High trait impulsivity predicts food addiction-like behavior in the rat

Short- and long-access palatable food self-administration results in different phenotypes of binge-type eating

Binge eating disorder (BED), the most common eating disorder in the United States, is characterized by binge-type eating and is associated with higher body mass index and greater motivation for food. This disorder tends to first appear in late adolescence or young adulthood and is more common in women than men. While some animal models of BED have recapitulated both the overeating and the excessive body weight / fat of BED, very few have examined the motivational aspects of this disorder or utilized young females as subjects. In the present study, female Long-Evans rats, starting in late adolescence, were trained in operant chambers to self-administer the highly palatable Milk Chocolate Ensure Plus®, in 30-minute ("short access") or 6-hour ("long access") sessions, 5 days per week, over 6.5 weeks. For comparison, other subjects were provided with Ensure ad libitum or maintained on chow and water only. Both short and long access to Ensure led rats to develop binge-type eating, measured as greater 30-minute caloric intake than rats with ad libitum or chow access and as increasing 30-minute intake across weeks. Compared to those with short access, rats with long access demonstrated moderately increased motivation for Ensure (measured by progressive ratio testing) and, compared to those with only chow access, they eventually showed significant hyperphagia on Ensure access days and hypophagia on non-access days. Rats with long access also showed greater body weight/fat than those maintained on chow. These findings suggest that, while both short and long operant access to Ensure causes young female rats to meet the definition of binge-type eating, they lead to different phenotypes of this behavior, with long access promoting the development of a greater number of features found in clinical BED. Ultimately, both models may be useful in future studies aimed at identifying the neurobiological basis of binge eating.

Convergent neural connectivity in motor impulsivity and high-fat food binge-like eating in male Sprague-Dawley rats

Food intake is essential for survival, but maladaptive patterns of intake, possibly encoded by a preexisting vulnerability coupled with the influence of environmental variables, can modify the reward value of food. Impulsivity, a predisposition toward rapid unplanned reactions to stimuli, is one of the multifaceted determinants underlying the etiology of dysregulated eating and its evolving pathogenesis. The medial prefrontal cortex (mPFC) is a major neural director of reward-driven behavior and impulsivity. Compromised signaling between the mPFC and nucleus accumbens shell (NAcSh) is thought to underlie the cognitive inability to withhold prepotent responses (motor impulsivity) and binge intake of high-fat food (HFF) seen in binge eating disorder. To explore the relationship between motor impulsivity and binge-like eating in rodents, we identified high (HI) and low impulsive (LI) rats in the 1-choice serial reaction time task and employed a rat model of binge-like eating behavior. HFF binge rats consumed significantly greater calories relative to control rats maintained on continual access to standard food or HFF. HI rats repeatedly exhibited significantly higher bingeing on HFF vs. LI rats. Next, we employed dual viral vector chemogenetic technology which allows for the targeted and isolated modulation of ventral mPFC (vmPFC) neurons that project to the NAcSh. Chemogenetic activation of the vmPFC to NAcSh pathway significantly suppressed motor impulsivity and binge-like intake for high-fat food. Thus, inherent motor impulsivity and binge-like eating are linked and the vmPFC to NAcSh pathway serves as a 'brake' over both behaviors.

Development of a Spatial Discount Task to Measure Impulsive Choices in Dogs

Simple Summary

Impulsivity is believed to play a role in problematic behaviors in dogs. In this study, we developed a test to assess dogs’ tendency to make impulsive choices, that is their preference for smaller immediate reward instead of larger, but harder to obtain ones. Dogs were first trained that a bowl presented on a certain side always contained a large food amount, whereas the one presented on the opposite side (although at the same distance from the dog) contained less food. Then, the bowl with less food was progressively placed closer to the dog. As expected, dogs’ choices to feed from the bowl with less food increased as the distance of the latter decreased. Choices did not depend on factors that could interfere, such as dogs’ level of motivation for food, training experience, or learning ability. This indicates that the test is likely to be actually assessing impulsivity, not other traits. Also, female dogs were more likely to make impulsive choices than males, in accordance with what is known in humans and rodents, supporting the validity of the test. The test was completed in less than 1 h, making it a valid option to assess impulsivity in dogs in various contexts.

Abstract

Impulsive choices reflect an individual’s tendency to prefer a smaller immediate reward over a larger delayed one. Here, we have developed a behavioural test which can be easily applied to assess impulsive choices in dogs. Dogs were trained to associate one of two equidistant locations with a larger food amount when a smaller amount was presented in the other location, then the smaller amount was placed systematically closer to the dog. Choices of the smaller amount, as a function of distance, were considered a measure of the dog’s tendency to make impulsive choices. All dogs (N = 48) passed the learning phase and completed the entire assessment in under 1 h. Choice of the smaller food amount increased as this was placed closer to the dog. Choices were independent from food motivation, past training, and speed of learning the training phase; supporting the specificity of the procedure. Females showed a higher probability of making impulsive choices, in agreement with analogue sex differences found in human and rodent studies, and supporting the external validity of our assessment. Overall, the findings support the practical applicability and represent a first indication of the validity of this method, making it suitable for investigations into impulsivity in dogs.

Impulsivity and body fat accumulation are linked to cortical and subcortical brain volumes among adolescents and adults

Obesity is associated not only with metabolic and physical health conditions, but with individual variations in cognition and brain health. This study examined the association between body fat (an index of excess weight severity), impulsivity (a vulnerability factor for obesity), and brain structure among adolescents and adults across the body mass index (BMI) spectrum. We used 3D T1 weighted anatomic magnetic resonance imaging scans to map the association between body fat and volumes in regions associated with obesity and impulsivity. Participants were 127 individuals (BMI: 18-40 kg/m2; M = 25.69 ± 5.15), aged 14 to 45 years (M = 24.79 ± 9.60; female = 64). Body fat was measured with bioelectric impendence technology, while impulsivity was measured with the UPPS-P Impulsive Behaviour Scale. Results showed that higher body fat was associated with larger cerebellar white matter, medial orbitofrontal cortex (OFC), and nucleus accumbens volume, although the latter finding was specific to adolescents. The relationship between body fat and medial OFC volume was moderated by impulsivity. Elevated impulsivity was also associated with smaller amygdala and larger frontal pole volumes. Our findings link vulnerability and severity markers of obesity with neuroanatomical measures of frontal, limbic and cerebellar structures, and unravel specific links between body fat and striatal volume in adolescence.

