[Neuropathic pain: experimental advances and clinical applications]

Differences and Similarities in Spontaneous Activity Between Animal Models of Cancer-Induced Pain and Neuropathic Pain

Background

Clinical data on cancer-induced pain (CIP) demonstrate widespread changes in sensory function. It is characterized in humans not only by stimulus-invoked pain, but also by spontaneous pain. In our previous studies in an animal model of CIP, we observed changes in intrinsic membrane properties and excitability of dorsal root ganglion (DRG) sensory neurons corresponding to mechanical allodynia and hyperalgesia, of which abnormal activities of Aβ-fiber sensory neurons are consistent in a rat model of peripheral neuropathic pain (NEP).

Objective

To investigate whether there are related peripheral neural mechanisms between the CIP and NEP models of spontaneous pain, we compared the electrophysiological properties of DRG sensory neurons at 2–3 weeks after CIP and NEP model induction.

Methods

CIP models were induced with metastasis tumour-1 rat breast cancer cells implanted into the distal epiphysis of the femur. NEP models were induced with a polyethylene cuff implanted around the sciatic nerve. Spontaneous pain in animals is measured by spontaneous foot lifting (SFL). After measurement of SFL, the animals were prepared for electrophysiological recordings of spontaneous activity (SA) in DRG neurons in vivo.

Results

Our data showed that SFL and SA occurred in both models. The proportion of SFL and SA of C-fiber sensory neurons in CIP was more significantly increased than in NEP models. There was no difference in duration of SFL and the rate of SA between the two models. The duration of SFL is related to the rate of SA in C-fiber in both models.

Conclusion

Thus, SFL may result from SA activity in C-fiber neurons in CIP and NEP rats. The differences and similarities in spontaneous pain between CIP and NEP rats is related to the proportion and rate of SA in C-fibers, respectively.

The analgesic effect of early hyperbaric oxygen treatment in chronic constriction injury rats and its influence on nNOS and iNOS expression and inflammatory factor production

Objective

To observe the analgesic effect of early hyperbaric oxygen (HBO) treatment in chronic constriction injury (CCI) rats, and to analyze the influence of HBO on the expression of neuronal nitric oxide synthase and inducible nitric oxide synthase and on the levels of inflammatory factors.

Methods

Rats were assigned into three groups randomly: sham, CCI, and HBO groups. The CCI rat model was established, and HBO treatment at 2.5 ATA (60 min) was given one day after surgery, lasting for five consecutive days. The pain behaviors of the rats were observed at predetermined time points, and the activation of astrocytes at dorsal horns as well as the changes of the synaptic ultrastructures were observed. The expressions of inducible nitric oxide synthase and neuronal nitric oxide synthase were detected by Western blot, and the levels of tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) were detected by quantitative real-time PCR.

Results

Rats in the CCI group developed hyperalgesia when compared with the sham group. Mechanical withdrawal threshold decreased and thermal withdrawal latency shortened in CCI group. Also, astrocytes at the dorsal horn were activated, the synaptic structure was disordered, the expressions of inducible nitric oxide synthase and neuronal nitric oxide synthase were increased significantly, and the release of inflammatory factor (TNF-α and IL-1β) was up-regulated. However, with early initiation of HBO treatment, rats in the HBO group showed significantly alleviated hyperalgesia, increased mechanical withdrawal threshold, and prolonged thermal withdrawal latency. HBO treatment inhibited astrocyte expression and maintained normal synaptic structure. The expressions of inducible nitric oxide synthase and neuronal nitric oxide synthase were decreased in the dorsal horn, and the release of inflammatory factor (TNF-α and IL-1β) was reduced.

Conclusions

Early HBO treatment significantly improves hyperalgesia in rats with neuropathic pain. The decreased expressions of inducible nitric oxide synthase and neuronal nitric oxide synthase and reduced levels of inflammatory factors are important mechanisms by which early HBO helps to alleviate neuropathic pain.

Reliability and validity of the Japanese translation of the DN4 Diagnostic Questionnaire in patients with neuropathic pain

BACKGROUND

The Douleur Neuropathique 4 questionnaire (DN4) is a simple and objective tool developed by the French Neuropathic Pain Group to screen for neuropathic pain.

METHODS

This prospective observational study was undertaken in three hospitals to assess the validity of a Japanese translation of the DN4. We first translated the DN4 into Japanese using a forward-backward method. Pain specialists then examined patients independently and diagnosed them with neuropathic or non-neuropathic pain, according to the International Association for the Study of Pain definitions. The Japanese version of the DN4 questionnaire was then given to each patient.

RESULTS

Of 187 patients that met our inclusion criteria, 100 and 87 were diagnosed with neuropathic and non-neuropathic pain, respectively. The test-retest intra-class correlation coefficient (95% confidence interval) was 0.827 (0.769-0.870). Among patients with identical diagnoses of neuropathic or non-neuropathic pain, receiver-operating characteristic curve analysis revealed an area under the curve of 0.89. A cut-off point of equal or greater than 4 resulted in a sensitivity of 71% and specificity of 92%.

