Animal models of painful diabetic neuropathy: the STZ rat model

6-Methoxyflavone antagonizes chronic constriction injury and diabetes associated neuropathic nociception expression

Neuropathy is a disturbance of function or a pathological change in nerves causing poor health and quality of life. A proportion of chronic pain patients in the community suffer persistent neuropathic pain symptoms because current drug therapies may be suboptimal so there is a need for new therapeutic modalities. This study investigated the neuroprotective flavonoid, 6-methoxyflavone (6MF), as a potential therapeutic agent and gabapentin as the standard comparator, against neuropathic models. Thus, neuropathic-like states were induced in Sprague-Dawley rats using sciatic nerve chronic constriction injury (CCI) mononeuropathy and systemic administration of streptozotocin (STZ) to induce polyneuropathy. Subsequent behaviors reflecting allodynia, hyperalgesia, and vulvodynia were assessed and any possible motoric side-effects were evaluated including locomotor activity, as well as rotarod discoordination and gait disruption. 6MF (25-75 mg/kg) antagonized neuropathic-like nociceptive behaviors including static- (pressure) and dynamic- (light brushing) hindpaw allodynia plus heat/cold and pressure hyperalgesia in the CCI and STZ models. 6MF also reduced static and dynamic components of vulvodynia in the STZ induced polyneuropathy model. Additionally, 6MF reversed CCI and STZ suppression of locomotor activity and rotarod discoordination, suggesting a beneficial activity on motor side effects, in contrast to gabapentin. Hence, 6MF possesses anti-neuropathic-like activity not only against different nociceptive modalities but also impairment of motoric side effects.

Preclinical Evaluation of Sigma 1 Receptor Antagonists as a Novel Treatment for Painful Diabetic Neuropathy

The global prevalence of diabetes is steadily rising, with an estimated 537 million adults affected by diabetes in 2021, projected to reach 783 million by 2045. A severe consequence of diabetes is the development of painful diabetic neuropathy (PDN), afflicting approximately one in every three diabetic patients and significantly compromising their quality of life. Current pharmacotherapies for PDN provide inadequate pain relief for many patients, underscoring the need for novel treatments that are both safe and effective. The Sigma 1 Receptor (S1R) is a ligand-operated chaperone protein that resides at the mitochondria-associated membrane of the endoplasmic reticulum. The S1R has been shown to play crucial roles in regulating cellular processes implicated in pain modulation. This study explores the potential of PW507, a novel S1R antagonist, as a therapeutic candidate for PDN. PW507 exhibited promising in vitro and in vivo properties in terms of ADME, toxicity, pharmacokinetics, and safety. In preclinical rat models of Streptozotocin-induced diabetic neuropathy, PW507 demonstrated significant efficacy in alleviating mechanical allodynia and thermal hyperalgesia following both acute and chronic (2-week) administration, without inducing tolerance and visual evidence of toxicity. To the best of our knowledge, this is the first report to evaluate an S1R antagonist in STZ-induced diabetic rats following both acute and 2-week chronic administration, offering compelling preclinical evidence for the potential use of PW507 as a promising therapeutic option for PDN.

Navigating Preclinical Models and Medications for Peripheral Neuropathy: A Review

Peripheral neuropathy (PN) is a multifaceted disorder characterised by peripheral nerve damage, manifesting in symptoms like pain, weakness, and autonomic dysfunction. This review assesses preclinical models in PN research, evaluating their relevance to human disease and their role in therapeutic development. The Streptozotocin (STZ)-induced diabetic rat model is widely used to simulate diabetic neuropathy but has limitations in faithfully replicating disease onset and progression. Cisplatin-induced PN models are suitable for studying chemotherapy-induced peripheral neuropathy (CIPN) and closely resemble human pathology. However, they may not fully replicate the spectrum of sensory and motor deficits. Paclitaxel-induced models also contribute to understanding CIPN mechanisms and testing neuroprotective agents. Surgical or trauma-induced models offer insights into nerve regeneration and repair strategies. Medications such as gabapentin, pregabalin, duloxetine, and fluoxetine have demonstrated promise in these models, enhancing our understanding of their therapeutic efficacy. Despite progress, developing models that accurately mirror human PN remains imperative due to its complex nature. Continuous refinement and innovative approaches are critical for effective drug discovery. This review underscores the strengths and limitations of current models and advocates for an integrated approach to address the complexities of PN better and optimise treatment outcomes.

