Leptin Suppresses the Rewarding Effects of Running via STAT3 Signaling in Dopamine Neurons

Activation of ventral pallidum-projecting neurons in the nucleus accumbens via 5-HT2C receptor stimulation regulates motivation for wheel running in male mice

Rodents have a strong motivation for wheel running; however, the neural mechanisms that regulate their motivation remain unknown. We investigated the possible involvement of serotonin (5-HT) systems in regulating motivation for wheel running in male mice. Systemic administration of a 5-HT1A receptor antagonist (WAY100635) increased the number of wheel rotations, whereas administration of a 5-HT2A or 5-HT2C receptor antagonist (volinanserin or SB242084, respectively) decreased it. In the open field test, neither WAY100635 nor volinanserin affected locomotor activity, whereas SB242084 increased locomotor activity. To identify the brain regions on which these antagonists act, we locally injected these into the motivational circuitry, including the nucleus accumbens (NAc), dorsomedial striatum (DM-Str), and medial prefrontal cortex (mPFC). Injection of SB242084 into the NAc, but not the DM-Str or mPFC, reduced the number of wheel rotations without altering locomotor activity. The local administration of WAY100635 or volinanserin to these brain regions did not affect the number of wheel rotations. Immunohistochemical analyses revealed that wheel running increased the number of c-Fos-positive cells in the NAc medial shell (NAc-MS), which was reduced by systemic SB242084 administration. In vitro slice whole-cell recordings showed that bath application of the 5-HT2C receptor agonist lorcaserin increased the frequency of spontaneous excitatory and inhibitory postsynaptic currents in the ventral tegmental area (VTA)-projecting neurons, whereas it only increased the frequency of spontaneous excitatory postsynaptic currents in ventral pallidum (VP)-projecting neurons in the NAc-MS. These findings suggest that the activation of VP-projecting NAc-MS neurons via 5-HT2C receptor stimulation regulates motivation for wheel running.

History and future of leptin: Discovery, regulation and signaling

The cloning of leptin 30 years ago in 1994 was an important milestone in obesity research. Prior to the discovery of leptin, obesity was stigmatized as a condition caused by lack of character and self-control. Mutations in either leptin or its receptor were the first single gene mutations found to cause severe obesity, and it is now recognized that obesity is caused mostly by a dysregulation of central neuronal circuits. Since the discovery of the leptin-deficient obese mouse (ob/ob) the cloning of leptin (ob aka lep) and leptin receptor (db aka lepr) genes, we have learned much about leptin and its action in the central nervous system. The first hope that leptin would cure obesity was quickly dampened because humans with obesity have increased leptin levels and develop leptin resistance. Nevertheless, leptin target sites in the brain represent an excellent blueprint to understand how neuronal circuits control energy homeostasis. Our expanding understanding of leptin function, interconnection of leptin signaling with other systems and impact on distinct physiological functions continues to guide and improve the development of safe and effective interventions to treat metabolic illnesses. This review highlights past concepts and current emerging concepts of the hormone leptin, leptin receptor signaling pathways and central targets to mediate distinct physiological functions.

Secondary metabolite profile of Streptomyces spp. changes when grown with the sub-lethal concentration of silver nanoparticles: possible implication in novel compound discovery

The decline of new antibiotics and the emergence of multidrug resistance in pathogens necessitates a revisit of strategies used for lead compound discovery. This study proposes to induce the production of bioactive compounds with sub-lethal concentrations of silver nanoparticles (Ag-NPs). A total of Forty-two Actinobacteria isolates from four Saudi soil samples were grown with and without sub-lethal concentration of Ag-NPs (50 µg ml-1). The spent broth grown with Ag-NPs, or without Ag-NPs were screened for antimicrobial activity against four bacteria. Interestingly, out of 42 strains, broths of three strains grown with sub-lethal concentration of Ag-NPs exhibit antimicrobial activity against Staphylococcus aureus and Micrococcus luteus. Among these, two strains S4-4 and S4-21 identified as Streptomyces labedae and Streptomyces tirandamycinicus based on 16S rRNA gene sequence were selected for detailed study. The change in the secondary metabolites profile in the presence of Ag-NPs was evaluated using GC-MS and LC-MS analyses. Butanol extracts of spent broth grown with Ag-NPs exhibit strong antimicrobial activity against M. luteus and S. aureus. While the extracts of the controls with the same concentration of Ag-NPs do not show any activity. GC-analysis revealed a clear change in the secondary metabolite profile when grown with Ag-NPs. Similarly, the LC-MS patterns also differ significantly. Results of this study, strongly suggest that sub-lethal concentrations of Ag-NPs influence the production of secondary metabolites by Streptomyces. Besides, LC-MS results identified possible secondary metabolites, associated with oxidative stress and antimicrobial activities. This strategy can be used to possibly induce cryptic biosynthetic gene clusters for the discovery of new lead compounds.

Does hypoleptinemia trigger entrapment in anorexia nervosa? Etiological and clinical considerations

Based on the recent observation that human recombinant leptin (r-Met-hu-leptin; metreleptin) may induce a profound alleviation of the complex symptomatology of patients with anorexia nervosa (AN), we examine the implications for our conceptualisation of this eating disorder. Hypoleptinemia as a core endocrine feature of AN serves as a central and peripheral trigger of tissue-specific adaptations to starvation. In this narrative review, we argue that leptin deficiency may explain many of the puzzling features of this eating disorder. Weight loss can be viewed as a two-step process, with only the second step entailing hypoleptinemia and thereby the entrapment characteristic of AN. We discuss the central and peripheral distribution of leptin receptors and consider possible functional implications of hypoleptinemia. We contrast the slow psychological recovery of patients with AN and of people who experienced starvation upon weight recovery with the rapid onset of improvements upon off-label metreleptin treatment. Characteristics of the sex and age dependent secretion of leptin may contribute to the elevated vulnerability of young females to develop AN.

NAPE-PLD in the ventral tegmental area regulates reward events, feeding and energy homeostasis

The N-acyl phosphatidylethanolamine-specific phospholipase D (NAPE-PLD) catalyzes the production of N-acylethanolamines (NAEs), a family of endogenous bioactive lipids, which are involved in various biological processes ranging from neuronal functions to energy homeostasis and feeding behaviors. Reward-dependent behaviors depend on dopamine (DA) transmission between the ventral tegmental area (VTA) and the nucleus accumbens (NAc), which conveys reward-values and scales reinforced behaviors. However, whether and how NAPE-PLD may contribute to the regulation of feeding and reward-dependent behaviors has not yet been investigated. This biological question is of paramount importance since NAEs are altered in obesity and metabolic disorders. Here, we show that transcriptomic meta-analysis highlights a potential role for NAPE-PLD within the VTA→NAc circuit. Using brain-specific invalidation approaches, we report that the integrity of NAPE-PLD is required for the proper homeostasis of NAEs within the midbrain VTA and it affects food-reward behaviors. Moreover, region-specific knock-down of NAPE-PLD in the VTA enhanced food-reward seeking and reinforced behaviors, which were associated with increased in vivo DA release dynamics in response to both food- and non-food-related rewards together with heightened tropism towards food consumption. Furthermore, midbrain knock-down of NAPE-PLD, which increased energy expenditure and adapted nutrient partitioning, elicited a relative protection against high-fat diet-mediated body fat gain and obesity-associated metabolic features. In conclusion, these findings reveal a new key role of VTA NAPE-PLD in shaping DA-dependent events, feeding behaviors and energy homeostasis, thus providing new insights on the regulation of body metabolism.

Influence of Bariatric Surgery on Gut Microbiota Composition and Its Implication on Brain and Peripheral Targets

Obesity remains a significant global health challenge, with bariatric surgery remaining as one of the most effective treatments for severe obesity and its related comorbidities. This review highlights the multifaceted impact of bariatric surgery beyond mere physical restriction or nutrient malabsorption, underscoring the importance of the gut microbiome and neurohormonal signals in mediating the profound effects on weight loss and behavior modification. The various bariatric surgery procedures, such as Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG), act through distinct mechanisms to alter the gut microbiome, subsequently impacting metabolic health, energy balance, and food reward behaviors. Emerging evidence has shown that bariatric surgery induces profound changes in the composition of the gut microbiome, notably altering the Firmicutes/Bacteroidetes ratio and enhancing populations of beneficial bacteria such as Akkermansia. These microbiota shifts have far-reaching effects beyond gut health, influencing dopamine-mediated reward pathways in the brain and modulating the secretion and action of key gut hormones including ghrelin, leptin, GLP-1, PYY, and CCK. The resultant changes in dopamine signaling and hormone levels contribute to reduced hedonic eating, enhanced satiety, and improved metabolic outcomes. Further, post-bariatric surgical effects on satiation targets are in part mediated by metabolic byproducts of gut microbiota like short-chain fatty acids (SCFAs) and bile acids, which play a pivotal role in modulating metabolism and energy expenditure and reducing obesity-associated inflammation, as well as influencing food reward pathways, potentially contributing to the regulation of body weight and reduction in hedonic eating behaviors. Overall, a better understanding of these mechanisms opens the door to developing non-surgical interventions that replicate the beneficial effects of bariatric surgery on the gut microbiome, dopamine signaling, and gut hormone regulation, offering new avenues for obesity treatment.

