The melanocortin system during fasting

peptides 27 (2006) 291–300

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Review

The melanocortin system during fasting Fabrice Bertile *, Thierry Raclot Centre d’Ecologie et Physiologie Energe´tiques, UPR 9010 CNRS, Associe´ a` l’Universite´ Louis Pasteur, 23 Rue Becquerel, 67087 Strasbourg Cedex 2, France

article info

abstract

Article history:

This paper sets out to review the implication of the melanocortin system in regulating

Received 30 July 2004

feeding behavior and energy balance during short- and long-term food deprivation. It is

Accepted 18 March 2005

discussed in relation to: (1) body fat exhaustion and the known enhanced drive for refeeding

Published on line 7 November 2005

in late fasting and (2) peripheral hormonal status with emphasis on the effect of leptin administration on melanocortin gene expression according to fat store mobilization. # 2005 Elsevier Inc. All rights reserved.

Keywords: Neuropeptides Leptin Feeding behavior Energy depletion Body lipid utilization Starvation

Contents 1. 2.

3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The melanocortin system in regulating energy homeostasis . 2.1. Melanocortin system components . . . . . . . . . . . . . . . . 2.2. Melanocortin system and energy balance. . . . . . . . . . . The long-term fasting paradigm . . . . . . . . . . . . . . . . . . . . . . . 3.1. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Metabolic phases of prolonged fasting . . . . . . . . . . . . . 3.2.1. Body mass changes . . . . . . . . . . . . . . . . . . . . . 3.2.2. Plasma metabolites and hormones . . . . . . . . . 3.3. Phase 2–phase 3 transition . . . . . . . . . . . . . . . . . . . . . . Leptin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Leptin and metabolic changes. . . . . . . . . . . . . . . . . . . . 4.2. Leptin and the melanocortin system . . . . . . . . . . . . . . Melanocortin system response to fasting . . . . . . . . . . . . . . . . Melanocortin system response to peripheral signals . . . . . . . 6.1. Response to leptin during prolonged fasting . . . . . . . . 6.2. Response to other signals during fasting . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: +33 3 88 10 69 00; fax: +33 3 88 10 69 06. E-mail address: [email protected] (F. Bertile). 0196-9781/$ – see front matter # 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.peptides.2005.03.063

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292 1.

peptides 27 (2006) 291–300

Introduction

The regulation of energy homeostasis is mediated by mechanisms that involve both key peripheral and central signals which regulate food intake and energy expenditure. The set-point theory of the regulation of body weight, as well as the lipostatic theory, hypothesize that any variation in body lipid content is signaled to the brain and results in adjustments in feeding behavior and/or in energy expenditure to restore adiposity [67,97]. Adiposity signals secreted in proportion to lipid reserves and acting in the central nervous system through a negative feedback loop, are then likely to be involved in such processes [98,117]. In this context, leptin appears to constitute the archetypal adiposity-signal that regulates body weight [9,20,55,74,86,117]. Among the several brain regions involved in energy balance regulation, the hypothalamus, responding to a wide range of nutritional and other signals, has been actively investigated. Numerous neurotransmitters and neuropeptides are found in the hypothalamus, including components of the melanocortin system [38,62,102,112,118]. A large body of evidence supports the role of melanocortins and their receptors in the regulation of energy balance [38,102,121]. In addition, the hypothalamic melanocortin system has emerged as a potentially important target for the effects of leptin on energy homeostasis [4]. This review will focus on the melanocortin system and its implication in regulating energy homeostasis and mediating the response to fasting. This will be discussed in relation with body lipid depletion and peripheral hormonal status, with emphasis on the effects of plasma leptin concentrations.

