**1. Introduction**

Feeding is one of the most important motivated behaviors for maintenance of body energy balance. Although obesity has become a pandemic in the modern world, young individuals are able to maintain their body weight over a long period, suggesting that body energy balance, at least at a young age, is precisely regulated.

Regulation of feeding is generally divided into homeostatic and nonhomeostatic mechanisms [1]. The hypothalamus and brain stem play important roles in homeostatic regulation (**Figure 1**). Nonhomeostatic regulation relates to "hedonic" feeding that manifests as hyperphagia for palatable diets rather than to the control of body energy balance. The reward system including dopaminergic neurons in the ventral tegmental area is associated with hedonic feeding. Homeostatic and nonhomeostatic systems are coordinately regulated under physiological and pathological conditions.

The recent introduction of new technologies including optogenetic and pharmacogenetic methods has led to the identification of neural circuits for the regulation of homeostatic feeding in the hypothalamus and other brain areas. In addition to hormones such as leptin, ghrelin, cholecystokinin (CCK), and glucagon-like peptide-1 (GLP-1), less well-known hormones such as asprosin and growth and differentiation factor 15 (GDF15) have also recently been implicated in the central regulation of homeostatic and hedonic feeding [2, 3].

#### **Figure 1.**

*Homeostatic and hedonic regulation of feeding. Feeding is regulated by homeostatic and hedonic mechanisms in the brain. The hypothalamus contributes to homeostatic regulation, and the reward system in the brain contributes to hedonic regulation.*

In this chapter, I will review regulatory mechanisms for homeostatic feeding in the hypothalamus, with a focus on the role of neuropeptide Y (NPY) and agouti-related peptide (AgRP) containing neurons in the arcuate nucleus of the hypothalamus (ARC). I will also address the role of novel regulatory hormones including asprosin and GDF15 in feeding. With regard to the molecular mechanisms of energy sensing in the hypothalamus, I will describe the role of 5′-adenosine monophosphate (AMP) activated protein kinase (AMPK)—a metabolic sensor and regulator of intermediate metabolism, autophagy, and mitochondrial function—in feeding regulation [4–7].

In addition to the regulation of total calorie intake with regard to whole-body energy balance, macronutrient intake plays an important role in cardiometabolic health, aging, and longevity [8, 9] and is regulated by the brain. We recently showed that AMPK-regulated neurons in the paraventricular nucleus of the hypothalamus (PVH) that express corticotropin-releasing hormone (CRH) are necessary and sufficient for the fasting-induced selection of carbohydrate over a basally preferred diet such as a high-fat diet (HFD) in mice [10]. Such consumption of a high-carbohydrate diet (HCD) after fasting resulted in a rapid reversal of the fasting-induced increase in the plasma concentration of ketone bodies. Whereas intake of an HFD can also improve ketone body metabolism, this occurs at a slower rate. These observations indicate that, when offered a choice of diets, rodents select an HCD as a means to achieve a rapid normalization of ketone and glucose metabolism during refeeding after fasting. I will thus also describe in more detail in this chapter our recent study regarding the role of AMPK-regulated CRH neurons of the PVH in carbohydrate selection.

## **2. Neural circuits for homeostatic regulation of feeding**

#### **2.1 NPY/AgRP and POMC neurons in the ARC**

The ARC contains neurons that express both NPY and AgRP as well as neurons that express pro-opiomelanocortin (POMC), with both of these types of neuron

**3**

**Figure 2.**

*feeding, leading to an increase in food intake.*

*Neural Control of Homeostatic Feeding and Food Selection*

persisting for ~3 h compared with ~24 h for AgRP.

humans [13].

