Neuronal systems and circuits involved in the control of food intake and adaptive thermogenesis

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1 Ann. N.Y. Acad. Sci. ISSN ANNALS OF THE NEW YORK ACADEMY OF SCIENCES Issue: The Year in Diabetes and Obesity REVIEW ARTICLE Neuronal systems and circuits involved in the control of food intake and adaptive thermogenesis Alexandre Caron and Denis Richard Institut Universitaire de Cardiologie et de Pneumologie de Quebec and Faculty of Medicine, Department of Medicine, Université Laval, Quebec City, Quebec, Canada Address for correspondence: Denis Richard, Ph.D., Institut Universitaire de Cardiologie et de Pneumologie de Québec and Department of Medicine, Université Laval, Pavillon Marguerite-d Youville, 2725 chemin Sainte-Foy, Quebec City, Quebec, Canada G1V 4G5. With the still-growing prevalence of obesity worldwide, major efforts are made to understand the various behavioral, environmental, and genetic factors that promote excess fat gain. Obesity results from an imbalance between energy intake and energy expenditure, which emphasizes the importance of deciphering the mechanisms behind energy balance regulation to understand its physiopathology. The control of energy balance is assured by brain systems/circuits capable of generating adequate ingestive and thermogenic responses to maintain the stability of energy reserves, which implies a proper integration of the homeostatic signals that inform about the status of the energy stores. In this article, we overview the organization and functionality of key neuronal circuits or pathways involved in the control of food intake and energy expenditure. We review the role of the corticolimbic (executive and reward) and autonomic systems that integrate their activities to regulate energy balance. We also describe the mechanisms and pathways whereby homeostatic sensing is achieved in response to variations of homeostatic hormones, such as leptin, insulin, and ghrelin, while putting some emphasis on the prominent importance of the mechanistic target of the rapamycin signaling pathway in coordinating the homeostatic sensing process. Keywords: hypothalamus; melanocortin; executive; reward; energy balance Introduction The understanding of energy balance regulation is a key element in our comprehension of the physiopathology of obesity, the prevalence of which has reached preoccupying proportions worldwide over the past four decades ( who.int/mediacentre/factsheets/fs311/en/). The excess fat deposition that characterizes obesity inescapably results from the imbalance between energy intake and energy expenditure, in line with the concept enunciated by the French scientist Antoine Lavoisier that nothing is lost, nothing is created, everything is transformed. 1 Energy balance or homeostasis is therefore regulated through controls exerted on both energy intake and energy expenditure. It is strongly dependent on genetic and environmental factors affecting both susceptibility and propensity to develop or resist energy imbalance and, hence, obesity. 2 4 These factors modulate the highly coordinated communications that exist between cognitive/affective corticolimbic entities, such as the brain executive and reward systems, and the hypothalamic and brainstem autonomic (as opposed to conscious cognitive/ affective) circuits. Altogether, those systems/circuits operate the regulation of energy balance by controlling food intake and energy expenditure while integrating several signals about the status of energy reserves (homeostatic signals). 5 The systems/ circuits involved in food intake control are not only sensitive to endogenous homeostatic signals about the status of energy reserves, but also to the organoleptic properties of food (taste and texture) and to emotional factors (stress, anxiety, and mood). 6 Energy expenditure, which includes a regulatory component, namely brown fat (brown adipose tissue, BAT) nonshivering adaptive thermogenesis, is under autonomic control and doi: /nyas

2 Neural control of energy balance Caron & Richard PFC ACC Ins. Hipp. Stri. NAc Amy. POA PVH DMH LH VMH VTA PBN DVC ARC RPa Autonomic Sensing of nutritional or energy reserve status (homeostatic signals) Relaying homeostatic signals to the forebrain Controlling adaptive thermogenesis Executive Decision to eat (and to commit to exercise) Reward Establishing the hedonic ("liking") and incentive or motivational salience ("wanting") properties of eating-related stimuli Figure 1. Brain structures involved in energy balance regulation. Structures of the hypothalamic and brainstem autonomic circuitry (orange) include the arcuate nucleus (ARC), the dorsomedial hypothalamus (DMH), the dorsal-vagal complex (DVC), the lateral hypothalamus (LH), the pontine parabrachial nucleus (PBN), the preoptic area (POA), the paraventricular hypothalamus (PVH), the raphe pallidus (RPa), and the ventromedial hypothalamus (VMH). Structures of the brain executive system (green) include the anterior cingulate cortex (ACC), the insula (Ins.), and the prefrontal cortex (PFC). Structures of the brain reward system include the amygdala (Amy.), hippocampus (Hipp.), Ins., LH, nucleus accumbens (NAc), striatum (Stri.), and ventral tegmental area (VTA). homeostatic influence. 7,8 Homeostatic information (changes in energy balance and nutritional status) is conveyed through circulating fluctuations of nutrients and hormones, such as the catabolic (promoting energy/fat loss) peptides leptin and insulin or the anabolic (promoting energy/fat gain) hormone ghrelin The overarching goal of this article is to review the current knowledge and emerging discoveries in the field of energy balance regulation. We overview the role of the brain executive and reward systems as well as that of the autonomic hypothalamic and brainstem circuits in the regulation of energy balance. We further highlight and summarize the current understanding of the brain action of the hormones leptin, insulin, and ghrelin, and emphasize the prominent importance of the mechanistic target of rapamycin (mtor) signaling pathway in energy homeostasis. The brain regulation of energy balance Several brain structures are involved in the regulation of energy balance through the control of food intake and energy expenditure (Fig. 1). They form extensive neuronal circuits capable of sensing and processing internal signals, such as those pertaining to energy reserve status. Those circuits couple a forebrain corticolimbic appetitive network to autonomic hypothalamic and brainstem neuronal circuits involved in energy balance. The understanding of the complex crosstalk between the rostral forebrain appetitive network and autonomic structures 36

