Identification of a receptor for gamma melanotropin and other proopiomelanocortin peptides in the hypothalamus and limbic system

Molecular cloning of the chicken melanocortin 2 (ACTH)-receptor gene

The chicken melanocortin 2-receptor (MC2-R) gene was isolated. It is found to be a single copy gene encoding a 357 amino acid protein, sharing 65.8-68.7% identity with mammalian counterparts. The chicken MC2-R mRNA is expressed in the adrenal and spleen, suggesting that the receptor mediates both endocrine and immunoregulatory functions of ACTH in the chicken. The amino acid sequence of the chicken MC2-R is collinear with those of other subtypes of MC-R, whereas all cloned mammalian MC2-Rs contain a gap in the third intracellular loop, suggesting that mammalian MC2-R molecules have evolved by lacking a part of the domain which determines the specificity of signal transduction in G-protein coupled receptors. Interestingly, the codon usage differs dramatically between MC1-R and MC2-R in the chicken; the GC-contents at the third codon position in MC1-R and MC2-R are 94.6 and 50.6%, respectively. It may reflect selective constraints on the usage of synonymous codons.

Melanocortin-4 receptor mRNA expression in the developing autonomic and central nervous systems

MC4-R mRNA expression was investigated in fetal rats (E14-E20) using in situ hybridisation. The autonomic nervous system showed the highest levels of MC4-R mRNA expression. In the spinal cord, dense signal was seen over the future intermediolateral cell column and dorsal horn. In the brain, MC4-R was expressed by E14 in diencephalon neuroepithelia, telencephalon, lamina terminalis and spinal trigeminal nucleus and was expressed by E19 throughout many regions of the brain.

Discovery of novel melanocortin4 receptor selective MSH analogues

1. We synthesized a novel series of cyclic melanocyte stimulating hormone (MSH) analogues and tested their binding properties on cells transiently expressing the human melanocortin1 (MC1), MC3, MC4 and MC5 receptors. 2. We discovered that compounds with 26 membered rings of [Cys4,D-Nal7,Cys11]alpha-MSH(4-11) displayed specific MC4 receptor selectivity. The preference order of the different MC receptor subtypes for the novel [Cys4D-Nal7Cys11]alpha-MSH(4-11) analogues are distinct from all other known MSH analogues, particularly as they bind the MC4 receptor with high and the MC1 receptor with low relative affinities. 3. HS964 and HS014 have 12 and 17 fold MC4/MC3 receptor selectivity, respectively, which is much higher than for the previously described cyclic lactam and [Cys4,Cys10]alpha-MSH analogues SHU9119 and HS9510. 4. HS964 is the first substance showing higher affinity for the MC5 receptor than the MC1 receptor. 5. HS014, which was the most potent and selective MC4 receptor ligand (Ki 3.2 nM, which is approximately 300 fold higher affinity than for alpha-MSH), was also demonstrated to antagonize alpha-MSH stimulation of cyclic AMP in MC4 receptor transfected cells. 6. We found that a compound with a 29 membered ring of [Cys3,Nle10,D-Nal7,Cys11]alpha-MSH(3-11) (HS010) had the highest affinity for the MC3 receptor. 7. This is the first study to describe ligands that are truly MC4 selective and a ligand having a high affinity for the MC3 receptor. The novel compounds may be of use in clarifying the physiological roles of the MC3, MC4 and MC5 receptors.

Selective antagonist for the melanocortin 4 receptor (HS014) increases food intake in free-feeding rats

Recently, we discovered a cyclic analogue of MSH (melanocyte stimulating hormone), HS014, which is the first selective antagonist of the MC4 receptor. We have here studied the effects of this peptide on food intake in non-deprived male rats. Vehicle or five doses of HS014 (0.1-10 nmol) were administered ICV at midday. HS014 (0.33-3.3 nmol) significantly and in a dose-dependent manner increased food intake for the first 1 h. At 4 h after the injections, food intake was also significantly increased in rats treated with 1 and 3.3 nmol of HS014, whereas the lowest dose tested (0.1 nmol) was without effect. Cumulative food intake increased to 100% at 4 h after the injections. The highest dose of HS014 (10 nmol) induced sedation and inhibited feeding for first hour of testing. However, this dose also increased food consumption later. These data demonstrate that attenuation of central melanocortinergic tone with HS014 induces disinhibition of feeding and provides additional evidence for the hypothesis that activation of the MC4 receptor inhibits food intake. HS014 may be a useful tool for elucidating the role of the MC receptor subtypes in vivo. This is the first report demonstrating an increase in daytime food intake in free-feeding animals caused by a MC receptor active agent.

Prevention of reflex natriuresis after acute unilateral nephrectomy by melanocortin receptor antagonists

gamma-Melanocyte-stimulating hormone (gamma-MSH), atrial natriuretic peptide (ANP), and oxytocin have been identified as candidate hormonal mediators of the reflex natriuresis that follows acute unilateral nephrectomy (AUN). Pharmacological characterization of the third melanocortin receptor (MC3-R) indicates that it uniquely responds to physiological concentrations of gamma-MSH. We tested the roles of gamma-MSH, ANP, and oxytocin in the postnephrectomy natriuresis by carrying out AUN during continuous intrarenal infusion of specific antagonists for their cognate receptors. In anesthetized Sprague-Dawley rats, urinary sodium excretion (UNaV) increased from 0.34 +/- 0.04 to 1.12 +/- 0.11 mu eq/min 90 min after AUN (P < 0.001). No change in UNaV occurred in rats undergoing a sham AUN procedure. Plasma immunoreactive gamma-MSH concentration was 53 +/- 8 fmol/ml after sham AUN but 112 +/- 17 fmol/ml after AUN (P < 0.01). SHU-9119 and SHU-9005 are substituted derivatives of alpha-MSH with potent antagonism at the MC3-R in vitro. Infusion of these compounds at 5 pmol/min completely blocked the natriuretic response to AUN despite a similar elevation in plasma gamma-MSH (111 +/- 12 vs. 49 +/- 8 fmol/ml in sham rats, P < 0.01). Intrarenal infusion of the ANP receptor antagonist A-71915 (5 pmol/min) or the oxytocin receptor antagonist [d(CH2)(5)1, Tyr(Me)2,Orn8] vasotocin (10 pmol/min) effectively inhibited the natriuresis induced by intravenous infusion of ANP or oxytocin (each at 1 pmol/min), respectively, but did not block the natriuresis after AUN. Plasma immunoreactivity of these peptides was not increased after AUN. These results indicate that reflex natriuresis after AUN is accompanied by an increase in plasma gamma-MSH but not ANP or oxytocin concentration and is prevented by intrarenal infusion of receptor antagonists with selectivity for MC3-R. The data indicate that gamma-MSH or a closely related peptide mediates postnephrectomy natriuresis and provide further support for the possibility that gamma-MSH may play a wider role in sodium homeostasis.