2. The melanocortin system in regulating energy homeostasis 2.1.

Melanocortin system components

The melanocortin system involves peptide hormones derived from post-translational cleavage of a polypeptide precursor, the proopiomelanocortin (POMC) gene product [1,21]. POMCderived peptides are among the most abundant neuropeptides in the brain [2,33,111]. The known melanocortin hormones include a-melanocyte-stimulating hormone (a-MSH), b-MSH, g-MSH, and adrenocorticotropin (ACTH), which share the core amino acid sequence His-Phe-Arg-Trp, a key pharmacophore necessary for their biological activity. To date, five melanocortin receptors (MC1R–MC5R) have been characterized with tissue-specific expression patterns and different binding affinities for each of the melanocortin hormones (reviews in [44,112]). MCRs are expressed in a few peripheral tissues [18,31,45,46,50,75,82]. MC3R and MC4R are both highly expressed in brain areas known to be involved in regulating energy balance, like hypothalamus nuclei [8,81,92]. Arcuate nucleus POMC and agouti-related protein (AGRP) neurons notably express MC3R (not MC4R), suggesting a role for this receptor in the feedback regulation of melanocortinergic circuitry [8,66]. The melanocortin system also consists of two naturally occurring antagonists, agouti and AGRP. Agouti is a high-

affinity competitive antagonist of the melanocortin peptides at MC1R and MC4R. AGRP is a central nervous system-specific homologue of agouti protein [85], which is a potent MC3R and MC4R competitive antagonist [84], suppressing the tonic inhibition induced by a-MSH. AGRP causes hyperphagia and obesity when over-expressed in transgenic mice [78]. Interestingly, melanin-concentrating hormone (MCH) has also been mentioned as a functional melanocortin antagonist in the hypothalamus [76]. In addition, two ancillary proteins, mahogany and syndecan-3, have been found that are supposed to be mediators in the melanocortin pathway for weight control [118].

2.2.

Melanocortin system and energy balance

Neural systems termed central effectors of energy balance represent major controllers of food intake and autonomic outflow that affect the storage and mobilization of energy. Briefly, central anabolic and catabolic pathways are supposed to respond to peripheral signals that reflect energy balance, adiposity and/or nutritional status. Regarding the melanocortin system, POMC-null mutant mice show altered lipid metabolism and hyperphagia and develop an obese phenotype, similar to the human POMC-null syndrome [119]. a-MSH is likely to inhibit food intake and to increase energy expenditure [41]. Activation of central melanocortin receptors inhibits feeding and leads to weight loss, whereas blockade of the central melanocortin signaling pathway increases food consumption and promotes weight gain. Indeed, targeted disruption of the MC4R produces hyperphagia and obesity in transgenic mice [63] and MC4R mutations have been found in obese humans [63,110]. Inactivation of the MC3R results in increased fat mass and decreased lean body mass [19,24]. AGRP is a potent orexigenic agent when administered intracerebroventricularly in rodents [93,105]. AGRP surexpression in mice induces an obese phenotype [78]. AGRP also has effects on energy expenditure and thermogenesis in relation with the hypothalamic–pituitary–thyroid axis. Moreover, AGRP seems to be involved in food selection [53,116].

3.

The long-term fasting paradigm

3.1.

Context

In comparison with pathophysiological obesity models, the prolonged fasting paradigm allows studying the physiological regulation of energy reserves in the context of marked fat depletion. The use of starvation as a treatment for severe obesity has ended up with the occurrence of several sudden deaths [72]. However, some wild animals regularly deplete their fat depots in an apparently safe way. Negative energy balance may then be experienced without becoming detrimental to the organism. Indeed, when engaged in activities incompatible with feeding, certain wild animals spontaneously fast for long periods [83]. For instance, the male emperor penguin Aptenodytes forsteri, an Antarctic seabird breeding and incubating the egg on the sea ice, fasts for 4 months during the austral winter [91]. In such a situation of no

peptides 27 (2006) 291–300

exogenous energy intake, carbohydrate and especially lipid energy reserves are intensively mobilized for the energy needs and the male emperor penguin loses about 40% of its body mass before leaving to feed at sea [90]. Such changes in specific body weight loss and body reserve composition recorded in the wildlife during prolonged fasting and subsequent refeeding are also found in experimental fasting/refeeding in laboratory rodents [13,14,47].

system adapts to utilize ketone bodies. The length of P2 has been shown to increase with larger initial adiposity [26,47,48]. As the fast is extended, P2 ends with the occurrence of a late and strong increase in protein breakdown, while lipid utilization decreases during P3 [11,13,14,23]. This latter phase is a reversible situation since rats have been refed successfully after 3 days of P3 [11,28].

