Anxiety-like behavior induced by IL-1beta is modulated by alpha-MSH through central melanocortin-4 receptors

The proinflammatory cytokine interleukin-1beta (IL-1beta) influences neuroendocrine activity and produces other effects, including fever and behavioral changes such as anxiety. The melanocortin neuropeptides, such as alpha-melanocyte-stimulating hormone (alpha-MSH), antagonize many actions of IL-1, including fever, anorexia and hypothalamic-pituitary-adrenal (HPA) axis activation through specific melanocortin receptors (MC-R) in the central nervous system. The objective of the present study was to establish the effect of MSH peptides on IL-1beta-induced anxiety-like behavior and the melanocortin receptors involved. We evaluated the effects of intracerebroventricular (i.c.v.) administration of IL-1beta (30 ng) and melanocortin receptor agonists: alpha-MSH, an MC3/MC4-R agonist (0.2 microg) or gamma-MSH, an MC3-R agonist (2 microg) or HS014, an MC4-R antagonist (2 microg), on an elevated plus-maze (EPM) test. Injection of IL-1beta induced an anxiogenic-like response, as indicated by reduced open arms entries and time spent on open arms. The administration of alpha-MSH reversed IL-1beta-induced anxiety with co-administration of HS014 inhibiting the effect of alpha-MSH. However, the associated treatment with gamma-MSH did not affect the anxiety response to IL-1beta. These data suggest that alpha-MSH, through central MC4-R can modulate the anxiety-like behavior induced by IL-1beta.     

Enterostatin inhibition of dietary fat intake is modulated through the melanocortin system

Enterostatin injected into the amygdala selectively reduces dietary fat intake by an action that involves a serotonergic component in the paraventricular nucleus. We have investigated the role of melanocortin signaling in the response to enterostatin by studies in melanocortin 4 receptor (MC4R) knock out mice and by the use of the MC4R and MC3R antagonist SHU9119, and by neurochemical phenotyping of enterostatin activated cells. We also determined the effect of enterostatin in vivo on the expression of AgRP in the hypothalamus and amygdala of rats and in culture on a GT1-7 neuronal cell line. Enterostatin had no effect on food intake in MC4R knock out mice. SHU9119 i.c.v. blocked the feeding response to amygdala enterostatin in rats. Amygdala enterostatin induced fos activation in alpha-melanocyte stimulating hormone (alpha-MSH) neurons in the arcuate nucleus. Enterostatin also reduced the expression of AgRP in the hypothalamus and amygdala and in GT1-7 cells. These data suggest enterostatin inhibits dietary fat intake through a melanocortin signaling pathway.     

Melanocortin-4 receptor-mediated inhibition of apoptosis in immortalized hypothalamic neurons via mitogen-activated protein kinase

The melanocortin-4 receptor (MC4R) is a seven transmembrane member of the melanocortin receptor family. The GT1-1 cell line exhibits endogenous expression of MC4R. In this study, GT1-1 cells were used to study MC4R signaling pathways and to examine the effects of melanocortin receptor agonist NDP-MSH on apoptosis. MC4R mRNA expression was demonstrated by RT-PCR. Functional melanocortin receptor expression was implied by specific binding of NDP-MSH and cAMP production. NDP-MSH-stimulated GnRH release in a dose-dependent manner. Serum deprivation-induced apoptosis in GT1-1 cells, and the NDP-MSH inhibited this effect. The melanocortin receptor antagonist SHU9119 blocked the antiapoptotic actions of NDP-MSH, and the MAP kinase inhibitor PD98059 significantly attenuated the antiapoptotic effect. NDP-MSH-stimulated ERK1/2 phosphorylation in a dose-dependent manner. ERK1/2 phosphorylation could be abolished by SHU9119. In GT1-1 cells, melanocortin receptor activation causes ERK1/2 phosphorylation. In these cells, MC4R activation is also associated with antiapoptotic effects.     