Neuropharmacology of compulsive eating

Compulsive eating behaviour is a transdiagnostic construct observed in certain forms of obesity and eating disorders, as well as in the proposed construct of 'food addiction'. Compulsive eating can be conceptualized as comprising three elements: (i) habitual overeating, (ii) overeating to relieve a negative emotional state, and (iii) overeating despite adverse consequences. Neurobiological processes that include maladaptive habit formation, the emergence of a negative affect, and dysfunctions in inhibitory control are thought to drive the development and persistence of compulsive eating behaviour. These complex psychobehavioural processes are under the control of various neuropharmacological systems. Here, we describe the current evidence implicating these systems in compulsive eating behaviour, and contextualize them within the three elements. A better understanding of the neuropharmacological substrates of compulsive eating behaviour has the potential to significantly advance the pharmacotherapy for feeding-related pathologies.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Chronic stress increases susceptibility to food addiction by increasing the levels of DR2 and MOR in the nucleus accumbens

Background: Stress-related obesity might be related to the suppression of the hypothalamic-pituitary- adrenocortical axis and dysregulation of the metabolic system. Chronic stress also induces the dysregulation of the reward system and increases the risk of food addiction, according to recent clinical findings. However, few studies have tested the effect of chronic stress on food addiction in animal models. Purpose: The objective of this study was to identify whether chronic stress promotes food addiction or not and explore the possible mechanisms. Method: We applied adaily 2 hrsflashing LED irradiation stress to mice fed chow or palatable food to mimic the effect of chronic stress on feeding. After 1 month of chronic stress exposure, we tested their binge eating behaviors, cravings for palatable food, responses for palatable food, and compulsive eating behaviors to evaluate the effect of chronic stress on food addiction-like behaviors. We detected changes in the levels of various genes and proteins in the nucleus accumbens (NAc), ventral tegmental area (VTA) and lateral hypothalamus using qPCR and immunofluorescence staining, respectively. Results: Behaviors results indicated chronic stress obviously increased food addiction score (FAS) in the palatable food feeding mice. Moreover, the FAS had astrong relationship with the extent of the increase in body weight. Chronic stress increased the expression of corticotropin-releasing factor receptor 1(CRFR1) was increased in the NAc shell and core but decreased in the VTA of the mice fed with palatable food. Chronic stress also increased expression of both dopamine receptor 2 (DR2) and mu-opioid receptor (MOR) in the NAc. Conclusion: Chronic stress aggravates the FAS and contributed to the development of stress-related obesity. Chronic stress drives the dysregulation of the CRF signaling pathway in the reward system and increases the expression of DR2 and MOR in the nucleus accumbens.

Pramipexole-induced impulsivity in mildparkinsonian rats: a model of impulse control disorders in Parkinson's disease

Treatment with dopaminergic agonists such as pramipexole (PPX) contributes to the development of impulse control disorders (ICDs) in patients with Parkinson's disease (PD). As such, animal models of abnormal impulse control in PD are needed to better study the pathophysiology of these behaviors. Thus, we investigated impulsivity and related behaviors using the 5-choice serial reaction time task, as well as FosB/ΔFosB expression, in rats with mild parkinsonism induced by viral-mediated substantia nigra overexpression of human A53T mutated α-synuclein, and following chronic PPX treatment (0.25 mg/kg/d) for 4 weeks. The bilateral loss of striatal dopamine transporters (64%) increased the premature response rate of these rats, indicating enhanced waiting impulsivity. This behavior persisted in the OFF state after the second week of PPX treatment and it was further exacerbated in the ON state throughout the treatment period. The enhanced rate of premature responses following dopaminergic denervation was positively correlated with the premature response rate following PPX treatment (both in the ON and OFF states). Moreover, the striatal dopaminergic deficit was negatively correlated with the premature response rate at all times (pretreatment, ON and OFF states) and it was positively correlated with the striatal FosB/ΔFosB expression. By contrast, PPX treatment was not associated with changes in compulsivity (perseverative responses rate). This model recapitulates some features of PD with ICD, namely the dopaminergic deficit of early PD and the impulsivity traits provoked by dopaminergic loss in association with PPX treatment, making this model a useful tool to study the pathophysiology of ICDs.

Better, Not Just More-Contrast in Qualitative Aspects of Reward Facilitates Impulse Control in Pigs

Delay-of-gratification paradigms, such as the famous "Marshmallow Test," are designed to investigate the complex cognitive concepts of self-control and impulse control in humans and animals. Such tests determine whether a subject will demonstrate impulse control by choosing a large, delayed reward over an immediate, but smaller reward. Documented relationships between impulsive behavior and aggression in humans and animals suggest important implications for farm animal husbandry and welfare, especially in terms of inadequate social behavior, tail biting and maternal behavior. In a preliminary study, we investigated whether the extent of impulse control would differ between quantitatively and qualitatively different aspects of reward in pigs. Twenty female piglets were randomly divided into two groups, with 10 piglets each. After a preference test to determine individual reward preference among six different food items, a discrimination test was conducted to train for successful discrimination between different amounts of reward (one piece vs. four pieces) and different qualitative aspects of reward (highly preferred vs. least preferred food item). Then, an increasing delay (2, 4, 8, 16, 24, 32 s) was introduced for the larger/highly preferred reward. Each piglet could choose to get the smaller/least preferred reward immediately or to wait for the larger/highly preferred reward. Piglets showed clear differences in their preference for food items. Moreover, the "quality group" displayed faster learning in the discrimination test (number of sessions until 90% of the animals completed the discrimination test: "quality group"-3 days vs. "quantity group"-5 days) and reached a higher level of impulse control in the delay-of-gratification test compared to the "quantity group" (maximum delay that was mastered: "quality group"-24 s vs. "quantity group"-8 s). These results demonstrate that impulse control is present in piglets but that the opportunity to get a highly preferred reward is more valued than the opportunity to get more of a given reward. This outcome also underlines the crucial role of motivation in cognitive test paradigms. Further investigations will examine whether impulse control is related to traits that are relevant to animal husbandry and welfare.

Dopamine D2 receptor-mediated circuit from the central amygdala to the bed nucleus of the stria terminalis regulates impulsive behavior

Impulsivity is closely associated with addictive disorders, and changes in the brain dopamine system have been proposed to affect impulse control in reward-related behaviors. However, the central neural pathways through which the dopamine system controls impulsive behavior are still unclear. We found that the absence of the D2 dopamine receptor (D2R) increased impulsive behavior in mice, whereas restoration of D2R expression specifically in the central amygdala (CeA) of D2R knockout mice (Drd2 ^-/- ) normalized their enhanced impulsivity. Inhibitory synaptic output from D2R-expressing neurons in the CeA underlies modulation of impulsive behavior because optogenetic activation of D2R-positive inhibitory neurons that project from the CeA to the bed nucleus of the stria terminalis (BNST) attenuate such behavior. Our identification of the key contribution of D2R-expressing neurons in the CeA → BNST circuit to the control of impulsive behavior reveals a pathway that could serve as a target for approaches to the management of neuropsychiatric disorders associated with impulsivity.

Impulsive choice does not predict binge-like eating in rats

Binge eating disorder is an addiction-like disorder characterized by recurrent, excessive food consumption within discrete periods of time, and it has been linked to increased trait impulsivity. Within impulsivity components, while impulsive action was shown to predict binge-like and addictive-like eating, the role of impulsive choice is instead unknown. The goal of this study was to determine if impulsive choice predicted, or was altered by binge-like eating of a sugary, highly palatable diet. We utilized a modified adjusting delay task procedure in free-fed rats to assess impulsive choice behavior, that is. the tendency to respond for a larger, delayed reward over a lesser, immediate reward. We found that baseline impulsive choice was not a predictor of binge-like eating in 1-h sessions of palatable diet operant self-administration. Furthermore, binge-like eating of the same palatable diet had no effect on later impulsive choice behavior. Thus, our data suggest that, unlike impulsive action, impulsive choice behavior does not predict binge-like eating in rats.