Differential alterations of amygdala nuclei volumes in acutely ill patients with anorexia nervosa and their associations with leptin levels

BACKGROUND

The amygdala is a subcortical limbic structure consisting of histologically and functionally distinct subregions. New automated structural magnetic resonance imaging (MRI) segmentation tools facilitate the in vivo study of individual amygdala nuclei in clinical populations such as patients with anorexia nervosa (AN) who show symptoms indicative of limbic dysregulation. This study is the first to investigate amygdala nuclei volumes in AN, their relationships with leptin, a key indicator of AN-related neuroendocrine alterations, and further clinical measures.

METHODS

T1-weighted MRI scans were subsegmented and multi-stage quality controlled using FreeSurfer. Left/right hemispheric amygdala nuclei volumes were cross-sectionally compared between females with AN (n = 168, 12-29 years) and age-matched healthy females (n = 168) applying general linear models. Associations with plasma leptin, body mass index (BMI), illness duration, and psychiatric symptoms were analyzed via robust linear regression.

RESULTS

Globally, most amygdala nuclei volumes in both hemispheres were reduced in AN v. healthy control participants. Importantly, four specific nuclei (accessory basal, cortical, medial nuclei, corticoamygdaloid transition in the rostral-medial amygdala) showed greater volumetric reduction even relative to reductions of whole amygdala and total subcortical gray matter volumes, whereas basal, lateral, and paralaminar nuclei were less reduced. All rostral-medially clustered nuclei were positively associated with leptin in AN independent of BMI. Amygdala nuclei volumes were not associated with illness duration or psychiatric symptom severity in AN.

CONCLUSIONS

In AN, amygdala nuclei are altered to different degrees. Severe volume loss in rostral-medially clustered nuclei, collectively involved in olfactory/food-related reward processing, may represent a structural correlate of AN-related symptoms. Hypoleptinemia might be linked to rostral-medial amygdala alterations.

NAPE-PLD in the ventral tegmental area regulates reward events, feeding and energy homeostasis

The N-acyl phosphatidylethanolamine-specific phospholipase D (NAPE-PLD) catalyzes the production of N-acylethanolamines (NAEs), a family of endogenous bioactive lipids, which are involved in various biological processes ranging from neuronal functions to energy homeostasis and feeding behaviors. Reward-dependent behaviors depend on dopamine (DA) transmission between the ventral tegmental area (VTA) and the nucleus accumbens (NAc), which conveys reward-values and scales reinforced behaviors. However, whether and how NAPE-PLD may contribute to the regulation of feeding and reward-dependent behaviors has not yet been investigated. This biological question is of paramount importance since NAEs are altered in obesity and metabolic disorders. Here, we show that transcriptomic meta-analysis highlights a potential role for NAPE-PLD within the VTA®NAc circuit. Using brain-specific invalidation approaches, we report that the integrity of NAPE-PLD is required for the proper homeostasis of NAEs within the midbrain VTA and it affects food-reward behaviors. Moreover, region-specific knock-down of NAPE-PLD in the VTA enhanced food-reward seeking and reinforced behaviors, which were associated with increased in vivo DA release dynamics in response to both food and non-food-related rewards together with heightened tropism towards food consumption. Furthermore, midbrain knock-down of NAPE-PLD, which increased energy expenditure and adapted nutrient partitioning, elicited a relative protection against high-fat diet-mediated body fat gain and obesity-associated metabolic features. In conclusion, these findings reveal a new key role of VTA NAPE-PLD in shaping DA-dependent events, feeding behaviors and energy homeostasis, thus providing new insights on the regulation of body metabolism.

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.

The Endocannabinoid System and Physical Exercise

The endocannabinoid system (ECS) is involved in various processes, including brain plasticity, learning and memory, neuronal development, nociception, inflammation, appetite regulation, digestion, metabolism, energy balance, motility, and regulation of stress and emotions. Physical exercise (PE) is considered a valuable non-pharmacological therapy that is an immediately available and cost-effective method with a lot of health benefits, one of them being the activation of the endogenous cannabinoids. Endocannabinoids (eCBs) are generated as a response to high-intensity activities and can act as short-term circuit breakers, generating antinociceptive responses for a short and variable period of time. A runner's high is an ephemeral feeling some sport practitioners experience during endurance activities, such as running. The release of eCBs during sustained physical exercise appears to be involved in triggering this phenomenon. The last decades have been characterized by an increased interest in this emotional state induced by exercise, as it is believed to alleviate pain, induce mild sedation, increase euphoric levels, and have anxiolytic effects. This review provides information about the current state of knowledge about endocannabinoids and physical effort and also an overview of the studies published in the specialized literature about this subject.

A critical update on the leptin-melanocortin system

The discovery of leptin in 1994 was an "eureka moment" in the field of neurometabolism that provided new opportunities to better understand the central control of energy balance and glucose metabolism. Rapidly, a prevalent model in the field emerged that pro-opiomelanocortin (POMC) neurons were key in promoting leptin's anorexigenic effects and that the arcuate nucleus of the hypothalamus (ARC) was a key region for the regulation of energy homeostasis. While this model inspired many important discoveries, a growing body of literature indicates that this model is now outdated. In this review, we re-evaluate the hypothalamic leptin-melanocortin model in light of recent advances that directly tackle previous assumptions, with a particular focus on the ARC. We discuss how segregated and heterogeneous these neurons are, and examine how the development of modern approaches allowing spatiotemporal, intersectional, and chemogenetic manipulations of melanocortin neurons has allowed a better definition of the complexity of the leptin-melanocortin system. We review the importance of leptin in regulating glucose homeostasis, but not food intake, through direct actions on ARC POMC neurons. We further highlight how non-POMC, GABAergic neurons mediate leptin's direct effects on energy balance and influence POMC neurons.

Associations of leptin and corticostriatal connectivity in bipolar disorder

Bipolar disorder (BD) and metabolic disturbance represent a chronic state of low-grade inflammation and corticostriatal circuitry alterations. Herein, we aimed to investigate whether plasma leptin, an adipokine that plays a key role in the interplay of metabolism and inflammation, is associated with corticostriatal connectivity in patients with BD. Twenty-eight BD I patients, 36 BD II patients and 66 healthy controls were enrolled and completed the Hamilton Depression Rating Scale, the Young Mania Rating Scale, and the Recent Life Change Questionnaire. Fasting plasma leptin and C-reactive protein (CRP) levels were measured, and corticostriatal connectivity was examined using functional magnetic resonance imaging (fMRI). The relationships between leptin, CRP and body mass index (BMI) identified in the controls and BD II patients were absent in the BD I patients. We did not find a significant group difference in the leptin level; nevertheless, the negative correlation between leptin level and corticostriatal connectivity (ventrolateral prefrontal cortex and inferior temporal gyrus) observed in the healthy controls was absent in the BD patients. The disproportionate increase in leptin level with increasing BMI in BD indicated a potential inflammatory role of white adipose tissue in BD. Furthermore, higher CRP levels in BD I patients might induce leptin resistance. Collectively, our results implied vulnerability to inflammatory and metabolic diseases in patients with BD, especially BD I.