Molecular determinants of human melanocortin-4 receptor responsible for antagonist SHU9119 selective activity

The hypothalamic melanocortin-4 receptor (MC4R), a seven transmembrane G-protein-coupled receptor, plays an important role in the regulation of body weight. The synthetic melanocortin analog SHU9119 has been widely used to characterize the physiological role of MC4R in feeding behavior and energy homeostasis. Previous studies indicated that SHU9119 is an agonist at the melanocortin-1 receptor (MC1R) but an antagonist at the MC4R. However, the molecular basis of the interaction between hMC4R and SHU9119 has not been clearly defined. To gain insight into the molecular determinants of hMC4R in the selectivity of SHU9119 chimeras and mutants hMC1R and hMC4R were expressed in cell lines and pharmacologically analyzed. A region of receptor containing the third transmembrane of hMC4R was found to be required for selective SHU9119 antagonism. Further mutagenesis studies of this region of hMC4R demonstrated that the amino acid residue leucine 133 in the third transmembrane was critical for the selective antagonist activity of SHU9119. The single substitution of leucine 133 to methionine did not affect SHU9119 binding to hMC4R. However, this substitution did convert SHU9119 from an antagonist to an agonist. Conversely, exchange of Met(128) in hMC1R to Leu, the homologous residue 133 of hMC4R, displayed a reduction in SHU9119 binding affinity and potency. This report provides the details of the molecular recognition of SHU9119 antagonism at hMC4R and shows that amino acid Leu(133) of hMC4R plays a key role in melanocortin receptor subtype specificity.     

Redundancy of a functional melanocortin 1 receptor in the anti-inflammatory actions of melanocortin peptides: studies in the recessive yellow (e/e) mouse suggest an important role for melanocortin 3 receptor

The issue of which melanocortin receptor (MC-R) is responsible for the anti-inflammatory effects of melanocortin peptides is still a matter of debate. Here we have addressed this aspect using a dual pharmacological and genetic approach, taking advantage of the recent characterization of more selective agonists/antagonists at MC1 and MC3-R as well as of the existence of a naturally defective MC1-R mouse strain, the recessive yellow (e/e) mouse. RT-PCR and ultrastructural analyses showed the presence of MC3-R mRNA and protein in peritoneal macrophages (M phi) collected from recessive yellow (e/e) mice and wild-type mice. This receptor was functional as Mphi incubation (30 min) with melanocortin peptides led to accumulation of cAMP, an effect abrogated by the MC3/4-R antagonist SHU9119, but not by the selective MC4-R antagonist HS024. In vitro M phi activation, determined as release of the CXC chemokine KC and IL-1 beta, was inhibited by the more selective MC3-R agonist gamma(2)-melanocyte stimulating hormone but not by the selective MC1-R agonist MS05. Systemic treatment of mice with a panel of melanocortin peptides inhibited IL-1 beta release and PMN accumulation elicited by urate crystals in the murine peritoneal cavity. MS05 failed to inhibit any of the inflammatory parameters either in wild-type or recessive yellow (e/e) mice. SHU9119 prevented the inhibitory actions of gamma(2)-melanocyte stimulating hormone both in vitro and in vivo while HS024 was inactive in vivo. In conclusion, agonism at MC3-R expressed on peritoneal M phi leads to inhibition of experimental nonimmune peritonitis in both wild-type and recessive yellow (e/e) mice.     

Role of hypothalamic interleukin-1beta (IL-1beta) in regulation of energy homeostasis by melanocortins

In the current study we sought to determine whether hypothalamic IL-1beta is regulated by melanocortin signaling and if melanocortin-induced changes in energy balance are dependent on IL-1beta. A melanocortin agonist, MTII, increased hypothalamic IL-1beta mRNA levels by two-fold, whereas a melanocortin antagonist, SHU9119, blunted lipopolysaccharide (LPS)-mediated increase of hypothalamic IL-1beta content. Pharmacological or genetic disruption of IL-1 receptor signaling prevented MTII-mediated reductions in locomotor activity, but did not reduce MTII-induced anorexia. These data suggest a potential role for central melanocortins in mediating the decrease of ambulation characteristic of the 'sickness' response.     

