Assessing binge eating. An analysis of data previously collected in bingeing rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) in the paraventricular nucleus of the thalamus: Influence on binge-type eating in male and female mice

Binge eating disorder, characterized by the overconsumption of food in a discrete time period, is the most common eating disorder in the United States, but its neurological basis is not fully understood. The paraventricular nucleus of the thalamus (PVT) is a limbic brain region implicated in eating, and the anorexigenic neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), is densely expressed in the PVT. This study sought to examine the possible involvement of PACAP in the PVT in binge-type eating. First, a model of binge-type eating was established in mice. Male and female C57BL/6J mice were given limited access to Milk Chocolate Ensure Plus® or had access only to chow and water. Under this model, while males and females both engaged in binge-type eating with Ensure, females engaged in this behavior to a greater degree than males. Next, the role of PACAP in the PVT was defined in relation to binge-type eating. Using quantitative real-time PCR, females were found to have higher baseline levels of PVT PACAP mRNA than males, but only males showed an increase in levels of PACAP after a history of binge-type eating, and only males showed a reduction in levels of PACAP immediately prior to a binge session. Using chemogenetics in PACAP-Cre transgenic mice on a C57BL/6J background, activation of PVT PACAP+ cells with a Cre-dependent Gq-DREADD was found to reduce binge-type eating, significantly in male but not female mice. These results indicate that PVT PACAP is involved in binge-type eating in a sex-dependent manner, with a decrease in PVT PACAP levels preceding binge-type eating in male mice, and enhanced PVT PACAP+ cell activity suppressing binge-type eating in male mice. Together, these results suggest that the PACAP system could be targeted in specific patient populations to help treat binge eating disorder.

Sex differences in regional gray matter density in pre-adolescent binge eating disorder: a voxel-based morphometry study

BACKGROUND

Binge eating disorder (BED) is a pernicious psychiatric disorder which is linked with broad medical and psychiatric morbidity, and obesity. While BED may be characterized by altered cortical morphometry, no evidence to date examined possible sex-differences in regional gray matter characteristics among those with BED. This is especially important to consider in children, where BED symptoms often emerge coincident with rapid gray matter maturation.

METHODS

Pre-adolescent, 9-10-year old boys (N = 38) and girls (N = 33) with BED were extracted from the 3.0 baseline (Year 0) release of the Adolescent Brain Cognitive Development Study. We investigated sex differences in gray matter density (GMD) via voxel-based morphometry. Control sex differences were also assessed in age and body mass index and developmentally matched control children (boys N = 36; girls N = 38). Among children with BED, we additionally assessed the association between dorsolateral prefrontal (dlPFC) GMD and parent-reported behavioral approach and inhibition tendencies.

RESULTS

Girls with BED uniquely demonstrate diffuse clusters of greater GMD (p < 0.05, Threshold Free Cluster Enhancement corrected) in the (i) left dlPFC (p = 0.003), (ii) bilateral dmPFC (p = 0.004), (iii) bilateral primary motor and somatosensory cortex (p = 0.0003) and (iv) bilateral precuneus (p = 0.007). Brain-behavioral associations suggest a unique negative correlation between GMD in the left dlPFC and behavioral approach tendencies among girls with BED.

CONCLUSIONS

Early-onset BED may be characterized by regional sex differences in terms of its underlying gray matter morphometry.

Dietary-induced binge-like eating impairs acoustic startle responses to acute nisoxetine in male mice

Sensorimotor gating disruptions have been noted in several psychiatric and neurodegenerative disorders. However, the involvement of sensorimotor gating processes in eating disorders has not been well characterized. Our objective was to examine the sensorimotor gating of the acoustic startle response following dietary-induced binge eating and high-fat diet (HFD) induced weight gain in male C57B/6J mice. Acute administration of the norepinephrine reuptake inhibitor, nisoxetine (0.5 and 5 mg/kg), and a dopamine reuptake inhibitor, GBR 12783 (1.6 and 16 mg/kg), were either given alone or in combination to assess norepinephrine and dopamine alterations, respectively. Male mice with repeated bouts of calorie restriction (Restrict) and with limited access to a sweetened fat food (Binge) demonstrated an escalation of intake over 2.5 weeks under standard chow conditions. Restrict Binge (RB) mice had a reduced startle response to the startle pulse (110 dB) compared with the Naive control group at 5 mg/kg nisoxetine. There was an overall effect of nisoxetine (0.5 and 5 mg/kg) to increase percent inhibition at pre-pulse (74 dB), %PP74. Under HFD conditions, the RB group did not demonstrate a binge-like eating phenotype. The RB group on HFD had a higher response to 74 dB with nisoxetine (5.0 mg/kg) compared with a combinational dose of nisoxetine (5.0 mg/kg) and GBR 12783 (1.6 mg/kg). These findings suggest that dietary conditions that promote binge-like eating can influence the central noradrenergic and dopaminergic controls of the acoustic startle response and potentially influence sensorimotor gating.

