Spinal endomorphins attenuate burn-injury pain in male mice by inhibiting p38 MAPK signaling pathway through the mu-opioid receptor

Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

The impact of burn injury on the central nervous system

Burn injuries can be devastating, with life-long impacts including an increased risk of hospitalization for a wide range of secondary morbidities. One area that remains not fully understood is the impact of burn trauma on the central nervous system (CNS). This review will outline the current findings on the physiological impact that burns have on the CNS and how this may contribute to the development of neural comorbidities including mental health conditions. This review highlights the damaging effects caused by burn injuries on the CNS, characterized by changes to metabolism, molecular damage to cells and their organelles, and disturbance to sensory, motor and cognitive functions in the CNS. This damage is likely initiated by the inflammatory response that accompanies burn injury, and it is often long-lasting. Treatments used to relieve the symptoms of damage to the CNS due to burn injury often target inflammatory pathways. However, there are non-invasive treatments for burn patients that target the functional and cognitive damage caused by the burn, including transcranial magnetic stimulation and virtual reality. Future research should focus on understanding the mechanisms that underpin the impact of a burn injury on the CNS, burn severity thresholds required to inflict damage to the CNS, and acute and long-term therapies to ameliorate deleterious CNS changes after a burn.

Proceedings of the 2021 American Burn Association State and Future of Burn Science Meeting

Periodically, the American Burn Association (ABA) has convened a State of the Science meeting on various topics representing multiple disciplines within burn care and research. In 2021 at the request of the ABA President, meeting development was guided by the ABA's Burn Science Advisory Panel (BSAP) and a subgroup of meeting chairs. The goal of the meeting was to produce both an evaluation of the current literature and ongoing studies, and to produce a research agenda and/or define subject matter-relevant next steps to advance the field(s). Members of the BSAP defined the topics to be addressed and subsequently solicited for nominations of expert speakers and topic leaders from the ABA's Research Committee. Current background literature for each topic was compiled by the meeting chairs and the library then enhanced by the invited topic and breakout discussion leaders. The meeting was held in New Orleans, LA on November 2nd and 3rd and was formatted to allow for 12 different topics, each with two subtopics, to be addressed. Topic leaders provided a brief overview of each topic to approximately 100 attendees, followed by expert-lead breakout sessions for each topic that allowed for focused discussion among subject matter experts and interested participants. The breakout and topic group leaders worked with the participants to determine research needs and associated next steps including white papers, reviews and in some cases collaborative grant proposals. Here, summaries from each topic area will be presented to highlight the main foci of discussion and associated conclusions.

Multifactorial pathways in burn injury-induced chronic pain: novel targets and their pharmacological modulation

Burn injuries are among the highly prevalent medical conditions worldwide that occur mainly in children, military veterans and victims of fire accidents. It is one of the leading causes of temporary as well as permanent disabilities in patients. Burn injuries are accompanied by pain that persists even after recovery from tissue damage which puts immense pressure on the healthcare system. The pathophysiology of burn pain is poorly understood due to its complex nature and lack of considerable preclinical and clinical shreds of evidence, that creates a substantial barrier to the development of new analgesics. Burns damage the skin layers supplied with nociceptors such as NAV1.7, TRPV1, and TRPA1. Burn injury-mediated co-localization and simultaneous activation of TRPA1 and TRPV1 in nociceptive primary afferent C-fibers which contributes to the development and maintenance of chronic pain. Burn injuries are accompanied by central sensitization, a key feature of pain pathophysiology mainly driven by a series of cascades involving aberrations in the glutamatergic system, microglial activation, release of neuropeptides, cytokines, and chemokines. Activation of p38 mitogen-activated protein kinase, altered endogenous opioid signaling, and distorted genomic expression are other pathophysiological factors responsible for the development and maintenance of burn pain. Here we discuss comprehensive literature on molecular mechanisms of burn pain and potential targets that could be translated into near future therapeutics.

Corydecumine G inhibits microglia activation via MAPK pathway in a rat model of neuropathic pain

BACKGROUND AND PURPOSE

Microglial activation plays an important role in the onset and progression of neuropathic pain by producing a variety of pro-inflammatory cytokines that interact with neurons to enhance neuronal hyperexcitability. Corydalis decumbens (Thunb.) pers., a traditional Chinese medicine has been used to treat mild cancer pain, dementia and to remit cerebral ischemia in clinics. Phenylphthalide isoquinolines are the major type of metabolites of C. decumbens and one of the derivatives, Corydecumine G (Cor G) has been shown to inhibit neuronal excitability. The present study aims to investigate the analgesic efficacy of Cor G in neuropathic pain rat model, the effects of Cor G on microglia activation and the possible mechanisms.