3 Caron & Richard Neural control of energy balance in the control of food intake and energy expenditure is currently one of the most challenging issues in the field of energy metabolism. The corticolimbic appetitive network The corticolimbic appetitive network includes forebrain cortical and subcortical limbic regions 13 that ultimately exert a decisional control on food intake. In relation to the feeding behavior, it essentially includes structures that constitute the executive and reward systems of the brain. The brain executive system and the decision to eat. The brain executive system integrates the activity of the prefrontal cortex (PFC), which covers the anterior part of the frontal lobe, as well as that of the anterior cingulate cortex (ACC), which surrounds the frontal part of the corpus callosum (Fig.1,ingreen). 14,15 The cortical executive circuits are close to and influenced by the adjacent olfactory, gustatory, and somatosensory cortices, which compose the insula. These cortices collect the sensory information associated with the organoleptic properties of food from the oral cavity and digestive tract. 16 The brain executive system plays a decisive role in the ingestive behavior; the ultimate decision of eating is indeed conscious and voluntary. 6,17 One could therefore choose to skip or postpone a meal, depending on several factors, even though food intake control is modulated to meet physiological needs dependent of the energy reserve status. Thebrainexecutivesystemiscriticaltorestrict lifestyle habits liable to cause obesity. 6,17 Hence, the integrity of the PFC and ACC is necessary to allow proper disciplined food intake 18 (and potentially commitment to physical activity). Executive dysfunction causes impulsive behaviors, and impulsivity (defined as a tendency to adopt a certain behavior without thinking about the consequences) is an important predictor of obesity. 19,20 The executive system is also modulated by emotions and personality traits, which can also provoke impulsive overconsumption of palatable energydense food items High-energy foods are readily available in an obesogenic environment and could per se weaken the ability of an individual to cognitively/voluntarily control their eating behavior. 24,25 The brain reward system and the desire/pleasure to eat. The neurons of the brain executive system interact with adjacent cortical or subcortical limbic structures in order to establish the motivational (incentive salience) and pleasurable (hedonic) values of energy balance associated stimuli and behaviors (Fig. 1, in blue). One important subcortical structure is the nucleus accumbens (NAc), which is found in the ventral striatum and constitutes a main component of the brain reward system. The NAc integrates the incentive salience ( wanting ) and hedonic ( liking ) aspects of the rewarding process associated with eating It is noteworthy that the brain circuits associated with the wanting and liking of food are the same as those implicated in other behaviors, such as drug and alcohol craving. 29 Electrical stimulation studies 13 have, in fact, linked eating- and drug-associated reward responses. 26 Indeed, these responses are controlled by common neuronal circuits that include the mesocorticolimbic dopaminergic pathway. 30 The latter links the NAc and brainstem ventral tegmental area (VTA) via dopaminergic neurons The importance of this circuit in eating is such that its disruption impairs reward responses. 34 Notably, dopamine-deficient mice are aphagic and die from starvation before weaning. 35 Dopamine release, ensuing from mesocorticolimbic dopaminergic pathway activation, is primarily associated with the wanting aspect of the reward response 36,37 and reinforces positive aspects of feeding. 38 Additionally, dopamine release in the NAc appears to be critical in learning about rewarding stimuli. 39,40 In fact, NAc s activity is modulated by hippocampal, amygdalar, and cortical areas, which play critical roles in the learning processes associated with the eating behavior. 16 Food ingestion through promoting reward responses imprints visual, olfactory, and gustatory effects/memories, which in turn modulate subsequent reward responses. 15,41 Finally, the mesocorticolimbic dopaminergic pathway also initiates and coordinates the reward-related motor activity linked to foraging for food. 42,43 The NAc is a critical interface between the brain executive system and autonomic hypothalamic circuits (Fig. 2). 44 The NAc integrates information from the PFC and gustatory circuits as well as signals emerging from viscera that reach the hindbrain dorsal-vagal complex (DVC). It additionally conveys reward-related information to the lateral 37

4 Neural control of energy balance Caron & Richard Energy intake Palatable food Executive (decision) PFC/ACC Motivation Muscles Brown fat Energy stores Homeostatic signals Leptin ( catabolic ) Ghrelin ( anabolic ) Insulin ( catabolic ) Other gut hormones Nutrients Reward ( liking and wanting ) Dopamine circuit (D2R, VTA, NAc, SN) Opioid system (enkephalin, beta-endorphin) Endocannabinoid system (CB1R, 2AG) Energy expenditure Palatable food Autonomic (homeostasis) MBH/DVC Pleasure Compulsion Figure 2. The regulation of energy balance. Energy homeostasis relies on controls exerted on both energy intake and energy expenditure. The control of energy expenditure can be exerted on the muscle activity as well as on BAT thermogenesis. Those controls are insured by different brain structures that compose the cortical executive circuits, reward circuits, and autonomic circuits. The activity of the brain networks is influenced by peripheral homeostatic signals that inform the central nervous system about energy stores. Palatable food also influences cognitive aspects including motivation and pleasure. 2AG, 2-arachidonoylglycerol; ACC, anterior cingulate cortex; CB1R, cannabinoid receptor 1; D2R, dopamine receptor 2; DVC, dorsal-vagal complex; MBH, mediobasal hypothalamus; NAc, nucleus accumbens; PFC, prefrontal cortex; SN, substancia nigra; VTA, ventral tegmental area. hypothalamus (LH), 45 which in turn receives learning-related inputs from the hippocampus and amygdala and also is a key modulator of the mesocorticolimbic dopaminergic pathway. 46 The opioids and endocannabinoids and the rewarding effects of food. Although the wanting aspect of reward has emerged as a dopamine-mediated process, the liking component appears partly determined by the opioid and endocannabinoid systems. 47,48 The brain opioid system includes widely scattered neurons that produce opioids, namely -endorphins, met- and leu-enkephalins, and dynorphins 49 and neurons that express the,, and receptors. 