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

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 literature review of dopamine in binge eating

OBJECTIVE

Binge eating, a core diagnostic symptom in binge eating disorder and bulimia nervosa, increases the risk of multiple physiological and psychiatric disorders. The neurotransmitter dopamine is involved in food craving, decision making, executive functioning, and impulsivity personality trait; all of which contribute to the development and maintenance of binge eating. The objective of this paper is to review the associations of dopamine levels/activities, dopamine regulator (e.g., dopamine transporter, degrading enzymes) levels/activities, and dopamine receptor availability/affinity with binge eating.

METHODS

A literature search was conducted in PubMed and PsycINFO to obtain human and animal studies published since 2010.

RESULTS

A total of 31 studies (25 human, six animal) were included. Among the human studies, there were 12 case-control studies, eight randomized controlled trials, and five cross-sectional studies. Studies used neuroimaging (e.g., positron emission tomography), genetic, and pharmacological (e.g., dopamine transporter inhibitor) techniques to describe or compare dopamine levels/activities, dopamine transporter levels/activities, dopamine degrading enzyme (e.g., catechol-O-methyltransferase) levels/activities, and dopamine receptor (e.g., D1, D2) availability/affinity among participants with and without binge eating. Most human and animal studies supported an altered dopaminergic state in binge eating (26/31, 83.9%); however, results were divergent regarding whether the altered state was hyperdopaminergic (9/26, 34.6%) or hypodopaminergic (17/26, 65.4%). The mixed findings may be partially explained by the variability in sample characteristics, study design, diagnosis criteria, and neuroimaging/genetic/pharmacological techniques used. However, it is possible that instead of being mutually exclusive, the hyperdopaminergic and hypodopaminergic state may co-exist, but in different stages of binge eating or in different individual genotypes.

CONCLUSIONS

For future studies to clarify the inconsistent findings, a homogenous sample that controls for confounders that may influence dopamine levels (e.g., psychiatric diseases) is preferable. Longitudinal studies are needed to evaluate whether the hyper- and hypo-dopaminergic states co-exist in different stages of binge eating or co-exist in individual phenotypes. Binge eating is characterized by eating a large amount of food in a short time and a feeling of difficulty to stop while eating. Binge eating is the defining symptom of binge eating disorder and bulimia nervosa, both of which are associated with serious health consequences. Studies have identified several psychological risk factors of binge eating, including a strong desire for food, impaired cognitive skills, and distinct personality traits (e.g., quick action without careful thinking). However, the physiological markers of binge eating remain unclear. Dopamine is a neurotransmitter that is heavily involved in feeding behavior, human motivation, cognitive ability, and personality. Therefore, dopamine is believed to play a critical role in binge eating. This review synthesized study findings related to the levels and activities of dopamine, dopamine regulators, and dopamine receptors in the context of binge eating. The primary finding is that most studies that used neuroimaging, genetic, or drug techniques found an altered dopaminergic state related to binge eating. However, the literature is inconsistent concerning the direction of the alteration. Considering the mixed findings and the limitations in study design, future studies, especially those that include repeated measurements, are needed to clarify the role of dopamine in binge eating.

Dopamine D1 receptor signalling in the lateral shell of the nucleus accumbens controls dietary fat intake in male rats

Central dopamine signaling regulates reward-related aspects of feeding behavior, and during diet-induced obesity dopamine receptor signaling is altered. Yet, the influence of dopamine signaling on the consumption of specific dietary components remains to be elucidated. We have previously shown that 6-hydroxydopamine-mediated lesions of dopamine neuron terminals in the lateral shell of the nucleus accumbens promotes fat intake in rats fed a multi-component free-choice high-fat high-sugar (fcHFHS) diet. It is however not yet determined which dopamine receptors are responsible for this shift towards fat preference. In this study, we assess the effects of D1-or D2 receptor acute inhibition in the lateral shell of the nucleus accumbens on fcHFHS diet consumption. We report that infusion of the D1 receptor antagonist SCH2 3390, but not the D2 receptor antagonist raclopride, promotes dietary fat consumption in male Sprague Dawley rats on a fcHFHS diet during 2 h after infusion. Furthermore, anatomical analysis of infusion sites revealed that the rostral region, but not the caudal region, of the lateral shell of the nucleus accumbens is sensitive to the D1 receptor inhibition effects on fat consumption. Our data highlight a role for D1 receptors in the rostral region of the lateral shell of the nucleus accumbens to control dietary fat consumption.