3.2.2. 3.2.

Metabolic phases of prolonged fasting

3.2.1.

Body mass changes

During long-term food deprivation, a sequential mobilization of energy reserves has been described. From changes in the loss of body mass, three distinct metabolic phases have been characterized in various species including rats [11,13,14,47,71,91]. The loss of body mass is rapid during phase 1 (P1), slower in phase 2 (P2) and faster again in phase 3 (P3). Accordingly, the specific daily body mass loss (dm/m dt) decreases during phase 1, remains at a minimum level in P2 and increases in P3 [90]. Body mass and dm/m dt variations during P1, P2 and P3 are depicted in Fig. 1. P1 is a short period of adaptation, marked by exhaustion of carbohydrate reserves, increasing mobilization of fat stores and lowering of protein utilization. P2 corresponds to a period of protein sparing, during which most of the energy requirements are derived from lipid oxidation. The central nervous

Plasma metabolites and hormones

The variations of plasma metabolite and hormone concentrations reflecting body reserve utilization in fasted rats [47,49] are similar to what has been described in fasting birds [27]. In comparison with fed animals, glycemia and uremia are decreased during P1, reflecting glycogen exhaustion and low levels of protein breakdown. Protein sparing in P2 leads to low plasma urea and low nitrogen excretion levels [13,29,47,51,91], while lipid utilization triggers a strong rise in circulating glycerol and especially fatty acids and ketone bodies [29,47]. Finally, the increased proteolysis in P3 produces increased uremia and nitrogen excretion [13,47,72,91]. It is of note that elevated plasma levels of amino acids have also been observed in P3 [52]. At this stage, the reduced lipid contribution to energy expenditure also explains diminished levels of plasma fatty acids and ketone bodies [47]. The classical profile of hormonal response to fasting includes increased plasma levels of glucocorticoids, catecholamines, ghrelin and glucagon and decreased levels of circulating insulin, growth hormone, gonadotropins and thyroid hormones [23]. Plasma leptin levels fall early during fasting, a fact which has been implicated in the adaptation to fasting [3,4,6].

3.3.

Fig. 1 – Illustration of the body mass and specific daily body mass loss (dm/m dt) variations in fasted rats. Values have been chosen arbitrarily to correspond to the profiles usually observed in 250–300 g male Sprague Dawley rats submitted to a total fast up to phases 1–3 (P1–P3) of fasting.

293

Phase 2–phase 3 transition

In various species, the transition from P2 to P3 is associated with behavioral changes. At this time, a rise in spontaneous locomotor activity has been measured in captive emperor penguins [89,91] and laboratory rats [70]. This has been postulated to reflect an increasing drive for refeeding to anticipate a lethal depletion of energy reserves and promoting food foraging [51,72]. Thus, wild animals that periodically fast for long periods spontaneously start to refeed when their body lipid reserves have been largely depleted [83]. The rise in the drive to refeed implies that the central nervous system is ‘‘informed’’ of decreased body adiposity or of changes in energy balance status. This is probably linked to a metabolic and hormonal shift [89]. Indeed, whole-body fuel utilization during fasting shifts from almost exclusively fat to both fat plus proteins. These adaptive responses lend support to the presence of fat store associated molecules that provide an input to the brain and regulate neuroendocrine systems and fuel utilization. All these observations have led to the proposition that there is probably an ‘‘alarm signal’’ that induces refeeding and promotes survival when energy stores are near exhaustion [70,89]. Corticosteronemia being gradually increased throughout P1 and P2 and then rising strongly in P3 [16,23], a key role for corticosterone has been hypothesized [23,115]. However, other peripheral signals, like leptin, are able to act on neural systems that motivate an organism to initiate eating behavior [59].

