Baclofen reduces fat intake under binge-type conditions

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

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

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.

Prospects for new drugs to treat binge-eating disorder: Insights from psychopathology and neuropharmacology

BACKGROUND

Binge-eating disorder (BED) is a common psychiatric condition with adverse psychological and metabolic consequences. Lisdexamfetamine (LDX) is the only approved BED drug treatment. New drugs to treat BED are urgently needed.

METHODS

A comprehensive review of published psychopathological, pharmacological and clinical findings.

RESULTS

The evidence supports the hypothesis that BED is an impulse control disorder with similarities to ADHD, including responsiveness to catecholaminergic drugs, for example LDX and dasotraline. The target product profile (TPP) of the ideal BED drug combines treating the psychopathological drivers of the disorder with an independent weight-loss effect. Drugs with proven efficacy in BED have a common pharmacology; they potentiate central noradrenergic and dopaminergic neurotransmission. Because of the overlap between pharmacotherapy in attention deficit hyperactivity disorder (ADHD) and BED, drug-candidates from diverse pharmacological classes, which have already failed in ADHD would also be predicted to fail if tested in BED. The failure in BED trials of drugs with diverse pharmacological mechanisms indicates many possible avenues for drug discovery can probably be discounted.

CONCLUSIONS

(1) The efficacy of drugs for BED is dependent on reducing its core psychopathologies of impulsivity, compulsivity and perseveration and by increasing cognitive control of eating. (2) The analysis revealed a large number of pharmacological mechanisms are unlikely to be productive in the search for effective new BED drugs. (3) The most promising areas for new treatments for BED are drugs, which augment noradrenergic and dopaminergic neurotransmission and/or those which are effective in ADHD.

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

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

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

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

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

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

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GABA_BR signaling in mesolimbic system does not affect energy balance

Pattern of access determines influence of junk food diet on cue sensitivity and palatability

AIMS

Like drug addiction, cues associated with palatable foods can trigger food-seeking, even when sated. However, whether susceptibility to the motivating influence of food-related cues is a predisposing factor in overeating or a consequence of poor diet is difficult to determine in humans. Using a rodent model, we explored whether a highly palatable 'junk food' diet impacts responses to reward-paired cues in a Pavlovian-to-instrumental transfer test, using sweetened condensed milk (SCM) as the reward. The hedonic impact of SCM consumption was also assessed by analyzing licking microstructure.

METHODS

To probe the effects of pattern and duration of junk food exposure, we provided rats with either regular chow ad libitum (controls) or chow plus access to junk food for either 2 or 24 h per day for 1, 3, or 6 weeks. We also examined how individual susceptibility to weight gain related to these measures.

RESULTS

Rats provided 24 h access to the junk food diet were insensitive to the motivational effects of a SCM-paired cue when tested sated even though their hedonic experience upon reward consumption was similar to controls. In contrast, rats provided restricted, 2 h access to junk food exhibited a cue generalization phenotype under sated conditions, lever-pressing with increased vigor in response to both a SCM-paired cue, and a cue not previously paired with reward. Hedonic response was also significantly higher in these animals relative to controls.

CONCLUSIONS

These data demonstrate that the pattern of junk food exposure differentially alters the hedonic impact of palatable foods and susceptibility to the motivating influence of cues in the environment to promote food-seeking actions when sated, which may be consequential for understanding overeating and obesity.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Junk Food Exposure Disrupts Selection of Food-Seeking Actions in Rats

There is growing evidence that repeated consumption of highly palatable, nutritionally poor "junk food" diets can produce deficits in cognition and behavioral control. We explored whether long-term junk-food diet exposure disrupts rats' ability to make adaptive choices about which foods to pursue based on (1) expected reward value (outcome devaluation test) and (2) cue-evoked reward expectations (Pavlovian-to-instrumental test). Rats were initially food restricted and trained on two distinct response-outcome contingencies (e.g., left press chocolate pellets, and right press sweetened condensed milk) and stimulus-outcome contingencies (e.g., white noise chocolate pellets, and clicker sweetened condensed milk). They were then given 6 weeks of unrestricted access to regular chow alone (controls) or chow and either 1 or 24 h access to junk food per day. Subsequent tests of decision making revealed that rats in both junk-food diet groups were impaired in selecting actions based on either expected food value or the presence of food-paired cues. These data demonstrate that chronic junk food consumption can disrupt the processes underlying adaptive control over food-seeking behavior. We suggest that the resulting dysregulation of food seeking may contribute to overeating and obesity.

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.

