Long-term, calorie-restricted intake of a high-fat diet in rats reduces impulse control and ventral striatal D2receptor signalling - two markers of addiction vulnerability

Repeated binge-like eating episodes in female rats alter adenosine A2A and dopamine D2 receptor genes regulation in the brain reward system

OBJECTIVE

Binge-eating disorder is an eating disorder characterized by recurrent binge-eating episodes, during which individuals consume excessive amounts of highly palatable food (HPF) in a short time. This study investigates the intricate relationship between repeated binge-eating episode and the transcriptional regulation of two key genes, adenosine A2A receptor (A2AAR) and dopamine D2 receptor (D2R), in selected brain regions of rats.

METHOD

Binge-like eating behavior on HPF was induced through the combination of food restrictions and frustration stress (15 min exposure to HPF without access to it) in female rats, compared to control rats subjected to only restriction or only stress or none of these two conditions. After chronic binge-eating episodes, nucleic acids were extracted from different brain regions, and gene expression levels were assessed through real-time quantitative PCR. The methylation pattern on genes' promoters was investigated using pyrosequencing.

RESULTS

The analysis revealed A2AAR upregulation in the amygdala and in the ventral tegmental area (VTA), and D2R downregulation in the nucleus accumbens in binge-eating rats. Concurrently, site-specific DNA methylation alterations at gene promoters were identified in the VTA for A2AAR and in the amygdala and caudate putamen for D2R.

DISCUSSION

The alterations on A2AAR and D2R genes regulation highlight the significance of epigenetic mechanisms in the etiology of binge-eating behavior, and underscore the potential for targeted therapeutic interventions, to prevent the development of this maladaptive feeding behavior. These findings provide valuable insights for future research in the field of eating disorders.

PUBLIC SIGNIFICANCE

Using an animal model with face, construct, and predictive validity, in which cycles of food restriction and frustration stress evoke binge-eating behavior, we highlight the significance of epigenetic mechanisms on adenosine A2A receptor (A2AAR) and dopamine D2 receptor (D2R) genes regulation. They could represent new potential targets for the pharmacological management of eating disorders characterized by this maladaptive feeding behavior.

Abnormal epigenetic memory of mesenchymal stem and progenitor cells caused by fetal malnutrition induces hypertension and renal injury in adulthood

Fetal malnutrition has been reported to induce hypertension and renal injury in adulthood. We hypothesized that this hypertension and renal injury would be associated with abnormal epigenetic memory of stem and progenitor cells contributing to organization in offspring due to fetal malnutrition. We measured blood pressure (BP) for 60 weeks in offspring of pregnant rats fed a normal protein diet (Control), low-protein diet (LP), and LP plus taurine (LPT) in the fetal period. We used western blot analysis to evaluate the expression of αSMA and renin in CD44-positive renal mesenchymal stem cells (MSCs) during differentiation by TGF-β1. We measured kidney label-retaining cells (LRCs) at 11 weeks of age and formation of endothelial progenitor cells (EPCs) at 60 weeks of age from the offspring with fetal malnutrition. Epigenetics of the renal MSCs at 14 weeks were investigated by ATAC-sequence and RNA-sequence analyses. BP was significantly higher in LP than that in Control and LPT after 45-60 weeks of age. Numbers of LRCs and EPC colonies were significantly lower in LP than in Control. Renal MSCs from LP already showed expression of h-caldesmon, αSMA, LXRα, and renin before their differentiation. Epigenetic analyses identified PAR2, Chac1, and Tspan6 genes in the abnormal differentiation of renal MSCs. These findings suggested that epigenetic abnormalities of stem and progenitor cell memory cause hypertension and renal injury that appear in adulthood of offspring with fetal malnutrition.

Operant Training for Highly Palatable Food Alters Translating Messenger RNA in Nucleus Accumbens D2 Neurons and Reveals a Modulatory Role of Ncdn

BACKGROUND

Highly palatable food triggers behavioral responses including strong motivation. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the long-lasting effects of highly palatable food on feeding behavior are poorly understood.

METHODS

We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral responses and dendritic spine modifications in the NAc. We compared the translating messenger RNA in NAc neurons identified by the type of dopamine receptors they express, depending on the kind of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn.

RESULTS

Operant conditioning for highly palatable food increased motivation for food even in well-fed mice. In wild-type mice, free choice between regular and highly palatable food increased weight compared with access to regular food only. Highly palatable food increased spine density in the NAc. In animals trained for highly palatable food, translating messenger RNAs were modified in NAc neurons expressing dopamine D2 receptors, mostly corresponding to striatal projection neurons, but not in neurons expressing D1 receptors. Knockout of Ncdn, an abundant downregulated gene, opposed the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting that downregulation may be a compensatory mechanism.

CONCLUSIONS

Our results emphasize the importance of messenger RNA alterations in D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

How are overweight and obesity associated with reinforcement learning deficits? A systematic review

Reinforcement learning (RL) refers to the ability to learn stimulus-response or response-outcome associations relevant to the acquisition of behavioral repertoire and adaptation to the environment. Research data from correlational and case-control studies have shown that obesity is associated with impairments in RL. The aim of the present study was to systematically review how obesity and overweight are associated with RL performance. More specifically, the relationship between high body mass index (BMI) and task performance was explored through the analysis of specific RL processes associated with different physiological, computational, and behavioral manifestations. Our systematic analyses indicate that obesity might be associated with impairments in the use of aversive outcomes to change ongoing behavior, as revealed by results involving instrumental negative reinforcement and extinction/reversal learning, but further research needs to be conducted to confirm this association. Hypotheses regarding how obesity might be associated with altered RL were discussed.

Involvement of the Receptor for Advanced Glycation End Products (RAGE) in high fat-high sugar diet-induced anhedonia in rats

Clinical and basic science investigation indicates a link between insulin resistance and anhedonia. Previous results of this laboratory point to impaired nucleus accumbens (NAc) insulin signaling as an underpinning of diet-induced anhedonia, based on use of a glucose lick microstructure assay. The present study evaluated whether advanced glycation end products (AGEs) and their receptor (RAGE), known to mediate obesogenic diet-induced inflammation and pathological metabolic conditions, are involved in this behavioral change. Six weeks maintenance of male and female rats on a high fat-high sugar liquid diet (chocolate Ensure) increased body weight gain, and markedly increased circulating insulin and leptin, but induced anhedonia (decreased first minute lick rate and lick burst size) in males only. In these subjects, anhedonia correlated with plasma concentrations of insulin. Although the diet did not alter plasma or NAc AGEs, or the expression of RAGE in the NAc, marginally significant correlations were seen between anhedonia and plasma content of several AGEs and NAc RAGE. Importantly, a small molecule RAGE antagonist, RAGE229, administered twice daily by oral gavage, prevented diet-induced anhedonia. This beneficial effect was associated with improved adipose function, reflected in the adiponectin/leptin ratio, and increased pCREB/total CREB in the NAc, and a shift in the pCREB correlation with pThr34-DARPP-32 from near-zero to strongly positive, such that both phospho-proteins correlated with the rescued hedonic response. This set of findings suggests that the receptor/signaling pathway and cell type underlying the RAGE229-mediated increase in pCREB may mediate anhedonia and its prevention. The possible role of adipose tissue as a locus of diet-induced RAGE signaling, and source of circulating factors that target NAc to modify hedonic reactivity are discussed.

