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

Current perspectives on brain circuits involved in food addiction-like behaviors

There is an emerging view that the increased availability of energy-dense foods in our society is contributing to excessive food consumption which could lead to food addiction-like behavior. Particularly, compulsive eating patterns are predominant in people suffering from eating disorders (binge-eating disorder, bulimia and anorexia nervosa) and obesity. Phenotypically, the behavioral pattern exhibits a close resemblance to individuals suffering from other forms of addiction (drug, sex, gambling). Growing body of evidence in neuroscience research is showing that excessive consumption of energy-dense foods alters the brain circuits implicated in reward, decision-making, control, habit formation, and emotions that are central to drug addiction. Here, we review the current understanding of the circuits of food addiction-like behaviors and highlight the future possibility of exploring those circuits to combat obesity and eating disorders.

The Role of the Endocannabinoid System in Binge Eating Disorder

Eating disorders are multifactorial disorders that involve maladaptive feeding behaviors. Binge eating disorder (BED), the most prevalent of these in both men and women, is characterized by recurrent episodes of eating large amounts of food in a short period of time, with a subjective loss of control over eating behavior. BED modulates the brain reward circuit in humans and animal models, which involves the dynamic regulation of the dopamine circuitry. The endocannabinoid system plays a major role in the regulation of food intake, both centrally and in the periphery. Pharmacological approaches together with research using genetically modified animals have strongly highlighted a predominant role of the endocannabinoid system in feeding behaviors, with the specific modulation of addictive-like eating behaviors. The purpose of the present review is to summarize our current knowledge on the neurobiology of BED in humans and animal models and to highlight the specific role of the endocannabinoid system in the development and maintenance of BED. A proposed model for a better understanding of the underlying mechanisms involving the endocannabinoid system is discussed. Future research will be necessary to develop more specific treatment strategies to reduce BED symptoms.

The emerging role of the endocannabinoidome-gut microbiome axis in eating disorders

Among the sources of chemical signals regulating food intake, energy metabolism and body weight, few have attracted recently as much attention as the expanded endocannabinoid system, or endocannabinoidome (eCBome), and the gut microbiome, the two systems on which this review article is focussed. Therefore, it is legitimate to expect that these two systems also play a major role in the etiopathology of eating disorders (EDs), in particular of anorexia nervosa, bulimia nervosa and binge-eating disorder. The major mechanisms through which, also via interactions with other endogenous signaling systems, the eCBome, with its several lipid mediators and receptors, and the gut microbiome, via its variety of microbial kingdoms, phyla and species, and armamentarium of metabolites, intervene in these disorders, are described here, based on several published studies in either experimental models or patients. Additionally, in view of the emerging multi-faceted cross-talk mechanisms between these two complex systems, we discuss the possibility that the eCBome-gut microbiome axis is also involved in EDs.

Binge sucrose-induced neuroadaptations: A focus on the endocannabinoid system

Binge eating, the defining feature of binge eating disorder (BED), is associated with a number of adverse health outcomes as well as a reduced quality of life. Animals, like humans, selectively binge on highly palatable food suggesting that the behaviour is driven by hedonic, rather than metabolic, signals. Given the links to both reward processing and food intake, this study examined the contribution of the endocannabinoid system (ECS) to binge-like eating in rats. Separate groups were given intermittent (12 h) or continuous (24 h) access to 10% sucrose and food over 28 days, with only the 12 h access group displaying excessive sucrose intake within a discrete period of time (i.e., binge eating). Importantly, this group also exhibited alterations in ECS transcripts and endocannabinoid levels in brain reward regions, including an increase in cannabinoid receptor 1 (CB1R) mRNA in the nucleus accumbens as well as changes in endocannabinoid levels in the prefrontal cortex and hippocampus. We then tested whether different doses (1 and 3 mg/kg) of a CB1R antagonist, Rimonabant, modify binge-like intake or the development of a conditioned place preference (CPP) to sucrose. CB1R blockade reduced binge-like intake of sucrose and blocked a sucrose CPP, but only in rats that had undergone 28 days of sucrose consumption. These findings indicate that sucrose bingeing alters the ECS in reward-related areas, modifications that exacerbate the effect of CB1R blockade on sucrose reward. Overall, our results broaden the understanding of neural alterations associated with bingeing eating and demonstrate an important role for CB1R mechanisms in reward processing. In addition, these findings have implications for understanding substance abuse, which is also characterized by excessive and maladaptive intake, pointing towards addictive-like properties of palatable food.

