Heterogeneity of brain melanocortin receptors suggested by differential ligand binding in situ

Recommended Tool Compounds for the Melanocortin Receptor (MCR) G Protein-Coupled Receptors (GPCRs)

The melanocortin receptors are a centrally and peripherally expressed family of Class A GPCRs with physiological roles, including pigmentation, steroidogenesis, energy homeostasis, and others yet to be fully characterized. There are five melanocortin receptor subtypes that, apart from the melanocortin-2 receptor (MC2R), are stimulated by a shared set of endogenous agonists. Until 2020, X-ray crystallographic and cryo-electron microscopic (cryo-EM) structures of these receptors were unavailable, and the investigation of their mechanisms of action and putative ligand-receptor interactions was driven by site-directed mutagenesis studies of the receptors and targeted structure-activity relationship (SAR) studies of the endogenous and derivative synthetic ligands. Synthetic derivatives of the endogenous agonist ligand α-MSH have evolved into a suite of powerful ligands such as NDP-MSH (melanotan I), melanotan II (MTII), and SHU9119. This suite of tool compounds now enables the study of the melanocortin receptors and serves as scaffolds for FDA-approved drugs, means of validating stably expressing melanocortin receptor cell lines, core ligands in assessing cryo-EM structures of active and inactive receptor complexes, and essential references for high-throughput discovery and mechanism of action studies. Herein, we review the history and significance of a finite set of these essential tool compounds and discuss how they are being utilized to further the field's understanding of melanocortin receptor physiology and greater druggability.

Control of goal-directed and inflexible actions by dorsal striatal melanocortin systems, in coordination with the central nucleus of the amygdala

The dorsomedial striatum (DMS) is associated with flexible goal seeking, as opposed to routinized habits. Whether local mechanisms brake this function, for instance when habits may be adaptive, is incompletely understood. We find that a sub-population of dopamine D1 receptor-containing striatal neurons express the melanocortin-4 receptor (MC4R) for α-melanocyte stimulating hormone. These neurons within the DMS are necessary and sufficient for controlling the capacity of mice to flexibly adjust actions based on the likelihood that they will be rewarded. In investigating MC4R function, we found that it suppresses immediate-early gene levels in the DMS and concurrently, flexible goal seeking. MC4R+ neurons receive input from the central nucleus of the amygdala, and behavioral experiments indicate that they are functionally integrated into an amygdalo-striatal circuit that suppresses action flexibility in favor of routine. Publicly available spatial transcriptomics datasets were analyzed for gene transcript correlates of Mc4r expression across the striatal subregions, revealing considerable co-variation in dorsal structures. This insight led to the discovery that the function of MC4R in the dorsolateral striatum complements that in the DMS, in this case suppressing habit-like behavior. Altogether, our findings suggest that striatal MC4R controls the capacity for goal-directed and inflexible actions alike.

Centrally Projecting Edinger-Westphal Nucleus in the Control of Sympathetic Outflow and Energy Homeostasis

The centrally projecting Edinger-Westphal nucleus (EWcp) is a midbrain neuronal group, adjacent but segregated from the preganglionic Edinger-Westphal nucleus that projects to the ciliary ganglion. The EWcp plays a crucial role in stress responses and in maintaining energy homeostasis under conditions that require an adjustment of energy expenditure, by virtue of modulating heart rate and blood pressure, thermogenesis, food intake, and fat and glucose metabolism. This modulation is ultimately mediated by changes in the sympathetic outflow to several effector organs, including the adrenal gland, heart, kidneys, brown and white adipose tissues and pancreas, in response to environmental conditions and the animal's energy state, providing for appropriate energy utilization. Classic neuroanatomical studies have shown that the EWcp receives inputs from forebrain regions involved in these functions and projects to presympathetic neuronal populations in the brainstem. Transneuronal tracing with pseudorabies virus has demonstrated that the EWcp is connected polysynaptically with central circuits that provide sympathetic innervation to all these effector organs that are critical for stress responses and energy homeostasis. We propose that EWcp integrates multimodal signals (stress, thermal, metabolic, endocrine, etc.) and modulates the sympathetic output simultaneously to multiple effector organs to maintain energy homeostasis under different conditions that require adjustments of energy demands.

