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Energy Balance
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance 1201
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance R M Twyman, University of York, York, UK ã 2009 Elsevier Ltd. All rights reserved.
Introduction Energy homeostasis – the balance between energy intake and energy expenditure – is the key principle underlying weight fluctuation in humans. Until the mid-1990s, very little was understood about the regulation of energy homeostasis. However, since the discovery of leptin in 1994, a network of hormonal mechanisms has been unraveled that links the control of appetite and energy expenditure to feeding behavior and adiposity. A host of signals produced in the digestive system, adipose tissue, and brain interact to modify behavior and physiological responses. Malfunctions in this delicately balanced system can lead to metabolic disorders and obesity.
Major Hormones Involved in Appetite Regulation Leptin
Leptin was the first hormone shown to have a direct role in the regulation of food intake. It was quickly recognized as a circulating index of peripheral energy stores that could directly signal hypothalamic neurons in a position to effect appropriate adjustments in energy intake and expenditure. Long before its discovery in 1994, researchers had been using a homozygous mutant strain of mice called obese (ob/ ob), which was characterized by excessive eating (hyperphagia) and obesity. When the ob gene was cloned, it was found to encode a circulating hormone, which was named leptin, secreted by adipocytes. The absence of leptin in ob mice was proposed to cause their characteristic phenotype since injections of recombinant leptin restored them to normal. Furthermore, the injection of leptin into wild-type mice resulted in fasting (hypophagia) and weight loss, leading to the proposal that leptin signals to the brain when excess fat is stored and promotes a reduction in food intake. In support of this conclusion, plasma leptin levels are usually proportional to total fat content, and the amount of leptin entering the central nervous system (CNS) is proportional to circulating levels. The leptin receptor gene encodes a protein with six known isoforms, but only one of these (LepRb) has
been shown to have an active intracellular signaling domain. Leptin receptors are richly expressed in the hypothalamus, which is the primary brain area responsible for energy homeostasis. This discovery correlated well with the results of earlier surgical experiments showing that lesions and electrical stimulations in different parts of the hypothalamus had a strong impact on food intake. Lesions in the ventromedial hypothalamus (VMH) and arcuate nucleus, or electrical stimulation of the lateral hypothalamus (LH), produce animals that tend toward morbid obesity and hyperphagia, whereas LH lesions or stimulation of the VMH produces unusually lean animals that are reluctant to eat. Such experiments formed the basis of Stellar’s dual-center model, in which the LH acts as the feeding center and the VMH as the satiety center. Leptin receptors are expressed in all these regions, as well as in the dorsomedial hypothalamus. There are extensive reciprocal connections among these brain regions and with other parts of the brain, including the nucleus tractus solitarius (NTS) and various higher brain centers. Since the discovery of leptin and other hormones that regulate appetite and food intake, the dual-center model has been progressively refined to include multiple hypothalamic nuclei and hormonal signaling pathways, as well as multiple roles for many of the regulators. For example, more-recent work with leptin has shown that its role is more complex than initially understood since it is also an indicator of general nutritional health and has a specific role in sexual development and the onset of fertility. Research on leptin has focused on the arcuate nucleus, a structure within the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence. This placement is probably significant since it makes the arcuate nucleus especially sensitive to leptin and other energy homeostasis regulators and indicators. The arcuate nucleus has the highest density of leptin receptors in the brain, and injections of leptin directly into this structure result in profound hypophagia, an effect which can be completely ameliorated by surgical ablation. Two groups of neurons have attracted particular interest, those containing neuropeptide Y (NPY) and those containing proopiomelanocortin (POMC). Centrally projecting NPY neurons, found in the most ventromedial part of the nucleus, project to the LH and paraventricular nucleus (PVN). Activation of these neurons stimulates the appetite, and they are acutely sensitive to leptin and ghrelin (see the section titled ‘Ghrelin’). Centrally
1202 Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance
projecting POMC neurons have widespread projections to many brain areas, including all hypothalamic nuclei. Activation of these neurons suppresses the appetite, and they are also responsive to leptin and ghrelin. There are direct projections between the NPY and POMC neurons. Insulin
Insulin is a pancreatic hormone that regulates blood glucose levels by stimulating the conversion of glucose to glycogen. In addition to this role in carbohydrate metabolism, insulin suppresses the appetite, as shown by the effect of direct insulin injections into the brain. Insulin receptors are distributed in the hypothalamus and NTS and are particularly concentrated on NPY and POMC neurons in the arcuate nucleus, where their distribution is similar to that of the leptin receptor. This additional role for insulin may be related to its function in lipid homeostasis since, as well as promoting the storage of carbohydrates, insulin promotes the storage of fats. Overall levels of insulin correlate with the degree of adiposity in a manner similar to the correlation shown by leptin. Ghrelin
