dizocilpine-maleate and Neuralgia

Convincing evidence has accumulated that indicates neuroplastic changes within the spinal cord in response to repeated exposure to opioids. Such neuroplastic changes occur at both cellular and intracellular levels. It has been generally acknowledged that the activation of N-methyl-D-aspartate (NMDA) receptors plays a pivotal role in the development of neuroplastic changes following repeated opioid exposure. Intracellular cascades can also be activated subsequent to NMDA receptor activation. In particular, protein kinase C has been shown to be a key intracellular element that contributes to the behavioral manifestation of neuroplastic changes. Moreover, interactions between NMDA and opioid receptors can lead to potentially irreversible degenerative neuronal changes in the spinal cord in association with the development of opioid tolerance. Interestingly, similar cellular and intracellular changes occur in the spinal cord following peripheral nerve injury. These findings indicate that interactions exist in the spinal cord neural structures between two seemingly unrelated conditions-chronic opioid exposure and a pathological pain state. These observations may help understand mechanisms of chemical intolerance and multiple chemical sensitivity as well as have significant clinical implications in pain management with opioid analgesics.

Topics: Analgesics; Analgesics, Opioid; Animals; Benzamides; Dizocilpine Maleate; Drug Tolerance; Hot Temperature; Humans; Hyperalgesia; Morphine; Multiple Chemical Sensitivity; N-Methylaspartate; Narcotics; Nerve Degeneration; Nerve Tissue Proteins; Neuralgia; Neuronal Plasticity; Nitric Oxide; Phosphorylation; Poly(ADP-ribose) Polymerases; Posterior Horn Cells; Protein Kinase C; Protein Processing, Post-Translational; Rats; Receptors, N-Methyl-D-Aspartate; Receptors, Opioid; Receptors, Opioid, mu; Sciatic Nerve; Spinal Cord

Trigeminal neuropathic pain (TNP) is an intense pain condition characterized by hyperalgesia and allodynia; however, its neural mechanisms are not completely understood. Its management is complex, and studies that investigate its biochemical mechanisms are important for improving clinical approaches. This study aimed to evaluate the involvement of GABAergic, glutamatergic, and opioidergic systems and brain-derived neurotrophic factor (BDNF) levels in the TNP process in rats. TNP is induced by chronic constriction injury of the infraorbital nerve (CCI-ION). Nociceptive responses were evaluated using the facial von Frey test before and after the administration of GABAergic and opioidergic agonists and glutamatergic antagonists. The rats were divided into vehicle-treated control (C), sham-surgery (SS), and CCI-ION groups, and then subdivided into the vehicle (V)-treated SS-V and CCI-ION-V groups, SS-MK801 and CCI-ION-MK801, treated with the N-methyl-d-aspartate receptor selective antagonist MK801; SS-PB and CCI-ION-PB, treated with phenobarbital; SS-BZD and CCI-ION-BZD, treated with diazepam; SS-MOR and CCI-ION-MOR, treated with morphine. BDNF levels were evaluated in the cerebral cortex, brainstem, trigeminal ganglion, infraorbital branch of the trigeminal nerve, and serum. CCI-ION induced facial mechanical hyperalgesia. Phenobarbital and morphine reversed the hyperalgesia induced by CCI-ION, and the CCI-BZD group had an increased nociceptive threshold until 60 min. CCI-ION-GLU increased the nociceptive threshold at 60 min. Cerebral cortex and brainstem BDNF levels increased in the CCI-ION and SS groups. Only the CCI group presented high levels of BDNF in the trigeminal ganglion. Our data suggest the involvement of GABAergic, glutamatergic, and opioidergic systems and peripheral BDNF in the TNP process.

Topics: Animals; Brain-Derived Neurotrophic Factor; Dizocilpine Maleate; GABAergic Neurons; Hyperalgesia; Morphine; Neuralgia; Phenobarbital; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Receptors, Opioid; Trigeminal Neuralgia

To investigate whether the Chinese massage system, Tuina, exerts analgesic effects in a rat model of chronic constriction injury (CCI) by remodeling the synaptic structure in the spinal cord dorsal horn (SCDH).. Sixty-nine male Sprague-Dawley rats were randomly and evenly divided into the normal group, sham group, CCI group, CCI + Tuina group, CCI + MK-801 [an -methyl D-aspartate receptor subtype 2B (NR2B) antagonist] group, and CCI + MK-801 + Tuina group. The neuropathic pain model was established using CCI with right sciatic nerve ligation. Tuina was administered 4 d after CCI surgery, using pressing manipulation for 10 min, once daily. Motor function was observed with the inclined plate test, and pain behaviors were observed by the Von Frey test and acetone spray test. At 19 d after surgery, the L3-L5 spinal cord segments were removed. Glutamate, interleukin 1β (IL-1β), and tumor necrosis factor-α (TNF-α) levels were detected by enzyme-linked immunosorbent assay. The protein expression levels of NR2B and postsynaptic density protein-95 (PSD-95) were detected by Western blot, and the synaptic structure was observed by transmission electron microscopy (TEM).. CCI reduced motor function and caused mechanical and cold allodynia in rats, increased glutamate concentration and TNF-α and IL-1β levels, and increased expression of synapse-related proteins NR2B and PSD-95 in the SCDH. TEM revealed that the synaptic structure of SCDH neurons was altered. Most of these disease-induced changes were reversed by Tuina and intrathecal injection of MK-801 ( < 0.05 or < 0.01). For the majority of experiments, no significant differences were found between the CCI + MK-801 and CCI + MK-801 + Tuina groups.. Chinese Tuina can alleviate pain by remodeling the synaptic structure, and NR2B and PSD-95 receptors in the SCDH may be among its targets.

Topics: Animals; Disks Large Homolog 4 Protein; Dizocilpine Maleate; Glutamates; Male; Massage; Neuralgia; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Spinal Cord Dorsal Horn; Tumor Necrosis Factor-alpha

Neuropathic pain is usually persistent due to maladaptive neuroplasticity-induced central sensitization and, therefore, necessitates long-term treatment. N-methyl-D-aspartate receptor (NMDAR)-mediated hypersensitivity in the spinal dorsal horn represents key mechanisms of central sensitization. Short-term use of NMDAR antagonists produces antinociceptive efficacy in animal pain models and in clinical practice by reducing central sensitization. However, how prolonged use of NMDAR antagonists affects central sensitization remains unknown. Surprisingly, we find that prolonged blockage of NMDARs does not prevent but aggravate nerve injury-induced central sensitization and produce analgesic tolerance, mainly due to reduced synaptic inhibition. The disinhibition that results from the continuous decrease in the production of nitric oxide from neuronal nitric oxide synthase, downstream signal of NMDARs, leads to the reduction of GABAergic inhibitory synaptic transmission by upregulating brain-derived neurotrophic factor expression and inhibiting the expression and function of potassium-chloride cotransporter. Together, our findings suggest that chronic blockage of NMDARs develops analgesic tolerance through the neuronal nitric oxide synthase-brain-derived neurotrophic factor-potassium-chloride cotransporter pathway. Thus, preventing the GABAergic disinhibition induced by nitric oxide reduction may be necessary for the long-term maintenance of the analgesic effect of NMDAR antagonists.

Topics: Analgesics; Animals; Dizocilpine Maleate; Drug Resistance; Excitatory Amino Acid Antagonists; GABAergic Neurons; Male; Mice; Mice, 129 Strain; Mice, Inbred C57BL; Mice, Knockout; Neural Inhibition; Neuralgia; Nitric Oxide; Nitric Oxide Synthase Type I; Organ Culture Techniques; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate

The central amygdaloid nucleus (CeA) is involved in processing and descending regulation of pain. Amygdaloid mechanisms underlying pain processing and control are poorly known. Here we tested the hypothesis that perioperative CeA administration of tetrapentylammonium (TPA), a non-selective THIK-1 channel blocker and thereby inhibitor of microglia, attenuates development of chronic neuropathic pain and comorbid anxiety-like behavior.. Rats with a spared nerve injury (SNI) model of neuropathy or sham operation had a chronic cannula for drug microinjections into the CeA or a control injection site. Monofilament test was used to evaluate pain, and light-dark box (LDB) to assess anxiety.. Perioperative CeA treatment with TPA (30 μg/day up to the third postoperative day, D3) significantly attenuated the development of pain and anxiety-like behavior. In the late phase (> D14), CeA administration of TPA (3-30 μg) failed to influence pain. Perioperative minocycline (microglia inhibitor; 25 μg), MK-801 (an N-Methyl-D-aspartate receptor antagonist; 0.1 μg), vehicle or TPA in a control injection site failed to attenuate pain development.. Perioperative treatment of the CeA with TPA delayed development of neuropathic pain and comorbid anxiety-like behavior, while TPA treatment failed to influence maintenance of established neuropathic pain. The failures to attenuate pain development with CeA administrations of minocycline or MK-801 do not support the hypothesis that the TPA-induced prophylactic effect was due to inhibition of amygdaloid microglia or N-methyl-D-aspartate receptors. While TPA in the CeA proved to have a prophylactic effect on SNI-induced pain behavior, the underlying mechanism still remains to be studied.

