The snacking rat as model of human obesity: effects of a free-choice high-fat high-sugar diet on meal patterns

Manipulation of feeding patterns in high fat diet fed rats improves microbiota composition dynamics, inflammation and gut-brain signaling

Chronic consumption of high fat (HF) diets has been shown to increase meal size and meal frequency in rodents, resulting in overeating. Reducing meal frequency and establishing periods of fasting, independently of caloric intake, may improve obesity-associated metabolic disorders. Additionally, diet-driven changes in microbiota composition have been shown to play a critical role in the development and maintenance of metabolic disorders. In this study, we used a pair-feeding paradigm to reduce meal frequency and snacking episodes while maintaining overall intake and body weight in HF fed rats. We hypothesized that manipulation of feeding patterns would improve microbiota composition and metabolic outcomes. Male Wistar rats were placed in three groups consuming either a HF, low fat diet (LF, matched for sugar), or pair-fed HF diet for 7 weeks (n = 11-12/group). Pair-fed animals received the same amount of food consumed by the HF fed group once daily before dark onset (HF-PF). Rats underwent oral glucose tolerance and gut peptide cholecystokinin sensitivity tests. Bacterial DNA was extracted from the feces collected during both dark and light cycles and sequenced via Illumina MiSeq sequencing of the 16S V4 region. Our pair-feeding paradigm reduced meal numbers, especially small meals in the inactive phase, without changing total caloric intake. This shift in feeding patterns reduced relative abundances of obesity-associated bacteria and maintained circadian fluctuations in microbial abundances. These changes were associated with improved gastrointestinal (GI) function, reduced inflammation, and improved glucose tolerance and gut to brain signaling. We concluded from these data that targeting snacking may help improve metabolic outcomes, independently of energy content of the diet and hyperphagia.

Circadian influences on feeding behavior

Feeding, like many other biological functions, displays a daily rhythm. This daily rhythmicity is controlled by the circadian timing system of which the central master clock is located in the hypothalamic suprachiasmatic nucleus (SCN). Other brain areas and tissues throughout the body also display rhythmic functions and contain the molecular clock mechanism known as peripheral oscillators. To generate the daily feeding rhythm, the SCN signals to different hypothalamic areas with the lateral hypothalamus, paraventricular nucleus and arcuate nucleus being the most prominent. With respect to the rewarding aspects of feeding behavior, the dopaminergic system is also under circadian influence. However the SCN projects only indirectly to the different reward regions, such as the ventral tegmental area where dopamine neurons are located. In addition, high palatable, high caloric diets have the potential to disturb the normal daily rhythms of physiology and have been shown to alter for example meal patterns. Around a meal several hormones and peptides are released that are also under circadian influence. For example, the release of postprandial insulin and glucagon-like peptide following a meal depend on the time of the day. Finally, we review the effect of deletion of different clock genes on feeding behavior. The most prominent effect on feeding behavior has been observed in Clock mutants, whereas deletion of Bmal1 and Per1/2 only disrupts the day-night rhythm, but not overall intake. Data presented here focus on the rodent literature as only limited data are available on the mechanisms underlying daily rhythms in human eating behavior.

High-Caloric Diets in Adolescence Impair Specific GABAergic Subpopulations, Neurogenesis, and Alter Astrocyte Morphology

We compared the effects of two different high-caloric diets administered to 4-week-old rats for 12 weeks: a diet rich in sugar (30% sucrose) and a cafeteria diet rich in sugar and high-fat foods. We focused on the hippocampus, particularly on the gamma-aminobutyric acid (GABA)ergic system, including the Ca2+-binding proteins parvalbumin (PV), calretinin (CR), calbindin (CB), and the neuropeptides somatostatin (SST) and neuropeptide Y (NPY). We also analyzed the density of cholinergic varicosities, brain-derived neurotrophic factor (BDNF), reelin (RELN), and cyclin-dependent kinase-5 (CDK-5) mRNA levels, and glial fibrillary acidic protein (GFAP) expression. The cafeteria diet reduced PV-positive neurons in the granular layer, hilus, and CA1, as well as NPY-positive neurons in the hilus, without altering other GABAergic populations or overall GABA levels. The high-sugar diet induced a decrease in the number of PV-positive cells in CA3 and an increase in CB-positive cells in the hilus and CA1. No alterations were observed in the cholinergic varicosities. The cafeteria diet also reduced the relative mRNA expression of RELN without significant changes in BDNF and CDK5 levels. The cafeteria diet increased the number but reduced the length of the astrocyte processes. These data highlight the significance of determining the mechanisms mediating the observed effects of these diets and imply that the cognitive impairments previously found might be related to both the neuroinflammation process and the reduction in PV, NPY, and RELN expression in the hippocampal formation.

A hamster model for stress-induced weight gain

This review addresses the translational relevance of animal models of stress and their effects on body weight. In humans, stress, whether chronic or acute, has often been associated with increased food intake and weight gain. In view of the current obesity epidemic, this phenomenon is especially relevant. Such observations contrast with reports with commonly used laboratory animals, especially rats and mice. In these species, it is common to find individuals gaining less weight under stress, even with potent social stressors. However, there are laboratory species that present increased appetite and weight gain under stress, such as golden hamsters. Furthermore, these animals also include metabolic and behavioral similarities with humans, including hoarding behavior which is also enhanced under stress. Consequently, we propose that our comparative perspective provides useful insights for future research on the development of obesity in humans as a consequence of chronic stress exposure.

Changes in reward-induced neural activity upon Cafeteria Diet consumption

Excessive consumption of highly palatable foods rich in sugar and fat, often referred to as "junk" or "fast" foods, plays a central role in the development of obesity. The highly palatable characteristics of these foods activate hedonic and motivational mechanisms to promote food-seeking behavior and overeating, which is largely regulated by the brain reward system. Excessive junk food consumption can alter the functioning of this reward system, but exact mechanisms of these changes are still largely unknown. This study investigated whether long-term junk food consumption, in the form of Cafeteria (CAF) diet, can alter the reward system in adult, female Long-Evans rats, and whether different regimes of CAF diet influence the extent of these changes. To this end, rats were exposed to a 6-week diet with either standard chow, or ad libitum daily access to CAF diet, 30 % restricted but daily access to CAF diet, or one-day-a-week (intermittent) ad libitum access to CAF diet, after which c-Fos expression in the Nucleus Accumbens (NAc), Prefrontal Cortex (PFC), and Ventral Tegmental Area (VTA) following consumption of a CAF reward of choice was examined. We found that all CAF diet regimes decreased c-Fos expression in the NAc-shell when presented with a CAF reward, while no changes in c-Fos expression upon the different diet regimes were found in the PFC, and possibly the VTA. Our data suggests that long-term junk food exposure can affect the brain reward system, resulting in an attenuated activity of the NAc-shell.

The Metabolic Syndrome, a Human Disease

This review focuses on the question of metabolic syndrome (MS) being a complex, but essentially monophyletic, galaxy of associated diseases/disorders, or just a syndrome of related but rather independent pathologies. The human nature of MS (its exceptionality in Nature and its close interdependence with human action and evolution) is presented and discussed. The text also describes the close interdependence of its components, with special emphasis on the description of their interrelations (including their syndromic development and recruitment), as well as their consequences upon energy handling and partition. The main theories on MS's origin and development are presented in relation to hepatic steatosis, type 2 diabetes, and obesity, but encompass most of the MS components described so far. The differential effects of sex and its biological consequences are considered under the light of human social needs and evolution, which are also directly related to MS epidemiology, severity, and relations with senescence. The triggering and maintenance factors of MS are discussed, with especial emphasis on inflammation, a complex process affecting different levels of organization and which is a critical element for MS development. Inflammation is also related to the operation of connective tissue (including the adipose organ) and the widely studied and acknowledged influence of diet. The role of diet composition, including the transcendence of the anaplerotic maintenance of the Krebs cycle from dietary amino acid supply (and its timing), is developed in the context of testosterone and β-estradiol control of the insulin-glycaemia hepatic core system of carbohydrate-triacylglycerol energy handling. The high probability of MS acting as a unique complex biological control system (essentially monophyletic) is presented, together with additional perspectives/considerations on the treatment of this 'very' human disease.

