Antipyretic role of endogenous melanocortins mediated by central melanocortin receptors during endotoxin-induced fever

Alpha-melanocyte-stimulating hormone contributes to an anti-inflammatory response to lipopolysaccharide

OBJECTIVE

During infection, metabolism and immunity react dynamically to promote survival through mechanisms that remain unclear. Pro-opiomelanocortin (POMC) cleavage products are produced and released in the brain and in the pituitary gland. One POMC cleavage product, alpha-melanocyte-stimulating hormone (α-MSH), is known to regulate food intake and energy expenditure and has anti-inflammatory effects. However, it is not known whether α-MSH is required to regulate physiological anti-inflammatory responses. We recently developed a novel mouse model with a targeted mutation in Pomc (Pomctm1/tm1 mice) to block production of all α-MSH forms which are required to regulate metabolism. To test whether endogenous α-MSH is required to regulate immune responses, we compared acute bacterial lipopolysaccharide (LPS)-induced inflammation between Pomctm1/tm1 and wild-type Pomcwt/wt mice.

METHODS

We challenged 10- to 14-week-old male Pomctm1/tm1 and Pomcwt/wt mice with single i.p. injections of either saline or low-dose LPS (100 μg/kg) and monitored immune and metabolic responses. We used telemetry to measure core body temperature (Tb), ELISA to measure circulating cytokines, corticosterone and α-MSH, and metabolic chambers to measure body weight, food intake, activity, and respiration. We also developed a mass spectrometry method to measure three forms of α-MSH produced in the mouse hypothalamus and pituitary gland.

RESULTS

LPS induced an exaggerated immune response in Pomctm1/tm1 compared to Pomcwt/wt mice. Both groups of mice were hypoactive and hypothermic following LPS administration, but Pomctm1/tm1 mice were significantly more hypothermic compared to control mice injected with LPS. Pomctm1/tm1 mice also had reduced oxygen consumption and impaired metabolic responses to LPS compared to controls. Pomctm1/tm1 mice had increased levels of key proinflammatory cytokines at 2 h and 4 h post LPS injection compared to Pomcwt/wt mice. Lastly, Pomcwt/wt mice injected with LPS compared to saline had increased total α-MSH in circulation 2 h post injection.

CONCLUSIONS

Our data indicate endogenous α-MSH contributes to the inflammatory immune responses triggered by low-dose LPS administration and suggest that targeting the melanocortin system could be a potential therapeutic for the treatment of sepsis or inflammatory disease.

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

Microbial and Host Metabolites at the Backstage of Fever: Current Knowledge about the Co-Ordinate Action of Receptors and Molecules Underlying Pathophysiology and Clinical Implications

Fever represents an elevation of body temperature, that exerts a protective effect against pathogens. Innate immune cells and neurons are implicated in the regulation of body temperature. Pathogen-associated molecular patterns, i.e., lipopolysaccharides from Gram-negative bacteria and peptidoglycan and lipoteichoic acid from Gram-positive bacteria are exogenous pyrogens, that bind to Toll-like receptors on immune and non-immune cells. The subsequent release of pro-inflammatory cytokines [interleukin-1 (IL-1), IL-6 and Tumor necrosis factor-alpha] and their passage through the brain trigger the febrile response. In fact, neurons of the pre-optic area produce prostaglandin E2 (PGE2), that, in turn, bind to the PGE2 receptors; thus, generating fever. Apart from classical non-steroidal anti-inflammatory drugs, i.e., aspirin and acetaminophen, various botanicals are currently used as antipyretic agents and, therefore, their mechanisms of action will be elucidated.

Comprehensive biology of antipyretic pathways

Pyrogens, the fever inducing substances accidently enter into a human body through contamination from medical or pharmaceutical products may create mild to severe complications including septicaemia and shocking syndromes. To avoid such drastic situations all the pharmaceuticals and medical devices are analysed for presence of pyrogens prior to their release into market. The entry of exogenous pyrogens like bacterial endotoxins induces the release of endogenous pyrogens or inflammatory cytokines that activate immune system to defend against these pathogens. Generation of heat is considered as one of the important defence mechanism of body achieved through receptor mediated interaction of endogenous pyrogens at the thermoregulatory centre of hypothalamus. However, uncontrolled fever and febrile reaction may cause lethal effects to the subject itself. So a well sophistically functioning antipyretic mechanism is necessary to achieve thermoregulation. The coordinated interaction of antipyretic cytokines and other mediators are active in human immune system which play a crucial role in maintaining thermal homeostasis. The multiple interacting antipyretic signals and their mechanism are the major subjects of this review.

Systemic N-terminal fragments of adrenocorticotropin reduce inflammation- and stress-induced anhedonia in rats

Emerging evidence implicates impaired self-regulation of the hypothalamic-pituitary-adrenal (HPA) axis and inflammation as important and closely related components of the pathophysiology of major depression. Antidepressants show anti-inflammatory effects and are suggested to enhance glucocorticoid feedback inhibition of the HPA axis. HPA axis activity is also negatively self-regulated by the adrenocorticotropic hormone (ACTH), a potent anti-inflammatory peptide activating five subtypes of melanocortin receptors (MCRs). There are indications that ACTH-mediated feedback can be activated by noncorticotropic N-terminal ACTH fragments such as a potent anti-inflammatory MC1/3/4/5R agonist α-melanocyte-stimulating hormone (α-MSH), corresponding to ACTH(1-13), and a MC3/5R agonist ACTH(4-10). We investigated whether intraperitoneal administration of rats with these peptides affects anhedonia, which is a core symptom of depression. Inflammation-related anhedonia was induced by a single intraperitoneal administration of a low dose (0.025mg/kg) of lipopolysaccharide (LPS). Stress-related anhedonia was induced by the chronic unpredictable stress (CUS) procedure. The sucrose preference test was used to detect anhedonia. We found that ACTH(4-10) pretreatment decreased LPS-induced increase in serum corticosterone and tumor necrosis factor (TNF)-α, and a MC3/4R antagonist SHU9119 blocked this effect. Both α-MSH and ACTH(4-10) alleviated LPS-induced anhedonia. In the CUS model, these peptides reduced anhedonia and normalized body weight gain. The data indicate that systemic α-MSH and ACTH(4-10) produce an antidepressant-like effect on anhedonia induced by stress or inflammation, the stimuli that trigger the release of ACTH and α-MSH into the bloodstream. The results suggest a counterbalancing role of circulating melanocortins in depression and point to a new approach for antidepressant treatment.