49 The role of opioids in energy homeostasis has been acknowledged since the early 1980s. 50 Of note, opioids influence the palatability of food; 51 mice lacking either enkephalin or -endorphin show a deficit in food reward 52 and a reduced hedonic response to food ingestion. 29 For its part, the brain endocannabinoid system includes neurons producing endocannabinoids and expressing the cannabinoid receptor type 1 (CB1R). CB1R is one of the two identified cannabinoid receptors and is expressed in energy balance structures 53 that produce or inactivaten-arachidonoyl ethanolamine (anandamide) and 2-arachidonoylglycerol, the most abundantly produced endocannabinoids. 54,55 CB1Rs are found in brain areas involved in reward, including those owing to the mesocorticolimbic dopaminergic pathway. CB1R activation stimulates the dopaminergic neurotransmission and produces rewarding effects associated with drugs and food. 56 Notably, the role of endocannabinoids in energy homeostasis goes beyond their involvement in reward circuits. Indeed, CB1R agonists have been shown to activate the hypothalamic proopiomelanocortin (POMC) neurons 57 (see next) and to interact with the melanocortin system in the control of thermogenesis. 58 It thus seems clear that endocannabinoids also affect energy balance through autonomic circuits. 38

5 Caron & Richard Neural control of energy balance The hypothalamic and brainstem autonomic controls The control of energy intake and expenditure (mainly through adaptive thermogenesis) implicates hypothalamic and brainstem circuits capable of modulating the activity of the corticolimbic appetitive network (Fig. 1, in orange). 17 The autonomic circuits involved in energy balance regulation mainly consist of groups of specialized neurons strategically located to control food intake and adaptive thermogenesis and to concomitantly sense homeostatic changes from blood levels of hormones and nutrients. Nutrients, while being utilized by neurons as energy substrates, can also be sensed as homeostatic signals to modulate neuronal activity (Fig. 2). 59 The role of nutrients in energy balance regulation has been reviewed elsewhere. 60,61 Nearly all hypothalamic nuclei participate in energy balance regulation, 62,63 with the arcuate (ARC) and ventromedial hypothalamic (VMH) nuclei being prominently involved in the control of energy intake and adaptive thermogenesis The ARC and VMH essentially constitute the mediobasal hypothalamus (MBH). Neurons from the ARC send projections to various nuclei, including the paraventricular hypothalamic nucleus (PVH), which is responsible for major neuroendocrine functions as well as autonomic controls on energy intake and expenditure (Fig. 3A). 63,67 Neurons from the ARC also terminate in the VMH, LH, dorsomedial hypothalamic nucleus (DMH), and preoptic area (POA), where they also influence food intake and adaptive thermogenesis (Fig. 3A). 63 The route whereby the ARC rostrally relays information on energy homeostasis to the corticolimbic appetite network has yet to be fully established, but there is evidence that it may implicate the hypothalamic thalamic striatal and hypothalamic brainstem striatal axes. 68,69 In addition to the hypothalamus, the brainstem is an important region involved in autonomic control of energy intake and expenditure (Fig. 3C). 10 The caudal brainstem includes three neighboring nuclei: the nucleus of the solitary tract, the area postrema (AP), and the dorsal motor nucleus of the vagus nerve. These nuclei constitute the DVC and are key structures integrating homeostatic signals about the nutritional and energy reserve status (Fig. 3C). 70,71 In addition, the brainstem includes nuclei, such as the pontine parabrachial nucleus (PBN), raphe pallidus (RPa), periaqueductal gray (PAG), pontine reticular nuclei, and lateral paragigantocellular (LPGi) nucleus, that have all been associated with either food intake or adaptive thermogenesis (Fig. 1). 72 TheRPa,PAG,andLPGiare,in fact, parts of circuits linking the hypothalamus to the intermediolateral nucleus (IML) of the spinal cord, from which originate the preganglionic neurons of the sympathetic nervous system (SNS) outflow to BAT. 72 The PBN is also an important relay that links the DVC to the corticolimbic appetite network. 73 The melanocortin system and energy homeostasis. The major role played by the ARC in the regulation of energy balance is attributable to the brain melanocortin system, which is one of the most critical components in the control of energy intake and expenditure (Fig. 3A). 74 It primarily consists of neurons producing melanocortins or agouti-related peptide (AgRP), together with neurons expressing the melanocortin receptors 3 (MC3R) and 4 (MC4R). 75 The activity of the melanocortin system is in part governed by two populations of ARC neurons markedly involved in the regulation of energy balance. 12 One population cosynthesizes AgRP, neuropeptide Y (NPY), and -aminobutyric acid (GABA), while the other synthesizes POMC and cocaine- and amphetamine-regulated transcript (CART). 11,76,77 POMC-containing neurons exert their catabolic effects apparently via the release of -melanocyte stimulating hormone ( -MSH), a peptidergic fragment ensuing from the cleavage of POMC. Within the brain, -MSH can bind to both MC3R and MC4R and primarily has a catabolic function. 74,78 81 MC4R is key to energy homeostasis; Mc4r knockout (KO) mice exhibit a massive and widespread increase in body fat. 81,82 Furthermore, human MC4R deficiency results in the most common form of monogenic obesity. 83 MC4R is abundantly expressed in the PVH, which also expresses Y1 and Y5 receptors, 84,85 the main receptors mediating the anabolic action of NPY on energy balance (Fig. 3A). NPY is a potent orexigenic peptide 86 whose expression in the ARC is markedly stimulated in obese 87 and food-deprived animals. 87,88 The functional importance of NPY and AgRP in the regulation of energy balance has been demonstrated in mice genetically engineered to lack AgRP/NPY/GABA-expressing neurons. In fact, ablation of AgRP/NPY/GABA neurons 39

