovian Conditioning to Explain and Treat Overeating

PURPOSE OF REVIEW

This review provides an overview of recent findings relating to the role of Pavlovian conditioning in food cue reactivity, including its application to overeating and weight loss interventions.

RECENT FINDINGS

Both in the laboratory and in real life, cue-elicited appetitive reactivity (e.g., eating desires) can be easily learned, but (long-term) extinction is more difficult. New findings suggest impaired appetitive learning in obesity, which might be causally related to overeating. The clinical analogue of extinction-cue exposure therapy-effectively reduces cue-elicited cravings and overeating. While its working mechanisms are still unclear, some studies suggest that reducing overeating expectancies is important.

SUMMARY

Pavlovian learning theory provides a still undervalued theoretical framework of how cravings and overeating can be learned and how they might be effectively tackled. Future studies should aim to elucidate inter-individual differences in Pavlovian conditioning, study ways to strengthen (long-term) extinction, and investigate the working mechanisms of cue exposure therapy.

Pattern of access determines influence of junk food diet on cue sensitivity and palatability

AIMS

Like drug addiction, cues associated with palatable foods can trigger food-seeking, even when sated. However, whether susceptibility to the motivating influence of food-related cues is a predisposing factor in overeating or a consequence of poor diet is difficult to determine in humans. Using a rodent model, we explored whether a highly palatable 'junk food' diet impacts responses to reward-paired cues in a Pavlovian-to-instrumental transfer test, using sweetened condensed milk (SCM) as the reward. The hedonic impact of SCM consumption was also assessed by analyzing licking microstructure.

METHODS

To probe the effects of pattern and duration of junk food exposure, we provided rats with either regular chow ad libitum (controls) or chow plus access to junk food for either 2 or 24 h per day for 1, 3, or 6 weeks. We also examined how individual susceptibility to weight gain related to these measures.

RESULTS

Rats provided 24 h access to the junk food diet were insensitive to the motivational effects of a SCM-paired cue when tested sated even though their hedonic experience upon reward consumption was similar to controls. In contrast, rats provided restricted, 2 h access to junk food exhibited a cue generalization phenotype under sated conditions, lever-pressing with increased vigor in response to both a SCM-paired cue, and a cue not previously paired with reward. Hedonic response was also significantly higher in these animals relative to controls.

CONCLUSIONS

These data demonstrate that the pattern of junk food exposure differentially alters the hedonic impact of palatable foods and susceptibility to the motivating influence of cues in the environment to promote food-seeking actions when sated, which may be consequential for understanding overeating and obesity.

Contexts Paired with Junk Food Impair Goal-Directed Behavior in Rats: Implications for Decision Making in Obesogenic Environments

The high prevalence of obesity and related metabolic diseases calls for greater understanding of the factors that drive excess energy intake. Calorie-dense palatable foods are readily available and often are paired with highly salient environmental cues. These cues can trigger food-seeking and consumption in the absence of hunger. Here we examined the effects of palatable food-paired environmental cues on control of instrumental food-seeking behavior. In Experiment 1, adult male rats received exposures to one context containing three “junk” foods (JFs context) and another containing chow (Chow context). Next, rats were food-deprived and trained to perform instrumental responses (lever-press) for two novel food rewards in a third, distinct context. Contextual influences on flexible control of food-seeking behavior were then assessed by outcome devaluation tests held in the JF, chow and training contexts. Devaluation was achieved using specific satiety and test order was counterbalanced. Rats exhibited goal-directed control over behavior when tested in the training and chow-paired contexts. Notably, performance was habitual (insensitive to devaluation) when tested in the JF context. In Experiment 2 we tested whether the impairment found in the JF context could be ameliorated by the presentation of a discrete auditory cue paired with the chow context, relative to a second cue paired with the JF context. Consistent with the results of Experiment 1, the devaluation effect was not significant when rats were tested in the JF context with the JF cue. However, presenting the chow cue increased the impact of the devaluation treatment leading to a robust devaluation effect. Further tests confirmed that performance in the chow context was goal-directed and that sensory-specific satiety in the JF context was intact. These results show that environments paired with palatable foods can impair goal-directed control over food-seeking behavior, but that this deficit was improved by a cue paired with chow. This has promising implications for assisting individuals in controlling their eating behavior in environments designed to dysregulate it.

