Neuropathic Pain Mechanical Allodynia Research Articles

Involvement of the spinal γ-aminobutyric acid receptor in the analgesic effects of intrathecally injected hypertonic saline in spinal nerve-ligated rats.

: Hypertonic saline is used for treating chronic pain; however, clinical studies that aid in optimizing therapeutic protocols are lacking. We aimed to determine the concentration of intrathecally injected hypertonic saline at which the effect reaches its peak as well as the underlying γ-aminobutyric acid (GABA) receptor-related antinociceptive mechanism. : Spinal nerve ligation (SNL; left L5 and L6) was performed to induce neuropathic pain in rats weighing 250-300 g. Experiment 1: one week after implanting the intrathecal catheter, 60 rats were assigned randomly to intrathecal injection with 0.45%, 0.9%, 2.5%, 5%, 10%, and 20% NaCl, followed by behavioral testing at baseline and after 30 minutes, 2 hours, 1 day, and 1 week to determine the minimal concentration which produced maximal analgesia. Experiment 2: after determining the optimal intrathecal hypertonic saline concentration, 60 rats were randomly divided into four groups: Sham, hypertonic saline without pretreatment, and hypertonic saline after pretreatment with one of two GABA receptor antagonists (GABAA [bicuculline], or GABAB [phaclofen]). Behavioral tests were performed at weeks 1 and 3 following each treatment. : Hypertonic saline at concentrations greater than 5% alleviated SNL-induced mechanical allodynia and had a significant therapeutic effect, while showing a partial time- and dose-dependent antinociceptive effect on thermal and cold hyperalgesia. However, pretreatment with GABA receptor antagonists inhibited the antinociceptive effect of 5% NaCl. : This study indicates that the optimal concentration of hypertonic saline for controlling mechanical allodynia in neuropathic pain is 5%, and that its analgesic effect is related to GABAA and GABAB receptors.

Ginsenoside Rb1, a principal effective ingredient of Panax notoginseng, produces pain antihypersensitivity by spinal microglial dynorphin A expression

Panax notoginseng (Chinese ginseng, Sanqi), one of the major ginseng species, has been traditionally used to alleviate different types of chronic pain. The raw P. notoginseng powder is commonly available in China as a non-prescription drug to treat various aliments including arthritic pain. However, strong scientific evidence is needed to illustrate its pain antihypersensitive effects, effective ingredients and mechanism of action. The oral P. notoginseng powder dose-dependently alleviated formalin-induced tonic hyperalgesia, and its total ginsenosides remarkably inhibited neuropathic pain hypersensitivity. Ginsenoside Rb1, the most abundant ginsenoside of P. notoginseng, dose-dependently produced neuropathic pain antihypersensitivity. Conversely, ginsenosides Rg1, Re and notoginseng R1, the other major saponins from P. notoginseng, failed to inhibit formalin-induced tonic pain or mechanical allodynia in neuropathic pain. Ginsenoside Rb1 metabolites ginsenosides Rg3, Compound-K and protopanaxadiol also had similar antineuropathic pain efficacy to ginsenoside Rb1. Additionally, intrathecal ginsenoside Rb1 specifically stimulated dynorphin A expression which was colocalized with microglia but not neurons or astrocytes in the spinal dorsal horn and primary cultured cells. Pretreatment with microglial metabolic inhibitor minocycline, dynorphin A antiserum and specific κ-opioid receptor antagonist GNTI completely blocked Rb1-induced mechanical antiallodynia in neuropathic pain. Furthermore, the specific glucocorticoid receptor (GR) antagonist Dex-21-mesylate (but not GPR30 estrogen receptor antagonist G15) also entirely attenuated ginsenoside Rb1-related antineuropathic pain effects. All these results, for the first time, show that P. notoginseng alleviates neuropathic pain and ginsenoside Rb1 is its principal effective ingredient. Furthermore, ginsenoside Rb1 inhibits neuropathic pain by stimulation of spinal microglial dynorphin A expression following GR activation.

