A spinal mechanism of action for duloxetine in a rat model of painful diabetic neuropathy

Specific transcription factors Ascl1 and Lhx6 attenuate diabetic neuropathic pain by modulating spinal neuroinflammation and microglial activation in mice

Gamma-aminobutyric acid (GABA) neuronal system-related transcription factors (TFs) play a critical role in GABA production, and GABA modulates diabetic neuropathic pain (DNP). The present study investigated the therapeutic effects of intrathecal delivery of two TFs achaete-scute homolog 1 (Ascl1) and LIM homeobox protein 6 (Lhx6) in a mouse model of DNP and elucidated their underlying mechanisms. GABA-related specific TFs, including Ascl1, Lhx6, distal-less homeobox 1, distal-less homeobox 5, the Nkx2.1 homeobox gene, and the Nkx2.2 homeobox gene, were investigated under normal and diabetic conditions. Among these, the expression of Ascl1 and Lhx6 was significantly downregulated in mice with diabetes. Therefore, a single intrathecal injection of combined lenti-Ascl1/Lhx6 was performed. Intrathecal delivery of lenti-Ascl1/Lhx6 significantly relieved mechanical allodynia and heat hyperalgesia in mice with DNP. Ascl1/Lhx6 delivery also reduced microglial activation, decreased the levels of pro-inflammatory cytokines including tumor necrosis factor-α and interleukin (IL)-1β, increased the levels of anti-inflammatory cytokines including IL-4, IL-10, and IL-13, and reduced the activation of p38, c-Jun N-terminal kinase, and NF-κB in the spinal cord of mice with DNP, thereby reducing DNP. The results of this study suggest that intrathecal Ascl1/Lhx6 delivery attenuates DNP via upregulating spinal GABA neuronal function and inducing anti-inflammatory effects.

Diabetic Neuropathic Pain and Serotonin: What Is New in the Last 15 Years?

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is involved in numerous physiological functions and plays a key role in pain modulation including neuropathic pain. Diabetic neuropathy is a common complication of diabetes mellitus often accompanied by chronic neuropathic pain. Animal models of diabetes offer relevant tools for studying the pathophysiological mechanisms and pharmacological sensitivity of diabetic neuropathic pain and for identifying new therapeutic targets. In this review, we report data from preclinical work published over the last 15 years on the analgesic activity of drugs acting on the serotonergic system, such as serotonin and noradrenaline reuptake inhibitor (SNRI) antidepressants, and on the involvement of certain serotonin receptors-in particular 5-HT_1A, 5-HT_2A/2c and 5-HT₆ receptors-in rodent models of painful diabetic neuropathy.

Spinal cord vascular degeneration impairs duloxetine penetration

Introduction

Chronic pain is a prevalent physically debilitating health-related morbidity. Frontline analgesics are inadequate, providing only partial pain relief in only a proportion of the patient cohort. Here, we explore whether alterations in spinal cord vascular perfusion are a factor in reducing the analgesic capability of the noradrenaline reuptake inhibitor, duloxetine.

Method

An established rodent model of spinal cord vascular degeneration was used. Endothelial-specific vascular endothelial growth factor receptor 2 knockout mouse was induced via hydroxytamoxifen administered via intrathecal injection. Duloxetine was administered via intraperitoneal injection, and nociceptive behavioural testing was performed in both WT and VEGFR2KO mice. LC-MS/MS was performed to explore the accumulation of duloxetine in the spinal cord in WT and VEGFR2KO mice.

Results

Spinal cord vascular degeneration leads to heat hypersensitivity and a decline in capillary perfusion. The integrity of noradrenergic projections (dopa - hydroxylase labelled) in the dorsal horn remained unaltered in WT and VEGFR2KO mice. There was an association between dorsal horn blood flow with the abundance of accumulated duloxetine in the spinal cord and analgesic capacity. In VEGFR2KO mice, the abundance of duloxetine in the lumbar spinal cord was reduced and was correlated with reduced anti-nociceptive capability of duloxetine.

Discussion

Here, we show that an impaired vascular network in the spinal cord impairs the anti-nociceptive action of duloxetine. This highlights that the spinal cord vascular network is crucial to maintaining the efficacy of analgesics to provide pain relief.

Spinal disinhibition: evidence for a hyperpathia phenotype in painful diabetic neuropathy

The dominant sensory phenotype in patients with diabetic polyneuropathy and neuropathic pain is a loss of function. This raises questions as to which mechanisms underlie pain generation in the face of potentially reduced afferent input. One potential mechanism is spinal disinhibition, whereby a loss of spinal inhibition leads to increased ascending nociceptive drive due to amplification of, or a failure to suppress, incoming signals from the periphery. We aimed to explore whether a putative biomarker of spinal disinhibition, impaired rate-dependent depression of the Hoffmann reflex, is associated with a mechanistically appropriate and distinct pain phenotype in patients with painful diabetic neuropathy. In this cross-sectional study, 93 patients with diabetic neuropathy underwent testing of Hoffmann reflex rate-dependent depression and detailed clinical and sensory phenotyping, including quantitative sensory testing. Compared to neuropathic patients without pain, patients with painful diabetic neuropathy had impaired Hoffmann reflex rate-dependent depression at 1, 2 and 3 Hz (P ≤ 0.001). Patients with painful diabetic neuropathy exhibited an overall loss of function profile on quantitative sensory testing. However, within the painful diabetic neuropathy group, cluster analysis showed evidence of greater spinal disinhibition associated with greater mechanical pain sensitivity, relative heat hyperalgesia and higher ratings of spontaneous burning pain. These findings support spinal disinhibition as an important centrally mediated pain amplification mechanism in painful diabetic neuropathy. Furthermore, our analysis indicates an association between spinal disinhibition and a distinct phenotype, arguably akin to hyperpathia, with combined loss and relative gain of function leading to increasing nociceptive drive.

The serotonergic system dysfunction in diabetes mellitus

Most peripheral serotonin (5-HT) is synthesized in enterochromaffin cells, and most circulating 5-HT is stored in platelets. As a monoamine, 5-HT has several functions in various non-neuronal and neuronal systems. In the central nervous system, it functions as a neurotransmitter to modulate feeding behavior and mood. Numerous clinical trials have focused on increasing 5-HT activation in the central nervous system, including those involving anti-obesity drugs currently in the market, although severe side effects on peripheral system can lead to the withdrawal of certain drugs. Recent studies have revealed that both the peripheral and central serotonergic systems play a vital role in diabetes and its complications. This review summarizes the roles of the serotonergic system in blood glucose regulation, diabetic macroangiopathy, diabetic peripheral neuropathy, and diabetic encephalopathy, indicating its potential clinical significance as a therapeutic target for the treatment of diabetes and its complications.

