Methods Used to Evaluate Pain Behaviors in Rodents

A novel model of paclitaxel-induced peripheral neuropathy produces a clinically relevant phenotype in mice

One of the most common adverse side effects of chemotherapeutics is chemotherapy-induced peripheral neuropathy (CIPN). Paclitaxel, a highly effective chemotherapeutic, is associated with a high incidence of paclitaxel-induced peripheral neuropathy (PIPN) that persists for over a year in 64% of patients and worsens with cumulative PTX dose. Patients experiencing PIPN may reduce the dosage of chemotherapy or halt treatment due to this pain. Current preclinical models have improved our understanding of PIPN but have been ineffective in generating translational therapeutic options. These models administer a single cycle of PTX to induce a PIPN phenotype of mechanical and cold hypersensitivity that resolves within 28 days. However, this does not mirror the clinical dosing regimen or the patient experience of CIPN. In this study, we conduct a comprehensive and longitudinal behavioral profile of our novel model of PIPN in mice where three consecutive cycles of PTX (4 mg/kg, 4 doses/cycle) are given to mimic the clinical administration. Repeated cycles of PTX caused long-lasting mechanical and cold hypersensitivity in male and female C57Bl/6J mice that mirrors clinical observations of persistent CIPN without causing detrimental effects to rodent overall health, normal rodent behavior, or motor function. Our findings support the use of this translational model to facilitate a better understanding of PIPN and the development of effective treatment options. Improved pain management will enable the completion of cancer treatment, decrease health care expenditure, decrease mortality, and improve the quality of life for cancer patients and survivors.

Hydrogels for Peripheral Nerve Repair: Emerging Materials and Therapeutic Applications

Peripheral nerve injuries pose a significant clinical challenge due to the complex biological processes involved in nerve repair and their limited regenerative capacity. Despite advances in surgical techniques, conventional treatments, such as nerve autografts, are faced with limitations like donor site morbidity and inconsistent functional outcomes. As such, there is a growing interest in new, novel, and innovative strategies to enhance nerve regeneration. Tissue engineering/regenerative medicine and its use of biomaterials is an emerging example of an innovative strategy. Within the realm of tissue engineering, functionalized hydrogels have gained considerable attention due to their ability to mimic the extracellular matrix, support cell growth and differentiation, and even deliver bioactive molecules that can promote nerve repair. These hydrogels can be engineered to incorporate growth factors, bioactive peptides, and stem cells, creating a conducive microenvironment for cellular growth and axonal regeneration. Recent advancements in materials as well as cell biology have led to the development of sophisticated hydrogel systems, that not only provide structural support, but also actively modulate inflammation, promote cell recruitment, and stimulate neurogenesis. This review explores the potential of functionalized hydrogels for peripheral nerve repair, highlighting their composition, biofunctionalization, and mechanisms of action. A comprehensive analysis of preclinical studies provides insights into the efficacy of these hydrogels in promoting axonal growth, neuronal survival, nerve regeneration, and, ultimately, functional recovery. Thus, this review aims to illuminate the promise of functionalized hydrogels as a transformative tool in the field of peripheral nerve regeneration, bridging the gap between biological complexity and clinical feasibility.

Analgesic efficacy of hydromorphone in American alligators (Alligator mississippiensis)

Background

American alligators (Alligator mississippiensis) are maintained in zoos, aquaria, and farms for educational, research, and production purposes. The standard of veterinary medical care and welfare for captive reptiles requires managing pain and discomfort under conditions deemed painful in mammals. While analgesic efficacy and pharmacokinetic data for several reptile species are published, data with respect to analgesic efficacy in crocodilians are clearly lacking.

Objective

The objective of this study was to determine the analgesic efficacy of hydromorphone in alligators.

Methods

Female American alligators (N = 9; 57 months of age) were exposed to mechanical noxious stimuli at multiple anatomic sites using von Frey filaments ranging in size from 1.65 to 6.65 grams-force, and their behavioral reactions recorded. In order to evaluate analgesic efficacy, hydromorphone (0.5 mg/kg SC) was administered in the axillary region to the same alligators and the mechanical noxious stimuli were repeated and behaviors recorded.

Results

Administration of hydromorphone contributed to a range from 62 to 92% reduced avoidance reactions to mechanical noxious stimuli for two anatomic sites (i.e., naris and lateral mandible, respectively).

Conclusion

Alligators did not appear to experience clinically relevant respiratory depression, hypothermia, or other adverse reactions. Therefore, hydromorphone shows promise as an analgesic option to be administered under painful conditions in American alligators.

Modeling Neuropathic Corneal Pain: Pulled Nerve Approach With Elevated Krt16 Gene Expression

Purpose

Neuropathic corneal pain (NCP) is a debilitating condition affecting millions of people worldwide. Despite their critical importance, currently available animal models for NCP research are limited by complex surgeries with high-risk strategy. To advance fundamental understanding of NCP, we developed a novel rodent model that explores both structural and functional mechanisms of the disease, offering a comprehensive approach.

Methods

By uplifting (2-3 mm transversely) the long ciliary nerve (LCN) with gentle force (0.09 ± 0.02 newton [N]) and pressure (0.18 ± 0.05 MPa), our pulled nerve model mimics human NCP conditions and was investigated alongside normal control, sham control, and full transection groups. Specifically, we quantified the NCP status by establishing a relationship between pain perception and chemical sensitivity, using Stevens' Power Law concept.

Results

Following surgery, the temporal patterns of heightened pain perception showed consistent trends across different stimulus methods, suggesting that von Frey and chemical tests could effectively evaluate pain progression. The discernable differences in Alpha values (exponent) of the pain-perception curves across the normal control, pulled nerve, and full transection groups (0.175 ± 0.035, 0.235 ± 0.015, and 0.275 ± 0.005, respectively) demonstrate the model's sensitivity to changes in NCP status. Histological analysis revealed LCN elongation, thickening, and corneal alterations in pulled nerve models, with reduced satellite glial cells (SGCs) in trigeminal ganglion compared to the normal control models. Krt16 gene expression was significantly upregulated following pulled nerve surgery.

Conclusions

Our model not only delineates the pathological landscape of NCP but also promises to accelerate the development of targeted therapies.

Humanin attenuates metabolic, toxic, and traumatic neuropathic pain in mice by protecting against oxidative stress and increasing inflammatory cytokine

Neuropathic pain is associated with diverse etiologies, including sciatica, diabetes, and the use of chemotherapeutic agents. Despite the varied origins, mitochondrial dysfunction, oxidative stress, and inflammatory cytokines are recognized as key contributing factors in both the initiation and maintenance of neuropathic pain. The effects of the mitochondrial-derived peptide humanin on neuropathic pain, however, remain unclear, despite its demonstrated influence on these mechanisms in numerous disease models. This study aimed to evaluate the effects of humanin on pain behavior in murine models of metabolic (streptozotocin/STZ), toxic (oxaliplatin/OXA), traumatic (sciatic nerve cuffing/cuff), and neuropathic pain. A secondary objective was to assess whether humanin modulates oxidative damage and inflammatory cytokine levels in these neuropathic pain models. Humanin (4 mg/kg) was administered intraperitoneally (i.p.) to BALB/c male mice with induced neuropathic pain over a period of 15 days, with pain thresholds assessed using hot plate, cold plate, and Von Frey tests. Serum levels of antioxidant enzymes, oxidative stress markers, and inflammatory/anti-inflammatory cytokines were measured via enzyme-linked immunosorbent assay (ELISA). In neuropathic pain-induced mice, humanin administration resulted in a statistically significant increase in pain threshold values in the STZ + Humanin, OXA + Humanin, and cuff + Humanin groups compared to their respective control groups (P < 0.05) over 15 days. Furthermore, humanin treatment significantly elevated antioxidant enzyme levels and anti-inflammatory cytokine concentrations, while reducing oxidative stress markers and pro-inflammatory cytokine levels compared to control groups (P < 0.01). These findings suggest that humanin exhibits therapeutic potential in the treatment of neuropathic pain induced by STZ, OXA, and cuff models. The ability of humanin to mitigate neuropathic pain through the suppression of oxidative stress and inflammatory cytokines indicates its promise as a novel therapeutic strategy.

Sex-Specific Contrasting Role of BECLIN-1 Protein in Pain Hypersensitivity and Anxiety-Like Behaviors

Chronic pain is a debilitative disease affecting one in five adults globally and is a major risk factor for anxiety ( Goldberg and McGee, 2011; Lurie, 2018). Given the current dearth of available treatments for both individuals living with chronic pain and mental illnesses, there is a critical need for research into the molecular mechanisms involved in order to discover novel treatment targets. Cellular homeostasis is crucial for normal bodily functions, and investigations of this process may provide better understanding of the mechanisms driving the development of chronic pain. Using the spared nerve injury (SNI) model of neuropathic pain, we found contrasting roles for BECLIN-1 in the development of pain hypersensitivity and anxiety-like behaviors in a sex-dependent manner. Remarkably, we found that male SNI mice with impaired BECLIN-1 function demonstrated heightened mechanical and thermal hypersensitivity compared with male wild-type SNI mice, while female SNI mice with impaired BECLIN-1 function demonstrated similar thresholds to the female wild-type SNI mice. We also found that disruptions of BECLIN-1 prevented SNI-induced increases in anxiety-like behaviors in male mice. Our data thus indicate that BECLIN-1 is differentially involved in the nociceptive and emotional components of chronic pain in male but not female mice.

SPHK1/S1PR1/PPAR-α axis restores TJs between uroepithelium providing new ideas for IC/BPS treatment

Interstitial cystitis/bladder pain syndrome (IC/BPS) represents a chronic, aseptic inflammatory bladder condition with an unclear etiology and few therapeutic options. A composite barrier structure composed of the uroepithelium and glycosaminoglycan layer forms on the bladder's inner surface to block urine and other harmful substances. Dysfunction of this barrier may initiate the pathogenesis of IC/BPS. Sphingosine-1-phosphate (S1P) plays a crucial role in forming tight junctions. Perfusion of S1P into the bladder restored uroepithelial tight junctions in mice with cyclophosphamide-induced acute cystitis and ameliorated symptoms of the lower urinary tract. Mice lacking sphingosine kinase 1 (SHPK1) exhibited more severe bladder injuries and dysfunction. Concurrent in vitro experiments elucidated S1P's protective effects and its role as a primary messenger through SPHK1 and S1P receptor 1 (S1PR1) knockdown. This study identifies a novel mechanism whereby S1P binding to S1PR1 activates the PPAR-α pathway, thereby enhancing cholesterol transport and restoring tight junctions between uroepithelial cells. These findings elucidate the regulatory role of S1P in the bladder epithelial barrier and highlight a promising therapeutic target for IC/BPS.