Melanocortin peptides: potential targets in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease resulting in loss of self-tolerance with multiple organs, such as the kidney, skin, joints, and the central nervous system (CNS), being targeted. Numerous immunosuppressant therapies are currently being used for the treatment of SLE, but their clinical utility is somewhat variable because of the clinical heterogeneity. Melanocortins are a family of peptides derived from the common precursor protein pro-opiomelanocortin. These multifunctional peptides activate five subtypes of melanocortin receptors expressed on immune, skin, muscle, bone, and kidney cells and cells within the CNS. Melanocortin peptides have demonstrated a variety of biologic actions including immunomodulation, melanogenesis, and renoprotection. This review aims to introduce the melanocortin system and explore the mechanisms by which they may be beneficial in diseases such as SLE.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Astrocytes: new targets of melanocortin 4 receptor actions

Astrocytes exert a wide variety of functions with paramount importance in brain physiology. After injury or infection, astrocytes become reactive and they respond by producing a variety of inflammatory mediators that help maintain brain homeostasis. Loss of astrocyte functions as well as their excessive activation can contribute to disease processes; thus, it is important to modulate reactive astrocyte response. Melanocortins are peptides with well-recognized anti-inflammatory and neuroprotective activity. Although melanocortin efficacy was shown in systemic models of inflammatory disease, mechanisms involved in their effects have not yet been fully elucidated. Central anti-inflammatory effects of melanocortins and their mechanisms are even less well known, and, in particular, the effects of melanocortins in glial cells are poorly understood. Of the five known melanocortin receptors (MCRs), only subtype 4 is present in astrocytes. MC4R has been shown to mediate melanocortin effects on energy homeostasis, reproduction, inflammation, and neuroprotection and, recently, to modulate astrocyte functions. In this review, we will describe MC4R involvement in anti-inflammatory, anorexigenic, and anti-apoptotic effects of melanocortins in the brain. We will highlight MC4R action in astrocytes and discuss their possible mechanisms of action. Melanocortin effects on astrocytes provide a new means of treating inflammation, obesity, and neurodegeneration, making them attractive targets for therapeutic interventions in the CNS.

Melanocortins in brain inflammation: the role of melanocortin receptor subtypes

The melanocortins (MC) are released from neurons and paracrine cells in the CNS where they are involved in important physiological functions, including regulation of body temperature and immune responses. MC bind to melanocortin receptors, a class of cell surface G-protein-coupled receptors. Of the five subtypes of MC receptors that have been cloned in mammals, the MC1, MC3, MC4 and MC5 receptors are expressed in brain tissues. Expression of MC receptors in both brain cells and cells of the immune system suggests direct involvement of MC in regulation of inflammatory processes in the brain. The binding of MC to MC receptors induces activation of adenylate cyclase, increase in intracellular cAMP level and, consequently, inhibition of the nuclear transcription factor kappaB (NF-κB) signalling. Inflammatory processes contribute to development of severe CNS diseases, both in acute and chronic conditions. Thus far, the anti-inflammatory effects of MC in the CNS have been mainly studied using peptides that are relatively unselective for individual MC receptor subtypes. Consequently, these studies do not allow identification of specific MC receptor(s) involved in the regulation of inflammatory processes. However, recently synthesized ligands selective for individual MC receptors indicated that both MC4 and MC3 agonists are promising anti-inflammatory agents in treatment of brain inflammation.

Identification of a physiological role for leptin in the regulation of ambulatory activity and wheel running in mice

Mechanisms regulating spontaneous physical activity remain poorly characterized despite evidence of influential genetic and acquired factors. We evaluated ambulatory activity and wheel running in leptin-deficient ob/ob mice and in wild-type mice rendered hypoleptinemic by fasting in both the presence and absence of subcutaneous leptin administration. In ob/ob mice, leptin treatment to plasma levels characteristic of wild-type mice acutely increased both ambulatory activity (by 4,000 ± 200 beam breaks/dark cycle, P < 0.05) and total energy expenditure (TEE; by 0.11 ± 0.01 kcal/h during the dark cycle, P < 0.05) in a dose-dependent manner and acutely increased wheel running (+350%, P < 0.05). Fasting potently increased ambulatory activity and wheel running in wild-type mice (AA: +25%, P < 0.05; wheel running: +80%, P < 0.05), and the effect of fasting was more pronounced in ob/ob mice (AA: +400%, P < 0.05; wheel running: +1,600%, P < 0.05). However, unlike what occurred in ad libitum-fed ob/ob mice, physiological leptin replacement attenuated or prevented fasting-induced increases of ambulatory activity and wheel running in both wild-type and ob/ob mice. Thus, plasma leptin is a physiological regulator of spontaneous physical activity, but the nature of leptin's effect on activity is dependent on food availability.

Organic chemistry and biology: chemical biology through the eyes of collaboration

From a scientific perspective, efforts to understand biology including what constitutes health and disease has become a chemical problem. However, chemists and biologists "see" the problems of understanding biology from different perspectives, and this has retarded progress in solving the problems especially as they relate to health and disease. This suggests that close collaboration between chemists and biologists is not only necessary but essential for progress in both the biology and chemistry that will provide solutions to the global questions of biology. This perspective has directed my scientific efforts for the past 45 years, and in this overview I provide my perspective of how the applications of synthetic chemistry, structural design, and numerous other chemical principles have intersected in my collaborations with biologists to provide new tools, new science, and new insights that were only made possible and fruitful by these collaborations.

Alpha-melanocyte-stimulating hormone and related tripeptides: biochemistry, antiinflammatory and protective effects in vitro and in vivo, and future perspectives for the treatment of immune-mediated inflammatory diseases

Alpha-MSH is a tridecapeptide derived from proopiomelanocortin. Many studies over the last few years have provided evidence that alpha-MSH has potent protective and antiinflammatory effects. These effects can be elicited via centrally expressed melanocortin receptors that orchestrate descending neurogenic antiinflammatory pathways. alpha-MSH can also exert antiinflammatory and protective effects on cells of the immune system and on peripheral nonimmune cell types expressing melanocortin receptors. At the molecular level, alpha-MSH affects various pathways implicated in regulation of inflammation and protection, i.e., nuclear factor-kappaB activation, expression of adhesion molecules and chemokine receptors, production of proinflammatory cytokines and mediators, IL-10 synthesis, T cell proliferation and activity, inflammatory cell migration, expression of antioxidative enzymes, and apoptosis. The antiinflammatory effects of alpha-MSH have been validated in animal models of experimentally induced fever; irritant and allergic contact dermatitis, vasculitis, and fibrosis; ocular, gastrointestinal, brain, and allergic airway inflammation; and arthritis, but also in models of organ injury. One obstacle limiting the use of alpha-MSH in inflammatory disorders is its pigmentary effect. Due to its preserved antiinflammatory effect but lack of pigmentary action, the C-terminal tripeptide of alpha-MSH, KPV, has been delineated as an alternative for antiinflammatory therapy. KdPT, a derivative of KPV corresponding to amino acids 193-195 of IL-1beta, is also emerging as a tripeptide with antiinflammatory effects. The physiochemical properties and expected low costs of production render both agents suitable for the future treatment of immune-mediated inflammatory skin and bowel disease, fibrosis, allergic and inflammatory lung disease, ocular inflammation, and arthritis.

