Rosuvastatin Synergistically Enhances the Antinociceptive Efficacy of Duloxetine in Paclitaxel-Induced Neuropathic Pain in Mice.

Paclitaxel, a widely used cancer chemotherapeutic agent, has high incidence of neurotoxicity associated with the production of neuropathic pain, for which only duloxetine has shown significant but moderate analgesic effect. Since statins, classically used to reduce hypercholesterolemia, have shown antinociceptive effect in preclinical studies on neuropathic pain, we studied whether the antinociceptive efficacy of duloxetine could be synergistically potentiated by rosuvastatin in a model of paclitaxel-induced neuropathy in mice. The astrocytic and microglial responses in the spinal cord of paclitaxel-treated mice were also assessed by measuring GFAP and CD11b proteins, respectively. Paclitaxel treatment did not impair motor coordination and balance in rotarod testing. Rosuvastatin, duloxetine, and the rosuvastatin/duloxetine combination (combined at equieffective doses) dose-dependently decreased mechanical allodynia (ED30, von Frey testing) and thermal hyperalgesia (ED50, hot plate testing) in paclitaxel-treated mice. Isobolographic analysis showed a superadditive interaction for rosuvastatin and duloxetine, as both the ED30 and ED50 for the rosuvastatin/duloxetine combination contained only a quarter of each drug compared to the individual drugs. The rosuvastatin/duloxetine combination reversed paclitaxel-induced GFAP overexpression, indicating that such effects might depend in part on astrocyte inactivation. Results suggest that statins could be useful in synergistically enhancing the efficacy of duloxetine in some chemotherapy-induced neuropathic conditions.

NMDA and P2X7 Receptors Require Pannexin 1 Activation to Initiate and Maintain Nociceptive Signaling in the Spinal Cord of Neuropathic Rats.

Pannexin 1 (Panx1) is involved in the spinal central sensitization process in rats with neuropathic pain, but its interaction with well-known, pain-related, ligand-dependent receptors, such as NMDA receptors (NMDAR) and P2X7 purinoceptors (P2X7R), remains largely unexplored. Here, we studied whether NMDAR- and P2X7R-dependent nociceptive signaling in neuropathic rats require the activation of Panx1 channels to generate spinal central sensitization, as assessed by behavioral (mechanical hyperalgesia) and electrophysiological (C-reflex wind-up potentiation) indexes. Administration of either a selective NMDAR agonist i.t. (NMDA, 2 mM) or a P2X7R agonist (BzATP, 150 µM) significantly increased both the mechanical hyperalgesia and the C-reflex wind-up potentiation, effects that were rapidly reversed (minutes) by i.t. administration of a selective pannexin 1 antagonist (10panx peptide, 300 µM), with the scores even reaching values of rats without neuropathy. Accordingly, 300 µM 10panx completely prevented the effects of NMDA and BzATP administered 1 h later, on mechanical hyperalgesia and C-reflex wind-up potentiation. Confocal immunofluorescence imaging revealed coexpression of Panx1 with NeuN protein in intrinsic dorsal horn neurons of neuropathic rats. The results indicate that both NMDAR- and P2X7R-mediated increases in mechanical hyperalgesia and C-reflex wind-up potentiation require neuronal Panx1 channel activation to initiate and maintain nociceptive signaling in neuropathic rats.

Stage-dependent C-reflex, pain-like behavior and opioid analgesia during the induction of chronic arthritis in rats.

Chronic arthritis (CA) is a common clinical entity associated with persistent pain and limited response to opioid analgesic therapy. However, it is unknown whether these features of CA change depending on its stage of evolution. To address this, in a well-established animal model of CA we studied the time course of electromyographic responses to electrical stimulation of C fibers (C-reflex), pain-like behavior as a response to mechanical nociceptive stimulation, and the inhibition of both responses by a prototypic opioid analgesic, morphine. To induce CA, rats received a single injection of complete Freund's adjuvant into the ankle joint and the C-reflex responses to electrical stimuli or the nociceptive response to paw pressure test were studied 2, 4 or 6 weeks later. The C-reflexes evoked by threshold and supra-threshold electrical stimulation exhibited progressive increases together with enhancement of the nociceptive behavior to mechanical stimulation during induction of monoarthritis. Notably, while systemic morphine produced antinociceptive effects upon both experimental approaches, the effects were markedly reduced during the early stages of CA but enhanced at later stages. These data indicate that C-reflex and pain-like responses evolve in parallel, and are inhibited by morphine in a stage-dependent manner through the induction of CA. The present results may contribute to explain the enhanced pain response and variable analgesic efficacy of opioids that characterize arthritic pain in humans.