Antinociceptive effect of LMH-2, a new sigma-1 receptor antagonist analog of haloperidol, on the neuropathic pain of diabetic mice

This study evaluates the antiallodynic and antihyperalgesic effects of LMH-2, a new haloperidol (HAL) analog that acts as sigma-1 receptor (σ1 R) antagonist, in diabetic mice using a model of neuropathic pain induced by chronic hyperglycemia. Additionally, we compared its effects with those of HAL. Hyperglycemia was induced in mice by nicotinamide-streptozotocin administration (NA-STZ, 50-130 mg/kg). Four weeks later, mechanical allodynia was assessed using the up-down method, and hyperalgesia was evoked with formalin 0.5%. We evaluated antiallodynic and antihyperalgesic effects of LMH-2 (5.6-56.2 mg/kg), HAL (0.018-0.18 mg/kg) and gabapentin (GBP, 5.6-56.2 mg/kg). The results showed that LMH-2 had a more significant antiallodynic effect compared to HAL and GBP (90.4±8.7 vs 75.1±3.1 and 41.9±2.3%, respectively; P<0.05), as well as an antihyperalgesic effect (96.3±1.2 vs 86.9±7.41 and 86.9±4.8%, respectively; P<0.05). Moreover, the antiallodynic and antihyperalgesic effect of both LMH-2 and HAL were completely abolished by PRE-084 (σ1 R agonist); and partially by pramipexole (a D2-like receptor agonist). Finally, the effect of all treatments on the rotarod test, barra, open field and exploratory behaviors showed that LMH-2 did not alter the animals' balance or the exploratory behavior, unlike as HAL or GBP. The molecular docking included indicate that LMH-2 has lower affinity to the D2R than HAL. These results provide evidence that LMH-2 exerts its antinociceptive effects as a σ1 R antagonist without the adverse effects induced by HAL or GBP. Consequently, LMH-2 can be considered a good and safe strategy for treating neuropathic pain caused by hyperglycemia in patients with diabetes.

Animal models for induction of diabetes and its complications

Objectives

Animal models are widely used to develop newer drugs for treatment of diabetes and its complications. We conducted a systematic review to find various animal models to induce diabetes and also the suitable methods in various diabetic complications. With an emphasis on the animal models of diabetes induction, this review provides a basic overview of diabetes and its various types. It focused on the use of rats and mice for chemical, spontaneous, surgical, genetic, viral, and hormonal induction approaches.

Methods

All observations and research conducted on Diabetes and its complications published up to 18 May 2023 in PubMed, Web of Science, Scopus and Conchrane Library databases were included. Main outcome measures were reporting the induction of diabetes in experimental animals, the various animal models for diabetic complications including diabetic nephropathy, diabetic retinopathy, diabetic neuropathy and diabetic osteopathy. The quality of reporting of included articles and risk of bias were assessed.

Results

We reached various articles and found that rats and mice are the most frequently used animals for inducing diabetes. Chemical induction is the most commonly used followed by spontaneous and surgical methods. With slight modification various breeds and species are developed to study and induce specific complications on eyes, kidneys, neurons and bones.

Conclusions

Our review suggested that rats and mice are the most suitable animals. Furthermore, chemical induction is the method frequently used by experimenters. Moreover, high quality studies are required to find the suitable methods for diabetic complications.

Neuroprotective Effects of a Hydrogen Sulfide Donor in Streptozotocin-Induced Diabetic Rats

Diabetic neuropathy is an important long-term complication of diabetes. This study explored the hypothesis that hydrogen sulfide (H2S) ameliorates neuropathic pain by controlling antiapoptotic and pro-apoptotic processes. The effects of a slow-releasing H2S donor, GYY4137, on the expression of antiapoptotic and pro-apoptotic genes and proteins, such as B-cell lymphoma 2 (Bcl2) and Bcl-2-like protein 4 (Bax), as well as caspases, cyclooxygenase (COX)-1 and COX-2, monocytes/macrophages, and endothelial cells, in the spinal cord of male Sprague-Dawley rats with streptozotocin-induced peripheral diabetic neuropathy, were investigated using reverse transcription-PCR, western blot and immunohistochemistry. The antihypoalgesic activities of GYY4137 on diabetic rats were evaluated using the tail flick test. Treatment of diabetic rats with GYY4137 attenuated thermal hypoalgesia and prevented both the diabetes-induced increase in Bax mRNA expression (p = 0.0032) and the diabetes-induced decrease in Bcl2 mRNA expression (p = 0.028). The GYY4137-treated diabetic group had increased COX-1 (p = 0.015), decreased COX-2 (p = 0.002), reduced caspase-7 and caspase-9 protein expression (p < 0.05), and lower numbers of endothelial and monocyte/macrophage cells (p < 0.05) compared to the non-treated diabetic group. In summary, the current study demonstrated the protective properties of H2S, which prevented the development of neuropathy related behavior, and suppressed apoptosis activation pathways and inflammation in the spinal cord. H2S-releasing drugs could be considered as possible treatment options of diabetic peripheral neuropathy.

Electroacupuncture may alleviate diabetic neuropathic pain by inhibiting the microglia P2X4R and neuroinflammation

Diabetic neuropathic pain (DNP) is a common and destructive complication of diabetes mellitus. The discovery of effective therapeutic methods for DNP is vitally imperative because of the lack of effective treatments. Although 2 Hz electroacupuncture (EA) was a successful approach for relieving DNP, the mechanism underlying the effect of EA on DNP is still poorly understood. Here, we established a rat model of DNP that was induced by streptozotocin (STZ) injection. P2X4R was upregulated in the spinal cord after STZ-injection. The upregulation of P2X4R was mainly expressed on activated microglia. Intrathecal injection of a P2X4R antagonist or microglia inhibitor attenuated STZ-induced nociceptive thermal hyperalgesia and reduced the overexpression of brain-derived neurotrophic factor (BDNF), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the spinal cord. We also assessed the effects of EA treatment on the pain hypersensitivities of DNP rats, and further investigated the possible mechanism underlying the analgesic effect of EA. EA relieved the hyperalgesia of DNP. In terms of mechanism, EA reduced the upregulation of P2X4R on activated microglia and decreased BDNF, IL-1β and TNF-α in the spinal cord. Mechanistic research of EA's analgesic impact would be beneficial in ensuring its prospective therapeutic effect on DNP as well as in extending EA's applicability.