TFOS DEWS II Sex, Gender, and Hormones Report

One of the most compelling features of dry eye disease (DED) is that it occurs more frequently in women than men. In fact, the female sex is a significant risk factor for the development of DED. This sex-related difference in DED prevalence is attributed in large part to the effects of sex steroids (e.g. androgens, estrogens), hypothalamic-pituitary hormones, glucocorticoids, insulin, insulin-like growth factor 1 and thyroid hormones, as well as to the sex chromosome complement, sex-specific autosomal factors and epigenetics (e.g. microRNAs). In addition to sex, gender also appears to be a risk factor for DED. "Gender" and "sex" are words that are often used interchangeably, but they have distinct meanings. "Gender" refers to a person's self-representation as a man or woman, whereas "sex" distinguishes males and females based on their biological characteristics. Both gender and sex affect DED risk, presentation of the disease, immune responses, pain, care-seeking behaviors, service utilization, and myriad other facets of eye health. Overall, sex, gender and hormones play a major role in the regulation of ocular surface and adnexal tissues, and in the difference in DED prevalence between women and men. The purpose of this Subcommittee report is to review and critique the nature of this role, as well as to recommend areas for future research to advance our understanding of the interrelationships between sex, gender, hormones and DED.

Melanocortins in brain inflammation: the role of melanocortin receptor subtypes

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

Terminal signal: anti-inflammatory effects of α-melanocyte-stimulating hormone related peptides beyond the pharmacophore

During the last two decades a significant number of investigations has established the fact that α-Melanocyte-stimulating hormone (α-MSH) is a potent anti-inflammatory mediator. The anti-inflammatory effects of α-MSH can be elicited via melanocortin receptors (MC-Rs) broadly expressed in a number of tissues ranging from the central nervous system to cells of the immune system and on resident somatic cells of peripheral tissues. α-MSH affects various pathways regulating inflammatory responses such as NF-κB activation, expression of adhesion molecules, inflammatory cytokines, chemokine receptors, T-cell proliferation and activity and inflammatory cell migration. In vivo α-MSH has been shown to be anti-inflammatory as well in animal models of fever, irritant and allergic contact dermatitis, cutaneous vasculitis, fibrosis, in ocular, gastrointestinal, brain and allergic airway inflammation and arthritis. A broad range of effects of α-MSH exerted beyond the field of inflammation, its pigmentory capacity being only the most visible aspect, has been one of the major impediments limiting the use of α-MSH in human inflammatory disorders. Interestingly KPV, C-terminal tripeptide of α-MSH, which lacks the entire sequence motif required for binding to any of the known MC-Rs, retains almost all of the anti-inflammatory capacity of the full hormone, but in its activities display a lack of any pigmentory action. While the exact signaling mechanism utilized by KPV and related peptides currently is unknown it has been demonstrated already that significant similarities between anti-inflammatory signaling of α-MSH and those short peptides exist. These α-MSH related tripeptides thus may be useful alternatives for anti-inflammatory peptide therapy. KdPT, a derivative of KPV corresponding to IL-1β(193-195), currently is emerging as another tripeptide with potent anti-inflammatory effects. A more limited spectrum of biologic activities, potentially advantageous physicochemical, pharmacokinetic and pharmacodynamic properties as well as the expectation of low costs for pharmaceutical production make these agents interesting candidates for the treatment of immune-mediated inflammatory skin and bowel diseases, allergic asthma and arthritis.

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

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

Central administration of selective melanocortin 4 receptor antagonist HS014 prevents morphine tolerance and withdrawal hyperalgesia

Major problem involved in treatment of chronic pain with morphine is the development of tolerance and dependence. Previous studies have demonstrated the participation of melanocortin (MC) system in the development of tolerance to antinociceptive effect of morphine. However, the impact of supraspinal MC4 receptors (MC4 R) modulation on this phenomenon and morphine withdrawal hyperalgesia remained unexplored. We investigated the role of central MC4 R in acute, chronic effects and withdrawal reactions of morphine using tail flick test. Acute intracerebroventricular (icv) administration of morphine (2-20 microg/rat) exhibited antinociceptive activity, which was antagonized by subeffective dose of nonselective MC R agonist NDP-MSH (0.04 ng/rat, icv), and potentiated by subeffective dose of MC4 R antagonist HS014 (0.008 ng/rat, icv). Isobolographic analysis revealed antagonistic interaction between NDP-MSH and morphine, and additive interaction between HS014 and morphine combinations. While chronic icv infusion of morphine (20 ng/microl/h) via osmotic pump for 7 days developed tolerance to its antinociceptive effect, its discontinuation produced hyperalgesia. Co-administration of HS014 (0.008 ng/rat, icv) with chronic morphine not only delayed the development of tolerance but also prevented withdrawal hyperalgesia. Furthermore, acute treatment with HS014 (0.008 and 0.04 ng/rat, icv) dose dependently attenuated the withdrawal hyperalgesia. This suggests the involvement of central MC4 R in the mechanism of development of tolerance and dependence following chronic morphine administration. We speculate that targeting this receptor may be a novel strategy to improve the effectiveness of morphine in the treatment of chronic pain.

