Stereochemical studies of the monocyclic agouti-related protein (103-122) Arg-Phe-Phe residues: conversion of a melanocortin-4 receptor antagonist into an agonist and results in the discovery of a potent and selective melanocortin-1 agonist

The agouti-related protein (AGRP) is an endogenous antagonist of the centrally expressed melanocortin receptors. The melanocortin-4 receptor (MC4R) is involved in energy homeostasis, food intake, sexual function, and obesity. The endogenous hAGRP protein is 132 amino acids in length, possesses five disulfide bridges at the C-terminus of the molecule, and is expressed in the hypothalamus of the brain. We have previously reported that a monocyclic hAGRP(103-122) peptide is an antagonist at the melanocortin receptors expressed in the brain. Stereochemical inversion from the endogenous l- to d-isomers of single or multiple amino acid modifications in this monocyclic truncated hAGRP sequence resulted in molecules that are converted from melanocortin receptor antagonists into melanocortin receptor agonists. The Asp-Pro-Ala-Ala-Thr-Ala-Tyr-cyclo[Cys-Arg-DPhe-DPhe-Asn-Ala-Phe-Cys]-Tyr-Ala-Arg-Lys-Leu peptide resulted in a 60 nM melanocortin-1 receptor agonist that is 100-fold selective versus the mMC4R, 1000-fold selective versus the mMC3R, and ca. 180-fold selective versus the mMC5R. In attempts to identify putative ligand-receptor interactions that may be participating in the agonist induced stimulation of the MC4R, selected ligands were docked into a homology molecular model of the mMC4R. These modeling studies have putatively identified hAGRP ligand DArg111-mMC4RAsn115 (TM3) and the hAGRP DPhe113-mMC4RPhe176 (TM4) interactions as important for agonist activity.     

Drug target discovery by pharmacogenetics: mutations in the melanocortin system and eating disorders

The identification of the genetic defect underlying the obese phenotype of the viable yellow mouse, ectopic overexpression of the agouti protein which acts as antagonist at the melanocortin-4 receptor, together with the demonstration that the brain melanocortin system was one major downstream effector pathway of leptin signaling has put forward melanocortin receptors as drug targets for obesity. The lack of compounds acting as melanocortin receptor antagonists was the reason why pharmacological studies had not recognized melanocortin receptors as important drug targets earlier. Blockade of brain melanocortin receptors results in increased food intake and body weight, whereas stimulation of the brain melanocortin system results in decreased food intake and activation of the hypothalamo-pituitary-adrenal axis. Anorexia nervosa is characterized by decreased body weight and food intake accompanied by changes in neuroendocrine systems such as strong activation of the hypothalamo-pituitary-adrenal axis. Since agouti-related protein suppresses the activity of the melanocortin system, the AgRP gene was investigated as candidate gene in anorexia nervosa. One variant of the AgRP gene was associated with anorexia nervosa, thus putting forward melanocortin receptor blockade as putative pharmacotherapy. Investigating variations in candidate genes in disease populations appears to be a fruitful approach towards the identification of drug targets.     

Melanocortin Antagonist Tetrapeptides with Minimal Agonist Activity at the Mouse Melanocortin-3 Receptor

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Characterization of melanocortin NDP-MSH agonist peptide fragments at the mouse central and peripheral melanocortin receptors

The central melanocortin receptors, melanocortin-4 (MC4R) and melanocortin-3 (MC3R), are involved in the regulation of satiety and energy homeostasis. The MC4R in particular has become a pharmaceutical industry drug target due to its direct involvement in the regulation of food intake and its potential therapeutic application for the treatment of obesity-related diseases. The melanocortin receptors are stimulated by the native ligand, alpha-melanocyte stimulating hormone (alpha-MSH). The potent and enzymatically stable analogue NDP-MSH (Ac-Ser-Tyr-Ser-Nle-Glu-His-DPhe-Arg-Trp-Gly-Lys-Pro-Val-NH(2)) is a lead peptide for the identification of melanocortin amino acids important for receptor molecular recognition and stimulation. We have synthesized nine peptide fragments of NDP-MSH, deleting N- and C-terminal amino acids to determine the "minimally active" sequence of NDP-MSH. Additionally, five peptides were synthesized to study stereochemical inversion at the Phe 7 and Trp 9 positions in attempts to increase tetra- and tripeptide potencies. These peptide analogues were pharmacologically characterized at the mouse melanocortin MC1, MC3, MC4, and MC5 receptors. This study has identified the Ac-His-DPhe-Arg-Trp-NH(2) tetrapeptide as possessing 10 nM agonist activity at the brain MC4R. The tripeptide Ac-DPhe-Arg-Trp-NH(2) possessed micromolar agonist activities at the MC1R, MC4R, and MC5R but only slight stimulatory activity was observed at the MC3R (at up to 100 microM concentration). This study has also examined to importance of both N- and C-terminal NDP-MSH amino acids at the different melanocortin receptors, providing information for drug design and identification of putative ligand-receptor interactions.     

