Augmentation of drug reward by chronic food restriction: behavioral evidence and underlying mechanisms

You are what you eat: influence of type and amount of food consumed on central dopamine systems and the behavioral effects of direct- and indirect-acting dopamine receptor agonists

The important role of dopamine (DA) in mediating feeding behavior and the positive reinforcing effects of some drugs is well recognized. Less widely studied is how feeding conditions might impact the sensitivity of drugs acting on DA systems. Food restriction, for example, has often been the focus of aging and longevity studies; however, other studies have demonstrated that mild food restriction markedly increases sensitivity to direct- and indirect-acting DA receptor agonists. Moreover, it is becoming clear that not only the amount of food, but the type of food, is an important factor in modifying the effects of drugs. Given the increased consumption of high fat and sugary foods, studies are exploring how consumption of highly palatable food impacts DA neurochemistry and the effects of drugs acting on these systems. For example, eating high fat chow increases sensitivity to some behavioral effects of direct- as well as indirect-acting DA receptor agonists. A compelling mechanistic possibility is that central DA pathways that mediate the effects of some drugs are regulated by one or more of the endocrine hormones (e.g. insulin) that undergo marked changes during food restriction or after consuming high fat or sugary foods. Although traditionally recognized as an important signaling molecule in regulating energy homeostasis, insulin can also regulate DA neurochemistry. Because direct- and indirect-acting DA receptor drugs are used therapeutically and some are abused, a better understanding of how food intake impacts response to these drugs would likely facilitate improved treatment of clinical disorders and provide information that would be relevant to the causes of vulnerability to abuse drugs. This article is part of a Special Issue entitled 'Central Control of Food Intake'.

Dietary factors affect food reward and motivation to eat

The propensity to indulge in unhealthy eating and overconsumption of palatable food is a crucial determinant in the rising prevalence of obesity in today's society. The tendency to consume palatable foods in quantities that exceed energy requirements has been linked to an addiction-like process. Although the existence of 'food addiction' has not been conclusively proven, evidence points to alterations in the brain reward circuitry induced by overconsumption of palatable foods that are similar to those seen in drug addiction. The diet-induced obesity paradigm is a common procedure to replicate features of human obesity in rodents. Here we review data on the effect of various obesogenic diets (high-fat, Ensure™, cafeteria type, sucrose) on the extent of leptin resistance, hypothalamic-neuropeptidergic adaptations and changes in feeding behavior. We also discuss to what extent such diets and properties such as macronutrient composition, physical structure, sensory stimuli, and post-ingestive effects influence the brain-reward pathways. Understanding the interaction between individual components of diets, feeding patterns, and brain reward pathways could facilitate the design of diets that limit overconsumption and prevent weight gain.

Food restriction increases acquisition, persistence and drug prime-induced expression of a cocaine-conditioned place preference in rats

Cocaine conditioned place preference (CPP) is more persistent in food-restricted than ad libitum fed rats. This study assessed whether food restriction acts during conditioning and/or expression to increase persistence. In Experiment 1, rats were food-restricted during conditioning with a 7.0 mg/kg (i.p.) dose of cocaine. After the first CPP test, half of the rats were switched to ad libitum feeding for three weeks, half remained on food restriction, and this was followed by CPP testing. Rats tested under the ad libitum feeding condition displayed extinction by the fifth test. Their CPP did not reinstate in response to overnight food deprivation or a cocaine prime. Rats maintained on food restriction displayed a persistent CPP. In Experiment 2, rats were ad libitum fed during conditioning with the 7.0 mg/kg dose. In the first test only a trend toward CPP was displayed. Rats maintained under the ad libitum feeding condition did not display a CPP during subsequent testing and did not respond to a cocaine prime. Rats tested under food-restriction also did not display a CPP, but expressed a CPP following a cocaine prime. In Experiment 3, rats were ad libitum fed during conditioning with a 12.0 mg/kg dose. After the first test, half of the rats were switched to food restriction for three weeks. Rats that were maintained under the ad libitum condition displayed extinction by the fourth test. Their CPP was not reinstated by a cocaine prime. Rats tested under food-restriction displayed a persistent CPP. These results indicate that food restriction lowers the threshold dose for cocaine CPP and interacts with a previously acquired CPP to increase its persistence. In so far as CPP models Pavlovian conditioning that contributes to addiction, these results suggest the importance of diet and the physiology of energy balance as modulatory factors.

Dietary conditions and highly palatable food access alter rat cannabinoid receptor expression and binding density

Endogenous cannabinoid signaling, mediated predominately by CB1 receptor activation, is involved in food intake control and body weight regulation. Despite advances in determining the role of the CB1 receptor in obesity, its involvement in the driven nature of eating pathologies has received little attention. The present study examined CB1 receptor alterations as a consequence of dietary-induced binge eating in female Sprague Dawley rats. Four control groups were used to control for calorie restriction and highly palatable food variables characterizing this behavioral model. All groups were kept on their respective feeding schedules for 6-weeks and were given a uniform 33% calorie restriction (~22 h food deprivation) prior to sacrifice. Our findings indicate that regional CB1 mRNA and density were influenced by dietary conditions, but were not specific to the dietary-induced binge eating paradigm used. An increase of approximately 50% (compared with naive controls) in CB1 receptor mRNA levels in the nucleus of the solitary tract as measured by in situ hybridization was found in animals receiving continuous access to a highly palatable food (i.e., vegetable shortening with 10% sucrose). This group also had a significant increase in body weight and adiposity. An approximate 20% reduction in CB1 mRNA was observed in the cingulate cortex (areas 1 and 2) in animals exposed to an intermittent schedule of feeding, compared with groups that had ad libitum feeding schedules (i.e., continuous access and naive controls). Receptor density as measured by [(3)H]CP55,940 autoradiography, was reduced by approximately 30% in the nucleus accumbens shell region in groups receiving repeated access to the highly palatable food. Taken together, these findings indicate that dietary conditions can differentially influence CB1 receptors in forebrain and hindbrain regions.