Midbrain and Lateral Nucleus Accumbens Dopamine Depletion Affects Free-choice High-fat high-sugar Diet Preference in Male Rats

Dopamine influences food intake behavior. Reciprocally, food intake, especially of palatable dietary items, can modulate dopamine-related brain circuitries. Among these reciprocal impacts, it has been observed that an increased intake of dietary fat results in blunted dopamine signaling and, to compensate this lowered dopamine function, caloric intake may subsequently increase. To determine how dopamine regulates food preference we performed 6-hydroxydopamine (6-OHDA) lesions, depleting dopamine in specific brain regions in male Sprague Dawley rats. Food preference was assessed by providing the rats with free choice access to control diet, fat, 20% sucrose and tap water. Rats with midbrain lesions targeting the substantia nigra (which is also a model of Parkinson's disease) consumed fewer calories, as reflected by a decrease in control diet intake, but they surprisingly displayed an increase in fat intake, without change in the sucrose solution intake compared to sham animals. To determine which of the midbrain dopamine projections may contribute to this effect, we next compared the impact of 6-OHDA lesions of terminal fields, targeting the dorsal striatum, the lateral nucleus accumbens and the medial nucleus accumbens. We found that 6-OHDA lesion of the lateral nucleus accumbens, but not of the dorsal striatum or the medial nucleus accumbens, led to increased fat intake. These findings indicate a role for lateral nucleus accumbens dopamine in regulating food preference, in particular the intake of fat.

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.

Stimuli predicting high-calorie reward increase dopamine release and drive approach to food in the absence of homeostatic need

Animals and humans are motivated to consume high-fat, high-calorie foods by cues predicting such foods. The neural mechanisms underlying this effect are not well understood.Objective: We tested the hypothesis that cues paired with a food reward, as compared to explicitly unpaired cues, increase rats' food-seeking behavior by potentiating dopamine release in the nucleus accumbens, and that this effect would be less evident under satiety.Methods: We used a simple discriminative stimulus task and electrochemical recordings of dopamine release in freely moving rats.Results: We found that both food-predictive cue and hunger increased conditioned approaches to the receptacle (food-seeking behavior indicated by movement to the food receptacle). In addition, we observed dopamine release when the food-predictive cue (but not the unpaired cue) was presented, independent of hunger or satiety. Finally, we found a positive correlation between dopamine release amplitude and the number of conditioned approaches to the food receptacle in the sated condition, but not in the hungry condition.Discussion: Our results suggest that dopamine could drive seeking behavior for calorie-dense food in absence of homeostatic need, a core aspect of binge eating disorders.

Dopamine D1 or D3 receptor modulators prevent morphine tolerance and reduce opioid withdrawal symptoms

The long-term treatment of chronic pain by opioids is limited by tolerance and risk of addiction/dependence. Previously, we have shown that combination treatment of morphine with a dopamine D₁ or D₃ receptor modulator restored morphine analgesia in morphine-resistant neuropathic pain and decreased morphine's reward potential in an acute setting. Here, we investigated whether such adjunct therapy with a dopamine D₁ receptor preferring antagonist (SCH 39166) or a dopamine D₃ receptor preferring agonist (pramipexole) could prevent morphine tolerance and reduce withdrawal symptoms. Initially, tolerance to the combination of morphine + pramipexole was assessed in mice. Mice receiving intraperitoneal injections of morphine showed reduced thermal thresholds on Day 7 whereas those receiving morphine + pramipexole maintained analgesia at Day 7. Next, tolerance and withdrawal to both combinations were tested over 14 days in rats. Rats were assigned one of four drug conditions, (1) saline, 2) morphine, 3) morphine + SCH 39166, 4) morphine + pramipexole), for chronic administration via osmotic pumps. Chronic administration of morphine over 14 days resulted in a significant reduction of morphine analgesia. However, analgesia was maintained when morphine was administered with either the dopamine D₁ receptor preferring antagonist or the D₃ receptor preferring agonist. Withdrawal symptoms peaked at 48 h and were decreased in rats receiving either combination compared to morphine alone. The data suggests that adjunct therapy with dopamine D₁ or D₃ receptor preferring modulators prevents morphine tolerance and reduces the duration of morphine withdrawal symptoms, and thus this combination has potential for long-term pain management therapy.

Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability

BACKGROUND

Insight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown.

METHODS

Bidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences.

RESULTS

Photoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning.

CONCLUSIONS

These results demonstrate a novel contribution of vHipp-NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp-NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people's food pleasantness ratings, hunger, and weight.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Local μ-Opioid Receptor Antagonism Blunts Evoked Phasic Dopamine Release in the Nucleus Accumbens of Rats

μ-opioid receptors (MORs) in the nucleus accumbens (NAc) can regulate reward-related behaviors that are dependent on mesolimbic dopamine, but the precise mechanism of this MOR regulation is unknown. We hypothesized that MORs within the NAc core regulate dopamine release. Specifically, we infused the MOR antagonist CTAP (d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2) into the NAc core while dopamine release was evoked by electrical stimulation of the ventral tegmental area and measured by fast-scan cyclic voltammetry. We report that CTAP dose-dependently inhibited evoked dopamine release, with full blockade achieved with the 8 μg infusion. In contrast, evoked dopamine release increased after nomifensine infusion and was unchanged after vehicle infusion. These findings demonstrate profound local control of dopamine release by MORs within the NAc core, which has implications for regulation of reward processing.

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.

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

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

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 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, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

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.

Activation of Dopamine Receptors in the Nucleus Accumbens Promotes Sucrose-Reinforced Cued Approach Behavior

Dopamine receptor activation in the nucleus accumbens (NAc) promotes vigorous environmentally-cued food-seeking in hungry rats. Rats fed ad libitum, however, respond to fewer food-predictive cues, particularly when the value of food reward is low. Here, we investigated whether this difference could be due to differences in the degree of dopamine receptor activation in the NAc. First, we observed that although rats given ad libitum access to chow in their home cages approached a food receptacle in response to reward-predictive cues, the number of such approaches declined as animals accumulated food rewards. Intriguingly, cued approach to food occurred in clusters, with several cued responses followed by successive non-responses. This pattern suggested that behavior was dictated by transitions between two states, responsive and non-responsive. Injection of D1 or D2 dopamine receptor agonists into the NAc dose-dependently increased cue responding by promoting transitions to the responsive state and by preventing transitions to the non-responsive state. In contrast, antagonists of either D1 or D2 receptors promoted long bouts of non-responding by inducing transitions to the non-responsive state and by preventing transitions to the responsive state. Moreover, locomotor behavior during the inter-trial interval was correlated with the responsive state, and was also increased by dopamine receptor agonists. These results suggest that activation of NAc dopamine receptors plays an important role in regulating the probability of approach to food under conditions of normative satiety.

Nociceptin receptor antagonist SB 612111 decreases high fat diet binge eating

Binge eating is a dysregulated form of feeding behavior that occurs in multiple eating disorders including binge-eating disorder, the most common eating disorder. Feeding is a complex behavioral program supported through the function of multiple brain regions and influenced by a diverse array of receptor signaling pathways. Previous studies have shown the overexpression of the opioid neuropeptide nociceptin (orphanin FQ, N/OFQ) can induce hyperphagia, but the role of endogenous nociceptin receptor (NOP) in naturally occurring palatability-induced hyperphagia is unknown. In this study we adapted a simple, replicable form of binge eating of high fat food (HFD). We found that male and female C57BL/6J mice provided with daily one-hour access sessions to HFD eat significantly more during this period than those provided with continuous 24h access. This form of feeding is rapid and entrained. Chronic intermittent HFD binge eating produced hyperactivity and increased light zone exploration in the open field and light-dark assays respectively. Treatment with the potent and selective NOP antagonist SB 612111 resulted in a significant dose-dependent reduction in binge intake in both male and female mice, and, unlike treatment with the serotonin selective reuptake inhibitor fluoxetine, produced no change in total 24-h food intake. SB 612111 treatment also significantly decreased non-binge-like acute HFD consumption in male mice. These data are consistent with the hypothesis that high fat binge eating is modulated by NOP signaling and that the NOP system may represent a promising novel receptor to explore for the treatment of binge eating.