A commentary on the "eating addiction" versus "food addiction" perspectives on addictive-like food consumption

The food addiction construct posits that vulnerable individuals may experience an addictive-like response to certain foods, such as those high in fat and refined carbohydrates. Recently, an alternative model to food addiction was proposed, suggesting that the act of eating may be a behavioral addiction that can trigger an addictive-like response in susceptible individuals. One major rationale for the eating addiction framework is that the assessment of food addiction is based on behavioral indicators, such as consuming greater quantities of food than intended and eating certain foods despite negative consequences. It is also suggested that the lack of investigation into which foods and food attributes (e.g., sugar) may have an addictive potential is evidence that food addiction does not parallel a substance-based addiction and more closely resembles a behavioral addiction. The present paper provides a commentary suggesting that the substance-based, food-addiction framework is more appropriate than the behavioral-addiction, eating-addiction perspective to conceptualize addictive-like food consumption. In order to illustrate this point, this manuscript will discuss behavioral components characteristic of all substance-use disorders, preliminary evidence to suggest that all foods are not equally associated with addictive-like eating, and key differences between the hypothesized eating addiction phenotype and the only existing behavioral addiction in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), gambling disorder. Further, this paper will consider implications of applying an addiction label to food versus eating and suggest future research directions to evaluate whether food addiction is a valid and clinically useful construct.

Food cue reactivity and craving predict eating and weight gain: a meta-analytic review

According to learning-based models of behavior, food cue reactivity and craving are conditioned responses that lead to increased eating and subsequent weight gain. However, evidence supporting this relationship has been mixed. We conducted a quantitative meta-analysis to assess the predictive effects of food cue reactivity and craving on eating and weight-related outcomes. Across 69 reported statistics from 45 published reports representing 3,292 participants, we found an overall medium effect of food cue reactivity and craving on outcomes (r = 0.33, p < 0.001; approximately 11% of variance), suggesting that cue exposure and the experience of craving significantly influence and contribute to eating behavior and weight gain. Follow-up tests revealed a medium effect size for the effect of both tonic and cue-induced craving on eating behavior (r = 0.33). We did not find significant differences in effect sizes based on body mass index, age, or dietary restraint. However, we did find that visual food cues (e.g. pictures and videos) were associated with a similar effect size to real food exposure and a stronger effect size than olfactory cues. Overall, the present findings suggest that food cue reactivity, cue-induced craving and tonic craving systematically and prospectively predict food-related outcomes. These results have theoretical, methodological, public health and clinical implications.

A Cognitive Profile of Obesity and Its Translation into New Interventions

Change your lifestyle: decrease your energy intake and increase your energy expenditure, is what obesity experts tell people who need to lose weight. Though the advice might be correct, it appears to be extremely difficult to change one’s lifestyle. Unhealthy habits usually are ingrained and hard to change, especially for people with an “obese cognitive profile.” Knowledge of the cognitive mechanisms that maintain unhealthy eating habits is necessary for the development of interventions that can change behavior effectively. This paper discusses some cognitive processes that might maintain unhealthy eating habits and make healthier eating difficult, like increased food cue reactivity, weak executive skills and attention bias. An effort is also done to translate these basic scientific findings into new interventions which aim to tackle the sabotaging cognitive processes. Preliminary studies into the effectiveness of these interventions, if available, are presented.

Integration of reward signalling and appetite regulating peptide systems in the control of food-cue responses

Understanding the neurobiological substrates that encode learning about food-associated cues and how those signals are modulated is of great clinical importance especially in light of the worldwide obesity problem. Inappropriate or maladaptive responses to food-associated cues can promote over-consumption, leading to excessive energy intake and weight gain. Chronic exposure to foods rich in fat and sugar alters the reinforcing value of foods and weakens inhibitory neural control, triggering learned, but maladaptive, associations between environmental cues and food rewards. Thus, responses to food-associated cues can promote cravings and food-seeking by activating mesocorticolimbic dopamine neurocircuitry, and exert physiological effects including salivation. These responses may be analogous to the cravings experienced by abstaining drug addicts that can trigger relapse into drug self-administration. Preventing cue-triggered eating may therefore reduce the over-consumption seen in obesity and binge-eating disorder. In this review we discuss recent research examining how cues associated with palatable foods can promote reward-based feeding behaviours and the potential involvement of appetite-regulating peptides including leptin, ghrelin, orexin and melanin concentrating hormone. These peptide signals interface with mesolimbic dopaminergic regions including the ventral tegmental area to modulate reactivity to cues associated with palatable foods. Thus, a novel target for anti-obesity therapeutics is to reduce non-homeostatic, reward driven eating behaviour, which can be triggered by environmental cues associated with highly palatable, fat and sugar rich foods.