Relation of addiction genes to hypothalamic gene changes subserving genesis and gratification of a classic instinct, sodium appetite

Sodium appetite is an instinct that involves avid specific intention. It is elicited by sodium deficiency, stress-evoked adrenocorticotropic hormone (ACTH), and reproduction. Genome-wide microarrays in sodium-deficient mice or after ACTH infusion showed up-regulation of hypothalamic genes, including dopamine- and cAMP-regulated neuronal phosphoprotein 32 kDa (DARPP-32), dopamine receptors-1 and -2, α-2C- adrenoceptor, and striatally enriched protein tyrosine phosphatase (STEP). Both DARPP-32 and neural plasticity regulator activity-regulated cytoskeleton associated protein (ARC) were up-regulated in lateral hypothalamic orexinergic neurons by sodium deficiency. Administration of dopamine D1 (SCH23390) and D2 receptor (raclopride) antagonists reduced gratification of sodium appetite triggered by sodium deficiency. SCH23390 was specific, having no effect on osmotic-induced water drinking, whereas raclopride also reduced water intake. D1 receptor KO mice had normal sodium appetite, indicating compensatory regulation. Appetite was insensitive to SCH23390, confirming the absence of off-target effects. Bilateral microinjection of SCH23390 (100 nM in 200 nL) into rats' lateral hypothalamus greatly reduced sodium appetite. Gene set enrichment analysis in hypothalami of mice with sodium appetite showed significant enrichment of gene sets previously linked to addiction (opiates and cocaine). This finding of concerted gene regulation was attenuated on gratification with perplexingly rapid kinetics of only 10 min, anteceding significant absorption of salt from the gut. Salt appetite and hedonic liking of salt taste have evolved over >100 million y (e.g., being present in Metatheria). Drugs causing pleasure and addiction are comparatively recent and likely reflect usurping of evolutionary ancient systems with high survival value by the gratification of contemporary hedonic indulgences. Our findings outline a molecular logic for instinctive behavior encoded by the brain with possible important translational-medical implications.

Impact of food restriction and cocaine on locomotion in ghrelin- and ghrelin-receptor knockout mice

Food restriction (FR) augments the behavioral and reinforcing effects of psychomotor stimulants such as cocaine or amphetamine; effects that may be related to the capacity of FR to increase plasma levels of ghrelin (GHR), a 28-amino acid orexigenenic peptide linked to activation of brain dopamine systems. The present study used wild-type (WT) mice or mutant mice sustaining knockout of either GHR [GHR((-/-)) ] or of the growth hormone secretagogue receptor [GHS-R((-/-)) ] and subjected to FR or not to evaluate the role of GHR and GHS-R in cocaine-stimulated locomotion. WT, GHR((-/-)) , and GHS-R((-/-)) mice were either restricted to 60% of baseline caloric intake or allowed to free-feed (FF). Mice were treated with 0, 1.25, 2.5 and 5.0 mg/kg cocaine on separate test days (in random dose order) and forward locomotion was recorded on each drug day for 45 minutes after drug dosing. Food (and water) was available immediately after (but not during) each activity test. For FF mice, there was no interaction between cocaine and GHR status on locomotion. FR-WT mice treated with saline exhibited significant increases in anticipatory locomotion (relative to FF-WT mice), whereas FR-GHS-R((-/-)) mice did not. Cocaine significantly increased locomotion in FR-GHR((-/-)) and FR-GHS-R((-/-)) mice to the levels noted in FR-WT mice. These results suggest that GHS-R activity, but not GHR activity, is required for FR to augment food-associated anticipatory locomotion, but do not support the contention that GHR pathways are required for the capacity of FR to augment the acute effect of cocaine on locomotion.

Food restriction dissociates sexual motivation, sexual performance, and the rewarding consequences of copulation in female Syrian hamsters

Animals can switch their behavioral priorities from ingestive to sex behaviors to optimize reproductive success in environments where energy fluctuates. We hypothesized that energy availability differentially affects the appetitive (motivation), consummatory (performance), and learned (rewarding) components of behavior. In Experiment 1, appetitive and consummatory aspects of sex behavior were dissociated in the majority of female Syrian hamsters restricted to 75% of their ad libitum food intake for between 8 and 11 days. Food restriction significantly inhibited vaginal scent marking, decreased the preference for spending time with male hamsters vs. spending time with food, and increased food hoarding with no significant effect on consummatory behaviors such as the incidence of lordosis or food intake. In Experiments 2 and 3, we attempted to use a similar level of food restriction to dissociate sexual appetite from sexual reward. In hamsters, formation of a conditioned place preference (CPP) for copulatory reward is reflected in increased nucleus accumbens (NAc) neural activation, measured as immunocytochemical staining for c-Fos, the protein product of the immediate-early gene, c-fos. In Experiment 2, neural activation increased 1h after copulation in the NAc, and did not differ significantly between 10-day food-restricted and ad libitum-fed females in any brain area examined. In Experiment 3, females were either food-restricted or fed ad libitum over 8-30 days of conditioning with copulatory stimuli. Food-restricted females showed significantly fewer appetitive behaviors, but no difference in formation of a CPP compared to females fed ad libitum. Together these data are consistent with the idea that mild levels of food restriction that inhibit appetitive behaviors fail to attenuate consummatory behaviors and the rewarding consequences of copulation. Thus, appetitive sex behaviors are, at least partially, neuroanatomically and behaviorally distinct from both consummatory behaviors and copulatory reward.