Spinal cord-wide structural disruption in type 2 diabetes rescued by exenatide "a glucagon-like peptide-1 analogue" via down-regulating inflammatory, oxidative stress and apoptotic signaling pathways

The mechanisms of spinal cord-wide structural and functional disruption in diabetic patients remain elusive. This study evaluated histopathological alterations of the spinal cord cytoarchitecture in T2DM model of rats and assessed the potential ameliorating effect of exenatide "a potent GLP-1 analogue". Thirty male rats were allocated into three groups; I (control), II (Diabetic): T2DM was induced by high fat diet for 8 weeks followed by a single I.P injection of STZ (25 mg/kg BW) and III (Diabetic/Exenatide): T2DM rats injected with exenatide (10 μg/Kg, S.C. twice daily for 2 weeks). Neurobehavioral sensory and motor tests were carried out and glycemic control biomarkers and indices of insulin resistance and sensitivity were measured. In addition, the spinal cord was processed for histological and immunohistochemical studies besides assessing its tissue homogenate levels of pro-inflammatory/anti-inflamatory cytokines and oxidant/antioxidant biomarkers. Moreover, RT-qPCR was performed to measure the expression of proapoptotic/antiapoptotic and neurotrophic genes. The diabetic rats exhibited thermal hyperalgesia, mechanical allodynia and decreased locomotor activity along with increased serum glucose, insulin, HbA1c, HOMA-IR while, quantitative insulin sensitivity check index (QUICKI) was decreased. Also, IL-1β NF-kB, MDA increased while IL-10, SOD activity and β-endorphin decreased in the spinal tissue. Up regulation of caspase-3 and down regulation of Bcl-2, nerve growth factor (NGF) and glial cell-derived neurotrophic (GDNF) in diabetic rats. Also, they exhibited histopathological changes and increased CD68 positive microglia and Bax immunoreactivity in the spinal cord. Subsequent to exenatide treatment, most biomolecular, structural and functional impairments of the spinal cord were restored in the diabetic rats. In conclusion, the neuro-modulating effect of exenatide against diabetic-induced spinal cord affection warrants the concern about its therapeutic relevance in confronting the devastating diabetic neuropathic complications.

Preoperative duloxetine on postoperative pain after laparoscopic gynecological surgeries: A systematic review and meta-analysis of randomized controlled trials

OBJECTIVE

To evaluate the efficacy and safety of preoperative duloxetine on postoperative pain management after gynecologic laparoscopic surgeries.

METHODS

A systematic search was done in Cochrane Library, PubMed, ISI web of science, and Scopus from inception to September 2021. We selected randomized clinical trials (RCTs) that compared preoperative duloxetine (intervention group) versus placebo (control group) among women undergoing gynecologic laparoscopic surgeries. Our primary outcomes were pain scores evaluated by the Visual Analog Scale (VAS) at 2, 6, 12, and 24 h postoperatively. Our secondary outcomes were the time required for the first analgesic request in minutes, postoperative analgesic consumption in milligrams, length of hospital stay in days, and side effects.

RESULTS

Four RCTs with a total number of 244 patients were included in our systematic review and meta-analysis. We found duloxetine was linked to a significant reduction in VAS pain scores at different time intervals. The first analgesic request was significantly earlier in the placebo group than in the duloxetine group (p = 0.03). In addition, duloxetine significantly reduced the postoperative analgesic consumption compared to placebo (MD= -41.97, 95% CI [-53.23, -30.72], p<0.001). However, both groups did not differ in the length of hospital stay and side effects.

CONCLUSIONS

Duloxetine administration prior to gynecological laparoscopic surgeries is safe and effective in improving postoperative pain and analgesia.

Application of PLGA nanoparticles to enhance the action of duloxetine on microglia in neuropathic pain

Duloxetine (DLX) is a selective serotonin and noradrenaline reuptake inhibitor (SNRI) used for the treatment of pain, but it has been reported to show side effects in 10-20% of patients. Its analgesic efficacy in central pain is putatively related to its influence on descending inhibitory neuronal pathways. However, DLX can also affect the activation of microglia. This study was performed to investigate whether PLGA nanoparticles (NPs), which are expected to enhance targeting to microglia, can improve the analgesic efficacy and limit the side effects of DLX. PLGA NPs encapsulating a low dose of DLX (DLX NPs) were synthesized and characterized and their localization was determined. The analgesic and anti-inflammatory effects of DLX NPs were evaluated in a spinal nerve ligation (SNL)-induced neuropathic pain model. The analgesic effect of DLX lasted for only a few hours and disappeared within 1 day. However, DLX NPs alleviated mechanical allodynia, and the effect was maintained for 1 week. DLX NPs were localized to the spinal microglia and suppressed microglial activation, phosphorylation of p38/NF-κB-mediated pathways and the production of inflammatory cytokines in the spinal dorsal horn of SNL rats. We demonstrated that DLX NPs can provide a prolonged analgesic effect by enhanced targeting of microglia. Our observations imply that DLX delivery through nanoparticle encapsulation allows drug repositioning with a prolonged analgesic effect, and reduces the potential side effects of abuse and overdose.

Optimal Utility of H-Reflex RDD as a Biomarker of Spinal Disinhibition in Painful and Painless Diabetic Neuropathy

Impaired rate-dependent depression of the Hoffman reflex (HRDD) is a potential biomarker of impaired spinal inhibition in patients with painful diabetic neuropathy. However, the optimum stimulus-response parameters that identify patients with spinal disinhibition are currently unknown. We systematically compared HRDD, performed using trains of 10 stimuli at five stimulation frequencies (0.3, 0.5, 1, 2 and 3 Hz), in 42 subjects with painful and 62 subjects with painless diabetic neuropathy with comparable neuropathy severity, and 34 healthy controls. HRDD was calculated using individual and mean responses compared to the initial response. At stimulation frequencies of 1, 2 and 3 Hz, HRDD was significantly impaired in patients with painful diabetic neuropathy compared to patients with painless diabetic neuropathy for all parameters and for most parameters when compared to healthy controls. HRDD was significantly enhanced in patients with painless diabetic neuropathy compared to controls for responses towards the end of the 1 Hz stimulation train. Receiver operating characteristic curve analysis in patients with and without pain showed that the area under the curve was greatest for response averages of stimuli 2-4 and 2-5 at 1 Hz, AUC = 0.84 (95%CI 0.76-0.92). Trains of 5 stimuli delivered at 1 Hz can segregate patients with painful diabetic neuropathy and spinal disinhibition, whereas longer stimulus trains are required to segregate patients with painless diabetic neuropathy and enhanced spinal inhibition.

