Neuropathy-induced spinal GAP-43 expression is not a main player in the onset of mechanical pain hypersensitivity

Therapeutic exercise interventions in rat models of arthritis

Arthritis is the leading cause of musculoskeletal pain and disability worldwide. Nearly 50% of individuals over the age of 65 have arthritis, which contributes to limited function, articular pain, physical inactivity, and diminished quality of life. Therapeutic exercise is often recommended in clinical settings for patients experiencing arthritic pain, however, there is little practical guidance regarding the use of therapeutic exercise to alleviate arthritic musculoskeletal pain. Rodent models of arthritis allow researchers to control experimental variables, which cannot be done with human participants, providing an opportunity to test therapeutic approaches in preclinical models. This literature review provides a summary of published findings in therapeutic exercise interventions in rat models of arthritis as well as gaps in the existing literature. We reveal that preclinical research in this field has yet to adequately investigate the impact of experimental variables in therapeutic exercise including their modality, intensity, duration, and frequency on joint pathophysiology and pain outcomes.

Neuronal plasticity of trigeminal ganglia in mice following nerve injury

Background

Nerve injury may induce neuropathic pain. In studying the mechanisms of orofacial neuropathic pain, attention has been paid to the plastic changes that occur in the trigeminal ganglia (TGs) and nucleus in response to an injury of the trigeminal nerve branches. Previous studies have explored the impact of sciatic nerve injury on dorsal root ganglia (DRGs) and it has shown dramatic changes in the expression of multiple biomarkers. In large, the changes in biomarker expression in TGs after trigeminal nerve injury are similar to that in DRGs after sciatic nerve injury. However, important differences exist. Therefore, there is a need to study the plasticity of biomarkers in TGs after nerve injury in the context of the development of neuropathic pain-like behaviors.

Aim

The aim of this study was to investigate the plasticity of biomarkers associated with chronic persistent pain in TGs after trigeminal nerve injury.

Materials and methods

To mimic the chronic nature of the disorder, we used an intraoral procedure to access the infraorbital nerve (ION) and induced a nerve injury in mice. Immunohistochemistry and quantification were used for revealing the expression level of each biomarker in TGs after nerve injury.

Results

Two weeks after partial ION injury, immunohistochemistry results showed strongly upregulated expressions of activating transcription factor 3 and neuropeptide Y (NPY) in the ipsilateral TGs. Microglial cells were also activated after nerve injury. In regard to positive neuronal profile counting, however, no significant difference in expression was observed in galanin, substance P, calcitonin gene-related peptide, neuronal nitric oxide synthase, phosphorylated AKT, or P2X3 in ipsilateral TGs when compared to contralateral TGs.

Conclusion

In this study, the expression and regulation of biomarkers in TGs have been observed in response to trigeminal nerve injury. Our results suggest that NPY and Iba1 might play crucial roles in the pathogenesis of orofacial neuropathic pain following this type of injury. Further investigations on the relevance of these changes may help to target suitable treatment possibilities for trigeminal neuralgia.

The Morphofunctional Effect of the Transplantation of Bone Marrow Stromal Cells and Predegenerated Peripheral Nerve in Chronic Paraplegic Rat Model via Spinal Cord Transection

Functional recovery following spinal cord injury (SCI) is limited by poor axonal and cellular regeneration as well as the failure to replace damaged myelin. Employed separately, both the transplantation of the predegenerated peripheral nerve (PPN) and the transplantation of bone marrow stromal cells (BMSCs) have been shown to promote the regrowth and remyelination of the damaged central axons in SCI models of hemisection, transection, and contusion injury. With the aim to test the effects of the combined transplantation of PPN and BMSC on regrowth, remyelination, and locomotor function in an adult rat model of spinal cord (SC) transection, 39 Fischer 344 rats underwent SC transection at T9 level. Four weeks later they were randomly assigned to traumatic spinal cord injury (TSCI) without treatment, TSCI + Fibrin Glue (FG), TSCI + FG + PPN, and TSCI + FG + PPN + BMSCs. Eight weeks after, transplantation was carried out on immunofluorescence and electron microscope studies. The results showed greater axonal regrowth and remyelination in experimental groups TSCI + FG + PPN and TSCI + FG + PPN + BMSCs analyzed with GAP-43, neuritin, and myelin basic protein. It is concluded that the combined treatment of PPN and BMSCs is a favorable strategy for axonal regrowth and remyelination in a chronic SC transection model.

