Escalation in high fat intake in a binge eating model differentially engages dopamine neurons of the ventral tegmental area and requires ghrelin signaling

Orexin-mediated motivated arousal and reward seeking

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

Evaluating the efficacy of the selective orexin 1 receptor antagonist nivasorexant in an animal model of binge-eating disorder

OBJECTIVE

Test the efficacy of the selective orexin 1 receptor (OX1R) antagonist (SO1RA) nivasorexant in an animal model of binge-eating disorder (BED) and study its dose-response relationship considering free brain concentrations and calculated OX1R occupancy. Compare nivasorexant's profile to that of other, structurally diverse SO1RAs. Gain understanding of potential changes in orexin-A (OXA) neuropeptide and deltaFosB (ΔFosB) protein expression possibly underlying the development of the binge-eating phenotype in the rat model used.

METHOD

Binge-like eating of highly palatable food (HPF) in rats was induced through priming by intermittent, repeated periods of dieting and access to HPF, followed by an additional challenge with acute stress. Effects of nivasorexant were compared to the SO1RAs ACT-335827 and IDOR-1104-2408. OXA expression in neurons and neuronal fibers as well as ΔFosB and OXA-ΔFosB co-expression was studied in relevant brain regions using immuno- or immunofluorescent histochemistry.

RESULTS

All SO1RAs dose-dependently reduced binge-like eating with effect sizes comparable to the positive control topiramate, at unbound drug concentrations selectively blocking brain OX1Rs. Nivasorexant's efficacy was maintained upon chronic dosing and under conditions involving more frequent stress exposure. Priming for binge-like eating or nivasorexant treatment resulted in only minor changes in OXA or ΔFosB expression in few brain areas.

DISCUSSION

Selective OX1R blockade reduced binge-like eating in rats. Neither ΔFosB nor OXA expression proved to be a useful classifier for their binge-eating phenotype. The current results formed the basis for a clinical phase II trial in BED, in which nivasorexant was unfortunately not efficacious compared with placebo.

PUBLIC SIGNIFICANCE

Nivasorexant is a new investigational drug for the treatment of binge-eating disorder (BED). It underwent clinical testing in a phase II proof of concept trial in humans but was not efficacious compared with placebo. The current manuscript investigated the drug's efficacy in reducing binge-like eating behavior of a highly palatable sweet and fat diet in a rat model of BED, which initially laid the foundation for the clinical trial.

Genetic or pharmacological GHSR blockade has sexually dimorphic effects in rodents on a high-fat diet

The stomach-derived hormone ghrelin regulates essential physiological functions. The ghrelin receptor (GHSR) has ligand-independent actions; therefore, GHSR gene deletion may be a reasonable approach to investigate the role of this system in feeding behaviors and diet-induced obesity (DIO). Here, we investigate the effects of a long-term (12-month) high-fat (HFD) versus regular diet on obesity-related measures in global GHSR-KO and wild-type (WT) Wistar male and female rats. Our main findings are that the GHSR gene deletion protects against DIO and decreases food intake during HFD in male but not in female rats. GHSR gene deletion increases thermogenesis and brain glucose uptake in male rats and modifies the effects of HFD on brain glucose metabolism in a sex-specific manner, as assessed with small animal positron emission tomography. We use RNA-sequencing to show that GHSR-KO rats have upregulated expression of genes responsible for fat oxidation in brown adipose tissue. Central administration of a novel GHSR inverse agonist, PF-5190457, attenuates ghrelin-induced food intake, but only in male, not in female mice. HFD-induced binge-like eating is reduced by inverse agonism in both sexes. Our results support GHSR as a promising target for new pharmacotherapies for obesity.

Genetic or pharmacological GHSR blockade has sexually dimorphic effects in rodents on a high-fat diet

The stomach-derived hormone ghrelin regulates essential physiological functions. The ghrelin receptor (GHSR) has ligand-independent actions, therefore, GHSR gene deletion may be a reasonable approach to investigate the role of this system in feeding behaviors and diet-induced obesity (DIO). Here we investigated the effects of a long-term (12 month) high-fat (HFD) versus regular diet on obesity-related measures in global GHSR-KO and wild type (WT) Wistar male and female rats. Our main findings were that the GHSR gene deletion protects against DIO and decreases food intake during HFD in male but not in female rats. GHSR gene deletion increased thermogenesis and brain glucose uptake in male rats and modified the effects of HFD on brain glucose metabolism in a sex-specific manner, as assessed with small animal positron emission tomography. RNA-sequencing was also used to show that GHSR-KO rats had upregulated expression of genes responsible for fat oxidation in brown adipose tissue. Central administration of a novel GHSR inverse agonist, PF-5190457, attenuated ghrelin-induced food intake, but only in male, not in female mice. HFD-induced binge-like eating was reduced by inverse agonism in both sexes. Our results support GHSR as a promising target for new pharmacotherapies for obesity.

Male mice engaging differently in emotional eating present distinct plasmatic and neurological profiles

Objective: Stressed individuals tend to turn to calorie-rich food, also known as 'comfort food' for the temporary relief it provides. The emotional eating drive is highly variable among subjects. Using a rodent model, we explored the plasmatic and neurobiological differences between 'high and low emotional eaters' (HEE and LEE).Methods: 40 male mice were exposed for 5 weeks to a protocol of unpredictable chronic mild stress. Every 3 or 4 days, they were submitted to a 1-h restraint stress, immediately followed by a 3-h period during which a choice between chow and chocolate sweet cereals was proposed. The dietary intake was measured by weighing. Plasmatic and neurobiological characteristics were compared in mice displaying high vs low intakes.Results: Out of 40 mice, 8 were considered as HEE because of their high post-stress eating score, and 8 as LEE because of their consistent low intake. LEE displayed higher plasma corticosterone and lower levels of NPY than HEE, but acylated and total ghrelin were similar in both groups. In the brain, the abundance of NPY neurons in the arcuate nucleus of the hypothalamus was similar in both groups, but was higher in the ventral hippocampus and the basal lateral amygdala of LEE. The lateral hypothalamus LEE had also more orexin (OX) positive neurons. Both NPY and OX are orexigenic peptides and mood regulators.Discussion: Emotional eating difference was reflected in plasma and brain structures implicated in emotion and eating regulation. These results concur with the psychological side of food consumption.

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.

Metabolic hormone action in the VTA: Reward-directed behavior and mechanistic insights

Dysfunctional signaling in midbrain reward circuits perpetuates diseases characterized by compulsive overconsumption of rewarding substances such as substance abuse, binge eating disorder, and obesity. Ventral tegmental area (VTA) dopaminergic activity serves as an index for how rewarding stimuli are perceived and triggers behaviors necessary to obtain future rewards. The evolutionary linking of reward with seeking and consuming palatable foods ensured an organism's survival, and hormone systems that regulate appetite concomitantly developed to regulate motivated behaviors. Today, these same mechanisms serve to regulate reward-directed behavior around food, drugs, alcohol, and social interactions. Understanding how hormonal regulation of VTA dopaminergic output alters motivated behaviors is essential to leveraging therapeutics that target these hormone systems to treat addiction and disordered eating. This review will outline our current understanding of the mechanisms underlying VTA action of the metabolic hormones ghrelin, glucagon-like peptide-1, amylin, leptin, and insulin to regulate behavior around food and drugs of abuse, highlighting commonalities and differences in how these five hormones ultimately modulate VTA dopamine signaling.

