Peripheral influences on central melanocortin neurons

Peptides 26 (2005) 1744–1752

Review

Peripheral influences on central melanocortin neurons K.G. Murphy, S.R. Bloom ∗ Department of Metabolic Medicine, Imperial College Faculty of Medicine, Hammersmith Campus, Du Cane Road, London W12 ONN, UK Received 30 July 2004; accepted 10 December 2004 Available online 20 June 2005

Abstract The melanocortins are peptide products of post-translational processing of the pro-opiomelanocortin precursor protein. Melanocortinexpressing neurons are found in the arcuate nucleus of the hypothalamus and the nucleus of the solitary tract in the brain stem. The central melanocortin system is involved in a number of biological functions, including regulation of energy homeostasis. Hypothalamic and brain stem circuits interpret and integrate a number of peripheral inputs to provide a coordinated central response. This review examines the effect of these peripheral signals on central melanocortin signaling. © 2005 Elsevier Inc. All rights reserved. Keywords: Melanocortin; Pro-opiomelanocortin; Hypothalamus; Brain stem; Peripheral

Contents 1. 2. 3.

4.

5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The melanocortin system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy homeostasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Long-term signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Leptin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Insulin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Short-term signals: gut hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Ghrelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Peptide YY(3–36) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Cholecystokinin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steroid hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Glucocorticoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Gonadal steroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exogenous influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The melanocortins are peptide products of post-translational processing of the pro-opiomelanocortin (POMC) precur∗

Corresponding author. Tel.: +44 20 8383 3242; fax: +44 20 8383 3142. E-mail address: [email protected] (S.R. Bloom).

0196-9781/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.peptides.2004.12.027

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sor protein. Melanocortin-expressing neurons are found in two main populations in the central nervous system (CNS). One set is located in the arcuate nucleus of the hypothalamus (ARC) and the other is found in the nucleus of the solitary tract (NTS) in the brain stem. The central melanocortin system appears to be involved in a number of biological functions, including energy homeostasis, neuroendocrine

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regulation, behavior, and regulation of the cardiovascular system [45]. Hypothalamic and brain stem circuits interpret and integrate a number of peripheral inputs to provide a coordinated central response. This review will examine the effect of these peripheral signals on central melanocortin signaling.

2. The melanocortin system The melanocortins include the peptides adrenocorticotrophic hormone (ACTH), alpha-melanocyte stimulating hormone (␣-MSH), beta-melanocyte stimulating hormone (␤-MSH) and gamma-melanocyte stimulating hormone (␥MSH). Melanocortins signal via five G-protein-coupled melanocortin receptors (MC-Rs), which have different affinities for the various melanocortins. The MC3R and MC4R are the major MCRs expressed in the CNS, although the MC1R and MC5R are expressed at low levels in specific regions [1,12,147]. The MC3R is highly expressed in the hypothalamus, septum, and ventral tegmental area. The MC4R is also highly concentrated in these regions, in addition to the neostriatum, nucleus accumbens, olfactory bulb, and the periaqueductal gray matter [4,52,87,108]. Alpha-MSH is believed to be the most important endogenous agonist for the MC3R and the MC4R. Melanocortin signaling is modulated by agouti-related protein (AGRP), an endogenous antagonist for the MC3R and MC4R synthesized in a distinct population of ARC neurons [46,93,119, 148]. Within the hypothalamus, melanocortin neurons extend to the paraventricular nucleus and the lateral hypothalamus. These nuclei are implicated in the central regulation of energy homeostasis and neuroendocrine function. The only POMCexpressing cell bodies in the brain stem are found in the NTS. However, a network of melanocortin-immunoreactive fibers projects throughout the NTS, lateral reticular formation, ventrolateral medulla and the nucleus ambiguous [94,95]. In addition to signaling within and between the hypothalamus and the brain stem, melanocortin neurons radiate fibers to the amygdala, cortex, hippocampus, medulla, mesencephalon, spinal cord, and thalamus [42,126].