Multiresidue Tetrapeptide Substitutions Yield a 140-fold Selective Melanocortin-3 over Melanocortin-4 Receptor Agonist

The five melanocortin receptors regulate numerous physiological functions. Although many ligands have been developed for the melanocortin-4 receptor (MC4R), the melanocortin-3 receptor (MC3R) has been less-well characterized, in part due to the lack of potent, selective tool compounds. Previously an Ac-His-Arg-(pI)DPhe-Tic-NH₂ scaffold, inverting the Phe-Arg motif of the native melanocortin signal sequence, was identified to possess mMC3R over mMC4R selective agonist activity. In this study, a library of 12 compounds derived from this scaffold was synthesized and assayed at the mouse melanocortin receptors (MCRs), utilizing substitutions previously shown to increase mMC3R agonist potency and/or selectivity. One compound (8, Ac-Val-Gln-DBip-DTic-NH₂) was identified as greater than 140-fold selective for the mMC3R over the mMC4R, possessed 70 nM potency at the mMC3R, and partially stimulated the mMC4R at 100 μM concentrations without antagonist activity. This pharmacological profile may be useful in developing new tool and therapeutic ligands that selective signal through the MC3R.     

Involvement of the melanocortin MC4 receptor in stress-related behavior in rodents

The melanocortin subtype 4 (MC4) receptor has been postulated to be involved in stress and stress-related behavior. We made use of melanocortin MC4 receptor agonists and antagonist to investigate the relationship between the melanocortin MC4 receptor and stress related disorders. The nonspecific melanocortin receptor agonist alpha-melanocyte stimulating hormone (alpha-MSH) and the melanocortin MC4 receptor agonist, Ac-[Nle4,Asp5,D-Phe7,Lys10]alpha-MSH-(4-10)-NH2 (MT II) dose-dependently and significantly reduced the number of licking periods in the rat Vogel conflict test, suggesting that stimulation of the melanocortin MC4 receptor causes anxiogenic-like activity in rats. We synthesized a peptidemimetic melanocortin MC4 receptor selective antagonist, Ac-D-2Nal-Arg-2Nal-NH2 (MCL0020), which has high affinity for the melanocortin MC4 receptor with IC50 values of 11.63 +/- 1.48 nM, in contrast, the affinities for melanocortin MC1 and MC3 receptors were negligible. In addition, MCL0020 significantly attenuated the cAMP formation induced by alpha-MSH in COS-1 cells expressing the melanocortin MC4 receptor without affecting basal cAMP contents. Thus, we considered MCL0020 to be a selective melanocrotin MC4 receptor antagonist among melanocortin receptors. Restraint stress significantly reduced food intake in rats, and i.c.v. administration of MCL0020 dose-dependently and significantly attenuated restraint stress-induced anorexia without affecting food intake. Swim stress induced reduction in the time spent in the light area in the mouse light/dark exploration test, and MCL0020 significantly prevented it. Taken together our findings suggest that the melanocortin MC4 receptor might be related to stress-induced changes in behavior, and blockade of the melanocortin MC4 receptor may prevent stress-induced disorders such as anxiety.     

Structure-activity relationships of the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH(2) at the mouse melanocortin receptors: part 2 modifications at the Phe position