Differentiation of rat hypothalamic dopaminergic neurons is stimulated in vitro by target cells: the melanotrophs

We have investigated in vitro the influence of pituitary intermediate lobe melanotrophs on the differentiation of their afferent hypothalamic dopaminergic neurons. The presence of melanotrophs in primary cultures of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unchanged) and induces a stimulation of their neuritic outgrowth. These effects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with intermediate lobe cells from newborn rats. Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alphaMSH) in the co-culture or conditioned medium, or by application of the peptide itself, we demonstrate that the neuritotrophic effect on dopaminergic neurons is mediated by alphaMSH, the main secretory product of melanotrophs, whereas the inductive effect on the number of dopaminergic neurons is attributable to another diffusible neurotrophic factor(s) present in foetal, but not adult, adenohypophysis. Similar effects are observed on cultures of newborn hypothalamic neurons. However, at this stage of neuronal development, alphaMSH also increases the number of dopaminergic neurons, which could be due to a change of neuronal receptivity. We show that the neuritotrophic influence of alphaMSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addition, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotrophs in situ. These findings indicate that the differentiation of dopaminergic hypothalamic neurons is influenced by the target cells, melanotrophs, and that this trophic influence implicates alphaMSH.

Melanin-concentrating hormone: a functional melanocortin antagonist in the hypothalamus

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and alpha-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, alpha-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of alpha-MSH. alpha-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block alpha-MSH binding or the ability of alpha-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain alpha-MSH receptor subtypes. These data suggest that MCH and alpha-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.

The role of central melanocortin receptors in the activation of the hypothalamus-pituitary-adrenal-axis and the induction of excessive grooming

1. In accord with previous studies intracerebroventricular (i.c.v.) injections of ACTH1-24 (1 microg) induced a display of excessive grooming, and increased the plasma concentrations of ACTH and corticosterone. Pituitary-adrenal activation was blocked by pretreatment with dexamethasone, indicating that the effect of the (i.c.v.) injected peptide was not caused by a peripheral effect on the adrenal cortex. 2. Doses of 1 and 3 microg of a non-selective melanocortin-3/4-receptor antagonist (SHU 9119), or of 5 and 10 microg of a selective melanocortin-4-receptor antagonist ([D-Arg8]ACTH4-10), coadministered (i.c.v.) with 1 microg ACTH1-24, inhibited the ACTH1-24-induced activation of the hypothalamus-pituitary-adrenal-axis and excessive grooming. 3. In addition, several doses of the selective melanocortin-3-receptor agonist Lys-gamma2-MSH were centrally administered, but neither neuroendocrine, nor excessive grooming responses were observed. 4. These results imply that the melanocortin-4-receptor, and not the melanocortin-3-receptor, is involved in the ACTH1-24-induced rise in plasma levels of ACTH and corticosterone, and excessive grooming.

The neuropharmacology of yawning

Yawning is a phylogenetically old, stereotyped event that occurs alone or associated with stretching and/or penile erection in humans and in animals from reptiles to birds and mammals under different conditions. Although its physiological function is still unknown, yawning is under the control of several neurotransmitters and neuropeptides at the central level as this short overview of the literature on the neurochemistry of yawning shows. Among these substances, the best known are dopamine, excitatory amino acids, acetylcholine, serotonin, nitric oxide, adrenocorticotropic hormone-related peptides and oxytocin, that facilitate yawning and opioid peptides that inhibit this behavioral response. Some of the above compounds interact in the paraventricular nucleus of the hypothalamus to control yawning. This hypothalamic nucleus contains the cell bodies of oxytocinergic neurons projecting to extra-hypothalamic brain areas that play a key role in the expression of this behavioral event. When activated by dopamine, excitatory amino acids and oxytocin itself, these neurons facilitate yawning by releasing oxytocin at sites distant form the paraventricular nucleus, i.e. the hippocampus, the pons and/or the medulla oblongata. Conversely, activation of these neurons by dopamine, oxytocin or excitatory amino acids, is antagonized by opioid peptides, that, in turn, prevent the yawning response. The activation and inhibition, respectively of these oxytocinergic neurons is related to a concomitant increase and decrease, respectively, of paraventricular nitric oxide synthase activity. However, other neuronal systems in addition to the central paraventricular oxytocinergic neurons are involved in the control of yawning, since they do not seem to be involved in the expression of yawning induced by the stimulation of acetylcholine or serotoninergic receptors, nor by adrenocorticotropic hormone (ACTH) and related peptides. Nitric oxide is also involved in the induction of yawning by the latter compounds and neuronal links, for instance between dopamine and acetylcholine and dopamine and serotonin, seem to be involved in the yawning response. Finally, other neurotransmitters, i.e. gamma-aminobutyric acid (GABA) and noradrenaline, and neuropeptides, i.e. neurotensin and luteinizing hormone-releasing hormone (LH-RH), influence this behavioral response. In conclusion, in spite of some recent progress, little is known of, and more has to be done to identify, the neurochemical mechanisms underlying yawning at the central level.

Different developmental patterns of melanocortin MC3 and MC4 receptor mRNA: predominance of Mc4 in fetal rat nervous system

Melanocortins are thought to be involved in neuronal development and regeneration. Pro-opiomelanocortin (POMC), the precursor of alpha-melanocyte stimulating hormone (alpha-MSH), gamma-MSH, ACTH, and beta-endorphin, becomes detectable in rat hypothalamic neurons from gestational day (E) 12.5. We recently described stage- and region-specific ontogenetic patterns of binding sites for the alpha-MSH analogue [125I]-Nle4,D-Phe7-alpha-MSH ([125I]-NDP), with the first localizations in epithalamus and sympathetic chain at E13. [125I]-NDP binds to all known melanocortin receptors, including MC3-R and MC4-R, the predominant melanocortin receptors in nervous system. To identify the receptor type expressed during ontogeny, the developmental pattern of MC3-R and MC4-R mRNA was investigated by in situ hybridization in fetuses and offspring of time-pregnant Long Evans rats between E14 and postnatal day (P) 27. MC4-R mRNA was found to be the predominant species during the entire fetal period. It was localized in all fetal areas exhibiting distinct [125I]-NDP binding, starting with sympathetic ganglia and epithalamus (E14), and including sensory trigeminal nuclei (E16), dorsal motor nucleus of vagus (E16) and cranial nerve ganglia, inferior olive (E18) and cerebellum (E18), striatal regions (E16), and entorhinal cortex (E22). In contrast, MC3-R mRNA was detectable only in the postnatal period, with a fast increase in expression in the ventromedial and arcuate nuclei. The early presence of MC4-R mRNA in central and peripheral nervous system and transient regional peaks of mRNA expression, often concomitant with periods of neural network formation, suggest a role of this receptor type in early ontogeny. The MC3 receptor may be involved in analogous processes during postnatal development.