Topics: Amygdala; Analgesics; Animals; Anti-Anxiety Agents; Anxiety; Behavior, Animal; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Locomotion; Male; Microglia; Microinjections; Minocycline; Neuralgia; Pain Perception; Pain Threshold; Peripheral Nerve Injuries; Potassium Channels, Tandem Pore Domain; Quaternary Ammonium Compounds; Rats, Wistar; Receptors, N-Methyl-D-Aspartate

Neuropathic pain is relatively common and occurs in approximately 6-8% of the population. It is associated with allodynia and hyperalgesia. Thus, non-pharmacological treatments, such as transcranial direct current stimulation (tDCS) may be useful for relieving pain.. This study aimed to investigate the antiallodynic effect of tDCS in a mice model of neuropathic pain, and the underlying neurotransmission systems that could drive these effects.. Male, Swiss mice, weighing 25-35 g, were subjected to partial sciatic nerve ligation (PSNL). Allodynia was assessed using a Von Frey filament (0.6 g). First, the behavioral time-course of these mice was assessed after 5, 10, 15 and 20 min of tDCS (0.5 mA). Second, the mice that underwent PSNL were assigned to either the tDCS (0.5 mA, 15 min) or tDCS sham group, and further assigned to receive either saline or a drug (i.e., naloxone, yohimbine, a-methyl-p-tyrosine, q-chlorophenylalanine methyl ester, caffeine, 1,3-dipropyl-8-cyclopentylxanthine, AM281, AM630, flumazenil, MK-801, or lidocaine).. The antiallodynic effect of tDCS lasted 2 h and 4 h, after 10 min and 15 or 20 min of treatment, respectively (P < .001, P < .01, and P < .05, respectively). The antiallodynic effect of tDCS was associated with all the systems that were analyzed, i.e., the opioidergic (P < .01), adenosinergic (P < .001), serotonergic (P < .01), noradrenergic (P < .001), cannabinoid (P < .001), GABAergic, and glutamatergic (P < .001) systems. Lidocaine did not reverse the antiallodynic effect of tDCS (P > .05).. The antiallodynic effect of tDCS was associated with different neurotransmitters systems; the duration of these after-effects depended on the time exposure to tDCS.

Topics: Adenosine A1 Receptor Antagonists; Animals; Caffeine; Central Nervous System Stimulants; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Flumazenil; GABA Modulators; Hyperalgesia; Male; Mice; Morpholines; Naloxone; Narcotic Antagonists; Neuralgia; Pain Threshold; Physical Stimulation; Pyrazoles; Transcranial Direct Current Stimulation; Xanthines

The persistent activation of N-methyl-d-aspartate acid receptors (NMDARs) seems to be responsible for a series of changes in neurons associated with neuropathic pain, including the failure of opioids that act through mu-opioid receptors (MORs) to provide efficacious pain relief. As the noradrenergic locus coeruleus (LC) forms part of the endogenous analgesic system, we explored how intra-LC administration of morphine, a MORs agonist, alone or in combination with MK-801, a NMDARs antagonist, affects the sensorial and affective dimension of pain in a rat model of neuropathic pain; chronic constriction injury (CCI). Intra-LC microinjection of morphine induced analgesia in CCI rats, as evident in the von Frey and cold plate test 7 and 30 days after surgery, although it was not able to reverse pain-related aversion when evaluated using the place escape/avoidance test. However, the thermal anti-nociception produced by morphine was enhanced when it was administered to the LC of CCI animals in combination with MK-801, without altering its effects on the mechanical thresholds. Furthermore, pain-related aversion was reduced by co-administration of these agents, yet only in the short-term CCI (7 day) rats. Overall the data indicate that administration of morphine to the LC produces analgesia in nerve injured animals and that this effect is potentiated in specific pain modalities by the co-administration of MK-801. While a combination of morphine and MK-801 could reduce pain-related aversion in short-term neuropathic animals, it was ineffective in the long-term, suggesting that its sensorial effects and its influence on the affective component of pain are regulated by different mechanisms.

Topics: Analgesics, Opioid; Animals; Anxiety; Constriction, Pathologic; Disease Models, Animal; Dizocilpine Maleate; Drug Therapy, Combination; Excitatory Amino Acid Antagonists; Locus Coeruleus; Male; Morphine; Neuralgia; Nociceptive Pain; Rats, Sprague-Dawley; Sciatic Neuropathy

Background and aims The clinical management of neuropathic pain remains a challenge. We examined the interaction between gabapentin and NMDA receptor antagonists dextromethrophan and MK-801 in alleviating neuropathic pain-like behaviors in rats after spinal cord or sciatic nerve injury. Methods Female and male rats were produced with Ischemic spinal cord injury and sciatic nerve injury. Gabapentin, dextromethorphan, MK-801 or drug combinations were injected with increasing doses. Mechanical response thresholds were tested with von Frey hairs to graded mechanical touch/pressure, and ethyl chloride spray was applied to assess the cold sensitivity before and after injuries. Results In spinally injured rats, gabapentin and dextromethorphan did not affect allodynia-like behaviors at doses of 30 and 20 mg/kg, respectively. In contrast, combination of 15 or 30 mg/kg gabapentin with dextromethorphan at 10 mg/kg produced total alleviation of allodynia to mechanical or cold stimulation. Further reducing the dose of gapapentin to 7.5 mg/kg and dextromethorphan to 5 mg/kg still produced significant effect. MK-801, another NMDA receptor antagonist, also enhanced the effect of gabapentin in spinally injured rats. Similar synergistic anti-allodynic effect between dextromethorphan and gabapentin was also observed in a rat model of partial sciatic nerve injury. No increased side effect was seen following the combination between gabapentin and dextromethorphan. Conclusions In conclusion, the present study suggested that combining NMDA receptor antagonists with gabapentin could provide synergistic effect to alleviate neuropathic pain and reduced side effects. Implications Combining NMDA receptor antagonists with gabapentin may provide a new approach in alleviating neuropathic pain with increased efficacy and reduced side effects.

Topics: Animals; Behavior, Animal; Dextromethorphan; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Female; Gabapentin; Hyperalgesia; Male; Neuralgia; Rats; Rats, Sprague-Dawley; Sciatic Nerve; Spinal Cord Injuries; Touch

Neuropathic pain is a chronic disease with hallmarks such as chronic allodynia and hyperalgesia. Previous studies have shown that the transforming growth factor-β superfamily acts as a protecting factor against neuropathic pain. In the current study, we found that growth and differentiation factor 10 (GDF10), which belongs to the transforming growth factor-β superfamily, is mainly expressed in the superficial layers of spinal dorsal horn neurons and it was dramatically downregulated after spinal nerve ligation and N-methyl-D-aspartate (NMDA) intrathecal infusion. Moreover, the decrease in GDF10 expression and increase in mechanical sensitivity could be blocked by MK-801, an antagonist of the NMDA receptor. These results suggest that the decreasing GDF10 may contribute toward neuropathic pain by facilitating NMDA receptor activation. Our findings shed new light on the understanding of the molecular mechanisms underlying neuropathic pain.

Topics: Animals; Dizocilpine Maleate; Growth Differentiation Factor 10; Male; Neuralgia; Posterior Horn Cells; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Spinal Nerves

Neuropathic pain is a chronic pain condition, which is resistant to therapy. Ascorbate was released because of the activation of glutaminergic neurons. Due to the important role of N-methyl-D-aspartate (NMDA) receptors in the pathophysiology of neuropathic pain, this study investigated the analgesic efficacy of ascorbic acid (AA) in neuropathic pain condition and the role of NMDA receptors in this effect.. For this purpose, adult male rats were randomly allocated to experimental groups (n=8 in each group). Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve. During the second week after CCI, animals received a single injection of 1, 3, 5, or 10 mg/kg of AA intraperitoneally and pain threshold was determined 15 and 60 min later. The antinociceptive effect of chronic administration was also evaluated by intraperitoneal injection (IP) of 3 mg/kg AA for 3 weeks. To determine the role of NMDA receptors, separate groups of animals 30 min after single injection of AA (1 mg/kg) animals received i.p. injection of ketamine (5 mg/kg), MK-801 (0.01 mg/kg), or glutamate (1000 nmol) and were tested 20 min afterwards. Data analyzed by ANOVA and Newman-Keuls tests and p<0.05 were considered as significant.. IP of 3, 5 and 10 mg/kg increased the pain threshold during the second week after CCI (p<0.05, F=3 in tactile allodynia and p<0.01, F=3.2 in thermal and mechanical hyperalgeisa). Chronic administration of AA also produced antinociceptive effect. Ascorbic acid (1 mg/kg, i.p.) inhibited MK-801 and ketamine-induced antinociception response significantly (p<0.001, F=2). It also prevented the analgesic effect of glutamate administration (p<0.001, F=2).. The results indicated that AA produced a dose-dependent antinociceptive effect that seems to mediate through its interaction with NMDA receptors.