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peptides 27 (2006) 291–300

4.

Leptin

4.1.

Leptin and metabolic changes

Leptin, an adipocyte-derived hormone, has been reported to play a major role in the maintenance of energy homeostasis [4]. In the fed state, leptin expression, secretion and plasma concentration reflect adipose mass in rodents and humans [34,42,56,77]. Rising with increasing adiposity, plasma leptin levels generate a signal that limits further weight gain. However, the true stimulus modulating leptin expression seems to be linked to metabolic activity of the adipocytes rather than to the amount of lipids stored [58,61]. For example, plasma leptin is: (1) augmented after a meal in rodents and after overfeeding in humans [68,94] and (2) decreased early (together with leptin expression) and out of proportion to the corresponding decrease in body fat mass during fasting [4,17,43,77,109]. Among metabolic, hormonal and behavioral variations associated with prolonged food deprivation [11,13,14,47,70,89–91], leptin is then likely to constitute an afferent signal to the integrating hypothalamus, thus signaling changes in nutritional status and energy balance to the brain. This is further supported by the known action of leptin on its central receptors in modulating hypothalamic neuropeptide gene expressions [4].

4.2.

Leptin and the melanocortin system

In a general way, hypothalamic neuropeptides have been classified into two categories: orexigenic peptides that are inhibited by leptin and anorexigenic peptides that are stimulated by leptin (review in [4]). An integrated control of appetite, fat metabolism and hence lipid reserves has been proposed that links the melanocortin system to leptin levels [41]. More precisely, leptin seems to regulate the genes encoding POMC and AGRP in a reciprocal manner [30,39,54]. Indeed, leptin receptor is highly expressed in both POMC and NPY/AGRP neurons [54,79,100]. AGRP expression is inhibited by leptin and stimulated in situations of altered leptin signaling like food restriction in wild type rodents (low levels of circulating leptin) or genetic mutations in ob/ob (leptin deficient) and db/db (leptin receptor deficient) mice [4,5,80,85,104]. Administration of leptin in genetically obese mice normalizes AGRP expression [3,80]. Moreover, the increase in hypothalamic AGRP expression in short-term fasted rodents does not occur in leptin-deficient ob/ob mice, suggesting that this effect is mediated by leptin [114]. POMC expression is stimulated by leptin, while it decreases when plasma leptin levels are low, like during short-term fasting [12,69,79,100]. Mice lacking functional leptin or leptin receptors are morbidly obese and have significantly reduced POMC mRNA levels in the hypothalamus, whereas POMC mRNA expression in leptin-treated ob/ob mice are indistinguishable from wild type controls [79,95,100,108]. In addition, neuroendocrine and endocrine responses, including the fall in POMC and the rise in AGRP mRNA levels in mice and rats fasted for 2– 3 days, can be prevented by administration of recombinant leptin during the fasting period to prevent the early fall in leptinemia [3–6,37,69,79,80,100]. Leptin deficiency may then be interpreted as reflecting a situation of nutritional and energy

depletion, which could trigger hyperphagia and subsequent morbid obesity in ob/ob (and db/db) mice. Also, falling leptin concentrations in the fasting state could mediate the hypothalamic response [6] and could possibly trigger the enhanced drive for refeeding in late fasting.

5.