Cold-seeking behavior as a thermoregulatory strategy in systemic inflammation

Systemic inflammation (SI) is a leading cause of hospital death. Although fever and hypothermia are listed as symptoms in every definition of SI, how SI affects thermoregulatory behavior is unclear. SI is often modeled by systemic administration of bacterial lipopolysaccharide (LPS) to rats. When rats are not allowed to regulate their body temperature (Tb) behaviorally, LPS causes either fever or hypothermia, and the direction of the response is determined by LPS dose and ambient temperature (Ta). However, in many studies in which rats were allowed to regulate Tb behaviorally (by selecting their preferred Ta in a thermogradient apparatus), they consistently expressed warmth-seeking behavior and developed fever. We hypothesized that SI can cause not only warmth-seeking behavior but also cold-seeking behavior; we then tested this hypothesis by studying LPS-induced thermoregulatory behavior in adult Wistar rats. A multichannel thermogradient apparatus, implantable data loggers and infrared thermography were used; multiple control experiments were conducted; and the ability of the apparatus to reliably register the changes in rats' preferred Ta induced by thermal (external cooling or heating) or chemical (TRPV1 or TRPM8 agonist) stimuli was confirmed. The rats responded to a low dose of LPS (10 microg/kg i.v.) with warmth-seeking behavior and a polyphasic fever, but to a high dose (5 mg/kg i.v.) with marked cold-seeking behavior and hypothermia followed by warmth-seeking behavior and fever. This is the first well-controlled study to report SI-associated cold-seeking behavior in rats. Cold-seeking behavior is likely to be an important defense response in severe SI.

Studies on the physiological functions of the melanocortin system

The melanocortin system refers to a set of hormonal, neuropeptidergic, and paracrine signaling pathways that are defined by components that include the five G protein-coupled melanocortin receptors; peptide agonists derived from the proopiomelanocortin preprohormone precursor; and the endogenous antagonists, agouti and agouti-related protein. This signaling system regulates a remarkably diverse array of physiological functions including pigmentation, adrenocortical steroidogenesis, energy homeostasis, natriuresis, erectile responses, energy homeostasis, and exocrine gland secretion. There are many complex and unique aspects of melanocortin signaling, such as the existence of endogenous antagonists, the agouti proteins, that act at three of the five melanocortin receptors. However, there is an aspect of melanocortin signaling that has facilitated highly reductionist approaches aimed at understanding the physiological functions of each receptor and peptide: in contrast to many peptides, the melanocortin agonists and antagonists are expressed in a limited number of very discrete locations. Similarly, the melanocortin receptors are also expressed in a limited number of discrete locations where they tend to be involved in rather circumscribed physiological functions. This review examines my laboratory's participation in the cloning of the melanocortin receptors and characterization of their physiological roles.

Targeting melanocortin receptors as potential novel therapeutics

Adrenocorticotrophic hormone (ACTH(1-39)) and the melanocortins (alpha, beta and gamma-melanocyte-stimulating hormone [MSH]) are derived from a larger precursor molecule known as the pro-opiomelanocortin (POMC) protein. They exert their numerous biological effects by activating 7 transmembrane G-protein coupled receptors (GPCR), leading to adenylyl cyclase activation and subsequent cAMP accumulation within the target cell. To date, 5 melanocortin receptors (MCR) have been identified and termed MC1R to MC5R, they have been shown to have a wide and varied distribution throughout the body, being found in the central nervous system (CNS), periphery and immune cells. Melanocortins have a multitude of actions including: (i) modulating disease pathologies including arthritis, asthma, obesity; (ii) affecting functions, for example erectile dysfunction, skin tanning; and (iii) organ systems, for example cardiovascular system. Recently a mechanistic approach has been identified with alpha-MSH preventing NF-kappaB activation via the preservation and expression of IkappaBalphaprotein. This leads to a reduction of pro-inflammatory mediators including cytokines and inhibition of adhesion molecule expression, with subsequent reduction in leukocyte emigration. Development of selective ligands with an appropriate pharmacokinetic profile will enable a pharmacological evaluation of the potential beneficial effects of the melanocortins. In this review I have discussed the potential mechanistic action for the melanocortins and some of the disease pathologies shown to be modulated. This review proposes targeting the MCR with the ultimate aim of controlling many of the diseases that we face today.

Peripheral influences on central melanocortin neurons

The melanocortins are peptide products of post-translational processing of the pro-opiomelanocortin precursor protein. Melanocortin-expressing 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.

Pituitary adenylate cyclase-activating polypeptide (PACAP) mimics neuroendocrine and behavioral manifestations of stress: Evidence for PKA-mediated expression of the corticotropin-releasing hormone (CRH) gene

The physiologic response to stress is highly dependent on the activation of corticotropin-releasing hormone (CRH) neurons by various neurotransmitters. A particularly rich innervation of hypophysiotropic CRH neurons has been detected by nerve fibers containing the neuropeptide PACAP, a potent activator of the cAMP-protein kinase A (PKA) system. Intracerebroventricular (icv) injections of PACAP also elevate steady-state CRH mRNA levels in the paraventricular nucleus (PVN), but it is not known whether PACAP effects can be associated with acute stress responses. Likewise, in cell culture studies, pharmacologic activation of the PKA system has stimulated CRH gene promoter activity through an identified cAMP response element (CRE); however, a direct link between PACAP and CRH promoter activity has not been established. In our present study, icv injection of 150 or 300 pmol PACAP resulted in robust phosphorylation of the transcription factor CREB in the majority of PVN CRH neurons at 15 to 30 min post-injection and induced nuclear Fos labeling at 90 min. Simultaneously, plasma corticosterone concentrations were elevated in PACAP-injected animals, and significant increases were observed in face washing, body grooming, rearing and wet-dog shakes behaviors. We investigated the effect of PACAP on human CRH promoter activity in alphaT3-1 cells, a PACAP-receptor expressing cell line. Cells were transiently transfected with a chloramphenicol acetyltransferase (CAT) reporter vector containing region - 663/+124 of the human CRH gene promoter then treated for with PACAP (100 nM) or with the adenylate cyclase activating agent, forskolin (2.5 muM). Both PACAP and forskolin significantly increased wild-type hCRH promoter activity relative to vehicle controls. The PACAP response was abolished in the CRE-mutant construct. Pretreatment of transfected cells with the PKA blocker, H-89, completely prevented both PACAP- and forskolin-induced increases in CRH promoter activity. Furthermore, CREB overexpression strongly enhanced PACAP-mediated stimulation of hCRH promoter activity, an effect which was also lost with mutation of the CRE. Thus, we demonstrate that icv PACAP administration to rats under non-stressed handling conditions leads to cellular, hormonal and behavioral responses recapitulating manifestations of the acute stress response. Both in vivo and in vitro data point to the importance of PACAP-mediated activation of the cAMP/PKA signaling pathway for stimulation of CRH gene transcription, likely via the CRE.

Attenuation of lipopolysaccharide anorexia by antagonism of caudal brain stem but not forebrain GLP-1-R

The central glucagon-like peptide-1 (GLP-1) system has been implicated in the control of feeding behavior. Here we explore GLP-1 mediation of the anorexic response to administration of systemic LPS and address the relative importance of caudal brain stem and forebrain GLP-1 receptor (GLP-1-R) for the mediation of the response. Fourth-intracerebroventricular delivery of the GLP-1-R antagonist exendin-(9-39) (10 microg) did not itself affect food intake in the 24 h after injection but significantly attenuated the otherwise robust (approximately 60%) reduction in food intake obtained after LPS (100 microg/kg) treatment. This result highlights a role for caudal brain stem GLP-1-R in the mediation of LPS anorexia but does not rule out the possibility that forebrain receptors also contribute to the response. Forebrain contribution was addressed by delivery of the GLP-1-R antagonist to the third ventricle with the caudal flow of cerebrospinal fluid blocked by occlusion of the cerebral aqueduct. Exendin-(9-39) delivery thus limited to forebrain did not attenuate the anorexic response to LPS. These data suggest that LPS anorexia is mediated, in part, by release of the native peptide acting on GLP-1-R within the caudal brain stem.