Topics: Analgesics; Animals; Ascorbic Acid; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Ketamine; Ligation; Male; Neuralgia; Pain Threshold; Peripheral Nerve Injuries; Rats; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve

Pain is an emotion and neuropathic pain symptoms are modulated by supraspinal structures such as the amygdala. The central nucleus of the amygdala is often called the 'nociceptive amygdala', but little is known about the role of the basolateral amygdala. Here, we monitored the mechanical nociceptive thresholds in a mouse model of neuropathic pain and infused modulators of the glutamate/GABAergic transmission in the basolateral nucleus of the amygdala (BLA) via chronically-implanted cannulas. We found that an N-methyl-D-aspartate-type glutamate receptor antagonist (MK-801) exerted a potent antiallodynic effect, whereas a transient allodynia was induced after perfusion of bicuculline, a GABA(A) receptor antagonist. Potentiating GABA(A) receptor function using diazepam or etifoxine (a non-benzodiazepine anxiolytic) fully but transiently alleviated mechanical allodynia. Interestingly, the antiallodynic effect of etifoxine disappeared in animals that were incapable of producing 3α-steroids. Diazepam had a similar effect but of shorter duration. As indicated by patch-clamp recordings of BLA neurons, these effects were mediated by a potentiation of GABA(A) receptor-mediated synaptic transmission. Together with a presynaptic elevation of miniature inhibitory postsynaptic current frequency, the duration and amplitude of GABA(A) miniature inhibitory postsynaptic currents were also increased (postsynaptic effect). The analgesic contribution of endogenous neurosteroid seemed to be exclusively postsynaptic. This study highlights the importance of the BLA and the local inhibitory/excitatory neuronal network activity while setting the mechanical nociceptive threshold. Furthermore, it appears that promoting inhibition in this specific nucleus could fully alleviate pain symptoms. Therefore, the BLA could be a novel interesting target for the development of pharmacological or non-pharmacological therapies.

Topics: Animals; Basolateral Nuclear Complex; Dizocilpine Maleate; GABA-A Receptor Agonists; GABA-A Receptor Antagonists; Inhibitory Postsynaptic Potentials; Male; Mice; Mice, Inbred C57BL; Neuralgia; Receptors, GABA-A

Amygdala is involved in processing of primary emotions and particularly its central nucleus (CeA) also in pain control. Here we studied mechanisms mediating the descending control of mechanical hypersensitivity by the CeA in rats with a peripheral neuropathy in the left hind limb. For drug administrations, the animals had a guide cannula in the right CeA and an intrathecal catheter or another guide cannula in the medullary raphe. Hypersensitivity was tested with monofilaments. Glutamate administration in the CeA produced a bidirectional effect on hypersensitivity that varied from an increase at a low-dose (9μg) to a reduction at high doses (30-100μg). The increase but not the reduction of hypersensitivity was prevented by blocking the amygdaloid NMDA receptor with a dose of MK-801 that alone had no effects. The glutamate-induced increase in hypersensitivity was reversed by blocking the spinal 5-HT3 receptor with ondansetron, whereas the reduction in hypersensitivity was reversed by blocking the spinal 5-HT1A receptor with WAY-100635. Both the increase and decrease of hypersensitivity induced by amygdaloid glutamate treatment were reversed by medullary administration of a 5-HT1A agonist, 8-OH-DPAT, that presumably produced autoinhibition of serotonergic cell bodies in the medullary raphe. The results indicate that depending on the dose, glutamate in the CeA has a descending facilitatory or inhibitory effect on neuropathic pain hypersensitivity. Serotoninergic raphe neurons are involved in mediating both of these effects. Spinally, the 5-HT3 receptor contributes to the increase and the 5-HT1A receptor to the decrease of neuropathic hypersensitivity induced by amygdaloid glutamate.

Topics: 8-Hydroxy-2-(di-n-propylamino)tetralin; Amygdala; Animals; Dizocilpine Maleate; Glutamic Acid; Male; Neuralgia; Ondansetron; Pain Measurement; Peripheral Nervous System Diseases; Piperazines; Pyridines; Rats; Rats, Wistar; Receptor, Serotonin, 5-HT1A; Receptors, N-Methyl-D-Aspartate; Receptors, Serotonin, 5-HT3; Serotonin 5-HT1 Receptor Agonists; Serotonin 5-HT1 Receptor Antagonists; Serotonin 5-HT3 Receptor Agonists; Serotonin 5-HT3 Receptor Antagonists; Spinal Cord

As previously reported, magnesium ions (Mg(2+)) administered in relatively low doses markedly potentiated opioid analgesia in neuropathic pain, in which the effectiveness of opioids is limited. Considering that Mg(2+) behaves like an N-methyl-D-aspartate receptor antagonist, the effect of this ion on the analgesic action of morphine was compared with that of MK-801. Acute pain was evoked by mechanical or thermal stimuli, whereas neuropathic hyperalgesia was induced by streptozotocin (STZ) administration. Magnesium sulphate (40 mg/kg i.p.) or MK-801 (0.05 mg/kg s.c.) administered alone did not modify the nociceptive threshold to acute stimuli or the streptozotocin hyperalgesia but significantly augmented the analgesic action of morphine (5 mg/kg i.p.). Furthermore, if these drugs (i.e. magnesium sulphate and MK-801) were applied concomitantly, a clear additive effect on the analgesic action of morphine occurred in both models of pain. Possible explanations of these observations are discussed.

Topics: Analgesics, Opioid; Animals; Disease Models, Animal; Dizocilpine Maleate; Drug Synergism; Excitatory Amino Acid Antagonists; Hyperalgesia; Magnesium Sulfate; Male; Morphine; Neuralgia; Pain; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Streptozocin

Paclitaxel, which is widely used for the treatment of solid tumors, causes neuropathic pain via poorly understood mechanisms. Previously, we have demonstrated that lysophosphatidic acid (LPA) and its receptors (LPA1 and LPA3) are required for the initiation of peripheral nerve injury-induced neuropathic pain. The present study aimed to clarify whether LPA and its receptors could mediate paclitaxel-induced neuropathic pain.. Intraperitoneal administration of paclitaxel triggered a marked increase in production of LPA species (18:1-, 16:0-, and 18:0-LPA) in the spinal dorsal horn. Also, we found significant activations of spinal cytosolic phospholipase A2 and calcium-independent phospholipase A2 after the paclitaxel treatment. The paclitaxel-induced LPA production was completely abolished not only by intrathecal pretreatment with neurokinin 1 (NK1) or N-methyl-D-aspartate (NMDA) receptor antagonist, but also in LPA1 receptor-deficient (Lpar1-/-) and LPA3 receptor-deficient (Lpar3-/-) mice. In addition, the pharmacological blockade of NK1 or NMDA receptor prevented a reduction in the paw withdrawal threshold against mechanical stimulation after paclitaxel treatments. Importantly, the paclitaxel-induced mechanical allodynia was absent in Lpar1-/- and Lpar3-/- mice.. These results suggest that LPA1 and LPA3 receptors-mediated amplification of spinal LPA production is required for the development of paclitaxel-induced neuropathic pain.

Topics: Animals; Antineoplastic Agents, Phytogenic; Disease Models, Animal; Dizocilpine Maleate; Lysophospholipids; Male; Mice; Mice, Inbred C57BL; Mice, Transgenic; Neuralgia; Paclitaxel; Pain Measurement; Phospholipases A2; Piperidines; Receptors, Lysophosphatidic Acid; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Spinal Cord Dorsal Horn; Time Factors

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.

Topics: Animals; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Hyperalgesia; Immunohistochemistry; Injections, Spinal; Ligation; Microglia; Neuralgia; Pain Threshold; Rats; Rats, Sprague-Dawley; Spinal Cord; Spinal Nerves; Time

Cytokines such as interleukins are known to be involved in the development of neuropathic pain through activation of neuroglia. However, the role of chemokine (C-C motif) ligand 1 (CCL-1), a well-characterized chemokine secreted by activated T cells, in the nociceptive transmission remains unclear. We found that CCL-1 was upregulated in the spinal dorsal horn after partial sciatic nerve ligation. Therefore, we examined actions of recombinant CCL-1 on behavioural pain score, synaptic transmission, glial cell function and cytokine production in the spinal dorsal horn. Here we show that CCL-1 is one of the key mediators involved in the development of neuropathic pain. Expression of CCL-1 mRNA was mainly detected in the ipsilateral dorsal root ganglion, and the expression of specific CCL-1 receptor CCR-8 was upregulated in the superficial dorsal horn. Increased expression of CCR-8 was observed not only in neurons but also in microglia and astrocytes in the ipsilateral side. Recombinant CCL-1 injected intrathecally (i.t.) to naive mice induced allodynia, which was prevented by the supplemental addition of N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801. Patch-clamp recordings from spinal cord slices revealed that application of CCL-1 transiently enhanced excitatory synaptic transmission in the substantia gelatinosa (lamina II). In the long term, i.t. injection of CCL-1 induced phosphorylation of NMDA receptor subunit, NR1 and NR2B, in the spinal cord. Injection of CCL-1 also upregulated mRNA level of glial cell markers and proinflammatory cytokines (IL-1β, TNF-α and IL-6). The tactile allodynia induced by nerve ligation was attenuated by prophylactic and chronic administration of neutralizing antibody against CCL-1 and by knocking down of CCR-8. Our results indicate that CCL-1 is one of the key molecules in pathogenesis, and CCL-1/CCR-8 signaling system can be a potential target for drug development in the treatment for neuropathic pain.

