Importance of glial activation in neuropathic pain

115. Diagnostic value of cytokine profiling in chronic pain disorders and as a biomarker of induced pain after capsaicin

Treatment of neuropathic pain, triggered by multiple insults to the nervous system, is a clinical challenge because the underlying mechanisms of neuropathic pain development remain poorly understood. Most treatments do not differentiate between different phases of neuropathic pain pathophysiology and simply focus on blocking neurotransmission, producing transient pain relief. Here, we report that early- and late-phase neuropathic pain development in rats and mice after nerve injury require different matrix metalloproteinases (MMPs). After spinal nerve ligation, MMP-9 shows a rapid and transient upregulation in injured dorsal root ganglion (DRG) primary sensory neurons consistent with an early phase of neuropathic pain, whereas MMP-2 shows a delayed response in DRG satellite cells and spinal astrocytes consistent with a late phase of neuropathic pain. Local inhibition of MMP-9 by an intrathecal route inhibits the early phase of neuropathic pain, whereas inhibition of MMP-2 suppresses the late phase of neuropathic pain. Further, intrathecal administration of MMP-9 or MMP-2 is sufficient to produce neuropathic pain symptoms. After nerve injury, MMP-9 induces neuropathic pain through interleukin-1beta cleavage and microglial activation at early times, whereas MMP-2 maintains neuropathic pain through interleukin-1beta cleavage and astrocyte activation at later times. Inhibition of MMP may provide a novel therapeutic approach for the treatment of neuropathic pain at different phases.

Studies showed a complex relationship between hydrogen sulfide (H2S) and neuropathic pain. In this study, the relationship between endogenous CBS-H2S pathway in L4-6 spinal cord and neuropathic pain was explored. A total of 163 adult Kunming mice were used in this study. CBS expression and H2S formation in L4-6 spinal cord were detected in the development of neuropathic pain firstly. Then, effect of AOAA, an CBS inhibitor, on treatment of neuropathic pain by chronic construction injury surgery (CCI) was detected. Pain thresholds and activation of NF-κB(p65), ERK1/2 and CREB were measured as biomarks of neuropathic pain. Results showed that CCI surgery significantly upregulated protein expression of CBS and H2S formation. Correlation analysis showed pain thresholds had negative relationships with protein expression of CBS and H2S formation. Treatment with AOAA, a CBS inhibitor, inhibited CCI-induced upregulation of CBS expression and H2S formation (P<0.05). Further, AOAA significantly decreased activation of NF-κB(p65), ERK1/2 and CREB pathway, and reversed CCI-induced allodynia (P<0.05). This indicated that CBS-H2S pathway promoted the development of neuropathic pain. CBS-H2S pathway could be a promising target for treatment of neuropathic pain.

Neuropathic pain is a challenging clinical problem and remains difficult to treat. Altered gene expression in peripheral sensory nerves and neurons due to nerve injury is well documented and contributes critically to the synaptic plasticity in the spinal cord and the initiation and maintenance of chronic pain. However, our understanding of the epigenetic mechanisms regulating the transcription of pro-nociceptive (e.g., NMDA receptors and α2δ-1) and antinociceptive (e.g., potassium channels and opioid and cannabinoid receptors) genes are still limited. In this review, we summarize recent studies determining the roles of histone modifications (including methylation, acetylation, and ubiquitination), DNA methylation, and noncoding RNAs in neuropathic pain development. We review the epigenetic writer, reader, and eraser proteins that participate in the transcriptional control of the expression of key ion channels and neurotransmitter receptors in the dorsal root ganglion after traumatic nerve injury, which is commonly used as a preclinical model of neuropathic pain. A better understanding of epigenetic reprogramming involved in the transition from acute to chronic pain could lead to the development of new treatments for neuropathic pain.

Gabapentin was initially developed as an antiepileptic drug but was later discovered to be an effective treatment of neuropathic pain. Gabapentin has been successfully used for the treatment of multiple neuropathic pain syndromes such as diabetic neuropathy and postherpetic neuralgia. However, limited data exist about its efficacy for other pain syndromes. The objective of the current review is to describe, from the literature, the role of gabapentin for the treatment of cancer-related pain syndromes. Studies were identified by searching the PubMed electronic databases. Additional review articles and article reference lists were used to identify other studies. Recent studies showed effectiveness of gabapentin in improving the pain control in patients with neuropathic cancer pain, already treated with opiates. Moreover, gabapentin appeared promising in reducing the need for high total doses of opioids and avoiding unplanned treatment interruptions for patients with head and neck malignancies treated with radiotherapy or concurrent chemoradiotherapy. Furthermore, the combination of gabapentin and morphine has been shown to effect better pain relief at lower doses of each drug when compared with gabapentin or morphine alone in patients with painful diabetic neuropathy or postherpetic neuralgia. The combination of both drugs was associated with a beneficial effect on pain-related interference with daily activity, mood, sleep and quality of life. Given the significant benefits of gabapentin and the combination of gabapentin with opioids for the treatment of neuropathic pain, randomized clinical trials are needed to establish the role of these analgesic regimens for the treatment of neuropathic cancer pain.

