Characterization of attenuated food motivation in high-fat diet-induced obesity: Critical roles for time on diet and reinforcer familiarity

Effects of Highly Palatable Diet on motivation for food and resistance to punishment in rats: Role of sex and age of exposure

Exposure to highly palatable food is believed to induce behavioral and neurobiological changes that may produce addiction-like behavior and increase the risks of obesity and overweight. Studies in rodents have led to conflicting results suggesting that several factors such as sex and age of exposure contribute to the development of maladaptive behaviors towards food. In addition, it is not clear whether effects of exposure to highly palatable diets (HPD) persist after their discontinuation, which would indicate long-term risks to develop addiction-like behavior. In this study, we investigated the persistent effects of an intermittent 8-week exposure to HPD in male and female rats as a function of age of exposure (adult and adolescent). We found that intermittent exposure to HPD did not alter body weight, but it affected consumption of standard food during the time of exposure in all groups. In addition, in adults, HPD produced a decrease in the initial baseline responding in FR1 schedules, an effect that persisted for 4 weeks in males but not in female rats. However, we found that exposure to HPD did not affect resistance to punishment measured by progressive shock strength break points or motivation for food as measured by progressive-ratio break points regardless of sex or age of exposure. Altogether, these results do not provide support for the hypothesis that intermittent exposure to HPD produce persistent increases in the vulnerability to develop addiction-like behaviors towards palatable food.

Translational science approaches for food insecurity research

Food insecurity is a pervasive problem that impacts health and well-being across the lifespan. The human research linking food insecurity to poor metabolic and behavioral health outcomes is inherently correlational and suffers from a high degree of variability both between households and even within the same household over time. Further, food insecurity is impacted by societal and political factors that are largely out of the control of individuals, which narrows the range of intervention strategies. Animal models of food insecurity are being developed to address some of the barriers to mechanistic research. However, animal models are limited in their ability to consider some of the more complex societal elements of the human condition. We believe that understanding the role that food insecurity plays in ingestive behavior and chronic disease requires a truly translational approach, and that understanding the health impacts of this complex social phenomenon requires understanding both its psychological and physiological dimensions. This brief review will outline some key features of food insecurity, highlighting those that are amenable to investigation with controlled animal models and identifying areas where integrating animal and human studies can improve our understanding of the psychological burden and health impacts of food insecurity. In the interest of brevity, this review will largely focus on food insecurity in the United States, as the factors that contribute to food insecurity vary considerably across the globe.

Diet and obesity effects on cue-driven food-seeking: insights from studies of Pavlovian-instrumental transfer in rodents and humans

Our modern environment is said to be obesogenic, promoting the consumption of calorically dense foods and reducing energy expenditure. One factor thought to drive excess energy intake is the abundance of cues signaling the availability of highly palatable foods. Indeed, these cues exert powerful influences over food-related decision-making. Although obesity is associated with changes to several cognitive domains, the specific role of cues in producing this shift and on decision-making more generally, remains poorly understood. Here we review the literature examining how obesity and palatable diets affect the ability of Pavlovian cues to influence instrumental food-seeking behaviors by examining rodent and human studies incorporating Pavlovian-instrumental transfer (PIT) protocols. There are two types of PIT: (a) general PIT that tests whether cues can energize actions elicited in the pursuit of food generally, and (b) specific PIT which tests whether cues can elicit an action that earns a specific food outcome when faced with a choice. Both types of PIT have been shown to be vulnerable to alterations as a result of changes to diet and obesity. However, effects appear to be driven less by increases in body fat and more by palatable diet exposure per se. We discuss the limitations and implications of the current findings. The challenges for future research are to uncover the mechanisms underlying these alterations to PIT, which appear unrelated to excess weight itself, and to better model the complex determinants of food choice in humans.

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

Obese-associated gut microbes and derived phenolic metabolite as mediators of excessive motivation for food reward

BACKGROUND

Excessive hedonic consumption is one of the main drivers for weight gain. Identifying contributors of this dysregulation would help to tackle obesity. The gut microbiome is altered during obesity and regulates host metabolism including food intake.

RESULTS

By using fecal material transplantation (FMT) from lean or obese mice into recipient mice, we demonstrated that gut microbes play a role in the regulation of food reward (i.e., wanting and learning processes associated with hedonic food intake) and could be responsible for excessive motivation to obtain sucrose pellets and alterations in dopaminergic and opioid markers in reward-related brain areas. Through untargeted metabolomic approach, we identified the 3-(3'-hydroxyphenyl)propanoic acid (33HPP) as highly positively correlated with the motivation. By administrating 33HPP in mice, we revealed its effects on food reward.

CONCLUSIONS

Our data suggest that targeting the gut microbiota and its metabolites would be an interesting therapeutic strategy for compulsive eating, preventing inappropriate hedonic food intake. Video Abstract.

