Mu opioid receptor antagonism in the nucleus accumbens shell blocks consumption of a preferred sucrose solution in an anticipatory contrast paradigm

Diet quality, nutritional status and taste recognition are impaired in men with substance use disorder

This study aimed to (1) evaluate the nutritional status, prevalence of malnutrition and dietary habits in individuals using substances and (2) examine the possible effects of substance use on the perception of five basic tastes. Ninety male individuals with substance use disorder (SUD) (heroin = 78, cocaine = 12) and 32 non-users participated in the study conducted at Manisa Alcohol and Substance Addiction Treatment Center (AMATEM), Turkey. To determine the quality of the diet, the mean nutrient adequacy ratio (MAR) was calculated based on 24-h recall food consumption records of the individuals. Subjective Global Assessment (SGA) was employed to determine nutritional status, and anthropometric measurements were also taken from the individuals. The taste detection and recognition thresholds were determined with solutions with different concentrations for bitter, sour, sweet, umami and salty tastes and scored, with higher scores indicating lower thresholds. Mild-moderate malnutrition was determined in 50% of the individuals with SUD based on SGA. The body mass index (BMI) of individuals with SUD was found to be 21.2 ± 1.88 kg/m2 , and 24.1 ± 1.64 kg/m2 for non-users (p < 0.001). Diet quality, evaluated by MAR, was lower in individuals with SUD (54.7 ± 18.9%) than in non-users (93.5 ± 9.0%) (p < 0.001). The taste detection and taste recognition thresholds of individuals with SUD were impaired, and the threshold scores for sour, salty, sweet and umami taste recognition were significantly lower compared with non-users, with the lowest substance user threshold scores observed for the sweet recognition threshold. Standardised nutritional and behavioural interventions designed by dietitians should be provided for drug users in treatment centres and integrated with medical treatment practices.

Ketogenic diet: a potential adjunctive treatment for substance use disorders

Substance use disorders (SUD) can lead to serious health problems, and there is a great interest in developing new treatment methods to alleviate the impact of substance abuse. In recent years, the ketogenic diet (KD) has shown therapeutic benefits as a dietary therapy in a variety of neurological disorders. Recent studies suggest that KD can compensate for the glucose metabolism disorders caused by alcohol use disorder by increasing ketone metabolism, thereby reducing withdrawal symptoms and indicating the therapeutic potential of KD in SUD. Additionally, SUD often accompanies increased sugar intake, involving neural circuits and altered neuroplasticity similar to substance addiction, which may induce cross-sensitization and increased use of other abused substances. Reducing carbohydrate intake through KD may have a positive effect on this. Finally, SUD is often associated with mitochondrial damage, oxidative stress, inflammation, glia dysfunction, and gut microbial disorders, while KD may potentially reverse these abnormalities and serve a therapeutic role. Although there is much indirect evidence that KD has a positive effect on SUD, the small number of relevant studies and the fact that KD leads to side effects such as metabolic abnormalities, increased risk of malnutrition and gastrointestinal symptoms have led to the limitation of KD in the treatment of SUD. Here, we described the organismal disorders caused by SUD and the possible positive effects of KD, aiming to provide potential therapeutic directions for SUD.

Lesions of nucleus accumbens shell abolish socially transmitted food preferences

Rats adapt their food choices to conform to their conspecifics' dietary preferences. The nucleus accumbens shell is a relevant brain region to process reward-related and motivated behaviours and social information. Here, we hypothesize that the integrity of the nucleus accumbens shell is necessary to show socially transmitted food preferences. We made excitotoxic and sham lesions of nucleus accumbens shell in male Long-Evans rats who performed a social transmission of food preference task. In this task, observer rats revealed their original preference for one out of two food options. Afterward, they were exposed to a demonstrator rat who was fed with the observer's originally non-preferred food, and the observer's food choices were sampled again. Sham lesioned observer rats changed their food preferences following interaction with the demonstrator, specifically by increasing the intake of their originally non-preferred food type. This interaction-related change in preference was not found after nucleus accumbens shell lesions. The lesion effects on choice were not the consequence of impaired social or non-social motivation, anxiety or sensory or motor function, suggesting that they reflected a genuine deficit in social reward revaluation. These results highlight the role of nucleus accumbens shell in revaluating food rewards to match a conspecific's preferences.

