Binge Eating in Rats with Limited Access to Vegetable Shortening

Stress hyper-reactivity increases vulnerability to developing binge-type eating and associated anxiety-like behavior; comparison between Wistar-Kyoto and Sprague-Dawley rats

Introduction

Binge eating disorder (BED) is a widespread eating disorder that primarily affects women worldwide, and it is characterized by the presence of binge eating episodes and the absence of any compensatory behavior to prevent weight gain. BED presents elevated comorbidity with other psychiatric disorders, such as anxiety, and it has been suggested that stress sensibility could be a vulnerability factor for the development of BED and the associated anxiety comorbidity. In this study, we aim to investigate whether the Wistar-Kyoto rat strain (WKY), which has a stress hyper-reactive phenotype, could develop both binge-type eating and anxiety-like behaviors simultaneously. We also aim to compare its vulnerability to developing both behaviors with the Sprague Dawley rat strain (SD), a rat strain commonly used in binge-eating models.

Methods

WKY and SD rats were subjected to the model of intermittent access to palatable food (sucrose solution 30% or shortening) without calorie restriction or stress exposure. We evaluated and compared the development of binge-type eating behavior, anxiety-like behavior, and serum corticosterone variation as an index of the stress response in both rat strains.

Results

WKY rats presented a higher percentage of binge-type eaters and required less time to develop binge-type eating behavior than SD rats. The WKY eating pattern emulated a binge-eating episode regardless of the palatable food. Although the development of sucrose binge-type eating was similar between strains, WKY developed more easily the shortening binge-type eating than SD and was more susceptible to developing anxiety-like behavior. Additionally, sucrose binge eating seems to differentially affect both strains' hypothalamic-pituitary-adrenal (HPA) axis response to stress since it facilitated its response in SD and blunted it in WKY.

Discussion

Our results show that high-stress sensitive phenotype is a common vulnerability factor for the development of binge-type eating and anxiety-like behavior. Regardless of the macronutrient composition of the palatable food, WKY is susceptible to developing a binge-type eating behavior and is more susceptible than SD to developing anxiety-like behavior simultaneously. In conclusion, results showed that a hyper-reactive stress phenotype predisposes the development of binge-type eating behavior and anxiety-like behavior in the absence of calorie restriction and stress exposure.

The organizational role of ovarian hormones during puberty on risk for binge-like eating in rats

Puberty is a high-risk period for the development of dysregulated eating, including binge eating. While risk for binge eating in animals and humans increases in both males and females during puberty, the increased prevalence is significantly greater in females. Emerging data suggest that the organizational effects of gonadal hormones may contribute to the female preponderance of binge eating. In this narrative review, we discuss studies conducted in animals that have examined these organizational effects as well as the neural systems that may serve as intermediary mechanisms. Relatively few studies have been conducted, but data thus far suggest that pubertal estrogens may organize risk for binge eating, potentially by altering key circuits in brain reward pathways. These promising results highlight the need for future studies to directly test organizational effects of pubertal hormones using hormone replacement techniques and circuit-level manipulations that can identify pathways contributing to binge eating across development.

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.

The Impact of Binge-Like Palatable Food Intake on the Endogenous Glucagon-Like Peptide-1 System in Female Rats

Binge eating involves consumption of large amounts of food and a loss of control over the amount consumed. The incidence of binge eating disorder is higher in females than males, hinting at important sex differences in binge eating behavior, but the neural underpinnings of binge eating still remain unresolved. Recent work in male rats has shown that a history of binge-like palatable food intake suppresses hindbrain expression of preproglucagon (PPG), the precursor for glucagon-like peptide-1 (GLP-1). Given the roles of GLP-1 in reducing feeding and food reward, this could be a mechanism underlying binge-like eating in rodents. However, whether similar effects occur in female rats is unknown. Here, we tested the hypothesis that a history of binge-like palatable food intake in female rats would reduce PPG expression in the nucleus tractus solitarius (NTS), a key central site of GLP-1 production. Female rats given access to vegetable shortening every fourth day (4D) engaged in binge-like feeding, demonstrated by consuming significantly more shortening during the first hour of fat access compared to counterparts with ad libitum (AL) fat access. After several weeks of fat access under these schedules, PPG and GLP-1 receptor (GLP-1R) expression were measured in the NTS and ileum. Surprisingly, and in contrast to previous findings in male rats, there were no significant differences in expression of PPG or GLP-1R in either site in 4D versus AL rats, nor were there effects on plasma GLP-1 levels. These findings highlight key differences in the effects of binge-like intake on the central GLP-1 system in female compared to male rats.

Fat rather than sugar diet leads to binge-type eating, anticipation, effort behavior and activation of the corticolimbic system

Objectives: One factor contributing to the development of obesity is overeating palatable food. The palatability of food is driven by specific energy yielding combinations and flavor profiles that may contribute to its overconsumption. In rodents, restricted access to palatable food (PF) is a strong stimulus to trigger binge-type eating behavior (BTE), food anticipatory activity (FAA), effort behaviors and withdrawal symptoms. This is accompanied by plastic changes in corticolimbic areas associated with motivation and reward responses. Palatable food contains mainly a mixture of fat and sugar, thus, the contribution of each macronutrient for the behavioral and neuronal changes is unclear.Methods: In this study, Wistar rats were exposed to restricted access to 50% fat rich diet (FRD) or 50% sugar rich diet (SRD) in order to compare the intensity of BTE, FAA, effort behaviors and withdrawal responses.Results: In corticolimbic areas, c-Fos activation and ΔFosB accumulation were evaluated. After an acute exposition, rats ate more SRD than FRD, but FDR stimulated higher c-Fos. After chronic administration, the FDR group exhibited higher levels of BTE and FAA; this was associated with higher c-Fos and accumulation of ΔFosB in the corticolimbic system. Similar effects in the FRD group were observed after one week of withdrawal.Discussion: Present data indicate that the fat rich diet is a stronger stimulus than the sugar rich diet for the development of wanting behavior for reward and the underlying plastic changes in the corticolimbic system. The differential effects may be due to the differing caloric density of the diets.

Binge eating behavior and incentive motivation with a cafeteria diet

Binge-like eating behavior (BLE) has been characterized as an eating disorder in which subjects have an enhanced intake of food, mainly fats. However, intake of fats and carbohydrates may have differential effects on motivation. Previously it was shown that BLE produces an increase in operant responding for vegetable shortening, but others were unable to replicate the finding using sucrose as the reinforcer. Our aim was to determine if BLE behavior induced with a cafeteria-like diet (CaLD) with several options with fat content would produce an increment in performance. Male Wistar rats were trained under an exponential progressive ratio schedule of sucrose reinforcement; thereafter, the limited access model was used to induce BLE using CaLD options. Finally, subjects were tested for increments in break points (BPs) in the progressive ratio schedule. Rats with intermittent access to CaLD options showed a clear BLE with an escalation in their intake; however they showed compensatory decrements of chow intake that rendered a similar body weight gain to a continuous access group. Although we were unable to observe an increase in BPs after BLE we were able to observe a protection against the decrements of BP previously observed with sugar. Different mechanisms for processing high fat and high carbohydrate reinforcers are variables worth exploring to gain a better understanding of BLE behavior in rodent models.

