Maintenance Of Mechanical Allodynia Research Articles

Melatonin Improves Mitochondrial Dysfunction and Attenuates Neuropathic Pain by Regulating SIRT1 in Dorsal Root Ganglions

Neuropathic pain is a debilitating chronic pain condition and is refractory to the currently available treatments. Emerging evidence suggests that melatonin exerts analgesic effects in rodent models of neuropathic pain. Nevertheless, the exact underlying mechanisms of the analgesic effects of melatonin on neuropathic pain are largely unknown. Here, we observed that spinal nerve ligation (SNL) in rats L5 and L6 induced an obvious decrease in the 50% paw withdrawal threshold (PWT) and paw withdrawal latency (PWL), indicating the induction of mechanical allodynia and the hyperalgesia, and melatonin prevented the genesis and maintenance of mechanical allodynia and the hyperalgesia. Notably, the inhibitory action of melatonin on SNL-induced mechanical allodynia and heat hypersensitivity was inhibited by a SIRT1 inhibitor (EX527). Melatonin treatment increased the expression of neuronal sirtuin1 (SIRT1) in DRGs following nerve injury. Furthermore, melatonin treatment restored the injury-dependent decrease in mitochondrial membrane potential and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and reduced the injury-dependent increase in hydrogen peroxide and 8-hydroxy-2-deoxyguanosine (8-OHdG), which was inhibited by EX527. In addition, we found that EX527 impeded the inhibitory effects of melatonin on the SNL-induced increased expression of cytokines tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). In conclusion, the above data demonstrated that melatonin alleviated mechanical allodynia and hyperalgesia induced by peripheral nerve injury via SIRT1 activation. Melatonin resolved mitochondrial dysfunction—oxidative stress-dependent and neuroinflammation mechanisms that were driven by SIRT1 after nerve injury.

Neuroinflammation in the anterior cingulate cortex: the potential supraspinal mechanism underlying the mirror-image pain following motor fiber injury

BackgroundPeripheral nerve inflammation or lesion can affect contralateral healthy structures, and thus result in mirror-image pain. Supraspinal structures play important roles in the occurrence of mirror pain. The anterior cingulate cortex (ACC) is a first-order cortical region that responds to painful stimuli. In the present study, we systematically investigate and compare the neuroimmune changes in the bilateral ACC region using unilateral- (spared nerve injury, SNI) and mirror-(L5 ventral root transection, L5-VRT) pain models, aiming to explore the potential supraspinal neuroimmune mechanism underlying the mirror-image pain.MethodsThe up-and-down method with von Frey hairs was used to measure the mechanical allodynia. Viral injections for the designer receptors exclusively activated by designer drugs (DREADD) were used to modulate ACC glutamatergic neurons. Immunohistochemistry, immunofluorescence, western blotting, protein microarray were used to detect the regulation of inflammatory signaling.ResultsIncreased expressions of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6) and chemokine CX3CL1 in ACC induced by unilateral nerve injury were observed on the contralateral side in the SNI group but on the bilateral side in the L5-VRT group, representing a stronger immune response to L5-VRT surgery. In remote ACC, both SNI and L5-VRT induced robust bilateral increase in the protein level of Nav1.6 (SCN8A), a major voltage-gated sodium channel (VGSC) that regulates neuronal activity in the mammalian nervous system. However, the L5-VRT-induced Nav1.6 response occurred at PO 3d, earlier than the SNI-induced one, 7 days after surgery. Modulating ACC glutamatergic neurons via DREADD-Gq or DREADD-Gi greatly changed the ACC CX3CL1 levels and the mechanical paw withdrawal threshold. Neutralization of endogenous ACC CX3CL1 by contralateral anti-CX3CL1 antibody attenuated the induction and the maintenance of mechanical allodynia and eliminated the upregulation of CX3CL1, TNF-α and Nav1.6 protein levels in ACC induced by SNI. Furthermore, contralateral ACC anti-CX3CL1 also inhibited the expression of ipsilateral spinal c-Fos, Iba1, CD11b, TNF-α and IL-6.ConclusionsThe descending facilitation function mediated by CX3CL1 and its downstream cascade may play a pivotal role, leading to enhanced pain sensitization and even mirror-image pain. Strategies that target chemokine-mediated ACC hyperexcitability may lead to novel therapies for the treatment of neuropathic pain.