A microbiome-dependent gut-brain pathway regulates motivation for exercise

Exercise exerts a wide range of beneficial effects for healthy physiology1. However, the mechanisms regulating an individual's motivation to engage in physical activity remain incompletely understood. An important factor stimulating the engagement in both competitive and recreational exercise is the motivating pleasure derived from prolonged physical activity, which is triggered by exercise-induced neurochemical changes in the brain. Here, we report on the discovery of a gut-brain connection in mice that enhances exercise performance by augmenting dopamine signalling during physical activity. We find that microbiome-dependent production of endocannabinoid metabolites in the gut stimulates the activity of TRPV1-expressing sensory neurons and thereby elevates dopamine levels in the ventral striatum during exercise. Stimulation of this pathway improves running performance, whereas microbiome depletion, peripheral endocannabinoid receptor inhibition, ablation of spinal afferent neurons or dopamine blockade abrogate exercise capacity. These findings indicate that the rewarding properties of exercise are influenced by gut-derived interoceptive circuits and provide a microbiome-dependent explanation for interindividual variability in exercise performance. Our study also suggests that interoceptomimetic molecules that stimulate the transmission of gut-derived signals to the brain may enhance the motivation for exercise.

The role of hypoleptinemia in the psychological and behavioral adaptation to starvation: implications for anorexia nervosa

This narrative review aims to pinpoint mental and behavioral effects of starvation, which may be triggered by hypoleptinemia and as such may be amenable to treatment with leptin receptor agonists. The reduced leptin secretion results from the continuous loss of fat mass, thus initiating a graded triggering of diverse starvation related adaptive functions. In light of leptin receptors located in several peripheral tissues and many brain regions adaptations may extend beyond those of the hypothalamus-pituitary-end organ-axes. We focus on gastrointestinal tract and reward system as relevant examples of peripheral and central effects of leptin. Despite its association with extreme obesity, congenital leptin deficiency with its many parallels to a state of starvation allows the elucidation of mental symptoms amenable to treatment with exogenous leptin in both ob/ob mice and humans with this autosomal recessive disorder. For starvation induced behavioral changes with an intact leptin signaling we particularly focus on rodent models for which proof of concept has been provided for the causative role of hypoleptinemia. For humans, we highlight the major cognitive, emotional and behavioral findings of the Minnesota Starvation Experiment to contrast them with results obtained upon a lesser degree of caloric restriction. Evidence for hypoleptinemia induced mental changes also stems from findings obtained in lipodystrophies. In light of the recently reported beneficial cognitive, emotional and behavioral effects of metreleptin-administration in anorexia nervosa we discuss potential implications for the treatment of this eating disorder. We postulate that leptin has profound psychopharmacological effects in the state of starvation.

Low leptin levels are associated with elevated physical activity among lean school children in rural Tanzania

BACKGROUND

In Sub-Saharan African countries, rapid urbanization and increasing socio-economic status are associated with a transition to decreased physical activity (PA). A more sedentary lifestyle is linked to increased body fat leading to increments in leptin levels. Since rodent and human studies in high-income countries have shown that starvation-induced hypoleptinemia triggers high PA, efforts are warranted to pursue the hypothesis that low leptin levels in lean children of low- and middle-income countries (LMIC) are also associated with high PA.

METHODS

In this cross-sectional study, we assessed seven-day PA with triaxial accelerometry (ActiGraph GT3X) among 223 primary school children (9 to 12 years of age) in rural Tanzania. Moderate-to-vigorous PA (MVPA) and total accelerometer counts per day were outcome variables. Leptin was determined using enzyme linked immunosorbent assay tests from dried blood spots. Anthropometric assessments were conducted and food insecurity and socio-demographic data were gathered using semi-structured interviews.

RESULTS

In this sample of school children in rural Tanzania, leptin concentrations (median: 0.91 ng/mL, P25: 0.55, P75: 1.69), body mass index z-scores (median: -1.35, P25: -1.93, P75: -0.82), and height-for-age-z-scores (median: -1.16, P25: -1.96, P75: -0.61) were low. In contrast, PA levels were high with a median MVPA time of 119 min/day. Linear regression confirmed that leptin levels were negatively associated with MVPA (beta: -18.1; 95%CI: -29.7; -6.5; p = 0.002) and total accelerometer counts (beta: -90,256; 95%CI: -154,146; -26,365; p = 0.006). Children residing in areas with better infrastructure had lower MVPA levels (p < 0.001) and tended to have higher leptin levels (p = 0.062) than children residing in areas only reachable via dirt roads.

CONCLUSION

Our cross-sectional field study is the first that supports the hypothesis of low leptin levels as a potential endocrine trigger of high PA in lean children of a LMIC. We observed early signs of a PA transition towards a less active lifestyle in a subgroup residing in areas with better infrastructure that concomitantly tended to have higher leptin concentrations. Considering that area-dependent PA differences were more pronounced among girls than boys, whereas differences in leptin levels were less pronounced, not only biological, but also external factors explain PA transition.

The Rise and Fall of Dopamine: A Two-Stage Model of the Development and Entrenchment of Anorexia Nervosa

Dopamine has long been implicated as a critical neural substrate mediating anorexia nervosa (AN). Despite nearly 50 years of research, the putative direction of change in dopamine function remains unclear and no consensus on the mechanistic role of dopamine in AN has been achieved. We hypothesize two stages in AN- corresponding to initial development and entrenchment- characterized by opposite changes in dopamine. First, caloric restriction, particularly when combined with exercise, triggers an escalating spiral of increasing dopamine that facilitates the behavioral plasticity necessary to establish and reinforce weight-loss behaviors. Second, chronic self-starvation reverses this escalation to reduce or impair dopamine which, in turn, confers behavioral inflexibility and entrenchment of now established AN behaviors. This pattern of enhanced, followed by impaired dopamine might be a common path to many behavioral disorders characterized by reinforcement learning and subsequent behavioral inflexibility. If correct, our hypothesis has significant clinical and research implications for AN and other disorders, such as addiction and obesity.

Vulnerable and Resilient Phenotypes in a Mouse Model of Anorexia Nervosa

BACKGROUND

Increased physical activity is a common feature of anorexia nervosa (AN). Although high activity levels are associated with greater risk of developing AN, particularly when combined with dieting, most individuals who diet and exercise maintain a healthy body weight. It is unclear why some individuals develop AN while most do not. A rodent model of resilience and vulnerability to AN would be valuable to research. Dopamine, which is believed to play a crucial role in AN, regulates both reward and activity and may modulate vulnerability.

METHODS

Adolescent and young adult female C57BL/6N mice were tested in the activity-based anorexia (ABA) model, with an extended period of food restriction in adult mice. ABA was also tested in dopamine transporter knockdown mice and wild-type littermates. Mice that adapted to conditions and maintained a stable body weight were characterized as resilient.

RESULTS

In adults, vulnerable and resilient phenotypes emerged in both the ABA and food-restricted mice without wheels. Vulnerable mice exhibited a pronounced increase in running throughout the light cycle, which dramatically peaked prior to requiring removal from the experiment. Resilient mice exhibited an adaptive decrease in total running, appropriate food anticipatory activity, and increased consumption, thereby achieving stable body weight. Hyperdopaminergia accelerated progression of the vulnerable phenotype.

CONCLUSIONS

Our demonstration of distinct resilient and vulnerable phenotypes in mouse ABA significantly advances the utility of the model for identifying genes and neural substrates mediating AN risk and resilience. Modulation of dopamine may play a central role in the underlying circuit.

Suggestive Evidence for Causal Effect of Leptin Levels on Risk for Anorexia Nervosa: Results of a Mendelian Randomization Study

Genetic correlations suggest a coexisting genetic predisposition to both low leptin levels and risk for anorexia nervosa (AN). To investigate the causality and direction of these associations, we performed bidirectional two-sample Mendelian randomization (MR) analyses using data of the most recent genome-wide association study (GWAS) for AN and both a GWAS and an exome-wide-association-study (EWAS) for leptin levels. Most MR methods with genetic instruments from GWAS showed a causal effect of lower leptin levels on higher risk of AN (e.g. IVW b = -0.923, p = 1.5 × 10^-4). Because most patients with AN are female, we additionally performed analyses using leptin GWAS data of females only. Again, there was a significant effect of leptin levels on the risk of AN (e.g. IVW b = -0.826, p = 1.1 × 10^-04). MR with genetic instruments from EWAS showed no overall effect of leptin levels on the risk for AN. For the opposite direction, MR revealed no causal effect of AN on leptin levels. If our results are confirmed in extended GWAS data sets, a low endogenous leptin synthesis represents a risk factor for developing AN.