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

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.

Insulin Bidirectionally Alters NAc Glutamatergic Transmission: Interactions between Insulin Receptor Activation, Endogenous Opioids, and Glutamate Release

Human fMRI studies show that insulin influences brain activity in regions that mediate reward and motivation, including the nucleus accumbens (NAc). Insulin receptors are expressed by NAc medium spiny neurons (MSNs), and studies of cultured cortical and hippocampal neurons suggest that insulin influences excitatory transmission via presynaptic and postsynaptic mechanisms. However, nothing is known about how insulin influences excitatory transmission in the NAc.

Human fMRI studies show that insulin influences brain activity in regions that mediate reward and motivation, including the nucleus accumbens (NAc). Insulin receptors are expressed by NAc medium spiny neurons (MSNs), and studies of cultured cortical and hippocampal neurons suggest that insulin influences excitatory transmission via presynaptic and postsynaptic mechanisms. However, nothing is known about how insulin influences excitatory transmission in the NAc. Furthermore, insulin dysregulation accompanying obesity is linked to cognitive decline, depression, anxiety, and altered motivation that rely on NAc excitatory transmission. Using whole-cell patch-clamp and biochemical approaches, we determined how insulin affects NAc glutamatergic transmission in nonobese and obese male rats and the underlying mechanisms. We find that there are concentration-dependent, bidirectional effects of insulin on excitatory transmission, with insulin receptor activation increasing and IGF receptor activation decreasing NAc excitatory transmission. Increases in excitatory transmission were mediated by activation of postsynaptic insulin receptors located on MSNs. However, this effect was due to an increase in presynaptic glutamate release. This suggested feedback from MSNs to presynaptic terminals. In additional experiments, we found that insulin-induced increases in presynaptic glutamate release are mediated by opioid receptor-dependent disinhibition. Furthermore, obesity resulted in a loss of insulin receptor-mediated increases in excitatory transmission and a reduction in NAc insulin receptor surface expression, while preserving reductions in transmission mediated by IGF receptors. These results provide the first insights into how insulin influences excitatory transmission in the adult brain, and evidence for a previously unidentified form of opioid receptor-dependent disinhibition of NAc glutamatergic transmission.

SIGNIFICANCE STATEMENT Data here provide the first insights into how insulin influences excitatory transmission in the adult brain, and identify previously unknown interactions between insulin receptor activation, opioids, and glutamatergic transmission. These data contribute to our fundamental understanding of insulin's influence on brain motivational systems and have implications for the use of insulin as a cognitive enhancer and for targeting of insulin receptors and IGF receptors to alter motivation.

Deletion of mu opioid receptors reduces palatable solution intake in a mouse model of binge eating

Binge eating in humans is driven by hedonic properties of food, suggesting that brain reward systems may contribute to this behaviour. We examined the role of mu opioid receptors (MOP) in binge eating by examining sweet solution intake in mice with genetic deletion of the MOP. Wildtype and MOP knockout mice had 4 hours access to food in the home cage combined with limited (4 hours) access to sucrose (17.1% w/v) or saccharin (0.09% w/v), or continuous (24 hours) access to sucrose. Only limited access groups exhibited binge intake, measured as increased solution consumption during the first hour. Knockout mice consumed less solution and food during the first hour as well as less food each day compared with wildtype mice. Limited access groups consumed more food and gained more weight than continuous access groups, and the effect was magnified in saccharin-consuming mice. Indeed, the increased food consumption in animals given limited access to saccharin was so excessive that caloric intake of this group was significantly higher than either of the sucrose groups (limited or continuous access). Within this group, females consumed more food per bodyweight than males, highlighting important sex differences in feeding behaviours under restricted access schedules.

Turkish version of the Yale Food Addiction Scale: preliminary results of factorial structure, reliability, and construct validity

BACKGROUND

Yale Food Addiction Scale (YFAS) was established to identify individuals exhibiting signs of addiction towards certain types of food. This study aimed to develop a Turkish version of the Yale Food Addiction Scale and test its psychometric properties.