Topics: Analgesics; Animals; Cells, Cultured; Chemokine CCL1; Dizocilpine Maleate; Ganglia, Spinal; Gene Expression; Gene Knockdown Techniques; Glutamic Acid; Hyperalgesia; Injections, Spinal; Male; Mice; Mice, Transgenic; Neuralgia; Neuroglia; Nociception; Peripheral Nerve Injuries; Phosphorylation; Protein Processing, Post-Translational; Receptors, CCR8; Receptors, N-Methyl-D-Aspartate; RNA, Small Interfering; Spinal Cord

Injuries can induce adaptations in pain processing that result in amplification of signaling. One mechanism may be analogous to long-term potentiation and involve the atypical protein kinase C, PKMζ. The possible contribution of PKMζ-dependent and independent amplification mechanisms to experimental neuropathic pain was explored in rats with spinal nerve ligation (SNL) injury. SNL increased p-PKMζ in the rostral anterior cingulate cortex (rACC), a site that mediates, in part, the unpleasant aspects of pain. Inhibition of PKMζ within the rACC by a single administration of ζ-pseudosubstrate inhibitory peptide (ZIP) reversed SNL-induced aversiveness within 24 hours, whereas N-methyl-d-aspartate receptor blockade with MK-801 had no effects. The SNL-induced aversive state (reflecting "spontaneous" pain), was re-established in a time-dependent manner, with full recovery observed 7 days post-ZIP administration. Neither rACC ZIP nor MK-801 altered evoked responses. In contrast, spinal ZIP or MK-801, but not scrambled peptide, transiently reversed evoked hypersensitivity, but had no effect on nerve injury-induced spontaneous pain. PKMζ phosphorylation was not altered by SNL in the spinal dorsal horn. These data suggest that amplification mechanisms contribute to different aspects of neuropathic pain at different levels of the neuraxis. Thus, PKMζ-dependent amplification contributes to nerve injury-induced aversiveness within the rACC. Moreover, unlike mechanisms maintaining memory, the consequences of PKMζ inhibition within the rACC are not permanent in neuropathic pain, possibly reflecting the re-establishment of amplification mechanisms by ongoing activity of injured nerves. In the spinal cord, however, both PKMζ-dependent and independent mechanisms contribute to amplification of evoked responses, but apparently not spontaneous pain.

Topics: Animals; Dizocilpine Maleate; Gyrus Cinguli; Male; Neuralgia; Peptides; Protein Kinase C; Rats; Rats, Sprague-Dawley; Signal Transduction; Spinal Cord; Spinal Nerves

The expression of NF-κB in the spinal cord is associated with neuropathic pain. However, little is known about its expression beyond the spinal cord. Here we examined a spatial and temporal pattern of the NF-κB expression in both spinal and supraspinal regions. After chronic constriction injury (CCI) of the sciatic nerve, NF-κB (p65) expression was significantly increased in the ipsilateral spinal cord. In contrast, the NF-κB expression in the contralateral primary somatosensory cortex was decreased with no significant differences seen in the thalamus. In the contralateral anterior cingulate cortex, the NF-κB expression was increased significantly on day 14 as compared with the sham group. In the contralateral amygdala, the NF-κB expression showed a time-dependent downregulation after CCI, which became significant on day 14. MK-801 reduced nociceptive behaviors and reversed the direction of NF-κB expression. These results indicate that the CCI-induced expression of p65 NF-κB is both time-dependent and region-specific, in areas that process both sensory-discriminative and motivational-affective dimensions of pain.. This article presents a spatiotemporal mapping of the NF-κB expression in spinal and supraspinal regions after peripheral nerve injury. These findings point to an involvement of NF-κB beyond the spinal cord in both the sensory discriminative and emotional affective aspects of neuropathic pain processing.

Topics: Animals; Disease Models, Animal; Dizocilpine Maleate; Functional Laterality; Gene Expression Regulation; Male; Neuralgia; Neuroprotective Agents; NF-kappa B; Pain Measurement; Peripheral Nervous System Diseases; Rats; Somatosensory Cortex; Spinal Cord; Synaptotagmin I; Thalamus

Dopaminergic and glutamatergic mechanisms are involved in the development and modulation of neuropathy. Cytokines and neurotrophins can be also involved in the supraspinal maintenance of neuropathic pain. We assessed the effects of chronic intraperitoneal (ip) injection of dizocilpine (MK-801), a N-methyl-d-Aspartate (NMDA) noncompetitive receptor antagonist, or apomorphine (APO), a dopamine (DA) D1 and D2 receptor agonist, on neuropathic manifestations in the chronic constriction injury (CCI) and the spared nerve injury (SNI) models of neuropathy in rats. Six groups of rats were subjected to SNI or CCI (3 groups each) neuropathy and 5-7 days later received daily ip injections of saline, MK-801, or APO for two weeks. An additional control group was subjected to sham surgery without nerve lesion or injections. Rats were then sacrificed, and levels of IL-1β, IL-6, NGF, BDNF and GDNF were determined in the cingulum, striatum, and hippocampus. In both models, the neuropathy seen in the saline group was associated with decreased BDNF and an increase in IL-1β, IL-6, NGF and GDNF in most brain regions when compared to sham group. Chronic systemic MK-801 or APO injections decreased the neuropathic manifestations in both models, increased the BDNF level and modulated the other cytokines and neurotrophins. This modulation depended on the neuropathy model and the region/side of the brain studied. Our results showed that the changes in surpraspinal cytokines and neurotrophins could parallel neuropathic manifestations. These changes and the observed hyperalgesia can be modulated by chronic systemic injections of NMDA antagonists or DA agonists.

Topics: Animals; Apomorphine; Brain; Cytokines; Disease Models, Animal; Dizocilpine Maleate; Female; Interleukin-1beta; Nerve Growth Factors; Neuralgia; Rats; Rats, Sprague-Dawley

Dopaminergic and glutamatergic inputs converge on nucleus accumbens (NAC) and affect the neuropathic pain. We tested the effects of daily systemic administration of dizocilpine (MK-801), a N-methyl-d-Aspartate (NMDA) noncompetitive receptor antagonist, or apomorphine (APO), a dopamine (DA) D1 and D2 receptor agonist, on neuropathic manifestations in the chronic constriction injury (CCI) and the spared nerve injury (SNI) models of mononeuropathy in rats. Six groups of rats were subjected to CCI or SNI neuropathy and 5-7 days later received daily intraperitoneal (ip) injections of saline, MK-801, or APO for two weeks. Tests for nociception and motor behaviors were performed at regular intervals. Tactile and cold allodynia were assessed using von Frey hairs or acetone drops, respectively. Heat hyperalgesia was assessed by the paw withdrawal test. Tests were performed before administering the daily injections. Another four groups of rats were subjected to SNI surgery, and then had their NAC (contralateral to the lesioned paw) perfused for two weeks with MK-801, saline, APO+ascorbic acid, or ascorbic acid alone using mini-osmotic pumps. Behavioral manifestations were assessed as above. Systemic daily injections of MK-801 and APO markedly attenuated the neuropathic manifestations in the CCI and SNI models with a minimal effect on cold allodynia. The same results were seen in the SNI model with chronic perfusion of NAC. Our results suggest that daily systemic administration of DA agonists and NMDA antagonists can attenuate neuropathic pain manifestations and that the NAC is involved in the modulation of neuropathic-like behaviors.

Topics: Animals; Apomorphine; Disease Models, Animal; Dizocilpine Maleate; Female; Mononeuropathies; Motor Activity; Neuralgia; Nucleus Accumbens; Pain Measurement; Random Allocation; Rats; Rats, Sprague-Dawley

The hippocampus plays an important role in learning and memory and possibly contributes to the formation of pain-related memory and emotional responses. However, there is currently little data linking the hippocampus to neuropathic pain. It has been reported that NF-κB is an important regulatory factor in memory consolidation within the hippocampus. This study aims to examine a possible relationship between the hippocampal NF-κB expression and nerve injury-induced thermal hyperalgesia using a rat model of constriction sciatic nerve injury (CCI). Immunofluorescence and Western blot analysis were performed to detect and quantify the hippocampal NF-κB expression. Thermal hyperalgesia was examined on day 0 and postoperative days 1, 7 and 14. The nuclear portion of the p65 NF-κB expression was significantly increased on the contralateral side on days 7 and 14 as well as significantly increased on the ipsilateral side on day 14 as compared to the sham control group. Intraperitoneal administration of MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist, reduced hyperalgesia and modulated the NF-κB expression in the contralateral side of hippocampus. These results suggest an association between the hippocampal NF-κB expression and the behavioral manifestation of thermal hyperalgesia, which is likely to be mediated through activation of the NMDA receptor.

Topics: Animals; Blotting, Western; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Fluorescent Antibody Technique; Functional Laterality; Hippocampus; Hot Temperature; Hyperalgesia; Ligation; Male; Neuralgia; NF-kappa B; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve

Recent studies have shown that leptin (an adipocytokine) played an important role in nociceptive behavior induced by nerve injury, but the cellular mechanism of this action remains unclear. Using the whole-cell patch-clamp recording from rat's spinal cord slices, we showed that superfusion of leptin onto spinal cord slices dose-dependently enhanced N-methyl-d-aspartate (NMDA) receptor-mediated currents in spinal cord lamina II neurons. At the cellular level, the effect of leptin on spinal NMDA-induced currents was mediated through the leptin receptor and the JAK2/STAT3 (but not PI3K or MAPK) pathway, as the leptin effect was abolished in leptin receptor-deficient (db/db) mice and inhibited by a JAK/STAT inhibitor. Moreover, we demonstrated in naïve rats that a single intrathecal administration of leptin enhanced spontaneous biting, scratching, and licking behavior induced by intrathecal NMDA and that repeated intrathecal administration of leptin elicited thermal hyperalgesia and mechanical allodynia, which was attenuated by the noncompetitive NMDA receptor antagonist MK-801. Intrathecal leptin also upregulated the expression of NMDA receptors and pSTAT3 within the rat's spinal cord dorsal horn, and intrathecal MK-801 attenuated this leptin effect as well. Our data demonstrate a relationship between leptin and NMDA receptor-mediated spinal neuronal excitation and its functional role in nociceptive behavior. Since leptin contributes to nociceptive behavior induced by nerve injury, the present findings suggest an important cellular link between the leptin's spinal effect and the NMDA receptor-mediated cellular mechanism of neuropathic pain. A functional link is demonstrated between leptin, an adipocytokine, and the cellular mechanisms of neuropathic pain via enhancement of function and expression of spinal N-methyl-d-aspartate receptors.