Effectiveness of pharmacotherapies for diabetes on nicotine, food, and water intake in insulin-resistant rats

The intersectionality between diabetes medications and nicotine consumption was assessed in female and male rats. Briefly, the rats were fed a high-fat diet (HFD) or regular diet (RD) for 4 weeks. Then separate groups received vehicle or a low dose of streptozotocin (STZ; 25 mg/kg). Three days later, insulin resistance was assessed by measuring plasma glucose levels for 180 min following an injection of insulin (0.75 U/kg). The rats were then prepared with jugular catheters, and they were given 23 h access to nicotine intravenous self-administration (IVSA) in 4 days cycles with 3 days of forced abstinence in their home cages where they consumed their respective diet. During the IVSA sessions, operant responses for food and water and changes in body weight were recorded. Prior to administration of the pharmacotherapies, the rats were given access to two doses of nicotine (0.015 then 0.03 mg/kg for the remainder of the study). Then, daily injections of the pharmacotherapies were given at the onset of dark cycle (6 p.m.) in the following order: 1) dapagliflozin (3.0 then 10.0 mg/kg), 2) insulin (0.75 U/kg twice), and 3) bromocriptine (3.0 then 10.0 mg/kg). The results suggest that our HFD+STZ regiment induced insulin resistance in female and male rats. Also, the HFD-fed rats displayed higher nicotine intake than RD controls, regardless of sex. Administration of insulin, but not dapagliflozin or bromocriptine, normalized nicotine intake in HFD-fed rats to control levels. These results have clinical implications regarding the potential efficacy of insulin to control excessive nicotine intake in persons with diabetes.

Exploring the link between hedonic overeating and prefrontal cortex dysfunction in the Ts65Dn trisomic mouse model

Individuals with Down syndrome (DS) have a higher prevalence of obesity compared to the general population. Conventionally, this has been attributed to endocrine issues and lack of exercise. However, deficits in neural reward responses and dopaminergic disturbances in DS may be contributing factors. To investigate this, we focused on a mouse model (Ts65Dn) bearing some triplicated genes homologous to trisomy 21. Through detailed meal pattern analysis in male Ts65Dn mice, we observed an increased preference for energy-dense food, pointing towards a potential "hedonic" overeating behavior. Moreover, trisomic mice exhibited higher scores in compulsivity and inflexibility tests when limited access to energy-dense food and quinine hydrochloride adulteration were introduced, compared to euploid controls. Interestingly, when we activated prelimbic-to-nucleus accumbens projections in Ts65Dn male mice using a chemogenetic approach, impulsive and compulsive behaviors significantly decreased, shedding light on a promising intervention avenue. Our findings uncover a novel mechanism behind the vulnerability to overeating and offer potential new pathways for tackling obesity through innovative interventions.

Stress, rhythm, choice and the munchies - tribute to Mary F. Dallman

In December 2021, we lost a pioneer in the field of stress research who inspired generations of scientists. Mary Dallman was an expert on the hypothalamic-pituitary-adrenal (HPA) axis, its interactions with a wide variety of other physiological systems and the impact of chronic changes of HPA function on energy metabolism and adiposity. She was not only an excellent scientist, she was a great role model and mentor for young scientists, especially women. She encouraged and supported many of her trainees even long after they left the lab. Her outside-the-box thinking, the fun and crazy discussions we had in the lab proved to be a beautiful basis for my own future research.

The administration of Exendin-4 and CCK affects food intake differentially in female and male rats tested on an alternate day fasting paradigm

Alternate day fasting (ADF) which involves the repetition of a 2-day cycle of a day of free access to food followed by a day of limited or no access to food, is an effective dietary intervention for weight loss in both humans and rats. We have previously reported that when presented with a high energy (HE) and standard chow diet, rats maintained on an ADF schedule displayed decreased HE diet preference compared to controls. Both male and female ADF rats increased overall intake of chow. However, this increase was driven by both meal size and meal number for males and only number of meals for females. Administration of cholecystokinin (CCK) or the glucagon-like peptide 1 (GLP-1) receptor agonist Exendin-4 (Ex-4) reduces food intake. It appears that CCK decreases food intake primarily through satiety signals whereas GLP-1 signaling may reduce intake by satiety and reward cues. Here, female and male rats were administered (i.p.) saline, 3.0 µg/kg Ex-4 (3 h before test), 3.0 µg/kg CCK (15 min before test) or a combination of both. Next, all rats were presented 23-h access to both HE diet and chow following food-restriction (ADF) or free access to chow (CON). Compared to saline-control sessions, administration of the combination of Ex-4 and CCK, but not Ex-4 or CCK alone, resulted in a decrease in both HE and chow intake early in the session for male ADF rats but the combination primarily decreased chow diet intake early in the session for female ADF rats. Thus, it appears that under these energy homeostatic conditions, administration of Ex-4 or CCK alone does not affect intake in ADF rats, but the combination produces decreases in feeding that are more than the sum of their individual effects. These findings support a role for the combination of GLP-1 and CCK signaling in the changes in diet preference induced by an alternate day fasting paradigm differentially in female and male rats.

New Insight into Selective Serotonin Receptor Agonists in the Central Nervous System, Studied with WAY163909 in Obese and Diabetic Wistar Rats

Background and aims: We investigated the effect of WAY-163909, a novel 5-hydroxytryptamine selective 2C receptor agonist on body weight, blood glucose levels, and insulin resistance in obese and diabetic Wistar rats. Materials and methods: We used twenty male Wistar rats with obesity and obesity-induced diabetes and twenty healthy Wistar rats as a control group. Each of these groups was separated into two subgroups: one with a daily intraperitoneal application of WAY-163909 (1 mg/kg) and one without. During the study, body weight, blood glucose levels, and immunoreactive insulin were tracked. Results: A reduction of 5.5% (p < 0.05) in body weight was registered in the rat group with diabetes and obesity and 2.56% in the control group with a daily application of WAY-163909 (1 mg/kg) at the end of the study. Decreases of 35.4% in blood glucose levels at week four in the diabetic and obese rat group with a daily application of WAY-163909 (1 mg/kg) were registered. A reduction of insulin levels of 4.1% (p < 0.05) in the diabetic and obese rats group using WAY-163909 was also observed. Conclusion: In our study, using WAY-163909 (1 mg/kg) led to a reduction of blood glucose levels, immunoreactive insulin, and body weight.

Food-grade titanium dioxide (E171) differentially affects satiation in mice fed a regular or a high fat diet

Food-grade titanium dioxide (E171) is a widely used food additive and the toxicity after oral consumption is still under research, although it has been already banned in some countries. The consumption of this additive occurs mainly through ultra-processed food products which also contain high amounts of fat. High fat diets (HFD) impair the physiological system controlling satiation and satiety, which are responsible for control of food intake and energy status. The impact of E171 on animal behavior has been poorly explored and here we hypothesize that E171 could worsen the effects on feeding behavior induced by HFD. Therefore, we aimed to evaluate the effects of E171 on the feeding pattern and the behavioral satiety sequence (BSS) of mice fed with a regular diet (RD) or a HFD after 1 and 16 weeks of exposure. The results showed that RD + E171 increased food intake and feeding time, but the prototypical structure of the BSS pattern (feeding→ grooming-activity → resting), was preserved. Conversely, food consumption was not altered in HFD + E171, but the BSS pattern was disrupted as the animals prolonged resting time and spent less time being active. Our findings suggest that E171 delayed the onset of satiation in mice fed with RD but induced the opposite effect in mice fed with HFD.