Obesity-mediated Lipoinflammation Modulates Food Reward Responses

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

The gut-brain axis mediates bacterial driven modulation of reward signaling

OBJECTIVE

Our goal is to investigate if microbiota composition modulates reward signaling and assess the role of the vagus in mediating microbiota to brain communication.

METHODS

Male germ-free Fisher rats were colonized with gastrointestinal contents from chow (low fat (LF) ConvLF) or HF (ConvHF) fed rats.

RESULTS

Following colonization, ConvHF rats consumed significantly more food than ConvLF animals. ConvHF rats displayed lower feeding-induced extracellular DOPAC levels (a metabolite of dopamine) in the Nucleus Accumbens (NAc) as well as reduced motivation for HF foods compared to ConvLF rats. Dopamine receptor 2 (DDR2) expression levels in the NAc were also significantly lower in ConvHF animals. Similar deficits were observed in conventionally raised HF fed rats, showing that diet-driven alteration in reward can be initiated via microbiota. Selective gut to brain deafferentation restored DOPAC levels, DRD2 expression, and motivational drive in ConvHF rats.

CONCLUSIONS

We concluded from these data that a HF-type microbiota is sufficient to alter appetitive feeding behavior and that bacteria to reward communication is mediated by the vagus nerve.

Self-reported intake of high-fat and high-sugar diet is not associated with cognitive stability and flexibility in healthy men

Animal studies indicate that a high-fat/high-sugar diet (HFS) can change dopamine signal transmission in the brain, which could promote maladaptive behavior and decision-making. Such diet-induced changes may also explain observed alterations in the dopamine system in human obesity. Genetic variants that modulate dopamine transmission have been proposed to render some individuals more prone to potential effects of HFS. The objective of this study was to investigate the association of HFS with dopamine-dependent cognition in humans and how genetic variations might modulate this potential association. Using a questionnaire assessing the self-reported consumption of high-fat/high-sugar foods, we investigated the association with diet by recruiting healthy young men that fall into the lower or upper end of that questionnaire (low fat/sugar group: LFS, n = 45; high fat/sugar group: HFS, n = 41) and explored the interaction of fat and sugar consumption with COMT Val¹⁵⁸Met and Taq1A genotype. During functional magnetic resonance imaging (fMRI) scanning, male participants performed a working memory (WM) task that probes distractor-resistance and updating of WM representations. Logistic and linear regression models revealed no significant difference in WM performance between the two diet groups, nor an interaction with COMT Val¹⁵⁸Met or Taq1A genotype. Neural activation in task-related brain areas also did not differ between diet groups. Independent of diet group, higher BMI was associated with lower overall accuracy on the WM task. This cross-sectional study does not provide evidence for diet-related differences in WM stability and flexibility in men, nor for a predisposition of COMT Val¹⁵⁸Met or Taq1A genotype to the hypothesized detrimental effects of an HFS diet. Previously reported associations of BMI with WM seem to be independent of HFS intake in our male study sample.

Habitual daily intake of a sweet and fatty snack modulates reward processing in humans

Western diets rich in fat and sugar promote excess calorie intake and weight gain; however, the underlying mechanisms are unclear. Despite a well-documented association between obesity and altered brain dopamine function, it remains elusive whether these alterations are (1) pre-existing, increasing the individual susceptibility to weight gain, (2) secondary to obesity, or (3) directly attributable to repeated exposure to western diet. To close this gap, we performed a randomized, controlled study (NCT05574660) with normal-weight participants exposed to a high-fat/high-sugar snack or a low-fat/low-sugar snack for 8 weeks in addition to their regular diet. The high-fat/high-sugar intervention decreased the preference for low-fat food while increasing brain response to food and associative learning independent of food cues or reward. These alterations were independent of changes in body weight and metabolic parameters, indicating a direct effect of high-fat, high-sugar foods on neurobehavioral adaptations that may increase the risk for overeating and weight gain.

Nucleus Accumbens D1 Receptor-Expressing Spiny Projection Neurons Control Food Motivation and Obesity

BACKGROUND

Obesity is a chronic relapsing disorder that is caused by an excess of caloric intake relative to energy expenditure. There is growing recognition that food motivation is altered in people with obesity. However, it remains unclear how brain circuits that control food motivation are altered in obese animals.

METHODS

Using a novel behavioral assay that quantifies work during food seeking, in vivo and ex vivo cell-specific recordings, and a synaptic blocking technique, we tested the hypothesis that activity of circuits promoting appetitive behavior in the core of the nucleus accumbens (NAc) is enhanced in the obese state, particularly during food seeking.

RESULTS

We first confirmed that mice made obese with ad libitum exposure to a high fat diet work harder than lean mice to obtain food, consistent with an increase in food motivation in obese mice. We observed greater activation of D₁ receptor-expressing NAc spiny projection neurons (NAc D1^SPNs) during food seeking in obese mice relative to lean mice. This enhanced activity was not observed in D₂ receptor-expressing neurons (D2^SPNs). Consistent with these in vivo findings, both intrinsic excitability and excitatory drive onto D1^SPNs were enhanced in obese mice relative to lean mice, and these measures were selective for D1^SPNs. Finally, blocking synaptic transmission from D1^SPNs, but not D2^SPNs, in the NAc core decreased physical work during food seeking and, critically, attenuated high fat diet-induced weight gain.

CONCLUSIONS

These experiments demonstrate the necessity of NAc core D1^SPNs in food motivation and the development of diet-induced obesity, establishing these neurons as a potential therapeutic target for preventing obesity.

Operant training for highly palatable food alters translating mRNA in nucleus accumbens D2 neurons and reveals a modulatory role of Neurochondrin

BACKGROUND

Highly palatable food triggers behavioral alterations reminiscent of those induced by addictive drugs. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the effects of highly palatable food on feeding behavior are poorly understood.