Can We Selectively Reduce Appetite for Energy-Dense Foods? An Overview of Pharmacological Strategies for Modification of Food Preference Behavior

Excessive intake of food, especially palatable and energy-dense carbohydrates and fats, is largely responsible for the growing incidence of obesity worldwide. Although there are a number of candidate antiobesity drugs, only a few of them have been proven able to inhibit appetite for palatable foods without the concurrent reduction in regular food consumption. In this review, we discuss the interrelationships between homeostatic and hedonic food intake control mechanisms in promoting overeating with palatable foods and assess the potential usefulness of systemically administered pharmaceuticals that impinge on the endogenous cannabinoid, opioid, aminergic, cholinergic, and peptidergic systems in the modification of food preference behavior. Also, certain dietary supplements with the potency to reduce specifically palatable food intake are presented. Based on human and animal studies, we indicate the most promising therapies and agents that influence the effectiveness of appetite-modifying drugs. It should be stressed, however, that most of the data included in our review come from preclinical studies; therefore, further investigations aimed at confirming the effectiveness and safety of the aforementioned medications in the treatment of obese humans are necessary.

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.

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

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

Environments predicting intermittent shortening access reduce operant performance but not home cage binge size in rats

When non-food-deprived rats are given brief access to vegetable shortening (a semi-solid fat used in baked products) on an intermittent basis (Monday, Wednesday, Friday), they consume significantly more and emit more operant responses for shortening than a separate group of rats given brief access to shortening every day. Since both groups are traditionally housed in the same room, it is possible that the environmental cues associated with placing shortening in the cages (e.g., investigator in room, cages opening and closing, etc.) provide predictable cues to the daily group, but unpredictable cues to the intermittent group. The present study examined the effects of providing predictable environmental cues to an isolated intermittent group in order to examine the independent contributions of intermittency and predictability on intake and operant performance. Two groups of rats were housed in the same room, with one group provided 30-min intermittent (INT) access and the second group provided 30-min daily access (D) to shortening. A third group (ISO) of rats was housed in a room by themselves in which all environmental cues associated with intermittent shortening availability were highly predictable. After five weeks of home cage shortening access, all rats were then exposed to several different operant schedules of reinforcement. The INT and ISO groups consumed significantly more shortening in the home cage than the D group. In contrast, the INT group earned significantly more reinforcers than both the ISO and D groups under all but one of the reinforcement schedules, while ISO and D did not differ. These data indicate that intermittent access will generate binge-type eating in the home cage independent of cue predictability. However, predictable cues in the home cage reduce operant responding independent of intermittent access.

Intermittent access to sweet high-fat liquid induces increased palatability and motivation to consume in a rat model of binge consumption

Binge eating disorders are characterized by discrete episodes of rapid and excessive food consumption. In rats, giving intermittent access to sweet fat food mimics this aspect of binge eating. These models typically employ solid food; however, the total amount consumed depends on motivation, palatability and satiety, which are difficult to dissociate with solid food. In contrast, lick microstructure analysis can be used to dissociate these parameters when the ingestant is a liquid. Therefore, we developed a binge model using a liquid emulsion composed of corn oil, heavy cream and sugar. We show that rats given intermittent access to this high-fat emulsion develop binge-like behavior comparable to that previously observed with solid high-fat food. One feature of this behavior was a gradual escalation in consumption across 2.5 weeks of intermittent access, which was not apparent in rats given lower-fat liquid on the same access schedule. Lick microstructure analysis suggests that this escalation was due at least in part to increases in both motivation to consume and palatability-driven consumption.

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

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

Effects of the GABAB receptor agonist baclofen on primary drinking in rats

The effects of subcutaneous (s.c.) administration of the GABA(B) receptor agonist baclofen were investigated on primary drinking in rats. Baclofen (1-4 mg/kg) produced a dose-related reduction in cumulative water intake in 16 h water-deprived rats during the 120 min measurement period (Experiment 1). The suppressant effect of baclofen (2mg/kg) on water intake 16 h water-deprived rats was significantly attenuated by pretreatment with the GABA(B) receptor antagonist CGP 35348 (3-aminopropyl (diethoxymethyl)-phosphinic acid; 50mg/kg; s.c., Experiment 2.), indicating that the hypodipsic effects of the drug in thirsty rats are mediated by an action at GABA(B) receptors. Experiment 3 was undertaken to investigate the effects of baclofen on volemic drinking induced in rats pretreated with propylene glycol. S.C. administration of polyethylene glycol induces volemic drinking in rats by reducing extracellular fluid. Baclofen (2mg/kg, s.c.) significantly reduced the volemic drinking in rats pretreated with polyethylene glycol (30% w/v solution). Experiment 4 was conducted to investigate the effects of baclofen on osmotic drinking in non-deprived rats pretreated with hypertonic sodium chloride (NaCl) solution. Hypertonic NaCl will draw out intracellular fluid to stimulate osmotic drinking. Baclofen (2mg/kg; s.c.) significantly reduced osmotic drinking in rats pretreated with 1 ml hypertonic NaCl (16% w/v). The results of this study indicate that (i) the hypodipsic effect of baclofen in water-deprived rats is mediated by an action at GABA(B) receptors and (ii) baclofen suppresses both volemic and osmotic drinking.