Food memory circuits regulate eating and energy balance

In mammals, learning circuits play an essential role in energy balance by creating associations between sensory cues and the rewarding qualities of food. This process is altered by diet-induced obesity, but the causes and mechanisms are poorly understood. Here, we exploited the relative simplicity and wealth of knowledge about the D. melanogaster reinforcement learning network, the mushroom body, in order to study the relationship between the dietary environment, dopamine-induced plasticity, and food associations. We show flies that are fed a high-sugar diet cannot make associations between sensory cues and the rewarding properties of sugar. This deficit was caused by diet exposure, not fat accumulation, and specifically by lower dopamine-induced plasticity onto mushroom body output neurons (MBONs) during learning. Importantly, food memories dynamically tune the output of MBONs during eating, which instead remains fixed in sugar-diet animals. Interestingly, manipulating the activity of MBONs influenced eating and fat mass, depending on the diet. Altogether, this work advances our fundamental understanding of the mechanisms, causes, and consequences of the dietary environment on reinforcement learning and ingestive behavior.

High Fat High Sucrose Diet Modifies Uterine Contractility and Cervical Resistance in Pregnant Rats: The Roles of Sex Hormones, Adipokines and Cytokines

Background: In obesity, the adipose tissue becomes a very significant endocrine organ producing different factors called adipokines, such as leptin, adiponectin and kisspeptin; however, no data are available about their actions on uterine contraction in obese pregnant rats. Our aim was to study the impact of obesity on pregnant uterine contraction in a rat model. Methods: Obesity was induced by the consumption of a high fat high sucrose diet (HFHSD) for 9 weeks, including pregnancy. Glucose tolerance, sex hormone, cytokine and adipokine levels were measured. Uterine contractions and cervical resistance, as well as their responses to adipokines, were tested along with the expressions of their uterine receptors. Results: HFHSD increased body weight, and altered glucose tolerance and fat composition. The uterine leptin and kisspeptin pathway affect increased. The levels of proinflammatory cytokines were reduced, while the plasma level of progesterone was increased, resulting in weaker uterine contractions, and improving the uterine relaxing effects of adipokines. HFHSD reduced cervical resistance, but the core effect of adipokines is difficult to determine. Conclusions: Obesity in pregnant rats reduces uterine contractility and cytokine-induced inflammatory processes, and therefore obese pregnant rat methods are partially applicable for modelling human processes.

Nutrients and the Circadian Clock: A Partnership Controlling Adipose Tissue Function and Health

White adipose tissue (WAT) is a metabolic organ with flexibility to retract and expand based on energy storage and utilization needs, processes that are driven via the coordination of different cells within adipose tissue. WAT is comprised of mature adipocytes (MA) and cells of the stromal vascular cell fraction (SVF), which include adipose progenitor cells (APCs), adipose endothelial cells (AEC) and infiltrating immune cells. APCs have the ability to proliferate and undergo adipogenesis to form MA, the main constituents of WAT being predominantly composed of white, triglyceride-storing adipocytes with unilocular lipid droplets. While adiposity and adipose tissue health are controlled by diet and aging, the endogenous circadian (24-h) biological clock of the body is highly active in adipose tissue, from adipocyte progenitor cells to mature adipocytes, and may play a unique role in adipose tissue health and function. To some extent, 24-h rhythms in adipose tissue rely on rhythmic energy intake, but individual circadian clock proteins are also thought to be important for healthy fat. Here we discuss how and why the clock might be so important in this metabolic depot, and how temporal and qualitative aspects of energy intake play important roles in maintaining healthy fat throughout aging.

Early Postoperative Exposure to High-Fat Diet Does Not Increase Long-Term Weight Loss or Fat Avoidance After Roux-en-Y Gastric Bypass in Rats

Background

Bariatric surgery alters food preferences in rats and reportedly decreases desire to consume high-fat high-sugar food in humans. The aim of this study was to investigate whether early post-operative exposure to high-fat food could increase body weight loss after Roux-en-Y gastric bypass (RYGB) by triggering fat avoidance.

Methods

Male Wistar rats underwent either RYGB (n = 15) or sham-operations (n = 16). Preoperatively a standardized 4-choice cafeteria diet [dietary options: low-fat/low-sugar (LFLS), low-fat/high-sugar (LFHS), high-fat/low-sugar (HFLS), high-fat/high-sugar (HFHS)] was offered. First, each option was available for 4 days, thereafter rats were offered the 4 options simultaneously for 3 days preoperatively. Post-surgery, 8 rats in the RYGB- and 8 in the sham-group were exposed to a high-fat content diet (Oatmeal + 30% lard, OM+L) for 10 days, while 7 RYGB rats and 8 sham-rats received OM alone. From the 11th postoperative day, the 4-choice cafeteria diet was reintroduced for 55-days. The intake of all available food items, macronutrients and body weight changes were monitored over 8 weeks. Main outcomes were long-term body-weight and daily change in relative caloric intake during the postoperative cafeteria period compared to the preoperative cafeteria.

Results

During the first 12 days of postoperative cafeteria access, RYGB-rats exposed to OM+L had a higher mean caloric intake per day than RYGB rats exposed to OM alone (Δ10 kCal, P_adj = 0.004), but this difference between the RYGB groups disappeared thereafter. Consequently, in the last 33 days of the postoperative cafeteria diet, the mean body weight of the RYGB+OM+L group was higher compared to RYGB+OM (Δ51 g, P_adj < 0.001). RYGB rats, independently from the nutritional intervention, presented a progressive decrease in daily consumption of calories from fat and increased their daily energy intake mainly from non-sugar carbohydrates. No such differences were detected in sham-operated controls exposed to low- or high fat postoperative interventions.

Conclusion

A progressive decrease in daily fat intake over time was observed after RYGB, independently from the nutritional intervention. This finding confirms that macronutrient preferences undergo progressive changes over time after RYGB and supports the role of ingestive adaptation and learning. Early postoperative exposure to high-fat food failed to accentuate fat avoidance and did not lead to superior weight loss in the long-term.

Early unhealthy eating habits underlie morpho-functional changes in the liver and adipose tissue in male rats

Early-life consumption of high-fat and sugar-rich foods is recognized as a major contributor for the onset of metabolic dysfunction and its related disorders, including diabetes and nonalcoholic fatty liver disease. The lifelong impact of early unhealthy eating habits that start at younger ages remains unclear. Therefore, to better understand the effects of diet, it is essential to evaluate the structural and functional changes induced in metabolic organs and potential mechanisms underlying those changes. To investigate the long-term effects of eating habits, young male rats were exposed to high-sugar and high-energy diets. After 14 weeks, body composition was assessed, and histopathological changes were analyzed in the liver and adipose tissue. Serum biochemical parameters were also determined. Expression of inflammatory markers in the liver was evaluated by immunohistochemistry. Our results revealed that serum levels of glucose, creatinine, aspartate transaminase (AST), alanine transaminase (ALT), and lipid profile were increased in rats red high-sugar and high-energy diets. Histopathological alterations were observed, including abnormal hepatocyte organization and lipid droplet accumulation in the liver, and abnormal structure of adipocytes. In both unhealthy diet groups, hepatic expression of Toll-like receptor 4 (TLR4), cyclooxygenase 2 (COX-2), and E-selectin were increased, as well as a biomarker of oxidative stress. Together, our data demonstrated that unhealthy diets induced functional and structural changes in the metabolic organs, suggesting that proinflammatory and oxidative stress mechanisms trigger the hepatic alterations and metabolic dysfunction.

Sex and Species Differences in the Development of Diet-Induced Obesity and Metabolic Disturbances in Rodents

Prevalence and health consequences of obesity differ between men and women. Yet, most preclinical studies investigating the etiology of obesity have, to date, been conducted in male rodents. Notably, diet is a major determinant of obesity, but sex differences in rodent models of diet-induced obesity, and the mechanisms that underlie such differences, are still understudied. Here, we aim to determine whether time course and characteristics of diet-induced obesity differ between sexes in rats and mice, and to investigate the potential causes of the observed divergence. To achieve this, we offered the most commonly tested rodents of both sexes, SD rats and C57BL/6 mice, a free choice of 60 % high-fat diet (HFD) and regular chow; body weight, food intake, fat mass, brown adipose responses, locomotor activity and glucose tolerance were assessed in a similar manner in both species. Our results indicate that overall diet-induced hyperphagia is greater in males but that females display a higher preference for the HFD, irrespective of species. Female rats, compared to males, showed a delay in diet-induced weight gain and less metabolic complications. Although male rats increased brown adipose tissue thermogenesis in response to the HFD challenge, this was not sufficient to counteract increased adiposity. In contrast to rats, female and male mice presented with a dramatic adiposity and impaired glucose tolerance, and a decreased energy expenditure. Female mice showed a 5-fold increase in visceral fat, compared to 2-fold increase seen in male mice. Overall, we found that male and female rodents responded very differently to HFD challenge, and engaged different compensatory energy expenditure mechanisms. In addition, these sex differences are divergent in rats and mice. We conclude that SD rats have a better face validity for the lower prevalence of overweight in women, while C57BL/6 mice may better model the increased prevalence of morbid obesity in women.

