Neuropathic pain: a possible role for the melanocortin system?

Effects of intra-nasal melanocortin-4 receptor antagonist on trigeminal neuropathic pain in male and female rats

Treatment of chronic orofacial pain remains a major therapeutic challenge despite available medications. Melanocortins have been implicated in pathologic pain. Intrathecal administration of MC4R antagonists has been shown to alleviate neuropathic pain (NP) in male rats. However, intrathecal delivery is very invasive and requires surgeon's intervention. Intra-nasal rout offers a non-invasive drug delivery method that can be self-administered making it very attractive clinically. In this study, we investigated the effects of intra-nasally delivered MC4R antagonist (HS014) on trigeminal neuropathic pain (TNP) in male and female rats. We also measured the MC4R protein levels in the trigeminal ganglia (TG) and infraorbital nerve (ION) of rats. We used ION chronic constriction injury (ION-CCI) to induce TNP in rats. We used von Frey and pinprick assays to measure the development of hypersensitivity in the face following ION-CCI. At 22 days post-ION-CCI, we delivered HS014 intra-nasally to measure its effects on TNP in rats. We used enzyme linked immunosorbent assay to measure MC4R protein levels in the TG and ION. ION-CCI resulted in a significant increase of MC4R protein levels in the ipsilateral TG and ION of male and female rats. Intra-nasal delivered HS014 resulted in a significant reduction of ION-CCI induced hypersensitivity in male and female rats. These results demonstrate that intranasal delivery of MC4R antagonist alleviated TNP in male and female rats and suggest that such treatment could be beneficial therapeutically for individuals with chronic NP.

Influence of Selective Melanocortin-4 Receptor Antagonist HS014 on Hypersensitivity After Nervous System Injuries in a Model of Rat Neuropathic Pain: A Narrative Review of the Literature

Background and Objective: The melanocortin-4 (MC4) receptor has been evaluated as a possible new therapeutic for neuropathic pain treatment. The purpose of this review article was to review and evaluate all recent in vivo studies on the effect of the MC4 receptor antagonist HS014 on rat hypersensitivity caused by neuropathic pain.

Methods: An electronic search was carried out using Scopus, Web of Science, PubMed, and Google Scholar. The following inclusion criteria were used: rat models of neuropathic pain-induced hypersensitivity, with investigated effects of the selective antagonist HS014. The included duration of the search was within the last ten years. Data regarding HS014, neuropathic pain model, post-treatment administration time and dose (days post-injury), behavior assessment assays, treatment frequency, and route of delivery were collected and subjected descriptively as complementary data in this narrative review.

Results: This narrative review included four papers that fulfilled the eligibility criteria. The findings demonstrate that as compared to vehicle-treated rats, administration of the MC4 receptor antagonist HS014 remarkably raised paw withdrawal threshold (PWT) in three studies and heat withdrawal latency in four studies among rat models subjected to neuropathic pain.

Conclusions: In rat neuropathic pain models, the MC4 receptor antagonist HS014 is helpful in reducing hypersensitivity. However, further studies are needed to determine the ideal treatment dosage and timing. In addition, further investigations are required for the role of this selective receptor antagonist (HS014) and compared with other types of MC4 receptors in neuropathic pain in humans.

Effects of melanocortin-4 receptor (MC4R) antagonist on neuropathic pain hypersensitivity in rats - A systematic review and meta-analysis

Melanocortin-4 receptor (MC4R) has been investigated as a potential drug target for the treatment of neuropathic pain. The objective of the study was to systematically identify the effects of MC4R antagonists on hypersensitivity in rat models of neuropathic pain. A systematic search was conducted using the following databases: WoS, PubMed, SCOPUS, and MEDLINE. Inclusion criteria were: rat hypersensitivity induced by models of neuropathic pain with reported effects of MC4R antagonist. Two researchers performed the selection process and data extraction. SYRCLE risk of bias tool was used. Standard mean differences (SMD) were calculated and pooled by meta-analysis using random effect models. Ten articles met the eligibility criteria and were included in the systematic review and meta-analysis. The results reveal that, in animals exposed to neuropathic pain, administration of MC4R antagonists significantly increased paw withdrawal threshold (SHU9119 SMD = 1.67, 95% CI: [0.91, 2.44], I²  = 0%; HS014 SMD = 2.2, 95% CI: [0.53, 3.87], I²  = 71%) and heat withdrawal latency (HS014 SMD = 3.35, 95% CI: [0.56, 6.14], I²  = 83%) compared to vehicle-treated animals. MC4R antagonists are effective in the alleviation of hypersensitivity in rodent neuropathic pain models. SHU9119 and HS014 antagonists showed the most prominent results. However, further investigation is needed to determine the optimal dose and time of treatment.

Mental imagery training for treatment of central neuropathic pain: a narrative review

Mental imagery is a quasi-perceptual experience in the absence of external stimuli. This concept has intrigued psychologists, sportspersons, neurologists and other scientists for over a decade now. Imagery has been used in rehabilitation and the results have been promising. Researchers refer to this as healing the body through the mind. However, the challenge is lack of standardized protocols, homogeneity and consistency in application of mental imagery in different populations. The purpose of this review is to discuss and understand the role of mental imagery in the treatment of central neuropathic pain (CNP). Treatment options of CNP are inadequate and their benefits are short lived. We conducted an extensive search on various databases using combinations of different keywords and reviewed the available literature in this area. We were able to finalize twelve studies where mental imagery was used for treating CNP in spinal cord injury (SCI), stroke and multiple sclerosis. However, the methodology and techniques of mental imagery training used in these studies were non-homogeneous and inconsistent. This review provides a guiding framework to further explore the different techniques of mental imagery and their roles in treating CNP.

A model of neuropathic pain in brachial plexus avulsion injury and associated spinal glial cell activation

BACKGROUND

Neuropathic pain is a common and intractable sequel of brachial plexus injury.

MATERIALS AND METHODS

To investigate the underlying mechanisms, we established a unique model of neuropathic pain in rats by creating brachial plexus avulsion injury.

RESULTS

Behavioral test of mechanical stimulation suggested that all rats developed neuropathic pain, and the pain thresholds of bilateral hind limbs significantly decreased. GFAP expression in the cervical spinal cord appeared on day 1 post-injury and increased on day 4. Ionized calcium-binding adaptor molecule 1 expression appeared on day 1 post-injury and increased until day 28. Therefore, the brachial plexus avulsion injury model can imitate the development of neuropathic pain and maintain it.

CONCLUSION

The activation of astrocyte and microglia in the spinal cord plays a key role in the mechanism and treatment of neuropathic pain.

Plate-Based Phenotypic Screening for Pain Using Human iPSC-Derived Sensory Neurons

Screening against a disease-relevant phenotype to identify compounds that change the outcome of biological pathways, rather than just the activity of specific targets, offers an alternative approach to find modulators of disease characteristics. However, in pain research, use of in vitro phenotypic screens has been impeded by the challenge of sourcing relevant neuronal cell types in sufficient quantity and developing functional end-point measurements with a direct disease link. To overcome these hurdles, we have generated human induced pluripotent stem cell (hiPSC)-derived sensory neurons at a robust production scale using the concept of cryopreserved "near-assay-ready" cells to decouple complex cell production from assay development and screening. hiPSC sensory neurons have then been used for development of a 384-well veratridine-evoked calcium flux assay. This functional assay of neuronal excitability was validated for phenotypic relevance to pain and other hyperexcitability disorders through screening a small targeted validation compound subset. A 2700-compound chemogenomics screen was then conducted to profile the range of target-based mechanisms able to inhibit veratridine-evoked excitability. This report presents the assay development, validation, and screening data. We conclude that high-throughput-compatible pain-relevant phenotypic screening with hiPSC sensory neurons is feasible and ready for application for the identification of new targets, pathways, mechanisms of action, and compounds for modulating neuronal excitability.

Structural Insights into Selective Ligand-Receptor Interactions Leading to Receptor Inactivation Utilizing Selective Melanocortin 3 Receptor Antagonists

Systematic N-methylated derivatives of the melanocortin receptor ligand, SHU9119, lead to multiple binding and functional selectivity toward melanocortin receptors. However, the relationship between N-methylation-induced conformational changes in the peptide backbone and side chains and melanocortin receptor selectivity is still unknown. We conducted comprehensive conformational studies in solution of two selective antagonists of the third isoform of the melanocortin receptor (hMC3R), namely, Ac-Nle-c[Asp-NMe-His⁶-d-Nal(2')⁷-NMe-Arg⁸-Trp⁹-Lys]-NH₂ (15) and Ac-Nle-c[Asp-His⁶-d-Nal(2')⁷-NMe-Arg⁸-NMe-Trp⁹-NMe-Lys]-NH₂ (17). It is known that the pharmacophore (His⁶-DNal⁷-Arg⁸-Trp⁹) of the SHU-9119 peptides occupies a β II-turn-like region with the turn centered about DNal⁷-Arg⁸. The analogues with hMC3R selectivity showed distinct differences in the spatial arrangement of the Trp⁹ side chains. In addition to our NMR studies, we also carried out molecular-level interaction studies of these two peptides at the homology model of hMC3R. Earlier chimeric human melanocortin 3 receptor studies revealed insights regarding the binding and functional sites of hMC3R selectivity. Upon docking of peptides 15 and 17 to the binding pocket of hMC3R, it was revealed that Arg⁸ and Trp⁹ side chains are involved in a majority of the interactions with the receptor. While Arg⁸ forms polar contacts with D154 and D158 of hMC3R, Trp⁹ utilizes π-π stacking interactions with F295 and F298, located on the transmembrane domain of hMC3R. It is hypothesized that as the frequency of Trp⁹-hMC3R interactions decrease, antagonistic activity increases. The absence of any interactions of the N-methyl groups with hMC3R suggests that their primary function is to modulate backbone conformations of the ligands.

Systematic Backbone Conformational Constraints on a Cyclic Melanotropin Ligand Leads to Highly Selective Ligands for Multiple Melanocortin Receptors

Human melanocortin receptors (hMCRs) have been challenging targets to develop ligands that are explicitly selective for each of their subtypes. To modulate the conformational preferences of the melanocortin ligands and improve the biofunctional agonist/antagonist activities and selectivities, we have applied a backbone N-methylation approach on Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2), a nonselective cyclic peptide antagonist at hMC3R and hMC4R and an agonist at hMC1R and hMC5R. Systematic N-methylated derivatives of Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2, with all possible backbone N-methylation combinations, have been synthesized and examined for their binding and functional activities toward melanocortin receptor subtypes 1, 3, 4, and 5 (hMCRs). Several N-methylated analogues are selective and potent agonists or antagonists for hMC1R or hMC5R or have selective antagonist activity for hMC3R. The selective hMC1R ligands show strong binding for human melanoma cells. We have also discovered the first universal antagonist (compound 19) for all subtypes of hMCRs.

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.

Melanocortin MC(4) receptor-mediated feeding and grooming in rodents

Decades ago it was recognized that the pharmacological profile of melanocortin ligands that stimulated grooming behavior in rats was strikingly similar to that of Xenopus laevis melanophore pigment dispersion. After cloning of the melanocortin MC1 receptor, expressed in melanocytes, and the melanocortin MC4 receptor, expressed mainly in brain, the pharmacological profiles of these receptors appeared to be very similar and it was demonstrated that these receptors mediate melanocortin-induced pigmentation and grooming respectively. Grooming is a low priority behavior that is concerned with care of body surface. Activation of central melanocortin MC4 receptors is also associated with meal termination, and continued postprandial stimulation of melanocortin MC4 receptors may stimulate natural postprandial grooming behavior as part of the behavioral satiety sequence. Indeed, melanocortins fail to suppress food intake or induce grooming behavior in melanocortin MC4 receptor-deficient rats. This review will focus on how melanocortins affect grooming behavior through the melanocortin MC4 receptor, and how melanocortin MC4 receptors mediate feeding behavior. This review also illustrates how melanocortins were the most likely candidates to mediate grooming and feeding based on the natural behaviors they induced.

Expression of activating transcription factor 3 (ATF3) in uninjured dorsal root ganglion neurons in a lower trunk avulsion pain model in rats

PURPOSE

Clinically, neuropathic pain is frequent and intense following brachial plexus injury. It is thought that brachial plexus pain is not generated by avulsed roots, but rather by non-avulsed roots, since the avulsed root could not possibly transmit action potentials to central nerves. The aim of this study was to evaluate pain behavior and activation of sensory neurons in a brachial plexus avulsion (BPA) model in rats.

METHODS

Fifteen male Wistar rats were used. In the BPA group, the C8-T1 roots were avulsed from the spinal cord with forceps at the lower trunk level (n = 5). In the naïve group, rats did not receive any procedures (n = 5). In the sham-operated group, the lower trunk was simply exposed (n = 5). Mechanical hyperalgesia of forelimbs corresponding to C6 and C7 dermatomes was measured using von Frey filaments every third day for 3 weeks. Activation of DRG neurons was immunohistochemically examined using anti-ATF3 (a marker for neuron activation) antibodies 21 days after surgery. Von Frey and immunohistochemical data between groups were analyzed using a Kruskal-Wallis test, followed by Mann-Whitney U tests. Bonferroni corrections were performed.

RESULTS

Animals in the BPA group displayed significant mechanical hyperalgesia at the dermatome innervated by uninjured nerves continuing through day 21 compared with animals in the sham-operated group. ATF3-immunoreactive small and large DRG neurons were significantly activated in the BPA group (10.6 ± 9.5 and 5.2 ± 4.1 %, 39.7 ± 6.7 and 25.2 ± 10.3 %, 78.0 ± 9.1 and 53.7 ± 29.3 %) compared with the sham-operated group (0.7 ± 0.9 and 0 ± 0 %, 2.8 ± 2.0 and 1.0 ± 2.0 %, 3.9 ± 2.7 and 8.6 ± 10.1 %) at every level of C5, 6, and 7. In the naïve group, no DRG neurons were activated. ATF3-immunoreactive small and large DRG neurons were significantly activated at the level of C7 compared with C6 and C5, and significantly activated at the level of C6 compared with C5 in the BPA group.

CONCLUSIONS

Expression of ATF3 in uninjured DRG neurons may contribute to pain following brachial plexus avulsion injury. Consequently, spared spinal sensory nerves may represent therapeutic targets for treatment of this pain.

Melanocortin 4 receptor activation protects against testicular ischemia-reperfusion injury by triggering the cholinergic antiinflammatory pathway

Melanocortins (MC) trigger a vagus nerve-mediated cholinergic-antiinflammatory pathway projecting to the testis. We tested whether pharmacological activation of brain MC receptors might protect the testis from the damage induced by ischemia-reperfusion. Adult male rats were subjected to 1-h testicular ischemia, followed by 24-h reperfusion [testicular ischemia-reperfusion (TI/R)]. Before TI/R, groups of animals were subjected to bilateral cervical vagotomy, or pretreated with the nicotinic acetylcholine receptor antagonist chlorisondamine or the selective MC(4) receptor antagonist HS024. Immediately after reperfusion, rats were ip treated with saline or the MC analog [Nle(4),D-Phe(7)]α-melanocyte-stimulating hormone (NDP-α-MSH) (340 μg/kg). We evaluated testicular IL-6 and TNF-α by Western blot analysis and organ damage by light microscopy. Some experimental groups were prepared for neural efferent activity recording along the vagus nerve starting 30 min after treatment with NDP-α-MSH or saline, and for a 30-min period. Additional groups of TI/R rats were treated for 30 d with saline, NDP-α-MSH, chlorisondamine plus NDP-α-MSH, or HS024 plus NDP-α-MSH to evaluate spermatogenesis, organ damage, and the apoptosis machinery. After a 24-h reperfusion, in TI/R saline-treated rats, there was an increase in IL-6 and TNF-α expression and a marked damage in both testes. NDP-α-MSH inhibited IL-6 and TNF-α expression, decreased histological damage, and increased neural efferent activity. Furthermore, NDP-α-MSH administration for 30 d greatly improved spermatogenesis, reduced organ damage, and inhibited apoptosis. All positive NDP-α-MSH effects were abrogated by vagotomy, chlorisondamine, or HS024. Our data suggest that selective MC(4) receptor agonists might be therapeutic candidates for the management of testicular torsion.

Gene therapy for chronic neuropathic pain: how does it work and where do we stand today?

OBJECTIVES

Chronic neuropathic pain has been an enigma to physicians and researchers for decades. A better understanding of its pathophysiology has given us more insight into its various mechanisms and possible treatment options. We now have an understanding of the role of various ionic channels, biologically active molecules involved in pain, and also the intricate pain pathways where possible interventions might lead to substantial pain relief. The recent research on laboratory animals using virus-based vectors for gene transfer at targeted sites is very promising and may lead to additional human clinical trials. However, one needs to be aware that this "novel" approach is still in its infancy and that many of its details need to be further elucidated. The purpose of this article is to thoroughly review the current available literature and analyze the deficiencies in our current knowledge.

DESIGN

Literature review.

METHODS

After an extensive online literature search, a total of 133 articles were selected to synthesize a comprehensive review about chronic neuropathic pain and gene therapy in order to understand the concepts and mechanisms.

RESULTS

Most of the studies have shown benefits of gene therapy in animal models, and recently, phase 1 human trials using herpes simplex virus vector have started for intractable cancer pain.

CONCLUSION

Although animal data have shown safety and efficacy, and initial human trials have been promising, additional studies in humans are required to more completely understand the actual benefits and risks of using gene therapy for the treatment of chronic neuropathic pain.

Central administration of neuronostatin induces antinociception in mice

Neuronostatin, a recently discovered endogenous bioactive peptide, was encoded by pro-mRNA of somatostatin that contributes to modulation of nociception. However, nociceptive effect of neuronostatin is still not fully known. The aim of this study was to evaluate effect of neuronostatin on nociception and elucidate its possible mechanism of action. Intracerebroventricular (i.c.v.) administration of neuronostatin (0.3, 3, 6, 12nmol/mouse) produced a dose- and time-related antinociceptive effect in the tail immersion assay in mice, an acute pain model. The antinociceptive effect of neuronostatin was significantly antagonized by naloxone, and was strongly inhibited by co-injection with β-funaltrexamine or nor-binaltorphimine, but not by naltrindole. Also, melanocortin 3/4 receptor antagonist, SHU9119, completely blocked the effect of neuronostatin. These data indicated the involvement of both μ- and κ-opioid receptors and central melanocortin system in the analgesic response induced by neuronostatin. In addition, neuronostatin (6nmol, i.c.v.) increased c-Fos protein expression in the periaqueductal gray (PAG) and the nucleus raphe magnus (NRM) that have a pivotal role in regulating descending pain pathways. Taken together, this study is the first to reveal that neuronostatin produces antinociceptive effect via opioid and central melanocortin systems, which is associated with an increase in neuronal activity the PAG and NRM.

Hypothalamic opioid-melanocortin appetitive balance and addictive craving

Whilst the parallels between drug and food craving are receiving increasing attention, the recently elucidated complex physiology of the hypothalamic appetite regulatory centres has been largely overlooked in the efforts to understand drug craving which is one of the most refractory and problematic aspects of drug and behavioural addictions. Important conceptual gains could be made by researchers from both appetite and addiction neuroscience if they were to have an improved understanding of each others' disciplines. It is well known in addiction medicine that the use of many substances is elevated in opiate dependency. There is voluminous evidence of very high rates of drug use in opiate agonist maintained patients, and the real possibility exists that opiate agonist therapy therefore increases drug craving. Conversely, opiate antagonist therapy with naloxone or naltrexone has been shown to reduce most chemical and behavioural addictions, and naltrexone is now being developed together with bupropion as the anti-obesity drug "Contrave". Hypothalamic melanocortins, particularly α-MSH, are known to constitute the main brake to consumptive behaviour of food. There is a well described antagonism between melanocortins and opioids at many loci including the hypothalamus. Administration of exogenous opiates is known to both suppress α-MSH and to stimulate hedonic food consumption. Opiate maintenance programs are associated with weight gain. As monoamines, opioids and cannabinoids are known to be involved in appetite regulation, and as endorphin opioids are known to be perturbed in other addictions, further exploration of the hypothalamic appetite regulatory centre would appear to be an obvious, albeit presently largely overlooked, locus in which to study drug and other craving mechanisms.

Melanocortin 4 receptor stimulation decreases pancreatitis severity in rats by activation of the cholinergic anti-inflammatory pathway

OBJECTIVE

Acute pancreatitis is an inflammatory condition that may lead to multisystemic organ failure. Melanocortin peptides have been successfully used in experimental models of organ failure and shock, and their protective effect occurs through the activation of a vagus nerve-mediated cholinergic anti-inflammatory pathway by acting at brain melanocortin 4 receptors. In the light of these observations, we studied the effects of the selective melanocortin 4 receptor agonist RO27-3225 in an experimental model of cerulein-induced pancreatitis.

DESIGN

Randomized experiment.

SETTING

Research laboratory at a university hospital.

SUBJECT

Experimental pancreatitis in rats.

INTERVENTIONS

Acute pancreatitis was induced in male Sprague-Dawley rats by intraperitoneal injections of cerulein (80 μg/kg, four injections at hourly intervals). Before pancreatitis induction, groups of animals were subjected to bilateral cervical vagotomy, pretreated with the nicotinic acetylcholine receptor antagonist chlorisondamine or the selective melanocortin 4 receptor antagonist HS024, or not pretreated. Thirty minutes after the first cerulein injection, rats were intraperitoneally treated with a nanomolar dose of RO27-3225 or vehicle. Some experimental groups were prepared for neural efferent activity recording along the vagus nerve starting 30 mins after treatment with RO27-3225 or vehicle, and for a 30-min period.

MEASUREMENTS AND MAIN RESULTS

Serum lipase and amylase activity, tumor necrosis factor-α and interleukin-6 expression, pancreatic myeloperoxidase activity, and histologic damage were evaluated; neural efferent activity of vagal fibers was also assessed. RO27-3225 reduced cerulein-induced serum lipase and amylase activity, blunted the expression of tumor necrosis factor-α and interleukin-6, abated the increase in pancreatic myeloperoxidase activity, and protected against histologic damage. Furthermore, RO27-3225 markedly increased neural efferent activity along the vagus nerve. Vagotomy, chlorisondamine, and HS024 abated these protective effects of RO27-3225.

CONCLUSIONS

Our data show that melanocortin 4 receptor agonists reduce pancreatitis severity through the activation of the cholinergic anti-inflammatory pathway. These findings could be of particular interest in the clinical setting.

Cyclic lactam hybrid α-MSH/Agouti-related protein (AGRP) analogues with nanomolar range binding affinities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the topographical similarities between the pharmacophores of Agouti related protein (AGRP) an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. When employed in two different 23-membered macrocyclic lactam peptide templates, the designed hybrid AGRP/MSH pharmacophore yielded non-competitive ligands with nanomolar range binding affinities. The topography-based pharmacophore hybridization strategy will prove useful in development of unique non-competitive melanocortin receptor modulators.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

Alpha-melanocyte stimulating hormone: production and degradation

Proopiomelanocortin (POMC) is a polypeptide hormone precursor that is expressed in the brain and in peripheral tissues such as in the pituitary gland, immune system, and skin. In the brain, POMC is processed to form several peptides including alpha-melanocyte stimulating hormone (α-MSH). alpha-MSH is expressed in the hypothalamic arcuate nucleus and in the nucleus tractus solitarius of the brainstem where it has a crucial role in the regulation of metabolic functions. Specifically, α-MSH is an anorexigenic peptide. Its production and maturation processes have been shown to be regulated according to the metabolic condition of the organism. This review summarizes our current knowledge on α-MSH processing including its maturation and degradation processes and pharmacological aspects of its manipulation.

Novel binding motif of ACTH analogues at the melanocortin receptors

The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily, and each of them has a different pharmacological profile with regard to the relative potency of the endogenous and synthetic melanocortin peptides. Alpha-MSH and ACTH are endogenous nonselective agonists for MC1R, MC3R, MC4R, and MC5R. In this study, we examined the role of Phe(7) in ACTH on human (h) MC1R, MC3R, and MC4R binding and signaling. Our results indicate that substitution of Phe(7) with d-Nal(2')(7) in ACTH1-24 yields a pharmacological profile different from that for substitution of Phe(7) with d-Nal(2')(7) in MSH in hMC1R, hMC3R, and hMC4R. N-d-Nal(2')(7)-ACTH1-24 is an agonist at hMC3R and hMC4R which did not change the peptide from an agonist to an antagonist at hMC3R and hMC4R. Further experiments indicate that N-d-Nal(2')(7)-ACTH1-17 is the minimal peptide required for hMC3R and hMC4R activation. Single-amino acid substitution studies of d-Nal(2')(7)-ACTH1-17 indicate that amino acid residues 15-17 in N-d-Nal(2')(7)-ACTH1-17 are crucial for hMC3R and hMC4R activation. Substitutions of these amino acid residues reduced or abolished agonist activity at hMC3R and hMC4R. Conformational studies revealed a new beta-turn (Arg(8)-Trp(9)-Gly(10)-Lys(11)) in N-d-Nal(2')(7)-ACTH1-17, compared to the beta-turn-like structure at NDP-alpha-MSH (His(6)-d-Phe(7)-Arg(8)-Trp(9)). Our results suggest that NDP-alpha-MSH and N-d-Nal(2')(7)-ACTH1-17 do not share the same binding site; the highly basic C-terminal fragment (Lys(15)-Lys(16)-Arg(17)) of N-d-Nal(2')(7)-ACTH1-17 induced a new beta-turn, and this shift contributed the selective agonist activity at hMC3R and hMC4R.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

Nicotine anxiogenic and rewarding effects are decreased in mice lacking beta-endorphin

The endogenous opioid system plays an important role in the behavioral effects of nicotine. Thus, micro-opioid receptor and the endogenous opioids derived from proenkephalin are involved in the central effects of nicotine. However, the role played by the different endogenous opioid peptides in the acute and chronic effects of nicotine remains to be fully established. Mice lacking beta-endorphin were acutely injected with nicotine at different doses to evaluate locomotor, anxiogenic and antinociceptive responses. The rewarding properties of nicotine were evaluated by using the conditioned place-preference paradigm. Mice chronically treated with nicotine were acutely injected with mecamylamine to study the behavioral expression of nicotine withdrawal. Mice lacking beta-endorphin exhibited a spontaneous hypoalgesia and hyperlocomotion and a reduction on the anxiogenic and rewarding effects induced by nicotine. Nicotine induced similar antinociception and hypolocomotion in both genotypes and no differences were found in the development of physical dependence. The dissociation between nicotine rewarding properties and physical dependence suggests a differential implication of beta-endorphin in these addictive related responses.

Minimal sex differences in gene expression in the rat superior olivary complex

A critical issue in large-scale gene expression analysis is the impact of sexually dimorphic genes, which may confound the results when sampling across sexes. Here, we assessed, for the first time, sex differences at the transcriptome level in the auditory brainstem. To this end, microarray experiments covering the whole rat genome were performed in the superior olivary complex (SOC) of 16-day-old Sprague-Dawley rats. Sexually dimorphic genes were identified using two criteria: a 2-fold change and a P-value < 0.05. Only 12 out of 41,374 probes (0.03%) showed sexually dimorphic expression. For comparison, pituitaries from 60-day-old female and male rats were analyzed, as this gland is known to display many sex-specific features. Indeed, almost 40 times more probes, i.e. 460 (1.1%), displayed sexual dimorphism. Quantitative RT-PCR confirmed 47 out of 48 microarray results from both tissues. Taking microarray and qRT-PCR data together, the expression of six genes (Prl, Eif2s3y, Gnrhr, Pomc, Ddx3y, Akr1c6) was higher in the male SOC, whereas two genes were upregulated in the female SOC (LOC302172, Xist). Four of these genes are sex-chromosome linked (Eif2s3y, Ddx3y, LOC302172, Xist). In summary, our data indicate only minor and negligible sex-specific differences in gene expression within the SOC at P16.

Opioid and melanocortin receptors: do they have overlapping pharmacophores?

We have identified compound 1 as a novel ligand for opioid and melanocortin (MC) receptors, which is derived from the overlapping of a well known structure for the delta opioid receptor, 2,6-dimethyltyrosine (Dmt)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic), and a small molecule for the MC receptor, Tic-DPhe(p-Cl)-piperidin-4-yl-N-phenyl-propionamide. Ligand 1 showed that there is an overlapping pharmacophore between opioid and MC receptors through the Tic residue. The ligand displayed high biological activities at the delta opioid receptor (Ki = 0.38 nM in binding assay, EC(50) = 0.48 nM in GTP-gamma-S binding assay, IC(50) = 74 nM in MVD) as an agonist instead of an antagonist and showed selective binding affinity (IC(50) = 2.3 muM) at the MC-3 receptor rather than at the MC-5 receptor. A study of the structure-activity relationships demonstrated that the residues in positions 2, 3, and the C-terminus act as a pharmacophore for the MC receptors, and the residues in positions 1 and 2 act as a pharmacophore for the opioid receptors. Thus, this structural construct can be used to prepare chimeric structures with adjacent or overlapping pharmacophores for opioid and MC receptors.

Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Pharmacotherapeutic Management of Pain with a Focus Directed at the Geriatric Patient

Twenty percent to 50% of geriatric noninstitutionalized patients are victimized by pain that can result from multiple chronic medical and psychiatric diseases. This article discusses recognizing the barriers that the clinician encounters in evaluating pain in geriatric patients, taking a pain and pain medication history, and establishing a treatment plan to address the patient's experience of pain. It discusses dosage modifications appropriate for the geriatric population and reviews the drugs available to alleviate pain.

Identification of differential melanocortin 4 receptor agonist profiles at natively expressed receptors in rat cortical astrocytes and recombinantly expressed receptors in human embryonic kidney cells

Using cAMP accumulation as a functional readout, we pharmacologically characterized the response of native melanocortin receptors in cultured rat astrocytes, and found this response to be mediated by the melanocortin 4 receptor (MC4R). Melancortin agonists stimulate cAMP in a concentration-dependent manner in both astrocytes and human embryonic kidney cells recombinantly expressing rat MC4R (HEK-rMC4R), however, the relative potency and intrinsic activity of both small molecule and peptide agonists are reduced in the native system. As such, the small molecules THIQ, NBI-702 and MB243 display 43, 30 and 18% of the maximal response elicited by alpha-MSH in astrocytes. Likewise, the peptides MTII and ACTH display 55 and 72% of the maximal response elicited by alpha-MSH in these cells. In contrast, all of these compounds elicit full agonist responses with similar intrinsic activity to alpha-MSH in HEK-rMC4R cells. MC4R mRNA was detected in astrocytes, however radioligand binding experiments failed to detect measurable MC4R in astrocyte membranes, in contrast to membranes from HEK-rMC4R cells that display a binding site density of 18.1+/-1.5 fmol/mg. We propose that the divergent observations in functional activity between the cell types reflect differences in receptor expression and that caution should be exercised when interpreting agonist activity in over-expression systems for the purposes of drug discovery.

The MC4 receptor and control of appetite

Mutations in the human melanocortin (MC)4 receptor have been associated with obesity, which underscores the relevance of this receptor as a drug target to treat obesity. Infusion of MC4R agonists decreases food intake, whereas inhibition of MC receptor activity by infusion of an MC receptor antagonist or with the inverse agonist AgRP results in increased food intake. This review addresses the role of the MC system in different aspects of feeding behaviour. MC4R activity affects meal size and meal choice, but not meal frequency, and the type of diet affects the efficacy of MC4R agonists to reduce food intake. The central sites involved in the different aspects of feeding behaviour that are affected by MC4R signalling are being unravelled. The paraventricular nucleus plays an important role in food intake per se, whereas MC signalling in the lateral hypothalamus is associated with the response to a high fat diet. MC4R signalling in the brainstem has been shown to affect meal size. Further genetic, behavioural and brain-region specific studies need to clarify how the MC4R agonists affect feeding behaviour in order to determine which obese individuals would benefit most from treatment with these drugs. Application of MCR agonists in humans has already revealed side effects, such as penile erections, which may complicate introduction of these drugs in the treatment of obesity.

Design, synthesis, and biological evaluation of a new class of small molecule peptide mimetics targeting the melanocortin receptors

A new bicyclic template has been developed for the synthesis of peptide mimetics. Straightforward synthetic steps, starting from amino acids, allow the facile construction of a wide range of analogs. This system was designed to target the melanocortin receptors (MCRs), with functional group selection based on a known pharmacophore and guidance from molecular modeling to rationally identify positional and stereochemical isomers likely to be active. The functions of hMCRs are critical to myriad biological activities, including pigmentation, steroidogenesis, energy homeostasis, erectile activity, and inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug discovery in a number of areas, including cancer, pain, and obesity therapeutics. All compounds from this series tested to date are antagonists which bind with high affinity. Importantly, many are highly selective for a particular MCR subtype, including some of the first completely hMC5R-selective antagonists reported.

Neuroprotective effects of neurotrophins and melanocortins in spinal cord injury: an experimental study in the rat using pharmacological and morphological approaches

Spinal cord injury (SCI) induces lifetime disability, and no suitable therapy is available to treat victims or to minimze their sufferings. Recently, neurotrophins and compounds acting at melanocortin receptors have been been identified as potential neuroprotective agents. In this investigation, the neuroprotective effects of neurotrophins and melanocortins on the pathophysiology of SCI were examined in a rat model. The SCI was produced by making a longitudinal incision into the right dorsal horn of the T10-11 segments under equithesin anesthesia. In separate groups, neurotrophins [BDNF or IGF-1 (0.1-1 microg/10 microL in saline)] or melanocortins (ME10092, ME10354, ME10393, ME10431, and ME10501, having affinities to melanocortin receptors; 1-10 mug in saline) were applied topically over the traumatized cord segment within 5-10 min after SCI and the rats were allowed to survive for 5 h. A focal SCI resulted in widespread disruption of the blood-spinal cord barrier (BSCB) to Evans blue albumin (EBA), ([131])iodine, or lanthanum tracers and exhibited profound edema formation and cell or tissue destruction. Topical application of BDNF, IGF-1, or ME10501 (having high affinity to melanocortin-4 receptor, MCR-4) in high quantity markedly attenuated BSCB disruption, edema formation, and nerve cell, glial cell, and axonal injuries. On the other hand, low doses of neurotrophins or melanocortins were not effective in attenuating pathophysiology of SCI. These observations suggest that neurotrophins (BDNF and IGF-1) and melanocortins (with high affinity to MCR-4) are capable of inducing neuroprotection if applied shortly after trauma in high doses. Taken together, the results indicate that neurotrophins and melanocortins participate in the pathophysiology of spinal cord cell and tissue injury following trauma.

Targeting melanocortin receptors as potential novel therapeutics

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

Perception, assessment, treatment, and management of pain in the elderly

Twenty to 50% of community elderly suffer from pain. Up to 80% of the institutionalized elderly report at least one pain problem. Multiple pain etiologies that occur in elderly patients may be the occurrence of multiple chronic diseases: osteoarthritis, RA, cancer, DJD, bone/joint disorders, osteoporosis, surgical pain, trauma, neuropathic pain, and nociceptive pain. The incidence of unrelieved pain inhibits respiration, decreases mobility, and decreases their functional status, which may lead to iatrogenic events, which include pneumonia, constipation and deep vein thrombosis. Prolonged inpatient stays and extended care facilities or nursing homes may decrease the elderly patient's expectations of quality of life and initiate social isolation. There exists some roadblocks or barriers to the detection of pain in the elderly client. These include social, emotional, cognitive, and subjective issues with the patient.

HS014, a selective melanocortin-4 (MC4) receptor antagonist, modulates the behavioral effects of morphine in mice

RATIONALE

Melanocortin and opioid systems regulate feeding as well as other behaviors; however, the relationship between the two systems is not yet defined. Since agonist-induced stimulation of melanocortin receptors blocks the behavioral effects of mu opioid receptor agonists, and melanocortin-4 (MC4) receptors and mu opioid receptors share a similar anatomical distribution in the central nervous system, MC4 receptor blockade may increase opioid responsiveness.

OBJECTIVES

The goal of this study was to test the hypothesis that blockade of MC4 receptors increases the behavioral effects of morphine.

METHODS

The effects of HS014 (0.0032, 0.032, and 1 nmol, i.c.v.), a selective MC4 antagonist, on morphine-induced (3.2, 10, and 32 mg/kg, i.p.) locomotor activity (measured in the open field for 15 min) and antinociception (measured in the hot plate at 55 degrees C) were assessed in C57Bl/6 mice. In addition, the effects of morphine were evaluated in A(y) mice, a genetic model for MC4 receptor blockade.

RESULTS

The dose-effect curve of morphine for locomotor activity was shifted downwards in C57Bl/6 mice pretreated with HS014 and in A(y) mice. The dose-effect curve of morphine for antinociception was shifted two- and threefold to the left in C57Bl/6 mice pretreated with HS014 and in A(y) mice, respectively.

CONCLUSIONS

These results indicate that blockade of MC4 receptors increases the antinociceptive effects of morphine without changing the potency of morphine for locomotor activity, suggesting that MC4 receptor antagonists may be candidate drugs that can be clinically used for the treatment of pain.

Gene expression profiling of melanocortin system in neuropathic rats supports a role in nociception

The melanocortin (MC) system is involved in several biological functions. Its possible role in nociception has recently attracted attention in the field. Published data suggest that melanocortin antagonists are analgesic and agonists are hyperalgesic. Gene expression information about the MC system components (receptor, agonist and antagonist) in pain relevant areas is at present limited. To deepen our knowledge, we studied the expression of MC system components in nai;ve, sham and neuropathic rat spinal cord and dorsal root ganglia (DRG) by PCR and quantitative real-time PCR. MC4 receptor, proopiomelanocortin (POMC) and agouti-related protein (AgRP) transcripts were detected in both spinal cord and DRG, whereas MC3 receptor was detected only in the spinal cord. To study the relationship between the MC system and chronic pain, we used the chronic constriction injury model and gene expression analysis was performed in rats showing both tactile allodynia and thermal hyperalgesia. MC4 and POMC transcript were upregulated in the spinal cord of neuropathic rats, whereas MC3 and AgRP expression were unaffected. Thus, this study demonstrates for the first time the presence of AgRP in the spinal cord and DRG, suggesting that it could play a role in the regulation of MC system activity. In addition, the upregulation of POMC and MC4, in parallel with the presence of tactile allodynia and thermal hyperalgesia, further supports the idea of MC system involvement in nociception.

Avulsion injury of the rat brachial plexus triggers hyperalgesia and allodynia in the hindpaws: a new model for the study of neuropathic pain

In the present study, we sought to characterise a behavioural model of persistent peripheral neuropathic pain produced by avulsion of the right brachial plexus in rats. In addition, we compared the effects of avulsion with those of ligation or crush injury of the brachial plexus. Avulsion and, to a lesser extent, ligation and crushing of brachial plexus caused a long-lasting (up to 90 days) and highly reproducible mechanical hyperalgesia, in both ipsilateral and contralateral hindpaws. However, the same injury did not produce thermal hyperalgesia. The avulsion and, to a lesser extent, ligation and crushing of the brachial plexus elicited a significant and long-lasting (up to 90 days) ipsilateral and contralateral cold and mechanical allodynia. Furthermore, the brachial plexus injury caused a significant decrease in functional activity of the forepaws as assessed in the grasping strength test, but did not alter the locomotor activity of the rats in the open field test in comparison with control or sham groups. Taken together these results show that avulsion of the brachial plexus in rat produces persistent mechanical and cold allodynia and mechanical hyperalgesia, and might represent a valuable method for understanding the mechanisms underlying the aetiology of neuropathic pain.

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

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

Characterization of aliphatic, cyclic, and aromatic N-terminally "capped" His-D-Phe-Arg-Trp-NH2 tetrapeptides at the melanocortin receptors

The melanocortin system is implicated in multiple physiological pathways including pigmentation, inflammation, erectile function, feeding behavior, energy homeostasis, weight homeostasis, and exocrine gland function, just to list a few. The endogenous agonists for the melanocortin receptors include the gene transcripts derived from the proopiomelanocortin gene and include the core tetrapeptide His-Phe-Arg-Trp sequence postulated to be important for melanocortin receptor selectivity and stimulation. Posttranslational processing of the proopiomelanocortin derived agonists results in the N-terminal acetylation and C-terminal amidation of alpha-melanocyte stimulation hormone (alpha-MSH). In this study we generated 25 N-terminally "capped" tetrapeptides containing the core sequence X-His-D-Phe-Arg-Trp-NH(2) and pharmacologically characterized them at the mouse melanocortin MC(1) receptor, melanocortin MC(3) receptor, melanocortin MC(4) receptor, and melanocortin MC(5) receptor. The N-terminal "capping" groups consisted of linear, cyclic, or aromatic moieties and all resulted in full agonist activity at the melanocortin receptors examined in this study. Increasing aliphatic chain length increased potency of the tetrapeptide derivatives, with the addition of octanoyl capping group resulting in 70- to 110-fold increased tetrapeptide potency at the melanocortin MC(1) receptor (EC(50)=0.4 nM), melanocortin MC(3) receptor (EC(50)=4.0 nM), and melanocortin MC(4) receptor (EC(50)=0.4 nM) while only enhancing potency at the melanocortin MC(5) receptor (EC(50)=0.8 nM) by 8-fold, compared to the tetrapeptide His-D-Phe-Arg-Trp-NH(2). This octanoyl derivative surprisingly resulted in a 14-fold greater potency than alpha-MSH (EC(50)=5.4 nM) at the mouse melanocortin MC(4) receptor implicated in feeding behavior and obesity. The 3,3,3-triphenylpropionyl derivative resulted in greater than 14 microM agonist potencies at the melanocortin MC(1) receptor, melanocortin MC(3) receptor, and melanocortin MC(4) receptor and possessed a 140 nM agonist EC(50) value at the melanocortin MC(5) receptor. This 3,3,3-triphenylpropionyl-His-D-Phe-Arg-Trp-NH(2) peptide is a 100-fold selective agonist for the melanocortin MC(5) receptor, versus the other melanocortin receptors studied herein, and is the first melanocortin MC(5) receptor selective tetrapeptide derivative reported to date with nanomolar potency.