A‐Fibers Mediate Cold Hyperalgesia in Patients with Oxaliplatin‐Induced Neuropathy

Cold hyperalgesia is a common side effect of oxaliplatin treatment; still, the pathophysiological and molecular mechanisms as well as the contribution of different primary afferent fiber systems are unclear. Therefore, patients with oxaliplatin‐induced acute neuropathy with (n = 6) and without (n = 7) cold hyperalgesia were tested by applying a preferential blockade of peripheral myelinated A‐fiber afferents in combination with quantitative sensory testing. Additionally, an interview‐based questionnaire assessed the severity of symptoms and the impact on daily activities. Results indicate a deficit of cold perception in patients without cold hyperalgesia compared to patients with cold hyperalgesia prior to A‐fiber blockade. In patients with cold hyperalgesia, a preferential blockade of A‐fibers abolished cold hyperalgesia. This suggests that oxaliplatin‐induced cold hyperalgesia is mediated by A‐fibers and that a deficit in A‐fiber function might prevent the development of cold hyperalgesia. The work supports findings in rodents and in human sural nerve biopsies indicating that oxaliplatin interferes with axonal ion conductance in intact A‐fibers by sensitizing potassium and/or sodium channels. Drugs that act on these molecular targets might be of potential value to treat oxaliplatin‐induced cold hyperalgesia.     

Inhibition of microglial activity alters spinal wide dynamic range neuron discharge and reduces microglial Toll‐like receptor 4 expression in neuropathic rats

It is believed that neuropathic pain results from aberrant neuronal discharges although some evidence suggests that the activation of glia cells contributes to pain after an injury to the nervous system. This study aimed to evaluate the role of microglial activation on the hyper‐responsiveness of wide dynamic range neurons (WDR) and Toll‐like receptor 4 (TLR4) expressions in a chronic constriction injury (CCI) model of neuropathic pain in rats. Adult male Wistar rats (230 ± 30 g) underwent surgery for induction of CCI neuropathy. Six days after surgery, administration of minocycline (10, 20, and 40 mg/kg, i.p.) was initiated and continued until day 14. After administration of the last dose of minocycline or saline, a behavioral test was conducted, then animals were sacrificed and lumbar segments of the spinal cord were collected for Western blot analysis of TLR4 expression. The electrophysiological properties of WDR neurons were investigated by single unit recordings in separate groups. The findings showed that after CCI, in parallel with thermal hyperalgesia, the expression of TLR4 in the spinal cord and the evoked response of the WDR neurons to electrical, mechanical, and thermal stimulation significantly increased. Post‐injury administration of minocycline effectively decreased thermal hyperalgesia, TLR4 expression, and hyper‐responsiveness of WDR neurons in CCI rats. The results of this study indicate that post‐injury, repeated administration of minocycline attenuated neuropathic pain by suppressing microglia activation and reducing WDR neuron hyper‐responsiveness. This study confirms that post‐injury modulation of microglial activity is a new strategy for treating neuropathic pain.     

Kv4.3 Channel Dysfunction Contributes to Trigeminal Neuropathic Pain Manifested with Orofacial Cold Hypersensitivity in Rats