Ghrelin was first identified as a growth hormonereleasing peptide (hence its name) but was later found to also have a role in energy homeostasis. Ghrelin is a circulating hormone and acts on the arcuate nucleus and hypothalamus in a manner antagonistic to leptin. Consistent with this role, injection of ghrelin either directly into the brain or into the blood results in increased food intake, and coinjection of ghrelin and leptin attenuates the effects of leptin. Ghrelin is produced by the empty stomach, specifically by P/D1 cells, and stimulates food intake. Levels of ghrelin fall when the stomach is full after a meal. However, it appears that ghrelin release is inhibited, not by stretching of the stomach, but by the presence of energy-rich food. Strategies to combat obesity such as gastric bypass work not only by limiting the capacity of the stomach but also by reducing the amount of circulating ghrelin. A treatment for obesity recently developed by the Scripps Research Institute is based on the use of antibodies against ghrelin to prevent its uptake by the CNS. The ghrelin receptor is expressed in the arcuate nucleus and is found on both NYP and POMC neurons. It is also expressed strongly in the VMH. NPY
Injections of NPY into the brain result in ravenous feeding, weight gain, and obesity, indicating that NPY is a potent appetite stimulator. In addition,
NPY reduces sympathetic impulses to brown adipose tissue, thus reducing the metabolic rate and increasing energy efficiency. NPY is produced by neurons in the arcuate nucleus, which project strongly to the LH and PVN. The synthesis and release of NPY by these neurons is strictly regulated according to the energy balance, and hypothalamic neurons have NPY receptors, indicating a complex feedback system is in place to control this pivotal component of energy homeostasis. Surprisingly, knockout mice lacking the ability to produce NPY appear to feed normally both under typical conditions and after a period of fasting, and they respond normally to chemical treatments that induce obesity. In ob/ob mice, NPY deficiency results in significant weight loss (although not a complete restoration to wild-type feeding behavior), which indicates that NPY may have a role in balancing the activity of leptin and may contribute to the obesity phenotype when leptin is absent. This is supported by the exaggerated effects of leptin injections in npy/ npy mice. Five major subtypes of NPY receptors have been described. Studies with antagonists suggest that NPY receptors 1 and 5 are the main receptors involved in the stimulation of food intake, although the putative autoreceptors 2 and 4 are also highly expressed in the hypothalamus, including both arcuate NPY and POMC neurons. Paradoxically, mice lacking the NPY1, NPY2, or NPY5 receptors have mild obesity phenotypes and varying degrees of hyperphagia. The failure to see a lean phenotype after the removal of a potent stimulator of feeding such as NPY suggests that the pathways involved in the control of food intake, like other hypothalamic systems, are sufficiently redundant to compensate for the loss of a major signaling system. Alpha-Melanocyte-Stimulating Hormone
The POMC gene encodes a precursor polypeptide that gives rise to multiple peptides in different cell types, depending on the way it is cleaved. In the arcuate nucleus, the main products of pro-opiomelanocortin are two melanocyte-stimulating hormones (aMSH and gMSH) and the opioid b-endorphin. Only aMSH has a major role in energy homeostasis, and it acts as a potent inhibitor of food intake when injected into the brain. Furthermore, the effects of aMSH on sympathetic outflow to brown adipose tissue are opposite those of NPY; that is, it increases energy expenditure and results in net weight loss. Mice expressing an antagonist of aMSH are morbidly obese, and several genetic forms of human obesity have been linked to mutations in the POMC gene.
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance 1203
There are five melanocortin receptors, and it appears that the effects of aMSH are mediated mainly through MC4R. This has been deduced primarily by studies of mc4r knockout mice, which are severely hyperphagic and obese, but it is also apparent that approximately 5% of cases of childhood-onset obesity are associated with mutations in the human MC4R gene. It is likely that aMSH may be partly responsible for the loss of appetite seen in chronic illnesses such as cancer, since mc4r knockout mice with cancer appear to feed normally, as do mice expressing an antagonist of aMSH. The specific role of MC4R, as opposed to the other receptors, is shown in experiments in which mc4r knockout mice are treated with aMSH agonists, which would increase signaling through the other receptors but still fail to influence feeding behavior. A mild obesity phenotype is seen in mc3r knockouts, but this is accompanied by feeding aversion, suggesting the underlying effect concerns energy utilization rather than food intake. Melanocortin receptor (MC4R) (blocked by AGRP)
Major Control Circuits Involved in Appetite Regulation The NPY and POMC neurons in the arcuate nucleus represent the primary integration site for circulating hormones and central signals involved in energy homeostasis (Figure 1). They carry a similar repertoire of receptors for the regulatory molecules discussed above, and they also project into many of the same brain regions, including the LH, PVN, and NTS, but they have antagonistic effects through the activities of NPY and aMSH. In addition to their nominal roles, NPY neurons produce agouti-related peptide (AGRP), which is an endogenous antagonist of the melanocortin receptors, and the inhibitory neurotransmitter g-aminobutyric acid. POMC neurons also produce cocaine- and amphetamine-regulated transcript (CART). Although NPY and POMC neurons have receptors for the major regulatory hormones, the effect of hormone signaling is distinct in each case. Leptin acts to
Hypothalamus
Ghrelin receptor Neuron
NPY/peptide YY3−36 receptor Y2R Melanocortin receptor (MC3R)
Food intake
Energy expenditure
NPY receptor Y1R Melanocortin
Leptin receptor or insulin receptor
NPY/ AGRP POMC/ CART +
−
Ghrelin Stomach
+
Leptin
Insulin Fat tissue
Pancreas
Colon
Figure 1 Simplified representation of the core regulatory circuit in energy homeostasis. AGRP, agouti-related peptide; CART, cocaineand amphetamine-regulated transcript; NPY, neuropeptide Y; POMC, pro-opiomelanocortin.