Sex-dependent role of orexin deficiency in feeding behavior and affective state of mice following intermittent access to a Western diet - Implications for binge-like eating behavior

Binge eating disorder is a debilitating disease characterized by recurrent episodes of excessive food consumption and associated with psychiatric comorbidities. Despite a growing body of research investigating the neurobiological underpinnings of eating disorders, specific treatments are lacking. Given its fundamental role in feeding behaviors, we investigated the role of the orexin (hypocretin) neuropeptide system in binge-like eating and associated phenotypes. Specifically, we submitted female and male orexin-deficient mice to a paradigm of intermittent access (once weekly for 24 h) to a Western diet (WD) to induce binge-like eating. Additionally, we measured their anxiety-like behavior and plasma corticosterone levels. All mice showed binge-like eating in response to the intermittent WD access, but females did so to a greater extent than males. While orexin deficiency did not affect binge-like eating in this paradigm, we found that female orexin-deficient mice generally weighed more, and they expressed increased hypophagia and stress levels compared to wild-type mice following binge-like eating episodes. These detrimental effects of orexin deficiency were marginal or absent in males. Moreover, male wild-type mice expressed post-binge anxiety, but orexin-deficient mice did not. In conclusion, these results extend our knowledge of orexin's role in dysregulated eating and associated negative affective states, and contribute to the growing body of evidence indicating a sexual dimorphism of the orexin system. Considering that many human disorders, and especially eating disorders, have a strong sex bias, our findings further emphasize the importance of testing both female and male subjects.

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.

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.

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.

The Disruptive Effects of Estrogen Removal before Puberty on Risk for Binge Eating in Female Rats

Recent research suggests that estrogen is protective against binge eating in adult females, and that pubertal estrogen may be critical for these effects. Nonetheless, to date, no study has examined the role of pubertal estrogen in adult binge eating phenotypes in females, potentially due to difficulties experimentally manipulating estrogen in humans to examine causal effects. We used a novel animal model to examine whether estrogen removal prior to puberty (via pre-pubertal ovariectomy (P-OVX)) increases rates of binge eating prone (BEP) phenotypes in adulthood in females. A total of 77 P-OVX and 79 intact rats were followed from pre-puberty into adulthood and phenotyped for BEP status in adulthood. Results showed significantly increased rates (~2-8x higher) of adult BEP phenotypes in P-OVX as compared to intact rats. Findings confirm that estrogen removal substantially increases later risk for binge eating in females, potentially by disrupting typical adolescent brain development.

The effect of the demyelinating agent cuprizone on binge-like eating of sweetened palatable food in female and male C57BL/6 substrains

Binge eating is a heritable symptom of eating disorders with an unknown genetic etiology. Rodent models for binge-like eating (BLE) of palatable food permit the study of genetic and biological mechanisms. We previously genetically mapped a coding mutation in Cyfip2 associated with increased BLE of sweetened palatable food in the C57BL/6NJ versus C57BL/6J substrain. The increase in BLE in C57BL/6NJ mice was associated with a decrease in transcription of genes enriched for myelination in the striatum. Here, we tested the hypothesis that decreasing myelin levels with the demyelinating agent cuprizone would enhance BLE. Mice were treated with a 0.3% cuprizone home cage diet for two weeks. Cuprizone induced similar weight loss in both substrains and sexes that recovered within 48 h after removal of cuprizone. Following a three-week recovery period, mice were trained for BLE in an intermittent, limited access procedure. Surprisingly, cuprizone significantly reduced BLE in male but not female C57BL/6NJ mice while having no effect in C57BL/6J mice. Cuprizone also reduced myelin basic protein (MBP) at seven weeks post-cuprizone removal while having no effect on myelin-associated glycoprotein at this time point. C57BL/6NJ mice also showed less MBP than C57BL/6J mice. There were no statistical interactions of Treatment with Sex on MBP levels, indicating that differences in MBP reduction are unlikely to account for sex differences in BLE. To summarize, cuprizone induced an unexpected, significant reduction in BLE in C57BL/6NJ males, which could indicate genotype-dependent sex differences in the biological mechanisms of BLE.