Appetitive Motivation and Associated Neurobiology Change Differentially across the Life Course of Mouse Offspring Exposed to Peri- and Postnatal High Fat Feeding

Alterations in neural pathways that regulate appetitive motivation may contribute to increased obesity risk in offspring born to mothers fed a high fat (HF) diet. However, current findings on the impact of maternal obesity on motivation in offspring are inconclusive, and there is no information about the long-lasting effects in aged animals. This study examined the longitudinal effect of perinatal and chronic postnatal HF intake on appetitive motivation in young and aged offspring. Female C57Bl/6 were fed either a control (C) or HF diet before mating through to lactation. At weaning, offspring were maintained on the C or HF diet, generating the following four diet groups: C/C, C/HF, HF/C, and HF/HF based on the pre/post weaning diet. At 6 months, motivation was higher in HF/C females, but lower in male and female C/HF and HF/HF mice. By 12 months, this difference was lost, as C-fed animals became less motivated, while motivation increased in HF-fed mice. The mRNA levels of dopamine receptor 1 and 2 increased with age, while cannabinoid receptor 1 and μ-opioid receptor expression remained stable or decreased in mesolimbic and mesocortical dopaminergic pathways. Results from this study suggest that perinatal and chronic postnatal HF feeding produced opposite effects on appetitive motivation in young adult offspring mice, which was also reflected in the shift in motivation over time. These results have significant implications for patterns of hedonic eating across the life course and the relative risk of obesity at different time points.

Exposure to a hypercaloric diet produces long lasting changes in motivation

Changes in motivation have been observed following induction of diet-induced obesity. However, to date, results have been contradictory, some authors reporting an increase in motivation to obtain palatable food, but others observing a decrease. Observed differences might be associated with the length of both the evaluation period and exposure to the diet. Therefore, the aim of this study was to evaluate changes in motivation during 20 weeks of exposure to a hypercaloric diet. Performance of the subjects in a progressive ratio schedule was evaluated before and during the exposure to a high-fat, high-sugar choice diet (HFHSc). A decrease in motivation was observed after 2 weeks of diet exposure, low levels of motivation remained throughout 20 weeks. A comparable decrease in motivation took longer (3 weeks) to develop using chow diet in the control group. Overall, our results suggest that, when changes in motivation are being evaluated, long periods of diet exposure made no further contribution, once motivation decreased, it remained low up to 18 weeks. Exposure to a HFHSc diet is a useful animal model of obesity, since it replicates some pathophysiological and psychological features of human obesity such as an increase in fasting glucose levels, body weight and the weight of adipose tissue.

Food Reward Alterations during Obesity Are Associated with Inflammation in the Striatum in Mice: Beneficial Effects of Akkermansia muciniphila

The reward system involved in hedonic food intake presents neuronal and behavioral dysregulations during obesity. Moreover, gut microbiota dysbiosis during obesity promotes low-grade inflammation in peripheral organs and in the brain contributing to metabolic alterations. The mechanisms underlying reward dysregulations during obesity remain unclear. We investigated if inflammation affects the striatum during obesity using a cohort of control-fed or diet-induced obese (DIO) male mice. We tested the potential effects of specific gut bacteria on the reward system during obesity by administrating Akkermansia muciniphila daily or a placebo to DIO male mice. We showed that dysregulations of the food reward are associated with inflammation and alterations in the blood–brain barrier in the striatum of obese mice. We identified Akkermansia muciniphila as a novel actor able to improve the dysregulated reward behaviors associated with obesity, potentially through a decreased activation of inflammatory pathways and lipid-sensing ability in the striatum. These results open a new field of research and suggest that gut microbes can be considered as an innovative therapeutic approach to attenuate reward alterations in obesity. This study provides substance for further investigations of Akkermansia muciniphila-mediated behavioral improvements in other inflammatory neuropsychiatric disorders.

Gut microbes and food reward: From the gut to the brain

Inappropriate food intake behavior is one of the main drivers for fat mass development leading to obesity. Importantly the gut microbiota-mediated signals have emerged as key actors regulating food intake acting mainly on the hypothalamus, and thereby controlling hunger or satiety/satiation feelings. However, food intake is also controlled by the hedonic and reward systems leading to food intake based on pleasure (i.e., non-homeostatic control of food intake). This review focus on both the homeostatic and the non-homeostatic controls of food intake and the implication of the gut microbiota on the control of these systems. The gut-brain axis is involved in the communications between the gut microbes and the brain to modulate host food intake behaviors through systemic and nervous pathways. Therefore, here we describe several mediators of the gut-brain axis including gastrointestinal hormones, neurotransmitters, bioactive lipids as well as bacterial metabolites and compounds. The modulation of gut-brain axis by gut microbes is deeply addressed in the context of host food intake with a specific focus on hedonic feeding. Finally, we also discuss possible gut microbiota-based therapeutic approaches that could lead to potential clinical applications to restore food reward alterations. Therapeutic applications to tackle these dysregulations is of utmost importance since most of the available solutions to treat obesity present low success rate.