De novo design, synthesis, and pharmacology of alpha-melanocyte stimulating hormone analogues derived from somatostatin by a hybrid approach

A number of alpha-melanotropin (alpha-MSH) analogues have been designed de novo, synthesized, and bioassayed at different melanocortin receptors from frog skin (fMC1R) and mouse/rat (mMC1R, rMC3R, mMC4R, and mMC5R). These ligands were designed from somatostatin by a hybrid approach, which utilizes a modified cyclic structure (H-d-Phe-c[Cys---Cys]-Thr-NH(2)) related to somatostatin analogues (e.g. sandostatin) acting at somatostatin receptors, CTAP which binds specifically to micro opioid receptors, and the core pharmacophore of alpha-MSH (His-Phe-Arg-Trp). Ligands designed were H-d-Phe-c[XXX-YYY-ZZZ-Arg-Trp-AAA]-Thr-NH(2) [XXX and AAA = Cys, d-Cys, Hcy, Pen, d-Pen; YYY = His, His(1'-Me), His(3'-Me); ZZZ = Phe and side chain halogen substituted Phe, d-Phe, d-Nal(1'), and d-Nal(2')]. The compounds showed a wide range of bioactivities at the frog skin MC1R; e.g. H-d-Phe-c[Hcy-His-d-Phe-Arg-Trp-Cys]-Thr-NH(2) (6, EC(50) = 0.30 nM) and H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-d-Cys]-Thr-NH(2) (8, EC(50) = 0.10 nM). In addition, when a lactam bridge was used as in H-d-Phe-c[Asp-His-d-Phe-Arg-Trp-Lys]-Thr-NH(2) (7, EC(50) = 0.10 nM), the analogue obtained is as potent as alpha-MSH in the frog skin MC1R assay. Interestingly, switching the bridge of 6 to give H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-Hcy]-Thr-NH(2) (5, EC(50) = 1000 nM) led to a 3000-fold decrease in agonist activity. An increase in steric size in the side chain of d-Phe(7) reduced the bioactivity significantly. For example, H-d-Phe-c[Cys-His-d-Nal(1')-Arg-Trp-d-Cys]-Thr-NH(2) (24) is 2000-fold less active than 9. On the other hand, H-d-Phe-c[Cys-His-d-Phe(p-I)-Arg-Trp-d-Cys]-Thr-NH(2) (23) lost all agonist activity and became a weak antagonist (IC(50) = 1 x 10(-5) M). Furthermore, the modified CTAP analogues with a d-Trp at position 7 all showed weak antagonist activities (EC(50) = 10(-6) to 10(-7) M). Compounds bioassayed at mouse/rat MCRs displayed intriguing results. Most of them are potent at all four receptors tested (mMC1R, rMC3R, mMC4R, and mMC5R) with poor selectivities. However, two of the ligands, H-d-Phe-c[Cys-His-d-Phe-Arg-Trp-Pen]-Thr-NH(2) (9, EC(50) = 6.9 x 10(-9) M, 6.4 x 10(-8) M, 2.0 x 10(-8) M, and 1.4 x 10(-10) M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) and H-d-Phe-c[Cys-His(3'-Me)-d-Phe-Arg-Trp-Cys]-Thr-NH(2) (16, EC(50) = 3.5 x 10(-8) M, 3.1 x 10(-8) M, 8.8 x 10(-9) M, and 5.5 x 10(-10) M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) showed significant selectivities for the mMC5R. Worthy of mention is that neither of these two ligands is potent in the frog skin MC1R assay (EC(50) = 10(-7) M for 9 and EC(50) = 10(-5) M for 16). These results clearly demonstrated that binding behaviors in rodent MCRs are quite different from those in the classical frog skin (R pipiens) assay.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

Roles of the melanocortin-4 receptor in antipyretic and hyperthermic actions of centrally administered α-MSH

Activation of central melanocortin receptors (MCR) inhibits fever but can also stimulate thermogenesis, and the mechanisms involved are unknown. To determine whether the long-recognized antipyretic effect of exogenous alpha-MSH is mediated by the melanocortin-4 receptor (MC4R), and what thermoeffector systems are involved, we tested the effects of intracerebroventricular (i.c.v.) injection of alpha-MSH on lipopolysaccharide (LPS, 30 microg/kg i.p.)-induced fever in rats, in the presence and absence of the selective MC4R antagonist HS014. Treatment with alpha-MSH (1 microg, i.c.v.) suppressed LPS-induced increases in core body temperature (Tc), whereas a lower dose (300 ng) was ineffective. Nevertheless, both alpha-MSH doses effectively inhibited LPS-induced peripheral vasoconstriction, the principal heat-conserving thermoeffector, as determined by changes in tail skin temperature (Tsk). This implies that the net antipyretic effect of alpha-MSH cannot be accounted for solely by modulation of heat loss effectors, but also involves other mechanisms. Surprisingly, central MC4-R blockade by coinjected HS014 (1 microg) not only prevented, but reversed the effect of alpha-MSH (1 microg) on Tc, thus resulting in augmented LPS-induced fever. In afebrile rats, alpha-MSH infusion caused a modest transient increase in Tc that was blocked by coinjected HS014, but was not accompanied by altered Tsk. Overall, the results support the hypothesis that the MC4R mediates the antipyretic effects of alpha-MSH. Paradoxically, in the presence of pharmacological MC4-R blockade during fever, exogenous alpha-MSH can exacerbate fever, probably by acting via other central MCR subtype(s). In normal animals, centrally injected alpha-MSH exerts a hyperthermic effect that is mediated by the MC4R, consistent with recent evidence that MC4R activation promotes energy expenditure in normal states through stimulation of thermogenesis.

Neuronal expression of caspase-1 immunoreactivity in the rat central nervous system

Caspase-1/interleukin-1beta (IL-1beta)-converting enzyme (ICE) cleaves IL-1beta and IL-18 precursor proteins to the active forms of these proinflammatory cytokines. Since both cytokines are constitutively expressed in the brain, we investigated whether this is also the case for caspase-1. Using an antibody raised against the p10-subunit of the active enzyme, constitutive expression of caspase-1 immunoreactivity was found in nerve cells in the arcuate nucleus and in nerve fibres throughout the brain. Co-localisation with alpha-melanocyte stimulating hormone was demonstrated. The distribution pattern of caspase-1 immunoreactive structures is consistent with a role to produce mature IL-1beta in regions where IL-1beta mediates fever and sleep.

Minireview: From anorexia to obesity--the yin and yang of body weight control

Over the past decade, there has been a tremendous increase in the understanding of the molecular and neural mechanisms that control food intake and body weight. Yet eating disorders and cachexia are still common, and obesity cases are rising at alarming rates. Thus, despite recent progress, an increased understanding of the molecular and neural substrates that control body weight homeostasis is a major public health goal. In this review, we discuss the mechanisms by which metabolic signals interact with key behavioral, neuroendocrine, and autonomic regulatory regions of the central nervous system. Additionally, we offer a model in which hormones such as leptin and ghrelin interact with similar central nervous system circuits and engage them in such a way as to maintain an appropriate and tight regulation of body weight and food intake. Our model predicts that overstimulation or understimulation of these central pathways can result in obesity, anorexia, or cachexia.