Effect of Hypericum perforatum Extract in an Experimental Model of Binge Eating in Female Rats

Purpose. The present study evaluated the effect of Hypericum perforatum dry extract in an experimental model of binge eating (BE). Methods. BE for highly palatable food (HPF) was evoked in female rats by three 8-day cycles of food restriction/re-feeding and acute stress on the test day (day 25). Stress was induced by preventing access to HPF for 15 min, while rats were able to see and smell it. Hypericum perforatum dry extract was given by gavage. Results. Only rats exposed to both food restrictions and stress exhibited BE. The doses of 250 and 500 mg/kg of Hypericum perforatum extract significantly reduced the BE episode, while 125 mg/kg was ineffective. The same doses did not affect HPF intake in the absence of BE. The dose of 250 mg/kg did not significantly modify stress-induced increase in serum corticosterone levels, suggesting that the effect on BE is not due to suppression of the stress response The combined administration of 125 mg/kg of Hypericum perforatum together with Salidroside, active principle of Rhodiola rosea, produced a synergic effect on BE. Conclusions. The present results indicate for the first time that Hypericum perforatum extracts may have therapeutic properties in bingeing-related eating disorders.

Fat emulsion composition alters intake and the effects of baclofen

Thickened oil-in-water emulsions are useful model foods in rat studies due to their high acceptance and similarity to foods consumed by humans. Previous work from this laboratory used oil-in-water emulsions thickened with a biopolymer blend containing starch. Intake and effects of baclofen, a GABA-B agonist that decreases fat intake and drug self-administration, were reported, but the contribution of starch was not assessed. In the present study, intake and effects of baclofen were assessed in rats using emulsions prepared with two fat types (32% vegetable shortening, 32% corn oil) and thickened with three biopolymer blends. One biopolymer blend contained starch and the other two did not. Daily 1-h intake of the vegetable shortening emulsion containing starch was significantly greater than the other emulsions. When starch was added to the emulsions originally containing no starch, intake significantly increased. Baclofen generally reduced intake of all emulsions regardless of starch content and stimulated intake of chow. However, effects were more often significant for vegetable shortening emulsions. This report: (1) demonstrates that products used to prepare thickened oil-in-water emulsions have significant effects on rat ingestive behavior, and (2) confirms the ability of baclofen to reduce consumption of fatty foods, while simultaneously stimulating intake of chow.

Memantine reduces consumption of highly palatable food in a rat model of binge eating

Excessive consumption of highly palatable food has been linked to the development of eating disorders and obesity, and can be modeled in non-food-deprived rats by offering them a limited (2-h daily) access to an optional dietary fat. Since the glutamatergic system has recently emerged as a viable target for binge-eating medication development, we compared the effects of subchronic treatment with glutamatergic receptor antagonists to the effects of a reference appetite-suppressing agent sibutramine on highly palatable food (lard) and normal chow intake. In three separate experiments, the consumption of a standard laboratory chow and lard were measured during 12 days of medication treatment and for 6 days afterwards. Generalized estimating equations analysis demonstrated that sibutramine (7.5 mg/kg, p.o.) significantly decreased lard consumption, with a concurrent increase in chow consumption. Sibutramine effects disappeared after treatment discontinuation. The NMDA receptor antagonist memantine (5 mg/kg, i.p.) significantly decreased lard consumption and increased chow consumption, comparable to effects of sibutramine; however, memantine's effects persisted after treatment discontinuation. The effects of the mGluR5 antagonist MTEP (7.5 mg/kg, i.p.) on food consumption were in the same direction as seen with memantine, but the observed differences were not significant. In an additional control experiment, sibutramine and memantine reduced unlimited (24 h) chow intake during the treatment phase. Present results provide evidence that glutamatergic neurotransmission might be involved in the regulation of excessive consumption of highly palatable foods, and suggest that NMDA receptor may be an attractive target for developing obesity and disordered eating pharmacotherapies.

