Preclinical Assessment of Inflammatory Pain

The synergistic anti-nociceptive effects of nefopam and gabapentinoids in inflammatory, osteoarthritis, and neuropathic pain mouse models

Pain is a common public health problem and remains as an unmet medical need. Currently available analgesics usually have limited efficacy or are accompanied by many adverse side effects. To achieve satisfactory pain relief by multimodal analgesia, new combinations of nefopam and gabapentinoids (pregabalin/gabapentin) were designed and assessed in inflammatory, osteoarthritis and neuropathic pain. Isobolographic analysis was performed to analyze the interactions between nefopam and gabapentinoids in carrageenan-induced inflammatory pain, mono-iodoacetate-induced osteoarthritis pain and paclitaxel-induced peripheral neuropathic pain in mice. The anti-inflammatory effect and motor performance of monotherapy or their combinations were evaluated in the carrageenan-induced inflammatory responses and rotarod test, respectively. Nefopam (1, 3, 5, 10, 30 mg/kg, p.o.), pregabalin (3, 6, 12, 24 mg/kg, p.o.) or gabapentin (25, 50, 75, 100 mg/kg, p.o.) dose-dependently reversed mechanical allodynia in three pain models. Isobolographic analysis indicated that the combinations of nefopam and gabapentinoids exerted synergistic anti-nociceptive effects in inflammatory, osteoarthritis, and neuropathic pain mouse models, as evidenced by the experimental ED50 (median effective dose) falling below the predicted additive line. Moreover, the combination of nefopam-pregabalin/gabapentin alleviated carrageenan-induced inflammation and edema, and also prevented gabapentinoids-related sedation or ataxia by lowering their effective doses. Collectively, the co-administration of nefopam and gabapentinoids showed synergistic analgesic effects and may result in improved therapeutic benefits for treating pain.

S1P/S1PR1 signaling is involved in the development of nociceptive pain

Background

Pain is a complex perception involving unpleasant somatosensory and emotional experiences. However, the underlying mechanisms that mediate its different components remain unclear. Sphingosine-1-phosphate (S1P), a metabolite of sphingomyelin and a potent lipid mediator, initiates signaling via G protein-coupled receptors (S1PRs) on cell surfaces. It serves as a second messenger in cellular processes such as proliferation and apoptosis. Nevertheless, the neuropharmacology of sphingolipid signaling in pain conditions within the central nervous system remains largely unexplored and controversial.

Methods

Chronic nociceptive pain models were induced in vivo by intraplantar injection of 20 μL complete Freund's adjuvant (CFA) into the left hind paws. We assessed S1P and S1PR1 expression in the spinal cords of CFA model mice. Functional antagonists of S1PR1 or S1PR1-specific siRNA were administered daily following CFA model establishment. Paw withdrawal response frequency (PWF) and paw withdrawal latency (PWL) were measured to evaluate mechanical allodynia and thermal hyperalgesia, respectively. RT-PCR assessed interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α levels. Western blotting and immunofluorescence were used to analyze glial fibrillary acidic protein (GFAP), ionized calcium-binding adapter molecule (Iba1), STAT3, ERK, and p38 MAPK protein expression.

Results

In the chronic nociceptive pain model induced by CFA, S1P and S1PR1 expression levels were significantly elevated, leading to activation of spinal cord glial cells. S1PR1 activation also promoted MMP2-mediated cleavage of mature IL-1β. Additionally, S1PR1 activation upregulated phosphorylation of STAT3, ERK, and p38 MAPK in glial cells, profoundly impacting downstream signaling pathways and contributing to chronic nociceptive pain.

Conclusion

The S1P/S1PR1 axis plays a pivotal role in the cellular and molecular mechanisms underlying nociceptive pain. This signaling pathway modulates glial cell activation and the expression of pain-related genes (STAT3, ERK, p38 MAPK) and inflammatory factors in the spinal dorsal horn. These findings underscore the potential of targeting the S1P system for developing novel analgesic therapies.

Hormone replacement therapy did not alleviate temporomandibular joint inflammatory pain in ovariectomized rats

This study had the aim of examining the relationships between variations in estrogen levels resulting from ovariectomy, and estrogen hormone replacement therapy (HRT) in rats subjected to an orofacial inflammatory pain model. Eighty adult female Wistar rats were initially divided into 2 groups: Sham or ovariectomy (OVX-D1). Seven days later (D7), the rats were subjected to an unilateral infiltration of Freund's Complete Adjuvant (CFA) or saline solution into the right temporomandibular joint (TMJ). Then, rats received 17β-estradiol (28 µg/kg/day) or placebo for 21 days (D10-D31). Nociception was evaluated by the von Frey (VF) and the Hot Plate (HP) tests, and depressive-like behavior by the Forced Swimming (FS) test. On D32 all rats were euthanized and serum, hippocampus and brainstem were collected. The CFA groups presented a mechanical hyperalgesia until day 21 (p ≤ 0.05). No differences were observed among groups in the HP (p = 0.735), and in the immobility and swimming time of the FS (p = 0.800; p = 0.998, respectively). In the brainstem, there was a significant difference in the TNF-ɑ levels (p = 0.043), and a marginal significant difference in BDNF levels (p = 0.054), without differences among groups in the hippocampal BDNF and TNF-ɑ levels (p = 0.232; p = 0.081, respectively). In conclusion, the hormone replacement therapy did not alleviate orofacial pain in ovariectomized rats. However, there is a decrease in brainstem TNF-ɑ levels in the animals submitted to both models, which was partially reverted by HRT.

Opuntia monacantha: Validation of the anti-inflammatory and anti-arthritic activity of its polyphenolic rich extract in silico and in vivo via assessment of pro- and anti-inflammatory cytokines

ETHNOPHARMACOLOGICAL RELEVANCE

Opuntia monacantha belongs to the cactus family Cactaceae and is also known by cochineal prickly pear, Barbary fig or drooping prickly pear. It was traditionally used to treat pain and inflammation. O. monacantha cladodes showed pharmacological effects such as antioxidant potential owing to the presence of certain polysaccharides, flavonoids, and phenols.

AIM OF THE STUDY

This research aimed to evaluate the anti-inflammatory as well as the anti-arthritic potential of ethanol extract of Opuntia monacantha (E-OM).

MATERIALS AND METHODS

In vivo edema in rat paw was triggered by carrageenan and used to evaluate anti-inflammatory activity, while induction of arthritis by Complete Freund's Adjuvant (CFA) rat model was done to measure anti-arthritic potential. In silico studies of the previously High performance liquid chromatography (HPLC) characterized metabolites of ethanol extract was performed by using Discovery Studio 4.5 (Accelrys Inc., San Diego, CA, USA) within active pocket of glutaminase 1 (GLS1) (PDB code: 3VP1; 2.30 Å).

RESULTS

EOM, particularly at 750 mg/kg, caused a reduction in the paw edema significantly and decreased arthritic score by 80.58% compared to the diseased group. It revealed significant results when histopathology of ankle joint was examined at 28th day as it reduced inflammation by 18.06%, bone erosion by 15.50%, and pannus formation by 24.65% with respect to the diseased group. It restored the altered blood parameters by 7.56%, 18.47%, and 3.37% for hemoglobin (Hb), white blood count (WBC), and platelets, respectively. It also reduced rheumatoid factor RF by 13.70% with concomitant amelioration in catalase (CAT) and superoxide dismutase (SOD) levels by 19%, and 34.16%, respectively, in comparison to the diseased group. It notably decreased mRNA expression levels of COX-2, IL-6, TNF-α, IL-1, NF-κβ and augmented the levels of IL-4 and IL-10 in real time PCR with respect to the diseased group and piroxicam. HPLC analysis previously performed showed that phenolic acids and flavonoids are present in E-OM. Molecular docking studies displayed pronounced inhibitory potential of these compounds towards glutaminase 1 (GLS1), approaching and even exceeding piroxicam.

CONCLUSIONS

Thus, Opuntia monacantha could be a promising agent to manage inflammation and arthritis and could be incorporated into pharmaceuticals.

C-176 reduces inflammation-induced pain by blocking the cGAS-STING pathway in microglia

OBJECTIVE

Inflammatory pain, is caused by lesions or diseases of the somatosensory tissue, is a prevalent chronic condition that profoundly impacts the quality of life. However, clinical treatment for this type of pain remains limited. Traditionally, the stimulation of microglia and subsequent inflammatory reactions are considered crucial elements to promote the worsening of inflammatory pain. Recent research has shown the crucial importance of the cGAS-STING pathway in promoting inflammation. It is still uncertain if the cGAS-STING pathway plays the role in the fundamental cause of inflammatory pain. We aim to explore the treatment of inflammatory pain by interfering with cGAS-STING signaling pathway.

METHODS

In this study, we established an inflammatory pain model by CFA into the plantar of mice. Activation of microglia, various inflammatory factors and cGAS-STING protein in the spinal dorsal horn were evaluated. Immunofluorescence staining was used to observe the cellular localization of cGAS and STING. The cGAS-STING pathway proteins expression and mRNA expression of indicated microglial M1/M2 phenotypic markers in the BV2 microglia were detected. STING inhibitor C-176 was intrathecal injected into mice with inflammatory pain, and the pain behavior and microglia were observed.

