Neural Control of Energy Expenditure

Brain lipid sensing and the neural control of energy balance

The central nervous system continuously detects circulating concentrations of lipids such as fatty acids and troglycerides. Once information has been detected, the central nervous system can in turn participate in the control of energy balance and blood sugar levels and in particular regulate the secretion and action of insulin. Neurons capable of detecting circulating lipid variations are located in the hypothalamus and in other regions such as the nucleus accumbens, the striatum or the hippocampus. An excess of lipids will have deleterious effects and may induce central lipotoxicity, in particular following local production of ceramides and the appearance of neuroinflammation which may lead to metabolic diseases such as obesity and type 2 diabetes.

Crosstalk between Gut Sensory Ghrelin Signaling and Adipose Tissue Sympathetic Outflow Regulates Metabolic Homeostasis

The stomach-derived orexigenic hormone ghrelin is a key regulator of energy homeostasis and metabolism in humans. The ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR), is widely expressed in the brain and gastrointestinal vagal sensory neurons, and neuronal GHSR knockout results in a profoundly beneficial metabolic profile and protects against diet-induced obesity (DIO) and insulin resistance. Here we show that in addition to the well characterized vagal GHSR, GHSR is robustly expressed in gastrointestinal sensory neurons emanating from spinal dorsal root ganglia. Remarkably, sensory neuron GHSR deletion attenuates DIO through increased energy expenditure and sympathetic outflow to adipose tissue independent of food intake. In addition, neuronal viral tract tracing reveals prominent crosstalk between gut non-vagal sensory afferents and adipose sympathetic outflow. Hence, these findings demonstrate a novel gut sensory ghrelin signaling pathway critical for maintaining energy homeostasis.

Impact of estrogens on resting energy expenditure: A systematic review

The fear of weight gain is one of the main reasons for women not to initiate or to early discontinue hormonal contraception or menopausal hormone therapy. Resting energy expenditure is by far the largest component and the most important determinant of total energy expenditure. Given that low resting energy expenditure is a confirmed predictive factor for weight gain and consecutively for the development of obesity, research into the influence of sex steroids on resting energy expenditure is a particularly exciting area. The objective of this systematic review was to evaluate the effects of medication with natural and synthetic estrogens on resting energy expenditure in healthy normal weight and overweight women. Through complex systematic literature searches, a total of 10 studies were identified that investigated the effects of medication with estrogens on resting energy expenditure. Our results demonstrate that estrogen administration increases resting energy expenditure by up to +208 kcal per day in the context of contraception and by up to +222 kcal per day in the context of menopausal hormone therapy, suggesting a preventive effect of circulating estrogen levels and estrogen administration on weight gain and obesity development.

Collateralizing ventral subiculum melanocortin 4 receptor circuits regulate energy balance and food motivation

Hippocampal dysfunction is associated with major depressive disorder, a serious mental illness characterized by not only depressed mood but also appetite disturbance and dysregulated body weight. However, the underlying mechanisms by which hippocampal circuits regulate metabolic homeostasis remain incompletely understood. Here we show that collateralizing melanocortin 4 receptor (MC4R) circuits in the ventral subiculum (vSUB), one of the major output structures of the hippocampal formation, affect food motivation and energy balance. Viral-mediated cell type- and projection-specific input-output circuit mapping revealed that the nucleus accumbens shell (NAcSh)-projecting vSUB^MC4R+ neurons send extensive collateral projections of to various hypothalamic nuclei known to be important for energy balance, including the arcuate, ventromedial and dorsomedial nuclei, and receive monosynaptic inputs mainly from the ventral CA1 and the anterior paraventricular nucleus of thalamus. Chemogenetic activation of NAcSh-projecting vSUB^MC4R+neurons lead to increase in motivation to obtain palatable food without noticeable effect on homeostatic feeding. Viral-mediated restoration of MC4R signaling in the vSUB partially restores obesity in MC4R-null mice without affecting anxiety- and depression-like behaviors. Collectively, these results delineate vSUB^MC4R+ circuits to the unprecedented level of precision and identify the vSUB^MC4R signaling as a novel regulator of food reward and energy balance.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

Exenatide increases CTRP3 gene expression in adipose cells by inhibiting adipogenesis and induces apoptosis