METHODS

The backward translation techniques were used to develop Turkish versions of the YFAS, and its reproducibility was assessed. Turkish version of the YFAS was administered to a total of 1033 participants (439 men and 594 women), aged 19-65 years. Exploratory factor analysis and confirmatory factor analysis were used to examine the factorial structure of the tool. Construct validity was assessed by principal component factor analysis with varimax rotation. Reliabilities were estimated with Cronbach's alpha coefficient. The criterion-related validity was tested by the administration of Eating Attitude Test-26 (EAT-26) to all participants.

RESULTS

The primary factor loadings for seven items were ranged between 0.45 and 0.79, and no items cross-loaded onto other factors. The fit indices showed that eight items of the YFAS were a good representation of the item responses and each item loaded significantly on the specified factor (p < 0.001 for each). YFAS subscales had a high internal consistency and test-retest reliability. The criterion-related validity of the tool showed a positive relationship with scales of the EAT-26.

CONCLUSION

Current study suggested that the Turkish version of the YFAS is a reliable, valid, and useful tool for assessing the signs of food addiction in a non-clinical sample.

Endogenous opioids in the nucleus accumbens promote approach to high-fat food in the absence of caloric need

When relatively sated, people (and rodents) are still easily tempted to consume calorie-dense foods, particularly those containing fat and sugar. Consumption of such foods while calorically replete likely contributes to obesity. The nucleus accumbens (NAc) opioid system has long been viewed as a critical substrate for this behavior, mainly via contributions to the neural control of consumption and palatability. Here, we test the hypothesis that endogenous NAc opioids also promote appetitive approach to calorie-dense food in states of relatively high satiety. We simultaneously recorded NAc neuronal firing and infused a µ-opioid receptor antagonist into the NAc while rats performed a cued approach task in which appetitive and consummatory phases were well separated. The results reveal elements of a neural mechanism by which NAc opioids promote approach to high-fat food despite the lack of caloric need, demonstrating a potential means by which the brain is biased towards overconsumption of palatable food.

Imagine that you have just finished Thanksgiving dinner. You are completely full, having eaten large portions of turkey, green beans and mashed potatoes. Yet, despite feeling full, you still find yourself tempted by a slice of pie for dessert, maybe even with ice cream on top. Why is it that in such a state of fullness, you desire a slice of pie but not, say, another helping of green beans?

The answer may lie in the way the brain responds to food when we do not need any more calories. At such times, your brain drives you to continue eating only those foods that are tasty and calorie-dense. This preference for fatty and sweet foods may have been helpful in the past when we could not be certain where our next meal would come from. But in modern times, the widespread availability of food makes this preference potentially harmful. For example, the drive to consume fatty and sweet foods even when not hungry may now be contributing to soaring levels of obesity and type 2 diabetes.

What exactly is happening inside the brain to produce this behavior? Previous work has implicated a structure called the nucleus accumbens. When scientists activated proteins called mu opioid receptors within the nucleus accumbens, animals ate more of the foods that they find tasty. However, they were not as interested in eating more of the foods that they are more ambivalent towards.

Caref and Nicola now show that preventing opioid binding makes rats unwilling to respond to a cue to obtain cream, an appetizing, high-fat reward. It also abolishes the brain activity that drives the rats to respond the cue. Crucially, however, this effect only occurs in rats that are not hungry.

It therefore appears that opioid binding in the nucleus accumbens drives animals to approach and eat high-fat foods, but only when they do not need the calories. That is, it increases fat consumption in animals that are not actually hungry. A drug that selectively blocks mu opioid receptors in the nucleus accumbens may reduce this behavior. Such a drug could potentially help to prevent obesity and the health problems associated with it.

Hyperpalatability and the Generation of Obesity: Roles of Environment, Stress Exposure and Individual Difference

PURPOSE OF REVIEW

This review investigates how exposure to palatable food and its associated cues alters appetite regulation and feeding behaviour to drive overeating and weight gain.