Antinociceptive effect of rat D-serine racemase inhibitors, L-serine-O-sulfate, and L-erythro-3-hydroxyaspartate in an arthritic pain model.

N-methyl-D-aspartic acid receptor (NMDAr) activation requires the presence of D-serine, synthesized from L-serine by a pyridoxal 5'-phosphate-dependent serine racemase (SR). D-serine levels can be lowered by inhibiting the racemization of L-serine. L-serine-O-sulfate (LSOS) and L-erythro-3-hydroxyaspartate (LEHA), among others, have proven to be effective in reducing the D-serine levels in culture cells. It is tempting then to try these compounds in their effectiveness to decrease nociceptive levels in rat arthritic pain. We measured the C-reflex paradigm and wind-up potentiation in the presence of intrathecally injected LSOS (100 µg/10 µL) and LEHA (100 µg/10 µL) in normal and monoarthritic rats. Both compounds decreased the wind-up activity in normal and monoarthritic rats. Accordingly, all the antinociceptive effects were abolished when 300 µg/10 µL of D-serine were injected intrathecally. Since no in vivo results have been presented so far, this constitutes the first evidence that SR inhibitions lower the D-serine levels, thus decreasing the NMDAr activity and the consequent development and maintenance of chronic pain.

Antihyperalgesic effects of clomipramine and tramadol in a model of posttraumatic trigeminal neuropathic pain in mice.

AIMS: To develop a behavioral model in mice that is capable of mimicking some distinctive symptoms of human posttraumatic trigeminal neuropathic pain such as spontaneous pain, cold allodynia, and chemical÷inflammatory hyperalgesia, and to use this model to investigate the antinociceptive effects of clomipramine and tramadol, two drugs used for the treatment of neuropathic pain. METHODS: A partial tight ligature of the right infraorbital nerve by an intraoral access or a sham procedure was performed. Fourteen days later, mice were subcutaneously injected with saline or drugs and the spontaneous nociceptive behavior, as well as the responses to topical acetone and to formalin or capsaicin injected into the ipsilateral vibrissal pad, were assessed. Data were analyzed by ANOVA. RESULTS: Neuropathic mice exhibited an increased spontaneous rubbing÷scratching of the ipsilateral vibrissal pad, together with enhanced responses to cooling (acetone) and the chemical irritants (formalin, capsaicin). Clomipramine and tramadol produced an antihyperalgesic effect on most of these nociceptive responses, but tramadol was ineffective on capsaicin-induced hyperalgesia. CONCLUSION: Nociceptive responses in this neuropathic pain model in mice exhibited a pattern consistent with the pain described by posttraumatic trigeminal neuropathic patients. The selective antihyperalgesic effect obtained with two commonly used drugs for treating neuropathic pain confirms the validity of this preclinical model.

Magnesium Salt, a Simple Strategy to Improve Methadone Analgesia in Chronic Pain: An Isobolographic Preclinical Study in Neuropathic Mice.

