Behavioral and hormonal responses to stress in binge-like eating prone female rats

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.

Linking drug and food addiction: an overview of the shared neural circuits and behavioral phenotype

Despite a lack of agreement on its definition and inclusion as a specific diagnosable disturbance, the food addiction construct is supported by several neurobiological and behavioral clinical and preclinical findings. Recognizing food addiction is critical to understanding how and why it manifests. In this overview, we focused on those as follows: 1. the hyperpalatable food effects in food addiction development; 2. specific brain regions involved in both food and drug addiction; and 3. animal models highlighting commonalities between substance use disorders and food addiction. Although results collected through animal studies emerged from protocols differing in several ways, they clearly highlight commonalities in behavioral manifestations and neurobiological alterations between substance use disorders and food addiction characteristics. To develop improved food addiction models, this heterogeneity should be acknowledged and embraced so that research can systematically investigate the role of specific variables in the development of the different behavioral features of addiction-like behavior in preclinical models.

Corticotropin-releasing factor system in the lateral septum: Implications in the pathophysiology of obesity

Obesity is a pandemic associated with lifestyles changes. These include excess intake of obesogenic foods and decreased physical activity. Brain areas, like the lateral hypothalamus (LH), ventral tegmental area (VTA), and nucleus accumbens (NAcc) have been linked in both homeostatic and hedonic control of feeding in experimental models of diet-induced obesity. Interestingly, these control systems are regulated by the lateral septum (LS), a relay of γ-aminobutyric (GABA) acid neurons (GABAergic neurons) that inhibit the LH and GABAergic interneurons of the VTA. Furthermore, the LS has a diverse receptor population for neurotransmitters and neuropeptides such as dopamine, glutamate, GABA and corticotropin-releasing factor (CRF), among others. Particularly, CRF a key player in the stress response, has been related to the development of overweight and obesity. Moreover, evidence shows that LS neurons neurophysiologically regulate reward and stress, although there is little evidence of LS taking part in homeostatic and hedonic feeding. In this review, we discuss the evidence that supports the role of LS and CRF on feeding, and how alterations in this system contribute to weight gain obesity.

A Framework for Developing Translationally Relevant Animal Models of Stress-Induced Changes in Eating Behavior

Stress often affects eating behaviors, leading to increased eating in some individuals and decreased eating in others. Identifying physiological and psychological factors that determine the direction of eating responses to stress has been a major goal of epidemiological and clinical studies. However, challenges of standardizing the stress exposure in humans hinder efforts to uncover the underlying mechanisms. The issue of what determines the direction of stress-induced feeding responses has not been directly addressed in animal models, but assays that combine stress with a feeding-related task are commonly used as readouts of other behaviors, such as anxiety. Sex, estrous cyclicity, circadian cyclicity, caloric restriction, palatable diets, elevated body weight, and properties of the stressors similarly influence feeding behavior in humans and rodent models. Yet, most rodent studies do not use conditions that are most relevant for studying feeding behavior in humans. This review proposes a conceptual framework for incorporating these influences to develop reproducible and translationally relevant assays to study effects of stress on food intake. Such paradigms have the potential to uncover links between emotional eating and obesity as well as to the etiology of eating disorders.

Compulsive Eating in a Rat Model of Binge Eating Disorder Under Conditioned Fear and Exploration of Neural Mechanisms With c-fos mRNA Expression

Compulsive eating is the most obstinate feature of binge eating disorder. In this study, we observed the compulsive eating in our stress-induced binge-like eating rat model using a conflicting test, where sucrose and an aversively conditioned stimulus were presented at the same time. In this conflicting situation, the binge-like eating prone rats (BEPs), compared to the binge-like eating resistant rats (BERs), showed persistent high sucrose intake and inhibited fear response, respectively, indicating a deficit in palatability devaluation and stronger anxiolytic response to sucrose in the BEPs. We further analyzed the neuronal activation with c-fos mRNA in situ hybridization. Surprisingly, the sucrose access under conditioned fear did not inhibit the activity of amygdala; instead, it activated the central amygdala. In the BEPs, sucrose reduced the response of the paraventricular hypothalamic nucleus (PVN), while enhancing activities in the lateral hypothalamic area (LHA) to the CS. The resistance to devaluating the palatable food in the BEPs could be a result of persistent Acb response to sucrose intake and attenuated recruitment of the medial prefrontal cortex (mPFC). We interpret this finding as that the reward system of the BEPs overcame the homeostasis system and the stress-responding system.

