Loss of central inhibition: implications for behavioral hypersensitivity after contusive spinal cord injury in rats

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

Body Weight-Supported Treadmill Training Ameliorates Motoneuronal Hyperexcitability by Increasing GAD-65/67 and KCC2 Expression via TrkB Signaling in Rats with Incomplete Spinal Cord Injury

Spasticity is a typical consequence after spinal cord injury (SCI). The critical reasons are reducing the synthesis of Gamma-Aminobutyric Acid (GABA), glycine and potassium chloride co-transporter 2 (KCC2) inside the distal spinal cord. The current work aimed to test whether exercise training could increase the expression of glutamic acid decarboxylase 65/67 (GAD-65/67, the key enzymes in GABA synthesis) and KCC2 in the distal spinal cord via tropomyosin-related kinase B (TrkB) signaling. The experimental rats were randomly assigned to the following five groups: Sham, SCI/phosphate-buffered saline (PBS), SCI-treadmill training (TT)/PBS, SCI/TrkB-IgG, and SCI-TT/TrkB-IgG. After that, the model of T10 contusion SCI was used, then TrkB-IgG was used to prevent TrkB activity at 7 days post-SCI. Body weight-supported treadmill training started on the 8^th day post-SCI for four weeks. The Hmax/Mmax ratio and the rate-dependent depression of H-reflex were used to assess the excitability of spinal motoneuronal networks. Western blotting and Immunohistochemistry techniques were utilized for measuring the expression of GAD-65, GAD-67, and KCC2. The findings revealed that exercise training could reduce motoneuronal excitability and boost GAD-65, GAD-67, and KCC2 production in the distal region of the spinal cord after SCI. The effects of exercise training were decreased after the TrkB signaling was inhibited. The present exploration demonstrated that exercise training increases GAD-65, GAD-67, and KCC2 expression in the spinal cord via TrkB signaling and that this method could also improve rats with motoneuronal hyperexcitability and spasticity induced by incomplete SCI.

Exercise training modulates glutamic acid decarboxylase-65/67 expression through TrkB signaling to ameliorate neuropathic pain in rats with spinal cord injury

Neuropathic pain is one of the most frequently stated complications after spinal cord injury. In post-spinal cord injury, the decrease of gamma aminobutyric acid synthesis within the distal spinal cord is one of the main causes of neuropathic pain. The predominant research question of this study was whether exercise training may promote the expression of glutamic acid decarboxylase-65 and glutamic acid decarboxylase-67, which are key enzymes of gamma aminobutyric acid synthesis, within the distal spinal cord through tropomyosin-related kinase B signaling, as its synthesis assists to relieve neuropathic pain after spinal cord injury. Animal experiment was conducted, and all rats were allocated into five groups: Sham group, SCI/PBS group, SCI-TT/PBS group, SCI/tropomyosin-related kinase B-IgG group, and SCI-TT/tropomyosin-related kinase B-IgG group, and then T10 contusion SCI model was performed as well as the tropomyosin-related kinase B-IgG was used to block the tropomyosin-related kinase B activation. Mechanical withdrawal thresholds and thermal withdrawal latencies were used for assessing pain-related behaviors. Western blot analysis was used to detect the expression of brain-derived neurotrophic factor, tropomyosin-related kinase B, CREB, p-REB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord. Immunohistochemistry was used to analyze the distribution of CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord dorsal horn. The results showed that exercise training could significantly mitigate the mechanical allodynia and thermal hyperalgesia in post-spinal cord injury and increase the synthesis of brain-derived neurotrophic factor, tropomyosin-related kinase B, CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord. After the tropomyosin-related kinase B signaling was blocked, the analgesic effect of exercise training was inhibited, and in the SCI-TT/tropomyosin-related kinase B-IgG group, the synthesis of CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord were also significantly reduced compared with the SCI-TT/PBS group. This study shows that exercise training may increase the glutamic acid decarboxylase-65 and glutamic acid decarboxylase-67 expression within the spinal cord dorsal horn through the tropomyosin-related kinase B signaling, and this mechanism may play a vital role in relieving the neuropathic pain of rats caused by incomplete SCI.

Pain inhibition through transplantation of fetal neuronal progenitors into the injured spinal cord in rats

Neuropathic pain after spinal cord injury (SCI) is a complex condition that responds poorly to usual treatments. Cell transplantation represents a promising therapy; nevertheless, the ideal cell type in terms of neurogenic potential and effectiveness against pain remains largely controversial. Here, we evaluated the ability of fetal neural stem cells (fNSC) to relieve chronic pain and, secondarily, their effects on motor recovery. Adult Wistar rats with traumatic SCI were treated, 10 days after injury, with intra-spinal injections of culture medium (sham) or fNSCs extracted from telencephalic vesicles (TV group) or the ventral medulla (VM group) of E/14 embryos. Sensory (von Frey filaments and hot plate) and motor (the Basso, Beattie, Bresnahan locomotor rating scale and inclined plane test) assessments were performed during 8 weeks. Thereafter, spinal cords were processed for immunofluorescence and transplanted cells were quantified by stereology. The results showed improvement of thermal hyperalgesia in the TV and VM groups at 4 and 5 weeks after transplantation, respectively. Moreover, mechanical allodynia improved in both the TV and VM groups at 8 weeks. No significant motor recovery was observed in the TV or VM groups compared with sham. Stereological analyses showed that ~70% of TV and VM cells differentiated into NeuN⁺ neurons, with a high proportion of enkephalinergic and GABAergic cells in the TV group and enkephalinergic and serotoninergic cells in the VM group. Our study suggests that neuronal precursors from TV and VM, once implanted into the injured spinal cord, maturate into different neuronal subtypes, mainly GABAergic, serotoninergic, and enkephalinergic, and all subtypes alleviate pain, despite no significant motor recovery. The study was approved by the Animal Ethics Committee of the Medical School of the University of São Paulo (protocol number 033/14) on March 4, 2016.