Metabolic vs. hedonic obesity: a conceptual distinction and its clinical implications

Body weight is determined via both metabolic and hedonic mechanisms. Metabolic regulation of body weight centres around the 'body weight set point', which is programmed by energy balance circuitry in the hypothalamus and other specific brain regions. The metabolic body weight set point has a genetic basis, but exposure to an obesogenic environment may elicit allostatic responses and upward drift of the set point, leading to a higher maintained body weight. However, an elevated steady-state body weight may also be achieved without an alteration of the metabolic set point, via sustained hedonic over-eating, which is governed by the reward system of the brain and can override homeostatic metabolic signals. While hedonic signals are potent influences in determining food intake, metabolic regulation involves the active control of both food intake and energy expenditure. When overweight is due to elevation of the metabolic set point ('metabolic obesity'), energy expenditure theoretically falls onto the standard energy-mass regression line. In contrast, when a steady-state weight is above the metabolic set point due to hedonic over-eating ('hedonic obesity'), a persistent compensatory increase in energy expenditure per unit metabolic mass may be demonstrable. Recognition of the two types of obesity may lead to more effective treatment and prevention of obesity.

How food cues can enhance and inhibit motivation to obtain and consume food

Learning may play an important role in over-eating. One example is Pavlovian-to-instrumental transfer (PIT), whereby reward cues facilitate responding to obtain that reward. Whilst there is increasing research indicating PIT for food in humans, these studies have exclusively tested PIT under instrumental extinction (i.e. when the food is no longer available), which may reduce their ecological validity. To address this, we conducted two experiments exploring PIT for food in humans when tested under instrumental reinforcement. Participants first underwent Pavlovian discrimination training with an auditory cue paired with a chocolate reward (CS+) and another auditory cue unpaired (CS-). In instrumental training participants learnt to press a button to receive the chocolate reward on a VR10 schedule. In the test phase, each CS was presented whilst participants maintained the opportunity to press the button to receive chocolate. In Experiment 1, the PIT test was implemented after up to 20 min of instrumental training (satiation) whereas in Experiment 2 it was implemented after only 4 min of instrumental training. In both experiments there was evidence for differential PIT, but the pattern differed according to the rate of responding at the time of the PIT test. In low baseline responders the CS+ facilitated both button press responding and consumption, whereas in high baseline responders the CS- suppressed responding. These findings suggest that both excitatory and inhibitory associations may be learnt during PIT training and that the expression of these associations depends on motivation levels at the time the cues are encountered. Particularly concerning is that a food-paired cue can elicit increased motivation to obtain and consume food even when the participant is highly satiated and no longer actively seeking food, as this may be one mechanism by which over-consumption is maintained.

A view of obesity as a learning and memory disorder

This articles describes how a cascade of associative relationships involving the sensory properties of foods, the nutritional consequences of their consumption, and perceived internal states may play an important role in the learned control of energy intake and body weight regulation. In addition, we describe ways in which dietary factors in the current environment can promote excess energy intake and body weight gain by degrading these relationships or by interfering with the neural substrates that underlie the ability of animals to use them to predict the nutritive or energetic consequences of intake. We propose that an expanded appreciation of the diversity of orosensory, gastrointestinal, and energy state signals about which animals learn, combined with a greater understanding of predictive relationships in which these cues are embedded, will help generate new information and novel approaches to addressing the current global problems of obesity and metabolic disease.

Co-sensitivity to the incentive properties of palatable food and cocaine in rats; implications for co-morbid addictions

Several lines of evidence suggest that there may be a shared vulnerability to acquire behaviors motivated by strong incentive stimuli. Non-food restricted male Sprague-Dawley rats (n = 78) underwent place conditioning with Oreos, and were subsequently tested on cocaine self-administration (SA) on fixed and progressive ratios, as well as extinction and reinstatement by cocaine primes and by consumption of Oreos. Although there was a group preference for the Oreo-paired compartment, at the individual level some rats (69%) displayed a preference and others did not. In cocaine SA, 'preference' rats achieved higher break points on a progressive ratio, and displayed greater responding during extinction and cocaine-induced reinstatement. Within the context of this study, Oreo-cocaine cross-reinstatement was not observed. In a control study, rats (n = 29) conditioned with a less palatable food (rice cakes) also displayed individual differences in place preference, but not on subsequent cocaine tests. These findings indicate that there is a relationship between incentive learning promoted by palatable foods and by cocaine. This supports the hypothesis that co-morbid food-drug addictions may result from a shared vulnerability to acquire behaviors motivated by strong incentives.

