Nerve Chronic Constriction Injury Research Articles

Medications in Treatment of Postherpetic Neuralgia

Among many various neuropathic pain syndromes, postherpetic neuralgia (PHN) is one of the most typical and difficult neuropathic pains to control in some patients. To prevent progression to PHN, vaccination against herpes zoster has been developed. There are reviews about herpes zoster vaccination in recently published articles in the Korean Journal of Pain (KJP) and other journals [1,2]. As is written in those and other reviews, once herpes zoster progresses to PHN, the syndrome could result in severe personal and social burdens for patients. Although PHN has been known for a long time and is a typical neuropathic pain, many treatment modalities and options continue to be reported. There have even been case reports on the effectiveness of systemic vitamin C administration and transforaminal epidural magnesium injection [3-6]. The treatment for PHN varies among hospitals, and commonly used pharmacological and interventional methods include anticonvulsants and interlaminar epidural blocks [7]. According to the suggested treatment algorithm for acute or subacute herpes zoster and PHN, adjuvant pharmacological medications are tricyclic antidepressants, anticonvulsants, opioids, tramadol, topical lidocaine, and capsaicin [8]. In this month's issue of KJP, Joo et al. [9] report on the controllability of PHN with nefopam. They used systemic nefopam during the titration of the oral medication. Although there have been many clinical reports on nefopam for the treatment of pain, to date, few reports may be found regarding the treatment of chronic neuropathic pain. Nefopam has been known to be effective in acute postoperative pain and postanesthetic shivering [10-13]. In animal research, intrathecal nefopam is reported to have an antinociceptive effect in an acute inflammatory rat pain model [14]. Systemic administration of nefopam was shown to enhance the analgesia with morphine and nimesulide and decrease pain behavior and reduce adverse effects in a chronic constrictive nerve injury rat model [15]. Nefopam's mechanism of action is suggested to be through the activities of the serotonin, glutamate, and dopamine circuits [16], and it might be successfully tried in the treatment of various chronic neuropathic pain conditions. If we consider the chronicity and intractability of many neuropathic pain conditions, and the continuity of medication in these patients, many trials for neuropathic treatments are anticipated.

Comprehensive analysis of neurobehavior associated with histomorphological alterations in a chronic constrictive nerve injury model through use of the CatWalk XT system

Neuropathic pain is debilitating, and when chronic, it significantly affects the patient physically, psychologically, and socially. The neurobehavior of animals used as a model for chronic constriction injury seems analogous to the neurobehavior of humans with neuropathic pain. However, no data depicting the severity of histomorphological alterations of the nervous system associated with graded changes in neurobehavior are available. To determine the severity of histomorphological alteration related to neurobehavior, the authors created a model of chronic constrictive injury of varying intensity in rats and used the CatWalk XT system to evaluate neurobehavior. A total of 60 Sprague-Dawley rats, weighing 250-300 g each, were randomly assigned to 1 of 5 groups that would receive sham surgery or 1, 2, 3, or 4 ligatures of 3-0 chromic gut loosely ligated around the left sciatic nerve. Neurobehavior was assessed by CatWalk XT, thermal hyperalgesia, and mechanic allodynia before injury and periodically after injury. The nerve tissue from skin to dorsal spinal cord was obtained for histomorphological analysis 1 week after injury, and brain evoked potentials were analyzed 4 weeks after injury. Significant differences in expression of nerve growth factor existed in skin, and the differences were associated with the intensity of nerve injury. After injury, expression of cluster of differentiation 68 and tumor necrosis factor-α was increased, and expression of S100 protein in the middle of the injured nerve was decreased. Increased expression of synaptophysin in the dorsal root ganglion and dorsal spinal cord correlated with the intensity of injury. The amplitude of sensory evoked potential increased with greater severity of nerve damage. Mechanical allodynia and thermal hyperalgesia did not differ significantly among treatment groups at various time points. CatWalk XT gait analysis indicated significant differences for print areas, maximum contact maximum intensity, stand phase, swing phase, single stance, and regular index, with sham and/or intragroup comparisons. Histomorphological and electrophysiological alterations were associated with severity of nerve damage. Subtle neurobehavioral differences were detected by the CatWalk XT system but not by mechanical allodynia or thermal hyperalgesia. Thus, the CatWalk XT system should be a useful tool for monitoring changes in neuropathic pain, especially subtle alterations.