Mechanical Allodynia Circuitry in the Dorsal Horn Is Defined by the Nature of the Injury

The spinal dorsal horn is a major site for the induction and maintenance of mechanical allodynia, but the circuitry that underlies this clinically important form of pain remains unclear. The studies presented here provide strong evidence that the neural circuits conveying mechanical allodynia in the dorsal horn differ by the nature of the injury. Calretinin (CR) neurons in lamina II inner convey mechanical allodynia induced by inflammatory injuries, while protein kinase C gamma (PKCγ) neurons at the lamina II/III border convey mechanical allodynia induced by neuropathic injuries. Cholecystokinin (CCK) neurons located deeper within the dorsal horn (laminae III-IV) are important for both types of injuries. Interestingly, the Maf+ subset of CCK neurons is composed of transient vesicular glutamate transporter 3 (tVGLUT3) neurons, which convey primarily dynamic allodynia. Identification of an etiology-based circuitry for mechanical allodynia in the dorsal horn has important implications for the mechanistic and clinical understanding of this condition.

CB1-cannabinoid-, TRPV1-vanilloid- and NMDA-glutamatergic-receptor-signalling systems interact in the prelimbic cerebral cortex to control neuropathic pain symptoms

Neuropathic pain (NP) is a challenge due to our limited understanding of the mechanisms that initiate and maintain chronic pain. The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is an important area of the emotional and cognitive components of pain and pharmacological systems can interact into the neocortex to elaborate the chronic pain. This work aimed to investigate the pharmacological cross-talk between synaptic neurotransmission, neuroanatomical approaches and NP conditions. A bidirectional neural tract tracer, the 3000-molecular-weight biodextran (BDA) was microinjected into the PrL cortex. The mechanical withdrawal threshold (MWT) was recorded by a von Frey test, and the effect of prelimbic cortex CB1, NMDA, and TRPV1 receptor modulation was evaluated 21 days after chronic constriction injury (CCI) of the sciatic nerve in male Wistar rats. Microinjection of a bidirectional neurotracer in the PrL cortex showed connections with the lateral division of the mediodorsal thalamic nucleus (MDL), central division of the mediodorsal thalamic nucleus (MDC), centrolateral thalamic nucleus (CL), ventromedial thalamic nucleus (VM), and the paracentral thalamic nucleus (PC). In detail, AM251, a CB1 receptor antagonist (at 50, 100 and 200 pmol) microinjections intra-PrL cortex decreased the MWT. Administrations of 6-iodonordihydrocapsaicin (6-I-CPS), a transient receptor potential vanilloid type 1 (TRPV1) antagonist, at 3 nmol and the endocannabinoid anandamide (AEA) at 50 and 100 pmol increased the MWT. AEA at 200 pmol injected in the PrL cortex decreased the MWT, and this hyperalgesic effect was blocked by 6-I-CPS at 3 nmol. The AEA (at 100 pmol) anti-allodynic effect was attenuated by AM251 (at 5 pmol). The TRPV1 selective agonist N-oleoyldopamine (OLDA) at 10 μM decreased the MWT. The blockade of the NMDA receptor with LY235959 (at 8 nmol) and 6-I-CPS (at 3 nmol) reversed the OLDA (at 10 μM) hyperalgesic effect. These findings showed that the PrL cortex sends pathways to thalamic nuclei that can mediate the nociception. We also suggest that the PrL cortex is involved in the potentiation and maintenance of mechanical allodynia by NMDA and TRPV1 receptor activation and that attenuation of this allodynia depends on CB1 receptor activation during NP.

Differential role of spinal progesterone in the induction and maintenance of mechanical allodynia in peripheral neuropathy: Involvement of cytochrome P450c17

Differential role of spinal progesterone in the induction and maintenance of mechanical allodynia in peripheral neuropathy: Involvement of cytochrome P450c17

Inhibition of the potassium channel KCa3.1 by senicapoc reverses tactile allodynia in rats with peripheral nerve injury

Neuropathic pain is a debilitating, chronic condition with a significant unmet need for effective treatment options. Recent studies have demonstrated that in addition to neurons, non-neuronal cells such as microglia contribute to the initiation and maintenance of allodynia in rodent models of neuropathic pain. The Ca2+- activated K+ channel, KCa3.1 is critical for the activation of immune cells, including the CNS-resident microglia. In order to evaluate the role of KCa3.1 in the maintenance of mechanical allodynia following peripheral nerve injury, we used senicapoc, a stable and highly potent KCa3.1 inhibitor. In primary cultured microglia, senicapoc inhibited microglial nitric oxide and IL-1β release. In vivo, senicapoc showed high CNS penetrance and when administered to rats with peripheral nerve injury, it significantly reversed tactile allodynia similar to the standard of care, gabapentin. In contrast to gabapentin, senicapoc achieved efficacy without any overt impact on locomotor activity. Together, the data demonstrate that the KCa3.1 inhibitor senicapoc is effective at reducing mechanical hypersensitivity in a rodent model of peripheral nerve injury.