Analgesic efficacy of methadone in cancer and chronic non-cancer pains is greater than that of other opioids, probably because of its unique pharmacokinetics properties and also because it targets glutamatergic receptors in addition to µ-opioid receptors. However, methadone has drawbacks which are clearly related to dosing and treatment duration. The authors hypothesized that the antinociceptive efficacy of methadone could be synergistically potentiated by magnesium and copper salts in a preclinical mouse model of chronic pain, using the intraplantar formalin test as algesimetric tool. The spared nerve injury mice model was used to generate mononeuropathy. A low dose (0.25%) formalin was injected in the neuropathic limb in order to give rise only to Phase I response, resulting from direct activation by formalin of nociceptive primary afferents. Licking/biting of the formalin-injected limb was evaluated as nociceptive behavior during a 35-min observation period. Dose-response curves for intraperitoneal magnesium sulfate (10, 30, 100, and 300 mg/kg i.p.), copper sulfate (0.1, 0.3, 1, and 3 mg/kg i.p.) and methadone (0.1, 0.3, 1, and 3 mg/kg i.p.) allowed to combine them in equieffective doses and to determine their interaction by isobolographic analysis. Magnesium sulfate, copper sulfate and methadone dose-dependently decreased the nociceptive response evoked by formalin injection, the respective ED50 being 76.38, 1.18, and 0.50 mg/kg i.p. Isobolographic analysis showed a superadditive interaction for magnesium and methadone. Indeed, despite that both ED50 are obviously equieffective, the ED50 for the MgSO4/methadone combination contained less than one third of the methadone having the ED50 for methadone alone. For the CuSO4/methadone combination, the interaction was only additive. Extrapolated to clinical settings, the results suggest that magnesium salts might be used to improve synergistically the efficacy of methadone in neuropathy, which would allow to reduce the dose of methadone and its associated side effects.

The antinociceptive effect of resveratrol in bone cancer pain is inhibited by the Silent Information Regulator 1 inhibitor selisistat.

OBJECTIVES: To study the antinociceptive effect of single and repeated doses of resveratrol in a bone cancer pain model, and whether this effect is prevented by the Silent Information Regulator 1 (SIRT1) inhibitor selisistat. METHODS: The femoral intercondylar bone of BALB/c mice was injected with 1 000 000 BJ3Z cancer cells. Bone resorption and tumour mass growth (measured by in vivo X-ray and fluorescence imaging), as well as mechanical nociceptive thresholds (von Frey device) and dynamic functionality (rotarod machine), were evaluated during the following 4 weeks. Acute resveratrol (100 mg/kg i.p.) and/or selisistat (10 mg/kg s.c.) were administered on day 14. Chronic resveratrol (100 mg/kg i.p., daily) and/or selisistat (0.5 µg/h s.c., Alzet pump) were administered between days 14 and 20. KEY FINDINGS: Tumour growth gradually incremented until day 31, while mechanical hyperalgesia started on day 3 after cancer cell injection. Acute resveratrol increased the mechanical threshold of pain (peaking at 1.5 h), while the dynamic functionality decreased. Chronic resveratrol produced a sustained antinociceptive effect on mechanical hyperalgesia and improved the loss of dynamic functionality induced by the bone cancer tumour. Selisistat prevented all the effects of resveratrol. CONCLUSIONS: Acute and chronic resveratrol induces antinociceptive effect in the model of metastatic osseous oncological pain, an effect that would be mediated by SIRT1 molecular signalling.

A pH-responsive nanoparticle targets the neurokinin 1 receptor in endosomes to prevent chronic pain.

Nanoparticle-mediated drug delivery is especially useful for targets within endosomes because of the endosomal transport mechanisms of many nanomedicines within cells. Here, we report the design of a pH-responsive, soft polymeric nanoparticle for the targeting of acidified endosomes to precisely inhibit endosomal signalling events leading to chronic pain. In chronic pain, the substance P (SP) neurokinin 1 receptor (NK1R) redistributes from the plasma membrane to acidified endosomes, where it signals to maintain pain. Therefore, the NK1R in endosomes provides an important target for pain relief. The pH-responsive nanoparticles enter cells by clathrin- and dynamin-dependent endocytosis and accumulate in NK1R-containing endosomes. Following intrathecal injection into rodents, the nanoparticles, containing the FDA-approved NK1R antagonist aprepitant, inhibit SP-induced activation of spinal neurons and thus prevent pain transmission. Treatment with the nanoparticles leads to complete and persistent relief from nociceptive, inflammatory and neuropathic nociception and offers a much-needed non-opioid treatment option for chronic pain.