Central administration of peptide and small molecule MC4 receptor antagonists induce hyperphagia in mice and attenuate cytokine-induced anorexia

We investigated the effect of melanocortin 4 receptor (MC4) antagonists on food intake in mice. Food intake during the light phase was significantly increased by ICV administration of mixed MC3/MC4 antagonists (AgRP and SHU9119) or MC4 selective antagonist peptide [(Cyclo (1-5)[Suc-D-Nal-Arg-Trp-Lys]NH2] (MBP10) and the small molecule antagonists THP and NBI-30. Both mixed and selective antagonists significantly reversed anorexia induced by ICV administration of the MC4 agonist (c (1-6) HfRWK-NH2) and the cytokine IL-1beta. These findings provide pharmacological evidence that the MC4 receptor mediates the effects of melanocortin agonists and antagonists on food intake in mice, and support the idea that selective small molecule MC4 antagonists may be useful as therapeutics for cachexia.     

Inhibition of the central melanocortin system decreases brown adipose tissue activity

The melanocortin system is an important regulator of energy balance, and melanocortin 4 receptor (MC4R) deficiency is the most common monogenic cause of obesity. We investigated whether the relationship between melanocortin system activity and energy expenditure (EE) is mediated by brown adipose tissue (BAT) activity. Therefore, female APOE*3-Leiden.CETP transgenic mice were fed a Western-type diet for 4 weeks and infused intracerebroventricularly with the melanocortin 3/4 receptor (MC3/4R) antagonist SHU9119 or vehicle for 2 weeks. SHU9119 increased food intake (+30%) and body fat (+50%) and decreased EE by reduction in fat oxidation (−42%). In addition, SHU9119 impaired the uptake of VLDL-TG by BAT. In line with this, SHU9119 decreased uncoupling protein-1 levels in BAT (−60%) and induced large intracellular lipid droplets, indicative of severely disturbed BAT activity. Finally, SHU9119-treated mice pair-fed to the vehicle-treated group still exhibited these effects, indicating that MC4R inhibition impairs BAT activity independent of food intake. These effects were not specific to the APOE*3-Leiden.CETP background as SHU9119 also inhibited BAT activity in wild-type mice. We conclude that inhibition of central MC3/4R signaling impairs BAT function, which is accompanied by reduced EE, thereby promoting adiposity. We anticipate that activation of MC4R is a promising strategy to combat obesity by increasing BAT activity.     

CNS melanocortin and leptin effects on stearoyl-CoA desaturase-1 and resistin expression

The objective of this study was to determine whether centrally administered leptin decreased liver and adipose SCD1 expression or adipose resistin expression, and whether these effects were mediated by central melanocortin receptors. Male Sprague-Dawley rats were injected into the lateral cerebral ventricle (i.c.v.) once daily for 4 days with artificial cerebrospinal fluid (aCSF, 5 microl), leptin (10 microg) or MTII (0.1 nmol); two other groups were pretreated icv with the melanocortin antagonist, SHU9119 (1.0 nmol), followed by leptin or MTII. Epididymal and inguinal adipose tissue and liver were collected after rats were killed and mRNA expression of SCD1 and resistin was measured. Both leptin and MTII reduced SCD1 expression and pretreatment with SHU9119 reversed their effects. Neither leptin nor MTII affected resistin expression, but it was increased by SHU9119. These results show that CNS melanocortin receptors are likely mediators of leptin's effects on SCD1 expression in liver and adipose tissue, The findings were inconclusive concerning the effects of leptin and melanocortins on adipose resistin expression.     