Novel and Emerging Electrophysiological Biomarkers of Diabetic Neuropathy and Painful Diabetic Neuropathy

PURPOSE

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes. Small and large peripheral nerve fibers can be involved in DPN. Large nerve fiber damage causes paresthesia, sensory loss, and muscle weakness, and small nerve fiber damage is associated with pain, anesthesia, foot ulcer, and autonomic symptoms. Treatments for DPN and painful DPN (pDPN) pose considerable challenges due to the lack of effective therapies. To meet these challenges, there is a major need to develop biomarkers that can reliably diagnose and monitor progression of nerve damage and, for pDPN, facilitate personalized treatment based on underlying pain mechanisms.

METHODS

This study involved a comprehensive literature review, incorporating article searches in electronic databases (Google Scholar, PubMed, and OVID) and reference lists of relevant articles with the authors' substantial expertise in DPN. This review considered seminal and novel research and summarizes emerging biomarkers of DPN and pDPN that are based on neurophysiological methods.

FINDINGS

From the evidence gathered from 145 papers, this submission describes emerging clinical neurophysiological methods with potential to act as biomarkers for the diagnosis and monitoring of DPN as well as putative future roles as predictors of response to antineuropathic pain medication in pDPN. Nerve conduction studies only detect large fiber damage and do not capture pathology or dysfunction of small fibers. Because small nerve fiber damage is prominent in DPN, additional biomarkers of small nerve fiber function are needed. Activation of peripheral nociceptor fibers using laser, heat, or targeted electrical stimuli can generate pain-related evoked potentials, which are an objective neurophysiological measure of damage along the small fiber pathways. Assessment of nerve excitability, which provides a surrogate of axonal properties, may detect alterations in function before abnormalities are detected by nerve conduction studies. Microneurography and rate-dependent depression of the Hoffmann-reflex can be used to dissect underlying pain-generating mechanisms arising from the periphery and spinal cord, respectively. Their role in informing mechanistic-based treatment of pDPN as well as facilitating clinical trials design is discussed.

IMPLICATIONS

The neurophysiological methods discussed, although currently not practical for use in busy outpatient settings, detect small fiber and early large fiber damage in DPN as well as disclosing dominant pain mechanisms in pDPN. They are suited as diagnostic and predictive biomarkers as well as end points in mechanistic clinical trials of DPN and pDPN.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Pharmacological Modulation of Rate-Dependent Depression of the Spinal H-Reflex Predicts Therapeutic Efficacy against Painful Diabetic Neuropathy

Impaired rate-dependent depression (RDD) of the spinal H-reflex occurs in diabetic rodents and a sub-set of patients with painful diabetic neuropathy. RDD is unaffected in animal models of painful neuropathy associated with peripheral pain mechanisms and diabetic patients with painless neuropathy, suggesting RDD could serve as a biomarker for individuals in whom spinal disinhibition contributes to painful neuropathy and help identify therapies that target impaired spinal inhibitory function. The spinal pharmacology of RDD was investigated in normal rats and rats after 4 and 8 weeks of streptozotocin-induced diabetes. In normal rats, dependence of RDD on spinal GABAergic inhibitory function encompassed both GABA_A and GABA_B receptor sub-types. The time-dependent emergence of impaired RDD in diabetic rats was preceded by depletion of potassium-chloride co-transporter 2 (KCC2) protein in the dorsal, but not ventral, spinal cord and by dysfunction of GABA_A receptor-mediated inhibition. GABA_B receptor-mediated spinal inhibition remained functional and initially compensated for loss of GABA_A receptor-mediated inhibition. Administration of the GABA_B receptor agonist baclofen restored RDD and alleviated indices of neuropathic pain in diabetic rats, as did spinal delivery of the carbonic anhydrase inhibitor acetazolamide. Pharmacological manipulation of RDD can be used to identify potential therapies that act against neuropathic pain arising from spinal disinhibition.

Preclinical Neuropathic Pain Assessment; the Importance of Translatability and Bidirectional Research

For patients suffering with chronic neuropathic pain the need for suitable novel therapies is imperative. Over recent years a contributing factor for the lack of development of new analgesics for neuropathic pain has been the mismatch of primary neuropathic pain assessment endpoints in preclinical vs. clinical trials. Despite continuous forward translation failures across diverse mechanisms, reflexive quantitative sensory testing remains the primary assessment endpoint for neuropathic pain and analgesia in animals. Restricting preclinical evaluation of pain and analgesia to exclusively reflexive outcomes is over simplified and can be argued not clinically relevant due to the continued lack of forward translation and failures in the clinic. The key to developing new analgesic treatments for neuropathic pain therefore lies in the development of clinically relevant endpoints that can translate preclinical animal results to human clinical trials. In this review we discuss this mismatch of primary neuropathic pain assessment endpoints, together with clinical and preclinical evidence that supports how bidirectional research is helping to validate new clinically relevant neuropathic pain assessment endpoints. Ethological behavioral endpoints such as burrowing and facial grimacing and objective measures such as electroencephalography provide improved translatability potential together with currently used quantitative sensory testing endpoints. By tailoring objective and subjective measures of neuropathic pain the translatability of new medicines for patients suffering with neuropathic pain will hopefully be improved.

Ammoxetine attenuates diabetic neuropathic pain through inhibiting microglial activation and neuroinflammation in the spinal cord

Background

Diabetic neuropathic pain (DNP) is a common and distressing complication in patients with diabetes, and the underlying mechanism remains unclear. Tricyclic antidepressants (TCAs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) are recommended as first-line drugs for DNP. Ammoxetine is a novel and potent SNRI that exhibited a strong analgesic effect on models of neuropathic pain, fibromyalgia-related pain, and inflammatory pain in our primary study. The present study was undertaken to investigate the chronic treatment properties of ammoxetine on DNP and the underlying mechanisms for its effects.