Polar fraction from Parkinsonia aculeata aerial parts extract improves imbalanced metabolic profile and reduces proinflammatory interleukin levels in white adipose tissue in obese rats induced by western diet

ETHNOPHARMACOLOGICAL RELEVANCE

Parkinsonia aculeata L. (Cesalpineaceae) is a medium tree found in the Xingó region (semi-arid area) in Northeast of Brazil, recognised by local population as an antidiabetic agent. According information from local community, the commonly traditional preparation is prepared as an infusion of the aerial part of the plant and consumed over the day to manage diabetes-related complications. Previous studies have described Parkinsonia aculeate as a product with both hypoglycemic and hypotriglyceridemic effects.

AIM OF THE STUDY

The objective of this study was to evaluate the effects of polar fraction obtained from the hydroethanolic extract of Parkinsonia aculeata (PfrHEPA) on the lipid profile of animals that consumed a westernized diet.

MATERIALS AND METHODS

Thirty-six Wistar rats (45-55 g) were fed either with standard control(C) or westernized diet(W) for 120 days. The food intake, body weight evolution and body size were also analyzed. From 120 to 150 days, they were orally treated according to their group with vehicle (distillated water, 10 mL/kg), PfrHEPA at three doses (35, 70 and 140 mg/kg/day) or Gemfibrozil (140 mg/kg/day) for 30 days.

RESULTS

The animals fed with westernized diet showed dyslipidemia when compared to animals receiving a standard diet. Treatment with PfrHEPA (140 mg/kg), even with the continued consumption of westernized diet by animals (from 120 to 150 days) promoted a significant reduction in total cholesterol, LDL and triglyceride levels, in relation to untreated W group. PfrHEPA 140 mg/kg reduced the key serum lipids and glycaemia as well as inflammatory cytokines known as important risk factors of cardiovascular diseases.

CONCLUSIONS

The observed evidence may contribute to the control of metabolic parameters as dyslipidemia corroborating the ethnopharmacological information concerning the antihyperlipidemic and hypoglycemic activities of P. aculeata.

The role of the nucleus accumbens and ventral pallidum in feeding and obesity

Obesity is a growing global epidemic that stems from the increasing availability of highly-palatable foods and the consequent enhanced calorie consumption. Extensive research has shown that brain regions that are central to reward seeking modulate feeding and evidence linking obesity to pathology in such regions have recently started to accumulate. In this review we focus on the contribution of two major interconnected structures central to reward processing, the nucleus accumbens and the ventral pallidum, to obesity. We first review the known literature linking these structures to feeding behavior, then discuss recent advances connecting pathology in the nucleus accumbens and ventral pallidum to obesity, and finally examine the similarities and differences between drug addiction and obesity in the context of these two structures. The understanding of how pathology in brain regions involved in reward seeking and consumption may drive obesity and how mechanistically similar obesity and addiction are, is only now starting to be revealed. We hope that future research will advance knowledge in the field and open new avenues to studying and treating obesity.

Crème de la Créature: Dietary Influences on Behavior in Animal Models

In humans, alterations in cognitive, motivated, and affective behaviors have been described with consumption of processed diets high in refined sugars and saturated fats and with high body mass index, but the causes, mechanisms, and consequences of these changes remain poorly understood. Animal models have provided an opportunity to answer these questions and illuminate the ways in which diet composition, especially high-levels of added sugar and saturated fats, contribute to brain physiology, plasticity, and behavior. Here we review findings from invertebrate (flies) and vertebrate models (rodents, zebrafish) that implicate these diets with changes in multiple behaviors, including eating, learning and memory, and motivation, and discuss limitations, open questions, and future opportunities.

Sucrose drinking mimics effects of nucleus accumbens µ-opioid receptor stimulation on fat intake and brain c-Fos-expression

Objectives: We have previously shown that the combined consumption of fat and a sucrose solution induces overeating, and there is evidence indicating that sucrose drinking directly stimulates fat intake. One neurochemical pathway by which sucrose may enhance fat intake is through the release of endogenous opioids in the nucleus accumbens (NAC).Methods: To test this hypothesis, we provided rats with a free-choice high-fat diet for two weeks. During the second week, rats had access to an additional bottle of water or a 30% sucrose solution for five minutes per day. After these two weeks, we infused vehicle or the μ-opioid receptor agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) into the NAC 30 min after their daily access to the additional bottle of water or the sucrose solution.Results: Sucrose drinking had two effects, (1) it stimulated fat intake in the absence of DAMGO infusion, (2) it diminished sensitivity to DAMGO, as it prevented the rapid increase in fat intake typically seen upon DAMGO infusion in the nucleus accumbens. In a second experiment, we confirmed that these results are not due to the ingested calories of the sucrose solution. Lastly, we investigated which brain areas are involved in the observed effects on fat intake by assessing c-Fos-expression in brain areas previously linked to DAMGO's effects on food intake. Both intra-NAC DAMGO infusion and sucrose consumption in the absence of DAMGO infusion had no effect on c-Fos-expression in orexin neurons and the central amygdala but increased c-Fos-expression in the NAC as well as the basolateral amygdala.Discussion: In conclusion, we confirm that sucrose drinking stimulates fat intake, likely through the release of endogenous opioids.

Accumbal TRH is downstream of the effects of isolation stress on hedonic food intake in rats

Emotional stress, through elevating corticosterone (CORT) levels may reduce feeding in rodents however when offered palatable food, stressed animals ingest more food compared to non-stressed controls. Nucleus accumbens (NAc) is part of the mesocorticolimbic system and participates in processing rewarding characteristics of food modulating palatable food intake, mainly when glucocorticoids are elevated. A possible mediator of CORT effects is accumbal thyrotropin-releasing hormone (TRH), which reduces chow intake in rats when administered into the NAc. We aimed to study the TRH role in hedonic feeding in stressed rats. For 14 days, animals with ad libitum access to chow or chow plus chocolate milk were either group-housed or singly-housed to induce stress. Rats with access to chocolate milk showed hyperphagia and decreased accumbal TRH mRNA levels, which were potentiated by stress. Results suggest that TRH downregulation was permissive of the increased palatable food intake. TRH injections into NAc of singly-housed animals with palatable food access reduced their food intake and increased serum CORT levels. The accumbal injections of a glucocorticoid receptor antagonist (mifepristone) in stressed rats with palatable food access, reduced only palatable food intake and increased accumbal TRH expression and serum CORT levels. This modulation of TRH mRNA when CORT signaling is modified suggests that accumbal TRH is downstream of glucocorticoids activity, which specifically increase palatable food intake. Our results strengthen the TRH involvement in regulating emotional aspects of hedonic feeding in stressed animals. Finding new therapies directed towards increasing TRHergic activity in NAc may be protective against overeating.