Exploring the Mechanisms of Recovery in Anorexia Nervosa through a Translational Approach: From Original Ecological Measurements in Human to Brain Tissue Analyses in Mice

Anorexia nervosa (AN) is a severe eating disorder where caloric restriction, excessive physical activity and metabolic alterations lead to life-threatening situations. Despite weight restoration after treatment, a significant part of patients experience relapses. In this translational study, we combined clinical and preclinical approaches. We describe preliminary data about the effect of weight gain on the symptomatology of patients suffering from acute AN (n = 225) and partially recovered (n = 41). We measured more precisely physical activity with continuous cardiac monitoring in a sub-group (n = 68). Using a mouse model, we investigated whether a long-term food restriction followed by nutritional recovery associated or not with physical activity may differentially impact peripheral and central homeostatic regulation. We assessed the plasma concentration of acyl ghrelin, desacyl ghrelin and leptin and the mRNA expression of hypothalamic neuropeptides and their receptors. Our data show an effect of undernutrition history on the level of physical activity in AN. The preclinical model supports an important role of physical activity in the recovery process and points out the leptin system as one factor that can drive a reliable restoration of metabolic variables through the hypothalamic regulation of neuropeptides involved in feeding behavior.

Anxiety-like behavior in female mice is modulated by STAT3 signaling in midbrain dopamine neurons

The central signaling actions of cytokines are mediated by signal transducer and activator of transcription (STAT3). STAT3 activation plays a pivotal role in the behavioral responses to the adiposity hormone leptin, including in midbrain dopamine (DA) neurons where it mediates the influence of leptin to diminish physical activity and running reward in male mice. Leptin also has anxiolytic effects which have been tied to the mesolimbic DA system. To assess the contribution of STAT3 signaling in mesolimbic DA neurons on feeding, mesolimbic DA tone and anxiodepressive behaviors in female mice, we generated DA-specific STAT3 knockout mice by crossing mice expressing Cre under the control of the dopamine transporter with STAT3-LoxP mice. Feeding, locomotion, wheel running, conditioned place preference for palatable food and amphetamine locomotor sensitization were unaffected by DA-specific STAT3 deletion. Conversely, knockout mice exhibited heightened anxiety-like behavior (open field test and elevated plus maze) along with increased basal and stress-induced plasma corticosterone, whereas indices of behavioral despair (forced swim and tail-suspension tasks) were unchanged. In accordance with biochemical evidence of increased D1 receptor signaling (phospho-DARPP32_Thr34) in the central nucleus of the amygdala (CeA) of knockout mice, local microinjections of a D1 receptor antagonist reversed the anxiogenic phenotype of knockout mice. In addition to alluding to sex differences in the signaling mechanisms mediating anxiety-like behavior, our findings suggest that activation of STAT3 in midbrain dopamine neurons projecting to the CeA dampens anxiety in a D1R-dependent manner in female mice.

Comparisons Between Serum Levels of Hepcidin and Leptin in Male College-Level Endurance Runners and Sprinters

Background: Hepcidin-25 is a 25 amino acid hepatokine and a key regulator of iron metabolism related to iron deficiency anemia. Recent studies have suggested that an elevated hepcidin level is correlated with low energy availability. Leptin is an appetite-suppressing adipokine and has been reported to stimulate hepcidin production in animals and cultured cells. While leptin is modulated by exercise, it is known that endurance runners and sprinters practice different types of exercise. This study investigated and compared the relationships between hepcidin and leptin levels, iron status, and body fat to understand better the risk of iron deficiency anemia in endurance runners and sprinters. Methods: Thirty-six male college track and field athletes (15 endurance runners and 21 sprinters) were recruited for this study. Dietary intake, body composition, and blood levels of ferritin, hepcidin-25, leptin, and adiponectin were measured. Correlations between hepcidin levels and ferritin, body fat, leptin, and adiponectin were evaluated using Pearson's correlation coefficient for each group. Results: The endurance runners had lower hepcidin levels and higher leptin and adiponectin levels compared with sprinters. Ferritin was positively correlated with hepcidin-25 levels in both the endurance and sprinter groups. A positive correlation was observed between hepcidin-25 and body fat or leptin levels only in sprinters. Conclusion: This is the first study investigating the relationship between blood levels of hepcidin and leptin in athletes. The positive correlation between hepcidin-25 and leptin was observed in sprinters but not endurance runners.

Is LRP2 Involved in Leptin Transport over the Blood-Brain Barrier and Development of Obesity?

The mechanisms underlying the transport of leptin into the brain are still largely unclear. While the leptin receptor has been implicated in the transport process, recent evidence has suggested an additional role of LRP2 (megalin). To evaluate the function of LRP2 for leptin transport across the blood-brain barrier (BBB), we developed a novel leptin-luciferase fusion protein (pLG), which stimulated leptin signaling and was transported in an in vitro BBB model based on porcine endothelial cells. The LRP inhibitor RAP did not affect leptin transport, arguing against a role of LRP2. In line with this, the selective deletion of LRP2 in brain endothelial cells and epithelial cells of the choroid plexus did not influence bodyweight, body composition, food intake, or energy expenditure of mice. These findings suggest that LRP2 at the BBB is not involved in the transport of leptin into the brain, nor in the development of obesity as has previously been described.

Exercise craving potentiates excitatory inputs to ventral tegmental area dopaminergic neurons

Physical exercise, which can be addictogenic on its own, is considered a therapeutic alternative for drug craving. Exercise might thus share with drugs the ability to strengthen excitatory synapses onto ventral tegmental area (VTA) dopaminergic neurones, as assessed by the ratio of AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) to NMDA receptor (NMDAR)-mediated EPSCs. As did acute cocaine, amphetamine, or Δ⁹ -tetrahydrocannabinol (THC) pretreatments, an acute 1-h wheel-running session increased the AMPAR/NMDAR ratio in VTA dopaminergic neurones. To dissect the respective influences of wheel-running seeking and performance, mice went through an operant protocol wherein wheel-running was conditioned by nose poking under fixed ratio schedules of reinforcement. Conditioned wheel-running increased the AMPAR/NMDAR ratio to a higher extent than free wheel-running, doing so although running performance was lower in the former paradigm than in the latter. Thus, the cue-reward association, rather than reward consumption, played a major role in this increase. The AMPAR/NMDAR ratio returned to baseline levels in mice that had extinguished the cued-running motivated task, but it increased after a cue-induced reinstatement session. The amplitude of this increase correlated with the intensity of exercise craving, as assessed by individual nose poke scores. Finally, cue-induced reinstatement of running seeking proved insensitive to acute cocaine or THC pretreatments. Our study reveals for the first time that the drive for exercise bears synaptic influences on VTA dopaminergic neurones which are reminiscent of drug actions. Whether these influences play a role in the therapeutic effects of exercise in human drug craving remains to be established.

Different baseline physical activity predicts susceptibility and resilience to chronic social defeat stress in mice: Involvement of dopamine neurons

Physical inactivity, the fourth leading mortality risk factor worldwide, is associated with chronic mental illness. Identifying the mechanisms underlying different levels of baseline physical activity and the effects of these levels on the susceptibility to stress is very important. However, whether different levels of baseline physical activity influence the susceptibility and resilience to chronic social defeat stress (CSDS), and the underlying mechanisms in the brain remain unclear. The present study segregated wild-type mice into low baseline physical activity (LBPA) and high baseline physical activity (HBPA) groups based on short term voluntary wheel running (VWR). LBPA mice showed obvious susceptibility to CSDS, while HBPA mice were resilient to CSDS. In addition, the expression of tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) was lower in LBPA mice than in HBPA mice. Furthermore, activation of TH neurons in the VTA of LBPA mice by chemogenetic methods increased the levels of VWR and resilience to CSDS. In contrast, inhibiting TH neurons in the VTA of HBPA mice lowered the levels of VWR and increased their susceptibility to CSDS. Thus, this study suggests that different baseline physical activities might be mediated by the dopamine system. This system also affects the susceptibility and resilience to CSDS, possibly via alteration of the baseline physical activity. This perspective on the neural control and impacts on VWR may aid the development of strategies to motivate and sustain voluntary physical activity. Furthermore, this can maximize the impacts of regular physical activity toward stress-reduction and health promotion.