Methods

The rat model of DNP was established by a single streptozocin (STZ) injection (60 mg/kg). Two weeks after STZ injection, the DNP rats were treated with ammoxetine (2.5, 5, and 10 mg/kg/day) for 4 weeks. The mechanical allodynia and locomotor activity were assayed to evaluate the therapeutic effect of ammoxetine. In mechanism study, the activation of microglia, astrocytes, the protein levels of pro-inflammatory cytokines, the mitogen-activated protein kinases (MAPK), and NF-κB were evaluated. Also, microglia culture was used to assess the direct effects of ammoxetine on microglial activation and the signal transduction mechanism.

Results

Treatment with ammoxetine for 4 weeks significantly relieved the mechanical allodynia and ameliorated depressive-like behavior in DNP rats. In addition, DNP rats displayed increased activation of microglia in the spinal cord, but not astrocytes. Ammoxetine reduced the microglial activation, accumulation of pro-inflammatory cytokines, and activation of p38 and c-Jun N-terminal kinase (JNK) in the spinal cord of DNP rats. Furthermore, ammoxetine displayed anti-inflammatory effects upon challenge with LPS in BV-2 microglia cells.

Conclusion

Our results suggest that ammoxetine may be an effective treatment for relieving DNP symptoms. Moreover, a reduction in microglial activation and pro-inflammatory release by inhibiting the p-p38 and p-JNK pathways is involved in the mechanism.

Neuroprotective effect of duloxetine in a mouse model of diabetic neuropathy: Role of glia suppressing mechanisms

AIMS

Painful diabetic neuropathy (PDN) is one of the most frequent complications of diabetes and the current therapies have limited efficacy. This study aimed to study the neuroprotective effect of duloxetine, a serotonin noradrenaline reuptake inhibitor (SNRI), in a mouse model of diabetic neuropathy.

MAIN METHODS

Nine weeks after developing of PDN, mice were treated with either saline or duloxetine (15 or 30 mg/kg) for four weeks. The effect of duloxetine was assessed in terms of pain responses, histopathology of sciatic nerve and spinal cord, sciatic nerve growth factor (NGF) gene expression and on the spinal expression of astrocytes (glial fibrillary acidic protein, GFAP) and microglia (CD₁₁b).

KEY FINDINGS

The present results highlighted that duloxetine (30 mg/kg) increased the withdrawal threshold in von-Frey test. In addition, both doses of duloxetine prolonged the licking time and latency to jump in the hot-plate test. Moreover, duloxetine administration downregulated the spinal expression of both CD₁₁b and GFAP associated with enhancement in sciatic mRNA expression of NGF.

SIGNIFICANCE

The current results highlighted that duloxetine provided peripheral and central neuroprotective effects in neuropathic pain is, at least in part, related to its downregulation in spinal astrocytes and microglia. Further, this neuroprotective effect was accompanied by upregulation of sciatic expression of NGF.

Giving priority to preclinical pain measures resistant to existing drugs for developing innovative analgesics

Preclinical Research & Development Chronic pain is a major health and socioeconomic burden because of its high prevalence, negative influence on patients' physical and/or emotional conditions, and huge costs to society. The responses of chronic pain patients to analgesic therapies vary substantially from individual to individual, and no more than a minority of chronic pain patients with various etiologies such as neuropathy and inflammation are, in fact, successfully relieved by existing drugs including opioid analgesics, nonopioid analgesics, antiepileptics, and antidepressants. The large primary unmet medical need would therefore be the patient domain that does not respond well to existing drugs. Accordingly, the expected profile for innovative analgesics would not be efficacy in the responder patient domain, but significant efficacy in patients with existing drug-resistant chronic pain. Meanwhile, the current gold standard in preclinical pain measures for the screening of analgesic candidates is existing drug-sensitive pain measures in animal models of chronic pain. Analgesic candidates screened using such preclinical pain measures during the last decades have been far from fulfilling the expected profile for innovative analgesics. Given that it is unlikely that such existing drug-sensitive pain measures are the best approach to developing innovative analgesics, one of the other approaches would be giving priority to existing drug-resistant pain measures in preclinical research. This review introduces potentially applicable existing drug-resistant pain measures published so far and suggests that the use of them would lead to the development of innovative analgesics.

A double blind, placebo controlled, phase II randomised cross-over trial investigating the use of duloxetine for the treatment of chemotherapy-induced peripheral neuropathy

BACKGROUND

Chemotherapy-induced peripheral neuropathy (CIPN) is a significant side effect of cancer treatment, potentially leading to early cessation of chemotherapy, enduring symptoms and long-lasting disability. Evidence from preclinical and clinical studies suggests that duloxetine, a serotonin-noradrenaline reuptake inhibitor, may be effective in the symptomatic treatment of CIPN. This double blind, placebo controlled, phase II randomised cross-over trial aims to determine whether treatment with duloxetine results in a reduction in chronic neuropathic symptoms experienced as a result of neurotoxic chemotherapy treatment.

METHODS/DESIGN

Participants who have received neurotoxic chemotherapy and experience daily symptoms as a consequence of peripheral neuropathy will be randomly allocated to control or experimental group with a 1:1 allocation, stratified by chemotherapy type. The primary endpoint will be patient-reported CIPN symptoms, as assessed via the FACT/GOG-Ntx. As a secondary objective, the trial will investigate whether duloxetine improves neurophysiological parameters and functional status in patients who have received neurotoxic chemotherapy treatment.

DISCUSSION

This trial will investigate the effectiveness of duloxetine in reducing neuropathic symptoms following chemotherapy treatment, and aims to provide insight into the mechanisms underlying the symptomatic relief that duloxetine may provide. These results will be informative in advancing clinical knowledge regarding the treatment of CIPN.

The H-Reflex as a Biomarker for Spinal Disinhibition in Painful Diabetic Neuropathy

PURPOSE OF REVIEW

Neuropathic pain may arise from multiple mechanisms and locations. Efficacy of current treatments for painful diabetic neuropathy is limited to an unpredictable subset of patients, possibly reflecting diversity of pain generator mechanisms, and there is a lack of targeted treatments for individual patients. This review summarizes preclinical evidence supporting a role for spinal disinhibition in painful diabetic neuropathy, the physiology and pharmacology of rate-dependent depression (RDD) of the spinal H-reflex and the translational potential of using RDD as a biomarker of spinally mediated pain.