Forebrain networks and the control of feeding by environmental learned cues

The motivation to eat is driven by a complex sum of physiological and non-physiological influences computed by the brain. Physiological signals that inform the brain about energy and nutrient needs are the primary drivers, but environmental signals unrelated to energy balance also control appetite and eating. The two components could act in concert to support the homeostatic regulation of food intake. Often, however, environmental influences rival physiological control and stimulate eating irrespective of satiety, or inhibit eating irrespective of hunger. If persistent, such maladaptive challenges to the physiological system could lead to dysregulated eating and ultimately to eating disorders. Nevertheless, the brain mechanisms underlying environmental contribution in the control of food intake are poorly understood. This paper provides an overview in recent advances in deciphering the critical brain systems using rodent models for environmental control by learned cues. These models use associative learning to compete with the physiological control, and in one preparation food cues stimulate a meal despite satiety, while in another preparation fear cues stop a meal despite hunger. Thus far, four forebrain regions have been identified as part of the essential cue induced feeding circuitry. These are telencephalic areas critical for associative learning, memory encoding, and decision making, the amygdala, hippocampus and prefrontal cortex and the lateral hypothalamus, which functions to integrate feeding, reward, and motivation. This circuitry also engages two orexigenic peptides, ghrelin and orexin. A parallel amygdalar circuitry supports fear cue cessation of feeding. These findings illuminate the brain mechanisms underlying environmental control of food intake and might be also relevant to aspects of human appetite and maladaptive overeating and undereating.

Environments predicting intermittent shortening access reduce operant performance but not home cage binge size in rats

When non-food-deprived rats are given brief access to vegetable shortening (a semi-solid fat used in baked products) on an intermittent basis (Monday, Wednesday, Friday), they consume significantly more and emit more operant responses for shortening than a separate group of rats given brief access to shortening every day. Since both groups are traditionally housed in the same room, it is possible that the environmental cues associated with placing shortening in the cages (e.g., investigator in room, cages opening and closing, etc.) provide predictable cues to the daily group, but unpredictable cues to the intermittent group. The present study examined the effects of providing predictable environmental cues to an isolated intermittent group in order to examine the independent contributions of intermittency and predictability on intake and operant performance. Two groups of rats were housed in the same room, with one group provided 30-min intermittent (INT) access and the second group provided 30-min daily access (D) to shortening. A third group (ISO) of rats was housed in a room by themselves in which all environmental cues associated with intermittent shortening availability were highly predictable. After five weeks of home cage shortening access, all rats were then exposed to several different operant schedules of reinforcement. The INT and ISO groups consumed significantly more shortening in the home cage than the D group. In contrast, the INT group earned significantly more reinforcers than both the ISO and D groups under all but one of the reinforcement schedules, while ISO and D did not differ. These data indicate that intermittent access will generate binge-type eating in the home cage independent of cue predictability. However, predictable cues in the home cage reduce operant responding independent of intermittent access.

The craving stops before you feel it: neural correlates of chocolate craving during cue exposure with response prevention

Cue reactivity and craving can be influenced by cue exposure with response prevention (CERP). This study investigated the neural correlates of CERP using functional magnetic resonance imaging, while participants smelled chocolate (17 participants) or a control object (17 participants). CERP was interrupted by 7 scanning sequences measuring the brain response to neutral and chocolate pictures. Chocolate craving was hypothesized to be mirrored by activation in brain reward regions. As expected, control group craving remained similar throughout the session. A short exposure (30 min) increased chocolate craving in the experimental group, which was mirrored by significant group differences in activation in brain reward regions. Unexpectedly, a long exposure (60 min) did not lead to craving extinction in the experimental group, although craving started to decrease at this point. On a neural level, however, activation in regions of interest in the experimental group seemed to have extinguished after the long exposure, as activation levels returned to or fell below control group levels. These results indicate that brain reward activation during CERP is linked to craving, at least for a short exposure. Regarding a longer exposure, the decline in brain reward activation in the experimental group may be a precursor of a decrease in craving.