Spinal cord retinoic acid receptor signaling gates mechanical hypersensitivity in neuropathic pain

Central sensitization caused by spinal disinhibition is a key mechanism of mechanical allodynia in neuropathic pain. However, the molecular mechanisms underlying spinal disinhibition after nerve injury remain unclear. Here, we show in mice that spared nerve injury (SNI), which induces mechanical hypersensitivity and neuropathic pain, triggers homeostatic reduction of inhibitory outputs from dorsal horn parvalbumin-positive (PV+) interneurons onto both primary afferent terminals and excitatory interneurons. The reduction in inhibitory outputs drives hyperactivation of the spinal cord nociceptive pathway, causing mechanical hypersensitivity. We identified the retinoic acid receptor RARα, a central regulator of homeostatic plasticity, as the key molecular mediator for this synaptic disinhibition. Deletion of RARα in spinal PV+ neurons or application of an RARα antagonist in the spinal cord prevented the development of SNI-induced mechanical hypersensitivity. Our results identify RARα as a crucial molecular effector for neuropathic pain and a potential target for its treatment.

Sensory neurons have an axon initial segment that initiates spontaneous activity in neuropathic pain.

The axon initial segment is a specialized compartment of the proximal axon of CNS neurons where action potentials are initiated. However, it remains unknown whether this domain is assembled in sensory dorsal root ganglion neurons, in which spikes are initiated in the peripheral terminals. Here we investigate whether sensory neurons have an axon initial segment and if it contributes to spontaneous activity in neuropathic pain. Our results demonstrate that myelinated dorsal root ganglion neurons assemble an axon initial segment in the proximal region of their stem axon, enriched in the voltage-gated sodium channels Nav1.1 and Nav1.7. Using correlative immunofluorescence and calcium imaging, we demonstrate that the Nav1.7 channels at the axon initial segment are associated with spontaneous activity. Computer simulations further indicate that the axon initial segment plays a key role in the initiation of spontaneous discharges by lowering their voltage threshold. Finally, using a Cre-based mouse model for time-controlled axon initial segment disassembly, we demonstrate that this compartment is a major source of spontaneous discharges causing mechanical allodynia in neuropathic pain. Thus, an axon initial segment domain is present in sensory neurons and facilitates their spontaneous activity. This study provides a new insight in the cellular mechanisms that cause pathological pain and identifies a new potential target for chronic pain management.

GPR177 in A-fiber sensory neurons drives diabetic neuropathic pain via WNT-mediated TRPV1 activation.

Diabetic neuropathic pain (DNP) is a common and devastating complication in patients with diabetes. The mechanisms mediating DNP are not completely elucidated, and effective treatments are lacking. A-fiber sensory neurons have been shown to mediate the development of mechanical allodynia in neuropathic pain, yet the molecular basis underlying the contribution of A-fiber neurons is still unclear. Here, we report that the orphan G protein-coupled receptor 177 (GPR177) in A-fiber neurons drives DNP via WNT5a-mediated activation of transient receptor potential vanilloid receptor-1 (TRPV1) ion channel. GPR177 is mainly expressed in large-diameter A-fiber dorsal root ganglion (DRG) neurons and required for the development of DNP in mice. Mechanistically, we found that GPR177 mediated the secretion of WNT5a from A-fiber DRG neurons into cerebrospinal fluid (CSF), which was necessary for the maintenance of DNP. Extracellular perfusion of WNT5a induced rapid currents in both TRPV1-expressing heterologous cells and nociceptive DRG neurons. Computer simulations revealed that WNT5a has the potential to bind the residues at the extracellular S5-S6 loop of TRPV1. Using a peptide able to disrupt the predicted WNT5a/TRPV1 interaction suppressed DNP- and WNT5a-induced neuropathic pain symptoms in rodents. We confirmed GPR177/WNT5A coexpression in human DRG neurons and WNT5A secretion in CSF from patients with DNP. Thus, our results reveal a role for WNT5a as an endogenous and potent TRPV1 agonist, and the GPR177-WNT5a-TRPV1 axis as a driver of DNP pathogenesis in rodents. Our findings identified a potential analgesic target that might relieve neuropathic pain in patients with diabetes.

5-HT1F Receptor Agonist Ameliorates Mechanical Allodynia in Neuropathic Pain via Induction of Mitochondrial Biogenesis and Suppression of Neuroinflammation.