Modulation of spinal glial reactivity by intrathecal PPF is not sufficient to inhibit mechanical allodynia induced by nerve crush

Spinal glial reactivity has been strongly implicated in pain that follows peripheral nerve injury. Among the many therapeutic agents that have been tested for anti-allodynia through immune modulation is the atypical methylxanthine propentofylline. While propentofylline shows a potent anti-allodynia effect after nerve transection injury, we here demonstrate that, when propentofylline is used intrathecally at the effective immune-modulatory dose, allodynia after rat nerve crush injury is completely preserved. Microglial/macrophage Iba-1 and astrocytic GFAP expression, increased in the dorsal horn of nerve crushed animals, was, however, effectively attenuated by propentofylline. Effective modulation of spinal glial reactivity is, thus, no assurance for anti-allodynia.

Heterogeneous responses of dorsal root ganglion neurons in neuropathies induced by peripheral nerve trauma and the antiretroviral drug stavudine

Background

Heterogeneity is increasingly recognized in clinical presentation of neuropathic pain (NP), but less often recognized in animal models. Neurochemical dysregulation in rodent dorsal root ganglia (DRG) is associated with peripheral nerve trauma, but poorly studied in non-traumatic NP conditions.

Methods

This study aimed to investigate the temporal expressions of activating transcription factor-3 (ATF-3), growth-associated protein-43 (GAP-43), neuropeptide Y (NPY) and galanin in traumatic and non-traumatic rat models of neuropathies associated with NP. Expressions of these markers were examined in the DRG at different time points following tibial nerve transection (TNT) injury and antiretroviral drug stavudine (d4T) administration using immunohistochemistry. The development of sensory gain following these insults was assessed by measuring limb withdrawal to a punctate mechanical stimulus.

Results

Both TNT-injured and d4T-treated rats developed hindpaw mechanical hypersensitivity. Robust expressions of ATF-3, GAP-43, NPY and galanin in both small- and large-sized L5 DRG neurons were observed in the DRG from TNT-injured rats. In contrast, d4T-treated rats did not exhibit any significant neurochemical changes in the DRG.

Conclusions

Taken together, the results suggest that ATF-3, GAP-43, NPY and galanin are likely indicators of nerve trauma-associated processes and not generic markers for NP. These experiments also demonstrate distinct expression patterns of neurochemical markers in the DRG and emphasize the mechanistic difference between nerve trauma and antiretroviral drug-associated NP.

Redistribution of voltage-gated sodium channels after nerve decompression contributes to relieve neuropathic pain in chronic constriction injury

Nerve decompression is an important therapeutic strategy to relieve neuropathic pain and promote the peripheral nerve regeneration. To address these issues, we investigated the effects of nerve decompression on relief of neuropathic pain behaviors, redistribution of voltage-gated sodium channels (VGSCs), and skin reinnervation with chronic constriction injury (CCI). At post-operative week (POW) 4, animals were divided into a decompression group, in which the ligatures were removed, and a CCI group, in which the ligatures remained. Thermal hyperalgesia and mechanical allodynia at POW 8 had distinct reductions in decompression group compared to CCI group. At that time in CCI group, morphological evidence of pan VGSCs (Pan Nav) and isoforms of VGSCs (Nav1.6, Nav1.9, except for Nav1.8) were shown the widely distribution along the injured sciatic nerve. All of the VGSCs in decompression group became clustering around the node of Ranvier, similar to the pattern of control sciatic nerve at POW 8. Skin reinnervation was demonstrated by epidermal nerve density (END) for protein gene product 9.5 (PGP 9.5)-immunoreactive (IR) nerve fibers and a significant difference between groups only at POW 24 (p=0.01). Growth-associated protein 43 (GAP-43) is participated in the nerve fiber growth and sprouting, a difference in END for GAP-43-IR nerve fibers at POW 24 between groups were also significant (p=0.02). These observations demonstrated that nerve decompression was accompanied with the disappearance of neuropathic pain behaviors after CCI. Morphological studies provided the evidence that redistribution of VGSCs along the injured sciatic nerve but still with an incomplete skin reinnervation. These significant findings demonstrated a role of VGSCs in the pathogenesis of neuropathic pain, and gave an approaching in pharmacological basis of therapeutics.

Up-regulation of spinal microglial Iba-1 expression persists after resolution of neuropathic pain hypersensitivity

Spinal microglial activation plays a major role in the development of neuropathic pain following peripheral nerve injury. We here provide evidence for an elevated expression of the microglial marker Iba-1 in the lumbar dorsal horn ipsilateral to L5 spinal nerve transection that persists for at least 14 weeks, a time at which mechanical hypersensitivity had fully resolved. Iba-1 expression was, however; significantly lower than at 4 weeks. We therefore conclude that microglia remain partly activated beyond the phase of pain hypersensitivity. Thus, the relation between microglial cells and neuropathic pain outcome is subject to change over time after nerve injury.

