Abstract
Mutations in the human melanocortin (MC)4 receptor have been associated with obesity, which underscores the relevance of this receptor as a drug target to treat obesity. Infusion of MC4R agonists decreases food intake, whereas inhibition of MC receptor activity by infusion of an MC receptor antagonist or with the inverse agonist AgRP results in increased food intake. This review addresses the role of the MC system in different aspects of feeding behaviour. MC4R activity affects meal size and meal choice, but not meal frequency, and the type of diet affects the efficacy of MC4R agonists to reduce food intake. The central sites involved in the different aspects of feeding behaviour that are affected by MC4R signalling are being unravelled. The paraventricular nucleus plays an important role in food intake per se, whereas MC signalling in the lateral hypothalamus is associated with the response to a high fat diet. MC4R signalling in the brainstem has been shown to affect meal size. Further genetic, behavioural and brain-region specific studies need to clarify how the MC4R agonists affect feeding behaviour in order to determine which obese individuals would benefit most from treatment with these drugs. Application of MCR agonists in humans has already revealed side effects, such as penile erections, which may complicate introduction of these drugs in the treatment of obesity.
In order to maintain stable body weight, energy intake (by ingestion) and energy expenditure (by exercise, basal metabolism and thermogenesis) need to be balanced. The constant availability of highly palatable foods and the lack of exercise strongly contribute to the development of obesity (Woods et al., 2004). Drugs that effectively reduce body weight are urgently needed to fight the obesity epidemic. The unravelling of the genetic defect underlying obesity in the ob/ob mouse, namely the absence of the leptin gene, was key towards identification of neural pathways and neuropeptides that control body weight. Leptin is an adipose tissue-derived hormone that is released into the circulation proportional to increased energy stores in fat. Leptin stimulates neural circuits that decrease food intake and increase energy expenditure. Both humans and rodents with mutations in the leptin gene or leptin receptor gene are obese.
The arcuate nucleus of the hypothalamus is an important relay centre for leptin's effects. The hypothalamic arcuate nucleus integrates and distributes peripheral information from hormonal and neural signals that reflect metabolic status further into the brain. Within the arcuate nucleus, neurons containing melanocortins (MCs), products of the pro-opiomelanocortin (POMC) gene, are activated by leptin. These neurons provide one of the systems via which the leptin signal is propagated further into the brain. Fasting results in loss of adipose tissue and low leptin levels, which causes diminished activation of POMC neurons, whereas overfeeding (high leptin levels) results in a stimulation of POMC neurons. Thus, POMC neurons are stimulated during a positive energy balance, and increased plasma leptin levels contribute to this stimulation.
The activity of the MC system is not only regulated by the endogenous MC receptor agonists, ?-melanocyte-stimulating hormone (?-MSH), ?-MSH and ?-MSH, which are all derived from the POMC precursor, but also by agouti-related protein (AgRP). AgRP is also expressed in the arcuate nucleus but in a different subset of neurons than those expressing POMC. In contrast to POMC neurons, AgRP neurons are inhibited by leptin and activated during negative energy balance. Although often described as a competitive antagonist, AgRP acts in fact as an inverse agonist on constitutively active MC3 and MC4 receptors, the main brain MC receptors. The unique presence of an endogenous agonist and an inverse agonist acting at the same receptor system implicates a tight regulation of MC function in the brain. Thus, during a negative energy balance, AgRP neurons are activated and AgRP acts to suppress MC receptor activity.
Although the MC system modulates energy expenditure and insulin sensitivity, this review focuses on the role of the MC system in regulation of energy intake. After summarizing the evidence that the MC4 receptor is a promising drug target for the treatment of obesity, it will be discussed how and via which neural circuits the MC system affects energy intake.
MTII activates similar neuronal circuits as leptin and is able to reduce adiposity in leptin-deficient ob/ob mice, which further underscores that MCs act downstream of leptin. MTII also counteracts starvation-induced hyperphagia, suggesting that reduction of MC receptor activity is part of the physiological response to starvation.* Moreover, MTII is not able to increase metabolic rate in MC4 receptor?/?, suggesting that the MC4 receptor is indeed necessary for the regulation of metabolism (Chen et al., 2000b).
Effects of melanocortinergic signalling on feeding behaviour
As described above, the central MC system is clearly involved in regulation of feeding behaviour. The question remains, however, which specific biological processes related to appetite control are regulated by MC signalling?