EXPERIMENTAL APPROACH

Neuropathic pain was modeled using chronic constriction sciatic nerve injury (CCI) in rats. Western blot, immunofluorescence, and qRT-PCR were used to evaluate the levels of protein and mRNA.

KEY RESULTS

Intraperitoneal administration of Cor G concentration-dependently ameliorates mechanical and thermo allodynia, suppresses CCI-induced p38/ERK phosphorylation and spinal cord microglia activation, and attenuates the expression levels of NO, inos, Tnf-α, Pge₂ in dorsal horn of L4-L6 spinal cord on the ligation side in CCI rats. Pretreatment with 30 μM Cor G decreased LPS-induced BV2 microglia activation, which occurred via the inos, Tnf-α, Il-1β, Il-6 and phospho-p38/ERK pathways.

CONCLUSIONS AND IMPLICATIONS

Taken together, we suggest that Cor G, the specific phthalide isoquinoline from traditional Chinese medicine Corydalis Decumbentis Rhizoma, may be promising for treatment of neuropathic pain.

BmK DKK13, A Scorpion Toxin, Alleviates Pain Behavior in a Rat Model of Trigeminal Neuralgia by Modulating Voltage-Gated Sodium Channels and MAPKs/CREB Pathway

BmK DKK13 (DKK13) is a mutated recombinant peptide, which has a significant antinociception in a rat model of the inflammatory pain. The purpose of this study was to evaluate the antinociceptive effect of DKK13 on trigeminal neuralgia (TN) in rats. Male Sprague-Dawley (SD) rats were treated with the chronic constriction injury of the infraorbital nerve (IoN-CCI) model to induce stable symptoms of TN. DKK13 (1.0 mg/kg, 2.0 mg/kg and 4.0 mg/kg, i.v.) or morphine (4.0 mg/kg, i.v.) was administered by tail vein once on day 14 after IoN-CCI injury. Behavioral tests, electrophysiology and western blotting were performed to investigate the role and underlying mechanisms of DKK13 on IoN-CCI model. Behavioral test results showed that DKK13 could significantly increase the mechanical pain and thermal radiation pain thresholds of IoN-CCI rats and inhibit the asymmetric spontaneous pain scratching behavior. Electrophysiological results showed that DKK13 could significantly reduce the current density of Nav1.8 in the ipsilateral side of trigeminal ganglion (TG) neurons in IoN-CCI rats, and the steady-state activation and inactivation curves of Nav1.8 shifted, respectively, to the direction of hyperpolarization and depolarization. Western blotting results showed that DKK13 significantly reduced the expression of Nav1.8 and the phosphorylation levels of key proteins of MAPKs/CREB pathway in TG tissues of IoN-CCI rats. In brief, DKK13 has a significant antinociceptive effect on IoN-CCI rats, which may be achieved by changing the dynamic characteristics of Nav1.8 channel and regulating the protein phosphorylation in MAPKs/CREB pathway.

Spinal microglia-derived TNF promotes the astrocytic JNK/CXCL1 pathway activation in a mouse model of burn pain

Burn injury-induced pain (BIP) is an extremely complicated condition usually resistant to analgesic drugs, while its pathogenesis remains unknown. Considerable attention has been attracted to elucidate the glial mechanisms in chronic pain. In this study, we initiatively used a mouse model of second-degree BIP to investigate the underlying non-neuronal mechanisms at the spinal cord level. Our behavioral results showed that hind-paw burn injury caused persistent allodynia and hyperalgesia for 2 weeks in mice. Further studies revealed that both microglia and astrocytes activated in a spatially- and temporally-dependent manner in spinal cord after burn injury. In addition, the phosphorylated p38 mitogen-activated protein kinase (MAPK)-mediated tumor necrosis factor (TNF) release in spinal microglia is essentially attributed to the early stage of BIP, while the c-Jun N-terminal kinase (JNK) MAPK-dependent chemokine CXCL1 expression is mainly involved in the maintenance of pain hypersensitivity. Most strikingly, burn injury-induced pain symptoms and the activation of astrocytes were significantly suppressed by TNF inhibitor Thalidomide. On the contrary, intrathecal injection of TNF caused apparent pain hypersensitivity, accompanied by the activation of astrocytes and the upregulation of CXCL1 via the JNK MAPK signaling pathway, indicating that TNF is the key cytokine in the interaction between microglia and astrocytes at the spinal level. Moreover, treatment with the CXCR2 receptor antagonist SB225002 to block the biological activities of CXCL1 significantly attenuated the mechanical allodynia and thermal hyperalgesia in this BIP model. Taken together, this study indicates that intervention of glial pathways provides a new perspective in the management of BIP.