Neuropathic pain is a devastating disease that affects millions of people worldwide. Serotonin (5-hydroxytryptamine, 5-HT) is involved in pain modulation. Several lines of evidence have indicated that 5-HT1F receptor agonists are potent inducers of mitochondrial biogenesis. In this study, we tested the hypothesis that 5-HT1F receptor agonists ameliorate mechanical allodynia in neuropathic pain via the induction of mitochondrial biogenesis and suppression of neuroinflammation. Male Sprague–Dawley rats were used to establish a neuropathic pain model via spared nerve injury (SNI). The paw withdrawal threshold (PWT) was used to evaluate mechanical allodynia. Real-time polymerase chain reaction was used to examine the mitochondrial DNA (mtDNA) copy number. Western blotting and immunofluorescence were used to examine the expression of target proteins. Our results showed that mitochondrial biogenesis was impaired in the spinal cord of rats with SNI. Moreover, activation of PGC-1α, the master regulator of mitochondrial biogenesis, attenuates established mechanical allodynia in rats with neuropathic pain. In addition, the neuronal 5-HT1F receptor is significantly downregulated in the spinal cord of rats with neuropathic pain. Furthermore, the selective 5-HT1F receptor agonist lasmiditan attenuated established mechanical allodynia in rats with neuropathic pain. Finally, lasmiditan (Las) treatment restored mitochondrial biogenesis and suppressed neuroinflammation in the spinal cord of rats with SNI. These results provide the first evidence that lasmiditan ameliorates mechanical allodynia in neuropathic pain by inducing mitochondrial biogenesis and suppressing neuroinflammation in the spinal cord. Inducers of mitochondrial biogenesis may be an encouraging therapeutic option for the management of neuropathic pain.

ErbB4 in Spinal PV Interneurons Regulates Mechanical Allodynia in Neuropathic Pain via Modulation of Glycinergic Inhibitory Tone.

BackgroundMechanical allodynia is the most common and challenging symptom associated with neuropathic pain; however, the underlying mechanisms are still unclear. The aim of this study was to investigate whether ErbB4, a receptor for neuregulin-1 (NRG1), participates in the modulation of mechanical allodynia.MethodsRadiant heat and von Frey filaments were applied to assess nociceptive behaviors. Real-time quantitative polymerase chain reaction, Western blotting, immunofluorescence, and small interfering RNA were used to identify the likely mechanisms.ResultsErbB4 was rapidly and persistently activated in spinal parvalbumin (PV) interneurons after chronic constriction injury (CCI) in mice. Knockdown of ErbB4 in the spinal cord prevented and reversed CCI-induced mechanical allodynia, and activation of ErbB4 by spinal application of NRG1 induced mechanical allodynia in naïve mice. Furthermore, we found that activation of ErbB4 decreased the glycine concentration in the spinal cord, contributing to modulation of mechanical allodynia.ConclusionErbB4 in spinal PV interneurons gates mechanical allodynia in neuropathic pain via regulation of glycinergic inhibitory tone, suggesting that a possible ErbB4-mediated process participates in the development of neuropathic pain.

Disinhibition of Touch-Evoked Itch in a Mouse Model of Psoriasis

Disinhibition of Touch-Evoked Itch in a Mouse Model of Psoriasis

Peripheral serotonin receptor 2B and transient receptor potential channel 4 mediate pruritus to serotonergic antidepressants in mice

Peripheral serotonin receptor 2B and transient receptor potential channel 4 mediate pruritus to serotonergic antidepressants in mice

Repetitive motor cortex stimulation reinforces the pain modulation circuits of peripheral neuropathic pain

Recent evidence indicates that motor cortex stimulation (MCS) is a potentially effective treatment for chronic neuropathic pain. However, the neural mechanisms underlying the attenuated hyperalgesia after MCS are not completely understood. In this study, we investigated the neural mechanism of the effects of MCS using an animal model of neuropathic pain. After 10 daily sessions of MCS, repetitive MCS reduced mechanical allodynia and contributed to neuronal changes in the anterior cingulate cortex (ACC). Interestingly, inhibition of protein kinase M zeta (PKMζ), a regulator of synaptic plasticity, in the ACC blocked the effects of repetitive MCS. Histological and molecular studies showed a significantly increased level of glial fibrillary acidic protein (GFAP) expression in the ACC after peripheral neuropathy, and neither MCS treatment nor ZIP administration affected this increase. These results suggest that repetitive MCS can attenuate the mechanical allodynia in neuropathic pain, and that the activation of PKMζ in the ACC may play a role in the modulation of neuropathic pain via MCS.