RECENT FINDINGS

Impaired RDD occurs in animal models of diabetes and was also detected in diabetic patients with painful vs painless neuropathy. RDD status can be determined using standard neurophysiological equipment. Loss of RDD may provide a clinical biomarker of spinal disinhibition, thereby enabling a personalized medicine approach to selection of current treatment options and enrichment of future clinical trial populations.

Levetiracetam synergizes with gabapentin, pregabalin, duloxetine and selected antioxidants in a mouse diabetic painful neuropathy model

RATIONALE

We have reported that levetiracetam, a novel anticonvulsant with analgesic properties, synergizes with ibuprofen/aspirin/paracetamol in a model of diabetic painful neuropathy (DPN). Most guidelines recommend gabapentin, pregabalin, and duloxetine as first- or second-line agents for DPN.

OBJECTIVE

We examined the effects of combination treatment of first-/second-line analgesics with levetiracetam in a model of DPN. Additionally, the levetiracetam's combinations with antioxidants, low dose of aspirin, coenzyme Q10, or α-lipoic acid were evaluated.

METHODS

Diabetes was induced in C57BL/6 mice with a single high dose of streptozotocin. The antinociceptive effects of orally administered levetiracetam, gabapentin, pregabalin, duloxetine (acute treatment) and aspirin, coenzyme Q10, and α-lipoic acid (preventive 7-day treatment), as well as combinations of levetiracetam with individual drugs were examined in the tail-flick test. In combination experiments, the drugs were coadministered in fixed-dose fractions of single-drug ED₅₀; the type of interaction was determined by isobolographic analysis.

RESULTS

About 60-, 32-, 30-, 26-, 18-, and 6-fold reductions of doses of both drugs in levetiracetam combinations with pregabalin, gabapentin, coenzyme Q10, aspirin, duloxetine, and α-lipoic acid, respectively, were detected.

CONCLUSIONS

Combinations of levetiracetam with gabapentin/pregabalin/duloxetine that target different mechanisms/sites of action involved in DPN, as well as combinations of levetiracetam and low-dose aspirin/coenzyme Q10/α-lipoic acid that target underlying causes of DPN, produce marked synergistic interactions in reducing nociception in diabetic mice. This suggests that these combination treatments might be of great benefit for diabetic patients and should be explored further in clinical trials.

Spinal Disinhibition in Experimental and Clinical Painful Diabetic Neuropathy

Impaired rate-dependent depression (RDD) of the Hoffman reflex is associated with reduced dorsal spinal cord potassium chloride cotransporter expression and impaired spinal γ-aminobutyric acid type A receptor function, indicative of spinal inhibitory dysfunction. We have investigated the pathogenesis of impaired RDD in diabetic rodents exhibiting features of painful neuropathy and the translational potential of this marker of spinal inhibitory dysfunction in human painful diabetic neuropathy. Impaired RDD and allodynia were present in type 1 and type 2 diabetic rats but not in rats with type 1 diabetes receiving insulin supplementation that did not restore normoglycemia. Impaired RDD in diabetic rats was rapidly normalized by spinal delivery of duloxetine acting via 5-hydroxytryptamine type 2A receptors and temporally coincident with the alleviation of allodynia. Deficits in RDD and corneal nerve density were demonstrated in patients with painful diabetic neuropathy compared with healthy control subjects and patients with painless diabetic neuropathy. Spinal inhibitory dysfunction and peripheral small fiber pathology may contribute to the clinical phenotype in painful diabetic neuropathy. Deficits in RDD may help identify patients with spinally mediated painful diabetic neuropathy who may respond optimally to therapies such as duloxetine.

The need for fundamental reforms in the pain research field to develop innovative drugs

INTRODUCTION

Chronic pain is a major healthcare issue owing to its high prevalence, significant physical and emotional burden on the patients, and huge financial burden on the society. The efficacy of currently available medications is unsatisfactory owing to their limited effect size and the low responder rate (less than 50%). Thus, there is a large unmet need for innovative therapies for chronic pain. Areas covered: In this review, the author points out the need for fundamental reforms in pain research. For the last several decades, drug discovery research has extensively focused on designing new therapies using animal models of chronic pain. It has, however, made insufficient progress with respect to the launch of innovative analgesic drugs, because the translation from preclinical to clinical stages has not been satisfactory. Thus, the strategies for developing innovative analgesic drugs are discussed. Expert opinion: Points to be considered in the discovery of drugs for pain relief include: (1) the exclusion of bias incorporation and the alignment of clinical and preclinical endpoints in the assessment of analgesic efficacy; (2) the understanding of primary unmet needs; (3) the assessment of new therapies by biomarker-prioritized frameworks, and (4) the stratification of chronic pain sufferers.

Duloxetine Inhibits Microglial P2X4 Receptor Function and Alleviates Neuropathic Pain after Peripheral Nerve Injury

P2X4 receptors (P2X4R) are a family of ATP-gated non-selective cation channels. We previously demonstrated that activation of P2X4R in spinal microglia is crucial for neuropathic pain, a highly debilitating chronic pain condition, suggesting that P2X4R is a potential therapeutic target for treating neuropathic pain. Thus, the identification of a compound that has a potent inhibitory effect on P2X4R is an important clinical challenge. In the present study, we screened a chemical library of clinically approved drugs and show for the first time that duloxetine, a serotonin and noradrenaline reuptake inhibitor, has an inhibitory effect on rodent and human P2X4R. In primary cultured microglial cells, duloxetine also inhibited P2X4R-, but not P2X7R-, mediated responses. Moreover, intrathecal administration of duloxetine in a model of neuropathic pain produced a reversal of nerve injury-induced mechanical allodynia, a cardinal symptom of neuropathic pain. In rats that were pretreated with a serotonin-depleting agent and a noradrenaline neurotoxin, the antiallodynic effect of duloxetine was reduced, but still remained. Based on these results, we suggest that, in addition to duloxetine’s primary inhibitory action on serotonin and noradrenaline transporters, an inhibitory effect on P2X4R may be involved at least in part in an antiallodynic effect of intrathecal duloxetine in a model of neuropathic pain.