Topics: Analysis of Variance; Animals; Behavior, Animal; Dizocilpine Maleate; Drug Interactions; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Hyperalgesia; In Vitro Techniques; Leptin; Male; Membrane Potentials; Mice; Mice, Knockout; N-Methylaspartate; Neuralgia; Pain Measurement; Patch-Clamp Techniques; Physical Stimulation; Rats; Rats, Sprague-Dawley; Receptors, Leptin; Receptors, N-Methyl-D-Aspartate; Sensory Receptor Cells; Signal Transduction; Spinal Cord; STAT3 Transcription Factor; Up-Regulation

Spinal N-methyl d-aspartate receptor (NMDAR) plays a pivotal role in nerve injury-induced central sensitization. Recent studies suggest that NMDAR also contributes to neuron-astrocyte signaling. c-Jun N-terminal kinase (JNK) is persistently and specifically activated (indicated by phosphorylation) in spinal cord astrocytes after nerve injury and thus it is considered as a dependable indicator of pain-related astrocytic activation. NMDAR-mediated JNK activation in spinal dorsal horn might be an important form of neuron-astrocyte signaling in neuropathic pain. In the present study, we observed that intrathecal injection of MK-801, a noncompetitive NMDA receptor antagonist, or Ro25-6981 and ifenprodil, which are selective antagonists of NR2B-containing NMDAR each significantly reduced nerve injury-induced JNK activation. Double immunostaining showed that NR2B was highly expressed in neurons, indicating the effect of NMDAR antagonists on JNK activation was indirect. We further observed that intrathecal injection of NMDA (twice a day for 3 days) significantly increased spinal JNK phosphorylation. Besides, NMDAR-related JNK activation could be blocked by a neuronal nitric oxide synthase (nNOS) selective inhibitor (7-nitroindazole sodium salt) but not by a nNOS sensitive guanylyl cyclase inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one). Finally, real-time RT-PCR and immunostaining showed that nerve injury-induced interleukin-1beta expression was dependent on astrocytic JNK activation. Treatments targeting NMDAR-nNOS pathway also influenced interleukin-1beta expression, which further confirmed our hypothesis. Taken together, our results suggest that neuronal NMDAR-nNOS pathway could activate astrocytic JNK pathway. Excitatory neuronal transmission initiates astrocytic activation-induced neuroinflammation in this way, which contributes to nerve injury-induced neuropathic pain.

Topics: Animals; Astrocytes; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Guanylate Cyclase; Hyperalgesia; Indazoles; JNK Mitogen-Activated Protein Kinases; Male; Neuralgia; Neurons; Nitric Oxide Synthase Type I; Pain Measurement; Phenols; Phosphorylation; Piperidines; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Signal Transduction; Spinal Cord

Plasticity in the spinal dorsal horn is thought to underlie the development of neuropathic pain. Calcineurin (protein phosphatase 3) plays an important role in plasticity in the brain. Here we examined whether chronic constriction injury (CCI) of the sciatic nerve modifies calcineurin expression in the spinal dorsal horn. Male rats were assigned to control (uninjured), sham-operated or CCI groups. CCI animals exhibited both a shift in weight bearing and a reduction in paw withdrawal latencies as signs of pain behavior. At 3 days (3D) the pain behavior was associated with a significant increase in calcineurin gene expression, enzyme activity and content of its Aα isoform in the ipsilateral spinal dorsal horn. In contrast, while the pain behavior persisted at 7 days (7D) calcineurin gene expression returned to control levels and activity and protein content decreased. A single intrathecal injection of MK-801 15 min before the ligation attenuated both signs of pain behavior in 3D but not 7D CCI animals. The same pre-treatment also prevented the CCI-associated increases in calcineurin in these animals. These data suggested an involvement of calcineurin in CCI-elicited neuropathic pain. The time-dependent divergent changes in calcineurin expression may underlie the different phases of neuropathic pain development.

Topics: Animals; Calcineurin; Constriction; Dizocilpine Maleate; Gene Expression; Immunoblotting; Male; Neuralgia; Neuronal Plasticity; Neuroprotective Agents; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Sciatic Nerve

Patients suffering from neuropathic pain are difficult to treat and many methods are used to resolve this issue. In this study, we used a model of neuropathic pain comprising rats with chronic constriction injury (CCI) on the left sciatic nerve to investigate the chronic effect of gabapentin via intrathecal administration. We also observed the expression of dorsal spinal protein kinase C gamma subunit (PKCγ) and other pain-related molecules in the spinal area which included cyclooxygenase 2 (COX2), c-Fos and cyclic AMP-dependent transcription factor (ATF3) in the neuropathic pain animals.. Male Sprague-Dawley (SD) rats (250-380 g) were randomly assigned to four groups, i.e., control, gabapentin (Gaba), MK801, and gabapentin plus MK801 (Gaba+M) groups. A PE-5 catheter was inserted into the lumbar spine area via the cervical spine area. CCI was performed the following day after the intrathecal catheter implant surgery. Gabapentin (1.05 μmol/day) was then given the following day after CCI surgery. Intrathecal gabapentin was administrated for 14 consecutive days. Pain-related behavior was assessed every 2 days thereafter by measuring the latency of foot withdrawal elicited by noxious radiant heat or Von Frey microfilament applied to the hind-paw plantar surface. MK801 (30 μg/day), an N-methyl-D-aspartate (NMDA) receptor blocker, was also added for potential effect. The tissue of dorsal horn of the lumbar spine was harvested on the 14(th) day for the expression of COX2, c-Fos, ATF3 and PKCγ with Western blotting, and positive finding protein was then checked on 7(th) day for further evaluation.. The beneficial effect of intrathecal gabapentin of statistic significance on thermal duration and mechanical microfilament appeared after 7-day and 11-day consecutive treatment, respectively. Furthermore, the NMDA receptor blocker also potentiated the effect on the behavior of thermal and mechanical stimulations. Gabapentin had no effect on the expression of COX2, c-Fos and ATF3. Interestingly, the expression of PKCγ in the spinal cord was initially inhibited by gabapentin on the 7(th) day but was potentiated on the 14(th) day.. Our results indicate that chronic intrathecal gabapentin has beneficial effects on the behaviors of both thermal and mechanical stimulations in the neuropathic pain animals and the NMDA blocker can potentiate this effect. Furthermore, gabapentin has biphasic effect on the expression of PKCγ in the spinal cord on Day 7 and Day 14 for the model rats with CCI.

Topics: Amines; Analgesics; Animals; Cyclohexanecarboxylic Acids; Dizocilpine Maleate; Gabapentin; gamma-Aminobutyric Acid; Injections, Spinal; Male; Neuralgia; Protein Kinase C; Rats; Rats, Sprague-Dawley; Spinal Cord

Plasticity in the spinal dorsal horn is thought to underlie, at least in part, pain behavior after peripheral nerve injury. Homer1 proteins play an important role in synaptic plasticity through an activity-dependent remodeling of the postsynaptic density (PSD). In this study, we examined the early consequences of the loose ligation of the sciatic nerve on the levels of Homer1a and Homer1b/c proteins in the PSD of spinal dorsal horn neurons.. Male rats were randomly assigned to control, sham-operated, or ligated groups. Four hours after sciatic exposure or ligation, the animals were anesthetized and killed. Dorsal horn ipsilateral and contralateral quadrants were homogenized and centrifuged to obtain a PSD-containing LP1 fraction. Homer1 isoforms were identified in Western immunoblots. In some animals, Homer1 small interfering RNA (siRNA), nontarget siRNA, MK-801, or U01026 was injected intrathecally before surgery to assess the effects of this treatment on the levels of Homer1 isoforms and on 2 signs of injury-associated pain behavior, a shift in weight-bearing distribution and thermal hyperalgesia.. In ligated animals, the protein levels of Homer1a increased and those of Homer1b/c decreased in the ipsilateral LP1 fraction of the spinal dorsal horn. In contrast, no changes were detected in the contralateral LP1 fraction of ligated animals or the ipsilateral or contralateral LP1 fraction of sham-operated animals. Intrathecal injections of Homer1 siRNA, but not nontarget siRNA, 2 h before the ligation prevented the accumulation of Homer1a and loss of Homer1b/c in the ipsilateral LP1 fraction. The same pretreatment with Homer1 siRNA also alleviated both a shift in weight-bearing behavior and thermal hyperalgesia in the ligated animals. Intrathecal injections of MK-801 or U0126 15 min before the ligation similarly prevented the injury-associated changes in Homer1 protein levels and the behavioral signs of pain.. The ligation-associated changes in the protein levels of Homer1a and Homer1b/c in the ipsilateral PSD of spinal dorsal horn neurons may be an important early reflection of the injury-associated plasticity that in time leads to the development of persistent pain.