1204 Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance
reduce food intake by stimulating POMC neurons (thus causing the release of aMSH) while inhibiting NPY neurons (and limiting the release of NPY). Conversely, ghrelin treatment stimulates the production of NPY and AGRP but has no impact on POMC messenger RNA (mRNA) levels. This is thought to reflect different intracellular signaling responses in each cell type. In the case of leptin signaling, binding to the leptin receptor activates the immediate early gene c-fos in POMC neurons, but not in NPY neurons. The activation of c-fos generally correlates with neuronal excitation. The ability of leptin to increase the activity of POMC neurons and decrease the activity of NPY neurons has been confirmed directly through electrophysiological recordings. There are also reciprocal connections between the NPY and POMC neurons, indicating they may directly modulate each other’s ability to release peptides. However, there is a further control mechanism mediated through the release of AGRP by NPY neurons. Like NPY, AGRP stimulates food intake and is thought to do so by blocking the effects of melanocortins. Thus, the release of both NPY and AGRP by NPY neurons has two simultaneous effects: a direct effect on downstream targets through the activity of NPY and indirect effects through the inhibition of aMSH and any other melanocyte-stimulating hormones released by POMC neurons. The PVN is a major candidate downstream target of the POMC and NPY neurons since both NPY and aMSH have been shown to influence food intake following direct injections into this structure, and thyrotropin-releasing hormone neurons located therein express the pivotal aMSH receptor, MC4R. It is possible that the inhibitory effects of aMSH on food intake are mediated by direct signaling to these neurons and the stimulatory effects of leptin reflect at least in part the blocking of melanocortin activity via AGRP. Regulation of thyrotropin-releasing hormone and other PVN neurons would regulate both the pituitary and the thyroid axes, thereby providing a mechanism by which leptin and the melanocortin system may modulate both food intake and energy homeostasis. Other potential downstream targets of the POMC and the NPY neurons are discussed in the next section.
Downstream Factors Involved in Appetite Regulation Factors That Promote Food Intake
A number of additional hormones and regulators have been shown to promote feeding when
administered to rodents, and research over the past few years has allowed many of these to be slotted into the primary control circuit involving leptin, ghrelin, NPY, and aMSH. Some of these factors are likely to be directly involved in energy homeostasis, while others have peripheral roles, such as general roles in arousal or in mediating the rewarding aspects of food intake. One of the principal downstream targets of NPY is the release of melanin-concentrating hormone (MCH), a potent stimulator of feeding when injected directly into the brain. MCH mRNA levels increase during fasting and decrease after meals (and after the administration of leptin). The hormone is found exclusively in so-called MCH neurons in the LH, which project to other hypothalamic nuclei as well as to the NTS and cortex. These neurons are likely to be targets of the NPY and POMC neurons in the arcuate nucleus, as discussed earlier, and possibly act as a relay system in the NPY circuit. Tampering with gene expression levels artificially has predictable effects: transgenic mice overexpressing the MCH gene are hyperphagic and obese, while mch/mch knockouts are hypophagic and lean. This contrasts with the phenotype of npy/npy mice and suggests that the functional redundancy exhibited in the NPY system is not present in its downstream target, MCH. Another possible target for NPY is the release of galanin, a 30-amino-acid neuropeptide, from discrete populations of neurons in the arcuate nucleus, dorsomedial hypothalamus, and PVN. Galanin neurons project to multiple areas of the hypothalamus and other brain regions involved in food intake, and they have synaptic links with both NPY and POMC neurons. Galanin levels increase during fasting, and the injection of the mature galanin peptide into the PVN, LH, VMH, and NTS stimulates food intake in rodents, although not to the levels seen with NPY injections. The precise role of galanin remains unclear since gal/gal knockout mice are neither obese nor unusually lean. Finally, it is likely that some of the effects of leptin are mediated by endogenous cannabinoids, at least two of which, anandamide and 2-arachidonoylglycerol, are normally present in the hypothalamus. Supportive evidence includes the increased levels of endogenous cannabinoids in the hypothalami of ob/ob mice and the reduction in levels that follows administration of leptin. Also, injections of cannabinoid agonists stimulate feeding, whereas mice lacking the CB1 cannabinoid receptor are hypophagic after fasting (although without fasting they eat normally). Research has focused on the CB1 receptor because prolonged administration
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance 1205
of a CB1 antagonist also results in hypophagia and leanness. Rimonabant (Acomplia) is a cannabinoid receptor antagonist which has been prescribed as an appetite suppressant in the countries of the European Union since 2006. Factors That Inhibit Food Intake