METHODS

We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral effects and dendritic spine modifications in the NAc. We compared the translating mRNA in NAc neurons identified by the type of dopamine receptors they express, depending on the type of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn (Neurochondrin).

RESULTS

Operant conditioning for highly palatable food increases motivation for food even in well-fed mice. In control mice, free access to regular or highly palatable food results in increased weight as compared to regular food only. Highly palatable food increases spine density in the NAc. In animals trained for highly palatable food, translating mRNAs are modified in NAc dopamine D2-receptor-expressing neurons, mostly corresponding to striatal projection neurons, but not in those expressing D1-receptors. Knock-out of Ncdn, an abundant down-regulated gene, opposes the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting down-regulation may be a compensatory mechanism.

CONCLUSIONS

Our results emphasize the importance of mRNA alterations D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

Neurophysiology, Neuropsychology, Epilepsy, 2022: Hills We Have Climbed and the Hills Ahead. Cognition and Sensory Systems in Healthy and Diseased Subjects

This article summarizes selected presentations from a session titled "Cognition and Sensory Systems in Healthy and Diseased Subjects", held to highlight and honor the work of Dr. Marilyn Jones-Gotman. The session was part of a two-day symposium, "Neurophysiology, Neuropsychology, Epilepsy, 2022: Hills We Have Climbed and the Hills Ahead". The session presented research on epilepsy and sensory systems by colleagues and former trainees of Dr. Jones-Gotman. The extended summaries provide an overview of historical and current work in the neuropsychology of epilepsy, neuropsychological and neuroimaging approaches to understanding brain organization, sex differences in brain mechanisms underlying neurological disorders, dietary influences on brain function and cognition, and expertise in olfactory training and language experiences and their implications for brain organization and structure.

Calorie restriction, but not Roux-en-Y gastric bypass surgery, increases [3 H] PK11195 binding in a rat model of obesity

Roux-en-Y gastric bypass surgery (RYGB) remains an effective weight-loss method used to treat obesity. While it is successful in combating obesity, there are many lingering questions related to the changes in the brain following RYGB surgery, one of them being its effects on neuroinflammation. While it is known that chronic high-fat diet (HFD) contributes to obesity and neuroinflammation, it remains to be understood whether bariatric surgery can ameliorate diet-induced inflammatory responses. To examine this, rats were assigned to either a normal diet (ND) or a HFD for 8 weeks. Rats fed a HFD were split into the following groups: sham surgery with ad libitum access to HFD (sham-HF); sham surgery with calorie-restricted HFD (sham-FR); RYGB surgery with ad libitum access to HFD (RYGB). Following sham or RYGB surgeries, rats were maintained on their diets for 9 weeks before being euthanized. [³ H] PK11195 autoradiography was then performed on fresh-frozen brain tissue in order to measure activated microglia. Sham-FR rats showed increased [³ H] PK11195 binding in the amygdala (63%), perirhinal (60%), and ectorhinal cortex (53%) compared with the ND rats. Obese rats who had the RYGB surgery did not show this increased inflammatory effect. Since the sham-FR and RYGB rats were fed the same amount of HFD, the surgery itself seems responsible for this attenuation in [³ H] PK11195 binding. We speculate that calorie restriction following obese conditions may be seen as a stressor and contribute to inflammation in the brain. Further research is needed to verify this mechanism.

Implication of saturated fats in the aetiology of childhood attention deficit/hyperactivity disorder - A narrative review

Attention Deficit/Hyperactivity Disorder (ADHD) is the most common mental health disorder in the paediatric population. ADHD is highly comorbid with obesity, and has also been associated with poor dietary patterns such as increased consumption of refined carbohydrates and saturated fats. Although ADHD in children was associated with high consumption of saturated fats, so far there has been no evidence-based attempt to integrate dietary strategies controlling for intake of saturated fats into the etiological framework of the disorder. Evidence from human studies and animal models has shown that diets high in saturated fats are detrimental for the development of dopaminergic neurocircuitries, synthesis of neurofactors (e.g. brain derived neurotrophic factor) and may promote brain inflammatory processes. Notably, animal models provide evidence that early life consumption of a high saturated fats diet may impair the development of central dopamine pathways. In the present paper, we review the impact of high saturated fats diets on neurobiological processes in human studies and animal models, and how these associations may be relevant to the neuropathophysiology of ADHD in children. The validation of this relationship and its underlying mechanisms through future investigative studies could have implications for the prevention or exacerbation of ADHD symptoms, improve the understanding of the pathogenesis of the disorder, and help design future dietary studies in patients with ADHD.

Association of Impulsivity With Food, Nutrients, and Fitness in a Longitudinal Birth Cohort Study

BACKGROUND

Impulsivity is a psychiatric vulnerability factor strongly associated with substance abuse but also with unhealthy diet. Whether these associations extend to specific nutrients is largely unknown. Therefore, we investigated the longitudinal association between diet, cardiorespiratory fitness, and 2 impulsivity dimensions in a representative sample of south Estonian adolescents and young adults. Impulsivity and dietary intake were measured 3 times in 2 birth cohorts at regular intervals in individuals aged 15 to 33 years.

METHODS

The sample included 2 birth cohorts of the longitudinal Estonian Children Personality Behaviour and Health Study. The analytic sample size consisted of 2883 observations (56.4% females). The primary outcomes were adaptive and maladaptive impulsivity scores measured by an original 24-item Likert-type questionnaire. Impulsivity scores were predicted from the food diaries data converted into nutrient categories. A linear mixed-effects approach was used to model the time dependence between observations.

RESULTS

Lower maladaptive impulsivity was associated with higher cardiorespiratory fitness (β = -.07; 95% CI = -0.12; -0.03). Higher maladaptive impulsivity was associated with lower dietary intake of zinc (β = -.10; -0.15; -0.06) and vegetables (β = -.04; -0.07; -0.01) and higher intake of sodium (β = .06; 0.02; 0.10). Vitamin B6 was positively associated with adaptive impulsivity (β = .04; 0.01; 0.07). Additionally, some of the adjusted models showed significant but weak associations with selenium, alcohol, fish, and cereal products.

CONCLUSIONS

Food choice may affect the neurochemistry and therefore regulate the manifestations of impulsivity. We identified associations between several (micro)nutrients and maladaptive impulsivity.