Trigeminal neuropathic pain is the most debilitating pain disorder but current treatments including opiates are not effective. A common symptom of trigeminal neuropathic pain is cold allodynia/hyperalgesia or cold hypersensitivity in orofacial area, a region where exposure to cooling temperatures are inevitable in daily life. Mechanisms underlying trigeminal neuropathic pain manifested with cold hypersensitivity are not fully understood. In this study, we investigated trigeminal neuropathic pain in male rats following infraorbital nerve chronic constrictive injury (ION-CCI). Assessed by the orofacial operant behavioral test, ION-CCI animals displayed orofacial cold hypersensitivity. The cold hypersensitivity was associated with the hyperexcitability of small-sized trigeminal ganglion (TG) neurons that innervated orofacial regions. Furthermore, ION-CCI resulted in a reduction of A-type voltage-gated K⁺ currents (IA currents) in these TG neurons. We further showed that these small-sized TG neurons expressed Kv4.3 voltage-gated K⁺ channels, and Kv4.3 expression in these cells was significantly downregulated following ION-CCI. Pharmacological inhibition of Kv4.3 channels with phrixotoxin-2 inhibited IA-currents in these TG neurons and induced orofacial cold hypersensitivity. On the other hand, pharmacological potentiation of Kv4.3 channels amplified IA currents in these TG neurons and alleviated orofacial cold hypersensitivity in ION-CCI rats. Collectively, Kv4.3 downregulation in nociceptive trigeminal afferent fibers may contribute to peripheral cold hypersensitivity following trigeminal nerve injury, and Kv4.3 activators may be clinically useful to alleviate trigeminal neuropathic pain.SIGNIFICANCE STATEMENT Trigeminal neuropathic pain, the most debilitating pain disorder, is often triggered and exacerbated by cooling temperatures. Here, we created infraorbital nerve chronic constrictive injury (ION-CCI) in rats, an animal model of trigeminal neuropathic pain to show that dysfunction of Kv4.3 voltage-gated K⁺ channels in nociceptive-like trigeminal ganglion (TG) neurons underlies the trigeminal neuropathic pain manifested with cold hypersensitivity in orofacial regions. Furthermore, we demonstrate that pharmacological potentiation of Kv4.3 channels can alleviate orofacial cold hypersensitivity in ION-CCI rats. Our results may have clinical implications in trigeminal neuropathic pain in human patients, and Kv4.3 channels may be an effective therapeutic target for this devastating pain disorder.     

Teleantagonism: A pharmacodynamic property of the primary nociceptive neuron

Previous work from our group showed that intrathecal (i.t.) administration of substances such as glutamate, NMDA, or PGE₂ induced sensitization of the primary nociceptive neuron (PNN hypernociception) that was inhibited by a distal intraplantar (i.pl.) injection of either morphine or dipyrone. This pharmacodynamic phenomenon is referred to in the present work as “teleantagonism”. We previously observed that the antinociceptive effect of i.t. morphine could be blocked by injecting inhibitors of the NO signaling pathway in the paw (i.pl.), and this effect was used to explain the mechanism of opioid-induced peripheral analgesia by i.t. administration. The objective of the present investigation was to determine whether this teleantagonism phenomenon was specific to this biochemical pathway (NO) or was a general property of the PNNs. Teleantagonism was investigated by administering test substances to the two ends of the PNN (i.e., to distal and proximal terminals; i.pl. plus i.t. or i.t. plus i.pl. injections). We found teleantagonism when: (i) inhibitors of the NO signaling pathway were injected distally during the antinociception induced by opioid agonists; (ii) a nonselective COX inhibitor was tested against PNN sensitization by IL-1β; (iii) selective opioid-receptor antagonists tested against antinociception induced by corresponding selective agonists. Although the dorsal root ganglion seems to be an important site for drug interactions, the teleantagonism phenomenon suggests that, in PNNs, a local sensitization spreads to the entire cell and constitutes an intriguing and not yet completely understood pharmacodynamic property of this group of neurons.     

Unmyelinated fiber sensory neuropathy differs in type 1 and type 2 diabetes

BACKGROUND: Neuropathic pain is common in diabetic patients. Degeneration of sensory C-fibers in peripheral nerve plays a prominent role in the generation of neuropathic pain. We examined degenerative changes of C-fibers in two rat models with type 1 and type 2 diabetes. METHODS: Type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor-rats of 8 months duration with equal exposure to hyperglycemia were examined. Thermal hyperalgesia was monitored using an infrared thermal probe. C-fiber size, number, frequencies of denervated Schwann cells, regenerating C-fibers, type 2 axon/Schwann cell relationship and collagen pockets in the sural nerve were examined morphometrically. Neurotrophic receptor expression was examined by Western blotting. Neurotrophins and neuropeptides were examined by ELISA. RESULTS: Type 1 rats showed increased thermal hyperalgesia followed by a decrease. Hyperalgesia in type 2 rats showed a slower progression. These findings were associated with a 50% (p < 0.001) loss of C-fibers, increased frequencies of denervated Schwann cells (p < 0.001), regenerating fibers (p < 0.001), collagen pockets (p < 0.001) and type 2 axon/Schwann cell relationship (p < 0.001) in type 1, but not in type 2 rats. Expression of insulin receptor, IGF-1R, TrkA and C was decreased in BB/Wor rats, whereas BBZDR/Wor rats showed milder or no deficits. NGF and NT-3 in sciatic nerve and substance P and calcitonin gene-related peptide in dorsal root ganglia were decreased in type 1, but not in type 2 rats. CONCLUSION: The more severe molecular, functional and morphometric abnormalities of nociceptive C-fibers in type 1 insulinopenic rats compared to type 2 hyperinsulinemic rats suggest that impaired insulin action may play a more important pathogenetic role than hyperglycemia per se.     