Risky decision-making after exposure to a food-choice task in excess weight adolescents: Relationships with reward-related impulsivity and hunger

OBJECTIVE

To assess the effects of exposure to a food-choice task (appetizing versus healthy food) on risky decision-making by excess versus normal weight adolescents. We also analyzed the influence of food visualization on hunger levels, as well as group differences in food choices and impulsivity.

METHODS

Fifty-six adolescents (aged 13-18 years) classified as excess (n = 27) or normal (n = 29) weight participated in the study. Risky-decision-making was assessed through the Balloon Analogue Risk Task, which was administered before and after a food-choice task. We also evaluated impulsivity traits through the UPPS-P Scale, and subjective hunger levels with a visual analogue scale.

RESULTS

Adolescents with excess weight showed enhanced risky decision-making after the food-choice task compared to normal weight adolescents, as well as increased hunger levels. Furthermore, excess weight adolescents made more appetizing choices, and showed greater scores for Positive Urgency and Sensation Seeking. Reward-related impulsivity measures were positively associated with the number of appetizing choices in the food-choice task. Several associations were found between impulsivity measures, hunger levels and risk-taking variables.

CONCLUSIONS

Excess weight adolescents increased their risky-decision-making after food exposure and this augmentation was associated with the increase in hunger levels. Increased hunger levels and risk-taking after food exposure could lead to overeating. Alterations in decision-making caused by food signals may be a long-term risk factor for the development of obesity in adulthood. In modern societies, with the high availability and continuous exposure to food cues, decision-making may be a crucial factor in maintain healthy eating habits in adolescents.

Ghrelin is Related to Personality Differences in Reward Sensitivity and Impulsivity

Aims

Ghrelin, a feeding-related peptide mainly produced in the stomach, has been linked to reward mechanisms for food and drugs of abuse in addition to traits of impulsivity. This study is a secondary analysis of an existing data set designed to examine the direct relationships between fasting ghrelin levels and reward sensitivity/impulsivity in healthy social drinkers.

Methods

Participants (n = 20) were recruited from an original study examining the subjective effects of alcohol among social drinkers. Fasting ghrelin levels were collected at baseline. Personality measures (Behavioral Inhibition, Behavioral Activation, and Affective Response to Impending Reward and Punishment and Barratt Impulsiveness Scale) were administered at baseline to evaluate sensitivity to reward and punishment, and measure traits of impulsivity, respectively.

Results

Fasting ghrelin levels were significantly related to reward sensitivity and impulsivity traits. Specifically, those with higher ghrelin levels were more sensitive to reward and were more impulsive (have lower self-control).

Conclusions

The results indicate that individuals with higher levels of ghrelin are more sensitive to reward. In addition, they are less able to exercise self-control and to an extent more likely to act without thinking. This is the first study to report on the direct relationship between fasting ghrelin levels and personality characteristics such as reward sensitivity and aspects of impulsivity among healthy social drinkers.

Short summary

Individuals with higher levels of fasting ghrelin are more sensitive to reward, but less sensitive to punishment. Higher ghrelin levels are also related to some aspects of impulsivity such as decreased self-control and increased likelihood of acting without thinking.

Pattern of access determines influence of junk food diet on cue sensitivity and palatability

AIMS

Like drug addiction, cues associated with palatable foods can trigger food-seeking, even when sated. However, whether susceptibility to the motivating influence of food-related cues is a predisposing factor in overeating or a consequence of poor diet is difficult to determine in humans. Using a rodent model, we explored whether a highly palatable 'junk food' diet impacts responses to reward-paired cues in a Pavlovian-to-instrumental transfer test, using sweetened condensed milk (SCM) as the reward. The hedonic impact of SCM consumption was also assessed by analyzing licking microstructure.

METHODS

To probe the effects of pattern and duration of junk food exposure, we provided rats with either regular chow ad libitum (controls) or chow plus access to junk food for either 2 or 24 h per day for 1, 3, or 6 weeks. We also examined how individual susceptibility to weight gain related to these measures.

RESULTS

Rats provided 24 h access to the junk food diet were insensitive to the motivational effects of a SCM-paired cue when tested sated even though their hedonic experience upon reward consumption was similar to controls. In contrast, rats provided restricted, 2 h access to junk food exhibited a cue generalization phenotype under sated conditions, lever-pressing with increased vigor in response to both a SCM-paired cue, and a cue not previously paired with reward. Hedonic response was also significantly higher in these animals relative to controls.

CONCLUSIONS

These data demonstrate that the pattern of junk food exposure differentially alters the hedonic impact of palatable foods and susceptibility to the motivating influence of cues in the environment to promote food-seeking actions when sated, which may be consequential for understanding overeating and obesity.

Impulse control disorders in Parkinson's disease

Impulse control disorders (ICD) are frequent side effects of dopamine replacement therapy (DRT) used in Parkinson's disease (PD) with devastating consequences on the patients and caregivers. ICD are behavioural addictions including compulsive gambling, shopping, sexual behaviour, and binge eating that are mainly associated with dopamine D2/D3 agonists. Their management is a real clinical challenge due to the lack of therapeutic alternative. Clinical studies have identified demographic and clinical risk factors for ICD such as younger age at disease onset, male gender, prior history of depression or substance abuse, REM sleep behaviour disorders and higher rate of dyskinesia. PD patients with ICD may also have a specific pattern of dopaminergic denervation in the ventral striatum. Specific evaluation tools have now been designed to better evaluate the severity and impact of ICD in PD. Patients with ICD display altered processing of reward and loss, and decisional bias associated with altered activity in cortical and subcortical areas such as the orbitofrontal cortex, amygdala, insula, anterior cingular cortex, and ventral striatum. Preclinical studies have demonstrated that D2/D3 agonists induce impairments in behavioural processes likely relevant to ICD such as risk-taking behaviour, preference for uncertainty, perseverative responding and sustained drive to engage in gambling-like behaviour. Whether interactions between dopamine denervation and DRT significantly contribute to the pathogenesis of ICD remains poorly understood so far, although features unique to PD have been identified in patients with ICD. Large-scale longitudinal studies are needed to better identify subjects with increased risk to develop ICD and develop therapeutic options.

A Review of Childhood Behavioral Problems and Disorders in the Development of Obesity: Attention Deficit/Hyperactivity Disorder, Autism Spectrum Disorder, and Beyond

PURPOSE OF REVIEW

Given the high rates of pediatric and adult obesity, it is imperative to identify early risk factors that might contribute to excess weight gain. This review aims to investigate the relationship between childhood behavioral problems with the development and persistence of obesity. Specifically, this review highlights the association of obesity with (1) neurocognitive constructs, such as executive functioning and inhibition/impulsivity, and (2) disorders commonly diagnosed in childhood, including attention deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD).