Melanocortin system response to fasting

The response of the melanocortin system to fasting may be of great importance since they have been evolutionarily conserved. In fact, results concerning AGRP and POMC expression in food-deprived rodents are commonly observed in nature and are similar in fasted zebrafish [106], goldfish [22] and Japanese quail [87]. During long-term fasting, the molecular basis underlying the neuropeptidergic control of feeding behavior were largely unknown. In a previous study [14], we hypothesized that hypothalamic neuropeptide expression could reflect the enhanced drive for refeeding in late fasting. To test this hypothesis, the expression of mRNA encoding key orexigenic (NPY, AGRP, MCH and prepro-orexin) and anorexigenic (POMC and cocaine and amphetamine-related transcript) peptides have been measured in fasted rats up to P2 and P3. A rise in orexigenic gene expression (MCH and especially AGRP and NPY) during food deprivation that was much higher in P3 than in P2 has been found (see Fig. 2 and [14]). Orexigenic neuropeptides are then differentially regulated according to the phase of fasting considered. The rise in orexigenic gene expression in P3 appears to be specific and could mean that a threshold has to be reached to trigger the late food-seeking behavior. In addition to their implication in feeding stimulation, hypothalamic NPY and AGRP also reduce energy expenditure and promote fat storage [65]. Hence, the fasting-induced stimulation of these latter two neuropeptidergic systems at the molecular level could also correspond to early responses directed at efficient body lipid reserve repletion in case of an eventual refeeding period. Concerning anorexigenic gene expressions, only little variations have been observed during long-term fasting (see Fig. 2 and [14]). Our data confirm previous results obtained on short-term fasting and support the view that responses are not so different between P2 and P3. However, although not significantly different from the fed state or from P2, the down-regulated mRNA expression of POMC in P3 could participate in the stimulation of food-seeking behavior. It has been shown recently that regulation of melanocortin secretion is an important mechanism by which the POMC system is controlled and that secretion of POMC-derived peptides is differentially down-regulated during negative energy balance [88]. POMC gene expression being only slightly altered during prolonged fasting, it could be that melanocortin secretion constitutes the regulated step in this situation. In order to examine whether neuropeptides are regulated in relation to fasting duration and/or body fat depletion, ratios between P2 and P3 have been calculated for these parameters (Fig. 3). The magnitude of increased AGRP mRNA levels occurring in P3 relative to P2 is out of proportion to that of increased fasting duration. It is rather similar to the magnitude of decreased adiposity levels. With some caution,

peptides 27 (2006) 291–300

295

Fig. 3 – Ratio of AGRP and POMC mRNA levels, adiposity and duration of fasting between P2 and P3. Values are computed from the results of a previous study [14]. Bars that do not share the same superscript letter are significantly different (P < 0.05). AGRP, agouti-related protein; POMC, proopiomelanocortin; P2 and P3, phase 2 and phase 3 of fasting.

Fig. 2 – AGRP and POMC mRNA levels in fed and fasted rats. Total RNA was extracted from hypothalamus, separated on a 1.2% agarose gel, transferred to a nylon membrane, and hybridized with specific DNA probes. Signals were quantified by densitometry. All data are normalized using 18S rRNA and are expressed relative to fed controls set to unity. Values are means W S.E.M. for 5–6 animals. Bars that do not share the same superscript letter are significantly different (P < 0.05). Representative Northern blots are shown above the histobars. Leptin (L) was infused at a rate of 25 mg/day in rats subjected to an experimental fast up to P2 and P3. AGRP, agouti-related protein; POMC, proopiomelanocortin; P2 and P3, phase 2 and phase 3 of fasting.

one can say that AGRP gene expression seems rather more linked to adiposity variations than to fasting duration while the reverse is observed for POMC. It was concluded that the up-regulation of orexigenic AGRP mRNA levels and the slight down-regulation of anorexigenic POMC expression constitute the main responses of the melanocortin system in P2 and especially in P3 of fasting. At this stage, AGRP has been proposed to be a constituent of a central signal that would mediate the late enhanced drive for refeeding during prolonged fasting.

