Wheel running eliminates high-fat preference and enhances leptin signaling in the ventral tegmental area

Voluntary wheel running access produces opposite effects in male and female rats on both palatable diet consumption and associated ventral striatal opioid- and dopamine-related gene expression

The relationship between physical activity levels and feeding behaviors has been a focus of preclinical research for decades, yet this interaction has only recently been explored for potential sex differences. The aim of the present study was to isolate sex-dependent effects of voluntary wheel running (RUN) vs. sedentary locked wheel (SED) home cage conditions on palatability-driven feeding behavior using a 2-diet choice task between standard chow and a high-fat diet. The sex-dependent effects of physical activity on feeding behavior were examined following a within-subject novel reversal design of physical activity conditions (i.e., RUN > SED > RUN), to assess temporal sensitivity of the interaction. Following the final 2 weeks of reestablished and sustained RUN vs. SED conditions in separate groups of both males and females, reward-related opioid and dopamine gene expression within the nucleus accumbens (Acb) brain region were analyzed. Results demonstrated that the initial RUN > SED transition led to sex-dependent effects of SED condition, as males increased, and females decreased their high fat consumption, compared to their respective high fat consumption during previous RUN condition phase. Following reintroduction to the RUN condition, males decreased, and females increased their high fat consumption, compared to their separate SED control group. Last, sex-dependent shifts in ventral striatal opioid- and dopamine-related gene expression were observed to parallel the behavioral effects. The major findings of the study reveal that SED and RUN home cage conditions shift palatability-driven feeding in the opposite direction for males and females, these effects are sensitive to reversal, and these sex-dependent feeding behaviors track sex-dependent changes to critical reward-related gene expression patterns in the Acb. Considering the present high rates of sedentary behavior and obesity, furthering our understanding of the interaction between physical activity (or lack thereof) and feeding behavior should be a priority, especially in the context of these divergent sex-dependent outcomes.

Metabolic hormone action in the VTA: Reward-directed behavior and mechanistic insights

Dysfunctional signaling in midbrain reward circuits perpetuates diseases characterized by compulsive overconsumption of rewarding substances such as substance abuse, binge eating disorder, and obesity. Ventral tegmental area (VTA) dopaminergic activity serves as an index for how rewarding stimuli are perceived and triggers behaviors necessary to obtain future rewards. The evolutionary linking of reward with seeking and consuming palatable foods ensured an organism's survival, and hormone systems that regulate appetite concomitantly developed to regulate motivated behaviors. Today, these same mechanisms serve to regulate reward-directed behavior around food, drugs, alcohol, and social interactions. Understanding how hormonal regulation of VTA dopaminergic output alters motivated behaviors is essential to leveraging therapeutics that target these hormone systems to treat addiction and disordered eating. This review will outline our current understanding of the mechanisms underlying VTA action of the metabolic hormones ghrelin, glucagon-like peptide-1, amylin, leptin, and insulin to regulate behavior around food and drugs of abuse, highlighting commonalities and differences in how these five hormones ultimately modulate VTA dopamine signaling.

Exercise modifies fatty acid perception and metabolism

AIM

Obesity is a major public health issue, which is associated with several chronic diseases. In rodents, voluntary wheel running (VWR) is a type of exercise that influences ingestive behavior. This study aims to investigate the possible function of VWR activity in the perception of fat taste and if it mitigates the immediate effects of fatty acid (FA) ingestion.

METHODS

Male C57BL/6 mice were arbitrarily assigned to either a sedentary (SED) lifestyle or free access to a running wheel after 5 weeks of dietary regimen. Later these mice groups were used in the investigations on fat preference, metabolic tolerance, and electrophysiology. Diet-induced alterations in CD36 and GPR120 expression that are related to fat perception and the capacitative calcium signaling caused by FA in taste bud cells (TBCs) were also examined.

RESULTS

In obese groups, VWR temporarily reduced body weight, demonstrated improvement in preference scores for FA, and recovered from a deterioration in glucose homeostasis. In CD36-positive TBCs, electrophysiological investigations showed alterations in [Ca2+ ]i caused by FA. Further, in the TBCs of circumvallate papillae, there are differences in the expression of the genes CD36 and GPR120 between the active and SED controls. Obese mice also show lower incentive salience for long-chain fatty acids (LCFA) and adapted to the reward system of VWR which may lead to improved incentive salience accredited to wheel running.

CONCLUSION

In conclusion, this study provides the first evidence that VWR causes orosensory adaptations to fat and appears to alter taste preference for LCFAs.