Effects of tianeptine on the development and maintenance of mechanical allodynia in a rat model of neuropathic pain

We validate the analgesic efficacy of tianeptine by different administration routes and timing in a rat model of neuropathic pain. Neuropathic pain was induced by ligating the L5 and L6 spinal nerves in male Sprague–Dawley rats, and mechanical allodynia was assessed using von Frey filaments. The effects of orally administered tianeptine and pretreatment with tianeptine (intrathecally or intraperitoneally) on mechanical allodynia were assessed. Oral and preemptive intrathecal administration of tianeptine significantly increased the paw withdrawal threshold but preemptive intraperitoneal administration did not. Nevertheless, intraperitoneal pretreatment of tianeptine potentiated the antiallodynic effects of subsequently administered tianeptine. These findings suggest that tianeptine may be effective for preventing and treating neuropathic pain and that it can be used more widely in clinical pain practice.

Involvement of the chemokine CCL3 and the purinoceptor P2X7 in the spinal cord in paclitaxel-induced mechanical allodynia.

BackgroundPaclitaxel is an effective chemotherapeutic agent widely used for the treatment of solid tumors. The major dose-limiting toxicity of paclitaxel is peripheral neuropathy. The mechanisms underlying the development and maintenance of paclitaxel-induced peripheral neuropathy are still unclear, and there are no currently established effective treatments. Accumulating evidence in models of neuropathic pain in which peripheral nerves are lesioned has implicated spinal microglia and chemokines in pain hypersensitivity, but little is know about their roles in chemotherapy-induced peripheral neuropathy. In the present study, we investigated the role of CC-chemokine ligand 3 (CCL3) in the spinal cord in the development and maintenance of mechanical allodynia using a rat model of paclitaxel-induced neuropathy.FindingsRepeated intravenous administration of paclitaxel induced a marked decrease in paw withdrawal threshold in response to mechanical stimulation (mechanical allodynia). In these rats, the number of microglia in the spinal dorsal horn (SDH) was significantly increased. Paclitaxel-treated rats showed a significant increase in the expression of mRNAs for CCL3 and its receptor CCR5 in the SDH. Intrathecal administration of a CCL3-neutralizing antibody not only attenuated the development of paclitaxel-induced mechanical allodynia but also reversed its maintenance. Paclitaxel also upregulated expression of purinoceptor P2X7 receptors (P2X7Rs), which have been implicated in the release of CCL3 from microglia, in the SDH. The selective P2X7R antagonist A438079 had preventive and reversal effects on paclitaxel-induced allodynia.ConclusionsOur findings suggest a contribution of CCL3 and P2X7Rs in the SDH to paclitaxel-induced allodynia and may provide new therapeutic targets for paclitaxel-induced painful neuropathy.

Chronological changes of mechanical allodynia and spinal microglia activation by an intrathecal injection of MK-801

The neuropathic pain that occurs after peripheral nerve injury may be related to abnormal central activity. The present experiments investigated the effects of MK-801 [N-methyl-D-aspartate (NMDA) receptor antagonist] on neuropathic pain behaviors and microglial activity in rats. Neuropathic pain was produced by L5 spinal nerve ligation of rats. MK-801 was injected to determine whether spinal microglial activation after nerve injury plays a crucial role in the development and/or maintenance of neuropathy through the NMDA receptor. Mechanical allodynia of the hind paw was examined with von Frey filaments postoperatively. Microglial activity was measured by observing changes in immunoreactivity with a microglia marker, OX-42. The MK-801, at a dose of 3 or 30 µg/5 µl, injection group showed higher neuropathic pain threshold and reduction of microglial activity. These results suggest that neuropathic pain behaviors following L5 spinal nerve ligation may be related to altered activity of the microglia involving the NMDA receptor, and chronological changes of microglial activation by MK-801 are related to maintenance of mechanical allodynia.

