Feed-forward mechanisms: addiction-like behavioral and molecular adaptations in overeating

Effect of intranasal oxytocin on palatable food consumption and c-Fos immunoreactivity in relevant brain areas in rats

Peripheral and central injections of oxytocin (OT) in laboratory animals decrease eating for energy and palatability, but the hypophagic response is dependent on the administration route. Human studies rely on intranasal (IN) administration of the peptide, the route underutilized in OT animal feeding studies thus far. Therefore, we examined the effect of IN OT on various aspects of food consumption in rats: (a) overnight deprivation-induced standard chow intake, (b) episodic (2-h) consumption of calorie-dense and palatable high-fat high-sugar (HFHS) chow, (c) 2-h episodic intake of palatable and calorie-dilute sucrose and Intralipid solutions, and (d) 2-h sucrose solution intake in rats habituated to ingesting this solution daily for several weeks. Finally, we assessed c-Fos changes in response to the acute IN OT administration in rats habituated to daily sugar consumption. We found that IN 20μg OT decreased deprivation-induced intake of standard chow and HFHS chow in nondeprived rats without affecting water consumption. IN OT also reduced 2-hour episodic fluid consumption of sucrose, but not Intralipid. In the habitual sugar consumption paradigm, acute IN OT diminished sucrose solution intake in animals accustomed to the 2-hour/day sucrose meal regimen. In rats habitually consuming sucrose, IN OT altered c-Fos immunoreactivity in brain areas related to energy homeostasis and reward, including the central nucleus of the amygdala, the hypothalamic paraventricular and the arcuate nuclei. We conclude that IN OT is an effective appetite suppressant for carbohydrate/sugar diets in rats and its effects involve feeding-related brain circuits.

Dynamic regulation of the extracellular matrix in reward memory processes: a question of time

Substance use disorders are a global health problem with increasing prevalence resulting in significant socioeconomic burden and increased mortality. Converging lines of evidence point to a critical role of brain extracellular matrix (ECM) molecules in the pathophysiology of substance use disorders. An increasing number of preclinical studies highlight the ECM as a promising target for development of novel cessation pharmacotherapies. The brain ECM is dynamically regulated during learning and memory processes, thus the time course of ECM alterations in substance use disorders is a critical factor that may impact interpretation of the current studies and development of pharmacological therapies. This review highlights the evidence for the involvement of ECM molecules in reward learning, including drug reward and natural reward such as food, as well as evidence regarding the pathophysiological state of the brain's ECM in substance use disorders and metabolic disorders. We focus on the information regarding time-course and substance specific changes in ECM molecules and how this information can be leveraged for the development of therapeutic strategies.

Highly processed foods can be considered addictive substances based on established scientific criteria

BACKGROUND

There is growing evidence that an addictive-eating phenotype may exist. There is significant debate regarding whether highly processed foods (HPFs; foods with refined carbohydrates and/or added fats) are addictive. The lack of scientifically grounded criteria to evaluate the addictive nature of HPFs has hindered the resolution of this debate.

ANALYSIS

The most recent scientific debate regarding a substance's addictive potential centered around tobacco. In 1988, the Surgeon General issued a report identifying tobacco products as addictive based on three primary scientific criteria: their ability to (1) cause highly controlled or compulsive use, (2) cause psychoactive (i.e. mood-altering) effects via their effect on the brain and (3) reinforce behavior. Scientific advances have now identified the ability of tobacco products to (4) trigger strong urges or craving as another important indicator of addictive potential. Here, we propose that these four criteria provide scientifically valid benchmarks that can be used to evaluate the addictiveness of HPFs. Then, we review the evidence regarding whether HPFs meet each criterion. Finally, we consider the implications of labeling HPFs as addictive.

CONCLUSION

Highly processed foods (HPFs) can meet the criteria to be labeled as addictive substances using the standards set for tobacco products. The addictive potential of HPFs may be a key factor contributing to the high public health costs associated with a food environment dominated by cheap, accessible and heavily marketed HPFs.

Obesity, Diabetes Mellitus, and Vascular Impediment as Consequences of Excess Processed Food Consumption

Regular intake of ready-to-eat meals is related to obesity and several noninfectious illnesses, such as cardiovascular diseases, hypertension, diabetes mellitus (DM), and tumors. Processed foods contain high calories and are often enhanced with excess refined sugar, saturated and trans fat, Na⁺ andphosphate-containing taste enhancers, and preservatives. Studies showed that monosodium glutamate (MSG) induces raised echelons of oxidative stress, and excessive hepatic lipogenesis is concomitant to obesity and type 2 diabetes mellitus (T2DM). Likewise, more than standard salt intake adversely affects the cardiovascular system, renal system, and central nervous system (CNS), especially the brain. Globally, excessive utilization of phosphate-containing preservatives and additives contributes unswervingly to excessive phosphate intake through food. In addition, communities and even health experts, including medical doctors, are not well-informed about the adverse effects of phosphate preservatives on human health. Dietary phosphate excess often leads to phosphate toxicity, ultimately potentiating kidney disease development. The mechanisms involved in phosphate-related adverse effects are not explainable. Study reports suggested that high blood level of phosphate causes vascular ossification through the deposition of Ca²⁺ and substantially alters fibroblast growth factor-23 (FGF23) and calcitriol.