Voluntary physical activity modulates self-selection of a high-caloric choice diet in male Wistar rats

Physical exercise training has been positioned as a behavioral strategy to prevent or alleviate obesity via promotion of energy expenditure as well as modulation of energy intake resulting from changes in dietary preference. Brain adaptations underlying the latter process are incompletely understood. Voluntary wheel running (VWR) is a self-reinforcing rodent paradigm that mimics aspects of human physical exercise training. Behavioral and mechanistic insight from such fundamental studies can help optimize therapies that improve body weight and metabolic health based on physical exercise training in humans. To assess the effects of VWR on dietary self-selection, male Wistar rats were given access to a two-component "no-choice" control diet (CD; consisting of prefabricated nutritionally complete pellets and a bottle with tap water) or a four-component free-choice high-fat high-sucrose diet (fc-HFHSD; consisting of a container with prefabricated nutritionally complete pellets, a dish with beef tallow, a bottle with tap water, and a bottle with 30% sucrose solution). Metabolic parameters and baseline dietary self-selection behavior during sedentary (SED) housing were measured for 21 days, after which half of the animals were allowed to run on a vertical running wheel (VWR) for another 30 days. This resulted in four experimental groups (SED^CD, SED^fc-HFHSD, VWR^CD, and VWR^fc-HFHSD). Gene expression of opioid and dopamine neurotransmission components, which are associated with dietary self-selection, was assessed in the lateral hypothalamus (LH) and nucleus accumbens (NAc), two brain regions involved in reward-related behavior, following 51 and 30 days of diet consumption and VWR, respectively. Compared to CD controls, consumption of fc-HFHSD before and during VWR did not alter total running distances. VWR and fc-HFHSD had opposite effects on body weight gain and terminal fat mass. VWR transiently lowered caloric intake and increased and decreased terminal adrenal and thymus mass, respectively, independent of diet. VWR during fc-HFHSD consumption consistently increased CD self-selection, had an acute negative effect on fat self-selection, and a delayed negative effect on sucrose solution self-selection compared to SED controls. Gene expression of opioid and dopamine neurotransmission components in LH and NAc were unaltered by fc-HFHSD or VWR. We conclude that VWR modulates fc-HFHSD component self-selection in a time-dependent manner in male Wistar rats.

Compensatory eating behaviors in male and female rats in response to exercise training

Exercise is often used as a strategy for weight loss maintenance. In preclinical models, we have shown that exercise may be beneficial because it counters the biological drive to regain weight. However, our studies have demonstrated sex differences in the response to exercise in this context. In the present study, we sought to better understand why females and males exhibit different compensatory food eating behaviors in response to regular exercise. Using a forced treadmill exercise paradigm, we measured weight gain, energy expenditure, food intake in real time, and the anorectic effects of leptin. The 4-wk exercise training resulted in reduced weight gain in males and sustained weight gain in females. In male rats, exercise decreased intake, whereas it increased food intake in females. Our results suggest that the anorectic effects of leptin were not responsible for these sex differences in appetite in response to exercise. If these results translate to the human condition, they may reveal important information for the use and application of regular exercise programs.

Role of glucocorticoid signaling in exercise-associated changes in high-fat diet preference in rats

The simultaneous introduction of wheel running (WR) and diet choice (high-carbohydrate chow vs. high-fat diet) results in sex-specific diet choice patterns in rats. WR induces a high-fat (HF) diet avoidance, and such avoidance persists in the majority of males, but not females, throughout a 2-wk period. Exercise is a physiological stressor that activates the hypothalamic-pituitary-adrenal (HPA) axis and stimulates glucocorticoid (GC) release, which can alter dietary preferences. Here, we examined the role of the HPA axis and GC signaling in mediating exercise-induced changes in diet preference and the associated neurobiological adaptations that may underlie sex differences in diet choice patterns. Experiment 1 revealed that adrenalectomy did not significantly alter the initiation and persistence of running-induced HF diet avoidance in male rats. Experiment 2 showed that acute WR resulted in greater neural activation than chronic WR in the medial prefrontal (mPFC) and insular cortices (IC) in male rats. Experiment 3 revealed sex differences in the molecular adaptation to exercise and diet preference. First, exercise increased gene expression of fkbp5 in the mPFC, IC, and hippocampus of WR females but had limited influence in males. Second, male and female WR rats that reversed or maintained HF diet avoidance showed distinct sex- and HF diet preference-dependent expression profiles of genes involved in cortical GC signaling (e.g., nr3c1, nr3c2, and src1). Taken together, our results suggest sex differences in region-specific neural adaptations may underlie sex differences in diet preference and the health benefits from exercise.

Voluntary wheel running effects on intra-accumbens opioid high-fat feeding and locomotor behavior in Sprague-Dawley and Wistar rat strains

The present study examined the influence of physical activity vs. sedentary home cage conditions on baseline and opioid-driven high-fat feeding behaviors in two common strains of laboratory rats. Sprague-Dawley and Wistar rats were singly housed with either access to a voluntary running wheel (RUN) or locked-wheel (SED) for 5 weeks, before being stereotaxically implanted with bilateral cannulae targeting the nucleus accumbens. Following recovery, with RUN or SED conditions continuing the duration of the experiment, all rats were given 2 h daily access to a high-fat diet for 6 consecutive days to establish a stable baseline intake. Over the next 2 weeks, all subjects were administered the μ-opioid agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO) (multiple dose range) or saline into the nucleus accumbens, immediately followed by 2 h access to a high-fat diet. Drug treatments were separated by at least 1 day and treatment order was counterbalanced. Baseline consumption of the high-fat diet during the 1-week baseline acclimation period did not differ between RUN and SED groups in either rat strain. Higher doses of DAMGO produced increased fat consumption in both strains of rats, yet no differences were observed between RUN vs. SED treated groups. However, SED treatment produced a greater locomotor response following intra-accumbens DAMGO administration, compared to the RUN condition, during the 2 h feeding session. The data suggest that the animals housed in sedentary versus voluntary wheel running conditions may differ in behavioral tolerance to the locomotor but not the orexigenic activating properties of intra-accumbens DAMGO treatment.