CONCLUSION

The Japanese version of the DN4 was found to be a helpful tool for discriminating between neuropathic and non-neuropathic pain.

Sensory Function and Chronic Pain in Multiple Sclerosis

Objective

To examine whether hypoesthesia and chronic pain are related in patients with MS.

Methods

Sixty-seven MS patients with pain and 80 persons without MS were included. Sensory functioning was tested by bedside neurological examination. Touch, joint position (dorsal column-medial lemniscus pathway), temperature sense, and pain (spinothalamic tract) were tested. Pain intensity was measured by the Colored Analogue Scale (CAS Intensity) and the Faces Pain Scale (FPS); pain affect was also measured by CAS Affect and Number of Words Chosen-Affective (NWC-A). Mood was assessed with the SCL-90 anxiety and depression subscales and the Beck Depression Inventory (BDI).

Results

A significant negative relationship was found between pain intensity and the function of the dorsal column-medial lemniscal pathway, but not with the spinothalamic tract.

Conclusion

In addition to the already known relation between hyperesthesia and pain, hypoesthesia for touch and joint position also seems to be related to chronic pain in MS patients.

The NO-cGMP-PKG signal transduction pathway is involved in the analgesic effect of early hyperbaric oxygen treatment of neuropathic pain

Background

Hyperbaric oxygen (HBO) has the potential to relieve neuropathic pain. The purpose of this study was to determine whether the NO-cGMP-PKG signaling pathway is involved in the analgesic effects of early hyperbaric oxygen treatment of neuropathic pain in rats.

Methods

Rats were randomly grouped for establishment of chronic constriction injury (CCI) models. Intrathecal catheters were inserted and 2.5ATA HBO therapy was administered from day 1 post-surgery for 60 minutes daily, continuously for 5 days; menstruum NS, DMSO, NO synthase(NOS) nonspecific inhibitor (L-NAME), soluble guanylyl cyclase(sGC) inhibitor (ODQ) and protein kinase G(PKG) inhibitor (KT5823) were administered intrathecally 30 minutes prior to HBO therapy. Pain-related behaviors in rats were observed at specific time points. Western blot and real-time RT-PCR were used to observe the expressions of PKG1 mRNA and protein in the spinal dorsal horn.

Results

Compared with the CCI group, HBO could significantly relieve mechanical and thermal hyperalgesia in rats. After intrathecal administration of L-NAME, ODQ and KT5823, effects of HBO on relieving hyperalgesia in rats were reversed (P < 0.05 vs. HBO), and expression of PKG1 mRNA and protein decreased in the spinal dorsal horn of the animals (P < 0.05 vs. HBO).

Conclusions

Early HBO therapy could significantly improve symptoms of hyperalgesia of neuropathic pain in rats, possibly via activation of the NO-cGMP-PKG signaling transduction pathway.

Puerarin alleviates neuropathic pain by inhibiting neuroinflammation in spinal cord

Neuropathic pain responds poorly to drug treatments, and partial relief is achieved in only about half of the patients. Puerarin, the main constituent of Puerariae Lobatae Radix, has been used extensively in China to treat hypertension and tumor. The current study examined the effects of puerarin on neuropathic pain using two most commonly used animal models: chronic constriction injury (CCI) and diabetic neuropathy. We found that consecutive intrathecal administration of puerarin (4–100 nM) for 7 days inhibited the mechanical and thermal nociceptive response induced by CCI and diabetes without interfering with the normal pain response. Meanwhile, in both models puerarin inhibited the activation of microglia and astroglia in the spinal dorsal horn. Puerarin also reduced the upregulated levels of nuclear factor-κB (NF-κB) and other proinflammatory cytokines, such as IL-6, IL-1β, and TNF-α, in the spinal cord. In summary, puerarin alleviated CCI- and diabetes-induced neuropathic pain, and its effectiveness might be due to the inhibition of neuroinflammation in the spinal cord. The anti-inflammation effect of puerarin might be related to the suppression of spinal NF-κB activation and/or cytokines upregulation. We conclude that puerarin has a significant effect on alleviating neuropathic pain and thus may serve as a therapeutic approach for neuropathic pain.