Effects of intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats measured under different feeding conditions

The effects of intraperitoneal (i.p.) administration of the GABA(B) receptor agonist baclofen were assessed in rats under different feeding conditions. In Experiment 1, it was observed that baclofen (1-4 mg/kg) significantly (at least, P<0.05) increased cumulative food intake in non-deprived rats during the 120 min measurement period during the early light phase of the light-dark cycle. By contrast, during the early dark phase of the light-dark cycle in non-deprived rats, the 1mg/kg doses of baclofen significantly increased cumulative feeding at 30, 60 and 120 min (at least P<0.05), the 2mg/kg dose significantly increased feeding at 30 and 60 min (at least P<0.05) and the 4 mg/kg dose had no effects on feeding. In Experiment 2, baclofen (1-4 mg/kg) was found to produce no significant effects on food intake in rats that were food-deprived for 22 h. In Experiment 3, the effects of baclofen were investigated on food intake in 16 h food-deprived rats that had received an oral preload for 2h prior to drug administration. Baclofen (1-4 mg/kg) significantly increased cumulative food consumption (at least, P<0.05) only during the first 30 min after administration in these animals. The results of this study indicate that the effects of baclofen on food intake may be related to the state of hunger or satiety of the animals and the time during the light-dark cycle when the drug is administered.

A model of binge-like eating behavior in mice that does not require food deprivation or stress

Binge eating disorder (BED) is characterized by excessive food intake during a short period of time and is often associated with obesity. Mouse models of binge-like eating behavior are lacking making it difficult to employ genetic models in the identification of mechanisms regulating excessive eating. We report a rapid and simple model to induce binge-like eating behavior in mice that does not require food deprivation or exogenous stressors. Weekly 24 h access to a nutritionally complete high energy diet (HED), along with continuous access to standard chow, resulted in a significant increase in HED intake following its presentation compared to mice that had continuous access to both diets. Mice exhibiting binge-like eating consumed one-third of their normal total daily caloric intake within 2.5 h of HED presentation. Moreover, total 24-h caloric intakes were increased by 50% in mice exhibiting binge-like eating. Following repeated cycles, binge-like eating of the HED was maintained over several weeks with no evidence of habituation or significant alterations in body weight and adiposity. Pharmacological evaluation of binge-like eating behavior was performed using clinically employed compounds. Interestingly, binge-like eating was dose-dependently decreased by fluoxetine, but not baclofen or topiramate. These data support clinical validation of this mouse model of binge-like eating behavior, as fluoxetine has been shown to reduce binge frequency in human subjects with BED. The availability of transgenic and knockout mice will allow for the determination of genes that are involved in the initiation and maintenance of binge-like eating behavior.

Food addiction and obesity: evidence from bench to bedside

Obesity has become a major health problem and epidemic. However, much of the current debate has been fractious and etiologies of obesity have been attributed to eating behavior or fast food, personality issues, depression, addiction, or genetics. One of the interesting new hypotheses for epidemic obesity is food addiction, which is associated with both substance-related disorder and eating disorder. Accumulating evidences have shown that there are many shared neural and hormonal pathways as well as distinct differences that may help researchers find why certain individuals overeat and become obese. Functional neuroimaging studies have further revealed that good or great smelling, looking, tasting, and reinforcing food has characteristics similar to that of drugs of abuse. Many of the brain changes reported for hedonic eating and obesity are also seen in various forms of addictions. Most importantly, overeating and obesity may have an acquired drive like drug addiction with respect to motivation and incentive; craving, wanting, and liking occur after early and repeated exposures to stimuli. The acquired drive for great food and relative weakness of the satiety signal would cause an imbalance between the drive and hunger/reward centers in the brain and their regulation.

Reinforcing efficacy of fat, as assessed by progressive ratio responding, depends upon availability not amount consumed

Intermittent limited access to an optional source of dietary fat can induce binge-type behavior in rats. However, the ability of such access to alter the reinforcing efficacy of fat has not been clearly demonstrated. In this study, performance under progressive ratio one (PR1) and three (PR3) schedules of shortening (fat) reinforcement was assessed in non-food deprived rats (n=15/group). One group of rats had intermittent access to a dietary fat option (INT, 1-hour shortening access in the home cage each Monday, Wednesday, and Friday), whereas the other group had daily access to the fat option (D, 1-hour shortening access daily). Chow and water were continuously available. After five weeks, the INT group consumed more shortening during the 1-hour access period than did the D group. Rats were then trained to lever press for a solid shortening reinforcer (0.1 gm). INT rats earned significantly more reinforcers than did D rats under PR1, but not under PR3. Subgroups of INT and D rats (n=7 each) were matched on the amount of shortening consumed in the home cage during week five of the protocol and the PR data were reanalyzed. The INT subgroup earned significantly more reinforcers than the D subgroup did under PR1, but not PR3. These results demonstrate that: (1) intermittent access to shortening in the home cage, but not the amount consumed during the access period (i.e. bingeing), increases the reinforcing efficacy of solid shortening; and (2) the type of PR schedule is critical in delineating differences between the groups.