Short- and long-access palatable food self-administration results in different phenotypes of binge-type eating

Binge eating disorder (BED), the most common eating disorder in the United States, is characterized by binge-type eating and is associated with higher body mass index and greater motivation for food. This disorder tends to first appear in late adolescence or young adulthood and is more common in women than men. While some animal models of BED have recapitulated both the overeating and the excessive body weight / fat of BED, very few have examined the motivational aspects of this disorder or utilized young females as subjects. In the present study, female Long-Evans rats, starting in late adolescence, were trained in operant chambers to self-administer the highly palatable Milk Chocolate Ensure Plus®, in 30-minute ("short access") or 6-hour ("long access") sessions, 5 days per week, over 6.5 weeks. For comparison, other subjects were provided with Ensure ad libitum or maintained on chow and water only. Both short and long access to Ensure led rats to develop binge-type eating, measured as greater 30-minute caloric intake than rats with ad libitum or chow access and as increasing 30-minute intake across weeks. Compared to those with short access, rats with long access demonstrated moderately increased motivation for Ensure (measured by progressive ratio testing) and, compared to those with only chow access, they eventually showed significant hyperphagia on Ensure access days and hypophagia on non-access days. Rats with long access also showed greater body weight/fat than those maintained on chow. These findings suggest that, while both short and long operant access to Ensure causes young female rats to meet the definition of binge-type eating, they lead to different phenotypes of this behavior, with long access promoting the development of a greater number of features found in clinical BED. Ultimately, both models may be useful in future studies aimed at identifying the neurobiological basis of binge eating.

Cyfip1 Haploinsufficiency Increases Compulsive-Like Behavior and Modulates Palatable Food Intake in Mice: Dependence on Cyfip2 Genetic Background, Parent-of Origin, and Sex

Binge eating (BE) is a heritable trait associated with eating disorders and involves episodes of rapid, large amounts of food consumption. We previously identified cytoplasmic FMR1-interacting protein 2 (Cyfip2) as a genetic factor underlying compulsive-like BE in mice. CYFIP2 is a homolog of CYFIP1 which is one of four paternally-deleted genes in patients with Type I Prader-Willi Syndrome (PWS), a neurodevelopmental disorder whereby 70% of cases involve paternal 15q11-q13 deletion. PWS symptoms include hyperphagia, obesity (if untreated), cognitive deficits, and obsessive-compulsive behaviors. We tested whether Cyfip1 haploinsufficiency (+/-) would enhance compulsive-like behavior and palatable food (PF) intake in a parental origin- and sex-dependent manner on two Cyfip2 genetic backgrounds, including the BE-prone C57BL/6N (Cyfip2N/N) background and the BE-resistant C57BL/6J (Cyfip2J/J) background. Cyfip1+/- mice showed increased compulsive-like behavior on both backgrounds and increased PF intake on the Cyfip2N/N background. In contrast, maternal Cyfip1 haploinsufficiency on the BE-resistant Cyfip2J/J background induced a robust escalation in PF intake in wild-type Cyfip1J/J males while having no effect in Cyfip1J/- males. Notably, induction of behavioral phenotypes in wild-type males following maternal Fmr1+/- has previously been reported. In the hypothalamus, there was a paternally-enhanced reduction in CYFIP1 protein whereas in the nucleus accumbens, there was a maternally-enhanced reduction in CYFIP1 protein. Nochange in FMR1 protein (FMRP) was observed in Cyfip1+/- mice, regardless of parental origin. To summarize, Cyfip1 haploinsufficiency increased compulsive-like behavior and induced genetic background-dependent, sex-dependent, and parent-of-origin-dependent effects on PF consumption and CYFIP1 expression that could have relevance for neurodevelopmental and neuropsychiatric disorders.

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.