Dietary adaptation for weight loss maintenance at Yale (DAWLY): Protocol and predictions for a randomized controlled trial

Background

Current therapies for obesity treatment are effective at producing short-term weight loss, but weight loss maintenance remains a significant challenge. Here we investigate the impact of pre-intervention dietary fat intake on the efficacy of a dietary supplement to support weight loss maintenance. Preclinical work demonstrates that a vagal afferent pathway critical for sensing dietary lipids is blunted by a high-fat diet (HFD), resulting in a reduced preference for a low-fat emulsion and severe blunting of the dopamine (DA) response to the gastric infusion of lipids. Infusion of the gut lipid messenger oleoylethanolamide (OEA), which is also depleted by HFD, immediately reverses this DA blunting and restores preference for the low-fat emulsion. Studies of OEA supplementation for weight loss in humans have had limited success. Given the strong effect of HFD on this pathway, we designed a study to test whether the efficacy of OEA as a weight loss treatment is related to pre-intervention habitual intake of dietary fat.

Methods/Design

We employed a randomized, double-blind, placebo-controlled trial in which 100 adults with overweight/obesity (OW/OB) were randomized to receive either OEA or placebo daily for 16 months. Following a baseline evaluation of diet, metabolic health, adiposity, and brain response to a palatable an energy dense food, participants in both groups underwent a 4-month behavioral weight loss intervention (LEARN^®) followed by a 1-year maintenance period. The study aims are to (1) determine if pre-intervention dietary fat intake moderates the ability of OEA to improve weight loss and weight loss maintenance after a gold standard behavioral weight loss treatment; (2) identify biomarkers that predict outcome and optimize a stratification strategy; and (3) test a model underlying OEA's effectiveness.

Discussion

Focusing on interventions that target the gut-brain axis is supported by mounting evidence for the role of gut-brain signaling in food choice and the modulation of this circuit by diet. If successful, this work will provide support for targeting the gut-brain pathway for weight loss maintenance using a precision medicine approach that is easy and inexpensive to implement.

Clinical Trial Registration

[www.ClinicalTrials.gov], identifier [NCT04614233].

Differential effects of lipopolysaccharide on cognition, corticosterone and cytokines in socially-housed vs isolated male rats

Social isolation is an established risk factor for mental illness and impaired immune function. Evidence suggests that neuroinflammatory processes contribute to mental illness, possibly via cytokine-induced modulation of neural activity. We examined the effects of lipopolysaccharide (LPS) administration and social home cage environment on cognitive performance in the 5-Choice Serial Reaction Time Task (5CSRTT), and their effects on corticosterone and cytokines in serum and brain tissue. Male Long-Evans rats were reared in pairs or in isolation before training on the 5CSRTT. The effects of saline and LPS (150 µg/kg i.p.) administration on sickness behaviour and task performance were then assessed. LPS-induced sickness behaviour was augmented in socially-isolated rats, translating to increased omissions and slower response times in the 5CSRTT. Both social isolation and LPS administration reduced impulsive responding, while discriminative accuracy remained unaffected. With the exception of reduced impulsivity in isolated rats, these effects were not observed following a second administration of LPS, revealing behavioural tolerance to repeated LPS injections. In a separate cohort of animals, social isolation potentiated the ability of LPS to increase serum corticosterone and IL-6, which corresponded to increased IL-6 in the orbitofrontal and medial prefrontal cortices and the nucleus accumbens. Basal IL-4 levels in the nucleus accumbens were reduced in socially-isolated rats. These findings are consistent with the adaptive response of reduced motivational drive following immune challenge, and identify social isolation as an exacerbating factor. Enhanced IL-6 signalling may play a role in mediating the potentiated behavioural response to LPS administration in isolated animals.

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

A Neuroeconomics Approach to Obesity

Obesity is a heterogeneous condition that is affected by physiological, behavioral, and environmental factors. Value-based decision making is a useful framework for integrating these factors at the individual level. The disciplines of behavioral economics and reinforcement learning provide tools for identifying specific cognitive and motivational processes that may contribute to the development and maintenance of obesity. Neuroeconomics complements these disciplines by studying the neural mechanisms underlying these processes. We surveyed recent literature on individual decision characteristics that are most frequently implicated in obesity: discounting the value of future outcomes, attitudes toward uncertainty, and learning from rewards and punishments. Our survey highlighted both consistent and inconsistent behavioral findings. These findings underscore the need to examine multiple processes within individuals to identify unique behavioral profiles associated with obesity. Such individual characterization will inform future studies on the neurobiology of obesity as well as the design of effective interventions that are individually tailored.

Obesity - A Matter of Motivation?

Excessive food intake and reduced physical activity have long been established as primary causes of obesity. However, the underlying mechanisms causing this unhealthy behavior characterized by heightened motivation for food but not for physical effort are unclear. Despite the common unjustified stigmatization that obesity is a result of laziness and lack of discipline, it is becoming increasingly clear that high-fat diet feeding and obesity cause alterations in brain circuits that are critical for the control of motivational behavior.

In this mini-review, we provide a comprehensive overview of incentive motivation, its neural encoding in the dopaminergic mesolimbic system as well as its metabolic modulation with a focus on derangements of incentive motivation in obesity. We further discuss the emerging field of metabolic interventions to counteract motivational deficits and their potential clinical implications.

Parsing the link between reinforcement sensitivity theory and eating behavior: A systematic review

The Reinforcement Sensitivity Theory (RST) is a widely studied psychobiological model of personality. RST factors seem to influence eating behavior, but how these personality traits are associated with distinct features of eating behavior is still unclear. In the present systematic review, we analyzed the relationship between RST personality factors and eating behavior using a parsing approach in which BMI-related results, self-reported results, and behavioral results were distinguished. Our analysis revealed that reward and punishment sensitivity seem to correlate and influence distinct features of eating behavior. The association between BMI and RST factors was uncertain, but nonlinear associations between reward sensitivity and weight need further testing. Reward sensitivity was linked to most eating behavior phenotypes (e.g., emotional eating and restrained eating), but only punishment sensitivity was primarily correlated with eating pathology. Reward sensitivity was the main factor linked with reactivity to food stimuli on many behavioral measures. The neurobiological personality factors of RST offer parsimonious concepts to understand eating behavior outcomes and the differential relationships observed are useful to translational research.

Dietary lipids as regulators of reward processes: multimodal integration matters

The abundance of energy-dense and palatable diets in the modern food environment tightly contributes to the obesity pandemic. The reward circuit participates to the regulation of body homeostasis by integrating energy-related signals with neural substrates encoding cognitive and motivational components of feeding behaviors. Obesity and lipid-rich diets alter dopamine (DA) transmission leading to reward dysfunctions and food overconsumption. Recent reports indicate that dietary lipids can act, directly and indirectly, as functional modulators of the DA circuit. This raises the possibility that nutritional or genetic conditions affecting 'lipid sensing' mechanisms might lead to maladaptations of the DA system. Here, we discuss the most recent findings connecting dietary lipid sensing with DA signaling and its multimodal influence on circuits regulating food-reward processes.