自噬与神经性疼痛

神经性疼痛是由外周和中枢神经元突触可塑性介导的疼痛超敏反应,由于神经细胞受体、酶、离子 通道的复杂变化,神经性疼痛的治疗仍是全世界健康问题的重大挑战。神经系统中,自噬对神经元的生存和动态平衡是至关重要的,其所需基因的特异性消耗,足以导致神经元的死亡,自噬功能障碍是神经病理性疼痛的基础。该文讨论神经病理性疼痛发生过程中自噬的可能作用机制,以及可用于治疗神经病理性疼痛的潜在 靶点。     

Spinal CCL2 and microglial activation are involved in paclitaxel-evoked cold hyperalgesia

The antineoplastic paclitaxel induces a sensory neuropathy that involves the spinal release of neuroinflammatory mediators and activation of glial cells. Although the chemokine CCL2 can evoke glial activation and its participation in neuropathic pain has been demonstrated in other models, its involvement in paclitaxel-evoked neuropathy has not been previously explored. Paclitaxel-evoked cold hypernociception was assessed in mice by the unilateral cold plate test and the effects on cold hyperalgesia of the CCR2 antagonist RS 504393, the CCR1 antagonist J113863, the microglial inhibitor minocycline or an anti-CCL2 antibody were tested. Furthermore, ELISA measurements of CCL2 concentration and immunohistochemical assays of Iba-1 and GFAP, markers of microglial and astroglial cells respectively, were performed in the lumbar spinal cord.Cold hypernociception measured 3 days after the administration of paclitaxel (10 mg/kg) was inhibited by the s.c. (0.3–3 mg/kg) or i.t. (1–10 μg) administration of RS 504393 but not of J113863 (3–30 mg/kg). CCL2 levels measured by ELISA in the lumbar spinal cord were augmented in mice treated with paclitaxel and the i.t. administration of an anti-CCL2 antibody completely suppressed paclitaxel-evoked cold hyperalgesia, strongly suggesting that CCL2 is involved in the hypernociception evoked by this taxane. Besides, the implication of microglial activation is supported by the increase in the immunolabelling of Iba-1, but not GFAP, in the spinal cord of paclitaxel-treated mice and by the inhibition of cold hyperalgesia produced by the i.t. administration of the microglial inhibitor minocycline (1–10 nmol). Finally, the neutralization of spinal CCL2 by the i.t. administration of a selective antibody for 3 days almost totally inhibited paclitaxel-evoked microglial activation.In conclusion, our results indicate that paclitaxel-evoked cold hypernociception depends on the activation of CCR2 due to the spinal release of CCL2 and the subsequent microglial activation.     

Visceral hyperalgesia induced by neonatal maternal separation is associated with nerve growth factor-mediated central neuronal plasticity in rat spinal cord

Neonatal maternal separation (NMS) has been shown to trigger alterations in neuroendocrine, neurochemical and sensory response to nociceptive stimuli along the brain-gut axis. These alterations may be the result of a cascade of events that are regulated by neurotrophic factors. Nerve growth factor (NGF), a member of the neurotrophin family, is essential for the development and maintenance of sensory neurons and for the formation of central pain circuitry. The present study aimed to investigate whether NMS causes changes in neuronal plasticity and the relationship of these changes in plasticity with the expression of NGF and its high affinity tyrosine kinase receptor A (TrkA) in the lumbosacral spinal cord in adult rats. Male Wistar rat pups were either subjected to 180 min daily of NMS or not handled (NH) for 13 consecutive days. The expression of NGF and TrkA was examined in NH and NMS rats with or without colorectal distention (CRD) as determined by Western blot analysis and immunohistochemistry. The present results of Western blot analysis indicated NMS and CRD have a significant effect on NGF protein level in the lumbosacral spinal cord of rats. Assessments of optical densities revealed that NMS enhanced TrkA-ir fiber densities in laminae I-III and laminae V-VI of rats in both conditions with or without CRD. Double immunofluorescence revealed that TrkA co-expressed with calcitonin gene-related peptide (CGRP) in afferent fibers, while no significant difference in terms of the intensity of TrkA-ir in these fibers was found among groups. Quantitative analysis of TrkA-ir neurons indicated a significant interactive effect of NMS and CRD on the mean number of TrkA-ir neurons in laminae V-VI of rats, in which significant difference was found between NMS+CRD and NH+CRD. Double immunofluorescence of TrkA and Fos showed that CRD has a significant effect on TrkA expression in Fos-positive neurons in laminae V-VI and lamina X of rats, while no significant difference was found between NMS+CRD and NH+CRD. These results demonstrate that NMS induced alterations in NGF protein level and TrkA expression in adult rat spinal cord and indicate that NGF is a crucial mediator for the changes in neuronal plasticity that occur in NMS-induced visceral hyperalgesia.     

