Excessive sugar intake alters binding to dopamine and mu-opioid receptors in the brain

Preliminary validation of the Yale Food Addiction Scale

Previous research has found similarities between addiction to psychoactive substances and excessive food consumption. Further exploration is needed to evaluate the concept of "food addiction," as there is currently a lack of psychometrically validated measurement tools in this area. The current study represents a preliminary exploration of the Yale Food Addiction Scale (YFAS), designed to identify those exhibiting signs of addiction towards certain types of foods (e.g., high fat and high sugar). Survey data were collected from 353 respondents from a stratified random sample of young adults. In addition to the YFAS, the survey assessed eating pathology, alcohol consumption and other health behaviors. The YFAS exhibited adequate internal reliability, and showed good convergent validity with measures of similar constructs and good discriminant validity relative to related but dissimilar constructs. Additionally, the YFAS predicted binge-eating behavior above and beyond existing measures of eating pathology, demonstrating incremental validity. The YFAS is a sound tool for identifying eating patterns that are similar to behaviors seen in classic areas of addiction. Further evaluation of the scale is needed, especially due to a low response rate of 24.5% and a non-clinical sample, but confirmation of the reliability and validity of the scale has the potential to facilitate empirical research on the concept of "food addiction".

Compensatory weight gain due to dopaminergic hypofunction: new evidence and own incidental observations

There is increasing evidence for a role of dopamine in the development of obesity. More specifically, dopaminergic hypofunction might lead to (over)compensatory food intake. Overeating and resulting weight gain may be induced by genetic predisposition for lower dopaminergic activity, but might also be a behavioral mechanism of compensating for decreased dopamine signaling after dopaminergic overstimulation, for example after smoking cessation or overconsumption of high palatable food. This hypothesis is in line with our incidental finding of increased weight gain after discontinuation of pharmaceutical dopaminergic overstimulation in rats. These findings support the crucial role of dopaminergic signaling for eating behaviors and offer an explanation for weight-gain after cessation of activities associated with high dopaminergic signaling. They further support the possibility that dopaminergic medication could be used to moderate food intake.

Relation between obesity and blunted striatal response to food is moderated by TaqIA A1 allele

The dorsal striatum plays a role in consummatory food reward, and striatal dopamine receptors are reduced in obese individuals, relative to lean individuals, which suggests that the striatum and dopaminergic signaling in the striatum may contribute to the development of obesity. Thus, we tested whether striatal activation in response to food intake is related to current and future increases in body mass and whether these relations are moderated by the presence of the A1 allele of the TaqIA restriction fragment length polymorphism, which is associated with dopamine D2 receptor (DRD2) gene binding in the striatum and compromised striatal dopamine signaling. Cross-sectional and prospective data from two functional magnetic resonance imaging studies support these hypotheses, which implies that individuals may overeat to compensate for a hypofunctioning dorsal striatum, particularly those with genetic polymorphisms thought to attenuate dopamine signaling in this region.

Bingeing, self-restriction, and increased body weight in rats with limited access to a sweet-fat diet

OBJECTIVE

Prior research has shown that fasting alternated with a diet of standard rodent chow and a 10% sucrose solution produces bingeing on the sucrose, but animals remain at normal body weight. The present study investigated whether restricted access to a highly palatable combination of sugar and fat, without food deprivation, would instigate binge eating and also increase body weight.

METHODS AND PROCEDURES

Male rats were maintained for 25 days on one of four diets: (i) sweet-fat chow for 2 h/day followed by ad libitum standard chow, (ii) 2-h sweet-fat chow only 3 days/week and access to standard chow the rest of the time, (iii) ad libitum sweet-fat chow, or (iv) ad libitum standard chow.

RESULTS

Both groups with 2-h access to the sweet-fat chow exhibited bingeing behavior, as defined by excessively large meals. The body weight of these animals increased due to large meals and then decreased between binges as a result of self-restricted intake of standard chow following binges. However, despite these fluctuations in body weight, the group with 2-h access to sweet-fat chow every day gained significantly more weight than the control group with standard chow available ad libitum.

DISCUSSION

These findings may have implications for the body weight fluctuations associated with binge-eating disorder, as well as the relationship between binge eating and the obesity epidemic.

Activation of mesolimbic dopamine neurons during novel and daily limited access to palatable food is blocked by the opioid antagonist LY255582

An analog of the trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine series (LY255582) exhibits high in vitro binding affinity and antagonist potency for the μ-, δ-, and κ-opioid receptors. In vivo, LY255582 exhibits potent effects in reducing food intake and body weight in several rodent models of obesity. In the present study, we evaluated the effects of LY255582 to prevent the consumption of a highly palatable (HP) diet (a high-fat/high-carbohydrate diet) both when the food was novel and following daily limited access to the HP diet. Additionally, we examined the effects of consumption of the HP diet and of LY255582 treatment on mesolimbic dopamine (DA) signaling by in vivo microdialysis. Consumption of the HP diet increased extracellular DA levels within the nucleus accumbens (NAc) shell. Increased DA in the NAc shell was not related to the quantity of the HP diet consumed, and the DA response did not habituate following daily scheduled access to the HP diet. Interestingly, treatment with LY255582 inhibited consumption of the HP diet and the HP diet-associated increase in NAc shell DA levels. Moreover, the increased HP diet consumption observed following daily limited access to the HP diet was completely prevented by LY255582 treatment. LY255582 may be a useful tool in understanding the neural mechanisms involved in the reinforcement mechanisms regulating food intake.

The taste of sugars

Sugars evoke a distinctive perceptual quality ("sweetness" in humans) and are generally highly preferred. The neural basis for these phenomena is reviewed for rodents, in which detailed electrophysiological measurements have been made. A receptor has been identified that binds sweeteners and activates G-protein-mediated signaling in taste receptor cells, which leads to changes in neural firing rates in the brain, where perceptions of taste quality, intensity, and palatability are generated. Most cells in gustatory nuclei are broadly tuned, so quality perception presumably arises from patterns of activity across neural populations. However, some manipulations affect only the most sugar-oriented cells, making it useful to consider them as a distinct neural subtype. Quality perception may also arise partly due to temporal patterns of activity to sugars, especially within sugar-oriented cells that give large but delayed responses. Non-specific gustatory neurons that are excited by both sugars and unpalatable stimuli project to ventral forebrain areas, where neural responses provide a closer match with behavioral preferences. This transition likely involves opposing excitatory and inhibitory influences by different subgroups of gustatory cells. Sweeteners are generally preferred over water, but the strength of this preference can vary across time or between individuals, and higher preferences for sugars are often associated with larger taste-evoked responses.