Therapeutic efficacy of brown propolis from Araucaria sp. in modulating rheumatoid arthritis

Rheumatoid arthritis is a systemic inflammatory autoimmune disease with prevalence estimated at 0.5% to 1% of the population. As one of the treatment routes of rheumatoid arthritis is based on the use of nonsteroidal anti-inflammatory drugs, the use of natural products with anti-inflammatory potential becomes relevant. Brown propolis has several biological properties, including immunomodulatory and anti-inflammatory effects. Thus, the present study evaluated the therapeutic efficacy of the crude extract of brown propolis from Araucaria sp. through experimental models of analgesia, anti-inflammatory activity, and rheumatoid arthritis. Hyperalgesia was evaluated by mechanical and thermal sensitivity. Anti-inflammatory activity was evaluated by plantar volume, cell migration and NF-kB expression in carrageenan-induced paw oedema. In collagen-induced rheumatoid arthritis, it was evaluated by mechanical and thermal nociception on the plantar surface, and mechanical nociception in the femorotibial and caudal joints, evaluation of plantar volume, radiography, weight gain and biochemical profile. The results demonstrated that the oral administration of brown propolis can modulate the course of rheumatoid arthritis, and it can inhibit pain through the modulation of mechanical sensitivity. The anti-arthritic effect of propolis may be due to its anti-inflammatory capacity, which includes inhibiting oedema formation, cell migration, and NF-kB expression, as well as preserving joint space and normalizing urea levels. This was an animal model study. Therefore, brown propolis should be evaluated in studies of human RA to determine efficacy.

Mu-opioid receptors in tachykinin-1-positive cells mediate the respiratory and antinociceptive effects of the opioid fentanyl

BACKGROUND AND PURPOSE

Opioid drugs are potent analgesics that carry the risk of respiratory side effects due to actions on μ-opioid receptors (MORs) in brainstem regions that control respiration. Substance P is encoded by the Tac1 gene and is expressed in neurons regulating breathing, nociception, and locomotion. Tac1-positive cells also express MORs in brainstem regions mediating opioid-induced respiratory depression. We determined the role of Tac1-positive cells in mediating the respiratory effects of opioid drugs.

EXPERIMENTAL APPROACH

In situ hybridization was used to determine Oprm1 mRNA expression (gene encoding MORs) in Tac1-positive cells in regions regulating respiratory depression by opioid drugs. Conditional knockout mice lacking functional MORs in Tac1-positive cells were produced and the respiratory and locomotor responses to the opioid analgesic fentanyl were assessed using whole-body plethysmography. A tail immersion assay was used to assess the antinociceptive response to fentanyl.

KEY RESULTS

Oprm1 mRNA was highly expressed (>80%) in subpopulations of Tac1-positive cells in the preBötzinger Complex, nucleus tractus solitarius, and Kölliker-Fuse/lateral parabrachial region. Conditionally knocking out MORs in Tac1-positive cells abolished the effects of fentanyl on respiratory rate, relative tidal volume, and relative minute ventilation compared with control mice. Importantly, the antinociceptive response of fentanyl was eliminated in mice lacking functional MORs in Tac1-positive cells, whereas locomotor effects induced by fentanyl were preserved.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that Tac1-positive cells mediate the respiratory depressive and antinociceptive effects of the opioid fentanyl, providing important insights for the development of pain therapies with reduced risk of respiratory side effects.

Sex differences in the affective-cognitive dimension of neuropathic pain: Insights from the spared nerve injury rat model

Over 40% of neuropathic pain patients experience mood and cognitive disturbances, often showing reduced response to analgesics, with most affected individuals being female. This highlights the critical role of biological sex in pain-related affective and cognitive disorders, making it essential to understand the emotional and cognitive circuits linked to pain for improving treatment strategies. However, research on sex differences in preclinical pain models is lacking. This study aimed to investigate these differences using the spared nerve injury (SNI) rat model, conducting a comprehensive series of behavioural tests over 100 days post-injury to identify key time points for observing sex-specific behaviours indicative of pain-related conditions. The findings revealed that female rats exhibited greater mechanical and cold hypersensitivity compared to males following nerve injury and showed earlier onset of depression-related behaviours, while males were more prone to anxiety, social, and memory-related alterations. Interestingly, by the 14th week post-injury, females displayed no signs of these emotional and cognitive impairments. Additionally, fluctuations in the oestrous cycle or changes in testosterone and oestradiol levels did not correlate with sex differences in pain sensitivity or negative affect. Recognizing the influence of biological sex on pain-induced affective and cognitive alterations, especially in later stages post-injury, is crucial for enhancing our understanding of this complex pain disorder. PERSPECTIVE: This manuscript reports the relevance of long-term investigations of sex differences in chronic pain. It shows differential development of somatosensory sensitivity, negative affective states and cognitive impairments in males and females. It emphasizes the importance of including subjects of both sexes in the investigation of pain-related mechanisms and therapeutic management.

Selective activation of SIGMAR1 in anterior cingulate cortex glutamatergic neurons facilitates comorbid pain in depression in male mice

Depression and comorbid pain are frequently encountered clinically, and the comorbidity complicates the overall medical management. However, the mechanism whereby depression triggers development of pain needs to be further elucidated. Here, by using the chronic restraint stress (CRS) mouse model of depression and comorbid pain, we showed that CRS hyperactivated the glutamatergic neurons in the anterior cingulate cortex (ACC), as well as increasing the dendrite complexity and number. Chemogenetic activation of these neurons can induce depression and pain, while chemogenetic blockade can reverse such depression-induced pain. Moreover, we utilized translating ribosome affinity purification (TRAP) in combination with c-Fos-tTA strategy and pharmacological approaches and identified SIGMAR1 as a potential therapeutic molecular target. These results revealed a previously unknown neural mechanism for depression and pain comorbidity and provided new mechanistic insights into the antidepressive and analgesic effects of the disease.

No More Evasion: Redefining Conflict Behaviour in Human-Horse Interactions

Euphemisms, anthropomorphisms, and equivocation are established characteristics of traditional equestrian language. 'Evasion', 'resistance', and 'disobedience' are common labels assigned to unwelcome equine behaviours, implying that the horse is at fault for not complying with the human's cues and expectations. These terms appear to overlook multiple motivations that may directly result in the horse offering unwelcome responses, which may then inadvertently be reinforced. This article revisits some of the anthropocentric inferences in these terms and explores the harmful consequences of such convenient but incorrect labels before proposing a redefinition of 'conflict behaviour' in human-horse interactions: Responses reflective of competing motivations for the horse that may exist on a continuum from subtle to overt, with frequencies that range from a singular momentary behavioural response to repetitive displays when motivational conflict is prolonged. Addressing how inadequate terms may mask pain, obscure the horse's motivation, and deflect human culpability, this commentary highlights the merits of a multidisciplinary approach to terminology across equine research. Acknowledging that variables contributing to behaviour can be biological, environmental and anthropogenic, it emphasises the need for more investigation into the relationships between equicentric motivations reflecting equine telos and problematic horse behaviours.

Peripheral nerve regeneration using a bioresorbable silk fibroin-based artificial nerve conduit fabricated via a novel freeze-thaw process

While silk fibroin (SF) obtained from silkworm cocoons is expected to become a next-generation natural polymer, a fabrication method for SF-based artificial nerve conduits (SFCs) has not yet been established. Here, we report a bioresorbable SFC, fabricated using a novel freeze-thaw process, which ensures biosafety by avoiding any harmful chemical additives. The SFC demonstrated favorable biocompatibility (high hydrophilicity and porosity with a water content of > 90%), structural stability (stiffness, toughness, and elasticity), and biodegradability, making it an ideal candidate for nerve regeneration. We evaluated the nerve-regenerative effects of the SFC in a rat sciatic-nerve-defect model, including its motor and sensory function recovery as well as histological regeneration. We found that SFC transplantation significantly promoted functional recovery and nerve regeneration compared to silicone tubes and was almost equally effective as autologous nerve transplantation. Histological analyses indicated that vascularization and M2 macrophage recruitment were pronounced inside the SFC. These results suggest that the unique properties of the SFC further enhanced the peripheral nerve regeneration mechanism. As no SFC has been applied in clinical practice, the SFC reported herein may be a promising candidate for repairing extensive peripheral nerve defects.

Hippocampal nicotinic acetylcholine receptor signaling mediates the anti-allodynic effect of ketamine and morphine on neuropathic pain

The present study investigated the involvement of hippocampal nicotinic acetylcholine receptors (nAChRs) in the anti-allodynic effect of ketamine/morphine on neuropathic pain in adult male Wistar rats. Morphine or ketamine administration decreased the percentage of maximum possible effect (MPE%), indicating an analgesic effect. The most significant decrease occurred with a 5 mg/kg dose of morphine (average MPE% = 98), while a 0.5 mg/kg dose of ketamine resulted in a high response (average MPE% = 91), using decision trees as a machine learning tool. Combining morphine and ketamine improved neuropathic pain (average MPE% = 91). Intra-CA1 microinjection of mecamylamine (2 μg/rat) with morphine (3 mg/kg) reduced neuropathic pain (average MPE% = 94). Co-administration of lower doses of ketamine (0.1 mg/kg, i.p.) and mecamylamine (0.5 or 1 μg/rat) with morphine (3 mg/kg) led to a considerable reduction in pain (average MPE% = 91). Utilizing the generalized least squares (GLS) model enabled the establishment of a continuous relation between drug dose and MPE% as the outcome of interest. There was a 19.60 higher average MPE% for each mg/kg increase in morphine dose. In contrast, there was a 17.05 higher average MPE% for every 0.1 mg/kg increase in ketamine dose. Each 0.1 mg/kg increase in ketamine dose, when combined with morphine (3 mg/kg), led to a 30.85 higher average MPE%. A tenfold impact of increasing mecamylamine dosage on MPE% was observed when paired with morphine. Thus, hippocampal nAChRs play a significant role in mediating the anti-allodynic effect of ketamine and morphine in neuropathic pain.