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.

Effect of superoxide on the development and maintenance of mechanical allodynia in a rat model of chronic post-ischemia pain.

BackgroundReactive oxygen species and inflammatory responses contribute to the development of neuropathic pain. Superoxide serves to mediate cell signaling processes and tissue injury during inflammation. We examined the effects of superoxide on the development and maintenance of mechanical allodynia, as well as its contribution to central sensitization in a superoxide-rich animal model of neuropathic pain.MethodsChronic post-ischemia pain (CPIP) was induced via the left hindpaw ischemia for 3 h, followed by reperfusion. Superoxide dismutase (4,000 U/kg, i.p.) was administered either 5 min before ischemia (BI), 5 min before reperfusion (BR), or 3 days after reperfusion (3AR). Withdrawal thresholds of the four paws were measured to assess the mechanical allodynia and the effects of circulating xanthine oxidase (XO)-mediated superoxide production. In addition, we measured the levels of N-methyl D-aspartate receptor subunit 1 phosphorylation (p-NR1) in the ipsilateral and contralateral spinal cord (L4-6), by Western blotting, to examine the superoxide-mediated central sensitization. Superoxide production was assessed by allopurinol-sensitive, XO-mediated lipid peroxidation of the spinal cord and gastrocnemius muscles.ResultsWithdrawal thresholds of forepaws did not vary across the 7 days of testing. In the hindpaws, both ipsilateral and contralateral mechanical allodynia was most attenuated in the BR group, followed by the BI and 3AR groups. The degree of NR1 activation was in contrast to the changes in the withdrawal thresholds.ConclusionsThese data suggest that superoxide is involved in the development and maintenance of mechanical allodynia, particularly via central sensitization in the spinal cord.

The Differential Effect of Intrathecal Nav1.8 Blockers on the Induction and Maintenance of Capsaicin- and Peripheral Ischemia-Induced Mechanical Allodynia and Thermal Hyperalgesia

It has been reported that the selective blockade of Nav1.8 sodium channels could be a possible target for the development of analgesics without unwanted side effects. However, the precise role of spinal Nav1.8 in the induction and maintenance of persistent pain, e.g., mechanical allodynia (MA) and thermal hyperalgesia (TH), is not clear. We designed this study to investigate whether spinal Nav1.8 contributes to capsaicin-induced and peripheral ischemia-induced MA and TH. The Nav1.8 blockers, A-803467 or ambroxol, were injected intrathecally either before or after intraplantar capsaicin injection. To evaluate capsaicin-induced neuronal activation in the spinal cord, we quantified the number of Fos-immunoreactive cells in the dorsal horn. In the thrombus-induced ischemic pain model, we determined the differential effect of A-803467 on the induction phase or maintenance phase of MA. Intrathecal injection of A-803467 (10, 30, 100 nmol) or ambroxol (241, 724, 2410 nmol) before intraplantar injection of capsaicin dose dependently prevented the induction of both MA and TH. However, posttreatment with A-803467 (100 nmol) and ambroxol (2410 nmol) did not reduce the MA that had already developed, but did significantly suppress capsaicin-induced TH. Moreover, the capsaicin-induced increase of spinal Fos-immunoreactive cells was significantly diminished by pretreatment, but not posttreatment with Nav1.8 blockers. In thrombus-induced ischemic pain rats, repetitive treatments of A-803467 during the induction period also prevented the development of MA, whereas A-803467 treatments during the maintenance period were ineffective in preventing or reducing MA. These results demonstrate that spinal activation of Nav1.8 mediates the early induction of MA, but not the maintenance of MA. However, both the induction and maintenance of TH are modulated by the intrathecal injection of Nav1.8 blockers. These findings suggest that early treatment with a Nav1.8 blocker can be an important factor in the clinical management of chronic MA associated with inflammatory and ischemic pain.