Melanocortin and MCH precursor-derived NEI effects on striatum-midbrain co-cultures

The possibility of developmental effects of POMC-derived melanocortins and analogs on neurons of fetal rat brain regions exhibiting marked developmental melanocortin receptor expression, was studied in serum-free co-cultures of gestational day 18 striatal and mesencephalic cells, and compared with NEI and NGE. These two peptide fragments of the melanin concentrating hormone precursor, occurring in brain areas devoid of POMC terminals, cross-react with alpha-MSH antibodies; NEI elicits grooming similar to alpha-MSH. Neurofilament protein (NF), growth-associated protein (GAP-43) and synaptophysin of the synaptosomal fraction were determined by ELISA as markers for neuritogenesis, growth cones, and nerve terminal differentiation. Cell survival was analyzed by MTT assay, proportions of major cell types by immunocytochemistry. alpha-Melanocyte-stimulating hormone (alpha-MSH, effective concentration 250-2500 nM), the analog Nle4-, D-Phe7-alpha-MSH (NDP, 3.1-750 nM), and NEI (250 nM) increased NF in 3 day cultures by 11%, 17%, and 22%, respectively, whereas ACTH(1-24) and ACTH(1-39) (25 2500 nM) were ineffective. In 11 day cultures, alpha-MSH (250-750 nM), but not NDP, ACTH(1-24) or ACTH(1-39), increased synaptosomal synaptophysin by 11%. GAP-43 and cell survival remained unaffected. These data indicate that selected melanocortins as well as NEI can influence differentiation of neural processes in brain neurons.

Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides

The effects of pituitary-derived melanocortin peptides are primarily attributed to ACTH-mediated adrenocortical glucocorticoid production. Identification of a widely distributed receptor for ACTH/MSH peptides, the melanocortin-5 receptor (MC5-R), suggested non-steroidally mediated systemic effects of these peptides. Targeted disruption of the MC5-R produced mice with a severe defect in water repulsion and thermoregulation due to decreased production of sebaceous lipids. High levels of MC5-R was found in multiple exocrine tissues, including Harderian, preputial, lacrimal, and sebaceous glands, and was also shown to be required for production and stress-regulated synthesis of porphyrins by the Harderian gland and ACTH/MSH-regulated protein secretion by the lacrimal gland. These data show a requirement for the MC5-R in multiple exocrine glands for the production of numerous products, indicative of a coordinated system for regulation of exocrine gland function by melanocortin peptides.

Phage display selection on whole cells yields a peptide specific for melanocortin receptor 1

A phage display system for the selection of peptides binding to heterologously expressed human melanocortin receptor 1 on the surface of insect cells has been established. It could be shown that phage particles displaying the natural ligand alpha-melanocyte-stimulating hormone bind selectively to cells expressing this receptor and that these phages exhibit biological activity on mouse B16F1 melanoma cells. Insect cells were superior to other cell lines tested and have been used to select binders from a small library, in which critical determinants (Phe7-Arg8-Trp9) were kept, whereas the flanking regions where allowed to variate freely. One peptide displaying little similarity with native hormone was found that binds to the receptor also in its free form with an affinity of 7 nM. It showed a remarkable selectivity for this receptor, because it binds to the other melanocortin receptor subtypes with a maximum affinity of 21 microM. This is the first time phage display has been used successfully with G-protein-coupled receptors lacking an extracellular binding domain.

Antagonism of central melanocortin receptors in vitro and in vivo by agouti-related protein

Expression of Agouti protein is normally limited to the skin where it affects pigmentation, but ubiquitous expression causes obesity. An expressed sequence tag was identified that encodes Agouti-related protein, whose RNA is normally expressed in the hypothalamus and whose levels were increased eightfold in ob/ob mice. Recombinant Agouti-related protein was a potent, selective antagonist of Mc3r and Mc4r, melanocortin receptor subtypes implicated in weight regulation. Ubiquitous expression of human AGRP complementary DNA in transgenic mice caused obesity without altering pigmentation. Thus, Agouti-related protein is a neuropeptide implicated in the normal control of body weight downstream of leptin signaling.

Identification of an obesity quantitative trait locus on mouse chromosome 2 and evidence of linkage to body fat and insulin on the human homologous region 20q

Chromosomal synteny between the mouse model and humans was used to map a gene for the complex trait of obesity. Analysis of NZB/BINJ x SM/J intercross mice located a quantitative trait locus (QTL) for obesity on distal mouse chromosome 2, in a region syntenic with a large region of human chromosome 20, showing linkage to percent body fat (likelihood of the odds [LOD] score 3.6) and fat mass (LOD score 4.3). The QTL was confirmed in a congenic mouse strain. To test whether the QTL contributes to human obesity, we studied linkage between markers located within a 52-cM region extending from 20p12 to 20q13.3 and measures of obesity in 650 French Canadian subjects from 152 pedigrees participating in the Quebec Family Study. Sib-pair analysis based on a maximum of 258 sib pairs revealed suggestive linkages between the percentage of body fat (P < 0.004), body mass index (P < 0.008), and fasting insulin (P < 0.0005) and a locus extending approximately from ADA (the adenosine deaminase gene) to MC3R (the melanocortin 3 receptor gene). These data provide evidence that a locus on human chromosome 20q contributes to body fat and insulin in a human population, and demonstrate the utility of using interspecies syntenic relationships to find relevant disease loci in humans.