The melanocortin pathway is an important participant in skin pigmentation, steroidogenesis, obesity, energy homeostasis and exocrine gland function. The centrally located melanocortin-3 and melanocortin-4 receptors (MC3R, MC4R) are involved in the metabolic and food intake aspects of energy homeostasis and are stimulated by melanocortin agonists such as alpha-melanocyte stimulation hormone (alpha-MSH). The melanocortin agonists contain the putative message sequence "His-Phe-Arg-Trp," and it has been well-documented that inversion of chirality of the Phe to DPhe results in a dramatic increase in melanocortin receptor potency. Herein, we report a tetrapeptide library, based upon the template Ac-His-DPhe-Arg-Trp-NH(2), consisting of 26 members that have been modified at the DPhe(7) position (alpha-MSH numbering) and pharmacologically characterized for agonist and antagonist activity at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R. The most notable results of this study include the identification of the tetrapeptide Ac-His-(pI)DPhe-Arg-Trp-NH(2) that is a full nanomolar agonist at the mMC1 and mMC5 receptors, a mMC3R partial agonist with potent antagonist activity (pA(2) = 7.25, K(i) = 56 nM) and, but unexpectedly, is a potent agonist at the mMC4R (EC(50) = 25 nM). This ligand possesses novel melanocortin receptor pharmacology, as compared to previously reported peptides, and is potentially useful for in vivo studies to differentiate MC3R vs MC4R physiological roles in animal models, such as primates, where "knockout" animals are not viable options. The DNal(2') substitution for DPhe resulted in a mMC3R partial agonist with antagonist activity (pA(2) = 6.5, K(i) = 295 nM) and a mMC4R (pA(2) = 7.8, K(i) = 17 nM) antagonist possessing 60- and 425-fold decreased potency, respectively, as compared with SHU9119 at these receptors. Examination of this DNal(2')-containing tetrapeptide at the F254S and F259S mutant mMC4Rs resulted in agonist activity of this mMC4R tetrapeptide antagonist, similar to that observed for the SHU9119 peptide, supporting our previously proposed hypothesis that the Phe 254 and 259 transmembrane six receptor residues are important for differentiating melanocortin sequence-based MC4R antagonists vs the agouti-related protein (AGRP) sequence-based antagonists.     

Discovery of a beta-MSH-derived MC-4R selective agonist

A series of novel, disulfide-constrained human beta-melanocyte stimulating hormone (beta-MSH)-derived peptides were optimized for in vitro melanocortin-4 receptor (MC-4R) binding affinity, agonist efficacy, and selectivity. The most promising of these, analogue 18, was further studied in vivo using chronic rat food intake and body weight models.     

Inverse agonism gains weight

Inverse agonism is emerging as a new endogenous principle for receptor regulation. Agouti-related protein (AgRP), following its release in the brain, stimulates food intake. AgRP binds to brain melanocortin receptors, which are involved in the regulation of body weight. In addition to antagonizing the effects of the melanocortin receptor agonist alpha-melanocyte-stimulating hormone (alpha-MSH), AgRP suppresses the constitutive activity of melanocortin MC(3) and MC(4) receptors, which characterizes AgRP as an inverse agonist rather than a neutral antagonist. The balance between the activity of AgRP-containing neurons and alpha-MSH-containing neurons determines the extent of activation of melanocortin receptors in neurons onto which they project. The identification of AgRP as an endogenous inverse agonist provides physiological relevance to inverse agonism in the control of body weight.     

In vitro and in vivo pharmacological characterization of the nociceptin/orphanin FQ receptor ligand Ac-RYYRIK-ol

It was recently reported that the hexapeptide Ac-RYYRIK-ol binds with high affinity nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors and competitively antagonizes N/OFQ actions in the mouse vas deferens assay. Here we further describe the in vitro and in vivo pharmacological features of this NOP receptor ligand. In mouse brain homogenate the degradation half life of Ac-RYYRIK-ol (2.48 min) was significantly higher than that of the parent compound Ac-RYYRIK-NH2 (1.20 min). In the electrically stimulated mouse vas deferens, Ac-RYYRIK-ol (10-1000 nM) competitively antagonized the inhibitory effect of N/OFQ (pA2=8.46), while in the isolated mouse colon the hexapeptide mimicked N/OFQ contractile effects thus behaving as a NOP receptor agonist (pEC50=9.09). This latter effect was no longer evident in colon tissues taken from mice knock out for the NOP receptor gene (NOP-/-). In vivo in mice, similarly to N/OFQ, Ac-RYYRIK-ol (dose range 0.001-1 nmol) produced: i) pronociceptive effects after intracerebroventricular (i.c.v.) administration and antinociceptive actions when given intrathecally (i.t.) in the tail withdrawal assay; ii) inhibition of locomotor activity and iii) stimulation of food intake after supraspinal administration. Finally, in the forced swimming test, Ac-RYYRIK-ol was inactive per se, but reversed the antidepressant-like effects elicited by the NOP receptor selective antagonist UFP-101 ([Nphe(1),Arg(14),Lys(15)]N/OFQ-NH2). Thus, in all these in vivo assays Ac-RYYRIK-ol mimicked the actions of N/OFQ showing however higher potency. In conclusion, Ac-RYYRIK-ol displayed a complex pharmacological profile which is likely due to the low efficacy agonist nature of this novel ligand of the NOP receptor. The high potency, selectivity of action, and in vivo effectiveness make Ac-RYYRIK-ol a useful pharmacological tool for future studies in the field of N/OFQ and its NOP receptor.     