Genetic differences in the behavioral organization of binge eating, conditioned food reward, and compulsive-like eating in C57BL/6J and DBA/2J strains

Binge eating (BE) is a heritable symptom of eating disorders associated with anxiety, depression, malnutrition, and obesity. Genetic analysis of BE could facilitate therapeutic discovery. We used an intermittent, limited access BE paradigm involving sweetened palatable food (PF) to examine genetic differences in BE, conditioned food reward, and compulsive-like eating between C57BL/6J (B6J) and DBA/2J (D2J) inbred mouse strains. D2J mice showed a robust escalation in intake and conditioned place preference for the PF-paired side. D2J mice also showed a unique style of compulsive-like eating in the light/dark conflict test where they rapidly hoarded and consumed PF in the preferred unlit environment. BE and compulsive-like eating exhibited narrow-sense heritability estimates between 56 and 73%. To gain insight into the genetic basis, we phenotyped and genotyped a small cohort of 133 B6J × D2J-F₂ mice at the peak location of three quantitative trait loci (QTL) previously identified in F₂ mice for sweet taste (chromosome 4: 156 Mb), bitter taste (chromosome 6: 133 Mb) and behavioral sensitivity to drugs of abuse (chromosome 11: 50 Mb). The D2J allele on chromosome 6 was associated with greater PF intake on training days and greater compulsive-like PF intake, but only in males, suggesting that decreased bitter taste may increase BE in males. The D2J allele on chromosome 11 was associated with an increase in final PF intake and slope of escalation across days. Future studies employing larger crosses and genetic reference panels comprising B6J and D2J alleles will identify causal genes and neurobiological mechanisms.

Cytoplasmic FMR1-Interacting Protein 2 Is a Major Genetic Factor Underlying Binge Eating

BACKGROUND

Eating disorders are lethal and heritable; however, the underlying genetic factors are unknown. Binge eating is a highly heritable trait associated with eating disorders that is comorbid with mood and substance use disorders. Therefore, understanding its genetic basis will inform therapeutic development that could improve several comorbid neuropsychiatric conditions.

METHODS

We assessed binge eating in closely related C57BL/6 mouse substrains and in an F₂ cross to identify quantitative trait loci associated with binge eating. We used gene targeting to validate candidate genetic factors. Finally, we used transcriptome analysis of the striatum via messenger RNA sequencing to identify the premorbid transcriptome and the binge-induced transcriptome to inform molecular mechanisms mediating binge eating susceptibility and establishment.

RESULTS

C57BL/6NJ but not C57BL/6J mice showed rapid and robust escalation in palatable food consumption. We mapped a single genome-wide significant quantitative trait locus on chromosome 11 (logarithm of the odds = 7.4) to a missense mutation in cytoplasmic FMR1-interacting protein 2 (Cyfip2). We validated Cyfip2 as a major genetic factor underlying binge eating in heterozygous knockout mice on a C57BL/6N background that showed reduced binge eating toward a wild-type C57BL/6J-like level. Transcriptome analysis of premorbid genetic risk identified the enrichment terms morphine addiction and retrograde endocannabinoid signaling, whereas binge eating resulted in the downregulation of a gene set enriched for decreased myelination, oligodendrocyte differentiation, and expression.

CONCLUSIONS

We identified Cyfip2 as a major significant genetic factor underlying binge eating and provide a behavioral paradigm for future genome-wide association studies in populations with increased genetic complexity.

Sex Differences in Binge Eating: Gonadal Hormone Effects Across Development

Eating disorders are highly sexually differentiated disorders that exhibit a female predominance in risk. Most theories focus on psychosocial explanations to the exclusion of biological/genetic influences. The purpose of this descriptive review is to evaluate evidence from animal and human studies in support of gonadal hormone effects on sex differences in binge eating. Although research is in its nascent stages, findings suggest that increased prenatal testosterone exposure in males appears to protect against binge eating. Although pubertal testosterone may exert additional protective effects, the prenatal period is likely critical for the decreased risk observed in males. By contrast, studies indicate that, in females, it is the lack of prenatal testosterone coupled with the organizational effects of pubertal ovarian hormones that may lead to increased binge eating. Finally, twin data suggest that changes in genetic risk may underlie these hormone influences on sex differences across development.