RECENT FINDINGS

Both supraphysiological and physiological feeding systems are affected by exposure to palatable foods and its associated cues. Preclinical research, largely using rodents, has demonstrated that palatable food modulates feeding-related neural systems and food-seeking behaviour by recruiting the mesolimbic reward pathway. This is supported by studies in adolescents which have shown that mesolimbic activity in response to palatable food cues and consumption predicts future weight gain. Additionally, stress exposure, environmental factors and individual susceptibility have been shown to modulate the effects of highly palatable foods on behaviour. Further preclinical research using free-choice diets modelling the modern obesogenic environment is needed to identify how palatable foods drive overeating. Moreover, future clinical research would benefit from more appropriate quantification of palatability, making use of rating systems and surveys.

Nucleus Accumbens MC4-R Stimulation Reduces Food and Ethanol Intake in Adult Rats Regardless of Binge-Like Ethanol Exposure during Adolescence

The melanocortin (MC) system regulates feeding and ethanol consumption. Recent evidence shows that melanocortin 4 receptor (MC4-R) stimulation within the nucleus accumbens (NAc) elicits anorectic responses and reduces ethanol consumption and ethanol palatability in adult rats. Ethanol exposure during adolescence causes long-lasting changes in neural pathways critically involved in neurobehavioral responses to ethanol. In this regard, binge-like ethanol exposure during adolescence reduces basal alpha-melanocyte-stimulating hormone (α-MSH) and alters the levels of agouti-related peptide (AgRP) in hypothalamic and limbic areas. Given the protective role of MC against excessive ethanol consumption, disturbances in the MC system induced by binge-like ethanol exposure during adolescence might contribute to excessive ethanol consumption during adulthood. In the present study, we evaluated whether binge-like ethanol exposure during adolescence leads to elevated ethanol intake and/or eating disturbance during adulthood. Toward that aim, Sprague-Dawley rats were treated with ethanol (3 g/kg i.p.; BEP group) or saline (SP group) for 14 days (PND 25 to PND 38). On PND73, all the groups were given access to 20% ethanol on an intermittent schedule. Our results showed that adult rats given intermittent access (IAE) to 20% ethanol achieved high spontaneous ethanol intake that was not significantly enhanced by binge-like ethanol pretreatment during adolescence. However, BEP group exhibited an increase in food intake without a parallel increase in body weight (BW) relative to SP group suggesting caloric efficiency disturbance. Additionally, we evaluated whether binge-like ethanol exposure during adolescence alters the expected reduction in feeding and ethanol consumption following NAc shell administration of a selective MC4-R agonist in adult rats showing high rates of ethanol consumption. For that, animals in each pretreatment condition (SP and BEP) were divided into three subgroups and given bilateral NAc infusions of the selective MC4-R agonist cyclo(NH-CH₂-CH₂-CO-His-D-Phe-Arg-Trp-Glu)-NH₂ (0, 0.75 or 1.5 μg). Results revealed that MC4-R stimulation within the NAc reduced feeding and ethanol intake in high ethanol-drinking adult rats, regardless of previous binge-like ethanol exposure during adolescence, which adds new evidence regarding the dual ability of MC compounds to control excessive ethanol and food intake.

Dopamine and μ-opioid receptor dysregulation in the brains of binge-eating female rats - possible relevance in the psychopathology and treatment of binge-eating disorder

Adult, female rats given irregular, limited access to chocolate develop binge-eating behaviour with normal bodyweight and compulsive/perseverative and impulsive behaviours similar to those in binge-eating disorder. We investigated whether (a) dysregulated central nervous system dopaminergic and opioidergic systems are part of the psychopathology of binge-eating and (b) these neurotransmitter systems may mediate the actions of drugs ameliorating binge-eating disorder psychopathology. Binge-eating produced a 39% reduction of striatal D₁ receptors with 22% and 23% reductions in medial and lateral caudate putamen and a 22% increase of striatal μ-opioid receptors. There was no change in D₁ receptor density in nucleus accumbens, medial prefrontal cortex or dorsolateral frontal cortex, striatal D₂ receptors and dopamine reuptake transporter sites, or μ-opioid receptors in frontal cortex. There were no changes in ligand affinities. The concentrations of monoamines, metabolites and estimates of dopamine (dopamine/dihydroxyphenylacetic acid ratio) and serotonin/5-hydroxyindolacetic acid ratio turnover rates were unchanged in striatum and frontal cortex. However, turnover of dopamine and serotonin in the hypothalamus was increased ~20% and ~15%, respectively. Striatal transmission via D₁ receptors is decreased in binge-eating rats while μ-opioid receptor signalling may be increased. These changes are consistent with the attenuation of binge-eating by lisdexamfetamine, which increases catecholaminergic neurotransmission, and nalmefene, a μ-opioid antagonist.

