Triple monoamine inhibitor tesofensine decreases food intake, body weight, and striatal dopamine D2/D3 receptor availability in diet-induced obese rats

Tesofensine, a novel antiobesity drug, silences GABAergic hypothalamic neurons

Obesity is a major global health epidemic that has adverse effects on both the people affected as well as the cost to society. Several anti-obesity drugs that target GLP-1 receptors have recently come to the market. Here, we describe the effects of tesofensine, a novel anti-obesity drug that acts as a triple monoamine neurotransmitter reuptake inhibitor. Using various techniques, we investigated its effects on weight loss and underlying neuronal mechanisms in mice and rats. These include behavioral tasks, DeepLabCut videotaped analysis, electrophysiological ensemble recordings, optogenetic activation, and chemogenetic silencing of GABAergic neurons in the Lateral Hypothalamus (LH). We found that tesofensine induces a greater weight loss in obese rats than lean rats, while differentially modulating the neuronal ensembles and population activity in LH. In Vgat-ChR2 and Vgat-IRES-cre transgenic mice, we found for the first time that tesofensine inhibited a subset of LH GABAergic neurons, reducing their ability to promote feeding behavior, and chemogenetically silencing them enhanced tesofensine's food-suppressing effects. Unlike phentermine, a dopaminergic appetite suppressant, tesofensine causes few, if any, head-weaving stereotypy at therapeutic doses. Most importantly, we found that tesofensine prolonged the weight loss induced by 5-HTP, a serotonin precursor, and blocked the body weight rebound that often occurs after weight loss. Behavioral studies on rats with the tastant sucrose indicated that tesofensine's appetite suppressant effects are independent of taste aversion and do not directly affect the perception of sweetness or palatability of sucrose. In summary, our data provide new insights into the effects of tesofensine on weight loss and the underlying neuronal mechanisms, suggesting that tesofensine may be an effective treatment for obesity and that it may be a valuable adjunct to other appetite suppressants to prevent body weight rebound.

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.

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.

Converging vulnerability factors for compulsive food and drug use

Highly palatable foods and substance of abuse have intersecting neurobiological, metabolic and behavioral effects relevant for understanding vulnerability to conditions related to food (e.g., obesity, binge eating disorder) and drug (e.g., substance use disorder) misuse. Here, we review data from animal models, clinical populations and epidemiological evidence in behavioral, genetic, pathophysiologic and therapeutic domains. Results suggest that consumption of highly palatable food and drugs of abuse both impact and conversely are regulated by metabolic hormones and metabolic status. Palatable foods high in fat and/or sugar can elicit adaptation in brain reward and withdrawal circuitry akin to substances of abuse. Intake of or withdrawal from palatable food can impact behavioral sensitivity to drugs of abuse and vice versa. A robust literature suggests common substrates and roles for negative reinforcement, negative affect, negative urgency, and impulse control deficits, with both highly palatable foods and substances of abuse. Candidate genetic risk loci shared by obesity and alcohol use disorders have been identified in molecules classically associated with both metabolic and motivational functions. Finally, certain drugs may have overlapping therapeutic potential to treat obesity, diabetes, binge-related eating disorders and substance use disorders. Taken together, data are consistent with the hypotheses that compulsive food and substance use share overlapping, interacting substrates at neurobiological and metabolic levels and that motivated behavior associated with feeding or substance use might constitute vulnerability factors for one another. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Nucleus Accumbens Functional Connectivity with the Frontoparietal Network Predicts Subsequent Change in Body Mass Index for American Children

Background: Nucleus accumbens (NAc) is a brain structure with a well-established role in the brain reward processing system. Altered function of the NAc is shown to have a role in the development of food addiction and obesity. However, less is known about sex differences in the role of NAc function as a predictor of children’s change in body mass index (BMI) over time. Aim: We used the Adolescent Brain Cognitive Development data (version 2.01) to investigate sex differences in the predictive role of the NAc functional connectivity with the frontoparietal network on children’s BMI change over a one-year follow-up period. Methods: This 1-year longitudinal study successfully followed 3784 9–10-year-old children. Regression models were used to analyze the data. The predictor variable was NAc functional connectivity with the frontoparietal network measured using resting-state functional magnetic resonance imaging (fMRI). The primary outcome was BMI at the end of the 1-year follow up. Covariates included race, ethnicity, age, socioeconomic factors, and baseline BMI. Sex was the effect modifier. Results: NAc functional connectivity with the frontoparietal network was predictive of BMI changes over time. This association remained significant above and beyond all covariates. The above association, however, was only significant in female, not male children. Conclusion: The epidemiological observation that NAc functional connectivity is associated with BMI changes in children is an extension of well-controlled laboratory studies that have established the role of the NAc in the brain reward processing. More research is needed on sex differences in the brain regions that contribute to childhood obesity.