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.

Voluntary wheel running reduces weight gain in mice by decreasing high-fat food consumption

We investigated whether wheel running for just 30 min on 5 days each week, an exercise routine based on recommended levels of physical activity for adults, regulates body weight and food intake in mice. Male C57BL/6 mice were divided into groups and given ad libitum access to high-fat food and standard chow or standard chow only. For 30 min on 5 days each week, mice were treated with an in-cage running wheel which was either open to allow voluntary exercise or locked and could not rotate for control. Wheel running reduced weight gain and fat mass among mice fed high-fat food and standard chow, but not mice fed standard chow only. Wheel running decreased high-fat food consumption. Standard chow intake was unchanged. Mice provided with a locked running wheel but pair-fed the same amount of food as wheel running mice also displayed reduced weight gain and fat mass. We conclude that voluntary wheel running for 30 min on 5 days each week reduced weight gain and fat mass in mice by preferentially decreasing high-fat food intake. This model of voluntary wheel running can be used to investigate mechanisms underlying the benefits of exercise on body weight and food intake, informing obesity intervention strategies for humans.

The effects of scheduled running wheel access on binge-like eating behavior and its consequences

Binge eating disorder (BED) is an eating disorder involving repeated, intermittent over consumption of food in brief periods of time, usually with no compensatory behaviors. There are few successful treatments and the underlying neural mechanisms remain unclear. In the current study, we hypothesized that voluntary running wheel (RW) activity could reduce binge-like eating behavior in a rat model. Rats were given intermittent (3 times/wk) limited (1hr) access to a high-fat food (Crisco), in addition to continuously available chow. Crisco was available every Mon, Wed, and Fri for 1hr before dark onset. Rats were divided into 2 groups: those with RW access during the first half of the experiment and sedentary during the second half (RW-SED) and those that were sedentary during the first half of the experiment and had RW access during the second half (SED-RW). Crisco intake was significantly less in both groups during the period of time with a RW present. Within the bingeing RW-SED rats, the gene expression of the orexigenic neuropeptides AgRP and NPY were similar to a non-bingeing sedentary control (CON) group, while the expression of the anorexigenic neuropeptide POMC was significantly increased relative to the SED-RW and CON groups. Despite elevated POMC, the rats continued to binge. Additionally, within both groups, the gene expression of the D2R and Oprm1 in the NAc and the VTA were altered suggesting that the reward system was stimulated by both the bingeing behavior and the running wheel activity. Overall, access to a RW and the resulting activity significantly reduced binge-like behavior as well as modulated the effects of binging on brain appetite and reward systems.

Ovarian hormones mediate running-induced changes in high fat diet choice patterns in female rats

Physical inactivity and increased consumption of energy dense, high fat (HF) foods often leads to a state of positive energy balance. Regular exercise can facilitate the maintenance of a healthy body weight and mediate changes in dietary selection. Past studies using a two-diet choice (chow vs. HF) and voluntary wheel running paradigm found that when a novel HF diet and wheel running are simultaneously introduced, male rats show complete and persistent HF diet avoidance whereas the majority of females show HF diet avoidance for a few days, but then revert to HF diet preference. Ovariectomy (OVX) appears to decrease preference for the HF diet bringing it closer to that of males. Given that estradiol but not progesterone mediates changes in food intake and energy balance, we hypothesized that estradiol signaling is required for the reversal of HF diet avoidance in female rats. Accordingly, Experiment 1 compared the persistency of running-induced HF diet avoidance in males, sham-operated females, and OVX rats with replacement of oil vehicle, estradiol benzoate (E), progesterone (P), or both (E + P). The number of wheel running rats that either avoided or preferred the HF diet varied with hormone treatment. The reversal of HF diet avoidance in running females and OVX E + P rats occurred more rapidly and frequently than male running rats. E + P but not E or P replaced OVX wheel running rats significantly reversed HF diet avoidance. OVX oil rats avoided HF diet to the same extent as male rats for the first 11 days of diet choice and then rapidly increased HF diet intake and began preferring it. This incomplete elimination of sex differences suggests that developmental factors or androgens might play a role in sustaining running-induced HF diet avoidance. Subsequently, Experiment 2 aimed to determine the role of androgens in the persistency of running-associated HF diet avoidance with sham-operated and orchiectomized (GDX) male rats. Both intact and GDX male running rats persistently avoided the HF diet to the same extent. Taken together, these results suggest that activational effects of ovarian hormones play a role in female specific running-induced changes in diet choice patterns. Furthermore, the activational effects of androgens are not required for the expression of HF diet avoidance in males.