having been shown to contribute to the monitoring of body energy balance and to the regulation of feeding [11] (**Figure 2**). The ARC does not have an effective bloodbrain barrier and possesses a specific transport system for the uptake of hormones such as leptin into the brain [12]. In addition to the neuropeptides NPY and AgRP, NPY/AgRP neurons release the inhibitory neurotransmitter γ-aminobutyric acid (GABA), with all three of these agents contributing to the induction of feeding. AgRP is an endogenous antagonist of the melanocortin 4 receptor (MC4R) and melanocortin 3 receptor (MC3R). AgRP and α-melanocyte-stimulating hormone (α-MSH) regulates feeding in a reciprocal manner by acting at MC4R and MC3R [11]. Mammals express five different NPY receptors, of which Y1 and Y5 regulate feeding [11]. Injection of NPY or GABA into the brain of mice induces a marked increase in feeding, but these effects are less long-lasting than is that of AgRP,

α-MSH is produced by cleavage of the precursor protein POMC [11]. POMC neurons thus release α-MSH and thereby inhibit food intake through activation of MC4R and, to a lesser extent, that of MC3R [11]. Loss of POMC or MC4R gives rise to pronounced obesity in both humans and mice, with mutations of the MC4R gene being the most common monogenic cause of human obesity [13]. The POMC gene encodes various proteins and peptides including adrenocorticotropic hormone (ACTH) and endorphin as well as α -MSH, and mutations of the POMC gene or of genes for the POMC-processing enzymes convertase 1 and 2 give rise to adrenal insufficiency and red hair pigmentation as well as early-onset obesity in

In contrast to the obesity associated with ablation of POMC or MC4R genes, mice lacking the NPY, AgRP, or both genes as well as those in which NPY/AgRP neurons were ablated by forced expression of diphtheria toxin during the neonatal

*Reciprocal regulation of NPY/AgRP neurons and POMC neurons of the ARC in the control of food intake. Food deprivation and ghrelin activate NPY/AgRP neurons and inhibit POMC neurons in the ARC. In contrast, satiety and leptin activate POMC neurons and inhibit NPY/AgRP neurons in this nucleus. Activation of NPY/ AgRP neurons and inhibition of POMC neurons result in inhibition of neurons in the PVH that suppress* 

*DOI: http://dx.doi.org/10.5772/intechopen.93413*

*Neural Control of Homeostatic Feeding and Food Selection DOI: http://dx.doi.org/10.5772/intechopen.93413*

*New Insights into Metabolic Syndrome*

In this chapter, I will review regulatory mechanisms for homeostatic feeding in the hypothalamus, with a focus on the role of neuropeptide Y (NPY) and agouti-related peptide (AgRP) containing neurons in the arcuate nucleus of the hypothalamus (ARC). I will also address the role of novel regulatory hormones including asprosin and GDF15 in feeding. With regard to the molecular mechanisms of energy sensing in the hypothalamus, I will describe the role of 5′-adenosine monophosphate (AMP) activated protein kinase (AMPK)—a metabolic sensor and regulator of intermediate metabolism, autophagy, and mitochondrial function—in feeding regulation [4–7]. In addition to the regulation of total calorie intake with regard to whole-body energy balance, macronutrient intake plays an important role in cardiometabolic health, aging, and longevity [8, 9] and is regulated by the brain. We recently showed that AMPK-regulated neurons in the paraventricular nucleus of the hypothalamus (PVH) that express corticotropin-releasing hormone (CRH) are necessary and sufficient for the fasting-induced selection of carbohydrate over a basally preferred diet such as a high-fat diet (HFD) in mice [10]. Such consumption of a high-carbohydrate diet (HCD) after fasting resulted in a rapid reversal of the fasting-induced increase in the plasma concentration of ketone bodies. Whereas intake of an HFD can also improve ketone body metabolism, this occurs at a slower rate. These observations indicate that, when offered a choice of diets, rodents select an HCD as a means to achieve a rapid normalization of ketone and glucose metabolism during refeeding after fasting. I will thus also describe in more detail in this chapter our recent study regarding the role of AMPK-regulated CRH neurons of the PVH in carbohydrate selection.