Painful neuropathy: Mechanisms

Painful neuropathy, like the other complications of diabetes, is a growing healthcare concern. Unfortunately, current treatments are of variable efficacy and do not target underlying pathogenic mechanisms, in part because these mechanisms are not well defined. Rat and mouse models of type 1 diabetes are frequently used to study diabetic neuropathy, with rats in particular being consistently reported to show allodynia and hyperalgesia. Models of type 2 diabetes are being used with increasing frequency, but the current literature on the progression of indices of neuropathic pain is variable and relatively few therapeutics have yet been developed in these models. While evidence for spontaneous pain in rodent models is sparse, measures of evoked mechanical, thermal and chemical pain can provide insight into the pathogenesis of the condition. The stocking and glove distribution of pain tantalizingly suggests that the generator site of neuropathic pain is found within the peripheral nervous system. However, emerging evidence demonstrates that amplification in the spinal cord, via spinal disinhibition and neuroinflammation, and also in the brain, via enhanced thalamic activity or decreased cortical inhibition, likely contribute to the pathogenesis of painful diabetic neuropathy. Several potential therapeutic strategies have emerged from preclinical studies, including prophylactic treatments that intervene against underlying mechanisms of disease, treatments that prevent gains of nociceptive function, treatments that suppress enhancements of nociceptive function, and treatments that impede normal nociceptive mechanisms. Ongoing challenges include unraveling the complexity of underlying pathogenic mechanisms, addressing the potential disconnect between the perceived location of pain and the actual pain generator and amplifier sites, and finding ways to identify which mechanisms operate in specific patients to allow rational and individualized choice of targeted therapies.

Impaired diffuse noxious inhibitory controls in specific alternation of rhythm in temperature-stressed rats

Fibromyalgia is characterized by chronic widespread musculoskeletal pain. A hypofunction in descending pain inhibitory systems is considered to be involved in the chronic pain of fibromyalgia. We examined functional changes in descending pain inhibitory systems in rats with specific alternation of rhythm in temperature (SART) stress, by measuring the strength of diffuse noxious inhibitory controls (DNIC). Hindpaw withdrawal thresholds to mechanical von Frey filament or fiber-specific electrical stimuli by the Neurometer system were used to measure the pain response. To induce DNIC, capsaicin was injected into the intraplantar of the forepaw. SART-stressed rats were established by exposure to repeated cold stress for 4 days. In the control rats, heterotopic intraplantar capsaicin injection increased withdrawal threshold, indicative of analgesia by DNIC. The strength of DNIC was reduced by naloxone (μ-opioid receptor antagonist, intraperitoneally and intracerebroventricularly), yohimbine (α2-adrenoceptor antagonist, intrathecally), and WAY-100635 (5-HT1A receptor antagonist, intrathecally) in the von Frey test. In SART-stressed rats, capsaicin injection did not increase withdrawal threshold in the von Frey test, indicating deficits in DNIC. In the Neurometer test, deficient DNIC in SART-stressed rats were observed only for Aδ- and C-fibers, but not Aβ-fibers stimulation. Analgesic effect of intracerebroventricular morphine was markedly reduced in SART-stressed rats compared with the control rats. Taken together, in SART-stressed rats, capsaicin-induced DNIC were deficient, and a hypofunction of opioid-mediated central pain modulation system may cause the DNIC deficit.

Effects of centrally acting analgesics on spinal segmental reflexes and wind-up

BACKGROUND

The spinal cord is a prime site of action for analgesia. Here we characterize the effects of established analgesics on segmental spinal reflexes. The aim of the study was to look for the pattern of action or signature of analgesic effects on these reflexes.

METHODS

We used a spinal cord in vitro preparation of neonate mice to record ventral root responses to dorsal root stimulation. Pregabalin, clonidine, morphine and duloxetine and an experimental sigma-1 receptor antagonist (S1RA) were applied to the preparation in a cumulative concentration protocol. Drug effects on the wind-up produced by repetitive stimulation of C-fibres and on responses to single A- and C-fibre intensity stimuli were analysed.

RESULTS

All compounds produced a concentration-dependent inhibition of total spikes elicited by repetitive stimulation. Concentrations producing ∼50% reduction in this parameter were (in μM) clonidine (0.01), morphine (0.1), pregabalin (1), duloxetine (10) and S1RA (30). At these concentrations clonidine, pregabalin and S1RA had significant effects on the wind-up index and little depressant effects on responses to single stimuli. Morphine and duloxetine did not depress wind-up index and showed large effects on responses to single stimuli. None of the compounds had strong effects on the amplitude of the non-nociceptive monosynaptic reflex.

CONCLUSIONS

morphine and duloxetine had general depressant effects on spinal reflexes, whereas the effects of clonidine, pregabalin and S1RA appeared to be restricted to signals originated by strong repetitive activation of C-fibres. Results are discussed in the context of reported behavioural effects of the compounds studied.

Mechanisms and pharmacology of diabetic neuropathy - experimental and clinical studies

Neuropathic pain is the most common chronic complication of diabetes mellitus. The mechanisms involved in the development of diabetic neuropathy include changes in the blood vessels that supply the peripheral nerves; metabolic disorders, such as the enhanced activation of the polyol pathway; myo-inositol depletion; and increased non-enzymatic glycation. Currently, much attention is focused on the changes in the interactions between the nervous system and the immune system that occur in parallel with glial cell activation; these interactions may also be responsible for the development of neuropathic pain accompanying diabetes. Animal models of diabetic peripheral neuropathy have been utilized to better understand the phenomenon of neuropathic pain in individuals with diabetes and to define therapeutic goals. The studies on the effects of antidepressants on diabetic neuropathic pain in streptozotocin (STZ)-induced type 1 diabetes have been conducted. In experimental models of diabetic neuropathy, the most effective antidepressants are tricyclic antidepressants, selective serotonin reuptake inhibitors, and serotonin norepinephrine reuptake inhibitors. Clinical studies of diabetic neuropathy indicate that the first line treatment should be tricyclic antidepressants, which are followed by anticonvulsants and then opioids. In this review, we will discuss the mechanisms of the development of diabetic neuropathy and the most common drugs used in experimental and clinical studies.