Considering the rapidly increasing prevalence of obesity worldwide, the number of weight control drugs is very few. Incretin-based therapies are currently being developed to achieve weight control, and Glucagon-Like Peptide-1 Receptor Agonists (GLP-1RA) are used in incretin-based therapies. This study aimed to investigate the cytotoxicity of exenatide, a GLP-1A, on 3T3-L1 adipocytes and the effect of exenatide on the expression of adipogenesis-related genes, insulin and glucose levels, and apoptosis. Cytotoxic activity of exenatide on 3T3-L1 adipocytes was determined by MTT method. Gene expression levels were determined by qPCR. Apoptosis studies were performed on the Muse Cell Analyzer. C1q/TNF-related protein-3 (CTRP3) expression levels were found to be higher in exenatide treated adipocyte cells than in control cells (p < 0.001). Adipocyte cells treated with exenatide were found to have lower PPAR-γ gene expression levels when compared to control adipocyte cells (p < 0.001). Intracellular insulin (p < 0.001) and glucose levels were higher in 3T3-L1 adipocytes treated with exenatide compared to control adipocyte cells. Total apoptosis increased approximately 1.5 times as a result of exenatide administration. The increase in CTRP3 gene expression, which is thought to be a new biomarker for obesity, and the decrease in PPAR-γ gene expression indicate that exenatide is a promising new pharmacotherapeutic agent in the treatment of obesity by regulating the expression of genes related to adipogenesis and lipogenesis and inducing apoptosis.

Competing paradigms of obesity pathogenesis: energy balance versus carbohydrate-insulin models

The obesity pandemic continues unabated despite a persistent public health campaign to decrease energy intake (“eat less”) and increase energy expenditure (“move more”). One explanation for this failure is that the current approach, based on the notion of energy balance, has not been adequately embraced by the public. Another possibility is that this approach rests on an erroneous paradigm. A new formulation of the energy balance model (EBM), like prior versions, considers overeating (energy intake > expenditure) the primary cause of obesity, incorporating an emphasis on “complex endocrine, metabolic, and nervous system signals” that control food intake below conscious level. This model attributes rising obesity prevalence to inexpensive, convenient, energy-dense, “ultra-processed” foods high in fat and sugar. An alternative view, the carbohydrate-insulin model (CIM), proposes that hormonal responses to highly processed carbohydrates shift energy partitioning toward deposition in adipose tissue, leaving fewer calories available for the body’s metabolic needs. Thus, increasing adiposity causes overeating to compensate for the sequestered calories. Here, we highlight robust contrasts in how the EBM and CIM view obesity pathophysiology and consider deficiencies in the EBM that impede paradigm testing and refinement. Rectifying these deficiencies should assume priority, as a constructive paradigm clash is needed to resolve long-standing scientific controversies and inform the design of new models to guide prevention and treatment. Nevertheless, public health action need not await resolution of this debate, as both models target processed carbohydrates as major drivers of obesity.

Phenylhydrazine-induced anemia reduces subcutaneous white and brown adipose tissues in hypothalamic obese rats

NEW FUNDINGS

What is the central question of this study? This study aims to assess whether an anemic state could modify adiposity and metabolic parameters in hypothalamic obese rats. What is the main finding and its importance? Our results indicate that hypothalamic obese rats do not display iron deficiency. However, the pharmacological induction of anemia in hypothalamic-obese rats resulted in reduced adiposity, characterized by a decrease in subcutaneous white and brown adipose tissue depots. These findings suggest that iron imbalance in obesity may elevate lipolysis.