RESULTS

This research showed that injecting CFA into the left hind paw of mice caused mechanical allodynia and increased inflammation in the spine. Our research results suggested that the cGAS-STING pathway had a function in the inflammation mediated by microglia in the spinal cord dorsal horn. Blocking the cGAS-STING pathway using STING antagonists (C-176) led to reduced release of inflammatory factors and prevented M1 polarization of BV2 microglia in a laboratory setting. Additionally, intrathecal administration of C-176 reduced the allodynia in CFA treated mice.

CONCLUSION

Our results suggest that inhibiting microglial polarization through the cGAS-STING pathway represents a potential novel therapeutic strategy for inflammatory pain.

Novel endomorphin analogues CEMR-1 and CEMR-2 produce potent and long-lasting antinociception with a favourable side effect profile at the spinal level

BACKGROUND AND PURPOSE

Endomorphins have shown great promise as pharmaceutics for the treatment of pain. We have previously confirmed that novel endomorphin analogues CEMR-1 and CEMR-2 behaved as potent μ agonists and displayed potent antinociceptive activities at the supraspinal and peripheral levels. The present study was undertaken to evaluate the antinociceptive properties of CEMR-1 and CEMR-2 following intrathecal (i.t.) administration. Furthermore, their antinociceptive tolerance and opioid-like side effects were also determined.

EXPERIMENTAL APPROACH

The spinal antinociceptive effects of CEMR-1 and CEMR-2 were determined in a series of pain models, including acute radiant heat paw withdrawal test, spared nerve injury-induced neuropathic pain, complete Freund's adjuvant-induced inflammatory pain, visceral pain and formalin pain. Antinociceptive tolerance was evaluated in radiant heat paw withdrawal test.

KEY RESULTS

Spinal administration of CEMR-1 and CEMR-2 produced potent and prolonged antinociceptive effects in acute pain. CEMR-1 and CEMR-2 may produce their antinociception through distinct μ receptor subtypes. These two analogues also exhibited significant analgesic activities in neuropathic, inflammatory, visceral and formalin pain at the spinal level. It is noteworthy that CEMR-1 showed non-tolerance-forming analgesic properties, while CEMR-2 exhibited substantially reduced antinociceptive tolerance. Furthermore, both analogues displayed no or reduced side effects on conditioned place preference response, physical dependence, locomotor activity and gastrointestinal transit.

CONCLUSIONS AND IMPLICATIONS

The present investigation demonstrated that CEMR-1 and CEMR-2 displayed potent and long-lasting antinociception with a favourable side effect profile at the spinal level. Therefore, CEMR-1 and CEMR-2 might serve as promising analgesic compounds with minimal opioid-like side effects.

An update of murine models and their methodologies in immune-mediated joint damage and pain research

Murine models have played an indispensable role in the understanding of rheumatic and musculoskeletal disorders (RMD), elucidating the genetic, endocrine and biomechanical pathways involved in joint pathology and associated pain. To date, the available models in RMD can be classified as induced or spontaneous, both incorporating transgenic alternatives that improve specific insights. It is worth noting that the selection of the most appropriate model together with the evaluation of their specific characteristics and technical capabilities are crucial when designing the experiments. Furthermore, it is also imperative to consistently adhere to the ethical standards concerning animal experimentation. Recognizing the inherent limitation that any model can entirely encapsulates the complexity of the pathophysiology of these conditions, the aim of this review is to provide an updated overview on the methodology of current murine models in major arthropathies and their immune-mediated pathways, addressing to basic, translational and pharmacological research in joint damage and pain.

Uncontrolled pain: a call for better study design

Studies assessing animal pain in veterinary research are often performed primarily for the benefit of animals. Frequently, the goal of these studies is to determine whether the analgesic effect of a novel treatment is clinically meaningful, and therefore has the capacity to improve the welfare of treated animals. To determine the treatment effect of a potential analgesic, control groups are necessary to allow comparison. There are negative control groups (where pain is unattenuated) and positive control groups (where pain is attenuated). Arising out of animal welfare concerns, there is growing reluctance to use negative control groups in pain studies. But for studies where pain is experimentally induced, the absence of a negative control group removes the opportunity to demonstrate that the study methods could differentiate a positive control intervention from doing nothing at all. For studies that are controlled by a single comparison group, the capacity to distinguish treatment effects from experimental noise is more difficult; especially considering that pain studies often involve small sample sizes, small and variable treatment effects, systematic error and use pain assessment measures that are unreliable. Due to these limitations, and with a focus on farm animals, we argue that many pain studies would be enhanced by the simultaneous inclusion of positive and negative control groups. This would help provide study-specific definitions of pain and pain attenuation, thereby permitting more reliable estimates of treatment effects. Adoption of our suggested refinements could improve animal welfare outcomes for millions of animals globally.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Central and peripheral mechanism of MOTS-c attenuates pain hypersensitivity in a mice model of inflammatory pain

BACKGROUND

Inflammatory pain is caused by damaged tissue or noxious stimuli, accompanied by the release of inflammatory mediators that often leads to severe hyperalgesia and allodynia with limited therapy options. Recently, a novel mitochondrial-derived peptide (named MOTS-c) was reported to regulate obesity, metabolic homeostasis and inflammatory response. The aim of this study was to investigate the effects of MOTS-c and its related regulatory mechanisms involved in inflammatory pain.

METHODS

Male Kunming mice (8-10 weeks-old) were intraplantar injected with formalin, capsaicin, λ-Carrageenan and complete Freund adjuvant (CFA) to establish acute and chronic inflammatory pain. The effects of MOTS-c on the above inflammatory pain mice and its underlying mechanisms were examined by behavioral tests, quantitative polymerase chain reaction (qPCR), western blotting, enzyme linked immunosorbent assay (ELISA), immunohistochemistry (IHC) and immunofluorescence (IF).

RESULTS

Behavioral experiments investigated the potential beneficial effects of MOTS-c on multiple acute and chronic inflammatory pain in mice. The results showed that MOTS-c treatment produced potent anti-allodynic effects in formalin-induced acute inflammatory pain, capsaicin-induced nocifensive behaviors and λ-Carrageenan/CFA-induced chronic inflammatory pain model. Further mechanistic studies revealed that central MOTS-c treatment significantly ameliorated CFA-evoked the release of inflammatory factors and activation of glial cells and neurons in the spinal dorsal horn. Moreover, peripheral MOTS-c treatment reduced CFA-evoked inflammatory responses in the surface structure of hindpaw skin, accompanied by inhibiting excitation of peripheral calcitonin gene-related peptide (CGRP) and P2X3 nociceptive neurons.

CONCLUSIONS

The present study indicates that MOTS-c may serve as a promising therapeutic target for inflammatory pain.

Spinal interleukin-16 mediates inflammatory pain via promoting glial activation

Proinflammatory cytokines are crucial contributors to neuroinflammation in the development of chronic pain. Here, we identified il16, which encodes interleukin-16 (IL-16), as a differentially expressed gene in spinal dorsal horn of a complete Freund's Adjuvant (CFA) inflammatory pain model in mice by RNA sequencing. We further investigated whether and how IL-16 regulates pain transmission in the spinal cord and contributes to the development of inflammatory pain hypersensitivity. RNA sequencing and bioinformatics analysis revealed elevated IL-16 transcript levels in the spinal dorsal horn after CFA injection. This increase was further confirmed by qPCR, immunofluorescence, and western blotting. Knockdown of IL-16 by intrathecal injection of IL-16 siRNA not only attenuated CFA-induced mechanical and thermal pain hypersensitivity, but also inhibited enhanced c-fos expression and glial activation in the spinal dorsal horn in male mice injected with CFA. Moreover, exogenous IL-16 induced nociceptive responses and increased c-fos expression and glial activation in spinal dorsal horn. This effect was largely impaired when CD4, the binding receptor for IL-16, was inhibited. In addition, CD4 expression was upregulated in the spinal dorsal horn after CFA injection and CD4 was present in microglia and in contact with astrocytes and activated spinal neurons. Taken together, these results suggest that enhanced IL-16-CD4 signaling triggers pain and activates microglia and astrocytes in the spinal dorsal horn, thus contributing to inflammatory pain. IL-16 may serve as a promising target for the treatment of inflammatory pain.

Optogenetic Neuromodulation in Inflammatory Pain

Inflammatory pain is one of the most prevalent forms of pain and negatively influences the quality of life. Neuromodulation has been an expanding field of pain medicine and is accepted by patients who have failed to respond to several conservative treatments. Despite its effectiveness, neuromodulation still lacks clinically robust evidence on inflammatory pain management. Optogenetics, which controls particular neurons or brain circuits with high spatiotemporal accuracy, has recently been an emerging area for inflammatory pain management and studying its mechanism. This review considers the fundamentals of optogenetics, including using opsins, targeting gene expression, and wavelength-specific light delivery techniques. The recent evidence on application and development of optogenetic neuromodulation in inflammatory pain is also summarised. The current limitations and challenges restricting the progression and clinical transformation of optogenetics in pain are addressed. Optogenetic neuromodulation in inflammatory pain has many potential targets, and developing strategies enabling clinical application is a desirable therapeutic approach and outcome.