Inhibition of GAP-43 by propentofylline in a rat model of neuropathic pain

Neural plasticity within the spinal nociceptive network may be fundamental to the chronic nature of neuropathic pain. The relation of growth-associated protein-43 (GAP-43), a protein involved in the nerve fiber growth and sprouting, to pain hypersensitivity has been investigated. Glial activation and inflammatory cytokines released by microglia and astrocytes are considered to be involved in the neural sprouting and plasticity. In the present study, the anti-nociception effect of propentofylline, a glial modulating agent, was investigated in a rat chronic constriction injury (CCI) model aiming to explore the role of GAP-43 expression. Our results demonstrated that propentofylline could attenuate the CCI-induced mechanical allodynia and thermal hyperalgesia and inhibit the astrocyte activation and production of IL-1β. GAP-43 expression was also down-regulated by intrathecal propentofylline. These findings suggest that astrocyte activation is involved in the regulation of GAP-43 expression and propentofylline might be used in the treatment of neuropathic pain.

Down-regulation of GAP-43 by inhibition of caspases-3 in a rat model of neuropathic pain

BACKGROUND

Neuropathic pain remains a prevalent and persistent clinical problem due to incomplete understanding of its pathogenesis.

OBJECTIVE

The present study aimed to investigate the role of caspase-3 in the neuropathic pain in rats with chronic constriction injury (CCI).

METHODS

SD rats were randomly assigned four groups (n=18 per group): sham group, normal saline group (NS group), Z-DEVD-FMK group (DEVD group) and RNA interference group (siRNA group). Z-DEVD-FMK (1 U/30 μl), siRNA targeting caspase-3 (10 μg/30 μl) and NS of equal volume were intrathecally administered once daily for 5 days starting 1 day before surgery in the DEVD, siRNA and NS group, respectively. Thermal hyperalgesia was assessed at one day before and 1, 2, 4, 5, 6, 7 and 10 days after surgery. The mRNA and protein expressions of caspase-3 were measured by real time PCR and immunofluorescence assay. Apoptosis was detected by TUNEL staining. GAP-43 expression was measured by immunofluorescence and western blot assays.

RESULTS

The right paw withdrawal latency (PWL) was decreased after CCI (P<0.05). TUNEL-positive neurons and the mRNA and protein expressions of caspase-3 in the spinal cord were increased significantly. After Z-DEVD-FMK or siRNA treatment, TUNEL-positive neurons were decreased, PWLs increased (P<0.05) and the mRNA and protein expressions of caspase-3 decreased. The expression of GAP-43, a sprouting related protein, was decreased in the DEVD and siRNA group as compared to NS group (P<0.05). Up-regulation of GAP-43 following CCI was decreased following caspase-3 inhibition. Following sciatic nerve ligation, the gene expression, translation and transcription are significantly changed in the neurons which finally results in neuron apoptosis. The neuron apoptosis induce the up-regulation of GAP-43 expression leading to hyperalgesia.

CONCLUSION

Caspase-3 mediated neuron apoptosis is probably responsible for the neuropathic pain in CCI rats. Inhibition of caspase-3 may serve as a treatment of neuropathic pain.

Neonatal repetitive needle pricking: plasticity of the spinal nociceptive circuit and extended postoperative pain in later life

Repetitive exposure of neonates to noxious events is inherent to their health status monitoring in neonatal intensive care units (NICU). Altered basal nociception in the absence of an injury in later life has been demonstrated in ex-NICU children, but the impact on pain hypersensitivity following an injury in later life is unknown. Also, underlying mechanisms for such long-term changes are relatively unknown. The objective of this study is to investigate acute and long-term effects of neonatal repetitive painful skin-breaking procedures on nociception and to investigate plasticity of the nociceptive circuit. The repetitive needle prick animal model was used in which neonatal rats received four needle pricks into the left hind paw per day during the first postnatal week and control animals received nonpainful tactile stimuli. Repetitive needle pricking during the first week of life induced acute hypersensitivity to mechanical stimuli. At the age of 8 weeks, increased duration of postoperative hypersensitivity to mechanical stimuli after ipsilateral hind paw incision was shown in needle prick animals. Basal nociception from 3 to 8 weeks of age was unaffected by neonatal repetitive needle pricking. Increased calcitonin gene-related peptide expression was observed in the ipsilateral and contralateral lumbar spinal cord but not in the hind paw of needle prick animals at the age of 8 weeks. Innervation of tactile Aβ-fibers in the spinal cord was not affected. Our results indicate both acute and long-term effects of repetitive neonatal skin breaking procedures on nociception and long-term plasticity of spinal but not peripheral innervation of nociceptive afferents.