Analgesic Effect of Moxibustion with Different Temperature on Inflammatory and Neuropathic Pain Mice: A Comparative Study.

The aim of this study was to determine whether variation of temperature during moxibustion would generate division of analgesic effect. The moxibustion with different temperatures (37°C, 42°C, 47°C, and 52°C) was applied to ST36 acupoint for 30 minutes in chronic inflammatory or neuropathic pain mice. The analgesic effect was evaluated by thermal hyperalgesia test in chronic inflammatory pain and by mechanical allodynia in neuropathic pain, respectively. The results indicated that interventions of moxibustion with different temperature caused different analgesic effect on either chronic inflammatory induced by injection of complete Freund's adjuvant (CFA) or neuropathic pain induced by spared nerve injury (SNI). In chronic inflammatory pain, different moxibustion temperature generated different intensity of analgesic effect: the higher the better. In chronic neuropathic pain, stronger analgesic effect was found in moxibustion with temperature 47°C or 52°C other than 37°C and 42°C. However, there is no significant difference displayed between moxibustion temperatures 47°C and 52°C or 37°C and 42°C. It implies that the temperature should be taken into account for moxibustion treatment to chronic inflammatory or neuropathic pain.

Elevated Neurosteroids in the Lateral Thalamus Relieve Neuropathic Pain in Rats with Spared Nerve Injury.

Neurosteroidsare synthesized in the nervous system from cholesterol or steroidal precursors imported from peripheral sources. These compounds are important allosteric modulators of γ-aminobutyric acid A receptors (GABAARs), which play a vital role in pain modulation in the lateral thalamus, a main gate where somatosensory information enters the cerebral cortex. Using high-performance liquid chromatography/tandem mass spectrometry, we found increased levels of neurosteroids (pregnenolone, progesterone, deoxycorticosterone, allopregnanolone, and tetrahydrodeoxycorticosterone) in the chronic stage of neuropathic pain (28days after spared nerve injury) in rats. The expression of the translocator protein TSPO, the upstream steroidogenesis rate-limiting enzyme, increased at the same time. In vivo stereotaxic microinjection of neurosteroids or the TSPO activator AC-5216 into the lateral thalamus (AP -3.0 mm, ML ±3.0 mm, DV 6.0 mm) alleviated the mechanical allodynia in neuropathic pain, while the TSPO inhibitor PK 11195 exacerbated it. The analgesic effects of AC-5216 and neurosteroids were significantly attenuated by the GABAAR antagonist bicuculline. These results suggested that elevated neurosteroids in the lateral thalamus play a protective role in the chronic stage of neuropathic pain.

Inhibition of mechanical allodynia in neuropathic pain by TLR5-mediated A-fiber blockade.

Mechanical allodynia, induced by normally innocuous low-threshold mechanical stimulation, represents a cardinal feature of neuropathic pain. Blockade or ablation of high-threshold, small-diameter unmyelinated group C nerve fibers (C-fibers) has limited effects on mechanical allodynia. Although large, myelinated group A fibers, in particular Aβ-fibers, have previously been implicated in mechanical allodynia, an A-fiber-selective pharmacological blocker is still lacking. Here we report a new method for targeted silencing of A-fibers in neuropathic pain. We found that Toll-like receptor 5 (TLR5) is co-expressed with neurofilament-200 in large-diameter A-fiber neurons in the dorsal root ganglion (DRG). Activation of TLR5 with its ligand flagellin results in neuronal entry of the membrane-impermeable lidocaine derivative QX-314, leading to TLR5-dependent blockade of sodium currents, predominantly in A-fiber neurons of mouse DRGs. Intraplantar co-application of flagellin and QX-314 (flagellin/QX-314) dose-dependently suppresses mechanical allodynia after chemotherapy, nerve injury, and diabetic neuropathy, but this blockade is abrogated in Tlr5-deficient mice. In vivo electrophysiology demonstrated that co-application of flagellin/QX-314 selectively suppressed Aβ-fiber conduction in naive and chemotherapy-treated mice. TLR5-mediated Aβ-fiber blockade, but not capsaicin-mediated C-fiber blockade, also reduced chemotherapy-induced ongoing pain without impairing motor function. Finally, flagellin/QX-314 co-application suppressed sodium currents in large-diameter human DRG neurons. Thus, our findings provide a new tool for targeted silencing of Aβ-fibers and neuropathic pain treatment.