*Homeostatic and hedonic regulation of feeding. Feeding is regulated by homeostatic and hedonic mechanisms in the brain. The hypothalamus contributes to homeostatic regulation, and the reward system in the brain* 

**2. Neural circuits for homeostatic regulation of feeding**

The ARC contains neurons that express both NPY and AgRP as well as neurons that express pro-opiomelanocortin (POMC), with both of these types of neuron

**2.1 NPY/AgRP and POMC neurons in the ARC**

**2**

**Figure 1.**

*contributes to hedonic regulation.*

having been shown to contribute to the monitoring of body energy balance and to the regulation of feeding [11] (**Figure 2**). The ARC does not have an effective bloodbrain barrier and possesses a specific transport system for the uptake of hormones such as leptin into the brain [12]. In addition to the neuropeptides NPY and AgRP, NPY/AgRP neurons release the inhibitory neurotransmitter γ-aminobutyric acid (GABA), with all three of these agents contributing to the induction of feeding. AgRP is an endogenous antagonist of the melanocortin 4 receptor (MC4R) and melanocortin 3 receptor (MC3R). AgRP and α-melanocyte-stimulating hormone (α-MSH) regulates feeding in a reciprocal manner by acting at MC4R and MC3R [11]. Mammals express five different NPY receptors, of which Y1 and Y5 regulate feeding [11]. Injection of NPY or GABA into the brain of mice induces a marked increase in feeding, but these effects are less long-lasting than is that of AgRP, persisting for ~3 h compared with ~24 h for AgRP.

α-MSH is produced by cleavage of the precursor protein POMC [11]. POMC neurons thus release α-MSH and thereby inhibit food intake through activation of MC4R and, to a lesser extent, that of MC3R [11]. Loss of POMC or MC4R gives rise to pronounced obesity in both humans and mice, with mutations of the MC4R gene being the most common monogenic cause of human obesity [13]. The POMC gene encodes various proteins and peptides including adrenocorticotropic hormone (ACTH) and endorphin as well as α -MSH, and mutations of the POMC gene or of genes for the POMC-processing enzymes convertase 1 and 2 give rise to adrenal insufficiency and red hair pigmentation as well as early-onset obesity in humans [13].

In contrast to the obesity associated with ablation of POMC or MC4R genes, mice lacking the NPY, AgRP, or both genes as well as those in which NPY/AgRP neurons were ablated by forced expression of diphtheria toxin during the neonatal

#### **Figure 2.**

*Reciprocal regulation of NPY/AgRP neurons and POMC neurons of the ARC in the control of food intake. Food deprivation and ghrelin activate NPY/AgRP neurons and inhibit POMC neurons in the ARC. In contrast, satiety and leptin activate POMC neurons and inhibit NPY/AgRP neurons in this nucleus. Activation of NPY/ AgRP neurons and inhibition of POMC neurons result in inhibition of neurons in the PVH that suppress feeding, leading to an increase in food intake.*

period were found to be able to maintain a normal body weight. However, ablation of NPY/AgRP neurons by diphtheria toxin during adulthood was shown to give rise to severe anorexia [14]. NPY/AgRP neurons are thus now recognized as key neurons in the regulation of food intake. The maintenance of a normal body weight after ablation of NPY/AgRP neurons during the neonatal period appears to reflect a compensatory rearrangement of neural circuits in the brain. Indeed, activation of certain other neuronal types has recently been found to induce hyperphagia to an extent similar to that triggered by activation of NPY/AgRP neurons. For example, somatostatin-expressing neurons in the tuberal nucleus of the hypothalamus increase food intake by releasing GABA and thereby inhibiting neurons in the PVH or the bed nucleus of the stria terminalis (BNST) [15]. In addition, GABAergic neurons in the zona incerta that project to the paraventricular nucleus of the thalamus also increase food intake [16]. However, in contrast to that of NPY/AgRP neurons, ablation of either of these two types of neuron during adulthood does not result in anorexia.