Activation of mu-opioid receptors in the ventral pallidum decreases the negative hedonic evaluation of a conditioned aversive taste in rats

Conditioned taste aversion (CTA) causes a shift in the hedonic evaluation of a conditioned stimulus (CS) from positive to negative, and reduces the CS intake. Mu-opioid receptors (MORs) in the ventral pallidum (VP) are known to be involved in the hedonic evaluation of positive rewarding stimuli; however, their involvement in evaluation of a negative aversive stimulus is still unclear. To explore the neural mechanisms of the negative hedonic evaluation of the CS in CTA, we examined the effects of the activation of VP MORs on the behavioral responses of rats to a CS. Rats implanted with guide cannulae into the bilateral VP received a pairing of 5mM saccharin solution as a CS with an intraperitoneal injection of 0.15M lithium chloride as an unconditioned stimulus. On the test day, after microinjections of MOR agonist [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) into the VP, we observed the behavioral responses to the intraorally infused CS solution. The DAMGO injections caused a larger number of ingestive taste reactivity responses to the CS solution. We also measured the consumption of the CS solution in a separate group of rats, using a single-bottle test. The DAMGO injected rats drank a higher volume of the CS solution than the saline injected rats. These results indicate that the activation of MORs in the VP results in the attenuation of aversion to the CS solution, thereby inducing the larger CS intake. Therefore, it is likely that VP MORs are involved in not only positive but also negative hedonic evaluation.

Reassessing wanting and liking in the study of mesolimbic influence on food intake

Humans and animals such as rats and mice tend to overconsume calorie-dense foods, a phenomenon that likely contributes to obesity. One often-advanced explanation for why we preferentially consume sweet and fatty foods is that they are more "rewarding" than low-calorie foods. "Reward" has been subdivided into three interdependent psychological processes: hedonia (liking a food), reinforcement (formation of associations among stimuli, actions, and/or the food), and motivation (wanting the food). Research into these processes has focused on the mesolimbic system, which comprises both dopamine neurons in the ventral tegmental area and neurons in their major projection target, the nucleus accumbens. The mesolimbic system and closely connected structures are commonly referred to as the brain's "reward circuit." Implicit in this title is the assumption that "rewarding" experiences are generally the result of activity in this circuit. In this review, I argue that food intake and the preference for calorie-dense foods can be explained without reference to subjective emotions. Furthermore, the contribution of mesolimbic dopamine to food intake and preference may not be a general one of promoting or coordinating behaviors that result in the most reward or caloric intake but may instead be limited to the facilitation of a specific form of neural computation that results in conditioned approach behavior. Studies on the neural mechanisms of caloric intake regulation must address how sensory information about calorie intake affects not just the mesolimbic system but also many other forms of computation that govern other types of food-seeking and food-oriented behaviors.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Insights revealed by rodent models of sugar binge eating

Binge eating is seen across the spectrum of eating disorder diagnoses as well as among individuals who do not meet diagnostic criteria. Analyses of the specific types of foods that are frequently binged upon reveal that sugar-rich items feature prominently in binge-type meals, making the effects of binge consumption of sugar an important focus of study. One avenue to do this involves the use of animal models. Foundational and recent studies of animal models of sugar bingeing, both outlined here, lend insight into the various neurotransmitters and neuropeptides that may participate in or be altered by this behavior. Further, several preclinical studies incorporating sugar bingeing paradigms have explored the utility of pharmacological agents that target such neural systems for reducing sugar bingeing in an effort to enhance clinical treatment. Indeed, the translational implications of findings generated using animal models of sugar bingeing are considered here, along with potential avenues for further study.