The role of the agouti gene in the yellow obese syndrome

The yellow obese syndrome in mice encompasses many pleiotropic effects including yellow fur, maturity-onset obesity, hyperinsulinemia, insulin resistance, hyperglycemia, increased skeletal length and lean body mass, and increased susceptibility to neoplasia. The molecular basis of this syndrome is beginning to be unraveled and may have implications for human obesity and diabetes. Normally, the agouti gene is expressed during the hair-growth cycle in the neonatal skin where it functions as a paracrine regulator of pigmentation. The secreted agouti protein antagonizes the binding of the alpha-melanocyte-stimulating hormone to its receptor (melanocortin 1 receptor) on the surface of hair bulb melanocytes, causing alterations in intracellular cAMP levels. Widespread, ectopic expression of the mouse agouti gene is central to the yellow obese phenotype, as demonstrated by the molecular cloning of several dominant agouti mutations and the ubiquitous expression of the wild-type agouti gene in transgenic mice. Recent experiments have revealed that the hypothalamus and adipose tissue are biologically active target sites for agouti in the yellow obese mutant lines.

Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R

[Nle4, DPhe7]-alpha-MSH (NDP-MSH), a highly potent analogue of alpha-melanocyte-stimulating hormone (alpha-MSH), possesses nanomolar efficacies at all the melanocortin receptor subtypes except the MC2R. Evaluation of the melanocortin "message" sequence of [Nle4, DPhe7]-alpha-MSH was performed on the human melanocortin receptor subtypes designated hMC1, hMC3R, hMC4R, and hMC5R. Tetrapeptides and tripeptides were stereochemically modified to explore topochemical preferences at these receptors and to identify lead peptides possessing agonist activity and subtype selectivity. Four peptides were discovered to only bind to the hMC1 and hMC4 receptor subtypes. The tetrapeptide Ac-His-DPhe-Arg-Trp-NH2 (1) possessed 0.6 microM binding affinity at the hMC1R, 1.2 microM binding affinity at the hMC4R, and agonist activity at both receptors. The tripeptides Ac-DPhe-Arg-Trp-NH2 (6) and Ac-DPhe-Arg-DTrp-NH2 (7) possessed 2.0 and 9.1 microM binding affinities, respectively, only at the hMC4R, and both compounds effected agonist activity. The tetrapeptide Ac-His-Phe-Arg-DTrp-NH2 (4) possessed 6.3 microM affinity and full agonist activity at the hMC1R, while only binding 7% at the hMC3R, 36% at the hMC4R, and 11% at the hMC5R at a maximal concentration of 10 microM. These data demonstrate that the His-Phe-Arg-Trp message sequence of the melanocortin peptides does not bind and stimulate each melanocortin receptor in a similar fashion, as previously hypothesized. Additionally, this study identified the simplest structural agonists for the hMC1R and hMC4R receptors reported to date.

Biological and conformational examination of stereochemical modifications using the template melanotropin peptide, Ac-Nle-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH2, on human melanocortin receptors

Examination of conformationally constrained melanotropin peptide (Ac-Nle4-c[Asp5-His-Phe7-Arg-Trp9-Ala-Lys]-NH2) on four human melanotropin receptors (hMC1R, hMC3R, hMC4R, and hMC5R) resulted in identifying the importance of ligand stereochemistry at positions 5, 7, and 9 for agonist binding affinity and receptor selectivity. A trend in ligand structure-activity relationships emerged for these peptides, with the hMC1R and hMC4R possessing similar tendencies, as did the hMC3R and hMC5R. alpha-MSH (Ac-Ser-Tyr-Ser-Met4-Glu-His-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2), NDP-MSH (Ac-Ser-Tyr-Ser-Nle4-Glu-His-D-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2), and MTII (Ac-Nle4-c[Asp5,D-Phe7,Lys10]-alpha-MSH(4-10)-NH2) were also examined at each of these melanocortin receptors. Interestingly, the linear NDP-MSH possessed greater binding affinity for the hMC3R and hMC5R than did the cyclic analogue MTII. The peptide Ac-Nle-c[Asp-His-Phe-Arg-D-Trp9-Ala-Lys]-NH2 demonstrated the greatest differentiation in binding affinity between the hMC1R and hMC4R (78-fold). Analogue Ac-Nle-c[Asp-His-Phe7-Arg-Trp-Ala-Lys]-NH2 resulted in micromolar binding affinity (or greater) at the hMC3R and hMC5R, demonstrating the importance of D-Phe7 for ligand binding potency at these receptors. Ac-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH2 resulted in loss of binding affinity at the hMC5R, implicating the importance of Nle4 (or a hydrophobic residue in this position) for binding to this receptor. Ac-Nle-c[D-Asp5-His-Phe-Arg-Trp-Ala-Lys]-NH2 was unable to competitively displace [125I]NDP-MSH binding at micromolar concentrations on the hMC3R and hMC5R, suggesting the importance of chirality of Asp5 either for ligand-receptor interactions or for orientation of the side chain lactam bridge and the structural integrity of the peptide conformation. Energy calculations performed for these peptides resulted in the identification of a low-energy ligand conformer family that is common to all the ligands. The differences in ligand binding affinities observed in this study are postulated to be a result of different ligand-receptor complexed interactions and not solely to the ligand structure.

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

Bacterial infection causes fever, an adaptive but potentially self-destructive response, in the host. Also activated are counterregulatory systems such as the pituitary-adrenal axis. Antipyretic roles have also been postulated for certain endogenous central neuropeptides, including the melanocortins (alpha-MSH-related peptides). To test the hypothesis that endogenous central melanocortins have antipyretic effects mediated by central melanocortin receptors (MCRs), we determined the effect of intracerebroventricular injection of a synthetic MCR antagonist, Ac-Nle4,c-[Asp5,DNal(2')7,Lys10]alpha-MSH(4-10)-NH2 (SHU-9119) in endotoxin-challenged rats. The efficacy and specificity of SHU-9119 as an MCR antagonist in the rat was first validated in vitro and in vivo. In vitro, in heterologous cells expressing either rat MC3-R or MC4-R, the major MCR subtypes expressed in brain, SHU-9119 showed no intrinsic agonism, but it inhibited alpha-MSH-induced cAMP accumulation (IC50 = 0.48 +/- 0.19 and 0.41 +/- 0.28 nM, respectively) and [125I]-[Nle4,DPhe7]-alpha-MSH binding (IC50 = 1.0 +/- 0.1 and 0.9 +/- 0.3 nM, respectively). In vivo, exogenous alpha-MSH (180 pmol) inhibited fever in rats when administered intracerebroventricularly 30 min after Escherichia coli lipopolysaccharide (LPS) (25 microg/kg, i.p.). When co-injected with alpha-MSH, SHU-9119 (168 pmol, i.c.v.) prevented the antipyretic action of exogenous alpha-MSH. In contrast, neither alpha-MSH nor SHU-9119, alone or in combination, affected body temperatures in afebrile rats. In LPS-treated rats, intracerebroventricular injection of SHU-9119 significantly increased fever, whereas intravenous injection of the same dose of SHU-9119 had no effect. Neither intracerebroventricular nor intravenous SHU-9119 significantly affected LPS-stimulated plasma ACTH or corticosterone levels. The results indicate that endogenous central melanocortins exert an antipyretic influence during fever by acting on MCRs located within the brain, independent of any modulation of the activity of the pituitary-adrenal axis.