Tearful relationships? Sex, hormones, the lacrimal gland, and aqueous-deficient dry eye

Sex and the endocrine system exert a significant influence on the physiology and pathophysiology of the lacrimal gland. The purpose of this article is to briefly review the nature and magnitude of these interactions between sex, hormones and lacrimal tissue, and to address how they may relate to the pathogenesis of aqueous-deficient dry eye. Towards this end, this article has a 3-fold approach: first, to summarize the influence of androgens, estrogens, glucocorticoids, mineralocorticoids, retinoic acid, prolactin, alpha-melanocyte stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, growth hormone, thyroid-stimulating hormone, arginine vasopressin, oxytocin, thyroxine, parathyroid hormone, insulin, glucagon, melatonin, human chorionic gonadotropin and cholecystokinin on the structure and function of the lacrimal gland; second, to discuss the mechanism of action of each hormone on lacrimal tissue; and third, to discuss the clinical relevance of the endocrine-lacrimal gland interrelationship, with a particular focus on each hormone's role (i.e. if relevant) in the development of aqueous-tear deficiency.

alpha-Melanocyte-stimulating hormone is neuroprotective in rat global cerebral ischemia

The aim of the study was to investigate the effects of alpha-melanocyte-stimulating hormone (alpha-MSH), a tridecapeptide derived from proopiomelanocortin (POMC), on the neurodegeneration following global cerebral ischemia and reperfusion in the rat. The biological activities of alpha-MSH include inhibition of inflammatory responses and anti-pyretic effects. Male Sprague-Dawley rats were subjected to four-vessel occlusion (4-VO) global cerebral ischemia followed by reperfusion, and treated with alpha-MSH (intraperitoneally, i.p.) at 30 min, and 24, 48, 72 and 96 h post-ischemia. Stereological quantification of the pyramidal cells in the CA1 area of the hippocampus showed that the number of viable neurons in ischemic rats was 96,945+/-18,610 (means+/-SD) as compared to 183,156+/-49,935 in sham-operated rats (P<0.05). The number of viable neurons after treatment of ischemic rats with alpha-MSH was 162,829+/-34,757, i.e. significantly different from the number of viable neurons in ischemic rats injected with saline (P<0.01). Astrocyte proliferation due to the ischemic insult was markedly reduced by the treatment with alpha-MSH, and the loss in body weight was reduced by alpha-MSH. In conclusion, post-ischemic administration of alpha-MSH was found to provide neuroprotection in the CA1 pyramidal cell layer in the hippocampus, concomitant with a reduction in glial activation, indicating that alpha-MSH or mimetics thereof may have a potential in the treatment of stroke or other neurodegenerative diseases. Further studies will be required to define the post-ischemic time window for administration of alpha-MSH.

The regulation of food intake by selective stimulation of the type 3 melanocortin receptor (MC3R)

High levels of binding sites for melanocortin peptides exist within the arcuate nucleus, and a functional response to melanocortin peptides has been demonstrated in arcuate POMC neurons. Because the MC3R is thought to function as an inhibitory autoreceptor on POMC neurons, we reasoned that peripheral injections of MC3R-specific agonists would act within the arcuate nucleus to inhibit POMC neurons and thereby stimulate feeding. We demonstrate that the peptidergic MC3R agonist, d-Trp(8)-gamma-MSH, stimulates feeding via the MC3R when injected peripherally. These data provide the first evidence that feeding can be stimulated by peripheral injection of MC3R-specific agonists.

Central Nervous System Agents in the Treatment of Erectile Dysfunction

In the last two decades, a better understanding of the mechanisms governing erectile function and the pathophysiologies underlying erectile dysfunction (ED) have led re-searchers to investigate novel treatment concepts. Selective type-5 phosphodiesterase inhibitors are recommended as first-line therapy because of their high efficacy, but 30% to 40% of patients who have ED do not respond adequately to these agents and require alternative methods. The central nervous system plays a fundamental role in sexual behavior. Animal models have advanced our understanding of the neuroanatomic and neuropharmacologic basis of centrally induced penile erections. Clinical research with apomorphine has demonstrated efficacy in men who have a range of ED. Recent interest has focused on other centrally acting agents for ED treatment, including the melanocortin receptor agonists.

Melanocortin Receptors and Erectile Function

OBJECTIVE

Review the historical and current evidence that suggests that activation of melanocortin receptors modulates erectile activity.

METHODS

The available literature was reviewed.