Reduced caloric intake allows access-induced consumption differences to emerge with concentrated sucrose solutions

Rats given intermittent access to 4% (w/v) sucrose solution elevate their consumption of solution relative to rats with continuous access, a difference that does not appear at higher concentrations. Here, we examined the hypothesis that a limit on the intake of sucrose calories prevents rats from demonstrating access-induced differences in consumption of a more concentrated sucrose solution. Energy-replete rats were given every day (ED) or every third day (E3D) access to sucrose solutions adulterated with bitter quinine which reduced solution palatability and consumption levels while intake was measured. In experiment 1, previously collected data were compiled to examine the trajectory of consumption of continuously available 4% sucrose solution which was shown to stabilize by day 3 and then informed group assignment. In experiment 2, daily consumption levels were higher for rats with E3D access to 4% sucrose solution than rats with ED access to the same solution, whereas rats consumed similar amounts of 8% sucrose solution across access schedules. In the first hour of solution availability rats with E3D access showed elevated sucrose solution consumption, relative to rats with ED access, for both 4% and 8% sucrose solution. Upon the addition of quinine (0.005%) sucrose solution consumption decreased and the E3D access group consumed more daily sucrose solution than the ED access group for both 4% and 8% sucrose solution. In experiment 3, four groups of rats were given ED or E3D access to 8% sucrose solution adulterated with 0.0025%, 0.005%, 0.01%, or 0.02% quinine. Quinine adulteration reduced 8% sucrose solution consumption and allowed rats with E3D access to elevate their consumption levels relative to rats with ED access; this effect persisted when all groups were switched to 8% sucrose + 0.02% quinine solution. Thus, daily access-induced consumption differences develop but do not emerge because of a caloric limit on sucrose solution intake. This work underscores the interaction of availability and caloric intake as determinants of sugar consumption and highlights an important distinction between animal models of food addiction and binge eating.

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.

Dissociation between binge eating behavior and incentive motivation

Binge-like eating behavior (BLE) has been characterized as an eating disorder in which subjects have an enhanced intake of food, mainly fats. However, intake of fats and carbohydrates may have differential effects on motivation. Previously it was shown that BLE produces an increase in operant responding for vegetable shortening. Our aim was to determine if BLE behavior induced with a sucrose solution would produce an increment in performance for sucrose reinforcers. Male Wistar rats were trained under an exponential progressive ratio schedule of sucrose reinforcement; thereafter, the limited access model was used to induce BLE. Finally, subjects were tested for increments in break points (BPs) in the progressive ratio schedule. We were unable to observe an increase in BPs after BLE. No increments in BPs were observed when a distinctive flavor (vanilla-flavored sucrose) was correlated with BLE induction and reinforcement, or when different types of ratio progression in the operant schedules were employed. However, rats adjusted their BPs according to reinforcer concentration after BLE induction, demonstrating that valuation (cost/benefit decision) of reinforcers was intact. Extent of training, alterations of reward processing after extended exposure to sucrose, and different mechanisms for processing high fat and high carbohydrate reinforcers are variables worth exploring to gain a better understanding of BLE behavior in rodent models.

Consummatory, Feeding Microstructural, and Metabolic Effects Induced by Limiting Access to Either a High-Sucrose or a High-Fat Diet

Background: Binge eating disorder (BED) is characterized by recurrent binge eating episodes consisting of rapid consumption of excessive amounts of highly palatable, energy-dense food within discrete periods of time. The aim of this study was to test the consummatory, food microstructural, and metabolic effects of a one hour limited access to either a high-sucrose diet (HSD) or a high-fat diet (HFD) in an operant rat model of binge-like eating. Methods: Female rats were subject to a binge-like eating procedure in which a HSD, a HFD, or a standard chow diet were provided in a fixed ratio 1 (FR1) operant schedule of reinforcement. Results: Limiting access to either a HSD or a HFD promoted binge-like eating as compared to the control chow diet. However, binge-like eating of HSD, but not HFD, was based on a true increase in the amount of food consumed, an increased eating rate, and a decrease in the intake of the home-cage standard chow, altogether suggesting an increase in palatability. Moreover, while HSD rats consumed overall less energy than HFD rats, the former were more energy efficient and gained more body weight than the latter. Conclusions: These results provide information on how the quality of food can deeply influence the behavioral and metabolic outcomes of binge-like eating.

Binge-like palatable food intake in rats reduces preproglucagon in the nucleus tractus solitarius

Binge eating involves eating larger than normal quantities of food within a discrete period of time. The neurohormonal controls governing binge-like palatable food intake are not well understood. Glucagon-like peptide-1 (GLP-1), a hormone produced peripherally in the intestine and centrally in the nucleus tractus solitarius (NTS), reduces food intake. Given that the NTS plays a critical role in integrating peripheral and central signals relevant for food intake, as well as the role of GLP-1 in motivated feeding, we tested the hypothesis that expression of the GLP-1 precursor preproglucagon (PPG) would be reduced in the NTS of rats with a history of binge-like palatable food intake. Adult male rats received access to fat for 1 h shortly before lights off, either every day (Daily, D) or only 3d/week (Intermittent, INT). INT rats ate significantly more fat than did D rats in sessions where all rats had fat access. After ~8.5 weeks of diet maintenance, we measured plasma GLP-1 as well as NTS PPG and GLP-1 receptor expression. INT rats had significantly lower NTS PPG mRNA expression compared to D rats. However, plasma GLP-1 was significantly increased in the INT group versus D rats. No significant differences were observed in NTS GLP-1 receptor expression. We also measured plasma insulin levels, fasted blood glucose, and plasma corticosterone but no differences were detected between groups. These results support the hypothesis that binge-like eating reduces NTS GLP-1 expression, and furthermore, demonstrate divergent impacts of binge-like eating on peripheral (plasma) versus central GLP-1.

Intermittent High-Fat Diet Intake Reduces Sensitivity to Intragastric Nutrient Infusion and Exogenous Amylin in Female Rats

OBJECTIVE

Intermittent (INT) access to a high-fat diet (HFD) can induce excessive-intake phenotypes in rodents. This study hypothesized that impaired satiation responses contribute to elevated intake in an INT-HFD access model.