3. Energy homeostasis The hypothalamus and the NTS are key CNS regions in the regulation of appetite and energy expenditure. A number of neurotransmitters are implicated in the regulation of energy balance [66]. However, transgenic models of disrupted melanocortin signaling and human genetic studies have shown that the central melanocortin system is crucial in the regulation of food intake and body weight [10,23,27,64,114,132,133,148]. The ARC plays a vital role in receiving and integrating humoral signals with central neural impulses. It is located at

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the base of the hypothalamus and is only partly isolated from the general circulation by the blood brain barrier, allowing direct access of circulating factors to ARC neurons. There are two main ARC neuronal populations involved in the regulation of energy balance. The first co-expresses the orexigenic peptides, neuropeptide Y (NPY) and AGRP. Acute central injection of NPY dramatically increases food intake, increases energy expenditure [17,124] and chronically increases body weight. AGRP also acutely increases food intake, though the effect is sustained for longer than NPY [109]. Chronic CNS injection of AGRP increases body weight and reduces energy expenditure [121,122]. The second neuronal population expresses POMC [42,141]. Acute central administration of ␣-MSH reduces food intake [45,96]. Both NPY/AGRP and POMC neurons project to other hypothalamic nuclei and to extra-hypothalamic regions [45]. Energy balance is regulated by both long-term and shortterm signals. Long-term signals communicate information regarding the body’s energy stores, endocrine status, and general health, and are predominantly humoral. Such signals include the hormones leptin and insulin. Short-term signals appear to control meal initiation and termination, and include gut hormones, such as peptide YY(3–36) (PYY(3–36)), ghrelin, and cholecystokinin (CCK) and neural signals from higher brain centers and the gut. Both long- and short-term signals can also affect energy expenditure [117]. 3.1. Long-term signals 3.1.1. Leptin Leptin is a 156 KDa protein secreted by adipocytes and found in plasma at concentrations proportional to body fat mass. It signals the extent of body adiposity to the CNS, acting as the afferent limb of a body fat regulation loop. Fasting acutely suppresses circulating leptin in rodents [57,74]. Central and peripheral administration of leptin to rodents reduces food intake and increases energy expenditure [49]. Central melanocortin neurons are important mediators of the effects of leptin on appetite and body weight. Leptin regulates ARC POMC expression. A 48 h fast reduces ARC POMC expression to approximately 50% of normal. In addition, ob/ob mice, which lack leptin, have a similarly reduced ARC POMC expression. ARC POMC expression is also lower in leptin receptor-deficient db/db mice. Treatment with exogenous leptin attenuates this reduction in ARC POMC expression in both fasted and ob/ob mice [116]. ARC AGRP levels are also regulated by leptin. AGRP expression is elevated 13-fold in the ARC by a 48 h fast, and ob/ob and db/db mice show a 5-fold increase. Leptin replacement in ob/ob mice reduces ARC AGRP expression by 35% [83]. Leptin, therefore, regulates AGRP and POMC in opposite directions in the ARC. Leptin signaling in the brain involves activation of the hypothalamic melanocortin system. The long-form of the

