Effects of Maternal Separation on Effort-based Responding for Sucrose Are Associated with c-Fos Expression in the Nucleus Accumbens Core

In both people and animals, exposure to adverse experiences early in life can alter neurodevelopment and lead to long-term behavioral effects, including effects on reward processing. In the current study, we use a well-validated rodent model of maternal neglect, maternal separation (MS), to investigate the impact of early life adversity on reward learning and motivation and identify associated modifications in cellular activation in reward-relevant areas. Litters of Long-Evans rats were separated from the dam for either 15 min (brief) or 180 min (prolonged)/day from postnatal day (PND)2 to PND14. As adults, offspring were trained to lever press for a sucrose pellet using fixed ratio (FR) schedules and motivation was tested using a progressive ratio (PR) schedule over 10 daily sessions to assess sustained effects on effort-based responding. Immunohistochemical staining for c-Fos was conducted in a subset of animals that underwent an additional PR session. While there were no effects on reward learning, both MS180 males and females demonstrated increased effort-based responding on the first day of PR testing, while only MS180 males demonstrated a sustained increase in effort across all 10 days. MS180-induced changes in c-Fos expression in the dorsal and ventral striatum were observed, with subregion-specific effects along the rostrocaudal axis. Moreover, regression analyses suggest that motivated responding for a sucrose food reward in MS180-exposed, but not MS15-exposed animals, was associated with increased c-Fos expression in the rostral nucleus accumbens core. These findings implicate specific striatal regions in sex-specific modulation of sustained effort-based reward behavior following early life adversity.

Behavioral responses to natural rewards in developing male and female rats

Reward deficits are a hallmark feature of multiple psychiatric disorders and often recapitulated in rodent models useful for the study of psychiatric disorders, including those employing early life stress. Moreover, rodent studies have shown sex differences during adulthood in response to natural and drug rewards under normative conditions and in stress-based rodent models. Yet, little is known about the development of reward-related responses under normative conditions, including how these may differ in rats of both sexes during early development. Comparing reward-related behavioral responses between developing male and female rats may be useful for understanding how these processes may be affected in rodent models relevant to psychiatric disorders. To this end, we tested behavioral responses to natural rewards in male and female rats using sucrose consumption, sweet palatable food intake and social play tests at two timepoints (peripuberty, adolescence). Our results suggest comparable responses to consummatory and social rewards in male and female rats during peripuberty and adolescence as no sex differences were found for sucrose preference, chocolate candy intake or a subset of play behaviors (dorsal contacts, pins). These findings suggest that sex differences in response to these natural rewards emerge and may be more robust during adulthood.

Obesity-mediated Lipoinflammation Modulates Food Reward Responses

Accumulation of white adipose tissue (WAT) during obesity is associated with the development of chronic low-grade inflammation, a biological process known as lipoinflammation. Systemic and central lipoinflammation accumulates pro-inflammatory cytokines including IL-6, IL-1β and TNF-α in plasma and also in brain, disrupting neurometabolism and cognitive behavior. Obesity-mediated lipoinflammation has been reported in brain regions of the mesocorticolimbic reward circuit leading to alterations in the perception and consumption of ultra-processed foods. While still under investigation, lipoinflammation targets two major outcomes of the mesocorticolimbic circuit during food reward: perception and motivation ("Wanting") and the pleasurable feeling of feeding ("Liking"). This review will provide experimental and clinical evidence supporting the contribution of obesity- or overnutrition-related lipoinflammation affecting the mesocorticolimbic reward circuit and enhancing food reward responses. We will also address neuroanatomical targets of inflammatory profiles that modulate food reward responses during obesity and describe potential cellular and molecular mechanisms of overnutrition linked to addiction-like behavior favored by brain lipoinflammation.

Examination of onset trajectories and persistence of binge-like eating behavior in mice after intermittent palatable food exposure

Binge eating is a persistent behavior associated with a chronic course of illness and poor treatment outcomes. While clinical research is unable to capture the full course of binge eating, preclinical approaches offer the opportunity to examine binge-like eating from onset through chronic durations, allowing identification of factors contributing to binge eating persistence. The present study quantified the trajectories of binge-like eating onset and modeled cycles of abstinence/relapse to develop a translational model for binge eating persistence. Adult male and female C57Bl6/J mice were randomized to a binge-like palatable food access schedule (daily 2-hr, 3×/week) or continuous, nonbinge like palatable food access for 12 days (Experiment 1). Persistence of palatable food consumption in both binge-like palatable food access groups was then examined across three cycles of forced abstinence and reexposure to palatable food (incubation) to model the persistence of binge eating in clinical populations. Mice with daily 2-hr palatable food access escalated their intake more than mice in the 3×/week or continuous groups (Experiment 1). This pattern was more pronounced in females. In addition, this pattern of palatable food intake reemerged across multiple cycles of behavioral incubation (Experiment 2). These findings provide a model of binge-like eating in mice that can be used in future studies examining both environmental factors and neural mechanisms contributing to binge eating persistence. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

The conserved endocannabinoid anandamide modulates olfactory sensitivity to induce hedonic feeding in C. elegans

The ability of cannabis to increase food consumption has been known for centuries. In addition to producing hyperphagia, cannabinoids can amplify existing preferences for calorically dense, palatable food sources, a phenomenon called hedonic amplification of feeding. These effects result from the action of plant-derived cannabinoids that mimic endogenous ligands called endocannabinoids. The high degree of conservation of cannabinoid signaling at the molecular level across the animal kingdom suggests hedonic feeding may also be widely conserved. Here, we show that exposure of Caenorhabditis elegans to anandamide, an endocannabinoid common to nematodes and mammals, shifts both appetitive and consummatory responses toward nutritionally superior food, an effect analogous to hedonic feeding. We find that anandamide's effect on feeding requires the C. elegans cannabinoid receptor NPR-19 but can also be mediated by the human CB1 cannabinoid receptor, indicating functional conservation between the nematode and mammalian endocannabinoid systems for the regulation of food preferences. Furthermore, anandamide has reciprocal effects on appetitive and consummatory responses to food, increasing and decreasing responses to inferior and superior foods, respectively. Anandamide's behavioral effects require the AWC chemosensory neurons, and anandamide renders these neurons more sensitive to superior foods and less sensitive to inferior foods, mirroring the reciprocal effects seen at the behavioral level. Our findings reveal a surprising degree of functional conservation in the effects of endocannabinoids on hedonic feeding across species and establish a new system to investigate the cellular and molecular basis of endocannabinoid system function in the regulation of food choice.