Arabinan-rich pectic polysaccharide fraction from Malpighia emarginata fruits alleviates inflammatory pain in mice

Malpighia emarginata (Malpighiaceae), popularly known as "acerola", is a tropical and subtropical fruit native to the Americas. Despite its high vitamin C content, which gives it a high antioxidant property, soluble dietary fibers, such as polysaccharides, are also abundant constituents of acerola (10% of the dried fruit). The acerola cold-water soluble (ACWS) fraction presented anti-fatigue and antioxidant effects in vivo and in vitro. To infer further systemic effects of ACWS, this study aimed to investigate the antinociceptive, anti-inflammatory, and antioxidant effects of ACWS in murine models of pain. In formalin-induced nociception, ACWS (0.1, 1, and 10 mg/kg) reduced only the inflammatory phase, and also (10 and 30 mg/kg) attenuated the acetic acid-induced writhing and leukocyte migration in the peritoneal cavity. The mechanical allodynia and paw edema induced by intraplantar injection of carrageenan were greatly reduced by ACWS (10 mg/kg). At the inflammatory pick induced by carrageenan (4 h), ACWS significantly reduced myeloperoxidase activity, TNF-α, IL-1β, and PGE2 levels, and restored IL-10 levels. ACWS also exhibited antioxidant properties by decreasing lipid hydroperoxides content, increasing GSH levels, and restoring superoxide dismutase and catalase activities in the carrageenan model and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging assay. Collectively, these results support the antinociceptive, anti-inflammatory, and antioxidant effects of ACWS and reveal a promising candidate for the treatment of inflammatory pain conditions.

Role of Cav2.3 (R-type) Calcium Channel in Pain and Analgesia: A Scoping Review

BACKGROUND

Voltage-gated calcium channels (VGCCs) play an important role in pain development and maintenance. As Cav2.2 and Cav3.2 channels have been identified as potential drug targets for analgesics, the participation of Cav2.3 (that gives rise to R-type calcium currents) in pain and analgesia remains incompletely understood.

OBJECTIVE

Identify the participation of Cav2.3 in pain and analgesia.

METHODS

To map research in this area as well as to identify any existing gaps in knowledge on the potential role of Cav2.3 in pain signalling, we conducted this scoping review. We searched PubMed and SCOPUS databases, and 40 articles were included in this study. Besides, we organized the studies into 5 types of categories within the broader context of the role of Cav2.3 in pain and analgesia.

RESULTS

Some studies revealed the expression of Cav2.3 in pain pathways, especially in nociceptive neurons at the sensory ganglia. Other studies demonstrated that Cav2.3-mediated currents could be inhibited by analgesic/antinociceptive drugs either indirectly or directly. Some articles indicated that Cav2.3 modulates nociceptive transmission, especially at the pre-synaptic level at spinal sites. There are studies using different rodent pain models and approaches to reduce Cav2.3 activity or expression and mostly demonstrated a pro-nociceptive role of Cav2.3, despite some contradictory findings and deficiencies in the description of study design quality. There are three studies that reported the association of single-nucleotide polymorphisms in the Cav2.3 gene (CACNA1E) with postoperative pain and opioid consumption as well as with the prevalence of migraine in patients.

CONCLUSION

Cav2.3 is a target for some analgesic drugs and has a pro-nociceptive role in pain.

Deciphering resveratrol's role in modulating pathological pain: From molecular mechanisms to clinical relevance

Pathological pain, a multifaceted and debilitating ailment originating from injury or post-injury inflammation of the somatosensory system, poses a global health challenge. Despite its ubiquity, reliable therapeutic strategies remain elusive. To solve this problem, resveratrol, a naturally occurring nonflavonoid polyphenol, has emerged as a potential beacon of hope owing to its anti-inflammatory, antioxidant, and immunomodulatory capabilities. These properties potentially position resveratrol as an efficacious candidate for the management of pathological pain. This concise review summaries current experimental and clinical findings to underscore the therapeutic potential of resveratrol in pathological pain, casting light on the complex underlying pathophysiology. Our exploration suggests that resveratrol may exert its analgesic effect by the modulating pivotal signaling pathways, including PI3K/Akt/mTOR, TNFR1/NF-κB, MAPKs, and Nrf2. Moreover, resveratrol appears to attenuate spinal microglia activation, regulate primary receptors in dorsal root sensory neurons, inhibit pertinent voltage-gated ion channels, and curb the expression of inflammatory mediators and oxidative stress responses. The objective of this review is to encapsulate the pharmacological activity of resveratrol, including its probable signaling pathways, pharmacokinetics, and toxicology pertinent to the treatment of pathological pain. Hopefully, we aim to map out promising trajectories for the development of resveratrol as a potential analgesic.

Adolescent morphine exposure changes the endogenous vlPAG opioid response to inflammatory pain in rats

Adolescence is one of the most critical periods for brain development, and exposure to morphine during this period can have long-life effects on pain-related behaviors. The opioid system in the periaqueductal gray (PAG) is highly vulnerable to drug exposure. However, the impact of adolescent morphine exposure (AME) on the endogenous opioid system in the PAG is currently unknown. This study aims to investigate the long-lasting effects of AME on the endogenous opioid system and its involvement in altering nociceptive behaviors. Adolescent rats were given escalating doses of morphine (2.5-25 mg/kg, subcutaneous) or an equal volume of saline twice daily for 10 consecutive days (PND 31-40). After a 30-day washout period, adult rats underwent formalin tests following microinjection of morphine, naloxone, or saline into the ventrolateral PAG (vlPAG) region. The results indicated that morphine microinjection into the vlPAG of the adolescent morphine-treated group significantly reduced the nociceptive score. However, the analgesic response to morphine in this group was significantly lower compared to the saline-treated group during adolescence. Additionally, the nociceptive score significantly increased following naloxone but not saline microinjection into the vlPAG of the saline-treated group during adolescence, rather than the morphine-treated one. These findings indicate that AME has long-lasting effects on the endogenous opioid system in the vlPAG, which can consequently alter behaviors related to inflammatory pain in adulthood.

Involvement of the Transient Receptor Channels in Preclinical Models of Musculoskeletal Pain

BACKGROUND

Musculoskeletal pain is a condition that affects bones, muscles, and tendons and is present in various diseases and/or clinical conditions. This type of pain represents a growing problem with enormous socioeconomic impacts, highlighting the importance of developing treatments tailored to the patient's needs. TRP is a large family of non-selective cation channels involved in pain perception. Vanilloid (TRPV1 and TRPV4), ankyrin (TRPA1), and melastatin (TRPM8) are involved in physiological functions, including nociception, mediation of neuropeptide release, heat/cold sensing, and mechanical sensation.

OBJECTIVE

In this context, we provide an updated view of the most studied preclinical models of muscle hyperalgesia and the role of transient receptor potential (TRP) in these models.

METHODS

This review describes preclinical models of muscle hyperalgesia induced by intramuscular administration of algogenic substances and/or induction of muscle damage by physical exercise in the masseter, gastrocnemius, and tibial muscles.

RESULTS

The participation of TRPV1, TRPA1, and TRPV4 in different models of musculoskeletal pain was evaluated using pharmacological and genetic tools. All the studies detected the antinociceptive effect of respective antagonists or reduced nociception in knockout mice.

CONCLUSION

Hence, TRPV1, TRPV4, and TRPA1 blockers could potentially be utilized in the future for inducing analgesia in muscle hypersensitivity pathologies.

Endomorphin-2 analogs with C-terminal esterification display potent antinociceptive effects in the formalin pain test in mice

Previously, we have investigated three C-terminal esterified endomorphin-2 (EM-2) analogs EM-2-Me, EM-2-Et and EM-2-Bu with methyl, ethyl and tert-butyl ester modifications, respectively. These analogs produced significant antinociception in acute pain at the spinal and supraspinal levels, with reduced tolerance and gastrointestinal side effects. The present study was undertaken to determine the analgesic effects and opioid mechanisms of these three analogs in the formalin pain test. Our results demonstrated that intracerebroventricular (i.c.v.) administration of 0.67-20 nmol EM-2 analogs EM-2-Me, EM-2-Et and EM-2-Bu produced dose-dependent antinociceptive effects in both phase Ⅰ and phase Ⅱ of formalin pain. EM-2-Me and EM-2-Bu displayed more potent antinociception than morphine. Especially, EM-2-Bu exhibited the highest antinociception in phase Ⅱ of formalin pain, with the ED50 value being 2.1 nmol. Naloxone (80 nmol, i.c.v.) completely antagonized the antinociceptive effects of EM-2-Me, EM-2-Et and EM-2-Bu (20 nmol, i.c.v.) in both phase I and phase Ⅱ of formalin pain, suggesting a central opioid mechanism. Nevertheless, the antinociception induced by EM-2-Me might be involved in the release of dynorphin A, which subsequently acted on κ- opioid receptor. EM-2-Bu produced the antinociception probably by the direct activation of both μ- and δ-opioid receptors. EM-2-Me, EM-2-Et and EM-2-Bu also produced significant analgesic effects after peripheral administration, and the central opioid receptors were involved. Furthermore, EM-2-Bu had no influence on the locomotor activity after i.c.v. injection. The present investigation demonstrated that C-terminal esterified modifications of EM-2 will be beneficial for developing novel therapeutics in formalin pain.