Cannabis and exercise: Effects of Δ9-tetrahydrocannabinol on preference and motivation for wheel-running in mice

Recent surveys have revealed close links between cannabis and exercise. Specifically, cannabis usage before and/or after exercise is an increasingly common habit primarily aimed at boosting exercise pleasure, motivation, and performance whilst facilitating post-exercise recovery. However, whether these beliefs reflect the true impact of cannabis on these aspects of exercise is unknown. This study has thus examined the effects of cannabis' main psychoactive ingredient, namely Δ⁹-tetrahydrocannabinol (THC), on (i) mouse wheel-running preference and performance and (ii) running motivation and seeking behaviour. Wheel-running preference and performance were investigated using a T-maze with free and locked wheels located at the extremity of either arm. Running motivation and seeking were assessed by a cued-running operant task wherein wheel-running was conditioned by nose poking. Moreover, because THC targets cannabinoid type 1 (CB₁) receptors, i.e. receptors previously documented to control running motivation, this study also assessed the role of these receptors in running preference, performance, and craving-like behaviour. Whilst acute blockade or genetic deletion of CB₁ receptors decreased running preference and performance in the T-maze, THC proved ineffective on either variable. The failure of THC to affect running variables in the T-maze extended to running motivation, as assessed by cued-running under a progressive ratio (PR) reinforcement schedule. This ineffectiveness of THC was not related to the treatment protocol because it successfully increased motivation for palatable food. Although craving-like behaviour, as indexed by a cue-induced reinstatement of running seeking, was found to depend on CB₁ receptors, THC again proved ineffective. Neither running motivation nor running seeking were affected when CB₁ receptors were further stimulated by increasing the levels of the endocannabinoid 2-arachidonoylglycerol. These results, which suggest that the drive for running is insensitive to the acute stimulation of CB₁ receptors, raise the hypothesis that cannabis is devoid of effect on exercise motivation. Future investigation using chronic administration of THC, with and without other cannabis ingredients (e.g. cannabidiol), is however required before conclusions can be drawn.

Sex-dependent alterations in behavior, drug responses and dopamine transporter expression in heterozygous DAT-Cre mice

Heterozygous mice that express Cre-recombinase under the dopamine transporter promoter (DAT-Cre knock in mice, or KI) are widely used for targeting midbrain dopamine neurons, under the assumption that their constitutive physiology is not affected. We report here that these mice display striking sex-dependent behavioral and molecular differences in relation to wildtypes (WT). Male and female KI mice were constitutively hyperactive, and male KI mice showed attenuated hyperlocomotor responses to amphetamine. In contrast, female KIs displayed a marked reduction in locomotion ("calming" effect) in response to the same dose of amphetamine. Furthermore, male and female DAT-Cre KI mice showed opposing differences in reinforcement learning, with females showing faster conditioning and males showing slower extinction. Other behavioral variables, including working memory and novelty preference, were not changed compared to WT. These effects were paralleled by differences in striatal DAT expression that disproportionately affected female KI mice. Our findings reveal clear limitations of the DAT-Cre line that must be considered when using this model.

Maternal high-fat diet during lactation reprograms the dopaminergic circuitry in mice

The maternal perinatal environment modulates brain formation, and altered maternal nutrition has been linked to the development of metabolic and psychiatric disorders in the offspring. Here, we showed that maternal high-fat diet (HFD) feeding during lactation in mice elicits long-lasting changes in gene expression in the offspring's dopaminergic circuitry. This translated into silencing of dopaminergic midbrain neurons, reduced connectivity to their downstream targets, and reduced stimulus-evoked dopamine (DA) release in the striatum. Despite the attenuated activity of DA midbrain neurons, offspring from mothers exposed to HFD feeding exhibited a sexually dimorphic expression of DA-related phenotypes, i.e., hyperlocomotion in males and increased intake of palatable food and sucrose in females. These phenotypes arose from concomitantly increased spontaneous activity of D1 medium spiny neurons (MSNs) and profoundly decreased D2 MSN projections. Overall, we have unraveled a fundamental restructuring of dopaminergic circuitries upon time-restricted altered maternal nutrition to induce persistent behavioral changes in the offspring.

Nicotine exposure during lactation causes disruption of hedonic eating behavior and alters dopaminergic system in adult female rats

Tobacco smoke during gestation is associated with increased consumption of palatable foods by the offspring in humans and rats. Postpartum relapse is observed in lactating women who quit smoking during pregnancy, putting their children at risk of adverse health outcomes caused by secondhand smoke. Nicotine is transferred through milk and alters the dopaminergic reward system of adult male rats, reducing dopamine action in the nucleus accumbens (NAc) and hypothalamic arcuate nucleus. Here, we evaluated the long-term effects of nicotine-only exposure during lactation on eating behavior, anxiety, locomotion, dopaminergic system, hypothalamic leptin signaling and nicotinic receptor in the adult female rat progeny. Two days after birth (PN2), Wistar rat dams were separated into control and nicotine (Nic) groups for implantation of osmotic minipumps that released respectively saline or 6 mg/kg nicotine. Lactating dams were kept with 6 pups. After weaning (PN21; nicotine withdrawal), only the female offspring were studied. Euthanasia occurred at PN180. Nic females showed hyperphagia, preference for a high-sucrose diet, increased anxiety-like behavior, lower tyrosine hydroxylase (TH), lower dopamine transporter and higher dopamine receptor (Drd2) in NAc; lower Drd1 in prefrontal cortex and lower TH in dorsal striatum (DS). These animals showed changes that can explain their hyperphagia, such as: lower leptin signaling pathway (Lepr^b, pJAK2, pSTAT3) and Chrna7 expression in hypothalamus. Neonatal nicotine exposure affects the brain reward system of the female progeny differently from males, mainly decreasing dopamine production in NAc and DS. Therefore, Nic females are more susceptible to develop food addiction and obesity.

Leptin Receptor Expressing Neurons in the Substantia Nigra Regulate Locomotion, and in The Ventral Tegmental Area Motivation and Feeding

Leptin is an anorexigenic hormone, important in the regulation of body weight. Leptin plays a role in food reward, feeding, locomotion and anxiety. Leptin receptors (LepR) are expressed in many brain areas, including the midbrain. In most studies that target the midbrain, either all LepR neurons of the midbrain or those of the ventral tegmental area (VTA) were targeted, but the role of substantia nigra (SN) LepR neurons has not been investigated. These studies have reported contradicting results regarding motivational behavior for food reward, feeding and locomotion. Since not all midbrain LepR mediated behaviors can be explained by LepR neurons in the VTA alone, we hypothesized that SN LepR neurons may provide further insight. We first characterized SN LepR and VTA LepR expression, which revealed LepR expression mainly on DA neurons. To further understand the role of midbrain LepR neurons in body weight regulation, we chemogenetically activated VTA LepR or SN LepR neurons in LepR-cre mice and tested for motivational behavior, feeding and locomotion. Activation of VTA LepR neurons in food restricted mice decreased motivation for food reward (p=0.032) and food intake (p=0.020), but not locomotion. In contrast, activation of SN LepR neurons in food restricted mice decreased locomotion (p=0.025), but not motivation for food reward or food intake. Our results provide evidence that VTA LepR and SN LepR neurons serve different functions, i.e. activation of VTA LepR neurons modulated motivation for food reward and feeding, while SN LepR neurons modulated locomotor activity.

The Promise of Automated Home-Cage Monitoring in Improving Translational Utility of Psychiatric Research in Rodents

Large number of promising preclinical psychiatric studies in rodents later fail in clinical trials, raising concerns about the efficacy of this approach to generate novel pharmacological interventions. In this mini-review we argue that over-reliance on behavioral tests that are brief and highly sensitive to external factors play a critical role in this failure and propose that automated home-cage monitoring offers several advantages that will increase the translational utility of preclinical psychiatric research in rodents. We describe three of the most commonly used approaches for automated home cage monitoring in rodents [e.g., operant wall systems (OWS), computerized visual systems (CVS), and automatic motion sensors (AMS)] and review several commercially available systems that integrate the different approaches. Specific examples that demonstrate the advantages of automated home-cage monitoring over traditional tests of anxiety, depression, cognition, and addiction-like behaviors are highlighted. We conclude with recommendations on how to further expand this promising line of preclinical research.

Diet containing stearic acid increases food reward-related behaviors in mice compared with oleic acid

Obesity is currently a worldwide phenomenon. The consumption of calorie-rich foods is responsible for most obesity cases, but not all humans exposed to high-calorie diets develop obesity. According to recent studies, exposure to fat-rich diets may be the actual cause of obesity. Dietary long-chain fatty acids affect brain function and are linked to food intake and motivation-related behaviors. Recently, many studies have shown that different types of fatty acids play different roles in animals. In our study, the effects of stearic acid (a saturated fatty acid) and oleic acid (a monounsaturated fatty acid) in diets on hedonic feeding behaviors were investigated, and changes of feeding-related protein levels in the brain were detected to explore the possible mechanism underlying the effects of these fatty acids. As a result, mice fed a diet containing stearic acid, compared to a diet containing oleic acid, exhibited increased food intake, hedonic eating, and an operant response to sucrose and locomotor activity. Furthermore, stearic acid corresponded to a higher level of leptin in serum than oleic acid. In addition, the stearic acid treated group had lower protein levels of p-JAK2 and p-STAT3 in the VTA and a higher dopamine concentration in the NAc than the oleic acid-treated group. Meanwhile, the protein level of TH in the NAc was higher and the protein level of the DA transporter in the VTA was lower in the stearic acid-fed group than in the oleic acid-fed group. In conclusion, these findings indicated that a diet containing stearic acid can increase hedonic feeding behavior and affect mesolimbic dopamine system signals in mice. Moreover, the lowering of serum leptin and leptin signaling in the VTA may contribute to this effect.