Current and emerging therapies for managing hyperphagia and obesity in Prader-Willi syndrome: A narrative review

In early childhood, individuals with Prader-Willi syndrome (PWS) experience excess weight gain and severe hyperphagia with food compulsivity, which often leads to early onset morbid obesity. Effective treatments for appetite suppression and weight control are currently unavailable for PWS. Our aim to further understand the pathogenesis of PWS led us to carry out a comprehensive search of the current and emerging therapies for managing hyperphagia and extreme weight gain in PWS. A literature search was performed using PubMed and the following keywords: "PWS" AND "therapy" OR "[drug name]"; reference lists, pharmaceutical websites, and the ClinicalTrials.gov registry were also reviewed. Articles presenting data from current standard treatments in PWS and also clinical trials of pharmacological agents in the pipeline were selected. Current standard treatments include dietary restriction/modifications, exercise, and growth hormone replacement, which appear to have limited efficacy for appetite and weight control in patients with PWS. The long-term safety and effectiveness of bariatric surgery in PWS remains unknown. However, many promising pharmacotherapies are in development and, if approved, will bring much needed choices into the PWS pharmacological armamentarium. With the progress that is currently being made in our understanding of PWS, an effective treatment may not be far off.

Parental Education and Nucleus Accumbens Response to Reward Anticipation: Minorities' Diminished Returns

Background

Considerable research has documented the effects of race and socioeconomic status (SES) on reward-seeking behaviors; however, less is known about the multiplicative effects of race and family SES on brain response to reward anticipation. Marginalization-related Diminished Returns (MDRs) suggest that family SES would show weaker effects on brain development of children in non-White families than in White families.

Objective

To test race by SES variation in Nucleus Accumbens (NAcc) response to reward anticipation (NAcc-RA) among American children.

Methods

For this cross-sectional analysis, data came from the Adolescent Brain Cognitive Development (ABCD) study which included 6,419, 9-10 year old children. The independent variable was parental education. The moderator was race. The primary outcome was the right NAcc-RA. Age, sex, ethnicity, household income, and family structure were the covariates. We used mixed effects regression models that adjusted for the nested nature of the ABCD data.

Results

While high parental education was associated with a higher amount of right NAcc-RA, this effect was stronger for White than non-White children. This finding was evident in the observed interactions between race and parental education on the right NAcc-RA.

Discussion

For American children, NAcc-RA is not shaped by race or family SES, but by their intersection. As a result of the interaction between race and SES (diminished return of SES for non-Whites), middle-class racial minority children may remain susceptible to high-risk behaviors. Disparities in high-risk behaviors in children should not be reduced to economic disparities. Structural inequalities may reduce the return of SES resources for non-White families.

Stronger Association between Nucleus Accumbens Density and Body Mass Index in Low-Income and African American Children

BACKGROUND

The nucleus accumbens' (NAc) size, function, and density influence individuals' body mass index (BMI). However, little is known about racial and socioeconomic status (SES) differences in the role of NAc density as a predictor of childhood BMI.

OBJECTIVES

We used the Adolescent Brain Cognitive Development (ABCD) data to investigate racial and SES differences in the effect of NAc density on childhood BMI.

METHODS

This cross-sectional study included 9497 children between ages 9 and 10. Mixed-effects regression models were used to analyze the data. The predictor variable was NAc density measured using diffusion MRI (dMRI). The outcome variable was BMI, operationalized as a continuous variable. Covariates included sex, age, ethnicity, family structure, and parental education. Race (White, African American, Asian, and Other/mixed) and household income (< 50k, 50-100 k, and 100+ k) were the moderators.

RESULTS

High NAc diffusion tension (density) was predictive of higher BMI, net of covariates. However, the positive association between NAc density and BMI was stronger in African Americans than in White, and in low-income than in high-income children.

CONCLUSIONS

Our findings suggest that although high NAc has implications for children's BMI, this effect varies across racial and SES groups. More research should be performed on the role of obesogenic environments in altering the effect of NAc on childhood BMI.