Positive effect of baclofen on body weight reduction in obese subjects: a pilot study

BACKGROUND AND OBJECTIVE

We previously reported that baclofen, a GABA(B) agonist, reduces body weight in obese mice. In this clinical study, we examined the possibility that baclofen might serve as an effective therapeutic agent for obesity in humans.

METHODS

Seven women and three men ranging from 25-51 years of age and 31.3-41.0 kg/m(2) body mass index were included in the study. Baclofen, which was started at a dose of 15 mg/day and increased to 30 mg/day in 10 days, was administered for 12 weeks.

RESULTS

Administration of baclofen for 12 weeks significantly decreased both body weight (baseline: 93.30 ± 9.80 kg, 12 weeks: 91.70 ± 10.30 kg) and waist circumference (baseline: 107.9 ± 9.0 cm, 12 weeks: 105.6 ± 10.0 cm). Serum leptin levels, which possibly reflect the amount of adipose stores, were also decreased significantly by the treatment. On the other hand, there were no significant changes in the blood pressure or metabolism of glucose and lipid. No adverse effects were observed.

CONCLUSION

Although the anti-obesity effects of baclofen in the present study were mild, this pilot study suggests the possibility that baclofen might serve as a new anti-obesity drug in humans.

Baclofen, raclopride, and naltrexone differentially affect intake of fat and sucrose under limited access conditions

Gamma-aminobutyric acid (GABA), dopamine, and opioids are implicated in impulse control, addiction and binge eating. Recent evidence suggests that sucrose alters the effects of GABAergic, dopaminergic, and opioid receptor ligands on consumption of a fatty food in a rat limited-access binge protocol. This study determined the independent effects of fat and sucrose on the efficacy of these ligands under limited-access conditions. Nonfood-deprived male Sprague-Dawley rats had 1 h access to fat (vegetable shortening) or sucrose (3.2, 10, or 32% w/v). Half had intermittent access (Monday, Wednesday, Friday) and half had daily access. Effects of baclofen (GABAB agonist), SCH 23390 (D1 antagonist), raclopride (D2 antagonist), and naltrexone (opioid antagonist) were assessed. Baclofen and naltrexone reduced fat intake regardless of the access schedule. Baclofen had no effect on sucrose intake; naltrexone reduced sucrose intake at higher doses than were required to reduce fat intake. Raclopride stimulated fat intake in intermittent-access rats and had no effect in daily-access rats; raclopride reduced sucrose intake in all groups. SCH 23390 reduced intake in a nonspecific manner. The results indicate the involvement of GABAB receptors in fat but not sucrose intake, and of D2 receptor dysfunction in rats with a history of bingeing on fat.

Baclofen suppresses binge eating of pure fat but not a sugar-rich or sweet-fat diet

Baclofen is a γ-aminobutyric acid-B agonist that is known to reduce the intake of some drugs of abuse. Binge eating of sugar or fat has been shown to have behavioral and neurochemical similarities to drug abuse, and may be special cases suggestive of natural addiction. To determine whether a treatment for drug abuse would have an effect on binge eating, and if so, which type of food intake might be affected, this study compared the effects of baclofen on binge eating sucrose, fat, and a sweet-fat combination. Rats were maintained for 21 days on a schedule of 12-h daily access to (i) a 10% sucrose solution, (ii) vegetable fat, or (iii) a commercially available sweet-fat chow. A fourth group had only 2-h daily access to vegetable fat. All four experimental groups, plus a control group, had ad libitum access to water and standard rodent chow. Food intake was then measured after intraperitoneal administration of baclofen (0, 0.6, 1.0, or 1.8 mg/kg). Results showed that although there was no effect of drug on standard chow intake of rats in any group, baclofen stimulated binge eating of sweet-fat food, suppressed binge eating of pure fat (vegetable shortening) in the group with 2-h access, and had no effect on sucrose binges. These results support earlier findings of a suppressive effect of baclofen on binge eating of fat and introduce a new finding that the drug differentially affects binge eating of sucrose and a sugar-fat combination.