Well-known effects of neurotrophic factors are related to supporting the survival and functioning of various neuronal populations in the body. However, these proteins seem to also play less well-documented roles in glial cells, thus, influencing neuroinflammation. This article summarizes available data on the effects of glial cell line derived neurotrophic factor (GDNF) family ligands (GFLs), proteins providing trophic support to dopaminergic, sensory, motor and many other neuronal populations, in non-neuronal cells contributing to the development and maintenance of neuropathic pain. The paper also contains our own limited data describing the effects of small molecules targeting GFL receptors on the expression of the satellite glial marker IBA1 in dorsal root ganglia of rats with surgery- and diabetes-induced neuropathy. In our experiments activation of GFLs receptors with either GFLs or small molecule agonists downregulated the expression of IBA1 in this tissue of experimental animals. While it can be a secondary effect due to a supportive role of GFLs in neuronal cells, growing body of evidence indicates that GFL receptors are expressed in glial and peripheral immune system cells. Thus, targeting GFL receptors with either proteins or small molecules may directly suppress the activation of glial and immune system cells and, therefore, reduce neuroinflammation. As neuroinflammation is considered to be an important contributor to the process of neurodegeneration these data further support research efforts to modulate the activity of GFL receptors in order to develop disease-modifying treatments for neurodegenerative disorders and neuropathic pain that target both neuronal and glial cells.

Background: Neuropathic pain is a common complaint after traumatic spinal cord injury (SCI). Gabapentin, a synthetic structural analogue of GABA, has been shown to have beneficial effects in the treatment of neuropathic pain in other diagnostic groups; however, no standardized clinical trial has been performed to evaluate its efficacy after SCI.Design: A 1 0-week, prospective, randomized, double-blind, crossover, and placebo-controlled clinical trial.Objective: To determine the efficacy of gabapentin in the treatment of SCI-related neuropathic pain.Methods: Seven subjects with neuropathic pain, who were more than 30 days post-SCI, completed the study. Two groups received a 4-week course of gabapentin and placebo in a randomized crossover design with a 2-week washout period. The Neuropathic Pain Scale was used to record daily pain levels. Data were analyzed using the Wilcoxon signed rank test.Results: Gabapentin has some beneficial effects on certain types of neuropathic pain. There was a significant decrease of "unpleasant feeling" and a trend toward a decrease in both the "pain intensity" and "burning sensation" at the fourth week of gabapentin treatment compared with those on the placebo. No significant difference was found among other pain descriptors during the gabapentin and placebo treatment, although this may have been limited by the small sample size and low maximum dosage of gabapentin.Conclusions: Gabapentin reduces certain types of neuropathic pain in the SCI population. Future studies with larger sample sizes, higher dosages, and quicker titration will help further determine the efficacy of gabapentin in the treatment of SCI-related neuropathic pain.

The treatment of neuropathic pain has always been a major challenge for clinicians. It has great significance to study the drug for the prevention and treatment of neuropathic pain which has definite curative effect and less adverse reaction. Therefore, it is necessary to study the mechanism of occurrence and development of neuropathic pain. Numerous experimental results show that microglia plays an important role in the pathogenesis of neuropathic pain. Nerve injury could induce the activation of microglia and increase the expression of P2X7 receptor in activated microglia, which causes the decrease of pain threshold and then results in pain. Key words: Microglia; Neuropathic pain; P2X7 receptor

Anticonvulsants are a class of medication FDA approved for the treatment of epilepsy but also used as first-line (sometimes FDA approved) treatments of neuropathic pain, trigeminal neuralgia, migraine headache, and myofascial pain. They act on a wide range of presynaptic and postsynaptic channels, ultimately decreasing neuronal excitability. Gabapentinoids are the primary first-line agents for neuropathic pain treatment with numerous studies supporting their efficacy and FDA approval for post-herpetic neuralgia (gabapentin and pregabalin) and painful diabetic peripheral neuropathy (pregabalin). Pregabalin is also FDA approved for the treatment of fibromyalgia and central neuropathic pain. Carbamazepine and oxcarbazepine are considered first-line treatments for trigeminal neuralgia. Data supporting the efficacy of other anticonvulsants in pain management is more limited. Despite their evidence of efficacy, anticonvulsants are underutilized and should be considered in combination with other medical and interventional therapeutic options to safely and effectively treat a patient’s pain.