A number of additional factors have also been described which inhibit feeding and generate a negative energy balance, and again it is possible to slot many into the central regulatory circuit controlling energy homeostasis. CART is a neuropeptide coexpressed with aMSH in POMC neurons of the arcuate nucleus. Like aMSH, CART can inhibit food intake when injected directly into the brain. CART expression is also dependent on leptin, which marks it out as an important component of the energy homeostasis network. However, mice deficient in the expression of this peptide show no significant changes in feeding behavior or body weight, indicating that whatever role CART plays in the system, it can be replaced by a redundant component. CART is expressed in several additional sets of neurons that do not express POMC, so it is possible these neurons have secondary roles in appetite control. While CART is expressed in the CNS, cholecystokinin (CCK) is a satiety factor expressed primarily in the gut. CCK is secreted by the gut after a meal, and its release stimulates CCK receptors located on the vagal nerve, which transmits the satiety signal through the NTS. There are two classes of CCK receptor, and studies using specific antagonists show that the satiety signal is transmitted primarily through the class A (CCKA) receptor. Such antagonists inhibit feeding after a period of fasting. Injections of CCK result in sudden but short-lived food aversion, which is enhanced in the presence of leptin. Additional studies have shown that CCK and its receptors are also produced in the brain, indicating a central role through direct action on neurons in the NTS and hypothalamus. The precise role of CCK is clouded by different results from studies in different animals. For example, CCKA knockout mice show no obvious feeding phenotype, whereas rats with the orthologous gene knocked out show mild obesity. Another gut peptide with activity similar to CCK’s is bombesin, and mice lacking the receptor BBR3 are obese. Finally, enterostatin is a peptide, produced by the pancreas and gastrointestinal tract, which reduces insulin secretion, increases sympathetic outflow to brown adipose tissue, and stimulates adrenal corticosteroid secretion. The overall impact of enterostatin secretion is a sensation of fullness leading to a selective reduction in fat intake. It is not yet clear how this
peptide interacts with the other components of the energy homeostasis system. Several other peptides are known to inhibit food intake following direct injection and to be regulated by leptin but to have little effect on feeding in rodents with the corresponding genes knocked out. Corticotropin-releasing hormone (CRH), for example, is produced mainly by neurons in the PVN and has a number of diverse effects, including the release of adrenocorticotropic hormone (another product derived from POMC) from the pituitary gland. CRH levels are regulated by leptin, and injections of CRH into the brain reduce food intake, even after induction by NPY, and stimulate thermogenesis via sympathetic pathways, but mice lacking the corresponding receptors, CRHR1 and CRHR2, have normal weight and feeding behavior. Urocortin is a related peptide which has a more potent anorexigenic effect than CRH’s when injected, but because it signals through the same receptors as CRH and there is no impact on feeding when these receptors are missing, its precise role is unclear. Another potent anorectic is glucagon-like peptide 1 (GLP-1), a gastrointestinal peptide that is released in response to food intake. GLP-1 plays an important role in glucose homeostasis by inhibiting the secretion of glucagon and augmenting glucose-induced insulin secretion. In the context of this article, GLP-1 is also proposed to act as a satiety factor. Consistent with this hypothesis, peripheral administration of GLP-1 inhibits food intake. In addition to being produced in the intestine, GLP-1 is expressed at hypothalamic sites, the caudal portion of the NTS, and the forebrain. Central administration of GLP-1 inhibits food intake through actions in the hypothalamus, including the PVN. Conversely, GLP-1 antagonists stimulate feeding in satiated rats. GLP-1 antagonists also attenuate the effects of both leptin and CCK to inhibit food intake. Leptin has been shown to increase hypothalamic GLP-1 levels and to activate GLP-1 neurons in the NTS, providing further evidence of a role for GLP-1 in food intake and energy homeostasis. However, mice lacking the GLP-1 receptor do not show any feeding abnormalities.
Summary The regulation of appetite and food intake is a complex process. It involves a coordinated response to many orexigenic and anorexigenic factors in multiple brain regions. Significant progress has occurred during the past decade, including the discovery of the adipostatic hormone leptin and some of the pathways required for its actions. Leptin produces its effects in part by a coordinated action to simultaneously
1206 Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance
activate POMC and inhibit NPY neurons in the arcuate nucleus to alter food intake and energy homeostasis. Downstream targets of the NPY and POMC neurons include specific neurons in the PVN, LH, NTS, and other brain regions. However, much is still unknown, and it is likely that some critical appetite factors remain to be discovered. Furthermore, as is the case with other functions regulated by the hypothalamus, there appears to be some redundancy between these appetite factors. See also: Appetitive Systems: Amygdala and Striatum; Eating Disorders; Energy Homeostasis: Adiposity Signals; Energy Homeostasis: Hypothalamic Development; Energy Homeostasis: Paraventricular Nucleus (PVN) System; Energy Homeostasis: Thermoregulation; Food and Water Intake: Regulation; Gastrointestinal Signals: Stimulation; Gastrointestinal Signals: Satiety; Neuroendocrine Control of Energy Balance (Central Circuits/Mechanisms).
Further Reading Balthasar N (2006) Genetic dissection of neuronal pathways controlling energy homeostasis. Obesity 14: 222S–227S. Butler AA and Cone RD (2001) Knockout models resulting in the development of obesity. Trends in Genetics 17: S50–S54. Dham S and Banerji MA (2006) The brain-gut axis in regulation of appetite and obesity. Pediatric Endocrinology Reviews 3: 544–554. Friedman JM (2000) Obesity in the new millennium. Nature 404: 632–634. Halford JC (2001) Pharmacology of appetite suppression: Implication for the treatment of obesity. Current Drug Targets 2: 353–370. Inui A (2001) Ghrelin: An orexigenic and somatotrophic signal from the stomach. Nature Reviews Neuroscience 2: 551–560. Klok MD, Jakobsdottir S, and Drent ML (2007) The role of leptin and ghrelin in the regulation of food intake and body weight in humans: A review. Obesity Reviews 8: 21–34. Murphy KG and Bloom SR (2006) Gut hormones and the regulation of energy homeostasis. Nature 444: 854–859. Spiegelman BM and Flier JS (2001) Obesity and the regulation of energy balance. Cell 104: 531–543.