After daily bingeing on a sucrose solution, food deprivation induces anxiety and accumbens dopamine/acetylcholine imbalance

Bingeing on sugar may activate neural pathways in a manner similar to taking drugs of abuse, resulting in related signs of dependence. The present experiments test whether rats that have been bingeing on sucrose and then fasted demonstrate signs of opiate-like withdrawal. Rats were maintained on 12-h deprivation followed by 12-h access to a 10% sucrose solution and chow for 28 days, then fasted for 36 h. These animals spent less time on the exposed arm of an elevated plus-maze compared with a similarly deprived ad libitum chow group, suggesting anxiety. Microdialysis revealed a concomitant increase in extracellular acetylcholine and decrease in dopamine release in the nucleus accumbens shell. These results did not appear to be due to hypoglycemia. The findings suggest that a diet of bingeing on sucrose and chow followed by fasting creates a state that involves anxiety and altered accumbens dopamine and acetylcholine balance. This is similar to the effects of naloxone, suggesting opiate-like withdrawal. This may be a factor in some eating disorders.

Sugar bingeing in rats

Bingeing behavior is characteristic of many eating disorders. This unit describes an animal model of sugar bingeing. This model has been used successfully to elicit behavioral and neurochemical signs of sugar dependence in rats, e.g., indices of bingeing, withdrawal, increased intake after abstinence (deprivation effect), cross-sensitization with amphetamine, and increases in dopamine release in the nucleus accumbens due to repeated bingeing.

Altered dopamine D2 receptor function and binding in obese OLETF rat

A decrease in D2-like receptor (D2R) binding in the striatum has been reported in obese individuals and drug addicts. Although natural and drug rewards share neural substrates, it is not clear whether such effects also contribute to overeating on palatable meals as an antecedent of dietary obesity. Therefore, we investigated receptor density and the effect of the D2R agonist quinpirole (0.05, 0.5 mg/kg, S.C.) on locomotor activity and sucrose intake in a rat model of diet-induced obesity, the CCK-1 receptor-deficient Otsuka Long Evans Tokushima Fatty (OLETF) rat. Compared to age-matched lean controls (LETO), OLETF rats expressed significantly lower [125I]-iodosulpride binding in the accumbens shell (-16%, p<0.02). Whereas the high dose of quinpirole increased motor activity in both strains equally, the low dose reduced activity more in OLETF. Both doses significantly reduced sucrose intake in OLETF but not LETO rats. These findings demonstrate an altered D2R signaling in obese OLETF rats similar to drug-induced sensitization and suggest a link between this effect and avidity for sucrose in this model.

Can artificial sweeteners help control body weight and prevent obesity?

The possible role played by artificial sweeteners in the long-term maintenance of body weight is considered. Although artificial sweeteners can play a role in a short-term energy-controlled diet, the evidence that they are helpful over a longer period is limited. In those in the recommended weight range there is evidence of compensation; that is, the consumption of low-energy foods is followed by an increased energy intake to make up the lost energy. Energy compensation is more likely in those not displaying dietary restraint. The desire to remove sugar from the diet reflects an assumption that its intake is associated with obesity. However, the consumption of energy-dense food, that almost entirely reflects a high fat and low water content, is the best predictor of obesity. Diets offering a high proportion of energy in the form of carbohydrate tend to contain low levels of fat. There are several reports that the use of artificial sweeteners leads to an increased consumption of fat. The weak ability of fat to satisfy hunger makes it easy to overeat fatty foods; in contrast, carbohydrates promote a feeling of ‘fullness’. Various short-term studies have found that carbohydrate consumed as a liquid, rather than a solid, is more likely to result in weight gain.

Accumbens dopamine-acetylcholine balance in approach and avoidance

Understanding systems for approach and avoidance is basic for behavioral neuroscience. Research on the neural organization and functions of the dorsal striatum in movement disorders, such as Huntington's and Parkinson's Disease, can inform the study of the nucleus accumbens (NAc) in motivational disorders, such as addiction and depression. We propose opposing roles for dopamine (DA) and acetylcholine (ACh) in the NAc in the control of GABA output systems for approach and avoidance. Contrary to DA, which fosters approach, ACh release is a correlate or cause of meal satiation, conditioned taste aversion and aversive brain stimulation. ACh may also counteract excessive DA-mediated approach behavior as revealed during withdrawal from drugs of abuse or sugar when the animal enters an ACh-mediated state of anxiety and behavioral depression. This review summarizes evidence that ACh is important in the inhibition of behavior when extracellular DA is high and the generation of an anxious or depressed state when DA is relatively low.

Higher locomotor response to cocaine in female (vs. male) rats selectively bred for high (HiS) and low (LoS) saccharin intake

Rats selectively bred for high saccharin consumption (HiS) self-administer more oral ethanol and i.v. cocaine than those selectively bred for low saccharin consumption (LoS). Male and female drug-seeking-prone (HiS) and -resistant (LoS) rats were used in the present experiment to test the prediction that cocaine-induced locomotor activity and sensitization varied with sex and their selective breeding status (HiS and LoS). All rats were intermittently exposed over 2 weeks to pairs of sequential saline and cocaine injections, separated by 45 min. The first 5 pairs of injections, each separated by 2-3 days (10-12 days total), were given to examine the development of cocaine-induced locomotor activity and the development of locomotor sensitization, which was determined by comparing the effects of cocaine injection 1 with injection 6 (given 2 weeks after the 5 pairs of intermittent injections). Results indicated that after the first injection pair (saline, cocaine) the HiS and LoS groups did not differ (saline vs. cocaine) in locomotor activity; however, after cocaine injection pairs 1, 5, and 6, HiS females were more active than HiS males and LoS females. There were also significant phenotype differences (HiS>LoS) in locomotor activity after cocaine injections 5 and 6. There was a weak sensitization effect in cocaine-induced locomotor activity in HiS females after cocaine injection 5 (compared to 1); however it was not present after injection 6 or in other groups. The lack of a strong sensitization effect under these temporal and dose conditions was inconsistent with previous reports. However, the results showing HiS>LoS and females>males on cocaine-induced activity measures are consistent with several measures of cocaine-seeking behavior (acquisition, maintenance, escalation, extinction, and reinstatement), and they suggest that cocaine-induced locomotor activity and sensitization are behavioral markers of drug-seeking phenotypes.