Neuronal activities during palatable food consumption in the reward system of binge-like eating female rats

Binge eating disorder (BED), characterized by bingeing episodes and compulsivity, is the most prevalent eating disorder; however, little is known about its neurobiological underpinnings. In humans, BED is associated with desensitization of the reward system, specifically, the medial prefrontal cortex (mPFC), nucleus accumbens (Acb), and ventral tegmental area (VTA). Additionally, BED patients feel relieved during bingeing, suggesting that bingeing helps to decrease the negative emotions they were feeling prior to the binge episode. However, the mechanisms that underlie this feeling of relief in BED patients have not been well investigated. To investigate neuronal activity before and during palatable food consumption in BED, we performed in vivo electrophysiological recordings in a binge-like eating rat model (bingeing, n = 12 and non-bingeing, n = 14) and analyzed the firing rate of neurons in the mPFC, Acb, and VTA before and during access to sucrose solution. We also investigated changes in the firing rate of neurons in these regions during and between active bingeing, which may underlie the feeling of relief in BED patients. We found that neuronal firing rates of mPFC and VTA neurons in bingeing rats were lower than those in non-bingeing rats before and during sucrose consumption. Palatable food consumption increased neuronal firing rates during and between active bingeing in bingeing rats. Our results suggest a desynchronization in the activity of reward system regions, specifically in the mPFC, in bingeing rats, which may also contribute to BED. These results are consistent with those of functional magnetic resonance imaging (fMRI) studies that reported decreased activity in the reward system in BED patients. We propose that increased neuronal activity in the mPFC, Acb, or VTA produces an antidepressant effect in rats, which may underlie the sense of relief patients express during bingeing episodes.

A systematic review and meta-analysis of animal models of binge eating - Part 1: Definitions and food/drink intake outcomes

Binge eating involves consuming excessive amounts of food within a discrete period of time and is associated with significant impairments in binge-eating disorder and bulimia nervosa. While research on clinical binge eating has provided valuable aetiological insights, animal models allow for closer examination of environmental, biological, and developmental risk factors. Numerous animal models of binge eating exist and differ widely in operational definitions of bingeing, animal characteristics and methodological parameters. The current review aimed to synthesise the available published evidence on these models. A systematic review of binge definitions in 170 articles found most studies displayed good face validity. Meta-analyses on 150 articles confirmed that the amount of food or drink consumed by animals under binge conditions was larger than that of non-binge conditions across many protocols. The meta-regression revealed species, strain, and sex moderated binge effect size, with the largest effect observed in studies with female animals and mice. Risk of bias assessment identified that improved reporting of allocation, baseline characteristics and outcome assessment is required in future studies.

A review of sex differences in the mechanisms and drivers of overeating

Disordered eating is often associated with marked psychological and emotional distress, and severe adverse impact on quality of life. Several factors can influence eating behavior and drive food consumption in excess of energy requirements for homeostasis. It is well established that stress and negative affect contribute to the aetiology of eating disorders and weight gain, and there is substantial evidence suggesting sex differences in sub-clinical and clinical types of overeating. This review will examine how negative affect and stress shape eating behaviors, and how the relationship between the physiological, endocrine, and neural responses to stress and eating behaviors differs between men and women. We will examine several drivers of overeating and explore possible mechanisms underlying sex differences in eating behavior.

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.