The role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception

The melanocortins are a family of bioactive peptides derived from proopiomelanocortin. Those peptides, included among hormones and comprising ACTH, alpha-MSH, beta-MSH and gamma-MSH, are best known mainly for their physiological effects, such as the control of skin pigmentation by alpha-MSH, and ACTH effects on pigmentation and steroidogenesis. Melanocortins are released in various sites in the central nervous system and in peripheral tissues, and participate in the regulation of multiple physiological functions. They are involved in grooming behavior, food intake and thermoregulation processes, and can also modulate the response of the immune system in inflammatory states. Research of the past decade provided evidence that melanocortins could elicit their diverse biological effects by binding to a distinct family of G protein-coupled receptors with seven transmembrane domains. To date, five melanocortin receptor genes have been cloned and characterized. Those receptors differ in their tissue distribution and in their ability to recognize various melanocortins. These advances have opened up new horizons for exploring the significance of melanocortins, their ligands and their receptors for a variety of important physiological functions. We reviewed the origin of MSH peptides, the function and distribution of melanocortin receptors and their endogenous and exogenous ligands and the role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception. Moreover, we analyzed their interaction with opioid peptides and finally, we discussed the postulated role of the melanocortin system in pain transmission at the spinal cord level.

Activation of central melanocortin-4 receptor suppresses lipopolysaccharide-induced fever in rats

Activation of central melanocortin receptors (MCR) inhibits fever, but the identity of the MCR subtype(s) mediating this antipyretic effect is unknown. To determine whether selective central melanocortin receptor-4 (MC4R) activation produces antipyretic effects, the MC4R selective agonist MRLOB-0001 (CO-His-d-Phe-Arg-Trp-Dab-NH(2)) was administered intracerebroventricularly to rats treated with Escherichia coli lipopolysaccharide (LPS, 30 microg/kg ip). Treatment with MRLOB-0001 (150 ng icv) did not lower core body temperature (T(c)) in afebrile rats but did suppress LPS-induced increases in T(c) and associated decreases in tail skin temperature (T(sk)), an indicator of vasomotor thermoeffector function. In contrast, systemic treatment with MRLOB-0001 (150 ng iv) did not produce similar antipyretic effects. Coadministration of the selective MC4R antagonist HS014 (1 microg icv) blocked the antipyretic effects of MRLOB-0001. HS014 alone (1 microg icv) had no significant effect on LPS-induced increases in T(c) or decreases in T(sk) and in afebrile rats had no significant effects on T(c) or T(sk). We conclude that pharmacological activation of central MC4R suppresses febrile increases in T(c) and that inhibition of heat conservation pathways may contribute to this effect. These findings suggest that the central MC4R may mediate the long-recognized antipyretic effects of centrally administered melanocortins.

The central melanocortin system and fever

Fever is a phylogenetically ancient response that is mounted upon exposure of the host to pathogens or inflammatory agents. Melanocortin agonists act centrally to inhibit fever by acting at receptors, including the melanocortin-4 receptor, which is prominently expressed in key hypothalamic thermoregulatory centers. Furthermore, endogenous melanocortins act centrally as physiological modulators of fever, recruited during the febrile response to restrain its intensity. Functionally, these actions lie at the interface between the anti-inflammatory effects of melanocortins, which involve suppression of the synthesis and actions of proinflammatory cytokines, and the central control of thermoregulation. Considering the extensive neuroanatomic and functional overlaps between central pathways and peripheral effectors involved in thermoregulation and energy balance, it is not surprising that melanocortins have been found to influence the metabolic economy profoundly in pathological as well as normal states. For example, despite suppressing endotoxin-induced fever, endogenous melanocortins appear to mediate the associated anorexia, a classic component of the "illness syndrome" accompanying acute infections, and promote a negative energy balance. The thermoregulatory actions of melanocortins are in several respects functionally opposed, and are remarkably dependent on physiological state, indicating that responsiveness to melanocortins is a physiologically modulated variable. Elucidating the anti-inflammatory and thermoregulatory roles of central melanocortin receptors during inflammatory states may lead to novel pharmacotherapeutic targets based on selective targeting of melanocortin receptor subtypes, for clinical benefit in human disease states involving neuroinflammatory components and metabolic wasting.

Agouti-related protein stimulates the hypothalamic-pituitary-adrenal (HPA) axis and enhances the HPA response to interleukin-1 in the primate

alpha-MSH antagonizes many of the immune and neuroendocrine effects induced by inflammatory cytokines. Studies have shown that alpha-MSH attenuates the stimulatory effect of IL-1 on the hypothalamic-pituitary-adrenal (HPA) axis and plays a physiological role in limiting the HPA response to IL-1. Recently an alpha-MSH antagonist, agouti-related protein (AGRP), has been identified in the hypothalamus, which stimulates food intake by antagonizing the effects of alpha-MSH at specific melanocortin receptors. It is unknown whether AGRP can also modulate neuroendocrine responses to inflammatory cytokines. We have therefore examined the effects of AGRP on the HPA axis and on prolactin (PRL) at baseline and in response to stimulation by IL-1 beta in nine ovariectomized rhesus monkeys. In the first study, the effects of intracerebroventricular (i.c.v) infusion of 20 microg (n = 6) and 50 micro g (n = 4) of human AGRP (83-132)-NH(2) were compared with icv saline infusion. There was a significant stimulatory effect of 20 microg AGRP on cortisol release over time (P < 0.001). The area under the hormone response curve (AUC) for cortisol increased by 29% after 20 microg AGRP vs. saline; the AUC for ACTH increased by 166% (P = 0.028); the AUC for PRL increased by 108% (P = 0.046). There was a significant stimulatory effect of 50 microg AGRP on ACTH (P < 0.001), cortisol (P < 0.001), and PRL (P < 0.001) release over time. The AUC for ACTH after 50 microg AGRP increased by 98%; the AUC for cortisol increased by 37%; the AUC for PRL increased by 161%. The effects of AGRP on ACTH, cortisol, and PRL release were prevented by alpha-MSH infusion. In the second study, animals received icv either 50 ng of human IL-1 beta or 20 microg of AGRP followed by 50 ng IL-1 beta. AGRP significantly enhanced the ACTH (P < 0.05) response to IL-1 beta. The peak ACTH response to IL-1 beta alone was 124 +/- 55 pg/ml vs. 430 +/- 198 pg/ml after IL-1 beta plus AGRP; the peak cortisol response was 70 +/- 8.2 microg/dl vs. 77 +/- 6.2 microg/dl, but this was not significantly different. In conclusion, AGRP stimulated ACTH, cortisol, and PRL release in the monkey and enhanced the ACTH response to IL-1 beta. These studies suggest that, in addition to its known orexigenic effects, AGRP may play a role in neuroendocrine regulation and specifically that AGRP may interact with alpha-MSH to modulate neuroendocrine responses to inflammation.