Saffron (Crocus sativus) and its constituents for pain management: A review of current evidence

Pain can become a chronic and deliberating experience with a significant burden. In preclinical and clinical studies, Saffron (Crocus sativus L.) has shown analgesic activities. Considering the unsatisfactory results of current therapeutic management for chronic pain conditions, we aimed to review saffron's analgesic activity and underlying mechanisms. Saffron showed antinociceptive activities in formalin-, carrageenan-, and capsaicin-induced experimental pain models. Saffron analgesic activities affected several targets, including ion channels of nociceptors; the adrenergic system and central histaminic system; inhibition of inflammatory pathways, apoptotic pathways, and oxidative stress; regulation of NO pathway, and the endocannabinoid system. Clinical studies showed analgesia of Saffron in rheumatoid arthritis, after-pain following childbirth, dysmenorrhea, and fibromyalgia. Our literature review showed that saffron can be beneficial as an adjunct therapy to commonly used analgesics in practice, particularly in chronic pain conditions.

Upregulation of lysine-specific demethylase 6B aggravates inflammatory pain through H3K27me3 demethylation-dependent production of TNF-α in the dorsal root ganglia and spinal dorsal horn in rats

AIMS

Lysine-specific demethylase 6B (KDM6B) serves as a key mediator of gene transcription. It regulates expression of proinflammatory cytokines and chemokines in variety of diseases. Herein, the role and the underlying mechanisms of KDM6B in inflammatory pain were studied.

METHODS

The inflammatory pain was conducted by intraplantar injection of complete Freund's adjuvant (CFA) in rats. Immunofluorescence, Western blotting, qRT-PCR, and chromatin immunoprecipitation (ChIP)-PCR were performed to investigate the underlying mechanisms.

RESULTS

CFA injection led to upregulation of KDM6B and decrease in the level of H3K27me3 in the dorsal root ganglia (DRG) and spinal dorsal horn. The mechanical allodynia and thermal hyperalgesia following CFA were alleviated by the treatment of intrathecal injection of GSK-J4, and by microinjection of AAV-EGFP-KDM6B shRNA in the sciatic nerve or in lumbar 5 dorsal horn. The increased production of tumor necrosis factor-α (TNF-α) following CFA in the DRGs and dorsal horn was inhibited by these treatments. ChIP-PCR showed that CFA-induced increased binding of nuclear factor κB with TNF-α promoter was repressed by the treatment of microinjection of AAV-EGFP-KDM6B shRNA.

CONCLUSIONS

These results suggest that upregulated KDM6B via facilitating TNF-α expression in the DRG and spinal dorsal horn aggravates inflammatory pain.

Efficacy of the Multi-Target Compound E153 in Relieving Pain and Pruritus of Different Origins

Itch and pain are closely related but distinct sensations that share largely overlapping mediators and receptors. We hypothesized that the novel, multi-target compound E153 has the potential to attenuate pain and pruritus of different origins. After the evaluation of sigma receptor affinity and pharmacokinetic studies, we tested the compound using different procedures and models of pain and pruritus. Additionally, we used pharmacological tools, such as PRE-084, RAMH, JNJ 5207852, and S1RA, to precisely determine the role of histamine H3 and sigma 1 receptors in the analgesic and antipruritic effects of the compound. In vitro studies revealed that the test compound had potent affinity for sigma 1 and sigma 2 receptors, moderate affinity for opioid kappa receptors, and no affinity for delta or μ receptors. Pharmacokinetic studies showed that after intraperitoneal administration, the compound was present at high concentrations in both the peripheral tissues and the central nervous system. The blood-brain barrier-penetrating properties indicate its ability to act centrally at the levels of the brain and spinal cord. Furthermore, the test compound attenuated different types of pain, including acute, inflammatory, and neuropathic. It also showed a broad spectrum of antipruritic activity, attenuating histamine-dependent and histamine-independent itching. Finally, we proved that antagonism of both sigma 1 and histamine H3 receptors is involved in the analgesic activity of the compound, while the antipruritic effect to a greater extent depends on sigma 1 antagonism.

Glutamatergic Neurons in the Zona Incerta Modulate Pain and Itch Behaviors in Mice

Emerging evidence suggest that parvalbumin neurons in zona incerta (ZI) modulate pain and itch behavior in opposite manners. However, the role of ZI glutamatergic neurons, a unique incertal neuronal subpopulation residing in the caudal division, in pain and itch modulation remains unknown. In the present study, by combining chemogenetic manipulation, fiber photometry, and behavioral tests, we proved that incertal glutamatergic neurons served as an endogenous negative diencephalic modulator for both pain and itch processing. We demonstrated that ZI vesicular glutamate transporter 2 (VGluT2) neurons exhibited increased calcium signal upon hindpaw withdrawal in response to experimental mechanical and thermal stimuli. Behavioral tests further showed that pharmacogenetic activation of this specific type of neurons reduced nocifensive withdrawal responses in both naïve and inflammatory pain mice. Similar neural activity and modulatory role of ZI VGluT2 neurons were also observed upon histaminergic and non-histaminergic acute itch stimuli. Together, our study would expedite our understandings of brain mechanisms underlying somatosensory processing and modulation, and supply a novel therapeutic target for the management of chronic pain and itch disorders.

Rab11a in the spinal cord: an essential contributor to complete Freund's adjuvant-induced inflammatory pain in mice

Inflammatory pain is a commonly observed clinical symptom in a range of acute and chronic diseases. However, the mechanism of inflammatory pain is far from clear yet. Rab11a, a small molecule guanosine triphosphate enzyme, is reported to regulate orofacial inflammatory pain in our previous works. However, the mechanism of Rab11a's involvement in the regulation of inflammatory pain remains obscure. Here, we aim to elucidate the potential mechanisms through which Rab11a contributes to the development of inflammatory pain in the spinal level. It's shown that neurons, rather than glial cells, were the primary cell type expressing Rab11a in the spinal dorsal horn (SDH). After intra-plantar injection of CFA, both the number of Fos/Rab11a-immunopositive neurons and the expression of Rab11a were increased. Administration of Rab11a-shRNA into the SDH resulted in significantly analgesic effect in mice with CFA injection. Application of Rab11a-shRNA also reduced the NMDA receptor-mediated excitatory post-synaptic current (EPSC) and the spike number of neurons in lamina II of the SDH in mice with CFA injection, without affecting the presynaptic glutamate release and the postsynaptic AMPA receptor-mediated EPSC. Our results thus suggest that the enhanced expression of neuronal Rab11a may be important for the process of inflammatory pain in mice with CFA injection, which is likely mediated by Rab11a's potentiation of the competence of post-synaptic NMDAR and spiking of SDH neurons.

Optimal lameness induction model development using amphotericin B in meat goats

Lameness continues to be a critical health and welfare concern associated with goat production. Amphotericin B (amp B) is an antimicrobial successful in inducing transient lameness for research purposes previously in livestock animals. The objectives of this study were to (1) identify which of three varying doses of amp B would be most effective in inducing lameness in meat type goats and (2) develop a facial grimace scale for goats. Lameness was produced by an intra-articular injection of amphotericin B into the left hind lateral claw distal interphalangeal joint with either a 5 mg/0.25 mL (high-low, 5 mg of amphotericin B in a volume of 0.25 mL), 5 mg/0.5 mL (high-high, 5 mg of amphotericin B in a volume of 0.5 mL), or a 2.5 mg/0.25 mL (low-low, 2.5 mg of amphotericin B in a volume of 0.25 mL). A saline treatment of 0.5 mL was used as control (0.9% sterile saline solution). Lameness response was analyzed by infrared thermography (IRT) at the induced joint, mechanical-nociception threshold (MNT), visual lameness scoring (VLS), a visual analogue scale (VAS), kinetic gait analysis (KGA), plasma cortisol (CORT), substance P (Sub P), and behavior scoring. The IRT and MNT values differed by timepoint (P ≤ 0.0001). Results from VLS showed the HL treatment was the most effective at inducing lameness (6/6 goats became lame compared to HH 4/6 and LL 2/6). At 24, 48, and 72 h, VAS scores were significantly higher when comparing HL to all other treatment groups (P = 0.0003). Both behavior observers (1 and 2) reported a significant time effect (P = 0.05), with goats exhibiting more facial grimacing at 24 h post-lameness induction. From these data, an optimal dose for a repeatable lameness induction model in goats was aquired. An effective Goat Grimace Scale (GGS) was also developed to evaluate pain responses in goats.

Advanced Dynamic Weight Bearing as an Observer-independent Measure of Hyperacute Hypersensitivity in Mice

Background

Standard methods assessing pain in rodents are often observer dependent, potentially resulting in biased outcomes. Advanced dynamic weight bearing (ADWB) offers an observer-independent approach that can provide objective, reliable data in preclinical pain research.