METHODS

First, this study characterized the intake and meal patterns of female rats that were subjected to an INT HFD in which a 45% HFD was presented for 20 hours every fourth day. To examine nutrient-induced satiation, rats received intragastric infusions of saline or Ensure Plus prior to darkness-onset food access. A similar design was used to examine sensitivity to the satiating effect of amylin. This study then examined whether an INT HFD influences amylin-induced c-Fos in feeding-relevant brain areas.

RESULTS

Upon INT HFD access, rats consumed meals of larger size. The anorexic response to intragastric Ensure infusion and exogenous amylin treatment was blunted in INT rats on both chow-only and INT-HFD days of the diet regimen, compared with chow-maintained and continuous-HFD rats. An INT HFD did not influence amylin-induced c-Fos in the area postrema, nucleus of the solitary tract, and lateral parabrachial nucleus.

CONCLUSIONS

Impaired satiation responses, mediated in part by reduced sensitivity to amylin, may explain the elevated intake observed upon INT HFD access and may play a role in disorders of INT overconsumption, including binge eating.

Expression of neural markers of gustatory signaling are differentially altered by continuous and intermittent feeding patterns

Food intake patterns are regulated by signals from the gustatory neural circuit, a complex neural network that begins at the tongue and continues to homeostatic and hedonic brain regions involved in eating behavior. The goal of the current study was to investigate the short-term effects of continuous access to a high fat diet (HFD) versus limited access to dietary fat on the gustatory neural circuit. Male Sprague-Dawley rats were fed a chow diet, a HFD (56% kcal from fat), or provided limited, daily (2 h/day) or limited, intermittent (2 h/day, 3 times/week) access to vegetable shortening for 2 weeks. Real time PCR was used to determine mRNA expression of markers of fat sensing/signaling (e.g. CD36) on the circumvallate papillae, markers of homeostatic eating in the mediobasal hypothalamus (MBH) and markers of hedonic eating in the nucleus accumbens (NAc). Continuous HFD increased mRNA levels of lingual CD36 and serotonin signaling, altered markers of homeostatic and hedonic eating. Limited, intermittent access to dietary fat selectively altered the expression of genes associated with the regulation of dopamine signaling. Overall, these data suggest that short-term, continuous access to HFD leads to altered fat taste and decreased expression of markers of homeostatic and hedonic eating. Limited, intermittent access, or binge-like, consumption of dietary fat led to an overall increase in markers of hedonic eating, without altering expression of lingual fat sensors or homeostatic eating. These data suggest that there are differential effects of meal patterns on gustatory neurocircuitry which may regulate the overconsumption of fat and lead to obesity.

Examining the Impact of Estrogen on Binge Feeding, Food-Motivated Behavior, and Body Weight in Female Rats

OBJECTIVE

Binge-eating disorder is associated with diminished self-control, emotional distress, and obesity. In this context, women are nearly twice as likely to develop binge-eating disorder and depression relative to men. Here, the physiological, psychological, and endocrine parameters were characterized in female rats subjected to a binge-eating protocol.

METHODS

Nonrestricted female Long Evans rats (n = 8/group) received 2-hour restricted access to a high-fat diet (HFD) (4.54 kcal/g) every day or every third day. The progression of estrous cycling, the functional relevance of estrogen signaling for binge feeding, and binge-induced changes in food motivation were measured.

RESULTS

Female rats developed a binge pattern of feeding that included alternation between caloric overconsumption and compensatory voluntary restriction without impacting estrous cycling. Notably, rats that received daily HFD exposure progressively decreased binge meals. Estrogen replacement in normal cycling or ovariectomized rats mimicked the reduction in body weight in female rats that received daily HFD access. Operant responding was unaffected by binge feeding; however, estrogen augmented operant performance in HFD-exposed rats.

CONCLUSIONS

Collectively, these data suggest that estrogen protects against binge-induced increases in body weight gain without affecting food motivation in female rats.

Convergent neural connectivity in motor impulsivity and high-fat food binge-like eating in male Sprague-Dawley rats

Food intake is essential for survival, but maladaptive patterns of intake, possibly encoded by a preexisting vulnerability coupled with the influence of environmental variables, can modify the reward value of food. Impulsivity, a predisposition toward rapid unplanned reactions to stimuli, is one of the multifaceted determinants underlying the etiology of dysregulated eating and its evolving pathogenesis. The medial prefrontal cortex (mPFC) is a major neural director of reward-driven behavior and impulsivity. Compromised signaling between the mPFC and nucleus accumbens shell (NAcSh) is thought to underlie the cognitive inability to withhold prepotent responses (motor impulsivity) and binge intake of high-fat food (HFF) seen in binge eating disorder. To explore the relationship between motor impulsivity and binge-like eating in rodents, we identified high (HI) and low impulsive (LI) rats in the 1-choice serial reaction time task and employed a rat model of binge-like eating behavior. HFF binge rats consumed significantly greater calories relative to control rats maintained on continual access to standard food or HFF. HI rats repeatedly exhibited significantly higher bingeing on HFF vs. LI rats. Next, we employed dual viral vector chemogenetic technology which allows for the targeted and isolated modulation of ventral mPFC (vmPFC) neurons that project to the NAcSh. Chemogenetic activation of the vmPFC to NAcSh pathway significantly suppressed motor impulsivity and binge-like intake for high-fat food. Thus, inherent motor impulsivity and binge-like eating are linked and the vmPFC to NAcSh pathway serves as a 'brake' over both behaviors.

Dopamine drives binge-like consumption of a palatable food in experimental Parkinsonism

BACKGROUND

Prolonged dopaminergic replacement therapy in PD results in pulsatile dopamine receptors stimulation in both dorsal and ventral striatum causing wearing off, motor fluctuations, and nonmotor side effects such as behavioral addictions. Among impulse control disorders, binge eating can be easily modeled in laboratory animals.

OBJECTIVES

We hypothesize that manipulation of dopamine levels in a 6-hydroxydopamine-lesioned rats, as a model of PD characterized by a different extent of dopamine denervation between dorsal and ventral striatum, would influence both synaptic plasticity of the nucleus accumbens and binge-like eating behavior.

METHODS

Food preference, food intake, and weight gain were monitored in sham-operated and unilaterally lesioned rats, subjected to a modified version of Corwin's limited access protocol, modelling binge eating disorder. Electrophysiological properties and long-term potentiation of GABAergic spiny projection neurons of the nucleus accumbens core were studied through ex vivo intracellular and patch-clamp recordings from corticostriatal slices of naïve and l-dopa-treated rats.