Topics: Animals; Behavior, Animal; Butadienes; Carrier Proteins; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Homer Scaffolding Proteins; Injections, Spinal; Ligation; Male; Mitogen-Activated Protein Kinase 1; Mitogen-Activated Protein Kinase 3; Neuralgia; Neuronal Plasticity; Nitriles; Posterior Horn Cells; Protein Kinase Inhibitors; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; RNA Interference; RNA, Small Interfering; Sciatic Nerve; Sciatic Neuropathy; Time Factors

Our previous study has reported that electroacupuncture (EA) at low frequency of 2 Hz had greater and more prolonged analgesic effects on mechanical allodynia and thermal hyperalgesia than that EA at high frequency of 100 Hz in rats with neuropathic pain. However, how EA at different frequencies produces distinct analgesic effects on neuropathic pain is unclear. Neuronal plastic changes in spinal cord might contribute to the development and maintenance of neuropathic pain. In the present study, we investigated changes of spinal synaptic plasticity in the development of neuropathic pain and its modulation by EA in rats with neuropathic pain. Field potentials of spinal dorsal horn neurons were recorded extracellularly in sham-operated rats and in rats with spinal nerve ligation (SNL). We found for the first time that the threshold for inducing long-term potentiation (LTP) of C-fiber-evoked potentials in dorsal horn was significantly lower in SNL rats than that in sham-operated rats. The threshold for evoking the C-fiber-evoked field potentials was also significantly lower, and the amplitude of the field potentials was higher in SNL rats as compared with those in the control rats. EA at low frequency of 2 Hz applied on acupoints ST 36 and SP 6, which was effective in treatment of neuropathic pain, induced long-term depression (LTD) of the C-fiber-evoked potentials in SNL rats. This effect could be blocked by N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 and by opioid receptor antagonist naloxone. In contrast, EA at high frequency of 100 Hz, which was not effective in treatment of neuropathic pain, induced LTP in SNL rats but LTD in sham-operated rats. Unlike the 2 Hz EA-induced LTD in SNL rats, the 100 Hz EA-induced LTD in sham-operated rats was dependent on the endogenous GABAergic and serotonergic inhibitory system. Results from our present study suggest that (1) hyperexcitability in the spinal nociceptive synaptic transmission may occur after nerve injury, which may contribute to the development of neuropathic pain; (2) EA at low or high frequency has a different effect on modulating spinal synaptic plasticities in rats with neuropathic pain. The different modulation on spinal LTD or LTP by low- or high-frequency EA may be a potential mechanism of different analgesic effects of EA on neuropathic pain. LTD of synaptic strength in the spinal dorsal horn in SNL rats may contribute to the long-lasting analgesic effects of EA at 2 Hz.

Topics: Action Potentials; Animals; Bicuculline; Dizocilpine Maleate; Drug Combinations; Electroacupuncture; Excitatory Amino Acid Antagonists; GABA Antagonists; Hyperesthesia; Ligation; Long-Term Synaptic Depression; Male; Methysergide; Naloxone; Narcotic Antagonists; Nerve Fibers, Unmyelinated; Neuralgia; Neuronal Plasticity; Pain Threshold; Posterior Horn Cells; Rats; Rats, Sprague-Dawley; Serotonin Antagonists; Spinal Nerves; Synapses; Time Factors

We have investigated the effect of treatment with N(omega)-nitro-l-arginine methylester (l-NAME), a non-selective nitric oxide synthase inhibitor (NOS), both before and after the induction of mechanical allodynia by tight ligation of the left L5 and L6 spinal nerves in rats (SNL rats). The degree of mechanical allodynia was measured by tactile threshold for paw flinching with von Frey filaments. Intraperitoneal (i.p.) administration of l-NAME (3-30 mg/kg) 1 week after the spinal nerve ligation produced a dose-dependent reduction of the behavioral signs of mechanical allodynia, but the effect was not reversed by pretreatment with l-arginine (300 mg/kg). N(omega)-Nitro-l-arginine (l-NNA, i.p., 30 mg/kg), aminoguanidine (AG, i.p., 30 mg/kg) and a potent neuronal NOS inhibitor (LY457963, i.p., 30 mg/kg) did not reduce mechanical sensitivity in the SNL rats. Furthermore, using an ex vivo NOS activity assay, l-NAME partially inhibited the spinal NOS activity, whereas LY457963 almost completely inhibited the spinal NOS activity. Prior administration of l-NAME (i.p., 30 mg/kg) or of MK-801 (0.5 mg/kg), an NMDA antagonist, 30 min before the spinal nerve ligation significantly prevented the development of mechanical allodynia after spinal nerve ligation for an extended period of time. High doses of l-arginine (100 mg/kg or 300 mg/kg, i.p.), however, did not reverse the preemptive effect of l-NAME. These results suggest that neither the anti-allodynic nor the preemptive effects of l-NAME are mediated by NOS inhibition.

Topics: Animals; Arginine; Dizocilpine Maleate; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Guanidines; Ligation; Male; Neuralgia; NG-Nitroarginine Methyl Ester; Nitric Oxide; Nitric Oxide Synthase; Pain Threshold; Rats; Rats, Sprague-Dawley; Spinal Nerves

Paclitaxel (Taxol) is one of the most effective and frequently used chemotherapeutics for the treatment of solid tumours. However, paclitaxel produces peripheral neurotoxicity with patients reporting sensory abnormalities and neuropathic pain during and often persisting after paclitaxel therapy. The mechanisms underlying this dose-limiting side effect are currently unknown and there are no validated drugs for its prevention or control. Male Sprague-Dawley rats received four intraperitoneal (i.p.) injections on alternate days of 2 mg/kg paclitaxel. Behavioural assessment using von Frey filaments and acetone showed that such paclitaxel treatment induced a pronounced mechanical and cold allodynia/hyperalgesia. Thus these studies aim to test potential analgesics on established paclitaxel-induced pain. Paclitaxel-induced pain appears to be relatively resistant to opioid therapy i.p. 4 mg/kg morphine was ineffective and i.p. 8 mg/kg morphine only elicited up to a 50% reversal of mechanical allodynia/hyperalgesia. Interestingly, a maximally tolerated dose (i.p. 0.2 mg/kg) of the potent NMDA receptor antagonist MK-801 produced no significant reversal of the mechanical allodynia/hyperalgesia suggesting that NMDA receptors have little role in paclitaxel-induced pain. Ethosuximide (i.p. 450 mg/kg) an anti-epileptic and relatively selective T-type calcium channel blocker elicited a near complete reversal of mechanical allodynia/hyperalgesia. Repetitive dosing with ethosuximide (i.p. 100 or 300 mg/kg daily for 3 days) showed a dose-related consistent reversal of mechanical allodynia/hyperalgesia with no evidence of tolerance. Ethosuximide (i.p. 300 mg/kg) also reversed paclitaxel-induced cold allodynia and vincristine-induced mechanical allodynia/hyperalgesia. These data suggest that T-type calcium channels may play a role in chemotherapy-induced neuropathy and moreover identify ethosuximide as a new potential treatment for chemotherapy-induced pain.

Topics: Animals; Behavior, Animal; Calcium Channel Blockers; Dizocilpine Maleate; Dose-Response Relationship, Drug; Drug Administration Schedule; Drug Interactions; Ethosuximide; Excitatory Amino Acid Antagonists; Male; Morphine; Narcotics; Neuralgia; Paclitaxel; Pain Measurement; Physical Stimulation; Rats; Rats, Sprague-Dawley; Time Factors; Vincristine

The central glutamatergic system has been implicated in the pathogenesis of neuropathic pain, and a highly active central glutamate transporter (GT) system regulates the uptake of endogenous glutamate. Here we demonstrate that both the expression and uptake activity of spinal GTs changed after chronic constriction nerve injury (CCI) and contributed to neuropathic pain behaviors in rats. CCI induced an initial GT upregulation up to at least postoperative day 5 primarily within the ipsilateral spinal cord dorsal horn, which was followed by a GT downregulation when examined on postoperative days 7 and 14 by Western blot and immunohistochemistry. Intrathecal administration of the tyrosine kinase receptor inhibitor K252a and the mitogen-activated protein kinase inhibitor PD98059 for postoperative days 1-4 reduced and nearly abolished the initial GT upregulation in CCI rats, respectively. Prevention of the CCI-induced GT upregulation by PD98059 resulted in exacerbated thermal hyperalgesia and mechanical allodynia reversible by the noncompetitive NMDA receptor antagonist MK-801, indicating that the initial GT upregulation hampered the development of neuropathic pain behaviors. Moreover, CCI significantly reduced glutamate uptake activity of spinal GTs when examined on postoperative day 5, which was prevented by riluzole (a positive GT activity regulator) given intrathecally twice a day for postoperative days 1-4. Consistently, riluzole attenuated and gradually reversed neuropathic pain behaviors when the 4 d riluzole treatment was given for postoperative days 1-4 and 5-8, respectively. These results indicate that changes in the expression and glutamate uptake activity of spinal GTs may play a critical role in both the induction and maintenance of neuropathic pain after nerve injury via the regulation of regional glutamate homeostasis, a new mechanism relevant to the pathogenesis of neuropathic pain.