Expression of melanocortin 4 receptor mRNA in the central nervous system of the rat

The melanocortin 4 receptor (MC4-R) plays a pivotal role in maintaining energy homeostasis in rodents and humans. For example, MC4-R deletion or mutation results in obesity, hyperphagia, and insulin resistance. Additionally, subsets of leptin-induced autonomic responses can be blocked by melanocortin receptor antagonism, suggesting that MC4-R-expressing neurons are downstream targets of leptin. However, the critical autonomic control sites expressing MC4-Rs are still unclear. In the present study, we systematically examined the distribution of MC4-R mRNA in the adult rat central nervous system, including the spinal cord, by using in situ hybridization histochemistry (ISHH) with a novel cRNA probe. Autonomic control sites expressing MC4-R mRNA in the hypothalamus included the anteroventral periventricular, ventromedial preoptic, median preoptic, paraventricular, dorsomedial, and arcuate nuclei. The subfornical organ, dorsal hypothalamic, perifornical, and posterior hypothalamic areas were also observed to express MC4-R mRNA. Within extrahypothalamic autonomic control sites, MC4-R-specific hybridization was evident in the infralimbic and insular cortices, bed nucleus of the stria terminalis, central nucleus of the amygdala, periaqueductal gray, lateral parabrachial nucleus, nucleus of the solitary tract, dorsal motor nucleus of the vagus (DMV), and intermediolateral nucleus of the spinal cord (IML). By using dual-label ISHH, we confirmed that the cells expressing MC4-R mRNA in the IML and DMV were autonomic preganglionic neurons as cells in both sites coexpressed choline acetyltransferase mRNA. The distribution of MC4-R mRNA is consistent with the proposed roles of central melanocortin systems in feeding and autonomic regulation.

Synthesis and biological evaluation on hMC3, hMC4 and hMC5 receptors of gamma-MSH analogs substituted with L-alanine

To elucidate the molecular basis of the interaction of the native dodecapeptide gamma-MSH with the melanocortin receptors, we performed a structure-activity study in which we systematically replaced l-Ala in each position of this peptide. Here we report the binding affinity and agonist potency on human MC3R, MC4R and MC5R. Intracellular cAMP concentration was measured on CHO cells, and binding assays were carried out using membranes prepared from these cell lines which stably express hMC3R, hMC4R and hMC5R. Our results indicate that the last four amino acids in the C-terminal region of gamma-MSH are not important determinants of biological activity and selectivity at human melanocortin receptors. Interesting results were obtained when l-Ala was substituted for His6, Phe7, Arg8 and Trp9. For these peptides, the affinity and activity at all three human receptors (MC3R, MC4R and MC5R) decreased significantly, demonstrating that the His-Phe-Arg-Trp sequence in gamma-MSH is important for activity at these three melanocortin receptors. Similar results were obtained when Met3 was replaced with l-Ala, suggesting the importance of this position in the interaction with all three receptors. This study highlights the role played by the His-Phe-Arg-Trp sequence in receptor binding and in agonist activity of gamma-MSH.

Design and synthesis of highly potent and selective melanotropin analogues of SHU9119 modified at position 6

The melanocortin receptors are involved in several important physiological functions. The potent and enzymatically stable analogues MT-II (Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH(2)) and SHU9119 (Ac-Nle-c[Asp-His-DNal(2')-Arg-Trp-Lys]-NH(2)) are important ligands of these receptors but are relatively nonselective. To differentiate between the physiological functions of these receptors, agonists, and antagonists with improved receptor selectivities are needed. We report here analogues of the well-characterized antagonist SHU9119 in which we replaced His(6) with conformationally constrained amino acids. By this structure-activity study we discovered two important compounds, PG-901 (Ac-Nle(4)-c[Asp(5)-Pro(6)-DNal(2')(7)-Arg(8)-Trp(9)-Lys(10)]-NH(2)) and PG-911 (Ac-Nle(4)-c[Asp(5)-Hyp(6)-DNal(2')(7)-Arg(8)-Trp(9)-Lys(10)]-NH(2)), characterized to be full agonists at the hMC5R (EC(50) = 0.072 nM and 0.031 nM, respectively), but full antagonists at the hMC3R and the hMC4R. We also demonstrated that the relative stereochemistry of the amino acid at the 6-position is critical for activity, and could play an important role in potency as well as in selectivity for the melanocortin receptors.

Effects of alpha-MSH on kainic acid induced changes in core temperature in rats

The effects of intraperitoneal (i.p.) administration of kainic acid (KA) and alpha-melanocyte-stimulating hormone (alpha-MSH) alone or in combination, on core temperature of freely moving rats were examined. KA or saline was administered once (10 mg/kg) and alpha-MSH or saline was given repeatedly i.e. 10 min before and 10, 30 and 60 min after the administration of saline or KA. Two doses of alpha-MSH were used: 0.5 and 2.5 mg/kg. KA alone produced a biphasic effect on core temperature, i.e. an initial short-lasting hypothermia followed by hyperthermia that lasted about 6 h. The higher dose of alpha-MSH had a potentiating effect on KA-induced hypothermia, while the lower dose of alpha-MSH increased the hyperthermia produced by KA. alpha-MSH administered alone produced a late (3 h), dose-dependent increase in core temperature. It is conceivable that repeated administration of alpha-MSH in the doses used in our study may cause a cumulative effect in raising body temperature for a limited period of time. The previously described interactions between KA and alpha-MSH, respectively, with dopaminergic and serotoninergic systems may account for the effects on core temperature in rats observed in our study.

Calcitonin induces IL-6 production via both PKA and PKC pathways in the pituitary folliculo-stellate cell line

It has been demonstrated that calcitonin-binding sites are present in a variety of tissue types, including in the pituitary gland. Interleukin-6 (IL-6) is also produced in the pituitary and it regulates the secretion of various hormones. In this study, we examined the expression of the calcitonin receptor and the mechanism of IL-6 production induced by calcitonin in the pituitary folliculo-stellate cell line (TtT/GF). The mRNA of calcitonin receptor subtype C1a, but not that of C1b, was detected by RT-PCR in TtT/GF cells and in the normal mouse pituitary. Calcitonin increased cAMP accumulation and IL-6 production in a concentration-dependent manner in TtT/GF cells. As calcitonin activates the PKA and PKC pathways, we investigated the contributions of PKA and PKC to IL-6 production. IL-6 production was only slightly increased by either 8-bromo-cAMP (1 mM) or phorbol 12-myristate 13-acetate (100 nM) alone. However, IL-6 was synergistically induced in the presence of both 8-bromo-cAMP (1 mM) and phorbol 12myristate 13-acetate (100 nM). Furthermore, calcitonin-induced IL-6 production was completely suppressed by H-89 (PKA inhibitor) or GF109203X (PKC inhibitor), indicating that the activation of both PKA and PKC is necessary for calcitonin-induced IL-6 production. On the other hand, pertussis toxin (G(i)/G(o) signaling inhibitor) treatment achieved an approximately 9-fold increase in calcitonin-induced IL-6 production. These results show that calcitonin-stimulated IL-6 production is mediated via both PKA- and PKC-signaling pathways, whereas calcitonin also suppresses IL-6 production by activating G(i)/G(o) proteins in folliculo-stellate cells.