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance R M Twyman, University of York, York, UK ã 2009 Elsevier Ltd. All rights reserved.
Introduction Energy homeostasis – the balance between energy intake and energy expenditure – is the key principle underlying weight fluctuation in humans. Until the mid-1990s, very little was understood about the regulation of energy homeostasis. However, since the discovery of leptin in 1994, a network of hormonal mechanisms has been unraveled that links the control of appetite and energy expenditure to feeding behavior and adiposity. A host of signals produced in the digestive system, adipose tissue, and brain interact to modify behavior and physiological responses. Malfunctions in this delicately balanced system can lead to metabolic disorders and obesity.
Major Hormones Involved in Appetite Regulation Leptin
Leptin was the first hormone shown to have a direct role in the regulation of food intake. It was quickly recognized as a circulating index of peripheral energy stores that could directly signal hypothalamic neurons in a position to effect appropriate adjustments in energy intake and expenditure. Long before its discovery in 1994, researchers had been using a homozygous mutant strain of mice called obese (ob/ ob), which was characterized by excessive eating (hyperphagia) and obesity. When the ob gene was cloned, it was found to encode a circulating hormone, which was named leptin, secreted by adipocytes. The absence of leptin in ob mice was proposed to cause their characteristic phenotype since injections of recombinant leptin restored them to normal. Furthermore, the injection of leptin into wild-type mice resulted in fasting (hypophagia) and weight loss, leading to the proposal that leptin signals to the brain when excess fat is stored and promotes a reduction in food intake. In support of this conclusion, plasma leptin levels are usually proportional to total fat content, and the amount of leptin entering the central nervous system (CNS) is proportional to circulating levels. The leptin receptor gene encodes a protein with six known isoforms, but only one of these (LepRb) has
been shown to have an active intracellular signaling domain. Leptin receptors are richly expressed in the hypothalamus, which is the primary brain area responsible for energy homeostasis. This discovery correlated well with the results of earlier surgical experiments showing that lesions and electrical stimulations in different parts of the hypothalamus had a strong impact on food intake. Lesions in the ventromedial hypothalamus (VMH) and arcuate nucleus, or electrical stimulation of the lateral hypothalamus (LH), produce animals that tend toward morbid obesity and hyperphagia, whereas LH lesions or stimulation of the VMH produces unusually lean animals that are reluctant to eat. Such experiments formed the basis of Stellar’s dual-center model, in which the LH acts as the feeding center and the VMH as the satiety center. Leptin receptors are expressed in all these regions, as well as in the dorsomedial hypothalamus. There are extensive reciprocal connections among these brain regions and with other parts of the brain, including the nucleus tractus solitarius (NTS) and various higher brain centers. Since the discovery of leptin and other hormones that regulate appetite and food intake, the dual-center model has been progressively refined to include multiple hypothalamic nuclei and hormonal signaling pathways, as well as multiple roles for many of the regulators. For example, more-recent work with leptin has shown that its role is more complex than initially understood since it is also an indicator of general nutritional health and has a specific role in sexual development and the onset of fertility. Research on leptin has focused on the arcuate nucleus, a structure within the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence. This placement is probably significant since it makes the arcuate nucleus especially sensitive to leptin and other energy homeostasis regulators and indicators. The arcuate nucleus has the highest density of leptin receptors in the brain, and injections of leptin directly into this structure result in profound hypophagia, an effect which can be completely ameliorated by surgical ablation. Two groups of neurons have attracted particular interest, those containing neuropeptide Y (NPY) and those containing proopiomelanocortin (POMC). Centrally projecting NPY neurons, found in the most ventromedial part of the nucleus, project to the LH and paraventricular nucleus (PVN). Activation of these neurons stimulates the appetite, and they are acutely sensitive to leptin and ghrelin (see the section titled ‘Ghrelin’). Centrally
1202 Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance
projecting POMC neurons have widespread projections to many brain areas, including all hypothalamic nuclei. Activation of these neurons suppresses the appetite, and they are also responsive to leptin and ghrelin. There are direct projections between the NPY and POMC neurons. Insulin
Insulin is a pancreatic hormone that regulates blood glucose levels by stimulating the conversion of glucose to glycogen. In addition to this role in carbohydrate metabolism, insulin suppresses the appetite, as shown by the effect of direct insulin injections into the brain. Insulin receptors are distributed in the hypothalamus and NTS and are particularly concentrated on NPY and POMC neurons in the arcuate nucleus, where their distribution is similar to that of the leptin receptor. This additional role for insulin may be related to its function in lipid homeostasis since, as well as promoting the storage of carbohydrates, insulin promotes the storage of fats. Overall levels of insulin correlate with the degree of adiposity in a manner similar to the correlation shown by leptin. Ghrelin