In order to survive, it is essential for an animal to search for food, remember food sources, prepare for consumption, ingest sufficient amounts of food and digest foods efficiently. The decision to eat is controlled by endogenous drives (hunger and satiety), and also by environmental cues such as availability of palatable or novel foods and predator exposure when searching for food. Furthermore, different factors involved in food intake influence each other. For example, meal termination is controlled by internal and external cues. Meal size is influenced by extent of negative energy balance preceding the meal as well as meal composition. Even when a high degree of satiety is achieved, the availability of a highly palatable food may overrule this and increase meal size. Both, anatomical and pharmacological evidence suggests that satiety, hunger and rewarding aspects associated with feeding are regulated in different anatomical sites, many of which contain MC receptors (Berthoud, 2004). To answer the question of how the MC system affects feeding behaviour, pharmacological and genetic interference studies have been combined with behavioural paradigms that address these different processes.
For example, it was found that similar to leptin, MTII reduces food intake by affecting meal size and duration as shown by meal pattern analysis studies, whereas meal frequency and inter-meal interval were unaffected. Oppositely, SHU9119 increases food intake by selectively increasing meal size. These data suggest that the MC system is involved in meal termination rather than meal initiation. This is consistent with the demonstration that MTII is less effective in suppressing food intake when rats expect a meal which underscores the lack of MC effects on affecting meal initiation. Other studies by Benoit et al. (2003) showed that the efficacy of MC receptor agonists appears attenuated during scheduled feeding, when rats have learned to consume large amounts of food in a short period of time, suggesting that the MC system is not involved in anticipation to food.
Data from analysis of MC4?/? mice do not always fit with these pharmacological data. In a paradigm where mice have to press a lever to get a meal, MC4 receptor?/? are not hyperphagic and lose body weight, while control animals show normal food intake and body weight gain. Meal size as well as frequency is normal in MC4 receptor?/? in this paradigm, suggesting that a functional MC4 receptor is not required for the normal regulation of meal patterns. However, data about meal patterns during hyperphagia ('abnormal increased appetite') and in a normal environment are necessary to clarify this. Furthermore, in MC4?/? mice, compensatory adaptation may mask a physiological role of the MC system in determining meal size.
Ligands that suppress food intake may do so because they induce a state of illness. This can be tested by pairing ligand infusion with a flavour, and subsequent measurement of the intake of the flavoured food in the absence of ligand infusion, as in conditioned taste aversion (CTA) tests. Although it has been reported that MTII, the mixed MC3/4 receptor agonist, induced CTA, more selective MC4 receptor agonists do not induce CTA, suggesting that the MC4 receptor is the candidate MC receptor for development of agonists to suppress energy intake (Butler, 2006). Thus, the MC system affects meal size, but not meal initiation, meal frequency or anticipation and a reduction of food intake by activation of the MC4 receptor is not caused by inducing nausea.
Relevant to the evaluation of MC receptor agonists as potential drugs to reduce food intake in obesity, is whether the efficacy of MC receptor agonists depends on the type of diet and whether MC receptor agonists are still effective in obese subjects.
When MC receptor agonists are infused over days to weeks in rats, the efficacy to suppress food intake is high in the first days but fades away over time. Rats that are fed a high-fat diet for several weeks also show an attenuated response to MTII. It was recently shown that chronic MTII treatment still reduces food intake in rats fed a high-fat diet as well as in food-deprived rats. MTII was, however, less effective in reducing food intake and body weight in rats with a lower body fat mass. This indicates that activity of the MC system is related to the defended level of body adiposity, suggesting that the main function of the MC system is regulating body adiposity rather than food intake.
Interestingly, when the high-fat diet is low in carbohydrates (mimicking the ‘Atkins’ diet), MTII sensitivity is maintained, although the level of adiposity on this diet is increased as compared to controls. It is important to note that not all laboratories that investigated the effects of MCs have used the same diet, which complicates direct comparisons. Further studies need to clarify whether the type of (high fat) diet affects the responsiveness of the MC system.