Neurobiology of overeating and obesity: the role of melanocortins and beyond

The alarming increase in the incidence of obesity and obesity-associated disorders makes the etiology of obesity a widely studied topic today. As opposed to 'homeostatic feeding', where food intake is restricted to satisfy one's biological needs, the term 'non-homeostatic' feeding refers to eating for pleasure or the trend to over-consume (palatable) food. Overconsumption is considered a crucial factor in the development of obesity. Exaggerated consumption of (palatable) food, coupled to a loss of control over food intake despite awareness of its negative consequences, suggests that overeating may be a form of addiction. At a molecular level, insulin and leptin resistance are hallmarks of obesity. In this review, we specifically address the question how leptin resistance contributes to enhanced craving for (palatable) food. Since dopamine is a key player in the motivation for food, the interconnection between dopamine, leptin and neuropeptides related to feeding will be discussed. Understanding the mechanisms by which these neuropeptidergic systems hijack the homeostatic feeding mechanisms, thus leading to overeating and obesity is the primary aim of this review. The melanocortin system, one of the crucial neuropeptidergic systems modulating feeding behavior will be extensively discussed. The inter-relationship between neuronal populations in the arcuate nucleus and other areas regulating energy homeostasis (lateral hypothalamus, paraventricular nucleus, ventromedial hypothalamus etc.) and reward circuitry (the ventral tegmental area and nucleus accumbens) will be evaluated and scrutinized.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Brain reward circuitry beyond the mesolimbic dopamine system: a neurobiological theory

Reductionist attempts to dissect complex mechanisms into simpler elements are necessary, but not sufficient for understanding how biological properties like reward emerge out of neuronal activity. Recent studies on intracranial self-administration of neurochemicals (drugs) found that rats learn to self-administer various drugs into the mesolimbic dopamine structures-the posterior ventral tegmental area, medial shell nucleus accumbens and medial olfactory tubercle. In addition, studies found roles of non-dopaminergic mechanisms of the supramammillary, rostromedial tegmental and midbrain raphe nuclei in reward. To explain intracranial self-administration and related effects of various drug manipulations, I outlined a neurobiological theory claiming that there is an intrinsic central process that coordinates various selective functions (including perceptual, visceral, and reinforcement processes) into a global function of approach. Further, this coordinating process for approach arises from interactions between brain structures including those structures mentioned above and their closely linked regions: the medial prefrontal cortex, septal area, ventral pallidum, bed nucleus of stria terminalis, preoptic area, lateral hypothalamic areas, lateral habenula, periaqueductal gray, laterodorsal tegmental nucleus and parabrachical area.

The influence of leptin on the dopamine system and implications for ingestive behavior

Food intake is regulated by many factors, including sensory information, metabolic hormones and the state of hunger. In modern humans, the drive to eat has proven to be incompatible with the excess food supply present in industrialized societies. A result of this imbalance is the dramatically increased rates of obesity during the last 20 years. The rise in obesity rates poses one of the most significant public health issues facing the United States and yet we do not understand the neural basis of ingestive behavior, and specifically, our motivation to eat. Understanding how the brain controls eating will lay the foundation for systematic dissection, understanding and treatment of obesity and related disorders. The lack of control over food intake bears resemblance to drug addiction, where loss of control over behavior leads to compulsive drug use. Work in laboratory animals has long suggested that there exist common neural substrates underlying both food and drug intake behaviors. Recent studies have shown direct leptin effects on dopamine neuron function and behavior. This provides a new mechanism by which peripheral hormones influence behavior and contribute to a more comprehensive model of neural control over food intake.

Leptin promotes dopamine transporter and tyrosine hydroxylase activity in the nucleus accumbens of Sprague-Dawley rats

Adipocytes produce the hormone, leptin, in proportion to fat mass to signal the status of body energy stores to the central nervous system, thereby modulating food intake and energy homeostasis. In addition to controlling satiety, leptin suppresses the reward value of food, which is controlled by the mesolimbic dopamine (DA) system. Previous results from leptin-deficient ob/ob animals suggest that chronic leptin deficiency decreases DA content in the mesolimbic DA system, thereby decreasing the response to amphetamine (AMPH). The extent to which these alterations in the mesolimbic DA system of ob/ob animals may mirror the leptin response of normal animals has remained unclear, however. We therefore examined the potential short-term modulation of the mesolimbic DA system by leptin in normal animals. We show that 4 h of systemic leptin treatment enhances AMPH-stimulated DA efflux in the nucleus accumbens (NAc) of Sprague-Dawley rats. While acute leptin treatment increased NAc tyrosine hydroxylase activity, total tyrosine hydroxylase and DA content were unchanged at this early time point. Leptin also increased NAc DA transporter activity in the absence of changes in cell surface or total DA transporter. Thus, leptin modulates the mesolimbic DA system via multiple acute mechanisms, and increases AMPH-mediated DA efflux in normal animals.

The tempted brain eats: pleasure and desire circuits in obesity and eating disorders

What we eat, when and how much, all are influenced by brain reward mechanisms that generate "liking" and "wanting" for foods. As a corollary, dysfunction in reward circuits might contribute to the recent rise of obesity and eating disorders. Here we assess brain mechanisms known to generate "liking" and "wanting" for foods and evaluate their interaction with regulatory mechanisms of hunger and satiety, relevant to clinical issues. "Liking" mechanisms include hedonic circuits that connect together cubic-millimeter hotspots in forebrain limbic structures such as nucleus accumbens and ventral pallidum (where opioid/endocannabinoid/orexin signals can amplify sensory pleasure). "Wanting" mechanisms include larger opioid networks in nucleus accumbens, striatum, and amygdala that extend beyond the hedonic hotspots, as well as mesolimbic dopamine systems, and corticolimbic glutamate signals that interact with those systems. We focus on ways in which these brain reward circuits might participate in obesity or in eating disorders.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Food seeking in spite of harmful consequences is under prefrontal cortical noradrenergic control

Background

Eating disorders are multifactorial psychiatric disorders. Chronic stressful experiences and caloric restriction are the most powerful triggers of eating disorders in human and animals. Although compulsive behavior is considered to characterize pathological excessive food intake, to our knowledge, no evidence has been reported of continued food seeking/intake despite its possible harmful consequences, an index of compulsive behavior. Brain monoamine transmission is considered to have a key role in vulnerability to eating disorders, and norepinephrine in medial prefrontal cortex has been shown to be critical for food-related motivated behavior.

Here, using a new paradigm of conditioned suppression, we investigated whether the ability of a foot-shock-paired conditioned stimulus to suppress chocolate-seeking behavior was reversed by previous exposure to a food restriction experience, thus modeling food seeking in spite of harmful consequences in mice. Moreover, we assessed the effects of selective norepinephrine inactivation in medial prefrontal cortex on conditioned suppression test in stressed and caloric restricted mice.