Short-term metreleptin treatment of patients with anorexia nervosa: rapid on-set of beneficial cognitive, emotional, and behavioral effects

To examine the hypothesis that normalization of low circulating leptin levels in patients with anorexia nervosa ameliorates hyperactivity, three seriously ill females with hyperactivity were treated off-label with metreleptin (recombinant human leptin) for up to 14 days. Drive for activity, repetitive thoughts of food, inner restlessness, and weight phobia decreased in two patients. Surprisingly, depression improved rapidly in all patients. No serious adverse events occurred. Due to obvious limitations of uncontrolled case series, placebo-controlled clinical trials are mandatory to confirm the observed rapid onset of beneficial effects. Our findings suggest an important role of hypoleptinemia in the mental and behavioral phenotype of anorexia nervosa.

Temperature but not leptin prevents semi-starvation induced hyperactivity in rats: implications for anorexia nervosa treatment

The hypothesis linking hyperactivity with weight loss associated hypoleptinemia in anorexia nervosa gained momentum after a study showing that leptin suppressed semi-starvation induced hyperactivity in rats. Alternatively, ambient temperature is a key modulating factor of activity in semi-starved rats. The aim of the study is to compare the efficacy of leptin with increased ambient temperature in the prevention of hyperactivity in semi-starved rats. 74 Sprague-Dawley male rats were employed in two experiments with the difference residing in the length of baseline. After an extended (28 days), or shorter (14 days) baseline with free access to food and the running wheel, housed at 21 °C, animals were either ad-lib feed or food restricted (60% of food ingested during previous week) and infused with same amount of leptin at 21 °C, 25 °C, or vehicle at 21 °C, 25 °C and 32 °C for a week. Animals housed at 32 °C significantly reduced wheel running and weight loss during food restriction while animals given leptin did not yield no differences in activity or weight loss. Moreover, unlike animals housed at 32 °C, body temperature of leptin infused animals housed at 21 °C was significantly reduced during food restriction. Furthermore, leptin treated rats without a preceding stable pattern of activity displayed a severe dysregulation of circadian rhythm in activity and a collapse of body temperature. Housing temperature plays a more critical role than leptin in the regulation of semi-starvation induced hyperactivity in rats, which may be of relevance for the management of hyperactivity in anorexia nervosa.

A role for leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction

RATIONAL

Caloric restriction increases the risk of relapse in abstinent drug users. Hormones involved in the regulation of energy balance and food intake, such as leptin and ghrelin, are implicated in drug-related behaviors.

OBJECTIVES

We investigated the role of leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction.

METHODS

Rats self-administered heroin (0.1 mg/kg/infusion) for 10 days followed by 14 days of drug withdrawal. During withdrawal, rats were food restricted to 90% of their original body weight or were given free access to food. In experiment 1, we measured the plasma concentrations of leptin and ghrelin following heroin self-administration and withdrawal. In experiment 2, leptin was administered centrally (2.0 or 4.0 μg; i.c.v.) prior to a heroin-seeking test under extinction conditions. High density of both leptin and ghrelin receptors was previously identified in the ventral tegmental area (VTA), suggesting a direct effect on reward and motivation. Hence, we administered leptin (experiment 3; 0.125 or 0.250 μg/side), or ghrelin receptor antagonist JMV 2959 (experiment 4; 2.0 or 10.0 μg/side) directly into the VTA prior to the heroin-seeking test.

RESULTS

Chronic food restriction significantly decreased plasma levels of leptin and elevated plasma levels of ghrelin. Central administration of leptin had no statistically significant effect on heroin seeking. Intra-VTA administration of either leptin or JMV 2959 dose-dependently and selectively decreased heroin seeking in the food-restricted rats.

CONCLUSIONS

Leptin and ghrelin transmission in the VTA can modulate the augmentation of heroin seeking induced by chronic food restriction.

DNA methylation in AgRP neurons regulates voluntary exercise behavior in mice

DNA methylation regulates cell type-specific gene expression. Here, in a transgenic mouse model, we show that deletion of the gene encoding DNA methyltransferase Dnmt3a in hypothalamic AgRP neurons causes a sedentary phenotype characterized by reduced voluntary exercise and increased adiposity. Whole-genome bisulfite sequencing (WGBS) and transcriptional profiling in neuronal nuclei from the arcuate nucleus of the hypothalamus (ARH) reveal differentially methylated genomic regions and reduced expression of AgRP neuron-associated genes in knockout mice. We use read-level analysis of WGBS data to infer putative ARH neural cell types affected by the knockout, and to localize promoter hypomethylation and increased expression of the growth factor Bmp7 to AgRP neurons, suggesting a role for aberrant TGF-β signaling in the development of this phenotype. Together, these data demonstrate that DNA methylation in AgRP neurons is required for their normal epigenetic development and neuron-specific gene expression profiles, and regulates voluntary exercise behavior.

AgRP neurons in the hypothalamic arcuate nucleus (ARH) are involved in regulating hunger and energy balance. Here the authors show that knockout of the DNA methyltransferase Dnmt3a in AgRP neurons of the ARH leads to a reduction in voluntary exercise along with numerous epigenetic and gene expression changes in ARH neurons.

A Metabolic Perspective on Reward Abnormalities in Anorexia Nervosa

Anorexia nervosa (AN) is the psychiatric disorder with the highest mortality rate; however, the mechanisms responsible for its pathogenesis remain largely unknown. Large-scale genome-wide association studies (GWAS) have identified genetic loci associated with metabolic features in AN. Metabolic alterations that occur in AN have been mostly considered as consequences of the chronic undernutrition state but until recently have not been linked to the etiology of the disorder. We review the molecular basis of AN based on human genetics, with an emphasis on the molecular components controlling energy homeostasis, highlight the main metabolic and endocrine alterations occurring in AN, and decipher the possible connection between metabolic factors and abnormalities of reward processes that are central in AN.

Neuronal cAMP/PKA Signaling and Energy Homeostasis

The brain plays a key role in the regulation of body weight and glucose metabolism. Peripheral signals including hormones, metabolites, and neural afferent signals are received and processed by the brain which in turn elicits proper behavioral and metabolic responses for maintaining energy and glucose homeostasis. The cAMP/protein kinase A (PKA) pathway acts downstream G-protein-coupled receptors (GPCR) to mediate the physiological effects of many hormones and neurotransmitters. Activated PKA phosphorylates various proteins including ion channels, enzymes, and transcription factors and regulates their activity. Recent studies have shown that neuronal cAMP/PKA activity in multiple brain regions are involved in the regulation of feeding, energy expenditure, and glucose homeostasis. In this chapter I summarize recent genetic and pharmacological studies concerning the regulation of body weight and glucose homeostasis by cAMP/PKA signaling in the brain.

NCB5OR Deficiency in the Cerebellum and Midbrain Leads to Dehydration and Alterations in Thirst Response, Fasted Feeding Behavior, and Voluntary Exercise in Mice

Cytosolic NADH-cytochrome-b5-oxidoreductase (NCB5OR) is ubiquitously expressed in animal tissues. We have previously reported that global ablation of NCB5OR in mice results in early-onset lean diabetes with decreased serum leptin levels and increased metabolic and feeding activities. The conditional deletion of NCB5OR in the mouse cerebellum and midbrain (conditional knock out, CKO mice) results in local iron dyshomeostasis and altered locomotor activity. It has been established that lesion to or removal of the cerebellum leads to changes in nutrient organization, visceral response, feeding behavior, and body weight. This study assessed whether loss of NCB5OR in the cerebellum and midbrain altered feeding or metabolic activity and had an effect on serum T3, cortisol, prolactin, and leptin levels. Metabolic cage data revealed that 16 week old male CKO mice had elevated respiratory quotients and decreased respiratory water expulsion, decreased voluntary exercise, and altered feeding and drinking behavior compared to wild-type littermate controls. Most notably, male CKO mice displayed higher consumption of food during refeeding after a 48-h fast. Echo MRI revealed normal body composition but decreased total water content and hydration ratios in CKO mice. Increased serum osmolality measurements confirmed the dehydration status of male CKO mice. Serum leptin levels were significantly elevated in male CKO mice while prolactin, T3, and cortisol levels remain unchanged relative to wild-type controls, consistent with elevated transcript levels for leptin receptors (short form) in the male CKO mouse cerebellum. Taken together, these findings suggest altered feeding response post starvation as a result of NCB5OR deficiency in the cerebellum.