Background: Transcutaneous electrical nerve stimulation (TENS) is presently one of the main methods of treatment for neuropathic pain in type II diabetes mellitus. The discussion about which TENS frequency is more effective in the treatment of neuropathic pain has been ongoing for many years. Despite this, the response of different aspects of neuropathic pain to various TENS modalities has not been sufficiently studied. Aim: To analyze changes in characteristics of neuropathic pain depending on the frequency of TENS. Materials and methods: Seventy-five Russian diabetic patients with painful distal axonal neuropathy were enrolled in the study. Patients were assigned to three groups: in the HF TENS group, 25 patients received standard drug therapy (Alpha-lipoic acid, Pentoxifylline, Vitamin B12, Gabapentin) + high-frequency TENS (HF); in the LF TENS group, 25 patients received standard drug therapy (Alpha-lipoic acid, Pentoxifylline, Vitamin B12, Gabapentin) + low-frequency TENS (LF); in the control group, 25 patients underwent just standard drug therapy (Alpha-lipoic acid, Pentoxifylline, Vitamin B12, Gabapentin). Pain intensity was calculated before and after treatment with visual analogue scale (VAS), McGill pain questionnaire (MPQ), Douleur Neuropathique 4 Questions (DN4) and Pain Drawing. Results: TENS increased the therapeutic effect of standard drug therapy, in the treatment of neuropathic pain, by 65.9% and prolonged its efficacy by 31% for up to 6 months after treatment. HF TENS had a more pronounced analgesic effect than LF TENS based on VAS (34.7%), sensory (57.6%) MPQ dimensions and DN4 (21%). Affective MPQ dimension with the use of LF TENS was lower than HF TENS by 34.7% immediately after treatment, by 47.3% after 2 months and by 34.8% after 6 months of the follow-up period. Conclusion: There are significant differences between HF and LF TENS based on pain assessment using various pain scales. This reflects the distinctive effects of different TENS modalities on different aspects of neuropathic pain.

Overexpression of miR-206 ameliorates chronic constriction injury-induced neuropathic pain in rats via the MEK/ERK pathway by targeting brain-derived neurotrophic factor

Background and AimsThe medial femoral cutaneous nerve (MFCN) is known to innervate the anteromedial knee area.1,2 A selective block of the anterior branch from the MFCN (MFCN-A) but not the...

Vanilloid receptor 1-positive neurons mediate thermal hyperalgesia and tactile allodynia

The use of cannabis and cannabinoid products for the treatment of neuropathic pain is a growing area of research. This type of pain has a high prevalence, limited response to available therapies and high social and economic costs. Systemic cannabinoid-based therapies have shown some unwanted side effects. Alternative routes of administration in the treatment of neuropathic pain may provide better acceptance for the treatment of multiple pathologies associated with neuropathic pain. To examine the efficacy, tolerability, and safety of cannabinoids (individualized formulations, phytocannabinoids, and synthetics) administered by routes other than oral or inhalation compared to placebo and/or conventional medications in the management of neuropathic pain. This systematic review of the literature reveals a lack of clinical research investigating cannabis by routes other than oral and inhalation as a potential treatment for neuropathic pain and highlights the need for further investigation with well-designed clinical trials. There is a significant lack of evidence indicating that cannabinoids administered by routes other than oral or inhaled may be an effective alternative, with better tolerance and safety in the treatment of neuropathic pain. Higher quality, long-term, randomized controlled trials are needed to examine whether cannabinoids administered by routes other than inhalation and oral routes may have a role in the treatment of neuropathic pain.

Aim of investigationPulsed radiofrequency (PRF) is a safe and effective approach for treating neuropathic pain. However, the optimal treatment conditions and analgesic mechanisms of PRF remain unclear. The aim of our study was to assess the beneficial and adverse effects of prolonged-duration PRF and the analgesic mechanisms of PRF treatment with neuropathic pain rats.MethodsMale Sprague Dawley rats received L5 spinal nerve ligation (SNL) for developing neuropathic pain. Fourteen days after L5 SNL surgery, they were divided into three groups according to duration of PRF current for 6 minutes, 12 minutes, and none. PRF current was delivered via direct visualization adjacent to the L5 dorsal root ganglion (DRG). Pain behavior was evaluated every week after L5 SNL surgery, until day 28. Seven days after PRF treatment, L5 DRG tissue was harvested to detect levels of activating translation factor 3 (ATF3; a marker of neuronal damage) and hyperpolarization-activated cyclic nucleotide (HCN)-gated cation channels (key factors in neuropathic pain) using quantitative PCR.ResultsBefore PRF application, withdrawal thresholds were significantly lower than at baseline and did not differ significantly between the three groups. After PRF application, withdrawal thresholds in the PRF6 and PRF12 groups were significantly increased compared to those in the sham group. However, those in the PRF6 and PRF12 groups did not differ significantly. The expression level of ATF3 mRNA in the PRF12 group was significantly higher than that in the sham group (P<0.01), but the expression of HCN1 and HCN2 channels did not differ significantly between the three groups.ConclusionProlonged PRF exposure, from 6 to 12 minutes, was not only ineffective but also associated with increased neuronal damage. These findings do not support prolonged PRF exposure as a helpful treatment for neuropathic pain. In this study, the involvement of HCN channels in the antiallodynic effects of PRF was uncertain.