Antinociception Induced by Copper Salt Revisited: Interaction with Ketamine in Formalin-Induced Intraplantar and Orofacial Pain in Mice.

AIMS: To evaluate in mice the antinociceptive effect of copper in spinal and trigeminal nociceptive pathways by using the intraplantar and orofacial formalin tests, respectively, and to examine whether this effect may interact synergistically with ketamine-induced antinociception. METHODS: Nociceptive behaviors (licking/biting of the formalin-injected limb and rubbing/scratching of the formalin-injected orofacial area) in male mice were evaluated during a 45-minute observation period post-formalin injection. Dose-response curves for intraperitoneal (ip) copper sulfate and ketamine allowed their combination in equi-effective doses, and their interaction was determined with isobolographic analysis. The results were examined with one-way analysis of variance followed by the Bonferroni post hoc test. Significance was accepted at an alpha level of .05. RESULTS: Irrespective of the region injected with formalin (upper lip or hindlimb), copper sulfate (0.3, 1.0, and 3.0 mg/kg) and ketamine (1.0, 3.0, and 10 mg/kg) dose-dependently decreased the nociceptive behaviors evoked by formalin injection. Isobolographic analysis showed a superadditive interaction between copper and ketamine at the spinal level, but this interaction was only additive at the trigeminal level. CONCLUSION: The results suggest that copper salts could be used to synergistically improve the efficacy of some commercial centrally acting analgesic agents, such as ketamine, while reducing the possibility of side effects. However, a synergistic effect probably should not be expected if treatment is for orofacial pain.

Burst-Like Subcutaneous Electrical Stimulation Induces BDNF-Mediated, Cyclotraxin B-Sensitive Central Sensitization in Rat Spinal Cord.

Intrathecal administration of brain derived neurotrophic factor (BDNF) induces long-term potentiation (LTP) and generates long-lasting central sensitization in spinal cord thus mimicking chronic pain, but the relevance of these observations to chronic pain mechanisms is uncertain. Since C-fiber activation by a high-frequency subcutaneous electrical stimulation (SES) protocol causes spinal release of BDNF and induces spinal cord LTP, we propose that application of such protocol would be a sufficient condition for generating long-lasting BDNF-mediated central sensitization. Results showed that application of burst-like SES to rat toes produced (i) rapid induction of hyperalgesia that lasted for more than 3 weeks, (ii) early increase of C-reflex activity followed by increased wind-up scores lasting for more than 1 week, and (iii) early increase followed by late decrease in BDNF protein levels and phosphorylated TrkB that lasted for more than 1 week. These changes were prevented by the TrkB antagonist cyclotraxin-B administered shortly before SES, while hyperalgesia was reversed by cyclotraxin-B administered 3 days after SES. Results suggest that mechanisms underlying central sensitization first involve BDNF release of probably neuronal origin, followed by brief increased expression of likely glial BDNF and pTrkB that could switch early phase sensitization into late one.

Biphasic effect of apomorphine on rat nociception and effect of dopamine D2 receptor antagonists.

Studies on the effect of dopaminergic agonists in behavioral measures of nociception have gathered numerous but rather conflicting data. We studied the effects of the D(1)/D(2) receptor agonist apomorphine, as well as the modulatory effects of (S)-(-)-sulpiride (selective D(2) receptor antagonist) and domperidone (peripheral D(2) receptor antagonist), on thermal, mechanical and chemical nociception on rats. Apomorphine induced a biphasic dose-response relationship, low doses producing hyperalgesia and high doses inducing antinociception. Tonic (chemical) pain was more sensitive to apomorphine than phasic (thermal and mechanical thresholds) pain. (S)-(-)-sulpiride, but not domperidone, fully antagonized the antinociceptive effect of apomorphine in all three measures of nociception, pointing to a participation of D(2) dopaminergic receptors for the antinociceptive action of apomorphine. Although spinal sites for dopaminergic ligands mechanistically may account for the effects observed, involvement of dopaminergic receptors of the forebrain could probably explain better the antinociceptive effects of apomorphine, especially in chemical tonic pain.

The orofacial capsaicin test in rats: effects of different capsaicin concentrations and morphine.