RESULTS

Sham-operated animals with intact nucleus accumbens core plasticity reliably developed food-addiction-like behavior when exposed to intermittent access to a highly palatable food. In contrast, parkinsonian rats were unresponsive to such restriction regimens, and also plasticity was lost in ventral spiny neurons. Chronic l-dopa reestablished long-term potentiation and compulsive eating, but with a different temporal dynamic that follows that of drug administration.

CONCLUSIONS

Our data indicate that endogenous and exogenous dopamine drive binge-like consumption of a palatable food in healthy and parkinsonian rats with distinct temporal dynamics, providing new insights into the complexity of l-dopa effects on the mesolimbic dopaminergic system. © 2019 International Parkinson and Movement Disorder Society.

Binge-Type Eating in Rats is Facilitated by Neuromedin U Receptor 2 in the Nucleus Accumbens and Ventral Tegmental Area

Binge-eating disorder (BED) is the most common eating disorder, characterized by rapid, recurrent overconsumption of highly palatable food in a short time frame. BED shares an overlapping behavioral phenotype with obesity, which is also linked to the overconsumption of highly palatable foods. The reinforcing properties of highly palatable foods are mediated by the nucleus accumbens (NAc) and the ventral tegmental area (VTA), which have been implicated in the overconsumption behavior observed in BED and obesity. A potential regulator of binge-type eating behavior is the G protein-coupled receptor neuromedin U receptor 2 (NMUR2). Previous research demonstrated that NMUR2 knockdown potentiates binge-type consumption of high-fat food. We correlated binge-type consumption across a spectrum of fat and carbohydrate mixtures with synaptosomal NMUR2 protein expression in the NAc and VTA of rats. Synaptosomal NMUR2 protein in the NAc demonstrated a strong positive correlation with binge intake of a “lower”-fat (higher carbohydrate) mixture, whereas synaptosomal NMUR2 protein in the VTA demonstrated a strong negative correlation with binge intake of an “extreme” high-fat (0% carbohydrate) mixture. Taken together, these data suggest that NMUR2 may differentially regulate binge-type eating within the NAc and the VTA.

Pimavanserin and Lorcaserin Attenuate Measures of Binge Eating in Male Sprague-Dawley Rats

Binge eating disorder (BED) is characterized by dysregulated feeding and reward-related processes, and treatment is often challenged by limited therapeutic options. The serotonin (5-HT) 5-HT_2A receptor (5-HT_2AR) and 5-HT_2CR are implicated in both feeding-related and reward-related behaviors and are thus poised to regulate BED-related behaviors. The purpose of this study was to assess the efficacy of the FDA-approved medications pimavanserin, a 5-HT_2AR antagonist/inverse agonist, and lorcaserin, a 5-HT_2CR agonist, in a rodent model of binge eating. The effects of pimavanserin (0.3–3.0 mg/kg), lorcaserin (0.25–1.0 mg/kg), and the lowest effective dose of pimavanserin (0.3 mg/kg) plus lorcaserin (1.0 mg/kg) were tested in a high-fat food (HFF) intermittent access binge eating model in adult male Sprague-Dawley rats (n = 64). We assessed three measures related to binge eating – binge episode occurrence, binge intake, and weight gain associated with HFF access. Pimavanserin decreased binge intake and weight gain associated with HFF access, but did not prevent binge episode occurrence. Lorcaserin decreased binge intake, but did not prevent binge episode occurrence or weight gain associated with HFF access. Combined pimavanserin plus lorcaserin prevented binge episode occurrence in addition to decreasing binge intake and weight gain associated with HFF access. These preclinical findings in male rats suggest that pimavanserin and lorcaserin may be effective in treating patients with BED. Our studies further indicate that administration of one or both drugs may be more effective in certain sub-populations of patients with BED because of the unique profile each treatment elicits. These data support future assessment in clinical populations with BED.

Anterior insula activity regulates the associated behaviors of high fat food binge intake and cue reactivity in male rats

Binge eating episodes are characterized by uncontrollable, excessive intake of food and are associated with binge eating disorder and some subtypes of obesity. One factor thought to contribute to binge episodes is a high level of reactivity to food-associated cues (i.e., cue reactivity). The insula is a neural node poised to regulate both binge eating and cue reactivity because of its prominent role in interpretation of internal and external cues. This work established a positive association between high fat food (HFF) binge intake and cue reactivity in male rats. Furthermore, we demonstrated that activation of the anterior insula suppressed both HFF binge intake and cue reactivity, without altering homeostatic intake of food. We further show that attenuation of HFF binge intake and cue reactivity is not due to decreased food-reward efficacy or deficits in motivation. Together, these data establish a key role for the anterior insula in the control of binge eating related-behaviors and support novel avenues for the treatment of binge eating.

Intermittent, extended access to preferred food leads to escalated food reinforcement and cyclic whole-body metabolism in rats: Sex differences and individual vulnerability

Compulsive binge eating is a hallmark of binge eating disorder and bulimia nervosa and is implicated in some obesity cases. Eating disorders are sexually dimorphic, with females more often affected than males. Animal models of binge-like eating based on intermittent access to palatable food exist; but, little is known regarding sex differences or individual vulnerability in these models with respect to the reinforcing efficacy of food, the development of compulsive- and binge-like eating, or associated changes in whole-body metabolism or body composition. Adolescent male (n = 24) and female (n = 32) Wistar rats were maintained on chow or a preferred, high-sucrose, chocolate-flavored diet in continuous or intermittent, extended access conditions. Body weight and composition, intake, fixed- and progressive-ratio operant self-administration, and whole body energy expenditure and respiratory exchange ratios were measured across an 11-week study period. Subgroup analyses were conducted to differentiate compulsive-like "high responder" intermittent access rats that escalated to extreme progressive-ratio self-administration performance vs. more resistant "low responders." Female rats had greater reinforcing efficacy of food than males in all diet conditions and were more often classified as "high responders". In both sexes, rats with intermittent access showed cycling of fuel substrate utilization and whole-body energy expenditure. Further, "high-responding" intermittent access female rats had especially elevated respiratory exchange ratios, indicating a fat-sparing phenotype. Future studies are needed to better understand the molecular and neurobiological basis of the sex and individual differences we have observed in rats and their translational impact for humans with compulsive, binge eating disorders.

Serotonin 5-HT2C Receptor Activation Suppresses Binge Intake and the Reinforcing and Motivational Properties of High-Fat Food

Binge eating disorder (BED) is characterized by dysfunctional hedonic food intake and reward-related processes. Activation of the serotonin (5-HT) 5-HT2C receptor (5-HT2CR) suppresses both food intake and reward-related behaviors and is thus poised to regulate BED. This study assessed the effects of 5-HT2CR activation via the selective 5-HT2CR agonist WAY163909 on binge eating-related behaviors in adult male Sprague-Dawley rats. Low doses of WAY163909 (1.0, 2.0 mg/kg) suppressed high-fat food (HFF) binge intake, but not standard food non-binge intake. WAY163909 (1.0 mg/kg) also attenuated operant responding for self-administered HFF pellets on fixed and progressive ratio schedules of reinforcement, indicating that 5-HT2CR activation suppresses the reinforcing and motivational properties of HFF, respectively. These findings suggest that activation of the 5-HT2CR may be effective at suppressing binge eating in patients with BED via suppression of the reinforcing and motivational properties of HFF. This work supports future studies targeting the 5-HT2CR in the treatment of BED.