Engineering spatially-confined conduits to tune nerve self-organization and allodynic responses via YAP-mediated mechanotransduction

Chronic allodynia stemming from peripheral stump neuromas can persist for extended periods, significantly compromising patients' quality of life. Conventional managements for nerve stumps have demonstrated limited effectiveness in ensuring their orderly termination. In this study, we present a spatially confined conduit strategy, designed to enhance the self-organization of regenerating nerves after truncation. This innovative approach elegantly enables the autonomous slowing of axonal outgrowth in response to the gradually constricting space, concurrently suppressing neuroinflammation through YAP-mediated mechanotransduction activation. Meanwhile, the decelerating axons exhibit excellent alignment and remyelination, thereby helping to prevent failure modes in nerve self-organization, such as axonal twisting in congested regions and overgrowth beyond the conduit's capacity. Additionally, proteins associated with mechanical allodynia, including TRPA1 and CGRP, exhibit a gradual reduction in expression as spatial constraints tighten, a trend inversely validated by the administration of the YAP-targeted inhibitor Verteporfin. This spatially confined conduit strategy significantly alleviates allodynia, thus preventing autotomy behavior and reducing pain-induced gait alterations.

Sodium butyrate restored TRESK current controlling neuronal hyperexcitability in a mouse model of oxaliplatin-induced peripheral neuropathic pain

Chemotherapy-induced peripheral neuropathy (CIPN) and its related pain are common challenges for patients receiving oxaliplatin chemotherapy. Oxaliplatin accumulation in dorsal root ganglion (DRGs) is known to impair gene transcription by epigenetic dysregulation. We hypothesized that sodium butyrate, a pro-resolution short-chain fatty acid, inhibited histone acetylation in DRGs and abolished K+ channel dysregulation-induced neuronal hyperexcitability after oxaliplatin treatment. Mechanical allodynia and cold hyperalgesia of mice receiving an accumulation of 15 ​mg/kg oxaliplatin, with or without intraperitoneal sodium butyrate supplementation, were assessed using von Frey test and acetone evaporation test. Differential expressions of histone deacetylases (HDACs) and pain-related K+ channels were quantified with rt-qPCR and protein assays. Immunofluorescence assays of histone acetylation at H3K9/14 were performed in primary DRG cultures treated with sodium butyrate. Current clamp recording of action potentials and persistent outward current of Twik-related-spinal cord K+ (TRESK) channel were recorded in DRG neurons with small diameters extract. Accompanied by mechanical allodynia and cold hyperalgesia, HDAC1 was upregulated in mice receiving oxaliplatin treatment. Sodium butyrate enhanced global histone acetylation at H3K9/14 in DRG neurons. In vivo sodium butyrate supplementation restored oxaliplatin-induced Kcnj9 and Kcnk18 expression and pain-related behaviors in mice for at least 14 days. Oxaliplatin-induced increase in action potentials frequencies and decrease in magnitudes of KCNK18-related current were reversed in mice receiving sodium butyrate supplementation. This study suggests that sodium butyrate was a useful agent to relieve oxaliplatin-mediated neuropathic pain.

GFRα1 Promotes Axon Regeneration after Peripheral Nerve Injury by Functioning as a Ligand

The neurotrophic factor, Glial cell line derived neurotrophi factor (GDNF), exerts a variety of biological effects through binding to its receptors, GDNF family receptor alpha-1 (GFRα1), and RET. However, the existence of cells expressing GFRα1 but not RET raises the possibility that GFRα1 can function independently from RET. Here, it is shown that GFRα1 released from repair Schwann cells (RSCs) functions as a ligand in a GDNF-RET-independent manner to promote axon regeneration after peripheral nerve injury (PNI). Local administration of GFRα1 into injured nerve promoted axon regeneration, even more when combined with GDNF blockade. GFRα1 bound to a receptor complex consisting of NCAM and integrin α7β1 of dorsal root ganglion neurons in a GDNF-RET independent manner. This is further confirmed by the Ret Y1062F knock-in mice, which cannot transmit most of GDNF-RET signaling. Finally, local administration of GFRα1 into injured sciatic nerve promoted functional recovery. These findings reveal a novel role of GFRα1 as a ligand, the molecular mechanism supporting axon regeneration by RSCs, and a novel therapy for peripheral nerve repair.

An Injectable Oil-Based Depot Formulation of N-Acyloxymethyl Prodrug of Ropivacaine for Long-Acting Local Analgesia: Formulation Development and In Vitro/In Vivo Evaluation

Objectives: The development of novel long-acting injectables for local anesthetics is necessary to effectively manage the acute postoperative pain. The aim of this study was to prepare an injectable oil-based formulation of ropivacaine (ROP) prodrug (ropivacaine stearoxil, ROP-ST) and to investigate the pharmacokinetics and pharmacodynamics after injectable administration. Methods: A novel N-acyloxymethyl prodrug of ROP, i.e., ROP-ST, was synthesized and its physicochemical properties such as log P, solubility and stability characterized. A soybean oil-based depot of ROP-ST was prepared, and the in-vitro release of ROP-ST was evaluated using an "inverted-cup" method. Pharmacokinetic profiles and tissue retention properties were investigated after intramuscular administration of the formulation in rats. The analgesic efficacy was assessed via a von Frey monofilaments test by measuring the paw withdrawal thresholds. Results: The structure of ROP-ST was ascertained with clear 1H NMR assignment and accurate mass-to-charge ratio. The high Log P value of ROP-ST (9.16) demonstrated extremely low aqueous solubility, but the prodrug is biolabile when in contact with plasma or liver esterase. Intramuscular injection of ROP-ST oil solution in rats provided a significantly higher mean residence time without a very clear plasma peak of ROP. In a postoperative pain model of rats, the injection of ROP-ST oil solution into the vicinity of the sciatic nerve in the right ankle effectively controlled the postoperative pain for at least 72 h. Conclusions: The injectable oil-based depot formulation of N-acyloxymethyl prodrug of ROP may provide a new opportunity of long-acting local analgesia for postoperative pain.

Loss of CHOP Prevents Joint Degeneration and Pain in a Mouse Model of Pseudoachondroplasia

Pseudoachondroplasia (PSACH), a severe dwarfing condition characterized by impaired skeletal growth and early joint degeneration, results from mutations in cartilage oligomeric matrix protein (COMP). These mutations disrupt normal protein folding, leading to the accumulation of misfolded COMP in chondrocytes. The MT-COMP mouse is a murine model of PSACH that expresses D469del human COMP in response to doxycycline and replicates the PSACH chondrocyte and clinical pathology. The basis for the mutant-COMP pathology involves endoplasmic reticulum (ER) stress signaling through the PERK/eIF2α/CHOP pathway. C/EBP homologous protein (CHOP), in conjunction with a TNFα inflammatory process, upregulates mTORC1, hindering autophagy clearance of mutant COMP protein. Life-long joint pain/degeneration diminishes quality of life, and treatments other than joint replacements are urgently needed. To assess whether molecules that reduce CHOP activity should be considered as a potential treatment for PSACH, we evaluated MT-COMP mice with 50% CHOP (MT-COMP/CHOP+/-), antisense oligonucleotide (ASO)-mediated CHOP knockdown, and complete CHOP ablation (MT-COMP/CHOP-/-). While earlier studies demonstrated that loss of CHOP in MT-COMP mice reduced intracellular retention, inflammation, and growth plate chondrocyte death, we now show that it did not normalize limb growth. ASO treatment reduced CHOP mRNA by approximately 60%, as measured by RT-qPCR, but did not improve limb length similar to MT-COMP/CHOP+/-. Interestingly, both 50% genetic reduction and complete loss of CHOP alleviated pain, while total ablation of CHOP in MT-COMP mice was necessary to preserve joint health. These results indicate that (1) CHOP reduction therapy is not an effective strategy for improving limb length and (2) pain and chondrocyte pathology are more responsive to intervention than the prevention of joint damage.

Beyond fur color: differences in socio-emotional behavior and the oxytocin system between male BL6 and CD1 mice in adolescence and adulthood

Introduction

The development of stress-related psychopathologies, often associated with socio-emotional dysfunctions, is crucially determined by genetic and environmental factors, which shape the individual vulnerability or resilience to stress. Especially early adolescence is considered a vulnerable time for the development of psychopathologies. Various mouse strains are known to age-dependently differ in social, emotional, and endocrine stress responses based on genetic and epigenetic differences. This highlights the importance of the qualified selection of an adequate strain and age for any biomedical research. Neuropeptides like oxytocin (OXT) can contribute to individual and strain-dependent differences in emotional and social behaviors.

Methods

In this study, we compared anxiety- and fear-related, as well as social behavior and pain perception between male adolescent and adult mice of two commonly used strains, C57BL/6N (BL6) and CD1.

Results

We revealed BL6 mice as being more anxious, less social, and more susceptible toward non-social and social trauma, both in adolescence and adulthood. Furthermore, during development from adolescence toward adulthood, BL6 mice lack the reduction in fear- and anxiety-related behavior seen in adult CD1 mice and show even higher social fear-responses and perception of noxious stimuli during adulthood. Analysis of the OXT system, by means of receptor autoradiography and immunohistochemistry, showed strain- and age-specific differences in OXT receptor (OXTR) binding in relevant brain regions, but no differences in the number of hypothalamic OXT neurons. However, intracerebroventricular infusion of OXT did neither reduce the high level of anxiety-related nor of social fear-related behavior in adult BL6 mice.

Discussion

In summary, we show that male BL6 mice present an anxious and stress vulnerable phenotype in adolescence, which further exacerbates in adulthood, whereas CD1 mice show a more resilient socio-emotional state both in adolescence as well as during adulthood. These consistent behavioral differences between the two strains might only be partly mediated by differences in the OXT system but highlight the influence of early-life environment on socio-emotional behavior.