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.

Naloxone attenuates incubated sucrose craving in rats

RATIONALE

Cue-induced craving precedes drug relapse and contributes to eating disorders. Opiate antagonists have been demonstrated to be effective at reducing cravings for drugs and food. Craving, as defined as responding for a stimulus previously associated with a reward, increases, or incubates, over forced abstinence in an animal model of relapse.

OBJECTIVES

This paper aims to determine anticraving effects of the opiate antagonist, naloxone, on the incubation of sucrose craving.

METHODS

106 male Long-Evans rats lever pressed for 10% sucrose solution 2 h/day for 10 days. On either day 1 or 30 of forced abstinence, rats responded in extinction for 6 h and then were injected (ip) with either saline or naloxone (0.001, 0.01, 0.1, 1, or 10 mg/kg). The rats then responded for 1 h for presentation of a tone + light cue previously presented with every sucrose delivery during self-administration training.

RESULTS

The rats responded more in extinction and following saline on day 30 vs day 1 (an incubation of craving). Except for a trend for a decrease in responding following 10 mg/kg on day 1, naloxone was primarily effective on day 30. On day 30, naloxone significantly reduced responding at all doses except for 0.1 mg/kg.

CONCLUSIONS

The time-dependent increase in sensitivity to an opiate antagonist is consistent with time-dependent changes in the opiate system following forced abstinence from sucrose. These changes may partly underlie the incubation of sucrose craving. In addition, these findings could be used to support the use of naloxone as an anticraving medication in protracted abstinence.

Neural mechanisms underlying obesity and drug addiction

Increasing rates of obesity have alarmed health officials and prompted much public dialogue. While the factors leading to obesity are numerous, an inability to control intake of freely available food is central to the problem. In order to understand this, we need to better define the mechanisms by which the brain regulates food intake, and why it is often difficult to control consumption. From this point of view, it seems valuable to consider the commonalities between food intake and drug abuse. While research in the two fields has historically emphasized different neural substrates, recent data have increased interest in better defining elements that may underlie both drug addiction and obesity. Here we discuss some of these shared elements with an emphasis on emerging areas of research that better define common mechanisms leading to overconsumption.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

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.

Decreases in dietary preference produce increased emotionality and risk for dietary relapse

BACKGROUND

Obesity is a modern health epidemic, with the overconsumption of highly palatable, calorically dense foods as a likely contributor. Despite the known consequences of obesity, behavioral noncompliance remains high, supporting the powerful rewarding properties of such foods. We hypothesized that exposure to preferred diets would result in an amelioration of stress responsivity via activation of reward pathways that would be reversed during dietary withdrawal, increasing the risk for relapse and treatment failure.

METHODS

Mice were exposed to preferred diets high in fat or carbohydrates for 4 weeks and then were withdrawn to house chow. Behavioral, physiologic, and biochemical assays were performed to examine changes in stress and reward pathways.

RESULTS

These studies revealed significant changes in arousal and anxiety-like behaviors, limbic corticotropin-releasing factor expression, and expression of reward-related signaling molecules in response to the highly preferred high-fat diet that was reversed by withdrawal. In a dietary-reinstatement model, mice withdrawn from the high-fat diet endured an aversive environment to gain access to the preferred food.

CONCLUSIONS

Exposure to a highly preferred diet high in fat reduces stress sensitivity, whereas acute withdrawal from such a diet elevates the stress state and reduces reward, contributing to the drive for dietary relapse.

Opioids for hedonic experience and dopamine to get ready for it

BACKGROUND AND RATIONALE

More than two decades ago, Wise proposed his "anhedonia hypothesis" to explain the role of dopamine in motivated behaviors. The hypothesis posits that dopamine mediates the pleasure experienced by reward obtainment. However, some experimental findings have contested this hypothesis and several authors have proposed alternative functions for dopamine with regard to motivation. Brain dopamine has been suggested to rather code for the preparatory aspects of behavior, while brain opioids seem to mediate the perception of the hedonic properties of rewards.

OBJECTIVES

The main goal of this review is to reexamine dopamine and opioids involvement in feeding when different aspects such as the anticipatory, motivational and consummatory components of this behavior are taken into account, but also when the physiologic state of the organism and the palatability of the food are considered.

RESULTS AND CONCLUSIONS

Altogether, the data presented point out for an implication of dopamine in the anticipatory/preparatory aspects of feeding more than on the motivational and consummatory aspects. However, dopamine involvement in the anticipatory/preparatory component of feeding seems specifically related to very relevant stimuli, such as highly palatable foods. On the other hand, our data, as well as those present in the literature, strongly suggest a role for opioids in food intake through their modulation of the hedonic perception of food. As a consequence, opioids are involved in those aspects of motivation driven by food palatability rather than by food homeostatic need.

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.

Central opioids and consumption of sweet tastants: when reward outweighs homeostasis

Numerous reports have described opioids as peptides involved in the regulation of food intake. The role of these endogenous substances appears to be linked with reward-dependent feeding, since injection of opioid receptor ligands alters consumption of palatable foods and solutions more readily than of non-palatable ones, and intake of such tastants affects the activity of the opioid system within the brain. Among a variety of available foods, those rich in sucrose and other sweet tastants, are extremely appealing to humans and laboratory animals. In the current review, we focus on the rewarding aspects of consummator behavior driven by opioids. We attempt to delineate opioid-dependent central mechanisms responsible for overconsumption of "rewarding" palatable diets, especially foods high in sugar that can potentially jeopardize homeostasis.

Modulation of parathion toxicity by glucose feeding: Is nitric oxide involved?