Results

While Control (non food deprived) animals showed a profound conditioned suppression of chocolate seeking during presentation of conditioned stimulus, previously food restricted animals showed food seeking/intake despite its possible harmful consequences. Moreover, food seeking in spite of harmful consequences was prevented by selective norepinephrine inactivation, thus showing that prefrontal cortical norepinephrine is critical also for maladaptive food-related behavior.

Conclusions

These findings indicate that adaptive food seeking/intake can be transformed into maladaptive behaviors and point to "top-down" influence on eating disturbances and to new targets for therapy of aberrant eating behaviors.

Ghrelin receptor antagonism attenuates cocaine- and amphetamine-induced locomotor stimulation, accumbal dopamine release, and conditioned place preference

INTRODUCTION

Recently we demonstrated that genetic or pharmacological suppression of the central ghrelin signaling system, involving the growth hormone secretagogue receptor 1A (GHS-R1A), lead to a reduced reward profile from alcohol. As the target circuits for ghrelin in the brain include a mesolimbic reward pathway that is intimately associated with reward-seeking behaviour, we sought to determine whether the central ghrelin signaling system is required for reward from drugs of abuse other than alcohol, namely cocaine or amphetamine.

RESULTS

We found that amphetamine-as well as cocaine-induced locomotor stimulation and accumbal dopamine release were reduced in mice treated with a GHS-R1A antagonist. Moreover, the ability of these drugs to condition a place preference was also attenuated by the GHS-R1A antagonist.

CONCLUSIONS

Thus GHS-R1A appears to be required not only for alcohol-induced reward, but also for reward induced by psychostimulant drugs. Our data suggest that the central ghrelin signaling system constitutes a novel potential target for treatment of addictive behaviours such as drug dependence.

Environmental enrichment reduces cocaine seeking and reinstatement induced by cues and stress but not by cocaine

Whereas earlier studies have focused on the preventive effects of enriched environments (EE) in drug addiction, in a recent study we suggested that EE can also have 'curative' effects. In fact, we found that cocaine addiction-related behaviors can be eliminated by housing cocaine-treated mice in EE during periods of forced abstinence. However, those results were obtained with two simple models of addiction, conditioned place preference (CPP), and behavioral sensitization. In this study, we used intravenous drug self-administration procedures in rats to further investigate the beneficial effects of EE on cocaine addiction in a reinstatement model of relapse. Singly housed rats learned to self-administer cocaine during 10 consecutive daily sessions (0.6 mg/injection, 6 h/day). They were then housed three per cage in either standard environments (SE) or EE and were kept abstinent in the animal facility until testing for extinction and reinstatement. We found that 30 days of EE significantly and consistently reduced cocaine seeking during a 6-h extinction session. In addition, EE significantly reduced cue- and stress-induced reinstatement. Surprisingly, given our earlier results in mice with CPP, EE did not reduce cocaine-induced reinstatement regardless of the level of exposure to cocaine and the duration of the period of abstinence and exposure to EE. Altogether, these results support the hypothesis that EE can reduce cocaine-induced craving and highlight the importance of positive life conditions in facilitating abstinence and preventing relapse to cocaine addiction.

An expanded view of energy homeostasis: neural integration of metabolic, cognitive, and emotional drives to eat

The traditional view of neural regulation of body energy homeostasis focuses on internal feedback signals integrated in the hypothalamus and brainstem and in turn leading to balanced activation of behavioral, autonomic, and endocrine effector pathways leading to changes in food intake and energy expenditure. Recent observations have demonstrated that many of these internal signals encoding energy status have much wider effects on the brain, particularly sensory and cortico-limbic systems that process information from the outside world by detecting and interpreting food cues, forming, storing, and recalling representations of experience with food, and assigning hedonic and motivational value to conditioned and unconditioned food stimuli. Thus, part of the metabolic feedback from the internal milieu regulates food intake and energy balance by acting on extrahypothalamic structures, leading to an expanded view of neural control of energy homeostasis taking into account the need to adapt to changing conditions in the environment. The realization that metabolic signals act directly on these non-traditional targets of body energy homeostasis brings opportunities for novel drug targets for the fight against obesity and eating disorders.

Relation of obesity to consummatory and anticipatory food reward

This report reviews findings from studies that have investigated whether abnormalities in reward from food intake and anticipated food intake increase risk for obesity. Self-report and behavioral data suggest that obese relative to lean individuals show elevated anticipatory and consummatory food reward. Brain imaging studies suggest that obese relative to lean individuals show greater activation of the gustatory cortex (insula/frontal operculum) and oral somatosensory regions (parietal operculum and Rolandic operculum) in response to anticipated intake and consumption of palatable foods. Yet, data also suggest that obese relative to lean individuals show less activation in the dorsal striatum in response to consumption of palatable foods and reduced striatal D2 dopamine receptor density. Emerging prospective data also suggest that abnormal activation in these brain regions increases risk for future weight gain and that genotypes associated with lowered dopamine signaling amplify these predictive effects. Results imply that individuals who show greater activation in the gustatory cortex and somatosensory regions in response to anticipation and consumption of food, but who show weaker activation in the striatum during food intake, may be at risk for overeating, particularly those at genetic risk for lowered dopamine receptor signaling.

SUBSEQUENTLY ENHANCED CPP TO MORPHINE FOLLOWING CHRONIC BUT NOT ACUTE FOOTSHOCK STRESS ASSOCIATED WITH CORTICOSTERONE MECHANISM IN RATS

The purpose of this study was to determine the morphine-induced place preference in rats pre-exposed to footshock stress and corticosterone (CORT). The authors also studied the involvement of dopamine mechanisms in the CORT-induced place preference. The results showed that chronic footshock or CORT exposure but not acute footshock or CORT exposure similarly potentiated the conditioned place preference to morphine. The CORT-induced conditioned place preference were established only with high and middle dose (5, 3 mg/kg) CORT. The dopamine levels in NAc of rats injected with CORT (5, 3, 1 mg/kg, i.p.) increased significantly. The findings suggest that the increase of dopamine levels in NAc induced by CORT might be the medium between stress and morphine.