Animal Models of Intervertebral Disc Diseases: Advantages, Limitations, and Future Directions

Animal models are valuable tools for studying the underlying mechanisms of and potential treatments for intervertebral disc diseases. In this review, we discuss the advantages and limitations of animal models of disc diseases, focusing on lumbar spinal stenosis, disc herniation, and degeneration, as well as future research directions. The advantages of animal models are that they enable controlled experiments, long-term monitoring to study the natural history of the disease, and the testing of potential treatments. However, they also have limitations, including species differences, ethical concerns, a lack of standardized protocols, and short lifespans. Therefore, ongoing research focuses on improving animal model standardization and incorporating advanced imaging and noninvasive techniques, genetic models, and biomechanical analyses to overcome these limitations. These future directions hold potential for improving our understanding of the underlying mechanisms of disc diseases and for developing new treatments. Overall, although animal models can provide valuable insights into pathophysiology and potential treatments for disc diseases, their limitations should be carefully considered when interpreting findings from animal studies.

Temporal changes of spinal microglia in murine models of neuropathic pain: a scoping review

Neuropathic pain (NP) is an ineffectively treated, debilitating chronic pain disorder that is associated with maladaptive changes in the central nervous system, particularly in the spinal cord. Murine models of NP looking at the mechanisms underlying these changes suggest an important role of microglia, the resident immune cells of the central nervous system, in various stages of disease progression. However, given the number of different NP models and the resource limitations that come with tracking longitudinal changes in NP animals, many studies fail to truly recapitulate the patterns that exist between pain conditions and temporal microglial changes. This review integrates how NP studies are being carried out in murine models and how microglia changes over time can affect pain behavior in order to inform better study design and highlight knowledge gaps in the field. 258 peer-reviewed, primary source articles looking at spinal microglia in murine models of NP were selected using Covidence. Trends in the type of mice, statistical tests, pain models, interventions, microglial markers and temporal pain behavior and microglia changes were recorded and analyzed. Studies were primarily conducted in inbred, young adult, male mice having peripheral nerve injury which highlights the lack of generalizability in the data currently being collected. Changes in microglia and pain behavior, which were both increased, were tested most commonly up to 2 weeks after pain initiation despite aberrant microglia activity also being recorded at later time points in NP conditions. Studies using treatments that decrease microglia show decreased pain behavior primarily at the 1- and 2-week time point with many studies not recording pain behavior despite the involvement of spinal microglia dysfunction in their development. These results show the need for not only studying spinal microglia dynamics in a variety of NP conditions at longer time points but also for better clinically relevant study design considerations.

P2X3 and P2X2/3 receptors inhibition produces a consistent analgesic efficacy: A systematic review and meta-analysis of preclinical studies

BACKGROUND

P2X3 and P2X2/3 receptors are promising therapeutic targets for pain treatment and selective inhibitors are under evaluation in ongoing clinical trials. Here we aim to consolidate and quantitatively evaluate the preclinical evidence on P2X3 and P2X2/3 receptors inhibitors for pain treatment.

METHODS

A literature search was conducted in PubMed, Scopus and Web-of-Science on August 5, 2023. Data was extracted and meta-analyzed using a random-effects model to estimate the analgesic efficacy of the intervention; then several subgroup analyses were performed.

RESULTS

67 articles were included. The intervention induced a consistent pain reduction (66.5 [CI95% = 58.5, 74.5]; p < 0.0001), which was highest for visceral pain (114.3), followed by muscle (79.8) and neuropathic pain (71.1), but lower for cancer (64.1), joint (57.5) and inflammatory pain (49.0). Further analysis showed a greater effect for mechanical hypersensitivity (70.4) compared to heat hypersensitivity (64.5) and pain-related behavior (54.1). Sex (male or female) or interspecies (mice or rats) differences were not appreciated (p > 0.05). The most used molecule was A-317491, but other such as gefapixant or eliapixant were also effective (p < 0.0001 for all). The analgesic effect was higher for systemic or peripheral administration than for intrathecal administration. Conversely, intracerebroventricular administration was not analgesic, but potentiated pain.

CONCLUSION

P2X3 and P2X2/3 receptor inhibitors showed a good analgesic efficacy in preclinical studies, which was dependent on the pain etiology, pain outcome measured, the drug used and its route of administration. Further research is needed to assess the clinical utility of these preclinical findings.

PROTOCOL REGISTRATION

PROSPERO ID CRD42023450685.

Novel insights into the molecular mechanisms underlying anti-nociceptive effect of myricitrin against reserpine-induced fibromyalgia model in rats: Implication of SIRT1 and miRNAs

ETHNOPHARMACOLOGICAL RELEVANCE

Manilkara zapota (L.) P. Royen, also termed sapodilla or chikoo, is a significant plant in ethnomedicine because of its long history of traditional medical applications. In diverse cultures, sapodilla is believed to protect against oxidative stress, inflammation, and some chronic diseases because of its high antioxidant content. The naturally occurring antioxidant myricitrin (MYR) flavonoid is primarily found in the leaves and other plant parts of sapodilla and it is well-known for having therapeutic qualities and possible health advantages.

AIM OF THE STUDY

To appraise the possible impact of MYR on a rat model of reserpine-induced fibromyalgia (FM) and explore its mechanism of action.

MATERIALS AND METHODS

Isolation and identification of MYR with more than 99% purity from Manilkara zapota leaves were primarily done and confirmed through chromatographic and spectrophotometric techniques. To develop FM model, reserpine (RSP) was injected daily (1 mg/kg, s.c.) for three successive days. Then, MYR (10 mg/kg, i.p.) and pregabalin (PGB, 30 mg/kg, p.o.) were given daily for another five days. Behavioral changes were assessed through open field test (OFT), hot plate test, and forced swimming test (FST). Further analyses of different brain parameters and signaling pathways were performed to assess monoamines levels, oxidative stress, inflammatory response, apoptotic changes as well as silent information regulator 1 (SIRT1) and micro RNAs (miRNAs) expressions.

RESULTS

From High-Performance Liquid Chromatography (HPLC) analysis, the methanol extract of sapodilla leaves contains 166.17 μg/ml of MYR. Results of behavioral tests showed a significant improvement in RSP-induced nociceptive stimulation, reduced locomotion and exploration and depressive-like behavior by MYR. Biochemical analyses showed that MYR significantly ameliorated the RSP-induced imbalance in brain monoamine neurotransmitters. In addition, MYR significantly attenuated oxidative stress elicited by RSP via up-regulating nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) protein expressions, enhancing superoxide dismutase (SOD) and catalase (CAT) activities, and reducing malondialdehyde (MDA) content in brain. The RSP-provoked inflammatory response was also diminished by MYR treatment as shown by a significant decreased NOD-like receptor protein 3 (NLRP3) inflammasome expression along with reduced levels of interleukin 1 beta (IL-1β) and nuclear factor-κB (NF-κB). Furthermore, the anti-apoptotic activity of MYR was demonstrated by a marked rise in Bcl-2-associated X protein (BAX)/B cell lymphoma-2 (Bcl-2) ratio by lowering Bcl-2 while increasing BAX levels. In addition, MYR treatment significantly boosted the expression of SIRT1 deacetylase in RSP-treated animals. Interestingly, molecular docking showed the ability of MYR to form a stable complex in the binding site of SIRT1. Regarding miRNAs, MYR effectively ameliorated RSP-induced changes in miR-320 and miR-107 gene expressions.

CONCLUSION

Our findings afford new insights into the anti-nociceptive profile of MYR in the RSP-induced FM model in rats. The underlying mechanisms involved direct binding and activation of SIRT1 to influence different signaling cascades, including Nrf2 and NF-κB/NLRP3 together with modulation of miRNAs. However, more in-depth studies are needed before proposing MYR as a new clinically relevant drug in the management of FM.

Pharmacological interventions for remifentanil-induced hyperalgesia: A systematic review and network meta-analysis of preclinical trials

BACKGROUND

To improve perioperative pain management, several interventions have been suggested for the prevention of increased pain sensitivity caused by opioids (called opioid-induced hyperalgesia). It is currently unclear which intervention is the most effective or appropriate in preventing opioid-induced hyperalgesia. Remifentanil is the most investigated opioid causing opioid-induced hyperalgesia. Thus, to guide future research, we conducted a systematic review and a network meta-analysis of preclinical trials investigating pharmacological interventions for remifentanil-induced hyperalgesia.

METHODS

To identify relevant articles, electronic database searches were conducted in Embase, PubMed, Web of Science, and Google Scholar. Study characteristics were extracted, and the risk of bias was evaluated. Studies were included in the network meta-analysis if they shared similar characteristics with at least one other study. The interventions were ranked based on P-scores.

RESULTS

Overall, the 62 eligible trials tested 86 individual interventions and 6 combination interventions. Thirty-five studies eligible in the network meta-analysis formed five groups which were further divided into subgroups based on the quantitative sensory tests used. The best-ranked interventions within the subgroups were Anxa12-26, MRS2179, salicylaldehyde isonicotinoyl hydrazone (SIH), ANA-12, TDZD-8, ketamine, dexmedetomidine, JWH015, and the combination of KN93 and ketamine.

DISCUSSION

The current literature is too heterogeneous to produce a clear answer on which intervention is the most effective in preventing remifentanil-induced hyperalgesia. Future research in this field should prioritise finding the most effective intervention over testing the efficacy of new options. The results of our work can be used in planning which comparisons should be included in new trials.

Docosahexaenoic Acid-Infused Core-Shell Fibrous Membranes for Prevention of Epidural Adhesions

Avoiding epidural adhesion following spinal surgery can reduce clinical discomfort and complications. As the severity of epidural adhesion is positively correlated with the inflammatory response, implanting a fibrous membrane after spinal surgery, which can act as a physical barrier to prevent adhesion formation while simultaneously modulates postoperative inflammation, is a promising approach to meet clinical needs. Toward this end, we fabricated an electrospun core-shell fibrous membrane (CSFM) based on polylactic acid (PLA) and infused the fiber core region with the potent natural anti-inflammatory compound docosahexaenoic acid (DHA). The PLA/DHA CSFM can continuously deliver DHA for up to 36 days in vitro and reduce the penetration and attachment of fibroblasts. The released DHA can downregulate the gene expression of inflammatory markers (IL-6, IL-1β, and TNF-α) in fibroblasts. Following an in vivo study that implanted a CSFM in rats subjected to lumbar laminectomy, the von Frey withdrawal test indicates the PLA/DHA CSFM treatment can successfully alleviate neuropathic pain-like behaviors in the treated rats, showing 3.60 ± 0.49 g threshold weight in comparison with 1.80 ± 0.75 g for the PLA CSFM treatment and 0.57 ± 0.37 g for the untreated control on day 21 post-implantation. The histological analysis also indicates that the PLA/DHA CSFM can significantly reduce proinflammatory cytokine (TNF-α and IL-1β) protein expression at the lesion and provide anti-adhesion effects, indicating its vital role in preventing epidural fibrosis by mitigating the inflammatory response.