Sex determines effect of physical activity on diet preference: Association of striatal opioids and gut microbiota composition

Previous studies suggest an interaction between the level of physical activity and diet preference. However, this relationship has not been well characterized for sex differences that may exist. The present study examined the influence of sex on diet preference in male and female Wistar rats that were housed under either sedentary (no wheel access) (SED) or voluntary wheel running access (RUN) conditions. Following a 1 week acclimation period to these conditions, standard chow was replaced with concurrent ad libitum access to a choice of 3 pelleted diets (high-fat, high-sucrose, and high-corn starch) in the home cage. SED and RUN conditions remained throughout the next 4 week diet preference assessment period. Body weight, running distance, and intake of each diet were measured daily. At the conclusion of the 4 week diet preference test, animals were sacrificed and brains were collected for mRNA analysis. Fecal samples were also collected before and after the 4 week diet preference phase to characterize microbiota composition. Results indicate sex dependent interactions between physical activity and both behavioral and physiological measures. Females in both RUN and SED conditions preferred the high-fat diet, consuming significantly more high-fat diet than either of the other two diets. While male SED rats also preferred the high-fat diet, male RUN rats consumed significantly less high-fat diet than the other groups, instead preferring all three diets equally. There was also a sex dependent influence of physical activity on both reward related opioid mRNA expression in the ventral striatum and the characterization of gut microbiota. The significant sex differences in response to physical activity observed through both behavioral and physiological measures suggest potential motivational or metabolic difference between males and females. The findings highlight the necessity for further exploration between male and female response to physical activity and feeding behavior.

The Determinants of Leptin Levels in Diabetic and Nondiabetic Saudi Males

Objective. This study aimed to identify the main determinants of serum leptin levels. Methods. A sample of 113 Saudi adult males (55 diabetic and 58 nondiabetic) was selected according to the inclusion and exclusion criteria identified below. Blood samples were taken from participants after fasting for 12 hours. For diabetic patients, the insulin dose was given 12 hours before. In general, the study instrument consisted of blood biochemical tests. Metabolic parameters, glycosylated hemoglobin (HbA1c), low-density lipoprotein (LDL), high-density lipoprotein (HDL), cholesterol, and triglyceride (TG), and adipokines, leptin, adiponectin, visfatin, and resistin, were measured. Multivariate model was utilized to identify the relationship between leptin levels and the independent variables. Results. When adjusted for resistin in the diabetic group, the results demonstrated a significant relationship between visfatin, LDL and TG, and leptin levels (p < 0.05). However, when controlled for resistin, the effect of LDL and TG disappeared while that of visfatin stayed in the model. For the nondiabetic group, the results indicated a significant relationship between insulin, BMI, and leptin levels when adjusted for resistin (p < 0.05). However, the effect of insulin disappeared when the model was controlled for resistin. The study results found no relationship between leptin and adiponectin levels in either the diabetic or nondiabetic group and whether adjusted or controlled for resistin. Conclusion. This study provided better understanding of the metabolism of leptin and unveiled the major determinants of leptin levels in diabetic and nondiabetic males. In conclusion, these results show that the association between leptin and metabolic parameters decreases with the progress of disease.

Voluntary wheel-running attenuates insulin and weight gain and affects anxiety-like behaviors in C57BL6/J mice exposed to a high-fat diet

It is widely accepted that lifestyle plays a crucial role on the quality of life in individuals, particularly in western societies where poor diet is correlated to alterations in behavior and the increased possibility of developing type-2 diabetes. While exercising is known to produce improvements to overall health, there is conflicting evidence on how much of an effect exercise has staving off the development of type-2 diabetes or counteracting the effects of diet on anxiety. Thus, this study investigated the effects of voluntary wheel-running access on the progression of diabetes-like symptoms and open field and light-dark box behaviors in C57BL/6J mice fed a high-fat diet. C57BL/6J mice were placed into either running-wheel cages or cages without a running-wheel, given either regular chow or a high-fat diet, and their body mass, food consumption, glucose tolerance, insulin and c-peptide levels were measured. Mice were also exposed to the open field and light-dark box tests for anxiety-like behaviors. Access to a running-wheel partially attenuated the obesity and hyperinsulinemia associated with high-fat diet consumption in these mice, but did not affect glucose tolerance or c-peptide levels. Wheel-running strongly increased anxiety-like and decreased explorative-like behaviors in the open field and light-dark box, while high-fat diet consumption produced smaller increases in anxiety. These results suggest that voluntary wheel-running can assuage some, but not all, of the physiological problems associated with high-fat diet consumption, and can modify anxiety-like behaviors regardless of diet consumed.