Activation of NPY/AgRP neurons in the ARC increases operant behaviors such as the pressing of a lever to get food. It also increased food intake even when mice were fed a noncaloric flavored diet [17, 18]. Furthermore, when mice were equipped with an optical fiber that allowed them to activate these NPY/AgRP neurons, they did so [19]. These observations suggest that activation of NPY/AgRP neurons increases the motivation for feeding. However, the optogenetic activation of NPY/ AgRP neurons appears to induce different behaviors depending on whether mice are anticipating the presentation of food or not. Food reward is necessary to increase the self-stimulation of NPY/AgRP neurons. A conditioned place preference test also revealed that mice avoid the place where NPY/AgRP neurons are stimulated in the absence of food reward [20].

Recent studies have shown that the activity of NPY/AgRP neurons in the ARC rapidly decreases after the onset of feeding [17–20]. Infusion of a liquid meal into the stomach was also found to reduce the activity of these neurons, as was the injection into the brain of feeding-suppressive hormones or neurotransmitters such as serotonin, CCK, and peptide YY (PYY). Anticipatory stimuli for food such as its smell induce a transient decrease in the activity of NPY/AgRP neurons. These changes in neuronal activity appear to be correlated with the reward value of food, and they suggest that NPY/AgRP neurons in the ARC are regulated by higher brain systems such as the reward system.

### **2.2 Hormonal regulation of NPY/AgRP neurons in the ARC**

## *2.2.1 Leptin, ghrelin, insulin, GLP-1, and CCK*

The activities of NPY/AgRP neurons and POMC neurons are regulated not only by nutrients such as glucose but also by hormones. Leptin and ghrelin control the activity of both of these types of neuron by eliciting intracellular signaling (**Figure 3**) [11, 21, 22]. Insulin also contributes to suppression of feeding by acting at the insulin receptor expressed in these neurons [23]. Nutrient signals in the gut and liver are also indirectly transmitted to NPY/AgRP and POMC neurons via afferent nerves in the vagus nerve trunk [21, 22]. Indeed, the gastrointestinal hormones ghrelin, CCK, GLP-1, and PYY have been shown to regulate NPY/AgRP neurons and POMC neurons directly by acting at receptors expressed in these neurons as well as indirectly through the afferent nerve fibers in the vagus nerve (**Figure 3**).

Leptin is an adipocyte hormone that reciprocally regulates the activities of NPY/ AgRP neurons and POMC neurons [11]. The plasma leptin concentration is correlated with the amount of adipose tissue in the body. Ablation of the leptin receptor

**5**

suppression of food intake.

**Figure 3.**

*Neural Control of Homeostatic Feeding and Food Selection*

in NPY/AgRP neurons or POMC neurons of mice during the fetal or neonatal period was found to have little effect on body weight and food intake, suggesting that these neurons might not play an important role in the regulation of whole-body energy balance by leptin. However, it was subsequently found that ablation of the leptin receptor in NPY/AgRP neurons of adult mice gives rise to obesity and diabetes similar to those of ob/ob (leptin-deficient) and db/db (leptin receptor-deficient) mice [24]. NPY/AgRP neurons are thus indeed an important target for leptin-induced

*Hormonal regulation of NPY/AgRP neurons in the ARC and of food intake. The adipocyte-derived hormones leptin and asprosin directly regulate the activity of NPY/AgRP neurons in the ARC. Gastrointestinal hormones also regulate the activity of these neurons by direct effects as well as indirectly through afferent nerves in the vagus trunk and the NTS pathway. GDF15 inhibits feeding through the AP-NTS-PBN neural pathway.*

Lipodystrophy is a congenital or acquired disease characterized by a reduction in the amount of adipose tissue in the body [25]. Some individuals with large reductions in the amount of adipose tissue have an increased appetite and develop type 2 diabetes associated with severe insulin resistance. However, leptin treatment was found to normalize food intake and the metabolic abnormalities of such individuals [25], and leptin is now the most effective medicine for patients with lipodystrophy. Given that injection of only a small amount of leptin into the brain is sufficient to inhibit food intake and to ameliorate metabolic abnormalities in lipodystrophy model mice, such effects of leptin are likely mediated by leptin receptors in the brain. The ventromedial nucleus of the hypothalamus (VMH) as well as ARC appears to be targets in the antidiabetic action of leptin (see Section 2.3.2).