Intrathecal administration of cyclic AMP response element-binding protein-antisense oligonucleotide attenuates neuropathic pain after peripheral nerve injury and decreases the expression of N-Methyl-D-Aspartic receptors in mice

The aim of the present study was to determine whether the cAMP response element binding protein (CREB) contributes to neuropathic pain during development stage. Adult (7-8 weeks old) male C57BL/6 mice weighing 20-25 g were used. Intrathecal catheter implantation and chronic constriction of the sciatic nerve of the animals were performed. Western blotting and reverse transcription PCR experiments were carried out. Our study demonstrated that the expression of spinal NMDAR after peripheral nerve injury was modulated by central CREB. Chronic constriction nerve injury (CCI) in mice induced thermal hyperalgesia and mechanical allodynia. The increase of NR1 and NR2B subunits of the NMDAR was significantly diminished by intrathecal administration of the CREB antisense oligonucleotide against CREB and pCREB. Additionally, nociceptive behavior induced by CCI was attenuated by intrathecal administration of the CREB antisense oligonucleotide during the period of injection, and the above effects of relieving pain lasted at least 12 days following the last injection. Our results suggested that central functional pCREB may contribute to the development of neuropathic pain and regulate the expression of the NR1 and NR2B subunits of the NMDAR in the process.

Capsaicin avoidance as a measure of chemical hyperalgesia in orofacial nerve injury models

Many patients suffer from trigeminal neuralgia and other types of orofacial pain that are poorly treated, necessitating preclininal animal models for development of mechanisms-based therapies. The present study assessed capsaicin avoidance and other nocifensive behavioral responses in three models of orofacial nerve injury in rats: chronic constriction injury (CCI) of the mental nerves, partial tight ligation of mental nerves, and CCI of lingual nerves. We additionally investigated if nerve injury resulted in enhanced capsaicin-evoked activation of neurons in trigeminal caudalis (Vc) or nucleus of the solitary tract (NTS) based on expression of Fos-like immunoreactivity (FLI). Mental nerve CCI resulted in an enhancement of capsaicin avoidance in a two-bottle preference paradigm, while neither mental nerve injury produced thermal hyperalgesia or mechanical allodynia. CCI of lingual nerves did not affect capsaicin avoidance. Counts of FLI in Vc were significantly higher in the lingual sham and mental nerve CCI groups compared to mental shams; FLI counts in NTS did not differ among groups. Mental nerve CCI may have induced central sensitization of chemical nociception since increased capsaicin avoidance was accompanied by greater activation of Vc neurons in response to oral capsaicin.

Swim Therapy Reduces Mechanical Allodynia and Thermal Hyperalgesia Induced by Chronic Constriction Nerve Injury in Rats

Neuropathic pain is common and often difficult to treat because it generally does not respond well to the currently available pain medications or nerve blocks. Recent studies in both humans and animals have suggested that exercise may induce a transient analgesia and reduce acute pain in normal healthy individuals. We examined whether swim therapy could alleviate neuropathic pain in rats. Rats were trained to swim over a 2-week period in warm water. After the rats were trained, neuropathic pain was induced by constricting the right sciatic nerve, and regular swimming was resumed. The sensitivity of each hind paw was monitored using the Hargreaves test and von Frey test to evaluate the withdrawal response thresholds to heat and touch. The paw ipsilateral to the nerve ligation expressed pain-like behaviors including thermal hyperalgesia and mechanical allodynia. Regular swim therapy sessions significantly reduced the mechanical allodynia and thermal hyperalgesia. Swim therapy had little effect on the withdrawal thresholds for the contralateral paw. In addition, swim therapy alone did not alter the thermal or mechanical thresholds of normal rats. The results suggest that regular exercise, including swim therapy, may be an effective treatment for neuropathic pain caused by nerve injuries. This study, showing that swim therapy reduces neuropathic pain behavior in rats, provides a scientific rationale for clinicians to test the efficacy of exercise in the management of neuropathic pain. It may prove to be a safe and cost-effective therapy in a variety of neuropathic pain states.