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Systematic Review of Binge Eating Rodent Models for Developing Novel or Repurposing Existing Pharmacotherapies

Recent advances in developing and screening candidate pharmacotherapies for psychiatric disorders have depended on rodent models. Eating disorders are a set of psychiatric disorders that have traditionally relied on behavioral therapies for effective long-term treatment. However, the clinical use of Lisdexamfatamine for binge eating disorder (BED) has furthered the notion of using pharmacotherapies for treating binge eating pathologies. While there are several binge eating rodent models, there is not a consensus on how to define pharmacological effectiveness within these models. Our purpose is to provide an overview of the potential pharmacotherapies or compounds tested in established rodent models of binge eating behavior. These findings will help provide guidance for determining pharmacological effectiveness for potential novel or repurposed pharmacotherapies.

Use of experimental medicine approaches for the development of novel psychiatric treatments based on orexin receptor modulation

Despite progress in understanding the pathological mechanisms underlying psychiatric disorders, translation from animal models into clinical use remains a significant bottleneck. Preclinical studies have implicated the orexin neuropeptide system as a potential target for psychiatric disorders through its role in regulating emotional, cognitive, and behavioral processes. Clinical studies are investigating orexin modulation in addiction and mood disorders. Here we review performance-outcome measures (POMs) arising from experimental medicine research methods which may show promise as markers of efficacy of orexin receptor modulators in humans. POMs provide objective measures of brain function, complementing patient-reported or clinician-observed symptom evaluation, and aid the translation from preclinical to clinical research. Significant challenges include the development, validation, and operationalization of these measures. We suggest that collaborative networks comprising clinical practitioners, academics, individuals working in the pharmaceutical industry, drug regulators, patients, patient advocacy groups, and other relevant stakeholders may provide infrastructure to facilitate validation of experimental medicine approaches in translational research and in the implementation of these approaches in real-world clinical practice.

Fragmented day-night cycle induces reduced light avoidance, excessive weight gain during early development, and binge-like eating during adulthood in mice

Fragmented day-night (FDN) cycles are environments in which multiple periods of light and dark alternate across a 24 h period. Exposure to FDN cycles disrupts circadian rhythms, resulting in period lengthening and alterations to mood in mice. A constant light environment, which also induces period lengthening, is linked to mood and metabolic disturbances and disruption to the development of the circadian clock. This study aims to determine how exposure to the FDN cycle impacts development in mice, with the hypothesis that there would be similar and adverse effects as observed in constant light conditions. Our study used CD-1 mice reared under the FDN cycle compared to the commonly used 12 h light: 12 h dark consolidated day-night cycle. During the first week of development, mouse pups reared under the FDN cycle gained bodyweight at a faster rate and did not avoid aberrant light exposure in comparison to 12:12 LD reared mouse pups. Developmental exposure to the FDN cycle lasted two weeks, and then mice were transferred to the 12:12 LD cycle, where after 2 weeks, bodyweight was similar between FDN reared and 12:12 LD reared mice at 1-month and 2-months old. When re-exposed to the FDN cycle during adulthood, FDN reared pups exhibited binge-like eating behaviors and reduced light avoidance. This study shows that the unnatural distribution of light and dark across the 24 h day can cause disruptions during early development that can reappear during adulthood when placed in the same stressful light-dark environment as adults.

Ghrelin deficiency sex-dependently affects food intake, locomotor activity, and adipose and hepatic gene expression in a binge-eating mouse model

Binge-eating disorder is the most prevalent eating disorder diagnosed, affecting three times more women than men. Ghrelin stimulates appetite and reward signaling, and loss of its receptor reduces binge-eating behavior in male mice. Here, we examined the influence of ghrelin itself on binge-eating behavior in both male and female mice. Five-wk-old wild-type (WT) and ghrelin-deficient (Ghrl^-/-) mice were housed individually in indirect calorimetry cages for 9 wks. Binge-like eating was induced by giving mice ad libitum chow, but time-restricted access to a Western-style diet (WD; 2 h access, 3 days/wk) in the light phase (BE); control groups received ad libitum chow (CO), or ad libitum access to both diets (CW). All groups of BE mice showed binge-eating behavior, eating up to 60% of their 24-h intake during the WD access period. Subsequent dark phase chow intake was decreased in Ghrl^-/- mice and remained decreased in Ghrl^-/- females on nonbinge days. Also, nonbinge day locomotor activity was lower in Ghrl^-/- than in WT BE females. Upon euthanasia, Ghrl^-/- BE mice weighed less and had a lower lean body mass percentage than WT BE mice. In BE and CW groups, ghrelin and sex altered the expression of genes involved in lipid processing, thermogenesis, and aging in white adipose tissue and livers. We conclude that, although ghrelin deficiency does not hamper the development of binge-like eating, it sex-dependently alters food intake timing, locomotor activity, and metabolism. These results add to the growing body of evidence that ghrelin signaling is sexually dimorphic.NEW & NOTEWORTHY Ghrelin, a peptide hormone secreted from the gut, is involved in hunger and reward signaling, which are altered in binge-eating disorder. Although sex differences have been described in both binge-eating and ghrelin signaling, this interaction has not been fully elucidated. Here, we show that ghrelin deficiency affects the behavior and metabolism of mice in a binge-like eating paradigm, and that the sex of the mice impacts the magnitude and direction of these effects.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

Growth hormone secretagogue receptor signaling in the supramammillary nucleus targets nitric oxide-producing neurons and controls recognition memory in mice

Ghrelin is a stomach-derived hormone that acts via the growth hormone secretagogue receptor (GHSR). Recent evidence suggests that some of ghrelin's actions may be mediated via the supramammillary nucleus (SuM). Not only does ghrelin bind to cells within the mouse SuM, but ghrelin also activates SuM cells and intra-SuM ghrelin administration induces feeding in rats. In the current study, we aimed to further characterize ghrelin action in the SuM. We first investigated a mouse model expressing enhanced green fluorescent protein (eGFP) under the promoter of GHSR (GHSR-eGFP mice). We found that the SuM of GHSR-eGFP mice contains a significant amount of eGFP cells, some of which express neuronal nitric oxide synthase. Centrally-, but not systemically-, injected ghrelin reached the SuM, where it induced c-Fos expression. Furthermore, a 5-day 40% calorie restriction protocol, but not a 2-day fast, increased c-Fos expression in non-eGFP+ cells of the SuM of GHSR-eGFP mice, whereas c-Fos induction by calorie restriction was not observed in GHSR-deficient mice. Exposure of satiated mice to a binge-like eating protocol also increased c-Fos expression in non-eGFP+ cells of the SuM of GHSR-eGFP mice in a GHSR-dependent manner. Finally, intra-SuM-injected ghrelin did not acutely affect food intake, locomotor activity, behavioral arousal or spatial memory but increased recognition memory. Thus, we provide a compelling neuroanatomical characterization of GHSR SuM neurons and its behavioral implications in mice.