Adult food choices depend on sex and exposure to early-life stress: Underlying brain circuitry, adipose tissue adaptations and metabolic responses

Exposure to early-life stress (ES) increases the risk to develop obesity later in life, and these effects may be sex-specific, but it is currently unknown what underlies the ES-induced metabolic vulnerability. We have previously shown that ES leads to a leaner phenotype under standard chow diet conditions, but to increased fat accumulation when exposed to an unhealthy obesogenic diet. However these diets were fed without a choice. An important, yet under investigated, element contributing to the development of obesity in humans is the choice of the food. There is initial evidence that ES leads to altered food choices but a thorough testing on how ES affects the choice of both the fat and sugar component, and if this is similar in males and females, is currently missing. We hypothesized that ES increases the choice for unhealthy foods, while it at the same time also affects the response to such a diet. In a mouse model for ES, in which mice are exposed to limited nesting and bedding material from postnatal day (P)2–P9, we investigated if ES exposure affected i) food choice with a free choice high-fat high-sugar diet (fcHFHS), ii) the response to such a diet, iii) the brain circuits that regulate food intake and food reward and iv) if such ES effects are sex-specific. We show that there are sex differences in food choice under basal circumstances, and that ES increases fat intake in females when exposed to a mild acute stressor. Moreover, ES impacts the physiologic response to the fcHFHS and the brain circuits regulating food intake in sex-specific manner. Our data highlight sex-specific effects of ES on metabolic functioning and food choice.

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Strong sex differences exist in food choice and metabolism in mice.

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Early-life stress (ES) increases fat intake in females after mild acute stress exposure.

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The physiological response to the diet is affected by ES in a sex-dependent manner.

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ES modulates the hedonic feeding circuitry.

The endocannabinoid system is modulated in reward and homeostatic brain regions following diet-induced obesity in rats: a cluster analysis approach

Objectives

Increased availability of high-calorie palatable food in most countries has resulted in overconsumption of these foods, suggesting that excessive eating is driven by pleasure, rather than metabolic need. The behavior contributes to the rise in eating disorders, obesity, and associated pathologies like diabetes, cardiac disease, and cancers. The mesocorticolimbic dopamine and homeostatic circuits are interconnected and play a central role in palatable food intake. The endocannabinoid system is expressed in these circuits and represents a potent regulator of feeding, but the impact of an obesogenic diet on its expression is not fully known.

Methods

Food intake and body weight were recorded in male Wistar rats over a 6-week free-choice regimen of high fat and sugar; transcriptional regulations of the endocannabinoid system were examined post-mortem in brain reward regions (prefrontal cortex, nucleus accumbens, ventral tegmental area, and arcuate nucleus). K-means cluster analysis was used to classify animals based on individual sensitivity to obesity and palatable food intake. Endocannabinoid levels were quantified in the prefrontal cortex and nucleus accumbens. Gene expression in dopamine and homeostatic systems, including ghrelin and leptin receptors, and classical homeostatic peptides, were also investigated.

Results

The free-choice high-fat -and sugar diet induced hyperphagia and obesity in rats. Cluster analysis revealed that the propensity to develop obesity and excessive palatable food intake was differently associated with dopamine and endocannabinoid system gene expression in reward and homeostatic brain regions. CB2 receptor mRNA was increased in the nucleus accumbens of high sugar consumers, whereas CB1 receptor mRNA was decreased in obesity prone rats.

Conclusions

Transcriptional data are consistent with observations of altered dopamine function in rodents that have access to an obesogenic diet and point to cannabinoid receptors as GPCR targets involved in neuroplasticity mechanisms associated with maladaptive intake of palatable food.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00394-021-02613-0.

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.

The Phantom Satiation Hypothesis of Bariatric Surgery

The excitation of vagal mechanoreceptors located in the stomach wall directly contributes to satiation. Thus, a loss of gastric innervation would normally be expected to result in abrogated satiation, hyperphagia, and unwanted weight gain. While Roux-en-Y-gastric bypass (RYGB) inevitably results in gastric denervation, paradoxically, bypassed subjects continue to experience satiation. Inspired by the literature in neurology on phantom limbs, I propose a new hypothesis in which damage to the stomach innervation during RYGB, including its vagal supply, leads to large-scale maladaptive changes in viscerosensory nerves and connected brain circuits. As a result, satiation may continue to arise, sometimes at exaggerated levels, even in subjects with a denervated or truncated stomach. The same maladaptive changes may also contribute to dysautonomia, unexplained pain, and new emotional responses to eating. I further revisit the metabolic benefits of bariatric surgery, with an emphasis on RYGB, in the light of this phantom satiation hypothesis.

Zona incerta neurons projecting to the ventral tegmental area promote action initiation towards feeding

KEY POINTS

The zona incerta (ZI) and ventral tegmental area (VTA) are brain areas that are both implicated in feeding behaviour. The ZI projects to the VTA, although it has not yet been investigated whether this projection regulates feeding. We experimentally (in)activated the ZI to VTA projection by using dual viral vector technology, and studied the effects on feeding microstructure, the willingness to work for food, general activity and body temperature. Activity of the ZI to VTA projection promotes feeding by facilitating action initiation towards food, as reflected in meal frequency and the willingness to work for food reward, without affecting general activity or directly modulating body temperature. We show for the first time that activity of the ZI to VTA projection promotes feeding, which improves the understanding of the neurobiology of feeding behaviour and body weight regulation.

ABSTRACT

Both the zona incerta (ZI) and the ventral tegmental area (VTA) have been implicated in feeding behaviour. The ZI provides prominent input to the VTA, although it has not yet been investigated whether this projection regulates feeding. Therefore, we investigated the role of ZI to VTA projection neurons in the regulation of several aspects of feeding behaviour. We determined the effects of (in)activation of ZI to VTA projection neurons on feeding microstructure, food-motivated behaviour under a progressive ratio schedule of reinforcement, locomotor activity and core body temperature. To activate or inactivate ZI neurons projecting to the VTA, we used a combination of canine adenovirus-2 in the VTA, as well as Cre-dependent designer receptors exclusively activated by designer drugs (DREADD) or tetanus toxin (TetTox) light chain in the ZI. TetTox-mediated inactivation of ZI to VTA projection neurons reduced food-motivated behaviour and feeding by reducing meal frequency. Conversely, DREADD-mediated chemogenetic activation of ZI to VTA projection neurons promoted food-motivated behaviour and feeding. (In)activation of ZI to VTA projection neurons did not affect locomotor activity or directly regulate core body temperature. Taken together, ZI neurons projecting to the VTA exert bidirectional control overfeeding behaviour. More specifically, activity of ZI to VTA projection neurons facilitate action initiation towards feeding, as reflected in both food-motivated behaviour and meal initiation, without affecting general activity.

The cafeteria diet: A standardized protocol and its effects on behavior

Obesity is a major health risk, with junk food consumption playing a central role in weight gain, because of its high palatability and high-energy nutrients. The Cafeteria (CAF) diet model for animal experiments consists of the same tasty but unhealthy food products that people eat (e.g. hot dogs and muffins), and considers variety, novelty and secondary food features, such as smell and texture. This model, therefore, mimics human eating patterns better than other models. In this paper, we systematically review studies that have used a CAF diet in behavioral experiments and propose a standardized CAF diet protocol. The proposed diet is ad libitum and voluntary; combines different textures, nutrients and tastes, including salty and sweet products; and it is rotated and varied. Our summary of the behavioral effects of CAF diet show that it alters meal patterns, reduces the hedonic value of other rewards, and tends to reduce stress and spatial memory. So far, no clear effects of CAF diet were found on locomotor activity, impulsivity, coping and social behavior.

Blue light at night acutely impairs glucose tolerance and increases sugar intake in the diurnal rodent Arvicanthis ansorgei in a sex-dependent manner

In our modern society, the exposure to light at night (LAN) has increased considerably, which may impact human health negatively. Especially exposure to light at night containing short wavelength emissions (~450–500 nm) can disrupt the normal function of the biological clock, altering sleep‐wake cycles and inducing metabolic changes. Recently, we reported that light at night acutely impairs glucose tolerance in nocturnal rats. However, light at night in nocturnal rodents coincides with their activity period, in contrast to artificial light at night exposure in humans. The aim of this study was to evaluate the acute effects of blue (λ = 490 ± 20 nm) artificial light at night (bALAN) on glucose metabolism and food intake in both male and female diurnal Sudanian grass rats (Arvicanthis ansorgei) fed either regular chow or a free choice high‐fat high sucrose diet (HFHS). In both chow and HFHS fed male Arvicanthis, 1‐hour of bALAN exposure induced a higher glucose response in the oral glucose tolerance test (OGTT) accompanied by a significant decrease in plasma insulin. Furthermore, in HFHS fed animals, bALAN induced an increase in sucrose intake during the dark phase in males but not in females. Additionally, 1‐h of bALAN increased the nonfasted glucose levels together with plasma corticosterone in female grass rats. These results provide new and further evidence for the deleterious effects of exposure to short wavelength emission‐containing artificial light at night on glucose metabolism in a diurnal rodent in a sex‐dependent manner.