The effects of scheduled running wheel access on binge-like eating behavior and its consequences

Binge eating disorder (BED) is an eating disorder involving repeated, intermittent over consumption of food in brief periods of time, usually with no compensatory behaviors. There are few successful treatments and the underlying neural mechanisms remain unclear. In the current study, we hypothesized that voluntary running wheel (RW) activity could reduce binge-like eating behavior in a rat model. Rats were given intermittent (3 times/wk) limited (1hr) access to a high-fat food (Crisco), in addition to continuously available chow. Crisco was available every Mon, Wed, and Fri for 1hr before dark onset. Rats were divided into 2 groups: those with RW access during the first half of the experiment and sedentary during the second half (RW-SED) and those that were sedentary during the first half of the experiment and had RW access during the second half (SED-RW). Crisco intake was significantly less in both groups during the period of time with a RW present. Within the bingeing RW-SED rats, the gene expression of the orexigenic neuropeptides AgRP and NPY were similar to a non-bingeing sedentary control (CON) group, while the expression of the anorexigenic neuropeptide POMC was significantly increased relative to the SED-RW and CON groups. Despite elevated POMC, the rats continued to binge. Additionally, within both groups, the gene expression of the D2R and Oprm1 in the NAc and the VTA were altered suggesting that the reward system was stimulated by both the bingeing behavior and the running wheel activity. Overall, access to a RW and the resulting activity significantly reduced binge-like behavior as well as modulated the effects of binging on brain appetite and reward systems.

Increased ethanol consumption after interruption of fat bingeing

There is a marked comorbidity between alcohol abuse and eating disorders, especially in the young population. We have previously reported that bingeing on fat during adolescence increases the rewarding effects of ethanol (EtOH). The aim of the present work was to study if vulnerability to EtOH persists after cessation of binge eating. OF1 mice binged on fat (HFB: high-fat binge) during adolescence (PND 25-43) and were tested for 15 days after the last access to HFB (on PND 59) using the self-administration paradigm, the conditioned place preference (CPP) and locomotor sensitization to ethanol. Our results showed that after 15 days of cessation of fat ingestion, mice increased their consumption of ethanol and showed greater motivation to obtain ethanol. On the other hand, no effects were observed in the CPP, while an increased locomotor response to ethanol was detected. The present results confirm and extend our previous study demonstrating that the compulsive intake of fat induces long-lasting effects on the reward system that lead to an increased consumption of EtOH.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Cognitive impairment and gene expression alterations in a rodent model of binge eating disorder

Binge eating disorder (BED) is defined as recurrent, distressing over-consumption of palatable food (PF) in a short time period. Clinical studies suggest that individuals with BED may have impairments in cognitive processes, executive functioning, impulse control, and decision-making, which may play a role in sustaining binge eating behavior. These clinical reports, however, are limited and often conflicting. In this study, we used a limited access rat model of binge-like behavior in order to further explore the effects of binge eating on cognition. In binge eating prone (BEP) rats, we found novel object recognition (NOR) as well as Barnes maze reversal learning (BM-RL) deficits. Aberrant gene expression of brain derived neurotrophic factor (Bdnf) and tropomyosin receptor kinase B (TrkB) in the hippocampus (HPC)-prefrontal cortex (PFC) network was observed in BEP rats. Additionally, the NOR deficits were correlated with reductions in the expression of TrkB and insulin receptor (Ir) in the CA3 region of the hippocampus. Furthermore, up-regulation of serotonin-2C (5-HT_2C) receptors in the orbitoprefrontal cortex (OFC) was associated with BM-RL deficit. Finally, in the nucleus accumbens (NAc), we found decreased dopamine receptor 2 (Drd2) expression among BEP rats. Taken together, these data suggest that binge eating vegetable shortening may induce contextual and reversal learning deficits which may be mediated, at least in part, by the altered expression of genes in the CA3-OFC-NAc neural network.

Patterned feeding induces neuroendocrine, behavioral and genetic changes that promote palatable food intake

BACKGROUND

Selection of a healthy diet is the cornerstone for treating obesity and metabolic disease. Unfortunately, the majority of diets fail leading to weight regain and in some cases, pathological feeding behavior. We hypothesize that alternating bouts of caloric overconsumption and caloric restriction, behavioral manifestations of dieting induce neuroendocrine, behavioral and genetic changes that promote future bouts of palatable food intake.

METHODS

To test this hypothesis, we subjected male Long-Evans rats to a high-fat diet (HFD) feeding paradigm that induced a pattern of caloric overconsumption and caloric restriction. Under these conditions we measured operant responding for sucrose, pre-meal ghrelin secretion, the effects of peripheral ghrelin blockade on patterned feeding, HFD intake in an aversive environment and mRNA expression of the ghrelin receptor, orexin, orexin-1 and 2 receptors, and FTO in the medial prefrontal cortex, lateral hypothalamus and ventral tegmental area.

RESULTS

Rats subjected to this feeding regimen displayed increased ghrelin levels prior to HFD exposure and blockade of this response attenuated patterned feeding behavior. In addition, patterned feeding promoted enhanced motivation for sucrose, diminished extinction of this response and increased HFD intake in an aversive environment. The neuroendocrine and behavioral changes correlated with increased hypothalamic expression of the ghrelin receptor and FTO.

CONCLUSION

Collectively, these data indicate that patterns of feeding that include caloric overconsumption and caloric restriction induce neuroendocrine and neurobiological changes that signify an enhanced drive for palatable food.