A selective α2 B adrenoceptor agonist (A-1262543) and duloxetine modulate nociceptive neurones in the medial prefrontal cortex, but not in the spinal cord of neuropathic rats

BACKGROUND

The noradrenergic system contributes to pain modulation, but the roles of its specific adrenoceptors are still being defined. We have identified a novel, potent (rat EC50  = 4.3 nM) and selective α2B receptor agonist, A-1262543, to further explore this adrenoceptor subtype's contribution to pathological nociception.

METHODS

Systemic administration of A-1262543 (1-10 mg/kg, intraperitoneal) dose-dependently attenuated mechanical allodynia in animals with a spinal nerve ligation injury. To further explore its mechanism of action, the activity of nociceptive neurones in the spinal cord and medial prefrontal cortex (mPFC) were examined after injection of 3 mg/kg of A-1262543 (intravenous, i.v.). These effects were compared with duloxetine (3 mg/kg, i.v.), a dual noradrenaline (NA) and serotonin (5-HT) reuptake inhibitor.

RESULTS

Systemic administration of A-1262543 or duloxetine did not alter the spontaneous or evoked firing of spinal wide dynamic range and nociceptive-specific neurones in the neuropathic rats, indicating that neither compound engaged spinal, peripheral or descending pathways. In contrast to the lack of effect on spinal neurones, both A-1262543 and duloxetine reduced the evoked and spontaneous firing of 'pain-responsive' (PR) neurones in the mPFC. Duloxetine, but not A-1262543, also inhibited the firing of pain non-responsive (nPR) neurones in the mPFC probably reflecting duloxetine's contribution to modulating non-pain endpoints.

CONCLUSIONS

These data highlight that activation of the α2B adrenoceptor as well as inhibiting NA and 5-HT reuptake can result in modulating the ascending nociceptive system, and in particular, dampening the firing of PR neurones in the mPFC.

Antinociceptive Effects of the Serotonin and Noradrenaline Reuptake Inhibitors Milnacipran and Duloxetine on Vincristine-Induced Neuropathic Pain Model in Mice

Vincristine is an anticancer drug used to treat a variety of cancer types, but it frequently causes peripheral neuropathy. Neuropathic pain is often associated with the appearance of abnormal sensory signs, such as allodynia. Milnacipran and duloxetine, serotonin/noradrenaline reuptake inhibitors, have shown efficacy against several chronic pain syndromes. In this study, we investigated the attenuation of vincristine-induced mechanical allodynia in mice by milnacipran and duloxetine. To induce peripheral neuropathy, vincristine was administered once per day (0.1 mg/kg, intraperitoneally (i.p.)) for 7 days. Mechanical allodynia was evaluated by measuring the withdrawal response to stimulation with a von Frey filament. In vincristine-treated mice, mechanical allodynia was observed on days 3-28 of vincristine administration. A single administration of milnacipran (40 mg/kg, i.p.) or duloxetine (20 mg/kg, i.p.) had no effect on vincristine-induced mechanical allodynia. However, repeated administration of milnacipran (20 or 40 mg/kg, once per day, i.p.) or duloxetine (5, 10, or 20 mg/kg, once per day, i.p.) for 7 days significantly reduced vincristine-induced mechanical allodynia. These results suggest that chronic vincristine administration induces mechanical allodynia, and that repeated milnacipran and duloxetine administration may be an effective approach for the treatment of neuropathic pain caused by vincristine treatment for cancer.

Animal models of diabetes-induced neuropathic pain

Neuropathy will afflict over half of the approximately 350 million people worldwide who currently suffer from diabetes and around one-third of diabetic patients with neuropathy will suffer from painful symptoms that may be spontaneous or stimulus evoked. Diabetes can be induced in rats or mice by genetic, dietary, or chemical means, and there are a variety of well-characterized models of diabetic neuropathy that replicate either type 1 or type 2 diabetes. Diabetic rodents display aspects of sensorimotor dysfunction such as stimulus-evoked allodynia and hyperalgesia that are widely used to model painful neuropathy. This allows investigation of pathogenic mechanisms and development of potential therapeutic interventions that may alleviate established pain or prevent onset of pain.

Impaired noradrenaline homeostasis in rats with painful diabetic neuropathy as a target of duloxetine analgesia

BACKGROUND

Painful diabetic neuropathy (PDN) is a serious complication of diabetes mellitus that affects a large number of patients in many countries. The molecular mechanisms underlying the exaggerated nociception in PDN have not been established. Recently, duloxetine (DLX), a serotonin and noradrenaline re-uptake inhibitor, has been recommended as one of the first-line treatments of PDN in the United States Food and Drug Administration, the European Medicines Agency and the Japanese Guideline for the Pharmacologic Management of Neuropathic pain. Because selective serotonin re-uptake inhibitors show limited analgesic effects in PDN, we examined whether the potent analgesic effect of DLX contributes toward improving the pathologically aberrant noradrenaline homeostasis in diabetic models.

RESULTS

In streptozotocin (STZ) (50 mg/kg, i.v.)-induced diabetic rats that exhibited robust mechanical allodynia and thermal hyperalgesia, DLX (10 mg/kg, i.p.) significantly and markedly increased the nociceptive threshold. The analgesic effect of DLX was nullified by the prior administration of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) (50 mg/kg, i.p.), which drastically eliminated dopamine-beta-hydroxylase- and norepinephrine transporter-immunopositive fibers in the lumbar spinal dorsal horn and significantly reduced the noradrenaline content in the lumbar spinal cord. The treatment with DSP-4 alone markedly lowered the nociceptive threshold in vehicle-treated non-diabetic rats; however, this pro-nociceptive effect was occluded in STZ-treated diabetic rats. Furthermore, STZ-treated rats exhibited a higher amount of dopamine-beta-hydroxylase- and norepinephrine transporter-immunopositive fibers in the dorsal horn and noradrenaline content in the spinal cord compared to vehicle-treated rats.

CONCLUSIONS

Impaired noradrenaline-mediated regulation of the spinal nociceptive network might underlie exaggerated nociception in PDN. DLX might exert its analgesic effect by selective enhancement of noradrenergic signals, thus counteracting this situation.