ABSTRACT

Iron imbalance is frequent in obesity. Herein, we evaluated the impact of anemia induced by phenylhydrazine on adiposity and metabolic state of hypothalamic obese rats. Hypothalamic obesity was induced by high doses of glutamate monosodium (MSG; 4g/Kg) administered to neonatal male rats (n = 20). Controls (CTL; non-obese rats) received saline equimolar (n = 20). Rats were weaned at 21 days of life. At 70 days, half of the rats received three intraperitoneal doses of phenylhydrazine (PHZ; 40mg/Kg/dose) or saline solution. Body weight and food intake were accompanied for four weeks after PHZ administration. At 92 days, rats were euthanized, blood was collected for microcapillary hematocrit (Hct) analysis and plasma quantification of glucose, triglycerides, total cholesterol, and iron levels. The liver, the spleen, and the white (WAT) and brown (BAT) adipose tissues were excised, weighed, and used for histology. MSG-treated rats developed obesity, hypertriglyceridemia, and insulin resistance, compared to CTL rats, without changes in iron levels and Hct. PHZ administration reduced iron plasma levels and promoted similar tissue injuries in the spleen and liver from MSG and CTL rats. However, in MSG-treated rats, PHZ decreased fasting glucose levels and Hct, as well as diminished the subcutaneous WAT and BAT mass. Although MSG-obesity does not affect iron plasma levels and Hct by itself, PHZ-induced anemia associated with obesity induces a marked drop in subcutaneous WAT and BAT mass, suggesting that iron imbalance may lead to increased lipolytic responses in obese rats, compared to lean rats. This article is protected by copyright. All rights reserved.

Adipose tissue-derived neurotrophic factor 3 regulates sympathetic innervation and thermogenesis in adipose tissue

Activation of brown fat thermogenesis increases energy expenditure and alleviates obesity. Sympathetic nervous system (SNS) is important in brown/beige adipocyte thermogenesis. Here we discover a fat-derived "adipokine" neurotrophic factor neurotrophin 3 (NT-3) and its receptor Tropomyosin receptor kinase C (TRKC) as key regulators of SNS growth and innervation in adipose tissue. NT-3 is highly expressed in brown/beige adipocytes, and potently stimulates sympathetic neuron neurite growth. NT-3/TRKC regulates a plethora of pathways in neuronal axonal growth and elongation. Adipose tissue sympathetic innervation is significantly increased in mice with adipocyte-specific NT-3 overexpression, but profoundly reduced in mice with TRKC haploinsufficiency (TRKC +/-). Increasing NT-3 via pharmacological or genetic approach promotes beige adipocyte development, enhances cold-induced thermogenesis and protects against diet-induced obesity (DIO); whereas TRKC + /- or SNS TRKC deficient mice are cold intolerant and prone to DIO. Thus, NT-3 is a fat-derived neurotrophic factor that regulates SNS innervation, energy metabolism and obesity.

Lifelong enhancement of body mass from adolescent stress in male hamsters

In hamsters, exposure to stress in adulthood causes increased body weight. We addressed how social stress during puberty would impact food intake and body weight. Stressed hamsters started gaining significantly more weight than controls after only two days of stress exposure. Over a two-week period, stressed subjects gained 10% more weight and consumed more food than controls. At the end of the stress period, stressed hamsters collected nearly twice as many palatable sugar pellets from an arena than controls. Stressed subjects presented 15-20% more body fat in mesenteric, inguinal, and retroperitoneal fat pads. In order to assess the duration of these effects, we analyzed data from previous studies keeping hamsters for over two months past the stress period in puberty. Our analysis shows that stressed hamsters stopped gaining more weight after the stress period, but their body weights remained elevated for over two months, consistently weighing 10% more than their non-stressed counterparts. We also analyzed conditioning training data collected after the period of stress in late puberty and early adulthood (P56 to P70) that was part of the original studies. Training consisted of lever pressing for palatable food rewards. At these times, previously stressed hamsters retrieved similar numbers of food pellets from the conditioning chambers, suggesting no difference in appetite after the stress period. These data showing a long-lasting effect of stress on body weight may be relevant to studies on the ontogeny of lifelong obesity.

The Arcuate Nucleus of the Hypothalamus and Metabolic Regulation: An Emerging Role for Renin-Angiotensin Pathways

Obesity is a chronic state of energy imbalance that represents a major public health problem and greatly increases the risk for developing hypertension, hyperglycemia, and a multitude of related pathologies that encompass the metabolic syndrome. The underlying mechanisms and optimal treatment strategies for obesity, however, are still not fully understood. The control of energy balance involves the actions of circulating hormones on a widely distributed network of brain regions involved in the regulation of food intake and energy expenditure, including the arcuate nucleus of the hypothalamus. While obesity is known to disrupt neurocircuits controlling energy balance, including those in the hypothalamic arcuate nucleus, the pharmacological targeting of these central mechanisms often produces adverse cardiovascular and other off-target effects. This highlights the critical need to identify new anti-obesity drugs that can activate central neurocircuits to induce weight loss without negatively impacting blood pressure control. The renin–angiotensin system may provide this ideal target, as recent studies show this hormonal system can engage neurocircuits originating in the arcuate nucleus to improve energy balance without elevating blood pressure in animal models. This review will summarize the current knowledge of renin–angiotensin system actions within the arcuate nucleus for control of energy balance, with a focus on emerging roles for angiotensin II, prorenin, and angiotensin-(1–7) pathways.