The aim of this study was to develop a rat model of capsaicin-induced pain in the orofacial region. We examined the effects of subcutaneous injection of different doses of capsaicin (0.25, 0.4, 0.8, 1.5, 2.5, 25, 50, 100, 500 microg) on the face-grooming response. Injection of capsaicin into the vibrissa pad produced an immediate grooming of the injected area with ipsilateral fore- or hindpaw. A positive relationship between the amplitude of the grooming response and the capsaicin dose was observed until 1.5 microg, but with the highest concentrations (ranging from 25 to 500 microg) the amplitude of the response decreased. Morphine administered either systemically (in the neck, 0.5-4.0 mg/kg) or locally (0.25-1.0 mg/kg) reduced in a dose-dependent fashion the face grooming provoked by subcutaneous capsaicin (1.5 microg). The systemic and local morphine effects could be reversed by systemic (0.1 mg/kg) and local (0.05 mg/kg) administration of naloxone, respectively. The local administration of morphine (ED(50): 0.65 mg/kg) was more potent than systemic injection (ED50: 2.54 mg/kg) in reducing the grooming behavior. In conclusion, the orofacial capsaicin test appears to be a valid and reliable method for studying trigeminal pain mechanisms and testing analgesic drugs. The results of the present study also support the clinical use of peripheral opioid administration for the treatment of orofacial painful conditions.

Evidence for an exclusive antinociceptive effect of nociceptin/orphanin FQ, an endogenous ligand for the ORL1 receptor, in two animal models of neuropathic pain.

Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor-like1), has been shown to be anti- or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 microg/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw-pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 microg/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunities in the treatment of neuropathic pain.

Evidence for an antihyperalgesic effect of venlafaxine in vincristine-induced neuropathy in rat.

Venlafaxine, a new antidepressant with fewer side effects, could be of interest to reduce neuropathic pain following antineoplasic drug treatment. In the present study, we demonstrated that venlafaxine inhibits hyperalgesia in a new rat model of neuropathy induced by the antineoplasic drug vincristine, and exerts its effect preferentially via supraspinal and spinal mechanisms.

Blockade of supraspinal 5-HT1A receptors potentiates the inhibitory effect of venlafaxine on wind-up activity in mononeuropathic rats.

In mononeuropathic rats submitted to a C-fiber reflex responses paradigm, repeated administration (five successive injections every half-life) of 10 mg/kg, s.c. of venlafaxine, but not of 2.5 mg/kg, s.c., a mixed monoamine reuptake inhibitor with preferential inhibitory activity in 5-HT reuptake, induced a progressive reduction of spinal wind-up. Repeated co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 i.c.v. (50 microg/injection) significantly increased the effect of venlafaxine s.c., indicating that venlafaxine-induced inhibition of spinal wind-up in mononeuropathic rats is potentiated by blockade of central 5-HT1A receptors.

Antinociceptive interactions of ketamine with morphine or methadone in mononeuropathic rats.

To study the antinociceptive synergy resulting from the combination of opioid receptor agonists and N-methyl-D-aspartate (NMDA) receptor antagonists on neuropathic pain, an isobolographic analysis of equianalgesic combinations of ketamine with methadone or morphine was performed in rats with mononeuropathy produced by placing four constrictive ligatures around the common sciatic nerve. Two weeks later, the antinociceptive effect of subcutaneous administration of the drugs alone or combined was evaluated by using the paw pressure test. Drugs and their combinations produced dose-dependent antinociception. Combinations produced synergy of a supra-additive nature in the neuropathic paw, but only additive antinociception in the normal paw. The ketamine/methadone combination was more effective to produce antinociception in the neuropathic paw than was the ketamine/morphine association, as revealed by the lower ED25. The results indicate supra-additive synergy between NMDA receptor antagonists and opioids, especially methadone, to produce antinociception in experimental neuropathy.

Nitric oxide synthase and soluble guanylate cyclase are involved in spinal cord wind-up activity of monoarthritic, but not of normal rats.