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

Bacterial infection causes fever, an adaptive but potentially self-destructive response, in the host. Also activated are counterregulatory systems such as the pituitary-adrenal axis. Antipyretic roles have also been postulated for certain endogenous central neuropeptides, including the melanocortins (alpha-MSH-related peptides). To test the hypothesis that endogenous central melanocortins have antipyretic effects mediated by central melanocortin receptors (MCRs), we determined the effect of intracerebroventricular injection of a synthetic MCR antagonist, Ac-Nle4,c-[Asp5,DNal(2')7,Lys10]alpha-MSH(4-10)-NH2 (SHU-9119) in endotoxin-challenged rats. The efficacy and specificity of SHU-9119 as an MCR antagonist in the rat was first validated in vitro and in vivo. In vitro, in heterologous cells expressing either rat MC3-R or MC4-R, the major MCR subtypes expressed in brain, SHU-9119 showed no intrinsic agonism, but it inhibited alpha-MSH-induced cAMP accumulation (IC50 = 0.48 +/- 0.19 and 0.41 +/- 0.28 nM, respectively) and [125I]-[Nle4,DPhe7]-alpha-MSH binding (IC50 = 1.0 +/- 0.1 and 0.9 +/- 0.3 nM, respectively). In vivo, exogenous alpha-MSH (180 pmol) inhibited fever in rats when administered intracerebroventricularly 30 min after Escherichia coli lipopolysaccharide (LPS) (25 microg/kg, i.p.). When co-injected with alpha-MSH, SHU-9119 (168 pmol, i.c.v.) prevented the antipyretic action of exogenous alpha-MSH. In contrast, neither alpha-MSH nor SHU-9119, alone or in combination, affected body temperatures in afebrile rats. In LPS-treated rats, intracerebroventricular injection of SHU-9119 significantly increased fever, whereas intravenous injection of the same dose of SHU-9119 had no effect. Neither intracerebroventricular nor intravenous SHU-9119 significantly affected LPS-stimulated plasma ACTH or corticosterone levels. The results indicate that endogenous central melanocortins exert an antipyretic influence during fever by acting on MCRs located within the brain, independent of any modulation of the activity of the pituitary-adrenal axis.

Obesity, diabetes and functions for proopiomelanocortin-derived peptides

Melanocortin peptides (adrenocorticotropin (ACTH), alpha-,beta-, and gamma-melanocyte stimulating hormone (MSH), and fragments thereof) derived from proopiomelanocortin (POMC) have a diverse array of biological activities, many of which have yet to be fully elucidated. The recent cloning of a family of five distinct melanocortin receptors through which these peptides act has provided the tools to further our understanding of melanocortin peptide functions. Early work on melanocortin peptides focused on their roles in pigmentation, adrenocortical function, the immune, central and peripheral nervous systems. Although melanocortin peptides have long been known to affect lipolysis, characterisation of the melanocortin receptors has opened up several lines of evidence for important roles in the development of obesity, insulin resistance and type II diabetes. We present here a review of the current evidence for melanocortin peptides playing such a role, and based on this evidence, a model for melanocortin peptides and their receptors in maintaining energy balance.

Different cardiovascular profiles of three melanocortins in conscious rats; evidence for antagonism between gamma 2-MSH and ACTH-(1-24)

1. We investigated the effects of [Nle4,D-Phe7]alpha-melanocyte-stimulating hormone (NDP-MSH), adrenocorticotropin-(1-24) (ACTH-(1-24)) and gamma 2-MSH, three melanocortins with different agonist selectivity for the five cloned melanocortin receptors, on blood pressure and heart rate in conscious, freely moving rats following intravenous administration. 2. As was previously found by other investigators as well as by us gamma 2-MSH, a peptide suggested to be an agonist with selectivity for the melanocortin MC3 receptor, caused a dose-dependent, short lasting pressor response in combination with a tachycardia. Despite the fact that NDP-MSH is a potent agonist of various melanocortin receptor subtypes, among which the melanocortin MC1 receptor, it did not affect blood pressure or heart rate, when administered i.v. in doses of up to 1000 nmol kg-1. 3. ACTH-(1-24) caused a dose-dependent decrease in blood pressure in combination with a dose-dependent increase in heart rate in a dose-range from 15 to 500 nmol kg-1. The cardiovascular effects of ACTH-(1-24) were independent of the presence of the adrenals. 4. Pretreatment with ACTH-(1-24) caused a pronounced, dose-dependent parallel shift to the right of the dose-response curve for the pressor and tachycardiac effects of gamma 2-MSH. The antagonistic effect of ACTH-(1-24) was already apparent following a dose of this peptide as low as 10 nmol kg-1, which when given alone had no intrinsic hypotensive activity. 5. These results form further support for the notion that it is not via activation of one of the as yet cloned melanocortin receptors that gamma-MSH-like peptides increase blood pressure and heart rate. The cardiovascular effects of ACTH-(1-24) seem not to be mediated by the adrenal melanocortin MC3 receptors, for which ACTH-(1-24) is a selective agonist, or by adrenal catecholamines. 6. There appears to be a functional antagonism between ACTH-(1-24) and gamma 2-MSH, two melanocortins derived from a common precursor, with respect to their effect on blood pressure and heart rate. Whether this antagonism plays a (patho)physiological role remains to be shown.

Hypothalamic expression of ART, a novel gene related to agouti, is up-regulated in obese and diabetic mutant mice

We have isolated cDNA clones that encode a novel human gene related to agouti. Sequence analysis of this gene, named ART, for agouti-related transcript, predicts a 132-amino-acid protein that is 25% identical to human agouti. The highest degree of identity is within the carboxyl terminus of both proteins. Like agouti, ART contains a putative signal sequence and a cysteine rich carboxyl terminus, but lacks the region of basic residues and polyproline residues found in the middle of the agouti protein. Both agouti and ART contain 11 cysteines, and 9 of these are conserved spatially. ART is expressed primarily in the adrenal gland, subthalamic nucleus, and hypothalamus, with a lower level of expression occurring in testis, lung, and kidney. The murine homolog of ART was also isolated and is predicted to encode a 131-amino-acid protein that shares 81% amino acid identity to humans. The mouse was found to have the same expression pattern as human when assessed by RT-PCR. Examination by in situ hybridization using mouse tissues showed localized expression in the arcuate nucleus of the hypothalamus, the median eminence, and the adrenal medulla. In addition, the hypothalamic expression of ART was elevated approximately 10-fold in ob/ob and db/db mice. ART was mapped to human chromosome 16q22 and to mouse chromosome 8D1-D2. The expression pattern and transcriptional regulation of ART, coupled with the known actions of agouti, suggests a role for ART in the regulation of melanocortin receptors within the hypothalamus and adrenal gland, and implicates this novel gene in the central control of feeding.