Aims

The aim of this study was to characterize the use of ADWB in assessing murine responses to allyl isothiocyanate (AITC)-induced hyperacute hypersensitivity and identify best practices for use of the device.

Methods

Male C57BL/6J mice received intraplantar injections of saline or 0.1% AITC solution and were assessed using the ADWB system; simultaneous observer-dependent durations of paw licking and biting were measured. ADWB data were analyzed using the proprietary software from Bioseb and correlated to observer-dependent results, with parameters assessed to optimize data collected.

Results

ADWB detected pain-directed changes in weight and surface area distribution in AITC-treated mice, with paw weight and surface area placement correlating to paw licking and biting. Optimization of adjustable threshold parameters allowed for reduced coefficients of variability and increased duration of validated data.

Conclusions

The ADWB assay provides an efficient and unbiased measure of chemical-induced hyperacute hypersensitivity in mice. ADWB detection parameters influence amount of validated data and variability, a consideration for data analysis in future studies.

Topical Analgesic and Anti-Inflammatory Properties of Bioengineered Juglans regia L. Silver Nanoparticles

<b>Background and Objective:</b> Research has demonstrated the antibacterial, anti-angiogenetic, antiviral, anti-inflammatory, anticancer and antioxidant properties of colloidal silver due to its biological, optical and electrical properties. The aim of this study was the anti-inflammatory effect of the silver bioengineered nanoparticles by using the acetonitrile-unripe fruit extract of <i>Juglans regia</i> L., on experimental animal model. <b>Materials and Methods:</b> The study uses various techniques to characterize nanoparticles, including ultraviolet spectra, dynamic light scattering and Fourier transform infrared. The study used carrageenan-induced rat paw edema as an induction model for inflammation and assessed its antinociceptive effects in mice using the formalin test. As well as evaluation of proinflammatory cytokines IL-6, TNF and IL-1. <b>Results:</b> The produced AgNPs were more compact and stable, according to physical characterization methods compared to chemical prepared nanoparticles. The formulation combining unripe fruit bio-reduced nanoparticles and extract, UF, shows a greater acute anti-inflammatory effect, while leaf extract has a better late anti-inflammatory effect. These bioengineered nanoparticles show efficient <i>in vivo</i> anti-acute inflammation, reducing skin inflammation through decreased cellular infiltrates and cytokine release. <b>Conclusion:</b> <i>Juglans regia</i> L., extract and silver nanoparticles show notable effects in both the early and late stages of the antinociceptive formalin test. While, bioengineered NP/UF and NP/LV can be used as topical analgesics. The potent topical anti-inflammatory and analgesic effects of these medications provide a sufficient basis for the use of this plant material in dermatological products.

Celastrol-encapsulated microspheres prepared by microfluidic electrospray for alleviating inflammatory pain

Inflammatory pain is induced by trauma, infection, chemical stimulation, etc. It causes severe physical and psychological agony to patients. Celastrol has powerful anti-inflammatory property and has achieved good results in various inflammation-related diseases. However, the low water solubility and multi-system toxicity limit its clinical application. Herein, we proposed alginate microspheres with core-shell structure which encapsulated celastrol by microfluidic electrospray to effectively overcome the shortcomings and improve the therapeutic effect. The microspheres had uniform size and good biocompatibility, and could release the loaded drugs in the gut. The behavioral tests showed that the celastrol-loaded microspheres effectively alleviated inflammatory pain, and the hematoxylin and eosin staining (HE staining), immunofluorescence and detection of inflammatory cytokines showed the anti-inflammatory effect. These results indicated that the microspheres could reduce dose and toxicity without affecting efficacy, and facilitate the application of celastrol in different clinical situations.

Cystathionine-γ-lyase attenuates inflammatory response and pain of osteoarthritis

The chronic articular disease osteoarthritis (OA) is characterized by osteophyte generation, subchondral bone remodeling, and cartilage deterioration. Low levels of H2S catalyzed by cystathionine-γ-lyase (CSE) encoded by Cthhas neuroprotective, cardioprotective, anti-apoptotic, and anti-inflammatory effects thus, Cth is being developed as a potential therapy for the management of the pathogenesis and symptoms of osteoarthritis. Single-cell RNA sequencing (scRNA-seq) and immunohistochemistry of human cartilage revealed that the expression of CTH was decreased in OA patients. We found that Cthoverexpression decrease IL-1β-induced overactivation of the NF-κB signaling pathway. In vivo, Cthoverexpression relieved pain response and cartilage damage in the anterior cruciate ligament transection (ACLT) rat model. In vitro, CSE alleviated chondrocytes catabolism, inflammation, apoptosis, and senescence, and suppressed the NF-κB pathway. We postulate that CSE has therapeutic effects in suppressing inflammation and degeneration in OA and should be further investigated clinically.

The bidirectional roles of the cGAS-STING pathway in pain processing: Cellular and molecular mechanisms

Pain is a common clinical condition. However, the mechanisms underlying pain are not yet fully understood. It is known that the neuroimmune system plays a critical role in the pathogenesis of pain. Recent studies indicated that the cyclic-GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway can activate the innate immune system by sensing both extrinsic and intrinsic double-stranded DNA in the cytoplasm, which is involved in pain processing. In this review, we summarise (1) the roles of the cGAS-STING pathway in different pain models, (2) the effect of the cGAS-STING pathway in different cells during pain regulation, and (3) the downstream molecular mechanisms of the cGAS-STING pathway in pain regulation. This review provides evidence that the cGAS-STING pathway has pro- and anti-nociceptive effects in pain models. It has different functions in neuron, microglia, macrophage, and T cells. Its downstream molecules include IFN-I, NF-κB, NLRP3, and eIF2α. The bidirectional roles of the cGAS-STING pathway in pain processing are mediated by regulating nociceptive neuronal sensitivity and neuroinflammatory responses. However, their effects in special brain regions, activation of astrocytes, and the different phases of pain require further exploration.

REV-ERB activation as a novel pharmacological approach for treating inflammatory pain

Pain is a complex problem affecting millions of people worldwide. The current therapies to reduce pain are limited as many treatment options inadequately address the causes of pain, lead to tolerance of the drug, or have adverse effects including abuse potential. While there are many causes of pain, one underlying mechanism to the pathogenesis and maintenance of pain conditions is chronic inflammation driven by the NLRP3 inflammasome. Several inflammasome inhibitors are currently under investigation however have the potential to suppress the functioning of the innate immune system, which may cause unwanted affects in patients. Here, we show that the nuclear receptor REV-ERB can suppress the activation of the inflammasome when pharmacologically activated with small molecule agonists. Additionally, REV-ERB activation appears to have analgesic potential in a model of acute inflammatory pain, likely as a result of inflammasome suppression.

Calcitonin gene-related peptide and its receptor plays important role in nociceptive regulation in the arcuate nucleus of hypothalamus of rats with inflammatory pain

The present study has explored the role of calcitonin gene-related peptide (CGRP) and its receptor in inflammatory pain modulation in arcuate nucleus of hypothalamus (ARC). Our study demonstrated that intra-ARC injection of CGRP induced antinociceptive effects to naïve rats and rats with inflammatory pain, the effect could be inhibited by the selective CGRP receptor antagonist CGRP8-37. Interestingly, the CGRP-induced antinociception effect was decreased in rats with inflammatory pain compared to naïve rats. Similarly, we found that calcitonin receptor like receptor (CLR), a main component of CGRP receptor, had a low decreased expression levels in the ARC regions of rats with inflammatory pain. The CGRP-induced antinociceptive effect was significantly impaired after reducing CLR expression by intra-ARC administration of CLR targeted siRNA. These findings demonstrated that CGRP might play a crucial role in nociceptive modulation in the ARC during inflammatory pain, which was mediated by CGRP receptor in the ARC. This study shed light upon CGRP and its receptor interaction might be valuable strategies for the alleviation of inflammatory pain.

Investigation of the chemical structure and analgesic and anti-inflammatory properties of polysaccharides that constitute the dietary fibers of soursop (Annona muricata) fruit

Soursop fruits are widely used in the folk medicine to treat a variety of health conditions. Once the chemical structure of dietary fibers from fruits is closely related to its biological functions in the human body, we aimed to explore structural features and biological activity of dietary fibers from soursop. Polysaccharides that constitute the soluble and insoluble fibers were extracted and further analyzed using monosaccharide composition, methylation, molecular weight determination and ¹³C NMR data. Soursop soluble fibers (SWa fraction) were characterized as having type II arabinogalactan and a highly methyl esterified homogalacturonan, while non-cellulosic insoluble fibers (SSKa fraction) were mainly composed by a pectic arabinan, a xylan-xyloglucan complex and a glucuronoxylan. The oral pre-treatment with SWa and SSKa promoted antinociception in mice writhing test, reducing the number of pain-like behaviors (in 84.2 % and 46.9 %, respectively, at 10 mg/kg) and peritoneal leucocyte migration (55.4 % and 59.1 %, at 10 mg/kg), effects possibly associated with the pectins present in fruit pulp extractions. SWa also significantly inhibited the plasmatic extravasation of Evans blue dye in 39.6 % at 10 mg/kg. This paper describes for the first time the structural features of soursop dietary fibers that may be of biological significance in future.