RESULTS

Melanocortin peptides derived from the pro-opiomelanocortin (POMC) precursor protein exert a host of diverse physiological effects in the periphery and in the CNS through interactions with one or more of the five cloned melanocortin receptors. Natural and synthetic melanocortin peptide agonists influence erectile and sexual function in a range of preclinical species. Emerging clinical evidence now suggests that the proerectile effects observed in preclinical species are evident in man as well.

CONCLUSIONS

Preclinical and clinical results support the involvement of melanocortins in the modulation of erectile and sexual function. Current evidence indicates that the melanocortin 4 receptor subtype contributes to the proerectile effects observed with pan-receptor agonists. However, the putative receptor subtypes, pathways and mechanisms implicated in mediating the proerectile effects of melanocortins remain to be fully elucidated.

The central melanocortin system and fever

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

beta-MSH: a functional ligand that regulated energy homeostasis via hypothalamic MC4-R?

alpha-Melanocyte stimulating hormone (MSH) has generally been assumed to be the endogenous ligand acting at the melanocortin-4 receptor (MC4-R), activation of which in the hypothalamus leads to reduced feeding. However, beta-MSH is also capable of activating MC4-R and inhibiting feeding. Here, we investigated the possibility that beta-MSH acts as an endogenous MC4-R agonist and that this melanocortin peptide plays a role in the regulation of feeding and energy balance. We found that beta-MSH had significantly higher affinities than alpha-MSH at both human MC4-R transfected into CHO cells (K(i): beta-MSH, 11.4+/-0.4 nmol/l versus alpha-MSH, 324+/-16 nmol/l, P<0.001) and MC4-R in rat hypothalamic homogenates (K(i): beta-MSH, 5.0+/-0.4 nmol/l versus alpha-MSH, 22.5+/-2.3 nmol/l, P<0.001). Incubation of brain slices with 5 microM beta-MSH significantly increased [35S]GTPgammaS binding by 140-160% (P<0.001), indicating activation of G-protein-coupled receptors (GPCRs), in the hypothalamic ventromedial (VMH), dorsomedial (DMH), arcuate (ARC) and paraventricular (PVN) nuclei. These sites match the distribution of beta-MSH immunoreactive fibres and also the distribution of MC4-R binding sites which we and others previously reported. Food-restriction significantly increased beta-MSH levels in the VMH, DMH and ARC (all P<0.05) above freely-fed controls, whilst alpha-MSH concentrations were unchanged. We propose that increased beta-MSH concentrations reflect blockade of the peptide's release in these sites, consistent with the increased hunger and the known up-regulation of MC4-R in the same nuclei. Thus, we conclude that (1). beta-MSH has higher affinity at MC4-R than alpha-MSH; (2). beta-MSH activates GPCR in these sites, which are rich in MC4-R; and (3). beta-MSH is present in hypothalamic nuclei that regulate feeding and its concentrations alter with nutritional state. We suggest that beta-MSH rather than alpha-MSH is the key ligand at the MC4-R populations that regulate feeding, and that inhibition of tonic release of beta-MSH is one mechanism contributing to hunger in under-feeding.

Long-term alterations in body weight do not affect the expression of melanocortin receptor-3 and -4 mRNA in the ovine hypothalamus

The pro-opiomelanocortin-derived peptides and the melanocortin receptors are implicated in various functions within the CNS including the regulation of food intake. In the present study, we used in situ hybridization, with specific 35S-labelled ovine riboprobes to map the expression of melanocortin receptor-3 (MC3-R) and -4 (MC4-R) mRNA in the diencephalon and brainstem of normal female sheep. Furthermore, we examined the effect of long-term alterations in energy balance on the distribution and expression of MC3-R and MC4-R mRNA in food-restricted and ad libitum-fed ovariectomized female sheep. The distribution of melanocortin receptors generally resembled that of the rat. A high number of MC3-R-labelled cells were seen in the ventral division of the lateral septum and the medial preoptic area. In the hypothalamus, a moderate number of MC3-R-labelled cells was observed in the lateral hypothalamic area while other nuclear groups had low to intermediate numbers of MC3-R-labelled cells. The distribution of MC4-R mRNA was generally similar to that of MC3-R mRNA in the septal/preoptic and hypothalamic regions, with a high number of labelled cells present in the intermediate division of the lateral septum. Within the hypothalamus, no MC4-R mRNA expression was observed in the arcuate nucleus. There was more widespread distribution of moderate to low numbers of MC4-R mRNA-expressing cells in the brainstem compared to that of MC3-R mRNA. Unlike findings in the rat, only a low number of cells expressed melanocortin receptor mRNA in the ovine hypothalamic nuclei associated with feeding behavior. The number of melanocortin receptor-labelled cells and the level of expression (silver grains/cell) in the hypothalamic feeding centers was similar in food-restricted and ad libitum-fed animals. These findings suggest that long-term alterations in metabolic status do not change the melanocortin receptor mRNA distribution and/or expression in the sheep hypothalamus.