Chronic experience with unpredictable food availability promotes food reward, overeating, and weight gain in a novel animal model of food insecurity

Ubiquitous, easy access to food is thought to promote obesity in the modern environment. However, people coping with food insecurity have limited, unpredictable food access and are also prone to obesity. Causal factors linking food insecurity and obesity are not understood. In this study we describe an animal model to investigate biopsychological impacts of the chronic unpredictability inherent in food insecurity. Female rats were maintained on a 'secure' schedule of highly predictable 4x/day feedings of uniform size, or an 'insecure' schedule delivering the same total food over time but frequently unpredictable regarding how much, if any, food would arrive at each scheduled feeding. Subgroups of secure and insecure rats were fed ordinary chow or high-fat/high-sugar (HFHS) chow to identify separate and combined effects of insecurity and diet quality. Insecure chow-fed rats, relative to secure chow-fed rats, were hyperactive and consumed more when provided a palatable liquid diet. Insecure HFHS-fed rats additionally had higher progressive ratio breakpoints for sucrose, increased meal size, and subsequently gained more weight during 8 days of ad libitum HFHS access. Insecurity appeared to maintain a heightened attraction to palatable food that habituated in rats with secure HFHS access. In a second experiment, rats fed ordinary chow on the insecure schedule subsequently gained more weight when provided ad libitum chow, showing that prior insecurity per se promoted short-term weight gain in the absence of HFHS food. We propose this to be a potentially useful animal model for mechanistic research on biopsychological impacts of insecurity, demonstrating that chronic food uncertainty is a factor promoting obesity.

Chronic exposure to cafeteria-style diet in rats alters sweet taste preference and reduces motivation for, but not 'liking' of sucrose

Obesity is associated with changes to taste perception and brain reward circuitry. It is important to understand how these effects alter the preference for palatable foods and drinks, given that these are widely consumed, and leading risk factors for obesity. This study examined the effects of diet-induced obesity on sweet taste preference by analysing the microstructure of licking for sugar solutions and assessing pERK expression in the nucleus accumbens shell and insula. Adult male Sprague-Dawley rats were fed standard chow (Control; n = 16) or a varied, palatable cafeteria diet (Caf; n = 16) for 12 weeks. Two-choice preference tests between 2%, 8% and 32% sucrose solutions were conducted at baseline and in weeks 11-12 of the diet. Rats in the Caf group trebled energy intake and doubled weight gain relative to controls. In tests held under water restriction after 11 weeks of diet, the Control group reliably preferred higher sucrose concentrations (i.e., 32% > 8% > 2%). Relative to controls, the Caf group showed a stronger preference for 32% vs. 2% sucrose, lower preference for 32% vs. 8% sucrose, and were indifferent to 8% vs. 2% sucrose. Testing without water restriction increased preference for higher sucrose concentrations in both groups. Chronic Caf diet increased the latency to lick, decreased total licks and reduced alternations between spouts, but did not alter lick cluster size, a measure of hedonic appraisal, on any test. Following a final exposure to a novel sucrose concentration, neuronal activity (pERK) in the insula and nucleus accumbens shell was significantly reduced in the Caf group. Results indicate that differences in 'liking' do not underlie obesity-induced changes to sweet taste preference.

Patterned Feeding of a Hyper-Palatable Food (Oreo Cookies) Reduces Alcohol Drinking in Rats

While a bidirectional positive link between palatable food intake and alcohol drinking has been suggested, several rodents studies report reduced alcohol drinking following palatable diets exposure. These studies utilized purified rodents’ diets high in sugar/fat; however, the effects of hyper-palatable food (HPF) rich in fat and sugar on alcohol drinking remain unclear. Furthermore, neural substrates involved in HPF-mediated changes in alcohol consumption are poorly understood. Therefore, the present study evaluated the effects of patterned feeding of a hyper-palatable food (Oreo cookies) on alcohol drinking as well as dopamine (DA) and serotonin (5-HT) content in rat’s mesocorticolimbic (medial-prefrontal cortex, orbitofrontal cortex, amygdala, and nucleus accumbens) circuitry. Male Long Evans rats received 8-weeks of intermittent (Mon, Tue, Wed) Oreo cookies access, which induced a patterned feeding, in which rats in the Oreo group overconsumed calories on HPF days whereas underconsumption was observed on chow only (Thu, Fri) days. Following HPF exposure, alcohol consumption was evaluated while patterned feeding continued. Alcohol intake in the Oreo group was significantly lower as compared to the chow controls. However, alcohol intake in the Oreo group increased to the levels seen in the group receiving chow following the suspension of patterned HPF feeding. Finally, DA levels in the nucleus accumbens were significantly greater, whereas its metabolite (DOPAC) levels were lower in the Oreo group compared to the chow controls. Surprisingly, 5-HT levels remained unaltered in all tested brain areas. Together, these data suggest that HPF-associated increased DA availability and reduced DA turnover within mesocorticolimbic circuitry may regulate alcohol drinking following patterned HPF feeding.