The role of the opioid system in binge eating disorder

Binge eating disorder is characterized by excessive, uncontrollable consumption of palatable food within brief periods of time. Excessive intake of palatable food is thought to be driven by hedonic, rather than energy homeostatic, mechanisms. However, reward processing does not only comprise consummatory actions; a key component is represented by the anticipatory phase directed at procuring the reward. This phase is highly influenced by environmental food-associated stimuli, which can robustly enhance the desire to eat even in the absence of physiological needs. The opioid system (endogenous peptides and their receptors) has been strongly linked to the rewarding aspects of palatable food intake, and perhaps represents the key system involved in hedonic overeating. Here we review evidence suggesting that the opioid system can also be regarded as one of the systems that regulates the anticipatory incentive processes preceding binge eating hedonic episodes.

Intermittent-access binge consumption of sweet high-fat liquid does not require opioid or dopamine receptors in the nucleus accumbens

Binge eating disorders are characterized by episodes of intense consumption of high-calorie food. In recently developed animal models of binge eating, rats given intermittent access to such food escalate their consumption over time. Consumption of calorie-dense food is associated with neurochemical changes in the nucleus accumbens, including dopamine release and alterations in dopamine and opioid receptor expression. Therefore, we hypothesized that binge-like consumption on intermittent access schedules is dependent on opioid and/or dopamine neurotransmission in the accumbens. To test this hypothesis, we asked whether injection of dopamine and opioid receptor antagonists into the core and shell of the accumbens reduced consumption of a sweet high-fat liquid in rats with and without a history of intermittent binge access to the liquid. Although injection of a μ opioid agonist increased consumption, none of the antagonists (including μ opioid, δ opioid, κ opioid, D1 dopamine and D2 dopamine receptor antagonists, as well as the broad-spectrum opioid receptor antagonist naltrexone) reduced consumption, and this was the case whether or not the animals had a prior history of intermittent access. These results suggest that consumption of sweet, fatty food does not require opioid or dopamine receptor activation in the accumbens even under intermittent access conditions that resemble human binge episodes.

Temporal regulation of peripheral BDNF levels during cocaine and morphine withdrawal: comparison with a natural reward

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that has long been studied in the field of addiction and its importance in regulating drug addiction-related behavior has been widely demonstrated. The aim of our study was to analyze the consequences of a repeated exposure to drugs of abuse or natural reward on plasma BDNF levels during withdrawal.

METHODS

Rats were chronically injected with morphine (subcutaneously, 5mg/kg) or cocaine (intraperitoneally, 20mg/kg) or fed with a butter biscuit (per os, 4g) once per day for 14 days. Blood collection was performed on the 1st (withdrawal day 1 or WD1) or on (WD14), either at the same time point rats had been exposed to drugs or natural reward or at a different time point (used to quantify basal brain-derived neurotrophic factor levels).

RESULTS

Cocaine treatment led to a rapid (WD1) and persistent (WD14) decrease of basal BDNF levels compared with saline-treated animals, whereas morphine induced an increase on WD14 without any alteration on WD1. On the contrary, the natural reward induced a significant increase of basal brain-derived neurotrophic factor levels only on WD1. The analysis of BDNF levels at the usual time point at which animals had been exposed showed that both drugs, but not the natural reward, increased BDNF levels compared with basal levels.

CONCLUSION

Our data highlight that only drugs of abuse are able to persistently alter BDNF levels and to induce specific variations of this neurotrophic factor at the usual hour of injection.

A psycho-genetic study of hedonic responsiveness in relation to "food addiction"

While food addiction has no formally-recognized definition, it is typically operationalized according to the diagnostic principles established by the Yale Food Addiction Scale-an inventory based on the symptom criteria for substance dependence in the DSM-IV. Currently, there is little biologically-based research investigating the risk factors for food addiction. What does exist has focused almost exclusively on dopaminergic reward pathways in the brain. While brain opioid signaling has also been strongly implicated in the control of food intake, there is no research examining this neural circuitry in the association with food addiction. The purpose of the study was therefore to test a model predicting that a stronger activation potential of opioid circuitry-as indicated by the functional A118G marker of the mu-opioid receptor gene-would serve as an indirect risk factor for food addiction via a heightened hedonic responsiveness to palatable food. Results confirmed these relationships. In addition, our findings that the food-addiction group had significantly higher levels of hedonic responsiveness to food suggests that this bio-behavioral trait may foster a proneness to overeating, to episodes of binge eating, and ultimately to a compulsive and addictive pattern of food intake.