Development of a homogeneous high throughput fluorescence polarization assay for G protein-coupled receptor binding

Traditional methods that follow receptor ligand interactions are competitive assays in which the test compound displaces a radiolabeled molecule. These assays require either a time-consuming step for separation of free ligands from bound ligands or immobilization of receptors and the scintillant on a solid-phase support. In this report, we describe the development of a homogeneous binding assay for a G protein-coupled receptor in the fluorescence polarization format. This homogeneous fluorescence polarization binding assay format is superior to the traditional binding methods because no radioisotope, separation step, or solid-phase support is required. The elimination of the separation step enhances detection of low-affinity ligands and enables a real-time, continuous readout of the binding activity in a high throughput 384-well microplate format.

Melanocortins and the brain: from effects via receptors to drug targets

The lack of specific receptors (and antagonists) has hampered the research on the neural mechanism of action of adrenocorticotropic hormone (ACTH)- and melanocyte-stimulating hormone (MSH)-like peptides. Yet the original observations in the 1970s already pointed to cAMP as a possible mediator of ACTH/MSH effects in neurons. The cloning of melanocortin receptors since 1992, the identification of at least two subtypes (melanocortin MC(3) and MC(4) receptors) that are present in neural tissue and the development of selective and potent agonists as well as antagonists have markedly furthered the position of melanocortins as important neuropeptides. In this paper we discuss the role of especially the receptor subtype melanocortin MC(4) in various behaviors including grooming behavior and feeding behavior and consider new insights in the interaction between the opioid and the melanocortin system at the level of the spinal cord (i.e. pain perception). Finally, based on new data obtained in molecular pharmacological studies on brain melanocortin receptors, we suggest a general concept for selective receptor-ligand interaction: ligand residues outside the peptide core-sequence may direct the conformation of the residues in the ligand core-sequence that interact directly with the receptor-binding pocket and thereby determine selectivity.

Conformation of the core sequence in melanocortin peptides directs selectivity for the melanocortin MC3 and MC4 receptors

Melanocortin peptides regulate a variety of physiological processes. Five melanocortin receptors (MC-R) have been cloned and the MC3R and MC4R are the main brain MC receptors. The aim of this study was to identify structural requirements in both ligand and receptor that determine gamma-melanocyte-stimulating hormone (MSH) selectivity for the MC3R versus the MC4R. Substitution of Asp10 in [Nle4]Lys-gamma2-MSH for Gly10 from [Nle4]alpha-MSH, increased both activity and affinity for the MC4R while the MC3R remained unaffected. Analysis of chimeric MC3R/MC4Rs and mutant MC4Rs showed that Tyr268 of the MC4R mainly determined the low affinity for [Nle4]Lys-gamma2-MSH. The data demonstrate that Asp10 determines selectivity for the MC3R, however, not through direct side chain interactions, but probably by influencing how the melanocortin core sequence is presented to the receptor-binding pocket. This is supported by mutagenesis of Tyr268 to Ile in the MC4R which increased affinity and activity for [Nle4]Lys-gamma2-MSH, but decreased affinity for two peptides with constrained cyclic structure of the melanocortin core sequence, MT-II and [D-Tyr4]MT-II, that also displayed lower affinity for the MC3R. This study provides a general concept for peptide receptor selectivity, in which the major determinant for a selective receptor interaction is the conformational presentation of the core sequence in related peptides to the receptor-binding pocket.

Autoradiographic discrimination of melanocortin receptors indicates that the MC3 subtype dominates in the medial rat brain

In the present study, we developed an autoradiographic method to visualize the distribution of melanocortin (MC) receptors 3 and 4 in sagittal sections of the rat brain. The method takes advantage of the MC3 and MC4 receptor selective compounds, gamma1-MSH and HS014. First, we characterized the binding of gamma1-MSH, HS014 and the radioligand [125I]NDP-MSH to the rat MC3 and MC4 receptors expressed in COS cells. [125I]NDP-MSH was found to be non-selective, whereas gamma1-MSH showed a 40-fold preference for the rat MC3 receptor, and HS014 an over 300-fold preference to the rat MC4 receptor. Second, to discriminate between the MC3 and MC4 receptors in rat brain sections, the sections were incubated with [125I]NDP-MSH in the presence of graded concentrations of the MC3 selective ligand, gamma1-MSH, or the MC4 selective ligand, HS014. From the autoradiograms thus made, competition curves of gamma1-MSH and HS014 could be constructed for different regions of the rat brain. Our results indicate that in the nucleus accumbens shell, the medial preoptic area, and the ventromedial nucleus of the hypothalamus, there is a clear dominance of the MC3 receptor, whereas in the lateral septum and the olfactory tubercle, there seem to be present both MC3 and MC4 receptors, although the MC3 receptor may still be the dominating subtype. In the optic layer of the superior colliculus, our data indicate a more abundant expression of the MC4 receptor. In the ventral tegmental area, there might be an additional MSH-peptide binding site of unknown origin.