Ultralow dose fentanyl prevents development of chronic neuropathic pain in rats

Opioids may cause progressive enhancement of pain sensitivity (opioid-induced hyperalgesia [OIH]) and thus, exacerbate existing pain. Animal studies also demonstrate paradoxical OIH with an ultralow dose (ULD, subanalgesic) of opioid; eg, the μ-opioid, morphine. Repeated administration of ULD-morphine resulted in tolerance to ULD-OIH. Prior exposure to ULD-morphine prolonged subsequent morphine antinociception in intact rats (delay of tolerance) and blocked neuropathic pain in nerve-injured rats (no hyperalgesia). Hence, pre-emptive desensitization of the excitatory function of opioid receptors may reduce further activation of a pain facilitatory system exerted by opioid or nerve injury. We determined if ULD-fentanyl (μ-opioid) and U50488H (κ-opioid) also affect post-nerve-injury neuropathy (a rat model of chronic constriction nerve injury [CCI]). Fentanyl (0.5-500 ng/kg) was administered acutely in noninjured rats. Chronic fentanyl (5 ng/kg/day) was initiated either immediately after CCI (day 1-28) or when neuropathy was established (day 7-14) in nerve-injured rats. U50488H (25 μg/kg/day) was given on day 1-28 post-CCI. Saline served as control. Responsiveness was assessed using tail-flick and paw-pressure tests, respectively, in intact and CCI Sprague-Dawley rats of both sexes. ULD-fentanyl evoked pain sensitization in noninjured rats. ULD-OIH was related to dose (inversely), gender (female > male), and was reversed by ketamine. Neuropathy developed after CCI in control (saline) rats. This was not observed in rats of either sex exposed to ULD-fentanyl on day 1-28 post-CCI. Rats treated with ULD-fentanyl from day 7 after CCI exhibited hyperalgesia similar to control rats. U50488H did not block post-CCI neuropathy (regardless of gender). Pre-emptive use of ULD μ-opioid (not κ-opiod) blocked initiation (not maintenance) of neuropathic pain after CCI in rats. These data may suggest a novel treatment approach in situations when the potential development of neuropathy can be anticipated.

Involvement of ERK phosphorylation of trigeminal spinal subnucleus caudalis neurons in thermal hypersensitivity in rats with infraorbital nerve injury.

To evaluate the involvement of the mitogen-activated protein kinase (MAPK) cascade in orofacial neuropathic pain mechanisms, this study assessed nocifensive behavior evoked by mechanical or thermal stimulation of the whisker pad skin, phosphorylation of extracellular signal-regulated kinase (ERK) in trigeminal spinal subnucleus caudalis (Vc) neurons, and Vc neuronal responses to mechanical or thermal stimulation of the whisker pad skin in rats with the chronic constriction nerve injury of the infraorbital nerve (ION-CCI). The mechanical and thermal nocifensive behavior was significantly enhanced on the side ipsilateral to the ION-CCI compared to the contralateral whisker pad or sham rats. ION-CCI rats had an increased number of phosphorylated ERK immunoreactive (pERK-IR) cells which also manifested NeuN-IR but not GFAP-IR and Iba1-IR, and were significantly more in ION-CCI rats compared with sham rats following noxious but not non-noxious mechanical stimulation. After intrathecal administration of the MEK1 inhibitor PD98059 in ION-CCI rats, the number of pERK-IR cells after noxious stimulation and the enhanced thermal nocifensive behavior but not the mechanical nocifensive behavior were significantly reduced in ION-CCI rats. The enhanced background activities, afterdischarges and responses of wide dynamic range neurons to noxious mechanical and thermal stimulation in ION-CCI rats were significantly depressed following i.t. administration of PD98059, whereas responses to non-noxious mechanical and thermal stimulation were not altered. The present findings suggest that pERK-IR neurons in the Vc play a pivotal role in the development of thermal hypersensitivity in the face following trigeminal nerve injury.

Modulation of Kv7 potassium channels by a novel opener pyrazolo[1,5‐a]pyrimidin‐7(4H)‐one compound QO‐58