Gut peptide regulation of food intake - evidence for the modulation of hedonic feeding

The number of people living with obesity has tripled worldwide since 1975 with serious implications for public health, as obesity is linked to a significantly higher chance of early death from associated comorbidities (metabolic syndrome, type 2 diabetes, cardiovascular disease and cancer). As obesity is a consequence of food intake exceeding the demands of energy expenditure, efforts are being made to better understand the homeostatic and hedonic mechanisms governing food intake. Gastrointestinal peptides are secreted from enteroendocrine cells in response to nutrient and energy intake, and modulate food intake either via afferent nerves, including the vagus nerve, or directly within the central nervous system, predominantly gaining access at circumventricular organs. Enteroendocrine hormones modulate homeostatic control centres at hypothalamic nuclei and the dorso-vagal complex. Additional roles of these peptides in modulating hedonic food intake and/or preference via the neural systems of reward are starting to be elucidated, with both peripheral and central peptide sources potentially contributing to central receptor activation. Pharmacological interventions and gastric bypass surgery for the treatment of type 2 diabetes and obesity elevate enteroendocrine hormone levels and also alter food preference. Hence, understanding of the hedonic mechanisms mediated by gut peptide action could advance development of potential therapeutic strategies for the treatment of obesity and its comorbidities.

Severe Hyperprolactinemia Promotes Brown Adipose Tissue Whitening and Aggravates High Fat Diet Induced Metabolic Imbalance

BACKGROUND

The association of high serum prolactin and increased body weight is positive but controversial, therefore we hypothesized that additional factors such as diets and the impact of prolactin on brown adipose tissue may condition its metabolic effects.

METHODS

We used LacDrd2KO females with lifelong severe hyperprolactinemia due dopamine-D2 receptor deletion from lactotropes, and slow onset of metabolic disturbances, and compared them to their respective controls (Drd2 ^loxP/loxP ). Food intake, and binge eating was evaluated. We then challenged mice with a High Fat (HFD) or a Control Diet (CD) for 8 weeks, beginning at 3 months of age, when no differences in body weight are found between genotypes. At the end of the protocol brown and white adipose tissues were weighed, and thermogenic and lipogenic markers studied, using real time PCR (Ucp1, Cidea, Pgc1a, Lpl, adiponectin, Prlr) or immunohistochemistry (UCP1). Histochemical analysis of brown adipose tissue, and glucose tolerance tests were performed.

RESULTS

Hyperprolactinemic mice had increased food intake and binge eating behavior. Metabolic effects induced by a HFD were exacerbated in lacDrd2KO mice. Hyperprolactinemia aggravated HFD-induced body weight gain and glucose intolerance. In brown adipose tissue pronounced cellular whitening as well as decreased expression of the thermogenic markers Ucp1 and Pgc1a were observed in response to high prolactin levels, regardless of the diet, and furthermore, hyperprolactinemia potentiated the decrease in Cidea mRNA expression induced by HFD. In subcutaneous white adipose tissue hyperprolactinemia synergistically increased tissue weight, while decreasing Prlr, Adiponectin and Lpl mRNA levels regardless of the diet.

CONCLUSIONS

Pathological hyperprolactinemia has a strong impact in brown adipose tissue, lowering thermogenic markers and evoking tissue whitening. Furthermore, it modifies lipogenic markers in subcutaneous white adipose, and aggravates HFD-induced glucose intolerance and Cidea decrease. Therefore, severe high prolactin levels may target BAT function, and furthermore represent an adjuvant player in the development of obesity induced by high fat diets.

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.

Gut hormones, adipokines, and pro- and anti-inflammatory cytokines/markers in loss of control eating: A scoping review

Loss of control (LOC) eating is the defining feature of binge-eating disorder, and it has particular relevance for bariatric patients. The biomarkers of LOC eating are unclear; however, gut hormones (i.e., ghrelin, cholecystokinin [CCK], peptide YY [PYY], glucagon-like peptide 1 [GLP-1], and pancreatic polypeptide [PP]), adipokines (i.e., leptin, adiponectin), and pro- and anti-inflammatory cytokines/markers (e.g., high-sensitivity C-reactive protein [hsCRP]) are candidates due to their involvement in the psychophysiological mechanisms of LOC eating. This review aimed to synthesize research that has investigated these biomarkers with LOC eating. Because LOC eating is commonly examined within the context of binge-eating disorder, is sometimes used interchangeably with subclinical binge-eating, and is the latent construct underlying disinhibition, uncontrolled eating, and food addiction, these eating behaviors were included in the search. Only studies among individuals with overweight or obesity were included. Among the identified 31 studies, 2 studies directly examined LOC eating and 4 studies were conducted among bariatric patients. Most studies were case-control in design (n = 16) and comprised female-dominant (n = 13) or female-only (n = 13) samples. Studies generally excluded fasting total ghrelin, fasting CCK, fasting PYY, and fasting PP as correlates of the examined eating behaviors. However, there was evidence that the examined eating behaviors were associated with lower levels of fasting acyl ghrelin (the active form of ghrelin) and adiponectin, higher levels of leptin and hsCRP, and altered responses of postprandial ghrelin, CCK, and PYY. The use of GLP-1 analog was able to decrease binge-eating. In conclusion, this review identified potential biomarkers of LOC eating. Future studies would benefit from a direct focus on LOC eating (especially in the bariatric population), using longitudinal designs, exploring potential mediators and moderators, and increased inclusion of the male population.

GABAB receptor signaling in the caudate putamen is involved in binge-like consumption during a high fat diet in mice

Previous studies suggest that signaling by the gamma-aminobutyric acid (GABA) type B receptor (GABABR) is involved in the regulation of binge eating, a disorder which might contribute to the development of obesity. Here, we show that intermittent access to a high fat diet (HFD) induced binge-like eating behavior with activation of dopamine receptor d1 (drd1)-expressing neurons in the caudate putamen (CPu) and nucleus accumbens (NAc) in wild-type (WT) mice. The activation of drd1-expressing neurons during binge-like eating was substantially increased in the CPu, but not in the NAc, in corticostriatal neuron-specific GABABR-deficient knockout (KO) mice compared to WT mice. Treatment with the GABABR agonist, baclofen, suppressed binge-like eating behavior in WT mice, but not in KO mice, as reported previously. Baclofen also suppressed the activation of drd1-expressing neurons in the CPu, but not in the NAc, during binge-like eating in WT mice. Thus, our data suggest that GABABR signaling in CPu neurons expressing drd1 suppresses binge-like consumption during a HFD in mice.

Assessing the role of ghrelin and the enzyme ghrelin O-acyltransferase (GOAT) system in food reward, food motivation, and binge eating behavior

The peripheral peptide hormone ghrelin is a powerful stimulator of food intake, which leads to body weight gain and adiposity in both rodents and humans. The hormone, thus, increases the vulnerability to obesity and binge eating behavior. Several studies have revealed that ghrelin's functions are due to its interaction with the growth hormone secretagogue receptor type 1a (GHSR1a) in the hypothalamic area; besides, ghrelin also promotes the reinforcing properties of hedonic food, acting at extra-hypothalamic sites and interacting with dopaminergic, cannabinoid, opioid, and orexin signaling. The hormone is primarily present in two forms in the plasma and the enzyme ghrelin O-acyltransferase (GOAT) allows the acylation reaction which causes the transformation of des-acyl-ghrelin (DAG) to the active form acyl-ghrelin (AG). DAG has been demonstrated to show antagonist properties; it is metabolically active, and counteracts the effects of AG on glucose metabolism and lipolysis, and reduces food consumption, body weight, and hedonic feeding response. Both peptides seem to influence the hypothalamic-pituitary-adrenal (HPA) axis and the corticosterone/cortisol level that drive the urge to eat under stressful conditions. These findings suggest that DAG and inhibition of GOAT may be targets for obesity and bingeing-related eating disorders and that AG/DAG ratio may be an important potential biomarker to assess the risk of developing maladaptive eating behaviors.