Associative and occasion-setting properties of contextual cues in flavor-nutrient learning in rats

This article studied the role of contextual cues, present at the time flavor conditioning occurs, on intake behavior in rats. In three experiments animals were given flavor-sucrose pairings in one distinctive context (Context A) whereas the flavor was presented unreinforced in an alternative context (Context B). Experiments 1 and 2 used a simple Pavlovian discrimination procedure (A: X+, B: X-) and tested consumption of flavor X in each context. Consumption of the flavor was higher in Context A than in Context B. In Experiment 2 rats were given a treatment (exposure to water in the context) designed to extinguish associations between the context and the reinforcer. This procedure did not affect the ability of the context to control intake of flavor X. Experiment 3 used a biconditional discrimination procedure (A: X+, Y-; B: X-, Y+; where X and Y were different flavors) in which no single context or flavor predicted reinforcement. The rats learned this discrimination, consuming more of each flavor in the context in which it had previously been reinforced. The results are interpreted in terms both of the effects of direct associations between context and events presented in them, and in terms of the modulatory or occasion-setting properties of the context.

Selective Fos induction in hypothalamic orexin/hypocretin, but not melanin-concentrating hormone neurons, by a learned food-cue that stimulates feeding in sated rats

Associative learning can enable cues from the environment to stimulate feeding in the absence of physiological hunger. How learned cues are integrated with the homeostatic regulatory system is unknown. Here we examined whether the underlying mechanism involves the hypothalamic orexigenic neuropeptide regulators orexin/hypocretin (ORX) and melanin-concentrating hormone (MCH). We used a Pavlovian conditioning procedure to train food-restricted rats to associate a discrete cue, a tone, with food pellets distinct from their regular lab chow diet. Rats in the conditioned group (Paired) received presentations of a tone immediately prior to food delivery, while the rats in the control group (Unpaired) received random presentations of the same number of tones and food pellets. After conditioning rats were allowed ad libitum access to lab chow for at least 10days before testing. At test sated rats were presented with the tones in their home cages, and then one group was allowed to consume food pellets, while another group was left undisturbed until sacrifice for Fos induction analysis. The tone cue stimulated food consumption in this setting; rats in the Paired group consumed larger amounts of food pellets than rats in the Unpaired group. To examine Fos induction we processed the brain tissue using fluorescent immunohistochemistry methods for combined detection of Fos and characterization of ORX and MCH neurons. We found a greater percentage of ORX and Fos double-labeled neurons in the Paired compared to the Unpaired condition, specifically in the perifornical area. In contrast, there were very few MCH neurons with Fos induction in both the Paired and Unpaired conditions. Thus, the food-cue selectively induced Fos in ORX but not in MCH neurons. These results suggest a role for ORX in cue-induced feeding that occurs in the absence of physiological hunger.

Effects of CB1 and CRF1 receptor antagonists on binge-like eating in rats with limited access to a sweet fat diet: lack of withdrawal-like responses

Positive reinforcement (e.g., appetitive, rewarding properties) has often been hypothesized to maintain excessive intake of palatable foods. Recently, rats receiving intermittent access to high sucrose diets showed binge-like intake with withdrawal-like signs upon cessation of access, suggesting negative reinforcement mechanisms contribute as well. Whether intermittent access to high fat diets also produces withdrawal-like syndromes is controversial. The present study therefore tested the hypothesis that binge-like eating and withdrawal-like anxiety would arise in a novel model of binge eating based on daily 10-min access to a sweet fat diet (35% fat kcal, 31% sucrose kcal). Within 2-3 weeks, female Wistar rats developed binge-like intake comparable to levels seen previously for high sucrose diets (~40% of daily caloric intake within 10 min) plus excess weight gain and adiposity, but absent increased anxiety-like behavior during elevated plus-maze or defensive withdrawal tests after diet withdrawal. Binge-like intake was unaffected by pretreatment with the corticotropin-releasing factor type 1 (CRF(1)) receptor antagonist R121919, and corticosterone responses to restraint stress did not differ between sweet-fat binge rats and chow-fed controls. In contrast, pretreatment with the cannabinoid type 1 (CB(1)) receptor antagonist SR147778 dose-dependently reduced binge-like intake, albeit less effectively than in ad lib chow or sweet fat controls. A priming dose of the sweet fat diet did not precipitate increased anxiety-like behavior, but rather increased plus-maze locomotor activity. The results suggest that CB(1)-dependent positive reinforcement rather than CRF(1)-dependent negative reinforcement mechanisms predominantly maintain excessive intake in this limited access model of sweet-fat diet binges.