Similarities and differences between Ziqin and Kuqin in anti-inflammatory, analgesic, and antioxidant activities and their core chemical composition based on the zebrafish model and spectrum-effect relationship

ETHNOPHARMACOLOGICAL RELEVANCE

Scutellaria baicalensis (SB) is a traditional Chinese medicine (TCM). In the clinical application of TCM, SB has been divided into two specifications (Ziqin and Kuqin) for a long time. At present, the Chinese Pharmacopoeia Commission no longer distinguishes between the two. However, the two specifications of medicinal materials and pieces are still in circulation in the market.

AIM OF THE STUDY

This work aimed at investigating the similarities and differences between Ziqin and Kuqin in anti-inflammatory, analgesic, and antioxidant activities and their material basis. It will provide a new angle for relevant regulations to formulate the specifications and standards of SB.

MATERIALS AND METHODS

Here we investigated the similarities and differences between Ziqin and Kuqin in anti-inflammatory, analgesic, and antioxidant activities related to four zebrafish models and three chemical tests. The chemical fingerprints of SB (Ziqin and Kuqin) were profiled by HPLC. Meanwhile, UHPLC-Q-TOF/MS was used to identify the chemical constituents of Ziqin and Kuqin. The main effect-related compounds of SB, Ziqin, and Kuqin were screened out by spectrum-effect relationship. Finally, six monomeric compounds were validated experimentally using the zebrafish inflammation model induced by CuSO₄.

RESULTS

Both Ziqin and Kuqin had significant anti-inflammatory, analgesic, and antioxidant activities. Kuqin had better anti-inflammatory and analgesic activities, while Ziqin had better antioxidant activity. HPLC fingerprint and UHPLC-Q-TOF/MS evaluation showed that the chemical composition types and main components of Ziqin and Kuqin were basically the same, while the contents and proportions of chemical components in Ziqin and Kuqin were different. By spectrum-effect relationship, compounds X1, X2 (luteoloside), X3, X4 (baicalin), X6 (wogonoside), X7 (baicalein), X8 (wogonin), and X9 (oroxylin A) were the same active chemical constituents of Ziqin and Kuqin. The core components of anti-inflammatory and analgesia activities in Kuqin were compounds X1, X2, X3, X5, X6, X7, X8, and X9. The antioxidant core active components of Ziqin were compounds X2, X3, X4, X6, X7, and X9. Among them, luteoloside, baicalin, wogonoside, baicalein, wogonin, and oroxylin A were validated successfully with good anti-inflammatory effects.

CONCLUSIONS

This study revealed that Ziqin and kuqin have high similarity in chemical composition, but their proportions and active core components are different. This may be one of the main reasons why they have the same activity but different activity trends. These findings will help to improve the understanding of the different clinical applications of Ziqin and Kuqin, and provide a reference for the formulation of quality standards and their further research.

P66shc in the spinal cord is an important contributor in complete Freund's adjuvant induced inflammatory pain in mice

PURPOSE

The aim of this study is to investigate whether p66shc is involved in inflammatory pain and the potential molecular mechanisms of p66shc in inflammatory pain.

METHODS

Inflammatory pain model was established by complete Freund's adjuvant (CFA) injection. Paw withdrawal latency (PWL) and paw withdrawal frequency (PWF) was recorded. The expression of spinal p66shc were determined by immunohistochemical staining, immunofluorescence staining. P66shc knockdown was performed by an adeno-associated virus (AAV) vector infusion. NLRP3 inflammasome complexes were determined by Western blot. DHE staining was used to evaluate reactive oxygen species (ROS) generation.

RESULTS

P66Shc expression was progressively elevated in spinal cord of inflammatory pain mice, and p66Shc knockdown in vivo significantly attenuated CFA injection triggers hyperalgesia. Furthermore, knockdown of p66Shc significantly inhibited ROS production and NOD-like receptor protein 3 (NLRP3) inflammasome activation, which were reversed by a ROS donor (t-BOOH). However, post-treatment with nigericin, a agonist of NLRP3, reversed AAV-shP66shc analgesic effect.

CONCLUSION

Spinal p66shc may facilitate the development of inflammatory pain by promoting the activation of NLRP3 inflammasome through ROS.

Advances in the role and mechanism of miRNA in inflammatory pain

Pain is a distressing experience associated with tissue damage or potential tissue damage, and its occurrence is related to sensory, emotional, cognitive and social factors. Inflammatory pain is one of the chronic pains where pain hypersensitivity are functional features of inflammation used to protect tissues from further damage. Pain has a serious impact on people's lives and has become a social problem that cannot be ignored. MiRNAs are small non-coding RNA molecules that exert directing effects on RNA silencing by complementary binding to the 3'UTR of target mRNA. MiRNAs can target a number of protein-coding genes and participate in almost all developmental and pathological processes in animals. Growing studies have suggested that miRNAs have significant implications for inflammatory pain via participating in multiple processes during the occurrence and development, such as affecting the activation of glial cells, regulating pro-inflammatory cytokines and inhibiting central and peripheral sensitization. In this review, the advances in the role of miRNAs in inflammatory pain were discussed. miRNAs as a class of micro-mediators are potential biomarkers and therapeutic targets for inflammatory pain, which provides a better diagnostic and treatment approach for inflammatory pain.

miR-26a-5p alleviates CFA-induced chronic inflammatory hyperalgesia through Wnt5a/CaMKII/NFAT signaling in mice

BACKGROUND

Inflammation often leads to the occurrence of chronic pain, and many miRNAs have been shown to play a key role in the development of inflammatory pain. However, whether miR-26a-5p relieves pain induced by inflammation and its possible mechanism are still unclear.

METHODS

The complete Freund's adjuvant (CFA)-induced inflammatory pain mouse model was employed. Intrathecal or subcutaneous injection of miR-26a-5p agomir was performed after modeling to study its antinociceptive effect and the comparison of different administration methods. Bioinformatics analysis of miRNAs was performed to study the downstream mechanisms of miR-26a-5p. HE staining, RT-qPCR, Western blotting, and immunofluorescence were used for further validation.

RESULTS

A single intrathecal and subcutaneous injection of miR-26a-5p both reversed mechanical hypersensitivity and thermal latency in the left hind paw of mice with CFA-induced inflammatory pain. HE staining and immunofluorescence studies found that both administrations of miR-26a-5p alleviated inflammation in the periphery and spinal cord. Bioinformatics analysis and dual-luciferase reporter gene analysis identified Wnt5a as a direct downstream target gene of miR-26a-5p. Wnt5a was mainly expressed in neurons and microglia in the spinal cord of mice with inflammatory pain. Intrathecal injection of miR-26a-5p could significantly reduce the expression level of Wnt5a and inhibit the downstream molecules of noncanonical Wnt signaling Camk2/NFAT, inhibiting the release of spinal cord inflammatory factors and alleviating the activation of microglia. In addition, miR-26a-5p could also inhibit lipopolysaccharide (LPS)-stimulated BV2 cell inflammation in vitro through a noncanonical Wnt signaling pathway.

CONCLUSIONS

miR-26a-5p is a promising therapy for CFA-induced inflammatory pain. Both intrathecal and subcutaneous injections provide relief for inflammatory pain. miR-26a-5p regulated noncanonical Wnt signaling to be involved in analgesia partly through antineuroinflammation, suggesting a pain-alleviating effect via noncanonical Wnt signaling pathway in the CFA-induced inflammatory pain model in vivo.

Modulation of inflammatory mediators involved in the antinociceptive and anti-inflammatory effects of a new thioamide derivative: thiocyanoacetamide

Inflammatory pain is part of the body's defense mechanism and plays an important role in the healing process. Although some drugs are efficient and intensively used for their potent anti-inflammatory properties, they present problematic side effects. The aim of this study was to evaluate the anti-nociceptive effect of the thiocyanoacetamide (Thm) compared to paracetamol (Para), dexamethasone (Dex) and morphine (Morph) and to study inflammatory mediators on models of acute inflammatory pain in rats using the formalin injection test in the hind paw of rats as chemical stimulus. The obtained results showed significant modulation of pain by Thm pretreatment with a maximum at an effective dose (10 mg/kg) proved by the absence of licking and biting of the affected paw during the early and late phases of inflammation. This effect was comparable to Dex at 10 mg/kg, Para at 400 mg/kg and less than Morph at 5 mg/kg pretreatment doses. The study of anti-inflammatory targets showed that Thm pretreatment maintained plasma serotonin release at normal level compared to the negative control group (T-) and corrected the decrease in the plasma level of prostaglandins after inflammatory induction with no variation in the level of histamine in different groups. The evaluation of inflammation mediators demonstrated that the pretreatment with Thm induced the decrease in the amount of both IL-1 Beta and TNF alpha in plasma and the increase in their amount in the tissue of the injection site. The Thm has been promoted as an anti-nociceptive drug that induces modulation of inflammatory mediators.