6 Neural control of energy balance Caron & Richard A Legend α-msh PVH PVH, VMH, DMH, LH, POA, POMC/CART neurons NPY/AgRP/GABA neurons vgat neurons NOS1 neurons ACBD7 neurons MC4R neurons Y1/Y5 neurons Second-order neurons Catabolic hormones ARC ARC Catabolic hormones B C dmvmh Catabolic hormones Forebrain SF-1 AP vlvmh ERα Hindbrain (RPa, IO, NST) NTS Vagus nerve DMV Liver GI tract Figure 3. Autonomiccircuitsinenergybalanceregulation.(A) Thearcuatenucleusincludessubsetsofso-calledfirst-orderneurons, including proopiomelanocortin/cocaine- and amphetamine-regulated transcript (POMC/CART), neuropeptide Y/agouti-related peptide/ -aminobutyric acid (NPY/AgRP/GABA), vesicular GABA transporter (vgat), neuronal nitric oxide synthase 1 (NOS1), and acyl-coa binding domain-containing 7 (ACBD7). These neurons sense fluctuations in levels of metabolic hormones and nutrients and send anabolic/catabolic signals to second-order effector neurons located downstream in the paraventricular nucleus (PVH), ventromedial hypothalamus (VMH), dorsomedial hypothalamus (DMH), lateral hypothalamus (LH), and preoptic nucleus (POA). Neurons expressing the melanocortin 4 receptor (MC4R) and the Y1/Y5 receptors are examples of well-characterized second-order neurons, whose activity is modulated by -melanocyte stimulating hormone ( -MSH) and NPY, respectively. (B) The dorsomedial part of the VMH (dmvmh) expresses steroidogenic factor 1 (SF1) neurons that are involved in the regulation of thermogenesis through an unclear circuitry that may involve the raphe pallidus (RPa), the inferior olive (IO), or the nucleus of the solitary tract (NTS). The ventrolateral VMH (vlvmh) expresses estrogen receptor (ER ), also involved in energy balance regulation. (C) The dorsal-vagal complex (DVC), which includes the area postrema (AP), the dorsal motor nucleus of the vagus nerve (DMV), and the NTS, integrates and sends information from/to visceral structures and the forebrain. in adults causes starvation and death. 89 AgRP, as a peptide, counteracts the MC4R-mediated catabolic action, thereby producing anabolic effects. 64 It has also been referred to as a biased agonist producing MC4R effects without interfering with the binding of -MSH. 90 AgRP/NPY/GABA-expressing neurons also produce GABA-mediated inhibition of ARC POMC and PBN glutamatergic excitatory neurons. 91 Finally, supporting the potential of targeting the activation of the brain melanocortin system to improve metabolism, the U.S. Food and Drug Administration recently approved the use of setmelanotide (RM-493), a novel MC4R agonist, for the treatment of Prader Willi syndrome, 92 acommon genetic cause of morbid obesity in children. 93 This drug has also been shown to be efficient in obese patients with defects in POMC. 94 Other neuronal populations in the ARC have been associated with the brain melanocortin system (Fig. 3A). Some cells, which apparently express neither POMC nor AgRP, have been shown to express the vesicular GABA transporter 95 or neuronal nitric oxide synthase 1 96 and to modulate the activity of the melanocortin system. We have 40

7 Caron & Richard Neural control of energy balance also recently demonstrated that some ARC neurons express acyl-coa binding domain containing 7 (ACBD7), a paralog gene of the diazepam-binding inhibitor/acyl-coa binding protein. 97 ACBD7- producing neurons are apparently involved in the control of both energy intake and expenditure through effects on the melanocortin system. 97 Further investigations are required to fully address the identity and functions of ACBD7-expressing neurons. These recent findings, together with the demonstration that segregated POMC/CART and NPY/AgRP/GABA neurons have distinct functions and express different receptors, 98 demonstrate the complexity of the ARC and the melanocortin system in the control of energy metabolism. VMH steroidogenic-factor 1 expressing neurons and energy expenditure. The VMH has long been implicated in a wide array of physiological and behavioral processes. 99,100 Seminal studies have, in fact, confirmed that VMH lesions (potentially spreading outside the VMH) affect both body weight and food intake. 101 Neurons from the VMH also project to the brainstem nuclei involved in the control of thermogenesis, including the RPa (Fig. 3B). 102 Recent investigation has suggested that VMH neurons could also connect with the ARC and modulate its neuronal activity. 103 The recent discovery of steroidogenic factor 1 (SF1) as a brain transcription factor selectively expressed in the VMH 104 has allowed for a deep understanding of the mechanisms linking the VMH to the regulation of energy balance. 105,106 SF1, which is also expressed in the testis and adrenal glands, has been demonstrated to be essential in establishing the cytoarchitecture of the VMH. 104 In addition, it was shown that mice lacking SF1 in the VMH develop obesity in the absence of increased food intake. 107 More recently, it was reported that targeted disruption of leptin signaling in SF1 neurons affects body weight and susceptibility to diet-induced obesity, primarily by affecting thermogenesis. 105,108 Notably, postnatal deletion of SF1 also impairs BAT thermogenesis. 109 In fact, the action of SF1 neurons in energy balance regulation seems to be rather specific to the control of energy expenditure, specifically on adaptive BAT thermogenesis through governing the SNS outflow to BAT. 7 There is indeed no evidence for a specific involvement of the VMH SF1 neurons in the control of food intake. 101 Sympathetic nervous system mediated adaptive BAT thermogenesis While addressing the autonomic regulation of energy balance, one cannot ignore the role of adaptive BAT thermogenesis in energy expenditure. BAT is a specialized organ with an incredible potential for thermogenesis 110 and hence for enhancing energy expenditure It is found in relative abundance in small eutherian mammals, allowing them to live in cold environments without relying on the uncomfortable shivering process to produce heat. Brown adipocytes are characterized by the presence in their cytoplasm of multilocular lipid droplets and numerous mitochondria intermingled with each other. BAT thermogenic potential owes to the presence of uncoupling protein 1, which disengages mitochondrial respiration from ATP production in a process that generates heat and dissipates energy. 114 BAT development is largely dependent on SNS activity 115 and is modulated by several mediators. 112,116,117 There currently is a therapeutic interest in targeting BAT thermogenesis to treat excess fat deposition and related metabolic complications. 118 Such interest was stirred by early and recent positron emission tomography (PET) and computed tomography imaging studies that revealed not only the presence 119 but also the thermogenic activity of BAT in adult humans The use of the PET tracers 18 F-fluorodeoxyglucose ( 18 FDG, glucose uptake), 18 F-fluoro-thia-heptadecanoic acid ( 18 FTHA, nonesterified fatty acid uptake), and 11 C-acetate (oxidative activity) have been instrumental in demonstrating brown fat activity in humans, 123,124 as well as in laboratory rodents. 125 Understanding and targeting BAT thermogenesis are worthy challenges, as increasing BAT activity could improve the metabolic profile of obese individuals. 118, Brain control of BAT thermogenesis. BAT thermogenesis is under well-organized autonomic brain control, which is essentially ensured by the hypothalamus and brainstem. 7,72 The brain control of thermogenesis has been largely described in connection with thermoregulation. 72 Thermoregulatory thermogenesis has been investigated in cold-exposed laboratory rodents and been reported to be governed by a POA DMH RPa pathway. 72 This pathway has proved to be sensitive to cold and to drive the activity of IML SNS preganglionic neurons innervating BAT depots. 72 The brain 41