Postnatal expression of melanocortin-3 receptor in rat diencephalon and mesencephalon

In situ hybridization was applied to examine the postnatal expression of melanocortin-3 (MC-3) receptor mRNA in the rat brain. Very weak and limited signals were seen in the hypothalamus on postnatal day 0 (P0) and in the dorsal lateral thalamus on P4. A marked increase was noted in several regions of the diencephalon and mesencephalon on P7. The highest levels were reached on P21, which was the time when an adult-like pattern was established. On P21, intense signals were seen in the ventromedial nucleus and the arcuate nucleus of the tuberal hypothalamus, the habenular nucleus of the epithalamus and the ventral tegmental area. [125I] Nle4, D-Phe7-alpha-MSH showed overlapping, but wider labelling of melanocortin receptors, that followed a similar developmental course. alpha-MSH-like immunoreactivity was seen widely in the forebrain and midbrain from P14. In contrast to the staining of alpha-MSH in neurons and their process, gamma 2-MSH-like immunoreactivity was detected strongly in the blood vessels. The neuronal localization of MC-3 receptor mRNA suggests that this receptor may mediate the neurotropic actions of melanocortin peptides in the developing brain.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Role of melanocortinergic neurons in feeding and the agouti obesity syndrome

Dominant alleles at the agouti locus (A) cause an obesity syndrome in the mouse, as a consequence of ectopic expression of the agouti peptide. This peptide, normally only found in the skin, is a high-affinity antagonist of the melanocyte-stimulating hormone receptor (MC1-R), thus explaining the inhibitory effect of agouti on eumelanin pigment synthesis. The agouti peptide is also an antagonist of the hypothalamic melanocortin-4 receptor (MC4-R). To test the hypothesis that agouti causes obesity by antagonism of hypothalamic melanocortin receptors, we identified cyclic melanocortin analogues that are potent agonists or antagonists of the neural MC3 (refs 11, 12) and MC4 receptors. Intracerebroventricular administration of the agonist, MTII, inhibited feeding in four models of hyperphagia: fasted C57BL/6J, ob/ob, and A(Y) mice, and mice injected with neuropeptide Y. Co-administration of the specific melanocortin antagonist and agouti-mimetic SHU9119 completely blocked this inhibition. Furthermore, administration of SHU9119 significantly enhanced nocturnal feeding, or feeding stimulated by a prior fast. Our data show that melanocortinergic neurons exert a tonic inhibition of feeding behaviour. Chronic disruption of this inhibitory signal is a likely explanation of the agouti obesity syndrome.

Brain melanocortin receptors: from cloning to function

The cloning of brain melanocortin (MC) receptors, the mapping of their expression pattern and the identification of MC receptor selective ligands have opened a new avenue towards elucidating the role of the melanocortin system in the brain. MC receptors have now been implicated in melanocortin-induced grooming behavior in rats, in the melanocortin-induced lowering of blood pressure and in the control of weight homeostasis. Functional opioid antagonism and the anti-pyretic and anti-inflammatory effects of melanocortins are probably also mediated via MC receptors. However, the effects of melanocortins on avoidance behavior and the effect of gamma 2-MSH on increasing blood pressure are not mediated via one of the cloned brain MC receptors. The structure of brain MC receptors, their expression pattern, the MC receptor selective ligands and the function of MC receptors are briefly reviewed.

Binding and biological activity of C-terminally modified melanocortin peptides: a comparison between their actions at rodent MC1 and MC3 receptors

Five subtypes of melanocortin receptors have to date been identified, but to date little is known about the different structural requirements for binding and biological activity at these receptors. In this study, the role of C-terminal melanocortin peptide residues in imparting selectivity for the receptor subtypes was examined. C-terminally modified analogues of alpha-MSH and gamma-MSH were synthesized and their interaction with MC1 and MC3 melanocortin receptors was investigated. This study provides further evidence for an important role of proline 12 (numbering with respect to alpha-MSH) for binding and activity at the MC1 receptor. Although the influence of C-terminal amino acids on binding and activity at MC3-R was less marked, some of them were nevertheless observed to be beneficial for the interaction with this receptor subtype.

Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones

Melanocortin peptides and at least two subtypes of melanocortin receptors (MC3-R and MC4-R) are present in brain regions involved in cardiovascular regulation. In urethane-anesthetized rats, unilateral microinjection of alpha-melanocyte-stimulating hormone (MSH) into the medullary dorsal-vagal complex (DVC) causes dose-dependent (125-250 pmol) hypotension and bradycardia, whereas gamma-MSH is less effective. The effects of alpha-MSH are inhibited by microinjection to the same site of the novel MG4-R/MC3-R antagonist SHU9119 (2-100 pmol) but not naloxone (270 pmol), whereas the similar effects of intra-DVC injection of beta-endorphin (1 pmol) are inhibited by naloxone and not by SHU9119. Hypotensive and bradycardic responses to electrical stimulation of the arcuate nucleus also are inhibited by ipsilateral intra-DVC microinjection of SHU9119. gamma-MSH and ACTH(4-10), but not alpha-MSH, elicit dose-dependent (0.1-12.5 nmol) pressor and tachycardic effects, which are much more pronounced after intracarotid than after intravenous administration. The effects of gamma-MSH (1.25 nmol) are not inhibited by the intracarotid injection of SHU9119 (1.25-12.5 nmol) or the novel MC3-R antagonist SHU9005 (1.25-12.5 nmol). We conclude that the hypotension and bradycardia elicited by the release of alpha-MSH from arcuate neurons is mediated by neural melanocortin receptors (MC4-R/MC3-R) located in the DVC, whereas the similar effects of beta-endorphin, a peptide derived from the same precursor, are mediated by opiate receptors at the same site. In contrast, neither MC3-R nor MC4-R is involved in the centrally mediated pressor and tachycardic actions of gamma-MSH, which, likely, are mediated by an as yet unidentified receptor.