Effect of LPS administration on the expression of POMC, NPY, galanin, CART and MCH mRNAs in the rat hypothalamus

Anorexia and weight loss are manifestations of inflammation seen both in patients and in experimental animal models such as the lipopolysaccharide (LPS)-treated rat. Using in situ hybridization, the levels of mRNAs encoding proopiomelanocortin (POMC), neuropeptide Y (NPY), galanin, melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART) were investigated in the rat hypothalamus after a single intraperitoneal dose (125 microg/kg) of LPS. Four hours after LPS injection the food intake was significantly decreased. POMC and CART mRNA levels were increased in the arcuate nucleus, and MCH, CART and galanin mRNAs were all decreased in the lateral hypothalamic area in LPS-treated rats. Levels of mRNAs for NPY and galanin in the arcuate nucleus, and for MCH and CART in the zona incerta did not change significantly after LPS treatment. These findings support the hypothesis that LPS-induced factors mediate signalling to the POMC/CART neurons in the arcuate nucleus which could lead to reduced food intake by decreasing MCH, CART and galanin synthesis in target lateral hypothalamic neurons.

The role of alpha-MSH as a modulator of cutaneous inflammation

Among various neuropeptides such as substance P, calcitonin gene-related peptide and others, alpha-melanocyte-stimulating hormone (alpha-MSH) was found to be produced in the skin. Moreover, melanocortin receptor 1 (MC-1R), which is specific for alpha-MSH and ACTH, is expressed in the skin on keratinocytes, dendritic cells, macrophages and endothelial cells. In monocytes, macrophages and dendritic cells alpha-MSH inhibits the production and activity of immunoregulatory and proinflammatory cytokines such as IL-2, IFN-gamma, TNF-alpha and IL-1. It downregulates the expression of costimulatory molecules such as CD86 and CD40 and induces the production of suppressor factors such as the cytokine synthesis inhibitory factor IL-10. On endothelial cells alpha-MSH is capable of downregulating the LPS-induced expression of adhesion molecules such as vascular cell adhesion molecule (VCAM) and E-selectin. Moreover, the LPS-induced activation of transcription factors such as NF kappa B is downregulated by alpha-MSH. In a mouse model i.v. or topical application of alpha-MSH was found to inhibit the induction phase as well as the effector phase of contact hypersensitivity (CHS) reactions and to induce hapten-specific tolerance. These findings indicate that the production of immunosuppressing neuropeptides such as alpha-MSH by epidermal cells may play an essential role during the pathogenesis of immune and inflammatory reactions in the skin.

Antagonism of the melanocortin system reduces cold and mechanical allodynia in mononeuropathic rats

The presence of both pro-opiomelanocortin-derived peptides and melanocortin (MC) receptors in nociception-associated areas in the spinal cord suggests that, at the spinal level, the MC system might be involved in nociceptive transmission. In the present study, we demonstrate that a chronic constriction injury (CCI) to the rat sciatic nerve, a lesion that produces neuropathic pain, results in changes in the spinal cord MC system, as shown by an increased binding of (125)I-NDP-MSH to the dorsal horn. Furthermore, we investigated whether intrathecal administration (in the cisterna magna) of selective MC receptor ligands can affect the mechanical and cold allodynia associated with the CCI. Mechanical and cold allodynia were assessed by measuring withdrawal responses of the affected limb to von Frey filaments and withdrawal latencies upon immersion in a 4.5 degrees C water bath, respectively. We show that treatment with the MC receptor antagonist SHU9119 has a profound anti-allodynic effect, suggesting that the endogenous MC system has a tonic effect on nociception. In contrast, administration of the MC4 receptor agonists MTII and d-Tyr-MTII primarily increases the sensitivity to mechanical and cold stimulation. No antinociceptive action was observed after administration of the selective MC3 receptor agonist Nle-gamma-MSH. Together, our data suggest that the spinal cord MC system is involved in neuropathic pain and that the effects of MC receptor ligands on the responses to painful stimuli are exerted through the MC4 receptor. In conclusion, antagonism of the spinal melanocortin system might provide a new approach in the treatment of neuropathic pain.

Endogenous antipyretics

Fever is the hallmark of the stereotyped host response to microbial infection, although it is just one of a number of high-risk strategies employed by the infected host to clear itself of invading pathogens. The febrile response is accompanied by activation of multiple endogenous antipyretic systems that serve to suppress its magnitude or duration. These include neuroactive substances of neural and humoral origin, some of which (e.g., glucocorticoids, melanocortins, and IL-10) have broad-ranging anti-inflammatory actions. Glucocorticoids, vasopressin, and melanocortins appear to exert their antipyretic effects by acting on receptors within the brain, but beyond this the mechanisms involved are unknown. It is hypothesized, but not proven, that endogenous antipyretic systems protect the host against the destructive consequences of unchecked fever. Importantly, pharmacological blockade of the actions of endogenous antipyretic systems increases fevers of even low to moderate intensity. Therefore, in addition to protecting against catastrophic consequences of high fever, endogenous antipyretic systems seem to play a fundamental physiological role in determining the normal course of fever. Elucidating the neural and biochemical mechanisms involved in suppression of fever by physiological antipyretic systems will yield a rich benefit, both by advancing the basic understanding of host defense strategies, and by permitting the design of novel antipyretic and anti-inflammatory strategies for therapeutic intervention in human disease.

alpha-Melanocyte stimulating hormone prevents fasting-induced suppression of corticotropin-releasing hormone gene expression in the rat hypothalamic paraventricular nucleus

During fasting, corticotropin-releasing hormone (CRH) mRNA decreases in the hypothalamic paraventricular nucleus (PVN), but the mechanism by which this takes place is not well understood. To test the hypothesis that the melanocortin system may be involved in the regulation of CRH mRNA in the PVN during fasting, the effect of intracerebroventricularly administered alpha-melanocyte stimulating hormone (MSH) on CRH mRNA in the PVN was studied in fasted animals by in situ hybridization histochemistry. Whereas fasting suppressed CRH mRNA levels in the PVN, alpha-MSH at doses of 150 and 300 ng every 6 h for 64 h prevented the fasting-induced suppression of CRH gene expression in the PVN. These data indicate that the suppression of alpha-MSH synthesis may be responsible for the decreased CRH gene expression in the PVN during fasting.

Centrally administered MTII affects feeding, drinking, temperature, and activity in the Sprague-Dawley rat

MTII, an agonist of melanocortinergic receptors, is a well-documented anorexigenic agent in rats. Many investigators have reported its effects on feeding without considering concurrent alterations in other behaviors. Accordingly, we performed studies to simultaneously measure nocturnal feeding, drinking, activity, and temperature of rats after intracerebroventricular (third ventricle) administration of a wide dose range of MTII (0.05-500 ng). We observed that MTII modulates these physiological parameters in a dose-dependent manner. Low doses of MTII (0.05 ng) caused reductions in feeding without alterations in body temperature, drinking, or activity. In contrast, hyperthermia and disrupted drinking patterns, along with food intake reductions, were evident at doses exceeding 50 ng. The fact that low doses altered only feeding, whereas higher doses affected a range of parameters, suggests that certain melanocortin-induced behavioral changes may be mediated by distinct populations of melanocortin receptors with varying affinities or that those changes seen at higher doses may be nonspecific in nature.