Effects of Intermittent Fasting on Hypothalamus-Pituitary-Thyroid Axis, Palatable Food Intake, and Body Weight in Stressed Rats

Dietary regimens that are focused on diminishing total caloric intake and restricting palatable food ingestion are the most common strategies for weight control. However, restrictive diet therapies have low adherence rates in obese patients, particularly in stressed individuals. Moreover, food restriction downregulates the hypothalamic-pituitary-thyroid axis (HPT) function, hindering weight loss. Intermittent fasting (IF) has emerged as an option to treat obesity. We compared the effects of IF to an all-day feeding schedule on palatable diet (PD)-stress (S)-induced hyperphagia, HPT axis function, accumbal thyrotropin-releasing hormone (TRH), and dopamine D2 receptor expression in association with adipocyte size and PPARƔ coactivator 1α (PGC1α) and uncoupling protein 1 (UCP1) expression in stressed vs. non-stressed rats. After 5 weeks, S-PD rats showed an increased energy intake and adipocyte size, fewer beige cells, and HPT axis deceleration-associated low PGC1α and UCP1 expression, as well as decreased accumbal TRH and D2 expression. Interestingly, IF reversed those parameters to control values and increased the number of beige adipocytes, UCP1, and PGC1α mRNAs, which may favor a greater energy expenditure and a reduced body weight, even in stressed rats. Our results showed that IF modulated the limbic dopaminergic and TRHergic systems that regulate feeding and HPT axis function, which controls the metabolic rate, supporting this regimen as a suitable non-pharmacologic strategy to treat obesity, even in stressed individuals.

The effects of the COVID-19 pandemic on disordered eating symptoms in women: A 49-day, daily study before and during the outbreak in the United States

Longitudinal data are needed to examine effects of the COVID-19 pandemic on disordered eating. We capitalized on an ongoing, longitudinal study collecting daily data to examine changes in disordered eating symptoms in women across 49 days that spanned the time before and during the COVID-19 outbreak in the United States. Women from the Michigan State University Twin Registry (N = 402) completed daily questionnaires assessing a range of symptoms (e.g., binge eating, weight/shape concerns, liking/wanting of palatable food (PF) and whole foods, hunger). Dates of the first US COVID-19 case, first case in each participant's state, and onset of the initial stay-at-home orders (SHOs) were used to categorize women into those who completed all daily assessments prior to, during, or after these dates. We used mixed linear models and specification-curve analysis to examine between-person (i.e., differences between women assessed before vs during/after COVID-19) and within-person (i.e., changes in symptoms from days before to days after the dates) effects of the pandemic. Results showed significantly higher levels of binge-related pathology (e.g., odds of binge eating, liking/wanting of PF) in women who completed assessments during/after COVID-19 events, and significantly increased liking/wanting of PF in the days following the pandemic onset. By contrast, minimal between- or within-person differences were observed for other variables, including weight/shape concerns, compensatory behaviors, hunger, or liking/wanting whole foods. Overall, results suggest a specific effect of the pandemic on binge-related phenotypes in women. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Measuring Ostracism-Induced Changes in Consumption of Palatable Food: Feasibility of a Novel Behavioral Task

Purpose

Ostracism is a highly aversive interpersonal experience. Previous research suggests that it can increase consumption of highly palatable food in some individuals, but decrease it in others. Thus, we developed the Cyberball-Milkshake Task (CMT), to facilitate research investigating individual differences in ostracism's effects on consumption of highly palatable food. We present data on feasibility for the CMT in a sample of young adult women.

Materials and Methods

Participants were 22 women, 18-30 years old, reporting very low or very high levels of emotional eating at screening. Participants performed the CMT, which consisted of 12 trials. Each trial included: playing a round of Cyberball (a computerized game of catch with fictitious "other participants" programmed to either include or exclude the participant); viewing a chocolate image; and then consuming a participant-determined amount of milkshake. Participants subsequently played an additional inclusion and exclusion round of Cyberball, each immediately followed by questionnaires assessing current mood and recent Cyberball experience.

Results

Cyberball exclusion (vs. inclusion) was associated with large, significant increases in reported ostracism and threats to self-esteem; exclusion's effects on affect were in the expected direction (e.g., increased negative affect), but generally small and non-significant. Milkshake intake was measurable for 95% of participants, on 96% of trials. Intake decreased quadratically across trials, with a steep negative slope for low trial numbers that decreased to the point of being flat for the highest trial numbers.

Discussion

The CMT is a generally feasible approach to investigating ostracism's effects on consumption of highly palatable food. The feasibility (and validity) of the CMT may benefit from modification (e.g., fewer trials and longer rounds of Cyberball). Future research should examine whether performance on a modified version of the CMT predicts real-world behavior in a larger sample.

Effects of chemotherapy on operant responding for palatable food in male and female mice

Patients treated with cancer chemotherapeutics frequently report chemotherapy-induced peripheral neuropathy (CIPN), changes in mood (depression and anxiety) and functional impairments. Rodent models of CIPN elicit limited alterations in functional behaviors, which pose challenges in developing preclinical models of chemotherapy-induced behavioral depression. The study examined the consequences of chemotherapy-induced mechanical hypersensitivity (paclitaxel: 32 or 64 mg/kg, cumulative; oxaliplatin: 30 mg/kg, cumulative) on behavioral depression, as measured with operant responding for palatable food during periods of food restriction and ad libitum chow, consumption of noncontingently available palatable food in the presence of ad libitum chow, and voluntary wheel running. The study employed two inbred mouse strains (C57BL/6J and Balb/cJ) and examined potential sex differences. All chemotherapeutic regimens caused profound mechanical hypersensitivity for the duration of the observation periods (up to 7 months), but no treatments changed voluntary wheel running or consumption of noncontingent palatable food. The high dose of paclitaxel temporarily reduced operant responding for palatable food in male C57BL/6J mice undergoing food restriction or maintained on ad libitum chow. However, paclitaxel failed to decrease operant responding for palatable food in free-feeding female C57BL/6J mice or Balb/cJ mice of either sex. Moreover, oxaliplatin did not significantly alter operant responding for palatable food in male or female C57BL/6J mice maintained on ad libitum chow. These findings demonstrate a dissociation between chemotherapy-induced mechanical hypersensitivity and behavioral depression. The transient effects of paclitaxel on operant responding in male C57BL/6J mice may represent a fleeting behavioral correlate of chemotherapy-associated pain-like behaviors.

Serum metabolic signature of binge-like palatable food consumption in female rats by nuclear magnetic resonance spectroscopy

Maladaptive eating behavior is a growing public health problem and compulsively eating excessive food in a short time, or binge eating, is a key symptom of many eating disorders. In order to investigate the binge-like eating behavior in female rats, induced by intermittent food restrictions/refeeding and frustration stress, we analyzed for the first time the metabolic profile obtained from serum of rats, through nuclear magnetic resonance (NMR) spectroscopy. In this experimental protocol, rats were exposed to chow food restricting/refeeding and frustration stress manipulation. This stress procedure consists of 15 min exposure to the odor and sight of a familiar chocolate paste, without access to it, just before offering the palatable food. In this model, a "binge-eating episode" was considered the significantly higher palatable food consumption within 2 h in restricted and stressed rats (R + S) than in the other three experimental groups: rats with no food restriction and no stress (NR + NS), only stressed rats (NR + S) or only restricted rats (R + NS). Serum samples from these four different rat groups were collected. The statistical analysis of the ¹ H NMR spectral profiles of the four sets of samples pointed to O- and N-acetyl glycoproteins as the main biomarkers for the discrimination of restriction effects. Other metabolites, such as threonine, glycine, glutamine, acetate, pyruvate and lactate, showed trends that may be useful to understand metabolic pathways involved in eating disorders. This study suggested that NMR-based metabolomics is a suitable approach to detect biomarkers related to binge-eating behavior.

