Innovative Opioid Peptides and Biased Agonism: Novel Avenues for More Effective and Safer Analgesics to Treat Chronic Pain

Targeting the JAK2/STAT3 signaling pathway for chronic pain

Chronic pain is a notable health concern because of its prevalence, persistence, and associated mental stress. Drugs targeting chronic pain with potent abirritation, and minimal side effects remain unidentified. Substantial evidence indicates that the Janus Kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway plays a distinct and critical role in different stages of chronic pain. Aberrant activation of the JAK2/STAT3 signaling pathway is evident in multiple chronic pain models. Moreover, an increasing number of studies have demonstrated that the downregulation of JAK2/STAT3 can attenuate chronic pain in different animal models. In this review, we investigated the mechanism and role of the JAK2/STAT3 signaling pathway in modulating chronic pain. The aberrant activation of JAK2/STAT3 can trigger chronic pain by interacting with microglia and astrocytes, releasing proinflammatory cytokines, inhibiting anti-inflammatory cytokines, and regulating synaptic plasticity. We also retrospectively reviewed current reports on JAK2/STAT3 pharmacological inhibitors that demonstrated their significant therapeutic potential in different types of chronic pain. In summary, our results provide strong evidence that the JAK2/STAT3 signaling pathway is a promising therapeutic target for chronic pain.

HCN2 Channels in the Ventral Hippocampal CA1 Regulate Nociceptive Hypersensitivity in Mice

Chronic pain is a significant health problem worldwide. Recent evidence has suggested that the ventral hippocampus is dysfunctional in humans and rodents, with decreased neuronal excitability and connectivity with other brain regions, parallel pain chronicity, and persistent nociceptive hypersensitivity. But the molecular mechanisms underlying hippocampal modulation of pain remain poorly elucidated. In this study, we used ex vivo whole-cell patch-clamp recording, immunofluorescence staining, and behavioral tests to examine whether hyperpolarization-activated cyclic nucleotide-gated channels 2 (HCN2) in the ventral hippocampal CA1 (vCA1) were involved in regulating nociceptive perception and CFA-induced inflammatory pain in mice. Reduced sag potential and firing rate of action potentials were observed in vCA1 pyramidal neurons from CFA-injected mice. Moreover, the expression of HCN2, but not HCN1, in vCA1 decreased in mice injected with CFA. HCN2 knockdown in vCA1 pyramidal neurons induced thermal hypersensitivity, whereas overexpression of HCN2 alleviated thermal hyperalgesia induced by intraplantar injection of CFA in mice. Our findings suggest that HCN2 in the vCA1 plays an active role in pain modulation and could be a promising target for the treatment of chronic pain.

Quantitative Systems Pharmacology and Biased Agonism at Opioid Receptors: A Potential Avenue for Improved Analgesics

Chronic pain is debilitating and represents a significant burden in terms of personal and socio-economic costs. Although opioid analgesics are widely used in chronic pain treatment, many patients report inadequate pain relief or relevant adverse effects, highlighting the need to develop analgesics with improved efficacy/safety. Multiple evidence suggests that G protein-dependent signaling triggers opioid-induced antinociception, whereas arrestin-mediated pathways are credited with modulating different opioid adverse effects, thus spurring extensive research for G protein-biased opioid agonists as analgesic candidates with improved pharmacology. Despite the increasing expectations of functional selectivity, translating G protein-biased opioid agonists into improved therapeutics is far from being fully achieved, due to the complex, multidimensional pharmacology of opioid receptors. The multifaceted network of signaling events and molecular processes underlying therapeutic and adverse effects induced by opioids is more complex than the mere dichotomy between G protein and arrestin and requires more comprehensive, integrated, network-centric approaches to be fully dissected. Quantitative Systems Pharmacology (QSP) models employing multidimensional assays associated with computational tools able to analyze large datasets may provide an intriguing approach to go beyond the greater complexity of opioid receptor pharmacology and the current limitations entailing the development of biased opioid agonists as improved analgesics.

Polymer Conjugates of Antimicrobial Peptides (AMPs) with d-Amino Acids (d-aa): State of the Art and Future Opportunities

In recent years, antimicrobial peptides (AMPs) have enjoyed a renaissance, as the world is currently facing an emergency in terms of severe infections that evade antibiotics’ treatment. This is due to the increasing emergence and spread of resistance mechanisms. Covalent conjugation with polymers is an interesting strategy to modulate the pharmacokinetic profile of AMPs and enhance their biocompatibility profile. It can also be an effective approach to develop active coatings for medical implants and devices, and to avoid biofilm formation on their surface. In this concise review, we focus on the last 5 years’ progress in this area, pertaining in particular to AMPs that contain d-amino acids, as well as their role, and the advantages that may arise from their introduction into AMPs.