Old Paradoxes and New Opportunities for Appetite Control in Obesity

Human obesity is accompanied by alterations in the blood concentrations of multiple circulating appetite regulators. Paradoxically, most of the appetite-inhibitory hormones are elevated in nonsyndromic obesity, while most of the appetite stimulatory hormones are reduced, perhaps reflecting vain attempts of regulation by inefficient feedback circuitries. In this context, it is important to understand which appetite regulators exhibit a convergent rather than paradoxical behavior and hence are likely to contribute to the maintenance of the obese state. Pharmacological interventions in obesity should preferentially consist of the supplementation of deficient appetite inhibitors or the neutralization of excessive appetite stimulators. Here, we critically analyze the current literature on appetite-regulatory peptide hormones. We propose a short-list of appetite modulators that may constitute the best candidates for therapeutic interventions.

The Role of Mediobasal Hypothalamic PACAP in the Control of Body Weight and Metabolism

Body energy homeostasis results from balancing energy intake and energy expenditure. Central nervous system administration of pituitary adenylate cyclase activating polypeptide (PACAP) dramatically alters metabolic function, but the physiologic mechanism of this neuropeptide remains poorly defined. PACAP is expressed in the mediobasal hypothalamus (MBH), a brain area essential for energy balance. Ventromedial hypothalamic nucleus (VMN) neurons contain, by far, the largest and most dense population of PACAP in the medial hypothalamus. This region is involved in coordinating the sympathetic nervous system in response to metabolic cues in order to re-establish energy homeostasis. Additionally, the metabolic cue of leptin signaling in the VMN regulates PACAP expression. We hypothesized that PACAP may play a role in the various effector systems of energy homeostasis, and tested its role by using VMN-directed, but MBH encompassing, adeno-associated virus (AAVCre) injections to ablate Adcyap1 (gene coding for PACAP) in mice (Adcyap1MBHKO mice). Adcyap1MBHKO mice rapidly gained body weight and adiposity, becoming hyperinsulinemic and hyperglycemic. Adcyap1MBHKO mice exhibited decreased oxygen consumption (VO2), without changes in activity. These effects appear to be due at least in part to brown adipose tissue (BAT) dysfunction, and we show that PACAP-expressing cells in the MBH can stimulate BAT thermogenesis. While we observed disruption of glucose clearance during hyperinsulinemic/euglycemic clamp studies in obese Adcyap1MBHKO mice, these parameters were normal prior to the onset of obesity. Thus, MBH PACAP plays important roles in the regulation of metabolic rate and energy balance through multiple effector systems on multiple time scales, which highlight the diverse set of functions for PACAP in overall energy homeostasis.

Top-Down Control of Sweet and Bitter Taste in the Mammalian Brain

Hardwired circuits encoding innate responses have emerged as an essential feature of the mammalian brain. Sweet and bitter evoke opposing predetermined behaviors. Sweet drives appetitive responses and consumption of energy-rich food sources, whereas bitter prevents ingestion of toxic chemicals. Here we identified and characterized the neurons in the brainstem that transmit sweet and bitter signals from the tongue to the cortex. Next we examined how the brain modulates this hardwired circuit to control taste behaviors. We dissect the basis for bitter-evoked suppression of sweet taste and show that the taste cortex and amygdala exert strong positive and negative feedback onto incoming bitter and sweet signals in the brainstem. Finally we demonstrate that blocking the feedback markedly alters responses to ethologically relevant taste stimuli. These results illustrate how hardwired circuits can be finely regulated by top-down control and reveal the neural basis of an indispensable behavioral response for all animals.