8 Neural control of energy balance Caron & Richard control of BAT activity has also been investigated in connection with energy homeostasis. Studies have demonstrated that BAT thermogenesis is controlled by neurons genuinely involved in energy balance, such as those attributable to the melanocortin system and expressing POMC/CART, AgRP/NPY/GABA, or MC4R. 7,17 Those neurons could act in close cooperation with those of the POA DMH RPa thermoregulatory pathway, as this axis is a potential site of the MC4R-mediated action of BAT thermogenesis. 130 In fact, injection of the nonselective MCR agonist melanotan 2 (MT2) into the POA activates interscapular BAT thermogenesis, an effect that is blocked by the caudal DMH lesion. 130 The melanocortin neurons could also influence BAT thermogenesis independent of the thermoregulatory pathway. In addition, the PVH, which includes MC4R neurons projecting to the IML, can activate BAT when stimulated with MT Another pathway that could control BAT thermogenesis independent of the thermoregulatory pathway involved the VMH SF1 neurons. 109 As mentioned previously, VMH neurons project to the brainstem PAG and rostroventrolateral medulla, which are two areas capable of governing interscapular BAT activity. 131 Homeostatic sensing and signaling Energy homeostasis depends on the brain reliability to integrate peripheral hormonal and nutritional signals and to generate adequate responses in order to maintain the stability of energy reserves. 9,61,132 The brain, in particular the hypothalamus, is well known to sense fluctuations in the levels of catabolic hormones, such as leptin and insulin, and the levels of anabolic hormones, such as ghrelin (Fig. 3). In addition, energy balance regulation depends on circulating levels of glucose, fatty acids, amino acids, and gastrointestinal hormones, which all reflect the nutritional status (for comprehensive reviews, see Refs ). One potential dysfunction in obesity could be the brain s inability to detect energy-storage fluctuations owing to inadequate sensing of metabolic signals by specific neuronal populations. 138 Over the last decades, it has become clear that obesity is exacerbated by the impaired central action of leptin, attributable to leptin resistance owing to excess fat accumulation. 139 Concomitantly, obesity leads to insulin resistance, which undoubtedly contributes to the metabolic complications, such as type 2 diabetes. 140 It is noteworthy that insulin resistance could also occur in the brain 141 and could exacerbate the obesity condition. 142 Finally, there is also evidence for ghrelin resistance in obesity. 143 The brain mechanisms whereby leptin, insulin, and ghrelin exert their effects in the regulation of energy balance are complex. These hormones affect both the corticolimbic and autonomic systems, 41 as described below in more detail. Leptin and the regulation of energy balance Leptin is a 16-kDa circulating protein with hormone/cytokine activities. It is released by the adipose tissue in proportion to the size of fat stores. Its discovery and that of its receptors has ensued from parabiotic (cross-circulation) experiments carried out in two obesity models, namely the ob/ob (Lep ob ) and db/db (Lepr db ) mice, which are deficient in leptin and its receptor, respectively. 144,145 Although leptin gene mutations in humans are rare, the reported cases exhibit severe obesity. 146 Administration of leptin, either peripherally or centrally, to Lep ob mice reduces body fat while reducing food intake and enhancing adaptive thermogenesis. 147,148 It also corrects obesity in infants presenting a congenital leptin deficiency. 149 Leptin acts centrally to exert its effects on energy balance. The neuronal action of leptin is mediated by the long isoform of the leptin receptor (LepRb), a splicing variant member of the class I cytokine receptor family. 150 This variant is characterized by a long cytoplasmic tail 151 allowing for the activation of the JAK STAT signal transduction pathway. 150 LepRb is predominantly located in hypothalamic nuclei known to be involved in the regulation of energy balance, including the ARC, where it is found at the surface of both NPY/AgRP/GABA and POMC/CART neurons It is also expressed in other structures including those of the corticolimbic appetitive network, where leptin could also act to influence food intake by modulating the food reward value. 155,156 Leptin appears as a genuine regulator of energy balance, 145,157 acting centrally to trigger catabolic effects in response to the enlargement of the fat Its circulating levels decrease during starvation (parallel to the reduction of adipose fat) to promote energy saving, whereas they increase after overfeeding (with the accretion of stores. 154,158 42

9 Caron & Richard Neural control of energy balance adipose fat) to supposedly prevent obesity. 159 Notably, fat-induced leptin production is rather ineffective to regulate energy balance, since leptin resistance develops consequent to the chronic tissue exposure to high levels of leptin. 160 Indeed, excess of leptin does not prevent obesity, as one would hypothetically expect. In the absence of leptin resistance, leptin may act on the ARC NPY/AgRP/GABA and POMC/CART neurons to reduce food intake and stimulate adaptive thermogenesis Leptin blunts the activity of NPY/AgRP/GABA neurons while it stimulates POMC cells. 164 The thermogenic effects of leptin are thought to be largely mediated through the melanocortin system, as the hormone appears ineffective in stimulating thermogenesis in Pomc KO and Mc4r KO mice. 165 Leptin also influences energy expenditure through effects exerted on VMH SF1 neurons. 106 As mentioned before, selective deletion of LepR in those neurons results in obesity. 105,106 Accumulating evidence indicates that leptin controls feeding through coordinated effects on both the autonomic and reward circuits. Indeed, leptin alters food intake by reducing the reward value or threshold for food, through modulating the activity of the mesocorticolimbic dopaminergic pathway. 166,167 These effects are likely mediated through direct inhibitory effects of leptin on VTA dopamine neurons. 