Clinical Trials Required to Assess Potential Benefits and Side Effects of Treatment of Patients With Anorexia Nervosa With Recombinant Human Leptin

The core phenotype of anorexia nervosa (AN) comprises the age and stage dependent intertwining of both its primary and secondary (i.e., starvation induced) somatic and mental symptoms. Hypoleptinemia acts as a key trigger for the adaptation to starvation by affecting diverse brain regions including the reward system and by induction of alterations of the hypothalamus-pituitary-“target-organ” axes, e.g., resulting in amenorrhea as a characteristic symptom of AN. Particularly, the rat model activity-based anorexia (ABA) convincingly demonstrates the pivotal role of hypoleptinemia in the development of starvation-induced hyperactivity. STAT3 signaling in dopaminergic neurons in the ventral tegmental area (VTA) plays a crucial role in the transmission of the leptin signal in ABA. In patients with AN, an inverted U-shaped relationship has been observed between their serum leptin levels and physical activity. Albeit obese and therewith of a very different phenotype, humans diagnosed with rare congenital leptin deficiency have starvation like symptoms including hypothalamic amenorrhea in females. Over the past 20 years, such patients have been successfully treated with recombinant human (rh) leptin (metreleptin) within a compassionate use program. The extreme hunger of these patients subsides within hours upon initiation of treatment; substantial weight loss and menarche in females ensue after medium term treatment. In contrast, metreleptin had little effect in patients with multifactorial obesity. Small clinical trials have been conducted for hypothalamic amenorrhea and to increase bone mineral density, in which metreleptin proved beneficial. Up to now, metreleptin has not yet been used to treat patients with AN. Metreleptin has been approved by the FDA under strict regulations solely for the treatment of generalized lipodystrophy. The recent approval by the EMA may offer, for the first time, the possibility to treat extremely hyperactive patients with AN off-label. Furthermore, a potential dissection of hypoleptinemia-induced AN symptoms from the primary cognitions and behaviors of these patients could ensue. Accordingly, the aim of this article is to review the current state of the art of leptin in relation to AN to provide the theoretical basis for the initiation of clinical trials for treatment of this eating disorder.

Identification of Leptin Receptor-Expressing Cells in the Nodose Ganglion of Male Mice

Leptin has been proposed to modulate viscerosensory information directly at the level of vagal afferents. In support of this view, broad expression for the leptin receptor (Lepr) has previously been reported in vagal afferents. However, the exact identity and distribution of leptin-sensitive vagal afferents has not been elucidated. Using quantitative PCR, we found that the whole mouse nodose ganglion was predominantly enriched in the short form of Lepr, rather than its long form. Consistent with this observation, the acute administration of leptin did not stimulate JAK-STAT signaling in the nodose ganglion. Using chromogenic in situ hybridization in wild-type mice and several reporter mouse models, we demonstrated that Lepr mRNA was restricted to nonneuronal cells in the epineurium and parenchyma of the nodose ganglion and a subset of vagal afferents, which accounted for only 3% of all neuronal profiles. Double labeling studies further established that Lepr-expressing vagal afferents were Nav1.8-negative fibers that did not supply the peritoneal cavity. Finally, double chromogenic in situ hybridization revealed that many Lepr-expressing neurons coexpressed the angiotensin 1a receptor (At1ar), which is a gene expressed in baroreceptors. Taken together, our data challenge the commonly held view that Lepr is broadly expressed in vagal afferents. Instead, our data suggest that leptin may exert a previously unrecognized role, mainly via its short form, as a direct modulator of a very small group of At1ar-positive vagal fibers.

The motivation for exercise over palatable food is dictated by cannabinoid type-1 receptors

The lack of intrinsic motivation to engage in, and adhere to, physical exercise has major health consequences. However, the neurobiological bases of exercise motivation are still unknown. This study aimed at examining whether the endocannabinoid system (ECS) is involved in this process. To do so, we developed an operant conditioning paradigm wherein mice unlocked a running wheel with nose pokes. Using pharmacological tools and conditional mutants for cannabinoid type-1 (CB1) receptors, we provide evidence that CB1 receptors located on GABAergic neurons are both necessary and sufficient to positively control running motivation. Conversely, this receptor population proved dispensable for the modulation of running duration per rewarded sequence. Although the ECS mediated the motivation for another reward, namely palatable food, such a regulation was independent from CB1 receptors on GABAergic neurons. In addition, we report that the lack of CB1 receptors on GABAergic neurons decreases the preference for running over palatable food when mice were proposed an exclusive choice between the two rewards. Beyond providing a paradigm that enables motivation processes for exercise to be dissected either singly or in concurrence, this study is the first to our knowledge to identify a neurobiological mechanism that might contribute to sedentary behavior.

Leptin Prevents Lipopolysaccharide-Induced Depressive-Like Behaviors in Mice: Involvement of Dopamine Receptors

Depression is a chronic and recurrent disorder, associated with high morbidity and risk of suicide. Leptin was firstly described as an anti-obesity hormone, but several actions of leptin in CNS have been reported. In fact, leptin regulates dopaminergic neurotransmission in mesolimbic areas and has antidepressant-like properties in stress-based models. In the present study, we investigated, for the first time, putative antidepressant-like effects of leptin in an animal model of depressive-like behaviors induced by lipopolysaccharide (LPS), and the potential involvement of dopamine receptors as mediators of those behavioral effects. Mice were injected leptin (1.5 mg/kg, IP) or imipramine prior to LPS administration. To evaluate the involvement of dopamine receptors, different experimental groups were pretreated with either the dopaminergic antagonist SCH23390, for D1 receptors or raclopride, for D2/D3 receptors, prior to leptin injection. Twenty-four hours post-LPS, mice were submitted to the forced swimming and sucrose preference tests. In addition, IL-1β levels were determined in the prefrontal cortex (PFC), hippocampus and striatum. BDNF levels were measured in the hippocampus. Our results showed that leptin, similarly to imipramine, prevented the core behavioral alterations induced by LPS (despair-like behavior and anhedonia), without altering locomotion. In neurochemical analysis, leptin restored LPS-induced changes in IL-1β levels in the PFC and striatum, and increased BDNF levels in the hippocampus. The blockade of dopamine D1 and D2/D3 receptors inhibited leptin's antidepressant-like effects, whilst only the blockade of D1-like receptors blunted leptin-induced increments in prefrontal IL-1β levels. Our results indicate that leptin has antidepressant-like effects in an inflammatory model of depression with the contribution, at least partial, of dopamine receptors.

Brown rice-specific γ-oryzanol as a promising prophylactic avenue to protect against diabetes mellitus and obesity in humans

Chronic overconsumption of animal fats causes a variety of health problems, including diabetes mellitus and obesity. Underlying molecular mechanisms encompass leptin resistance, a decrease in rewarding effects of physical activities, xanthine oxidase-induced oxidative stress in vasculature and peripheral tissue, impaired activation of incretin signaling, deviation in food preference, and dysbiosis of gut microbiota. Based on our clinical observation that daily intake of brown rice effectively ameliorates bodyweight gain, impaired glucose tolerance/insulin resistance and dependence on fatty foods in obese, prediabetes men, a line of research on brown rice (rice bran)-derived γ-oryzanol in mice experiments, cultured cells and human clinical trials is underway in our laboratory. Our works in mice showed that γ-oryzanol, an ester mixture of ferulic acid and several kinds of phytosterols, acts as a molecular chaperone, thereby attenuating the strong preference for animal fats through suppression of endoplasmic reticulum stress in the hypothalamus. In pancreatic islets from both high-fat diet-induced and streptozotocin-induced diabetic mice, γ-oryzanol ameliorates endoplasmic reticulum stress and protects β-cells against apoptosis. Noticeably, γ-oryzanol also acts as a potent inhibitor against deoxyribonucleic acid methyltransferases in the brain reward system (striatum) in mice, thereby attenuating, at least partly, the preference for a high-fat diet through the epigenetic modulation of striatal dopamine D2 receptor. Because dopamine D2 receptor signaling in the brain reward system is considerably attenuated in obese humans and rodents, γ-oryzanol might represent a unique property to ameliorate both hedonic and metabolic dysregulation of feeding behavior, highlighting a promising prophylactic avenue to protect against metabolic derangement.