6. Melanocortin system response to peripheral signals 6.1.

Response to leptin during prolonged fasting

Hypothalamic gene expression seems to be sensitive to fat store depletion, suggesting that leptin could constitute an

essential mediator of the central (including the melanocortin system) response to late fasting (P3). Indeed, recombinant leptin perfusion tended to normalize most of the hypothalamic orexigenic neuropeptide mRNA levels in P3 (see Fig. 2 and [15]). Among possible explanations, it can be proposed that: (1) leptin could tonically suppress AGRP expression in the fed state and (2) fasting could release AGRP neurons from leptin’s inhibitory tone. During the P2–P3 transition, the central nervous system, informed of the low adiposity level when leptinemia reaches a low threshold, would elaborate a hypothalamic response that involves the strong up-regulation of AGRP (and NPY) mRNA levels. This response could then promote feeding behavior before the complete exhaustion of body energy reserves. Such a system would therefore be of primary importance for animal survival. One can cautiously propose that leptin and particularly low levels of plasma leptin can reasonably be considered as a constituent of a peripheral signal triggering the fasting-induced enhanced drive for refeeding in P3. On the contrary, recombinant leptin infusion had no effect on anorexigenic mRNA, including POMC. This does not lend support for a key role of POMC in P3 of fasting. Leptin effects being dose-dependent [36,64], the various neural pathways could be implicated differentially as a function of plasma leptin levels. Therefore, dose-dependent effects of chronic leptin infusion have been tested on hypothalamic gene expression in fasted rats, with emphasis on P2 and P3 of fasting [15]. To discriminate between fasting and leptin effects, fasted rats were subcutaneously infused with recombinant leptin at doses within the physiological range found in ad libitum fed rats and compared with control rats infused with vehicle. We found that AGRP expression was: (1) unchanged in all leptin-infused rats in P2 of fasting and (2) dose-dependently decreased in leptin-infused rats in P3 relative to vehicle-treated animals while no effect was observed for POMC mRNA levels (Fig. 4).

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Fig. 4 – AGRP and POMC mRNA levels in fasted rats. Total RNA was extracted from hypothalamus, separated on a 1.2% agarose gel, transferred to a nylon membrane, and hybridized with specific DNA probes. Signals were quantified by densitometry. All data are normalized using 18S rRNA and are expressed relative to vehicle-treated rats (P2 and P3 groups) set to unity. Values are means W S.E.M., for four animals. Bars that do not share the same superscript letter are significantly different (P < 0.05). Representative Northern blots are shown above the histobars. Leptin (L) was infused at a rate of 12.5 and 50 mg/day in rats subjected to an experimental fast until P2 (P2L12.5 and P2L50 groups) and P3 (P3L12.5 and P3L50 groups) and compared to vehicle-treated animals. AGRP, agouti-related protein; POMC, proopiomelanocortin; P2 and P3, phase 2 and phase 3 of fasting.

It was concluded that leptin plays a key role in the regulation of neuronal pathways involved in feeding behavior, including a dose-dependent regulation of AGRP expression in P3 but not in P2. Such a mechanism could allow for the correct and fine adjustment of the melanocortin system response as the fast is extended and body reserves are depleted.

6.2.

Response to other signals during fasting

AGRP (and NPY) mRNA levels in P3 leptin-infused rats were still higher than in fed control animals [15]. Although essential, leptin might then not be the sole regulator of hypothalamic gene expression during fasting. As reviewed in [32,59], few peripheral signals can convey metabolic information to the brain and thus are likely to constitute regulators of energy homeostasis and food intake, and more precisely of orexigenic and anorexigenic neuropeptide expression in hypothalamic nuclei in the fasted state. The regulation of hypothalamic genes during prolonged fasting should result from the coordinate action of various factors including fuel availability and the associated endocrine changes.