Refractory hypothalamic alpha-mSH satiety and AGRP feeding systems in rats bearing MCA sarcomas

In pre-anorectic tumor-bearing (TB: methylcholanthrene-induced sarcoma) rats, injection of alpha-melanocyte stimulating hormone (alpha-MSH) into the perifornical hypothalamus (PFH) had no significant effect on food intake at a dose (5 microg) that reduced feeding in non-TB control rats. Following the development of anorexia, injection of alpha-MSH MC3/MC4 receptor antagonists, SHU9119 (1 microg) or 4 microg agouti-related protein (AGRP), stimulated feeding in non-TB rats, while having no significant effect in TB rats. Concentrations of alpha-MSH were not altered significantly in ventromedial, dorsomedial or lateral hypothalamic areas of TB rats, and proopiomelanocortin (POMC) messenger RNA was not changed in TB rats in these hypothalamic areas. Determination of cytokines by ELISA in non-operated TB and non-TB rats revealed elevated IL-2 in plasma and hypothalamus as well as increased TNF-alpha in the hypothalamus of anorectic TB rats. IL-1B was not detectable in plasma and was not altered significantly in hypothalamus of TB rats. These results suggest that the POMC alpha-MSH satiety system is refractory in TB rats, even prior to the onset of anorexia. This change in MC3/MC4 receptor response does not appear to be secondary to alterations of endogenous alpha-MSH in TB rats. Cytokine involvement in the altered response to MC3/MC4 receptor stimulation and blockade is a possibility, since TNF-alpha and IL-2 were increased in hypothalamus of anorectic TB rats. Therefore, these results suggest major alterations in POMC neuropeptide systems in TB rats as anorexia progresses. Although these changes do not appear to have occurred due to grossly-altered concentrations of alpha-MSH, elevated cytokine activity in the hypothalamus may be an important factor. Due to the complex multi-factorial nature of feeding control, additional factors are likely to be involved in cancer anorexia.     

Design of novel melanotropin agonists and antagonists with high potency and selectivity for human melanocortin receptors

alpha-MSH and gamma-MSH are the natural endogenous hormones for the human melanocortin-1, 3, 4 and 5 receptors (hMC1R, hMC3R, hMC4R and hMC5R). These and more potent, stable and prolonged acting analogues such as NDP-alpha-MSH, MT-II and SHU-9119 are not very receptor selective. To develop potent and selective agonist and antagonist ligands for the melanocortin receptors we have used state-of-the-art biophysical studies, computational chemistry, and design of conformational and topographical constraints with novel templates.     

Structure activity studies of the melanocortin-4 receptor by in vitro mutagenesis: identification of agouti-related protein (AGRP), melanocortin agonist and synthetic peptide antagonist interaction determinants

In vitro mutagenesis of the mouse melanocortin-4 receptor (mMC4R) has been performed, based upon homology molecular modeling and previous melanocortin receptor mutagenesis studies that identified putative ligand-receptor interactions. Twenty-three mMC4 receptor mutants were generated and pharmacologically characterized using several melanocortin-based ligands [alpha-MSH, NDP-MSH, MTII, DNal (1')(7)-MTII, Nal(2')(7)-MTII, SHU9119, and SHU9005]. Selected mutant receptors possessing significant differences in the melanocortin-based peptide agonist and/or antagonist pharmacology were further evaluated using the endogenous antagonist agouti-related protein fragment hAGRP(83-132) and hAGRP(109-118) molecules. These studies of the mouse MC4R provide further experimental data suggesting that the conserved melanocortin receptor residues Glu92 (TM2), Asp114 (TM3), and Asp118 (TM3) (mouse MC4R numbering) are important for melanocortin-based peptide molecular recognition. Additionally, the Glu92 and Asp118 mMC4R residues are important for molecular recognition and binding of AGRP(83-132). We have identified the Phe176 (TM4), Tyr179 (TM4), Phe254 (TM6), and Phe259 (TM6) receptor residues as putatively interacting with the melanocortin-based ligand Phe(7) by differences between alpha-MSH and NDP-MSH agonist potencies. The Glu92, Asp118, and Phe253 mMC4R receptor residues appear to be critical for hAGRP(83-132) molecular recognition and binding while Phe176 appears to be important for functional antagonism of AGRP(83-132) and AGRP(109-118) but not molecular recognition. The Phe253 mMC4R residue appears to be important for AGRP(83-132) molecular recognition and general mMC4 receptor stimulation. The Phe254 and Phe259 mMC4R amino acids may participate in the differentiation of agonist versus antagonist activity of the melanocortin-based peptide antagonists SHU9119 and SHU9005, but not AGRP(83-132) or AGRP(109-118). The Met192 side chain when mutated to a Phe results in a constitutively active mMC4R that does not effect agonist ligand binding or potency. Melanocortin-based peptides modified at the 7 position of MTII with DPhe, DNal(1'), Nal(2'), and DNal(2') have been pharmacologically characterized at these mutant mouse MC4Rs. These data suggest a revised hypothesis for the mechanism of SHU9119 antagonism at the MC4R which may be attributed to the presence of a "bulky" naphthyl moiety at the 7 position (original hypothesis), and additionally that both the stereochemistry and naphthyl ring position (2' versus 1') are important for positioning of the ligand Arg(8) residue with the corresponding mMC4R amino acids.     