167 It is noteworthy that the selective ablation of LepR in VTA neurons decreases overall food intake but increases consumption of palatable food and dopamine transporter activity in the NAc. 155,156 Furthermore, dopamine seems required for hyperphagia in Lep ob mice. 38 Leptin also affects the motivational aspects of feeding through effects on the endocannabinoid system. 168 By acting on VTA dopamine neurons, leptin could also affect physical activity. In that regard, Lep ob mice exhibit reduced locomotor responses to amphetamine, an effect that is readily reversed by leptin administration. 169 Interestingly, reexpression of STAT3 specifically in VTA dopamine neurons normalized the running behavior, suggesting that leptin through STAT3 signaling in VTA dopamine neurons modulates physical activity and enhances the motivation to seek for food. 170 It is unclear whether leptin-induced physical activity enhances energy expenditure, but this is a possibility. Insulin and the regulation of energy balance One acknowledged role of pancreatic insulin is to reduce circulating levels of glucose by facilitating its absorption mainly in skeletal muscle and adipose tissue and by reducing its production by the liver. 171 Another role of insulin is to promote the storage of nutrients mainly as glycogen and fat. 172 To achieve those actions, insulin must first bind to its membrane receptor (insulin receptor (IR)) to induce a cascade of well-characterized intracellular events. 173 In the brain, insulin acts as a catabolic hormone through reducing food intake and stimulating energy expenditure. 140,174 The importance of insulin in energy balance regulation has been highlighted in a mouse model of neuron-specific disruption of IR. 175 Mice with such a disruption present a diet-sensitive obesity with increased body fat and insulin resistance. 175 Pharmacological studies have also shown that the intracerebroventricular (icv) or intranasal administration of insulin reduces food intake through effects on the MBH neuropeptides However, selective inactivation of IR in either POMC- or AgRP-expressing neurons does not alter energy homeostasis, suggesting that the anorexigenic effect of insulin is independent of these neurons. 183,184 Likewise, deletion of IR in SF1 neurons of the VMH does not impair body weight gain, even though it protects mice from diet-induced fat gain. 185 The anorexigenic effect of insulin is also thought to be mediated through the mesocorticolimbic dopaminergic pathway. 167 In fact, dopaminergic neurons of the VTA express IR, and insulin dose dependently reduced somatodentritic dopamine in VTA. 186 Additionally, insulin injection into the VTA reduces feeding of sweetened high-fat food, suggesting that it may suppress the incentive salience for fatty food. 186 Moreover, it has also been observed that pathological hyperinsulinemia results in impaired insulin action in the VTA, which contributes to hyperphagia and obesity. 187 Finally, brain-specifickoofirhasalsobeendemonstrated to increase levels of lipid and protein oxidation in the striatum and NAc, suggesting that brain insulin resistance leads to dopaminergic dysfunction and behavioral disorders. 188 Ghrelin and the regulation of energy balance Ghrelin is an anabolic hormone mainly produced by the stomach. Fasting stimulates its release from 43

10 Neural control of energy balance Caron & Richard the upper stomach cells, which include the fundic P/D1 cells. 189 Ghrelin increases both food intake and adiposity in rats and mice, demonstrating that it has an anabolic role. 190 Ghrelin also blunts adaptive BAT thermogenesis. 191 The observation that ghrelin null mice resist diet-induced obesity emphasizes the obesogenic role of ghrelin 192 andhenceitsrolein energy homeostasis. Within the central nervous system, ghrelin binds to the growth hormone secretagogue receptor type 1 (GHSR-1 ), which is expressed in a few brain regions, including the ARC. 193 In that respect, the orexigenic actions of ghrelin appear to be primarily mediated through effects in the ARC. 189 Central administration of ghrelin increases the expression of Npy and Agrp while inhibiting the transcription of Pomc. 194,195 It was recently demonstrated that adult re-expression of GHSR-1 selectively in AgRP neurons partially restored the orexigenic response to ghrelin, indicating that these neurons are important in mediating ghrelin s effects. 196 Similar to LepR and IR, GHSR-1 is also found on dopaminergic neurons of the VTA, 197,198 where it could influence energy balance. Exogenous ghrelin appears to mimic fasting in that it increases the motivation to eat. 198,199 These effects contrast with those of leptin, which counteracts the effects of food restriction by blunting the wanting for palatable foods. 166,200 In addition, ghrelin injections into the VTA stimulate food reward value and increase dopamine concentration in the NAc Similarly, ghrelin has also been shown to enhance the rewarding value of high-fat feeding, an effect that is apparently dependent on orexin. 198,204 Altogether, these studies highlight the importance of ghrelin in both reward-based and hedonic eating behaviors. 205 Brain mtor signaling and homeostatic sensing Being expressed in the MBH, the brain mtor is a key molecule in sensing the circulating homeostatic hormones, leptin, insulin, and ghrelin. 206 mtor is a serine/threonine kinase that belongs to the PI3K kinase family. 173 This kinase is part of a well-conserved molecular pathway that regulates many cellular processes, including growth and metabolism. 173 The mtor kinase nucleates two large protein complexes, namely mtor complex 1 (mtorc1) and mtor complex 2 (mtorc2) (Fig. 4A), whose biological functions are numerous. 173, mtor is part of the insulin-pi3k signaling pathway, 173 which senses the fluctuations in the levels of metabolic hormones and initiates signaling cascades that ultimately affect energy homeostasis (Fig. 4B) Previous studies have shown that PI3K activity in the hypothalamus increases upon central administration of insulin and leptin In this regard, it is noteworthy that central inhibition of PI3K signaling blunts the metabolic effects of leptin 212,213,215 and attenuates insulin-induced glucose lowering effects in diabetic rats. 216 Many of the studies on hypothalamic PI3K activity have focused on its role in the ARC. 98,217,218 Notably, insulin activates the PI3K pathway in the ARC, and inhibition of PI3K signaling blunts insulin-induced suppression of food intake. 211 Similarly, brain PI3K inhibition attenuates the improvement in insulin sensitivity induced by re-expression of LepR in the ARC. 210 In addition, PI3K signaling in POMC neurons has been shown to be required for the acute leptin and insulin effects on food intake. 