Pairing Binge Drinking and a High-Fat Diet in Adolescence Modulates the Inflammatory Effects of Subsequent Alcohol Consumption in Mice

Alcohol binge drinking (BD) and poor nutritional habits are two frequent behaviors among many adolescents that alter gut microbiota in a pro-inflammatory direction. Dysbiotic changes in the gut microbiome are observed after alcohol and high-fat diet (HFD) consumption, even before obesity onset. In this study, we investigate the neuroinflammatory response of adolescent BD when combined with a continuous or intermittent HFD and its effects on adult ethanol consumption by using a self-administration (SA) paradigm in mice. The inflammatory biomarkers IL-6 and CX3CL1 were measured in the striatum 24 h after BD, 3 weeks later and after the ethanol (EtOH) SA. Adolescent BD increased alcohol consumption in the oral SA and caused a greater motivation to seek the substance. Likewise, mice with intermittent access to HFD exhibited higher EtOH consumption, while the opposite effect was found in mice with continuous HFD access. Biochemical analyses showed that after BD and three weeks later, striatal levels of IL-6 and CX3CL1 were increased. In addition, in saline-treated mice, CX3CL1 was increased after continuous access to HFD. After oral SA procedure, striatal IL-6 was increased only in animals exposed to BD and HFD. In addition, striatal CX3CL1 levels were increased in all BD- and HFD-exposed groups. Overall, our findings show that adolescent BD and intermittent HFD increase adult alcohol intake and point to neuroinflammation as an important mechanism modulating this interaction.

Adaptive Changes in the Central Control of Energy Homeostasis Occur in Response to Variations in Energy Status

Energy homeostasis is regulated in coordinate fashion by the brain-gut axis, the homeostatic energy balance circuitry in the hypothalamus and the hedonic energy balance circuitry comprising the mesolimbcortical A10 dopamine pathway. Collectively, these systems convey and integrate information regarding nutrient status and the rewarding properties of ingested food, and formulate it into a behavioral response that attempts to balance fluctuations in consumption and food-seeking behavior. In this review we start with a functional overview of the homeostatic and hedonic energy balance circuitries; identifying the salient neural, hormonal and humoral components involved. We then delve into how the function of these circuits differs in males and females. Finally, we turn our attention to the ever-emerging roles of nociceptin/orphanin FQ (N/OFQ) and pituitary adenylate cyclase-activating polypeptide (PACAP)-two neuropeptides that have garnered increased recognition for their regulatory impact in energy homeostasis-to further probe how the imposed regulation of energy balance circuitry by these peptides is affected by sex and altered under positive (e.g., obesity) and negative (e.g., fasting) energy balance states. It is hoped that this work will impart a newfound appreciation for the intricate regulatory processes that govern energy homeostasis, as well as how recent insights into the N/OFQ and PACAP systems can be leveraged in the treatment of conditions ranging from obesity to anorexia.

Opioid System Contributes to the Trifluoromethyl-Substituted Diselenide Effectiveness in a Lifestyle-Induced Depression Mouse Model

Energy-dense foods and ethanol consumption are associated with mood disorders. m-Trifluoromethyl-diphenyl diselenide [(m-CF₃-PhSe)₂] has been a prominent pharmacological target due to its antidepressant-like effects. This study investigated if the modulation of opioid and glucocorticoid receptors and its well-known antioxidant property contribute to the (m-CF₃-PhSe)₂ antidepressant-like effect in young mice subjected to an energy-dense diet and ethanol intake. Swiss male mice [postnatal day (PND) 25] were exposed to an energy-dense diet (containing 20% fat and 20% carbohydrate) or standard chow until the PND 67. Mice received ethanol (2 g/kg) or water administration (3 times a week, intragastrically [i.g.]) from PND 45 to PND 60. After that, mice received (m-CF₃-PhSe)₂ (5 mg/kg/day; i.g) or vegetal oil administration from PND 60 to 66. Mice performed the behavioral tests to evaluate the depressive-like phenotype. The results showed that individually neither an energy-dense diet nor ethanol group induced a depressive-like phenotype, but the association of both induced this phenotype in young mice. Oxidative stress was characterized by the increase of malondialdehyde, the decrease in the superoxide dismutase activity, and non-protein sulfhydryl levels in the cerebral cortex of depressive-like mice. Depressive-like mice showed an increase in the protein levels of opioid receptors and depletion in those of glucocorticoid. (m-CF₃-PhSe)₂ abolished depressive-like phenotype and oxidative stress as well as modulated the levels of glucocorticoid and opioid receptors. In conclusion, the modulation of opioid and glucocorticoid receptors and the antioxidant property contributed to the (m-CF₃-PhSe)₂ antidepressant-like effect in young mice exposed to an energy-dense diet and ethanol intake.

Nociceptin/orphanin FQ neurons in the Arcuate Nucleus and Ventral Tegmental Area Act via Nociceptin Opioid Peptide Receptor Signaling to Inhibit Proopiomelanocortin and A10 Dopamine Neurons and Thereby Modulate Ingestion of Palatable Food

The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits neuronal activity via its cognate nociceptin opioid peptide (NOP) receptor throughout the peripheral and central nervous systems, including those areas involved in the homeostatic and hedonic regulation of energy homeostasis. We thus tested the hypothesis that N/OFQ neurons in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA) act via NOP receptor signaling to inhibit nearby anorexigenic proopiomelanocortin (POMC) and A10 dopamine neuronal excitability, respectively, and thereby modulate ingestion of palatable food. Electrophysiologic recordings were performed in slices prepared from transgenic male and ovariectomized (OVX) female N/OFQ-cre/enhanced green fluorescent protein-POMC, N/OFQ-cre and tyrosine hydroxylase-cre animals to see if optogenetically-stimulated peptide release from N/OFQ neurons could directly inhibit these neuronal populations. Binge-feeding behavioral experiments were also conducted where animals were exposed to a high-fat-diet (HFD) for one hour each day for five days and monitored for energy intake. Photostimulation of ARC and VTA N/OFQ neurons produces an outward current in POMC and A10 dopamine neurons receiving input from these cells. This is associated with a hyperpolarization and decreased firing. These features are also sex hormone- and diet-dependent; with estradiol-treated slices from OVX females being less sensitive, and obese males being more sensitive, to N/OFQ. Limited access to HFD causes a dramatic escalation in consumption, such that animals eat 25-45% of their daily intake during that one-hour exposure. Moreover, the NOP receptor-mediated regulation of these energy balance circuits are engaged, as N/OFQ injected directly into the VTA or ARC respectively diminishes or potentiates this binge-like increase in a manner heightened by diet-induced obesity or dampened by estradiol in females. Collectively, these findings provide key support for the idea that N/OFQ regulates appetitive behavior in sex-, site- and diet-specific ways, along with important insights into aberrant patterns of feeding behavior pertinent to the pathogenesis of food addiction.

Understanding the Influence of Eating Patterns on Binge Drinking: A Mediation Model

BACKGROUND

Binge drinking is an important health problem, and it has been related to binge eating and fat intake in animal models, but this relationship has not been tested in humans. The first objective of this study was to analyze whether binge eating and fat intake are related to binge drinking in a youth sample. The second objective was to analyze whether binge eating and fat intake mediate the relationship between individual factors associated with binge eating and fat intake (sex, body mass index (BMI), drive for thinness, body dissatisfaction, eating styles, impulsivity, and food addiction) and binge drinking.