Repeated binge access to a palatable food alters feeding behavior, hormone profile, and hindbrain c-Fos responses to a test meal in adult male rats

Repetitive cycles of palatable food access and chronic calorie restriction alter feeding behaviors and forebrain neural systems. The purpose of this study was to determine the behavioral, endocrine, and meal-related hindbrain neural activation in adult male Sprague-Dawley rats exposed to a binge-access feeding schedule. The binge-access schedule consisted of repeated twice-per-week episodes of acute calorie restriction (to one-third of the previous day's intake) followed by 2 h of concurrent access to high-calorie palatable food (sweetened fat: 90% vegetable shortening-10% sucrose) and chow. The binge-access rats consumed more calories during the "binge" period than rats with continuous access to sweetened fat (continuous-access group) or subjected to repeated acute calorie restriction only (chow-restricted group). The binge-access group also exhibited a approximately 25% increase in sweetened fat intake from week 1 to week 6. Persistence of the binge phenotype in the binge-access animals was demonstrated 2 wk, but not 4 wk, after ad libitum chow. The binge-access and chow-restricted groups maintained a similar normal body composition and hormonal profiles, whereas the continuous-access animals developed an obese phenotype. Terminal ghrelin levels were significantly higher in the binge-access group than in the continuous-access group. Consumption of a standardized meal resulted in more c-Fos-positive cells along the anterior-posterior nucleus of the solitary tract regions in the binge-access group than in naive controls. These results suggest that repeated cycles of acute calorie restriction followed by palatable food produce physiological alterations that may facilitate overconsumption of a highly palatable food during limited-access periods.

Early malnutrition favours heavy ethanol intake in weaning rats without long-lasting effects

INTRODUCTION

Although food restriction is well known to increase ethanol intake, the subject has not been extensively studied in developing animals which could be more vulnerable to long-lasting effects. Therefore, the aim of this study was to show some findings concerning this subject.

MATERIALS AND METHODS

Food restriction was used to produce malnutrition either during lactation (lactating dams food restricted 60%) or after weaning (pups food restricted 60%). At weaning, day 25, males were assigned to one of the following groups: C, food ad libitum throughout the experiment - control group; MW, malnourished only after weaning; ML, malnourished only during lactation period; and MLW, malnourished throughout the experiment, during lactation and after weaning. All rats were kept isolated in cages in which they could choose to drink either a 10% ethanol solution or tap water (from days 25 to 45). Re-exposure to this model was performed on day 49. Between exposure and re-exposure, rats drank tap water for 4 days.

RESULTS

There was a significant effect of malnutrition during lactation, up to day 35, with heavy drinking patterns (ethanol intake day 2: C, 8 g/kg; MW, 9 g/kg; ML, 19 g/kg; and MLW, 22 g/kg). This heavy drinking pattern lasted until the recovery of body weight. Food restriction after weaning had significant effects after 14 days, when a statistically significant decrease in body weight occurred (body weight day 39: C, 147.8 g; MW, 98.5 g). Only rats which were persistently malnourished (MLW and MW) drank ethanol more significantly than their ad libitum-fed counterparts during the re-exposure period (ethanol intake: malnourished, 5 g/kg; and well-nourished, 2.5 g/kg). Adulteration of the ethanol solution with quinine (0.1 g/l) precluded the effect of malnutrition.

CONCLUSIONS

Malnutrition during early development had no long-lasting effects on ethanol consumption. In addition, malnutrition increased ethanol consumption as long as it kept body weight low, which was apparently more significant in young animals.

Effects of rimonabant on the reinstatement of nicotine-conditioned place preference by drug priming in rats

Drug addiction is a chronic disorder characterized by a relatively high rate of relapse even after long period of abstinence. In the present study, we used the conditioned place preference (CPP) paradigm to investigate the establishment, extinction, reinstatement and cross-reinstatement of nicotine-induced place conditioning in rats. Nicotine produced a place preference to the initially less-preferred compartment paired with its injections during conditioning (0.5mg/kg, i.p., three drug sessions). Once established, nicotine CPP was extinguished by repeated testing. Following this extinction phase, the reinstatement of CPP was investigated. Nicotine-experienced rats were challenged with nicotine (0.5mg/kg, i.p.) or morphine (10mg/kg, i.p.). These priming injections of both drugs induced a marked preference for the compartment previously paired with nicotine. Furthermore, the objective of the present study was to evaluate the efficacy of CB1 cannabinoid receptor antagonist rimonabant (0.5, 1 and 2mg/kg, i.p.) in blocking the reinstatement of nicotine-induced CPP provoked by nicotine and morphine. It was shown that rimonabant attenuated the reinstatement of nicotine-conditioned response induced by both drugs. The outcome of our studies may suggest that CB1 receptor antagonists may become a promising target for effective pharmacotherapy of tobacco addiction and polydrug abuse.

Neuropeptide Y augments cocaine self-administration and cocaine-induced hyperlocomotion in rats

Food restriction and deprivation are known to modulate drug-related behaviors. However, the mechanisms through which metabolic manipulations intercede the rewarding effects of drug reward are unknown. Neuropeptide Y (NPY) is thought to be critically involved in the regulation of energy balance. Central administration of NPY induces feeding in sated animals, and importantly, is reported to increase the rewarding properties of food. NPY has also been shown to be involved in drug-related behavior. We have recently demonstrated that NPY injections augmented on-going heroin self-administration and induced a reinstatement of heroin seeking. The present study sought to support and expand our previous finding on NPY's role in addictive drugs-related behaviors by examining the effects of NPY on cocaine-induced locomotor hyperactivity and cocaine self-administration. In Experiment 1, rats received NPY injections (0.0, 2.5, 5.0microg/rat, ICV), followed by cocaine administration (0.0, 1.0, 5.0, and 10.0mg/kg, IP) and their locomotor activity was monitored over 90min. In Experiment 2, rats were trained to self-administer cocaine (0.50mg/kg/infusion) during one 3-h session per day for 12 days. Once trained, NPY (0.0, 4.0, 10.0microg/rat, ICV) was administered 15min prior to the self-administration session. Results revealed that NPY injections augmented cocaine-induced hyperactivity and moderately increased cocaine self-administration. Together with our previous findings, these results suggest that NPY is involved, albeit to a limited extent, in the augmenting effect of food deprivation on drug-related behaviors.