Pharmacotherapy of Obesity: Limits and Perspectives

Obesity is a severe worldwide epidemic. Obesity comorbidities, such as type 2 diabetes mellitus, hypertension, and atherosclerosis, are costly for patients and governments. The treatment of obesity involves several facets, including lifestyle changes, bariatric surgery, and pharmacotherapy. As changes in lifestyle require considerable patient commitment that is sometimes unachievable, and surgery is expensive and invasive, pharmacotherapy is the primary option for most patients. This review describes the pharmacotherapy currently available in the USA, Europe, and Brazil, focusing on its limitations. We then analyze the results from clinical trials of new drug candidates. Most drugs cause weight loss of < 4 kg compared with controls, and severe adverse effects have caused a number of drugs to be withdrawn from the market in several countries. Drugs under development have not shown more significant weight loss or reduced adverse effects. We conclude that a significant portion of obese patients have few treatment options because of the adverse effects and minimal weight loss associated with current pharmacotherapy. However, drugs currently under development appear unable to change this scenario in the near future. Thus, it is essential that new compounds are developed and new molecular targets studied so obesity can be efficiently treated in all patients in the future.

Pharmacotherapy of obesity: Available medications and drugs under investigation

Obesity is a chronic disease with a continuously rising prevalence that currently affects more than half a billion people worldwide. Energy balance and appetite are highly regulated via central and peripheral mechanisms, and weight loss triggers a homeostatic response leading to weight regain. Lifestyle and behavioral modifications are the cornerstones of obesity management; however, they often fail to achieve or sustain long-term weight loss. Pharmacotherapy added onto lifestyle modifications results in an additional, albeit limited, weight reduction. Regardless, this weight reduction of 5-10% conveys multiple cardiovascular and metabolic benefits. In this review, evidence on the food and drug administration (FDA)-approved medications, i.e., orlistat, lorcaserin, phentermine/topiramate, liraglutide and naltrexone/bupropion, is summarized. Furthermore, anti-obesity agents in the pipeline for potential future therapeutic use are presented.

Low Dopamine D2 Receptor Increases Vulnerability to Obesity Via Reduced Physical Activity, Not Increased Appetitive Motivation

BACKGROUND

The dopamine D2 receptor (D2R) has received much attention in obesity studies. Data indicate that D2R is reduced in obesity and that the TaqA1 D2R variant may be more prevalent among obese persons. It is often suggested that reduced D2R generates a reward deficiency and altered appetitive motivation that induces compulsive eating and contributes to obesity. Although dopamine is known to regulate physical activity, it is often neglected in these studies, leaving open the question of whether reduced D2R contributes to obesity through alterations in energy expenditure and activity.

METHODS

We generated a D2R knockdown (KD) mouse line and assessed both energy expenditure and appetitive motivation under conditions of diet-induced obesity.

RESULTS

The KD mice did not gain more weight or show increased appetitive motivation compared with wild-type mice in a standard environment; however, in an enriched environment with voluntary exercise opportunities, KD mice exhibited dramatically lower activity and became more obese than wild-type mice, obtaining no protective benefit from exercise opportunities.

CONCLUSIONS

These data suggest the primary contribution of altered D2R signaling to obesity lies in altered energy expenditure rather than the induction of compulsive overeating.

Size-Dependent Toxicity Differences of Intratracheally Instilled Manganese Oxide Nanoparticles: Conclusions of a Subacute Animal Experiment

Incomplete information on toxicological differences of micro- and nanometer-sized particles raised concerns about the effects of the latter on health and environment. Besides chemical composition, size and surface-to-volume ratio of nanoparticles (NPs) can affect toxicity. To investigate size-dependent toxicity differences, we used particles made of dioxide of the neurotoxic heavy metal manganese (Mn), typically found in inhaled metal fumes, in three size ranges (size A, 9.14 ± 1.98 nm; size B, 42.36 ± 8.06 nm; size C, 118.31 ± 25.37 nm). For modeling the most frequent route of exposure to Mn, NPs were given to rats for 6 weeks by intratracheal instillation. Of each NP size, 3 or 6 mg/kg body weight was given while control animals were vehicle treated. Neurotoxicity was assessed by measuring spontaneous locomotor activity in an open field and by recording spontaneous and evoked electrical activity from the somatosensory cortical area. Mn content of brain, lung, and blood, measured by ICP-MS, were correlated to the observed functional alterations to see the relationship between Mn load and toxic effects. Body weight gain and organ weights were measured as general toxicological indices. The toxicity of size A and size B NPs proved to be stronger compared to size C NPs, seen most clearly in decreased body weight gain and altered spontaneous cortical activity, which were also well correlated to the internal Mn dose. Our results showed strong effect of size on NP toxicity, thus, beyond inappropriateness of toxicity data of micrometer-sized particles in evaluation of NP exposure, differentiation within the nano range may be necessary.