Characterization of a novel and selective cannabinoid CB1 receptor inverse agonist, Imidazole 24b, in rodents

We document in vitro and in vivo effects of a novel, selective cannabinoid CB(1) receptor inverse agonist, Imidazole 24b (5-(4-chlorophenyl)-N-cyclohexyl-4-(2,4-dichlorophenyl)-1-methyl-imidazole-2-carboxamide). The in vitro binding affinity of Imidazole 24b for recombinant human and rat CB(1) receptor is 4 and 10 nM, respectively. Imidazole 24b binds to human cannabinoid CB(2) receptor with an affinity of 297 nM; in vitro, it is a receptor inverse agonist at both cannabinoid CB(1) and CB(2) receptors as it causes a further increase of forskolin-induced cAMP increase. Oral administration of Imidazole 24b blocked CP-55940-induced hypothermia, demonstrating cannabinoid CB(1) receptor antagonist efficacy in vivo. Using ex vivo autoradiography, Imidazole 24b resulted in dose-dependent increases in brain cannabinoid CB(1) receptor occupancy (RO) at 2h post-dosing in rats, indicating that approximately 50% receptor occupancy is sufficient for attenuation of receptor agonist-induced hypothermia. Imidazole 24b administered to C57Bl/6 mice and to dietary-induced obese (DIO) Sprague-Dawley rats attenuated overnight food intake with a minimal effective dose of 10 mg/kg, p.o. Administration had no effect in cannabinoid CB(1) receptor-deficient mice. DIO rats were dosed orally with vehicle, Imidazole 24b (1, 3 or 10 mg/kg), or dexfenfluramine (3 mg/kg) for 2 weeks. At 3 mg/kg, Imidazole 24b reduced cumulative food intake, leading to a non-significant decrease in weight gain. Imidazole 24b at 10 mg/kg and dexfenfluramine treatment inhibited food intake and attenuated weight gain. These findings suggest that selective cannabinoid CB(1) receptor inverse agonists such as Imidazole 24b have potential for the treatment of obesity.     

Design and pharmacology of N-[(3R)-1,2,3,4-tetrahydroisoquinolinium- 3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)- 2-[4-cyclohexyl-4-(1H-1,2,4-triazol- 1-ylmethyl)piperidin-1-yl]-2-oxoethylamine (1), a potent,selective, melanocortin subtype-4 receptor agonist

Synthetic and natural peptides that act as nonselective melanocortin receptor agonists have been found to be anorexigenic and to stimulate erectile activity. We report the design and development of 1, a potent, selective (1184-fold vs MC3R, 350-fold vs MC5R), small-molecule agonist of the MC4 receptor. Pharmacological testing confirms the food intake lowering effects of MC4R agonism and suggests another role for the receptor in the stimulation of erectile activity.     

Structure-activity relationships of the unique and potent agouti-related protein (AGRP)-melanocortin chimeric Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH2 peptide template

The melanocortin receptor system consists of endogenous agonists, antagonists, G-protein coupled receptors, and auxiliary proteins that are involved in the regulation of complex physiological functions such as energy and weight homeostasis, feeding behavior, inflammation, sexual function, pigmentation, and exocrine gland function. Herein, we report the structure-activity relationship (SAR) of a new chimeric hAGRP-melanocortin agonist peptide template Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) that was characterized using amino acids previously reported in other melanocortin agonist templates. Twenty peptides were examined in this study, and six peptides were selected for (1)H NMR and computer-assisted molecular modeling structural analysis. The most notable results include the identification that modification of the chimeric template at the His position with Pro and Phe resulted in ligands that were nM mouse melanocortin-3 receptor (mMC3R) antagonists and nM mouse melanocortin-4 receptor (mMC4R) agonists. The peptides Tyr-c[beta-Asp-His-DPhe-Ala-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) and Tyr-c[beta-Asp-His-DNal(1')-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) resulted in 730- and 560-fold, respectively, mMC4R versus mMC3R selective agonists that also possessed nM agonist potency at the mMC1R and mMC5R. Structural studies identified a reverse turn occurring in the His-DPhe-Arg-Trp domain, with subtle differences observed that may account for the differences in melanocortin receptor pharmacology. Specifically, a gamma-turn secondary structure involving the DPhe(4) in the central position of the Tyr-c[beta-Asp-Phe-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) peptide may differentiate the mixed mMC3R antagonist and mMC4R agonist pharmacology.     

