Functional Characterization of Ovine Dorsal Root Ganglion Neurons Reveal Peripheral Sensitization after Osteochondral Defect

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

Role of Human Mesenchymal Stem Cells and Derived Extracellular Vesicles in Reducing Sensory Neuron Hyperexcitability and Pain Behaviors in Murine Osteoarthritis

OBJECTIVE

Mesenchymal stem/stromal cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) have been reported to alleviate pain in patients with knee osteoarthritis (OA). We undertook this study to determine whether MSCs and/or MSC-EVs reduce OA pain through influencing sensory neuron excitability in OA joints.

METHODS

We induced knee OA in adult male C57BL/6J mice through destabilization of the medial meniscus (DMM) surgery. Mice were sorted into 4 experimental groups with 9 mice per group as follows: unoperated sham, untreated DMM, DMM plus MSC treatment, and DMM plus MSC-EV treatment. Treated mice received either MSCs at week 14 postsurgery or MSC-EVs at weeks 12 and 14 postsurgery. Mouse behavior was evaluated by digging and rotarod tests and the Digital Ventilated Cage system. At week 16, mouse knee joints were harvested for histology, and dorsal root ganglion (DRG) neurons were isolated for electrophysiology. Furthermore, we induced hyperexcitability in DRG neurons in vitro using nerve growth factor (NGF) then treated these neurons with or without MSC-EVs and evaluated neuron excitability.

RESULTS

MSC- and MSC-EV-treated DMM-operated mice did not display pain-related behavior changes (in locomotion, digging, and sleep) that occurred in untreated DMM-operated mice. The absence of pain-related behaviors in MSC- and MSC-EV-treated mice was not the result of reduced joint damage but rather a lack of knee-innervating sensory neuron hyperexcitability that was observed in untreated DMM-operated mice. Furthermore, we found that NGF-induced sensory neuron hyperexcitability is prevented by MSC-EV treatment (P < 0.05 versus untreated NGF-sensitized neurons when comparing action potential threshold).

CONCLUSION

MSCs and MSC-EVs may reduce pain in OA by direct action on peripheral sensory neurons.

Gefapixant, a Novel P2X3 Antagonist, Protects against Post Myocardial Infarction Cardiac Dysfunction and Remodeling Via Suppressing NLRP3 Inflammasome

OBJECTIVE

The ATP responsive P2 purinergic receptors can be subdivided into metabotropic P2X family and ionotropic P2Y family. Among these, P2X3 is a type of P2X receptor which is specifically expressed on nerves, especially on pre-ganglionic sensory fibers. This study investigates whether gefapixant possesses the potential of inhibiting cardiac sympathetic hypersensitivity to protect against cardiac remodeling in the context of myocardial infarction.

METHODS

The Sprague-Dawley rats were divided randomly into three groups: sham group-myocardial infarction group, and myocardial infarction with gefapixant treatment group. Myocardial infarction was induced by left anterior descending branch ligation. The gefapixant solution was intraperitoneally injected each time per day for 7 days and the appropriate dosage of gefapixant was determined according to the results of hematoxylin-eosin (HE) staining and myocardial injury biomarkers. Conditions of cardiac function were assessed by echocardiograph and cardiac fibrosis was evaluated by Western blotting and immunofluorescence staining of collagen I and collagen III. The sympathetic innervation was detected by norepinephrine concentration (pg/mL), in-vivo electrophysiology, and typical sympathetic biomarkers. Inflammatory cell infiltration was shown from immunofluorescence staining and pro-inflammatory signaling pathway activation was checked by immunohistology, quantitative realtime PCR (qPCR) and Western blotting.

RESULTS

It was found that gefapixant injection of 10 mg/kg per day had the highest dosage-efficacy ratio. Furthermore, gefapixant treatment improved cardiac pump function as shown by increased LVEF and LVFS, and decreased LVIDd and LVIDs. The expression levels of collagen I and collagen III, and TNF-α were all decreased by P2X3 inhibition. Mechanistically, the decreased activation of nucleotide-binding and oligomerization domain-like receptors family pyrin-domain-containing 3 (NLRP3) inflammasome and subsequent cleavage of caspase-1 which modulated interleukin-1β (IL-1β) and IL-18 level in heart after gefapixant treatment were associated with the suppressed cardiac inflammation.

CONCLUSION

It is suggested that P2X3 inhibition by gefapixant ameliorates post-infarct autonomic nervous imbalance, cardiac dysfunction, and remodeling possibly via inactivating NLRP3 inflammasome.

The Neuroimmune Interface and Chronic Pain Through the Lens of Production Animals

Communication between the central nervous system (CNS) and the immune system has gained much attention for its fundamental role in the development of chronic and pathological pain in humans and rodent models. Following peripheral nerve injury, neuroimmune signaling within the CNS plays an important role in the pathophysiological changes in pain sensitivity that lead to chronic pain. In production animals, routine husbandry procedures such as tail docking and castration, often involve some degree of inflammation and peripheral nerve injury and consequently may lead to chronic pain. Our understanding of chronic pain in animals is limited by the difficulty in measuring this pathological pain state. In light of this, we have reviewed the current understanding of chronic pain in production animals. We discuss our ability to measure pain and the implications this has on animal welfare and production outcomes. Further research into the neuroimmune interface in production animals will improve our fundamental understanding of chronic pain and better inform human clinical pain management and animal husbandry practices and interventions.