Paeoniflorin alleviates CFA-induced inflammatory pain by inhibiting TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway

BACKGROUND

Treatment of chronic inflammatory pain remains a major goal in the clinic. It is thus of prime importance to characterize inherent pathophysiological pathways to design new therapeutic strategies and analgesics for pain management. Paeoniflorin (PF), a monoterpenoid glycoside from Paeonia lactiflora Pallas plants, possesses promising anti-nociceptive property. However, therapeutic effect and underlying mechanism of action of PF on inflammatory pain have not yet been fully elucidated. In this study, we aim to investigate the analgesic effect further and clarify its mechanism of action of PF on complete freund's adjuvant (CFA)-evoked inflammatory pain.

METHODS

Twenty-four male mice were divided into 3 groups: sham, CFA, and CFA + PF groups (n = 8/group). Mice were treated with normal saline or PF (30 mg/kg) for 11 days. Footpad swelling (n = 8/group), mechanical (n = 8/group) and thermal hypersensitivity (n = 8/group) were measured to evaluate the analgesic effect of PF on CFA-injected mice. At the end of the animal experiment, blood and L4-L6 dorsal root ganglion neurons were collected to assess the therapeutic effect of PF on CFA-induced inflammatory pain. Next, hematoxylin and eosin, quantitative realtime PCR, ELISA, capsaicin and dimethyl succinate induced pain test (n = 8/group), motor coordination test (n = 8/group), tail flicking test (n = 8/group), pyruvate and succinate dehydrogenase assay (n = 6/group), immunohistochemical staining, were performed to clarify the action mechanism of PF on CFA-evoked inflammatory pain. Besides, the effect of PF on TRPV1 was evaluated by whole-cell patch clamp recording on primary neurons (n = 7). Finally, molecular docking further performed to evaluate the binding ability of PF to TRPV1.

RESULTS

PF significantly relieved inflammatory pain (P < 0.001) and paw edema (P < 0.001) on a complete Freund adjuvant (CFA)-induced peripheral inflammatory pain model. Furthermore, PF inhibited neutrophil infiltration (P < 0.01), IL-1β increase (P < 0.01), and pain-related peptide substance P release (P < 0.001). Intriguingly, CFA-induced succinate aggregation was notably reversed by PF via modulating pyruvate and SDH activity (P < 0.01). In addition, PF dampened the high expression of subsequent succinate receptor SUCNR1 (P < 0.01), HIF-1α (P < 0.05), as well as the activation of NLPR3 inflammasome (P < 0.05) and TRPV1 (P < 0.05). More importantly, both capsaicin and dimethyl succinate supplementation obviously counteracted the pain-relieving effect of PF and TRPV1 (P < 0.01 or P < 0.001).

CONCLUSION

Our findings suggest that PF can significantly relieve CFA-induced paw swelling, as well as mechanical and thermal hyperalgesia. PF alleviated inflammatory pain partly through inhibiting the activation of TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway. Furthermore, we found that PF exerted its analgesic effect without affecting motor coordination and pain-related cold ion-channels. In summary, this study may provide valuable evidence for the potential application of PF as therapeutic strategy for inflammatory pain treatment.

Daphnetin inhibits spinal glial activation via Nrf2/HO-1/NF-κB signaling pathway and attenuates CFA-induced inflammatory pain

Daphnetin (7, 8-dihydroxycoumarin, DAPH), a coumarin derivative isolated from Daphne odora var., recently draws much more attention as a promising drug candidate to treat neuroinflammatory diseases due to its protective effects against neuroinflammation. However, itscontribution to chronic inflammatory pain is largely unknown. In the current work, we investigated the effects of DAPH in a murine model of inflammatory pain induced by complete Freund's adjuvant (CFA) and its possible underlying mechanisms. Our results showed that DAPH treatment significantly attenuated mechanical allodynia provoked by CFA. A profound inhibition of spinal glial activation, followed by attenuated expression levels of spinal pro-inflammatory cytokines, was observed in DAPH-treated inflammatory pain mice. Further study demonstrated that DAPH mediated negative regulation of spinal NF-κB pathway, as well as its preferential activation of Nrf2/HO-1 signaling pathway in inflammatory pain mice. This study, for the first time, indicated that DAPH might preventthe development of mechanical allodynia in mice with inflammatory pain. And more importantly, these data provide evidence for the potential application of DAPH in the treatment of chronic inflammatory pain.