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leptin receptor (Ob-Rb) mediates the effects of leptin on energy homeostasis via a number of intracellular signaling mechanisms [18,50,92,150], including signal transducer and activator of transcription 3 (STAT3). Ob-Rb is highly expressed in the ARC [81], and radiolabelled leptin binds in the ARC following peripheral administration [11]. Ob-Rb is also expressed in the NTS [82,118]. It is not known whether Ob-Rb and POMC are co-expressed in the NTS. Leptin is thought to directly stimulate ARC POMC neurons and directly inhibit NPY/AGRP neurons. Dual in situ hybridization studies show that the majority of ARC NPY/AGRP and POMC neurons express Ob-Rb mRNA [29,55]. Exogenous leptin treatment induces expression of the early gene c-fos, a marker of neuronal activation, in ARC POMC neurons but not NPY neurons [44]. However, leptin stimulates expression of suppressor of cytokine signaling-3 (SOCS3) in both POMC and NPY neurons. Leptin also stimulates the release of ␣-MSH, and has been reported to suppress the release of NPY and AGRP from in vitro hypothalamic explants [69,75]. Cowley et al. used transgenic mice with targeted expression of green fluorescent protein to identify POMC neurons for electrophysiological studies. Their studies showed that leptin increases action potential frequency in POMC neurons directly by depolarization through a nonspecific cation channel. NPY/AGRP neurons densely innervate POMC cells [33,37] and are thought to tonically suppress the activation of POMC neurons via gammaaminobutyric acid (GABA)-ergic terminals. In addition to its direct depolarization of the POMC neuron, leptin also hyperpolarizes NPY/AGRP neurons, inhibiting the release of GABA from these terminals and hence indirectly increasing POMC signaling [26,29]. Recently, besides increasing the excitatory tone and reducing the inhibitory tone onto POMC neurons, leptin has also been shown to increase inhibitory tone and decrease excitatory tone onto NPY neurons [98]. Leptin may mediate its effects on distinct neuronal populations via different intracellular signaling cascades. Hypothalamic NPY expression is increased and POMC expression decreased in db/db mice, which are hyperphagic, obese, infertile, short, and diabetic. Transgenic mice with chronically disrupted Ob-Rb mediated activation of STAT3 are also hyperphagic and obese, but are fertile, long and less hyperglycemic [14]. While they also show decreased hypothalamic POMC expression, their hypothalamic NPY expression is normal. This suggests that Ob-Rb STAT3 signaling regulates leptin’s effects on melanocortin expression and energy homeostasis, and that other pathways control NPY expression, fertility, growth, and blood glucose levels [41,88]. Peripheral leptin administration has also been reported to suppress plasma insulin levels via a CNS pathway. This effect can be blocked by a centrally administered MC3R and MC4R antagonist, suggesting the melanocortin system may play a role in this effect [90]. The central melanocortin system is not simply a conduit for the actions of leptin. The leptin-deficient ob/ob

mouse rapidly increases diet-induced thermogenesis and physical activity in response to a diet with increased fat content. However, mice lacking the MC4R do not, suggesting the melanocortin system has a leptin-independent effect on behavioral and metabolic responses to diet [24]. The effects of leptin deficiency and agouti antagonism of central POMC signaling have been shown to be independent and additive [20]. Thus, the central melanocortin system may have a specific role in energy homeostasis, independent of leptin. Additionally, the melanocortins are thought to have, for example, central neurotrophic, and antipyretic roles that are not regulated by leptin [43,62]. Similarly, leptin influences a number of processes that are not thought to be regulated by the melanocortin system. For example, the ob/ob mouse is infertile [127] and has raised circulating glucocorticoids [48,49], unlike mice lacking the MC3R or MC4R [27,64]. Such effects may be mediated by other hypothalamic neuropeptides [72,77,102,110]. 3.1.2. Insulin Like leptin, insulin is thought to signal body adiposity levels to the brain. Although insulin is not released from the adipocytes themselves, circulating levels of insulin correlate with body adiposity level [100,144]. In addition, disruptions of insulin sensitivity are associated with obesity and diabetes mellitus [65,106]. The ARC contains a high concentration of insulin receptors [79]. Hypothalamic POMC-expressing neurons co-express the insulin receptor and are responsive to insulin [16,33]. When activated, the insulin receptor has intrinsic tyrosine kinase activity, which results in receptor autophosphorylation and phosphorylation of several substrates, including insulin receptor substrate proteins [99,139]. CNS injection of insulin reduces food intake [3], and this effect is blocked by a melanocortin antagonist. Central administration of insulin increases hypothalamic POMC expression [16] and mice deficient in insulin receptor substrate-2 have lower hypothalamic POMC expression. Insulin is also thought to inhibit NPY/AGRP neurons in the ARC, either directly or via interneurons [13,66,115,120]. Central insulin administration decreases hypothalamic NPY [16] and streptozotocininduced insulinopenia increases ARC NPY expression. Inhibition of NPY/AGRP neurons by insulin may lead, as it does after leptin administration, to a secondary stimulatory effect on POMC neurons [114,120]. These results suggest that the central effects of insulin may be mediated by the melanocortin system. Whether the melanocortin system has a physiological role in mediating central effects of peripheral insulin is currently unknown. 3.2. Short-term signals: gut hormones 3.2.1. Ghrelin Ghrelin is a recently discovered gastric peptide hormone. Central and peripheral administration of ghrelin stimulates growth hormone release and appetite [71,91,131,145,146].