AgRP(83-132) acts as an inverse agonist on the human-melanocortin-4 receptor

The central melanocortin (MC) system has been demonstrated to act downstream of leptin in the regulation of body weight. The system comprises alpha-MSH, which acts as agonist, and agouti-related protein (AgRP), which acts as antagonist at the MC3 and MC4 receptors (MC3R and MC4R). This property suggests that MCR activity is tightly regulated and that opposing signals are integrated at the receptor level. We here propose another level of regulation within the melanocortin system by showing that the human (h) MC4R displays constitutive activity in vitro as assayed by adenylyl cyclase (AC) activity. Furthermore, human AgRP(83-132) acts as an inverse agonist for the hMC4R since it was able to suppress constitutive activity of the hMC4R both in intact B16/G4F melanoma cells and membrane preparations. The effect of AgRP(83-132) on the hMC4R was blocked by the MC4R ligand SHU9119. Also the hMC3R and the mouse(m)MC5R were shown to be constitutively active. AgRP(83-132) acted as an inverse agonist on the hMC3R but not on the mMC5R. Thus, AgRP is able to regulate MCR activity independently of alpha-MSH. These findings form a basis to further investigate the relevance of constitutive activity of the MC4R and of inverse agonism of AgRP for the regulation of body weight.     

Pharmacological comparison of rat and human melanocortin 3 and 4 receptors in vitro

The melanocortin 3 and 4 receptors are G-protein-coupled receptors found in the hypothalamus with important role in regulation of the energy balance. In this study, we performed pharmacological comparison of the rat and human melancortin (MC) 3 and MC4 receptors. We transiently expressed the genes for these receptors individually in a mammalian cell line and determined the binding affinities to several MSH peptides. The results showed no major difference between the rat and human MC3 receptors while the rat MC4 receptor had higher affinity to several peptides compared with the human MC4 receptor. NDP-, alpha-, beta-, gamma-MSH, ACTH(1-24), HS014 and MTII had from 5- to 34-fold higher affinity for the rat MC4 receptor, while SHU9119, HS024 and HS028 had similar affinity for both the MC4 receptors. Pharmacological species difference have earlier been reported for the MC1 and MC5 receptors but this is the first report showing important differences between the rat and human MC4 receptors.     