Trait and state binge eating predispose towards cocaine craving

Binge eating (BE) and drug seeking share similar behavioral features, including loss of control over consumption and compulsive seeking of the craved substance. Previous studies in animal models have demonstrated a complex interaction between 'state' BE, produced by intermittent access to a palatable diet, and 'trait' BE, a phenotypical proneness towards overeating. In the present study, we examined the relationship between state and trait BE and cocaine seeking. We used Otsuka Long Evans Tokushima Fatty rats, a genetic model for obesity that demonstrates BE-like behavior, and Long Evans Tokushima Otsuka controls. They received a schedule of limited access to a palatable diet (3 days/week or 5 days/week access to Ensure for a month). Next, they underwent cocaine self-administration training (1 mg/kg, 1 hour/day for 10 days) followed by extinction sessions (7 days). We found that the degree of BE-like behavior and the state and trait BE combination predicted cocaine craving patterns. Lower levels of dopamine D2 receptors in the prefrontal cortex were correlated with increased drug craving. Moreover, restricted access to an attractive diet was found to be a risk factor for heightened cocaine craving, particularly in trait binge eaters, as rats on the 3 days/week access schedule persistently failed to cease cocaine seeking throughout extinction. Hence, we postulate a joint role of state and trait BE as risk factors for heightened cocaine craving.

Development of bingeing in rats altered by a small operant requirement

Previous studies have shown that providing an optional food for a brief period of time to non-food deprived rats on an intermittent basis in the home cage engenders significantly more intake (binge-type behavior) than when the optional food is provided for a brief period on a daily basis. Experiment 1 examined the effects of placing a small operant response requirement on access to an optional food (vegetable shortening) on the establishment of binge-type behavior. Experiment 2 examined the effects of different schedules of reinforcement, a period of abstinence from shortening, and 24h of food deprivation on established binge-type behavior. In Experiment 1 the group of rats with 30-min access to shortening on an intermittent basis in their home cages (IC) consumed significantly more shortening than the group with 30-min daily access in the home cage (DC). The group with 30-min intermittent access in an operant chamber (IO group) earned significantly more reinforcers than the group with 30-min daily access in an operant chamber (DO). In Experiment 2, the IO group earned significantly more reinforcers than the DO group regardless of the response cost, the period of shortening abstinence, and overnight food deprivation. These results demonstrate that while intermittent access generates binge-type eating, the size of the binge (intake) can be altered by different contingency arrangements.

Developmental emergence of an obsessive-compulsive phenotype and binge behavior in rats

RATIONALE

Obsessive-compulsive disorder (OCD) gradually emerges and reaches clinical significance during early adulthood. Whether a predisposition for OCD manifests as binge eating disorder earlier during adolescence is proposed.

OBJECTIVES

To further characterize how OCD-like behaviors increase across maturation and to determine whether an OCD-like predisposition increases the likelihood of binge eating during adolescence.

METHODS

Male and female Sprague-Dawley rats were injected with the tricyclic antidepressant clomipramine (CMI, 15 mg/kg) or saline vehicle twice daily between postnatal days 9-15. Both groups were tested for perseverative (spontaneous alternation) and anxiety-like (elevated plus maze; marble burying) behaviors during juvenility (day 28), adolescence (day 60), and adulthood (day 90). Both motivations to eat sucrose pellets and binge eating on fat were investigated.

RESULTS

Sex- and age-dependent increases in anxiety-like and perseverative behavior were observed in CMI subjects. Differences in consummatory behaviors emerged during late adolescence, while no significant differences in alternation or anxiety-like behaviors were detected between CMI and vehicle animals until adulthood. Adolescent CMI females consumed more sucrose pellets in 30 min relative to vehicle females, whereas adolescent CMI males consumed approximately half as much as vehicle males. Sucrose consumption did not differ between groups in adulthood. Adolescent CMI rats demonstrated more fat bingeing than vehicles, independent of sex.

CONCLUSIONS

OCD-like behaviors are emerging during adolescence, but sucrose consumption and fat bingeing in CMI-treated animals significantly precedes the appearance of anxiety and perseveration. This OCD-like phenotype emerges fully during adulthood, suggesting that eating may likely serve as a coping strategy in these animals.