METHODS

A sample of 428 undergraduate students filled out several questionnaires on binge drinking, binge eating, fat intake, drive for thinness, body dissatisfaction, eating styles, food addiction, and impulsivity.

RESULTS

Results showed an excellent model fit: χ2(25) = 30.342 (p = 0.212), comparative fit index (CFI) = 0.992, root mean squared error of approximation (RMSEA) = 0.022 [90% CI = 0.000, 0.047]. Binge eating and fat intake were positively related to binge drinking. Furthermore, emotional eating, external eating, and food addiction showed positive and statistically significant indirect relationships with binge drinking, whereas the relationship with restrained eating was negative.

CONCLUSIONS

These findings point to the need to use a broader approach in understanding and preventing binge drinking in the youth population by showing the influence of the eating pattern on this problem. This information could be helpful in preventing future behaviors and improving interventions that address health risk behaviors.

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

Polycystic Ovary Syndrome: A Brain Disorder Characterized by Eating Problems Originating during Puberty and Adolescence

Polycystic ovary syndrome (PCOS) is an endocrine condition associated with reproductive and psychiatric disorders, and with obesity. Eating disorders, such as bulimia and recurrent dieting, are also linked to PCOS. They can lead to the epigenetic dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis, thereby impacting on ovarian folliculogenesis. We postulate that PCOS is induced by psychological distress and episodes of overeating and/or dieting during puberty and adolescence, when body dissatisfaction and emotional distress are often present. We propose that upregulated activation of the central HPG axis during this period can be epigenetically altered by psychological stressors and by bulimia/recurrent dieting, which are common during adolescence and which can lead to PCOS. This hypothesis is based on events that occur during a largely neglected stage of female reproductive development. To date, most research into the origins of PCOS has focused on the prenatal induction of this disorder, particularly in utero androgenization and the role of anti-Müllerian hormone. Establishing causality in our peripubertal model requires prospective cohort studies from infancy. Mechanistic studies should consider the role of the gut microbiota in addition to the epigenetic regulation of (neuro) hormones. Finally, clinicians should consider the importance of underlying chronic psychological distress and eating disorders in PCOS.

Diet containing stearic acid increases food reward-related behaviors in mice compared with oleic acid

Obesity is currently a worldwide phenomenon. The consumption of calorie-rich foods is responsible for most obesity cases, but not all humans exposed to high-calorie diets develop obesity. According to recent studies, exposure to fat-rich diets may be the actual cause of obesity. Dietary long-chain fatty acids affect brain function and are linked to food intake and motivation-related behaviors. Recently, many studies have shown that different types of fatty acids play different roles in animals. In our study, the effects of stearic acid (a saturated fatty acid) and oleic acid (a monounsaturated fatty acid) in diets on hedonic feeding behaviors were investigated, and changes of feeding-related protein levels in the brain were detected to explore the possible mechanism underlying the effects of these fatty acids. As a result, mice fed a diet containing stearic acid, compared to a diet containing oleic acid, exhibited increased food intake, hedonic eating, and an operant response to sucrose and locomotor activity. Furthermore, stearic acid corresponded to a higher level of leptin in serum than oleic acid. In addition, the stearic acid treated group had lower protein levels of p-JAK2 and p-STAT3 in the VTA and a higher dopamine concentration in the NAc than the oleic acid-treated group. Meanwhile, the protein level of TH in the NAc was higher and the protein level of the DA transporter in the VTA was lower in the stearic acid-fed group than in the oleic acid-fed group. In conclusion, these findings indicated that a diet containing stearic acid can increase hedonic feeding behavior and affect mesolimbic dopamine system signals in mice. Moreover, the lowering of serum leptin and leptin signaling in the VTA may contribute to this effect.

Growth hormone secretagogue receptor in dopamine neurons controls appetitive and consummatory behaviors towards high-fat diet in ad-libitum fed mice

Growth hormone secretagogue receptor (GHSR), the receptor for ghrelin, is expressed in key brain nuclei that regulate food intake. The dopamine (DA) pathways have long been recognized to play key roles mediating GHSR effects on feeding behaviors. Here, we aimed to determine the role of GHSR in DA neurons controlling appetitive and consummatory behaviors towards high fat (HF) diet. For this purpose, we crossed reactivable GHSR-deficient mice with DA transporter (DAT)-Cre mice, which express Cre recombinase under the DAT promoter that is active exclusively in DA neurons, to generate mice with GHSR expression limited to DA neurons (DAT-GHSR mice). We found that DAT-GHSR mice show an increase of c-Fos levels in brain areas containing DA neurons after ghrelin treatment, in a similar fashion as seen in wild-type mice; however, they did not increase food intake or locomotor activity in response to systemically- or centrally-administered ghrelin. In addition, we found that satiated DAT-GHSR mice displayed both anticipatory activity to scheduled HF diet exposure and HF intake in a binge-like eating protocol similar to those in wild-type mice, whereas GHSR-deficient mice displayed impaired responses. We conclude that GHSR expression in DA neurons is sufficient to both mediate increased anticipatory activity to a scheduled HF diet exposure and fully orchestrate binge-like HF intake, but it is insufficient to restore the acute orexigenic or locomotor effects of ghrelin treatment. Thus, GHSR in DA neurons affects appetitive and consummatory behaviors towards HF diet that take place in the absence of caloric needs.

Access schedules mediate the impact of high fat diet on ethanol intake and insulin and glucose function in mice

Alcoholism and high fat diet (HFD)-induced obesity individually promote insulin resistance and glucose intolerance in clinical populations, increasing risk for metabolic diseases. HFD can also stimulate alcohol intake in short-term clinical studies. Unfortunately, there is currently a disconnect between animal models and the clinical findings, as animal studies typically show that HFD decreases ethanol intake while ethanol intake mitigates HFD-induced effects on insulin and glucose dysfunction. However, most previous animal studies utilized forced or continuous HFD and/or ethanol. In three experiments we sought to determine whether HFD (HFD = 60% calories from fat) vs. control diet (chow = 16% fat) alters voluntary two-bottle choice ethanol intake in male C57Bl/6J mice given differing access schedules for 6-7 weeks, and we assessed the resultant impact on metabolic function via insulin and glucose tolerance tests. Experiment 1: Unlimited Access Ethanol + HFD (UAE + HFD; n = 15; 10% ethanol v/v, ad libitum diet and ethanol) or UAE + Chow (n = 15). Experiment 2: Limited Access Ethanol + HFD (LAE + HFD; n = 15; ethanol = 4 h/day; 3 days/week, ad libitum diet) or LAE + Chow (n = 15) with increasing ethanol concentrations (10%, 15%, 20%). Experiment 3: Intermittent HFD with limited access to ethanol (iHFD-E; HFD = single 24-h session/week; ethanol = 4 h/day; 4 days/week) (n = 10). UAE + HFD mice consumed significantly less ethanol and were insulin-resistant and hyperglycemic compared with UAE + Chow mice. LAE + HFD mice consumed ethanol similarly to LAE + Chow mice, but exhibited hyperglycemia, insulin resistance, and glucose intolerance. iHFD-E mice displayed binge eating-like behaviors and consumed significantly more ethanol than mice given ad libitum chow or HFD. iHFD-E mice did not have significantly altered body composition, but developed insulin insensitivity and glucose intolerance. These findings suggest that access schedules influence HFD effects on ethanol consumption and resultant metabolic dysfunction, ethanol intake does not improve HFD-induced metabolic dysfunction, and binge eating-like behaviors can transfer to binge drinking behaviors.