'Liking' and 'wanting' food rewards: brain substrates and roles in eating disorders

What brain reward systems mediate motivational 'wanting' and hedonic 'liking' for food rewards? And what roles do those systems play in eating disorders? This article surveys recent findings regarding brain mechanisms of hedonic 'liking', such as the existence of cubic-millimeter hedonic hotspots in nucleus accumbens and ventral pallidum for opioid amplification of sensory pleasure. It also considers brain 'wanting' or incentive salience systems important to appetite, such as mesolimbic dopamine systems and opioid motivation circuits that extend beyond the hedonic hotspots. Finally, it considers some potential ways in which 'wanting' and 'liking' might relate to eating disorders.

High fat diet and food restriction differentially modify the behavioral effects of quinpirole and raclopride in rats

Nutritional status can impact dopamine systems in a manner that might be important to understanding possible common neurobiological mechanisms that mediate abnormal compulsive food (e.g., obesity) and drug taking. Limiting food intake, for example, can increase sensitivity to the behavioral effects of indirect-acting dopamine receptor agonists. Much less is known regarding possible diet-induced changes in sensitivity to direct-acting dopamine receptor drugs. The present study investigated the effects of a high fat diet and of food restriction on sensitivity of rats to the behavioral effects of a direct-acting dopamine receptor agonist and a dopamine receptor antagonist. Free access to high fat chow increased sensitivity to quinpirole-induced yawning without changing sensitivity to raclopride-induced catalepsy or quinpirole-induced hypothermia. Food restriction (10 g/day) decreased sensitivity to quinpirole-induced yawning and raclopride-induced catalepsy without affecting sensitivity to quinpirole-induced hypothermia. Free access to a standard chow restored sensitivity to the behavioral effects of both drugs in rats that were previously food-restricted but not in rats that previously ate a high fat diet. These data confirm that food restriction can decrease sensitivity to behavioral effects of direct-acting dopamine receptor drugs, they provide evidence (i.e., no change in hypothermic effects) indicating that these changes are not due to pharmacokinetic mechanisms, and they provide initial evidence showing enhanced sensitivity to behavioral effects of dopamine receptor drugs in rats eating a high fat diet. These changes in sensitivity of dopamine systems could be relevant to understanding the impact of nutrition on therapeutic and recreational drug use.

Changes in feeding and locomotion induced by amphetamine analogs in rats

Studies of the biobehavioral actions of psychostimulants commonly focus on locomotion and less commonly on feeding, and only rarely are these measures considered in conjunction within the same animal. The present study compared the impact of (+)-amphetamine and three amphetamine analogs, PAL-287, PAL-313, and PAL-353, on eating and locomotion assessed concurrently using an automated activity/feeding chamber during a daily 45 min session. Each analog is a potent releaser of norepinephrine and of dopamine, but exerts differential serotonin-releasing activity (PAL-287>PAL-313>amphetamine>PAL-353). Rats were tested with each of five doses of drug (0, 2, 4, 8, or 16 micromol/kg, i.p.), given in equimolar concentrations and in random dose order. PAL-353, an analog with minimal serotonin-releasing capacity, markedly stimulated forward locomotion at 2, 4, 8 and 16 micromol/kg, as did amphetamine, whereas PAL-287 and PAL-313 did not. In contrast to the locomotor findings, all four amphetamine-like drugs exerted similar effects on the suppression of food intake. These results suggest that the capacity of an amphetamine analog (i.e. amphetamine and PAL-353) to stimulate serotonin release can diminish its psychostimulant action on locomotion, but does not reliably augment drug-induced hypophagia.

Underweight rats have enhanced dopamine release and blunted acetylcholine response in the nucleus accumbens while bingeing on sucrose

The present study tested whether rats release more accumbens dopamine (DA) during a sugar binge when they are underweight vs. normal weight. Since acetylcholine (ACh) in the nucleus accumbens (NAc) normally increases as a meal progresses and satiety ensues, we also tested whether ACh release is altered when an animal has lost weight. Rats were maintained on daily 8-h access to chow, with 10% sucrose solution available for the first 2 h. Microdialysis performed on day 21, at normal body weight, revealed an increase in extracellular DA to 122% of baseline in response to drinking sucrose. Extracellular ACh peaked at the end of the meal. Next, the rats were food and sucrose restricted so that by day 28 they were at 85% body weight. When retested, these animals released significantly more DA when drinking sucrose (179%), but ACh release failed to rise. A control group was tested in the same manner but given sugar only on days 1, 21 and 28. At normal body weight, control animals showed a non-significant rise in DA when drinking sucrose on day 21. On day 28, at 85% body weight, the controls showed a small increase (124%) in DA release; however, this was significantly lower than the 179% observed in the underweight rats with daily sugar access. These findings suggest that when an animal binges on sugar and then loses weight, the binge releases significantly more DA and less ACh than when animals are at a normal body weight.

Insulin, leptin, and food reward: update 2008

The hormones insulin and leptin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis at medial hypothalamic sites. In a previous review, we described new research demonstrating that, in addition to these direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and motivation is also a direct and an indirect target for insulin and leptin action. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, i.e., midbrain dopamine and opioidergic pathways. Here we summarize new behavioral, systems, and cellular evidence in support of this hypothesis and in the context of research into the homeostatic roles of both hormones in the CNS. We discuss some current issues in the field that should provide additional insight into this hypothetical model. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

The effect of food deprivation on cigarette smoking in females

Food deprivation is associated with increased self-administration of nicotine and other substances in laboratory animals, although little is known about the effects of food deprivation on substance use in humans. A within-subjects design was utilized to test the primary hypotheses that smoking rate and expired carbon monoxide (CO) levels would be greater among 15 female participants following a 24-hour fasting period than when participants were not food-deprived. Food-deprived participants were also expected to endorse greater negative mood, smoking urges, and pleasure from smoking, as well as less positive mood. Analyses indicated that CO levels were significantly greater in the food-deprived condition than in the non-deprived condition (p=.05), although no differences were found in the number of cigarettes smoked during the laboratory session. Participants reported significantly lower Vigor-Activity in the food-deprived condition (p=.02). No differences were found between conditions in urges to smoke or pleasure from smoking. Findings suggest that nicotine intake may increase among females during acute food deprivation.