Glucose feeding can markedly exacerbate the toxicity of the anticholinesterase insecticide, parathion. We determined the effects of parathion on brain nitric oxide and its possible role in potentiation of toxicity by glucose feeding. Adult rats were given water or 15% glucose in water for 3 days and challenged with vehicle or parathion (18 mg/kg, s.c.) on day 4. Functional signs, plasma glucose and brain cholinesterase, citrulline (an indicator of nitric oxide production) and high-energy phosphates (HEPs) were measured 1-3 days after parathion. Glucose feeding exacerbated cholinergic toxicity. Parathion increased plasma glucose (15-33%) and decreased cortical cholinesterase activity (81-90%), with no significant differences between water and glucose treatment groups. In contrast, parathion increased brain regional citrulline (40-47%) and decreased HEPs (18-40%) in rats drinking water, with significantly greater changes in glucose-fed rats (248-363% increase and 31-61% decrease, respectively). We then studied the effects of inhibiting neuronal nitric oxide synthase (nNOS) by 7-nitroindazole (7NI, 30 mg/kg, i.p. x4) on parathion toxicity and its modulation by glucose feeding. Co-exposure to parathion and 7NI led to a marked increase in cholinergic signs of toxicity and lethality, regardless of glucose intake. Thus, glucose feeding enhanced the accumulation of brain nitric oxide following parathion exposure, but inhibition of nitric oxide synthesis was ineffective at counteracting increased parathion toxicity associated with glucose feeding. Evidence is therefore presented to suggest that nitric oxide may play both toxic and protective roles in cholinergic toxicity, and its precise contribution to modulation by glucose feeding requires further investigation.

Acute methylphenidate treatments reduce sucrose intake in restricted-fed bingeing rats

Recent evidence suggests that methylphenidate HCl may be effective at limiting the frequency and the amount of binge eating. The present study investigated if daily treatments with methylphenidate reduced the bingeing-like behavior observed in restricted-fed adult male rats. Three groups (n = 6) received peripheral injections of methylphenidate in doses of 1.5 or 0.75 mg/kg/day, or saline, 3 days prior and 7 days during a previously characterized intermittent feeding regimen that results in a gradual increase of sucrose and food intake. The higher, but not the lower, dose of methylphenidate reduced sucrose intake to an asymptotic level starting after 3 days of the feeding protocol and concurrently led to an increase in the intake of chow. The high dose methylphenidate group also had two-fold lower plasma insulin levels compared with the saline-treated animals at the time of sacrifice on the last day of the feeding regimen. Further histological assays revealed that the methylphenidate treatments, irrespective of the dose used, resulted in selectively higher dopamine transporter and D2-like receptor labeled bindings in the shell region of the nucleus accumbens. These results suggest that relatively low-dose methylphenidate treatments may be effective for the management of binge eating by reducing the intake of palatable foods and may not interfere with short-term regulation of energy balance. These findings further support the notion that the mesoaccumbens dopamine system plays an important role in restricted access-induced sucrose bingeing in this rat model.

The anxiogenic drug yohimbine reinstates palatable food seeking in a rat relapse model: a role of CRF1 receptors

The major problem in treating excessive eating is high rates of relapse to maladaptive eating habits during diet treatments; this relapse is often induced by stress or anxiety states. Preclinical studies have not explored this clinical problem. Here, we adapted a reinstatement model (commonly used to study relapse to abused drugs) to examine the role of stress and anxiety in relapse to palatable food seeking during dieting. Rats were placed on restricted diet (75-80% of daily standard food) and for 12 intermittent training days (9 h/day, every other day) lever-pressed for palatable food pellets (25% fat, 48% carbohydrate) under a fixed ratio 1 (20-s timeout) reinforcement schedule. Subsequently, the rats were given 10 daily extinction sessions during which lever presses were not reinforced, and were then injected with yohimbine (an alpha-2 adrenoceptor antagonist that induces stress and anxiety in humans and non-humans) or given a single food pellet to assess reinstatement of food seeking. The rats rapidly learned to lever press for the palatable pellets and across the training days the ratio of timeout nonreinforced lever presses to reinforced lever presses progressively increased more than three-fold, suggesting the development of compulsive eating behavior. After extinction, yohimbine injections and pellet priming reliably reinstated food seeking. The corticotropin-releasing factor1 (CRF1) receptor antagonist antalarmin attenuated the reinstatement induced by yohimbine, but not pellet priming. Antalarmin also reversed yohimbine's anxiogenic effects in the social interaction test. These data suggest that CRF is involved in stress-induced relapse to palatable food seeking, and that CRF1 antagonists should be considered for the treatment of maladaptive eating habits.

Food intake and reward mechanisms in patients with schizophrenia: implications for metabolic disturbances and treatment with second-generation antipsychotic agents

Obesity is highly prevalent among patients with schizophrenia and is associated with detrimental health consequences. Although excessive consumption of fast food and pharmacotherapy with such second-generation antipsychotic agents (SGAs) as clozapine and olanzapine has been implicated in the schizophrenia/obesity comorbidity, the pathophysiology of this link remains unclear. Here, we propose a mechanism based on brain reward function, a relevant etiologic factor in both schizophrenia and overeating. A comprehensive literature search on neurobiology of schizophrenia and of eating behavior was performed. The collected articles were critically reviewed and relevant data were extracted and summarized within four key areas: (1) energy homeostasis, (2) food reward and hedonics, (3) reward function in schizophrenia, and (4) metabolic effects of the SGAs. A mesolimbic hyperdopaminergic state may render motivational/incentive reward system insensitive to low salience/palatability food. This, together with poor cognitive control from hypofunctional prefrontal cortex and enhanced hedonic impact of food, owing to exaggerated opioidergic drive (clinically manifested as pain insensitivity), may underlie unhealthy eating habits in patients with schizophrenia. Treatment with SGAs purportedly improves dopamine-mediated reward aspects, but at the cost of increased appetite and worsened or at least not improved opiodergic capacity. These effects can further deteriorate eating patterns. Pathophysiological and therapeutic implications of these insights need further validation via prospective clinical trials and neuroimaging studies.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

The animal model in food intake regulation: Examples from the opioid literature

Animal models allow us to investigate the basic mechanisms by which food intake is regulated. There are a host of neuroregulators distributed across a complex central network that control eating behavior. The opioid peptides represent one family of such regulators that have been studied extensively in animals. Using anatomical, biochemical and behavioral methods investigators have found that opioids play an important role in reward-related eating. In this brief review we summarize representative animal studies that utilize a variety of experimental techniques to help explain the role of opioids in ingestive behavior.