Role of relaxin-3/RXFP3 system in stress-induced binge-like eating in female rats

Binge eating is frequently stimulated by stress. The neuropeptide relaxin-3 (RLN3) and its native receptor RXFP3 are implicated in stress and appetitive behaviors. We investigated the dynamics of the central RLN3/RXFP3 system in a newly established model of stress-induced binge eating. Female Sprague-Dawley rats were subjected to unpredictable intermittent 1-h access to 10% sucrose. When sucrose intake stabilized, rats were assessed for consistency of higher or lower sucrose intake in response to three unpredictable episodes of foot-shock stress; and assigned as binge-like eating prone (BEP) or binge-like eating resistant (BER). BEP rats displayed elevated consumption of sucrose under non-stressful conditions (30% > BER) and an additional marked increase in sucrose intake (60% > BER) in response to stress. Conversely, sucrose intake in BER rats was unaltered by stress. Chow intake was similar in both phenotypes on 'non-stress' days, but was significantly reduced by stress in BER, but not BEP, rats. After stress, BEP, but not BER, rats displayed a significant increase in RLN3 mRNA levels in the nucleus incertus. In addition, in response to stress, BEP, but not BER, rats had increased RXFP3 mRNA levels in the paraventricular and supraoptic nuclei of the hypothalamus. Intracerebroventricular administration of a selective RXFP3 antagonist, R3(B1-22)R, blocked the stress-induced increase in sucrose intake in BEP rats and had no effect on sucrose intake in BER rats. These results provide important evidence for a role of the central RLN3/RXFP3 system in the regulation of stress-induced binge eating in rats, and have therapeutic implications for eating disorders.

Differential expression of CART in feeding and reward circuits in binge eating rat model

Binge eating (BE) disrupts feeding and subverts reward mechanism. Since cocaine- and amphetamine-regulated transcript peptide (CART) mediates satiety as well as reward, its role in BE justifies investigation. To induce BE, rats were provided restricted access to high fat sweet palatable diet (HFSPD) for a period of 4 weeks. Immunoreactivity profile of the CART elements, and accompanying neuroplastic changes were studied in satiety- and reward-regulating brain nuclei. Further, we investigated the effects of CART, CART-antibody or rimonabant on the intake of normal chow or HFSPD. Rats fed on HFSPD showed development of BE-like phenotype as reflected by significant consumption of HFSPD in short time frame, suggestive of dysregulated satiety mechanisms. At the mid-point during BE, CART-immunoreactivity was significantly increased in hypothalamic arcuate (ARC), lateral (LH), nucleus accumbens shell (AcbSh) and paraventricular nucleus of thalamus (PVT). However, for next 22-h post-binge time-period, the animals showed no interest in food, and low CART expression. Pre-binge treatment with rimonabant, a drug recommended for the treatment of BE, produced anorexia, increased CART expression in ARC and LH, but not in AcbSh and PVT. Higher dose of CART was required to produce anorexia in binged rats. While neuronal tracing studies confirmed CART fiber connectivity from ARC and LH to AcbSh, increase in CART and synaptophysin immunostaining in this pathway in BE rats suggested strengthening of the CART connectivity. We conclude that CART bearing ARC-LH-PVT-AcbSh reward circuit may override the satiety signaling in ARC-PVN pathway in BE rats.

Animal models of compulsive eating behavior

Eating disorders are multifactorial conditions that can involve a combination of genetic, metabolic, environmental, and behavioral factors. Studies in humans and laboratory animals show that eating can also be regulated by factors unrelated to metabolic control. Several studies suggest a link between stress, access to highly palatable food, and eating disorders. Eating "comfort foods" in response to a negative emotional state, for example, suggests that some individuals overeat to self-medicate. Clinical data suggest that some individuals may develop addiction-like behaviors from consuming palatable foods. Based on this observation, "food addiction" has emerged as an area of intense scientific research. A growing body of evidence suggests that some aspects of food addiction, such as compulsive eating behavior, can be modeled in animals. Moreover, several areas of the brain, including various neurotransmitter systems, are involved in the reinforcement effects of both food and drugs, suggesting that natural and pharmacological stimuli activate similar neural systems. In addition, several recent studies have identified a putative connection between neural circuits activated in the seeking and intake of both palatable food and drugs. The development of well-characterized animal models will increase our understanding of the etiological factors of food addiction and will help identify the neural substrates involved in eating disorders such as compulsive overeating. Such models will facilitate the development and validation of targeted pharmacological therapies.

Pharmacological modulation of the endocannabinoid signalling alters binge-type eating behaviour in female rats

BACKGROUND AND PURPOSE

Binge eating disorder (BED) is characterized by excessive food intake during short periods of time. Recent evidence suggests that alterations in the endocannabinoid signalling could be involved in the pathophysiology of BED. In this study, we investigated whether pharmacological manipulation of endocannabinoid transmission may be effective in modulating the aberrant eating behaviour present in a validated rat model of BED.

EXPERIMENTAL APPROACH

Binge-type eating was induced in female rats by providing limited access to an optional source of dietary fat (margarine). Rats were divided into three groups, all with ad libitum access to chow and water: control (C), with no access to margarine; low restriction (LR), with 2 h margarine access 7 days a week; high restriction (HR), with 2 h margarine access 3 days a week.

KEY RESULTS

Compared with the LR group, the HR group consumed more margarine and this was accompanied by an increase in body weight. The cannabinoid CB₁/CB₂ receptor agonist Δ⁹-tetrahydrocannabinol significantly increased margarine intake selectively in LR rats, while the fatty acid amide hydrolase inhibitor URB597 showed no effect. The CB₁ receptor inverse agonist/antagonist rimonabant dose-dependently reduced margarine intake in HR rats. Notably, in HR rats, chronic treatment with a low dose of rimonabant induced a selective long-lasting reduction in margarine intake that did not develop tolerance, and a significant and persistent reduction in body weight.

CONCLUSIONS AND IMPLICATIONS

Chronic pharmacological blockade of CB₁ receptors reduces binge eating behaviour in female rats and may prove effective in treating BED, with an associated significant reduction in body weight.

Pharmacological manipulations in animal models of anorexia and binge eating in relation to humans

Eating disorders, such as anorexia nervosa (AN), bulimia nervosa (BN) and binge eating disorders (BED), are described as abnormal eating habits that usually involve insufficient or excessive food intake. Animal models have been developed that provide insight into certain aspects of eating disorders. Several drugs have been found efficacious in these animal models and some of them have eventually proven useful in the treatment of eating disorders. This review will cover the role of monoaminergic neurotransmitters in eating disorders and their pharmacological manipulations in animal models and humans. Dopamine, 5-HT (serotonin) and noradrenaline in hypothalamic and striatal regions regulate food intake by affecting hunger and satiety and by affecting rewarding and motivational aspects of feeding. Reduced neurotransmission by dopamine, 5-HT and noradrenaline and compensatory changes, at least in dopamine D2 and 5-HT(2C/2A) receptors, have been related to the pathophysiology of AN in humans and animal models. Also, in disorders and animal models of BN and BED, monoaminergic neurotransmission is down-regulated but receptor level changes are different from those seen in AN. A hypofunctional dopamine system or overactive α2-adrenoceptors may contribute to an attenuated response to (palatable) food and result in hedonic binge eating. Evidence for the efficacy of monoaminergic treatments for AN is limited, while more support exists for the treatment of BN or BED with monoaminergic drugs.