Learned food-cue stimulates persistent feeding in sated rats

Cues that predict food can stimulate appetite and feeding independent of physiological hunger. How long such effects might last is currently unknown. Here we began to characterize long-term effects in a rodent model of cue-potentiated feeding. Rats were conditioned to associate a tone with food pellets distinct from their regular laboratory chow, and then were tested along with controls for food consumption following tone presentations. In Experiment 1, rats were tested under sated or food-deprived conditions to determine whether fasting would augment cue-driven feeding. Rats in the control group regulated intake based on physiological state, while conditioned rats consumed similar large amounts of food regardless. Experiment 2 tested the durability of cue-potentiated feeding to repeated testing in sated rats. We observed robust cue-potentiated feeding during the first two tests, while in the third and fourth tests both groups ate similar large amounts of pellets. In both experiments the conditioned tone-cue induced binge-like consumption of the cued food and persistent feeding for the duration of 4-h tests. Rats then failed to adjust daily chow consumption to account for their increased intake post-cue. In summary, brief cue priming stimulated substantial intake in sated states that was behaviorally uncompensated for by homeostatic mechanisms.

Binge eating disorder and food addiction

Binge eating disorder (BED) shares many characteristics with addictive behaviors (e.g., diminished control, continued use despite negative consequences), and a body of scientific literature is building to support addiction conceptualizations of problematic eating. Despite similarities, BED and "food addiction" may represent unique yet overlapping conditions. Although the exploration of food addiction is relatively new, understanding the relationship between food addiction and BED may be informative in understanding the mechanisms underlying the development and maintenance of problematic eating. In the following paper, we 1) examine the theoretical similarities and differences between BED and addiction, 2) review recent empirical evidence that speak to the relationship between BED and food addiction and 3) discuss the implications of associations between BED and food addiction with respect to clinical interventions.

Towards an animal model of food addiction

The dramatically increasing prevalence of obesity, associated with potentially life-threatening health problems, including cardiovascular diseases and type II diabetes, poses an enormous public health problem. It has been proposed that the obesity epidemic can be explained by the concept of 'food addiction'. In this review we focus on possible similarities between binge eating disorder (BED), which is highly prevalent in the obese population, and drug addiction. Indeed, both behavioral and neural similarities between addiction and BED have been demonstrated. Behavioral similarities are reflected in the overlap in DSM-IV criteria for drug addiction with the (suggested) criteria for BED and by food addiction-like behavior in animals after prolonged intermittent access to palatable food. Neural similarities include the overlap in brain regions involved in food and drug craving. Decreased dopamine D2 receptor availability in the striatum has been found in animal models of binge eating, after cocaine self-administration in animals as well as in drug addiction and obesity in humans. To further explore the neurobiological basis of food addiction, it is essential to have an animal model to test the addictive potential of palatable food. A recently developed animal model for drug addiction involves three behavioral characteristics that are based on the DSM-IV criteria: i) extremely high motivation to obtain the drug, ii) difficulty in limiting drug seeking even in periods of explicit non-availability, iii) continuation of drug-seeking despite negative consequences. Indeed, it has been shown that a subgroup of rats, after prolonged cocaine self-administration, scores positive on these three criteria. If food possesses addictive properties, then food-addicted rats should also meet these criteria while searching for and consuming food. In this review we discuss evidence from literature regarding food addiction-like behavior. We also suggest future experiments that could further contribute to our understanding of behavioral and neural commonalities and differences between obesity and drug addiction.

Contextual control of appetite. Renewal of inhibited food-seeking behavior in sated rats after extinction

Obesity and overeating have become fundamental problems in modern society. This article studies the inhibition of food-seeking behavior, and how contextual cues can control it. Rats that had free food in the home cage nevertheless learned to lever press for sucrose or high-fat pellets in a distinctive context (a Skinner box). Lever pressing was then inhibited by extinction, in which lever presses no longer produced food. After extinction, inhibited responding was "renewed" when the rats were switched to a different context: in the new context, the rats lever-pressed again, and worked more for food when food was made available. These effects were observed when conditioning, extinction and testing occurred in contexts A, B, and A (respectively) or in A, A, and B. Thus, mere removal from the context in which food-seeking was inhibited initiated a return to food-seeking. The contextual control of extinction may help explain why food seeking and consumption seem so persistent.