Melanocortins and cardiovascular regulation

The melanocortins form a family of pro-opiomelanocortin-derived peptides that have the melanocyte-stimulating hormone (MSH) core sequence, His-Phe-Arg-Trp, in common. Melanocortins have been described as having a variety of cardiovascular effects. We review here what is known about the sites and mechanisms of action of the melanocortins with respect to their effects on cardiovascular function, with special attention to the effects of the gamma-melanocyte-stimulating hormones (gamma-MSHs). This is done in the context of present knowledge about agonist selectivity and localisation of the five melanocortin receptor subtypes cloned so far. gamma2-MSH, its des-Gly12 analog (= gamma1-MSH) and Lys-gamma2-MSH are 5-10 times more potent than adrenocorticotropic hormone-(4-10)(ACTH-(4-10)) to induce a pressor and tachycardiac effect following intravenous administration. The Arg-Phe sequence near the C-terminal seems to be important for full in vivo intrinsic activity. Related peptides with a C-terminal extension with (gamma3-MSH) or without the Arg-Phe sequence (alpha-MSH, as well as the potent alpha-MSH analog, [Nle4,D-Phe7]alpha-MSH), are, however, devoid of these effects. In contrast, ACTH-(1-24) has a depressor effect combined with a tachycardiac effect, effects which are not dependent on the presence of the adrenals. Although the melanocortin MC3 receptor is the only melanocortin receptor subtype for which gamma2-MSH is selective, in vivo and in vitro structure-activity data indicate that it is not via this receptor that this peptide and related peptides exert either their pressor and tachycardiac effects or their extra- and intracranial blood flow increasing effect. We review evidence that the pressor and tachycardiac effects of the gamma-MSHs are due to an increase of sympathetic outflow to the vasculature and the heart, secondary to activation of centrally located receptors. These receptors are most likely localised in the anteroventral third ventricle (AV3V) region, a brain region situated outside the blood-brain barrier, and to which circulating peptides have access. These receptors might be melanocortin receptors of a subtype yet to be identified. Alternatively, they might be related to other receptors for which peptides with a C-terminal Arg-Phe sequence have affinity, such as the neuropeptide FF receptor and the recently discovered FMRFamide receptor. Melanocortin MC4 receptors and still unidentified receptors are part of the circuitry in the medulla oblongata which is involved in the depressor and bradycardiac effect of the melanocortins, probably via interference with autonomic outflow. Regarding the effects of the gamma-MSHs on cortical cerebral blood flow, it is not yet clear whether they involve activation of the sympathetic nervous system or activation of melanocortin receptors located on the cerebral vasculature. The depressor effect observed following intravenous administration of ACTH-(1-24) is thought to be due to activation of melanocortin MC2 receptors whose location may be within the peripheral vasculature. Melanocortins have been observed to improve cardiovascular function and survival time in experimental hemorrhagic shock in various species. Though ACTH-(1-24) is the most potent melanocortin in this model, alpha-MSH and [Nle4,D-Phe7]alpha-MSH and ACTH-(4-10) are quite effective as well. As ACTH-(4-10) is a rather weak agonist of all melanocortin receptors, it is difficult to determine via which of the melanocortin receptors the melanocortins bring about this effect. Research into the nature of the receptors involved in the various cardiovascular effects of the melanocortins would greatly benefit from the availability of selective melanocortin receptor antagonists.

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.

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.

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.

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.

Melanocortins and opiate addiction

Adrenocorticotropic hormone (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) are centrally acting melanocortin peptides with numerous reported functions, including induction of excessive grooming and antipyresis, among others. Also reported is a role for melanocortins in aspects of opiate action. Although early work examined the effects of ACTH and MSH on opiate-induced behaviors, further progress has been limited. Recently, however, advances in the identification and characterization of melanocortin receptor (MC-R) subtypes have provided novel tools with which to study interactions between melanocortins and addiction. The present review discusses the effects of ACTH and MSH on opiate-induced behaviors and relates these findings to more recent reports on the regulation of melanocortin systems by exogenous opiates. Emerging from these data is the possibility that melanocortin receptor activation, specifically at the MC4-R subtype, may act to antagonize certain properties of exogenous opiates, including perhaps addiction.