RECENT FINDINGS

Consistent evidence supports a relationship between childhood behavioral problems, executive functioning, inhibition/impulsivity, ADHD, and ASD with obesity across the lifespan. Longitudinal studies suggest behavior problems, neurocognitive functioning deficits, and ADHD symptoms in childhood predict weight gain over time. Identifying risk factors in childhood that promote obesity may help develop targeted intervention and prevention programs. Additional research should elucidate mechanisms that account for these relationships.

Dietary influences on cognition

Obesity is a world-wide crisis with profound healthcare and socio-economic implications and it is now clear that the central nervous system (CNS) is a target for the complications of metabolic disorders like obesity. In addition to decreases in physical activity and sedentary lifestyles, diet is proposed to be an important contributor to the etiology and progression of obesity. Unfortunately, there are gaps in our knowledge base related to how dietary choices impact the structural and functional integrity of the CNS. For example, while chronic consumption of hypercaloric diets (increased sugars and fat) contribute to increases in body weight and adiposity characteristic of metabolic disorders, the mechanistic basis for neurocognitive deficits in obesity remains to be determined. In addition, studies indicate that acute consumption of hypercaloric diets impairs performance in a wide variety of cognitive domains, even in normal non-obese control subjects. These results from the clinical and basic science literature indicate that diet can have rapid, as well as long lasting effects on cognitive function. This review summarizes our symposium at the 2017 Society for the Study of Ingestive Behavior (SSIB) meeting that discussed these effects of diet on cognition. Collectively, this review highlights the need for integrated and comprehensive approaches to more fully determine how diet impacts behavior and cognition under physiological conditions and in metabolic disorders like type 2 diabetes mellitus (T2DM) and obesity.

Hyperpalatability and the Generation of Obesity: Roles of Environment, Stress Exposure and Individual Difference

PURPOSE OF REVIEW

This review investigates how exposure to palatable food and its associated cues alters appetite regulation and feeding behaviour to drive overeating and weight gain.

RECENT FINDINGS

Both supraphysiological and physiological feeding systems are affected by exposure to palatable foods and its associated cues. Preclinical research, largely using rodents, has demonstrated that palatable food modulates feeding-related neural systems and food-seeking behaviour by recruiting the mesolimbic reward pathway. This is supported by studies in adolescents which have shown that mesolimbic activity in response to palatable food cues and consumption predicts future weight gain. Additionally, stress exposure, environmental factors and individual susceptibility have been shown to modulate the effects of highly palatable foods on behaviour. Further preclinical research using free-choice diets modelling the modern obesogenic environment is needed to identify how palatable foods drive overeating. Moreover, future clinical research would benefit from more appropriate quantification of palatability, making use of rating systems and surveys.

Trace Amine Associated Receptor 1 (TAAR1) Modulation of Food Reward

Eating disorders and some forms of obesity are characterized by addictive-like, compulsive eating behavior which contains numerous similarities with compulsive drug use. Food intake is in part mediated by reward and reinforcement processes that can become dysregulated in these disorders. Additionally, impairments in inhibitory control regulation of reward-related responding can cause or further exacerbate binge and compulsive eating. Dysfunctions in two neurotransmitter systems in the mesocorticolimbic pathway, dopamine and glutamate, are thought to contribute to maladaptive eating behaviors. The trace amine associated receptor 1 (TAAR1) system is a promising therapeutic target for compulsive eating behavior due to the role of TAAR1 in synaptic transmission and in the modulation of dopaminergic and glutamatergic signaling. In support of this notion, the TAAR1 agonist RO5256390 was found to decrease the reinforcing effects of palatable food-cues and to reduce binge-like and compulsive-like eating of palatable food. Additionally, prolonged, intermittent access to palatable food was shown to downregulate TAAR1 in the prefrontal cortex, suggesting a potential role for TAAR1 signaling in inhibitory control processes. Research into the role of TAAR1 in addiction, including TAAR1’s ability to modulate psychostimulant reward and reinforcement, bolsters support for TAAR1 agonism as a pharmacological treatment for compulsive eating and other addictive behaviors. This review summarizes the evidence for TAAR1 agonism as a promising therapeutic for compulsive eating behavior as well as the hypothesized mechanism responsible for these effects.

Urban Mind: Using Smartphone Technologies to Investigate the Impact of Nature on Mental Well-Being in Real Time

Existing evidence on the beneficial effects of nature on mental health comes from studies using cross-sectional designs. We developed a smartphone-based tool (Urban Mind; www.urbanmind.info) to examine how exposure to natural features within the built environment affects mental well-being in real time. The tool was used to monitor 108 individuals who completed 3013 assessments over a 1-week period. Significant immediate and lagged associations with mental well-being were found for several natural features. These associations were stronger in people with higher trait impulsivity, a psychological measure of one's tendency to behave with little forethought or consideration of the consequences, which is indicative of a higher risk of developing mental-health issues. Our investigation suggests that the benefits of nature on mental well-being are time-lasting and interact with an individual's vulnerability to mental illness. These findings have potential implications from the perspectives of global mental health as well as urban planning and design.

Intra-accumbens Raclopride Administration Prevents Behavioral Changes Induced by Intermittent Access to Sucrose Solution

Overeating is one of the most relevant clinical features in Binge Eating Disorder and in some obesity patients. According to several studies, alterations in the mesolimbic dopaminergic transmission produced by non-homeostatic feeding behavior may be associated with changes in the reward system similar to those produced by drugs of abuse. Although it is known that binge-eating is related with changes in dopaminergic transmission mediated by D2 receptors in the nucleus accumbens shell (NAcS), it has not been determined whether these receptors may be a potential target for the treatment of eating pathology with binge-eating. Accordingly, the aim of the present study was to evaluate whether sugar binging induced by intermittent access to a sucrose solution produced changes in the structure of feeding behavior and whether blocking D2 receptors prevented these changes. We used the intermittent access model to a 10% sucrose solution (2 h/day for 4 weeks) to induce sugar binging in Sprague Dawley female rats. Experimental subjects consumed in a 2-h period more than 50% of the caloric intake consumed by the subjects with ad-lib access to the sweetened solution without any increase in body weight or fat accumulation. Furthermore, we evaluated whether sugar binging was associated to the estrous cycle and we did not find differences in caloric intake (estrous vs. diestrus). Subsequently, we characterized the structure of feeding behavior (microstructural analysis) and the motivation for palatable food (breakpoints) of the subjects with sugar binging and found that feeding episodes had short latencies, high frequencies, as well as short durations and inter-episode intervals. The intermittent access model did not increase breakpoints, as occurred in subjects with ad-lib access to the sucrose. Finally, we evaluated the effects of D2 receptor blockade in the NAcS, and found that raclopride (18 nM) prevented the observed changes in the frequency and duration of episodes induced by intermittent access to the sucrose solution. Our results suggest that alterations in behavioral patterns associated with binge-eating behavior depend in part on the dopaminergic transmission in the NAcS and that the antagonism of D2 receptors may be a therapeutic tool for feeding pathology with binge-eating.