The Neurobiology of Binge-eating Disorder Compared with Obesity: Implications for Differential Therapeutics

PURPOSE

Emerging work indicates divergence in the neurobiologies of binge-eating disorder (BED) and obesity despite their frequent co-occurrence. This review highlights specific distinguishing aspects of BED, including elevated impulsivity and compulsivity possibly involving the mesocorticolimbic dopamine system, and discusses implications for differential therapeutics for BED.

METHODS

This narrative review describes epidemiologic, clinical, genetic, and preclinical differences between BED and obesity. Subsequently, this review discusses human neuroimaging work reporting differences in executive functioning, reward processing, and emotion reactivity in BED compared with obesity. Finally, on the basis of the neurobiology of BED, this review identifies existing and new therapeutic agents that may be most promising given their specific targets based on putative mechanisms of action relevant specifically to BED.

FINDINGS

BED is characterized by elevated impulsivity and compulsivity compared with obesity, which is reflected in divergent neurobiological characteristics and effective pharmacotherapies. Therapeutic agents that influence both reward and executive function systems may be especially effective for BED.

IMPLICATIONS

Greater attention to impulsivity/compulsivity-related, reward-related, and emotion reactivity-related processes may enhance conceptualization and treatment approaches for patients with BED. Consideration of these distinguishing characteristics and processes could have implications for more targeted pharmacologic treatment research and interventions.

The endocannabinoidome as a substrate for noneuphoric phytocannabinoid action and gut microbiome dysfunction in neuropsychiatric disorders

The endocannabinoid (eCB) system encompasses the eCBs anandamide and 2-arachidonoylglycerol, their anabolic/catabolic enzymes, and the cannabinoid CB1 and CB2 receptors. Its expansion to include several eCB-like lipid mediators, their metabolic enzymes, and their molecular targets, forms the endocannabinoidome (eCBome). This complex signaling system is deeply involved in the onset, progress, and symptoms of major neuropsychiatric disorders and provides a substrate for future therapeutic drugs against these diseases. Such drugs may include not only THC, the major psychotropic component of cannabis, but also other, noneuphoric plant cannabinoids. These compounds, unlike THC, possess a wide therapeutic window, possibly due to their capability of hitting several eCBome and non-eCBome receptors. This is particularly true for cannabidiol, which is one of the most studied cannabinoids and shows promise for the treatment of a wide range of mental and mood disorders. The eCBome plays a role also in the microbiota-gut-brain axis, which is emerging as an important actor in the control of affective and cognitive functions and in their pathological alterations.
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Targeting the Endocannabinoid CB1 Receptor to Treat Body Weight Disorders: A Preclinical and Clinical Review of the Therapeutic Potential of Past and Present CB1 Drugs

Obesity rates are increasing worldwide and there is a need for novel therapeutic treatment options. The endocannabinoid system has been linked to homeostatic processes, including metabolism, food intake, and the regulation of body weight. Rimonabant, an inverse agonist for the cannabinoid CB1 receptor, was effective at producing weight loss in obese subjects. However, due to adverse psychiatric side effects, rimonabant was removed from the market. More recently, we reported an inverse relationship between cannabis use and BMI, which has now been duplicated by several groups. As those results may appear contradictory, we review here preclinical and clinical studies that have studied the impact on body weight of various cannabinoid CB1 drugs. Notably, we will review the impact of CB1 inverse agonists, agonists, partial agonists, and neutral antagonists. Those findings clearly point out the cannabinoid CB1 as a potential effective target for the treatment of obesity. Recent preclinical studies suggest that ligands targeting the CB1 may retain the therapeutic potential of rimonabant without the negative side effect profile. Such approaches should be tested in clinical trials for validation.

Transcriptional regulation of the endocannabinoid system in a rat model of binge-eating behavior reveals a selective modulation of the hypothalamic fatty acid amide hydrolase gene

OBJECTIVE

Binge-eating episodes are recurrent and are defining features of several eating disorders. Thus binge-eating episodes might influence eating disorder development of which exact underlying mechanisms are still largely unknown.