Functional magnetic resonance imaging has been applied to examine the brain of lipodystrophy patients before and after feeding and with and without leptin treatment [26]. Control subjects showed a strong response of the reward system including the striatum when presented with photographs of palatable food after food deprivation, but the response rapidly declined after they were allowed to eat. In contrast, lipodystrophy patients continued to show increased activity in the striatum after feeding, whereas administration of leptin greatly improved the brain response to food. Unfortunately, such imaging, even with the most advanced machines, is

*DOI: http://dx.doi.org/10.5772/intechopen.93413*

#### **Figure 3.**

*New Insights into Metabolic Syndrome*

anorexia.

absence of food reward [20].

systems such as the reward system.

*2.2.1 Leptin, ghrelin, insulin, GLP-1, and CCK*

the afferent nerve fibers in the vagus nerve (**Figure 3**).

**2.2 Hormonal regulation of NPY/AgRP neurons in the ARC**

period were found to be able to maintain a normal body weight. However, ablation of NPY/AgRP neurons by diphtheria toxin during adulthood was shown to give rise to severe anorexia [14]. NPY/AgRP neurons are thus now recognized as key neurons in the regulation of food intake. The maintenance of a normal body weight after ablation of NPY/AgRP neurons during the neonatal period appears to reflect a compensatory rearrangement of neural circuits in the brain. Indeed, activation of certain other neuronal types has recently been found to induce hyperphagia to an extent similar to that triggered by activation of NPY/AgRP neurons. For example, somatostatin-expressing neurons in the tuberal nucleus of the hypothalamus increase food intake by releasing GABA and thereby inhibiting neurons in the PVH or the bed nucleus of the stria terminalis (BNST) [15]. In addition, GABAergic neurons in the zona incerta that project to the paraventricular nucleus of the thalamus also increase food intake [16]. However, in contrast to that of NPY/AgRP neurons, ablation of either of these two types of neuron during adulthood does not result in

Activation of NPY/AgRP neurons in the ARC increases operant behaviors such as the pressing of a lever to get food. It also increased food intake even when mice were fed a noncaloric flavored diet [17, 18]. Furthermore, when mice were equipped with an optical fiber that allowed them to activate these NPY/AgRP neurons, they did so [19]. These observations suggest that activation of NPY/AgRP neurons increases the motivation for feeding. However, the optogenetic activation of NPY/ AgRP neurons appears to induce different behaviors depending on whether mice are anticipating the presentation of food or not. Food reward is necessary to increase the self-stimulation of NPY/AgRP neurons. A conditioned place preference test also revealed that mice avoid the place where NPY/AgRP neurons are stimulated in the

Recent studies have shown that the activity of NPY/AgRP neurons in the ARC rapidly decreases after the onset of feeding [17–20]. Infusion of a liquid meal into the stomach was also found to reduce the activity of these neurons, as was the injection into the brain of feeding-suppressive hormones or neurotransmitters such as serotonin, CCK, and peptide YY (PYY). Anticipatory stimuli for food such as its smell induce a transient decrease in the activity of NPY/AgRP neurons. These changes in neuronal activity appear to be correlated with the reward value of food, and they suggest that NPY/AgRP neurons in the ARC are regulated by higher brain

The activities of NPY/AgRP neurons and POMC neurons are regulated not only by nutrients such as glucose but also by hormones. Leptin and ghrelin control the activity of both of these types of neuron by eliciting intracellular signaling (**Figure 3**) [11, 21, 22]. Insulin also contributes to suppression of feeding by acting at the insulin receptor expressed in these neurons [23]. Nutrient signals in the gut and liver are also indirectly transmitted to NPY/AgRP and POMC neurons via afferent nerves in the vagus nerve trunk [21, 22]. Indeed, the gastrointestinal hormones ghrelin, CCK, GLP-1, and PYY have been shown to regulate NPY/AgRP neurons and POMC neurons directly by acting at receptors expressed in these neurons as well as indirectly through