Blockade of Orexin Receptors in the Posterior Paraventricular Nucleus of the Thalamus Prevents Stress-Induced Reinstatement of Reward-Seeking Behavior in Rats With a History of Ethanol Dependence

Neural systems involved in processing natural rewards and drugs of abuse overlap and exposure to drugs of abuse induce neuroadaptations that can cause compulsive-like behavior. For example, the recruitment of the orexin (Orx) system by drugs of abuse has been proposed to induce neuroadaptations that in turn alter its function, reflected by maladaptive, compulsive, and addictive behavior. Orexin neurons project to the paraventricular nucleus of the thalamus (PVT)—particularly the posterior part (pPVT), a structure that plays a key role in stress regulation. This study investigated whether Orx transmission in the pPVT plays a role in stress-induced reinstatement of reward-seeking behavior toward ethanol (EtOH) and a highly palatable food reward [sweetened condensed milk (SCM)] in rats and whether this role changes with EtOH dependence. After being trained to orally self-administer EtOH or SCM, the rats were made dependent (EtOH_D and SCM_D) by chronic intermittent EtOH vapor exposure. The control nondependent groups (EtOH_ND and SCM_ND) were exposed to air. Following extinction, the rats were tested for stress-induced reinstatement of EtOH- and SCM-seeking behavior. Stress reinstated EtOH- and SCM-seeking behavior in all groups (EtOH_D/ND and SCM_D/ND). Administration of the dual Orx receptor (OrxR) antagonist TCS1102 (15 μg) in the pPVT prevented stress-induced reinstatement only in dependent rats (EtOH_D and SCM_D). In parallel, the qPCR analysis showed that Orx mRNA expression in the hypothalamus and OrxR1/R2 mRNA expression in the pPVT were increased at the time of testing in the EtOH_D and SCM_D groups. These results are the first to implicate Orx transmission in the pPVT in the stress-induced reinstatement of reward-seeking behavior in EtOH dependent rats and indicate the maladaptive recruitment of Orx transmission in the pPVT by EtOH dependence.

Multi-method assessment of palatable food exposure in women with and without eating disorders

OBJECTIVE

Eating disorders (EDs) are characterized by dysregulated responses to palatable food. Using a multi-method approach, this study examined responses to palatable food exposure and subsequent ad libitum eating in women with binge-eating disorder (BED: n = 64), anorexia nervosa (AN: n = 16), and bulimia nervosa (BN: n = 35) and 26 healthy controls (HCs).

METHOD

Participants were exposed to palatable food followed by an ad libitum eating opportunity. Affective and psychophysiological responses were measured before and during the task.

RESULTS

Participants with EDs reported greater negative affect, particularly fear, following the food cue exposure, whereas HCs reported no change. BN and BED groups reported greater urge to binge after the food cue exposure, whereas AN and HC groups reported no change. Respiratory sinus arrhythmia levels, skin conductance and tonic skin conductance levels increased during food exposure for all groups. Across baseline and during the food exposure, the BED group had lower respiratory sinus arrhythmia levels relative to the BN and HC groups. The BED group consumed significantly more palatable food than the AN group.

CONCLUSIONS

'Palatable' food stimuli elicited more negative affect, particularly fear, in individuals with EDs; and this, rather than psychophysiological responses, distinguishes individuals with EDs from those without.

Factors affecting emotional eating and eating palatable food in adults

BACKGROUND/OBJECTIVES

The aim of this study was to determine the relationship among emotional eating behavior, tendency to eat palatable foods, and several risk factors.

SUBJECTS/METHODS

This study was carried out on 2,434 persons (1,736 women and 698 men) aged between 19 and 64 years. A questionnaire form was used as a data collection tool, which consisted of items for the socio-demographic characteristics, anthropometric measurements, Emotional Appetite Questionnaire (EMAQ), and the Palatable Eating Motives Scale (PEMS).

RESULTS

A positive significant correlation was observed between the BMI groups and the negative emotions, negative situations, and negative total scores of EMAQ (P < 0.01). The regression results on negative subscales showed that a one unit increase in BMI resulted in a 0.293 unit increase in negative situations scores, a 0.626 unit increase in negative emotions scores, and a 0.919 unit increase in negative total EMAQ scores. When the BMI groups and PEMS subscale scores were examined, a significant relationship was found in the social motives, rewarding, and conformity subscales (P < 0.01). A one unit increase in BMI increased the coping motives scores by 0.077 units.

CONCLUSIONS

The emotional states have a significant effect on the eating behavior. On the other hand, an increase in eating attacks was observed, particularly in people who were under the effect of a negative emotion or situation. Nevertheless, there were some limitations of the study in terms of quantitative determination of the effects of this eating behavior depending on the BMI.

Regulation of adenosine A2A receptor gene expression in a model of binge eating in the amygdaloid complex of female rats

BACKGROUND

Pharmacological treatment approaches for eating disorders, such as binge eating disorder and bulimia nervosa, are currently limited.

METHODS AND AIMS

Using a well-characterized animal model of binge eating, we investigated the epigenetic regulation of the A_2A Adenosine Receptor (A_2AAR) and dopaminergic D2 receptor (D2R) genes.

RESULTS

Gene expression analysis revealed a selective increase of both receptor mRNAs in the amygdaloid complex of stressed and restricted rats, which exhibited binge-like eating, when compared to non-stressed and non-restricted rats. Consistently, pyrosequencing analysis revealed a significant reduction of the percentage of DNA methylation but only at the A_2AAR promoter region in rats showing binge-like behaviour compared to the control animals. Focusing thus on A_2AAR agonist (VT 7) administration (which inhibited the episode of binge systemically at 0.1 mg/kg or intra-central amygdala (CeA) injection at 900 ng/side) induced a significant increase of A_2AAR mRNA levels in restricted and stressed rats when compared to the control group. In addition, we observed a significant decrease in A_2AAR mRNA levels in rats treated with the A_2AAR antagonist (ANR 94) at 1 mg/kg. Consistent changes in the DNA methylation status of the A_2AAR promoter were found in restricted and stressed rats after administration of VT 7 or ANR 94.

CONCLUSION

We confirm the role of A_2AAR in binge eating behaviours, and we underline the importance of epigenetic regulation of the A_2AAR gene, possibly due to a compensatory mechanism to counteract the effect of binge eating. We suggest that A_2AAR activation, inducing receptor gene up-regulation, could be relevant to reduction of food consumption.