Whey-Adapted versus Natural Cow's Milk Formulation: Distinctive Feeding Responses and Post-Ingestive c-Fos Expression in Laboratory Mice

The natural 20:80 whey:casein ratio in cow’s milk (CM) for adults and infants is adjusted to reflect the 60:40 ratio of human milk, but the feeding and metabolic consequences of this adjustment have been understudied. In adult human subjects, the 60:40 CM differently affects glucose metabolism and hormone release than the 20:80 CM. In laboratory animals, whey-adapted goat’s milk is consumed in larger quantities. It is unknown whether whey enhancement of CM would have similar consequences on appetite and whether it would affect feeding-relevant brain regulatory mechanisms. In this set of studies utilizing laboratory mice, we found that the 60:40 CM was consumed more avidly than the 20:80 control formulation by animals motivated to eat by energy deprivation and by palatability (in the absence of hunger) and that this hyperphagia stemmed from prolongation of the meal. Furthermore, in two-bottle choice paradigms, whey-adapted CM was preferred against the natural 20:80 milk. The intake of the whey-adapted CM induced neuronal activation (assessed through analysis of c-Fos expression in neurons) in brain sites promoting satiation, but importantly, this activation was less pronounced than after ingestion of the natural 20:80 whey:casein CM. Activation of hypothalamic neurons synthesizing anorexigenic neuropeptide oxytocin (OT) was also less robust after the 60:40 CM intake than after the 20:80 CM. Pharmacological blockade of the OT receptor in mice led to an increase in the consumption only of the 20:80 CM, thus, of the milk that induced greater activation of OT neurons. We conclude that the whey-adapted CM is overconsumed compared to the natural 20:80 CM and that this overconsumption is associated with weakened responsiveness of central networks involved in satiety signalling, including OT.

Acute Hypophagia and Changes in c-Fos Immunoreactivity in Adolescent Rats Treated with Low Doses of Oxytocin and Naltrexone

A recent case report has shown that an adjunctive oxytocin + naltrexone (OT + NTX) treatment promoted more robust hypophagia and body weight reduction than OT alone in an adolescent male with hypothalamic obesity after craniopharyngioma resection. Thus far, there has been no basic research in adolescent laboratory animals that would examine whether the benefit of OT + NTX on appetite extends onto adolescent individuals without surgically induced overeating. Thus, here we examined whether low doses of combined OT + NTX acutely affect post-deprivation intake of energy-dense, standard chow; intake of energy-dense and palatable high-fat high-sugar (HFHS) diet; or calorie-dilute, palaTable 10% sucrose solution without deprivation in adolescent male rats. We assessed whether OT + NTX decreases water intake after water deprivation or produces a conditioned taste aversion (CTA). Finally, by using c-Fos immunoreactivity, we determined changes in activity of feeding-related brain areas after OT + NTX. We found that individual subthreshold doses of OT and NTX decreased feeding induced by energy and by palatability. Significant c-Fos changes were noted in the arcuate and dorsomedial hypothalamic nuclei. The hypophagic doses of OT + NTX did not suppress water intake in thirsty rats and did not cause a CTA, which suggests that feeding reduction is not a secondary effect of gastrointestinal discomfort or changes in thirst processing. We conclude that OT + NTX is an effective drug combination to reduce appetite in adolescent male rats.

The impact of food addiction behaviours on the treatment of overweight students

The present study evaluated the association of food addiction (FA), the change of the BMI/age z-score and the consumption of ultra-processed foods in overweight students undergoing a 16-month, multicomponent intervention in the school environment. FA was investigated using the Yale Food Addiction Scale for Children, and the dietary assessment was estimated using the semi-quantitative FFQ in overweight 9-11-year-old students (BMI/age z-score ≥ 1) of both sexes at their baseline and after the intervention (n 120). Among the schoolchildren, 33·4 % had FA in at least one of the two assessments. The analysis of mixed-effects models to assess the effect of the intervention and the change of the BMI/age z-score between evaluations showed that the occurrence of FA influenced the maintenance of weight (time#FA, β = 0·30, 95 % CI 0·05, 0·54, P = 0·016). Weight loss was observed only in individuals who did not present FA (BMI/age z-score = -0·3). When evaluating the effect of the intervention and the dietary variables, we verified a reduction in the consumption of sugary milk-based drinks -71·13 kJ (-17 kcal), P = 0·04 only in non-FA students at the end of the study. FA has been identified as an underlying factor with therapeutic relevance, and an enhanced understanding of FA can open new paths for the prevention and management of obesity.

Clusterin levels in undernourished SH-SY5Y cells

Food-related disorders are increasingly common in developed societies, and the psychological component of these disorders has been gaining increasing attention. Both overnourishment with high-fat diets and perinatal undernourishment in mice have been linked to a higher motivation toward food, resulting in an alteration in food intake. Clusterin (CLU), a multifaced protein, is overexpressed in the nucleus accumbens (NAc) of over-fed rats, as well as in those that suffered chronic undernutrition. Moreover, an increase of this protein was observed in the plasma of obese patients with food addiction, suggesting the implication of CLU in this eating disorder. To characterize CLU’s cellular mechanisms, in vitro experiments of undernutrition were performed using dopaminergic SH-SY5Y cells. To mimic in vivo dietary conditions, cells were treated with different fetal bovine serum (FBS) concentrations, resulting in control (C group) diet (10% FBS), undernourishment (U group) diet (0.5% FBS), and undernourishment diet followed by restoration of control diet (UC group) (0.5 + 10% FBS). Undernourishment compromised cell viability and proliferation, and concomitantly increased CLU secretion as well as the cytosolic pool of the protein, while decreasing the mitochondrial level. The restoration of normal conditions tended to recover cell physiology, and the normal levels and distribution of CLU. This research study is a step forward toward the characterization of clusterin as a potential marker for food addiction and nutritional status.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

The accelerator, the brake, and the terrain: associations of reward-related eating, self-regulation, and the home food environment with diet quality during pregnancy and postpartum in the pregnancy eating attributes study (PEAS) cohort

Background

Neurobehavioral factors, including reward-related eating and self-regulation, in conjunction with the food environment, may influence dietary behaviors. However, these constructs have not been examined in pregnancy and postpartum, a time of changing appetite and eating behaviors, and when dietary intake has implications for maternal and child health. This study examined associations of reward-related eating, self-regulation, and the home food environment with pregnancy and postpartum diet quality.