Prevention of ACTH- and adrenalectomy-induced muricidal behavior: by benzodiazepinic ligands

We investigated the effect of treatment with central (neuronal and glial) benzodiazepine binding site-active molecules on ACTH- or adrenalectomy (ADX)-induced muricidal behavior in male Wistar rats. Pretreatment (IP) with either flumazenil or clonazepam prevented the subsequent induction of ADX-induced behavior, but only flumazenil protected against ACTH-induced behavior; posttreatment in both cases induced no significant modifications. Using 4'-chloro-diazepam or PK 11195, both pre- and posttreatment afforded protection, the effect lasting longer (> 1 week) than that induced by flumazenil or clonazepam (2 days). Pretreatment with the GABAA agonist, muscimol, also resulted in complete protection, whereas posttreatment had only a slight effect.

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.

Region- and stage-specific patterns of melanocortin receptor ontogeny in rat central nervous system, cranial nerve ganglia and sympathetic ganglia

Observations on developmental actions of melanotropic peptides in nervous system have been difficult to interpret in the absence of data on receptor ontogeny. We investigated binding of [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP) in developing Long Evans rats from gestational day (E) 13 by quantitative autoradiography. Regional [125I]NDP binding characteristics were assessed by competition experiments in early postnatal brain. The study revealed region- and stage-specific, often transient ontogenetic patterns. Sympathetic ganglia exhibit high [125I]NDP binding from E13, with a peak in superior cervical ganglion at E16-E18. The first central [125I]NDP binding sites transiently appear in parts of thalamus between E13 and E15. The early fetal period is characterized by prominent peaks of receptor density in somatosensory and viscerosensory nuclei (trigeminal sensory nuclei, solitary tract nucleus), paralleled by receptor expression in 5th, 7th, 9th and 10th cranial nerve ganglia. During late fetal life, receptor density peaks in dorsal motor nucleus of vagus and inferior olive; binding sites transiently appear in cerebellum. Caudate-putamen, nucleus accumbens, olfactory tubercle and septohippocampal nucleus show a high perinatal maximum. Starting with late fetal piriform cortex, [125I]NDP binding peaks sequentially in cerebral cortical areas, with highest levels in entorhinal cortex. Preoptic, septal, hypothalamic and amygdaloid areas known for elevated receptor densities in adulthood, exhibit a slow, peri- and postnatal receptor ontogeny. Temporal relations to regional developmental processes support the idea of a role of melanocortins during ontogeny.

Opioid peptide inhibition of endogenous norepinephrine release from the A2 noradrenergic cell group in vitro

We investigated the potential influence of opioid and melanotropic peptides on endogenous norepinephrine (NE) release from the A2 noradrenergic cell group of rats using a static, fixed-volume incubation procedure. Norepinephrine release from slices of caudal dorsomedial medulla (CDMM) was evoked by high potassium concentrations (20 and 60 mM) in a Ca(2+)-dependent and dose-related manner. Treatment with the potent melanotropin agonist [Nle4,D-Phe7] alpha-MSH(NDP-MSH) had no effect on K(+)-induced NE release. In contrast, human beta-endorphin1-31 significantly reduced K(+)-stimulated NE release, but not in the presence of naloxone. The highly-selective mu-opioid agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) also significantly reduced evoked NE release. The inhibitory effect of DAMGO was completely blocked by naloxone. Naloxone alone did not alter evoked NE release. The inhibitory effect of DAMGO was not enhanced by reducing the stimulatory concentration of K+. None of the peptides tested influenced basal NE release. These data indicate that melanotropin receptors do not regulate NE release in CDMM. In contrast, the opioid peptides DAMGO and beta-endorphin inhibit K(+)-stimulated release of endogenous NE. These data suggest a role for mu-opioid receptors in controlling NE release from A2 noradrenergic neurons.

ACTH-(1-24) blocks the decompensatory phase of the haemodynamic response to acute hypovolaemia in conscious rabbits

Graded caval occlusion in conscious rabbits caused a biphasic cardiovascular response. Phase I was characterized by a fall in systemic vascular conductance so that arterial pressure was maintained. When cardiac output had fallen to 64 +/- 3% of its baseline level, phase II supervened. During phase II, conductance rose abruptly and arterial pressure fell to a life-threatening level (< 40 mm Hg). Intravenous (i.v.) or central (fourth ventricular) administration of the adrenocorticotrophin (ACTH) fragment ACTH-(1-24) prevented the occurrence of phase II. The central dose of ACTH-(1-24) needed to block the occurrence of phase II was approximately 39 times less than the i.v. dose. Central administration of the delta 1-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) reversed this effect of both central and i.v. ACTH-(1-24). I.v. ACTH-(1-24) also lowered arterial pressure while raising cardiac output and vascular conductance. These effects were insensitive to propranolol and hyoscine methyl bromide, and were not mimicked by cortisol or adrenaline. It is concluded that ACTH-(1-24) has an acute, adrenal-independent, peripheral vasodilator effect as well as a central, anti-shock, effect.