Targeting HMGB1-TLR4 signaling by miR-216a-5p elevation alleviates the inflammatory behavioral hypersensitivity

Neuroinflammation induced by microglial activation has a critical role in inflammatory pain. In this study, we detected the function of miR-216a-5p in the progression of inflammatory behavioral hypersensitivity. Here, decreases of miR-216a-5p and up-regulation of high-mobility group box1 (HMGB1) were observed in complete freund's adjuvant (CFA)-induced inflammatory pain model in mice and LSP-activated BV2 microglia. HMGB1 was identified as a target of miR-216a-5p by luciferase reporter system. Ectopic expression of miR-216a-5p suppressed microglial marker IBA-1 expression and subsequent pro-inflammatory cytokine releases (IL-1β, IL-6 and TNF-α) from LPS-activated microglia. Additionally, LPS exposure enhanced the protein expression levels of HMGB1, TLR4 and p-p65 NF-kB in microglia, which were abrogated following miR-216a-5p overexpression. Intriguingly, transfection of HMGN1 cDNA into BV2 microglial cells reversed the inhibitory effects of miR-216a-5p elevation on microglial activation-triggered inflammatory response. Intrathecal delivery of LV-miR-216a-5-p ameliorated CFA-evoked mechanical and thermal hyperalgesia in mice. Concomitantly, overexpressing miR-216a-5p also restrained the inflammatory response and microglia activation in CFA-induced inflammatory mouse models, concomitant with the decreases in the expression of HMGB1, TLR4 and p-p65 NF-kB in spinal cord. Thus, these findings highlight that miR-216a-5p may alleviate inflammatory behavioral hypersensitivity by blocking microglia-mediated neuroinflammation via targeting the HMGB1-TLR4-NF-kB pathway, supporting miR-216a-5p as a potential therapeutic avenue for inflammatory pain.

Cyclizing Painkillers: Development of Backbone-Cyclic TAPS Analogs

Painkillers are commonly used medications. Native peptide painkillers suffer from various pharmacological disadvantages, while small molecule painkillers like morphine are highly addictive. We present a general approach aimed to use backbone-cyclization to develop a peptidomimetic painkiller. Backbone-cyclization was applied to transform the linear peptide Tyr-Arg-Phe-Sar (TAPS) into an active backbone-cyclic peptide with improved drug properties. We designed and synthesized a focused backbone-cyclic TAPS library with conformational diversity, in which the members of the library have the generic name TAPS c(n-m) where n and m represent the lengths of the alkyl chains on the nitrogens of Gly and Arg, respectively. We used a combined screening approach to evaluate the pharmacological properties and the potency of the TAPS c(n-m) library. We focused on an in vivo active compound, TAPS c(2-6), which is metabolically stable and has the potential to become a peripheral painkiller being a full μ opioid receptor functional agonist. To prepare a large quantity of TAPS c(2-6), we optimized the conditions of the on-resin reductive alkylation step to increase the efficiency of its SPPS. NMR was used to determine the solution conformation of the peptide lead TAPS c(2-6).

The Quest for More Effective Analgesics with Reduced Abuse Liability and Fewer Adverse Effects: Promises, Pitfalls, and Future Perspectives of Biased Agonists at Opioid Receptors

Chronic pain is a relevant health condition affecting one out of five individuals that is often not adequately treated by currently available analgesics. This, together with the dramatic increase in addicted people within the dramatic "opioid epidemics," significantly spurs the quest for innovative analgesics provided with increased efficacy, reduced abuse liability, and fewer adverse effects.Within this frame, biased agonists at opioid receptors have attracted increasing interest in the last decade as they have emerged as more effective and safer candidate analgesics.In this chapter, promises, pitfalls, and future perspective of biased agonists at mu (MOR) and kappa (KOR) opioid receptors are discussed. Moreover, methodological insights are provided with regard to the most appropriate experimental settings to be employed aiming at developing novel biased KOR agonists.

Optimizing Perioperative Use of Opioids: A Multimodal Approach

Purpose of Review

The main purpose of this article is to review recent literature regarding multimodal analgesia medications, citing their recommended doses, efficacy, and side effects. The second part of this report will provide a description of drugs in different stages of development which have novel mechanisms with less side effects such as tolerance and addiction.

Recent Findings

Multimodal analgesia is a technique that facilitates perioperative pain management by employing two or more systemic analgesics along with regional anesthesia, when possible. Even though opioids and non-opioid analgesics remain the most common medication used for acute pain management after surgery, they have many undesirable side effects including the potential for misuse. Newer analgesics including peripheral acting opioids, nitric oxide inhibitors, calcitonin gene-related peptide receptor antagonists, interleukin-6 receptor antagonists and gene therapy are under intensive investigation.