Growth hormone secretagogue receptor signalling affects high-fat intake independently of plasma levels of ghrelin and LEAP2, in a 4-day binge eating model

The growth hormone secretagogue receptor (GHSR) is a G protein-coupled receptor that is highly expressed in the central nervous system. GHSR acts as a receptor for ghrelin and for liver-expressed antimicrobial peptide 2 (LEAP2), which blocks ghrelin-evoked activity. GHSR also displays ligand-independent activity, including a high constitutive activity that signals in the absence of ghrelin and is reduced by LEAP2. GHSR activity modulates a variety of food intake-related behaviours, including binge eating. Previously, we reported that GHSR-deficient mice daily and time-limited exposed to a high-fat (HF) diet display an attenuated binge-like HF intake compared to wild-type mice. In the present study, we aimed to determine whether ligand-independent GHSR activity affects binge-like HF intake in a 4-day binge-like eating protocol. We found that plasma levels of ghrelin and LEAP2 were not modified in mice exposed to this binge-like eating protocol. Moreover, systemic administration of ghrelin or LEAP2 did not alter HF intake in our experimental conditions. Interestingly, we found that central administration of LEAP2 or K-(D-1-Nal)-FwLL-NH₂ , which are both blockers of constitutive GHSR activity, reduced binge-like HF intake, whereas central administration of ghrelin or the ghrelin-evoked GHSR activity blockers [D-Lys3]-GHRP-6 and JMV2959 did not modify binge-like HF intake. Taken together, current data indicate that GHSR activity in the brain affects binge-like HF intake in mice independently of plasma levels of ghrelin and LEAP2.

Investigation of the Role of Chirality in the Interaction with σ Receptors and Effect on Binge Eating Episode of a Potent σ1 Antagonist Analogue of Spipethiane

The enantiomers of the potent σ1 receptor antagonist (±)-1 were synthesized and evaluated for their affinity at σ1, σ2 receptors and dopamine transporter (DAT). Analogously to (±)-1, both of the enantiomers showed very high affinity for the σ1 receptor and unprecedented selectivity over both the σ2 receptor and DAT. The lack of enantioselectivity between (+)-1 and (-)-1 indicated that the center of chirality in the 2-position of the benzothiochromane nucleus does not play a crucial role in the interaction with any of the studied targets. Docking studies confirmed that the configuration of the enantiomers has only marginal effects on the molecular interactions with the σ1 receptor. In in vivo studies in a female rat model of binge eating, (±)-1 dose-dependently decreased the binge eating episode elicited by a history of intermittent food restriction and stress, confirming and strengthening the important role played by the σ1 receptor in bingeing-related eating disorders.

Beyond the Activity-Based Anorexia Model: Reinforcing Values of Exercise and Feeding Examined in Stressed Adolescent Male and Female Mice

Anorexia nervosa (AN), mostly observed in female adolescents, is the most fatal mental illness. Its core is a motivational imbalance between exercise and feeding in favor of the former. The most privileged animal model of AN is the "activity-based anorexia" (ABA) model wherein partly starved rodents housed with running wheels exercise at the expense of feeding. However, the ABA model bears face and construct validity limits, including its inability to specifically assess running motivation and feeding motivation. As infant/adolescent trauma is a precipitating factor in AN, this study first analyzed post-weaning isolation rearing (PWIR) impacts on body weights and wheel-running performances in female mice exposed to an ABA protocol. Next, we studied through operant conditioning protocols i) whether food restriction affects in a sex-dependent manner running motivation before ii) investigating how PWIR and sex affect running and feeding drives under ad libitum fed conditions and food restriction. Besides amplifying ABA-elicited body weight reductions, PWIR stimulated wheel-running activities in anticipation of feeding in female mice, suggesting increased running motivation. To confirm this hypothesis, we used a cued-reward motivated instrumental task wherein wheel-running was conditioned by prior nose poke responses. It was first observed that food restriction increased running motivation in male, but not female, mice. When fed grouped and PWIR mice were tested for their running and palatable feeding drives, all mice, excepted PWIR males, displayed increased nose poke responses for running over feeding. This was true when rewards were proposed alone or within a concurrent test. The increased preference for running over feeding in fed females did not extend to running performances (time, distance) during each rewarded sequence, confirming that motivation for, and performance during, running are independent entities. With food restriction, mice displayed a sex-independent increase in their preference for feeding over running in both group-housed and PWIR conditions. This study shows that the ABA model does not specifically capture running and feeding drives, i.e. components known to be affected in AN.

Palatable Food Dampens the Long-Term Behavioral and Endocrine Effects of Juvenile Stressor Exposure but May Also Provoke Metabolic Syndrome in Rats

The juvenile period is marked by a reorganization and growth of important brain regions including structures associating with reward seeking behaviors such as the nucleus accumbens (NA) and prefrontal cortex (PFC). These changes are impacted by stressors during the juvenile period and may lead to a predisposition to stress induced psychopathology and abnormal development of brain reward systems. Like in humans, adult rodents engage certain coping mechanisms such as increases in the consumption of calorie-rich palatable foods to reduce stress, but this behavior can lead to obesity and metabolic disorders. In this study, we examined whether stressors during the juvenile period led to increased caloric intake when a palatable diet was accessible, and whether this diet attenuated adult stress responses. In addition, we examined if the stress buffering effects produced by the palatable diet were also accompanied by an offset propensity towards obesity, and by alterations in mRNA expression of dopamine (DA) receptors in the NA and PFC in adulthood. To this end, juvenile male Wistar rats underwent episodic stressor exposure (forced swim, elevated platform stress and restraint) on postnatal days (PD) 27-29 and received access to regular chow or daily limited access to a palatable diet until adulthood. At the age of 2 months, rats were tested on a social interaction test that screens for anxiety-like behaviors and their endocrine responses to an acute stressor. Animals were sacrificed, and their brains processed to detect differences in DA receptor subtype expression in the PFC and NA using qPCR. Results showed that rats that were stressed during the juvenile period displayed higher social anxiety and a sensitized corticosterone response as adults and these effects were attenuated by access to the palatable diet. Nevertheless, rats that experienced juvenile stress and consumed a palatable diet showed greater adiposity in adulthood. Interestingly, the same group displayed greater mRNA expression of DA receptors at the NA. This suggests that access to a palatable diet mitigates the behavioral and endocrine effects of juvenile stressor exposure in adulthood, but at the cost of metabolic imbalances and a sensitized dopaminergic system.

Reducing Effect of Saikosaponin A, an Active Ingredient of Bupleurum falcatum, on Intake of Highly Palatable Food in a Rat Model of Overeating

Recent lines of experimental evidence have indicated that saikosaponin A (SSA)-a bioactive ingredient of the medicinal plant, Bupleurum falcatum L.-potently and effectively reduced operant self-administration of chocolate and reinstatement of chocolate-seeking behavior in rats. The present study was designed to assess whether the protective properties of SSA on addictive-like, food-related behaviors generalize to a rat model of overeating of palatable food. To this end, rats were habituated to feed on a standard rat chow for 3 h/day; every 4 days, the 3-h chow-feeding session was followed by a 1-h availability of highly palatable, calorie-rich Danish butter cookies or Oreo chocolate cookies. Even though fed, rats consumed large amounts of cookies; intake of calories from cookies (consumed in 1 h) was even larger than that of calories from chow (consumed in 3 h). SSA (0, 0.25, 0.5, and 1 mg/kg, i.p.) was administered 10 min before cookie presentation. Treatment with SSA resulted in a dose-related decrease in intake of both butter and chocolate cookies. Administration of the cannabinoid CB₁ receptor antagonist/inverse agonist, rimonabant (0, 0.3, 1, and 3 mg/kg, i.p.; tested as reference compound), produced a similar reduction in intake of butter cookies. These results (a) contribute to the set-up and validation of a rat model of overeating, characterized by the intake of large amounts of unnecessary calories and (b) provide an additional piece of evidence to the anorectic profile of SSA in rats.