Role of food type in yohimbine- and pellet-priming-induced reinstatement of food seeking

We have recently adapted a reinstatement model, commonly used to study relapse to drugs of abuse, to study the role of stress and anxiety in relapse to palatable food seeking [Ghitza UE, Gray SM, Epstein DH, Rice KC, Shaham Y. The anxiogenic drug yohimbine reinstates palatable food seeking in a rat relapse model: a role of CRF(1) receptors. Neuropsychopharmacology [in press]]. We found that the anxiogenic drug yohimbine, as well as pellet-priming, reinstate food seeking in food restricted rats previously trained to lever press for palatable food pellets (25% fat, 48% carbohydrate). Here, we studied the generality of the effect of yohimbine and pellet priming on reinstatement of food seeking by using three distinct pellet types: non-sucrose carbohydrate (NSC) (5.5% fat, 60% carbohydrate, 4.5% fiber), fiber (0% fat, 0% carbohydrate, 91% fiber) and sucrose (0% fat, 91% carbohydrate, 4% fiber). Rats were placed on a restricted diet (75-80% of daily standard food) and for 9-12 intermittent training days (9 h/day, every other day) lever-pressed for the food pellets under a fixed ratio-1 (20-s timeout) reinforcement schedule. Subsequently, the rats were given 9-10 daily extinction sessions during which lever-presses were not reinforced, and were then injected with yohimbine (0, 0.5, 1.0, 2.0 mg/kg, i.p.) or given a single food pellet to induce reinstatement of food seeking. Yohimbine reinstated food seeking previously reinforced by NSC and sucrose pellets, but had a minimal effect on food seeking in rats previously trained to lever press for fiber pellets. Pellet priming produced a greater degree of reinstatement of lever pressing in rats previously trained on NSC pellets than in rats trained on fiber or sucrose pellets. Results suggest that the magnitude of the effect of yohimbine and pellet priming on reinstatement of food seeking depends in part on the composition of the food pellets used during training.

Implications of an animal model of sugar addiction, withdrawal and relapse for human health

The effect of intermittent glucose administration on the circadian rhythm of body temperature was studied in rats to provide evidence of sugar addiction, withdrawal and relapse. Metabolic and behavioral phenomena were also observed. Biotelemetry transmitters recorded body temperature for the duration of the 4-week experiment. Rats were divided into an experimental and a control group, which were maintained on the same habituation conditions for the duration of the experiment, with the exception of weeks 2 and 4, when the experimental group was presented with a 25% glucose solution. Experimental animals displayed a precipitous drop in body temperature and behavioral changes associated with withdrawal during week 3, when sugar was removed. There was an increase in kilocalories (kcal) consumed during weeks 2 and 4 by experimental animals and, by the end of the experiment, these animals showed a greater percent increase in body weight. Elevated blood glucose levels were found in experimental animals. The study demonstrates that the effects of sugar addiction, withdrawal and relapse are similar to those of drugs of abuse. Implications of the rewarding and addicting effects of sugar are related to weight gain, obesity and Type II diabetes. Furthermore, pitfalls related to dieting are elucidated.

Combined dieting and stress evoke exaggerated responses to opioids in binge-eating rats

The authors developed an animal model of binge eating where history of caloric restriction with footshock stress (R + S) causes rats to consume twice the normal amount of palatable food. The authors tested the hypothesis that binge eating is mediated by changes in opioid control of feeding by comparing rats' anorectic and orexigenic responses to naloxone and butorphanol, respectively, and by testing the ability of butorphanol to elicit binge eating of chow when palatable food was absent. Mu/kappa opioid-receptor blockade and activation had exaggerated responses in the R + S rats with naloxone suppressing binge eating to control levels, and although butorphanol did not trigger chow binge eating, it enhanced binge eating of palatable food. These responses in sated normal-weight rats strengthen evidence that reward, over metabolic need, drives binge eating.

Quinpirole-induced behavioral sensitization is enhanced by prior scheduled exposure to sucrose: A multi-variable examination of locomotor activity

Sensitization of dopaminergic neural reward circuits has been hypothesized to be involved in the development of drug addiction. Highly palatable foods activate these same brain areas, specifically the nucleus accumbens. In this study, the effects of a highly palatable food (sucrose) on these circuits were investigated using the dopamine D(2)/D(3) receptor agonist quinpirole. Male Long-Evans rats received 30 min daily access to 0.3 M sucrose solution or water over nine consecutive days, followed by nine daily injections of quinpirole (0.5 mg/kg, s.c.) or saline. Locomotor activity was assessed using an automated open-field system. Locomotor sensitization developed, as quinpirole-treated rats traveled significantly more, and exhibited a greater number of movements than saline controls. A characteristic pattern of an initial suppression of locomotor activity, followed by excitation of activity was observed in quinpirole-treated rats. Pre-exposure to sucrose attenuated the initial suppression of activity, and facilitated excitation of activity. Rats that were pre-exposed to sucrose exhibited a reduced suppression of activity as compared to rats pre-exposed to water. Rats receiving sucrose and quinpirole also displayed a significantly greater enhancement of locomotor activity as compared to rats receiving water and quinpirole. These results support the hypothesis that highly palatable foods can alter the same neural reward circuits as drugs of abuse, and may facilitate sensitization-related addiction. This may aid in further understanding the neural basis of eating disorders.

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.

Hormonal and metabolic effects of olanzapine and clozapine related to body weight in rodents

OBJECTIVE

To characterize a model of atypical antipsychotic drug-induced obesity and evaluate its mechanism.

RESEARCH METHODS AND PROCEDURES

Chronically, olanzapine or clozapine was self-administered via cookie dough to rodents (Sprague-Dawley or Wistar rats; C57Bl/6J or A/J mice). Chronic studies measured food intake, body weight, adiponectin, active ghrelin, leptin, insulin, tissue wet weights, glucose, clinical chemistry endpoints, and brain dopaminergic D2 receptor density. Acute studies examined food intake, ghrelin, leptin, and glucose tolerance.