Protective effects of Boswellia and Curcuma extract on oxaliplatin-induced neuropathy via modulation of NF-κB signaling

Oxaliplatin is a third-generation anticancer agent with better efficacy, lower toxicity, and a broad spectrum of antineoplastic activity. Its use is frequently associated with chronic oxaliplatin-induced neuropathy (OIN), a cumulative phenomenon manifesting as loss of sensation, paresthesia, dysesthesia, and irresolvable fluctuations in proprioception that greatly affect the patients' quality of life. The inevitable nature and high incidence of OIN, along with the absence of efficacious preventive agents, necessitate the development of effective and reliable protective options for limiting OIN while maintaining anticancer activity. The pathogenesis of chronic OIN involves neuroinflammation and oxidative stress. This study aimed to explore the neuroprotective effects of Boswellia serrata and Curcuma longa via modulation of nuclear factor-kappa B (NF-κB) signaling. Behavioral tests were conducted to assess cold allodynia, heat hyperalgesia, mechanical allodynia, mechanical hyperalgesia, and slowed nerve conduction velocity associated with chronic oxaliplatin administration. The modulation of NF-κB signaling and the subsequent activation of cytokines were evaluated through quantitative analysis of inflammatory cytokines in sciatic nerve homogenates. Additional assessments included oxidative stress parameters, serum neuronal biomarkers, and examination of sciatic nerve cross-sections. The findings indicate improvements in behavioral and biochemical parameters, as well as nerve histology, with the combined extract of Boswellia serrata and Curcuma longa at doses of 50 mg/kg and 75 mg/kg. Thus, this study presents evidence for the protective potential of the combined extract of Boswellia serrata and Curcuma longa in OIN through modulation of NF-κB signaling.

Spinal Cord Stimulation Paradigms and Alleviation of Neuropathic Pain Behavior in Experimental Painful Diabetic Polyneuropathy

OBJECTIVES

Spinal cord stimulation (SCS) is an alternative treatment option for painful diabetic polyneuropathy (PDPN). Differential target multiplexed (DTM)-SCS is proposed to be more effective than conventional (Con)-SCS. Animal studies are essential for understanding SCS mechanisms in PDPN pain relief. Although the Von Frey (VF) test is the gold standard for preclinical pain research, it has limitations. Operant testing using the conditioned place preference (CPP) test provides insights into spontaneous neuropathic pain relief and enhances the translatability of findings. This study aims to 1) use the CPP test to evaluate Con- and DTM-SCS effects on spontaneous neuropathic pain relief in PDPN animals and 2) investigate the correlation between mechanical hypersensitivity alleviation and spontaneous neuropathic pain relief.

MATERIAL AND METHODS

Diabetes was induced through streptozotocin injection in 32 rats; 16 animals developed PDPN and were implanted with a quadripolar lead. Rats were conditioned for Con-SCS (n = 8) or DTM-SCS (n = 7), and a preference score compared with sham was determined. After conditioning, a 30-minute SCS protocol was conducted. Mechanical sensitivity was assessed using VF before, during, and after SCS.

RESULTS

There were no significant chamber preference changes for DTM-SCS (p = 0.3449) or Con-SCS (p = 0.3632). Subgroups of responders and nonresponders were identified with significant increases in preference score for responders for both DTM-SCS (-266.6 to 119.8; p = 0.0238; n = 4) and Con-SCS (-350.7 to 88.46; p = 0.0148; n = 3). No strong correlation between SCS-induced spontaneous neuropathic pain relief and effects on mechanical hypersensitivity in PDPN animals is noted.

CONCLUSIONS

The CPP test is a valuable tool to test the efficacy of the pain-relieving potential of various SCS paradigms in PDPN animals. The results of this study show no differences in spontaneous neuropathic pain relief between DTM- and Con-SCS in PDPN animals. Furthermore, there is no correlation between the effect of SCS in spontaneous pain relief and hind paw mechanical hypersensitivity.

The relationship between changes in inflammation and locomotor function in sensory phenotypes of central neuropathic pain after spinal cord injury

Introduction

Central neuropathic pain (CNP) commonly develops in patients after spinal cord injury (SCI), causing debilitating symptoms and sensory abnormalities to mechanical and thermal stimuli. The biological variability of pain phenotypes in individuals has limited the number of positive outcomes. Thus, it is necessary to investigate the physiological processes contributing to sensory changes that develop over time.

Objective

To investigate the physiological processes contributing to neuropathic pain sensory changes and locomotor impairments with sensory phenotypes that develop over time.

Methods

Using the tail flick and von Frey tests, we performed hierarchical clustering to determine the subpopulation of rats that developed thermal and mechanical sensory abnormalities. To measure inflammation as a potential mediator of CNP phenotypes, we used flow cytometry and immunohistochemistry. Finally, to assess the secondary effects on locomotor recovery, up to 8 weeks after injury, we used the CatWalk test to assess multiple parameters of gait.

Results

The von Frey test showed a subpopulation of SCI rats that were hyposensitive to mechanical stimuli from 6 to 8 weeks after injury. The tail flick test showed a subpopulation of SCI rats that were hypersensitive to thermal stimuli at 1 week and 3 to 8 weeks after injury. Although there were no differences in inflammatory cells between subpopulations, we did see significant changes in locomotor recovery between rats with and without sensory abnormalities.

Conclusion

The myeloid cell population at large is not affected by mechanical or thermal phenotypes of pain in this model; however, locomotor recovery is impaired depending on the pain phenotype present. Further investigation into acute inflammatory cells may be insightful for predicting the development of pain phenotypes.

Promotive effect of skin precursor-derived Schwann cells on brachial plexus neurotomy and motor neuron damage repair through milieu-regulating secretome

Brachial plexus injury (BPI) with motor neurons (MNs) damage still remain poor recovery in preclinical research and clinical therapy, while cell-based therapy approaches emerged as novel strategies. Previous work of rat skin precursor-derived Schwann cells (SKP-SCs) provided substantial foundation for repairing peripheral nerve injury (PNI). Given that, our present work focused on exploring the repair efficacy and possible mechanisms of SKP-SCs implantation on rat BPI combined with neurorrhaphy post-neurotomy. Results indicated the significant locomotive and sensory function recovery, with improved morphological remodeling of regenerated nerves and angiogenesis, as well as amelioration of target muscles atrophy and motor endplate degeneration. Besides, MNs could restore from oxygen-glucose-deprivation (OGD) injury upon SKP-SCs-sourced secretome treatment, implying the underlying paracrine mechanisms. Moreover, rat cytokine array assay detected 67 cytokines from SKP-SC-secretome, and bioinformatic analyses of screened 32 cytokines presented multiple functional clusters covering diverse cell types, including inflammatory cells, Schwann cells, vascular endothelial cells (VECs), neurons, and SKP-SCs themselves, relating distinct biological processes to nerve regeneration. Especially, a panel of hypoxia-responsive cytokines (HRCK), can participate into multicellular biological process regulation for permissive regeneration milieu, which underscored the benefits of SKP-SCs and sourced secretome, facilitating the chorus of nerve regenerative microenvironment. Furthermore, platelet-derived growth factor-AA (PDGF-AA) and vascular endothelial growth factor-A (VEGF-A) were outstanding cytokines involved with nerve regenerative microenvironment regulating, with significantly elevated mRNA expression level in hypoxia-responsive SKP-SCs. Altogether, through recapitulating the implanted SKP-SCs and derived secretome as niche sensor and paracrine transmitters respectively, HRCK would be further excavated as molecular underpinning of the neural recuperative mechanizations for efficient cell therapy; meanwhile, the analysis paradigm in this study validated and anticipated the actions and mechanisms of SKP-SCs on traumatic BPI repair, and was beneficial to identify promising bioactive molecule cocktail and signaling targets for cell-free therapy strategy on neural repair and regeneration.

Repressing iron overload ameliorates central post-stroke pain via the Hdac2-Kv1.2 axis in a rat model of hemorrhagic stroke

JOURNAL/nrgr/04.03/01300535-202412000-00027/figure1/v/2024-04-08T165401Z/r/image-tiff Thalamic hemorrhage can lead to the development of central post-stroke pain. Changes in histone acetylation levels, which are regulated by histone deacetylases, affect the excitability of neurons surrounding the hemorrhagic area. However, the regulatory mechanism of histone deacetylases in central post-stroke pain remains unclear. Here, we show that iron overload leads to an increase in histone deacetylase 2 expression in damaged ventral posterolateral nucleus neurons. Inhibiting this increase restored histone H3 acetylation in the Kcna2 promoter region of the voltage-dependent potassium (Kv) channel subunit gene in a rat model of central post-stroke pain, thereby increasing Kcna2 expression and relieving central pain. However, in the absence of nerve injury, increasing histone deacetylase 2 expression decreased Kcna2 expression, decreased Kv current, increased the excitability of neurons in the ventral posterolateral nucleus area, and led to neuropathic pain symptoms. Moreover, treatment with the iron chelator deferiprone effectively reduced iron overload in the ventral posterolateral nucleus after intracerebral hemorrhage, reversed histone deacetylase 2 upregulation and Kv1.2 downregulation, and alleviated mechanical hypersensitivity in central post-stroke pain rats. These results suggest that histone deacetylase 2 upregulation and Kv1.2 downregulation, mediated by iron overload, are important factors in central post-stroke pain pathogenesis and could serve as new targets for central post-stroke pain treatment.