While increasing evidence points to a role for the nitric oxide (NO)/cyclic guanosine 3,5-monophosphate (GMPc) cascade in hyperalgesia and allodynia, participation of the NO/GMPc pathway in synaptic processing in the spinal cord, i.e. wind-up activity, is less clear. We studied the effects of intrathecal administration of Nomega-nitro-L-arginine methyl ester (L-NAME) and methylene blue, inhibitors of NO synthase and guanylate cyclase respectively, on wind-up activity developed in a C-fiber reflex response paradigm. 5, 10 and 20 microg i.t. of L-NAME or methylene blue did not modify spinal wind-up in normal rats, while a dose-dependent inhibition of wind-up was observed in monoarthritic rats. Results suggest that the NO/GMPc pathway plays a non-significant role in wind-up activity evoked in normal animals, while it may be essential in chronic pain processing.

The antihyperalgesic effect of venlafaxine in diabetic rats does not involve the opioid system.

Venlafaxine (VFX) is a structurally novel antidepressant that inhibits reuptake of serotonin and norepinephrine but, unlike tricyclic antidepressants, has few side effects. The present work studies the antihyperalgesic effect of repeated administrations of VFX (five successive injections of 2.5, 5 or 10 mg/kg, s.c., every half-life) in diabetic rats with the paw pressure test and the effect of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) because an opioidergic mechanism is usually considered to be involved in the analgesic effect of antidepressants. VFX induced a significant dose-dependent increase in vocalization thresholds. This effect was not reversed by naloxone. Thus, we demonstrate a clear antinociceptive effect of VFX which, unlike that of most mixed tricyclic antidepressants, does not involve the endogenous opioid system.

Interleukin-1beta increases spinal cord wind-up activity in normal but not in monoarthritic rats.

Cytokines produced by spinal cord glia after peripheral inflammation, infection or trauma have a relevant role in the maintenance of pain states. The effect of intrathecally administered interleukin-1beta (IL-1beta) on spinal cord nociceptive transmission was studied in normal and monoarthritic rats by assessing wind-up activity in a C-fiber-mediated reflex paradigm evoked by repetitive (1 Hz) electric stimulation. Low i.t. doses of IL-1beta (0.03, 0.12, 0.5 and 2.0 ng) dose-dependently enhanced wind-up activity in normal rats, while higher doses (8.0 ng) only produced a marginal unsignificant effect. IL-1beta administration to monoarthritic rats did not significantly change wind-up scores at any dose. Adaptive changes developed in the spinal cord during chronic pain may underlie the ineffectiveness of exogenous IL-1beta to up-regulate nociceptive transmission.

Pannexin 1: a novel participant in neuropathic pain signaling in the rat spinal cord.

Pannexin 1 (panx1) is a large-pore membrane channel expressed in many tissues of mammals, including neurons and glial cells. Panx1 channels are highly permeable to calcium and adenosine triphosphatase (ATP); on the other hand, they can be opened by ATP and glutamate, two crucial molecules for acute and chronic pain signaling in the spinal cord dorsal horn, thus suggesting that panx1 could be a key component for the generation of central sensitization during persistent pain. In this study, we examined the effect of three panx1 blockers, namely, 10panx peptide, carbenoxolone, and probenecid, on C-reflex wind-up activity and mechanical nociceptive behavior in a spared nerve injury neuropathic rat model involving sural nerve transection. In addition, the expression of panx1 protein in the dorsal horn of the ipsilateral lumbar spinal cord was measured in sural nerve-transected and sham-operated control rats. Sural nerve transection resulted in a lower threshold for C-reflex activation by electric stimulation of the injured hindpaw, together with persistent mechanical hypersensitivity to pressure stimuli applied to the paw. Intrathecal administration of the panx1 blockers significantly depressed the spinal C-reflex wind-up activity in both neuropathic and sham control rats, and decreased mechanical hyperalgesia in neuropathic rats without affecting the nociceptive threshold in sham animals. Western blotting showed that panx1 was similarly expressed in the dorsal horn of lumbar spinal cord from neuropathic and sham rats. The present results constitute the first evidence that panx1 channels play a significant role in the mechanisms underlying central sensitization in neuropathic pain.