Synergistic Effect of Feeding Time and Diet on Hepatic Steatosis and Gene Expression in Male Wistar Rats

OBJECTIVE

Eating out of phase with the endogenous biological clock alters clock and metabolic gene expression in rodents and can induce obesity and type 2 diabetes mellitus. Diet composition can also affect clock gene expression. This study assessed the combined effect of diet composition and feeding time on (1) body composition, (2) energy balance, and (3) circadian expression of hepatic clock and metabolic genes.

METHODS

Male Wistar rats were fed a chow or a free-choice high-fat, high-sugar (fcHFHS) diet, either ad libitum or with food access restricted to either the light or dark period. Body weight, adiposity, and hepatic fat accumulation as well as hepatic clock and metabolic mRNA expression were measured after 5 weeks of the diet. Energy expenditure was measured using calorimetric cages.

RESULTS

Animals with access to the fcHFHS diet only during the light period showed more hepatic fat accumulation than fcHFHS dark-fed animals despite less calories consumed. In contrast, within the chow-fed groups, light-fed animals showed the lowest hepatic fat content, but they also showed the lowest caloric intake. Locomotor activity and heat production followed feeding times, except in the fcHFHS light-fed group. Hepatic clock and metabolic gene expression rhythms also followed timing of food intake. Yet, in the fcHFHS light-fed animals, clock gene expression appeared 3 hours advanced compared with chow light-fed animals, an effect not observed in the fcHFHS dark-fed animals.

CONCLUSIONS

An fcHFHS diet consumed in the light period promotes hepatic fat accumulation and advances clock gene expression in male Wistar rats, likely because of a mismatch between energy intake and expenditure.

Analysis of Western diet, palmitate and BMAL1 regulation of neuropeptide Y expression in the murine hypothalamus and BMAL1 knockout cell models

Western diets that are high in saturated fat and sugar disrupt circadian rhythms, induce weight gain, and lead to metabolic diseases including obesity. However, the mechanistic link between altered circadian rhythms and energy homeostasis remains poorly understood. In C57BL/6J mice, consuming a Western diet for 16 weeks significantly reduced food intake (at zeitgeber 12-16), in association with decreases in hypothalamic expression of the orexigenic neuropeptides, neuropeptide Y (Npy) and agouti-related peptide (AgRP). To examine the acute effects of the most prevalent saturated fatty acid in a Western diet, palmitate, and the role of the core clock gene, Bmal1, in the regulation of hypothalamic feeding neuropeptides, we used heterogeneous and clonal BMAL1 knockout (KO) immortalized hypothalamic cell lines, expressing specific neuropeptides, derived from male (M) and female (F) mice. Both mHypoA-BMAL1-KO/F and mHypoA-BMAL1-KO/M cells demonstrated a loss of circadian rhythmicity in expression of the clock gene, Per2, as compared to wild-type (control) cultures. Loss of BMAL1 also altered the time-dependent expression of Npy and proopiomelanocortin, and disrupted AgRP rhythmicity. Furthermore, palmitate increased BMAL1 binding to the Npy promotor region, and palmitate treatment (50 μM for 24 h) stimulated Npy expression in a BMAL1-dependent manner in both heterogeneous and clonal NPY-expressing female-derived cell models. The results of this study demonstrate that circadian expression of Bmal1 serves as a mechanistic link between Western diet- and palmitate-induced disruptions of the normal rhythmic patterns in hypothalamic feeding-related neuropeptides.

Ileal Transposition in Rats Reduces Energy Intake, Body Weight, and Body Fat Most Efficaciously When Ingesting a High-Protein Diet

Purpose

Ileal transposition (IT) allows exploration of hindgut effects of bariatric procedures in inducing weight loss and reducing adiposity. Here we investigated the role of dietary macronutrient content on IT effects in rats.

Methods

Male Lewis rats consuming one of three isocaloric liquid diets enriched with fat (HF), carbohydrates (HC), or protein (HP) underwent IT or sham surgery. Body weight, energy intake, energy efficiency, body composition, and (meal-induced) changes in plasma GIP, GLP-1, PYY, neurotensin, and insulin levels were measured.

Results

Following IT, HC intake remained highest leading to smallest weight loss among dietary groups. IT in HF rats caused high initial weight loss and profound hypophagia, but the rats caught up later, and finally had the highest body fat content among IT rats. HP diet most efficaciously supported IT-induced reduction in body weight and adiposity, but (as opposed to other diet groups) lean mass was also reduced. Energy efficiency decreased immediately after IT irrespective of diet, but normalized later. Energy intake alone explained variation in post-operative weight change by 80%. GLP-1, neurotensin, and PYY were upregulated by IT, particularly during (0–60 min) and following 17-h post-ingestive intake, with marginal diet effects. Thirty-day post-operative cumulative energy intake was negatively correlated to 17-h post-ingestive PYY levels, explaining 47% of its variation.

Conclusion

Reduction in energy intake underlies IT-induced weight loss, with highest efficacy of the HP diet. PYY, GLP-1, and neurotensin levels are upregulated by IT, of which PYY may be most specifically related to reduced intake and weight loss after IT.

A systematic review on different models of inducing obesity in animals: Advantages and limitations

Several animals have been in the limelight of basic research associated with metabolic diseases like obesity. Obesity can be considered as a significant public health concern in the world. It raises the chances for a variety of disease conditions that includes diabetes, hypertension, liver disease, and cancers, which, in turn, decreases the overall lifespan of adult men and women. The World Health Organization has considered obesity as a global epidemic. Researchers have made several attempts to classify human obesity, but none have been successful. Animal obesity can be classified based on their etiology; however, till now, no animal model of obesity can replicate models of the human condition, they have only provided clues into the causes, aftermaths, and preventive remedy to human adiposity. Over the years, there are varieties of animal models used to induce obesity. Some of them include monogenic, polygenic, surgical, seasonal, and other models of obesity. Apart from the advantages of these models, most of them are accompanied by limitations. The primary purpose of this review is, therefore, to highlight the several models with their advantages and limitations. By knowing the benefits and limitations of animal models of obesity, researchers may be at liberty to select the appropriate one for the study of obesity.

Metabolically healthy obese and metabolic syndrome of the lean: the importance of diet quality. Analysis of MAGNETIC cohort

BACKGROUND

Obesity is considered as an indispensable component of metabolic health assessment and metabolic syndrome diagnosis. The associations between diet quality and metabolic health in lean, young adults have not been yet established whilst data addressing this issue in overweight and obese subjects is scarce. Our analysis aimed to establish the link between diet quality (measured with data-driven dietary patterns and diet quality scores) and metabolic syndrome (MS) in young adults, regardless of their adiposity status.

METHODS

A total of 797 participants aged 18-35 years old were included in the study. Participants were assigned into metabolic syndrome (MS) group if at least two abnormalities within the following parameters were present: blood pressure, triglycerides, total cholesterol, HDL cholesterol, blood glucose. Participants with one or none abnormalities were considered as metabolically healthy subjects (MH), Diet quality was assessed with two approaches: 1) a posteriori by drawing dietary patterns (DPs) with principal component analysis (PCA) and 2) a priori by establishing diet quality scores and the adherence to pro-Healthy-Diet-Index (pHDI) and non-Healthy-Diet-Index (nHDI). Logistic regression with backward selection based on Akaike information criterion was carried out, to identify factors independently associated with metabolic health.