Role of impulsivity and reward in the anti-obesity actions of 5-HT2C receptor agonists

The selective 5-HT_2C receptor agonist lorcaserin entered clinical obesity trials with the prevalent view that satiety was a primary mechanism of action. Subsequent Phase II and III trials demonstrated efficacy in terms of weight loss, although the overall effect size (~3% placebo-corrected change) is considered modest. Lorcaserin has been approved by the FDA for the treatment of obesity with lifestyle modification, but since its introduction in 2013 its sales are in decline, probably due to its overall modest effect. However, in some individuals, lorcaserin has a much more clinically significant effect (i.e. >10% placebo-corrected change), although what common features, if any, define these high responders is presently unknown. In the present article we highlight the evidence that alternative mechanisms to satiety may contribute to the anti-obesity effect of lorcaserin, namely effects on constructs of primary and conditioned reward and impulsivity. This may better inform the clinical evaluation of lorcaserin (and any future 5-HT_2C receptor agonists) to subgroups of obese subjects characterized by overeating due to maladaptive impulsivity and reward mechanisms. One such population might be individuals diagnosed with binge eating disorder.

Different Molecular/Behavioral Endophenotypes in C57BL/6J Mice Predict the Impact of OX1 Receptor Blockade on Binge-Like Ethanol Intake

Ethanol (EtOH) research has focused on stages of dependence. It is of paramount importance to more deeply understand the neurobehavioral factors promoting increased risk for EtOH binge drinking during the early stages of the addiction cycle. The first objective of this study was to evaluate whether C57BL/6J mice showing high drinking in the dark (DID) exhibit neurobehavioral traits known to contribute to EtOH binge-drinking disorders. Comparing high vs. low drinkers (HD/LD), we evaluated different types of basal anxiety-like responses, EtOH preference and sensitivity to the reinforcing properties of EtOH, and basal mRNA expression of the OX1/OX2 receptors (OX1r/OX2r) within the prefrontal cortex (PFC) and the nucleus accumbens (NAcc). Additionally, we tested binge drinking by LD/HD in response to a selective OX1r antagonist following intermittent episodes of DID (iDID). We report that DID consistently segregates two neurobehavioral endophenotypes, HD vs. LD, showing differences in neophobia and/or impulsivity/compulsivity traits. Additionally, HD mice show decreased basal OX1r and OX2r mRNA expression within the NAcc and elevated OX1r within the PFC. Exposure to several intermittent episodes of EtOH DID triggered a rapid increase in EtOH intake over time in LD mice matching that observed in HD mice. Despite HD/LD endophenotypes did not show differences in EtOH intake, they still predicted the response to a pharmacological challenge with a selective OX1r antagonist. The present data underscore the relevance of HD/LD endophenotypes stemming from DID procedures for exploring neurobehavioral processes underlying the early stages of the addiction cycle and EtOH binge-drinking disorders.

A preclinical model for identifying rats at risk of alcohol use disorder

Alcohol use is one of the world's leading causes of death and disease, although only a small proportion of individuals develop persistent alcohol use disorder (AUD). The identification of vulnerable individuals prior to their chronic intoxication remains of highest importance. We propose here to adapt current methodologies for identifying rats at risk of losing control over alcohol intake by modeling diagnostic criteria for AUD: inability to abstain during a signaled period of reward unavailability, increased motivation assessed in a progressive effortful task and persistent alcohol intake despite aversive foot shocks. Factor analysis showed that these three addiction criteria loaded on one underlying construct indicating that they represent a latent construct of addiction trait. Further, not only vulnerable rats displayed higher ethanol consumption, and higher preference for ethanol over sweetened solutions, but they also exhibited pre-existing higher anxiety as compared to resilient rats. In conclusion, the present preclinical model confirms that development of an addiction trait not only requires prolonged exposure to alcohol, but also depends on endophenotype like anxiety that predispose a minority of individuals to lose control over alcohol consumption.

Neuroscience of Compulsive Eating Behavior

A systematic characterization of compulsivity in pathological forms of eating has been proposed in the context of three functional domains: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. In this review, we provide evidence supporting this hypothesis and we differentiate the nascent field of neurocircuits and neurochemical mediators of compulsive eating through their underlying neuropsychobiological processes. A better understanding of the neurobiological mechanisms that lead to compulsive eating behavior can improve behavioral and pharmacological intervention for disorders of pathological eating.

A Potential Animal Model of Maladaptive Palatable Food Consumption Followed by Delayed Discomfort

Introduction: Binging is the consumption of larger amounts of food in a briefer period of time than would normally be consumed under similar circumstances. Binging requires palatable food (PF) to trigger abnormal eating, probably reflecting gene × environment interactions. In this study we examined the impact of trait binge eating (BE) and its compulsive nature on the conflict between hedonic eating of PF and anticipation of a delayed aversive effect. We used female rats as an animal model similar to other models of BE. A novel aspect of this model in this paper is the use of a delayed internal aversive effect produced by lactose ingestion. Establishing this model will allow us to better understand the nature of the conflict between immediate reward and its delayed aversive implications. We hypothesized that BE prone (BEP) rats will demonstrate maladaptive decision making, presenting higher motivation toward PF even when this is associated with delayed discomfort. Method: (Phase 1) 52 female adult Wistar rats were divided to two eating profiles: resistant and prone binge eaters (BER/BEP) based on intake of liquid PF (Ensure). Next, all subjects underwent a Lactose Conditioning Protocol (LCP) that included 4 h tests, one baseline and 3 conditioning days (Phase 2), in which solid PF (Oreo cookies) was paired with glucose (control-no internal aversive effect) or lactose, dissolved in liquid PF. Index for PF motivation was PF consumption during the 4 h LCP. To test for memory of lactose conditioning, we performed another LCP with glucose only (anticipation, but no actual lactose-induced discomfort), a week after the last conditioning session. Results: Lactose conditioned BEP showed higher motivation toward PF compared to lactose conditioned BER faced with delayed aversive effects. Only lactose conditioned BER rats devaluated the PF over LCP days, indicating an association between PF and abdominal discomfort. In addition, only lactose conditioned BER presented an adaptive dynamic behavior, by varying PF intake according to consequences. Furthermore, solid PF consumption was predicted by binge size of liquid PF, only for lactose conditioned rats. Conclusions: We established an animal model for a common eating conflict in humans using delayed internal aversive unconditional stimuli.

Pathological Overeating: Emerging Evidence for a Compulsivity Construct

Compulsive eating behavior is a transdiagnostic construct that is characteristic of medical and psychiatric conditions such as forms of obesity and eating disorders. Although feeding research is moving toward a better understanding of the proposed addictive properties of food, the components and the mechanisms contributing to compulsive eating are not yet clearly defined or understood. Current understanding highlights three elements of compulsive behavior as it applies to pathological overeating: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. These elements emerge through mechanisms involving pathological habit formation through an aberrant learning process, the emergence of a negative emotional state, and dysfunctions in behavioral control. Dysfunctions in systems within neurocircuitries that comprise the basal ganglia, the extended amygdala, and the prefrontal cortex result in compulsive eating behaviors. Here, we present evidence to relate compulsive eating behavior and addiction and to characterize their underlying neurobiological mechanisms. A major need to improve understanding of compulsive eating through the integration of complex motivational, emotional, and cognitive constructs is warranted.