Summary

A patient's first exposure to opioids is often in the perioperative setting, a vulnerable time when multimodal therapy can play a large role in decreasing opioid exposure. Additionally, the current shift towards faster recovery times, fewer post-operative complications and improved cost-effectiveness during the perioperative period has made multimodal analgesia a central pillar of Enhanced Recovery After Surgery (ERAS) protocols.

Functional Selectivity and Antinociceptive Effects of a Novel KOPr Agonist

Kappa opioid receptor (KOPr) agonists represent alternative analgesics for their low abuse potential, although relevant adverse effects have limited their clinical use. Functionally selective KOPr agonists may activate, in a pathway-specific manner, G protein-mediated signaling, that produces antinociception, over β-arrestin 2-dependent induction of p38MAPK, which preferentially contributes to adverse effects. Thus, functionally selective KOPr agonists biased toward G protein-coupled intracellular signaling over β-arrestin-2-mediated pathways may be considered candidate therapeutics possibly devoid of many of the typical adverse effects elicited by classic KOPr agonists. Nonetheless, the potential utility of functionally selective agonists at opioid receptors is still highly debated; therefore, further studies are necessary to fully understand whether it will be possible to develop more effective and safer analgesics by exploiting functional selectivity at KOPr. In the present study we investigated in vitro functional selectivity and in vivo antinociceptive effects of LOR17, a novel KOPr selective peptidic agonist that we synthesized. LOR17-mediated effects on adenylyl cyclase inhibition, ERK1/2, p38MAPK phosphorylation, and astrocyte cell proliferation were studied in HEK-293 cells expressing hKOPr, U87-MG glioblastoma cells, and primary human astrocytes; biased agonism was investigated via cAMP ELISA and β-arrestin 2 recruitment assays. Antinociception and antihypersensitivity were assessed in mice via warm-water tail-withdrawal test, intraperitoneal acid-induced writhing, and a model of oxaliplatin-induced neuropathic cold hypersensitivity. Effects of LOR17 on locomotor activity, exploratory activity, and forced-swim behavior were also assayed. We found that LOR17 is a selective, G protein biased KOPr agonist that inhibits adenylyl cyclase and activates early-phase ERK1/2 phosphorylation. Conversely to classic KOPr agonists as U50,488, LOR17 neither induces p38MAPK phosphorylation nor increases KOPr-dependent, p38MAPK-mediated cell proliferation in astrocytes. Moreover, LOR17 counteracts, in a concentration-dependent manner, U50,488-induced p38MAPK phosphorylation and astrocyte cell proliferation. Both U50,488 and LOR17 display potent antinociception in models of acute nociception, whereas LOR17 counteracts oxaliplatin-induced thermal hypersensitivity better than U50,488, and it is effective after single or repeated s.c. administration. LOR17 administered at a dose that fully alleviated oxaliplatin-induced thermal hypersensitivity did not alter motor coordination, locomotor and exploratory activities nor induced pro-depressant-like behavior. LOR17, therefore, may emerge as a novel KOPr agonist displaying functional selectivity toward G protein signaling and eliciting antinociceptive/antihypersensitivity effects in different animal models, including oxaliplatin-induced neuropathy.

Muscone relieves inflammatory pain by inhibiting microglial activation-mediated inflammatory response via abrogation of the NOX4/JAK2-STAT3 pathway and NLRP3 inflammasome

Previous studies have shown that muscone, a pharmacologically active ingredient isolated from musk, has excellent effects on anti-inflammation. However, its effect on microglia activation-induced inflammatory pain is not known yet. In the present study, a mouse BV2 microglia cell activation-mediated inflammatory model was developed with LPS induction, and a mouse inflammatory pain model was established with CFA injection. The inhibitory effect of muscone on microglia inflammatory activation was verified by measuring pro-inflammatory cytokines expression (interleukin-6, tumor necrosis factor-α, and interleukin-1β; IL-6, TNF-α and IL-1β). We found that muscone suppressed microglial activation-mediated inflammatory response through the NADPH oxidase 4 (NOX4)/janus kinase 2-signal transducer and activator of transcription 3 (JAK2-STAT3) pathway and pyrin-domain-containing 3 (NLRP3) inflammasome. Notably, muscone mitigated CFA-induced pain hypersensitivity and inflammation, as well as microglia cell activation in vivo. Furthermore, muscone inhibited the CFA-induced NOX4, p-JAK2/p-STAT3, and NLRP3 inflammasome expression in spinal cord of mice. In conclusion, this study uncovered that muscone relieved inflammatory pain by inhibiting microglial activation-mediated inflammatory response via abrogation of the NOX4/JAK2-STAT3 pathway and NLRP3 inflammasome. This finding of muscone is promising for treating inflammatory pain.