Parathyroid hormone-related protein and lung injury after pulmonary thromboendarterectomy

Agouti-related protein (AGRP) is one of two naturally occurring antagonists of G-Protein coupled receptors (GPCRs) identified to date, and has been physiologically implicated in regulating food intake, body weight, and energy homeostasis. AGRP has been identified in vitro, as competitively antagonizing the brain melanocortin-4 (MC4R) and melanocortin-3 (MC3R) receptors, and when over expressed in transgenic mice, results in an obese phenotype. Emerging data propose that AGRP has additional targets in the hypothalamus and/or physiologically functions via a mechanism in addition to competitive antagonism of alpha-MSH at the brain melanocortin receptors. We report data herein supporting an alternative mechanism for AGRP involvement in feeding behavior. A constitutively active MC4R has been generated which possess EC(50) values for melanocortin agonists (alpha-MSH, NDP-MSH, and MTII) and a pA2 value for the synthetic peptide antagonist SHU9119 identical to the wildtype receptor, but increases basal activity to 50% maximal response. AGRP possesses inverse agonist activity at this constitutively active MC4R. These data support the hypothesis for an additional physiological mechanism for AGRP action in feeding behavior and energy homeostasis.     

The melanocortin 4 receptor mediates leptin stimulation of luteinizing hormone and prolactin surges in steroid-primed ovariectomized rats

We have previously reported that leptin, the product of the obese (ob) gene, may play a physiologically relevant role in the generation of estradiol/progesterone-induced luteinizing hormone (LH) and prolactin (PRL) surges in female rats. In the present study, we examined whether the stimulatory effect of leptin on the hormonal surges is mediated through the melanocortin (MC) 4 receptor in the brain, as is leptin's effect on feeding behavior. We also explored whether the MC4 receptor participates in tonic stimulation of steroid-induced LH and PRL surges. Experiments were performed on both normally fed and 3-day starved rats, which were ovariectomized and primed with estradiol and progesterone. At 11:00 h on the day of the experiments, the normally fed rats received an intracerebroventricular administration of artificial cerebrospinal fluid (vehicle), SHU 9119 (a nonselective MC3/MC4 receptor antagonist, 1.0 nmol), or HS014 (a selective MC4 receptor antagonist, 1.0 nmol). The 3-day starved rats were given vehicle, recombinant mouse leptin (0.3 nmol), leptin (0.3 nmol) + SHU9119 (1.0 nmol), or leptin (0.3 nmol) + HS014 (1.0 nmol). From 11:00 to 18:00 h, blood was collected every 30 min to measure LH and PRL. The 3-day starvation completely abolished both LH and PRL surges, but leptin significantly reinstated these hormonal surges. Both SHU9119 and HS014 significantly decreased the magnitude of LH and PRL surges in normally fed rats and also significantly blocked the leptin stimulation of the hormonal surges in starved rats. These results suggest that the MC4 receptor may be the pivotal subtype of MC receptors mediating the leptin stimulation of LH and PRL surges. The data also suggest that endogenous MC(s) may tonically stimulate the hormonal surges in normally fed rats via the MC4 receptor. This is the first report describing a physiological role of a specific MC receptor in regulating the reproductive axis.     

Peripheral effect of alpha-melanocyte-stimulating hormone on fatty acid oxidation in skeletal muscle

To study the peripheral effects of melanocortin on fuel homeostasis in skeletal muscle, we assessed palmitate oxidation and AMP kinase activity in alpha-melanocyte-stimulating hormone (alpha-MSH)-treated muscle cells. After alpha-MSH treatment, carnitine palmitoyltransferase-1 and fatty acid oxidation (FAO) increased in a dose-dependent manner. A strong melanocortin agonist, NDP-MSH, also stimulated FAO in primary culture muscle cells and C2C12 cells. However, [Glu6]alpha-MSH-ND, which has ample MC4R and MC3R agonistic activity, stimulated FAO only at high concentrations (10(-5) M). JKC-363, a selective MC4R antagonist, did not suppress alpha-MSH-induced FAO. Meanwhile, SHU9119, which has both antagonistic activity on MC3R and MC4R and agonistic activity on both MC1R and MC5R, increased the effect of alpha-MSH on FAO in both C2C12 and primary muscle cells. Small interference RNA against MC5R suppressed the alpha-MSH-induced FAO effectively. cAMP analogues mimicked the effect of alpha-MSH on FAO, and the effects of both alpha-MSH and cAMP analogue-mediated FAO were antagonized by a protein kinase A inhibitor (H89) and a cAMP antagonist ((Rp)-cAMP). Acetyl-CoA carboxylase activity was suppressed by alpha-MSH and cAMP analogues by phosphorylation through AMP-activated protein kinase activation in C2C12 cells. Taken together, these results suggest that alpha-MSH increases FAO in skeletal muscle, in which MC5R may play a major role. Furthermore, these results suggest that alpha-MSH-induced FAO involves cAMP-protein kinase A-mediated AMP-activated protein kinase activation.     