Melanocortin receptors: identification and characterization by melanotropic peptide agonists and antagonists

Hormones are chemical messengers released from cells to act on and control the activity of other cells. Hormonal ligands initiate their actions by interacting with receptive substances (Langley, 1906) of the target cells. These receptors are proteins that are either integral components of the cell membrane or are localized cytoplasmically within cells. Ligand-receptor interaction results in either the stimulation or inhibition of cellular activity. Since most hormones bind rather specifically to receptors possessed by their target cells, labeling of hormonal ligands can be utilized to identify and localize cells within an animal. In this report we discuss what is presently known about melanocortin receptors (MCRs) as studied by the use of labeled melanotropic peptide ligands.

Characterisation of the melanocortin 4 receptor by radioligand binding

The DNA encoding the human melanocortin 4 receptor was expressed in COS (CV-1 origin, SV 40) cells and its radioligand binding properties was tested by using the [125I][Nle4, D-Phe7] alpha-melanocyte stimulating hormone (MSH). The radioligand was found to bind to a single saturable site with a Kd of 3.84 +/- 0.57 nmol/l in the MC4 receptor expressing cells. The order of potency of a number of substance competing for the [125I][Nle4, D-Phe7] alpha-MSH binding was the following; [Nle4, D-Phe7] alpha-MSH > [Nle4]-alpha-MSH > beta-MSH > desacetyl-alpha-MSH > alpha-MSH > ACTH (1-39) > ACTH (4-10) > gamma 1-MSH > gamma 2-MSH. This order of potency is unique for the melanocortin 4 receptor when compared to our previously published data for the other melanocortin receptor subtypes. Most notably the melanocortin 4 receptor shows highest affinity for beta-MSH, among the endogenous MSH-peptides. Furthermore the melanocortin 4 receptor shows very low affinity for the gamma-MSH peptides. This distinguishes the melanocortin 4 receptor from the melanocortin 3 receptor, which is the other major central nervous system melanocortin-receptor, as melanocortin 3 receptor shows high affinity for gamma-MSH. Our finding might indicate a specific role for beta-MSH for the melanocortin 4 receptor.

Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones

Melanocortin peptides and at least two subtypes of melanocortin receptors (MC3-R and MC4-R) are present in brain regions involved in cardiovascular regulation. In urethane-anesthetized rats, unilateral microinjection of alpha-melanocyte-stimulating hormone (MSH) into the medullary dorsal-vagal complex (DVC) causes dose-dependent (125-250 pmol) hypotension and bradycardia, whereas gamma-MSH is less effective. The effects of alpha-MSH are inhibited by microinjection to the same site of the novel MG4-R/MC3-R antagonist SHU9119 (2-100 pmol) but not naloxone (270 pmol), whereas the similar effects of intra-DVC injection of beta-endorphin (1 pmol) are inhibited by naloxone and not by SHU9119. Hypotensive and bradycardic responses to electrical stimulation of the arcuate nucleus also are inhibited by ipsilateral intra-DVC microinjection of SHU9119. gamma-MSH and ACTH(4-10), but not alpha-MSH, elicit dose-dependent (0.1-12.5 nmol) pressor and tachycardic effects, which are much more pronounced after intracarotid than after intravenous administration. The effects of gamma-MSH (1.25 nmol) are not inhibited by the intracarotid injection of SHU9119 (1.25-12.5 nmol) or the novel MC3-R antagonist SHU9005 (1.25-12.5 nmol). We conclude that the hypotension and bradycardia elicited by the release of alpha-MSH from arcuate neurons is mediated by neural melanocortin receptors (MC4-R/MC3-R) located in the DVC, whereas the similar effects of beta-endorphin, a peptide derived from the same precursor, are mediated by opiate receptors at the same site. In contrast, neither MC3-R nor MC4-R is involved in the centrally mediated pressor and tachycardic actions of gamma-MSH, which, likely, are mediated by an as yet unidentified receptor.

The interaction of agouti signal protein and melanocyte stimulating hormone to regulate melanin formation in mammals

Important regulatory controls of melanogenesis that operate at the subcellular level to modulate the structural and/or the functional nature of the melanins and melanin granules produced in melanocytes are reviewed. Melanocyte stimulating hormone and agouti signal protein have antagonistic roles and possibly opposing mechanisms of action in the melanocyte. In the mouse, melanocyte stimulating hormone promotes melanogenic enzyme function and elicits increases in the amount of eumelanins produced, while agouti signal protein reduces total melanin production and elicits the synthesis of pheomelanin rather than eumelanin. We are now beginning to understand the complex controls involved in regulating this switch at the molecular and biochemical levels. The quality and quantity of melanins produced by melanocytes have important physiological consequences for melanocyte function and undoubtedly play important roles in the various functions of the melanins per se, including hair and skin coloration and photoprotection.

Molecular modelling of beta turns in a cyclic melanotropin

Alpha-Melanocyte stimulating hormone (alpha-MSH) is a tridecapeptide which interacts with a family of G protein-coupled receptors, the melanocortin receptors, to cause its biological effects. We have modelled the low energy conformations of the alpha-MSH derivatives as part of a project to probe the receptor binding conformation of melanocortins, and also to design ligands for targeting cytotoxic drugs to MC1 receptors expressed by melanoma cells. Here we report a molecular dynamics study of beta turns in a cyclic lactam analogue [Nle4, Asp5, D-Phe7, Lys10]alpha-MSH. The data show that it is possible for a beta turn to exist in the ring portion of this molecule which contains the melanocortin conserved sequence - His-Phe-Arg-Trp-, even though the lowest energy conformers lack a beta turn.

Receptor binding affinities and biological activities of linear and cyclic melanocortins in B16 murine melanoma cells expressing the native MC1 receptor

Cyclic alpha-melanocyte-stimulating hormone (alpha-MSH) analogues produced by disulphide bridging (e.g. [Cys4,Cys10] alpha-MSH) are known to be almost equipotent to the native hormone in amphibian skin bioassays and as a consequence have been proposed as a paradigm for the active conformation of native MSH at the pigment cell MC1 receptor. However this proposal has been somewhat speculative as there is no published data comparing biological activity of cyclic MSH analogues with data on receptor binding. This study addresses this problem by comparing tyrosinase stimulatory activity with their receptor binding affinity in B16 murine melanoma cells expressing the native MC1 melanocortin receptor. Cyclic [Cys4,Cys10] alpha-MSH showed almost the same affinity for the MC1 receptor as alpha-MSH, but the linear analogue [Cys4,Cys10] alpha-MSH bound less strongly. Both had biological activities similar to that of the natural ligand. Introduction of D-Phe into the ring in position 7 increased both affinity and activity of the cyclic compound. The study suggests that the intrinsic efficacy of cyclic [Cys4,Cys10] alpha-MSH analogues is similar to native alpha-MSH. Our studies support the proposal that the cyclic structure serves as a good model for the active conformation of linear alpha-MSH.