G Protein-Gated Inwardly Rectifying Potassium Channel Subunit 3 is Upregulated in Rat DRGs and Spinal Cord After Peripheral Nerve Injury

Background

G protein-gated inwardly rectifying potassium (GIRK) channels are involved in the regulation of neuronal excitability. Four GIRK subunits (GIRK1-4) are expressed in rat dorsal root ganglia (DRGs). Recently, we have characterized the expression of GIRK1 and −2, and both are downregulated in rat DRGs and spinal cord after a complete sciatic nerve transection (axotomy). Here, we aimed to study the neurochemical characteristics of GIRK3, and its regulation in rat DRGs and spinal cord induced by nerve injury.

Methods

A sciatic nerve axotomy was performed to study the influences of injury on GIRK3 expression in DRGs and spinal cord. A dorsal root rhizotomy and a sciatic nerve crush were employed to study the axonal transport of GIRK3 protein, respectively. Immunohistochemistry analysis was employed for investigating the neurochemical characteristics of GIRK3.

Results

In control DRGs, ~18% of neuron profiles (NPs) were GIRK3-positive (⁺), and ~41%, ~48% and ~45% of GIRK3⁺ NPs were CGRP⁺, IB4⁺ and NF200⁺, respectively. GIRK3-like immunoreactivity was observed in glabrous skin of hind paws and axons originating from DRG neurons. Fourteen days after axotomy, more than one-third of DRG NPs were GIRK3⁺, and among these ~51% and ~56% coexpressed galanin and neuropeptide Y, respectively. In control animals, a small group of interneurons found in the dorsal horn was GIRK3⁺. In addition, GIRK3⁺ processes could be observed in superficial laminae of spinal dorsal horn. After nerve injury, the intensity of GIRK3-like immunoreactivity in the superficial layers was increased. Evidence based on rhizotomy and sciatic nerve crush indicated both anterograde and retrograde transport of GIRK3.

Conclusion

Our study demonstrates that GIRK3 is expressed in sensory neurons and spinal cord. GIRK3 has both anterograde and retrograde axonal transport. GIRK3 expression can be regulated by peripheral nerve injury.

Oleanolic acid administration alleviates neuropathic pain after a peripheral nerve injury by regulating microglia polarization-mediated neuroinflammation

Neuropathic pain caused by a peripheral nerve injury constitutes a great challenge in clinical treatments due to the unsatisfactory efficacy of the current strategy. Microglial activation-mediated neuroinflammation is a major characteristic of neuropathic pain. Oleanolic acid is a natural triterpenoid in food and medical plants, and fulfills pleiotropic functions in inflammatory diseases. Nevertheless, its role in neuropathic pain remains poorly elucidated. In the current study, oleanolic acid dose-dependently suppressed LPS-evoked IBA-1 expression (a microglial marker) without cytotoxicity to microglia, suggesting the inhibitory efficacy of oleanolic acid in microglial activation. Moreover, oleanolic acid incubation offset LPS-induced increases in the iNOS transcript and NO releases from microglia, concomitant with the decreases in pro-inflammatory cytokine transcripts and production including IL-6, IL-1β, and TNF-α. Simultaneously, oleanolic acid shifted the microglial polarization from the M1 phenotype to the M2 phenotype upon LPS conditions by suppressing LPS-induced M1 marker CD16, CD86 transcripts, and enhancing the M2 marker Arg-1 mRNA and anti-inflammatory IL-10 levels. In addition, the LPS-induced activation of TLR4-NF-κB signaling was suppressed in the microglia after the oleanolic acid treatment. Restoring this signaling by the TLR4 plasmid transfection overturned the suppressive effects of oleanolic acid on microglial polarization-evoked inflammation. In vivo, oleanolic acid injection alleviated allodynia and hyperalgesia in SNL-induced neuropathic pain mice. Concomitantly, oleanolic acid facilitated microglial polarization to M2, accompanied by inhibition in inflammatory cytokine levels and activation of TLR4-NF-κB signaling. Collectively, these findings confirm that oleanolic acid may ameliorate neuropathic pain by promoting microglial polarization from pro-inflammatory M1 to anti-inflammatory M2 phenotype via the TLR4-NF-κB pathway, thereby indicating its usefulness as therapeutic intervention in neuropathic pain.

Neuropathic pain caused by a peripheral nerve injury constitutes a great challenge in clinical treatments due to the unsatisfactory efficacy of the current strategy.