Estrous Cycle Modulation of Feeding and Relaxin-3/Rxfp3 mRNA Expression: Implications for Estradiol Action

INTRODUCTION

Food intake varies during the ovarian hormone/estrous cycle in humans and rodents, an effect mediated mainly by estradiol. A potential mediator of the central anorectic effects of estradiol is the neuropeptide relaxin-3 (RLN3) synthetized in the nucleus incertus (NI) and acting via the relaxin family peptide-3 receptor (RXFP3).

METHODS

We investigated the relationship between RLN3/RXFP3 signaling and feeding behavior across the female rat estrous cycle. We used in situ hybridization to investigate expression patterns of Rln3 mRNA in NI and Rxfp3 mRNA in the hypothalamic paraventricular nucleus (PVN), lateral hypothalamic area (LHA), medial preoptic area (MPA), and bed nucleus of the stria terminalis (BNST), across the estrous cycle. We identified expression of estrogen receptors (ERs) in the NI using droplet digital PCR and assessed the electrophysiological responsiveness of NI neurons to estradiol in brain slices.

RESULTS

Rln3 mRNA reached the lowest levels in the NI pars compacta during proestrus. Rxfp3 mRNA levels varied across the estrous cycle in a region-specific manner, with changes observed in the perifornical LHA, magnocellular PVN, dorsal BNST, and MPA, but not in the parvocellular PVN or lateral LHA. G protein-coupled estrogen receptor 1 (Gper1) mRNA was the most abundant ER transcript in the NI. Estradiol inhibited 33% of type 1 NI neurons, including RLN3-positive cells.

CONCLUSION

These findings demonstrate that the RLN3/RXFP3 system is modulated by the estrous cycle, and although further studies are required to better elucidate the cellular and molecular mechanisms of estradiol signaling, current results implicate the involvement of the RLN3/RXFP3 system in food intake fluctuations observed across the estrous cycle in female rats.

Differential Expression of DeltaFosB in Reward Processing Regions Between Binge Eating Prone and Resistant Female Rats

Binge eating (BE) is characterized by the consumption of large amounts of palatable food in a discrete period and compulsivity. Even though BE is a common symptom in bulimia nervosa (BN), binge eating disorder (BED), and some cases of other specified feeding or eating disorders, little is known about its pathophysiology. We aimed to identify brain regions and neuron subtypes implicated in the development of binge-like eating in a female rat model. We separated rats into binge eating prone (BEP) and binge eating resistant (BER) phenotypes based on the amount of sucrose they consumed following foot-shock stress. We quantified deltaFosB (ΔFosB) expression, a stably expressed Fos family member, in different brain regions involved in reward, taste, or stress processing, to assess their involvement in the development of the phenotype. The number of ΔFosB-expressing neurons was: (1) higher in BEP than BER rats in reward processing areas [medial prefrontal cortex (mPFC), nucleus accumbens (Acb), and ventral tegmental area (VTA)]; (2) similar in taste processing areas [insular cortex, IC and parabrachial nucleus (PBN)]; and (3) higher in the paraventricular nucleus of BEP than BER rats, but not different in the locus coeruleus (LC), which are stress processing structures. To study subtypes of ΔFosB-expressing neurons in the reward system, we performed in situ hybridization for glutamate decarboxylase 65 and tyrosine hydroxylase (TH) mRNA after ΔFosB immunohistochemistry. In the mPFC and Acb, the proportions of γ-aminobutyric acidergic (GABAergic) and non-GABAergic ΔFosB-expressing neurons were similar in BER and BEP rats. In the VTA, while the proportion of dopaminergic ΔFosB-expressing neurons was similar in both phenotypes, the proportion of GABAergic ΔFosB-expressing neurons was higher in BER than BEP rats. Our results suggest that reward processing brain regions, particularly the VTA, are important for the development of binge-like eating.