Feeding and reward: perspectives from three rat models of binge eating

Research has focused on understanding how overeating can affect brain reward mechanisms and subsequent behaviors, both preclinically and in clinical research settings. This work is partly driven by the need to uncover the etiology and possible treatments for the ongoing obesity epidemic. However, overeating, or non-homeostatic feeding behavior, can occur independent of obesity. Isolating the variable of overeating from the consequence of increased body weight is of great utility, as it is well known that increased body weight or obesity can impart its own deleterious effects on physiology, neural processes, and behavior. In this review, we present data from three selected animal models of normal-weight non-homeostatic feeding behavior that have been significantly influenced by Bart Hoebel's 40+-yr career studying motivation, feeding, reinforcement, and the neural mechanisms that participate in the regulation of these processes. First, a model of sugar bingeing is described (Avena/Hoebel), in which animals with repeated, intermittent access to a sugar solution develop behaviors and brain changes that are similar to the effects of some drugs of abuse, serving as the first animal model of food addiction. Second, another model is described (Boggiano) in which a history of dieting and stress can perpetuate further binge eating of palatable and non-palatable food. In addition, a model (Boggiano) is described that allows animals to be classified as having a binge-prone vs. binge-resistant behavioral profile. Lastly, a limited access model is described (Corwin) in which non-food deprived rats with sporadic limited access to a high-fat food develop binge-type behaviors. These models are considered within the context of their effects on brain reward systems, including dopamine, the opioids, cholinergic systems, serotonin, and GABA. Collectively, the data derived from the use of these models clearly show that behavioral and neuronal consequences of bingeing on a palatable food, even when at a normal body weight, are different from those that result from simply consuming the palatable food in a non-binge manner. These findings may be important in understanding how overeating can influence behavior and brain chemistry.

Learning and the motivation to eat: forebrain circuitry

Appetite and eating are not only controlled by energy needs, but also by extrinsic factors that are not directly related to energy balance. Environmental signals that acquire motivational properties through associative learning-learned cues-can override homeostatic signals and stimulate eating in sated states, or inhibit eating in states of hunger. Such influences are important, as environmental factors are believed to contribute to the increased susceptibility to overeating and the rise in obesity in the developed world. Similarly, environmental and social factors contribute to the onset and maintenance of anorexia nervosa and other eating disorders through interactions with the individual genetic background. Nevertheless, how learning enables environmental signals to control feeding, and the underlying brain mechanisms are poorly understood. We developed two rodent models to study how learned cues are integrated with homeostatic signals within functional forebrain networks, and how these networks are modulated by experience. In one model, a cue previously paired with food when an animal was hungry induces eating in sated rats. In the other model, food-deprived rats inhibit feeding when presented with a cue that signals danger, a tone previously paired with footshocks. Here evidence will be reviewed that the forebrain network formed by the amygdala, lateral hypothalamus and medial prefrontal cortex mediates cue-driven feeding, while a parallel amygdalar circuitry mediates suppression of eating by the fear cue. Findings from the animal models may be relevant for understanding aspects of human appetite and eating, and maladaptive mechanisms that could lead to overeating and anorexia.

Motivation for palatable food despite consequences in an animal model of binge eating

OBJECTIVE

Binge eating involves an abnormal motivation for highly palatable food in that these foods are repeatedly consumed despite their binge-triggering effects and life-affecting consequences associated with binge eating. We determined if rats identified as binge-eating prone (BEP) similarly display abnormal motivation for palatable food.

METHOD

Food-sated BEP and binge-eating resistant (BER) rats were given voluntary access to palatable food paired with increasing intensity of footshock. Later, they were exposed to a period of cyclic caloric restriction-refeeding.

RESULTS

BEPs consumed significantly more and tolerated higher levels of footshock for palatable food than BERs. Cyclic restriction-refeeding increased BERs' tolerance of shock for palatable food.

DISCUSSION

Previously observed parallels of the rat BEP model to human binge eating can now be extended to include an abnormal motivation for palatable food. This model should prove useful in identifying specific genes that interact with the nutritional environment to mediate binge eating and may point to novel physiological targets to treat compulsive overeating.