Addicted to palatable foods: comparing the neurobiology of Bulimia Nervosa to that of drug addiction

RATIONALE

Bulimia nervosa (BN) is highly comorbid with substance abuse and shares common phenotypic and genetic predispositions with drug addiction. Although treatments for the two disorders are similar, controversy remains about whether BN should be classified as addiction.

OBJECTIVES

Here, we review the animal and human literature with the goal of assessing whether BN and drug addiction share a common neurobiology.

RESULTS

Similar neurobiological features are present following administration of drugs and bingeing on palatable food, especially sugar. Specifically, both disorders involve increases in extracellular dopamine (DA), D1 binding, D3 messenger RNA (mRNA), and ΔFosB in the nucleus accumbens (NAc). Animal models of BN reveal increases in ventral tegmental area (VTA) DA and enzymes involved in DA synthesis that resemble changes observed after exposure to addictive drugs. Additionally, alterations in the expression of glutamate receptors and prefrontal cortex activity present in human BN or following sugar bingeing in animals are comparable to the effects of addictive drugs. The two disorders differ in regards to alterations in NAc D2 binding, VTA DAT mRNA expression, and the efficacy of drugs targeting glutamate to treat these disorders.

CONCLUSIONS

Although additional empirical studies are necessary, the synthesis of the two bodies of research presented here suggests that BN shares many neurobiological features with drug addiction. While few Food and Drug Administration-approved options currently exist for the treatment of drug addiction, pharmacotherapies developed in the future, which target the glutamate, DA, and opioid systems, may be beneficial for the treatment of both BN and drug addiction.

Caloric restriction increases the sensitivity to the hyperphagic effect of nociceptin/orphanin FQ limiting its ability to reduce binge eating in female rats

RATIONALE

Nociceptin/orphanin FQ (N/OFQ) is a functional antagonist of corticotrophin-releasing factor, the main mediator of the stress response. Stress represents a key determinant of binge eating (BE) for highly palatable food (HPF).

OBJECTIVES

In relation to the antistress properties of N/OFQ, we evaluated its effect on BE. After the observation that episodes of food restriction increase the sensitivity to its hyperphagic effects, the function of NOP receptor and N/OFQ was investigated after cycles of food restrictions.

MATERIALS AND METHODS

In BE experiments, four groups were used: rats fed normally and not stressed or stressed, rats exposed to cycles of restriction/refeeding and then stressed, or not stressed. In the other experiments, two groups were used: rats exposed or not to food restriction.

RESULTS

Only restricted and stressed rats exhibited BE for HPF (containing chocolate cream). Intracerebroventricular injections of N/OFQ of 0.5 nmol/rat significantly reduced BE. N/OFQ 1 nmol/rat did not reduce BE but significantly increased HPF intake following food restrictions. Cycles of food restriction increased animals' sensitivity to the hyperphagic effect of N/OFQ for HPF. In situ hybridization studies following food restrictions showed decreased ppN/OFQ mRNA expression in the bed nucleus of the stria terminalis and increased expression of ppN/OFQ and NOP receptor mRNA in the ventral tegmental area and in the ventromedial hypothalamus, respectively.

CONCLUSIONS

These findings indicate that N/OFQ slightly reduces BE at low doses, while higher doses increase HPF intake, due to increased sensitivity to its hyperphagic effect following a history of caloric restrictions.

Intermittent access to sweet high-fat liquid induces increased palatability and motivation to consume in a rat model of binge consumption

Binge eating disorders are characterized by discrete episodes of rapid and excessive food consumption. In rats, giving intermittent access to sweet fat food mimics this aspect of binge eating. These models typically employ solid food; however, the total amount consumed depends on motivation, palatability and satiety, which are difficult to dissociate with solid food. In contrast, lick microstructure analysis can be used to dissociate these parameters when the ingestant is a liquid. Therefore, we developed a binge model using a liquid emulsion composed of corn oil, heavy cream and sugar. We show that rats given intermittent access to this high-fat emulsion develop binge-like behavior comparable to that previously observed with solid high-fat food. One feature of this behavior was a gradual escalation in consumption across 2.5 weeks of intermittent access, which was not apparent in rats given lower-fat liquid on the same access schedule. Lick microstructure analysis suggests that this escalation was due at least in part to increases in both motivation to consume and palatability-driven consumption.

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.

Dysregulation of brain reward systems in eating disorders: neurochemical information from animal models of binge eating, bulimia nervosa, and anorexia nervosa

Food intake is mediated, in part, through brain pathways for motivation and reinforcement. Dysregulation of these pathways may underlay some of the behaviors exhibited by patients with eating disorders. Research using animal models of eating disorders has greatly contributed to the detailed study of potential brain mechanisms that many underlie the causes or consequences of aberrant eating behaviors. This review focuses on neurochemical evidence of reward-related brain dysfunctions obtained through animal models of binge eating, bulimia nervosa, or anorexia nervosa. The findings suggest that alterations in dopamine (DA), acetylcholine (ACh) and opioid systems in reward-related brain areas occur in response to binge eating of palatable foods. Moreover, animal models of bulimia nervosa suggest that while bingeing on palatable food releases DA, purging attenuates the release of ACh that might otherwise signal satiety. Animal models of anorexia nervosa suggest that restricted access to food enhances the reinforcing effects of DA when the animal does eat. The activity-based anorexia model suggests alterations in mesolimbic DA and serotonin occur as a result of restricted eating coupled with excessive wheel running. These findings with animal models complement data obtained through neuroimaging and pharmacotherapy studies of clinical populations. Information on the neurochemical consequences of the behaviors associated with these eating disorders will be useful in understanding these complex disorders and may inform future therapeutic approaches, as discussed here. This article is part of a Special Issue entitled 'Central Control of Food Intake'.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

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.

Intermittent access to sucrose increases sucrose-licking activity and attenuates restraint stress-induced activation of the lateral septum

Intermittent access to palatable food can attenuate anorectic and hormonal responses to stress in rats. The neuronal mechanisms of modulation of stress response by diets are not fully understood. The present study was conducted to create rat models with intermittent access to sucrose that demonstrate resistance to stress-induced hypophagia, to study the pattern of sucrose consumption by these rat models, and to investigate in which brain structures intermittent sucrose regimens modify stress-induced neuronal activation. The obtained results demonstrate that 6-wk intermittent access to sucrose without food restriction (4 day/wk ad libitum access to sucrose in addition to chow, and following 3 day/wk exclusive feeding of chow; SIA rats) and combined with food restriction (4 day/wk access to chow and sucrose restricted to 2 h/day, and following 3 days/wk on unrestricted chow; SIR rats) increased sucrose-licking activity. The alterations in the rats' feeding behavior were accompanied by a resistance of their body weight gain and food intake to 1-h restraint stress applied once per week. The chronic intermittent sucrose consumption significantly lowered, in the SIA and SIR rats, the levels of expression of corticotropin-releasing factor type 2 receptor and restraint stress-induced expression of c- fos mRNA in the medioventral part of the lateral septum. Conversely, the levels of the corticotropin-releasing factor type 2 receptor transcript in the ventromedial hypothalamic nucleus were decreased only in the food-restricted SIR rats. The lower stress-induced neuronal activation in the medioventral part of the lateral septum may contribute to the attenuated anorectic stress response in the rats maintained on intermittent sucrose regimens.