Baclofen, raclopride, and naltrexone differentially affect intake of fat/sucrose mixtures under limited access conditions

This study assessed the effects of the opioid antagonist naltrexone, the dopamine 2-like (D2) antagonist raclopride, and the GABA(B) agonist baclofen on consumption of fat/sucrose mixtures (FSM) using a limited access protocol. Sixty male Sprague-Dawley rats were grouped according to two schedules of access (Daily [D] or Intermittent [I]) to an optional FSM. Each FSM was created by whipping 3.2% (L), 10% (M), or 32% (H) powdered sugar into 100% vegetable shortening in a w/w manner (n=10 per group). One-hour intakes of the IL and IM groups were significantly greater than intakes of the respective DL and DM groups, thus fulfilling our operational definition of binge-type eating in these groups. Baclofen reduced intakes of the L and M mixtures regardless of access schedule, but failed to reduce intake of the H mixture. Naltrexone reduced intake in all groups, but potency was greater in IL rats than in DL rats. Furthermore, potency was attenuated in Intermittent rats, but enhanced in Daily rats, at higher sucrose concentrations. Raclopride reduced intake in the DL and stimulated intake in the IL groups, reduced intake in both M groups, and was without effect in both H groups. These results indicate that fat/sucrose mixtures containing relatively low concentrations of sucrose allow distinctions to be made between: 1) intakes stimulated by different access schedules and 2) opioid and dopaminergic modulation of those intakes. These results also suggest that brief bouts of food consumption involving fatty, sugar-rich foods may prove to be particularly resistant to pharmacological intervention.

Ovarian hormones inhibit fat intake under binge-type conditions in ovariectomized rats

Binge eating is more common in females than in males. This study investigated the effects of ovarian hormones on binge-eating behavior in a diet-related rat model. Six groups of ovariectomized Sprague-Dawley rats were used (n=13/group). All rats had continuous access to chow and water throughout the study. One half of the rats were injected every fourth day with estradiol benzoate (2 microg/100 microl sesame oil) and progesterone (500 microg/100 microl sesame oil); the other half received only the sesame oil vehicle. Three feeding protocols were tested in each hormone injection condition: (1) chow only: no additional dietary fat access; (2) low-restriction: 1-h fat access every day; (3) high-restriction: 1-h fat access on Monday, Wednesday, and Friday. As previously reported in intact male and female rats, the high-restriction groups exhibited binge-like increases in 1-h energy intake during fat access. The major new finding of this study is that 1-h energy intake was tonically, but not cyclically, reduced in the hormone-treated high-restriction (binge) rats. Specifically, both low- and high-restriction hormone-treated rats consumed significantly less energy than did the oil-treated rats during the 1-h fat period (p<0.0001) and overall (p<0.0001), indicating a tonic inhibition of eating. However, food intake during the 1-h fat access period was also cyclically reduced in the hormone-treated low-restriction rats, but not in the hormone-treated high-restriction rats. These results indicate that the normal cyclic inhibitory influence of ovarian hormones on eating, but not their normal tonic inhibitory influence, is disrupted by conditions leading to binge-type eating.

The effects of chronic intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats

This study was undertaken to examine the effects of repeated administration of the GABA(B) receptor agonist baclofen on food intake in male Wistar rats. In the 1st Experiment, the effects of daily administration of physiological saline and baclofen (2 mg/kg, i.p.) for 27 days were investigated on food intake and body weight in non-deprived rats (n=6 in each group). Baclofen significantly (P<0.05) increased cumulative food intake each day over the treatment period during the 60 min measurement period following administration. Tolerance did not develop to the short-term hyperphagic effect of baclofen over the course of the experiment. In addition, treatment with baclofen did not alter body weight of the animals over the 27 day treatment period when compared with the saline control rats. In the 2nd Experiment, the effects of acute and chronic administration of baclofen (2 mg/kg) were investigated on 24 h food intake in rats. The rats were injected daily for 21 days with either saline (n=6) or baclofen (n=6). Food intake was measured in 30 min time bins for 24 h on treatment Days 1, 12 and 21 following injection. The results showed that while baclofen produced short-term increases in food consumption following injection on treatment Days 1, 12 and 21, the daily (24 h) food intake of the animals was not significantly different from those of control rats. Thus, these data reveal that while chronic administration of baclofen (2 mg/kg) produces short-term increases in feeding without the development of tolerance, daily (24 h) food consumption is not affected. These findings are consistent with the observation that chronic administration of baclofen (2 mg/kg) had no effect on the body weight of these animals.