Use of a combination strategy to improve neuroprotection and neuroregeneration in a rat model of acute spinal cord injury

Spinal cord injury is a very common pathological event that has devastating functional consequences in patients. In recent years, several research groups are trying to find an effective therapy that could be applied in clinical practice. In this study, we analyzed the combination of different strategies as a potential therapy for spinal cord injury. Immunization with neural derived peptides (INDP), inhibition of glial scar formation (dipyridyl: DPY), as well as the use of biocompatible matrix (fibrin glue: FG) impregnated with bone marrow mesenchymal stem cells (MSCs) were combined and then its beneficial effects were evaluated in the induction of neuroprotection and neuroregeneration after acute SCI. Sprague-Dawley female rats were subjected to a moderate spinal cord injury and then randomly allocated into five groups: 1) phosphate buffered saline; 2) DPY; 3) INDP + DPY; 4) DPY+ FG; 5) INDP + DPY + FG + MSCs. In all rats, intervention was performed 72 hours after spinal cord injury. Locomotor and sensibility recovery was assessed in all rats. At 60 days after treatment, histological examinations of the spinal cord (hematoxylin-eosin and Bielschowsky staining) were performed. Our results showed that the combination therapy (DPY+ INDP + FG + MSCs) was the best strategy to promote motor and sensibility recovery. In addition, significant increases in tissue preservation and axonal density were observed in the combination therapy group. Findings from this study suggest that the combination theapy (DPY+ INDP + FG + MSCs) exhibits potential effects on the protection and regeneration of neural tissue after acute spinal cord injury. All procedures were approved by the Animal Bioethics and Welfare Committee (approval No. 178544; CSNBTBIBAJ 090812960) on August 15, 2016.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

Botulinum Toxin for Central Neuropathic Pain

Botulinum toxin (BTX) is widely used to treat muscle spasticity by acting on motor neurons. Recently, studies of the effects of BTX on sensory nerves have been reported and several studies have been conducted to evaluate its effects on peripheral and central neuropathic pain. Central neuropathic pain includes spinal cord injury-related neuropathic pain, post-stroke shoulder pain, multiple sclerosis-related pain, and complex regional pain syndrome. This article reviews the mechanism of central neuropathic pain and assesses the effect of BTX on central neuropathic pain.

Cobra neurotoxin produces central analgesic and hyperalgesic actions via adenosine A1 and A2A receptors

Cobra neurotoxin, a short-chain peptide isolated from snake venom of Naja naja atra, showed both a central analgesic effect and a hyperalgesic effect in mice tests. In order to explore mechanisms, a hypothesis is put forward that cobra neurotoxin takes effect through adenosine receptor pathway. The central effects of cobra neurotoxin were evaluated using the hot plate test (a model of acute pain) and the spinal cord injury (a model of central pain) in mice and using A₁ receptor antagonist (DPCPX) and A_2A receptor antagonist (ZM241385); behaviors were scored and signal molecules such as reactive oxygen species and adenosine triphosphate levels and mitogen-activated protein kinases/extracellular signal-regulated protein kinase expression were measured. Low dose of cobra neurotoxin (25 µg/kg) had analgesic effects which were inhibited by DPCPX, while high dose of cobra neurotoxin (100 µg/kg) had hyperalgesic effects which were blocked by ZM241385. Cobra neurotoxin reduced reactive oxygen species and increased adenosine triphosphate in brain tissues, and extracellular signal-regulated protein kinase expression was markedly inhibited by cobra neurotoxin. Cobra neurotoxin may take effect through mitogen-activated protein kinases/extracellular signal-regulated protein kinase pathway inhibition by activating adenosine A₁Rs and cause changes of reactive oxygen species and adenosine triphosphate through feedback mechanisms. Overdose cobra neurotoxin further activates the adenosine A_2ARs to generate pain sensitization. This research proposes a new central analgesic mechanism of cobra neurotoxin and discloses dual regulation of pain.

CXCR4 antagonist AMD3100 elicits analgesic effect and restores the GlyRα3 expression against neuropathic pain

Objective

Chemokine CXCL12 and its receptor CXCR4 have been reported to play a critical role in neurogenesis and neuronal differentiation. Recently, some reports have implicated this chemokine signaling in the pathogenesis of many kinds of pain. However, its role in neuropathic pain (NP) is still largely unclear. This study explored the distribution and function of CXCR4 in spinal cord (SC) dorsal horn (DH) in a rat L5 spinal nerve ligation (SNL) model.

Methods

Rats received repeated intrathecal injection of CXCR4 antagonist AMD3100. Behavioral assessments were conducted using a traditional “up–down” method. The spinal CXCL12 contents were measured by enzyme linked immunosorbent assay. The expression and distribution of CXCR4 in the SC were determined by immunoflurescence and Western blot. GlyRα3 expressions were also measured by Western blot or immunofluorescence.

Results

SNL induced CXCL12–CXCR4 activation in the spinal DH. Intrathecal administration of AMD3100 alleviated the chronic NP against SNL (P<0.01). CXCR4 was colocalized with GlyRα3-positive neurons in the spinal DH at ratio >97%. Meanwhile, AMD3100 rescued the decrease of GlyRα3 expression (P<0.01 vs the SNL group on Day 14 and Day 21).

Conclusion

CXCR4 antagonist can elicit analgesic effects and restore the inhibitory neurotransmission such as GlyRα3 against NP.