Obesity and dietary fat influence dopamine neurotransmission: exploring the convergence of metabolic state, physiological stress, and inflammation on dopaminergic control of food intake

The aim of this review is to explore how metabolic changes induced by diets high in saturated fat (HFD) affect nucleus accumbens (NAc) dopamine neurotransmission and food intake, and to explore how stress and inflammation influence this process. Recent evidence linked diet-induced obesity and HFD with reduced dopamine release and reuptake. Altered dopamine neurotransmission could disrupt satiety circuits between NAc dopamine terminals and projections to the hypothalamus. The NAc directs learning and motivated behaviours based on homeostatic needs and psychological states. Therefore, impaired dopaminergic responses to palatable food could contribute to weight gain by disrupting responses to food cues or stress, which impacts type and quantity of food consumed. Specifically, saturated fat promotes neuronal resistance to anorectic hormones and activation of immune cells that release proinflammatory cytokines. Insulin has been shown to regulate dopamine neurotransmission by enhancing satiety, but less is known about effects of diet-induced stress. Therefore, changes to dopamine signalling due to HFD warrant further examination to characterise crosstalk of cytokines with endocrine and neurotransmitter signals. A HFD promotes a proinflammatory environment that may disrupt neuronal endocrine function and dopamine signalling that could be exacerbated by the hypothalamic-pituitary-adrenal and κ-opioid receptor stress systems. Together, these adaptive changes may dysregulate eating by changing NAc dopamine during hedonic versus homeostatic food intake. This could drive palatable food cravings during energy restriction and hinder weight loss. Understanding links between HFD and dopamine neurotransmission will inform treatment strategies for diet-induced obesity and identify molecular candidates for targeted therapeutics.

Clusterin overexpression as a potential neuroprotective response to the pathological effects of high fat dieting on the brain reward system

High-fat diets (HFDs) can lead to pathological changes in the brain underlying several behavioral disturbances (e.g., reward deficiency). To further increase our knowledge of these associations, we studied the sucrose reward and the brain expression of clusterin, a protein that is overexpressed after several kind of brain damaging conditions. C57BL/6J male mice were differentially fed on an HFD or standard chow for 41 days and underwent 11 sucrose place conditioning sessions followed by 4 extinction sessions to monitor the effects of HFD on sucrose reward by means of free choice tests. We quantified clusterin expression by immunochemistry in the nucleus accumbens, dorsal striatum and cingulate cortex. HFD tended to provoke a transient potentiation in the acquisition of sucrose-conditioned place preference, but this effect was followed by a much more consistent reduction in sucrose preference, which spontaneously disappeared after 31 days of an HFD with no need for extinction learning. The HFD mice showed higher clusterin expression in the nucleus accumbens but not in the other brain areas studied. The results confirm that HFDs strongly influence the rewarding properties of palatable foods and suggest a direct connection with neurotoxic alterations in the brain reward system tagged by clusterin overexpression.

Midbrain and Lateral Nucleus Accumbens Dopamine Depletion Affects Free-choice High-fat high-sugar Diet Preference in Male Rats

Dopamine influences food intake behavior. Reciprocally, food intake, especially of palatable dietary items, can modulate dopamine-related brain circuitries. Among these reciprocal impacts, it has been observed that an increased intake of dietary fat results in blunted dopamine signaling and, to compensate this lowered dopamine function, caloric intake may subsequently increase. To determine how dopamine regulates food preference we performed 6-hydroxydopamine (6-OHDA) lesions, depleting dopamine in specific brain regions in male Sprague Dawley rats. Food preference was assessed by providing the rats with free choice access to control diet, fat, 20% sucrose and tap water. Rats with midbrain lesions targeting the substantia nigra (which is also a model of Parkinson's disease) consumed fewer calories, as reflected by a decrease in control diet intake, but they surprisingly displayed an increase in fat intake, without change in the sucrose solution intake compared to sham animals. To determine which of the midbrain dopamine projections may contribute to this effect, we next compared the impact of 6-OHDA lesions of terminal fields, targeting the dorsal striatum, the lateral nucleus accumbens and the medial nucleus accumbens. We found that 6-OHDA lesion of the lateral nucleus accumbens, but not of the dorsal striatum or the medial nucleus accumbens, led to increased fat intake. These findings indicate a role for lateral nucleus accumbens dopamine in regulating food preference, in particular the intake of fat.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Preliminary evidence for an association between intake of high-fat high-sugar diet, variations in peripheral dopamine precursor availability and dopamine-dependent cognition in humans

Obesity is associated with alterations in dopaminergic transmission and cognitive function. Rodent studies suggest that diets rich in saturated fat and refined sugars (HFS), as opposed to diets diets low in saturated fat and refined sugars (LFS), change the dopamine system independent of excessive body weight. However, the impact of HFS on the human brain has not been investigated. Here, we compared the effect of dietary dopamine depletion on dopamine-dependent cognitive task performance between two groups differing in habitual intake of dietary fat and sugar. Specifically, we used a double-blind within-subject cross-over design to compare the effect of acute phenylalanine/tyrosine depletion on a reinforcement learning and a working memory task, in two groups that are on opposite ends of the spectrum of self-reported HFS intake (low vs high intake: LFS vs HFS group). We tested 31 healthy young women matched for body mass index (mostly normal weight to overweight) and IQ. Depletion of peripheral precursors of dopamine reduced the working memory specific performance on the operation span task in the LFS, but not in the HFS group (P = 0.016). Learning from positive- and negative-reinforcement (probabilistic selection task) was increased in both diet groups after dopamine depletion (P = 0.049). As a secondary exploratory research question, we measured peripheral dopamine precursor availability (pDAP) at baseline as an estimate for central dopamine levels. The HFS group had a significantly higher pDAP at baseline compared to the LFS group (P = 0.025). Our data provide the first evidence indicating that the intake of HFS is associated with changes in dopamine precursor availability, which is suggestive of changes in central dopamine levels in humans. The observed associations are present in a sample of normal to overweight participants (ie, in the absence of obesity), suggesting that the consumption of a HFS might already be associated with altered behaviours. Alternatively, the effects of HFS diet and obesity might be independent.