Opiates, overeating and obesity: a psychogenetic analysis

OBJECTIVE

This study provides an original perspective on the associations among endogenous opiates, overeating and obesity. The aim was to assess whether variability in the OPRM1 gene, as assessed by seven single-nucleotide polymorphisms, relates to individual differences in the preference for sweet and fatty foods. We also anticipated that these food preferences would be positively associated with binge eating, hedonic eating and emotionally driven eating-patterns of overeating that would, in turn, predict higher body mass index (BMI).

DESIGN

Analysis of variance procedures examined genotype differences in food preferences; bivariate correlation coefficients examined the relationships among food preferences and the overeating variables; and a regression analysis tested the combined influences of the overeating variables on BMI. DNA was extracted from whole blood for the genotyping, and measures of food preferences and eating behaviours were obtained from well-validated self-report questionnaires.

SUBJECTS

Participants were 300 healthy adult men and women recruited from the community.

RESULTS

All the predicted associations were supported by statistically significant results. In particular, the G/G genotype group of the functional A118G marker of the OPRM1 gene reported higher preferences for sweet and fatty foods compared with the other two groups. Food preferences were also related to all overeating measures, which in turn accounted for a substantial proportion of the variance in BMI.

CONCLUSIONS

Our findings suggest that some of the diversity in the preference for highly palatable foods can be explained by genotypic differences in the regulation of mu opioid receptors. The associations reported in this paper are important from a public-health perspective because of the abuse potential of sweet-fat foods and their strong relationship with obesity.

Rats' learned preferences for flavors encountered early or late in a meal paired with the postingestive effects of glucose

Rats learn to prefer flavors that are followed by postingestive effects of nutrients. This experiment investigated whether the timing of a flavor (specifically, in the first or second half of the meal) influences learning about that flavor. Stronger learning about earlier or later flavors would indicate when the rewarding postingestive effects of nutrients are sensed. Rats with intragastric (IG) catheters drank saccharin-sweetened, calorically-dilute solutions with distinct flavors added, accompanied by IG infusion of glucose (+sessions) or water (-sessions). In both types of sessions, an "Early" flavor was provided for the first 8 min and a "Late" flavor for the last 8 min. Thus, rats were trained with Early(+) and Late(+) in high-calorie meals, and Early(-) and Late(-) in low-calorie meals. Strength of the learned preference for Early(+) and Late(+) was then assessed in a series of two-bottle choice tests between Early(+) vs. Early(-), Late(+) vs. Late(-), Early(+) vs. Late(+), and Early(-) vs. Late(-). Rats preferred both Early(+) and Late(+) over the respective (-) flavors. But Early(+) was only preferred when rats were tested hungry. Late(+) was preferred when rats were tested hungry or recently satiated. This indicates qualitatively different associations learned about flavors at different points in the meal. While not supporting the idea that postingestive effects become most strongly associated with later-occurring ("dessert") flavors, it does suggest a reason dessert flavors may remain attractive in the absence of hunger.

Memantine reduces consumption of highly palatable food in a rat model of binge eating

Excessive consumption of highly palatable food has been linked to the development of eating disorders and obesity, and can be modeled in non-food-deprived rats by offering them a limited (2-h daily) access to an optional dietary fat. Since the glutamatergic system has recently emerged as a viable target for binge-eating medication development, we compared the effects of subchronic treatment with glutamatergic receptor antagonists to the effects of a reference appetite-suppressing agent sibutramine on highly palatable food (lard) and normal chow intake. In three separate experiments, the consumption of a standard laboratory chow and lard were measured during 12 days of medication treatment and for 6 days afterwards. Generalized estimating equations analysis demonstrated that sibutramine (7.5 mg/kg, PO) significantly decreased lard consumption, with a concurrent increase in chow consumption. Sibutramine effects disappeared after treatment discontinuation. The NMDA receptor antagonist memantine (5 mg/kg, IP) significantly decreased lard consumption and increased chow consumption, comparable to effects of sibutramine; however, memantine’s effects persisted after treatment discontinuation. The effects of the mGluR5 antagonist MTEP (7.5 mg/kg, IP) on food consumption were in the same direction as seen with memantine, but the observed differences were not significant. In an additional control experiment, sibutramine and memantine reduced unlimited (24 h) chow intake during the treatment phase. Present results provide evidence that glutamatergic neurotransmission might be involved in the regulation of excessive consumption of highly palatable foods, and suggest that NMDA receptor may be an attractive target for developing obesity and disordered eating pharmacotherapies.