Another aspect of different diets is that these not only differ in caloric density but also in palatability. MC receptors are expressed in brain centres that relay information on taste and palatability, such as the amygdala, nucleus of the tractus solitarius and parabrachial nucleus. This provides an anatomical basis for an effect of MCs on food choice. As MCs may affect taste processing, this might contribute to an overall effect on food intake. There are several reports on the role of the MC system in preference for certain foods. MTII specifically reduces intake of fat (but not of protein or carbohydrates) on a three-choice macronutrient diet (Samama et al., 2003) and the inverse agonist AgRP enhances the intake of specifically high-fat diets in rats (Hagan et al., 2001). In addition, obese mice with ectopic overexpression of Agouti (which mimics the action of AgRP) have enhanced preference for fat meals (Koegler et al., 1999).
Also, experiments using MC4?/? mice indicate an involvement of the MC system in fat preference. When exposed to a high-fat diet, MC4 receptor?/? display an increased caloric intake, which is, unlike in control animals, not normalized after 48 h. Together with an enlarged feed efficiency, this results in an even more increased body weight gain. Additionally, whereas normal animals increase their oxygen consumption on a high-fat diet, this effect is absent in MC4 receptor?/?. This indicates that the MC4 receptor is required for a normal metabolic and behavioural response to increased dietary fat. Thus, MC4?/? have a deficit in the normal response (reduced intake) to a high-fat diet, which may, besides a reduced metabolic response to high-fat diet, be explained by a deficit in sensing foods with an increased caloric density or by increased liking of fat foods. When given the choice between a high-fat, high-protein and high-carbohydrate diet, wild-type animals treated with MTII reduce specifically the intake of the high-fat diet, whereas the intake of high protein and high carbohydrate derived calories remains unchanged. This effect is absent in MC4 receptor?/?, suggesting that the MC4 receptor is necessary for the MTII-induced reduction of fat intake (Samama et al., 2003).
Taken together, accumulating evidence in rodents, but also in humans carrying mutations in the MC system, indicate that reduction of MC receptor activity is associated with preference for fat meals. However, there are long-term effects of different diets, with the ratio of fat to protein and carbohydrates in diets as important factors affecting the sensitivity for MC receptor agonists. It might be that the level of adiposity (which is increased by high-fat diets) sets the sensitivity of the brain MC system. Future studies need to clarify whether treatment with MC4 receptor agonists are able to shift this adiposity set point.
Interference within the central MC system (e.g. at the level of AgRP, POMC, MC3 or of MC4 receptors) revealed a wide variety of energy balance phenotypes. Hyperphagia and obesity are observed in both mice and humans with mutations in MC system. Reduction of MC receptor activity is associated with pushing the energy balance towards positive. The MC system affects multiple factors affecting energy balance, such as meal size, food choice and energy expenditure, which are controlled in different brain sites expressing MC receptors. The PVN plays a major role in MC4 receptor-mediated hyperphagia, whereas interactions between hypothalamic (e.g., LH) and mesolimbic signalling may play a role in the normal response to high-fat diets. The role of the MC system in feeding behaviour needs to be unravelled further, in order to select those groups of obese individuals that may benefit from treatment with MC4 receptor agonists. Careful analysis of feeding behaviour in humans treated with MC4 receptor agonists (when they become available for clinical studies) provides an interesting approach to achieve this.
Stress, Neuropeptides, and Feeding Behavior: A Comparative Perspective
Evidence supporting a physiological role of melanocortins in feeding behavior came initially from interest in the biological basis for coat color in agouti mice. These mice ubiquitously over-express the agouti protein, an endogenous melanocortin receptor antagonist (Lu et al., 1994). As a result, they develop a yellow coat. However, agouti mice also develop a form of late-onset obesity similar to that occurring in mice deficient in the melanocortin MC-4 receptor (Huszar et al., 1997). MC-4 receptor antagonists block leptin effects on weight gain (Seeley et al., 1997) while direct intracerebroventricular (i.c.v) administration of an MC-4 agonist inhibits feeding in a wide variety of hyperphagic models (Fan et al., 1997). Leptin increases POMC expression in arcuate neurons while leptin deficient mice express 50% less POMC in the arcuate nucleus than wild-type mice (Schwartz et al., 1997). A similar reduction in POMC expression is observed in fasted mice (Schwartz et al., 1997). These findings, with evidence that leptin receptors are expressed within arcuate POMC neurons (Cheung et al., 1997; Håkansson et al., 1998), point to a critical role for melanocortins in regulating feeding behavior. Melanocortins inhibit feeding in amphibians and birds (Table 2), suggesting an evolutionarily ancient role as anorexigenic neuropeptides. Although disruption of ?E signaling does not result in the dramatic effects on weight gain observed with targeted disruption of melanocortin receptors (Huszar et al., 1997), direct i.c.v administration of this peptide increases feeding (McKay et al., 1981; Table 2).