Circulating Triglycerides Gate Dopamine-Associated Behaviors through DRD2-Expressing Neurons

Energy-dense food alters dopaminergic (DA) transmission in the mesocorticolimbic (MCL) system and can promote reward dysfunctions, compulsive feeding, and weight gain. Yet the mechanisms by which nutrients influence the MCL circuitry remain elusive. Here, we show that nutritional triglycerides (TGs), a conserved post-prandial metabolic signature among mammals, can be metabolized within the MCL system and modulate DA-associated behaviors by gating the activity of dopamine receptor subtype 2 (DRD2)-expressing neurons through a mechanism that involves the action of the lipoprotein lipase (LPL). Further, we show that in humans, post-prandial TG excursions modulate brain responses to food cues in individuals carrying a genetic risk for reduced DRD2 signaling. Collectively, these findings unveil a novel mechanism by which dietary TGs directly alter signaling in the reward circuit to regulate behavior, thereby providing a new mechanistic basis by which energy-rich diets may lead to (mal)adaptations in DA signaling that underlie reward deficit and compulsive behavior.

Blunted leptin sensitivity during hedonic overeating can be reinstated by activating galanin 2 receptors (Gal2R) in the lateral hypothalamus

AIM

Since foods with high hedonic value are often consumed in excess of energetic needs, this study was designed to identify the mechanisms that may counter anorexigenic signalling in the presence of hedonic foods in lean animals.

METHODS

Mice, in different states of satiety (fed/fasted, or fed/fasted and treated with ghrelin or leptin, respectively), were allowed to choose between high-fat/high-sucrose and standard foods. Intake of each food type and the activity of hypothalamic neuropetidergic neurons that regulate appetite were monitored. In some cases, food choice was monitored in leptin-injected fasted mice that received microinjections of galanin receptor agonists into the lateral hypothalamus.

RESULTS

Appetite-stimulating orexin neurons in the lateral hypothalamus are rapidly activated when lean, satiated mice consume a highly palatable food (PF); such activation (upregulated c-Fos expression) occurred even after administration of the anorexigenic hormone leptin and despite intact leptin signalling in the hypothalamus. The ability of leptin to restrain PF eating is restored when a galanin receptor 2 (Gal2R) agonist is injected into the lateral hypothalamus.

CONCLUSION

Hedonically-loaded foods interrupt the inhibitory actions of leptin on orexin neurons and interfere with the homeostatic control of feeding. Overeating of palatable foods can be curtailed in lean animals by activating Gal2R in the lateral hypothalamus.

Behavioral and Neurobiological Consequences of Hedonic Feeding on Alcohol Drinking

A complex interplay of peripheral and central signaling mechanisms within the body of an organism maintains energy homeostasis. In addition, energy/food intake is modified by various external factors (e.g., palatability, food availability, social and environmental triggers). Highly palatable foods can provoke maladaptive feeding behavior, which in turn disrupts normal homeostatic regulation resulting in numerous health consequences. Furthermore, neuroendocrine peptides, traditionally considered to regulate appetite and energy homeostasis, also control the intake and reinforcing properties of alcohol and drugs of abuse. Therefore, dysregulated eating as a result of a hedonic/binge-like intake of hyper-palatable food may impact alcohol drinking behavior. Relevant in this case is the fact that eating disorders are highly comorbid with several neuropsychiatric conditions, including alcohol use disorder. The present review is intended to summarize the neurobiological and functional consequences of hedonic feeding on alcohol intake.

Random access to palatable food stimulates similar addiction-like responses as a fixed schedule, but only a fixed schedule elicits anticipatory activation

Restricted intermittent food access to palatable food (PF) induces addiction-like behaviors and plastic changes in corticolimbic brain areas. Intermittent access protocols normally schedule PF to a fixed time, enabling animals to predict the arrival of PF. Because outside the laboratory the presence of PF may occur in a random unpredictable manner, the present study explored whether random access to PF would stimulate similar addiction-like responses as observed under a fixed scheduled. Rats were randomly assigned to a control group without chocolate access, to ad libitum access to chocolate, to fixed intermittent access (CH-F), or to random unpredictable access (CH-R) to chocolate. Only the CH-F group developed behavioral and core temperature anticipation to PF access. Both groups exposed to intermittent access to PF showed binge eating, increased effort behaviors to obtain chocolate, as well as high FosB/ΔFosB in corticolimbic areas. Moreover, FosB/ΔFosB in all areas correlated with the intensity of binge eating and effort behaviors. We conclude that both conditions of intermittent access to PF stimulate addiction-like behaviors and FosB/ΔFosB accumulation in brain reward areas; while only a fixed schedule, which provides a time clue, elicited anticipatory activation, which is strongly associated with craving behaviors and may favor relapse during withdrawal.

Neuroendocrinology of reward in anorexia nervosa and bulimia nervosa: Beyond leptin and ghrelin

The pathophysiology of anorexia nervosa (AN) and bulimia nervosa (BN) are still poorly understood, but psychobiological models have proposed a key role for disturbances in the neuroendocrines that signal hunger and satiety and maintain energy homeostasis. Mounting evidence suggests that many neuroendocrines involved in the regulation of homeostasis and body weight also play integral roles in food reward valuation and learning via their interactions with the mesolimbic dopamine system. Neuroimaging data have associated altered brain reward responses in this system with the dietary restriction and binge eating and purging characteristic of AN and BN. Thus, neuroendocrine dysfunction may contribute to or perpetuate eating disorder symptoms via effects on reward circuitry. This narrative review focuses on reward-related neuroendocrines that are altered in eating disorder populations, including peptide YY, insulin, stress and gonadal hormones, and orexins. We provide an overview of the animal and human literature implicating these neuroendocrines in dopaminergic reward processes and discuss their potential relevance to eating disorder symptomatology and treatment.

Growth hormone secretagogue receptor signalling affects high-fat intake independently of plasma levels of ghrelin and LEAP2, in a 4-day binge eating model

The growth hormone secretagogue receptor (GHSR) is a G protein-coupled receptor that is highly expressed in the central nervous system. GHSR acts as a receptor for ghrelin and for liver-expressed antimicrobial peptide 2 (LEAP2), which blocks ghrelin-evoked activity. GHSR also displays ligand-independent activity, including a high constitutive activity that signals in the absence of ghrelin and is reduced by LEAP2. GHSR activity modulates a variety of food intake-related behaviours, including binge eating. Previously, we reported that GHSR-deficient mice daily and time-limited exposed to a high-fat (HF) diet display an attenuated binge-like HF intake compared to wild-type mice. In the present study, we aimed to determine whether ligand-independent GHSR activity affects binge-like HF intake in a 4-day binge-like eating protocol. We found that plasma levels of ghrelin and LEAP2 were not modified in mice exposed to this binge-like eating protocol. Moreover, systemic administration of ghrelin or LEAP2 did not alter HF intake in our experimental conditions. Interestingly, we found that central administration of LEAP2 or K-(D-1-Nal)-FwLL-NH₂ , which are both blockers of constitutive GHSR activity, reduced binge-like HF intake, whereas central administration of ghrelin or the ghrelin-evoked GHSR activity blockers [D-Lys3]-GHRP-6 and JMV2959 did not modify binge-like HF intake. Taken together, current data indicate that GHSR activity in the brain affects binge-like HF intake in mice independently of plasma levels of ghrelin and LEAP2.