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Plasma ghrelin levels are suppressed post-prandially and are inversely correlated to body mass index [38,131]. Ghrelin is the endogenous peptide ligand for the growth hormone secretagogue receptor (GHS-R), which is expressed in the majority of NPY/AGRP neurons and a smaller population of POMC neurons in the ARC [140]. Ghrelin’s orexigenic effects appear to be primarily mediated by NPY/AGRP neurons. Chronic central ghrelin administration increases hypothalamic NPY and AGRP expression [67], and NPY/AGRP double knockouts do not respond to ghrelin [28]. Ghrelin post-synaptically stimulates neurons in the ventromedial ARC, where the majority of NPY neurons are located, and presynaptically inhibits a population of neurons in the ventrolateral ARC, where most POMC neurons are found [107]. Ghrelin causes depolarization of ARC NPY/AGRP neurons and hyperpolarization of POMC neurons [35,36]. Peripheral ghrelin administration activates c-fos in NPY/AGRP neurons, but not in POMC neurons, suggesting that the effect of ghrelin on POMC neurons is probably caused by the inhibitory actions of GABA released by NPY/AGRP neurons [36,136]. Whether the direct effect of ghrelin on NPY/AGRP release is more physiologically important than secondary inhibition of POMC neurons is currently unknown. However, it is worth noting that ghrelin induces a pattern of NPY/AGRP neuronal firing that favors peptide release. In addition to its expression in the periphery, ghrelin has been found to be present in the hypothalamus [36], where ghrelin-containing neurons synapse with both NPY/AGRP and POMC ARC neurons. It is, therefore, difficult to determine whether central or peripheral ghrelin regulates ARC neuronal activity under normal circumstances. 3.2.2. Peptide YY(3–36) The NPY Y2 receptor (Y2R), a putative inhibitory presynaptic receptor, is highly expressed on NPY neurons in the ARC. PYY(3–36) is an endogenous Y2R agonist released from the gut following a meal [2]. Peripheral administration of PYY(3–36) to rats inhibits food intake and reduces weight gain. Unlike other gastric and gut hormones released post-prandially, peripheral administration of PYY(3–36) does not activate brainstem neurons [56], but increases c-fos immunoreactivity in POMC-expressing and other ARC neurons. It also decreases hypothalamic ARC NPY expression and increases ARC POMC expression [15,25]. A specific Y2R agonist stimulates the release of ␣-MSH and decreases NPY release from hypothalamic explants [15]. The same study showed that PYY(3–36) inhibits electrical activity of NPY nerve terminals, disinhibiting adjacent POMC neurons, which then show an increase in action potential frequency [15]. The anorectic effects of PYY(3–36) therefore appear to be mediated in the ARC by direct inhibition of NPY/AGRP neurons and secondary stimulation of POMC neurons. However, recent work suggests that the melanocortin system is not vital for the anorectic effects of PYY(3–36). Peripheral PYY(3–36) administration still acutely suppresses food