Decreased motor impulsivity following chronic lithium treatment in male rats is associated with reduced levels of pro-inflammatory cytokines in the orbitofrontal cortex

Lithium's efficacy in reducing both symptom severity in bipolar disorder (BD) and suicide risk across clinical populations may reflect its ability to reduce impulsivity. Changes in immune markers are associated with BD and suicidality yet their exact role in symptom expression remains unknown. Evidence also suggests that lithium may decrease levels of pro-inflammatory cytokines in the periphery and central nervous system, and that such changes are related to its therapeutic efficacy. However, issues of cause and effect are hard to infer from clinical data alone. Here, we investigated the effects of chronic dietary lithium treatment on rats' performance of the 5-Choice Serial Reaction Time Task (5CSRTT), a well-validated operant behavioural task measuring aspects of impulsivity, attention and motivation. Male Long-Evans rats received a diet supplemented with 0.3% LiCl (n = 13), or the equivalent control diet (n = 16), during behavioural testing. Blood and brain tissue samples were assayed for a wide range of cytokines once any changes in impulsivity became significant. After 12 weeks, chronic lithium treatment reduced levels of motor impulsivity, as indexed by premature responses in the 5CSRTT; measures of sustained attention and motivation were unaffected. Plasma levels of IL-1β, IL-10 and RANTES (CCL-5) were reduced in lithium-treated rats at this time point. IL-1β, IL-6 and RANTES were also reduced selectively within the orbitofrontal cortex of lithium-treated rats, whereas cytokine levels in the medial prefrontal cortex and nucleus accumbens were comparable with control subjects. These results are consistent with the hypothesis that lithium may improve impulse control deficits in clinical populations by minimising the effects of pro-inflammatory signalling on neuronal activity, particularly within the orbitofrontal cortex.

Latent classes of lifetime drug use disorder in national epidemiological survey on alcohol and related conditions - III

AIMS/BACKGROUND

People diagnosed with multiple drug use disorders are high-risk subpopulations, but changes in diagnostic classification and drug use prevalence mean patterning of drug use disorders has changed in the past decade. We analyzed comorbidity patterns of lifetime drug use disorder in a general population sample.

DESIGN

Using latent class analysis, we derived lifetime drug use disorder classes based on dichotomous indicators of sedative, cannabis, opioid, cocaine, stimulant, hallucinogen, inhalant/solvent, club drug, heroin, and other drug use disorders in the National Epidemiologic Survey on Alcohol and Related Conditions-III (n = 36,309). Multinomial models assessed associations between sociodemographic and clinical correlates and latent class status.

RESULTS

Four latent classes of lifetime substance use disorder were identified: A very low risk class, a class with high opioid, sedative and heroin comorbidity, a class based on cocaine and stimulant comorbidity, and class with high likelihood of multiple lifetime drug use disorders. All higher risk classes were associated with higher risk of lifetime personality disorder and mood disorder. Conduct disorder was also associated with higher risk, but level of risk varied by class. Opioid and sedative class was associated with higher odds of lifetime eating disorder diagnosis.

CONCLUSIONS

Comorbidity of drug use disorders is associated with a range of lifetime mental health disorder diagnoses. Unlike previous research, we did not identify a cannabis use class, possibly due to changes in diagnostic criteria and cannabis prevalence rates.

Circulating Triglycerides Gate Dopamine-Associated Behaviors through DRD2-Expressing Neurons

Energy-dense food alters dopaminergic (DA) transmission in the mesocorticolimbic (MCL) system and can promote reward dysfunctions, compulsive feeding, and weight gain. Yet the mechanisms by which nutrients influence the MCL circuitry remain elusive. Here, we show that nutritional triglycerides (TGs), a conserved post-prandial metabolic signature among mammals, can be metabolized within the MCL system and modulate DA-associated behaviors by gating the activity of dopamine receptor subtype 2 (DRD2)-expressing neurons through a mechanism that involves the action of the lipoprotein lipase (LPL). Further, we show that in humans, post-prandial TG excursions modulate brain responses to food cues in individuals carrying a genetic risk for reduced DRD2 signaling. Collectively, these findings unveil a novel mechanism by which dietary TGs directly alter signaling in the reward circuit to regulate behavior, thereby providing a new mechanistic basis by which energy-rich diets may lead to (mal)adaptations in DA signaling that underlie reward deficit and compulsive behavior.

Food Intake Recruits Orosensory and Post-ingestive Dopaminergic Circuits to Affect Eating Desire in Humans

Pleasant taste and nutritional value guide food selection behavior. Here, orosensory features of food may be secondary to its nutritional value in underlying reinforcement, but it is unclear how the brain encodes the reward value of food. Orosensory and peripheral physiological signals may act together on dopaminergic circuits to drive food intake. We combined fMRI and a novel [11C]raclopride PET method to assess systems-level activation and dopamine release in response to palatable food intake in humans. We identified immediate orosensory and delayed post-ingestive dopamine release. Both responses recruit segregated brain regions: specialized integrative pathways and higher cognitive centers. Furthermore, we identified brain areas where dopamine release reflected the subjective desire to eat. Immediate dopamine release in these wanting-related regions was inversely correlated with, and presumably inhibited, post-ingestive release in the dorsal striatum. Our results highlight the role of brain and periphery in interacting to reinforce food intake in humans.

Uncontrolled eating: a unifying heritable trait linked with obesity, overeating, personality and the brain

Many eating-related psychological constructs have been proposed to explain obesity and overeating. However, these constructs, including food addiction, disinhibition, hedonic hunger, emotional eating, binge eating and the like all have similar definitions, emphasizing loss of control over intake. As questionnaires measuring the constructs correlate strongly (r > 0.5) with each other, we propose that these constructs should be reconsidered to be part of a single broad phenotype: uncontrolled eating. Such an approach enables reviewing and meta-analysing evidence obtained with each individual questionnaire. Here, we describe robust associations between uncontrolled eating, body mass index (BMI), food intake, personality traits and brain systems. Reviewing cross-sectional and longitudinal data, we show that uncontrolled eating is phenotypically and genetically intertwined with BMI and food intake. We also review evidence on how three psychological constructs are linked with uncontrolled eating: lower cognitive control, higher negative affect and a curvilinear association with reward sensitivity. Uncontrolled eating mediates all three constructs' associations with BMI and food intake. Finally, we review and meta-analyse brain systems possibly subserving uncontrolled eating: namely, (i) the dopamine mesolimbic circuit associated with reward sensitivity, (ii) frontal cognitive networks sustaining dietary self-control and (iii) the hypothalamus-pituitary-adrenal axis, amygdala and hippocampus supporting stress reactivity. While there are limits to the explanatory and predictive power of the uncontrolled eating phenotype, we conclude that treating different eating-related constructs as a single concept, uncontrolled eating, enables drawing robust conclusions on the relationship between food intake and BMI, psychological variables and brain structure and function.