Inflammation in pathogenesis of chronic pain: Foe and friend

Chronic pain is a refractory health disease worldwide causing an enormous economic burden on individuals and society. Accumulating evidence suggests that inflammation in the peripheral nervous system (PNS) and central nervous system (CNS) is the major factor in the pathogenesis of chronic pain. The inflammation in the early- and late phase may have distinctive effects on the initiation and resolution of pain, which can be viewed as friend or foe. On the one hand, painful injuries lead to the activation of glial cells and immune cells in the PNS, releasing pro-inflammatory mediators, which contribute to the sensitization of nociceptors, leading to chronic pain; neuroinflammation in the CNS drives central sensitization and promotes the development of chronic pain. On the other hand, macrophages and glial cells of PNS and CNS promote pain resolution via anti-inflammatory mediators and specialized pro-resolving mediators (SPMs). In this review, we provide an overview of the current understanding of inflammation in the deterioration and resolution of pain. Further, we summarize a number of novel strategies that can be used to prevent and treat chronic pain by controlling inflammation. This comprehensive view of the relationship between inflammation and chronic pain and its specific mechanism will provide novel targets for the treatment of chronic pain.

Effect of nitric oxide-cyclic GMP-K+ channel pathway blockers, naloxone and metformin, on the antinociception induced by the diuretic pamabrom

Pamabrom is a diuretic that is effective in treating premenstrual syndrome and primary dysmenorrhea. The aim of this study was to examine the effect of metformin and modulators of the opioid receptor-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP)-K⁺ channel pathway on the local antinociception induced by pamabrom. The rat paw 1% formalin test was used to assess the effects. Rats were treated with local administration of pamabrom (200-800 µg/paw) or indomethacin (200-800 µg/paw). The antinociception of pamabrom or indomethacin was evaluated with and without the local pretreatment of the blockers. Local administration of pamabrom and indomethacin produced dose-dependent antinociception during the second phase of the test. Local pretreatment of the paws with naloxone (50 µg/paw), l-nitro-arginine methyl ester (10-100 µg/paw), or 1H-(1,2,4)-oxadiazolo[4,2-a]quinoxalin-1-one (10-100 µg/paw) reverted the antinociception induced by local pamabrom, but not of indomethacin. Similarly, the K⁺ channel blockers glibenclamide, glipizide, 4-aminopyridine, tetraethylammonium, charybdotoxin, or apamin reverted the pamabrom-induced antinociception, but not of indomethacin. Metformin significantly blocked the antinociception of pamabrom and indomethacin. Our data suggest that pamabrom could activate the opioid receptor-NO-cGMP-K⁺ channel pathway to produce its peripheral antinociception in the formalin test. Likewise, a biguanide-dependent mechanism could be activated by pamabrom and indomethacin to generate antinociception.

Mechanisms of acupuncture-electroacupuncture on inflammatory pain

Acupuncture, as a traditional treatment, has been extensively used in China for thousands of years. According to the World Health Organization (WHO), acupuncture is recommended for the treatment of 77 diseases. And 16 of these diseases are related to inflammatory pain. As a combination of traditional acupuncture and modern electrotherapy, electroacupuncture (EA) has satisfactory analgesic effects on various acute and chronic pain. Because of its good analgesic effects and no side effects, acupuncture has been widely accepted all over the world. Despite the increase in the number of studies, the mechanisms via which acupuncture exerts its analgesic effects have not been conclusively established. A literature review of related research is of great significance to elaborate on its mechanisms and to inform on further research directions. We elucidated on its mechanisms of action on inflammatory pain from two levels: peripheral and central. It includes the mechanisms of acupuncture in the periphery (immune cells and neurons, purinergic pathway, nociceptive ion channel, cannabinoid receptor and endogenous opioid peptide system) and central nervous system (TPRV1, glutamate and its receptors, glial cells, GABAergic interneurons and signaling molecules). In this review, we collected relevant recent studies to systematically explain the mechanisms of acupuncture in treating inflammatory pain, with a view to providing direction for future applications of acupuncture in inflammatory pain and promoting clinical development.

Electroacupuncture activated local sympathetic noradrenergic signaling to relieve synovitis and referred pain behaviors in knee osteoarthritis rats

Introduction

Recent research has focused on the local control of articular inflammation through neuronal stimulation to avoid the systemic side effects of conventional pharmacological therapies. Electroacupuncture (EA) has been proven to be useful for inflammation suppressing and pain reduction in knee osteoarthritis (KOA) patients, yet its mechanism remains unclear.

Methods

In the present study, the KOA model was established using the intra-articular injection of sodium monoiodoacetate (MIA) (1 mg/50 μL) into the knee cavity. EA was delivered at the ipsilateral ST36-GB34 acupoints. Hind paw weight-bearing and withdrawl thresholds were measured. On day 9, the histology, dep enrichment proteins, cytokines contents, immune cell population of the synovial membrane of the affected limbs were measured using HE staining, Masson staining, DIA quantitative proteomic analysis, flow cytometry, immunofluorescence staining, ELISA, and Western Blot. The ultrastructure of the saphenous nerve of the affected limb was observed using transmission electron microscopy on the 14th day after modeling.

Results

The result demonstrated that EA intervention during the midterm phase of the articular inflammation alleviated inflammatory pain behaviors and cartilage damage, but not during the early phase. Mid-term EA suppressed the levels of proinflammatory cytokines TNF-α, IL-1β, and IL-6 in the synovium on day 9 after MIA by elevating the level of sympathetic neurotransmitters Norepinephrine (NE) in the synovium but not systemic NE or systemic adrenaline. Selective blocking of the sympathetic function (6-OHDA) and β2-adrenergic receptor (ICI 118,551) prevented the anti-inflammatory effects of EA. EA-induced increment of the NE in the synovium inhibited the CXCL1-CXCR2 dependent overexpression of IL-6 in the synovial macrophages in a β2-adrenergic receptor (AR)-mediated manner.

Discussion

These results revealed that EA activated sympathetic noradrenergic signaling to control local inflammation in KOA rats and contributed to the development of novel therapeutic neurostimulation strategies for inflammatory diseases.

Benzophenone Derivatives with Histamine H3 Receptor Affinity and Cholinesterase Inhibitory Potency as Multitarget-Directed Ligands for Possible Therapy of Alzheimer's Disease

The multitarget-directed ligands demonstrating affinity to histamine H₃ receptor and additional cholinesterase inhibitory potency represent a promising strategy for research into the effective treatment of Alzheimer's disease. In this study, a novel series of benzophenone derivatives was designed and synthesized. Among these derivatives, we identified compound 6 with a high affinity for H₃R (K_i = 8 nM) and significant inhibitory activity toward BuChE (IC₅₀ = 172 nM and 1.16 µM for eqBuChE and hBuChE, respectively). Further in vitro studies revealed that compound 6 (4-fluorophenyl) (4-((5-(piperidin-1-yl)pentyl)oxy)phenyl)methanone) displays moderate metabolic stability in mouse liver microsomes, good permeability with a permeability coefficient value (P_e) of 6.3 × 10^-6 cm/s, and its safety was confirmed in terms of hepatotoxicity in the HepG2 cell line. Therefore, we investigated the in vivo activity of compound 6 in the Passive Avoidance Test and the Formalin Test. While compound 6 did not show a statistically significant influence on memory and learning, it showed analgesic properties in both acute (ED₅₀ = 20.9 mg/kg) and inflammatory (ED₅₀ = 17.5 mg/kg) pain.

The Double Insult of Neonatal Cystitis Plus Adult Somatic Inflammation Results in Corticotropin Releasing Factor Type II Receptor-Dependent Bladder Hypersensitivity in Female Rats

The spinal mechanisms of visceral hypersensitivity are poorly understood, particularly when there is an interaction with somatic systems. Recently we demonstrated that rats which were pretreated with neonatal bladder inflammation (NBI) and subsequently pretreated as adults with bladder re-inflammation had augmented reflex and neuronal responses to urinary bladder distension via a corticotropin-releasing factor receptor type II (CRFR2) mechanism. Another insult producing similar augmented responses is somatic inflammation induced by Complete Freund's Adjuvant (CFA) in the hindlimb. Using neurochemical measures and both reflex and neuronal responses to urinary bladder distension as endpoints, the present study probed the role of CRFR2-related mechanisms in bladder hyperalgesia secondary to NBI and CFA-induced hindlimb inflammation. ELISA measures of the lumbosacral spinal cord demonstrated increased CRFR2 protein following pretreatment with NBI+CFA. Intrathecal CRFR2 antagonists blocked the augmentation of visceromotor responses to distension following pretreatment with both NBI+CFA. Lumbosacral dorsal horn neuronal responses to bladder distension in rats pretreated with NBI+CFA were attenuated by the spinal topical administration of a CRFR2 antagonist. These findings are the first demonstration of a somatovisceral interaction working via CRFR2 receptors and support the therapeutic value of these agents in the treatment of painful bladder disorders, particularly when triggered by somatic events. (Word Count 199). PERSPECTIVE: Bladder hypersensitivity occurs following neonatal cystitis and an adult insult such as somatic inflammation. This paper demonstrates that CRFR2-related mechanisms are associated with this hypersensitivity. This supports the therapeutic value of these agents in the treatment of painful bladder disorders, particularly when triggered by somatic events.