Dopamine and binge eating behaviors

Central dopaminergic mechanisms are involved in the motivational aspects of eating and food choices. This review focuses on human and animal data investigating the importance of dopamine on binge eating behaviors. Early work examining dopamine metabolites in the cerebrospinal fluid and plasma of bulimic individuals suggested decreased dopamine turnover during the active phase of the illness. While neuroimaging studies of dopamine mechanisms in bulimia nervosa (BN) and binge eating disorder (BED) are limited, genetic studies in humans have implicated an increased frequency of dopamine transporter and associated D2 receptor polymorphisms with binge pathology. Recent studies in rodent models of dietary-induced binge eating (DIBE) have investigated plausible dopamine mechanisms involved in sustaining binge eating behaviors. In DIBE models, highly palatable foods (fats, sugars and their combination), as well as restricted access conditions appear to promote ingestive responses and result in sustained dopamine stimulation within the nucleus accumbens. Taken together with studies on the comorbidity of illicit drug use and eating disorders, the data reviewed here support a role for dopamine in perpetuating the compulsive feeding patterns of BN and BED. As such, we propose that sustained stimulation of the dopamine systems by bingeing promoted by preexisting conditions (e.g., genetic traits, dietary restraint, stress, etc.) results in progressive impairments of dopamine signaling. To disrupt this vicious cycle, novel research-based treatment options aiming at the neural substrates of compulsive eating patterns are necessary.

Brain-derived neurotrophic factor regulates hedonic feeding by acting on the mesolimbic dopamine system

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, play prominent roles in food intake regulation through central mechanisms. However, the neural circuits underlying their anorexigenic effects remain largely unknown. We showed previously that selective BDNF depletion in the ventromedial hypothalamus (VMH) of mice resulted in hyperphagic behavior and obesity. Here, we sought to ascertain whether its regulatory effects involved the mesolimbic dopamine system, which mediates motivated and reward-seeking behaviors including consumption of palatable food. We found that expression of BDNF and TrkB mRNA in the ventral tegmental area (VTA) of wild-type mice was influenced by consumption of palatable, high-fat food (HFF). Moreover, amperometric recordings in brain slices of mice depleted of central BDNF uncovered marked deficits in evoked release of dopamine in the nucleus accumbens (NAc) shell and dorsal striatum but normal secretion in the NAc core. Mutant mice also exhibited dramatic increases in HFF consumption, which were exacerbated when access to HFF was restricted. However, mutants displayed enhanced responses to D(1) receptor agonist administration, which normalized their intake of HFF in a 4 h food intake test. Finally, in contrast to deletion of Bdnf in the VMH of mice, which resulted in increased intake of standard chow, BDNF depletion in the VTA elicited excessive intake of HFF but not of standard chow and increased body weights under HFF conditions. Our findings indicate that the effects of BDNF on eating behavior are neural substrate-dependent and that BDNF influences hedonic feeding via positive modulation of the mesolimbic dopamine system.

Reduction of extinction and reinstatement of cocaine seeking by wheel running in female rats

RATIONALE AND OBJECTIVES

Previous work has shown that wheel running reduced the maintenance of cocaine self-administration in rats. In the present study, the effect of wheel running on extinction and reinstatement of cocaine seeking was examined. Female rats were trained to run in a wheel during 6-h sessions, and they were then catheterized and placed in an operant conditioning chamber where they did not have access to the wheel but were allowed to self-administer iv cocaine. Subsequently, rats were divided into four groups and were tested on the extinction and reinstatement of cocaine seeking while they had varying access to a wheel in an adjoining compartment. The four groups were assigned to the following wheel access conditions: (1) wheel running during extinction and reinstatement (WER), (2) wheel running during extinction and a locked wheel during reinstatement (WE), (3) locked wheel during extinction and wheel running during reinstatement (WR), and (4) locked wheel during extinction and reinstatement (WL). WE and WR were retested later to examine the effect of one session of wheel access on cocaine-primed reinstatement.

RESULTS

There were no group differences in wheel revolutions, in rate of acquisition of cocaine self-administration, or in responding during maintenance when there was no wheel access. However, during extinction, WE and WER responded less than WR and WL. WR and WER had lower cocaine-primed reinstatement than WE and WL. One session of wheel exposure in WE also suppressed cocaine-primed reinstatement.

CONCLUSIONS

Wheel running immediately and effectively reduced cocaine-seeking behavior, but concurrent access to running was necessary. Thus, exercise is a useful and self-sustaining intervention to reduce cocaine-seeking behavior.

Repeated binge access to a palatable food alters feeding behavior, hormone profile, and hindbrain c-Fos responses to a test meal in adult male rats

Repetitive cycles of palatable food access and chronic calorie restriction alter feeding behaviors and forebrain neural systems. The purpose of this study was to determine the behavioral, endocrine, and meal-related hindbrain neural activation in adult male Sprague-Dawley rats exposed to a binge-access feeding schedule. The binge-access schedule consisted of repeated twice-per-week episodes of acute calorie restriction (to one-third of the previous day's intake) followed by 2 h of concurrent access to high-calorie palatable food (sweetened fat: 90% vegetable shortening-10% sucrose) and chow. The binge-access rats consumed more calories during the "binge" period than rats with continuous access to sweetened fat (continuous-access group) or subjected to repeated acute calorie restriction only (chow-restricted group). The binge-access group also exhibited a approximately 25% increase in sweetened fat intake from week 1 to week 6. Persistence of the binge phenotype in the binge-access animals was demonstrated 2 wk, but not 4 wk, after ad libitum chow. The binge-access and chow-restricted groups maintained a similar normal body composition and hormonal profiles, whereas the continuous-access animals developed an obese phenotype. Terminal ghrelin levels were significantly higher in the binge-access group than in the continuous-access group. Consumption of a standardized meal resulted in more c-Fos-positive cells along the anterior-posterior nucleus of the solitary tract regions in the binge-access group than in naive controls. These results suggest that repeated cycles of acute calorie restriction followed by palatable food produce physiological alterations that may facilitate overconsumption of a highly palatable food during limited-access periods.