Characterisation of melanocortin receptor subtypes by radioligand binding analysis

The DNAs encoding three melanocortin receptor subtypes (melanocortin MC1 receptor, melanocortin MC3 receptor and melanocortin MC5 receptor) were expressed individually in COS (CV-1 Origin, SV40) cells to characterise their ligand binding properties. The results indicated that [125I][Nle4, D-Phe7]alpha-MSH (melanocyte stimulating hormone) bound to a single saturable site with Kd values of 85.1 +/- 8.0 pmol/l (mean +/- S.E.M), 396 +/- 65 pmol/l and 5.05 +/- 1.00 nmol/l for melanocortin MC1 receptor, melanocortin MC3 receptor and melanocortin MC3 receptor, respectively. The melanocortin MC1 receptor and the melanocortin MC5 receptor showed a similar potency order to the melanocortic peptides examined which was markedly different from the potency order of the melanocortin MC3 receptor. The melanocortin MC1 receptor and melanocortin MC5 receptor had a relatively higher affinity for alpha-MSH than gamma-MSH and beta-MSH, whereas the melanocortin MC3 receptor had higher affinity for desacetyl-alpha-MSH, gamma-MSH and beta-MSH compared to alpha-MSH. The inclusion of the endopeptidase inhibitor phosphoramidon to prevent the breakdown of ACTH-(1-39) (adrenocorticotrophic hormone) to alpha-MSH, decreased ACTH-(1-39) binding affinity showing that ACTH-(1-39) had a much lower affinity for melanocortin MC1 receptor than reported earlier.

Melanocortin analogue Org2766 binds to rat Schwann cells, upregulates NGF low-affinity receptor p75, and releases neurotrophic activity

Binding of the stable melanocortin(4-9) analogue, Org2766 [Met(O2)-Glu-His-Phe-D-Lys-Phe] to cultured rat sciatic nerve Schwann cells was demonstrated using a biotinylated derivative in semiquantitative histochemical and CELISA assays. Org2766 bound to Schwann cells, but not to fibroblasts, and was displaced maximally by unlabeled Org2766, alpha-MSH and ACTH(1-24). Displacement of Org2766 from the binding sites was considerably reduced by N- and C-truncation of the peptide. Specific binding of Org2766 was also demonstrated in the immortal rat Schwann cell line SCL4.1/F7 and was more pronounced in cells displaying a differentiated morphology. Org2766 and alpha-MSH increased cyclic AMP content of Schwann cells but neither stimulated DNA synthesis when applied alone. However, in the presence of a priming (subthreshold) concentration of the mitogen, cholera toxin, Org2766 and alpha-MSH caused a delayed increase in DNA synthesis. Org2766 did not modulate the expression of several differentiation-related Schwann cell markers. However, Org2766 increased immunoreactivity for p75 low-affinity NGF receptor on Schwann cells and evoked the release of neurotrophic factor(s) that synergized with NGF in stimulating neurite outgrowth in rat DRG neurons. The results indicate that Schwann cells are a primary target for the action of Org2766 and provide evidence for an indirect mechanism by which melanocortins might stimulate neurite sprouting in regenerating peripheral nerve axons.

ACTH-related peptides: receptors and signal transduction systems involved in their neurotrophic and neuroprotective actions

ACTH-related peptides are promising neurotrophic and neuroprotective agents, as demonstrated in many in vivo and in vitro studies. They accelerate nerve repair after injury, improving both sensor and motor function. Furthermore, ACTH-related peptides have neuroprotective properties against cisplatin- and taxol-induced neurotoxicity, they improve neuronal function in animals with neuropathy due to experimental diabetes, and they prevent degeneration of myelinated axons in rats suffering from experimental allergic neuritis, a model of peripheral demyelinating neuropathy. Studies in neuronal cultures have corroborated these clinical observations and serve to investigate the mechanism of action of the ACTH-related peptide effects. This paper reviews both in vitro and in vivo effects and emphasizes the mechanism of action. Recent data on melanotrophic receptors and signal transduction systems will be discussed in this context.