Leptin is an adipocyte hormone that reciprocally regulates the activities of NPY/ AgRP neurons and POMC neurons [11]. The plasma leptin concentration is correlated with the amount of adipose tissue in the body. Ablation of the leptin receptor

**4**

*Hormonal regulation of NPY/AgRP neurons in the ARC and of food intake. The adipocyte-derived hormones leptin and asprosin directly regulate the activity of NPY/AgRP neurons in the ARC. Gastrointestinal hormones also regulate the activity of these neurons by direct effects as well as indirectly through afferent nerves in the vagus trunk and the NTS pathway. GDF15 inhibits feeding through the AP-NTS-PBN neural pathway.*

in NPY/AgRP neurons or POMC neurons of mice during the fetal or neonatal period was found to have little effect on body weight and food intake, suggesting that these neurons might not play an important role in the regulation of whole-body energy balance by leptin. However, it was subsequently found that ablation of the leptin receptor in NPY/AgRP neurons of adult mice gives rise to obesity and diabetes similar to those of ob/ob (leptin-deficient) and db/db (leptin receptor-deficient) mice [24]. NPY/AgRP neurons are thus indeed an important target for leptin-induced suppression of food intake.

Lipodystrophy is a congenital or acquired disease characterized by a reduction in the amount of adipose tissue in the body [25]. Some individuals with large reductions in the amount of adipose tissue have an increased appetite and develop type 2 diabetes associated with severe insulin resistance. However, leptin treatment was found to normalize food intake and the metabolic abnormalities of such individuals [25], and leptin is now the most effective medicine for patients with lipodystrophy. Given that injection of only a small amount of leptin into the brain is sufficient to inhibit food intake and to ameliorate metabolic abnormalities in lipodystrophy model mice, such effects of leptin are likely mediated by leptin receptors in the brain. The ventromedial nucleus of the hypothalamus (VMH) as well as ARC appears to be targets in the antidiabetic action of leptin (see Section 2.3.2).

Functional magnetic resonance imaging has been applied to examine the brain of lipodystrophy patients before and after feeding and with and without leptin treatment [26]. Control subjects showed a strong response of the reward system including the striatum when presented with photographs of palatable food after food deprivation, but the response rapidly declined after they were allowed to eat. In contrast, lipodystrophy patients continued to show increased activity in the striatum after feeding, whereas administration of leptin greatly improved the brain response to food. Unfortunately, such imaging, even with the most advanced machines, is

not able to reliably detect changes in neuronal activity in the hypothalamus. Indeed, NPY/AgRP neurons and POMC neurons are reciprocally regulated in the ARC, which itself constitutes only a small proportion of the hypothalamus, making it difficult to study such changes in neuronal activity. However, given that individuals with a mutation of the POMC gene manifest hyperphagia and obesity similar to those of mutant mice, NPY/AgRP neurons and POMC neurons play an important role in feeding behavior and metabolic control in humans as well as rodents.

Ghrelin is a peptide released from the stomach and has a unique structure in that it is octanoylated at its third amino acid residue (serine) [22], with this acylation being essential for the orexigenic and metabolic effects of ghrelin. Ghrelin induces feeding by actions in the brain, including the activation of NPY/AgRP neurons and suppression of POMC neurons in the ARC. Until the discovery of asprosin, ghrelin was the only orexigenic peripheral hormone known. In humans, the plasma level of ghrelin increases immediately before breakfast, lunch, and dinner and declines after feeding. Fasting and anorexia nervosa are associated with an increased circulating concentration of ghrelin. Although ghrelin markedly stimulates feeding, its physiological function remains unknown because feeding and energy metabolism are largely unaffected in ghrelin knockout mice. It may contribute to the alert system for promotion of feeding at scheduled times such as breakfast, lunch, and dinner.