The role of impulsive behavior in drug abuse

BACKGROUND

Impulsivity is a multifaceted construct that has recently been recognized as a factor contributing to enhanced vulnerability to drug abuse.

OBJECTIVES

In the present review, we focus on two facets of impulsivity (and tasks that measure them): (1) impulsive choice (delay discounting task) and (2) inhibitory failure (go/no-go, stop signal reaction time, and five-choice serial reaction time tasks). We also describe how performance on each of these tasks is associated with drug-related behavior during phases of drug abuse that capture the essential features of addiction (acquisition, escalation, and reinstatement of drug-seeking after drug access has terminated). Three hypotheses (H) regarding the relationship between impulsivity and drug abuse are discussed: (1) increased levels of impulsivity lead to drug abuse (H1), (2) drugs of abuse increase impulsivity (H2), and (3) impulsivity and drug abuse are associated through a common third factor (H3).

CONCLUSION

Impulsivity expressed as impulsive choice or inhibitory failure plays a role in several key transition phases of drug abuse. There is evidence to support all three nonexclusive hypotheses. Increased levels of impulsivity lead to acquisition of drug abuse (H1) and subsequent escalation or dysregulation of drug intake. Drugs of abuse may increase impulsivity (H2), which is an additional contributor to escalation/dysregulation. Abstinence, relapse, and treatment may be influenced by both H1 and H2. In addition, there is a relationship between impulsivity and other drug abuse vulnerability factors, such as sex, hormonal status, reactivity to nondrug rewards, and early environmental experiences that may impact drug intake during all phases of addiction (H3). Relating drug abuse and impulsivity in phases of addiction via these three hypotheses provides a heuristic model from which future experimental questions can be addressed.

Dopamine signaling in the dorsal striatum is essential for motivated behaviors: lessons from dopamine-deficient mice

Genetically engineered mice that lack tyrosine hydroxylase in all dopaminergic neurons become hypoactive and aphagic, and they starve by 4 weeks of age. However, they can be rescued by daily treatment with l-dopa, which restores activity and feeding for about 10 hours. Thus, these mice can be examined in both dopamine-depleted and dopamine-replete states. A series of behavioral experiments lead to the primary conclusion that in the dopamine-depleted state these mice are not motivated to engage in goal-directed behaviors. Nevertheless, they still have a preference for sucrose, they can learn the location of food rewards, and they can form a conditioned-place preference for drugs. Dopamine signaling can be restored to the striatum by several different viral gene-therapy procedures. Restoring dopamine signaling selectively to the dorsal striatum is sufficient to allow feeding, locomotion, and reward-based learning. The rescued mice appear to have normal motivation to engage in all goal-directed behaviors that have been tested. The results suggest that dopamine facilitates the output from dorsal striatum, which provides a permissive signal allowing feeding and other goal-directed behaviors.

Food deprivation-like effects of neuropeptide Y on heroin self-administration and reinstatement of heroin seeking in rats

Numerous findings suggest that drug seeking and ingestive behaviors share common neurobiological mechanisms, but the relevant pathways are unknown. Dietary manipulations result in changes in endocrine the and/or neuropeptide signals, such as the hormones leptin and ghrelin, which are dynamically linked to energy balance and the regulation of feeding behavior. We have recently demonstrated that food deprivation-induced reinstatement of heroin seeking can be blocked with leptin, and others have suggested a role for ghrelin in drug-related behaviors. The feeding-relevant effects of leptin and ghrelin involve the inhibition or activation, respectively, of neuropeptide Y/agouti-related peptide (NPY/AGRP) neurons in the hypothalamus. However, the effects of NPY, a highly potent orexigenic peptide, on drug-related behaviors have not been thoroughly studied. Here we examined the effect of acute NPY administration on the rate of heroin self-administration and the reinstatement of extinguished heroin-seeking behavior. Heroin intake (0.05mg/kg/infusion) was tested using a self-administration procedure (FR-1), 10-min post-NPY injections (0.0, 4.0, and 10microg/rat, ICV). In a different group of rats, NPY-induced reinstatement (0.0, 4.0, and 10microg/rat, ICV) of extinguished heroin seeking was assessed. NPY injections increased on-going heroin self-administration, and induced a reinstatement of extinguished heroin-seeking behavior. These findings suggest that NPY can modulate the rewarding and conditioned reinforcing effects of drugs of abuse.

Effects of moderate chronic ethanol consumption on hippocampal nicotinic receptors and associative learning

A number of studies have reported that ethanol exposure induces changes in different brain systems. The hippocampus is a brain region that is very vulnerable to ethanol exposition, which functionally results in impairment of learning and memory processes reported in heavy drinkers. Hippocampal nicotinic receptors are involved in learning and memory. In this study, we determined the effects of ethanol on the main hippocampal subtypes of neural nicotinic receptors (alpha7 and alpha4beta2) in rats non-selected for alcohol consumption, in order to check for possible changes on these receptors that could be linked with alterations in learning acquisition. Binding assays were carried out with [3H]methyllycaconitine ([3H]MLA) to study the alpha7 and [3H]nicotine to study alpha4beta2 receptors. Auto-shaping, continuous ratio and extinction procedures were used as behavioral tests. The results show that moderate chronic ethanol consumption for 10 weeks produces: (a) a decrease of both hippocampal nicotinic receptor subtypes without alterations in affinity; (b) no differences in behavioral performance between control rats and ethanol-drinking rats in auto-shaping and continuous ratio; (c) an improvement of performance of extinction paradigm. These results indicate that chronic ethanol consumption, at moderate levels, induces changes in hippocampal nicotinic receptors but does not impair acquisition and performance of new associative learning and even improves some kind of paradigms. These results may have implications in the biochemical basis of interactions between alcohol and nicotine and the effects of these drugs on behavior.