Synthesis and Pharmacology of α/β3-Peptides Based on the Melanocortin Agonist Ac-His-dPhe-Arg-Trp-NH2 Sequence

dPhe-Arg-Trp-NH₂. The most potent mouse melanocortin-4 receptor (mMC4R) agonist, Ac-His-dPhe-Arg-β³hTrp-NH₂ (8) showed 35-fold selectivity versus the mMC3R. The study presented here has identified a new template with heterogeneous backbone for designing potent and selective melanocortin receptor ligands.     

Activation of melanocortin MC(4) receptors increases erectile activity in rats ex copula

Melanocortin peptide agonists, alpha-melanocyte stimulating hormone (alpha-MSH) and melanotan-II, stimulate erectile activity in a variety of species, including man. Since neither peptide discriminates amongst melanocortin receptors, it is not clear which subtype mediates these pro-erectile effects. Here, we present data that melanocortin-induced erectogenesis is mediated by melanocortin MC(4) receptors. Systemic administration of a melanocortin MC(4) receptor agonist (N-[(3R)-1,2,3,4-tetrahydroisoquinolinium-3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)-2-[4-cyclohexyl-4-(1H-1,2,4-triazol-1ylmethyl)piperidin-1-yl]-2-oxoethylamine; THIQ) with high selectivity over other melanocortin receptors enhanced intracavernosal pressure and stimulated erectile activity in rats ex copula. THIQ dose-dependently (1-5 mg/kg, i.v.) increased the total number of erections, to an extent comparable or greater than that produced by apomorphine (0.025 mg/kg, s.c.). Central administration of THIQ (20 microg, intracerebroventricular (i.c.v.)) increased the number of reflexive penile erections; whereas administration of both a nonselective endogenous melanocortin MC(4) receptor antagonist (agouti-related protein (AgRP), 5.5. microg, i.c.v.) and a melanocortin MC(4) receptor preferring antagonist (MPB10, 1 mg/kg, i.v.) blocked THIQ-induced erectogenesis. These pro-erectile effects were also attenuated by systemic or central administration of an oxytocin antagonist (L-368899, 1 mg/kg, i.v.). Thus, melanocortin MC(4) receptor activation is sufficient for erectogenesis and these effects may involve oxytocinergic pathways.     

Structure-activity relationships of the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH(2) at the mouse melanocortin receptors. 1. Modifications at the His position

The melanocortin pathway is an important participant in obesity and energy homeostasis. The centrally located melanocortin-3 and melanocortin-4 receptors (MC3R, MC4R) are involved in the metabolic and food intake aspects of energy homeostasis and are stimulated by melanocortin agonists such as alpha-melanocyte stimulation hormone (alpha-MSH). The melanocortin agonists contain the putative message sequence "His-Phe-Arg-Trp", and it has been well documented that inversion of chirality of the Phe to DPhe results in a dramatic increase in melanocortin receptor potency. Herein, we report a tetrapeptide library based on the template Ac-His-DPhe-Arg-Trp-NH(2), consisting of 17 members that have been modified at the His(6) position (alpha-MSH numbering) and pharmacologically characterized for agonist activity at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R. These studies provide further experimental evidence that the His(6) position can determine MC4R versus MC3R agonist selectivity and that chemically nonreactive side chains may be substituted for the imidazole ring (generally needs to be side chain protected in synthetic schemes) in the design of MC4R-selective, small-molecule, non-peptide agonists. Specifically, the tetrapeptide containing the amino-2-naphthylcarboxylic acid (Anc) amino acid at the His position resulted in a potent agonist at the mMC4R (EC(50) = 21 nM), was a weak mMC3R micromolar antagonist (pA(2) = 5.6, K(i) = 2.5 microM), and possessed >4700-fold agonist selectivity for the MC4R versus the MC3R. Substitution of the His(6) amino acid in the tetrapeptide template by the Phe, Anc, 3-(2-thienyl)alanine (2Thi), and 3-(4-pyridinyl)alanine (4-Pal) resulted in equipotency or only up to a 7-fold decrease in potency, compared to the His(6)-containing tetrapeptide at the mMC4R, demonstrating that these amino acid side chains may be substituted for the imidazole in the design of MC4R-selective non-peptide molecules.     