Hedonic sensitivity to natural rewards is affected by prenatal stress in a sex-dependent manner

Palatable food is a strong activator of the reward circuitry and may cause addictive behavior leading to eating disorders. How early life events and sex interact in shaping hedonic sensitivity to palatable food is largely unknown. We used prenatally restraint stressed (PRS) rats, which show abnormalities in the reward system and anxious/depressive-like behavior. Some of the hallmarks of PRS rats are known to be sex-dependent. We report that PRS enhanced and reduced milk chocolate-induced conditioned place preference in males and females, respectively. Male PRS rats also show increases in plasma dihydrotestosterone (DHT) levels and dopamine (DA) levels in the nucleus accumbens (NAc), and reductions in 5-hydroxytryptamine (5-HT) levels in the NAc and prefrontal cortex (PFC). In male rats, systemic treatment with the DHT-lowering drug finasteride reduced both milk chocolate preference and NAc DA levels. Female PRS rats showed lower plasma estradiol (E₂ ) levels and lower DA levels in the NAc, and 5-HT levels in the NAc and PFC. E₂ supplementation reversed the reduction in milk chocolate preference and PFC 5-HT levels. In the hypothalamus, PRS increased ERα and ERβ estrogen receptor and CARTP (cocaine-and-amphetamine receptor transcript peptide) mRNA levels in males, and 5-HT_2C receptor mRNA levels in females. Changes were corrected by treatments with finasteride and E₂ , respectively. These new findings show that early life stress has a profound impact on hedonic sensitivity to high-palatable food via long-lasting changes in gonadal hormones. This paves the way to the development of hormonal strategies aimed at correcting abnormalities in the response to natural rewards.

Epigenetic regulation of nociceptin/orphanin FQ and corticotropin-releasing factor system genes in frustration stress-induced binge-like palatable food consumption

Evidence suggests that binge eating may be caused by a unique interaction between dieting and stress. We developed a binge-eating model in which female rats with a history of intermittent food restriction show binge-like palatable food consumption after a 15-minute exposure to the sight of the palatable food (frustration stress). The aim of the present study was to investigate the regulation of the stress neurohormone corticotropin-releasing factor (CRF) system and of the nociceptin/orphanin FQ (N/OFQ) system genes in selective rat brain regions, using our animal model. Food restriction by itself seems to be responsible in the hypothalamus for the downregulation on messenger RNA levels of CRF-1 receptor, N/OFQ and its receptor (NOP). For the latter, this alteration might be due to selective histone modification changes. Instead, CRF gene appears to be upregulated in the hypothalamus as well as in the ventral tegmental area only when rats are food restricted and exposed to frustration stress, and, of relevance, these changes appear to be due to a reduction in DNA methylation at gene promoters. Moreover, also CRF-1 receptor gene resulted to be differentially regulated in these two brain regions. Epigenetic changes may be viewed as adaptive mechanisms to environmental perturbations concurring to facilitate food consumption in adverse conditions, that is, in this study, under food restriction and stressful conditions. Our data on N/OFQ and CRF signaling provide insight on the use of this binge-eating model for the study of epigenetic modifications in controlled genetic and environmental backgrounds.

Addiction: From Context-Induced Hedonia to Appetite, Based on Transition of Micro-behaviors in Morphine Abstinent Tree Shrews

Drug addiction is viewed as a maladaptive memory induced by contextual cues even in the abstinent state. However, the variations of hedonia and appetite induced by the context during the abstinence have been neglected. To distinguish the representative behaviors between hedonia and appetite, micro-behaviors in abstinent animal such as psycho-activity and drug seeking behaviors were observed in morphine conditioned place preference (CPP). To confirm the different effects of reward between drug and natural reward, a palatable food CPP paradigm was compared in current work. After a 10-day training in CPP with morphine or food, the preference was tested on day 1, 14, 28, and the changes of micro-behaviors were analyzed further. Our data showed that tree shrews treated with morphine performed more jumps on day 1 and more visits to saline paired side on day 28, which indicated a featured behavioral transition from psycho-activity to seeking behavior during drug abstinence. Meanwhile, food-conditioned animals only displayed obvious seeking behaviors in the three tests. The results suggest that the variations of micro-behaviors could imply such a transition from hedonic response to appetitive behaviors during morphine abstinence, which provided a potential behavioral basis for further neural mechanism studies.

Endogenous GLP1 and GLP1 analogue alter CNS responses to palatable food consumption

Glucagon-like peptide-1 (GLP1) affects appetite, supposedly mediated via the central nervous system (CNS). In this study, we investigate whether modulation of CNS responses to palatable food consumption may be a mechanism by which GLP1 contributes to the central regulation of feeding. Using functional MRI, we determined the effects of endogenous GLP1 and treatment with the GLP1 analogue liraglutide on CNS activation to chocolate milk receipt. Study 1 included 20 healthy lean individuals and 20 obese patients with type 2 diabetes (T2DM). Scans were performed on two occasions: during infusion of the GLP1 receptor antagonist exendin 9-39 (blocking actions of endogenous GLP1) and during placebo infusion. Study 2 was a randomised, cross-over intervention study carried out in 20 T2DM patients, comparing treatment with liraglutide to insulin, after 10 days and 12 weeks. Compared with lean individuals, T2DM patients showed reduced activation to chocolate milk in right insula (P = 0.04). In lean individuals, blockade of endogenous GLP1 effects inhibited activation in bilateral insula (P ≤ 0.03). Treatment in T2DM with liraglutide, vs insulin, increased activation to chocolate milk in right insula and caudate nucleus after 10 days (P ≤ 0.03); however, these effects ceased to be significant after 12 weeks. Our findings in healthy lean individuals indicate that endogenous GLP1 is involved in the central regulation of feeding by affecting central responsiveness to palatable food consumption. In obese T2DM, treatment with liraglutide may improve the observed deficit in responsiveness to palatable food, which may contribute to the induction of weight loss observed during treatment. However, no long-term effects of liraglutide were observed.