Long-term high fat diet consumption reversibly alters feeding behavior via a dopamine-associated mechanism in mice

Obesity is a costly, global epidemic that is perpetuated by an unhealthy diet. A significant factor in the initial consumption and maintenance of an unhealthy diet is the abundance of highly palatable, calorically dense foods. The aim of the present study is to better understand the effects of high fat diet (HFD) consumption on food valuation and preference, and to elucidate the neurobiological mechanisms mediating these effects. By using a novel food preference assay, we found that prolonged consumption of a HFD diminishes preference for and consumption of the more calorically dense food choice when two lab diets are presented. Additionally, we demonstrated that prolonged HFD consumption dampens ventral tegmental c-fos induction during hedonic feeding, implicating the mesolimbic dopamine signaling pathway as a target of HFD. Notably, both the changes in food preference and this reduced c-fos induction were reversed during withdrawal from HFD. Further, HFD-induced alterations in food preference were attenuated by exercise. Our findings suggest that prolonged HFD consumption leads to anhedonia and altered feeding choices, and this is associated with changes in mesolimbic dopamine signaling.

Resting activity of the hippocampus and amygdala in obese individuals predicts their response to food cues

Obese individuals exhibit brain functional abnormalities in multiple regions implicated in reward/motivation, emotion/memory, homeostatic regulation, and executive control when exposed to food cues and during rest. However, it remains unclear whether abnormal brain responses to food cues might account for or relate to their abnormal activity in resting state. This information would be useful for understanding the neural mechanisms behind hyperactive responses to food cues, a critical marker of obesity. Resting-state functional magnetic resonance imaging (RS-fMRI) and a cue-reactivity fMRI task with high- (HiCal) and low-caloric (LoCal) food cues were employed to investigate brain baseline activity and food cue-induced activation differences in 44 obese participants (OB), in 37 overweight participants (OW), and in 37 normal weight (NW) controls. One-way analyses of variance showed there was a group difference in the left hippocampus/amygdala activity during resting state and during food-cue stimulation (p_FWE < 0.05); post-hoc tests showed the OB group had both greater basal activity and greater food cue-induced activation than the OW and NW groups; OW had higher activity in the hippocampus/amygdala than the NW group, which was only significant during resting state. In the OB group, resting-state activity in the left hippocampus/amygdala was positively correlated with activation induced by HiCal food cues, and both of these measures correlated with body mass index (BMI). Mediation analysis showed that the relationship between BMI and hippocampus/amygdala response to HiCal food cues was mediated by their resting-state activity. These findings suggest a close association between obesity and brain functional abnormality in the hippocampus/amygdala. They also indicate that resting-state activity in the hippocampus/amygdala may impact these regions' responses to food cues.

Fat and Carbohydrate Interact to Potentiate Food Reward in Healthy Weight but Not in Overweight or Obesity

Prior work suggests that actual, but not estimated, energy density drives the reinforcing value of food and that energy from fat and carbohydrate can interact to potentiate reward. Here we sought to replicate these findings in an American sample and to determine if the effects are influenced by body mass index (BMI). Thirty participants with healthy weight (HW; BMI 21.92 ± 1.77; M ± SD) and 30 participants with overweight/obesity (OW/OB; BMI 29.42 ± 4.44) rated pictures of common American snacks in 120-kcal portions for liking, familiarity, frequency of consumption, expected satiety, healthiness, energy content, energy density, and price. Participants then completed an auction task where they bid for the opportunity to consume each food. Snacks contained either primarily carbohydrate, primarily fat, or roughly equal portions of fat and carbohydrate (combo). Replicating prior work, we found that participants with HW bid the most for combo foods in linear mixed model analyses. This effect was not observed among individuals with OW/OB. Additionally, in contrast with previous reports, our linear regression analyses revealed a negative relationship between the actual energy density of the snacks and bid amount that was mediated by food price. Our findings support altered macronutrient reinforcement in obesity and highlight potential influences of the food environment on the regulation of food reward.

Effects of a high-fat diet on impulsive choice in rats

INTRODUCTION

Obesity and binge eating disorder are associated with high levels of impulsivity, but the causal role of eating and palatable food in these associations is unclear. Studies in rodents show that a high-fat diet can increase one aspect of impulsivity (impulsive action); it is less clear, however, whether a dissociable aspect of impulsivity (impulsive choice) is similarly affected. Hence, the aim of this study was to ascertain whether chronic exposure to a high-fat diet would alter impulsive choice.