Projections from the nucleus accumbens shell to the ventral pallidum are involved in the control of sucrose intake in adult female rats

In rodents, stimulation of the nucleus accumbens shell (AcbSh) directly or via its projection to the lateral hypothalamus (LH) attenuates food intake. The ventral pallidum (VP) receives dense projections from the AcbSh and is sensitive to the hedonic aspect of food and motivation for reward. However, the role of accumbal projections to the VP in the regulation of food intake was not well investigated. In the present study conducted on female rats, we examined the effects of stimulation of the AcbSh using optogenetics, or pharmacological inhibition of the rostral VP, or stimulation of projections from the AcbSh to the rostral VP using optogenetics on the consumption of 10% sucrose, lick microstructure and the expression of c-fos mRNA. Stimulation of the AcbSh, inhibition of the rostral VP with muscimol, or stimulation of axonal terminals from the AcbSh to the rostral VP resulted in a decrease in sucrose intake, meal duration, and total number of licks. The licking microstructure analysis showed that optogenetic stimulation of AcbSh or axonal terminals from the AcbSh to the rostral VP decreased the hedonic value of the sucrose. However, inhibition of the rostral VP decreased the motivation, whereas stimulation of the accumbal projections in the rostral VP increased the motivation to drink. This difference could be due to differential involvement of GABAergic and glutamatergic VP neurons. Stimulation of the AcbSh resulted in a decrease of c-fos mRNA expression in the LH and rostral VP, and stimulation of axonal terminals from the AcbSh to the rostral VP decreased c-fos mRNA expression only in the rostral VP. This study demonstrates that in adult female rats, in addition to the already known role of the AcbSh projections to the LH, AcbSh projections to the VP play a major role in the regulation of sucrose intake.

Mid-adolescent stress differentially affects binge-like intake of sucrose across estrous cycles in female rats✰

Binge eating disorder (BED), characterized by excessive food consumption within a discrete period of time, is the most prevalent of all eating disorders, with higher rates in women than men. Chronic stress, particularly during adolescence, is a significant risk factor for BED in women, but the mechanism underlying this relationship remains elusive. We investigated the phenomenon by testing the impact of mid-adolescent intermittent physical stress (IPS) on binge-like intake of sucrose in adult female rats, assessing how the behavior changed across reproductive cycles. One hundred and nineteen Long-Evans rats were exposed to IPS (n = 59) or no stress (NS; n = 60) for 12 days during mid-adolescence (PD35-46). Binge-like eating was induced in adult animals using an intermittent access protocol: animals were provided with 12 h or 24 h access to sucrose, 12 h access to saccharin, or 12 h access to food over 28 days. After 1- or 28-day abstinence, compulsive responding for sucrose was measured using a conditioned suppression paradigm. Rats given 12 h access to sucrose developed binge-like intake, measured as increased consumption during the first hour; the effect was magnified in IPS animals and most pronounced during proestrous. Solution intake in IPS rats was predicted by open arm entries in the elevated plus maze, suggesting that increased risk-taking behavior is associated with greater binge-like eating. IPS blocked conditioned suppression after 28 days of abstinence, pointing to a role of mid-adolescent stress in compulsivity. Collectively, these findings emphasize the impact of stress on the emergence of binge eating in females and suggest that intervention programs for women with a history of adolescent adversity should be investigated as a means to reduce risk for BED.

Insula to ventral striatal projections mediate compulsive eating produced by intermittent access to palatable food

Compulsive eating characterizes many binge-related eating disorders, yet its neurobiological basis is poorly understood. The insular cortex subserves visceral-emotional functions, including taste processing, and is implicated in drug craving and relapse. Here, via optoinhibition, we implicate projections from the anterior insular cortex to the nucleus accumbens as modulating highly compulsive-like food self-administration behaviors that result from intermittent access to a palatable, high-sucrose diet. We identified compulsive-like eating behavior in female rats through progressive ratio schedule self-administration and punishment-resistant responding, food reward tolerance and escalation of intake through 24-h energy intake and fixed-ratio operant self-administration sessions, and withdrawal-like irritability through the bottle brush test. We also identified an endocrine profile of heightened GLP-1 and PP but lower ghrelin that differentiated rats with the most compulsive-like eating behavior. Measures of compulsive eating severity also directly correlated to leptin, body weight and adiposity. Collectively, this novel model of compulsive-like eating symptoms demonstrates adaptations in insula-ventral striatal circuitry and metabolic regulatory hormones that warrant further study.