Isocaloric low protein diet in a mouse model for vanishing white matter does not impact ISR deregulation in brain, but reveals ISR deregulation in liver

Objective: Vanishing white matter (VWM) is a genetic brain white matter disorder caused by mutations in eIF2B. eIF2B is central in the integrated stress response (ISR), during which its activity is inhibited by various cellular stresses. VWM is a chronic progressive disease with episodes of rapid neurological deterioration provoked by stresses. VWM patients and VWM mouse models show ISR deregulation in brain, correlating with chronic disease development. ISR inhibition ameliorates the chronic disease in VWM mice. The subacute deteriorations have not been modeled yet. We hypothesized that ISR activation could worsen disease progression in mice and model the episodic neurological deterioration.Method: We chose to activate the ISR by subjecting wild-type (wt) and VWM mice to an isocaloric low protein diet. This model would allow us to investigate the contribution of ISR activation in subacute decline in VWM.Results: We found that the low protein diet did not significantly affect amino acid levels nor ISR levels in wt and VWM mouse brain. Our study serendipitously led to the discovery of increased levels of glycine, asparagine and Fgf21 mRNA in VWM mouse brain irrespective of the dietary protein content. Strikingly, the ISR was not activated by the low protein diet in the liver of VWM in contrast to wt mice, due to a modest ISR deregulation in this organ.Discussion: A model for subacute neurological deterioration in VWM was not established. Possibly, ISR deregulation in VWM results in reduced ISR responsiveness.

Brain-gut-microbiome interactions in obesity and food addiction

Normal eating behaviour is coordinated by the tightly regulated balance between intestinal and extra-intestinal homeostatic and hedonic mechanisms. By contrast, food addiction is a complex, maladaptive eating behaviour that reflects alterations in brain-gut-microbiome (BGM) interactions and a shift of this balance towards hedonic mechanisms. Each component of the BGM axis has been implicated in the development of food addiction, with both brain to gut and gut to brain signalling playing a role. Early-life influences can prime the infant gut microbiome and brain for food addiction, which might be further reinforced by increased antibiotic usage and dietary patterns throughout adulthood. The ubiquitous availability and marketing of inexpensive, highly palatable and calorie-dense food can further shift this balance towards hedonic eating through both central (disruptions in dopaminergic signalling) and intestinal (vagal afferent function, metabolic endotoxaemia, systemic immune activation, changes to gut microbiome and metabolome) mechanisms. In this Review, we propose a systems biology model of BGM interactions, which incorporates published reports on food addiction, and provides novel insights into treatment targets aimed at each level of the BGM axis.

What is bipolar disorder? A disease model of dysregulated energy expenditure

Advances in the understanding and management of bipolar disorder (BD) have been slow to emerge. Despite notable recent developments in neurosciences, our conceptualization of the nature of this mental disorder has not meaningfully progressed. One of the key reasons for this scenario is the continuing lack of a comprehensive disease model. Within the increasing complexity of modern research methods, there is a clear need for an overarching theoretical framework, in which findings are assimilated and predictions are generated. In this review and hypothesis article, we propose such a framework, one in which dysregulated energy expenditure is a primary, sufficient cause for BD. Our proposed model is centered on the disruption of the molecular and cellular network regulating energy production and expenditure, as well its potential secondary adaptations and compensatory mechanisms. We also focus on the putative longitudinal progression of this pathological process, considering its most likely periods for onset, such as critical periods that challenges energy homeostasis (e.g. neurodevelopment, social isolation), and the resulting short and long-term phenotypical manifestations.

Postnatal leptin surge is critical for the transient induction of the developmental beige adipocytes in mice

Beige adipocytes have become a promising therapeutic target to combat obesity. Our senior author Dr. B. Xue previously discovered a transient but significant induction of beige adipocytes in mice during early postnatal development, which peaked at postnatal day (P) 20 and then disappeared thereafter. However, the physiological mechanism underlying the transient induction of the developmental beige cells remains mystery. Interestingly, there exists a postnatal surge of leptin in mice at P10 before the appearance of the developmental beige adipocytes. Given the neurotropic effect of leptin during neuronal development and its role in activating the sympathetic nervous system (SNS), we tested the hypothesis that postnatal leptin surge is required for the transient induction of developmental beige adipocytes through sympathetic innervation. Unlike wild-type (WT) mice that were able to acquire the developmentally induced beige adipocytes at P20, ob/ob mice had much less uncoupling protein 1 (UCP1)-positive multilocular cells in inguinal white adipose tissue at the same age. This was consistent with reduced expression of UCP1 mRNA and protein levels in white fat of ob/ob mice. In contrast, daily injection of ob/ob mice with leptin between P8 and P16, mimicking the postnatal leptin surge, largely rescued the ability of these mice to acquire the developmentally induced beige adipocytes at P20, which was associated with enhanced sympathetic nerve innervation assessed by whole mount adipose tissue immunostaining of tyrosine hydroxylase. Our data demonstrate that the postnatal leptin surge is essential for the developmentally induced beige adipocyte formation in mice, possibly through increasing sympathetic nerve innervation.