Ghrelin was first identified as a growth hormonereleasing peptide (hence its name) but was later found to also have a role in energy homeostasis. Ghrelin is a circulating hormone and acts on the arcuate nucleus and hypothalamus in a manner antagonistic to leptin. Consistent with this role, injection of ghrelin either directly into the brain or into the blood results in increased food intake, and coinjection of ghrelin and leptin attenuates the effects of leptin. Ghrelin is produced by the empty stomach, specifically by P/D1 cells, and stimulates food intake. Levels of ghrelin fall when the stomach is full after a meal. However, it appears that ghrelin release is inhibited, not by stretching of the stomach, but by the presence of energy-rich food. Strategies to combat obesity such as gastric bypass work not only by limiting the capacity of the stomach but also by reducing the amount of circulating ghrelin. A treatment for obesity recently developed by the Scripps Research Institute is based on the use of antibodies against ghrelin to prevent its uptake by the CNS. The ghrelin receptor is expressed in the arcuate nucleus and is found on both NYP and POMC neurons. It is also expressed strongly in the VMH. NPY
Injections of NPY into the brain result in ravenous feeding, weight gain, and obesity, indicating that NPY is a potent appetite stimulator. In addition,
NPY reduces sympathetic impulses to brown adipose tissue, thus reducing the metabolic rate and increasing energy efficiency. NPY is produced by neurons in the arcuate nucleus, which project strongly to the LH and PVN. The synthesis and release of NPY by these neurons is strictly regulated according to the energy balance, and hypothalamic neurons have NPY receptors, indicating a complex feedback system is in place to control this pivotal component of energy homeostasis. Surprisingly, knockout mice lacking the ability to produce NPY appear to feed normally both under typical conditions and after a period of fasting, and they respond normally to chemical treatments that induce obesity. In ob/ob mice, NPY deficiency results in significant weight loss (although not a complete restoration to wild-type feeding behavior), which indicates that NPY may have a role in balancing the activity of leptin and may contribute to the obesity phenotype when leptin is absent. This is supported by the exaggerated effects of leptin injections in npy/ npy mice. Five major subtypes of NPY receptors have been described. Studies with antagonists suggest that NPY receptors 1 and 5 are the main receptors involved in the stimulation of food intake, although the putative autoreceptors 2 and 4 are also highly expressed in the hypothalamus, including both arcuate NPY and POMC neurons. Paradoxically, mice lacking the NPY1, NPY2, or NPY5 receptors have mild obesity phenotypes and varying degrees of hyperphagia. The failure to see a lean phenotype after the removal of a potent stimulator of feeding such as NPY suggests that the pathways involved in the control of food intake, like other hypothalamic systems, are sufficiently redundant to compensate for the loss of a major signaling system. Alpha-Melanocyte-Stimulating Hormone
The POMC gene encodes a precursor polypeptide that gives rise to multiple peptides in different cell types, depending on the way it is cleaved. In the arcuate nucleus, the main products of pro-opiomelanocortin are two melanocyte-stimulating hormones (aMSH and gMSH) and the opioid b-endorphin. Only aMSH has a major role in energy homeostasis, and it acts as a potent inhibitor of food intake when injected into the brain. Furthermore, the effects of aMSH on sympathetic outflow to brown adipose tissue are opposite those of NPY; that is, it increases energy expenditure and results in net weight loss. Mice expressing an antagonist of aMSH are morbidly obese, and several genetic forms of human obesity have been linked to mutations in the POMC gene.
Hormonal Signaling to the Brain for the Control of Feeding/Energy Balance 1203
There are five melanocortin receptors, and it appears that the effects of aMSH are mediated mainly through MC4R. This has been deduced primarily by studies of mc4r knockout mice, which are severely hyperphagic and obese, but it is also apparent that approximately 5% of cases of childhood-onset obesity are associated with mutations in the human MC4R gene. It is likely that aMSH may be partly responsible for the loss of appetite seen in chronic illnesses such as cancer, since mc4r knockout mice with cancer appear to feed normally, as do mice expressing an antagonist of aMSH. The specific role of MC4R, as opposed to the other receptors, is shown in experiments in which mc4r knockout mice are treated with aMSH agonists, which would increase signaling through the other receptors but still fail to influence feeding behavior. A mild obesity phenotype is seen in mc3r knockouts, but this is accompanied by feeding aversion, suggesting the underlying effect concerns energy utilization rather than food intake. Melanocortin receptor (MC4R) (blocked by AGRP)
Major Control Circuits Involved in Appetite Regulation The NPY and POMC neurons in the arcuate nucleus represent the primary integration site for circulating hormones and central signals involved in energy homeostasis (Figure 1). They carry a similar repertoire of receptors for the regulatory molecules discussed above, and they also project into many of the same brain regions, including the LH, PVN, and NTS, but they have antagonistic effects through the activities of NPY and aMSH. In addition to their nominal roles, NPY neurons produce agouti-related peptide (AGRP), which is an endogenous antagonist of the melanocortin receptors, and the inhibitory neurotransmitter g-aminobutyric acid. POMC neurons also produce cocaine- and amphetamine-regulated transcript (CART). Although NPY and POMC neurons have receptors for the major regulatory hormones, the effect of hormone signaling is distinct in each case. Leptin acts to
Hypothalamus
Ghrelin receptor Neuron
NPY/peptide YY3−36 receptor Y2R Melanocortin receptor (MC3R)
Food intake
Energy expenditure
NPY receptor Y1R Melanocortin
Leptin receptor or insulin receptor
NPY/ AGRP POMC/ CART +
−
Ghrelin Stomach
+
Leptin
Insulin Fat tissue
Pancreas
Colon
Figure 1 Simplified representation of the core regulatory circuit in energy homeostasis. AGRP, agouti-related peptide; CART, cocaineand amphetamine-regulated transcript; NPY, neuropeptide Y; POMC, pro-opiomelanocortin.