RESULTS

Olanzapine (1 to 8 mg/kg), but not clozapine, increased body weight in female rats only. Weight changes were detectable within 2 to 3 days and were associated with hyperphagia starting approximately 24 hours after the first dose. Chronic administration (12 to 29 days) led to adiposity, hyperleptinemia, and mild insulin resistance; no lipid abnormalities or changes in D2 receptor density were observed. Topiramate, which has reversed weight gain from atypical antipsychotics in humans, attenuated weight gain in rats. Acutely, olanzapine, but not clozapine, lowered plasma glucose and leptin. Increases in glucose, insulin, and leptin following a glucose challenge were also blunted.

DISCUSSION

A model of olanzapine-induced obesity was characterized which shares characteristics of patients with atypical antipsychotic drug-induced obesity; these characteristics include hyperphagia, hyperleptinemia, insulin resistance, and weight gain attenuation by topiramate. This model may be a useful and inexpensive model of uncomplicated obesity amenable to rapid screening of weight loss drugs. Olanzapine-induced weight gain may be secondary to hyperphagia associated with acute lowering of plasma glucose and leptin, as well as the inability to increase plasma glucose and leptin following a glucose challenge.

Cannabinoids, opioids and eating behavior: the molecular face of hedonism?

Obesity represents nowadays one of the most devastating health threats. Published reports even project a decline in life expectancy of US citizens due to the rapidly increasing prevalence of obesity. This alarming increase is intimately linked with recent changes of environment and lifestyle in western countries. In this context, the rewarding or even addictive properties of popular food may represent one of the most serious obstacles to overcome for an effective anti-obesity therapy. Therefore, in addition to molecular networks controlling energy homeostasis, now researchers are starting to define central nervous mechanisms governing hedonic and addictive components of food intake. A recently emerging body of data suggests that the endogenous cannabinoid and opioid systems both represent key circuits responding to the rewarding value of food. This review focuses on the role of these two systems for the homeostatic and hedonic aspects of eating behavior and includes their anatomical and functional interactions. Independent from the degree to which eating can be considered an addiction, cannabinoid and opioid receptor antagonists are promising anti-obesity drugs, since they are targeting both hedonic and homeostatic components of energy balance control.

Sucrose-induced analgesia is related to sweet preferences in children but not adults

The present study tested the hypothesis that the efficacy of sucrose in reducing pain during the Cold Pressor Test (CPT) was related to its hedonic value. To this aim, we determined the most preferred level of sucrose and the analgesic properties of 24% w/v sucrose during the CPT in 242, 5- to 10-year-old children and their mothers. Outcome measures included pain thresholds (the time at which discomfort was first indicated) and pain tolerance (the length of time the hand was kept in the cold water bath). Although children, as a group, preferred significantly higher sucrose concentrations than adults, there were individual differences that allowed us to group them on the basis of those who preferred sucrose concentrations below that used in the CPT (24% w/v) and those who preferred levels >or=24% w/v sucrose. Regardless of such groupings, sucrose was not an effective analgesic in adult women. Unlike adults, the more children liked sucrose, the better its efficacy as an analgesic. That is, children who preferred >or=24%w/v sucrose exhibited an increased latency to report pain and tolerated pain for significantly longer periods of time when sucrose was held in their mouths relative to water. This effect was more pronounced among normal weight when compared to overweight/at risk for overweight children. The role that dietary habits and individual differences contribute to the preferences for sweet taste and its physiological consequences in children is an important area for future research.

A Proteomics Approach to Identify Long-Term Molecular Changes in Rat Medial Prefrontal Cortex Resulting from Sucrose Self-Administration

The medial prefrontal cortex (mPFC) is involved in the processing and retrieval of reward-related information. Here, we investigated long-lasting changes in protein composition of the mPFC in rats with a history of sucrose self-administration. Protein levels were analyzed using 2-D PAGE and MALDI-TOF sequencing. From approximately 1500 spots, 28 regulated proteins were unambiguously identified and were involved in cytoskeleton organization, energy metabolism, oxidative stress, neurotransmission, and neuronal outgrowth and differentiation. For several proteins, this change was also found as a long-lasting alteration in gene expression. We show that self-administration of sucrose produces long-lasting molecular neuroadaptations in the mPFC that may be involved in reward-related information processing.

Wheel running as a predictor of cocaine self-administration and reinstatement in female rats

Avidity for behaviors mediated by nondrug rewards, such as novelty seeking or intake of sweets or fats, is predictive of enhanced vulnerability to the locomotor-activating and rewarding effects of drugs of abuse. The purpose of the present study was to determine whether avidity for wheel running was predictive of subsequent cocaine-induced locomotor activity, cocaine self-administration, and cocaine-seeking behavior in rats. Rats with high (HiR) and low (LoR) levels of wheel running were selected from an outbred sample of Wistar rats. These rats were first tested for their locomotor response to an acute injection of cocaine (10 mg/kg, i.p.). Subsequently, a multi-phase self-administration procedure was used to examine the effect of wheel running on the maintenance, extinction, and cocaine-induced reinstatement of cocaine-seeking behavior in HiR and LoR rats. The results indicate no significant differences between HiR and LoR rats in the cocaine-induced stimulation of locomotor activity. During maintenance, HiR rats self-administered more cocaine than LoR rats. While there were no group differences in saline self-administration behavior during extinction, HiR rats showed higher cocaine-induced reinstatement than LoR rats. Rats that were previously high responders to novelty (day 1 in locomotor track) also showed significantly higher reinstatement than low novelty responders. These results suggest that a propensity for wheel running is associated with increased vulnerability for cocaine self-administration and reinstatement and that HiR rats are more motivated than LoR rats to seek cocaine.

Nutritional aspects modulating brain development and the responses to stress in early neonatal life

Nutrition is one of the critical factors insuring adequate growth and development in all species. In particular, brain development is sensitive to specific nutrient intake such as proteins and lipids, which are important for cell membrane formation and myelinization. Carbohydrate intake insures adequate short-term energy supply, but has important effects on the activity of the hypothalamic-pituitary-adrenal (HPA) axis to regulate stress responsiveness. This review focuses on the effects of carbohydrates and fat on the activity of the HPA axis as well as other brain-related functions such as pain modulation, neuropeptide and neurotransmitters release, and some aspects related to cognitive functions. The role of leptin, DHA and AA as mediators of the effects of fat on the brain is discussed.

Brief intermittent access to sucrose differentially modulates prepulse inhibition and acoustic startle response in obese CCK-1 receptor deficient rats

Otsuka Long-Evans Tokushima Fatty (OLETF) rats lack the CCK-1 receptor and are hyperphagic and obese. CCK-1 receptors play a role in prepulse inhibition (PPI) by modulating mesolimbic dopamine transmission, a modulator of sensorimotor gating. Therefore, the present study assessed the effects of brief, daily sucrose access on PPI and acoustic startle response (ASR) in OLETF rat and age-matched non-mutant Long-Evans Tokushima Otsuka (LETO) rats. The results revealed that OLETF rats with sucrose access showed an increased ASR [F(1,16) = 6.84; P < 0.01)], relative to sucrose receiving LETO rats. No significant sucrose effect (P = 0.283) on PPI was noted in OLETF rats, whereas sucrose receiving LETO rats had a significantly lower (P < 0.05) PPI percentage than non-sucrose controls. In contrast, sucrose-receiving OLETF rats expressed significantly higher PPI percentage than LETO rats with identical sucrose presentation (P < 0.01). Taken together, these results suggest that sucrose access alters PPI and ASR in general, and the CCK-1 receptors play a modulatory role in facilitating or inhibiting these responses, respectively. A similar effect may be contributory to the hyperphagic behavioral phenotype of obese animal models with altered central dopamine regulation.

Central administration of melanin-concentrating hormone increases alcohol and sucrose/quinine intake in rats

BACKGROUND

Alcohol is a caloric compound that can contribute to energy intake. Therefore, peptides that regulate energy balance likely modify the motivation to consume alcohol. Melanin-concentrating hormone (MCH) regulates energy homeostasis and has been implicated in other behaviors that impact alcohol consumption (i.e., anxiety, fluid balance, and reward). We tested the hypothesis that MCH would decrease the motivation to consume alcohol secondarily to reducing anxiety.

METHODS

Rats were trained to drink 10% ethanol or an isocaloric concentration of sucrose with use of a sucrose-fading technique. MCH (1, 5, or 10 microg) or its saline vehicle was administered into the third cerebral ventricle (i3vt), and intake of ethanol or sucrose and chow was assessed for 2 hr. Alcohol-naïve rats were evaluated in an elevated plus maze after i3vt MCH (10 microg), neuropeptide Y, or saline administration.

RESULTS

Contrary to the hypothesis, MCH dose-dependently increased alcohol intake: saline = 0.7 +/- 0.1 g/kg, 1 microg MCH = 1.0 +/- 0.1 g/kg, 5 microg MCH = 1.2 +/- 0.1 g/kg, and 10 microg MCH = 1.8 +/- 0.3 g/kg (p < 0.01), and this was true whether water was simultaneously available or not. MCH also significantly increased sucrose intake (saline = 1.0 +/- 0.3 g/kg, 10 mug MCH = 1.4 +/- 0.5 g/kg; p < 0.05). MCH had no effect on time spent in the open arms (54.3 +/- 11.5 sec) relative to saline (58.2 +/- 23.8 sec), whereas neuropeptide Y, a known anxiolytic, increased time spent on the open arms (119.2 +/- 22 sec, p < 0.05).

CONCLUSIONS

We conclude that MCH nonspecifically increases ingestive behavior. Furthermore, MCH had no apparent effect on anxiety. The ability of MCH to increase alcohol and/or sucrose intake may be explained by the effect of MCH on energy balance and/or reward processes.

The effect of low carbohydrate on energy metabolism

OBJECTIVE

To investigate whether low-carbohydrate diets are efficient for reduction of body weight and through which mechanism.

DESIGN

A couple of studies using low-carbohydrate diets in the treatment of obesity are reviewed. Mechanisms for explaining the reduced appetite are described in relation to knowledge on regulation of appetite for fat and carbohydrate.

RESULTS

Studies with low-carbohydrate diets demonstrate a rapid weight loss, being more pronounced after 3 and 6 months compared to low-fat diets. After 12 months there is no difference between the low-carbohydrate and the conventional low-fat diet on weight loss. Both diets lead to improvements in risk factors for coronary heart disease, the low-carbohydrate diet leading to a greater decrease in serum triglycerides and increase in HDL cholesterol compared to the low-fat diet. Blood pressure, insulin sensitivity and LDL cholesterol were improved to a similar degree by the two diets. The mechanism for the rapid weight loss with the low-carbohydrate diet is a suppressed appetite, first through the high-protein content of the diet, second through the ketogenic nature of the diet with satiety signals for fat being active and third through the absence of hunger-promoting carbohydrate components like sucrose and/or fructose.

CONCLUSION

A rapid initial weight loss occurs with a low-carbohydrate diet due to a suppressed appetite. There is as yet no indication of an increased metabolic rate and an increased thermogenesis by the low-carbohydrate diet. The safety and efficacy of low-carbohydrate diets have to await further studies.

Translational approaches to understanding anorexia nervosa

Translational research has fostered significant gains in neuroscience and psychiatry and has been identified by the NIMH as a "priority area" for further funding. As applied to anorexia nervosa (AN), "translational research" describes the application of principles and methodologies employed in related fields to advance the understanding, and ultimately treatment and prevention, of this disorder. Several promising areas of translational research in AN are identified. Such research, particularly when linked to a conceptual framework, offers the potential of lending much-needed novel insight into this challenging and enigmatic disorder.

Chronic sucrose ingestion enhances mu-opioid discriminative stimulus effects

Sucrose affects a variety of opioid-related behaviors. We hypothesized that, if sucrose ingestion alters opioidergic circuitry, opioid-induced discriminative stimulus effects would be enhanced following sucrose intake. In the present study, rats were trained to discriminate nalbuphine (3.2 mg/kg, s.c.) from saline in an operant choice procedure. After acquiring the discrimination, subjects were injected with a single nalbuphine dose (0.1-3.2 mg/kg) and given 30-min access to 30% sucrose or water. Sucrose consumption did not alter nalbuphine's discriminative stimulus effects under these conditions. During subsequent tests, training was suspended, and rats received continuous access to sucrose (9 days) or water (8 days). Chronic sucrose consumption increased the potency of nalbuphine to produce its discriminative stimulus effects by 3-fold. These findings suggest chronic sucrose consumption results in changes in opioid-system function that modulates the effects of exogenously administered opioids.