GABAB Receptor Signaling in the Mesolimbic System Suppresses Binge-like Consumption of a High-Fat Diet

Binge eating could contribute to the development of obesity, and previous studies suggest that gamma-aminobutyric acid (GABA) type B receptor (GABA_BR) signaling is involved in the regulation of binge eating. Here, we show that time-restricted access to a high-fat diet (HFD) induces binge-like eating behavior in wild-type mice. HFD consumption during restricted time was significantly increased in corticostriatal neuron-specific GABA_BR-deficient mice compared with wild-type mice. Furthermore, the GABA_BR agonist baclofen suppressed HFD intake during restricted time in wild-type mice but not in corticostriatal or dopaminergic neuron-specific GABA_BR-deficient mice. In contrast, there were no significant differences in food consumption among genotypes under ad libitum access to HFD. Thus, our data show that the mesolimbic system regulates food consumption under time-restricted but not ad libitum access to HFD and have identified a mechanism by which GABA_BR signaling suppresses binge-like eating of HFD.

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GABA_BR KO in corticostriatal neurons enhances binge-like feeding of HFD

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Baclofen suppresses binge-like feeding of HFD via the mesolimbic system

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GABA_BR signaling in mesolimbic system does not affect energy balance

Inter-individual Variability for High Fat Diet Consumption in Inbred C57BL/6 Mice

Since inbred C57BL/6 mice are known to show inter-individual phenotypic variability for some traits, we tested the hypothesis that inbred C57BL/6 mice display a different tendency to consume a high fat (HF) diet. For this purpose, we used a compilation of HF intake data from an experimental protocol in which satiated mice were exposed to a HF pellet every morning for 2-h over 4 consecutive days. We found that mice displayed a large degree of variability in HF intake. Since day 1 HF intake significantly correlated with HF intake in successive days, we applied a hierarchical clustering algorithm on HF intake measurements in days 2, 3, and 4 in order to classify mice into “low” or “high” HF intake groups. “Low” HF intake group showed a day 1 HF intake similar to that seen in mice exposed to regular chow, while “high” HF intake group showed a higher day 1 HF intake as compared to “low” HF intake group. Both groups of mice increased HF consumption over the successive days, but “high” HF intake group always displayed a higher HF consumption than the “low” HF intake group. As compared to “low” HF intake group, “high” HF intake group showed a higher number of dopamine neurons positive for c-Fos in the VTA after the last event of HF intake. Thus, inbred C57BL/6 mice show inter-individual variability for HF intake and such feature may be linked to a different response to the rewarding properties of the HF diet.

Behavioral profile of intermittent vs continuous access to a high fat diet during adolescence

Over the past few years, the effects of a high-fat diet (HFD) on cognitive functions have been broadly studied as a model of obesity, although no studies have evaluated whether these effects are maintained after the cessation of this diet. In addition, the behavioral effects of having a limited access to an HFD (binge-eating pattern) are mostly unknown, although they dramatically increase the vulnerability to drug use in contrast to having continuous access. Thus, the aim of the present study was to compare the effects of an intermittent versus a continuous exposure to an HFD during adolescence on cognition and anxiety-like behaviors, as well as to study the changes observed after the interruption of this diet. Adolescent male mice received for 40 days a standard diet, an HFD with continuous access or an HFD with sporadic limited access (2 h, three days a week). Two additional groups were fed with intermittent or continuous access to the HFD and withdrawn from this diet 15 days before the behavioral tests. Only the animals with a continuous access to the HFD showed higher circulating leptin levels, increased bodyweight, marked memory and spatial learning deficits, symptoms that disappeared after 15 days of HFD abstinence. Mice that binged on fat only showed hyperlocomotion, which normalized after 15 days of HFD cessation. However, discontinuation of fat, either in a binge or a continuous pattern, led to an increase in anxiety-like behavior. These results highlight that exposure to a high-fat diet during adolescence induces alterations in brain functions, although the way in which this diet is ingested determines the extent of these behavioral changes.

The orexin-1 receptor antagonist SB-334867 reduces motivation, but not inhibitory control, in a rat stop signal task

There is considerable clinical interest in the neuropeptide orexin/hypocretin for its ability to regulate motivation and reward as well as arousal and wakefulness. For instance, antagonists for the orexin-1 receptor (OxR1) are thought to hold great promise for treating drug addiction and disorders associated with overeating, as these compounds repeatedly have been found to suppress seeking of various drugs of abuse as well as highly palatable foods in preclinical models. Given the hypothesized role of OxR1 signaling in cue-driven motivation, an outstanding question is whether pharmacologically blocking this receptor affects cognitive functioning. Response inhibition - the ability to cancel ongoing behavior - is one aspect of cognitive control that may be particularly relevant. Response inhibition deficits are commonly associated with a range of psychiatric disorders and neurological diseases, including substance use disorders and obesity. Moreover, OxR1 signaling recently has been implicated in waiting impulsivity, another aspect of inhibitory control. Here, we investigated the effects of the OxR1 antagonist SB-334867 on response inhibition in a rat version of the stop-signal reaction time task. Results show that acutely blocking OxR1 had minimal effects on response inhibition or attentional functioning. In contrast, this manipulation reduced motivation to perform the task and earn food rewards, consistent with other recent findings. These results add to the growing body of literature implicating OxR1 in the regulation of motivation and suggest that effects of pharmacological compounds such as SB-334867 on drug-seeking behavior are not related to effects on response inhibition.

Attenuation of satiety gut hormones increases appetitive behavior after curative esophagectomy for esophageal cancer

Background

Reduced appetite and weight loss are common after esophagectomy (ES), and this cohort demonstrates an exaggerated postprandial satiety gut hormone response. Satiety gut hormones modulate food reward, resulting in reduced energy intake.

Objectives

This study aimed to determine the effect of satiety gut hormone modulation by measuring the effect of the somatostatin analog octreotide on appetitive behavior among patients after ES.

Methods

In this randomized, double-blind, placebo-controlled crossover study, patients ≥1 y after ES and matched controls received either 1 mL 0.9% saline or 1 mL (100 μg) octreotide subcutaneously before completing a progressive ratio task. A measure of appetitive behavior, this task requires subjects to undertake progressively increasing amounts of work to obtain a sweet-fat reinforcer; the final completed increment (breakpoint) represents reinforcer reward value. Separate cohorts were studied in the fasted or 1-h postprandial states.

Results

Thirty-six subjects (ES, n = 18; matched controls, n = 18) were studied. The ES subjects were 2.5 ± 0.3 y postoperation and had a weight loss of 14.6% ± 2.6% and elevated postprandial glucagon-like peptide 1 compared with controls (49.2 ± 13.4 compared with 20.2 ± 2.3 pM; P = 0.04). Octreotide did not alter the breakpoint among ES or control subjects when tested in a fasting condition (ES: 980 ± 371 compared with 1700 ± 584 clicks; P = 0.16; controls: 1056 ± 274 compared with 1124 ± 273 clicks; P = 0.81). When tested 1 h postprandially, octreotide was associated with an increased breakpoint compared with placebo among ES subjects (322 ± 143 compared with 246 ± 149 clicks; P = 0.04) but not controls (248 ± 119 compared with 247 ± 120 clicks; P = 0.97).

Conclusions

Attenuation of the exaggerated postprandial satiety gut hormone response is associated with increased appetitive behavior toward a sweet-fat stimulus among patients post-ES. Suppression of satiety gut hormones may be a novel target to increase appetite, food intake, and body weight among patients after ES. This study was registered at clinicaltrials.gov as NCT02381249.