RESULTS

Within the MS group, 31% were of normal weight. Three PCA-driven DPs were identified, in total explaining 30.0% of the variance: "Western" (11.8%), "Prudent" (11.2%) and "Dairy, breakfast cereals & treats" (7.0%). In the multivariate models which included PCA-driven DPs, higher adherence to middle and upper tertiles of "Western" DP (Odds Ratios [OR] and 95% Confidence Intervals [95% CI]: 1.72, 1.07-2.79 and 1.74, 1.07-2.84, respectively), was associated with MS independently of clinical characteristics including BMI and waist-hip ratio (WHR). Similar results were obtained in the multivariate model with diet quality scores - MS was independently associated with higher scores within nHDI (2.2, 0.92-5.28).

CONCLUSIONS

Individuals with MS were more likely to adhere to the western dietary pattern and have a poor diet quality in comparison to metabolically healthy peers, independently of BMI and WHR. It may imply that diet composition, as independent factor, plays a pivotal role in increasing metabolic risk. Professional dietary advice should be offered to all metabolically unhealthy patients, regardless of their body mass status.

Every-Other-Day Feeding Decreases Glycolytic and Mitochondrial Energy-Producing Potentials in the Brain and Liver of Young Mice

Intermittent fasting is used to reduce body mass in obese adult humans and animals. However, information on the impact of one type of intermittent fasting (IF) called every-other-day feeding (EODF) on young animals is scarce. In this study, 1-month-old mice of both sexes were subjected to a 4-week regimen of EODF using age-matched counterparts fed ad libitum as controls. At the end of EODF exposure, experimental male and female mice weighed 14 and 13% less than the control counterparts. The EODF regimen resulted in lower liver levels of glycogen, glucose, and lactate, but did not affect lactate level in mouse cerebral cortex of both sexes. Activities of key glycolytic enzymes (hexokinase, phosphofructokinase, and pyruvate kinase) in liver of experimental mice were lower than those in controls. In the cerebral cortex, only hexokinase and pyruvate kinase activities were lower than in controls, but phosphofructokinase activity was not affected in IF females and was higher in IF males as compared with ad libitum fed males. Mitochondria isolated from liver of IF mice had lower respiratory control ratios, but those from the cortex had the same values as control animals. The concentration of β-hydroxybutyrate and the activity of β-hydroxybutyrate dehydrogenase were lower in the IF mouse liver, but not changed or enhanced in the IF cerebral cortex. Thus, animal responses to IF do not depend significantly on sex and are directed to decrease energy metabolism to save resources, and the effects are more pronounced in the liver than in the brain.

Rodent models of obesity

Obese or overweight people exceed one-third of the global population and obesity along with diabetes mellitus consist basic components of metabolic syndrome, both of which are known cardio-cerebrovascular risk factors with detrimental consequences. These data signify the pandemic character of obesity and the necessity for effective treatments. Substantial advances have been accomplished in preclinical research of obesity by using animal models, which mimic the human disease. In particular, rodent models have been widely used for many decades with success for the elucidation of the pathophysiology of obesity, since they share physiological and genetic components with humans and appear advantageous in their husbandry. The most representative rodents include the laboratory mouse and rat. Within this review, we attempted to consolidate the most widely used mice and rat models of obesity and highlight their strengths as well as weaknesses in a critical way. Our aim was to bridge the gap between laboratory facilities and patient's bed and help the researcher find the appropriate animal model for his/her obesity research. This tactful selection of the appropriate model of obesity may offer more translational derived results. In this regard, we included, the main diet induced models, the chemical/mechanical ones, as well as a selection of monogenic or polygenic models.

Gastric bypass surgery in lean adolescent mice prevents diet-induced obesity later in life

Gastric bypass surgery is the most effective treatment and is often the only option for subjects with severe obesity. However, investigation of critical molecular mechanisms involved has been hindered by confounding of specific effects of surgery and side effects associated with acute surgical trauma. Here, we dissociate the two components by carrying out surgery in the lean state and testing its effectiveness to prevent diet-induced obesity later in life. Body weight and composition of female mice with RYGB performed at 6 weeks of age were not significantly different from sham-operated and age-matched non-surgical mice at the time of high-fat diet exposure 12 weeks after surgery. These female mice were completely protected from high-fat diet-induced obesity and accompanying metabolic impairments for up to 50 weeks. Similar effects were seen in male mice subjected to RYGB at 5-6 weeks, although growth was slightly inhibited and protection from diet-induced obesity was less complete. The findings confirm that RYGB does not indiscriminately lower body weight but specifically prevents excessive diet-induced obesity and ensuing metabolic impairments. This prevention of obesity model should be crucial for identifying the molecular mechanisms underlying gastric bypass surgery.

Influence of the fat/carbohydrate component of snack food on energy intake pattern and reinforcing properties in rodents

Hedonic intake of strongly rewarding foods is independent from biological needs and, thus, a common cause of obesity. The effect of potato chips on energy intake in a snacking model could be explained by their fat/carbohydrate content (FCHc). The present study investigated if the FCHc shapes energy intake patterns and reward processing of satiated rodents. Modulation of energy intake patterns was studied in an established snacking model offering FCHc rich food for 3 × 10 min/day to satiated rats. Reward processing was analyzed by a previously established conditioned place preference tests in satiated mice. The limited access to FCHc rich food led to higher daily energy intake compared to days without access (110 ± 10 vs. 96 ± 5 kcal/day) indicating that fat/carbohydrate intake was not fully compensated by reducing standard chow intake during the rest of the day. Furthermore, fat/carbohydrate snacking led to binge eating episodes with up to 55% of the daily energy intake consumed during limited access. Forced withdrawal from fat/carbohydrate snacking opportunities for six weeks increased the total daily energy intake and the relative amount of energy consumed by FCHc after reintroducing fat/carbohydrate snacking. Snack food and fat/carbohydrate food were powerful food reinforcers in satiated mice in contrast to standard chow. Altogether, these data suggest that the FCHc of snack food has strong reinforcing properties, which are probably responsible for the significant modulation of the amount and pattern of food intake in ad libitum fed animals.

Maternal overnutrition programs hedonic and metabolic phenotypes across generations through sperm tsRNAs

Obesity is a major public health issue worldwide. Easy accessibility of junk food is considered a major contributor to the current obesity epidemic. Thus, the impact of maternal overnutrition in determining disease susceptibility in offspring has received wide attention. It has also been shown that the effects of maternal overnutrition are not limited to the immediate offspring but can also be transmitted to successive generations. Among different epigenetic marks, sperm small noncoding RNAs (sncRNAs) have recently been reported as a direct mediator of acquired traits to the progeny following postnatal trauma or paternal diet. Here, we investigate whether sperm sncRNAs contributes to the transmission of metabolic and hedonic phenotypes across generations following maternal overnutrition.

There is a growing body of evidence linking maternal overnutrition to obesity and psychopathology that can be conserved across multiple generations. Recently, we demonstrated in a maternal high-fat diet (HFD; MHFD) mouse model that MHFD induced enhanced hedonic behaviors and obesogenic phenotypes that were conserved across three generations via the paternal lineage, which was independent of sperm methylome changes. Here, we show that sperm tRNA-derived small RNAs (tsRNAs) partly contribute to the transmission of such phenotypes. We observe increased expression of sperm tsRNAs in the F1 male offspring born to HFD-exposed dams. Microinjection of sperm tsRNAs from the F1-HFD male into normal zygotes reproduces obesogenic phenotypes and addictive-like behaviors, such as increased preference of palatable foods and enhanced sensitivity to drugs of abuse in the resultant offspring. The expression of several of the differentially expressed sperm tsRNAs predicted targets such as CHRNA2 and GRIN3A, which have been implicated in addiction pathology, are altered in the mesolimbic reward brain regions of the F1-HFD father and the resultant HFD-tsRNA offspring. Together, our findings demonstrate that sperm tsRNA is a potential vector that contributes to the transmission of MHFD-induced addictive-like behaviors and obesogenic phenotypes across generations, thereby emphasizing its role in diverse pathological outcomes.