Tanycytes Regulate Lipid Homeostasis by Sensing Free Fatty Acids and Signaling to Key Hypothalamic Neuronal Populations via FGF21 Secretion

The hypothalamus plays a key role in the detection of energy substrates to regulate energy homeostasis. Tanycytes, the hypothalamic ependymo-glia, are located at a privileged position to integrate multiple peripheral inputs. We observed that tanycytes produce and secrete Fgf21 and are located close to Fgf21-sensitive neurons. Fasting, likely via the increase in circulating fatty acids, regulates this central Fgf21 production. Tanycytes store palmitate in lipid droplets and oxidize it, leading to the activation of a reactive oxygen species (ROS)/p38-MAPK signaling pathway, which is essential for tanycytic Fgf21 expression upon palmitate exposure. Tanycytic Fgf21 deletion triggers an increase in lipolysis, likely due to impaired inhibition of key neurons during fasting. Mice deleted for tanycytic Fgf21 exhibit increased energy expenditure and a reduction in fat mass gain, reminiscent of a browning phenotype. Our results suggest that tanycytes sense free fatty acids to maintain body lipid homeostasis through Fgf21 signaling within the hypothalamus.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Regulation of Energy Expenditure by Brainstem GABA Neurons

Homeostatic control of core body temperature is essential for survival. Temperature is sensed by specific neurons, in turn eliciting both behavioral (i.e., locomotion) and physiologic (i.e., thermogenesis, vasodilatation) responses. Here, we report that a population of GABAergic (Vgat-expressing) neurons in the dorsolateral portion of the dorsal raphe nucleus (DRN), hereafter DRN^Vgat neurons, are activated by ambient heat and bidirectionally regulate energy expenditure through changes in both thermogenesis and locomotion. We find that DRN^Vgat neurons innervate brown fat via a descending projection to the raphe pallidus (RPa). These neurons also densely innervate ascending targets implicated in the central regulation of energy expenditure, including the hypothalamus and extended amygdala. Optogenetic stimulation of different projection targets reveals that DRN^Vgat neurons are capable of regulating thermogenesis through both a "direct" descending pathway through the RPa and multiple "indirect" ascending pathways. This work establishes a key regulatory role for DRN^Vgat neurons in controlling energy expenditure.

Brain Trauma Disrupts Hepatic Lipid Metabolism: Blame It on Fructose?

SCOPE

The action of brain disorders on peripheral metabolism is poorly understood. The impact of traumatic brain injury (TBI) on peripheral organ function and how TBI effects can be influenced by the metabolic perturbation elicited by fructose ingestion are studied.

METHODS AND RESULTS

It is found that TBI affects glucose metabolism and signaling proteins for insulin and growth hormone in the liver; these effects are exacerbated by fructose ingestion. Fructose, principally metabolized in the liver, potentiates the action of TBI on hepatic lipid droplet accumulation. Studies in isolated cultured hepatocytes identify GH and fructose as factors for the synthesis of lipids. The liver has a major role in the synthesis of lipids used for brain function and repair. TBI results in differentially expressed genes in the hypothalamus, primarily associated with lipid metabolism, providing cues to understand central control of peripheral alterations. Fructose-fed TBI animals have elevated levels of markers of inflammation, lipid peroxidation, and cell energy metabolism, suggesting the pro-inflammatory impact of TBI and fructose in the liver.

CONCLUSION

Results reveal the impact of TBI on systemic metabolism and the aggravating action of fructose. The hypothalamic-pituitary-growth axis seems to play a major role in the regulation of the peripheral TBI pathology.