The Trace Amine-Associated Receptor 1 Agonist RO5256390 Blocks Compulsive, Binge-like Eating in Rats

Compulsive, binge eating of highly palatable food constitutes a core feature of some forms of obesity and eating disorders, as well as of the recently proposed disorder of food addiction. Trace amine-associated receptor 1 (TAAR1) is a highly conserved G-protein-coupled receptor bound by endogenous trace amines. TAAR1 agonists have been shown to reduce multiple behavioral effects of drugs of abuse through their actions on the mesocorticolimbic system. In this study, we hypothesized that TAAR1 may have a role in compulsive, binge-like eating; we tested this hypothesis by assessing the effects of a TAAR1 agonist, RO5256390, in multiple excessive feeding-related behaviors induced by limiting access to a highly palatable diet in rats. Our results show that RO5256390 blocked binge-like eating in rats responding 1 h per day for a highly palatable sugary diet. Consistent with a palatability-selective effect, drug treatment selectively reduced the rate and regularity of palatable food responding, but it did not affect either baseline intake or food restriction-induced overeating of the standard chow diet. Furthermore, RO5256390 fully blocked compulsive-like eating when the palatable diet was offered in an aversive compartment of a light/dark conflict box, and blocked the conditioned rewarding properties of palatable food, as well as palatable food-seeking behavior in a second-order schedule of reinforcement. Drug treatment had no effect on either anxiety-like or depressive-like behavior, and it did not affect control performance in any of the tests. Importantly, rats exposed to palatable food showed decreased TAAR1 levels in the medial prefrontal cortex (mPFC), and RO5256390 microinfused into the infralimbic, but not prelimbic, subregion of the mPFC-reduced binge-like eating. Altogether, these results provide evidence for TAAR1 agonism as a novel pharmacological treatment for compulsive, binge eating.

Addiction-like Synaptic Impairments in Diet-Induced Obesity

BACKGROUND

There is increasing evidence that the pathological overeating underlying some forms of obesity is compulsive in nature and therefore contains elements of an addictive disorder. However, direct physiological evidence linking obesity to synaptic plasticity akin to that occurring in addiction is lacking. We sought to establish whether the propensity to diet-induced obesity (DIO) is associated with addictive-like behavior, as well as synaptic impairments in the nucleus accumbens core considered hallmarks of addiction.

METHODS

Sprague Dawley rats were allowed free access to a palatable diet for 8 weeks then separated by weight gain into DIO-prone and DIO-resistant subgroups. Access to palatable food was then restricted to daily operant self-administration sessions using fixed ratio 1, 3, and 5 and progressive ratio schedules. Subsequently, nucleus accumbens brain slices were prepared, and we tested for changes in the ratio between α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate currents and the ability to exhibit long-term depression.

RESULTS

We found that propensity to develop DIO is linked to deficits in the ability to induce long-term depression in the nucleus accumbens, as well as increased potentiation at these synapses as measured by AMPA/N-methyl-D-aspartate currents. Consistent with these impairments, we observed addictive-like behavior in DIO-prone rats, including 1) heightened motivation for palatable food; 2) excessive intake; and 3) increased food seeking when food was unavailable.

CONCLUSIONS

Our results show overlap between the propensity for DIO and the synaptic changes associated with facets of addictive behavior, supporting partial coincident neurological underpinnings for compulsive overeating and drug addiction.

Adenosine A2A receptor and ERK-driven impulsivity potentiates hippocampal neuroblast proliferation

Dampened adenosine A_2A receptor (A_2AR) function has been implicated in addiction through enhancement of goal-directed behaviors. However, the contribution of the A_2AR to the control of impulsive reward seeking remains unknown. Using mice that were exposed to differential reward of low rate (DRL) schedules during Pavlovian-conditioning, second-order schedule discrimination, and the 5-choice serial reaction time task (5-CSRTT), we demonstrate that deficits of A_2AR function promote impulsive responses. Antagonism of the A_2AR lowered ERK1 and ERK2 phosphorylation in the dorsal hippocampus (dHip) and potentiated impulsivity during Pavlovian-conditioning and the 5-CSRTT. Remarkably, inhibition of ERK1 and ERK2 phosphorylation by U0126 in the dHip prior to Pavlovian-conditioning exacerbated impulsive reward seeking. Moreover, we found decreased A_2AR expression, and reduced ERK1 and ERK2 phosphorylation in the dHip of equilibrative nucleoside transporter type 1 (ENT1^-/-) null mice, which displayed exacerbated impulsivity. To determine whether impulsive response behavior is associated with hippocampal neuroblast development, we investigated expression of BrdU⁺ and doublecortin (DCX⁺) following 5-CSRTT testing. These studies revealed that impulsive behavior driven by inhibition of the A_2AR is accompanied by increased neuroblast proliferation in the hippocampus.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Patterned feeding induces neuroendocrine, behavioral and genetic changes that promote palatable food intake

BACKGROUND

Selection of a healthy diet is the cornerstone for treating obesity and metabolic disease. Unfortunately, the majority of diets fail leading to weight regain and in some cases, pathological feeding behavior. We hypothesize that alternating bouts of caloric overconsumption and caloric restriction, behavioral manifestations of dieting induce neuroendocrine, behavioral and genetic changes that promote future bouts of palatable food intake.

METHODS

To test this hypothesis, we subjected male Long-Evans rats to a high-fat diet (HFD) feeding paradigm that induced a pattern of caloric overconsumption and caloric restriction. Under these conditions we measured operant responding for sucrose, pre-meal ghrelin secretion, the effects of peripheral ghrelin blockade on patterned feeding, HFD intake in an aversive environment and mRNA expression of the ghrelin receptor, orexin, orexin-1 and 2 receptors, and FTO in the medial prefrontal cortex, lateral hypothalamus and ventral tegmental area.

RESULTS

Rats subjected to this feeding regimen displayed increased ghrelin levels prior to HFD exposure and blockade of this response attenuated patterned feeding behavior. In addition, patterned feeding promoted enhanced motivation for sucrose, diminished extinction of this response and increased HFD intake in an aversive environment. The neuroendocrine and behavioral changes correlated with increased hypothalamic expression of the ghrelin receptor and FTO.

CONCLUSION

Collectively, these data indicate that patterns of feeding that include caloric overconsumption and caloric restriction induce neuroendocrine and neurobiological changes that signify an enhanced drive for palatable food.