218 While leptin and insulin signaling converge toward PI3K, there is evidence that leptin and insulin activate distinct subpopulations of POMC neurons, 98 highlighting the complexity of these intracellular signaling pathways in the regulation of energy balance. PI3K signaling in the VMH has also been shown to participate in the control of energy expenditure. 108 While the role of hypothalamic PI3K has been acknowledged, little is known about the downstream effectors mediating its effects on energy balance. One important target of PI3K is the mtor kinase, whose involvement in energy balance regulation has recently been demonstrated 206, but whose mechanisms of action are still unclear. Several studies have highlighted the importance of hypothalamic mtorc1 in the regulation of energy balance. 220,222,223 Within neurons of the MBH, mtorc1 activity is induced by food intake and repressed by fasting, 220 and evidence keeps accumulating to suggest that MBH activation of mtorc1 is catabolic (Fig. 4B). 219, Cota et al. 220 initially observed that mtorc1 was present in both AgRP/NPY/GABA and POMC/CART neurons in the ARC, where its activity was reduced by fasting and induced by refeeding. There is also evidence that insulin, 227 leptin, 220 and ghrelin 228 modulate mtor signaling in the MBH. In fact, 44

11 Caron & Richard Neural control of energy balance A mtorc1 mtorc2 B Leptin Insulin Amino acids Fatty acids Glucose mtor Tti1/Tel2 mlst8 Deptor Raptor PRAS40 mtor Tti1/Tel2 mlst8 Deptor Rictor msin1 Protor Jak2 P Stat3 P Stat3 IRS1 PI3K PIP3 PIP2 PDPK1 PTEN FoxO1 mtorc2 Akt Feedback inhibition FoxO1 P mtorc1 Plasma Membrane Cytoplasm 4E-BP1 S6K1 PKC SGK1 P Stat3 P Stat3 Nucleus FoxO1 S6K1 Akt S6 Food intake Energy expenditure? Obesity Obesity C Amino acids Fatty acids Glucose Insulin D Amino acids Fatty acids Glucose Insulin Plasma Membrane Cytoplasm Plasma Membrane Cytoplasm mtorc1 Akt mtorc1 Akt FoxO1 Deptor Degradation S6K1 Feedback inhibition Deptor Overexpression S6K1 Feedback inhibition Food intake Energy expenditure Figure 4. Brain mtor signaling and homeostatic sensing. (A) The mtor complexes. (B) Hormonal and nutritional regulation of mtor signaling in the hypothalamus. (C) Obesity and nutrient overload constitutively activate mtorc, leading to insulin resistance. (D) Overexpression of DEPTOR rewires mtor signaling, preventing the development of obesity and metabolic complications. 4E-BP1, eukaryotic translation initiation factor 4E-binding protein 1; Akt, protein kinase B; DEPTOR, dishevelled, Egl-10, and pleckstrin (DEP) domain containing mtor-interacting protein; FoxO1, forkhead box protein O1; IRS1, insulin receptor substrate 1; Jak2, Janus kinase 2; mlst8, mammalian lethal with Sec 13 protein 8; msin1, mammalian stress-activated protein kinase interacting protein; mtorc1/2, mtor complex 1/2; PDPK1, 3-phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3-kinase; PIP2, phosphatidylinositol (4,5)-biphosphate; PIP3, phosphatidylinositol (3,4,5)-triphosphate; PKC,protein kinase C-alpha; PRAS40, proline-rich Akt substrate 40 kda; Protor1/2, protein observed with Rictor-1 and -2; PTEN, phosphatase and tensin homolog deleted on chromosome 10; Raptor, regulatory-associated protein of mtor; Rictor, rapamycin-insensitive companion of mtor; S6K1, ribosomal protein S6 kinase 1; S6, ribosomal protein S6; SGK1, serum- and glucocorticoid-induced protein kinase 1; Stat3, signal transducer and activator of transcription 3; Tel2, telomere length regulation protein TEL2 homolog; Tti1, TELO2-interacting protein 1 homolog. Red arrow: inhibition; dashed arrow: attenuation. icv administration of leptin promotes mtorc1 activity and reduces food intake in rodents. 220 Additionally, the anorexic effects of leptin are blocked by the mtor-specific inhibitor rapamycin. 220 Furthermore, studies have revealed that S6K1, a downstream effector of mtorc1, is involved in energy balance regulation (Fig. 4A & B). 222,229 In addition, activation of hypothalamic mtorc1 increases arterial pressure and SNS activity, 230 suggesting that it may also regulate SNS-dependent activation of BAT thermogenesis. It is noteworthy that not all the studies have described mtorc1 as a direct regulator of energy balance. 231,232 Most studies have, however, demonstrated a role of mtorc1 on components of 45

12 Neural control of energy balance Caron & Richard energy metabolism, such as a glucose metabolism. Notably, gene deletion studies on mtorc1 components have been done using constitutive prenatal manipulations, which may have induced developmental compensations. 233,234 The generation of adult models allowing for the selective cell manipulation of mtorc1 components appears crucial to comprehensively understand the role of hypothalamic mtorc1 in energy balance. While the role of hypothalamic mtorc1 in the regulation of energy balance regulation has been decently studied, that of mtorc2 remains largely unexplored. It has nonetheless recently been demonstrated that the reduction in hypothalamic Akt/PKB caused by the loss of Rictor, an essential component of mtorc2, impairs glucose homeostasis in mice and causes obesity. 221 mtorc2, through its effects on Akt/PKB, would be an important regulator of energy balance. The potential function of Akt/PKB signaling as an energy sensor that controls food intake has been suggested previously. 235 One of the downstream targets of Akt/ PKB is the transcriptional factor FoxO Once activated, Akt/PKB phosphorylates FoxO1, preventing its translocation to the nucleus. Hence, Akt/PKB reduces the transcriptional activity of FoxO1 (Fig. 4B). Together with STAT3, FoxO1 regulates the expression of Agrp and Pomc and is an important regulator of energy homeostasis It is noteworthy that mice lacking FoxO1 in SF1 neurons are lean owing to an increase in energy expenditure, 241 suggesting that FoxO1 could also affect the transcription of VMH metabolic genes. Whether mtorc2 affects energy balance through effects on FoxO1 is unknown but plausible, and further investigations are required to identify the neurons upon which mtorc2 acts to influence energy homeostasis, even though the involvement of POMC and SF1 neurons seems likely. 221 DEPTOR and the brain rewiring of mtor signaling DEP domain containing mtor-interacting protein (DEPTOR) was discovered as an endogenous regulator of mtorc1 and mtorc Arolefor DEPTOR in controlling peripheral physiological processes has recently begun to emerge In fact, studies have shown that DEPTOR promotes insulin sensitivity and Akt/PKB activation both in vitro and in vivo. 242,243, Mechanistically, DEPTOR apparently reduces the negative feedback inhibition of IR and improves insulin signaling by reducing mtorc1 activity (Fig. 4C). 242,243,246 There is also evidence that DEPTOR can act in the brain. 