Eating Rewards the Gears of the Clock

Eating behavior is regulated by metabolic and hedonic brain networks, which interact with each other to balance the physiological regulation of hunger and satiety. The daily balance of this regulation is controlled by the central circadian clock. Importantly, metabolic and reward properties of food impact the functioning of circadian clocks, altering the oscillatory activity of the molecular clockwork and circadian rhythms. However, when feeding (metabolic or reward) is timed, the whole circadian system is entrained. Furthermore, besides synchronizing the clock, the timing of both metabolic and reward eating might be crucial for health, to improve circadian physiology, as well as to treat metabolic (e.g., diabetes, obesity) and neurological diseases (e.g., mental, neurodegenerative).

Stressing the importance of choice: Validity of a preclinical free-choice high-caloric diet paradigm to model behavioural, physiological and molecular adaptations during human diet-induced obesity and metabolic dysfunction

Humans have engineered a dietary environment that has driven the global prevalence of obesity and several other chronic metabolic diseases to pandemic levels. To prevent or treat obesity and associated comorbidities, it is crucial that we understand how our dietary environment, especially in combination with a sedentary lifestyle and/or daily-life stress, can dysregulate energy balance and promote the development of an obese state. Substantial mechanistic insight into the maladaptive adaptations underlying caloric overconsumption and excessive weight gain has been gained by analysing brains from rodents that were eating prefabricated nutritionally-complete pellets of high-fat diet (HFD). Although long-term consumption of HFDs induces chronic metabolic diseases, including obesity, they do not model several important characteristics of the modern-day human diet. For example, prefabricated HFDs ignore the (effects of) caloric consumption from a fluid source, do not appear to model the complex interplay in humans between stress and preference for palatable foods, and, importantly, lack any aspect of choice. Therefore, our laboratory uses an obesogenic free-choice high-fat high-sucrose (fc-HFHS) diet paradigm that provides rodents with the opportunity to choose from several diet components, varying in palatability, fluidity, texture, form and nutritive content. Here, we review recent advances in our understanding how the fc-HFHS diet disrupts peripheral metabolic processes and produces adaptations in brain circuitries that govern homeostatic and hedonic components of energy balance. Current insight suggests that the fc-HFHS diet has good construct and face validity to model human diet-induced chronic metabolic diseases, including obesity, because it combines the effects of food palatability and energy density with the stimulating effects of variety and choice. We also highlight how behavioural, physiological and molecular adaptations might differ from those induced by prefabricated HFDs that lack an element of choice. Finally, the advantages and disadvantages of using the fc-HFHS diet for preclinical studies are discussed.

Physiology of energy homeostasis: Models, actors, challenges and the glucoadipostatic loop

The aim of this review is to discuss the physiology of energy homeostasis (EH), which is a debated concept. Thus, we will see that the set-point theory is highly challenged and that other models integrating an anticipative component, such as energy allostasis, seem more relevant to experimental reports and life preservation. Moreover, the current obesity epidemic suggests that EH is poorly efficient in the modern human dietary environment. Non-homeostatic phenomena linked to hedonism and reward seem to profoundly impair EH. In this review, the apparent failed homeostatic responses to energy challenges such as exercise, cafeteria diet, overfeeding and diet-induced weight loss, as well as their putative determinants, are analyzed to highlight the mechanisms of EH. Then, the hormonal, neuronal, and metabolic factors of energy intake or energy expenditure are briefly presented. Last, this review focuses on the contributions of two of the most pivotal and often overlooked determinants of EH: the availability of endogenous energy and the pattern of energy intake. A glucoadipostatic loop model is finally proposed to link energy stored in adipose tissue to EH through changes in eating behavior via leptin and sympathetic nervous system activity.

Impact of Free-Choice Diets High in Fat and Different Sugars on Metabolic Outcome and Anxiety-Like Behavior in Rats

OBJECTIVE

Rats were exposed to free-choice diets (fat plus one of two different sugar solutions, glucose or sucrose), and the metabolic consequences and impact on locomotor activity and anxiety-like behavior were explored.

METHODS

For 3 weeks, 7-week-old male rats were offered either chow only or free-choice high-fat diets differing in their added sugar: no sugar, sucrose, or glucose. In a second experiment, after 2 weeks on the diets, rats were switched from high sucrose to high glucose for two additional weeks. Metabolic end points included body weight, food intake, food choice, glycemic control, metabolic hormones, fat pad weight, brown adipose tissue weight, and gene expression. Behavioral analysis included locomotor and anxiety-like activity in the open field and elevated plus maze.

RESULTS

Both sugar diets enhanced adiposity and induced hyperphagia, favoring unhealthier dietary selection above that of the control diets (chow or free-choice high-fat with no sugar). Despite isocaloric intake in the sugar-containing diets, offering glucose instead of sucrose was associated with improved insulin sensitivity. The sugar-containing diets reduced activity (but with movements of increased velocity) and induced an anxiety-like phenotype.

CONCLUSIONS

Although free-choice diets negatively impacted on metabolism and anxiety-like behavior, replacing sucrose with glucose improved insulin sensitivity and may therefore be better for health.

Effects of long-term high-fat food or methamphetamine intake and serotonin 2C receptors on reversal learning in female rhesus macaques

Perseverative behavior has been highly implicated in addiction. Activation of serotonin 2C receptors (5-HT_2CRs) attenuates cocaine and high caloric food intake, but whether a 5-HT_2CR agonist can reduce high caloric diet (HCD) or methamphetamine (METH) intake and response perseveration remains unknown. Clarifying the role of 5-HT_2CRs in these behaviors will improve knowledge of neurochemical processes that regulate flexible decision-making and whether improvements in decision-making are accompanied by decreases in HCD or METH intake. This study evaluated the effects of long-term HCD and METH intake on reversal learning in female rhesus monkeys. The effects of the 5-HT_2CR agonist WAY163909 on reversal learning before and after extended HCD or METH intake, and on food intake, was also tested. Moreover, we examined whether the 5-HT_2CR is necessary for the effects of WAY163909. WAY163909 was given prior to reversal learning at baseline and after extended HCD or METH intake, and prior to measures of food intake. Extended intake of METH or the HCD increased perseverative errors during reversal. WAY163909 increased correct responses and decreased perseverative errors, both before and after extended HCD or METH intake. Similarly, WAY163909 decreased consumption of a HCD, but not a low caloric diet. The effects of WAY163909 on all these measures were blocked by co-administration with a 5-HT_2CR antagonist. These data indicate that long-term HCD or METH intake disrupts flexible decision-making. Further, the results suggest that reductions in food intake produced by WAY163909 are associated with parallel improvements in decision-making strategies, underscoring the role of the 5-HT_2CR for these behavioral effects.

Maternal High-Protein Diet during Pregnancy Modifies Rat Offspring Body Weight and Insulin Signalling but Not Macronutrient Preference in Adulthood

Diet of mothers during gestation may impact offspring phenotype. This study evaluated the consequences of a maternal High-Protein (HP) diet during gestation on food preferences and phenotypic characteristics in adult rat offspring. Dams were fed a HP or a Normal-Protein (NP) isocaloric diet during gestation only. Weaned female pups were divided into 3 diet groups: NP control or one of two dietary self-selection (DSS) conditions. In DSS1, offspring had a free choice between proteins (100%) or a mix of carbohydrates (88%) and lipids (12%). In DSS2, the choice was between proteins (100%), carbohydrate (100%) or lipids (100%). DSS2 groups consumed more of their energy from protein and lipids, with a decreased carbohydrate intake (p < 0.0001) compared to NP groups, regardless of the maternal diet. Offspring from HP gestation dams fed the DSS2 diet (HPDSS2) had a 41.2% increase of total adiposity compared to NPDSS2 (p < 0.03). Liver Insulin receptor and Insulin substrate receptor 1 expression was decreased in offspring from HP compared to NP gestation dams. These results showed the specific effects of DSS and maternal diet and data suggested that adult, female offspring exposed to a maternal HP diet during foetal life were more prone to adiposity development, in response to postweaning food conditions.