Preweaning iron deficiency increases non-contingent responding during cocaine self-administration in rats

Iron deficiency (ID) is the most prevalent single-nutrient deficiency worldwide. There is evidence that ID early in development (preweaning in rat) causes irreversible neurologic, behavioral, and motor development deficits. Many of these effects have been attributed to damage to dopamine systems, including ID-induced changes in transporter and receptor numbers in the striatum and nucleus accumbens. These mesolimbic dopaminergic neurons are, in part, responsible for mediating reward and thus play a key role in addiction. However, there has been relatively little investigation into the behavioral effects of ID on drug addiction. In 2002, we found that rats made ID from weaning (postnatal day 21) and throughout the experiment acquired cocaine self-administration significantly more slowly than controls and failed to increase responding when the dose of the drug was decreased. In the present study, we assessed addiction for self-administered cocaine in rats with a history of preweaning ID only during postnatal days 4 through 21, and iron replete thereafter. The results showed that while ID did not affect the number of cocaine infusions or the overall addiction-like behavior score, ID rats scored higher on a measure of continued responding for drug than did iron replete controls. This increase in responding, however, was less goal-directed as ID rats also responded more quickly to the non-rewarded manipulandum than did control rats. Thus, while ID early in infancy did not significantly increase addiction-like behaviors for cocaine in this small study, the pattern of data suggests a possible underlying learning or performance impairment. Future studies will be needed to elucidate the exact neuro-behavioral deficits that lead to the increase in indiscriminate responding for drug in rats with a history of perinatal ID.

Lorcaserin and CP-809101 reduce motor impulsivity and reinstatement of food seeking behavior in male rats: Implications for understanding the anti-obesity property of 5-HT2C receptor agonists

RATIONALE

The 5-HT2C receptor agonist lorcaserin (Belviq®) has been approved by the FDA for the treatment of obesity. Impulsivity is a contributory feature of some eating disorders.

OBJECTIVE

Experiments investigated the effect of lorcaserin and the highly selective 5-HT2C agonist CP-809101 on measures of impulsivity and on reinstatement of food-seeking behaviour, a model of dietary relapse. The effect of both drugs on 22-h deprivation-induced feeding was also examined, as was the effect of prefeeding in each impulsivity test.

RESULTS

Lorcaserin (0.3-0.6 mg/kg SC) and CP-809101 (0.6-1 mg/kg SC) reduced premature responding in rats trained on the 5-CSRTT and improved accuracy in a Go-NoGo task by reducing false alarms. At equivalent doses, both drugs also reduced reinstatement for food-seeking behaviour. Neither drug altered impulsive choice measured in a delay-discounting task. Lorcaserin (1-3 mg/kg SC) and CP-809101 (3-6 mg/kg SC) reduced deprivation-induced feeding but only at higher doses.

CONCLUSIONS

These results suggest that in addition to previously reported effects on satiety and reward, altered impulse control may represent a contributory factor to the anti-obesity property of 5-HT2C receptor agonists. Lorcaserin may promote weight loss by improving adherence to dietary regimens in individuals otherwise prone to relapse and may be beneficial in cases where obesity is associated with eating disorders tied to impulsive traits, such as binge eating disorder.

The Neurobiology of "Food Addiction" and Its Implications for Obesity Treatment and Policy

There is a growing view that certain foods, particularly those high in refined sugars and fats, are addictive and that some forms of obesity can usefully be treated as a food addiction. This perspective is supported by a growing body of neuroscience research demonstrating that the chronic consumption of energy-dense foods causes changes in the brain's reward pathway that are central to the development and maintenance of drug addiction. Obese and overweight individuals also display patterns of eating behavior that resemble the ways in which addicted individuals consume drugs. We critically review the evidence that some forms of obesity or overeating could be considered a food addiction and argue that the use of food addiction as a diagnostic category is premature. We also examine some of the potential positive and negative clinical, social, and public policy implications of describing obesity as a food addiction that require further investigation.

The Stomach-Derived Hormone Ghrelin Increases Impulsive Behavior

Impulsivity, defined as impaired decision making, is associated with many psychiatric and behavioral disorders, such as attention-deficit/hyperactivity disorder as well as eating disorders. Recent data indicate that there is a strong positive correlation between food reward behavior and impulsivity, but the mechanisms behind this relationship remain unknown. Here we hypothesize that ghrelin, an orexigenic hormone produced by the stomach and known to increase food reward behavior, also increases impulsivity. In order to assess the impact of ghrelin on impulsivity, rats were trained in three complementary tests of impulsive behavior and choice: differential reinforcement of low rate (DRL), go/no-go, and delay discounting. Ghrelin injection into the lateral ventricle increased impulsive behavior, as indicated by reduced efficiency of performance in the DRL test, and increased lever pressing during the no-go periods of the go/no-go test. Central ghrelin stimulation also increased impulsive choice, as evidenced by the reduced choice for large rewards when delivered with a delay in the delay discounting test. In order to determine whether signaling at the central ghrelin receptors is necessary for maintenance of normal levels of impulsive behavior, DRL performance was assessed following ghrelin receptor blockade with central infusion of a ghrelin receptor antagonist. Central ghrelin receptor blockade reduced impulsive behavior, as reflected by increased efficiency of performance in the DRL task. To further investigate the neurobiological substrate underlying the impulsivity effect of ghrelin, we microinjected ghrelin into the ventral tegmental area, an area harboring dopaminergic cell bodies. Ghrelin receptor stimulation within the VTA was sufficient to increase impulsive behavior. We further evaluated the impact of ghrelin on dopamine-related gene expression and dopamine turnover in brain areas key in impulsive behavior control. This study provides the first demonstration that the stomach-produced hormone ghrelin increases impulsivity and also indicates that ghrelin can change two major components of impulsivity-motor and choice impulsivity.

Seeking behavior, place conditioning, and resistance to conditioned suppression of feeding in rats intermittently exposed to palatable food

Binge eating disorder is characterized by excessive consumption of highly palatable food within short periods of time accompanied by loss of control over eating. Extensive evidence provides support for the consideration of binge eating disorder as an addiction-like disorder. In this study, we wanted to determine whether rats undergoing an operant binge-like eating procedure could develop maladaptive forms of conditioned feeding behaviors. For this purpose, we trained male rats to self-administer either a sugary, highly palatable diet ("Palatable" rats) or a chow diet ("Chow" rats) for 1 hour a day. After escalation and stabilization of palatable food intake, we tested Chow and Palatable rats in (a) a conditioned place preference test, (b) a second-order schedule of reinforcement, (c) a cue-induced suppression of feeding test. In the conditioned place preference task, Palatable rats spent significantly more time in the compartment that was previously paired with the palatable food, compared to Chow controls. Furthermore, in the second-order schedule of reinforcement task, Palatable rats exhibited active lever responding 4- to 6-fold higher than Chow control rats. Finally, in the cue-induced suppression of feeding test, although Chow control subjects reduced responding by 32% in the presence of the conditioned punishment, Palatable rats persevered in responding despite the aversive cue. These results further characterize this animal model of binge-like eating and provide additional evidence for the addictive properties of highly palatable food.