Involvement of EphB1 receptor/ephrinB1 ligand in bone cancer pain

In prior studies, Eph/ephrin system was demonstrated to be involved in inflammatory and neuropathic pain modulation. The present study was to investigate whether the spinal Eph/ephrin signaling was involved in modulation of spinal inflammatory cytokines in bone cancer pain (BCP) of rats. BCP was induced by intra-tibial inoculation of Walker 256 mammary gland carcinoma cells. The expressions of EphB1/ephrinB1 in spinal cord (SC) and dorsal root ganglia (DRG) were determined. At 16 days post inoculation, the pain relieving effect and the mRNA levels of inflammatory cytokines were detected after intrathecal administration of EphB1-Fc (blocker of EphB1 receptor, 10μg). The results showed that the EphB1/ephrinB1 expression was significantly increased in SC, but ephrinB1 was decreased in DRG after Walker 256 inoculation. The mechanical allodynia induced by bone cancer was significantly alleviated by intrathecal administration of EphB1-Fc. Furthermore, the RT-PCR analysis showed that the mRNA levels of IL-1β, IL-6 and TNF-α were significantly increased at 16 days post Walker 256 inoculation and were significantly suppressed by intrathecal administration of EphB1-Fc in SC. We concluded that Eph/ephrin might be involved in the maintenance of mechanical allodynia, via modulating the expression of spinal inflammatory cytokines, in the present rat model of BCP. This study suggested that Eph/ephrin signaling would be a potential target for the treatment of BCP.

Chronic blockade of interleukin-1 (IL-1) prevents and attenuates neuropathic pain behavior and spontaneous ectopic neuronal activity following nerve injury

Neuropathic pain is a chronic pain state resulting from peripheral nerve injury, characterized by hyperalgesia and allodynia. We have reported that mice with genetic impairment of IL-1 signaling display attenuated neuropathic pain behavior and ectopic neuronal activity. In order to substantiate the role of IL-1 in neuropathic pain, WT mice were implanted subcutaneously with osmotic micropumps containing either IL-1ra or vehicle. Two days following the implantation, two models of neuropathic pain were used; partial nerve injury (spinal nerve transection, SNT), or complete nerve cut (spinal neuroma model). Mechanosensitivity was assessed seven consecutive days following SNT, and on day 7 recordings of spontaneous ectopic activity were performed. In the spinal nerve neuroma model, autotomy scores were recorded up to 35 days. Vehicle-treated mice developed significant allodynia and autotomy, and clear ectopic activity (4.1 ± 1.1% of the axons); whereas IL-1ra-treated mice did not display allodynic response, displayed delayed onset of autotomy and markedly reduced severity of autotomy scores, and displayed reduced spontaneous activity (0.8 ± 0.4% of the axons). To test whether IL-1 is involved in maintenance of mechanical allodynia, a separate group of WT mice was treated with a single injection of either saline or IL-1ra four days following SNT, after the allodynic response was already manifested. Whereas saline-treated mice displayed robust allodynia, acute IL-1ra treatment induced long-lasting attenuation of the allodynic response. The results support our hypothesis that IL-1 signaling plays an important role in neuropathic pain and in the ectopic neuronal activity that underling its development.

Peripheral antinociceptive effects of µ- and δ-opioid receptor agonists in NOS2 and NOS1 knockout mice during chronic inflammatory pain

The aim of this study is to investigate the involvement of nitric oxide synthesized by the inducible (NOS2) or neuronal (NOS1) nitric oxide synthases in the local antinociceptive effects produced by µ- and δ-opioid receptor agonists during chronic inflammatory pain. Peripheral inflammatory pain was induced in NOS2 and NOS1 knockout mice and their wild type littermates by the subplantar administration of complete Freund's adjuvant (30 µl). The presence of paw inflammation, mechanical allodynia and thermal hyperalgesia induced by complete Freund's adjuvant were assessed by measuring paw diameter and using the von Frey filaments and plantar tests, respectively. During chronic inflammation, NOS2 deficient mice have a more rapid recovery of paw edema and a reduced thermal hyperalgesia compared to wild type. In contrast, a reduced paw edema and mechanical allodynia, as well as a modest rapid recovery from thermal hyperalgesia were observed in NOS1 knockout mice compared to wild type. The thermal hyperalgesia induced by complete Freund's adjuvant was not completely reversed by the subplantar administration of morphine (days 4 and 7) or [ d-Pen 2,5] enkephalin (DPDPE) (days 1 and 4) in NOS2 knockout mice as occurs in wild type mice. Moreover, the local administration of morphine or DPDPE also failed to reverse the decrease of ipsilateral paw withdrawal latency induced by complete Freund's adjuvant in NOS1 knockout mice throughout 10 days of peripheral inflammation. These results indicate the different roles played by nitric oxide synthesized by NOS2 or NOS1 in the maintenance of mechanical allodynia and thermal hyperalgesia induced by chronic inflammatory pain as well as, in the antinociceptive effects produced by µ- and δ-opioid receptor agonists during peripheral inflammatory pain.