Chemotherapy-evoked painful peripheral neuropathy: analgesic effects of gabapentin and effects on expression of the alpha-2-delta type-1 calcium channel subunit

Chemotherapeutics in the taxane and vinca-alkaloid classes sometimes produce a painful peripheral neuropathy for which there is no validated treatment. Experiments with rat models of paclitaxel- and vincristine-evoked pain suggest that these conditions may not respond to all of the analgesics that have efficacy in other models of painful peripheral neuropathy. We tested gabapentin as a potential analgesic for paclitaxel- and vincristine-evoked pain. We used a repeated dosing paradigm because there are precedents showing that repeated drug exposure may be necessary to demonstrate analgesia in neuropathic pain models. Gabapentin is believed to work via binding to voltage-gated calcium channels that contain the alpha-2-delta type-1 (alpha(2)delta-1) subunit, and the expression of this subunit is known to be increased in some painful peripheral neuropathy models. Thus we also examined whether the paclitaxel-evoked pain syndrome was accompanied by an alpha(2)delta-1 increase, and whether gabapentin had any effect on subunit expression. We found that the paclitaxel- and vincristine-evoked mechano-allodynia and mechano-hyperalgesia were significantly reduced by gabapentin, but only with repeated dosing. Paclitaxel-evoked painful peripheral neuropathy was associated with an increased expression of the alpha(2)delta-1 subunit in the spinal dorsal horn, but not in the dorsal root ganglia. The spinal cord increase was normalized by repeated gabapentin injections. Together, these findings suggest that repeated dosing with gabapentin may be beneficial in patients with chemotherapy-evoked painful peripheral neuropathy and that gabapentin's mechanisms of action may include normalization of the nerve injury-evoked increase in calcium channel alpha(2)delta-1 subunit expression.     

Severity of alcohol-induced painful peripheral neuropathy in female rats: role of estrogen and protein kinase (A and Cepsilon)

Small-fiber painful peripheral neuropathy, a complication of chronic ethanol ingestion, is more severe in women. In the present study, we have replicated this clinical finding in the rat and evaluated for a role of estrogen and second messenger signaling pathways. The alcohol diet (6.5% ethanol volume:volume in Lieber-DeCarli formula) induced hyperalgesia with more rapid onset and severity in females. Following ovariectomy, alcohol failed to induce hyperalgesia in female rats, well past its time to onset in gonad intact males and females. Estrogen replacement reinstated alcohol neuropathy in the female rat. The protein kinase A (PKA) inhibitor (Walsh inhibitor peptide, WIPTIDE) only attenuated alcohol-induced hyperalgesia in female rats. Inhibitors of protein kinase Cepsilon (PKCepsilon-I) and extracellular-signal related kinase (ERK) 1/2 (2'-amino-3'-methoxyflavone (PD98059) and 1,4-diamino-2, 3-dicyano-1, 4-bis (2-aminophenylthio) butadiene (U0126)) attenuated hyperalgesia in males and females, however the degree of attenuation produced by PKCepsilon-I was much greater in females. In conclusion, estrogen plays an important role in the expression of pain associated with alcohol neuropathy in the female rat. In contrast to inflammatory hyperalgesia, in which only the contribution of PKCepsilon signaling is sexually dimorphic, in alcohol neuropathy PKA as well as PKCepsilon signaling is highly sexually dimorphic.