Sympathetic nerve innervation is required for beigeing in white fat

It is increasingly recognized that activation of beige adipocyte thermogenesis by pharmacological or genetic approaches increases energy expenditure and alleviates obesity. Sympathetic nervous system (SNS) directly innervating brown adipose tissue (BAT) and white adipose tissue (WAT) plays a key role in promoting nonshivering thermogenesis. However, direct evidence that supports the importance of SNS innervation for beige adipocyte formation is still lacking, and the significance of beige adipocyte thermogenesis in protection of body temperature during cold challenge is not clear. Here we tested the necessity of SNS innervation into WAT for beige adipocyte formation in mice with defective brown fat thermogenesis via interscapular BAT (iBAT) SNS denervation. SNS denervation was achieved by microinjection of 6-hydroxydopamine (6-OHDA), a selective neurotoxin to SNS nerves, into iBAT, inguinal WAT (iWAT), or both. The partial chemical denervation of iBAT SNS down-regulated UCP-1 protein expression in iBAT demonstrated by immunoblotting and immunohistochemical measurements. This was associated with an up-regulation of UCP1 protein expression and enhanced formation of beige cells in iWAT of mice with iBAT SNS denervation. In contrast, the chemical denervation of iWAT SNS completely abolished the upregulated UCP-1 protein and beige cell formation in iWAT of mice with iBAT SNS denervation. Our data demonstrate that SNS innervation in WAT is required for beige cell formation during cold-induced thermogenesis. We conclude that there exists a coordinated thermoregulation for BAT and WAT thermogenesis via a functional cross talk between BAT and WAT SNS.

Leptin and brain-adipose crosstalks

Interactions between the brain and distinct adipose depots have a key role in maintaining energy balance, thereby promoting survival in response to metabolic challenges such as cold exposure and starvation. Recently, there has been renewed interest in the specific central neuronal circuits that regulate adipose depots. Here, we review anatomical, genetic and pharmacological studies on the neural regulation of adipose function, including lipolysis, non-shivering thermogenesis, browning and leptin secretion. In particular, we emphasize the role of leptin-sensitive neurons and the sympathetic nervous system in modulating the activity of brown, white and beige adipose tissues. We provide an overview of advances in the understanding of the heterogeneity of the brain regulation of adipose tissues and offer a perspective on the challenges and paradoxes that the community is facing regarding the actions of leptin on this system.

Insulin action in the brain: Roles in energy and glucose homeostasis

A growing body of evidence from research in rodents and humans has identified insulin as an important neuoregulatory peptide in the brain, where it coordinates diverse aspects of energy balance and peripheral glucose homeostasis. This review discusses where and how insulin interacts within the brain and evaluates the physiological and pathophysiological consequences of central insulin signalling in metabolism, obesity and type 2 diabetes.

High-fat diet increases pain behaviors in rats with or without obesity

Obesity is associated with increased risk for chronic pain. Basic mechanisms for this association are poorly understood. Using a milder version of a radicular pain model, local inflammation of the dorsal root ganglion (DRG), we observed marked increases in mechanical and cold allodynia in rats of both sexes that were maintained on a high-fat diet (HFD) for 6 weeks prior to DRG inflammation. Notably, this increase in pain-related behaviors occurred in both Long-Evans and Sprague-Dawley rats despite the fact that the 6-week HFD exposure induced obesity (e.g., increased insulin, leptin, weight, and percent body fat) in the Long-Evans, but not Sprague-Dawley, strains. This suggested that HFD, rather than obesity per se, increased pain behaviors. Increased pain behaviors were observed even after a much shorter (1 week) exposure to the HFD but the effect was smaller. HFD also increased behavioral responses and paw swelling to paw injection of complete Freund’s adjuvant, a model of peripheral inflammatory pain. No change was detected in plasma cytokine levels in HFD rats. However, increased macrophage infiltration of the DRG was observed in response to the HFD, absent any pain model. The results suggest that HFD can increase pain even when it does not cause obesity.