Modulation of K(v)7/M channel function represents a relatively new strategy to treat neuronal excitability disorders such as epilepsy and neuropathic pain. We designed and synthesized a novel series of pyrazolo[1,5-a] pyrimidin-7(4H)-one compounds, which activate K(v)7 channels. Here, we characterized the effects of the lead compound, QO-58, on K(v)7 channels and investigated its mechanism of action. A perforated whole-cell patch technique was used to record K(v)7 currents expressed in mammalian cell lines and M-type currents from rat dorsal root ganglion neurons. The effects of QO-58 in a rat model of neuropathic pain, chronic constriction injury (CCI) of the sciatic nerve, were also examined. QO-58 increased the current amplitudes, shifted the voltage-dependent activation curve in a more negative direction and slowed the deactivation of K(v)7.2/K(v)7.3 currents. QO-58 activated K(v)7.1, K(v)7.2, K(v)7.4 and K(v)7.3/K(v)7.5 channels with a more selective effect on K(v)7.2 and K(v)7.4, but little effect on K(v)7.3. The mechanism of QO-58's activation of K(v)7 channels was clearly distinct from that used by retigabine. A chain of amino acids, Val(224)Val(225)Tyr(226), in K(v)7.2 was important for QO-58 activation of this channel. QO-58 enhanced native neuronal M currents, resulting in depression of evoked action potentials. QO-58 also elevated the pain threshold of neuropathic pain in the sciatic nerve CCI model. The results indicate that QO-58 is a potent modulator of K(v)7 channels with a mechanism of action different from those of known K(v)7 openers. Hence, QO-58 shows potential as a treatment for diseases associated with neuronal hyperexcitability.

Operant Behavioral Responses to Orofacial Cold Stimuli in Rats with Chronic Constrictive Trigeminal Nerve Injury: Effects of Menthol and Capsazepine

Both spinal and trigeminal somatosensory systems use the TRPM8 channel as a principal transducer for detecting cold stimuli. It is currently unclear whether this cold transducer may play a role in trigeminal neuropathic pain manifesting cold allodynia and hyperalgesia. In the present study, trigeminal neuropathy was induced by chronic constrictive nerve injury of the infraorbital nerve (ION-CCI). Behavioral responses to cold stimuli in orofacial regions were assessed by the newly developed orofacial operant test in the ION-CCI rats. We tested menthol and capsazepine, two compounds that can activate and inhibit TRPM8 respectively, on orofacial operant responses to cold stimuli in ION-CCI rats. Testing animals performed operant tasks by voluntarily contacting their orofacial regions to a cold stimulation module in order to access sweetened milk as a reward, and contact time and number of the operant behaviors were automatically recorded. Total contact time was significantly reduced at the cooling temperatures of 17°C and 12°C in ION-CCI group in comparison with sham group, indicating the presence of cold allodynia and hyperalgesia in ION-CCI rats. When menthol was administered to ION-CCI rats, total contact time was further reduced and total contact number increased at the cooling temperatures. In contrast, after administration of capsazepine to ION-CCI rats, total contact time was significantly increased at the cooling temperatures. The behavioral outcomes support the idea that TRPM8 plays a role in cold allodynia and hyperalgesia following chronic trigeminal nerve injury.

Neuroprotective Activity of Thioctic Acid in Central Nervous System Lesions Consequent to Peripheral Nerve Injury

Peripheral neuropathies are heterogeneous disorders presenting often with hyperalgesia and allodynia. This study has assessed if chronic constriction injury (CCI) of sciatic nerve is accompanied by increased oxidative stress and central nervous system (CNS) changes and if these changes are sensitive to treatment with thioctic acid. Thioctic acid is a naturally occurring antioxidant existing in two optical isomers (+)- and (−)-thioctic acid and in the racemic form. It has been proposed for treating disorders associated with increased oxidative stress. Sciatic nerve CCI was made in spontaneously hypertensive rats (SHRs) and in normotensive reference cohorts. Rats were untreated or treated intraperitoneally for 14 days with (+/−)-, (+)-, or (−)-thioctic acid. Oxidative stress, astrogliosis, myelin sheets status, and neuronal injury in motor and sensory cerebrocortical areas were assessed. Increase of oxidative stress markers, astrogliosis, and neuronal damage accompanied by a decreased expression of neurofilament were observed in SHR. This phenomenon was more pronounced after CCI. Thioctic acid countered astrogliosis and neuronal damage, (+)-thioctic acid being more active than (+/−)- or (−)-enantiomers. These findings suggest a neuroprotective activity of thioctic acid on CNS lesions consequent to CCI and that the compound may represent a therapeutic option for entrapment neuropathies.