Differential role of melanocortins in mediating leptin's central effects on feeding and reproduction

Leptin serves as a humoral link coupling the status of energy reserves to the functional activity of the reproductive system. Leptin is thought to act through melanocortinergic pathways in the brain to regulate ingestive behaviors; however, whether melanocortins mediate leptin's actions on the neuroendocrine-reproductive axis is unknown. We tested this hypothesis first by determining whether the effects of leptin on feeding behavior and reproduction in the ob/ob mouse could be blocked by the melanocortin receptor (MC-R) antagonist SHU9119 and second, by examining the effects of the MC-R agonist MTII on feeding and the endocrine-reproductive system. Administered by intracerebroventricular injections, leptin inhibited food intake, raised plasma gonadotropin levels, and increased seminal vesicle weights compared with controls; SHU9119 (intracerebroventricularly) attenuated leptin's effects on food intake and body weight but did not alter leptin's stimulatory effect on the reproductive axis. MTII (intracerebroventricularly and intraperitoneally) decreased food intake and increased body temperature compared with controls but had no effect on the reproductive-endocrine axis. These results suggest that although leptin acts centrally through melanocortinergic pathways to inhibit ingestive behaviors and stimulate metabolism, leptin's activational effect on the reproductive axis is likely to be mediated by other, unknown neuroendocrine circuits.

Inhibition of peripheral NF-kappaB activation by central action of alpha-melanocyte-stimulating hormone

With the rise in the field of neuroimmunomodulation research, there is increased recognition of the influence of the nervous system and neuropeptides in peripheral disease. The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuroimmunomodulatory agent that modulates production of proinflammatory cytokines and inhibits peripheral inflammation via actions on CNS receptors. We examined whether central alpha-MSH operates by inhibiting activation of the nuclear factor kappa B (NF-kappaB) that is essential to the expression of proinflammatory cytokines and development of inflammation in the periphery. Electrophoretic mobility shift assays of nuclear extracts from the murine foot pad injected with TNF-alpha demonstrated that centrally administered alpha-MSH does inhibit NF-kappaB activation. Western blot analysis revealed that this inhibition was linked to central alpha-MSH-induced preservation of expression of IkappaBalpha protein in the peripheral tissue. The NF-kappaB and IkappaBalpha effects were inhibited in mice with spinal cord transection. Intraperitoneal (i.p.) injection of the nonspecific beta-adrenergic receptor blocker propranolol, and of a specific beta2-adrenergic receptor antagonist, likewise prevented these effects of central alpha-MSH; blockade of cholinergic, alpha-adrenergic, or beta1-adrenergic receptors did not. Centrally administered alpha-MSH inhibited peripheral NF-kappaB activation and IkappaBalpha degradation even in mice with nonfunctional melanocortin 1 receptors (MC1R). These findings indicate that alpha-MSH can act centrally to inhibit NF-kappaB activation in peripheral acute inflammation via a descending neural pathway. The pathway involves beta2-adrenergic receptors, but does not require activation of MC1R within the brain.

The proopiomelanocortin system

POMC (31,000 MW) is localized to the pituitary, brain, skin, and other peripheral sites. The particular enzyme profile present within a cell dictates the nature of the hormonal ligand (melanocortin) synthesized and secreted: melanotropic peptides (alpha-MSH beta-lipotropin, lambda-MSH), corticotropin (ACTH), several endorphins (e.g., met-enkephalin). These POMC-derived peptides mediate their actions through typical seven-spanning membrane receptors (MCRs; MCR1, 2, 3, 4, and 5). A specific melanocortin acting on a specific MCR regulates a particular biological response; for example, alpha-MSH on MCR1 increases melanogenesis within melanocytes, ACTH on MCR2 increases cortisol production within adrenal zona fasciculata cells. Within the brain melanocortins regulate satiety (MCR4) and erectile activity (MCR?). MCRs have been localized by melanocortin macromolecular probes, for example, fluorescent to human epidermal melanocytes and also to keratinocytes, suggesting that systemic melanocortins or localized POMC products might regulate these integumental cellular elements in synchrony to enhance skin pigmentation and/or immunological responses. Superpotent, prolonged acting melanotropic peptides have been synthesized and their application in clinical medicine has been demonstrated. MCR antagonists have been used to discover and further delineate other roles of melanocortin ligands. For example, melanocortin-induced satiety can be antagonized by a melanocortin antagonist. Defects in melanocortin ligand biosynthesis, secretion, and melanocortin receptor function can lead to a diverse number of pathological states.

Two important systems in energy homeostasis: melanocortins and melanin-concentrating hormone

Our understanding of the regulation of appetite and energy balance has advanced significantly over the past decade as several peptides, centrally or peripherally expressed, have been characterized and shown to profoundly influence food intake and energy expenditure. (1)The growing number of putative appetite-regulating neuropeptides includes peptides that are orexigenic (appetite-stimulating) signals and anorectic peptides. Neuropeptide Y (NPY), melanin concentrating hormone (MCH), orexins A and B, galanin, and agouti -related peptide (AgRP) all act to stimulate feeding while alpha-melanocyte stimulating hormone (alphaMSH), corticotropin releasing hormone (CRH), cholecystokinin (CCK), cocaine and amphetamine regulated transcript (CART), neurotensin, glucagon-like peptide 1 (GLP 1), and bombesin have anorectic actions.(1) Leptin, expressed in the periphery in white adipose tissue, acts in the CNS to modulate the expression of several of these hypothalamic peptides.(1) This creates a functional link between the adipose tissue and the brain that translates the information on body fat provided by leptin to input into energy balance regulating processes. In the current review we examine the significant role of the melanocortin system (alphaMSH, agouti and AgRP peptides, and their receptors and mahogany protein) and melanin concentrating hormone in the regulation of energy balance.

Characterization of melanocortin receptor ligands on cloned brain melanocortin receptors and on grooming behavior in the rat

Since the melanocortin MC3 and melanocortin MC4 receptors are the main melanocortin receptor subtypes expressed in rat brain, we characterized the activity and affinity of nine melanocortin receptor ligands using these receptors in vitro, as well as their activity in a well-defined melanocortin-induced behavior in the rat: grooming behavior. We report here that [D-Tyr4]melanotan-II and RMI-2001 (Ac-cyclo-[Cys4, Gly5, D-Phe7, Cys10]alpha-MSH-NH2) have significantly higher affinity and potency on the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 receptor. Nle-gamma-MSH (melanocyte-stimulating hormone) was the only ligand with higher affinity and potency on the rat melanocortin MC3 receptor. The potency order of melanocortin MC4 receptor agonists, but not that of melanocortin MC3 receptor agonists, fitted with the potency of these ligands to stimulate grooming behavior, when administered intracerebroventricularly. SHU9119 (Ac-cyclo-[Nle4, Asp5, D-Nal(2)7, Lys10]alpha-MSH-(4-10)-NH2) and RMI-2005 (Ac-cyclo-[Cys4, Gly5, D-Na](2)7, Nal(2)9, Cys10]alpha-MSH-(4-10)-NH2) were able to inhibit alpha-MSH-induced melanocortin receptor activity in vitro, as well as alpha-MSH-induced grooming behavior. Melanotan-II, [Nle4-D-Phe7]alpha-MSH and RMI-2001 were also effective in inducing grooming behavior when administered intravenously. In the absence of purely selective melanocortin MC(3/4) receptor ligands, we demonstrated that careful comparison of ligand potencies in vitro with ligand potencies in vivo, could identify which melanocortin receptor subtype mediated alpha-MSH-induced grooming behavior. Furthermore, blockade of novelty-induced grooming behavior by SHU9119 demonstrated that this physiological stress response is mediated via activation of the melanocortin system.