Analogs of lactam derivatives of alpha-melanotropin with basic and acidic residues

A role of the aromatic and of the basic residues of the potent agonist (MTII) and antagonist (SHU9119) at the human melanocortin receptors 4 in the formation and stabilization of ligand-receptor complexes was examined. Analogs of MTII and SHU9119 with glutamic acid replacing one amino acid at a time were synthesized and tested for their ability to bind to and activate human melanocortin receptors 3, 4, and 5. Replacement of Phe (Nal) or Trp with Glu resulted in analogs of MTII and SHU9119 which were practically inactive at the receptors studied. The rather large (and unexpected) tolerance toward the presence of Glu in the position of His or Arg of MTII and SHU9119 clearly suggested that in the ligand receptor complexes these basic residues are not in contact with the receptors but probably face the extracellular environment. This identified the aromatic residues of MTII and SHU9119 as the primary structural features determining interactions of the agonist/antagonist with hMCR3-5.     

Solution structures and molecular interactions of selective melanocortin receptor antagonists

The solution structures and inter-molecular interaction of the cyclic melanocortin antagonists SHU9119, JKC363, HS014, and HS024 with receptor molecules have been determined by NMR spectroscopy and molecular modeling. While SHU9119 is known as a nonselective antagonist, JKC363, HS014, and HS024 are selective for the melanocortin subtype-4 receptor (MC4R) involved in modulation of food intake. Data from NMR and molecular dynamics suggest that the conformation of the Trp9 sidechain in the three MC4R-selective antagonists is quite different from that of SHU9119. This result strongly supports the concept that the spatial orientation of the hydrophobic aromatic residue is more important for determining selectivity than the presence of a basic, “arginine-like” moiety responsible for biological activity. We propose that the conformation of hydrophobic residues of MCR antagonists is critical for receptor-specific selectivity.     

Effects of high-fat diets with different carbohydrate-to-protein ratios on energy homeostasis in rats with impaired brain melanocortin receptor activity

Changes in dietary macronutrient composition and/or central nervous system neuronal activity can underlie obesity and disturbed fuel homeostasis. We examined whether switching rats from a diet with high carbohydrate content (HC; i.e., regular chow) to diets with either high fat (HF) or high fat/high protein content at the expense of carbohydrates (LC-HF-HP) causes differential effects on body weight and glucose homeostasis that depend on the integrity of brain melanocortin (MC) signaling. In vehicle-treated rats, switching from HC to either HF or LC-HF-HP feeding caused similar reductions in food intake without alterations in body weight. A reduced caloric intake (-16% in HF and LC-HF-HP groups) required to maintain or increase body weight underlay these effects. Chronic third cerebroventricular infusion of the MC receptor antagonist SHU9119 (0.5 nmol/day) produced obesity and hyperphagia with an increased food efficiency again observed during HF (+19%) and LC-HF-HP (+33%) feeding. In this case, however, HF feeding exaggerated SHU9119-induced hyperphagia and weight gain relative to HC and LC-HF-HP feeding. Relative to vehicle-treated controls, SHU9119 treatment increased plasma insulin (2.8-4 fold), leptin (7.7-15 fold), and adiponectin levels (2.4-3.7 fold), but diet effects were only observed on plasma adiponectin (HC and LC-HF-HP相似文献