METHODS

Here we focused on the transcriptional regulation of the endocannabinoid system, a potent regulator of feeding behavior, in relevant rat brain regions, using a rat model in which a history of intermittent food restriction and a frustration stress induce binge-like palatable food consumption.

RESULTS

We observed a selective down-regulation of fatty acid amide hydrolase (faah) gene expression in the hypothalamus of rats showing the binge-eating behavior with a consistent reduction in histone 3 acetylation at lysine 4 of the gene promoter. No relevant changes were detected for any other endocannabinoid system components in any brain regions under study, as well as for the other epigenetic mechanisms investigated (DNA methylation and histone 3 lysine 27 methylation) at the faah gene promoter.

DISCUSSION

Our findings suggest that faah transcriptional regulation is a potential biomarker of binge-eating episodes, with a relevant role in the homeostatic regulation of food intake.

Animal Models of Eating Disorders

Eating disorders (EDs) include a range of chronic and disabling pathologies characterized by persistent maladaptive eating habits and/or behaviors aimed at controlling body shape and size, with important consequences on physical health. Different animal models of EDs have been developed to investigate pharmacological, environmental, and genetic determinants that contribute to the development and maintenance of these disorders as well as for the identification of potential therapeutic targets. In this chapter, we will provide an overview of the most useful animal models of EDs, focusing mainly on those used to study anorexia nervosa and binge eating disorder.

Progress in Developing Pharmacologic Agents to Treat Bulimia Nervosa

This paper reviews past and current progress in developing pharmacologic agents for the treatment of individuals with bulimia nervosa (BN). We searched the literature and clinical trial registries for compounds studied in BN, the related condition, binge eating disorder (BED), and preclinical models of binge-eating behavior. Drug classes evaluated included antidepressants, antiepileptic drugs, stimulants and other medications for attention-deficit/hyperactivity disorder, opioid antagonists, and weight loss agents, among others. The only available drugs with established efficacy in BN at this time include antidepressants (especially selective serotonin reuptake inhibitors [SSRIs]) and the antiepileptic topiramate, though the efficacy of these compounds is modest at best. The only medications we found currently receiving empirical study in people with BN were fluoxetine, other serotonergic antidepressants, intranasal naloxone, lisdexamfetamine dimesylate, phentermine-topiramate combination, the antiandrogenic oral contraceptive ethinyl estradiol plus drospirenone, and prazosin. Preclinical models suggest that nociceptin receptor antagonists, the selective serotonin 5-HT2C receptor agonist lorcaserin, monoamine stabilizers, and selective orexin-1 receptor antagonists might be helpful. We found no evidence of a drug developed specifically for the treatment of individuals with BN. Future areas for research in the pharmacotherapy of BN are suggested. Importantly, until drugs are developed specifically for eating disorders, drugs developed for other conditions that are centrally acting and associated with beneficial psychotropic effects and/or reduced appetite or weight loss might be considered for repurposing in BN.

Hedonic Eating and the "Delicious Circle": From Lipid-Derived Mediators to Brain Dopamine and Back

Palatable food can be seductive and hedonic eating can become irresistible beyond hunger and negative consequences. This is witnessed by the subtle equilibrium between eating to provide energy intake for homeostatic functions, and reward-induced overeating. In recent years, considerable efforts have been devoted to study neural circuits, and to identify potential factors responsible for the derangement of homeostatic eating toward hedonic eating and addiction-like feeding behavior. Here, we examined recent literature on “old” and “new” players accountable for reward-induced overeating and possible liability to eating addiction. Thus, the role of midbrain dopamine is positioned at the intersection between selected hormonal signals involved in food reward information processing (namely, leptin, ghrelin, and insulin), and lipid-derived neural mediators such as endocannabinoids. The impact of high fat palatable food and dietary lipids on endocannabinoid formation is reviewed in its pathogenetic potential for the derangement of feeding homeostasis. Next, endocannabinoid signaling that regulates synaptic plasticity is discussed as a key mechanism acting both at hypothalamic and mesolimbic circuits, and affecting both dopamine function and interplay between leptin and ghrelin signaling. Outside the canonical hypothalamic feeding circuits involved in energy homeostasis and the notion of “feeding center,” we focused on lateral hypothalamus as neural substrate able to confront food-associated homeostatic information with food salience, motivation to eat, reward-seeking, and development of compulsive eating. Thus, the lateral hypothalamus-ventral tegmental area-nucleus accumbens neural circuitry is reexamined in order to interrogate the functional interplay between ghrelin, dopamine, orexin, and endocannabinoid signaling. We suggested a pivotal role for endocannabinoids in food reward processing within the lateral hypothalamus, and for orexin neurons to integrate endocrine signals with food reinforcement and hedonic eating. In addition, the role played by different stressors in the reinstatement of preference for palatable food and food-seeking behavior is also considered in the light of endocannabinoid production, activation of orexin receptors and disinhibition of dopamine neurons. Finally, type-1 cannabinoid receptor-dependent inhibition of GABA-ergic release and relapse to reward-associated stimuli is linked to ghrelin and orexin signaling in the lateral hypothalamus-ventral tegmental area-nucleus accumbens network to highlight its pathological potential for food addiction-like behavior.