Baclofen, raclopride, and naltrexone differentially reduce solid fat emulsion intake under limited access conditions

Previous work in rats has demonstrated that an Intermittent (Monday, Wednesday, Friday) schedule of access promotes binge-type consumption of 100% vegetable shortening during a 1-h period of availability. The present study used novel shortening-derived stable solid emulsions of various fat concentrations. These emulsions were the consistency of pudding and did not demonstrate oil and water phase separation previously reported with oil-based liquid emulsions. Male Sprague-Dawley rats were grouped according to schedule of access (Daily or Intermittent) to one of three concentrations (18%, 32%, 56%) of solid fat emulsion. There were no significant Intermittent vs. Daily differences in amount consumed, due to high intakes in all groups. This indicated the acceptability of the emulsions. Baclofen (GABA(B) agonist) and raclopride (D2-like antagonist) both significantly reduced emulsion intake in all Daily groups, but only in the 56% fat Intermittent group. Naltrexone (opioid antagonist), in contrast, significantly reduced 32% and 56% fat emulsion intake in the Intermittent, as well as the Daily groups. These results indicate that the fat intake-reducing effects of GABA(B) activation and D(2) blockade depend upon fat concentration and schedule of fat access, while the fat intake-reducing effects of opioid blockade depend upon fat concentration but not schedule of access.

Baclofen for binge eating: an open-label trial

OBJECTIVE

Baclofen is a GABA-B agonist that may be useful in the treatment of substance use disorders, and also reduces 'binge-like' eating in rodents. We hypothesized that baclofen might be effective in reducing binge eating episodes in binge eating disorder (BED) and bulimia nervosa (BN).

METHOD

Seven women with BED (n = 4) or BN (n = 3) took baclofen (60 mg/day) for 10 weeks.

RESULTS

Six out of seven patients completed the full 10-week trial. Five out of seven participants (3 BED; 2 BN) demonstrated 50% or greater reduction of frequency of binge eating from beginning to end of the study. Three out of seven participants (2 BED; 1 BN) were free of binge eating at study end. Four out of seven participants elected to continue baclofen at study end. Baclofen was well tolerated by the participants.

CONCLUSION

In this open-label trial, baclofen was associated with decreased binge eating frequency in patients with BED and BN.

Liquid sucrose bingeing in rats depends on the access schedule, concentration and delivery system

Previous studies have reported binge-type consumption of solid vegetable shortening in non-food deprived rats maintained on schedules of limited shortening access. The current study determined if limited access would promote binge-type consumption of sucrose solutions. Adult male rats (6 groups, n = 10 each) were provided with one of three different sucrose concentrations (3.2%, 10%, 32% w/v) for 2 h either everyday (Daily) or Monday, Wednesday, and Friday (Intermittent). A 'binge' during the 2-h access periods was operationally defined as Intermittent intakes significantly greater than Daily intakes. Sucrose initially was provided in a 100 ml glass tube equipped with a stainless-steel drinking spout. Under these conditions, there were no differences in sucrose intake between Daily and Intermittent groups at any of the concentrations. In contrast, when sucrose was provided in a modified 60 ml plastic syringe with the same drinking spout, intakes of the Intermittent groups consuming 3.2% and 10% sucrose were greater than those of the respective Daily groups, indicating that binge-type consumption of sucrose occurred. These results demonstrate that brief, intermittent access to low and moderate concentrations of sucrose can promote binge-type behavior, and the characteristics of the drinking apparatus can affect sucrose intake.

Evidence for sugar addiction: behavioral and neurochemical effects of intermittent, excessive sugar intake

[Avena, N.M., Rada, P., Hoebel B.G., 2007. Evidence for sugar addiction: Behavioral and neurochemical effects of intermittent, excessive sugar intake. Neuroscience and Biobehavioral Reviews XX(X), XXX-XXX]. The experimental question is whether or not sugar can be a substance of abuse and lead to a natural form of addiction. "Food addiction" seems plausible because brain pathways that evolved to respond to natural rewards are also activated by addictive drugs. Sugar is noteworthy as a substance that releases opioids and dopamine and thus might be expected to have addictive potential. This review summarizes evidence of sugar dependence in an animal model. Four components of addiction are analyzed. "Bingeing," "withdrawal," "craving" and "cross-sensitization" are each given operational definitions and demonstrated behaviorally with sugar bingeing as the reinforcer. These behaviors are then related to neurochemical changes in the brain that also occur with addictive drugs. Neural adaptations include changes in dopamine and opioid receptor binding, enkephalin mRNA expression and dopamine and acetylcholine release in the nucleus accumbens. The evidence supports the hypothesis that under certain circumstances rats can become sugar dependent. This may translate to some human conditions as suggested by the literature on eating disorders and obesity.