Assessment of metabolic and hormonal profiles and striatal dopamine D2 receptor expression following continuous or scheduled high-fat or high-sucrose diet in rats

BACKGROUND

Obesity has reached global epidemic proportions and is associated with serious medical comorbidities and economic consequences. In this preclinical study, we characterized how the palatable diet changed food intake pattern, caloric intake, metabolic profile and hormone levels. We also evaluated the expression of dopamine D2 receptors in the rat striatum.

METHODS

Male Wistar rats were fed with either high-fat or high-sucrose diet for 5 weeks according to different feeding regimes: ad libitum access or scheduled for a 2-h period each day without caloric restriction during the remainder of the day.

RESULTS

Both diets resulted in an enhancement in caloric intake and total body weight. Post-meal data showed that high-fat diet increased cholesterol, triglycerides and glucose concentrations. Animals fed on high sucrose diet were only hyperglycemic. High-fat diet schedules resulted in the enhancement of leptin concentrations, while increases in blood levels of ghrelin were noted after intermitted high-fat or continuous high-sucrose diet. Finally, we report that only ad libitum high-sucrose evoked a significant enhancement of the dopamine D2 receptor protein level and a reduction in the D2 mRNA and receptor affinity in the rat striatum. Independently of the diet type, a similar reduction in dopamine D2 receptor affinity (decrease in KD value) was found in the striatum of rats with intermittent food access.

CONCLUSION

The findings provide a better understanding of eating disorders and indicate that diet composition leading to obesity induces distinct changes in dopamine D2 receptor signaling in the striatum.

A high-refined carbohydrate diet facilitates compulsive-like behavior in mice through the nitric oxide pathway

Obesity is characterized by abnormal adipose tissue expansion and is associated with chronic inflammation. Obesity itself may induce several comorbidities, including psychiatric disorders. It has been previously demonstrated that proinflammatory cytokines are able to up-regulate inducible nitric oxide synthase (iNOS) and nitric oxide (NO) release, which both have a role in compulsive related behaviors.

OBJECTIVE

To evaluate whether acute or chronic consumption of a high-refined carbohydrate-containing (HC) diet will modify burying-behavior in the Marble Burying Test (MBT) through augmentation of NO signaling in the striatum, a brain region related to the reward system. Further, we also verified the effects of chronic consumption of a HC diet on the reinforcing effects induced by cocaine in the Conditioned Place Preference (CPP) test.

METHODS

Male BALB/c mice received a standard diet (control diet) or a HC diet for 3 days or 12 weeks.

RESULTS

An increase in burying behavior occurred in the MBT after chronic consumption of a HC diet that was associated with an increase of nitrite levels in the striatum. The pre-treatment with Aminoguanidine (50 mg/kg), a preferential inhibitor of iNOS, prevented such alterations. Additionally, a chronic HC diet also induced a higher expression of iNOS in this region and higher glutamate release from striatal synaptosomes. Neither statistical differences were observed in the expression levels of the neuronal isoform of NOS nor in microglia number and activation. Finally, the reinforcing effects induced by cocaine (15 mg/kg, i.p.) during the expression of the conditioned response in the CPP test were not different between the chronically HC diet fed mice and the control group. However, HC diet-feeding mice presented impairment of cocaine-preference extinction.

CONCLUSION

Altogether, our results suggest that the chronic consumption of a HC diet induces compulsive-like behavior through a mechanism possibly associated with NO activation in the striatum.

Functional and structural plasticity contributing to obesity: roles for sex, diet, and individual susceptibility

The role of cortico-striatal pathways in cue-triggered motivational processes have been extensively studied. However, recent work has begun to examine the potential contribution of plasticity in these circuits to obesity. Despite the inclusion of women in human obesity studies examining neurobehavioral alterations in cue-triggered motivation, preclinical studies have focused mainly on male subjects. This lack of female subjects in preclinical research had led to a gap in the basic understanding of the neural mechanisms underlying over-eating in females. In this review, we highlight recent work from our lab and others that has begun to elucidate how diet, obesity, and individual susceptibility to weight gain influence functional and structural plasticity within the nucleus accumbens and prefrontal cortex in adult rats. As is the case throughout neuroscience, studies of females or sex differences are largely lacking in this area. Thus, below we describe preliminary neurobehavioral results from female studies in our labs and point out areas for future investigation.

Roux-en-Y gastric bypass surgery normalizes dopamine D1, D2, and DAT levels

Roux-en-Y gastric bypass surgery (RYGB) is one of the most effective treatments for morbid obesity. However, increased substance abuse following RYGB has been observed clinically. This study examined the effects of RYGB on the dopamine system to elucidate these observed changes in reward-related behavior. Rats were assigned to four groups: normal diet with sham surgery, ad libitum high fat (HF) diet with sham surgery, restricted HF diet with sham surgery, and HF diet with RYGB surgery. Following surgeries, rats were kept on their respective diets for 9 weeks before they were sacrificed. [3 H]SCH 23390, [3 H]Spiperone, and [3 H]WIN35 428 autoradiography was performed to quantify the effects of diet and RYGB surgery on dopamine type 1-like receptor (D1R)-like, dopamine type 2-like receptor (D2R)-like, and dopamine transporter (DAT) binding. Rats on a chronic HF diet became obese with reduced D1R-like binding within the ventrolateral striatum and the nucleus accumbens core, reduced D2R-like binding in all areas of the striatum and nucleus accumbens core and shell, and reduced DAT binding in the dorsomedial striatum. Restricted HF diet rats showed similar reductions in D1R-like and D2-R-like binding as the obese rats, and reduced DAT binding within all areas of the striatum. Both RYGB and restricted HF diet rats showed similar weight reductions, with RYGB rats showing no difference in binding compared to controls. The observed changes in binding between non-treated obese rats and RYGB rats demonstrates that HF dietary effects on the dopamine system were reversed by RYGB.

Persistent effects of obesity: a neuroplasticity hypothesis

The obesity epidemic is a leading cause of health problems in the United States, increasing the risk of cardiovascular, endocrine, and psychiatric diseases. Although many people lose weight through changes in diet and lifestyle, keeping the weight off remains a challenge. Here, we discuss a hypothesis that seeks to explain why obesity is so persistent. There is a great degree of overlap in the circuits implicated in substance use disorder and obesity, and neural plasticity of these circuits in response to drugs of abuse is well documented. We hypothesize that obesity is also associated with neural plasticity in these circuits, and this may underlie persistent changes in behavior, energy balance, and body weight. Here, we discuss how obesity-associated reductions in motivation and physical activity may be rooted in neurophysiological alterations in these circuits. Such plasticity may alter how humans and animals use, expend, and store energy, even after weight loss.