Low-Carbohydrate High-Fat Diet: A SWOC Analysis

Insulin resistance (IR) plays a role in the pathogenesis of many diseases, such as type 2 diabetes mellitus, cardiovascular disease, non-alcoholic fatty liver disease, obesity, and neurodegenerative diseases, including Alzheimer's disease. The ketogenic diet (KD) is a low-carbohydrate/high-fat diet that arose in the 1920s as an effective treatment for seizure control. Since then, the KD has been studied as a therapeutic approach for various IR-related disorders with successful results. To date, the use of the KD is still debatable regarding its safety. Some studies have acknowledged its usefulness, while others do not recommend its long-term implementation. In this review, we applied a SWOC (Strengths, Weaknesses, Opportunities, and Challenges) analysis that revealed the positive, constructive strengths of the KD, its potential complications, different conditions that can make used for it, and the challenges faced by both physicians and subjects throughout a KD. This SWOC analysis showed that the KD works on the pathophysiological mechanism of IR-related disorders such as chronic inflammation, oxidative stress and mitochondrial stress. Furthermore, the implementation of the KD as a potential adjuvant therapy for many diseases, including cancer, neurodegenerative disorders, polycystic ovary syndrome, and pain management was proven. On the other hand, the short and long-term possible undesirable KD-related effects, including nutritional deficiencies, growth retardation and nephrolithiasis, should be considered and strictly monitored. Conclusively, this review provides a context for decision-makers, physicians, researchers, and the general population to focus on this dietary intervention in preventing and treating diseases. Moreover, it draws the attention of scientists and physicians towards the opportunities and challenges associated with the KD that requires attention before KD initiation.

The emerging power and promise of non-coding RNAs in chronic pain

Chronic pain (CP) is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage lasting longer than 3 months. CP is the main reason why people seek medical care and exerts an enormous economic burden. Genome-wide expression analysis has revealed that diverse essential genetic elements are altered in CP patients. Although many possible mechanisms of CP have been revealed, we are still unable to meet all the analgesic needs of patients. In recent years, non-coding RNAs (ncRNAs) have been shown to play essential roles in peripheral neuropathy and axon regeneration, which is associated with CP occurrence and development. Multiple key ncRNAs have been identified in animal models of CP, such as microRNA-30c-5p, ciRS-7, and lncRNA MRAK009713. This review highlights different kinds of ncRNAs in the regulation of CP, which provides a more comprehensive understanding of the pathogenesis of the disease. It mainly focuses on the contributions of miRNAs, circRNAs, and lncRNAs to CP, specifically peripheral neuropathic pain (NP), diabetic NP, central NP associated with spinal cord injury, complex regional pain syndrome, inflammatory pain, and cancer-induced pain. In addition, we summarize some potential ncRNAs as novel biomarkers for CP and its complications. With an in-depth understanding of the mechanism of CP, ncRNAs may provide novel insight into CP and could become new therapeutic targets in the future.

Treatment with Hydrogen-Rich Water Improves the Nociceptive and Anxio-Depressive-like Behaviors Associated with Chronic Inflammatory Pain in Mice

Chronic inflammatory pain is manifested in many diseases. The potential use of molecular hydrogen (H2) as a new therapy for neurological disorders has been demonstrated. Recent studies prove its analgesic properties in animals with neuropathic pain, but the possible antinociceptive, antidepressant, and/or anxiolytic actions of H2 during persistent inflammatory pain have not been investigated. Therefore, using male mice with chronic inflammatory pain incited by the subplantar injection of complete Freud's adjuvant (CFA), we assessed the actions of hydrogen-rich water (HRW) systemically administered on: (1) the nociceptive responses and affective disorders associated and (2) the oxidative (4-hydroxy-2-nonenal; 4-HNE), inflammatory (phosphorylated-NF-kB inhibitor alpha; p-IKBα), and apoptotic (Bcl-2-like protein 4; BAX) changes provoked by CFA in the paws and amygdala. The role of the antioxidant system in the analgesia induced by HRW systemically and locally administered was also determined. Our results revealed that the intraperitoneal administration of HRW, besides reducing inflammatory pain, also inhibited the depressive- and anxiolytic-like behaviors associated and the over expression of 4-HNE, p-IKBα, and BAX in paws and amygdala. The contribution of the nuclear factor erythroid 2-related factor 2/heme oxygenase 1 and NAD(P)H: quinone oxidoreductase 1 pathway in the analgesic activities of HRW, systemically or locally administered, was also shown. These data revealed the analgesic, antidepressant, and anxiolytic actions of HRW. The protective, anti-inflammatory, and antioxidant qualities of this treatment during inflammatory pain were also demonstrated. Therefore, this study proposes the usage of HRW as a potential therapy for chronic inflammatory pain and linked comorbidities.

Targeting the nitric oxide/cGMP signaling pathway to treat chronic pain

Nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP) signaling has been shown to act as a mediator involved in pain transmission and processing. In this review, we summarize and discuss the mechanisms of the NO/cGMP signaling pathway involved in chronic pain, including neuropathic pain, bone cancer pain, inflammatory pain, and morphine tolerance. The main process in the NO/cGMP signaling pathway in cells involves NO activating soluble guanylate cyclase, which leads to subsequent production of cGMP. cGMP then activates cGMP-dependent protein kinase (PKG), resulting in the activation of multiple targets such as the opening of ATP-sensitive K⁺ channels. The activation of NO/cGMP signaling in the spinal cord evidently induces upregulation of downstream molecules, as well as reactive astrogliosis and microglial polarization which participate in the process of chronic pain. In dorsal root ganglion neurons, natriuretic peptide binds to particulate guanylyl cyclase, generating and further activating the cGMP/PKG pathway, and it also contributes to the development of chronic pain. Upregulation of multiple receptors is involved in activation of the NO/cGMP signaling pathway in various pain models. Notably the NO/cGMP signaling pathway induces expression of downstream effectors, exerting both algesic and analgesic effects in neuropathic pain and inflammatory pain. These findings suggest that activation of NO/cGMP signaling plays a constituent role in the development of chronic pain, and this signaling pathway with dual effects is an interesting and promising target for chronic pain therapy.

(2R,6R)-hydroxynorketamine (HNK) reverses mechanical hypersensitivity in a model of localized inflammatory pain

The ketamine metabolite (2R,6R)-hydroxynorketamine, or (2R,6R)-HNK, was recently reported to evoke antinociception in response to a noxious thermal stimulus in healthy mice and reverse mechanical hypersensitivity in a murine model of neuropathic pain. This study reports the behavioral effects of (2R,6R)-HNK in male and female C57BL/6J mice exposed to a localized inflammatory pain condition and the broad pharmacological mechanism underlying this effect. Hind paw intraplantar injection of λ-carrageenan (CARR) caused inflammation and mechanical hypersensitivity in mice within 2 hours, lasting at least 48 hours. Intraperitoneal administration of (2R,6R)-HNK (10-30 mg/kg i.p.) 2 hours following CARR injection significantly reversed mechanical hypersensitivity within 1 hour in male and female mice, and the effect persisted for 24 hours following a single dose. The magnitude and timing of the analgesic effect of (2R,6R)-HNK were comparable to the non-steroidal anti-inflammatory drug carprofen. The reversal of hypersensitivity by (2R,6R)-HNK was blocked at 4 and 24 hours after administration by pretreatment with the AMPA receptor antagonist NBQX and was not accompanied by changes in locomotor activity. These findings reinforce the growing evidence supporting (2R,6R)-HNK as a novel analgesic in multiple preclinical pain models and further support an AMPAR-dependent mechanism of action. SIGNIFICANCE: The ketamine metabolite (2R,6R)-HNK reversed mechanical hypersensitivity associated with localized inflammation with onset less than one hour and duration greater than 24 hours in an effect comparable to the NSAID carprofen. Reversal of mechanical hypersensitivity by (2R,6R)-HNK is AMPAR-dependent.

Lysophosphatidylcholine: Potential Target for the Treatment of Chronic Pain

The bioactive lipid lysophosphatidylcholine (LPC), a major phospholipid component of oxidized low-density lipoprotein (Ox-LDL), originates from the cleavage of phosphatidylcholine by phospholipase A2 (PLA2) and is catabolized to other substances by different enzymatic pathways. LPC exerts pleiotropic effects mediated by its receptors, G protein-coupled signaling receptors, Toll-like receptors, and ion channels to activate several second messengers. Lysophosphatidylcholine (LPC) is increasingly considered a key marker/factor positively in pathological states, especially inflammation and atherosclerosis development. Current studies have indicated that the injury of nervous tissues promotes oxidative stress and lipid peroxidation, as well as excessive accumulation of LPC, enhancing the membrane hyperexcitability to induce chronic pain, which may be recognized as one of the hallmarks of chronic pain. However, findings from lipidomic studies of LPC have been lacking in the context of chronic pain. In this review, we focus in some detail on LPC sources, biochemical pathways, and the signal-transduction system. Moreover, we outline the detection methods of LPC for accurate analysis of each individual LPC species and reveal the pathophysiological implication of LPC in chronic pain, which makes it an interesting target for biomarkers and the development of medicine regarding chronic pain.