Corticosterone administration in drinking water decreases high-fat diet intake but not preference in male rats

One of the mechanisms through which regular exercise contributes to weight maintenance could be by reducing intake and preference for high-fat (HF) diets. Indeed, we previously demonstrated that wheel-running rats robustly reduced HF diet intake and preference. The reduced HF diet preference by wheel running can be so profound that the rats consumed only the chow diet and completely avoided the HF diet. Because previous research indicates that exercise activates the hypothalamic-pituitary-adrenal axis and increases circulating levels of corticosterone, this study tested the hypothesis that elevation of circulating corticosterone is involved in wheel running-induced reduction in HF diet preference in rats.Experiment 1 measured plasma corticosterone levels under sedentary and wheel-running conditions in the two-diet-choice (high-carbohydrate chow vs. HF) feeding regimen. The results revealed that plasma corticosterone is significantly increased and positively correlated with the levels of running in wheel-running rats with two-diet choice.Experiments 2 and 3 determined whether elevated corticosterone without wheel running is sufficient to reduce HF diet intake and preference. Corticosterone was elevated by adding it to the drinking water. Compared with controls, corticosterone-drinking rats had reduced HF diet intake and body weight, but the HF diet preference between groups did not differ. The results of this study support a role for elevated corticosterone on the reduced HF diet intake during wheel running. The elevation of corticosterone alone, however, is not sufficient to produce a robust reduction in HF diet preference.

Moderate voluntary exercise attenuates the metabolic syndrome in melanocortin-4 receptor-deficient rats showing central dopaminergic dysregulation

Objective

Melanocortin-4 receptors (MC4Rs) are highly expressed by dopamine-secreting neurons of the mesolimbic tract, but their functional role has not been fully resolved. Voluntary wheel running (VWR) induces adaptations in the mesolimbic dopamine system and has a myriad of long-term beneficial effects on health. In the present experiments we asked whether MC4R function regulates the effects of VWR, and whether VWR ameliorates MC4R-associated symptoms of the metabolic syndrome.

Methods

Electrically evoked dopamine release was measured in slice preparations from sedentary wild-type and MC4R-deficient Mc4r^K314X (HOM) rats. VWR was assessed in wild-type and HOM rats, and in MC4R-deficient loxTB^Mc4r mice, wild-type mice body weight-matched to loxTB^Mc4r mice, and wild-type mice with intracerebroventricular administration of the MC4R antagonist SHU9119. Mesolimbic dopamine system function (gene/protein expression) and metabolic parameters were examined in wheel-running and sedentary wild-type and HOM rats.

Results

Sedentary obese HOM rats had increased electrically evoked dopamine release in several ventral tegmental area (VTA) projection sites compared to wild-type controls. MC4R loss-of-function decreased VWR, and this was partially independent of body weight. HOM wheel-runners had attenuated markers of intracellular D1-type dopamine receptor signaling despite increased dopamine flux in the VTA. VWR increased and decreased ΔFosB levels in the nucleus accumbens (NAc) of wild-type and HOM runners, respectively. VWR improved metabolic parameters in wild-type wheel-runners. Finally, moderate voluntary exercise corrected many aspects of the metabolic syndrome in HOM runners.

Conclusions

Central dopamine dysregulation during VWR reinforces the link between MC4R function and molecular and behavioral responding to rewards. The data also suggest that exercise can be a successful lifestyle intervention in MC4R-haploinsufficient individuals despite reduced positive reinforcement during exercise training.

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MC4R-deficiency causes metabolic syndrome.

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Loss of MC4R signaling decreases voluntary wheel running (VWR).

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Despite moderate amounts of VWR, MC4R-associated metabolic syndrome is severely attenuated.

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MC4R-deficiency is associated with mesolimbic dopamine dysregulation during VWR.

Effects of physical exercise on central nervous system functions: a review of brain region specific adaptations

Pathologies of central nervous system (CNS) functions are involved in prevalent conditions such as Alzheimer's disease, depression, and Parkinson's disease. Notable pathologies include dysfunctions of circadian rhythm, central metabolism, cardiovascular function, central stress responses, and movement mediated by the basal ganglia. Although evidence suggests exercise may benefit these conditions, the neurobiological mechanisms of exercise in specific brain regions involved in these important CNS functions have yet to be clarified. Here we review murine evidence about the effects of exercise on discrete brain regions involved in important CNS functions. Exercise effects on circadian rhythm, central metabolism, cardiovascular function, stress responses in the brain stem and hypothalamic pituitary axis, and movement are examined. The databases Pubmed, Web of Science, and Embase were searched for articles investigating regional brain adaptations to exercise. Brain regions examined included the brain stem, hypothalamus, and basal ganglia. We found evidence of multiple regional adaptations to both forced and voluntary exercise. Exercise can induce molecular adaptations in neuronal function in many instances. Taken together, these findings suggest that the regional physiological adaptations that occur with exercise could constitute a promising field for elucidating molecular and cellular mechanisms of recovery in psychiatric and neurological health conditions.