Antihyperalgesic activity of chlorogenic acid in experimental neuropathic pain

Chlorogenic acid (CGA) is a natural organic phenolic compound that is found in many plants, fruits and vegetables. CGA has beneficial bioactivities and strong therapeutic effects in inflammatory processes. CGA-rich fractions have analgesic activity but CGA has not been tested previously in neuropathic pain, which results from tissue damage, inflammation or injury of the nervous system. Chronic constrictive nerve injury (CCI) is a peripheral neuropathic pain model which initiates an inflammatory cascade. We aimed to determine possible antihyperalgesic effects of CGA in neuropathic pain. Our study showed for the first time that CGA [50, 100 and 200 mg/kg; intraperitoneally (i.p.)] produced significant dose- and time-dependent antihyperalgesic activity in CCI-induced neuropathic pain. In addition, chronic administration of CGA (100 mg/kg/day; i.p. for 14 days) prevented the development of mechanical hyperalgesia and attenuated CCI-induced histopathological changes. On the other hand, CGA (200 mg/kg) did not affect falling latencies of rats in the rota rod test. Hence, CGA might represent a novel potential therapeutic option for the management of neuropathic pain.

Assessment of chronic trigeminal neuropathic pain by the orofacial operant test in rats

Classical behavioral tests in animal models of trigeminal neuropathic pain measure reflexive responses that are not necessarily measures of pain. To overcome the problem, we created a chronic constrictive nerve injury (CCI) rat model of pain by ligation of the infraorbital nerve (ION), and applied the orofacial operant test to assess behavioral responses to mechanical and cold stimulation in these rats. Animals were trained to voluntarily contact their facial region to a mechanical or a cold stimulation module in order to access sweetened milk as a positive reward. ION-CCI rats displayed aversive behaviors to innocuous mechanical stimuli, as indicated by a significant decrease in both contact time and the numbers of long contact events in comparison with sham group. For cold stimulation, ION-CCI rats displayed aversive behaviors to both innocuous (17°C) and noxious cold temperatures (12°C and 5°C), as indicated by a significant decrease in both contact time and the numbers of long contact events at the cooling temperatures. The decreases of the contact time and numbers in ION-CCI rats were partially abolished by morphine. Our orofacial operant test demonstrates mechanical allodynia, cold allodynia, and hyperalgesia in rats with chronic trigeminal nerve injury. The neuropathic pain in ION-CCI rats was partially alleviated by morphine. Thus, orofacial operant test provides a desirable behavioral assessment method for preclinical studies of chronic trigeminal neuropathic pain.

TRPV1 in GABAergic Interneurons Mediates Neuropathic Mechanical Allodynia and Disinhibition of the Nociceptive Circuitry in the Spinal Cord

Neuropathic pain and allodynia may arise from sensitization of central circuits. We report a mechanism of disinhibition-based central sensitization resulting from long-term depression (LTD) of GABAergic interneurons as a consequence of TRPV1 activation in the spinal cord. Intrathecal administration of TRPV1 agonists led to mechanical allodynia that was not dependent on peripheral TRPV1 neurons. TRPV1 was functionally expressed in GABAergic spinal interneurons and activation of spinal TRPV1 resulted in LTD of excitatory inputs and a reduction of inhibitory signaling to spinothalamic tract (STT) projection neurons. Mechanical hypersensitivity after peripheral nerve injury was attenuated in TRPV1(-/-) mice but not in mice lacking TRPV1-expressing peripheral neurons. Mechanical pain was reversed by a spinally applied TRPV1 antagonist while avoiding the hyperthermic side effect of systemic treatment. Our results demonstrate that spinal TRPV1 plays a critical role as a synaptic regulator and suggest the utility of central nervous system-specific TRPV1 antagonists for treating neuropathic pain.

English

  There are many evidences which indicate that calcium channel related voltage plays an important role on the mechanism of neuropathic pain, but no strong evidence has been found yet on the role played by type-L calcium channel. The aim of this study was to evaluate this role in using a properly designed experiment. This cohort study was carried out on seven groups (eight  animals each) of male Wistar rats (200 to300 g). Chronic constriction nerve injury was performed on rats by loosely ligating their common sciatic nerve. The impact of corticosteron at 15 mg/kg on neuropathic pain behaviors was assessed in the presence or absence of verapamil as an L-VSC channel blocker at 5, 10 or 20 mg/kg. Standard procedures were employed to evaluate the behavioral pain responses including the thermal hyperalgesia and the thermal and mechanical allodynia. Our findings indicate that peripheral administration of corticosterone suppressed both hyperalgesia, and thermal and mechanical allodynia; however, verapamil pretreatment attenuated the effects of corticosterone on both thermal hyperalgesia and mechanical allodynia. In addition, the administration of verapamil alone at high dose (20 mg) suppressed both thermal and mechanical allodynia. These findings show that the inhibitory effects of glucocorticoids on neuropathic pain behaviors, at least in part, may be mediated through L-type VSC channels; but our findings suggest a potential role for glucocorticoids receptors agonists in combination with L-type VSC channels antagonists in the clinical management of neuropathic pain.   Key words: Corticosterone, L-type, calcium channel, mechanical allodynia, thermal allodynia, thermal hyperalgesia.