Involvement of an increased spinal TRPV1 sensitization through its up-regulation in mechanical allodynia of CCI rats

Vanilloid receptor 1 (TRPV1) antagonists are known to attenuate the neuropathic pain symptoms in peripheral nerve injury models, but the mechanism(s) of their effect remains unclear. At the same time, the role of spinal TRPV1 in pain transduction system has not been fully understood. In this study, the role of spinal TRPV1 in mechanical allodynia in rat chronic constriction injury (CCI) model was investigated. Intrathecal administration of a selective TRPV1 antagonist, N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropryazine-1(2H)-carbox-amide (BCTC) significantly attenuated mechanical allodynia in CCI rats at 100 and 300 nmol. In vitro, BCTC inhibited capsaicin (300 nM)-induced releases of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) and substance P-like immunoreactivity (SP-LI) from the rat spinal cord slice preparations with IC 50s of 37.0 and 36.0 nM, respectively, confirming that BCTC potently inhibits TRPV1 function in the rat spinal cord. TRPV1 expression levels in the spinal cord following CCI were quantified in by Western blot analysis. TRPV1 protein levels were significantly increased in the ipsilateral side of the lumbar spinal cord at 7 and 14 days following CCI surgery, but not in the contralateral side. Furthermore, capsaicin (300 nM)-evoked release of CGRP-LI was significantly higher in the ipsilateral spinal cord of CCI rats (14 days after surgery) than that of sham-operated rats. These findings suggest that an increased sensitization of the spinal TRPV1 through its up-regulation is involved in the development and/or maintenance of mechanical allodynia in rat CCI model.

The Effect of an Intraperitoneal Injection of Ketamine and Ketorolac on Mechanical Allodynia in Rats with Spinal Nerve Ligation

Background: It is difficult to manage the symptoms of neuropathic pain, especially alloynia. The mechanism of the induction and maintenance of mechanical allodynia has been extensively researched for several decades. N-methyl-D-aspartate (NMDA) receptor antagonists are known to reduce mechanical allodynia. Recently, the role of prostaglandins in spinal nociceptive processing has been the focus of attention. Therefore, the present study was designed to investigate the effect of a combination of ketamine, a non-competitive NMDA antagonist, and of ketorolac, non-selective cyclooxygenase (COX) inhibitor on mechanical allodynia. Methods: Male SD rats were prepared by tightly ligating the left L5 and L6 spinal nerves. All rats developed mechanical allodynia 7 days after surgery. N group (control, n = 6) received 5 ml of 0.9% normal saline intraperitoneally. K group (n = 6) received ketamine 1 mg/kg. T group received ketorolac 30 mg/kg, and KT group received ketamine 1 mg/kg and ketorolac 30 mg/kg simultaneously. Paw withdrawal thresholds to von Frey hairs were measured before and at 15 min, 30 min, 60 min and 120 min after drug administration. Results: Normal saline and ketamine 1 mg/kg did not increase the paw withdrawal threshold from baseline. Ketorolac 30 mg/kg increased the paw withdrawal threshold only at 120 min after intraperitoneal injection. However, the co-administration of ketamine 1 mg/kg and ketorolac 30 mg/kg increased the paw withdrawal threshold significantly from baseline for 120 min. Conclusions: Intraperitoneal injection of ketamine and ketorolac attenuated the mechanical allodynia developed by spinal nerve ligation. Therefore, we suggest that combination of ketamine and ketorolac might be useful for the management of neuropathic pain.

The role of calcitonin gene-related peptide (CGRP) in the generation and maintenance of mechanical allodynia and hyperalgesia in rats after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the generation and maintenance of secondary mechanical allodynia and hyperalgesia induced by intradermal injection of capsaicin in rats. Paw withdrawal responses (PWRs) to von Frey hairs with different bending forces applied on the rat paw were tested in this study. CGRP 8–37, a specific antagonist of CGRP 1 receptors, was delivered through a microdialysis fiber inserted across the dorsal horn. Post- and pretreatment paradigms were followed. When CGRP 8–37 was administered 1 h after capsaicin injection, the mechanical allodynia and hyperalgesia were partially reversed in a dose-dependent manner. On the other hand, when rats were treated with CGRP 8–37 prior to capsaicin injection, the PWRs to von Frey applications were significantly reduced as compared to control animals. Collectively, these results suggest that CGRP receptors present in the dorsal horn are involved in the generation and maintenance of nociceptive behaviors associated with cutaneous inflammation.