Wheel running decreases palatable diet preference in Sprague-Dawley rats

Physical activity has beneficial effects on not only improving some disease conditions but also by preventing the development of multiple disorders. Experiments in this study examined the effects of wheel running on intakes of chow and palatable diet e.g. high fat (HF) or high sucrose (HS) diet in male and female Sprague-Dawley rats. Experiment 1 demonstrated that acute wheel running results in robust HF or HS diet avoidance in male rats. Although female rats with running wheel access initially showed complete avoidance of the two palatable diets, the avoidance of the HS diet was transient. Experiment 2 demonstrated that male rats developed decreased HF diet preferences regardless of the order of diet and wheel running access presentation. Running associated changes in HF diet preference in females, on the other hand, depended on the testing schedule. In female rats, simultaneous presentation of the HF diet and running access resulted in transient complete HF diet avoidance whereas running experience prior to HF diet access did not affect the high preference for the HF diet. Ovariectomy in females resulted in HF diet preference patterns that were similar to those in male rats during simultaneous exposure of HF and wheel running access but similar to intact females when running occurred before HF exposure. Overall, the results demonstrated wheel running associated changes in palatable diet preferences that were in part sex dependent. Furthermore, ovarian hormones play a role in some of the sex differences. These data reveal complexity in the mechanisms underlying exercise associated changes in palatable diet preference.

Wheel running reduces high-fat diet intake, preference and mu-opioid agonist stimulated intake

The ranges of mechanisms by which exercise affects energy balance remain unclear. One potential mechanism may be that exercise reduces intake and preference for highly palatable, energy dense fatty foods. The current study used a rodent wheel running model to determine whether and how physical activity affects HF diet intake/preference and reward signaling. Experiment 1 examined whether wheel running affected the ability of intracerebroventricular (ICV) μ opioid receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO) to increase HF diet intake. Experiment 2 examined the effects of wheel running on the intake of and preference for a previously preferred HF diet. We also assessed the effects of wheel running and diet choice on mesolimbic dopaminergic and opioidergic gene expression. Experiment 1 revealed that wheel running decreased the ability of ICV DAMGO administration to stimulate HF diet intake. Experiment 2 showed that wheel running suppressed weight gain and reduced intake and preference for a previously preferred HF diet. Furthermore, the mesolimbic gene expression profile of wheel running rats was different from that of their sedentary paired-fed controls but similar to that of sedentary rats with large HF diet consumption. These data suggest that alterations in preference for palatable, energy dense foods play a role in the effects of exercise on energy homeostasis. The gene expression results also suggest that the hedonic effects of exercise may substitute for food reward to limit food intake and suppress weight gain.

Differential modulation of arcuate nucleus and mesolimbic gene expression levels by central leptin in rats on short-term high-fat high-sugar diet

OBJECTIVE

Leptin resistance is a common hallmark of obesity. Rats on a free-choice high-fat high-sugar (fcHFHS) diet are resistant to peripherally administered leptin. The aim of this study was to investigate feeding responses to central leptin as well as the associated changes in mRNA levels in hypothalamic and mesolimbic brain areas.

DESIGN AND METHODS

Rats on a CHOW or fcHFHS diet for 8 days received leptin or vehicle intracerebro(lateral)ventricularly (ICV) and food intake was measured 5 h and 24 h later. Four days later, rats were sacrificed after ICV leptin or vehicle and mRNA levels were quantified for hypothalamic pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) and for preproenkephalin (ppENK) in nucleus accumbens and tyrosine hydroxylase (TH) in ventral tegmental area (VTA).

RESULTS

ICV leptin decreased caloric intake both in CHOW and fcHFHS rats. In fcHFHS, leptin preferentially decreased chow and fat intake. Leptin increased POMC and decreased NPY mRNA in CHOW, but not in fcHFHS rats. In CHOW rats, leptin had no effect on ppENK mRNA and decreased TH mRNA. In fcHFHS, leptin decreased ppENK mRNA and increased TH mRNA.

CONCLUSION

Despite peripheral and arcuate leptin resistance, central leptin suppresses feeding in fcHFHS rats. As the VTA and nucleus accumbens are still responsive to leptin, these brain areas may therefore, at least partly, account for the leptin-induced feeding suppression in rats on a fcHFHS diet.

Effects of early-life exposure to Western diet and wheel access on metabolic syndrome profiles in mice bred for high voluntary exercise

Experimental studies manipulating diet and exercise have shown varying effects on metabolic syndrome components in both humans and rodents. To examine the potential interactive effects of diet, exercise and genetic background, we studied mice from four replicate lines bred (52 generations) for high voluntary wheel running (HR lines) and four unselected control lines (C). At weaning, animals were housed for 60 days with or without wheels and fed either a standard chow or Western diet (WD, 42% kcal from fat). Four serial (three juvenile and one adult) blood samples were taken to measure fasting total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), triglycerides and glucose. Western diet was obesogenic for all mice, even after accounting for the amount of wheel running and kilojoules consumed. Western diet significantly raised glucose as well as TC and HDL-C concentrations. At the level of individual variation (repeatability), there was a modest correlation (r = 0.3-0.5) of blood lipids over time, which was reduced with wheel access and/or WD. Neither genetic selection history nor wheel access had a statistically significant effect on blood lipids. However, HR and C mice had divergent ontogenetic trajectories for body mass and caloric intake. HR mice also had lower adiposity, an effect that was dependent on wheel access. The environmental factors of diet and wheel access had pronounced effects on body mass, food consumption and fasting glucose concentrations, interacting with each other and/or with genetic strain. These data underscore the importance (and often unpredictable nature) of genotype-by-environment and environment-by-environment interactions when studying body weight regulation.