Factors affecting the ability of baclofen to reduce fat intake in rats

The GABA-B agonist baclofen has been reported to reduce the consumption of vegetable shortening, but not lard, in rats. This study sought to examine some of the factors that could account for these differences. Baclofen (0, 1.0, 1.8, 3.2 mg/kg, intraperitoneal) was tested: (i) on shortening and lard intake, (ii) under 'binge-type' and non-'binge-type' conditions, (iii) when each fat was presented alone or simultaneously, and (iv) with a 30-min or no pretreatment time. With a 30-min pretreatment time, baclofen (3.2 mg/kg, intraperitoneal) consistently reduced shortening intake under 'binge-type' and non-'binge-type' conditions, as well as when shortening was presented alone or when lard was simultaneously available. Baclofen also reduced lard intake under 'binge-type' and non-'binge-type' conditions, but only when lard was presented alone. Baclofen had no effect on chow intake. When each fat was presented alone, and with no pretreatment time, the results were less consistent; baclofen reduced shortening intake only under non-'binge-type' conditions, and lard intake only under 'binge-type' conditions, and also stimulated chow intake. In summary, the type of fat, the presentation mode (one fat presented alone or two fats simultaneously), and the time between baclofen administration and intake all influence the ability of baclofen to reduce fat intake.

Dietary-induced binge eating increases prefrontal cortex neural activation to restraint stress and increases binge food consumption following chronic guanfacine

Binge eating is a prominent feature of bulimia nervosa and binge eating disorder. Stress or perceived stress is an often-cited reason for binge eating. One notion is that the neural pathways that overlap with stress reactivity and feeding behavior are altered by recurrent binge eating. Using young adult female rats in a dietary-induced binge eating model (30 min access to binge food with or without 24-h calorie restriction, twice a week, for 6 weeks) we measured the neural activation by c-Fos immunoreactivity to the binge food (vegetable shortening mixed with 10% sucrose) in bingeing and non-bingeing animals under acute stress (immobilization; 1 h) or no stress conditions. There was an increase in the number of immunopositive cells in the dorsal medial prefrontal cortex (mPFC) in stressed animals previously exposed to the binge eating feeding schedules. Because attention deficit hyperactive disorder (ADHD) medications target the mPFC and have some efficacy at reducing binge eating in clinical populations, we examined whether chronic (2 weeks; via IP osmotic mini-pumps) treatment with a selective alpha-2A adrenergic agonist (0.5 mg/kg/day), guanfacine, would reduce binge-like eating. In the binge group with only scheduled access to binge food (30 min; twice a week; 8 weeks), guanfacine increased total calories consumed during the 30-min access period from the 2-week pre-treatment baseline and increased binge food consumption compared with saline-treated animals. These experiments suggest that mPFC is differentially activated in response to an immobilization stress in animals under different dietary conditions and chronic guanfacine, at the dose tested, was ineffective at reducing binge-like eating.

Binge-like eating attenuates nisoxetine feeding suppression, stress activation, and brain norepinephrine activity

Stress is often associated with binge eating. A critical component of the control of stress is the central norepinephrine system. We investigated how dietary-induced binge eating alters central norepinephrine and related behaviors. Young male Sprague Dawley rats received calorie deprivation (24 h) and /or intermittent sweetened fat (vegetable shortening with sucrose; 30 min) twice a week for 10 weeks. The groups were Restrict Binge (calorie deprivation/sweetened fat), Binge (sweetened fat), Restrict (calorie deprivation), and Naive (no calorie deprivation/no sweetened fat). Dietary-induced binge eating was demonstrated by Restrict Binge and Binge, which showed an escalation in 30-min intake over time. Feeding suppression following nisoxetine (3 mg/kg; IP), a selective norepinephrine reuptake inhibitor, was not evident in Restrict Binge (Restrict Binge: 107±13, Binge: 52±9, Restrict: 80±8, Naive: 59±13% of saline injection at 1 h). In subsequent experiments with Restrict Binge and Naive, Restrict Binge had reduced corticosterone (Restrict Binge: 266±25; Naive: 494±36 ng/ml) and less feeding suppression (Restrict Binge: 81±12, Naive: 50±11% of non-restraint intake at 30 min) following restraint stress (1 h). Dietary-induced binge eating in Restrict Binge was not altered by a dorsal noradrenergic bundle lesion caused by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), but frontal cortex norepinephrine was positively correlated with the average 30-min intake post-lesion (0.69; p<0.01). In a separate set of animals, single-unit in vivo electrophysiological recording of locus coeruleus–norepinephrine neural activity demonstrated reduced sensory-evoked response as a consequence of the Restrict Binge schedule (Restrict Binge: 8.1±0.67, Naive: 11.9±1.09 Hz). These results, which suggest that a consequence of dietary-induced binge eating is to attenuate the responsiveness of the brain norepinephrine system, will further our understanding of how highly palatable foods dampen the stress neuraxis.