Consumption of palatable food primes food approach behavior by rapidly increasing synaptic density in the VTA

In an environment with easy access to highly palatable and energy-dense food, food-related cues drive food-seeking regardless of satiety, an effect that can lead to obesity. The ventral tegmental area (VTA) and its mesolimbic projections are critical structures involved in the learning of environmental cues used to predict motivationally relevant outcomes. Priming effects of food-related advertising and consumption of palatable food can drive food intake. However, the mechanism by which this effect occurs, and whether these priming effects last days after consumption, is unknown. Here, we demonstrate that short-term consumption of palatable food can prime future food approach behaviors and food intake. This effect is mediated by the strengthening of excitatory synaptic transmission onto dopamine neurons that is initially offset by a transient increase in endocannabinoid tone, but lasts days after an initial 24-h exposure to sweetened high-fat food (SHF). This enhanced synaptic strength is mediated by a long-lasting increase in excitatory synaptic density onto VTA dopamine neurons. Administration of insulin into the VTA, which suppresses excitatory synaptic transmission onto dopamine neurons, can abolish food approach behaviors and food intake observed days after 24-h access to SHF. These results suggest that even a short-term exposure to palatable foods can drive future feeding behavior by "rewiring" mesolimbic dopamine neurons.

The role of the opioid system in binge eating disorder

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

Anticipatory and consummatory effects of (hedonic) chocolate intake are associated with increased circulating levels of the orexigenic peptide ghrelin and endocannabinoids in obese adults

BACKGROUND

Hedonic hunger refers to consumption of food just for pleasure and not to maintain energy homeostasis. Recently, consumption of food for pleasure was reported to be associated with increased circulating levels of both the orexigenic peptide ghrelin and the endocannabinoid 2-arachidonoyl-glycerol (2-AG) in normal-weight subjects. To date, the effects of hedonic hunger, and in particular of chocolate craving, on these mediators in obese subjects are still unknown.

METHODS

To explore the role of some gastrointestinal orexigenic and anorexigenic peptides and endocannabinoids (and some related congeners) in chocolate consumption, we measured changes in circulating levels of ghrelin, glucagon-like peptide 1 (GLP-1), peptide YY (PYY), anandamide (AEA), 2-AG, palmitoylethanolamide (PEA), and oleoylethanolamide (OEA) in 10 satiated severely obese subjects after consumption of chocolate and, on a separate day, of a non-palatable isocaloric food with the same bromatologic composition. Evaluation of hunger and satiety was also performed by visual analogic scale.

RESULTS

The anticipatory phase and the consumption of food for pleasure were associated with increased circulating levels of ghrelin, AEA, 2-AG, and OEA. In contrast, the levels of GLP-1, PYY, and PEA did not differ before and after the exposure/ingestion of either chocolate or non-palatable foods. Hunger and satiety were higher and lower, respectively, in the hedonic session than in the non-palatable one.

CONCLUSIONS

When motivation to eat is generated by exposure to, and consumption of, chocolate a peripheral activation of specific endogenous rewarding chemical signals, including ghrelin, AEA, and 2-AG, is observed in obese subjects. Although preliminary, these findings predict the effectiveness of ghrelin and endocannabinoid antagonists in the treatment of obesity.

Persistent palatable food preference in rats with a history of limited and extended access to methamphetamine self-administration

Recent studies have shown that when given a mutually exclusive choice between cocaine and palatable foods, most rats prefer the non-drug rewards over cocaine. Here, we used a discrete choice procedure to assess whether palatable food preference generalizes to rats with a history of limited (3 hours/day) or extended (6 or 9 hours/day) access to methamphetamine self-administration. On different daily sessions, we trained rats to lever-press for either methamphetamine (0.1-0.2 mg/kg/infusion) or palatable food (five pellets per reward delivery) for several weeks; regular food was freely available. We then assessed food-methamphetamine preference either during training, after priming methamphetamine injections (0.5-1.0 mg/kg), following a satiety manipulation (palatable food exposure in the home cage) or after 21 days of withdrawal from methamphetamine. We also assessed progressive ratio responding for palatable food and methamphetamine. We found that independent of the daily drug access conditions and the withdrawal period, the rats strongly preferred the palatable food over methamphetamine, even when they were given free access to the palatable food in the home cage. Intake of methamphetamine and progressive ratio responding for the drug, both of which increased or escalated over time, did not predict preference in the discrete choice test. Results demonstrate that most rats strongly prefer palatable food pellets over intravenous methamphetamine, confirming previous studies using discrete choice procedures with intravenous cocaine. Results also demonstrate that escalation of drug self-administration, a popular model of compulsive drug use, is not associated with a cardinal feature of human addiction of reduced behavioral responding for non-drug rewards.

Food reward without a timing component does not alter the timing of activity under positive energy balance

Circadian clocks drive daily rhythms in physiology and behavior which allow organisms to anticipate predictable daily changes in the environment. In most mammals, circadian rhythms result in nocturnal activity patterns although plasticity of the circadian system allows activity patterns to shift to different times of day. Such plasticity is seen when food access is restricted to a few hours during the resting (light) phase resulting in food anticipatory activity (FAA) in the hours preceding food availability. The mechanisms underlying FAA are unknown but data suggest the involvement of the reward system and homeostatic regulation of metabolism. We previously demonstrated the isolated effect of metabolism by inducing diurnality in response to energetic challenges. Here the importance of reward timing in inducing daytime activity is assessed. The daily activity distribution of mice earning palatable chocolate at their preferred time by working in a running wheel was compared with that of mice receiving a timed palatable meal at noon. Mice working for chocolate (WFC) without being energetically challenged increased their total daily activity but this did not result in a shift to diurnality. Providing a chocolate meal at noon each day increased daytime activity, identifying food timing as a factor capable of altering the daily distribution of activity and rest. These results show that timing of food reward and energetic challenges are both independently sufficient to induce diurnality in nocturnal mammals. FAA observed following timed food restriction is likely the result of an additive effect of distinct regulatory pathways activated by energetic challenges and food reward.

Food reward system: current perspectives and future research needs

This article reviews current research and cross-disciplinary perspectives on the neuroscience of food reward in animals and humans, examines the scientific hypothesis of food addiction, discusses methodological and terminology challenges, and identifies knowledge gaps and future research needs. Topics addressed herein include the role of reward and hedonic aspects in the regulation of food intake, neuroanatomy and neurobiology of the reward system in animals and humans, responsivity of the brain reward system to palatable foods and drugs, translation of craving versus addiction, and cognitive control of food reward. The content is based on a workshop held in 2013 by the North American Branch of the International Life Sciences Institute.

Frustrated expected reward induces differential transcriptional changes in the mouse brain

Frustration represents a particular aspect of the addictive process that is related to loss of control when the expected reward is not obtained. We aim to study the consequences of frustrated expected reward on gene expression in the mouse brain. For this purpose, we used an operant model of frustration using palatable food as reward combined with microarrays. Transcriptomic profiles of frontal cortex, ventral striatum and hippocampus were analysed in five groups of mice: (1) positive control receiving palatable food and the cue light as conditioned stimulus; (2) frustrated group only receiving the cue light; (3) extinction learning group that did not receive palatable food nor the light; (4) negative control that never received the reinforcer nor the light during the whole experiment; and (5) yoked that received palatable food passively. Gene expression changes produced by frustration were revealed in the frontal cortex and ventral striatum, but not in the hippocampus. Most of the changes, such as the modification of the dopamine-DARPP-32 signalling pathway, were common in both areas and estimated to have neuronal origin. Extinction learning induced transcriptional changes only in the ventral striatum, with most genes showing down-regulation and without alteration in the dopamine-DARPP-32 signalling pathway. Active palatable food-seeking behaviour induced changes in gene expression in ventral striatum mainly affecting cell communication. In conclusion, frustration behaviour-induced changes in frontal cortex and ventral striatum mainly related to dopamine-DARPP-32 signalling that could play an important role in the loss of behavioural control during the addictive processes.