The Antinociceptive Effects and Sex-Specific Neurotransmitter Modulation of Metformin in a Mouse Model of Fibromyalgia

Fibromyalgia (FM) is a chronic and debilitating condition characterized by diffuse pain, often associated with symptoms such as fatigue, cognitive disturbances, and mood disorders. Metformin, an oral hypoglycemic agent, has recently gained attention for its potential benefits beyond glucose regulation. It has shown promise in alleviating neuropathic and inflammatory pain, suggesting that it could offer a novel approach to managing chronic pain conditions like FM. This study aimed to further explore metformin's analgesic potential by evaluating its effects in an experimental FM model induced by reserpine in both male and female mice. After the administration of 200 mg/kg metformin to male and female mice, the FM-related symptoms were assessed, including mechanical allodynia, thermal hyperalgesia, and depressive-like behaviors. A histological examination of the thalamus, hippocampus, and spinal cord was conducted using haematoxylin and eosin staining. The neurotransmitter and proinflammatory cytokines levels were measured in the brains and spinal cords. Our results have shown that metformin treatment for seven days significantly reversed these FM-like symptoms, reducing pain sensitivity and improving mood-related behaviors in both the male and female mice. Additionally, metformin exhibited neuroprotective effects, mitigating reserpine-induced damage in the hippocampus, thalamus, and spinal cord. It also significantly lowered the levels of the proinflammatory cytokine interleukin 1-beta (IL-1β) in the brain and spinal cord. Notably, metformin modulated the neurotransmitter levels differently between the sexes, decreasing glutamate and increasing serotonin and norepinephrine in the male mice, but not in the females. These findings underscore metformin's potential as an alternative therapy for FM, with sex-specific differences suggesting distinct mechanisms of action.

Microparticles Mediate Lipopolysaccharide-induced Inflammation and Chronic Pain in Mouse Model

Recent evidence highlights microparticles (MPs) as crucial players in intercellular communication among immune cells, yet their role in inflammation-induced chronic pain remains unexplored. In this study, we investigated the involvement of MPs in the progression of inflammation and associated pain using mouse models of chronic neuroinflammation induced by repeated intraperitoneal injections of lipopolysaccharide (LPS; 1 mg/kg for four consecutive days) in C57BL/6 mice. Chronic pain was analyzed at baseline (day 0) and on day 21 post-LPS injection using von Frey and the hot metal plate tests. We found a significant increase in the levels of proinflammatory mediators and activation of the TLR4-NFκB signaling pathways following LPS administration.  Additionally, transcriptional upregulation of chronic pain-associated TRP channels and glutamate receptors, including TRPA1, TRPM2, and mGluR2 in the cortex and hippocampus as well as mGluR5 in the cortex, was noted on day 21 post-LPS injection. Moreover, upregulation of TRPM2, mGluR2, and mGluR5 was found in the spinal cord, along with increased TRPA1 protein expression in the brain cortex. Plasma-derived MPs were isolated, revealing a significant increase in concentration 21 days after LPS injection, accompanied by TNF-α DNA encapsulation and increased TNF-α mRNA expression within MPs. Furthermore, MPs concentration positively correlated with the expression of TRPA1, TRPM2, mGluR2, and mGluR5. These findings suggest that MPs contribute to inflammation-induced chronic pain, highlighting their potential as therapeutic targets.

In-vivo evaluation of analgesic and anti-inflammatory activities of the 80% methanol extract of Acacia seyal stem bark in rodent models

BACKGROUND

Pain and inflammation are the major medical condition commonly addressed with traditional remedies. Acacia seyal is a traditional herb widely used in Ethiopian folk medicine for pain management. However, its effectiveness has yet to be validated through scientific or experimental research. Therefore, the current study aims at evaluating the in vivo analgesic and anti-inflammatory effects of 80% methanolic stem bark extract of Acacia seyal in rodent models.

METHODS

After successful extractions of the stem barks of Acacia seyal with 80% methanol, the pain relieving effects of 100, 200 and 400 mg/kg extract were evaluated using acetic acid-induced writhing test and hot plate method whereas the anti-inflammatory profile was determined by carrageenan induced paw-edema model and cotton pellet induced granuloma technique.

RESULTS

The 80% methanol Acacia seyal stem bark extract exhibited substantial (p < 0.001) analgesic effect in acetic acid induced writing test (p < 0.001). The plant extract also witnessed significant central analgesic effect in hot plate method beginning at 30 min with maximum % elongation time occurred at 120 min. Furthermore, the acacia stem bark extract produced anti-inflammatory effect against carrageenan induced paw-edema model. In cotton pellet induced granuloma model, the 200 and 400 mg/kg doses of the current plant material appeared to inhibit granuloma mass formation and exudate reduction significantly (p < 0.001).

CONCLUSION

The collective findings of the current study revealed that 80% methanol extracts of Acacia seyal exhibited considerable analgesic and anti-inflammatory activities, supporting the plant's traditional use for management of pain and inflammatory disorders.

Overactive PKA signaling underlies the hyperalgesia in an ADHD mouse model

There is an intimate relationship between pain hypersensitivity and attention deficit hyperactivity disorder (ADHD); however, the underlying mechanisms are still elusive. Individuals carrying the mutation in CRY1 (c. 1657 + 3A > C), which leads to deletion of exon 11 expression in the CRY1 protein (CRY1Δ11), exhibit ADHD symptoms. Here, we demonstrate that the responses to thermal and mechanical stimuli were amplified in the Cry1Δ11 mice. RNA-sequencing analysis identified protein kinase A (PKA) signaling as being overactive in the spinal cords of Cry1Δ11 mice. The neuronal excitability was significantly enhanced in the spinal cords of Cry1Δ11 mice as determined by in vitro electrophysiology. The PKA inhibitor H89 normalized hyperalgesia in Cry1Δ11 mice, underscoring the causative effect of overactive PKA signaling. Our results thus point to the PKA signaling pathway as the underlying mechanism and a potential therapeutic target for pain hypersensitivity in a validated ADHD mouse model.

Lactobacillus Helveticus Improves Controlled Cortical Impact Injury-Generated Neurological Aberrations by Remodeling of Gut-Brain Axis Mediators

Considerable studies augured the potential of gut microbiota-based interventions in brain injury-associated complications. Based on our earlier study results, we envisaged the sex-specific neuroprotective effect of Lactobacillus helveticus by remodeling of gut-brain axis. In this study, we investigated the effect of L. helveticus on neurological complications in a mouse model of controlled cortical impact (CCI). Adult, male and female, C57BL/6 mice underwent CCI surgery and received L. helveticus treatment for six weeks. Sensorimotor function was evaluated via neurological severity score and rotarod test. Long-term effects on anxiety-like behavior and cognition were assessed using the elevated-zero maze (EZM) and novel object recognition test (NORT). Brain perilesional area, blood, colon, and fecal samples were collected post-CCI for molecular biology analysis. CCI-operated mice displayed significant neurological impairments at 1-, 3-, 5-, and 7-days post-injury (dpi) and exhibited altered behavior in EZM and NORT compared to sham-operated mice. However, these behavioral changes were ameliorated in mice receiving L. helveticus. GFAP, Iba-1, TNF-α, and IL-1β expressions and corticotrophin-releasing hormone (CRH) levels were elevated in the perilesional cortex of CCI-operated male/female mice. These elevated biomarkers and decreased BDNF levels in both male/female mice were modified by L. helveticus treatment. Additionally, L. helveticus treatment restored altered short-chain fatty acids (SCFAs) levels in fecal samples and improved intestinal integrity but did not affect decreased plasma levels of progesterone and testosterone in CCI mice. These results indicate that L. helveticus exerts beneficial effects in the CCI mouse model by mitigating inflammation and remodeling of gut microbiota-brain mediators.

Encoding of Vibrotactile Stimuli by Mechanoreceptors in Rodent Glabrous Skin

Somatosensory coding in rodents has been mostly studied in the whisker system and hairy skin, whereas the function of low-threshold mechanoreceptors (LTMRs) in the rodent glabrous skin has received scant attention, unlike in primates where the glabrous skin has been the focus. The relative activation of different LTMR subtypes carries information about vibrotactile stimuli, as does the rate and temporal patterning of LTMR spikes. Rate coding depends on the probability of a spike occurring on each stimulus cycle (reliability), whereas temporal coding depends on the timing of spikes relative to the stimulus cycle (precision). Using in vivo extracellular recordings in male rats and mice of either sex, we measured the reliability and precision of LTMR responses to tactile stimuli including sustained pressure and vibration. Similar to other species, rodent LTMRs were separated into rapid-adapting (RA) or slow-adapting based on their response to sustained pressure. However, unlike the dichotomous frequency preference characteristic of RA1 and RA2/Pacinian afferents in other species, rodent RAs fell along a continuum. Fitting generalized linear models to experimental data reproduced the reliability and precision of rodent RAs. The resulting model parameters highlight key mechanistic differences across the RA spectrum; specifically, the integration window of different RAs transitions from wide to narrow as tuning preferences across the population move from low to high frequencies. Our results show that rodent RAs can support both rate and temporal coding, but their heterogeneity suggests that coactivation patterns play a greater role in population coding than for dichotomously tuned primate RAs.

Melatonin mitigates vincristine-induced peripheral neuropathy by inhibiting TNF-α/astrocytes/microglial cells activation in the spinal cord of rats, while preserving vincristine's chemotherapeutic efficacy in lymphoma cells

Vincristine (VCR), an anti-tubulin chemotherapy agent, is known to cause peripheral and central nerve damage, inducing severe chemotherapy-induced peripheral neuropathy (CIPN). Although melatonin has been recently recognized for its potential anti-neuropathic effects, its efficacy in countering VCR-induced neuropathy remains unclear. This study examines the neuroprotective potential of melatonin against VCR-induced neuropathy using a rat model. Neuropathic pain was induced through 10 VCR injections (0.1 mg/kg/day i.p.), administered in two five-day cycles with a two-day break. Melatonin treatment started two days before VCR administration and continued daily throughout the experiment. Rats were assigned to five groups: control, VCR, and three melatonin-treated groups receiving VCR with melatonin (5, 10, or 20 mg/kg/day i.p.). We assessed mechanical (von-Frey and Randall-Selitto tests) and thermal (hot-plate and tail-flick tests) hyperalgesia, motor coordination (rotarod test), and sciatic nerve conduction velocity (NCV). Changes in body weight, spinal cord histopathology (H&E), and proinflammatory markers (TNF-α, IL-1β, and IL-6), reactive astrocytes (GFAP) and microglial cells (IBA-1) were also assessed, as well as spinal cord degeneration (Nissl stain) and demyelination (LFB stain and MBP). Finally, the effect of melatonin on the cytotoxic activity of VCR against EL4 lymphoma cells was assessed using an MTT assay. Our results indicated that melatonin coadministration with VCR preserved spinal cord architecture, elevated nociceptive thresholds, improved motor coordination, enhanced NCV, and maintained normal body weight gain. Melatonin also reduced inflammation, decreased reactive astrocytes and microglia, and prevented neurodegeneration and demyelination in the spinal cord. Importantly, melatonin did not affect VCR's cytotoxic activity in cancer cells.