Hypothalamic neuropeptides mediate stress-induced changes in feeding behavior
Given the involvement of melanocortins and CRH in mediating leptin action, it is not surprising to see that some of these same neuropeptides are involved in stress-induced affects on feeding. Restraint-stress induced anorexia in rats is partially-reduced after administration of the CRH-receptor antagonist ?-helical CRH (9–41) (Krahn et al., 1986) and is completely inhibited by immunoneutralization of CRH (Shibasaki et al., 1988). CRH is also involved in anorexia nervosa, as CSF CRH is elevated in patients with this disorder (Hotta et al., 1986). A role for melanocortins is suggested by evidence that administration of the MC4 antagonist HS014 reduces stress-induced anorexia (Vergoni et al., 1999a). The effects of CRH are not mediated by the melanocortins, as HS014 does not block CRH-induced anorexia (Vergoni et al., 1999b).
An interesting problem arises when one considers that ?E and ?MSH are produced from the same pro-hormone (POMC) but have opposite effects on feeding. Nonetheless, feeding-related changes in arcuate POMC expression generally support an anorexigenic role for these neurons. Neurotoxin-induced transient hyperphagia and weight gain are both associated with decreased POMC expression and levels of ?MSH in the PVN, effects that can be reversed with administration of the non-selective MC-3/MC-4 agonist Melanotan 2 (MT-II). Obese Zucker rats have lower arcuate POMC gene expression and reduced ?MSH content in the PVN compared to lean Zucker controls (Kim et al., 2000). Interestingly, ?E content in the PVN is unchanged in obese Zucker rats, suggesting differential secretion and/or metabolism of POMC end-products in the PVN (Kim et al., 2000).
Neuroanatomy of melanocortin-4 receptor pathway in the lateral hypothalamic area.
2012
Abstract
The central melanocortin system regulates body energy homeostasis including the melanocortin-4 receptor (MC4R). The lateral hypothalamic area (LHA) receives dense melanocortinergic inputs from the arcuate nucleus of hypothalamus and regulates multiple processes including food intake, reward behaviors and autonomic function. Using a mouse line in which green fluorescent protein (GFP) is expressed under control of MC4R gene promoter, we systemically investigated MC4R signaling in the LHA by combining double immunohistochemistry, electrophysiology and retrograde tracing techniques. We found that LHA MC4R-GFP neurons co-express neurotensin as well as the leptin receptor but not with other peptide neurotransmitters found in the LHA including orexin, melanin concentrating hormone and nesfatin-1. Furthermore, electrophysiological recording demonstrated that leptin, but not the MC4R agonist melanotan II, hyperpolarizes the majority of LHA MC4R-GFP neurons in an ATP-sensitive potassium channel-dependent manner. Retrograde tracing revealed that LHA MC4R-GFP neurons do not project to the ventral tegmental area, dorsal raphe nucleus, nucleus accumbens and spinal cord, and only limited number of neurons project to the nucleus of solitary tract and parabrachial nucleus. Our findings provide new insight into MC4R signaling in the LHA and its potential implication in homeostatic regulation of body energy balance.
PMID:* * 22605619
Effects of Peptide Melanocortins
The melanocortin pathway is involved in controlling food intake and autonomic activity. Neurons within the ARC express POMC, from which ?-melanocyte stimulating hormone (?-MSH) is cleaved. POMC neurons are activated by both leptin and insulin and are suppressed in states of negative energy balance or genetically defective leptin signaling. ?-MSH acts as a melanocortin 4 receptor (MC4R) agonist and is induced by leptin. Interestingly, AgRP, whose expression is activated by leptin, acts as an endogenous MC4R antagonist. Although MC4R (H2396) pathways are regulated by leptin, MC4R regulates 5-HT2C serotonin receptors, activation of which induces weight loss. The MC4R also activates brain-derived neurotrophic factor (BDNF) (B3795) through TrkB receptors in the ventromedial (VMH) region of the hypothalamus. Central administration of BDNF in db/db mice decreases food intake and increases energy expenditure, demonstrating that BDNF plays a role in regulating food balance, possibly mediated through MC4R.
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