Blaming the Brain for Obesity: Integration of Hedonic and Homeostatic Mechanisms

The brain plays a key role in the controls of energy intake and expenditure, and many genes associated with obesity are expressed in the central nervous system. Technological and conceptual advances in both basic and clinical neurosciences have expanded the traditional view of homeostatic regulation of body weight by mainly the hypothalamus to include hedonic controls of appetite by cortical and subcortical brain areas processing external sensory information, reward, cognition, and executive functions. Hedonic controls interact with homeostatic controls to regulate body weight in a flexible and adaptive manner that takes environmental conditions into account. This new conceptual framework has several important implications for the treatment of obesity. Because much of this interactive neural processing is outside awareness, cognitive restraint in a world of plenty is made difficult and prevention and treatment of obesity should be more rationally directed to the complex and often redundant mechanisms underlying this interaction.

Brain nuclear receptors and body weight regulation

Neural pathways, especially those in the hypothalamus, integrate multiple nutritional, hormonal, and neural signals, resulting in the coordinated control of body weight balance and glucose homeostasis. Nuclear receptors (NRs) sense changing levels of nutrients and hormones, and therefore play essential roles in the regulation of energy homeostasis. Understanding the role and the underlying mechanisms of NRs in the context of energy balance control may facilitate the identification of novel targets to treat obesity. Notably, NRs are abundantly expressed in the brain, and emerging evidence indicates that a number of these brain NRs regulate multiple aspects of energy balance, including feeding, energy expenditure and physical activity. In this Review we summarize some of the recent literature regarding effects of brain NRs on body weight regulation and discuss mechanisms underlying these effects.

Amylin and Leptin: Co-Regulators of Energy Homeostasis and Neuronal Development

While the regulation of energy homeostasis by amylin is already well-characterized, emerging data suggest that amylin is also crucial for the development of neural pathways in the hypothalamus and caudal hindbrain (area postrema, AP; nucleus tractus solitarius, NTS). Exciting new findings demonstrate crucial amylin-leptin interactions in altering the activity of specific hypothalamic and AP neurons, and a role for amylin as a novel class of 'leptin sensitizers' which enhance leptin signaling in both leptin-sensitive and -resistant individuals, in part by stimulating IL-6 production by hypothalamic microglia. This review summarizes these findings and provides a hypothetical framework for future studies to elucidate the mechanisms by which amylin and leptin act individually and as co-conspirators to alter energy homeostasis and neuronal development.

Emerging drugs for the treatment of obesity

INTRODUCTION

The increasing prevalence of obesity represents a huge threat to public health and the current pharmacological treatment options are limited. Bariatric surgery is by far the most effective treatment for severe obesity, highlighting the urgent need for new and improved drug therapies. Areas covered: Based on the physiological regulation of energy homeostasis, pharmacological strategies to treat obesity are evaluated with focus on drugs in phase 2 and 3 clinical development. The potential impact of these drugs on current treatment standards and the barriers for development are discussed and set in a historical perspective of previous antiobesity medications. Expert opinion: The radical effects of bariatric surgery have extended our understanding of the mechanisms controlling appetite and boosted the search for new drug targets in obesity treatment. Accordingly, several compounds targeting the central nervous system and/or periphery are in pipeline for obesity. These drugs should be evaluated over a wide array of end-points; in particular, long-term safety monitoring is necessary as serious adverse events may appear. Combination therapy targeting more than one pathway controlling energy balance might be necessary to achieve substantial weight loss while minimising side effects.

Physiology, pathophysiology and therapeutic implications of enteroendocrine control of food intake

With the increasing prevalence of obesity and its associated comorbidities, strides to improve treatment strategies have enhanced our understanding of the function of the gut in the regulation of food intake. The most successful intervention for obesity to date, bariatric surgery effectively manipulates enteroendocrine physiology to enhance satiety and reduce hunger. Areas covered: In the present article, we provide a detailed overview of the physiology of enteroendocrine control of food intake, and discuss its pathophysiologic correlates and therapeutic implications in both obesity and gastrointestinal disease. Expert commentary: Ongoing research in the field of nutrient sensing by L-cells, as well as understanding the role of the microbiome and bile acid signaling may facilitate the development of novel strategies to combat the rising population health threat associated with obesity. Further refinement of post-prandial satiety gut hormone based therapies, including the development of chimeric peptides exploiting the pleiotropic nature of the gut hormone response, and identification of novel methods of delivery may hold the key to optimization of therapeutic modulation of gut hormone physiology in obesity.