Behavioral and neural correlates of diverse conditioned fear responses in male and female rats

Pavlovian fear conditioning is a widely used tool that models associative learning in rodents. For decades the field has used predominantly male rodents and focused on a sole conditioned fear response: freezing. However, recent work from our lab and others has identified darting as a female-biased conditioned response, characterized by an escape-like movement across a fear conditioning chamber. It is also accompanied by a behavioral phenotype: Darters reliably show decreased freezing compared to Non-darters and males and reach higher velocities in response to the foot shock ("shock response"). However, the relationship between shock response and conditioned darting is not known. This study investigated if this link is due to differences in general processing of aversive stimuli between Darters, Non-darters and males. Across a variety of modalities, including corticosterone measures, the acoustic startle test, and sensitivity to thermal pain, Darters were found not to be more reactive or sensitive to aversive stimuli, and, in some cases, they appear less reactive to Non-darters and males. Analyses of cFos activity in regions involved in pain and fear processing following fear conditioning identified discrete patterns of expression among Darters, Non-darters, and males exposed to low and high intensity foot shocks. The results from these studies further our understanding of the differences between Darters, Non-darters and males and highlight the importance of studying individual differences in fear conditioning as indicators of fear state.

10 kHz spinal cord stimulation improves metrics of spinal sensory processing in a male STZ rat model of diabetes

To explore why clinical 10 kHz spinal cord stimulation (10 kHz SCS) might improve neurological function in a model of painful diabetic neuropathy (PDN), the short-term behavioral, electrophysiological, and histological effects of 10 kHz SCS were studied using adult male streptozotocin (STZ)-induced diabetic Sprague-Dawley rats. Four testing groups were established: Naïve controls (N = 8), STZ controls (N = 7), STZ+Sham SCS (N = 9), and STZ+10 kHz SCS (N = 11). After intraperitoneal injection (60 mg/kg) of STZ caused the rats to become hyperglycemic, SCS electrodes were implanted in the dorsal epidural space over the L5-L6 spinal segments in the STZ+Sham SCS and STZ+10 kHz SCS groups and were stimulated for 14 days. The von Frey filament paw withdrawal threshold was measured weekly. At termination, animals were anesthetized and the electrophysiologic response of dorsal horn neurons (receptive field size, vibration, radiant warmth) of the ipsilateral foot was measured. Tissue from the plantar paw surface was obtained post-euthanization for intraepidermal nerve fiber density measurements. In comparison to other control groups, while no significant effect of 10 kHz SCS on peripheral intraepidermal nerve fiber density was observed, 10 kHz SCS 'normalized' the central neural response to vibration, receptive field, and paw withdrawal threshold, and elevated the neural response to tissue recovery from warm stimuli. These results suggest that short-term, low intensity 10 kHz SCS operates in the spinal cord to ameliorate compromised sensory processing, and may compensate for reduced peripheral sensory functionality from chronic hyperglycemia, thereby treating a broader spectrum of the sensory symptoms in diabetic neuropathy.

Disrupted stress-induced analgesia in a neuropathic pain state is rescued by the endocannabinoid degradation inhibitor JZL195

Acute stress normally engages descending brain pathways to produce an antinociceptive response, known as stress-induced analgesia. Paradoxically, these descending pain modulatory pathways are also involved in the maintenance of the abnormal pain associated with chronic neuropathic pain. It remains unclear how stress-induced analgesia is affected by neuropathic pain states. We therefore examined the impact of a chronic constriction nerve-injury (CCI) model of neuropathic pain on restraint stress-induced analgesia in C57BL/6 mice. Thirty minutes of restraint stress produced analgesia in the hotplate thermal nociceptive assay that was less in CCI compared to control mice who underwent a sham-surgery. In sham but not CCI mice, stress-induced analgesia was reduced by the opioid receptor antagonist naltrexone. The cannabinoid CB1 receptor antagonist AM281 did not affect stress-induced analgesia in either sham or CCI mice. Low-dose pre-treatment with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195 increased stress-induced analgesia in CCI but not sham mice. The JZL195 enhancement of stress-induced analgesia in CCI mice was abolished by AM281 but was unaffected by naltrexone. These findings indicate that the acute opioid-mediated analgesic response to a psychological stressor is disrupted in a nerve-injury model of neuropathic pain. Importantly, this impairment of stress-induced analgesia was rescued by blockade of endocannabinoid breakdown via a cannabinoid CB1 receptor dependent mechanism. These findings suggest that subthreshold treatment with endocannabinoid degradation blockers could be used to alleviate the disruption of endogenous pain control systems in a neuropathic pain state.

Early signs of osteoarthritis in differing rat osteochondral defects

Preclinical models of osteochondral defects (OCDs) are fundamental test beds to evaluate treatment modalities before clinical translation. To increase the rigor and reproducibility of translational science for a robust "go or no-go," we evaluated disease progression and pain phenotypes within the whole joint for two OCD rat models with same defect size (1.5 x 0.8 mm) placed either in the trochlea or medial condyle of femur. Remarkably, we only found subtle transitory changes to gaits of rats with trochlear defect without any discernible effect to allodynia. At 8-weeks post-surgery, anatomical evaluations of joint showed early signs of osteoarthritis with EPIC-microCT. For the trochlear defect, cartilage attenuation was increased in trochlear, medial, and lateral compartments of the femur. For condylar defect, increased cartilage attenuation was isolated to the medial condyle of the femur. Further, the medial ossicle showed signs of deterioration as indicated with decreased bone mineral density and increased bone surface area to volume ratio. Thus, OCD in a weight-bearing region of the femur gave rise to more advanced osteoarthritis phenotype within a unilateral joint compartment. Subchondral bone remodeling was evident in both models without any indication of closure of the articular cartilage surface. We conclude that rat OCD, placed in the trochlear or condylar region of the femur, leads to differing severity of osteoarthritis progression. As found herein, repair of the defect with fibrous tissue and subchondral bone is insufficient to alleviate onset of osteoarthritis. Future therapies using rat OCD model should address joint osteoarthritis in addition to repair itself.

Bay 11-7082 mitigates oxidative stress and mitochondrial dysfunction via NLRP3 inhibition in experimental diabetic neuropathy

OBJECTIVE

Diabetic neuropathy is associated with mitochondrial dysfunction and neuroinflammation. Chronic hyperglycemia triggers inflammatory responses and oxidative stress, causing peripheral neuropathy, whereas mitochondrial dysfunction caused by increased ROS generation and reduced bioenergetics maintains the inflammatory cycle. The purpose of this study is to evaluate the pharmacological efficacy of Bay 11-7082 (B11) against diabetic neuropathy in rats.

METHODS

B11 was administered at doses of 1 and 3 mg/kg to STZ-induced diabetic animals (55 mg/kg, i.p). Behavioral and functional assessments were conducted to assess neuropathy. Molecular protein expressions were evaluated for B11's efficacy against STZ-induced diabetic neuropathic rats and in SHSY5Y cells exposed to 175 mM of d-glucose.

RESULTS

Diabetic rats exhibited deficits in nerve functions, altered nociceptive parameters, and increased expression of NLRP3, ASC, Caspase-1, and NF-κB. Additionally, diabetic animals showed reduced levels of PGC1α/Nrf2/HO-1, with an overexpression of PARP1. Compromised mitochondrial function was evident through increased mitochondrial dynamic marker DRP1 and elevated levels of inflammatory cytokines TNF-α, IL-1β, IL-18, and IL-6. However, B11 administration significantly ameliorated these changes, suggesting that B11's NLRP3 inhibition may be attributed to the activation of the mitochondrial biogenesis pathway via PGC1α/Nrf2/HO-1, along with improved mitochondrial health. In high glucose exposed SHSY5Y cells, B11 treatment attenuated neuroinflammation by inhibiting NLRP3 activation and reducing mitochondrial damage.

CONCLUSION

B11, showed a protective effect against diabetic neuropathy by inhibiting oxidative stress, NLRP3 activation, and improving mitochondrial health in experimental diabetic neuropathy. This study provides new mechanistic insights into the neuroprotective role of Bay 11-7082 against diabetic neuropathy.

Advancing Pain Understanding and Drug Discovery: Insights from Preclinical Models and Recent Research Findings

Despite major advancements in our understanding of its fundamental causes, pain-both acute and chronic-remains a serious health concern. Various preclinical investigations utilizing diverse animal, cellular, and alternative models are required and frequently demanded by regulatory approval bodies to bridge the gap between the lab and the clinic. Investigating naturally occurring painful disorders can speed up medication development at the preclinical and clinical levels by illuminating molecular pathways. A wide range of animal models related to pain have been developed to elucidate pathophysiological mechanisms and aid in identifying novel targets for treatment. Pain sometimes drugs fail clinically, causing high translational costs due to poor selection and the use of preclinical tools and reporting. To improve the study of pain in a clinical context, researchers have been creating innovative models over the past few decades that better represent pathological pain conditions. In this paper, we provide a summary of traditional animal models, including rodents, cellular models, human volunteers, and alternative models, as well as the specific characteristics of pain diseases they model. However, a more rigorous approach to preclinical research and cutting-edge analgesic technologies may be necessary to successfully create novel analgesics. The research highlights from this review emphasize new opportunities to develop research that includes animals and non-animals using proven methods pertinent to comprehending and treating human suffering. This review highlights the value of using a variety of modern pain models in animals before human trials. These models can help us understand the different mechanisms behind various pain types. This will ultimately lead to the development of more effective pain medications.