Methods

Participants in the Pregnancy Eating Attributes Study observational cohort were enrolled at ≤12 weeks gestation and followed through one-year postpartum. Pregnancy and postpartum Healthy Eating Index-2015 (HEI-total), and adequacy and moderation scores, respectively, were calculated by pooling 24-h diet recalls administered each trimester and during 2, 6, and 12 months postpartum. Participants completed four measures of reward-related eating – Modified Yale Food Addiction Scale (mYFAS), Power of Food Scale (PFS), Multiple Choice Procedure (MCP), and Reinforcing Value of Food Questionnaire (RVFQ); two measures of self-regulation – Barratt Impulsiveness Scale (BIS) and Delay of Gratification Inventory (DGI); and a Home Food Inventory (HFI), yielding obesogenic (OBES) and fruit/vegetables (FV) scores. Linear regression analyses estimated associations of reward-related eating, self-regulation, and home food environment with diet quality during pregnancy and postpartum, adjusting for sociodemographic characteristics.

Results

Pregnancy HEI-total was inversely associated with PFS (β = − 0.14 ± 0.05, p = 0.009), mYFAS(β = − 0.14 ± 0.06, p = 0.02), 2 of the 5 RVFQ indices, MCP (β = − 0.14 ± 0.05, p = 0.01), and DGI food subscale (β = 0.23 ± 0.05, p < 0.001), but associations of postpartum HEI-total with reward-related eating measures and self-regulation were small and not statistically significant. Pregnancy and postpartum HEI-total were associated inversely with HFI-OBES (β = − 0.17 ± 0.06, p = 0.004 and β = − 0.19 ± 0.07, p = 0.006, respectively), and positively with HFI-FV (β = 0.21 ± 0.05, p < 0.001 and β = 0.17 ± 0.06, p = 0.009, respectively).

Conclusions

Associations of poorer diet quality with greater reward-related eating during pregnancy but not postpartum suggests the need to better understand differences in the determinants of eating behaviors and approaches to circumvent or moderate reward-related eating to facilitate more optimal diet quality across this critical period.

Trial registration

Clinicaltrials.gov. URL – Registration ID – NCT02217462. Date of registration – August 13, 2014.

Transient Administration of Dopaminergic Precursor Causes Inheritable Overfeeding Behavior in Young Drosophila melanogaster Adults

Imbalances in dopaminergic signaling during development have been indicated as part of the underlying neurobiology of several psychiatric illnesses, including schizophrenia, major depression, bipolar disorder, and food addiction. Yet, how transient manipulation of dopaminergic signaling influences long-lasting behavioral consequences, or if these modifications can induce inheritable traits, it is still not understood. In this study, we used the Drosophila melanogaster model to test if transient pharmacological activation of the dopaminergic system leads to modulations of feeding and locomotion in adult flies. We observed that transient administration of a dopaminergic precursor, levodopa, at 6 h, 3 days or 5 days post-eclosion, induced overfeeding behavior, while we did not find significant effects on locomotion. Moreover, this phenotype was inherited by the offspring of flies treated 6 h or 3 days post-eclosion, but not the offspring of those treated 5 days post-eclosion. These results indicate that transient alterations in dopaminergic signaling can produce behavioral alterations in adults, which can then be carried to descendants. These findings provide novel insights into the conditions in which environmental factors can produce transgenerational eating disorders.

Food craving, cortisol and ghrelin responses in modeling highly palatable snack intake in the laboratory

Overeating of highly palatable (HP) foods in the ubiquitous HP food cue environment and under stress is associated with weight gain and contributes to the global obesity epidemic. However, subjective and biobehavioral processes that may increase HP overeating are not clear. Using a novel experimental approach, we examined HP food motivation and intake and neuroendocrine responses in the context of food cues, stress and a control neutral relaxing cue exposure in healthy individuals.

METHODS

Twenty individuals (12 M; 8F; ages 18-45) with body mass index (BMI) in the lean (LN: N = 8; 3/8 female BMI: 18-24.9) or overweight/obese (OW: N = 12; 5/12 female; BMI: 25-37) range were enrolled in a controlled, hospital-based, 3-day laboratory experiment. On each day, subjects were exposed to a brief 5-min individualized guided imagery of stress, food cue or an active neutral-relaxing control cue script, followed by a food snack test (FST), with one imagery condition per day and order of imagery exposure randomized and counterbalanced across subjects. Subjective HP food craving and caloric intake, anxiety, cortisol and total ghrelin was assessed repeatedly during each test day.

RESULTS

Significant condition and condition × group effects for food craving, anxiety and HP calorie intake were observed, with food cue relative to neutral condition increasing HP food craving and intake across all subjects (p < .001), but stress relative to neutral condition increased HP food craving and intake in the OW but not LN group (p < .01). Pre-snack increases in food craving after exposure to food cues and to stress predicted greater subsequent HP food intake (p's < 0.01). Furthermore, ghrelin increased in the food cue and stress conditions (p < .01), but stress-induced increases in ghrelin was associated with HP food intake only in the OW/OB condition (p < .01). Finally, cortisol increased during food cue exposure and increased cortisol responses were associated with greater HP food caving and with intake (p's < 0.05).

CONCLUSIONS

These findings, while preliminary, validate a laboratory model of HP food motivation and intake and identify specific subjective and neuroendocrine responses that may play a role in HP snacking with implications for weight gain and obesity risk. (342 words).