Topics: Amino Acid Transport System X-AG; Animals; Behavior, Animal; Biological Transport; Carbazoles; Constriction; Dizocilpine Maleate; Enzyme Inhibitors; Excitatory Amino Acid Transporter 2; Flavonoids; Glutamate Plasma Membrane Transport Proteins; Glutamic Acid; Hyperalgesia; Indole Alkaloids; Kinetics; Male; Mitogen-Activated Protein Kinases; Neuralgia; Peripheral Nerves; Rats; Rats, Sprague-Dawley; Receptor Protein-Tyrosine Kinases; Riluzole; Spinal Cord; Symporters; Up-Regulation

Whereas tissue injury increases spinal dynorphin expression, the functional relevance of this upregulation to persistent pain is unknown. Here, mice lacking the prodynorphin gene were studied for sensitivity to non-noxious and noxious stimuli, before and after induction of experimental neuropathic pain. Prodynorphin knock-out (KO) mice had normal responses to acute non-noxious stimuli and a mild increased sensitivity to some noxious stimuli. After spinal nerve ligation (SNL), both wild-type (WT) and KO mice demonstrated decreased thresholds to innocuous mechanical and to noxious thermal stimuli, indicating that dynorphin is not required for initiation of neuropathic pain. However, whereas neuropathic pain was sustained in WT mice, KO mice showed a return to baselines by post-SNL day 10. In WT mice, SNL upregulated lumbar dynorphin content on day 10, but not day 2, after injury. Intrathecal dynorphin antiserum reversed neuropathic pain in WT mice at post-SNL day 10 (when dynorphin was upregulated) but not on post-SNL day 2; intrathecal MK-801 reversed SNL-pain at both times. Opioid (mu, delta, and kappa) receptor density and G-protein activation were not different between WT and KO mice and were unchanged by SNL injury. The observations suggest (1) an early, dynorphin-independent phase of neuropathic pain and a later dynorphin-dependent stage, (2) that upregulated spinal dynorphin is pronociceptive and required for the maintenance of persistent neuropathic pain, and (3) that processes required for the initiation and the maintenance of the neuropathic pain state are distinct. Identification of mechanisms that maintain neuropathic pain appears important for strategies to treat neuropathic pain.

Topics: Animals; Chronic Disease; Disease Models, Animal; Dizocilpine Maleate; Dynorphins; Excitatory Amino Acid Antagonists; Hyperesthesia; Immune Sera; Injections, Spinal; Ligation; Lumbosacral Region; Male; Mice; Mice, Knockout; Neuralgia; Pain Measurement; Pain Threshold; Physical Stimulation; Reaction Time; Receptors, Opioid; Spinal Cord; Spinal Nerves

Considering that magnesium and non-competitive NMDA receptor antagonists inhibit the opening of the channel linked to the NMDA receptor, we assessed their effects on mechanical hyperalgesia in two animal models of neuropathic pain (rats with a sciatic nerve ligature and diabetic rats). Our data show that magnesium reverses the hyperalgesia, as does MK-801. These results suggest that magnesium could be an alternative for the treatment of neuropathic pain in patients.

Topics: Animals; Diabetes Mellitus, Experimental; Diabetic Neuropathies; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Hyperalgesia; Magnesium Sulfate; Male; Neuralgia; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve

The unilateral sciatic nerve chronic constriction injury (CCI) model of Bennett and Xie [G.J. Bennett, Y.-K. Xie, A peripheral neuropathy in rat that produces disorders of pain sensation like those seen in man, Pain, 33 (1988) 87-108] shows features of a neuropathic pain state. We examined mechanical hyperalgesia and Fos protein staining in the lumbar spinal cord 1, 7, 14 and 28 days after unilateral CCI to the sciatic nerve or sham operation. In addition, we examined the effect of the NMDA antagonist MK-801 (0.3 mg/kg s.c. administered 30 min prior to and 6 h following operation) on Fos expression and hyperalgesia at 28 days. CCI animals were hyperalgesic compared to the sham operated animals at 14 and 28 days post injury. MK-801 reduced hyperalgesia by 68% in CCI animals on day 28 (p=0.0001). In the spinal cord, Fos positive cells were present bilaterally in deeper laminae in both sham and CCI animals at all time points examined. Relatively few Fos positive cells were present in laminae 1-2 at any time point examined. At days 1 and 7, there were increased numbers of Fos positive cells ipsilaterally in the deeper laminae of the spinal cord in CCI animals compared to sham animals, but by 14 and 28 days Fos counts were similar in sham and CCI despite the obvious behavioural differences between the two groups. Fos counts ipsilateral to the injury in laminae 3-10 correlated with hyperalgesia scores in the CCI but not sham animals. Analysis at the 28-day time point showed that MK-801 differentially affected Fos expression: MK-801 significantly reduced the Fos count bilaterally in laminae 3-10 in the CCI but not in the sham group animals. These results indicate that Fos expression is initiated by different peripheral and central mechanisms following nerve injury or sham operation.

Topics: Animals; Behavior, Animal; Chronic Disease; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Hyperalgesia; Male; Nerve Compression Syndromes; Neuralgia; Peripheral Nerve Injuries; Proto-Oncogene Proteins c-fos; Rats; Rats, Inbred Strains; Receptors, N-Methyl-D-Aspartate; Spinal Cord

Chronic constriction injury (CCI) of the sciatic nerve results in persistent mechanical hyperalgesia together with Fos protein expression in the lumbar spinal cord. We have examined the relationship between mechanical hyperalgesia and Fos expression within the lumbar spinal cord on days 14, 35 and 55 after either CCI or sham operation. To determine the role of NMDA receptor mechanisms in the maintenance of hyperalgesia and Fos expression, the NMDA antagonist MK-801 (0.3 mg kg-1 s.c.) was administered daily on days 28 to 34 after operation. CCI animals developed unilateral hind limb hyperalgesia that persisted unchanged from days 14 to 55 of the study. MK-801 treatment reduced hyperalgesia by 57% (p=0.02) on day 35 in CCI animals but did influence hyperalgesia at day 55. In the spinal cord, Fos positive cells were present bilaterally throughout laminae 3-10 at all time points examined in both CCI and sham group animals. Fos counts ipsilateral to the side of injury in laminae 3-10 correlated significantly with hyperalgesia scores in the CCI but not sham animals. MK-801 treatment resulted in a suppression of Fos expression in ipsilateral laminae 3-4 (p=0.0017) and laminae 5-10 (p=0.0026) of CCI animals on day 35. Fos expression in sham group animals was not inhibited by MK-801 treatment at day 35. These results indicate that Fos expression is maintained by differing mechanisms following nerve injury or sham operation. The functional consequences of Fos expression following nerve injury and sham operation are discussed.

Topics: Animals; Behavior, Animal; Chronic Disease; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Genes, Immediate-Early; Hyperalgesia; Male; Nerve Compression Syndromes; Neuralgia; Peripheral Nerve Injuries; Proto-Oncogene Proteins c-fos; Rats; Rats, Inbred Strains; Receptors, N-Methyl-D-Aspartate; Spinal Cord

The co-administration of morphine at spinal (i.th.) and supraspinal (i.c.v.) sites to the same rat produces antinociceptive synergy, a phenomenon which may underlie the clinical analgesic utility of this drug. In animals with peripheral nerve injury, however, the antinociceptive potency and efficacy of i.th. morphine is significantly decreased. Here, the possible loss of spinal/supraspinal morphine antinociceptive synergy and relationship to elevation of spinal dynorphin content was studied. Ligation of lumbar spinal nerves resulted in elevated dynorphin in the ipsilateral lumbar and sacral spinal cord. In sham-operated rats supraspinal/spinal co-administration of morphine produced synergistic antinociception which was unaffected by i.th. MK-801 or dynorphin A((1-17)) antiserum. In nerve-injured rats, i.th. morphine was inactive against tactile allodynia and showed diminished in potency against acute nociception without supraspinal/spinal antinociceptive synergy. Antiserum to dynorphin A((1-17)) or the non-competitive NMDA antagonist MK-801 increased the antinociceptive potency of i.th. morphine, restored supraspinal/spinal morphine antinociceptive synergy and elicited a dose-related i.th. morphine antiallodynic action. These agents did not demonstrate antinociceptive or antiallodynic activity alone and did not alter morphine actions in sham-operated animals. The loss of spinal/supraspinal antinociceptive synergy and lack of antiallodynic activity of spinal morphine appear to be due to the elevation across multiple spinal segments of dynorphin following nerve injury. Pathological actions of elevated dynorphin may directly or indirectly modulate the NMDA receptor, result in a loss of supraspinal/spinal morphine synergy and may thus account for the decreased clinical analgesic efficacy of morphine in peripheral neuropathies.

Topics: Analgesics, Opioid; Animals; Dizocilpine Maleate; Drug Synergism; Dynorphins; Excitatory Amino Acid Antagonists; Immune Sera; Injections, Intraventricular; Injections, Spinal; Male; Morphine; Neuralgia; Pain Measurement; Peripheral Nerve Injuries; Rats; Rats, Sprague-Dawley; Touch