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intake in the POMC-deficient mouse, the MC4R-deficient mouse and the agouti mouse [26,56,80]. 3.2.3. Cholecystokinin The brainstem melanocortin system also plays a role in energy homeostasis. Injection of a melanocortin receptor agonist or leptin into the fourth ventricle or the dorsal vagal complex inhibits food intake. Administration of an MC3R and MC4R antagonist into the same regions increases food intake [53,54,142]. However, the mechanisms by which brain stem melanocortin neurons regulate food intake are largely unknown. It appears that AGRP plays a much more limited role in the brain stem than it does in ARC melanocortin circuits. AGRP is not synthesized in the brain stem, and only a limited number of ARC AGRP neurons extend to the brain stem [9,45]. The influence of anorectic gut hormones on the melanocortin system is not limited to the hypothalamus. Cholecystokinin (CCK) dose-dependently suppresses feeding in rats and humans [51,70,86,89]. The anorectic effects of CCK are thought to be mediated by the CCKA receptor via the vagal nerve, and are abolished by vagotomy [123]. Vagal afferent fibers transmit signals from the gastrointestinal system to the brain stem and NTS neurons are activated by both electrical and CCK stimulation of these fibers [135]. CCK activates POMC neurons in the mouse NTS, as does feeding-induced satiety. Activation of the MC4-R appears to be required for CCK-induced suppression of feeding [47]. CCK may, therefore, reduce food intake at least partly via the brain stem melanocortin system. In addition, CCK has been shown to electrically modulate the activity of ARC neurons [22]. The identity of these neurons is, however, currently unknown.

4. Steroid hormones 4.1. Glucocorticoids Melanocortins stimulate the hypothalamo–pituitary– adrenal axis [40]. Interestingly, circulating glucocorticoids appear to influence central melanocortin signaling. ARC POMC neurons express glucocorticoid receptors and glucocorticoids stimulate POMC neurons [31,60]. Adrenalectomy reduces hypothalamic POMC mRNA and ␣-MSH peptide [111,138]. These changes can be reversed by glucocorticoid replacement. The POMC neurons responsive to glucocorticoids may be distinct from those that stimulate the HPA axis. In addition, the anorectic effects of central leptin and melanocortin administration are increased in adrenalectomised (ADX) rats. However, the orexigenic effect of AGRP is suppressed. These effects are normalized by glucocorticoid replacement [41,97]. Circulating glucocorticoids, therefore, appear able to modulate central melanocortin signaling, but the exact mechanism by which they do this is unknown. ADX animals show reduced food intake and body weight, and

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changes in several hypothalamic neuropeptide systems implicated in appetite and energy expenditure [73]. The importance of the melanocortin system in mediating the effects of glucocorticoids on energy homeostasis is, therefore, difficult to ascertain. 4.2. Gonadal steroids There is strong evidence for a link between the CNS systems controlling body weight homeostasis and the reproductive axis [113]. However, the specific mechanisms involved are poorly understood. Leptin appears likely to be involved, and as the melanocortin system mediates some of the downstream effects of leptin, central melanocortin circuits may also play a role [32,127]. There is evidence that the melanocortin system is involved in the regulation of the hypothalamo–pituitary–gonadal (HPG) axis. Central injection of AGRP has been reported to stimulate the HPG axis [125]. However, the reported effects of ␣-MSH on gonadotrophin secretion are contradictory [6,68,78]. In addition, sex steroids modulate hypothalamic POMC expression. Orchidectomy increases hypothalamic levels of POMC derived peptides in male rats. These increases are prevented by testosterone replacement [19]. Testosterone appears to regulate POMC gene expression only in a select group of POMC neurons located in the rostral portion of the ARC [30]. Ovariectomy similarly increases POMC mRNA levels in the rostral ARC, an effect that can be reversed by estradiol and dihydrotestosterone. Administering estrogen to ovariectomized animals suppresses ARC POMC mRNA [130]. Given that neither the MC3R or MC4R knockout mice have any obvious reproductive malfunctions, and that deletion of the POMC gene does not prevent reproduction, any role for the melanocortin system in the regulation of reproduction might be thought to be minor [23,27,64,149]. However, it is possible that other neuropeptides compensate for the disruption in melanocortin signaling. The importance of the melanocortins in the control of reproduction, therefore, requires further investigation.