Regulation of intravenous cocaine self-administration in rats selectively bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (i.v.) cocaine self-administration more rapidly than their low saccharin (LoS)-consuming counterparts. The present experiment was designed to determine whether HiS and LoS rats differed in other aspects of drug abuse.

OBJECTIVE

The purpose of the present experiment was to use a two-lever dose self-selection procedure to investigate the regulation/dysregulation of i.v. cocaine self-administration in female HiS and LoS rats.

MATERIALS AND METHODS

HiS and LoS rats were trained to self-administer eight different doses of cocaine during daily 5-h sessions, with the cocaine doses ranging from 0.2 to 1.6 mg/kg in steps of 0.2 mg/kg. The dose size increased after a response on one lever (infusion duration lengthened by 3 s) and decreased after a response on the other lever (infusion duration shortened by 3 s), with a lower limit of 0 s and with an upper limit of 24 s (a corresponding range of 0 to 1.6 mg/kg); the animals increased or decreased their self-administered dose in nine discrete steps.

RESULTS

The HiS rats showed less precise regulation of their postinfusion interval (PII) than LoS rats based on the size of the previously self-administered cocaine dose. Correlations between these variables were lower for HiS than for LoS rats during the acquisition and maintenance phases, and HiS rats had lower PIIs than LoS rats after many of the cocaine doses. The HiS rats also self-administered significantly more cocaine infusions during the maintenance phase than the LoS rats, especially at the highest dose.

CONCLUSIONS

These data indicate that HiS rats are more likely to self-administer more cocaine infusions, at higher doses, and with less precise dose-time regulation than LoS rats. Thus, rats selectively bred for HiS showed less inhibitory control over their cocaine intake than LoS rats, suggesting that a genetic predisposition for saccharin preference is related to the rewarding effects of i.v. cocaine.

Effects of baclofen on operant performance for food pellets and vegetable shortening after a history of binge-type behavior in non-food deprived rats

Operant performance of non-food deprived rats (n=8) was assessed under progressive ratio (PR) and concurrent PR-fixed ratio schedules of food pellet and/or vegetable shortening reinforcement. Post operant baselines, rats were matched and divided into 2 groups based upon the schedule of shortening availability: High restriction binge group (H, 1-hr home cage shortening access each week on Monday, Wednesday, and Friday) and Low restriction (L, 1-hr shortening access daily). Chow and water were continuously available; only access to the shortening was restricted. After 8 weeks, operant performance was reassessed. Lever pressing for shortening increased in the H rats for all schedules, but was either unaffected or decreased in the L rats. Pellet responding under the concurrent schedules increased for both groups. The effects of four dosages of (R)-baclofen (0.3-1.8 mg/kg, i.p.) on operant performance were also assessed. For both groups, 1.0 mg/kg baclofen significantly reduced shortening responding relative to saline for all schedules except one, but had no or minimal effect on pellet responding. This suggests a specific effect of baclofen on responding maintained by fat. These results indicate that intermittent episodes of bingeing on fat can increase the reinforcing efficacy of fat and that GABAB receptor activation can attenuate this effect.

Sucrose sham feeding on a binge schedule releases accumbens dopamine repeatedly and eliminates the acetylcholine satiety response

Drinking a sugar solution on an intermittent schedule can promote sugar bingeing and cause signs of dependence while releasing dopamine repeatedly like a drug of abuse. It is hypothesized that sweet taste alone is sufficient for this effect in sucrose bingeing rats. On the theory that acetylcholine in the nucleus accumbens plays a role in satiety, it is further hypothesized that purging the stomach contents will delay acetylcholine release. Rats with gastric fistulas and nucleus accumbens guide shafts for microdialysis were fed 12 h each day. During the first hour, fistulas were open for the sham-feeding group and closed for the real-feeding group, and 10% sucrose was the only food source. For the remaining 11 h, liquid rodent diet was available as well as the 10% sucrose to provide a balanced diet. In microdialysis tests during the first sugar meal on days 1, 2 and 21, extracellular dopamine increased at least 30% each day in both groups. Acetylcholine also increased during the sugar meals for the real-feeding animals, but not during sham feeding. In conclusion, the taste of sugar can increase extracellular dopamine in the nucleus accumbens without fail in animals on a dietary regimen that causes bingeing and sugar dependency. During sham feeding, the acetylcholine satiation signal is eliminated, and the animals drink more. These findings support the hypothesis that dopamine is released repeatedly in response to taste when bingeing on sweet food, and the acetylcholine satiety effect is greatly reduced by purging; this may be relevant to bulimia nervosa in humans.