METHODS

Male rats were maintained on either a high-fat or control chow diet for two weeks ad libitum. They then underwent equi-caloric food restriction for the duration of the experiment, with each group maintained on their respective diet. To measure impulsive choice, rats were trained on a delay discounting task (DDT) in which they made discrete choices between a lever that delivered a small food reward immediately and a lever that delivered a large food reward accompanied by systematically increasing delays. Upon reaching stable performance on the DDT, rats were given acute systemic injections of amphetamine prior to testing in the DDT to determine whether increased monoamine transmission affected impulsive choice differently in the two diet groups. Lastly, subjects were tested on a progressive ratio schedule of reinforcement to assess motivation for a sucrose reward.

RESULTS

There was no significant effect of the high-fat diet on impulsive choice. Further, amphetamine decreased choice of the large, delayed reward (increased impulsive choice) to the same extent in both groups. Exposure to the high-fat diet did, however, increase motivation to obtain a sucrose reward.

CONCLUSIONS

These experiments reveal that, under conditions that do not promote weight gain, a chronic high-fat diet does not affect impulsive choice in a delay discounting task. The data are surprising in light of findings showing that this same diet alters impulsive action, and highlight the necessity of further research to elucidate relationships between palatable food consumption and impulsivity.

Dissociation between binge eating behavior and incentive motivation

Binge-like eating behavior (BLE) has been characterized as an eating disorder in which subjects have an enhanced intake of food, mainly fats. However, intake of fats and carbohydrates may have differential effects on motivation. Previously it was shown that BLE produces an increase in operant responding for vegetable shortening. Our aim was to determine if BLE behavior induced with a sucrose solution would produce an increment in performance for sucrose reinforcers. Male Wistar rats were trained under an exponential progressive ratio schedule of sucrose reinforcement; thereafter, the limited access model was used to induce BLE. Finally, subjects were tested for increments in break points (BPs) in the progressive ratio schedule. We were unable to observe an increase in BPs after BLE. No increments in BPs were observed when a distinctive flavor (vanilla-flavored sucrose) was correlated with BLE induction and reinforcement, or when different types of ratio progression in the operant schedules were employed. However, rats adjusted their BPs according to reinforcer concentration after BLE induction, demonstrating that valuation (cost/benefit decision) of reinforcers was intact. Extent of training, alterations of reward processing after extended exposure to sucrose, and different mechanisms for processing high fat and high carbohydrate reinforcers are variables worth exploring to gain a better understanding of BLE behavior in rodent models.

High fat diet consumption restricted to adolescence has minimal effects on adult executive function that vary by sex

Early life environment can have a lasting effect on brain development and behavior. Diet is a potent environmental factor that can positively or negatively affect neurodevelopment, and unfortunately, the likelihood of a poor diet is high during adolescence. Adverse effects of adolescent high fat diet have been observed on reward-related behaviors, reversal learning, and hippocampal-dependent learning tasks in rodents when tested in adulthood. The prefrontal cortex (PFC) continues to develop throughout adolescence and is thus vulnerable to environmental insults such as poor diet. Therefore, we sought to examine the effects of a high fat diet (HFD) consumed only during adolescence on later life adult PFC-dependent executive function. Male and female mice were fed a HFD (60% energy from fat) during either early or late adolescence then switched to standard chow and tested in a battery of touchscreen-based operant tests of executive function in adulthood. Contrary to our prediction of an adverse effect of HFD, there was no effect of adolescent HFD in males, and females showed faster learning and decreased inattention in adulthood. We conclude that the effects of adolescent-limited HFD on adult executive function are mild, positive, and vary by sex.

Reduced alcohol drinking following patterned feeding: Role of palatability and acute contingent availability

Recent studies from our lab have demonstrated that intermittent high-fat diet access reduces alcohol drinking in rats. However, it was unclear if caloric overload, palatability, or diet itself triggered reduced alcohol drinking. It is also unknown if a similar paradigm could reduce relapse-like alcohol drinking. The presented study tested the hypothesis that acute intermittent palatable diet (PD) access would rescue relapse-like drinking and palatability, but not diet itself contributes to reduced drinking. Male Long Evans rats received six-weeks intermittent or chronic chow (controls) or PDs (high-fat diet, high-sugar diet) exposure, and alcohol testing occurred following PDs suspension. Alcohol intake was not significantly different among groups in either condition, suggesting that diet itself did not impact alcohol drinking. A subset of these rats received two-weeks intermittent PDs (Int-PDs) exposure and alcohol testing reinitiated while Int-PDs access continued. Alcohol intake significantly escalated (~137% compared to baseline; alcohol deprivation effect) in the chow controls, whereas it remained unchanged in PD groups. These data demonstrate the critical importance of acute intermittent PDs availability and its protective effect in relapse-like drinking. To assess the contribution of palatability in reduced alcohol drinking, a separate group of rats received two-weeks intermittent high-sugar diet (Int-HSD) or saccharin (Int-SAC) access and tested for alcohol drinking while Int-HSD/SAC continued. Alcohol drinking significantly decreased (~30%) in both HSD and SAC groups compared to the controls. These data identify the critical parameters by which acute intermittent PD access reduces alcohol drinking and could have important therapeutic implications in the management of alcoholism.

Behavioral and Neurobiological Consequences of Hedonic Feeding on Alcohol Drinking

A complex interplay of peripheral and central signaling mechanisms within the body of an organism maintains energy homeostasis. In addition, energy/food intake is modified by various external factors (e.g., palatability, food availability, social and environmental triggers). Highly palatable foods can provoke maladaptive feeding behavior, which in turn disrupts normal homeostatic regulation resulting in numerous health consequences. Furthermore, neuroendocrine peptides, traditionally considered to regulate appetite and energy homeostasis, also control the intake and reinforcing properties of alcohol and drugs of abuse. Therefore, dysregulated eating as a result of a hedonic/binge-like intake of hyper-palatable food may impact alcohol drinking behavior. Relevant in this case is the fact that eating disorders are highly comorbid with several neuropsychiatric conditions, including alcohol use disorder. The present review is intended to summarize the neurobiological and functional consequences of hedonic feeding on alcohol intake.

Random access to palatable food stimulates similar addiction-like responses as a fixed schedule, but only a fixed schedule elicits anticipatory activation

Restricted intermittent food access to palatable food (PF) induces addiction-like behaviors and plastic changes in corticolimbic brain areas. Intermittent access protocols normally schedule PF to a fixed time, enabling animals to predict the arrival of PF. Because outside the laboratory the presence of PF may occur in a random unpredictable manner, the present study explored whether random access to PF would stimulate similar addiction-like responses as observed under a fixed scheduled. Rats were randomly assigned to a control group without chocolate access, to ad libitum access to chocolate, to fixed intermittent access (CH-F), or to random unpredictable access (CH-R) to chocolate. Only the CH-F group developed behavioral and core temperature anticipation to PF access. Both groups exposed to intermittent access to PF showed binge eating, increased effort behaviors to obtain chocolate, as well as high FosB/ΔFosB in corticolimbic areas. Moreover, FosB/ΔFosB in all areas correlated with the intensity of binge eating and effort behaviors. We conclude that both conditions of intermittent access to PF stimulate addiction-like behaviors and FosB/ΔFosB accumulation in brain reward areas; while only a fixed schedule, which provides a time clue, elicited anticipatory activation, which is strongly associated with craving behaviors and may favor relapse during withdrawal.

Eating "junk food" has opposite effects on intrinsic excitability of nucleus accumbens core neurons in obesity-susceptible versus -resistant rats

The nucleus accumbens (NAc) plays critical roles in motivated behaviors, including food seeking and feeding. Differences in NAc function contribute to overeating that drives obesity, but the underlying mechanisms are poorly understood. In addition, there is a fair degree of variation in individual susceptibility versus resistance to obesity that is due in part to differences in NAc function. For example, using selectively bred obesity-prone and obesity-resistant rats, we have found that excitability of medium spiny neurons (MSNs) within the NAc core is enhanced in obesity-prone versus -resistant populations, before any diet manipulation. However, it is unknown whether consumption of sugary, fatty "junk food" alters MSN excitability. Here whole cell patch-clamp recordings were conducted to examine MSN intrinsic excitability in adult male obesity-prone and obesity-resistant rats with and without exposure to a sugary, fatty junk food diet. We replicated our initial finding that basal excitability is enhanced in obesity-prone versus obesity-resistant rats and determined that this is due to a lower fast transient potassium current (I_A) in prone versus resistant groups. In addition, the junk food diet had opposite effects on excitability in obesity-prone versus obesity-resistant rats. Specifically, junk food enhanced excitability in MSNs of obesity-resistant rats; this was mediated by a reduction in I_A. In contrast, junk food reduced excitability in MSNs from obesity-prone rats; this was mediated by an increase in inward-rectifying potassium current. Thus individual differences in obesity susceptibility influence both basal excitability and how MSN excitability adapts to junk food consumption.NEW & NOTEWORTHY Medium spiny neurons (MSNs) in the nucleus accumbens of obesity-prone rats are hyperexcitable compared with MSNs from obesity-resistant rats. We found that 10 days of "junk food" exposure reduces MSN excitability in obesity-prone rats by increasing inward-rectifying potassium current and increases MSN excitability in obesity-resistant rats by decreasing fast transient potassium current. These data show that there are basal and junk food diet-induced differences in MSN excitability in obesity-prone and obesity-resistant individuals; this may contribute to previously observed differences in incentive motivation.

Cafeteria diet increased adiposity in comparison to high fat diet in young male rats

Background

Dietary intervention studies in animal models of obesity are crucial to elucidate the mechanistic effects of specific nutrients and diets. Although several models of diet induced obesity have been examined in rodents to assess obesity, there are few studies that have researched influence of different high fat and/or westernized diets. The aim of this study was to compare a high fat diet and a cafeteria diet on obesity related biochemical and physiological parameters in young male rats.

Methods

Five week old Wistar male rats were fed a control chow diet (C), butter-based high fat diet (HF) or cafeteria diet (CAF) for twelve weeks. In HF, 40% of energy came from fat and this ratio was 46% in CAF. CAF composed of highly energetic and palatable human foods along with chow diet. At the end of the feeding protocol all animals were culled using CO₂ asphyxia and cervical dislocation after an overnight fasting.

Results

Total energy and fat intake of CAF was significantly higher than C and HF. CAF was more effective in inducing obesity, as demonstrated by increased weight gain, Lee index, fat depot weights and total body fat in comparison to C and HF. Despite increased adiposity in CAF, plasma glucose, insulin and HOMA-IR levels were similar between the groups. Plasma leptin and cholesterol levels were markedly higher in CAF than C and HF.

Discussion

We have demonstrated that there are differential effects of high fat diet and cafeteria diet upon obesity and obesity-related parameters, with CAF leading to a more pronounced adiposity in comparison to high fat diet in young male rats. Future studies should consider the varied outcomes of different diet induced obesity models and development of a standardized approach in similar research practices.

Reduced plasma ghrelin concentrations are associated with decreased brain reactivity to food cues after laparoscopic sleeve gastrectomy

The "hunger" hormone ghrelin regulates food-intake and preference for high-calorie (HC) food through modulation of the mesocortico-limbic dopaminergic pathway. Laparoscopic sleeve gastrectomy (LSG) is an effective bariatric surgery to treat morbid obesity. We tested the hypothesis that LSG-induced reductions in appetite and total ghrelin levels in blood are associated with reduced prefrontal brain reactivity to food cues. A functional magnetic resonance imaging (fMRI) cue-reactivity task with HC and low-calorie (LC) food pictures was used to investigate brain reactivity in 22 obese participants tested before and one month after bariatric surgery (BS). Nineteen obese controls (Ctr) without surgery were also tested at baseline and one-month later. LSG significantly decreased (1) fasting plasma concentrations of total ghrelin, leptin and insulin, (2) craving for HC food, and (3) brain activation in the right dorsolateral prefrontal cortex (DLPFC) in response to HC vs. LC food cues (P_FWE < 0.05). LSG-induced reduction in DLPFC activation to food cues were positively correlated with reduction in ghrelin levels and reduction in craving ratings for food. Psychophysiological interaction (PPI) connectivity analyses showed that the right DLPFC had stronger connectivity with the ventral anterior cingulate cortex (vACC) after LSG, and changes in BMI were negatively correlated with changes in connectivity between the right DLPFC and vACC in the LSG group only. These findings suggest that LSG-induced weight-loss may be related to reductions in ghrelin, possibly leading to decreased food craving and hypothetically reducing DLPFC response to the HC food cues.

Decrease in Operant Responding Under Obesogenic Diet Exposure is not Related to Deficits in Incentive or Hedonic Processes

OBJECTIVE

A growing body of evidence suggests that obesity could result from alterations in reward processing. In rodent models, chronic exposure to an obesogenic diet leads to blunted dopamine signaling and related incentive responding. This study aimed to determine which reward-related behavioral dimensions are actually impacted by obesogenic diet exposure.

METHODS

Mice were chronically exposed to an obesogenic diet. Incentive and hedonic processes were tested through operant conditioning and licking microstructures, respectively. In parallel, mesolimbic dopamine transmission was assessed using microdialysis.

RESULTS

Prolonged high-fat (HF) diet exposure led to blunted mesolimbic dopamine release, paralleled by a decrease in operant responding in all schedules tested. HF-fed and control animals similarly decreased their operant responding in an effort-based choice task, and HF-fed animals displayed an overall lower calorie intake in this task. Analysis of the licking microstructures during consumption of a freely accessible reward suggested a decrease in basal hunger and a potentiation of gastrointestinal inhibition in HF-fed animals, without changes in hedonic reactivity.

CONCLUSIONS

These results suggest that the decrease in operant responding under prolonged HF diet exposure is mainly driven by decrease in hunger as well as stronger postingestive negative feedback mechanisms, rather than by a decrease in incentive or hedonic responses.

Intermittent access to a nutritionally complete high-fat diet attenuates alcohol drinking in rats

Binge eating disorder and alcohol use disorder (AUD) frequently co-occur in the presence of other psychiatric conditions. Data suggest that binge eating engages similar behavioral and neurochemical processes common to AUD, which might contribute to the etiology or maintenance of alcoholism. However, it is unclear how binge feeding behavior and alcohol intake interact to promote initiation or maintenance of AUD. We investigated the impact of binge-like feeding on alcohol intake and anxiety-like behavior in male Long Evans rats. Rats received chow (controls) or extended intermittent access (24h twice a week; Int-HFD) to a nutritionally complete high-fat diet for six weeks. Standard rodent chow was available ad-libitum to all groups and food intake was measured. Following HFD exposure, 20.0% ethanol, 2.0% sucrose intake and endocrine peptide levels were evaluated. Anxiety-like behavior was measured using a light-dark (LD) box apparatus. Rats in the Int-HFD group displayed a binge-like pattern of feeding (alternations between caloric overconsumption and voluntary caloric restriction). Surprisingly, alcohol intake was significantly attenuated in the Int-HFD group whereas sugar consumption was unaffected. Plasma acyl-ghrelin levels were significantly elevated in the Int-HFD group, whereas glucagon-like peptide-1 levels did not change. Moreover, rats in the Int-HFD group spent more time in the light side of the LD box compared to controls, indicating that binge-like feeding induced anxiolytic effects. Collectively, these data suggest that intermittent access to HFD attenuates alcohol intake through reducing anxiety-like behavior, a process potentially controlled by elevated plasma ghrelin levels.

Binge-like intake of HFD attenuates alcohol intake in rats

Binge eating and binge alcohol intake are behavioral manifestations of pathological feeding and alcohol use disorder (AUD), respectively. Binge-feeding and AUD have high comorbidity with other psychiatric disorders such as depression, which could have important implications for the management of these conditions. Importantly, these behaviors share many common features suggesting a singular etiology. However, the nature by which binge-feeding affects the development or maintenance of AUD is unclear. The present study examined the impact of a binge-feeding from a nutritionally complete high-fat diet (HFD) on initiation and maintenance of alcohol intake, anxiolytic behavior and central genetic changes in brain regions that control alcohol-reinforced behaviors. To do this, male Long-Evans rats received chow (controls) or HFD every three days (HFD-3D) or every day (HFD-ED) for 5weeks. Rodent chow and water were available ad-libitum to all groups throughout the experiment. Following 5weeks of HFD cycling, 20.0% ethanol or 2.0% sucrose intake was evaluated. In addition, anxiety-like behavior was measured using a light-dark box apparatus. Both HFD-3D and -ED groups of rats consumed significantly large amount of food during 2h HFD access sessions and reduced their chow intake in the next 22h. Surprisingly, binge-fed rats displayed attenuated acquisition of alcohol intake whereas sucrose consumption was unaffected. Rats exposed to HFD spent more time in the light side compared to chow controls, indicating that binge-feeding induced anxiolytic effects. In addition, alterations in the brain neurotensin system were observed following HFD exposure. These data indicate that binge-feeding behavior induces behavioral and genetic changes that help explain how alcohol intake is influenced by co-morbid eating disorders.

Behavioral consequences of exposure to a high fat diet during the post-weaning period in rats

We explored the impact of exposure to an obesogenic diet (High Fat-High Sucrose; HFS) during the post-weaning period on sweet preference and behaviors linked to reward and anxiety. All rats were fed chow. In addition a HFS-transient group had access to this diet for 10days from post-natal (PN) day 22 and a HFS-continuous group continued access until adult. Behavioral tests were conducted immediately after PN 32 (adolescence) or after PN 60 (adult) and included: the condition place preference (CPP) test for chocolate, sugar and saccharin preference (anhedonia), the elevated plus maze (anxiety-like behavior) and the locomotor response to quinpirole in the open field. Behavior was unaltered in adult rats in the HFS-transient group, suggesting that a short exposure to this obesogenic food does not induce long-term effects in food preferences, reward perception and value of palatable food, anxiety or locomotor activity. Nevertheless, rats that continued to have access to HFS ate less chocolate during CPP training and consumed less saccharin and sucrose when tested in adolescence, effects that were attenuated when these rats became adult. Moreover, behavioral effects linked to transient HFS exposure in adolescence were not sustained if the rats did not remain on that diet until adult. Collectively our data demonstrate that exposure to fat and sucrose in adolescence can induce immediate reward hypofunction after only 10days on the diet. Moreover, this effect is attenuated when the diet is extended until the adult period, and completely reversed when the HFS diet is removed.