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Exploring the consumption of ultra-processed foods and its association with food addiction in overweight children

The present study explored the consumption of ultra-processed foods and its association with food addiction in overweight children. The prevalence of food addiction was investigated using the Yale Food Addiction Scale for Children in overweight 9-11 year-old children (BMI/age ≥1 Z score) of both sexes from two schools (n = 139). Food intake was estimated by a food frequency questionnaire and the food items were classified into 4 categories: minimally processed, culinary ingredients, processed foods and ultra-processed foods (UPF), based on their degree of processing. Among the children, 95% showed at least one of the seven symptoms of food addiction and 24% presented with a diagnosis of food addiction. In analysis of covariance adjusted for age and sex, a tendency of higher consumption of added sugar (refined sugar, honey, corn syrup) and UPF was found among those diagnosed with food addiction. Multiple logistic regression adjusted for sugar, sodium and fat ingestion showed that consumption of cookies/biscuits (OR = 4.19, p = 0.015) and sausages (OR = 11.77, p = 0.029) were independently associated with food addiction. The identification of foods that may be associated with addictive behavior is very important for correctly treating and preventing childhood obesity, which continues to be one of the greatest health problems in the world.

Neuronal SIRT1 regulates macronutrient-based diet selection through FGF21 and oxytocin signalling in mice

Diet affects health through ingested calories and macronutrients, and macronutrient balance affects health span. The mechanisms regulating macronutrient-based diet choices are poorly understood. Previous studies had shown that NAD-dependent deacetylase sirtuin-1 (SIRT1) in part influences the health-promoting effects of caloric restriction by boosting fat use in peripheral tissues. Here, we show that neuronal SIRT1 shifts diet choice from sucrose to fat in mice, matching the peripheral metabolic shift. SIRT1-mediated suppression of simple sugar preference requires oxytocin signalling, and SIRT1 in oxytocin neurons drives this effect. The hepatokine FGF21 acts as an endocrine signal to oxytocin neurons, promoting neuronal activation and Oxt transcription and suppressing the simple sugar preference. SIRT1 promotes FGF21 signalling in oxytocin neurons and stimulates Oxt transcription through NRF2. Thus, neuronal SIRT1 contributes to the homeostatic regulation of macronutrient-based diet selection in mice.

Hypothalamic Integration of the Endocrine Signaling Related to Food Intake

Hypothalamic integration of gastrointestinal and adipose tissue-derived hormones serves as a key element of neuroendocrine control of food intake. Leptin, adiponectin, oleoylethanolamide, cholecystokinin, and ghrelin, to name a few, are in a constant "cross talk" with the feeding-related brain circuits that encompass hypothalamic populations synthesizing anorexigens (melanocortins, CART, oxytocin) and orexigens (Agouti-related protein, neuropeptide Y, orexins). While this integrated neuroendocrine circuit successfully ensures that enough energy is acquired, it does not seem to be equally efficient in preventing excessive energy intake, especially in the obesogenic environment in which highly caloric and palatable food is constantly available. The current review presents an overview of intricate mechanisms underlying hypothalamic integration of energy balance-related peripheral endocrine input. We discuss vulnerabilities and maladaptive neuroregulatory processes, including changes in hypothalamic neuronal plasticity that propel overeating despite negative consequences.

Multisensory influence on eating behavior: Hedonic consumption

Research in obesity has traditionally focused on prevention strategies and treatments aimed at changing lifestyle habits. However, recent research suggests that eating behavior is a habit regulated not only by homeostatic mechanisms, but also by the hedonic pathway that controls appetite and satiety processes. Cognitive, emotional, social, economic, and cultural factors, as well as organoleptic properties of food, are basic aspects to consider in order to understand eating behavior and its impact on health. This review presents a multisensory integrative view of food at both the homeostatic and non-homeostatic levels. This information will be of scientific interest to determine behavior drivers leading to overeating and, thus, to propose effective measures, at both the individual and population levels, for the prevention of obesity and associated metabolic diseases.

Systematic review and meta-analysis of the efficacy of interventions for people with Type 1 diabetes mellitus and disordered eating

AIM

To examine the types of interventions currently available for people with Type 1 diabetes mellitus and their effectiveness.

BACKGROUND

The prevalence of disordered eating in people with Type 1 diabetes mellitus is twice that in their counterparts without diabetes, and is associated with worse biomedical outcomes and greater mortality.

METHODS

Medline, Embase, PsycINFO, the Cochrane Library, PubMed and OpenGrey databases were searched up to August 2016 to identify studies on interventions in people with Type 1 diabetes-associated disordered eating. For the systematic review, intervention components were identified and their effectiveness was examined. For the meta-analysis, the pooled effect sizes of glycaemic control (HbA_1c ) between pre- and post-treatment in treatment and comparison groups were calculated using a random effects model.

RESULTS

Of 91 abstracts reviewed, six studies met the inclusion criteria, of which three had appropriate data for the meta-analysis (n = 118). The pooled effect size was -0.21 95% CI (-0.58 to 0.16; where negative values represent an improvement in HbA_1c levels), indicating no statistically significant improvement in the treatment group compared with comparison group. Inpatient therapy appeared to be the most effective treatment, and this had multiple components including cognitive behavioural therapy, psychoeducation and family therapy.

CONCLUSION

Limited or no improvement in glycaemic control and disordered eating symptoms was observed in people with Type 1 diabetes-associated disordered eating who were receiving currently available interventions. The present review suggests that developing an intensive intervention with a joint focus on both disordered eating and diabetes management is needed for this complex patient group.

Changes in gene expression and sensitivity of cocaine reward produced by a continuous fat diet

RATIONALE

Preclinical studies report that free access to a high-fat diet (HFD) alters the response to psychostimulants.

OBJECTIVES

The aim of the present study was to examine how HFD exposure during adolescence modifies cocaine effects. Gene expression of CB1 and mu-opioid receptors (MOr) in the nucleus accumbens (N Acc) and prefrontal cortex (PFC) and ghrelin receptor (GHSR) in the ventral tegmental area (VTA) were assessed.