5. Exogenous influences In addition to physiological peripheral signals, peripherally administered exogenous substances can influence central melanocortin signaling. Central serotonin signaling has long been identified as a factor in central regulation of energy homeostasis [59]. The now withdrawn oral anti-obesity drug, fenfluramine, promotes the release of serotonin at synapses. Recent research suggests fenfluramine can act on POMC neurons via the serotonin 2C receptor. Fenfluramine stimulates c-fos in central POMC neurons expressing the serotonin 2C receptor [37,58]. Fenfluramine depolarizes POMC neurons in vitro via activation of the serotonin 2C receptor, increasing their firing rate. It is thought brain stem serotonin neurons

may project to the ARC where their signals can be integrated with those of peripheral factors. The melanocortin system may also be involved in the CNS response to drugs of abuse. The endogenous opiate, ␤-endorphin is another product of POMC processing. Alpha-MSH modulates opiate-induced behaviors and exogenous opiates and neurostimulant drugs can affect the central melanocortin system [5]. Chronic peripheral morphine administration reduces hypothalamic POMC expression [21,84,85]. This is consistent with down regulation of ␤-endorphin by exogenous opiates. However, morphine administration effects CNS MC4R expression, suggesting it also influences melanocortin pathways [4,7]. Cocaine also appears to up-regulate MC4R in the striatum via a D1 receptor mediated mechanism [7]. MC4-R activation may antagonize certain properties of exogenous opiates, including addiction [5,34,128,134]. Ethanol also effects central POMC expression. Administration of ethanol has been reported to increase or decrease hypothalamic POMC expression depending upon the length of exposure [8,39,104,105,112]. There is also direct evidence that ethanol administration influences central ␣-MSH levels. Feeding rats 6.5% ethanol diet for three weeks reduces ␣MSH immunoreactivity in the ARC and the substantia nigra [103]. However, the precise role of the melanocortin system in addictive pathways remains to be elucidated.

6. Additional factors In addition to those mentioned above, a number of other factors may regulate the melanocortin system. Adiponectin is an adipocyte hormone involved in glucose and lipid metabolism. CNS administration of ADP decreases body weight, primarily by increasing energy expenditure. However, agouti mice do not respond to central adiponectin, suggesting the melanocortin pathway may mediate the central effects of adiponectin [101]. Glucose increases or decreases action potential frequencies in different ARC neurons in vitro. The identity of these neurons is unknown, though glucose-inhibited neurons were found predominantly in the medial ARC, and glucose-excited neurons in the lateral ARC [137]. ARC glucose-excited neurons are reported not to show POMC immunoreactivity, though these neurons may secondarily modulate POMC neuron activity. However, another report mentions that POMC neurons do respond to glucose [33]. The ARC has also been reported as being sensitive to the intermediates of fatty acid metabolism. Central and systemic administration of the fatty acid synthase inhibitor C75 to mice inhibits feeding and reduces body weight [76]. Central C75 blocks the increase in hypothalamic NPY and AGRP and the decrease in hypothalamic POMC seen in response to fasting. Hypothalamic malonyl-CoA, a substrate of fatty acid synthase also regulates feeding in mice [61]. However, C75 has been shown to be a non-specific neuronal activator, which

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may mean that its effects on energy homeostasis have little physiological relevance [129]. Studies have suggested that the melanocortin system is necessary for anorexia and weight loss induced by injected lipopolysaccharide [63,143]. Exactly how peripheral inflammatory factors influence central POMC neurons is not known. However, POMC neurons have been reported to respond to interleukin-1␤ in vitro [33].

7. Concluding remarks Central melanocortin neurons are, therefore, the target of a number of peripheral signals. Much of the data presently available regarding the regulation of central melanocortin neurons by peripheral factors concentrates on ARC POMC neurons. A high percentage of POMC fibers in the brain stem actually project from the ARC, suggesting that a good deal of brain stem melanocortin signaling may be influenced by peripheral factors modulating ARC neurons. Further studies are required to elucidate the functions of these NTS POMC neurons.

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