Many plasma changes occur during P2 and P3 of fasting. They include a slight diminution in glycemia (P2 and P3) and a strong drop in plasma non-esterified fatty acids (NEFA) (P3). It has been reported that neurons expressing AGRP are regulated by changes in glucose but not in NEFA utilization [103]. Therefore, diminished glucose utilization could be at the origin (at least in part) of the variations in AGRP expression during fasting. In particular, the reduction of glycemia in P2 and mainly in P3 could contribute to explaining the enhanced AGRP expression. On the contrary, fatty acids are unlikely to exert any essential control on AGRP gene expression during long-term food deprivation. Insulinemia is known to drop early and even more when reaching P3 in rats. Insulin could then be involved in the fasting hypothalamic response. Insulin is known to inhibit AGRP and NPY expression [99,101] and insulin administration to short-term fasted or streptozotocin diabetic rats has been shown to normalize AGRP (and NPY) as well as POMC mRNA levels [60,101]. The over-expression of AGRP and the slightly reduced POMC mRNA levels during fasting (Fig. 2) could then be induced by a mechanism dependent on low insulin levels. However, leptinemia also tended to be restored by insulin treatment in diabetic rats [60], providing a possible indirect mechanism of the regulation of hypothalamic gene expressions. This would support the view that insulin and leptin interact to reduce food intake and body weight in rats [7]. Glucocorticoids can promote food intake and obesity [107,120]. However, it has been shown that elevated corticosteronemia is not required for the fasting-induced up-regulation in hypothalamic AGRP and NPY gene expressions [57,96]. In our study [15], corticosteronemia rose during prolonged fasting to the same extent in rats with or without constant infusion of recombinant leptin, while AGRP was differentially regulated in control rats compared to leptin-treated animals. In other respects, adrenalectomy has been shown to suppress POMC mRNA, although leptin levels were similar in sham operated and adrenalectomized fasted rats [96]. However, POMC mRNA levels in fasted rats compared to fed controls were only slightly affected despite large variations in plasma corticosterone. Corticosterone is then unlikely to constitute an essential regulator of the melanocortin system during fasting, even in P3. This view is supported by recent data showing that leptin seems to influence POMC mRNA expression independently of circulating corticosterone as shown in adrenalectomized leptin-deficient mice receiving exogenous corticosterone as well as recombinant leptin injections [113]. Although corticosterone has been thought to be important in the changes of locomotor activity that reflect a food-seeking behavior at the P2– P3 transition of fasting [23], our data do not support this hypothesis at the molecular level. High corticosteronemia would be rather related to the need for protein breakdown to ensure gluconeogenesis during prolonged fasting. Nevertheless, effects of glucocorticoids on systems independent of NPY or melanocortin systems should be explored during long-term fasting to determine if the rise in corticosteronemia is able to combine with the drop in leptinemia to trigger refeeding. For example, experiments should be performed on adrenalectomized and control animals in late fasting (P3) perfused or not with leptin to provide insights into the role of corticosterone in the induction of feeding behavior in late fasting.

peptides 27 (2006) 291–300

It has been reported that hypothalamic AGRP and/or POMC neurons may be able to respond to signals like satiety factors including ghrelin, the gut hormone peptide YY3–36, and cholecystokinin [38]. Ghrelin does in fact have inhibitory effects on hypothalamic POMC neurons and simultaneously activates AGRP neurons [25,35]. POMC neurons are activated by peptide YY3–36 in the hypothalamus [10] and by cholecystokinin in the brainstem [40]. Moreover, cholecystokinin’s appetite-suppressing properties require the activation of neuronal MC4R. In contrast to many other gastrointestinal hormones like peptide YY3–36 or cholecystokinin, plasma levels of ghrelin increase significantly in response to fasting and return promptly to basal levels upon refeeding [73]. These variations are then expected to stimulate feeding behavior during long-term food deprivation, potentially through regulation of the melanocortin system. All the data mentioned here argue for a convergence of numerous metabolic and energy state related signals to the melanocortin system. Results of further investigations are still needed to determine: (1) other central effectors involved in the hypothalamic response to long-term fasting and (2) to what extent humoral signals mediate these effects according to body reserves utilization. Indeed, the improved understanding of the mechanisms and the finding of signs indicating the safety limits when energy intake is no longer maintained should help in gaining further insight into severe eating disorders associated with marked body energy depletion.

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