Early postweaning exercise improves central leptin sensitivity in offspring of rat dams fed high-fat diet during pregnancy and lactation

Maternal high-fat (HF) diet has long-term consequences on the metabolic phenotype of the offspring. Here, we determined the effects of postweaning exercise in offspring of rat dams fed HF diet during gestation and lactation. Pregnant Sprague-Dawley rats were maintained on chow or HF diet throughout gestation and lactation. All pups were weaned onto chow diet on postnatal day (PND) 21. At 4 wk of age, male pups were given free access to running wheels (RW) or remained sedentary (SED) for 3 wk, after which all rats remained sedentary, resulting in four groups: CHOW-SED, CHOW-RW, HF-SED, and HF-RW. Male HF offspring gained more body weight by PND7 compared with CHOW pups and maintained this weight difference through the entire experiment. Three weeks of postweaning exercise did not affect body weight gain in either CHOW or HF offspring, but reduced adiposity in HF offspring. Plasma leptin was decreased at the end of the 3-wk running period in HF-RW rats but was not different from HF-SED 9 wk after the exercise period ended. At 14 wk of age, intracerebroventricular injection of leptin suppressed food intake in CHOW-SED, CHOW-RW, and HF-RW, while it did not affect food intake in HF-SED group. At death, HF-RW rats also had higher leptin-induced phospho-STAT3 level in the arcuate nucleus than HF-SED rats. Both maternal HF diet and postweaning exercise had effects on hypothalamic neuropeptide and receptor mRNA expression in adult offspring. Our data suggest that postweaning exercise improves central leptin sensitivity and signaling in this model.

Leptin overexpression in VTA trans-activates the hypothalamus whereas prolonged leptin action in either region cross-desensitizes

High-fat feeding or CNS leptin overexpression in chow-fed rats results in a region-specific cellular leptin resistance in medial basal hypothalamic regions and the ventral tegmental area (VTA). The present investigation examined the effects of targeted chronic leptin overexpression in the VTA as compared with the medial basal hypothalamus on long-term body weight homeostasis. The study also examined if this targeted intervention conserves regional leptin sensitivity or results in localized leptin resistance. Cellular leptin resistance was assessed by leptin-stimulated phosphorylation of signal transducers and activators of transcription 3 (STAT3). Tyrosine hydroxylase was measured in hypothalamus and VTA along with brown adipose tissue uncoupling protein 1. Leptin overexpression in VTA tempered HF-induced obesity, but to a slightly lesser extent than that with leptin overexpression in the hypothalamus. Moreover, the overexpression of leptin in the VTA stimulated cellular STAT3 phosphorylation in several regions of the medial basal hypothalamus, whereas verexpression in the hypothalamus did not activate STAT3 signaling in the VTA. This unidirectional trans-stimulation did not appear to involve migration of either the vector or the gene product. Long-term leptin overexpression in either the medial basal hypothalamus or VTA caused desensitization of leptin signaling in the treated region and cross-desensitization of leptin signaling in the untreated region. These results demonstrate a role of leptin receptors in the VTA in long-term body weight regulation, but the trans-activation of the hypothalamus following VTA leptin stimulation suggests that an integrative response involving both brain regions may account for the observed physiological outcomes.

Anorexic effects of intra-VTA leptin are similar in low-fat and high-fat-fed rats but attenuated in a subgroup of high-fat-fed obese rats

Leptin is an adiposity hormone that plays an important role in regulating food intake and energy homeostasis. This study investigated the effects of a high-fat (HF) and a low-fat, high-carbohydrate/sugar (LF) diet on leptin sensitivity in the ventral tegmental area (VTA) in rats. The animals were exposed to a HF or LF diet for 16 weeks. Then the effects of intra-VTA leptin (150 and 500 ng/side, unilateral dose) on food intake and body weights were investigated while the animals were maintained on the HF or LF diet. Long-term exposure to the HF or LF diet led to similar body weight gain in these groups. The HF-fed animals consumed a smaller amount of food by weight than the LF-fed animals but both groups consumed the same amount of calories. The bilateral administration of leptin into the VTA decreased food intake (72 h) and body weights (48 h) to a similar degree in the HF and LF-fed animals. When the HF-fed animals were ranked by body weight gain it was shown that the diet-induced obese rats (HF-fed DIO, upper quartile for weight gain) were less sensitive to the effects of leptin on food intake and body weights than the diet-resistant rats (HF-fed DR, lower quartile for weight gain). A control experiment with fluorescent Cy3-labeled leptin showed that leptin did not spread beyond the borders of the VTA. This study indicates that leptin sensitivity in the VTA is the same in animals that are exposed to a HF or LF diet. However, HF-fed DIO rats are less sensitive to the effects of leptin in the VTA than HF-fed DR rats. Leptin resistance in the VTA might contribute to overeating and weight gain when exposed to a HF diet.