Dietary modulation of parathion-induced neurotoxicity in adult and juvenile rats

Previous studies indicated that dietary glucose (15% in drinking water) could markedly exacerbate the toxicity of parathion in adult rats. The present study evaluated the effect of consumption of the commonly used sweetener, high fructose corn syrup (HFCS), on parathion toxicity in adult and juvenile rats. Animals were given free access to either water or 15% HFCS in drinking water for a total of 10 days and challenged with parathion (6 or 18 mg/kg, s.c., for juveniles or adults, respectively) on the 4th day. Signs of cholinergic toxicity, body weight and chow/fluid intake were recorded daily. Acetylcholinesterase (AChE) activity and immunoreactivity (AChE-IR) in frontal cortex and diaphragm were measured at 2, 4, and 7 days after parathion. As HFCS was associated with significant reduction in chow intake, adult rats were also pair-fed to evaluate the effect of similar reduced chow intake alone on parathion toxicity. The results indicated that the cholinergic toxicity of parathion was significantly increased by HFCS feeding in both age groups. The excess sugar consumption, however, did not significantly affect parathion-induced AChE inhibition in either tissue or either age group. Enzyme immunoreactivity in frontal cortex was generally not affected in either age group while diaphragm AChE-IR was significantly reduced by parathion and HFCS alone in adult animals at 2 and 4 days timepoints, and more so by the combination of sugar feeding and parathion exposure in both age groups. Food restriction alone did not exacerbate parathion toxicity. While the mechanism(s) remains unclear, we conclude that voluntary consumption of the common sweetener HFCS can markedly amplify parathion acute toxicity in both juvenile and adult rats.

Sucrose ingestion decreases seizure onset time in female rats treated with lithium and pilocarpine

To extend previous work concerning diet and overt seizures in rats, we tested the hypothesis that ingestion of 10% sucrose-water could reduce seizure onset time (SOT) in rats given lithium and pilocarpine. We found that female but not male rats given free access to a 10% sucrose-water solution for 3 weeks exhibited shorter SOTs than age- and sex-matched control subjects. A separate experiment determined that SOT was significantly reduced whether female rats were provided 1, 2, 3, or 4 weeks of free access to sucrose. Moreover, the daily volume of sucrose ingested was significantly correlated (r=-0.42) with SOT regardless of the duration of sucrose treatment (in weeks). These findings suggest that a diet supplemented with sugar can facilitate the emergence of behavioral seizures in female rats given lithium and pilocarpine. We discuss the potential role of dopamine in mediating the sucrose-induced changes in SOT.

Sugar-dependent rats show enhanced responding for sugar after abstinence: Evidence of a sugar deprivation effect

Studies have shown that intermittent sugar availability (12 h/day) produces signs of dependence in rats, including escalation of intake, mu-opioid and dopamine receptor changes, behavioral and neurochemical indices of withdrawal, and cross-sensitization with amphetamine. "Deprivation-effect" paradigms, whereby abstinence from a substance results in enhanced intake, are often used to measure "craving" for drugs of abuse, such as alcohol. The present study used operant conditioning to investigate consumption of sugar after abstinence in rats selected for glucose avidity. The experimental group was trained on a fixed ratio (FR-1) schedule for 25% glucose for 30 min/day for 28 days and also had glucose access in the home cages for an additional 11.5 h daily. The control group had only the 30-min/day access to glucose in the operant chambers. Then, both groups were deprived of glucose for 2 weeks. After this period of abstinence, animals were put back in the operant chambers. The experimental group responded significantly more than ever before, and significantly more than the control group. In conclusion, daily 12-h access to sugar, in the paradigm used, can result in an altered neural state that lasts throughout 2 weeks of abstinence, leading to enhanced intake. Together with previous results, this deprivation effect supports the theory that animals may become dependent on sugar under selected dietary circumstances.

Daily bingeing on sugar repeatedly releases dopamine in the accumbens shell

Most drugs of abuse increase dopamine (DA) in the nucleus accumbens (NAc), and do so every time as a pharmacological response. Palatable food also releases accumbens-shell DA, but in naïve rats the effect can wane during a long meal and disappears with repetition. Under select dietary circumstances, sugar can have effects similar to a drug of abuse. Rats show signs of DA sensitization and opioid dependence when given intermittent access to sucrose, such as alterations in DA and mu-opioid receptors, cross-sensitization with amphetamine and alcohol, and behavioral and neurochemical signs of naloxone-precipitated withdrawal. The present experiment asks whether sucrose-dependent rats release DA each time they binge. We also predict that acetylcholine (ACh), which rises as the end of a meal, will be delayed in rats with intermittent access to sucrose. To create dependency, the experimental group (Daily Intermittent Sucrose) was maintained on a diet of 12-h food deprivation that extended 4 h into the dark, followed by 12-h access to a 10% sucrose solution and chow, daily, for 21 days. As the main result, these rats gradually increased their sucrose intake from 37 to 112 ml per day (from 13 to 20 ml in the first hour of access), and repeatedly increased extracellular DA to 130% of baseline as measured in the NAc shell by microdialysis during the first hour of sucrose access on day 1, day 2 and day 21. Three control groups failed to show a significant increase in extracellular DA on day 21: Sucrose only for 1 h on days 1 and 21 (Sucrose Twice), ad libitum access to sucrose and chow (Daily Ad libitum Sucrose), and intermittent chow instead of sucrose (Daily Intermittent Chow). Acetylcholine measured at the same time as DA, increased significantly toward the end and after each test meal in all groups. In the Daily Intermittent Sucrose group, the highest ACh levels (133%) occurred during the first sample after the sucrose meal ended. In summary, sucrose-dependent animals have a delayed ACh satiation response, drink more sucrose, and release more DA than sucrose- or binge-experienced, but non-dependent animals. These results suggest another neurochemical similarity between intermittent bingeing on sucrose and drugs of abuse: both can repeatedly increase extracellular DA in the NAc shell.