Antagonists of NMDA glutamate receptors have been shown to alleviate neuropathic pain in rats and humans. However, NMDA antagonists can cause significant side effects ranging from behavioral disturbances to injury of neurons in the posterior cingulate/retrosplenial (PC/RS) cortex. We have found that alpha-2 adrenergic agonists prevent the PC/RS neurotoxic side effects of NMDA antagonists. In the present study of adult female rats subjected to sciatic nerve ligation (Bennett neuropathic pain model) and tested for paw withdrawal latency (PWL) following a thermal stimulus, we evaluated the ability of the NMDA antagonist, MK-801, to alleviate neuropathic pain either by itself or when administered together with the alpha-2 adrenergic agonist, clonidine. We found that MK-801, at a dose (0.05 mg/kg s.c.) that is known to cause mild hyperactivity but is subthreshold for producing PC/RS neurotoxic changes, relieved the neuropathic pain state associated with sciatic nerve ligation. However, the relief at this dose was very transient, and no neuropathic pain-relieving effect was observed at a lower dose (0. 025 mg/kg s.c.) of MK-801. Clonidine, at a dose (0.05 mg/kg s.c.) that prevents the cerebrocortical neurotoxic effects of MK-801, decreased sensitivity to the thermal stimulus equally under all conditions (ligated, sham ligated, unoperated), but did not specifically relieve neuropathic pain in the ligated limb. Combining this dose of clonidine with an ineffective dose (0.025 mg/kg s.c.) of MK-801 provided specific, complete and long lasting (up to 4 h) relief from neuropathic pain. Rats receiving this drug combination did not display hyperactivity or any other behavioral disturbance typically associated with MK-801 treatment, nor show neurotoxic changes in cerebrocortical neurons. In separate experiments on normal unoperated rats, we found that clonidine (0.05 mg/kg s.c.) counteracted the hyperactivity induced by MK-801 (0.05 mg/kg s.c.) and returned activity levels to a normal range. These findings signify that clonidine, which does not specifically relieve neuropathic pain, can potentiate the neuropathic pain-relieving action of MK-801, while also protecting against neurotoxicity and hyperactivity side effects of MK-801. The potentiation is of a sufficient magnitude that it permits cutting the MK-801 dose requirement in half, thereby achieving prolonged neuropathic pain relief while doubling the margin of safety against any type of side effect that might be med

Topics: Adrenergic alpha-Agonists; Analysis of Variance; Animals; Clonidine; Dizocilpine Maleate; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Female; Hindlimb; Hyperesthesia; Hyperkinesis; Neuralgia; Neuroprotective Agents; Rats; Rats, Sprague-Dawley; Sciatic Nerve

Neuropathic pains arising from peripheral nerve injury can result in increased sensitivity to both noxious and non-noxious stimuli and are accompanied by a number of neuroplastic alterations at the level of the spinal cord including upregulation of neurotransmitters including dynorphin, cholecystokinin and neuropeptide Y. Additionally, such pain states appear to be associated with activation of excitatory amino acid receptors including the N-methyl-D-aspartate (NMDA) receptor. Neuropathic pains have often been classified as 'opioid resistant' in both clinical and laboratory settings. As it is known that dynorphin produces 'non-opioid' effects through interaction with NMDA receptors and this peptide is upregulated after peripheral nerve injury, the present studies were undertaken to determine the possible importance of this substance in the neuropathic state. Nerve injury was produced in rats by tight ligation of the L5 and L6 spinal roots of the sciatic nerve. Catheters were inserted for the intrathecal (i.t.) delivery of drug to the lumbar spinal cord. Tactile allodynia was determined by measuring responses to probing the plantar surface of the affected limb with von Frey filaments, and acute nociception was determined in the 55 degrees C hot-water tail-flick test in nerve-ligated and sham-operated subjects. Intrathecal administration of MK-801 or antisera to dynorphin A (1-13) did not alter the tactile allodynia associated with nerve-ligation injury or the baseline tail-flick latency in either sham-operated or nerve-injured animals. As previously reported, i.t. morphine did not alter tactile allodynia and showed reduced potency and efficacy to block the tail-flick reflex in nerve-injured animals. Co-administration, however, of i.t. morphine with MK-801, or i.t. antisera to dynorphin A (1-13) given prior to morphine elicited both a full antiallodynic response and a complete block of the tail-flick reflex in nerve-injured animals. These results suggest that tonic activation of NMDA receptors, following peripheral nerve injury, is involved with the attenuation of the effectiveness of spinal morphine in a model of neuropathic pain. Additionally, this tonic NMDA activity may be mediated, in part, by increased levels of endogenous dynorphin associated with peripheral nerve injury.

Topics: Analgesics, Opioid; Animals; Constriction; Dizocilpine Maleate; Dynorphins; Excitatory Amino Acid Antagonists; Immune Sera; Injections, Spinal; Male; Morphine; Neuralgia; Pain Measurement; Peptide Fragments; Peripheral Nervous System Diseases; Rats; Rats, Sprague-Dawley; Touch

We have previously reported that enhanced glycine release is produced by epidural spinal cord stimulation, a clinical method for treating neuropathic pain. Our current hypothesis is that glycine administered intrathecally reduces neuropathic pain as measured by the Randall-Selitto method. Neuropathic rats created by unilateral partial ligation of the sciatic nerve were treated with intrathecal infusion of glycine, strychnine, MK-801, or 5,7-DKA at 0.1 mumol, or artificial CSF for 2 hours at a rate of 10 microliters/min. Force required to produce the pain response was significantly increased after glycine administration and reduced using strychnine, a specific glycine receptor (Gly l) antagonist. Strychnine blocked the response to glycine when infused together. Administration of the non-specific NMDA receptor MK-801 antagonist and 5,7-DKA, a specific glycine-NMDA receptor (Gly 2) antagonist, however, failed to block the response to glycine. Our results provide evidence for the use of glycine and related compounds to treat neuropathic pain.

Topics: Animals; Cerebrospinal Fluid; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Glycine; Glycine Agents; Injections, Spinal; Kynurenic Acid; Male; Mechanoreceptors; Neuralgia; Nociceptors; Rats; Rats, Sprague-Dawley; Sciatic Nerve; Strychnine; Transcutaneous Electric Nerve Stimulation

Neuropathic pain following nerve injury is believed to involve excitatory amino acids (EAAs) and Ca2+-mediated neuronal plastic changes in the central nervous system (CNS). This study was designed to investigate the changes in glutamate and aspartate contents in the dorsal half of the spinal cord following chronic constrictive injury (CCI) of the rat common sciatic nerve. We also examined the changes in intracellular calcium ion concentration ([Ca2+]i) of the spinal dorsal horn in transverse spinal slices in the same animal model. Thermal and mechanical hyperalgesia were observed on day 2 and thereafter following CCI (P < 0.0001). In the CCI rats to which 0.5 mg/kg of i.p. MK-801 was given 30 min prior to CCI and subsequently three daily treatments with 0.5 mg/kg of i.p. MK-801, the development of thermal and mechanical hyperalgesia was suppressed for a period of up to 7 days; however, hyperalgesia appeared on day 10 and day 14 (P < 0.001). In CCI rats, significant increases were observed in glutamate and aspartate contents on the ipsilateral side of the dorsal horn to nerve ligation on days 4, 7 and 14 (P < 0.001). Moreover, significant increases in [Ca2+]i in the spinal dorsal horn were also observed in the superficial (lamina I-II) and deep layers (lamina V-VI) on the ipsilateral side to nerve ligation on days 4, 7 and 14 after nerve ligation in the spinal slices (P < 0.0001). The treatment with i.p. MK-801 suppressed the increases in the contents of glutamate and aspartate and in [Ca2+]i on days 4 and 7. However, the ipsilateral contents of glutamate and aspartate significantly increased on day 14 (P < 0.001 and 0.003, respectively); the increased [Ca2+]i was also observed on day 14 (P < 0.001), and the spatial pattern of the increased regions was similar to untreated CCI rats. We interpret these results to indicate that neuropathic hyperalgesia induced by CCI in the rat is associated with an increase in glutamate and aspartate contents and the subsequent activation of NMDA receptors, followed by an increase in [Ca2+]i within dorsal horn of the spinal cord.

Topics: Animals; Calcium; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Excitatory Amino Acids; Hyperalgesia; Male; Neuralgia; Peripheral Nervous System Diseases; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve; Spinal Cord; Stress, Mechanical; Temperature

Recent evidence suggests that hyperalgesia and morphine tolerance, two seemingly unrelated phenomena, have in common certain neural substrates such as activation of the N-methyl-D-aspartate (NMDA) receptor and the subsequent intracellular activation of protein kinase C and nitric oxide. Should common cellular elements be involved in hyperalgesia and morphine tolerance, these cellular and intracellular commonalities might be expected to result in interactions between these two phenomena. Indeed, our previous studies have shown that thermal hyperalgesia develops when animals are made tolerant to the antinociceptive effects of morphine. In this study, we examined the hypothesis that reduction of morphine antinociception occurs following unilateral ligation of the rats's sciatic nerve, a procedure which produces symptoms of a neuropathic pain syndrome including thermal hyperalgesia. When tested using the paw-withdrawal test on day 8 (D8) after either nerve ligation or sham operation, a single intrathecal treatment with 10 micrograms morphine sulfate (30 min after administration) produced significant antinociception in sham-operated rats but not in nerve-injured ones. These results also were obtained when thermal hyperalgesia was reversed in nerve-injured rats by the non-competitive NMDA receptor antagonist MK-801. Consistently, 8 days after sciatic nerve ligation but not after a sham operation, an approximately 6-fold rightward shift occurred in the morphine antinociceptive dose-response curve. This rightward shift of the morphine antinociceptive dose-response curve did not occur at 24 h after either nerve ligation or sham operation. In addition, once daily treatment with 10 nmol MK-801 from D2 to D7 after nerve ligation prevented both the development of thermal hyperalgesia and the rightward shift of the morphine antinociceptive dose-response curve on D8. The results indicate that the antinociceptive effects of morphine are reduced in nerve-injured rats in the absence of daily exposure to morphine and that the NMDA receptor activation may have a critical role in mechanisms of this phenomenon. These data provide further evidence indicating that interactions do occur between neural mechanisms underlying thermal hyperalgesia and morphine tolerance.

Topics: Analgesics, Opioid; Animals; Dizocilpine Maleate; Dose-Response Relationship, Drug; Drug Administration Schedule; Hot Temperature; Hyperalgesia; Male; Morphine; Neuralgia; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Sciatic Nerve