Leptin and insulin do not exert redundant control of metabolic or emotive function via dopamine neurons

Leptin and insulin's hunger-suppressing and activity-promoting actions on hypothalamic neurons are well characterized, yet the mechanisms by which they modulate the midbrain dopamine system to influence energy balance remain less clear. A subset of midbrain dopamine neurons express receptors for leptin (Lepr) and insulin (Insr). Leptin-dopamine signaling reduces running reward and homecage activity. However, dopamine-specific deletion of Lepr does not affect body weight or food intake in mice. We hypothesized insulin-dopamine signaling might compensate for disrupted leptin-dopamine signaling. To investigate the degree to which insulin and leptin exert overlapping (i.e. redundant) versus discrete control over dopamine neurons, we generated transgenic male and female mice exhibiting dopamine-specific deletion of either Lepr (Lepr KO), Insr (Insr KO) or both Lepr and Insr (Dbl KO) and assessed their feeding behavior, voluntary activity, and energy expenditure compared to control mice. No differences in body weight, daily food intake, energy expenditure or hyperphagic feeding of palatable chow were observed between Lepr, Insr or Dbl KO mice and control mice. However, consistent with previous findings, Lepr KO (but not Insr or Dbl KO) male mice exhibited significantly increased running wheel activity compared to controls. These data demonstrate that insulin and leptin do not exert redundant control of dopamine neuron-mediated modulation of energy balance. Furthermore, our results indicate neither leptin nor insulin plays a critical role in the modulation of dopamine neurons regarding hedonic feeding behavior or anxiety-related behavior.

Development of leptin resistance in sucrose drinking rats is associated with consuming carbohydrate-containing solutions and not calorie-free sweet solution

Rats offered 30% sucrose solution plus chow or a sucrose-free diet develop leptin resistance within 4 weeks. This experiment tested whether leptin resistance was associated with the reward of sweet taste or the pre- or post-absorptive effects of consumption of simple carbohydrate. Male Sprague Dawley rats were offered a sucrose-free diet (NS), a diet containing 67% calories as sucrose (HS) or NS diet plus 30% sucrose (LS), 0.03% saccharin (Sacc) or 20% SolCarb^® solution for 38 days. SolCarb^® is a maltodextrin powder. Sacc rats initially drank more than LS rats, but intakes were the same after Day 20. SolCarb^® and LS rats drank the same number of calories from Day 15 to the end of the experiment. SolCarb^® and LS rats ate less dry food than other groups, but total energy intake was greater than that of NS, HS and Sacc groups and over 80% of their energy intake was from carbohydrate. Leptin responsiveness was tested on Days 27 and 32 with each rat acting as its own control. An i.p. injection of 2 mg/kg leptin inhibited food intake of NS, HS and Sacc rats, but had no effect on energy intake of LS or SolCarb^® rats or on consumption of Sacc, sucrose or SolCarb^® solution. At the end of the experiment all of the rats were insulin sensitive, had the same body composition and serum leptin concentrations. These data indicate that consumption of a calorie containing carbohydrate solution and not sweet taste drives the development of leptin resistance and suggest that there is lower threshold for inhibition of hunger than for inhibition of reward by leptin.

Evaluating anhedonia in the activity-based anorexia (ABA) rat model

Patients suffering anorexia nervosa (AN) become anhedonic, in other words, unable or unwilling to derive normal pleasures and avoid rewarding outcomes, most profoundly in food intake. The neurobiological underpinnings of anhedonia are likely to involve mesolimbic reward circuitry. We propose here that this circuitry and its involvement in AN can be investigated using the activity-based anorexia (ABA) rodent model that recapitulates many of the characteristics of the human condition, most notably rapid weight loss. Preference for sweetened water was used to assay hedonic processing in female Sprague-Dawley rats exposed to the ABA protocol, which involves free access to running wheels paired with time-limited access to food. This protocol uncovered a transient anhedonia in only one quarter of cases; however, exposure to running wheels alone was associated with a rapid aversion to sweetened water (F_{1.833, 20.17} = 78.29, p < .0001), and time-limited food access alone did not impact preference (F_{2.205, 24.25} = 0.305, p = .761). High levels of running wheel activity prior to the onset of food restriction increased susceptibility to body weight loss in ABA (F_10,196.129 = 2.069, p = .029) and food anticipatory activity predicted subsequent food intake only for rats that were resistant to body weight loss (r = 0.44, p = .001). These data are inconsistent with the hypothesis that anhedonia underscores the precipitous weight loss in ABA, however, they highlight the predictive nature of hyperactivity in susceptibility to the ABA paradigm. These results will help inform the neurobiological framework of ABA and provide insight into the mechanisms of reward relevant to feeding and weight loss.

Physical Fitness and Its Relationship to Plasma Leptin, Leptin Soluble Receptor, and Free Leptin Index in a Saudi Population: A Comparison Between Diabetic and Non-Diabetic Individuals

Background

Low physical activity is considered to be a risk factor for type 2 diabetes mellitus (T2DM). One theory suggest that leptin resistance is involved in the pathophysiology of impaired glucose metabolism. In this study we aimed to assess the correlation of physical fitness scores (PFS) with serum total leptin (TL), serum leptin soluble receptor (LSR), and free leptin index (FLI) in a group of Saudi patients with T2DM.

Material/Methods

This cross-sectional study involved 115 subjects: 52 healthy control subjects and 63 patients with T2DM. All subjects underwent body composition analysis. Blood samples were analyzed for fasting blood glucose (FBG), glycosylated hemoglobin (HbA1c), serum total leptin (TL), and serum leptin soluble receptor (LSR). Based on ideal body composition and our previous studies, physical fitness scores (PFS) were recorded for each subject.

Results

In patients with T2DM, levels of LSR were positively correlated with PFS (r=0.281, p=0.025), while the levels of TL (r=−0.425, p=0.001) and FLI (r=−0.439, p=0.001) were negatively correlated with PFS. In control subjects, TL and FLI levels were negatively correlated (r=−0.612, p=0.001and r=−0.543, p=0.001 respectively) with PFS. In linear regression analysis, after adjustment for age and BMI, TL and FLI were independent predictors of PFS.

Conclusions

Serum TL and FLI were negatively correlated while LSR was positively correlated with PFS in patients with T2DM. Therefore, they may be important biomarkers for predicting the outcomes of physical fitness and exercise programs.

Voluntary wheel running promotes resilience to chronic social defeat stress in mice: a role for nucleus accumbens ΔFosB

Elucidating mechanisms by which physical exercise promotes resilience, the brain's ability to cope with prolonged stress exposure while maintaining normal psychological functioning, is a major research challenge given the high prevalence of stress-related mental disorders, including major depressive disorder. Chronic voluntary wheel running (VWR), a rodent model that mimics aspects of human physical exercise, induces the transcription factor ΔFosB in the nucleus accumbens (NAc), a key reward-related brain area. ΔFosB expression in NAc modulates stress susceptibility. Here, we explored whether VWR induction of NAc ΔFosB promotes resilience to chronic social defeat stress (CSDS). Male young-adult C57BL/6J mice were single housed for up to 21 d with or without running wheels and then subjected to 10 d of CSDS. Stress-exposed sedentary mice developed a depressive-like state, characterized by anhedonia and social avoidance, whereas stress-exposed mice that had been wheel running showed resilience. Functional inhibition of NAc ΔFosB during VWR, by viral-mediated overexpression of a transcriptionally inactive JunD mutant, reinstated susceptibility to CSDS. Within the NAc, VWR induction of ΔFosB was CREB-dependent, associated with altered dendritic morphology, and medium spiny neuron (MSN) subtype specific in the NAc core and shell subregions. Finally, when mice performed VWR following the onset of CSDS-induced social avoidance, VWR normalized such behavior. These data indicate that VWR promoted resilience to CSDS, and suggest that sustained induction of ΔFosB in the NAc underlies, at least in part, the stress resilience mediated by VWR. These findings provide a potential framework for the development of treatments for stress-associated mental illnesses based on physical exercise.