Elevated prepronociceptin, nociceptin/orphanin FQ and nocistatin concentrations in rat chronic constriction nerve injury and diabetic neuropathic pain models

Nociceptin/orphanin FQ (N/OFQ) and nocistatin are derived from the same precursor peptide, prepronociceptin. N/OFQ and nocistatin have been postulated to participate in pain modulation. In this study, we investigated whether the prepronociceptin, N/OFQ and nocistatin concentrations in the brain and spinal cord would be altered in chronic constriction injury and diabetic rat neuropathic pain models. Total brain and spinal cord lysates as well as serum from rats that had undergone chronic constriction injury and streptozocin-induced diabetic neuropathy were used to determine the concentrations of three peptides using competitive radioimmunoassay. We found that N/OFQ and prepronociceptin concentrations were significantly raised in both rat neuropathic pain models. Nocistatin was raised in the brains of post traumatic neuropathy pain rats. Overall, our data have demonstrated for the first time that prepronociceptin, N/OFQ and nocistatin concentrations are significantly altered at different tissues of two rat neuropathy pain models.

The novel small molecule α9α10 nicotinic acetylcholine receptor antagonist ZZ-204G is analgesic

Chronic pain is inadequately managed with currently available classes of analgesic drugs. Recently, peptide antagonists of the α9α10 nicotinic acetylcholine receptor were shown to be analgesic. The present study was conducted to characterize a novel small molecule, non-peptide antagonist at nicotinic receptors. The tetrakis-quaternary ammonium compound ZZ-204G was evaluated for functional activity on cloned nicotinic receptors expressed in Xenopus oocytes. In-vivo efficacy was assessed in rat models of tonic inflammatory pain (formalin test), neuropathic pain (chronic constriction nerve injury), and thermal nociception (tail flick test). ZZ-204G was an antagonist at nicotinic receptors inhibiting the α9α10 subtype with an IC₅₀ of 0.51 (0.35-0.72) nM. Antagonist activity at other nicotinic subtypes (α1β1δε, α2β2, α2β4, α3β2, α3β4, α4β2, α4β4, α6/α3β2β3, α6/α3β4 and α7) was 10-1000-fold lower than at the α9α10 subtype. In competition binding assays, the k(i) of ZZ-204G at γ-aminobutyric acid(A), serotonin(3), γ-aminobutyric acid(B), κ- and μ-opioid receptors was 1000- to >10,000-fold lower than at α9α10 nicotinic receptors. Parenteral administration of ZZ-204G dose-dependently decreased nociceptive behaviors (paw flinches) in the formalin test and mechanical hyperalgesia in the chronic constriction nerve injury model of neuropathic pain. ZZ-204G was not antinociceptive in the tail flick assay. Results from the rotarod assay indicated that lower doses of ZZ-204G that were analgesic did not alter motor function. In summary, ZZ-204G represents a prototype small molecule antagonist for α9α10 nicotinic receptors and provides a novel molecular scaffold for analgesic agents with the potential to treat chronic inflammatory or neuropathic pain.

TNF-α expression, not iNOS expression, is correlated with NF-κB activation in the spinal cord of rats following peripheral nerve injury

Previous studies have shown that pro-inflammatory cytokines were involved in the genesis and persistence of neuropathic pain. Nuclear factor kappa B (NF-κB) plays a crucial role in regulating proinflammatory cytokine gene expression. In this study, we examined the hypothesis that NF-κB would regulate the expression of spinal tumor necrosis factor α (TNF-α) and inducible nitric oxide synthase (iNOS) following chronic constriction nerve injury (CCI) in rats. CCI induced a significant upregulation of NF-κB in the ipsilateral spinal cord on postoperative Day 7 as revealed by Western blot, immunohistochemistry and real time polymerase chain reaction (real time-PCR). Moreover, TNF-α mRNA expression was markedly upregulated by CCI, whereas only a low level of iNOS mRNA was detected in the ipsilateral spinal cord. The immunohistochemical data indicated a pattern of colocalization between NF-κB and TNF-α within spinal cord dorsal horn. These results show that NF-κB activation is correlated with TNF-α expression, but not iNOS expression, in the spinal cord after peripheral nerve injury. Key words: Neuropathic pain, chronic constriction nerve injury, NF-κB; TNF-α, iNOS.