Neonatal handling causes impulsive behavior and decreased pharmacological response to methylphenidate in male adult wistar rats

Neonatal handling has an impact on adult behavior of experimental animals and is associated with rapid and increased palatable food ingestion, impaired behavioral flexibility, and fearless behavior to novel environments. These symptoms are characteristic features of impulsive trait, being controlled by the medial prefrontal cortex (mPFC). Impulsive behavior is a key component of many psychiatric disorders such as attention deficit hyperactivity disorder (ADHD), manic behavior, and schizophrenia. Others have reported a methylphenidate (MPH)-induced enhancement of mPFC functioning and improvements in behavioral core symptoms of ADHD patients. The aims of the present study were: (i) to find in vivo evidence for an association between neonatal handling and the development of impulsive behavior in adult Wistar rats and (ii) to test whether neonatal handling could have an impact on monoamine levels in the mPFC and the pharmacological response to MPH in vivo. Therefore, experimental animals (litters) were classified as: "non-handled" and "handled" (10[Formula: see text]min/day, postnatal days 1-10). After puberty, they were exposed to either a larger and delayed or smaller and immediate reward (tolerance to delay of reward task). Acute MPH (3[Formula: see text]mg/Kg. i.p.) was used to suppress and/or regulate impulsive behavior. Our results show that only neonatally handled male adult Wistar rats exhibit impulsive behavior with no significant differences in monoamine levels in the medial prefrontal cortex, together with a decreased response to MPH. On this basis, we postulate that early life interventions may have long-term effects on inhibitory control mechanisms and affect the later response to pharmacological agents during adulthood.

Back by Popular Demand: A Narrative Review on the History of Food Addiction Research

In recent years, the concept of food addiction has gained more and more popularity. This approach acknowledges the apparent parallels between substance use disorders and overeating of highly palatable, high-caloric foods. Part of this discussion includes that "hyperpalatable" foods may have an addictive potential because of increased potency due to certain nutrients or additives. Although this idea seems to be relatively new, research on food addiction actually encompasses several decades, a fact that often remains unrecognized. Scientific use of the term addiction in reference to chocolate even dates back to the 19th century. In the 20th century, food addiction research underwent several paradigm shifts, which include changing foci on anorexia nervosa, bulimia nervosa, obesity, or binge eating disorder. Thus, the purpose of this review is to describe the history and state of the art of food addiction research and to demonstrate its development and refinement of definitions and methodologies.

Diet-induced obesity and diet-resistant rats: differences in the rewarding and anorectic effects of D-amphetamine

RATIONALE

Obesity is a leading public health problem worldwide. Multiple lines of evidence associate deficits in the brain reward circuit with obesity.

OBJECTIVE

Whether alterations in brain reward sensitivity precede or are a consequence of obesity is unknown. This study aimed to investigate both innate and obesity-induced differences in the sensitivity to the effects of an indirect dopaminergic agonist.

METHODS

Rats genetically prone to diet-induced obesity (DIO) and their counterpart diet-resistant (DR) were fed a chow diet, and their response to D-amphetamine on intracranial self-stimulation and food intake were assessed. The same variables were then evaluated after exposing the rats to a high-fat diet, after DIO rats selectively developed obesity. Finally, gene expression levels of dopamine receptors 1 and 2 as well as tyrosine hydroxylase were measured in reward-related brain regions.

RESULTS

In a pre-obesity state, DIO rats showed innate decreased sensitivity to the reward-enhancing and anorectic effects of D-amphetamine, as compared to DR rats. In a diet-induced obese state, the insensitivity to the potentiating effects of D-amphetamine on intracranial self-stimulation (ICSS) threshold persisted and became more marked in DIO rats, while the anorectic effects were comparable between genotypes. Finally, innate and obesity-induced differences in the gene expression of dopamine receptors were observed.

CONCLUSIONS

Our results demonstrate that brain reward deficits antedate the development of obesity and worsen after obesity is fully developed, suggesting that these alterations represent vulnerability factors for its development. Moreover, our data suggests that the reward-enhancing and anorectic effects of D-amphetamine are dissociable in the context of obesity.

Therapeutic Potential of 5-HT2C Receptor Agonists for Addictive Disorders

The neurotransmitter 5-hydroxytryptamine (5-HT; serotonin) has long been associated with the control of a variety of motivated behaviors, including feeding. Much of the evidence linking 5-HT and feeding behavior was obtained from studies of the effects of the 5-HT releaser (dex)fenfluramine in laboratory animals and humans. Recently, the selective 5-HT2C receptor agonist lorcaserin received FDA approval for the treatment of obesity. This review examines evidence to support the use of selective 5-HT2C receptor agonists as treatments for conditions beyond obesity, including substance abuse (particularly nicotine, psychostimulant, and alcohol dependence), obsessive compulsive, and excessive gambling disorder. Following a brief survey of the early literature supporting a role for 5-HT in modulating food and drug reinforcement, we propose that intrinsic differences between SSRI and serotonin releasers may have underestimated the value of serotonin-based pharmacotherapeutics to treat clinical forms of addictive behavior beyond obesity. We then highlight the critical involvement of the 5-HT2C receptor in mediating the effect of (dex)fenfluramine on feeding and body weight gain and the evidence that 5-HT2C receptor agonists reduce measures of drug reward and impulsivity. A recent report of lorcaserin efficacy in a smoking cessation trial further strengthens the idea that 5-HT2C receptor agonists may have potential as a treatment for addiction. This review was prepared as a contribution to the proceedings of the 11th International Society for Serotonin Research Meeting held in Hermanus, South Africa, July 9-12, 2014.

Pituitary Adenylate Cyclase-Activating Peptide in the Central Amygdala Causes Anorexia and Body Weight Loss via the Melanocortin and the TrkB Systems

Growing evidence suggests that the pituitary adenylate cyclase-activating polypeptide (PACAP)/PAC1 receptor system represents one of the main regulators of the behavioral, endocrine, and autonomic responses to stress. Although induction of anorexia is a well-documented effect of PACAP, the central sites underlying this phenomenon are poorly understood. The present studies addressed this question by examining the neuroanatomical, behavioral, and pharmacological mechanisms mediating the anorexia produced by PACAP in the central nucleus of the amygdala (CeA), a limbic structure implicated in the emotional components of ingestive behavior. Male rats were microinfused with PACAP (0-1 μg per rat) into the CeA and home-cage food intake, body weight change, microstructural analysis of food intake, and locomotor activity were assessed. Intra-CeA (but not intra-basolateral amygdala) PACAP dose-dependently induced anorexia and body weight loss without affecting locomotor activity. PACAP-treated rats ate smaller meals of normal duration, revealing that PACAP slowed feeding within meals by decreasing the regularity and maintenance of feeding from pellet-to-pellet; postprandial satiety was unaffected. Intra-CeA PACAP-induced anorexia was blocked by coinfusion of either the melanocortin receptor 3/4 antagonist SHU 9119 or the tyrosine kinase B (TrKB) inhibitor k-252a, but not the CRF receptor antagonist D-Phe-CRF(12-41). These results indicate that the CeA is one of the brain areas through which the PACAP system promotes anorexia and that PACAP preferentially lessens the maintenance of feeding in rats, effects opposite to those of palatable food. We also demonstrate that PACAP in the CeA exerts its anorectic effects via local melanocortin and the TrKB systems, and independently from CRF.