Role of bed nucleus of the stria terminalis corticotrophin-releasing factor receptors in frustration stress-induced binge-like palatable food consumption in female rats with a history of food restriction

We developed recently a binge-eating model in which female rats with a history of intermittent food restriction show binge-like palatable food consumption after 15 min exposure to the sight of the palatable food. This "frustration stress" manipulation also activates the hypothalamic-pituitary-adrenal stress axis. Here, we determined the role of the stress neurohormone corticotropin-releasing factor (CRF) in stress-induced binge eating in our model. We also assessed the role of CRF receptors in the bed nucleus of the stria terminalis (BNST), a brain region implicated in stress responses and stress-induced drug seeking, in stress-induced binge eating. We used four groups that were first exposed or not exposed to repeated intermittent cycles of regular chow food restriction during which they were also given intermittent access to high-caloric palatable food. On the test day, we either exposed or did not expose the rats to the sight of the palatable food for 15 min (frustration stress) before assessing food consumption for 2 h. We found that systemic injections of the CRF1 receptor antagonist R121919 (2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7 dipropylamino pyrazolo[1,5-a]pyrimidine) (10-20 mg/kg) and BNST (25-50 ng/side) or ventricular (1000 ng) injections of the nonselective CRF receptor antagonist D-Phe-CRF(12-41) decreased frustration stress-induced binge eating in rats with a history of food restriction. Frustration stress also increased Fos (a neuronal activity marker) expression in ventral and dorsal BNST. Results demonstrate a critical role of CRF receptors in BNST in stress-induced binge eating in our rat model. CRF1 receptor antagonists may represent a novel pharmacological treatment for bingeing-related eating disorders.

Evidence for a compulsive-like behavior in rats exposed to alternate access to highly preferred palatable food

Converging evidence suggests that recurrent excessive calorie restriction causes binge eating by promoting behavioral disinhibition and overeating. This interpretation suggests that cognitive adaptations may surpass physiological regulations of metabolic needs after recurrent cycles of dieting and binging. Intermittent access to palatable food has long been studied in rats, but the consequences of such diet cycling procedures on the cognitive control of food seeking remain unclear. Female Wistar rats were divided in two groups matched for food intake and body weight. One group received standard chow pellets 7 days/week, whereas the second group was given chow pellets for 5 days and palatable food for 2 days over seven consecutive weeks. Rats were also trained for operant conditioning. Intermittent access to palatable food elicited binging behavior and reduced intake of normal food. Rats with intermittent access to palatable food failed to exhibit anxiety-like behaviors in the elevated plus maze, but displayed reduced locomotor activity in the open field and developed a blunted corticosterone response following an acute stress across the diet procedure. Trained under a progressive ratio schedule, both groups exhibited the same motivation for sweetened food pellets. However, in contrast to controls, rats with a history of dieting and binging exhibited a persistent compulsive-like behavior when access to preferred pellets was paired with mild electrical foot shock punishments. These results highlight the intricate development of anxiety-like disorders and cognitive deficits leading to a loss of control over preferred food intake after repetitive cycles of intermittent access to palatable food.

Controversies about a common etiology for eating and mood disorders

Obesity and depression represent a growing health concern worldwide. For many years, basic science and medicine have considered obesity as a metabolic illness, while depression was classified a psychiatric disorder. Despite accumulating evidence suggesting that obesity and depression may share commonalities, the causal link between eating and mood disorders remains to be fully understood. This etiology is highly complex, consisting of multiple environmental and genetic risk factors that interact with each other. In this review, we sought to summarize the preclinical and clinical evidence supporting a common etiology for eating and mood disorders, with a particular emphasis on signaling pathways involved in the maintenance of energy balance and mood stability, among which orexigenic and anorexigenic neuropeptides, metabolic factors, stress responsive hormones, cytokines, and neurotrophic factors.

Addicted to palatable foods: comparing the neurobiology of Bulimia Nervosa to that of drug addiction

RATIONALE

Bulimia nervosa (BN) is highly comorbid with substance abuse and shares common phenotypic and genetic predispositions with drug addiction. Although treatments for the two disorders are similar, controversy remains about whether BN should be classified as addiction.

OBJECTIVES

Here, we review the animal and human literature with the goal of assessing whether BN and drug addiction share a common neurobiology.

RESULTS

Similar neurobiological features are present following administration of drugs and bingeing on palatable food, especially sugar. Specifically, both disorders involve increases in extracellular dopamine (DA), D1 binding, D3 messenger RNA (mRNA), and ΔFosB in the nucleus accumbens (NAc). Animal models of BN reveal increases in ventral tegmental area (VTA) DA and enzymes involved in DA synthesis that resemble changes observed after exposure to addictive drugs. Additionally, alterations in the expression of glutamate receptors and prefrontal cortex activity present in human BN or following sugar bingeing in animals are comparable to the effects of addictive drugs. The two disorders differ in regards to alterations in NAc D2 binding, VTA DAT mRNA expression, and the efficacy of drugs targeting glutamate to treat these disorders.

CONCLUSIONS

Although additional empirical studies are necessary, the synthesis of the two bodies of research presented here suggests that BN shares many neurobiological features with drug addiction. While few Food and Drug Administration-approved options currently exist for the treatment of drug addiction, pharmacotherapies developed in the future, which target the glutamate, DA, and opioid systems, may be beneficial for the treatment of both BN and drug addiction.

Co-sensitivity to the incentive properties of palatable food and cocaine in rats; implications for co-morbid addictions

Several lines of evidence suggest that there may be a shared vulnerability to acquire behaviors motivated by strong incentive stimuli. Non-food restricted male Sprague-Dawley rats (n = 78) underwent place conditioning with Oreos, and were subsequently tested on cocaine self-administration (SA) on fixed and progressive ratios, as well as extinction and reinstatement by cocaine primes and by consumption of Oreos. Although there was a group preference for the Oreo-paired compartment, at the individual level some rats (69%) displayed a preference and others did not. In cocaine SA, 'preference' rats achieved higher break points on a progressive ratio, and displayed greater responding during extinction and cocaine-induced reinstatement. Within the context of this study, Oreo-cocaine cross-reinstatement was not observed. In a control study, rats (n = 29) conditioned with a less palatable food (rice cakes) also displayed individual differences in place preference, but not on subsequent cocaine tests. These findings indicate that there is a relationship between incentive learning promoted by palatable foods and by cocaine. This supports the hypothesis that co-morbid food-drug addictions may result from a shared vulnerability to acquire behaviors motivated by strong incentives.