Subchronic administration of short-acting naltrexone has no effect on striatal dopamine transporter availability, food intake or body weight gain in rats

The opioid receptor antagonist naltrexone is successfully used in the treatment of opioid and alcohol dependence. However, questions have been raised about possible anhedonic side effects, because the opioid system is directly involved in hedonic responses to natural rewarding activities, possibly due to its indirect effects on the striatal dopamine transporter (DAT). In order to test this hypothesis, 30 rats were randomized to either a 10-day treatment with 3 mg/kg short-acting naltrexone or vehicle. No significant differences between the groups were found in striatal DAT availability, cumulative food intake (for 48 or 72 h), body weight gain and abdominal fatpad weight. Thus, the results of this study suggest that (sub)chronic treatment with short-acting naltrexone does not induce possible anhedonic effects. However, it cannot be ruled out the anhedonic effect of naltrexone is only short-lived and thus not detected in the current study. Therefore, future studies are needed to study possible acute anhedonic effects at several time points shortly after short-acting naltrexone administration and to directly compare the possible anhedonic effects of long-acting with those of short-acting opioid antagonists.

Hormonal and neural mechanisms of food reward, eating behaviour and obesity

With rising rates of obesity, research continues to explore the contributions of homeostatic and hedonic mechanisms related to eating behaviour. In this Review, we synthesize the existing information on select biological mechanisms associated with reward-related food intake, dealing primarily with consumption of highly palatable foods. In addition to their established functions in normal feeding, three primary peripheral hormones (leptin, ghrelin and insulin) play important parts in food reward. Studies in laboratory animals and humans also show relationships between hyperphagia or obesity and neural pathways involved in reward. These findings have prompted questions regarding the possibility of addictive-like aspects in food consumption. Further exploration of this topic may help to explain aberrant eating patterns, such as binge eating, and provide insight into the current rates of overweight and obesity.

Tesofensine, a novel triple monoamine re-uptake inhibitor with anti-obesity effects: dopamine transporter occupancy as measured by PET

Tesofensine (TE) is a novel triple monoamine re-uptake inhibitor inducing a potent inhibition of the re-uptake process in the synaptic cleft of the neurotransmitters dopamine, norepinephrine, and serotonin. In recent preclinical and clinical evaluations TE showed a robust anti-obesity effect, but the specific mechanism of this triple monoamine re-uptake inhibitor still needs to be further elucidated. This positron emission tomography (PET) study, using [¹¹C]βCIT-FE, aimed to assess the degree of the dopamine transporter (DAT) occupancy, at constant TE plasma levels, following different oral, multiple doses of TE during totally 8-12 days. In addition, the relationships between DAT occupancy and TE plasma concentrations, or doses, were investigated to enable assessment of DAT occupancies in subsequent clinical trials. The results demonstrated that TE induced a dose-dependent blockade of DAT following multiple doses of 0.125-1 mg TE at anticipated steady-state conditions. The mean striatal DAT occupancy varied dose-dependently between 18% and 77%. A sigmoid E(max) model well described the relationship between striatal DAT occupancy and TE plasma concentrations or doses. It was estimated that the maximum achievable DAT occupancy was about 80% and that half of this effect was accomplished by approximately 0.25 mg TE and a plasma drug concentration of 4 ng/ml. The results indicated an important mechanism of action of TE on DAT. Further, these results suggest that the previously reported dose-dependent weight loss, in TE treated subjects, was in part mediated by an up-regulation of dopaminergic pathways due to enhanced amounts of synaptic dopamine after blockade of DAT.

High fat/carbohydrate ratio but not total energy intake induces lower striatal dopamine D2/3 receptor availability in diet-induced obesity

High-energy diets that induce obesity decrease striatal dopamine D2/3 receptor (DRD2/3) availability. It is however poorly understood which components of these diets are underlying this decrease. This study assessed the role of saturated fat intake on striatal DRD2/3 availability. Forty rats were randomized to a free-choice high-fat high-sugar diet (HFHS) or a standard chow diet for 28 days. Striatal DRD2/3 availability was measured using (123)I-IBZM storage phosphor imaging at day 29. The HFHS group was split in a HFHS-high-fat (HFHS-hf) and HFHS-low-fat (HFHS-lf) group based on the percentage energy intake from fat. Rats of both HFHS subgroups had increased energy intake, abdominal fat stores and plasma leptin levels compared with controls. DRD2/3 availability in the nucleus accumbens (NAcc) was significantly lower in HFHS-hf than in HFHS-lf rats, whereas it was similar for HFHS-lf and control rats. Furthermore, DRD2/3 availability in the NAcc was positively correlated with the percentage energy intake from sugar. Total energy intake was lower for HFHS-hf than for HFHS-lf rats. Together these results suggest that a diet with a high fat/carbohydrate ratio, but not total energy intake or the level of adiposity, is the best explanation for the decrease in striatal DRD2/3 availability observed in diet-induced obesity.