Hormonal and neural mechanisms of food reward, eating behaviour and obesity

With rising rates of obesity, research continues to explore the contributions of homeostatic and hedonic mechanisms related to eating behaviour. In this Review, we synthesize the existing information on select biological mechanisms associated with reward-related food intake, dealing primarily with consumption of highly palatable foods. In addition to their established functions in normal feeding, three primary peripheral hormones (leptin, ghrelin and insulin) play important parts in food reward. Studies in laboratory animals and humans also show relationships between hyperphagia or obesity and neural pathways involved in reward. These findings have prompted questions regarding the possibility of addictive-like aspects in food consumption. Further exploration of this topic may help to explain aberrant eating patterns, such as binge eating, and provide insight into the current rates of overweight and obesity.

The effects of nucleus accumbens μ-opioid and adenosine 2A receptor stimulation and blockade on instrumental learning

Prior research has shown that glutamate and dopamine receptors in the nucleus accumbens (NAcc) core are critical for the learning of an instrumental response for food reinforcement. It has also been demonstrated that μ-opioid and adenosine A2A receptors within the NAcc impact feeding and motivational processes. In these experiments, we examined the potential roles of NAcc μ-opioid and A2A receptors on instrumental learning and performance. Sprague-Dawley rats were food restricted and trained to lever press following daily intra-accumbens injections of the A2A receptor agonist CGS 21680 (at 0.0, 6.0, or 24.0ng/side), the A2A antagonist pro-drug MSX-3 (at 0.0, 1.0, or 3.0μg/side), the μ-opioid agonist DAMGO (at 0.0, 0.025, or 0.025μg/side), or the opioid receptor antagonist naltrexone (at 0.0, 2.0 or 20.0μg/side). After five days, rats continued training without drug injections until lever pressing rates stabilized, and were then tested with a final drug test to assess potential performance effects. Stimulation, but not inhibition, of NAcc adenosine A2A receptors depressed lever pressing during learning and performance tests, but did not impact lever pressing on non-drug days. Both μ-opioid receptor stimulation and blockade inhibited learning of the lever-press response, though only naltrexone treatment caused impairments in lever-pressing after the task had been learned. The effect of A2A receptor stimulation on learning and performance were consistent with known effects of adenosine on effort-related processes, whereas the pattern of lever presses, magazine approaches, and pellet consumption following opioid receptor manipulations suggested that their effects may have been driven by drug-induced shifts in the incentive value of the sugar reinforcer.

Opioid hedonic hotspot in nucleus accumbens shell: mu, delta, and kappa maps for enhancement of sweetness "liking" and "wanting"

A specialized cubic-millimeter hotspot in the rostrodorsal quadrant of medial shell in nucleus accumbens (NAc) of rats may mediate opioid enhancement of gustatory hedonic impact or "liking". Here, we selectively stimulated the three major subtypes of opioid receptors via agonist microinjections [mu (DAMGO), delta (DPDPE), or kappa (U50488H)] and constructed anatomical maps for functional localizations of consequent changes in hedonic "liking" (assessed by affective orofacial reactions to sucrose taste) versus "wanting" (assessed by changes in food intake). Results indicated that the NAc rostrodorsal quadrant contains a shared opioid hedonic hotspot that similarly mediates enhancements of sucrose "liking" for mu, delta, and kappa stimulations. Within the rostrodorsal hotspot boundaries each type of stimulation generated at least a doubling or higher enhancement of hedonic reactions, with comparable intensities for all three types of opioid stimulation. By contrast, a negative hedonic coldspot was mapped in the caudal half of medial shell, where all three types of opioid stimulation suppressed "liking" reactions to approximately one-half normal levels. Different anatomical patterns were produced for stimulation of food "wanting", reflected in food intake. Altogether, these results indicate that the rostrodorsal hotspot in medial shell is unique for generating opioid-induced hedonic enhancement, and add delta and kappa signals to mu as hedonic generators within the hotspot. Also, the identification of a separable NAc caudal coldspot for hedonic suppression, and separate NAc opioid mechanisms for controlling food "liking" versus "wanting" further highlights NAc anatomical heterogeneity and localizations of function within subregions of medial shell.