Attenuation of saccharin-seeking in rats by orexin/hypocretin receptor 1 antagonist

RATIONALE

The orexin (Orx)/hypocretin system has been implicated in reward-seeking, especially for highly salient food and drug rewards. We recently demonstrated that signaling at the OxR1 receptor is involved in sucrose reinforcement and reinstatement of sucrose-seeking elicited by sucrose-paired cues in food-restricted rats. Because sucrose reinforcement has both a hedonic and caloric component, it remains unknown what aspect of this reward drives its reinforcing value.

OBJECTIVES

The present study examined the involvement of the Orx system in operant responding for saccharin, a noncaloric, hedonic (sweet) reward, and in cue-induced reinstatement of extinguished saccharin-seeking in ad libitum-fed vs food-restricted male subjects.

METHODS

Male Sprague Dawley rats were fed ad libitum or food-restricted and trained to self-administer saccharin. We determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) saccharin self-administration and on cue-induced reinstatement of extinguished saccharin-seeking.

RESULTS

SB decreased responding and number of reinforcers earned during FR responding for saccharin and decreased cue-induced reinstatement of extinguished saccharin-seeking. All of these effects were obtained similarly in food-restricted and ad libitum-fed rats.

CONCLUSIONS

These results indicate that signaling at the OxR1 receptor is involved in saccharin reinforcement and reinstatement of saccharin-seeking elicited by saccharin-paired cues regardless of food restriction. These findings lead us to conclude that the Orx system contributes to the motivational effects of hedonic food rewards, independently of caloric value and homeostatic needs.

Role of orexin/hypocretin in conditioned sucrose-seeking in rats

RATIONALE

The orexin/hypocretin system has recently been implicated in reward-seeking, especially for highly salient food and drug rewards. We reasoned that this system may be strongly engaged during periods of reward restriction, including food restriction.

OBJECTIVES

This study examined the involvement of the orexin (Orx) system in responding for sucrose, and in cue-induced reinstatement of extinguished sucrose-seeking, in ad libitum fed versus food-restricted male subjects.

METHODS

Sprague-Dawley rats (n = 108) were trained to self-administer sucrose, and we determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) or progressive ratio (PR) sucrose self-administration, as well as on cue-induced reinstatement of sucrose-seeking. Finally, expression of the immediate early gene c-fos in Orx neurons was examined after self-administration, late extinction or cue-induced reinstatement of sucrose seeking.

RESULTS

SB decreased lever responding (by about 1/3) and the number of reinforcers earned during FR, and less so during PR, schedules and decreased cue-induced reinstatement to sucrose-seeking to extinction levels, predominately in food-restricted rats. Additionally, Fos expression in Orx neurons in perifornical and dorsomedial hypothalamus was increased during extinction.

CONCLUSIONS

These results indicate that signaling at the OxR1 receptor is involved in pronounced sucrose reinforcement, and reinstatement of sucrose-seeking elicited by sucrose-paired cues, in food-restricted subjects. These findings lead us to conclude that conditioned activation of Orx neurons increases motivation for food reward during food restriction.

Food reward in the obese and after weight loss induced by calorie restriction and bariatric surgery

Increased availability of tasty, energy-dense foods has been blamed as a major factor in the alarmingly high prevalence of obesity, diabetes, and metabolic disease, even in young age. A heated debate has started as to whether some of these foods should be considered addictive, similar to drugs and alcohol. One of the main arguments for food addiction is the similarity of the neural mechanisms underlying reward generation by foods and drugs. Here, we will discuss how food intake can generate reward and how behavioral and neural reward functions are different in obese subjects. Because most studies simply compare lean and obese subjects, it is not clear whether predisposing differences in reward functions cause overeating and weight gain, or whether repeated exposure or secondary effects of the obese state alter reward functions. While studies in both rodents and humans demonstrate preexisting differences in reward functions in the obese, studies in rodent models using calorie restriction and gastric bypass surgery show that some differences are reversible by weight loss and are therefore secondary to the obese state.

Feeding and reward: perspectives from three rat models of binge eating

Research has focused on understanding how overeating can affect brain reward mechanisms and subsequent behaviors, both preclinically and in clinical research settings. This work is partly driven by the need to uncover the etiology and possible treatments for the ongoing obesity epidemic. However, overeating, or non-homeostatic feeding behavior, can occur independent of obesity. Isolating the variable of overeating from the consequence of increased body weight is of great utility, as it is well known that increased body weight or obesity can impart its own deleterious effects on physiology, neural processes, and behavior. In this review, we present data from three selected animal models of normal-weight non-homeostatic feeding behavior that have been significantly influenced by Bart Hoebel's 40+-yr career studying motivation, feeding, reinforcement, and the neural mechanisms that participate in the regulation of these processes. First, a model of sugar bingeing is described (Avena/Hoebel), in which animals with repeated, intermittent access to a sugar solution develop behaviors and brain changes that are similar to the effects of some drugs of abuse, serving as the first animal model of food addiction. Second, another model is described (Boggiano) in which a history of dieting and stress can perpetuate further binge eating of palatable and non-palatable food. In addition, a model (Boggiano) is described that allows animals to be classified as having a binge-prone vs. binge-resistant behavioral profile. Lastly, a limited access model is described (Corwin) in which non-food deprived rats with sporadic limited access to a high-fat food develop binge-type behaviors. These models are considered within the context of their effects on brain reward systems, including dopamine, the opioids, cholinergic systems, serotonin, and GABA. Collectively, the data derived from the use of these models clearly show that behavioral and neuronal consequences of bingeing on a palatable food, even when at a normal body weight, are different from those that result from simply consuming the palatable food in a non-binge manner. These findings may be important in understanding how overeating can influence behavior and brain chemistry.

Effects of estradiol on food intake and meal patterns for diets that differ in flavor and fat content

Apart from the well known inhibitory effects of estradiol on food intake, meal size, and body weight in female rats that have been documented over the past thirty years, a more recent report presents the opposite finding; that a large dose of estradiol can increase food intake and weight gain in gonadally intact female rats presented with a palatable diet. The purpose of the present experiment was to further examine this hypothesis by evaluating the ability of estradiol to influence feeding behavior in ovariectomized rats presented with diets that differ in flavor and fat content. Female rats were given a cyclic regimen of estradiol benzoate treatment (5.0 or 20.0 microg) or the oil vehicle and were presented with the standard chow diet or a diet with a higher fat content and chocolate flavor. Food intake, meal size, and meal number were monitored three days after the first injection of estradiol or oil. Compared to the chow diet, food intake increased when animals had access to the chocolate/fat diet during the vehicle treatment condition. Both doses of estradiol significantly decreased food intake, meal size, and body weight gain when animals were presented with either the standard chow diet or the chocolate/fat diet. These findings indicate that estradiol does not stimulate the intake of a palatable diet in ovariectomized rats, and suggest that previous results showing that estradiol enhanced eating and weight gain stemmed from a disruption of the hypothalamic-pituitary-gonadal axis when intact females received a large dose of exogenous estradiol.

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.