Brief exposure to obesogenic diet disrupts brain dopamine networks

Objective

We have previously demonstrated that insulin signaling, through the downstream signaling kinase Akt, is a potent modulator of dopamine transporter (DAT) activity, which fine-tunes dopamine (DA) signaling at the synapse. This suggests a mechanism by which impaired neuronal insulin receptor signaling, a hallmark of diet-induced obesity, may contribute to impaired DA transmission. We tested whether a short-term (two-week) obesogenic high-fat (HF) diet could reduce striatal Akt activity, a marker of central insulin, receptor signaling and blunt striatal and dopaminergic network responsiveness to amphetamine (AMPH).

Methods

We examined the effects of a two-week HF diet on striatal DAT activity in rats, using AMPH as a probe in a functional magnetic resonance imaging (fMRI) assay, and mapped the disruption in AMPH-evoked functional connectivity between key dopaminergic targets and their projection areas using correlation and permutation analyses. We used phosphorylation of the Akt substrate GSK3α in striatal extracts as a measure of insulin receptor signaling. Finally, we confirmed the impact of HF diet on striatal DA D2 receptor (D2R) availability using [¹⁸F]fallypride positron emission tomography (PET).

Results

We found that rats fed a HF diet for only two weeks have reductions in striatal Akt activity, a marker of decreased striatal insulin receptor signaling and blunted striatal responsiveness to AMPH. HF feeding also reduced interactions between elements of the mesolimbic (nucleus accumbens–anterior cingulate) and sensorimotor circuits (caudate/putamen–thalamus–sensorimotor cortex) implicated in hedonic feeding. D2R availability was reduced in HF-fed animals.

Conclusion

These studies support the hypothesis that central insulin signaling and dopaminergic neurotransmission are already altered after short-term HF feeding. Because AMPH induces DA efflux and brain activation, in large part via DAT, these findings suggest that blunted central nervous system insulin receptor signaling through a HF diet can impair DA homeostasis, thereby disrupting cognitive and reward circuitry involved in the regulation of hedonic feeding.

What Is the Evidence for "Food Addiction?" A Systematic Review

The diagnostic construct of “food addiction” is a highly controversial subject. The current systematic review is the first to evaluate empirical studies examining the construct of “food addiction” in humans and animals. Studies were included if they were quantitative, peer-reviewed, and in the English language. The 52 identified studies (35 articles) were qualitatively assessed to determine the extent to which their findings indicated the following addiction characteristics in relation to food: brain reward dysfunction, preoccupation, risky use, impaired control, tolerance/withdrawal, social impairment, chronicity, and relapse. Each pre-defined criterion was supported by at least one study. Brain reward dysfunction and impaired control were supported by the largest number of studies (n = 21 and n = 12, respectively); whereas risky use was supported by the fewest (n = 1). Overall, findings support food addiction as a unique construct consistent with criteria for other substance use disorder diagnoses. The evidence further suggests that certain foods, particularly processed foods with added sweeteners and fats, demonstrate the greatest addictive potential. Though both behavioral and substance-related factors are implicated in the addictive process, symptoms appear to better fit criteria for substance use disorder than behavioral addiction. Future research should explore social/role impairment, preoccupation, and risky use associated with food addiction and evaluate potential interventions for prevention and treatment.

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.

Deep brain stimulation as a therapeutic option for obesity: A critical review

Despite a better understanding of obesity pathophysiology, treating this disease remains a challenge. New therapeutic options are needed. Targeting the brain is a promising way, considering both the brain abnormalities in obesity and the effects of bariatric surgery on the gut-brain axis. Deep brain stimulation could be an alternative treatment for obesity since this safe and reversible neurosurgical procedure modulates neural circuits for therapeutic purposes. We aimed to provide a critical review of published clinical and preclinical studies in this field. Owing to the physiology of eating and brain alterations in people with obesity, two brain areas, namely the hypothalamus and the nucleus accumbens are putative targets. Preclinical studies with animal models of obesity showed that deep brain stimulation of hypothalamus or nucleus accumbens induces weight loss. The mechanisms of action remain to be fully elucidated. Preclinical data suggest that stimulation of nucleus accumbens reduces food intake, while stimulation of hypothalamus could increase resting energy expenditure. Clinical experience with deep brain stimulation for obesity remains limited to six patients with mixed results, but some clinical trials are ongoing. Thus, drawing clear conclusions about the effectiveness of this treatment is not yet possible, even if the results of preclinical studies are encouraging. Future clinical studies should examine its efficacy and safety, while preclinical studies could help understand its mechanisms of action. We hope that our review will provide ways to design further studies.

To Do or Not to Do: Dopamine, Affordability and the Economics of Opportunity

Five years ago, we introduced the thrift hypothesis of dopamine (DA), suggesting that the primary role of DA in adaptive behavior is regulating behavioral energy expenditure to match the prevailing economic conditions of the environment. Here we elaborate that hypothesis with several new ideas. First, we introduce the concept of affordability, suggesting that costs must necessarily be evaluated with respect to the availability of resources to the organism, which computes a value not only for the potential reward opportunity, but also the value of resources expended. Placing both costs and benefits within the context of the larger economy in which the animal is functioning requires consideration of the different timescales against which to compute resource availability, or average reward rate. Appropriate windows of computation for tracking resources requires corresponding neural substrates that operate on these different timescales. In discussing temporal patterns of DA signaling, we focus on a neglected form of DA plasticity and adaptation, changes in the physical substrate of the DA system itself, such as up- and down-regulation of receptors or release probability. We argue that changes in the DA substrate itself fundamentally alter its computational function, which we propose mediates adaptations to longer temporal horizons and economic conditions. In developing our hypothesis, we focus on DA D2 receptors (D2R), arguing that D2R implements a form of “cost control” in response to the environmental economy, serving as the “brain’s comptroller”. We propose that the balance between the direct and indirect pathway, regulated by relative expression of D1 and D2 DA receptors, implements affordability. Finally, as we review data, we discuss limitations in current approaches that impede fully investigating the proposed hypothesis and highlight alternative, more semi-naturalistic strategies more conducive to neuroeconomic investigations on the role of DA in adaptive behavior.