Partial nerve injury induces electrophysiological changes in conducting (uninjured) nociceptive and nonnociceptive DRG neurons: Possible relationships to aspects of peripheral neuropathic pain and paresthesias

Partial nerve injury leads to peripheral neuropathic pain. This injury results in conducting/uninterrupted (also called uninjured) sensory fibres, conducting through the damaged nerve alongside axotomised/degenerating fibres. In rats seven days after L5 spinal nerve axotomy (SNA) or modified-SNA (added loose-ligation of L4 spinal nerve with neuroinflammation-inducing chromic-gut), we investigated a) neuropathic pain behaviours and b) electrophysiological changes in conducting/uninterrupted L4 dorsal root ganglion (DRG) neurons with receptive fields (called: L4-receptive-field-neurons). Compared to pretreatment, modified-SNA rats showed highly significant increases in spontaneous-foot-lifting duration, mechanical-hypersensitivity/allodynia, and heat-hypersensitivity/hyperalgesia, that were significantly greater than after SNA, especially spontaneous-foot-lifting. We recorded intracellularly in vivo from normal L4/L5 DRG neurons and ipsilateral L4-receptive-field-neurons. After SNA or modified-SNA, L4-receptive-field-neurons showed the following: a) increased percentages of C-, Ad-, and Ab-nociceptors and cutaneous Aa/b-low-threshold mechanoreceptors with ongoing/spontaneous firing; b) spontaneous firing in C-nociceptors that originated peripherally; this was at a faster rate in modified-SNA than SNA; c) decreased electrical thresholds in A-nociceptors after SNA; d) hyperpolarised membrane potentials in A-nociceptors and Aa/b-low-threshold-mechanoreceptors after SNA, but not C-nociceptors; e) decreased somatic action potential rise times in C- and A-nociceptors, not Aa/b-low-threshold-mechanoreceptors. We suggest that these changes in subtypes of conducting/uninterrupted neurons after partial nerve injury contribute to the different aspects of neuropathic pain as follows: spontaneous firing in nociceptors to ongoing/spontaneous pain; spontaneous firing in Aa/b-low-threshold-mechanoreceptors to dysesthesias/paresthesias; and lowered A-nociceptor electrical thresholds to A-nociceptor sensitization, and greater evoked pain.

Spontaneous pain, both neuropathic and inflammatory, is related to frequency of spontaneous firing in intact C-fiber nociceptors

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL). To determine whether SFL is caused by spontaneous firing in nociceptive neurons, we studied the following groups of rats: (1) untreated; (2) spinal nerve axotomy (SNA), L5 SNA 1 week earlier; (3) mSNA (modified SNA), SNA plus loose ligation of the adjacent L4 spinal nerve with inflammation-inducing chromic gut; and (4) CFA (complete Freund's adjuvant), intradermal complete Freund's adjuvant-induced hindlimb inflammation 1 and 4 d earlier. In all groups, recordings of SFL and of spontaneous activity (SA) in ipsilateral dorsal root ganglion (DRG) neurons (intracellularly) were made. Evoked pain behaviors were measured in nerve injury (SNA/mSNA) groups. Percentages of nociceptive-type C-fiber neurons (C-nociceptors) with SA increased in intact L4 but not axotomized L5 DRGs in SNA and mSNA (to 35%), and in L4/L5 DRGs 1-4 d after CFA (to 38-25%). SFL occurred in mSNA but not SNA rats. It was not correlated with mechanical allodynia, extent of L4 fiber damage [ATF3 (activation transcription factor 3) immunostaining], or percentage of L4 C-nociceptors with SA. However, L4 C-nociceptors with SA fired faster after mSNA (1.8 Hz) than SNA (0.02 Hz); estimated L4 total firing rates were approximately 5.0 and approximately 0.6 kHz, respectively. Similarly, after CFA, faster L4 C-nociceptor SA after 1 d was associated with SFL, whereas slower SA after 4 d was not. Thus, inflammation causes L4 C-nociceptor SA and SFL. Overall, SFL was related to SA rate in intact C-nociceptors. Both L5 degeneration and chromic gut cause inflammation. Therefore, both SA and SFL/spontaneous pain after nerve injury (mSNA) may result from cumulative neuroinflammation.

A behavioral model of neuropathic pain induced by ligation of the common peroneal nerve in mice

Different laboratory animal models of neuropathic pain that replicate pathophysiological changes in patients have been developed. In most animal models of neuropathic pain, both sensory and motor nerves are injured. Thus, animals usually show both abnormal sensory and motor responses. Assessment of the sensory system is likely to be affected by the motor defects, although motor functions have not been evaluated in previous neuropathic pain models. An ideal neuropathic pain model to assess behavioral nociceptive responses in animals is one without affecting motor function and without muscle injury. Here, we report a novel mouse model of neuropathic pain with normal motor functions. Ligation of the common peroneal nerve near the head of fibula was performed by a less invasive procedure. Long-lasting behavioral allodynia and thermal hyperalgesia was observed in mice after the ligation. Furthermore, behavioral allodynia is resistant to morphine treatment at 5 mg/kg body weight, as reported in some cases of neuropathic pain. Standard rotarod test analysis confirmed intact motor functions. Our results show that ligation of the common peroneal nerve can be used as an efficacious mouse model for assessing behavioral nociceptive responses in neuropathic pain.

PERSPECTIVE

Tests to assess behavioral responses in a neuropathic pain model depend on intact motor functions. Here we report a less invasive procedure to ligate common peroneal nerve of leg to induce neuropathic pain with least motor defects.