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.

Endocannabinoid Regulation of Reward and Reinforcement through Interaction with Dopamine and Endogenous Opioid Signaling

The endocannabinoid system (eCB) is implicated in the mediation of both reward and reinforcement. This is evidenced by the ability of exogenous cannabinoid drugs to produce hedonia and maintain self-administration in both human and animal subjects. eCBs similarly facilitate behaviors motivated by reward through interaction with the mesolimbic dopamine (DA) and endogenous opioid systems. Indeed, eCB signaling in the ventral tegmental area stimulates activation of midbrain DA cells and promotes DA release in terminal regions such as the nucleus accumbens (NAc). DA transmission mediates several aspects of reinforced behavior, such as motivation, incentive salience, and cost-benefit calculations. However, much research suggests that endogenous opioid signaling underlies the hedonic aspects of reward. eCBs and their receptors functionally interact with opioid systems within the NAc to support reward, most likely through augmenting DA release. This review explores the interaction of these systems as it relates to reward and reinforcement and examines current literature regarding their role in food reward.

Peripheral Endocannabinoid Responses to Hedonic Eating in Binge-Eating Disorder

Reward mechanisms are likely implicated in the pathophysiology of binge-eating behaviour, which is a key symptom of binge-eating disorder (BED). Since endocannabinoids modulate food-related reward, we aimed to investigate the responses of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) to hedonic eating in patients with BED. Peripheral levels of AEA and 2-AG were measured in 7 obese BED patients before and after eating favorite (hedonic eating) and non-favorite (non-hedonic eating) foods. We found that plasma levels of AEA progressively decreased after eating the non-favorite food and significantly increased after eating the favorite food, whereas plasma levels of 2-AG did not differ significantly between the two test conditions, although they showed a trend toward significantly different time patterns. The changes in peripheral AEA levels were positively correlated to the subjects’ sensations of the urge to eat and the pleasantness while eating the presented food, while changes in peripheral 2-AG levels were positively correlated to the subjects’ sensation of the pleasantness while eating the presented food and to the amount of food they would eat. These results suggest the occurrence of distinctive responses of endocannabinoids to food-related reward in BED. The relevance of such findings to the pathophysiology of BED remains to be elucidated.

Binge-eating disorder: Clinical and therapeutic advances

Binge-eating disorder (BED) is the most prevalent eating disorder with estimates of 2-5% of the general adult population. Nonetheless, its pathophysiology is poorly understood. Furthermore, there exist few therapeutic options for its effective treatment. Here we review the current state of binge-eating neurobiology and pharmacology, drawing from clinical therapeutic, neuroimaging, cognitive, human genetic and animal model studies. These studies, which are still in their infancy, indicate that while there are many gaps in our knowledge, several key neural substrates appear to underpin binge-eating and may be conserved between human and animals. This observation suggests that behavioral intermediate phenotypes or endophenotypes relevant to BED may be modeled in animals, facilitating the identification and testing of novel pharmacological targets. The development of novel, safe and effective pharmacological therapies for the treatment of BED will enhance the ability of clinicians to provide optimal care for people with BED.