248,249 DEPTOR is present in the MBH, where its expression is affected by fasting in obese rodents. 248 The brain presence of DEPTOR in the ARC, VMH, and other brain regions, such as the circumventricular organs, suggests that this protein may play a role in the regulation of energy balance. 248 In that regard, we recently observed that the overexpression of DEPTOR specifically in the MBH prevented the development of obesity and related metabolic complications. 249 In agreement with previous studies, 242,243,245,246 we observed an increase in MBH Akt/PKB phosphorylation in response to DEPTOR overexpression, indicative of an increase in hypothalamic insulin signaling (Fig. 4C & D). Interestingly, this DEPTOR-induced increase in MBH Akt/PKB phosphorylation prevented diet-induced obesity and was accompanied by an improvement in systemic glucose metabolism, mimicking the effects associated with an elevated central insulin action. 249 Whether DEPTOR also is a target for leptin and ghrelin signaling or whether it also participates in influencing the executive and reward circuits, however, needs further investigation. In addition, the neuronal populations responsible for the beneficial effects of DEPTOR need to be identified, although it appears that POMC neurons are not involved. 250 The possibility that DEPTOR can act on the ARC AgRP/NPY/ GABA neurons or else on VMH SF1 neurons deserves attention. Summary In this review, we highlighted the importance of considering the neuronal circuits and determinants of energy balance regulation in an integrated way to better decipher the physiopathology of obesity. We overviewed the integrated roles of the corticolimbic appetitive network and hypothalamic/ brainstem autonomic circuits involved in the regulation of energy balance. We further reviewed the recent literature on the homeostatic sensing of the catabolic (leptin and insulin) and anabolic (ghrelin) hormones within the central nervous system and emphasized the role played by the mtor signaling pathway in energy homeostasis. Acquiring knowledge about energy balance regulation, 46

13 Caron & Richard Neural control of energy balance hence the physiopathology of excess fat deposition, appears imperative to envision effective prevention and treatment strategies to reduce obesity, whose prevalence worldwide could reach 20% in the adult population by While there have been major advances in obesity research over the past decades, there is still much research to achieve before efficiently combatting obesity and its complications. Acknowledgments D.R. is supported by the Canadian Institutes of Health Research (MOP and MOP ) and the Natural Sciences and Engineering Research Council of Canada ( ). A.C. is a Canadian Diabetes Association postdoctoral fellow currently at the Division of Hypothalamic Research at UT Southwestern Medical Center, Dallas. Conflicts of interest The authors declare no conflicts of interest. References 1. de Lavoisier, A.-L Traité élémentaire de chimie. Deterville. 2. Bell, C.G., A.J. Walley & P. Froguel. The genetics of human obesity. Nat. Rev. Genet. 6: Clement, K Genetics of human obesity. Proc. Nutr. Soc. 64: Willyard, C. Heritability: the family roots of obesity. Nature 508: S58-S Woods, S.C. & D.A. D Alessio Central control of body weight and appetite. J. Clin. Endocrinol. Metab. 93: S37 S Berthoud, H.R Interactions between the cognitive and metabolic brain in the control of food intake. Physiol. Behav. 91: Labbe, S.M. et al Hypothalamic control of brown adipose tissue thermogenesis. Front. Syst. Neurosci. 9: Morrison, S.F., C.J. Madden & D. Tupone Central neural regulation of brown adipose tissue thermogenesis and energy expenditure. Cell Metab. 19: Gautron, L., J.K. Elmquist & K.W. Williams Neural control of energy balance: translating circuits to therapies. Cell 161: Schneeberger, M., R. Gomis & M. Claret Hypothalamic and brainstem neuronal circuits controlling homeostatic energy balance. J. Endocrinol. 220: T25 T Dietrich, M.O. & T.L. Horvath Hypothalamic control of energy balance: insights into the role of synaptic plasticity.trends Neurosci. 36: Schwartz, M.W., S.C. Woods, D. Porte, Jr., et al Central nervous system control of food intake. Nature 404: Dagher, A Functional brain imaging of appetite. Trends Endocrinol. Metab. 23: Vainik, U., A. Dagher, L. Dubé & L.K. Fellows Neurobehavioural correlates of body mass index and eating behaviours in adults: a systematic review. Neurosci. Biobehav. Rev. 37: Swanson, L.W Cerebral hemisphere regulation of motivated behavior. Brain Res. 886, Berthoud, H.R Multiple neural systems controlling food intake and body weight. Neurosci. Biobehav. Rev. 26: Richard, D Cognitive and autonomic determinants of energy homeostasis in obesity. Nat. Rev. Endocrinol. 11: Volkow, N.D., G.J. Wang, D. Tomasi & R.D. Baler Obesity and addiction: neurobiological overlaps. Obes. Rev. 14: Jansen, A. et al High-restrained eaters only overeat when they are also impulsive. Behav. Res. Ther. 47: Sutin, A., L. Ferrucci, A. Zonderman & A. Terracciano Personality and obesity across the adult life span. J. Pers. Soc. Psychol. 101: Greeno, C.G. & R.R. Wing Stress-induced eating. Psychol. Bull. 115: Tice, D.M., E. Bratslavsky & R.F. Baumeister Emotional distress regulation takes precedence over impulse control:ifyoufeelbad,doit!j. Pers. Soc. Psychol. 80: Riggs, N.R., D. Spruijt-Metz, K.L. Sakuma, et al Executive cognitive function and food intake in children. J. Nutr. Educ. Behav. 42: Dietrich, A., M. Federbusch, C. Grellmann, et al Body weight status, eating behavior, sensitivity to reward/punishment, and gender: relationships and interdependencies. Front. Psychol. 5: Bongers, P. et al Being impulsive and obese increases susceptibility to speeded detection of high-calorie foods. Health Psychol. 34: Dagher, A The neurobiology of appetite: hunger as addiction.int. J. Obes. 33(Suppl. 2): S30 S Berridge, K.C Liking and wanting food rewards: brain substrates and roles in eating disorders. Physiol. Behav. 97: Meye, F.J. & R.A. Adan Feelings about food: the ventral tegmental area in food reward and emotional eating. Trends Pharmacol. Sci. 35: Saper, C.B., T.C. Chou & J.K. Elmquist The need to feed: homeostatic and hedonic control of eating. Neuron 36: Tang, D.W., L.K. Fellows, D.M. Small & A. Dagher Food and drug cues activate similar brain regions: a metaanalysis of functional MRI studies. Physiol. Behav. 106: Carlezon, W.A., Jr. & M.J. Thomas Biological substrates of reward and aversion: a nucleus accumbens activity hypothesis. Neuropharmacology 56(Suppl. 1): Yun, I.A., K.T. Wakabayashi, H.L. Fields & S.M. Nicola The ventral tegmental area is required for the behavioral and nucleus accumbens neuronal firing responses to incentive cues. J. Neurosci. 24: