Melanocortin-4 receptor regulation of pain

The Skin-Brain Axis: From UV and Pigmentation to Behaviour Modulation

The skin-brain axis has been suggested to play a role in several pathophysiological conditions, including opioid addiction, Parkinson's disease and many others. Recent evidence suggests that pathways regulating skin pigmentation may directly and indirectly regulate behaviour. Conversely, CNS-driven neural and hormonal responses have been demonstrated to regulate pigmentation, e.g., under stress. Additionally, due to the shared neuroectodermal origins of the melanocytes and neurons in the CNS, certain CNS diseases may be linked to pigmentation-related changes due to common regulators, e.g., MC1R variations. Furthermore, the HPA analogue of the skin connects skin pigmentation to the endocrine system, thereby allowing the skin to index possible hormonal abnormalities visibly. In this review, insight is provided into skin pigment production and neuromelanin synthesis in the brain and recent findings are summarised on how signalling pathways in the skin, with a particular focus on pigmentation, are interconnected with the central nervous system. Thus, this review may supply a better understanding of the mechanism of several skin-brain associations in health and disease.

Melanocortin-4 receptor signaling in the central amygdala mediates chronic inflammatory pain effects on nociception

Chronic inflammatory pain represents one of the largest subsets of chronic pain diagnoses, which affect nearly a quarter of individuals in the United States and cost nearly $600 billion dollars annually. Chronic pain leads to persistent sensory hypersensitivities, as well as emotional and cognitive disturbances. Evidence suggests that melanocortin 4 receptors (MC4Rs) mediate pain-signaling and pain-like behaviors via actions at various nodes in the pain-neural axis, but the field lacks a complete understanding of the potential role of MC4Rs in chronic inflammatory pain in males and females. The central amygdala (CeA) expresses high quantities of MC4R and receives pain-related information from the periphery, and in vivo CeA manipulations alter nociceptive behavior in pain-naïve and in animals with chronic pain. Here, we tested the hypothesis that MC4Rs in the CeA modulate thermal nociception and mechanical sensitivity, as well as pain avoidance, in male and female Wistar rats, using a model of chronic inflammatory pain (Complete Freud's Adjuvant; CFA). First, we report that CFA produces long-lasting hyperalgesia in adult male and female Wistar rats, and long-lasting pain avoidance in male Wistar rats. Second, we report that MC4R antagonism in the CeA reduces thermal nociception and mechanical sensitivity in male and female Wistar rats treated with CFA. Finally, we report that MC4R antagonism in the CeA reduces pain avoidance in male, and that this effect is not due to drug effects on locomotor activity. Our results indicate that a model of chronic inflammatory pain produces long-lasting increases in pain-like behaviors in adult male and female Wistar rats, and that antagonism of MC4Rs in the CeA reverses those effects.

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.

Novel bifunctional hybrid compounds designed to enhance the effects of opioids and antagonize the pronociceptive effects of nonopioid peptides as potent analgesics in a rat model of neuropathic pain

ABSTRACT

The purpose of our work was to determine the role of nonopioid peptides derived from opioid prohormones in sensory hypersensitivity characteristics of neuropathic pain and to propose a pharmacological approach to restore the balance of these endogenous opioid systems. Nonopioid peptides may have a pronociceptive effect and therefore contribute to less effective opioid analgesia in neuropathic pain. In our study, we used unilateral chronic constriction injury (CCI) of the sciatic nerve as a neuropathic pain model in rats. We demonstrated the pronociceptive effects of proopiomelanocortin- and proenkephalin-derived nonopioid peptides assessed by von Frey and cold plate tests, 7 to 14 days after injury. The concentration of proenkephalin-derived pronociceptive peptides was increased more robustly than that of Met-enkephalin in the ipsilateral lumbar spinal cord of CCI-exposed rats, as shown by mass spectrometry, and the pronociceptive effect of one of these peptides was blocked by an antagonist of the melanocortin 4 (MC4) receptor. The above results confirm our hypothesis regarding the possibility of creating an analgesic drug for neuropathic pain based on enhancing opioid activity and blocking the pronociceptive effect of nonopioid peptides. We designed and synthesized bifunctional hybrids composed of opioid (OP) receptor agonist and MC4 receptor antagonist (OP-linker-MC4). Moreover, we demonstrated that they have potent and long-lasting antinociceptive effects after a single administration and a delayed development of tolerance compared with morphine after repeated intrathecal administration to rats subjected to CCI. We conclude that the bifunctional hybrids OP-linker-MC4 we propose are important prototypes of drugs for use in neuropathic pain.

The Role of Melanocortin Plasticity in Pain-Related Outcomes After Alcohol Exposure

The global COVID-19 pandemic has shone a light on the rates and dangers of alcohol misuse in adults and adolescents in the US and globally. Alcohol exposure during adolescence causes persistent molecular, cellular, and behavioral changes that increase the risk of alcohol use disorder (AUD) into adulthood. It is established that alcohol abuse in adulthood increases the likelihood of pain hypersensitivity and the genesis of chronic pain, and humans report drinking alcohol to relieve pain symptoms. However, the longitudinal effects of alcohol exposure on pain and the underlying CNS signaling that mediates it are understudied. Specific brain regions mediate pain effects, alcohol effects, and pain-alcohol interactions, and neural signaling in those brain regions is modulated by neuropeptides. The CNS melanocortin system is sensitive to alcohol and modulates pain sensitivity, but this system is understudied in the context of pain-alcohol interactions. In this review, we focus on the role of melanocortin signaling in brain regions sensitive to alcohol and pain, in particular the amygdala. We also discuss interactions of melanocortins with other peptide systems, including the opioid system, as potential mediators of pain-alcohol interactions. Therapeutic strategies that target the melanocortin system may mitigate the negative consequences of alcohol misuse during adolescence and/or adulthood, including effects on pain-related outcomes.

Novel hybrid compounds, opioid agonist+melanocortin 4 receptor antagonist, as efficient analgesics in mouse chronic constriction injury model of neuropathic pain

When the nerve tissue is injured, endogenous agonist of melanocortin type 4 (MC4) receptor, α-MSH, exerts tonic pronociceptive action in the central nervous system, contributing to sustaining the neuropathic pain state and counteracting the analgesic effects of exogenous opioids. With the intent of enhancing opioid analgesia in neuropathy by blocking the MC4 activation, so-called parent compounds (opioid agonist, MC4 antagonist) were joined together using various linkers to create novel bifunctional hybrid compounds. Analgesic action of four hybrids was tested after intrathecal (i.t.) administration in mouse models of acute and neuropathic pain (chronic constriction injury model, CCI). Under nerve injury conditions, one of the hybrids, UW3, induced analgesia in 1500 times lower i.t. dose than the opioid parent (ED50: 0.0002 nmol for the hybrid, 0.3 nmol for the opioid parent) and in an over 16000 times lower dose than the MC4 parent (ED50: 3.33 nmol) as measured by the von Frey test. Two selected hybrids were tested for analgesic properties in CCI mice after intravenous (i.v.) and intraperitoneal (i.p.) administration. Opioid receptor antagonists and MC4 receptor agonists diminished the analgesic action of these two hybrids studied, though the extent of this effect differed between the hybrids; this suggests that linker is of key importance here. Further results indicate a significant advantage of hybrid compounds over the physical mixture of individual pharmacophores in their analgesic effect. All this evidence justifies the idea of synthesizing a bifunctional opioid agonist-linker-MC4 antagonist compound, as such structure may bring important benefits in neuropathic pain treatment.

Mitogen- and stress-activated protein kinase-1 activation is involved in melanocortin-induced BDNF expression in Neuro2a neuronal cells

Melanocortins are neuropeptides exerting versatile functions in the nervous system. Melanocortin 4 receptor (MC4R) is primarily expressed in the brain and is thought to be a major mediator for melanocortin. Brain-derived neurotrophic factor (BDNF) may be a crucial downstream molecule of MC4R activation, to yield neurite outgrowth, neuroregenerative, anorexigenic and other actions. In this study, we stimulated Neuro2a murine neuronal cells with an α-melanocyte stimulating hormone (α-MSH) analog, [Nle(4), D-Phe(7)]melanocyte-stimulating hormone (NDP-MSH). In Neuro2a cells, NDP-MSH promoted neurite outgrowth. Upon NDP-MSH administration, BDNF expression was greatly enhanced. Furthermore, this effect was effectively reversed by the MC4R antagonist, JKC-363. We found that NDP-MSH treatment activated the ERK cascade and its downstream kinase MSK1 (mitogen- and stress-activated protein kinase-1). Antagonism of the MSK1 cascade by a specific inhibitor or overexpression of a defective MSK1 mutant interrupted the phosphorylation of the transcription factor cAMP-response element binding protein (CREB), blocking BDNF upregulation. In addition, MSK1 activation triggered an epigenetic alteration in histone H3 (Ser10), facilitating the expression of the BDNF gene. Taken together, our results showed that MSK1 kinase positively activates MC4R-induced BDNF expression via modulating the phosphorylation of CREB and histone H3 in Neuro2a neuronal cells.

Distinct binding and signaling activity of Acthar Gel compared to other melanocortin receptor agonists

PURPOSE

To compare the binding and agonistic activity of Acthar^® Gel and synthetic melanocortin receptor (MCR) agonists and examine how the activity of select agonists affects the in vivo production of corticosterone.

MATERIALS AND METHODS

In vitro binding was determined using concentration-dependent displacement of the ligand [¹²⁵I]Nle⁴, D-Phe⁷-α-melanocyte-stimulating hormone (α-MSH) on cells expressing MC1R, MC3R, MC4R, or MC5R. Functional activity was determined using a time-resolved fluorescence cyclic adenosine monophosphate (cAMP) assay in cells expressing MC1R, MC2R, MC3R, MC4R, or MC5R. In vivo corticosterone analyses were performed by measuring plasma corticosterone levels in Sprague Dawley rats.

RESULTS

Acthar Gel and synthetic MCR agonists exhibited the highest binding at MC1R, lowest binding at MC5R, and moderate binding at MC3R and MC4R. Acthar Gel stimulated the production of cAMP in all 5 MCR-expressing cell lines, with MC2R displaying the lowest level of full agonist activity, 3-, 6.6-, and 10-fold lower than MC1R, MC3R, and MC4R, respectively. Acthar Gel was a partial agonist at MC5R. The synthetic MCR agonists induced full activity at all 5 MCRs, with the exception of α-MSH having no activity at MC2R. Acthar Gel treatment had less of an impact on in vivo production of corticosterone compared with synthetic ACTH_1-24 depot.

CONCLUSIONS

Acthar Gel bound to and activated each MCR tested in this study, with partial agonist activity at MC5R and the lowest level of full agonist activity at MC2R, which distinguished it from synthetic MCR agonists. The minimal activity of Acthar Gel at MC2R corresponded to lower endogenous corticosteroid production.

MC4R Is Involved in Neuropathic Pain by Regulating JNK Signaling Pathway After Chronic Constriction Injury

Background

Neuropathic pain can develop after nerve injury, when deleterious changes occur in injured neurons and glia cells. Melanocortin 4 receptor (MC4R) is involved in the regulation of pain due to its high expressions in brain. Moreover, MC4R could mediate the c-Jun N-terminal kinase (JNK) signaling pathway, but whether the MC4R-regulated JNK signaling pathway participated in neuropathic pain after chronic constriction injury (CCI) is still unclear.

Methods

A total of 128 Sprague-Dawley rats were allocated into four experiment groups: the SHAM group, CCI + NaCl group, CCI + HS group, and CCI + SP + HS group. For the CCI + NaCl group, the sciatic nerves were ligated. For the SHAM group, an identical manner to the CCI without ligation was performed. For CCI + HS and CCI + SP + HS groups, rats were injected with MC4R inhibitor (HS014) and HS014 plus JNK inhibitor (SP600125), respectively, from days 3 to 14 after CCI. Paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) were used to assess the nociceptive behavior. ELISA was used to detect the levels of inflammatory cytokines. qRT-PCR and Western blots (WB) were utilized to examine the mRNA and protein expressions of JNK signaling pathway-related genes. Meanwhile, the expression levels of MC4R and p-JNK were further evaluated by immunohistochemistry (IHC) and immunofluorescence (IF) experiments. Finally, in order to confirm the in vivo results, astrocytes were isolated and transfected with MC4R-overexpression plasmid. Furthermore, the protein expressions of JNK signaling pathway-related genes were tested by WB.

Results

It was showed that the values of PWL and PWT were significantly increased in CCI + HS group and CCI + SP + HS group compared with CCI + NaCl group. The increased interleukin-6 (IL-6), IL-1β, and tumor necrosis factor-α (TNF-α) secretion in CCI + NaCl group was lowered by HS and SP + HS. MC4R, p-JNK, ATF3, and c-Jun levels were up-regulated with CCI surgery, but down-regulated with HS and SP + HS treatments. Moreover, the IHC and IF results further revealed that MC4R and p-JNK expressions in CCI + NaCl group were remarkably higher than those in HS group and HS + SP group. In vitro data also indicated that HS, SP, and SP + HS could down-regulate the expressions of MC4R, p-JNK, ATF3, and c-Jun in M1830 astrocytes.

Conclusion

Our findings indicated that MC4R is involved in neuropathic pain by regulating JNK signaling pathway after CCI.

Altered expression of differential gene and lncRNA in the lower thoracic spinal cord on different time courses of experimental obstructive jaundice model accompanied with altered peripheral nociception in rats

The spinal origin of jaundice-induced altered peripheral nociceptive response poorly understood. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a jaundice model accompanied by altered peripheral nociceptive response, and then to analyze differential gene and lncRNA expression patterns in the lower thoracic spinal cord on different time courses after BDL operation by using high-throughput RNA sequencing. The differentially expressed genes (DEGs) identified using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis, followed by clustering analysis, Gene Ontology analysis and pathway analysis. As a result, a total of 2033 lncRNAs were differentially expressed 28d after BDL, in which 1545 probe sets were up-regulated and 488 probe sets were down-regulated, whereas a total of 2800 mRNAs were differentially expressed, in which 1548 probe sets were up-regulated and 1252 probe sets were down-regulated. The RNAseq data of select mRNAs and lncRNAs was validated by RT-qPCR. 28d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated whereas the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. 14d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated; the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. In conclusion, the present study showed that jaundice accompanied with decreased peripheral nociception involved in the changes of gene and lncRNA expression profiles in spinal cord. These findings extend current understanding of spinal mechanism for obstructive jaundice accompanied by decreased peripheral nociception.