Analgesic effect of intrathecally γ-aminobutyric acid transporter-1 inhibitor NO-711 administrating on neuropathic pain in rats

To investigate the analgesic effect of intrathecally administered γ-aminobutyric acid (GABA) transporter-1 inhibitor NO-711 on the sciatic nerve chronic constriction injury (CCI) rats. 5 days after intrathecal catheter placement, neuropathic pain model was established by CCI of sciatic nerve on rats. Withdrawal thresholds for mechanical allodynia and latency for thermal hyperalgesia were measured in all animals. All rats operated upon for CCI displayed decreased withdrawal thresholds for mechanical allodynia and latency for thermal hyperalgesia, which has significant difference compared with sham groups. After intrathecal NO-711 administration, withdrawal thresholds and latency were significantly increased on CCI rats compared with control group after 1 day. The results show that GABA transporter-1 inhibitor could effectively develop analgesic effect in sciatic nerve CCI rats’ model.

Effects of local lidocaine treatment before and after median nerve injury on mechanical hypersensitivity and microglia activation in rat cuneate nucleus

This study examined the relationship between microglia activation in the cuneate nucleus (CN) and behavioral hypersensitivity after chronic constriction injury (CCI) of the median nerve. We also investigated effects of local lidocaine pre- and post-treatment on microglia activation and development of hypersensitivity in this model. By immunohistochemistry and immunoblotting, little immunoreactivity of OX-42, a microglia activation marker, was detected in the CN of normal rats. As early as 1 day after CCI, there was a significant increase in OX-42 immunoreactivity in the lesion side of CN, which reached a maximum at 14 days. Microinjection of minocycline, a microglia activation inhibitor, into the CN 1 day after CCI attenuated injury-induced behavioral hypersensitivity in a dose-dependent manner. Furthermore, the animals received 1%, 2% or 5% lidocaine 15 min prior to median nerve CCI (pre-treatment), 5 h (early post-treatment) or 1 day (late post-treatment) after median nerve CCI. Pre-treatment and early post-treatment with 2% and 5% lidocaine, but not 1% lidocaine, attenuated OX-42 immunoreactivity and behavioral hypersensitivity following median nerve injury. Late post-treatment with 1%, 2%, or 5% lidocaine failed to decrease OX-42 immunoreactivity and mechanical hypersensitivity in CCI rats. In conclusion, median nerve injury-induced microglia activation in the CN modulated development of behavioral hypersensitivity. High-concentration lidocaine was effective in decreasing microglia activation in the CN and in attenuating neuropathic pain sensations at the early stage following nerve injury, when microglia had not yet been activated.

Antinociceptive effects and toxicity of morphine-6- O-sulfate sodium salt in rat models of pain

Mu-opioids (i.e. morphine, oxycodone, hydrocodone) are considered to be the primary drugs for treatment of moderate to severe acute, chronic and cancer pain. Despite their analgesic effectiveness they have several clinically significant side-effects (cognitive, motor, respiratory, cardiovascular, gastrointestinal). They also have a limited spectrum of action, being more effective for nociceptive than neuropathic pain. In an effort to identify other opioid analgesics with greater effectiveness in mixed pain states and with a better side-effect profile compared to the classical mu-opioid agonist, morphine, a relatively little-known morphine derivative, morphine-6- O-sulfate, was characterized using a range of well-established rodent pain models. The present data demonstrated that morphine-6- O-sulfate was efficacious after several routes of administration, including neuroaxial (intrathecal), parenteral (intraperitoneal) and oral in the rat. It showed potent, dose-related, analgesic activities against acute nociceptive pain (the tail flick test), neuropathic pain (chronic constriction nerve injury hyperalgesia and allodynia) and inflammatory pain (formalin test). It had a good separation based on dose (at least 10-fold) between side-effects (incoordination, hypolocomotion, inhibition of gastrointestinal motility) and analgesia in all models of pain tested. In addition, morphine-6- O-sulfate had a more favorable potency ratio for delay of gastrointestinal transit and analgesia when compared to morphine. These preclinical findings suggest that morphine-6- O-sulfate is a potential candidate for development as a novel opioid for management of nociceptive, neuropathic and mixed pain states.