Mechanical allodynia in neuropathic pain. Where are the brain representations located? A positron emission tomography (PET) study

Brain areas involved in nociception have been repeatedly investigated. Therefore, brain responses to physiological pain conditions are well identified. The same is not true for allodynic pain in patients with neuropathic pain since the cortical reorganizations that are involved in the conversion of non-noxious stimuli into painful sensations still remain unknown. The present positron emission tomography (PET) study enrolled 19 patients with dynamic mechanical allodynia to brushing or to cold rubbing of the skin. PET activations during allodynic stimulation were compared to those obtained with the same innocuous stimulation applied outside the neuropathic pain area (control). In a second comparison, they were compared with responses to a noxious heat stimulation applied outside the neuropathic pain area (experimental pain). Common responses to allodynia and control stimulations were found in contralateral SI, SII and insula and in ipsilateral cerebellum. Not surprisingly, heat pain condition was associated with activations in contralateral prefrontal and SII cortices and, bilaterally, in the anterior insular cortices. Distinctive cortical responses between control and allodynic conditions were restricted to one activation within the contralateral anterior insula, a region also activated by experimental heat pain. The insular subdivision was inappropriately activated considering the innocuous nature of the stimulus, but adequately activated with regard to pain-evoked sensation. Subcortically, the hypothesis of reorganization at any level of the somatosensory and pain pathways underlying such insular activity was supported by the observed shift of thalamic activation from a lateral-posterior to an anterior-medial position.

Sigma-1 receptor-mediated increase in spinal p38 MAPK phosphorylation leads to the induction of mechanical allodynia in mice and neuropathic rats

The direct activation of the spinal sigma-1 receptor (Sig-1R) produces mechanical allodynia (MA) and thermal hyperalgesia (TH) in mice. In addition, the blockade of the spinal Sig-1R prevents the induction of MA, but not TH in chronic constriction injury (CCI)-induced neuropathic rats. The present study was designed to investigate whether the increase in spinal p38 MAPK phosphorylation (p-p38 MAPK) mediates Sig-1R-induced MA or TH in mice and the induction of MA in neuropathic rats. MA and TH were evaluated using von Frey filaments and a hot-plate apparatus, respectively. Neuropathic pain was produced by CCI of the right sciatic nerve in rats. Western blot assay and immunohistochemistry were performed to determine the changes of p-p38 MAPK expression in the spinal cord. Intrathecal (i.t.) injection of PRE084, a selective Sig-1R agonist, into naïve mice time-dependently increased the expression of p-p38 MAPK, which was blocked by pretreatment with BD1047, a Sig-1R antagonist. I.t. pretreatment with SB203580, a p38 MAPK inhibitor also dose-dependently inhibited PRE084-induced MA, whereas TH induction was not affected. In CCI rats, i.t. injection of BD1047 during the induction phase (postoperative days 0 to 5) reduced the CCI-induced increase in p-p38 MAPK. In addition, i.t. SB203580 treatment during the induction phase also suppressed the development of CCI-induced MA, but not TH. Conversely, i.t. SB203580 treatment during the maintenance phase (postoperative days 15 to 20) had no effect on CCI-induced MA or TH. These results demonstrate that the increase in spinal p-p38 MAPK is closely associated with the induction of Sig-1R mediated MA, but not TH. Sigma-1 receptor modulation of p-p38 MAPK also plays an important role in the induction, but not the maintenance, of MA in neuropathic pain.

Activation of notch signaling mediates the induction and maintenance of mechanical allodynia in a rat model of neuropathic pain.