Differential effects of CCK-JMV-180 on food intake in rats and mice.

Boc-Tyr(SO3)-Nle-Gly-Trp-Nle-Asp-2-phenylether ester (CCK-JMV-180) has been reported to be a CCK-based heptipeptide with novel in vitro properties. Based on studies conducted in rat and mouse pancreatic acini, it has been proposed that the compound acts as an agonist at the high-affinity site and an antagonist at the low-affinity site in the rat, but as an agonist at both sites in the mouse. In the present study, we examined the effects of CCK-JMV-180 on locomotor activity in the rat and on intake of a liquid diet in the rat and mouse. Although CCK-JMV-180 slightly reduced activity on its own in the rat, it completely reversed the suppression produced by coadministration of CCK-8. In rat feeding studies, CCK-JMV-180 failed to suppress intakes of a liquid diet, but was able to antagonize the anorectic effects of CCK-8. In contrast, in the mouse CCK-JMV-180 potently suppressed intakes on its own, and this effect was blocked by pretreatment with the selective CCK-A receptor antagonist, A-70104. The results of these studies suggest that similar receptor mechanisms are involved in CCK's ability to inhibit food intake in vivo and its effects on pancreatic function in vitro.     

Novel agouti-related-protein-based melanocortin-1 receptor antagonist.

The melanocortin receptors are G-protein coupled receptors (GPCRs) that activate the cAMP signal transduction pathway and are stimulated by the melanocortin agonist alpha-melanocyte stimulating hormone (alpha-MSH). Members of these melanocortin receptors are antagonized by agouti (ASP) and agouti-related protein (AGRP), which are the only known endogenous antagonists of GPCRs identified to date. Structure-function studies of the hAGRP(109-118) decapeptide, Tyr-c[Cys-Arg-Phe-Phe-Asn-Ala-Phe-Cys]-Tyr-NH(2), by replacing the 26-membered disulfide Cys(2)-Cys(9) ring with lactam bridges resulted in the identification of a novel peripheral skin melanocortin-1 receptor (MC1R) antagonist. This antagonist, Tyr-c[Glu-Arg-Phe-Phe-Asn-Ala-Phe-Dpr]-Tyr-NH(2), possesses a 27-membered ring with the lactam bridge being formed from the Calpha-carboxyl moiety of Glu (instead of the typical side chain carboxyl moiety) with the amine of the diaminopropionic acid (Dpr) residue. This mouse MC1 receptor antagonist (pA(2) = 5.9) is also an antagonist at the brain melanocortin-4 receptor (pA(2) = 6.9), with no observable pharmacology at the melanocortin-3 or -5 receptors. This MC1R hAGRP(109-118) based decapeptide is novel in that AGRP(83-132) itself does not bind to, agonize, or antagonize the skin MC1R. Structural analysis has been performed using two-dimensional (1)H NMR and computer-assisted molecular modeling (CAMM) techniques in attempts to identify structural features of this Tyr-c[Glu-Arg-Phe-Phe-Asn-Ala-Phe-Dpr]-Tyr-NH(2) (cyclo Glu alphaCOOH-Dpr betaNH) peptide that can differentially result in antagonist versus agonist properties at the mMC1R.     

Agrp, a novel gene implicated in the control of feeding

Rapid progress has recently been made to characterise a new pathway involved in the control of feeding behaviour, referred to as the melanocortinergic pathway. Studies of the obese phenotype of the yellow mouse mutant (Ay) suggested the existence of this feeding circuit for some time, but the precise molecular and biochemical regulation was unclear. Through pharmacological and genetic approaches, specific melanocortin receptors expressed within the brain are now known to play a pivotal role. Since receptor agonists decrease food intake, and blocking receptor activity (through antagonists or mutations) increases food intake, this pathway is thought to provide a tonic inhibitory effect on food consumption. A unique feature of this pathway seems to be the regulation of receptor activity by an endogenous protein antagonist, agouti-related protein (AGRP). The molecular characterisation and biochemical activity of this gene suggest that it plays a role in the regulation of feeding behaviour.