The novel cannabinoid antagonist SM-11 reduces hedonic aspect of food intake through a dopamine-dependent mechanism

Cannabinoids, endogenous and exogenously administered, are known to positively regulate food intake and energy balance. Since CB1 receptor antagonists reduce food intake and antagonize overweight, we developed a new CB1 receptor antagonist in an attempt to identify a compound with potential application in overeating disorders. The newly developed SM-11 compound dose-dependently decreases food intake in rats by 15-20%. Moreover, SM-11 reduces self-administration of palatable food in both food restricted and ad libitum fed rats, suggesting an action on the hedonic component of food intake. Thus, we next tested the effect of SM-11 on the stimulating properties of the CB1 receptor agonist WIN55,212-2 (WIN) on the electrophysiological activity of Nucleus Accumbens-projecting dopaminergic neurons of the ventral tegmental area (VTA). SM-11 fully and readily antagonized the WIN-induced increments in single spiking and burst firing of antidromically-identified dopamine neurons. When administered to naïve (no WIN-pretreated) rats, SM-11 did not alter basal neuronal activity, thereby suggesting a pure antagonistic profile. SM-11 thus appears as a promising candidate in the search of potential anti-obesity medications.

Endocannabinoid signalling in reward and addiction

Brain endocannabinoid (eCB) signalling influences the motivation for natural rewards (such as palatable food, sexual activity and social interaction) and modulates the rewarding effects of addictive drugs. Pathological forms of natural and drug-induced reward are associated with dysregulated eCB signalling that may derive from pre-existing genetic factors or from prolonged drug exposure. Impaired eCB signalling contributes to dysregulated synaptic plasticity, increased stress responsivity, negative emotional states and cravings that propel addiction. Understanding the contributions of eCB disruptions to behavioural and physiological traits provides insight into the eCB influence on addiction vulnerability.

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

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

Endocannabinoids and Metabolic Disorders

The endocannabinoid system (ECS) is known to exert regulatory control on essentially every aspect related to the search for, and the intake, metabolism and storage of calories, and consequently it represents a potential pharmacotherapeutic target for obesity, diabetes and eating disorders. While the clinical use of the first generation of cannabinoid type 1 (CB(1)) receptor blockers has been halted due to the psychiatric side effects that their use occasioned, recent research in animals and humans has provided new knowledge on the mechanisms of actions of the ECS in the regulation of eating behavior, energy balance, and metabolism. In this review, we discuss these recent advances and how they may allow targeting the ECS in a more specific and selective manner for the future development of therapies against obesity, metabolic syndrome, and eating disorders.

Reproductive impacts and physiological adaptations of zebrafish to elevated dietary nickel

Nickel (Ni) concentrations in the environment can rise due to human industrial activities. The toxicity of waterborne Ni to aquatic animals has been examined in a number of previous studies; however, little is known about the impacts of elevated dietary Ni. In the present study, zebrafish were chronically fed diets containing two concentrations of Ni [3.7 (control) and 116 μg Ni/g diet]. Ni-exposed males, but not females, were significantly smaller (26%) compared to controls at 80 days. In addition, total egg production was decreased by 65% in the Ni treatment at 75-78 days of the experiment. Ni was ubiquitously distributed in control animals (similar to previous studies), and concentrations varied between tissues by 15-fold. Ni exposure resulted in modest but significant Ni accumulation in some tissues (increases were highest in brain, vertebrae and gut; 44%, 34% and 25%, respectively), an effect observed only at 80 days. The limited Ni accumulation may be due to (1) the lack of an acidified stomach in zebrafish and/or (2) the efficient upregulation of Ni transport and excretion mechanisms, as indicated by the 4.5-fold increase in waterborne (63)Ni uptake by Ni-exposed fish. Eggs from Ni-exposed adults had Ni concentrations that were 5.2-fold higher than controls. However, by 4 days post fertilization, larvae had similar Ni concentrations as controls, demonstrating a capacity for rapid Ni depuration. Larvae from Ni-exposed adults were also more resistant to waterborne Ni (35% increase in the 96-h LC50 over controls). In conclusion, elevated dietary Ni significantly affected zebrafish reproduction despite only modest tissue Ni accumulation. There were also indications of adaptation, including increased Ni uptake rates and increased Ni tolerance of offspring from Ni-exposed adults. Ni concentrations were particularly elevated in the brain with exposure; possible relations to growth and reproductive impacts require further study.