Inhibition of bladder overactivity by duloxetine in combination with foot stimulation or WAY-100635 treatment in cats

The purpose of this study was to determine whether duloxetine [a serotonin (5-HT)-norepinephrine reuptake inhibitor] combined with transcutaneous foot stimulation or WAY-100635 (a 5-HT1A antagonist) can enhance inhibition of bladder overactivity in cats. Cystometrograms were performed on eight cats under α-chloralose anesthesia by infusing saline and then 0.25% acetic acid (AA) to induce bladder overactivity. To inhibit bladder overactivity, foot stimulation (5 Hz) was applied via transcutaneous pad electrodes to the right hindfoot at two and four times the threshold intensity for inducing a toe twitch. Duloxetine (0.003-3 mg/kg) was administered intravenously to determine the effect of combination treatment. After the 3 mg/kg dose of duloxetine, WAY-100635 (0.5 mg/kg) was given intravenously. AA irritation significantly (P < 0.0001) reduced bladder capacity to 42.7 ± 7.4% of the saline control capacity. Foot stimulation alone at both two and four times the threshold intensity significantly (P < 0.0001) inhibited bladder overactivity and increased bladder capacity to 66.7 ± 6.3% and 85.7 ± 6.5% of the saline control, respectively. Duloxetine alone dose dependently inhibited bladder overactivity and completely restored bladder capacity to the saline control (109 ± 15.5%) at 3 mg/kg. Although duloxetine combined with foot stimulation did not further enhance inhibition, WAY-100635 (0.5 mg/kg) given after 3 mg/kg duloxetine further increased (P = 0.008) bladder capacity to 162.2 ± 22.5% of the saline control. Although duloxetine and foot stimulation independently inhibited bladder overactivity, combined treatment did not enhance inhibition. Duloxetine combined with WAY-100635, however, synergistically enhanced bladder inhibition, indicating a potential novel treatment for overactive bladder if duloxetine is combined with a 5-HT1A receptor antagonist drug.

Synergistic analgesia of duloxetine and celecoxib in the mouse formalin test: a combination analysis

Duloxetine, a serotonin and noradrenaline reuptake inhibitor, and celecoxib, a non-steroidal anti-inflammatory drug, are commonly used analgesics for persistent pain, however with moderate gastrointestinal side effects or analgesia tolerance. One promising analgesic strategy is to give a combined prescription, allowing the maximal or equal efficacy with fewer side effects. In the current study, the efficacy and side effects of combined administration of duloxetine and celecoxib were tested in the mouse formalin pain model. The subcutaneous (s.c.) injection of formalin into the left hindpaw induced significant somatic and emotional pain evaluated by the biphasic spontaneous flinching of the injected hindpaw and interphase ultrasonic vocalizations (USVs) during the 1 h after formalin injection, respectively. Pretreatment with intraperitoneal (i.p.) injection of duloxetine or celecoxib at 1 h before formalin injection induced the dose-dependent inhibition on the second but not first phase pain responses. Combined administration of duloxetine and celecoxib showed significant analgesia for the second phase pain responses. Combination analgesia on the first phase was observed only with higher dose combination. A statistical difference between the theoretical and experimental ED₅₀ for the second phase pain responses was observed, which indicated synergistic interaction of the two drugs. Concerning the emotional pain responses revealed with USVs, we assumed that the antinociceptive effects were almost completely derived from duloxetine, since celecoxib was ineffective when administered alone or reduced the dosage of duloxetine when given in combination. Based on the above findings, acute concomitant administration of duloxetine and celecoxib showed synergism on the somatic pain behavior but not emotional pain behaviors.

Evidence for the participation of peripheral 5-HT₂A, 5-HT₂B, and 5-HT₂C receptors in formalin-induced secondary mechanical allodynia and hyperalgesia

The role of 5-HT₂A/₂B/₂C receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pre-treatment for five consecutive days with compound 48/80 (1, 3, 10, 10, and 10 μg/paw) prevented formalin-induced secondary allodynia and hyperalgesia. Ipsilateral, but not contralateral, peripheral pre-treatment (nmol/paw) with the 5-HT₂ receptor agonist DOI (3-30), 5-HT (10-100) or fluoxetine (0.3-3) significantly increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of DOI (10 nmol/paw), 5-HT (100 nmol/paw) and fluoxetine (1 nmol/paw) was blocked by selective 5-HT₂A (ketanserin), 5-HT₂B (RS-127445), and 5-HT₂C (RS-102221) receptor antagonists. Furthermore, ipsilateral pre-treatment (nmol/paw) with ketanserin (1, 10, and 100), RS-127445 (0.01, 0.1 and 1) or RS-102221 (1, 10 and 100) prevented while post-treatment reversed 1% formalin-induced secondary allodynia and hyperalgesia in both paws. In marked contrast, contralateral injection of the greatest tested dose of 5-HT₂A/₂B/₂C receptor antagonists did not modify long-lasting secondary allodynia and hyperalgesia. These results suggest that 5-HT released from mast cells after formalin injection sensitizes primary afferent neurons via 5-HT₂A/₂B/₂C receptors leading to the development and maintenance of secondary allodynia and hyperalgesia.

Pain modality and spinal glia expression by streptozotocin induced diabetic peripheral neuropathy in rats

Pain symptoms are a common complication of diabetic peripheral neuropathy or an inflammatory condition. In the most experiments, only one or two evident pain modalities are observed at diabetic peripheral neuropathy according to experimental conditions. Following diabetic peripheral neuropathy or inflammation, spinal glial activation may be considered as an important mediator in the development of pain. For this reason, the present study was aimed to address the induction of pain modalities and spinal glial expression after streptozotocin injection as compared with that of zymosan inflammation in the rat. Evaluation of pain behavior by either thermal or mechanical stimuli was performed at 3 weeks or 5 hours after either intravenous streptozotocin or zymosan. Degrees of pain were divided into 4 groups: severe, moderate, mild, and non-pain induction. On the mechanical allodynia test, zymosan evoked predominantly a severe type of pain, whereas streptozotocin induced a weak degree of pain (severe+moderate: 57.1%). Although zymosan did not evoke cold allodynia, streptozotocin evoked stronger pain behavior, compared with zymosan (severe+moderate: 50.0%). On the other hand, the high incidence of thermal hyperalgesia (severe+moderate: 90.0%) and mechanical hyperalgesia (severe+moderate: 85.7%) by streptozotocin was observed, as similar to that of zymosan. In the spinal cord, the increase of microglia and astrocyte was evident by streptozotocin, only microglia was activated by zymosan. Therefore, it is recommended that the selection of mechanical and thermal hyperalgesia is suitable for the evaluation of streptozotocin induced diabetic peripheral neuropathy. Moreover, spinal glial activation may be considered an important factor.