Ghrelin and food reward

Food intake is tightly regulated by homeostatic and reward mechanisms and the adequate function of both is necessary for the proper maintenance of energy balance. Ghrelin impacts on these two levels to induce feeding. In this review, we present the actions of ghrelin in food reward, including their dependence on other relevant modulators implicated in the motivational aspects of feeding, including dopamine, opioid peptides, and endocannabinoids. We also describe the interaction between brain areas involved in homeostatic regulation of feeding and the reward system, with a special emphasis on the role of arcuate nucleus melanocortins and lateral hypothalamus orexins in ghrelin function. Finally, we briefly discuss the actions of ghrelin in food reward in obesity. We propose that new insights into the mechanism of action of ghrelin in the rewarding and motivational control of food intake will help to understand food-related disorders including obesity and anorexia.

Orexin/hypocretin and dysregulated eating: Promotion of foraging behavior

At its discovery, orexin/hypocretin (OX) was hypothesized to promote food intake. Subsequently, with the identification of the participation of OX in numerous other phenomena, including arousal and drug seeking, this neuropeptide was proposed to be involved in highly motivated behaviors. The present review develops the hypothesis that the primary evolutionary function of OX is to promote foraging behavior, seeking for food under conditions of limited availability. Thus, it will first describe published literature on OX and homeostatic food intake, which shows that OX neurons are activated by conditions of food deprivation and in turn stimulate food intake. Next, it will present literature on excessive and binge-like food intake, which demonstrates that OX stimulates both intake and willingness to work for palatable food. Importantly, studies show that binge-like eating can be inhibited by OX antagonists at doses far lower than those required to suppress homeostatic intake (3 mg/kg vs. 30 mg/kg), suggesting that an OX-based pharmacotherapy, at the right dose, could specifically control dysregulated eating. Finally, the review will discuss the role of OX in foraging behavior, citing literature which shows that OX neurons, which are activated during the anticipation of food reward, can promote a number of phenomena involved in successful foraging, including food-anticipatory locomotor behavior, olfactory sensitivity, visual attention, spatial memory, and mastication. Thus, OX may promote homeostatic eating, as well as binge eating of palatable food, due to its ability to stimulate and coordinate the activities involved in foraging behavior.

A spontaneous binge-like eating model in mice using unpredictable once weekly access to palatable diets

Many pre-clinical models of binge-like eating involve predictable, scheduled, access to a palatable diet high in fat (HF), where access may be preceded by anticipatory behaviour. Here, to introduce spontaneity into the binge-type consumption of palatable diets, mice were allowed 2 h access on a random day once per week and at a random time within an 8 h window either side of the transition from dark phase to light phase. Despite normal intake of a stock diet prior to unpredictable access to HF diet, mice immediately initiated a substantial eating episode when presented with HF diet. Following this consumption, compensatory hypophagia was observed relative to stock diet-fed controls, and cumulative energy intakes converged. There were no effects of HF diet on body weight or body composition over a 12-week period. Binge-like consumption was also observed on unpredictable access to the complete liquid diet, chocolate Ensure, but not with a 10% sucrose solution. Binge-like responses to unpredictable access to HF diet or Ensure were similar in male and female mice, although there were effects of sex on caloric consumption from stock diet in the compensatory period following palatable diet intake, with higher intakes in females. The timing of the 2h access period relative to light phase transition affected intake of palatable diets, but less robustly than the equivalent effect on stock diet intake during the same timed periods - the diurnal patterning of energy intake was diet sensitive. The large spontaneous binge-like consumption on unpredictable access to either solid or liquid palatable diets in mice of either sex offers the potential to combine these attributes with other manipulations where a developing obesity is part of the binge-like eating phenotype.

Limited Access to a High Fat Diet Alters Endocannabinoid Tone in Female Rats

Emerging evidence suggest an impaired endocannabinoid activity in the pathophysiology of binge eating disorder (BED). Herein, we investigated whether endocannabinoid tone could be modified as a consequence of dietary-induced binge eating in female rats. For this purpose, brain levels of the endocannabinoids anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), as well as two endocannabinoid-like lipids, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), were assessed in different brain areas involved in the hedonic feeding (i.e., prefrontal cortex, nucleus accumbens, amygdala, hippocampus, and hypothalamus). The brain density of cannabinoid type-1 receptors (CB₁) was also evaluated. Furthermore, we determined plasma levels of leptin, ghrelin, and corticosterone hormones, which are well-known to control the levels of endocannabioids and/or CB1 receptors in the brain. To induce binge eating behavior, rats were subject to an intermittent and limited access to a high fat diet (HFD) (margarine). Three experimental groups were used, all with ad libitum access to chow: control (CTRL), with no access to margarine; low restriction (LR), with 2 h margarine access 7 days/week; high restriction (HR), with 2 h margarine access 3 days/week. Bingeing was established when margarine intake in the HR group exceeded that of the LR group. Our results show that, compared to CTRL, AEA significantly decreased in the caudate putamen, amygdala, and hippocampus of HR group. In contrast, 2-AG significantly increased in the hippocampus while OEA decreased in the hypothalamus. Similar to the HR group, AEA and OEA decreased respectively in the amygdala and hypothalamus and 2-AG increased in the hippocampus of LR group. Moreover, LR group also had AEA decreased in the prefrontal cortex and increased in the nucleus accumbens. In both groups we found the same reduction of CB₁ receptor density in the prefrontal cortex compared to CTRL. Also, LR and HR groups showed alterations in both ghrelin and corticosterone levels, while leptin remained unaltered. In conclusion, our findings show a modified endocannabinoid tone due to margarine exposure, in several brain areas that are known to influence the hedonic aspect of food. Even if not uniquely specific to binge eating, margarine-induced changes in endocannabinoid tone could contributes to the development and maintenance of this behavior.

Evidence Supporting a Role for Constitutive Ghrelin Receptor Signaling in Fasting-Induced Hyperphagia in Male Mice

Ghrelin is a potent orexigenic peptide hormone that acts through the growth hormone secretagogue receptor (GHSR), a G protein-coupled receptor highly expressed in the hypothalamus. In vitro studies have shown that GHSR displays a high constitutive activity, whose physiological relevance is uncertain. As GHSR gene expression in the hypothalamus is known to increase in fasting conditions, we tested the hypothesis that constitutive GHSR activity at the hypothalamic level drives the fasting-induced hyperphagia. We found that refed wild-type (WT) mice displayed a robust hyperphagia that continued for 5 days after refeeding and changed their food intake daily pattern. Fasted WT mice showed an increase in plasma ghrelin levels, as well as in GHSR expression levels and ghrelin binding sites in the hypothalamic arcuate nucleus. When fasting-refeeding responses were evaluated in ghrelin- or GHSR-deficient mice, only the latter displayed an ∼15% smaller hyperphagia, compared with WT mice. Finally, fasting-induced hyperphagia of WT mice was significantly smaller in mice centrally treated with the GHSR inverse agonist K-(D-1-Nal)-FwLL-NH2, compared with mice treated with vehicle, whereas it was unaffected in mice centrally treated with the GHSR antagonists D-Lys3-growth hormone-releasing peptide 6 or JMV2959. Taken together, genetic models and pharmacological results support the notion that constitutive GHSR activity modulates the magnitude of the compensatory hyperphagia triggered by fasting. Thus, the hypothalamic GHSR signaling system could affect the set point of daily food intake, independently of plasma ghrelin levels, in situations of negative energy balance.