Receptors for melanocortin peptides in the hypothalamic-pituitary-adrenal axis and skin

The syndromes of hereditary IGD and triple A syndrome are potentially life threatening and severely disabling diseases. Clinical awareness of these syndromes is of considerable prognostic and therapeutic importance. The defects in the ACTH receptor causing IGD help illuminate the mechanisms of ligand binding and signal transduction by this receptor. Identification of the molecular defect(s) responsible for IGD cases with a normal ACTH receptor structural gene and for the triple A syndrome remains a challenge, which will hopefully eventually provide further insight into the mechanisms of adrenocortical function. The cloning of the melanocortin receptors has been a giant step towards a better insight into the physiological role of the POMC-derived peptides. It is now becoming apparent that the differential processing of POMC in the brain, the pituitary, and peripheral tissues, yielding a number of different biologically active melanocortin peptides, combined with the distinct tissue distribution and pharmacological profile of the melanocortin receptors will help elucidate the molecular basis of these functions.

Evidence of autocrine modulation of macrophage nitric oxide synthase by alpha-melanocyte-stimulating hormone

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a potent inhibitory agent in all major forms of inflammation. To identify a potential mechanism of antiinflammatory action of alpha-MSH, we tested its effects on production of nitric oxide (NO), believed to be a mediator common to all forms of inflammation. We measured NO and alpha-MSH production in RAW 264.7 cultured murine macrophages stimulated with bacterial lipopolysaccharide and interferon gamma. alpha-MSH inhibited production of NO, as estimated from nitrite production and nitration of endogenous macrophage proteins. This occurred through inhibition of production of NO synthase II protein; steady-state NO synthase II mRNA abundance was also reduced. alpha-MSH increased cAMP accumulation in RAW cells, characteristic of alpha-MSH receptors in other cell types. RAW cells also expressed mRNA for the primary alpha-MSH receptor (melanocortin 1). mRNA for proopiomelanocortin, the precursor molecular of alpha-MSH, was expressed in RAW cells, and tumor necrosis factor alpha increased production and release of alpha-MSH. These results suggest that the proinflammatory cytokine tumor necrosis factor alpha can induce macrophages to increase production of alpha-MSH, which then becomes available to act upon melanocortin receptors on the same cells. Such stimulation of melanocortin receptors could modulate inflammation by inhibiting the production of NO. The results suggest that alpha-MSH is an autocrine factor in macrophages which modulates inflammation by counteracting the effects of proinflammatory cytokines.

Beneficial effect of an ACTH4-9 analogue on experimentally induced diabetic autonomic neuropathy in the eye of the rat under general anaesthesia

While peripheral polyneuropathy is a well-known complication in diabetes mellitus, and the subject of a great deal of study, the clinical importance of autonomic diabetic neuropathy is increasingly recognised. Using an animal model, where the pupil diameter of the eye serves as a parameter of autonomic function, we produced an age and weight curve of pupil diameter and studied the development of autonomic neuropathy in rats with streptozotocin-induced diabetes. We show that diabetic rats develop significantly (P < 0.009) smaller pupils compared with controls, most probably due to a defective sympathetic input, caused by sympathetic neuropathy. Treatment with the neurotrophic peptide Org 2766, a synthetic ACTH4-9 analogue, prevents the occurrence of this sympathetic neuropathy, as the pupil diameters in the ACTH4-9 analogue-treated group are significantly (P < 0.05) larger than the pupils of placebo-treated rats, and are comparable to the pupil diameters of the rats in the control group.

alpha-MSH receptor autoradiography on mouse and human melanoma tissue sections and biopsies

MSH receptors and their binding characteristics of [125I]-labelled derivatives of alpha-MSH have been studied extensively on various mouse and human melanoma cell lines in culture. The aim of this study was to determine the binding characteristics of alpha-MSH radioligands to MSH receptors occurring in experimental mouse and human melanoma tumours as well as in human melanoma biopsies. For this reason, solid tumours were grown on experimental animals by inoculation of murine B16-F1 and human D10 and HBL melanoma cells. After excision and cryosectioning of the tumours, frozen tissue sections were incubated with [(125I)Tyr2]-alpha-MSH or [(125I)Tyr2,Nle4,D-Phe7]-alpha-MSH and specific alpha-MSH binding sites were visualized by subsequent autoradiography. The presence of increasing concentrations of unlabelled alpha-MSH during incubation with tracer led to a dose-dependent displacement of the radioligand. Quantitative analysis of the autoradiograms produced dissociation constants which were comparable with those obtained with cell binding assays: KD = 1.87 and 1.31 nmol/l for B16 tumours and cells, respectively; 0.32 and 0.33 nmol/l for D10, and 2.24 and 1.36 nmol/l for HBL tumours and cells, respectively. This indicates similar binding properties of alpha-MSH radioligands to both cultured melanoma cells and tissue sections of melanoma tumours from experimental animals. Similar binding characteristics were also observed with human melanoma tissue sections originating from biopsies of melanoma patients.

Cloning, expression, and tissue distribution of a fifth melanocortin receptor subtype

The melanocortin (MC) peptides mediate a diverse spectrum of biological activities in both the central nervous system and peripheral tissues by interacting with specific guanine nucleotide binding (G protein)-coupled receptors. Previously, four human melanocortin receptor subtypes have been cloned and characterized. In this study, we have isolated mouse complementary DNA (cDNA) and human genomic clones encoding a fifth melanocortin receptor subtype, MC5. Melanocortin peptide stimulation of human MC5, transiently expressed in COS1 cells, results in activation of adenylate cyclase with the following rank order of potency: [Nle4, D-Phe7]-alpha-MSH (melanocyte stimulating hormone) > ACTH (1-24) (adrenocorticotropic hormone) > alpha-MSH > beta-MSH > gamma-MSH. Northern blot hybridization, ribonuclease protection, and reverse transcription/polymerase chain reaction assays indicate that mouse MC5 mRNA is most abundant in skeletal muscle and brain. Lower but detectable levels of MC5 mRNA are also found in RT2-2 retinal neuronal cells, lung, testis, spleen, heart, kidney, and liver.