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reduce food intake by stimulating POMC neurons (thus causing the release of aMSH) while inhibiting NPY neurons (and limiting the release of NPY). Conversely, ghrelin treatment stimulates the production of NPY and AGRP but has no impact on POMC messenger RNA (mRNA) levels. This is thought to reflect different intracellular signaling responses in each cell type. In the case of leptin signaling, binding to the leptin receptor activates the immediate early gene c-fos in POMC neurons, but not in NPY neurons. The activation of c-fos generally correlates with neuronal excitation. The ability of leptin to increase the activity of POMC neurons and decrease the activity of NPY neurons has been confirmed directly through electrophysiological recordings. There are also reciprocal connections between the NPY and POMC neurons, indicating they may directly modulate each other’s ability to release peptides. However, there is a further control mechanism mediated through the release of AGRP by NPY neurons. Like NPY, AGRP stimulates food intake and is thought to do so by blocking the effects of melanocortins. Thus, the release of both NPY and AGRP by NPY neurons has two simultaneous effects: a direct effect on downstream targets through the activity of NPY and indirect effects through the inhibition of aMSH and any other melanocyte-stimulating hormones released by POMC neurons. The PVN is a major candidate downstream target of the POMC and NPY neurons since both NPY and aMSH have been shown to influence food intake following direct injections into this structure, and thyrotropin-releasing hormone neurons located therein express the pivotal aMSH receptor, MC4R. It is possible that the inhibitory effects of aMSH on food intake are mediated by direct signaling to these neurons and the stimulatory effects of leptin reflect at least in part the blocking of melanocortin activity via AGRP. Regulation of thyrotropin-releasing hormone and other PVN neurons would regulate both the pituitary and the thyroid axes, thereby providing a mechanism by which leptin and the melanocortin system may modulate both food intake and energy homeostasis. Other potential downstream targets of the POMC and the NPY neurons are discussed in the next section.
Downstream Factors Involved in Appetite Regulation Factors That Promote Food Intake
A number of additional hormones and regulators have been shown to promote feeding when
administered to rodents, and research over the past few years has allowed many of these to be slotted into the primary control circuit involving leptin, ghrelin, NPY, and aMSH. Some of these factors are likely to be directly involved in energy homeostasis, while others have peripheral roles, such as general roles in arousal or in mediating the rewarding aspects of food intake. One of the principal downstream targets of NPY is the release of melanin-concentrating hormone (MCH), a potent stimulator of feeding when injected directly into the brain. MCH mRNA levels increase during fasting and decrease after meals (and after the administration of leptin). The hormone is found exclusively in so-called MCH neurons in the LH, which project to other hypothalamic nuclei as well as to the NTS and cortex. These neurons are likely to be targets of the NPY and POMC neurons in the arcuate nucleus, as discussed earlier, and possibly act as a relay system in the NPY circuit. Tampering with gene expression levels artificially has predictable effects: transgenic mice overexpressing the MCH gene are hyperphagic and obese, while mch/mch knockouts are hypophagic and lean. This contrasts with the phenotype of npy/npy mice and suggests that the functional redundancy exhibited in the NPY system is not present in its downstream target, MCH. Another possible target for NPY is the release of galanin, a 30-amino-acid neuropeptide, from discrete populations of neurons in the arcuate nucleus, dorsomedial hypothalamus, and PVN. Galanin neurons project to multiple areas of the hypothalamus and other brain regions involved in food intake, and they have synaptic links with both NPY and POMC neurons. Galanin levels increase during fasting, and the injection of the mature galanin peptide into the PVN, LH, VMH, and NTS stimulates food intake in rodents, although not to the levels seen with NPY injections. The precise role of galanin remains unclear since gal/gal knockout mice are neither obese nor unusually lean. Finally, it is likely that some of the effects of leptin are mediated by endogenous cannabinoids, at least two of which, anandamide and 2-arachidonoylglycerol, are normally present in the hypothalamus. Supportive evidence includes the increased levels of endogenous cannabinoids in the hypothalami of ob/ob mice and the reduction in levels that follows administration of leptin. Also, injections of cannabinoid agonists stimulate feeding, whereas mice lacking the CB1 cannabinoid receptor are hypophagic after fasting (although without fasting they eat normally). Research has focused on the CB1 receptor because prolonged administration
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of a CB1 antagonist also results in hypophagia and leanness. Rimonabant (Acomplia) is a cannabinoid receptor antagonist which has been prescribed as an appetite suppressant in the countries of the European Union since 2006. Factors That Inhibit Food Intake