Reduced Medial Prefrontal Control of Palatable Food Consumption Is Associated With Binge Eating Proneness in Female Rats

Binge eating is the core, maladaptive eating behavior that cuts across several major types of eating disorders. Binge eating is associated with a significant loss of control over palatable food (PF) intake, and deficits in behavioral control mechanisms, subserved by the prefrontal cortex (PFC), may underlie binge eating. Few studies, to date, have examined whether the PFC is directly involved in the expression of binge eating. As such, the present study investigated the functional role of the medial PFC (mPFC) in PF consumption, using an individual differences rat model of binge eating proneness. Here, we tested the hypothesis that binge eating proneness (i.e., high levels of PF consumption) is associated with reduced mPFC-mediated behavioral control over PF intake. In experiment 1, we quantified PF-induced Fos expression in both excitatory and inhibitory neurons within the mPFC in binge eating prone (BEP) and binge eating resistant (BER) female rats. In experiment 2, we pharmacologically inactivated the mPFC of BEP and BER female rats, just prior to PF exposure, and subsequently quantified PF intake and scores of feeding behavior. While most Fos-expressing neurons of the mPFC in both BEPs and BERs were of the excitatory phenotype, fewer excitatory neurons were engaged by PF in BEPs than in BERs. Moreover, pharmacological inactivation of the mPFC led to a significant increase in PF intake in both BEPs and BERs, but the rise in PF consumption was stronger in BEPs than in BERs. Thus, these data suggest that lower, PF-induced excitatory tone in the mPFC of BEP rats may lead to a weaker, mPFC-mediated behavioral “brake” over excessive PF intake.

A Specialized Medical Management Program to Address Post-operative Weight Regain in Bariatric Patients

INTRODUCTION

Although bariatric surgery results in massive weight loss, weight regain over time up to as much as 25% is not uncommon. Weight regain in this population often leads to long-term weight loss failure and non-compliance in clinical follow-up and program recommendations.

METHODS

We analyzed early weight outcomes at 3 and 6 months of 48 bariatric patients referred to an individualized, multidisciplinary medical management program at the Center for Obesity Medicine (COM) to address weight regain in 2015 and compared to a group of matched non-bariatric patients. The medical management center, under the direction of a medical obesity specialist and complementary to the surgical program and multidisciplinary team, addressed weight regain with intensive lifestyle (diet, activity, anti-stress therapy, behavioral counseling, sleep) and with medical intervention (one or more anti-obesity medications).

RESULTS

According to early findings, the average percentage post-operative weight regain of patients entering the weight management program was 20% above nadir and time since surgery averaged 6 years (range = 1 to 20 years) with a mean weight loss of - 2.3 kg after 3 months and - 4.4 kg at 6 months into the program. Individuals most successful with weight loss were those treated with anorexigenic pharmaceuticals. Weight and percent weight loss were significantly greater for the non-surgical than the surgical patients at 3 and 6 months (p < 0.05).

CONCLUSIONS

A medically supervised weight management program complementary to surgery is beneficial for the treatment of weight regain and may prove important in assisting the surgical patient achieve long-term weight loss success.

Compulsive eating behaviors in Parkinson's disease

PURPOSE

Eating disorders are common in Parkinson's disease (PD) patients and often class in Impulse control disorders, however, little is known about their phenomenology. Specific symptoms and comorbidities were described in a group of PD patients in this preliminary study.

METHODS

Over a period of 6 months, 51 PD patients who experienced significant changes in eating habits following diagnosis of PD and were interviewed during regularly scheduled follow-up visits. We assessed each patient's height and weight, impulsivity, psychological distress, current eating disorder symptoms, food addiction, food habits and craving.

RESULTS

Among the PD patients who experienced modified dietary habits following diagnosis, few exhibited binge eating disorders (BED) full criteria (3.9%). However, 21.6% of patients experienced episodes of out-of-control eating with a large quantity of food in short time and 39.2% satisfied food addiction (FA) criteria without binge eating disorder. Food cravings more than once a week were experienced in approximately half of the population including all FA patients. Regarding comorbidities, FA PD patients present impulsive features and anxiety.

CONCLUSIONS

This study confirms the existence of FA profile in PD patients. Eating disorders even in PD are complex and have a cross-cutting criteria related to out-of-control eating, FA, and BED. The association of anxiety with PD-related food addiction, contrary to L-dopa equivalent daily dose mean score or the presence of dopamine agonists, underline the complex sustainability of the dopaminergic brainstem support. A study on their detailed prevalence in this population could be helpful to better understand unspecified feeding or eating disorder.

CLINICAL TRIAL NUMBER

DR-2012-007.

NAME OF THE REGISTRY

French Committee for the Protection of Persons (CPP) & French National Commission on Computing and Liberty (CNIL).

LEVEL OF EVIDENCE

Level V, descriptive study.

Stress as a potential moderator of ovarian hormone influences on binge eating in women

Previous research has demonstrated significant associations between increased levels of ovarian hormones and increased rates of binge eating (BE) in women. However, whereas all women experience fluctuations in ovarian hormones across the menstrual cycle, not all women binge eat in response to these fluctuations, suggesting that other factors must contribute. Stress is one potential contributing factor. Specifically, it may be that hormone-BE associations are stronger in women who experience high levels of stress, particularly as stress has been shown to be a precipitant to BE episodes in women. To date, no studies have directly examined stress as a moderator of hormone-BE associations, but indirect data (that is, associations between BE and stress and between ovarian hormones and stress) could provide initial clues about moderating effects. Given the above, the purpose of this narrative review was to evaluate these indirect data and their promise for understanding the role of stress in hormone-BE associations. Studies examining associations between all three phenotypes (that is, ovarian hormones, stress, and BE) in animals and humans were reviewed to provide the most thorough and up-to-date review of the literature on the potential moderating effects of stress on ovarian hormone-BE associations. Overall, current evidence suggests that associations between hormones and BE may be stronger in women with high stress levels, possibly via altered hypothalamic-pituitary-adrenal axis response to stress and increased sensitivity to and altered effects of ovarian hormones during stress. Additional studies are necessary to directly examine stress as a moderator of ovarian hormone-BE associations and identify the mechanisms underlying these effects.

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.

Intra-accumbens Raclopride Administration Prevents Behavioral Changes Induced by Intermittent Access to Sucrose Solution

Overeating is one of the most relevant clinical features in Binge Eating Disorder and in some obesity patients. According to several studies, alterations in the mesolimbic dopaminergic transmission produced by non-homeostatic feeding behavior may be associated with changes in the reward system similar to those produced by drugs of abuse. Although it is known that binge-eating is related with changes in dopaminergic transmission mediated by D2 receptors in the nucleus accumbens shell (NAcS), it has not been determined whether these receptors may be a potential target for the treatment of eating pathology with binge-eating. Accordingly, the aim of the present study was to evaluate whether sugar binging induced by intermittent access to a sucrose solution produced changes in the structure of feeding behavior and whether blocking D2 receptors prevented these changes. We used the intermittent access model to a 10% sucrose solution (2 h/day for 4 weeks) to induce sugar binging in Sprague Dawley female rats. Experimental subjects consumed in a 2-h period more than 50% of the caloric intake consumed by the subjects with ad-lib access to the sweetened solution without any increase in body weight or fat accumulation. Furthermore, we evaluated whether sugar binging was associated to the estrous cycle and we did not find differences in caloric intake (estrous vs. diestrus). Subsequently, we characterized the structure of feeding behavior (microstructural analysis) and the motivation for palatable food (breakpoints) of the subjects with sugar binging and found that feeding episodes had short latencies, high frequencies, as well as short durations and inter-episode intervals. The intermittent access model did not increase breakpoints, as occurred in subjects with ad-lib access to the sucrose. Finally, we evaluated the effects of D2 receptor blockade in the NAcS, and found that raclopride (18 nM) prevented the observed changes in the frequency and duration of episodes induced by intermittent access to the sucrose solution. Our results suggest that alterations in behavioral patterns associated with binge-eating behavior depend in part on the dopaminergic transmission in the NAcS and that the antagonism of D2 receptors may be a therapeutic tool for feeding pathology with binge-eating.

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.

Pathological Overeating: Emerging Evidence for a Compulsivity Construct

Compulsive eating behavior is a transdiagnostic construct that is characteristic of medical and psychiatric conditions such as forms of obesity and eating disorders. Although feeding research is moving toward a better understanding of the proposed addictive properties of food, the components and the mechanisms contributing to compulsive eating are not yet clearly defined or understood. Current understanding highlights three elements of compulsive behavior as it applies to pathological overeating: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. These elements emerge through mechanisms involving pathological habit formation through an aberrant learning process, the emergence of a negative emotional state, and dysfunctions in behavioral control. Dysfunctions in systems within neurocircuitries that comprise the basal ganglia, the extended amygdala, and the prefrontal cortex result in compulsive eating behaviors. Here, we present evidence to relate compulsive eating behavior and addiction and to characterize their underlying neurobiological mechanisms. A major need to improve understanding of compulsive eating through the integration of complex motivational, emotional, and cognitive constructs is warranted.

Polymorphisms of the oxytocin receptor gene and overeating: the intermediary role of endophenotypic risk factors

Background/Objectives:

Oxytocin (OXT) is an evolutionarily ancient neuropeptide with strong links to affiliative and prosocial behaviors, and the management of stress. Increases in OXT also tend to decrease food intake, especially of sweet carbohydrates. The social correlates of low OXT levels mesh with the social deficits and stress proneness identified in interpersonal models of overeating, as well as the increased appetite for highly palatable foods typically seen in chronic overeaters. The objectives of this study were to investigate links between polymorphisms of the oxytocin receptor (OXTR) gene and overeating, and to examine OXTR links with relevant endophenotypes of overeating related to reward and stress sensitivity, and to food preferences.

Subject/Methods:

The sample comprised 460 adults between the ages of 25 and 50 years recruited from the community, and representing a broad range of body weights. Overeating, reward and punishment sensitivity, and food preferences, were quantified as composite variables using well-validated questionnaires. In addition, seven single-nucleotide polymorphisms (rs237878, rs237885, rs2268493, rs2268494, rs2254298, rs53576, rs2268498) of the OXTR gene were genotyped.

Results:

Analyses identified a four-marker haplotype that was significantly related to food preferences. Individual genotype analyses also found that at least one of the markers was related to each of the phenotypic variables. In addition, an empirically derived structural equation model linking genetic and phenotype variables produced a good fit to the data.

Conclusions:

The results of this preliminary study have demonstrated that OXTR variation is associated with overeating, and with endophenotypic traits such as sweet and fatty food preferences, and reward and punishment sensitivity. In general, the genetic findings also favor the view that overeating may be associated with relatively low basal OXT levels.

Wistar-Kyoto Female Rats Are More Susceptible to Develop Sugar Binging: A Comparison with Wistar Rats

The hedonic component of the feeding behavior involves the mesolimbic reward system and resembles addictions. Nowadays, the excessive consumption of sucrose is considered addictive. The Wistar-Kyoto (WKY) rat strain is prone to develop anxiety and addiction-like behavior; nevertheless, a lack of information regarding their vulnerability to develop sugar binging-like behavior (SBLB) and how it affects the reward system persist. Therefore, the first aim of the present study was to compare the different predisposition of two rat strains, Wistar (W) and WKY to develop the SBLB in female and male rats. Also, we studied if the SBLB-inducing protocol produces changes in anxiety-like behavior using the plus-maze test (PMT) and, analyzed serotonin (5-HT) and noradrenaline (NA) concentrations in brain areas related to anxiety and ingestive behavior (brain stem, hypothalamus, nucleus accumbens, and amygdala). Finally, we evaluated whether fluoxetine, a drug that has been effective in reducing the binge-eating frequency, body weight, and severity of binge eating disorder, could also block this behavior. Briefly, WKY and W female rats were exposed to 30% sucrose solution (2 h, 3 days/week for 4 weeks), and fed up ad libitum. PMT was performed between the last two test periods. Immediately after the last test where sucrose access was available, rats were decapitated and brain areas extracted for high-performance liquid chromatography analysis. The results showed that both W and WKY female and male rats developed the SBLB. WKY rats consumed more calories and ingested a bigger amount of sucrose solution than their W counterpart. This behavior was reversed by using fluoxetine, rats exposed to the SBLB-inducing protocol presented a rebound effect during the washout period. On female rats, the SBLB-inducing protocol induced changes in NA concentrations on WKY, but not on W rats. No changes were found in 5-HT levels. Finally, animals that developed SBLB showed increased anxiety-like behavior in the PMT. In conclusion, WKY female rats can be considered as a more susceptible rat strain to develop SBLB.

Sex-specific effects of relaxin-3 on food intake and body weight gain

Relaxin-3 (RLN3) is a neuropeptide that is strongly expressed in the pontine nucleus incertus (NI) and binds with high affinity to its cognate receptor RXFP3. Central administration of RLN3 in rats increases food intake and adiposity. In humans, RLN3 polymorphism has been associated with obesity and hypercholesterolaemia. Emerging evidence suggests that the effects of RLN3 may have sex-specific aspects. Thus, the RLN3 knockout female but not male mice are hypoactive. RLN3 produced stronger orexigenic and obesogenic effects in female rats compared with male rats. In addition, female rats demonstrated higher sensitivity to lower doses of RLN3. Repeated cycles of food restriction and stress were accompanied by an increase in RLN3 expression and hyperphagia in female but not in male rats. Furthermore, stress-induced binge eating in female rats was blocked by an RXFP3 receptor antagonist. RLN3 increased the expression of corticotropin releasing factor in the paraventricular hypothalamic nucleus in male but not in female rats. Conversely, in female rats, RLN3 increased the expression of orexin in the lateral hypothalamus. There is evidence that orexin directly activates the RLN3 neurons in the NI. The positive reinforcement of the RLN3 effects by orexin may intensify behavioural activation and feeding in females. Sex-specific effects of RLN3 may also depend on differential expression of RXFP3 receptors in the brain. Given the higher sensitivity of females to the orexigenic effects of RLN3 and the stress-induced activation of RLN3, the overall data suggest a possible role for RLN3 in eating disorders that show a higher propensity in women.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Transient inactivation of the nucleus accumbens (NAc) shell prominently ameliorates responses to acute stress in female rats

It is clear that the stress response differs between females and males. Moreover, the nucleus accumbens shell is considered a critical area in the brain's response to stress. In the present study we investigate the effect of the transient inactivation of the NAc shell on the metabolic and hormonal disturbance induced by acute stress in female rats. Female Wistar rats were canulated uni- or bilaterally in the NAc shell before stress induction. Electric foot shock was applied to the animals and lidocaine hydrochloride was administered intra-acumbally 5min before each stress session. Food and water intake, delay to eating time, plasma corticosterone, estradiol and progesterone were measured after stress administration. Our results indicate that food intake increased whereas water intake decreased in stress group, furthermore, delay in the onset of eating also was observed. In addition, plasma corticosterone and progesterone concentrations were increased whereas estradiol was decreased. Transient inactivation of right side of NAc shell inhibited the stress effect on food intake; however, the effect of stress was exacerbated when the left part of the NAc shell was inhibited. Transient inactivation of both NAc shell sides augmented the stress effect on water intake. The same results also obtained for the effect of stress on delay to eating time. Lidocaine administration inhibited the effects of stress on all of the hormones we tested. It can be concluded that the NAc shell plays an important role in the mediation of stress responses in female rats and this influence shows asymmetrical manner.