From passive overeating to "food addiction": a spectrum of compulsion and severity

A psychobiological dimension of eating behaviour is proposed, which is anchored at the low end by energy intake that is relatively well matched to energy output and is reflected by a stable body mass index (BMI) in the healthy range. Further along the continuum are increasing degrees of overeating (and BMI) characterized by more severe and more compulsive ingestive behaviours. In light of the many similarities between chronic binge eating and drug abuse, several authorities have adopted the perspective that an apparent dependence on highly palatable food-accompanied by emotional and social distress-can be best conceptualized as an addiction disorder. Therefore, this review also considers the overlapping symptoms and characteristics of binge eating disorder (BED) and models of food addiction, both in preclinical animal studies and in human research. It also presents this work in the context of the modern and "toxic" food environment and therein the ubiquitous triggers for over-consumption. We complete the review by providing evidence that what we have come to call "food addiction" may simply be a more acute and pathologically dense form of BED.

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.

Baclofen-induced reductions in optional food intake depend upon food composition

Baclofen reduces intake of some foods but stimulates intake or has no effect on others. The reasons for these differences are not known. The present study examined effects of baclofen when composition, energy density, preference, presentation and intake of optional foods varied. Semi-solid fat emulsions and sucrose products were presented for brief periods to non-food-deprived rats. In Experiment 1, fat and sucrose composition were varied while controlling energy density. In Experiment 2A, schedule of access and the number of optional foods were varied. In Experiment 2B, the biopolymer (thickener) was examined. Baclofen reduced intake of fat and/or sugar options with different energy densities (1.28-9kcal/g), when presented daily or intermittently, and when intakes were relatively high or low. However, the efficacy of baclofen was affected by the biopolymer used to thicken the options: baclofen had no effect when options were thickened with one biopolymer (3173), but reduced intake when options were thickened with another biopolymer (515). Baclofen failed to reduce intake of a concentrated sugar option (64% sucrose), regardless of biopolymer. Based upon these results, caution is urged when interpreting results obtained with products using different thickening agents. Systematic research is needed when designing products used in rat models of food intake.

U, R.L.W. C. 2-Hydroxyestradiol enhances binge onset in female rats and reduces prefrontal cortical dopamine in male rats

Women are more likely to suffer from a bingeing-related eating disorder, which is surprising, since estradiol reduces meal size and is associated with reduced binge frequency. This apparent contradiction may involve the estradiol metabolite, 2-hydroxyestradiol. We previously reported that female rats had faster escalations in shortening intake during the development of bingeing than did males, but acute administration of 2-hydroxyestradiol increased the intake of vegetable shortening to a greater extent in male rats once bingeing was established. Here, we report two separate studies that follow up these previous findings. In the first, we hypothesized that chronic exposure to 2-hydroxyestradiol would promote escalation of bingeing during binge development in ovariectomized female rats. In the second, we hypothesized that acute exposure to 2-hydroxyestradiol would enhance dopamine signaling in the prefrontal cortex after bingeing was established in male rats. In study 1, non-food-deprived female rats were separated into 3 groups: ovariectomized (OVX) with chronic 2-hydroxyestradiol supplementation (E), OVX with vehicle supplementation (O), and intact with vehicle (I). Each group was given access to an optional source of dietary fat (shortening) on Mon, Wed, and Fri for 4 weeks. 2-hydroxyestradiol supplementation prevented OVX-induced weight gain and enhanced escalation of shortening intake over the four-week period (ps<0.05). Additionally, in week 4, rats in the E group ate significantly more shortening than I controls, less chow than either the O or I group, and had a higher shortening to chow ratio than O or I (ps<0.05). Study 2 indicated that acute injection of 2-hydroxyestradiol abolished shortening-evoked dopamine efflux in the prefrontal cortex of bingeing male rats (p<0.05). Together, these studies indicate that 2-hydroxyestradiol can exacerbate bingeing as it develops and can suppress dopamine signaling in the prefrontal cortex once bingeing is established.