A number of additional factors have also been described which inhibit feeding and generate a negative energy balance, and again it is possible to slot many into the central regulatory circuit controlling energy homeostasis. CART is a neuropeptide coexpressed with aMSH in POMC neurons of the arcuate nucleus. Like aMSH, CART can inhibit food intake when injected directly into the brain. CART expression is also dependent on leptin, which marks it out as an important component of the energy homeostasis network. However, mice deficient in the expression of this peptide show no significant changes in feeding behavior or body weight, indicating that whatever role CART plays in the system, it can be replaced by a redundant component. CART is expressed in several additional sets of neurons that do not express POMC, so it is possible these neurons have secondary roles in appetite control. While CART is expressed in the CNS, cholecystokinin (CCK) is a satiety factor expressed primarily in the gut. CCK is secreted by the gut after a meal, and its release stimulates CCK receptors located on the vagal nerve, which transmits the satiety signal through the NTS. There are two classes of CCK receptor, and studies using specific antagonists show that the satiety signal is transmitted primarily through the class A (CCKA) receptor. Such antagonists inhibit feeding after a period of fasting. Injections of CCK result in sudden but short-lived food aversion, which is enhanced in the presence of leptin. Additional studies have shown that CCK and its receptors are also produced in the brain, indicating a central role through direct action on neurons in the NTS and hypothalamus. The precise role of CCK is clouded by different results from studies in different animals. For example, CCKA knockout mice show no obvious feeding phenotype, whereas rats with the orthologous gene knocked out show mild obesity. Another gut peptide with activity similar to CCK’s is bombesin, and mice lacking the receptor BBR3 are obese. Finally, enterostatin is a peptide, produced by the pancreas and gastrointestinal tract, which reduces insulin secretion, increases sympathetic outflow to brown adipose tissue, and stimulates adrenal corticosteroid secretion. The overall impact of enterostatin secretion is a sensation of fullness leading to a selective reduction in fat intake. It is not yet clear how this
peptide interacts with the other components of the energy homeostasis system. Several other peptides are known to inhibit food intake following direct injection and to be regulated by leptin but to have little effect on feeding in rodents with the corresponding genes knocked out. Corticotropin-releasing hormone (CRH), for example, is produced mainly by neurons in the PVN and has a number of diverse effects, including the release of adrenocorticotropic hormone (another product derived from POMC) from the pituitary gland. CRH levels are regulated by leptin, and injections of CRH into the brain reduce food intake, even after induction by NPY, and stimulate thermogenesis via sympathetic pathways, but mice lacking the corresponding receptors, CRHR1 and CRHR2, have normal weight and feeding behavior. Urocortin is a related peptide which has a more potent anorexigenic effect than CRH’s when injected, but because it signals through the same receptors as CRH and there is no impact on feeding when these receptors are missing, its precise role is unclear. Another potent anorectic is glucagon-like peptide 1 (GLP-1), a gastrointestinal peptide that is released in response to food intake. GLP-1 plays an important role in glucose homeostasis by inhibiting the secretion of glucagon and augmenting glucose-induced insulin secretion. In the context of this article, GLP-1 is also proposed to act as a satiety factor. Consistent with this hypothesis, peripheral administration of GLP-1 inhibits food intake. In addition to being produced in the intestine, GLP-1 is expressed at hypothalamic sites, the caudal portion of the NTS, and the forebrain. Central administration of GLP-1 inhibits food intake through actions in the hypothalamus, including the PVN. Conversely, GLP-1 antagonists stimulate feeding in satiated rats. GLP-1 antagonists also attenuate the effects of both leptin and CCK to inhibit food intake. Leptin has been shown to increase hypothalamic GLP-1 levels and to activate GLP-1 neurons in the NTS, providing further evidence of a role for GLP-1 in food intake and energy homeostasis. However, mice lacking the GLP-1 receptor do not show any feeding abnormalities.
Summary The regulation of appetite and food intake is a complex process. It involves a coordinated response to many orexigenic and anorexigenic factors in multiple brain regions. Significant progress has occurred during the past decade, including the discovery of the adipostatic hormone leptin and some of the pathways required for its actions. Leptin produces its effects in part by a coordinated action to simultaneously
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activate POMC and inhibit NPY neurons in the arcuate nucleus to alter food intake and energy homeostasis. Downstream targets of the NPY and POMC neurons include specific neurons in the PVN, LH, NTS, and other brain regions. However, much is still unknown, and it is likely that some critical appetite factors remain to be discovered. Furthermore, as is the case with other functions regulated by the hypothalamus, there appears to be some redundancy between these appetite factors. See also: Appetitive Systems: Amygdala and Striatum; Eating Disorders; Energy Homeostasis: Adiposity Signals; Energy Homeostasis: Hypothalamic Development; Energy Homeostasis: Paraventricular Nucleus (PVN) System; Energy Homeostasis: Thermoregulation; Food and Water Intake: Regulation; Gastrointestinal Signals: Stimulation; Gastrointestinal Signals: Satiety; Neuroendocrine Control of Energy Balance (Central Circuits/Mechanisms).
Further Reading Balthasar N (2006) Genetic dissection of neuronal pathways controlling energy homeostasis. Obesity 14: 222S–227S. Butler AA and Cone RD (2001) Knockout models resulting in the development of obesity. Trends in Genetics 17: S50–S54. Dham S and Banerji MA (2006) The brain-gut axis in regulation of appetite and obesity. Pediatric Endocrinology Reviews 3: 544–554. Friedman JM (2000) Obesity in the new millennium. Nature 404: 632–634. Halford JC (2001) Pharmacology of appetite suppression: Implication for the treatment of obesity. Current Drug Targets 2: 353–370. Inui A (2001) Ghrelin: An orexigenic and somatotrophic signal from the stomach. Nature Reviews Neuroscience 2: 551–560. Klok MD, Jakobsdottir S, and Drent ML (2007) The role of leptin and ghrelin in the regulation of food intake and body weight in humans: A review. Obesity Reviews 8: 21–34. Murphy KG and Bloom SR (2006) Gut hormones and the regulation of energy homeostasis. Nature 444: 854–859. Spiegelman BM and Flier JS (2001) Obesity and the regulation of energy balance. Cell 104: 531–543.