METHODS

Mice were allowed continuous access to fat from PND 29, and the locomotor (10 mg/kg) and reinforcing effects of cocaine (1 and 6 mg/kg) on conditioned place preference (CPP) were evaluated on PND 69. Another group of mice was exposed to a standard diet until the day of post-conditioning, on which free access to the HFD began.

RESULTS

HFD induced an increase of MOr gene expression in the N Acc, but decreased CB1 receptor in the N Acc and PFC. After fat withdrawal, the reduction of CB1 receptor in the N Acc was maintained. Gene expression of GHSR in the VTA decreased during the HFD and increased after withdrawal. Following fat discontinuation, mice exhibited increased anxiety, augmented locomotor response to cocaine, and developed CPP for 1 mg/kg cocaine. HFD reduced the number of sessions required to extinguish the preference and decreased sensitivity to drug priming-induced reinstatement.

CONCLUSION

Our results suggest that consumption of a HFD during adolescence induces neurobiochemical changes that increased sensitivity to cocaine when fat is withdrawn, acting as an alternative reward.

A high-fat diet combined with food deprivation increases food seeking and the expression of candidate biomarkers of addiction

A mouse model has been developed to study the effect of dietary fat combined with food deprivation periods on palatable food seeking and on the expression of three potential addiction biomarkers in the nucleus accumbens: fumarate hydratase (FH), ATP synthase subunit alpha (ATP5a1) and transketolase (TKT). Forty C57BL/6 J male mice, four-week old, were fed either with a high-fat (HF) diet or standard diet along the experiment. After 3 weeks of differential feeding, animals underwent a two-week training period of two daily sessions where visual cues were paired either to palatable food (chocolate cereals) or no food at all. This training was prolonged one more week with similar, one daily sessions preceded by 12 hours of food deprivation. A behavioural test was finally conducted where mice were confined for 30 minutes either in food unpaired compartments or in compartments previously paired with cereals, but now with empty food trays. Total activity during this behavioural test and serum corticosterone levels right after it were similar in all experimental groups. Mice tested in food-paired compartments showed a marked preference for the empty food tray that gradually disappeared in standard diet-fed individuals but persisted in HF-fed mice. HF-fed mice also overexpressed FH, ATP5a1 and TKT, which positively correlated with the persistence of preference for the empty food tray. It is suggested that HF diets combined with food deprivation may enhance food seeking behaviours while upregulating FH/ATP5a1/TKT, which are further envisaged as biomarkers of addiction.

Role of addiction and stress neurobiology on food intake and obesity

The US remains at the forefront of a global obesity epidemic with a significant negative impact on public health. While it is well known that a balance between energy intake and expenditure is homeostatically regulated to control weight, growing evidence points to multifactorial social, neurobehavioral and metabolic determinants of food intake that influence obesity risk. This review presents factors such as the ubiquitous presence of rewarding foods in the environment and increased salience of such foods that stimulate brain reward motivation and stress circuits to influence eating behaviors. These rewarding foods via conditioned and reinforcing effects stimulate not only metabolic, but also stress hormones, that, in turn, hijack the brain emotional (limbic) and motivational (striatal) pathways, to promote food craving and excessive food intake. Furthermore, the impact of high levels of stress and trauma and altered metabolic environment (e.g. higher weight, altered insulin sensitivity) on prefrontal cortical self-control processes that regulate emotional, motivational and visceral homeostatic mechanisms of food intake and obesity risk are also discussed. A heuristic framework is presented in which the interactive dynamic effects of neurobehavioral adaptations in metabolic, motivation and stress neurobiology may further support food craving, excessive food intake and weight gain in a complex feed-forward manner. Implications of such adaptations in brain addictive-motivational and stress pathways and their effects on excessive food intake and weight gain are discussed to highlight key questions that requires future research attention in order to better understand and address the growing obesity epidemic.

Suppressed Fat Appetite after Roux-en-Y Gastric Bypass Surgery Associates with Reduced Brain μ-opioid Receptor Availability in Diet-Induced Obese Male Rats

Brain μ-opioid receptors (MORs) stimulate high-fat (HF) feeding and have been implicated in the distinct long term outcomes on body weight of bariatric surgery and dieting. Whether alterations in fat appetite specifically following these disparate weight loss interventions relate to changes in brain MOR signaling is unknown. To address this issue, diet-induced obese male rats underwent either Roux-en-Y gastric bypass (RYGB) or sham surgeries. Postoperatively, animals were placed on a two-choice diet consisting of low-fat (LF) and HF food and sham-operated rats were further split into ad libitum fed (Sham-LF/HF) and body weight-matched (Sham-BWM) to RYGB groups. An additional set of sham-operated rats always only on a LF diet (Sham-LF) served as lean controls, making four experimental groups in total. Corresponding to a stage of weight loss maintenance for RYGB rats, two-bottle fat preference tests in conjunction with small-animal positron emission tomography (PET) imaging studies with the selective MOR radioligand [¹¹C]carfentanil were performed. Brains were subsequently collected and MOR protein levels in the hypothalamus, striatum, prefrontal cortex and orbitofrontal cortex were analyzed by Western Blot. We found that only the RYGB group presented with intervention-specific changes: having markedly suppressed intake and preference for high concentration fat emulsions, a widespread reduction in [¹¹C]carfentanil binding potential (reflecting MOR availability) in various brain regions, and a downregulation of striatal and prefrontal MOR protein levels compared to the remaining groups. These findings suggest that the suppressed fat appetite caused by RYGB surgery is due to reduced brain MOR signaling, which may contribute to sustained weight loss unlike the case for dieting.

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

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

Greater Food Reward Sensitivity Is Associated with More Frequent Intake of Discretionary Foods in a Nationally Representative Sample of Young Adults

Food reward sensitivity may influence individual susceptibility to an environment replete with highly palatable foods of minimal nutritional value. These foods contain combinations of added sugar, fat, and/or salt that may enhance their motivational salience. This study examined associations of food reward sensitivity with eating behaviors in the NEXT Generation Health Study, a nationally representative sample of U.S. young adults. Participants (n = 2202) completed self-report measures including the Power of Food Scale, assessing food reward sensitivity, and intake frequency of 14 food groups. Multiple linear regressions estimated associations of food reward sensitivity with each of the eating behaviors adjusting for covariates. Higher food reward sensitivity was associated with more frequent intake of fast food (b ± linearized SE = 0.24 ± 0.05, p < 0.001), sweet and salty snacks (0.21 ± 0.05, p < 0.001), foods made with cheese (0.14 ± 0.06, p = 0.03), soda (0.12 ± 0.04, p = 0.009), processed meats (0.12 ± 0.05, p = 0.045), and fish (0.08 ± 0.03, p = 0.03) but was not associated with intake frequency of fruit or juice, green or orange vegetables, beans, whole grains, nuts/seeds, or dairy products. Food reward sensitivity was associated with greater intake of discretionary foods but was not associated with intake of most health-promoting foods, suggesting food reward sensitivity may lead to preferential intake of unhealthful foods.

Pregnancy eating attributes study (PEAS): a cohort study examining behavioral and environmental influences on diet and weight change in pregnancy and postpartum

BACKGROUND

The rising prevalence of maternal overweight/obesity and excessive gestational weight gain poses a serious public health concern due to the contribution of these factors to increased risk of negative health outcomes for both mother and child. Scant intervention research has indicated moderate short-term improvement in maternal diet and gestational weight gain, with little evidence of long-term behavior change, in parallel with findings from interventions outside of pregnancy. Recent laboratory-based findings from neuroscience implicate aberrant reward processing of food at the brain level ("food reward sensitivity," the between-individual variation in the response to food stimuli) as a contributor to eating beyond energy needs. However, scant research has examined the influence of these processes on weight change in population-based settings, and the relevance of these processes to pregnancy-related weight change has not been explored. The purpose of the Pregnancy Eating Attributes Study (PEAS) is to examine the role of food reward sensitivity in maternal diet and weight change during pregnancy and postpartum. The study examines the interplay of food reward sensitivity with behavioral control, home food environment, and related aspects of eating behavior in the context of weight-related biomedical, psychosocial, genetic and behavioral factors including physical activity, stress, sleep and depression.

METHODS

Women of varying baseline weight status (n = 450) are enrolled early in pregnancy and followed, along with their infants, until 1 year postpartum. Assessments occur during each trimester of pregnancy, and postpartum at approximately 2 months, 6 months, 9 months and 12 months. Maternal food reward, self-control, home food environment, eating behaviors, dietary intake, health behaviors, and anthropometrics are assessed along with maternal and infant clinical and biological data, infant anthropometrics, and feeding practices. Primary exposures of interest include food reward sensitivity, behavioral control, and home food environment. Primary outcomes include gestational weight gain, postpartum weight retention and maternal diet quality.

DISCUSSION

With increasing evidence suggesting the relevance of food reward sensitivity for understanding eating behavior, PEAS aims to advance understanding of the determinants of eating behavior during pregnancy, informing future interventions for improving maternal diet and weight change, and leading to improved maternal and child health and weight trajectories.

TRIAL REGISTRATION

Clinicaltrials.gov, NCT02217462. Date of registration: August 13, 2014.

A refined high carbohydrate diet is associated with changes in the serotonin pathway and visceral obesity

Consumption of palatable foods high in refined carbohydrate has been implicated as a contributing factor to the epidemic levels of obesity. Such foods may disrupt appetite regulation in the hypothalamus through alterations in hunger and satiety signalling. This investigation examined whether a palatable high refined carbohydrate (HRC) diet with the potential to induce obesity was linked to modulation of serotonin and dopamine signalling within the hypothalamus of rats. Male Wistar rats were allowed ad libitum access to either a palatable refined carbohydrate enriched (HRC) diet or standard chow (SC). Visceral fat percentage was used as a measure of the animals' weight gain during the trial. Real-time PCR was applied to determine any variation in levels of expression of the serotonin (Slc6A4 or Sert) and dopamine transporter (Slc6A3 or Dat) genes. After 29 weeks, the HRC group showed a significant increase in visceral fat percentage accompanied by increased expression of Sert. Higher levels of circulating triglycerides were also seen. This investigation determined that a refined high carbohydrate diet is associated with visceral obesity, increased circulating lipids in the blood and distorted serotonergic signalling, which possibly alters satiety and hunger signals.

Developing a theoretical maintenance model for disordered eating in Type 1 diabetes

BACKGROUND

According to the literature, eating disorders are an increasing problem for more than a quarter of people with Type 1 diabetes and they are associated with accentuated diabetic complications. The clinical outcomes in this group when given standard eating disorder treatments are disappointing. The Medical Research Council guidelines for developing complex interventions suggest that the first step is to develop a theoretical model.

AIM

To review existing literature to build a theoretical maintenance model for disordered eating in people with Type 1 diabetes.

METHOD

The literature in diabetes relating to models of eating disorder (Fairburn's transdiagnostic model and the dual pathway model) and food addiction was examined and assimilated.

RESULTS

The elements common to all eating disorder models include weight/shape concern and problems with mood regulation. The predisposing traits of perfectionism, low self-esteem and low body esteem and the interpersonal difficulties from the transdiagnostic model are also relevant to diabetes. The differences include the use of insulin mismanagement to compensate for breaking eating rules and the consequential wide variations in plasma glucose that may predispose to 'food addiction'. Eating disorder symptoms elicit emotionally driven reactions and behaviours from others close to the individual affected and these are accentuated in the context of diabetes.

CONCLUSION

The next stage is to test the assumptions within the maintenance model with experimental medicine studies to facilitate the development of new technologies aimed at increasing inhibitory processes and moderating environmental triggers.

Genetic Effects on Longitudinal Changes from Healthy to Adverse Weight and Metabolic Status – The HUNT Study

Introduction

The complexity of obesity and onset and susceptibility of cardio-metabolic disorders are still poorly understood and is addressed here through studies of genetic influence on weight gain and increased metabolic risk longitudinally.

Subjects/Methods

Twenty seven previously identified obesity, eating disorder or metabolic risk susceptibility SNPs were tested for association with weight or metabolically related traits longitudinally in 3999 adults participating both in the HUNT2 (1995–97) and HUNT3 (2006–08) surveys. Regression analyses were performed with changes from normal weight to overweight/obesity or from metabolically healthy to adverse developments with regards to blood pressure, glucose, HDL cholesterol, triglycerides or metabolic syndrome as outcomes. Additionally, a sub-sample of 1380 adolescents was included for testing association of nine SNPs with longitudinal weight gain into young adulthood.

Results

The most substantial effect on BMI-based weight gain from normal to overweight/obesity in adults was observed for the DRD2 variant (rs6277)(OR: 0.79, 95% CI: 0.69–0.90, P = 3.9x10^-4, adj. P = 0.015). DRD2 was not associated with BMI on a cross-sectional level. In the adolescent sample, FTO (rs1121980) was associated with change to overweight at adulthood in the combined male-female sample (OR: 1.27, 95% CI: 1.09–1.49, P = 3.0x10^-3, adj. P = 0.019) and in females (OR: 1.53, 95% CI: 1.23–1.91, P = 1.8x10^-4, adj. P = 0.003). When testing for association to longitudinal adverse developments with regard to blood pressure, blood lipids and glucose, only rs964184 (ZNF259/APOA5) was significantly associated to unfavourable triglyceride changes (OR: 1.66, 95% CI: 1.36–2.03, P = 5.7x10^-7, adj. P = 0.001). Pleiotropic effects on metabolic traits, however, were observed for several genetic loci cross-sectionally, ZNF259/APOA5, LPL and GRB14 being the most important.

Conclusions

DRD2 exhibits effects on weight gain from normal weight to overweight/obesity in adults, while, FTO is associated to weight gain from adolescence to young adulthood. Unhealthy longitudinal triglyceride development is strongly affected by ZNF259/APOA. Our main finding, linking the DRD2 variant directly to the longitudinal weight gain observed, has not previously been identified. It suggests a genetic pre-disposition involving the dopaminergic signalling pathways known to play a role in food reward and satiety linked mechanisms.

Enhanced flavor-nutrient conditioning in obese rats on a high-fat, high-carbohydrate choice diet

Through flavor-nutrient conditioning rats learn to prefer and increase their intake of flavors paired with rewarding, postingestive nutritional consequences. Since obesity is linked to altered experience of food reward and to perturbations of nutrient sensing, we investigated flavor-nutrient learning in rats made obese using a high fat/high carbohydrate (HFHC) choice model of diet-induced obesity (ad libitum lard and maltodextrin solution plus standard rodent chow). Forty rats were maintained on HFHC to induce substantial weight gain, and 20 were maintained on chow only (CON). Among HFHC rats, individual differences in propensity to weight gain were studied by comparing those with the highest proportional weight gain (obesity prone, OP) to those with the lowest (obesity resistant, OR). Sensitivity to postingestive food reward was tested in a flavor-nutrient conditioning protocol. To measure initial, within-meal stimulation of flavor acceptance by post-oral nutrient sensing, first, in sessions 1-3, baseline licking was measured while rats consumed grape- or cherry-flavored saccharin accompanied by intragastric (IG) water infusion. Then, in the next three test sessions they received the opposite flavor paired with 5 ml of IG 12% glucose. Finally, after additional sessions alternating between the two flavor-infusion contingencies, preference was measured in a two-bottle choice between the flavors without IG infusions. HFHC-OP rats showed stronger initial enhancement of intake in the first glucose infusion sessions than CON or HFHC-OR rats. OP rats also most strongly preferred the glucose-paired flavor in the two-bottle choice. These differences between OP versus OR and CON rats suggest that obesity is linked to responsiveness to postoral nutrient reward, consistent with the view that flavor-nutrient learning perpetuates overeating in obesity.

Physiological mechanisms by which non-nutritive sweeteners may impact body weight and metabolism

Evidence linking sugar-sweetened beverage (SSB) consumption to weight gain and other negative health outcomes has prompted many individuals to resort to artificial, non-nutritive sweetener (NNS) substitutes as a means of reducing SSB intake. However, there is a great deal of controversy regarding the biological consequences of NNS use, with accumulating evidence suggesting that NNS consumption may influence feeding and metabolism via a variety of peripheral and central mechanisms. Here we argue that NNSs are not physiologically inert compounds and consider the potential biological mechanisms by which NNS consumption may impact energy balance and metabolic function, including actions on oral and extra-oral sweet taste receptors, and effects on metabolic hormone secretion, cognitive processes (e.g. reward learning, memory, and taste perception), and gut microbiota.

Distinctive striatal dopamine signaling after dieting and gastric bypass

Highly palatable and/or calorically dense foods, such as those rich in fat, engage the striatum to govern and set complex behaviors. Striatal dopamine signaling has been implicated in hedonic feeding and the development of obesity. Dieting and bariatric surgery have markedly different outcomes on weight loss, yet how these interventions affect central homeostatic and food reward processing remains poorly understood. Here, we propose that dieting and gastric bypass produce distinct changes in peripheral factors with known roles in regulating energy homeostasis, resulting in differential modulation of nigrostriatal and mesolimbic dopaminergic reward circuits. Enhancement of intestinal fat metabolism after gastric bypass may also modify striatal dopamine signaling contributing to its unique long-term effects on feeding behavior and body weight in obese individuals.

When chocolate seeking becomes compulsion: gene-environment interplay

BACKGROUND

Eating disorders appear to be caused by a complex interaction between environmental and genetic factors, and compulsive eating in response to adverse circumstances characterizes many eating disorders.

MATERIALS AND METHODS

We compared compulsion-like eating in the form of conditioned suppression of palatable food-seeking in adverse situations in stressed C57BL/6J and DBA/2J mice, two well-characterized inbred strains, to determine the influence of gene-environment interplay on this behavioral phenotype. Moreover, we tested the hypothesis that low accumbal D2 receptor (R) availability is a genetic risk factor of food compulsion-like behavior and that environmental conditions that induce compulsive eating alter D2R expression in the striatum. To this end, we measured D1R and D2R expression in the striatum and D1R, D2R and α1R levels in the medial prefrontal cortex, respectively, by western blot.

RESULTS

Exposure to environmental conditions induces compulsion-like eating behavior, depending on genetic background. This behavioral pattern is linked to decreased availability of accumbal D2R. Moreover, exposure to certain environmental conditions upregulates D2R and downregulates α1R in the striatum and medial prefrontal cortex, respectively, of compulsive animals. These findings confirm the function of gene-environment interplay in the manifestation of compulsive eating and support the hypothesis that low accumbal D2R availability is a "constitutive" genetic risk factor for compulsion-like eating behavior. Finally, D2R upregulation and α1R downregulation in the striatum and medial prefrontal cortex, respectively, are potential neuroadaptive responses that parallel the shift from motivated to compulsive eating.

Animal models of compulsive eating behavior

Eating disorders are multifactorial conditions that can involve a combination of genetic, metabolic, environmental, and behavioral factors. Studies in humans and laboratory animals show that eating can also be regulated by factors unrelated to metabolic control. Several studies suggest a link between stress, access to highly palatable food, and eating disorders. Eating "comfort foods" in response to a negative emotional state, for example, suggests that some individuals overeat to self-medicate. Clinical data suggest that some individuals may develop addiction-like behaviors from consuming palatable foods. Based on this observation, "food addiction" has emerged as an area of intense scientific research. A growing body of evidence suggests that some aspects of food addiction, such as compulsive eating behavior, can be modeled in animals. Moreover, several areas of the brain, including various neurotransmitter systems, are involved in the reinforcement effects of both food and drugs, suggesting that natural and pharmacological stimuli activate similar neural systems. In addition, several recent studies have identified a putative connection between neural circuits activated in the seeking and intake of both palatable food and drugs. The development of well-characterized animal models will increase our understanding of the etiological factors of food addiction and will help identify the neural substrates involved in eating disorders such as compulsive overeating. Such models will facilitate the development and validation of targeted pharmacological therapies.

A possible link between BDNF and mTOR in control of food intake

Food intake is intricately regulated by glucose, amino acids, hormones, neuropeptides, and trophic factors through a neural circuit in the hypothalamus. Brain-derived neurotrophic factor (BDNF), the most prominent neurotrophic factor in the brain, regulates differentiation, maturation, and synaptic plasticity throughout life. Among its many roles, BDNF exerts an anorexigenic function in the brain. However, the intracellular signaling induced by BDNF to control food intake is not fully understood. One candidate for the molecule involved in transducing the anorexigenic activity of BDNF is the mammalian target of rapamycin (mTOR). mTOR senses extracellular amino acids, glucose, growth factors, and neurotransmitters, and regulates anabolic reactions response to these signals. Activated mTOR increases protein and lipid synthesis and inhibits protein degradation. In the hypothalamus, mTOR activation is thought to reduce food intake. Here we summarize recent findings regarding BDNF- and mTOR-mediated feeding control, and propose a link between these molecules in eating behavior.

Snack food intake in ad libitum fed rats is triggered by the combination of fat and carbohydrates

Snack food like potato chips substantially contributes to energy intake in humans. In contrast to basic food, snacks are consumed additionally to other meals and may thereby lead to non-homeostatic energy intake. Snack food is also frequently associated with hedonic hyperphagia, a food intake independent from hunger. Analysis of brain activity patterns by manganese-enhanced MRI has previously revealed that the intake of potato chips in ad libitum fed rats strongly activates the reward system of the rat brain, which may lead to hedonic hyperphagia. The purpose of the present study was to develop a two-choice preference test to identify molecular determinants of snack food triggering extra food intake in ad libitum fed rats. Different kinds of test food were presented three times a day for 10 min each time. To minimize the influence of organoleptic properties, each test food was applied in a homogenous mixture with standard chow. Food intake as well as food intake-related locomotor activity were analyzed to evaluate the effects induced by the test foods in the two-choice preference test. In summary, fat (F), carbohydrates (CH), and a mixture of fat and carbohydrates (FCH) led to a higher food intake compared to standard chow. Notably, potato chip test food (PC) was highly significantly preferred over standard chow (STD) and also over their single main macronutrients F and CH. Only FCH induced an intake comparable to PC. Despite its low energy density, fat-free potato chip test food (ffPC) was also significantly preferred over STD and CH, but not over F, FCH, and PC. Thus, it can be concluded that the combination of fat and carbohydrates is a major molecular determinant of potato chips triggering hedonic hyperphagia. The applied two-choice preference test will facilitate future studies on stimulating and suppressive effects of other food components on non-homeostatic food intake.

A novel procedure for evaluating the reinforcing properties of tastants in laboratory rats: operant intraoral self-administration

This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.

A novel procedure for evaluating the reinforcing properties of tastants in laboratory rats: operant intraoral self-administration

This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.