Neuropathic pain is a major health problem caused by dysfunction or damage of the nerve fibers in the peripheral or central nervous system. Notch signaling is a highly conserved evolutionary pathway, which regulates the fate of cells in the developing nervous system and is important in synaptic plasticity and inflammation in the adult central nervous system. The aim of the present study was to investigate the potential roles of the notch signaling pathway in the induction and maintenance of mechanical allodynia in neuropathic pain. Neuropathic pain was induced through spared nerve injury (SNI) in rats. DAPT, a γ-secretase inhibitor of the notch signaling pathway, was intrathecally administered at different concentrations (5, 15, 50 and 150 µM), and time-points. In addition, Jagged-1 (JAG-1) peptide, a ligand of the notch signaling pathway, was administered intrathecally to normal rats. The mechanical allodynia was assessed using a von Frey test. The results demonstrated that administering DAPT prior to the appearance of pain sensitivity significantly prevented the decrease of mechanical paw withdrawal threshold (PWT) for >4 weeks (P<0.05 vs. SNI group). Administering DAPT following the appearance of pain sensitivity significantly reversed the decrease of mechanical PWT (P<0.05 vs. SNI group). Furthermore, early and late administration of DAPT resulted in dose-dependent antinociceptive effects. In addition, administration of JAG-1 induced a dose-dependent increase in the mechanical PWT of the normal rats. In conclusion, the results of the present study demonstrated that activation of notch signaling contributed to the induction and maintenance of mechanical allodynia in neuropathic pain, suggesting a potential novel therapeutic target for the treatment of neuropathic pain.

Brain-derived neurotrophic factor–activated astrocytes produce mechanical allodynia in neuropathic pain

Neuropathic pain management is challenging for physicians and a vexing problem for basic researchers. Recent studies reveal that activated spinal astrocytes may play a vital role in nerve injury-induced neuropathic pain, although the mechanisms are not fully understood. We have found increased glial fibrillary acidic protein (GFAP) expression, a hallmark of reactive gliosis, and elevated brain-derived neurotrophic factor (BDNF) expression in the dorsal horn in a rat model of allodynia induced by spinal nerve ligation (SNL). The high GFAP expression and mechanical allodynia that SNL induces were prevented by the intrathecal injection of the BDNF-sequestering fusion protein TrkB/Fc. Additionally, mechanical allodynia and GFAP overexpression was induced by the spinal administration of exogenous BDNF to naive rats, and exogenous BDNF given together with fluorocitrate, an astrocytic metabolism inhibitor, inhibited allodynia and GFAP upregulation. Exogenous BDNF also activated the astrocytes directly when tested in vitro. Furthermore, intrathecal administration of BDNF-stimulated astrocytes also induced mechanical allodynia in naive rats. All of these results indicate that astrocytes activated by BDNF might contribute to mechanical allodynia development in neuropathic pain in rats.

Exposure to inhomogeneous static magnetic field ceases mechanical allodynia in neuropathic pain in mice

Magnetic therapy as a self-care intervention has led to the conduct of numerous human trials and animal experiments. Results concerning the analgesic efficacy of magnetic exposure, however, are inconsistent. By using a magnetic device generating an inhomogeneous static magnetic field (iSMF), here we studied how the whole-body exposure to iSMF may influence the mechanical withdrawal threshold (MWT) of the hind paw in different stages of neuropathic pain evoked by partial ligation of the sciatic nerve in mice. It was found that iSMF exposure did not prevent the decrease of MWT in the first postoperative week. A 2-week long iSMF treatment that was started just after the nerve ligation elevated MWT values to a modest extent. However, the effectiveness of a daily exposure to iSMF was much more prominent when it was applied between postoperative days 15 and 28. In this case, MWT was already noticeably increased after the first treatment and it practically reached the control values by the end of the 2-week long exposure period. The results suggest that exposure to iSMF cannot prevent the development of mechanical allodynia, but can inhibit processes that maintain the increased sensitivity to mechanical stimuli in neuropathic pain.

Dynamic mechanical allodynia in neuropathic pain

Thirty-four patients with various forms of neuropathic pain have been examined with respect to two parameters of dynamic mechanical allodynia: the effect of repetitive stimulation on pain intensity; and refractory period. Pain intensity increased with repetitive stimulation (‘windup’) in most patients with neuropathic pain of peripheral origin, while it was not observed in patients with central neuropathic pain. While a non-responsive period occurs after tactile allodynic elicitation in patients with trigeminal neuralgia ( Kugelberg and Lindblom, 1959), it was not seen in any case of neuropathic pain, including trigeminal neuropathy. The findings have implications for diagnosis, and require pathophysiological elucidation in terms of revealed differences.