Activation of TGR5 in the injured nerve site according to a prevention protocol mitigates partial sciatic nerve ligation-induced neuropathic pain by alleviating neuroinflammation

ABSTRACT

Neuropathic pain is a pervasive medical challenge currently lacking effective treatment options. Molecular changes at the site of peripheral nerve injury contribute to both peripheral and central sensitization, critical components of neuropathic pain. This study explores the role of the G-protein-coupled bile acid receptor (GPBAR1 or TGR5) in the peripheral mechanisms underlying neuropathic pain induced by partial sciatic nerve ligation in male mice. TGR5 was upregulated in the injured nerve site and predominantly colocalized with macrophages. Perisciatic nerve administration of the TGR5 agonist, INT-777 according to a prevention protocol (50 μg/μL daily from postoperative day [POD] 0 to POD6) provided sustained relief from mechanical allodynia and spontaneous pain, whereas the TGR5 antagonist, SBI-115 worsened neuropathic pain. Transcriptome sequencing linked the pain relief induced by TGR5 activation to reduced neuroinflammation, which was further evidenced by a decrease in myeloid cells and pro-inflammatory mediators (eg, CCL3, CXCL9, interleukin [IL]-6, and tumor necrosis factor [TNF] α) and an increase in CD86-CD206+ anti-inflammatory macrophages at POD7. Besides, myeloid-cell-specific TGR5 knockdown in the injured nerve site exacerbated both neuropathic pain and neuroinflammation, which was substantiated by bulk RNA-sequencing and upregulated expression levels of inflammatory mediators (including CCL3, CCL2, IL-6, TNF α, and IL-1β) and the increased number of monocytes/macrophages at POD7. Furthermore, the activation of microglia in the spinal cord on POD7 and POD14 was altered when TGR5 in the sciatic nerve was manipulated. Collectively, TGR5 activation in the injured nerve site mitigates neuropathic pain by reducing neuroinflammation, while TGR5 knockdown in myeloid cells worsens pain by enhancing neuroinflammation.

The role of neutrophils in pain: systematic review and meta-analysis of animal studies

ABSTRACT

The peripheral inflammatory response is an attractive therapeutic target for pain treatment. Neutrophils are the first circulating inflammatory cells recruited to sites of injury, but their contribution to pain outcomes is unclear. We performed a systematic review and meta-analysis of original preclinical studies, which evaluated the effect of preemptive neutrophil depletion on pain outcomes (PROSPERO registration number: CRD42022364004). Literature search (PubMed, January 19, 2023) identified 49 articles, which were meta-analyzed using a random-effects model. The risk of bias was evaluated using SYRCLE's tool. The pooled effect considering all studies showed that neutrophil depletion induced a consistent pain reduction. Inflammatory, joint, neuropathic, and visceral pain showed significant pain alleviation by neutrophil depletion with medium-large effect sizes. However, muscle and postoperative pain were not significantly alleviated by neutrophil depletion. Further analysis showed a differential contribution of neutrophils to pain outcomes. Neutrophils had a higher impact on mechanical hyperalgesia, followed by nociceptive behaviors and mechanical allodynia, with a smaller contribution to thermal hyperalgesia. Interspecies (mice or rats) differences were not appreciated. Analyses regarding intervention unveiled a lower pain reduction for some commonly used methods for neutrophil depletion, such as injection of antineutrophil serum or an anti-Gr-1 antibody, than for other agents such as administration of an anti-Ly6G antibody, fucoidan, vinblastine, CXCR1/2 inhibitors, and etanercept. In conclusion, the contribution of neutrophils to pain depends on pain etiology (experimental model), pain outcome, and the neutrophil depletion strategy. Further research is needed to improve our understanding on the mechanisms of these differences.

Morphine-induced hyperalgesia impacts small extracellular vesicle miRNA composition and function

Morphine and other synthetic opioids are widely prescribed to treat pain. Prolonged morphine exposure can paradoxically enhance pain sensitivity in humans and nociceptive behavior in rodents. To better understand the molecular mechanisms underlying opioid-induced hyperalgesia, we investigated changes in miRNA composition of small extracellular vesicles (sEVs) from the serum of mice after a morphine treatment paradigm that induces hyperalgesia. We observed significant differential expression of 18 miRNAs in sEVs from morphine-treated mice of both sexes compared to controls. Several of these miRNAs were bioinformatically predicted to regulate cyclic AMP response element binding protein (CREB), a well-characterized transcription factor implicated in pain and drug addiction. We confirmed the binding and repression of Creb mRNA by miR-155 and miR-10a. We tested if serum-derived sEVs from morphine-treated mice could elicit nociceptive behavior in naïve recipient mice. Intrathecal injection of 1 μg sEVs did not significantly impact basal mechanical and thermal threshold in naïve recipient mice. However, prophylactic 1 μg sEV administration in recipient mice resulted in faster resolution of complete Freund's adjuvant-induced mechanical and thermal inflammatory hypersensitivity. Other behaviors assayed following administration of these sEVs were not impacted including sEV conditioned place preference and locomotor sensitization. These results indicate that morphine regulation of serum sEV composition can contribute to analgesia and suggest a potential for sEVs to be a non-opioid therapeutic intervention strategy to treat pain.

TLR2/NF-κB signaling in macrophage/microglia mediated COVID-pain induced by SARS-CoV-2 envelope protein

Pain has become a major symptom of long COVID-19 without effective therapy. Apart from viral infection pathological process, SARS-CoV-2 membranal proteins (envelope [S2E], spike [S2S] and membrane [S2M]) also present pro-inflammatory feature independently. Here, we aim to uncover the neuroinflammatory mechanism of COVID-pain induced by SARS-CoV-2 membranal proteins. We detected the three proteins in both peripheral sensory ganglions and spinal dorsal horn of COVID-19 donors. After intradermal and intrathecal injection, only S2E triggered pain behaviors, accompanied with upregulated-phosphorylation nuclear factor kappa B (NF-κB), which was significantly attenuated by minocycline in mice. We further identified Toll-like receptor 2 (TLR2) among TLRs as the target of S2E to evoke inflammatory responses leading to COVID-pain. This study identified the nociceptive effect of S2E through directly interacting with macrophage/microglia TLR2 and inducing the following NF-κB inflammatory storm. Clearing away S2E and inhibiting macrophage/microglia TLR2 served as perspective therapeutic strategies for COVID-19 pain.

Δ9-Tetrahydrocannabinol Alleviates Hyperalgesia in a Humanized Mouse Model of Sickle Cell Disease

People with sickle cell disease (SCD) often experience chronic pain as well as unpredictable episodes of acute pain, which significantly affects their quality of life and life expectancy. Current treatment strategies for SCD-associated pain primarily rely on opioid analgesics, which have limited efficacy and cause serious adverse effects. Cannabis has emerged as a potential alternative, yet its efficacy remains uncertain. In this study, we investigated the antinociceptive effects of Δ9-tetrahydrocannabinol (THC), cannabis' intoxicating constituent, in male HbSS mice, which express >99% human sickle hemoglobin, and male HbAA mice, which express normal human hemoglobin A, as a control. Acute THC administration (0.1-3 mg/kg-1, i.p.) dose-dependently reduced mechanical and cold hypersensitivity in human sickle hemoglobin (HbSS) but not human normal hemoglobin A (HbAA) mice. In the tail-flick assay, THC (1 and 3 mg/kg-1, i.p.) produced substantial antinociceptive effects in HbSS mice. By contrast, THC (1 mg/kg-1, i.p.) did not alter anxiety-like behavior (elevated plus maze) or long-term memory (24-hour novel object recognition). Subchronic THC treatment (1 and 3 mg/kg-1, i.p.) provided sustained relief of mechanical hypersensitivity but led to tolerance in cold hypersensitivity in HbSS mice. Together, the findings identify THC as a possible therapeutic option for the management of chronic pain in SCD. Further research is warranted to elucidate its mechanism of action and possible interaction with other cannabis constituents. SIGNIFICANCE STATEMENT: The study explores Δ9-tetrahydrocannabinol (THC)'s efficacy in alleviating pain in sickle cell disease (SCD) using a humanized mouse model. Findings indicate that acute THC administration reduces mechanical and cold hypersensitivity in SCD mice without impacting emotional and cognitive dysfunction. Subchronic THC treatment offers sustained relief of mechanical hypersensitivity but leads to cold hypersensitivity tolerance. These results offer insights into THC's potential as an alternative pain management option in SCD, highlighting both its benefits and limitations.

Antinociceptive Behavior, Glutamine/Glutamate, and Neopterin in Early-Stage Streptozotocin-Induced Diabetic Neuropathy in Liraglutide-Treated Mice under a Standard or Enriched Environment

Diabetic neuropathy (DN) is a common complication of long-lasting type 1 and type 2 diabetes, with no curative treatment available. Here, we tested the effect of the incretin mimetic liraglutide in DN in mice with early-stage type 1 diabetes bred in a standard laboratory or enriched environment. With a single i.p. injection of streptozotocin 150 mg/kg, we induced murine diabetes. Liraglutide (0.4 mg/kg once daily, i.p. for ten days since the eighth post-streptozotocin day) failed to decrease the glycemia in the diabetic mice; however, it alleviated their antinociceptive behavior, as tested with formalin. The second phase of the formalin test had significantly lower results in liraglutide-treated mice reared in the enriched environment vs. liraglutide-treated mice under standard conditions [2.00 (0.00-11.00) vs. 29.00 (2.25-41.50) s, p = 0.016]. Liraglutide treatment, however, decreased the threshold of reactivity in the von Fray test. A significantly higher neopterin level was demonstrated in the diabetic control group compared to treatment-naïve controls and the liraglutide-treated diabetic mice (p < 0.001). The glutamine/glutamate ratio in both liraglutide-treated groups, either reared under standard conditions (p = 0.003) or an enriched environment (p = 0.002), was significantly higher than in the diabetic controls. This study demonstrates an early liraglutide effect on pain sensation in two streptozotocin-induced diabetes mouse models by reducing some inflammatory and oxidative stress parameters.