Simultaneous introduction of a novel high fat diet and wheel running induces anorexia

Voluntary wheel running (WR) is a form of physical activity in rodents that influences ingestive behavior. The present report describes an anorexic behavior triggered by the simultaneous introduction of a novel diet and WR. This study examined the sequential, compared with the simultaneous, introduction of a novel high-fat (HF) diet and voluntary WR in rats of three different ages and revealed a surprising finding; the simultaneous introduction of HF food and voluntary WR induced a behavior in which the animals chose not to eat although food was available at all times. This phenomenon was apparently not due to an aversion to the novel HF diet because introduction of the running wheels plus the HF diet, while continuing the availability of the normal chow diet did not prevent the anorexia. Moreover, the anorexia was prevented with prior exposure to the HF diet. In addition, the anorexia was not related to extent of WR but dependent on the act of WR. The introduction a HF diet and locked running wheels did not induce the anorexia. This voluntary anorexia was accompanied by substantial weight loss, and the anorexia was rapidly reversed by removal of the running wheels. Moreover, the HF/WR-induced anorexia is preserved across the age span despite the intrinsic decrease in WR activity and increased consumption of HF food with advancing age. The described phenomenon provides a new model to investigate anorexia behavior in rodents.

Metabolic and hedonic drives in the neural control of appetite: who is the boss?

Obesity is on the rise in all developed countries, and a large part of this epidemic has been attributed to excess caloric intake, induced by ever present food cues and the easy availability of energy dense foods in an environment of plenty. Clearly, there are strong homeostatic regulatory mechanisms keeping body weight of many individuals exposed to this environment remarkably stable over their adult life. Other individuals, however, seem to eat not only because of metabolic need, but also because of excessive hedonic drive to make them feel better and relieve stress. In the extreme, some individuals exhibit addiction-like behavior toward food, and parallels have been drawn to drug and alcohol addiction. However, there is an important distinction in that, unlike drugs and alcohol, food is a daily necessity. Considerable advances have been made recently in the identification of neural circuits that represent the interface between the metabolic and hedonic drives of eating. We will cover these new findings by focusing first on the capacity of metabolic signals to modulate processing of cognitive and reward functions in cortico-limbic systems (bottom-up) and then on pathways by which the cognitive and emotional brain may override homeostatic regulation (top-down).

Reward mechanisms in obesity: new insights and future directions

Food is consumed in order to maintain energy balance at homeostatic levels. In addition, palatable food is also consumed for its hedonic properties independent of energy status. Such reward-related consumption can result in caloric intake exceeding requirements and is considered a major culprit in the rapidly increasing rates of obesity in developed countries. Compared with homeostatic mechanisms of feeding, much less is known about how hedonic systems in brain influence food intake. Intriguingly, excessive consumption of palatable food can trigger neuroadaptive responses in brain reward circuitries similar to drugs of abuse. Furthermore, similar genetic vulnerabilities in brain reward systems can increase predisposition to drug addiction and obesity. Here, recent advances in our understanding of the brain circuitries that regulate hedonic aspects of feeding behavior will be reviewed. Also, emerging evidence suggesting that obesity and drug addiction may share common hedonic mechanisms will also be considered.

Leptin reduces hyperactivity in an animal model for anorexia nervosa via the ventral tegmental area

Hyperactivity in anorexia nervosa (AN) is associated with low plasma leptin levels and negatively impacts on disease outcome. Using an animal model that mimics features of AN including food-restriction induced hyperlocomotion, we demonstrate that central leptin injections in the lateral ventricle and local injections of leptin into the ventral tegmental area (VTA) suppress running wheel activity. The results support that falling levels of leptin, that accompany caloric restriction, result in increased activity levels because of decreased leptin signaling in the VTA, part of the mesolimbic reward system.

Region-specific diet-induced and leptin-induced cellular leptin resistance includes the ventral tegmental area in rats

Diet-induced obesity (DIO) results in region-specific cellular leptin resistance in the arcuate nucleus (ARC) of the hypothalamus in one strain of mice and in several medial basal hypothalamic regions in another. We hypothesized that the ventral tegmental area (VTA) is also likely susceptible to diet-induced and leptin-induced leptin resistance in parallel to that in hypothalamic areas. We examined two forms of leptin resistance in F344xBN rats, that induced by 6-months of high fat (HF) feeding and that induced by 15-months of central leptin overexpression by use of recombinant adeno-associated viral (rAAV)-mediated gene delivery of rat leptin. Cellular leptin resistance was assessed by leptin-stimulated phosphorylation of signal transducers and activators of transcription 3 (STAT3) in medial basal hypothalamic areas and the VTA. The regional pattern and degree of leptin resistance with HF was distinctly different than that with leptin overexpression. Chronic HF feeding induced a cellular leptin resistance that was identified in the ARC and VTA, but absent in the lateral hypothalamus (LH), ventromedial hypothalamus (VMH), and dorsomedial hypothalamus (DMH). In contrast, chronic central leptin overexpression induced cellular leptin resistance in all areas examined. The identification of leptin resistance in the VTA, in addition to the leptin resistance in the hypothalamus, provides one potential mechanism, underlying the increased susceptibility of leptin resistant rats to HF-induced obesity.