Stress and Rodent Models of Drug Addiction: Role of VTA-Accumbens-PFC-Amygdala Circuit

Stress can trigger, intensify, and prolong drug consumption, as well as reinstate previously extinguished drug-taking behavior by directly impacting a neural circuit often referred to as a reward pathways. Animal models of drug abuse have been used to understand these neural circuits mediating stress-induced drug intake and relapse through examination of cellular and subcellular molecular mechanisms. Several types of intermittent stressors have been shown to induce cross-sensitization to psychomotor stimulants, enhance conditioned place preference under most conditions, increase self-administration of cocaine and amphetamine and induce reinstatement of heroin and cocaine seeking via activation of the mesocorticolimbic dopamine system.

Lobeline attenuates progressive ratio breakpoint scores for intracranial self-stimulation in rats

The alkaloid lobeline inhibits the function of vesicular monoamine and dopamine transporters and diminishes the behavioral and neurochemical effects of nicotine and amphetamines. In the present study, we examined the interaction of systemic administration of lobeline on breakpoint scores on a progressive ratio (PR) schedule of intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB). Rats were run in two 30 min sessions, separated by a 10 min timeout period. At the end of the first session, each rat was injected with either 0, 0.5, 1.0 or 2.0 mg/kg (i.p.) lobeline. Positive controls known to suppress and to augment ICSS responding included the adrenergic antagonist prazosin (0, 0.5 and 2.0 mg/kg, i.p.) and the psychostimulant cocaine (0, 1.25, and 5.0 mg/kg, i.p.). Analyses of changes in average PR breakpoint scores between the 2 sessions revealed that lobeline significantly suppressed PR scores at doses of 0.5, 1.0 and 2.0 mg/kg, as did 0.5 mg/kg and 2.0 mg/kg prazosin. These changes are unlikely to reflect motoric effects of these drugs inasmuch as neither lobeline nor prazosin alter locomotion at these doses. In contrast, PR breakpoint scores were significantly increased at 5.0 mg/kg cocaine, a dose that is sufficient to elevate locomotion in the rat. These results are consistent with the view that lobeline modulates brain reinforcement processes.

Food reinforcement and eating: a multilevel analysis

Eating represents a choice among many alternative behaviors. The purpose of this review is to provide an overview of how food reinforcement and behavioral choice theory are related to eating and to show how this theoretical approach may help organize research on eating from molecular genetics through treatment and prevention of obesity. Special emphasis is placed on how food reinforcement and behavioral choice theory are relevant to understanding excess energy intake and obesity and how they provide a framework for examining factors that may influence eating and are outside of those that may regulate energy homeostasis. Methods to measure food reinforcement are reviewed, along with factors that influence the reinforcing value of eating. Contributions of neuroscience and genetics to the study of food reinforcement are illustrated by using the example of dopamine. Implications of food reinforcement for obesity and positive energy balance are explored, with suggestions for novel approaches to obesity treatment based on the synthesis of behavioral and pharmacological approaches to food reinforcement.

Behavioral effects of amphetamine in streptozotocin-treated rats

Experimentally-induced diabetes can modify the behavioral and neurochemical effects of drugs acting on dopamine systems, possibly through insulin-related regulation of dopamine transporter activity. In this study, several behavioral procedures were used to examine possible changes in sensitivity to amphetamine and other drugs in rats rendered diabetic by a single injection of streptozotocin. Conditioned place preference developed to food (Froot Loops) in both control and diabetic rats, demonstrating that conditioned place preference with tactile stimuli can occur in streptozotocin-treated rats. Baseline locomotion was lower in streptozotocin-treated as compared to control rats, although amphetamine significantly increased locomotion in all rats. Conditioned place preference developed to amphetamine regardless of whether rats had received streptozotocin or saline. A second study compared the potency of drugs to decrease lever pressing maintained by food, before and after streptozotocin treatment. Gamma-hydroxybutyrate and amphetamine were less potent after streptozotocin while the potency of raclopride, quinpirole, ketamine, haloperidol and cocaine was not significantly changed by streptozotocin. While markedly affecting locomotion, body weight and blood glucose, streptozotocin only modestly affected sensitivity to the behavioral effects of amphetamine and other drugs; these results fail to confirm previous reports of decreased behavioral actions of stimulants in diabetic rats.

Peptide YY3-36 decreases reinstatement of high-fat food seeking during dieting in a rat relapse model

A major problem in treating obesity is high rates of relapse to maladaptive food-taking habits during dieting. This relapse is often provoked by acute re-exposure to palatable food, food-associated cues, or stress. We used a reinstatement model, commonly used to study relapse to abused drugs, to explore the effect of peptide YY3-36 (PYY3-36) on reinstatement of high-fat (35%, 45 mg pellets) food seeking induced by acute exposure to the pellets (pellet priming), a cue previously associated with pellet delivery (pellet cue), or yohimbine (2 mg/kg, a pharmacological stressor). Rats were placed on a restricted diet (16 g of chow per day) and lever-pressed for the pellets for 9-12 sessions (6 h/d, every 48 h); pellet delivery was paired with a tone-light cue. They were then given 10-20 extinction sessions wherein lever presses were not reinforced with the pellets and subsequently tested for reinstatement of food seeking. Systemic PYY3-36 injections (100-200 microg/kg) decreased pellet priming- and pellet cue-induced reinstatement of food seeking but not yohimbine-induced reinstatement. Arcuate nucleus (Arc) injections of PYY3-36 (0.4 microg per side) decreased pellet priming-induced reinstatement. The attenuation of pellet priming-induced reinstatement by systemic PYY3-36 was reversed by systemic (2 mg/kg) but not Arc (0.5 microg per side) injections of the Y2 receptor antagonist BIIE0246. Arc PYY3-36 injections did not decrease pellet cue-induced reinstatement. Finally, systemic PYY3-36 injections had minimal effects on ongoing food self-administration or heroin priming- or heroin cue-induced reinstatement of heroin seeking. These data identify an effect of systemic PYY3-36 on relapse to food seeking that is independent of Y2 receptor activation in Arc and suggest that PYY3-36 should be considered for the treatment of relapse to maladaptive food-taking habits during dieting.