The Role of Spinal NO on the Mechanical Allodynia in the Inflammation Model by Freund's Complete Adjuvant

Background : The effect of spinal nitric oxide (NO) on mechanical allodynia brought about by Freund''s complete adjuvant (FCA)-induced inflammation is not known. From our previous experiment nitric oxide synthase (NOS) inbibitor nitro -L-arginine methyl ester (L-NAME) given intraplantarly during the induction period decreased a mechanical hyperalgesia occurring because of FCA-induced inflammation. Therefore, we investigated the spinal effect of NO on mechanical allodynia after the development of allodynia pmduced by FCA-induced inflammation in rats. Methods : Male Sprague Dawley rats were prepared with lumbar intrathecal catheter implantation. Inflammation was induced in the rats by injecting 0.1 ml of FCA under halothane anesthesia. Behavioral tests were done 1, 3, 6, 24, and 48 hours after injection of FCA. In the other group, inlrathecal L-NAME (10 μg) was given prior to FCA injection to examine the effect of pretreatment. On postinjection day 2, either L-NAME (10 μg) or methylene blue (10 and 30 μg) was administered intrathecally after the baseline measurement. The withdrawal response on mechanical allodynia was assessed by applying von Frey filarnents to th right lesioned hindpaw and contralateral paw (as control) at 15, 30, 45, 60, 90, and 120 minutes. Sodium niiraprusside was administered intrathecally to determine the reversal effect of increased threshold in the L-NAME group. Results : Injection of FCA produced a significant mechanical allodynia over time. Pretreatment with L-NAME did not prevcnt such a mechanical allodynia. Intrathecal L-NAME, but not methylene blue, reduced the mechanical allodynia, which was reversed by sodium nitroprusside. Conclusions: Spinal NO is likely invloved in the mechanism of the development and maintenance of mechanical allodynia in a state of FCA-induced inflammation. (Korean J Anesthesiol 2000; 38: 1068 ~ 1074)

The kainate receptor antagonist 2 S,4 R-4-methylglutamate attenuates mechanical allodynia and thermal hyperalgesia in a rat model of nerve injury

Opioids and receptor antagonists of excitatory amino acids attenuate mechanical allodynia and thermal hyperalgesia in animal models of neuropathic pain. Recently, a kainate receptor antagonist, 2 S,4 R-4-methylglutamate, has been developed but has not been tested for antinociceptive effects in animal models of neuropathic pain. We evaluated whether 2 S,4 R-4-methylglutamate attenuated responses to mechanical and thermal stimuli in uninjured (control) rats and increased responsiveness in rats with chronic constriction injury. Rats were tested for a number of withdrawal responses using a calibrated von Frey filament (mechanical stimulus) and withdrawal latencies from a radiant heat source (thermal stimulus). In control rats, 2 S,4 R-4-methylglutamate produced a small but significant decrease in responses from the mechanical stimulus (25 mg/kg) and significantly increased withdrawal latencies from the thermal stimulus at the highest dose administered (100 mg/kg). In addition, 2 S,4 R-4-methylglutamate greatly attenuated increased responsiveness in rats with chronic constriction injury. At four to eight days following chronic constriction injury, animals that displayed increased responsiveness to mechanical and thermal stimuli were injected intraperitoneally with either dizocilpine maleate (0.1 mg/kg), morphine (4 mg/kg), vehicle as controls, or 2 S,4 R-4-methylglutamate (25, 50, 75 or 100 mg/kg). 2 S,4 R-4-Methylglutamate (25, 50, 75 and 100 mg/kg) significantly attenuated the frequency of responses to mechanical stimuli (Wilcoxon, P<0.05) and the latency of responses to thermal stimuli (analysis of variance and Duncan's, P<0.05). Dizocilpine maleate and morphine, as expected, also reduced these responses. These results suggest that, in addition to opioid and N-methyl- d-aspartate receptors, kainate receptors may play a role in the maintenance of mechanical allodynia and thermal hyperalgesia associated with peripheral nerve injury.