Isolation, characterization, development and evaluation of phytoconstituent based formulation for diabetic neuropathy

Background

Morus alba Linn, referred to as white mulberry, is a potential traditional medicine for diabetes and neuroprotection.

Aim

Isolation, characterization, development and evaluation of phytoconstituent based formulation for diabetic neuropathy.

Material and methods

The stem Bark of M. alba was peeled and subjected to extraction. A phytoconstituent was then isolated by column chromatography and characterized using Mass spectroscopy, FTIR, and NMR. The isolated phytoconstituent was used to formulate a nanoemulsion. Nanoemulsion was also characterized for viscosity, surface tension, refractive index, pH, and particle size. Selected nanoemulsion formulations were then tested for acute oral toxicity and diabetic neuropathy, including behavioral, hematological, histopathological, and biomarker examinations.

Results

The spectral analysis affirmed that the isolated compound was found to be chrysin. A nanoemulsion formulation was made using the chrysin and was characterized and found to be stable during the stability testing and fulfilled all other testing parameters. Then acute oral toxicity study of the formulations was found to be safe. Formulations were found to possess significant results against diabetic neuropathy in rats. Biomarkers were analyzed for their mechanistic involvement in reducing neuropathy in rats, and it was found that the oxidative pathway was considerably restored, suggesting that chrysin causes these effects via this pathway.

Conclusions

Results suggests that isolated phytoconstituent (chrysin) from the bark of Morus alba derived nanoemulsion has protective and beneficial effects by diminishing the oxidative damage against alloxan-induced diabetic neuropathy in rats.

Withametelin, a steroidal lactone, isolated from datura innoxa attenuates STZ-induced diabetic neuropathic pain in rats through inhibition of NF-kB/MAPK signaling

Diabetic neuropathic pain is one of the microvascular complications of diabetes mellitus characterized by symmetrical pain and sensory abnormalities. A steroidal lactone isolated from the datura innoxa plant, withametelin (WMT), exhibited significant neuroprotective, anti-inflammatory, antioxidant, and anticancer properties. The current study aimed to investigate anti-neuropathic pain activity and the molecular mechanism of WMT against streptozotocin (STZ)-induced diabetic neuropathy. Rats were given a single injection of STZ (60 mg/kg, intraperitoneally (i.p.)) for induction of diabetes on the first day of the study. After the onset of diabetic neuropathy, pregabalin (10 mg/kg, i.p.) and WMT (0.1 and 1 mg/kg, i.p.) treatments were started from day 14 up to day 42. It was found that STZ-induced neuropathic pain behaviors were markedly reduced by WMT. It inhibited the STZ-associated histopathological changes and genotoxicity in the sciatic nerve and spinal cord. Additionally, Fourier transforms infrared (FTIR) spectroscopy results revealed that STZ-induced alterations in the biochemical components of the sciatic nerve's myelin sheath were inhibited by WMT. In the spinal cord, it markedly reduced the immunoreactivity of mitogen-activated protein kinases (MAPKs) signaling components such as p38-MAPK, c-Jun N-terminal kinase (JNK), extracellular-signal-regulated-kinase (ERK), and activator-protein 1 (AP-1). It also reduced the expression levels of nuclear factor-kappa-B (NF-κB), cyclooxygenase-2 (COX-2), and inducible nitric oxide synthase (iNOS). The production of inflammatory cytokines was considerably reduced by WMT. This study provides convincing evidence that WMT treatment attenuated STZ-induced diabetic neuropathic pain by inhibition of MAPK/NF-κB signaling.

Corinthian Currants Supplementation Restores Serum Polar Phenolic Compounds, Reduces IL-1beta, and Exerts Beneficial Effects on Gut Microbiota in the Streptozotocin-Induced Type-1 Diabetic Rat

The present study aimed at investigating the possible benefits of a dietary intervention with Corinthian currants, a rich source of phenolic compounds, on type 1 diabetes (T1D) using the animal model of the streptozotocin-(STZ)-induced diabetic rat. Male Wistar rats were randomly assigned into four groups: control animals, which received a control diet (CD) or a diet supplemented with 10% w/w Corinthian currants (CCD), and diabetic animals, which received a control diet (DCD) or a currant diet (DCCD) for 4 weeks. Plasma biochemical parameters, insulin, polar phenolic compounds, and inflammatory factors were determined. Microbiota populations in tissue and intestinal fluid of the caecum, as well as fecal microbiota populations and short-chain fatty acids (SCFAs), were measured. Fecal microbiota was further analyzed by 16S rRNA sequencing. The results of the study showed that a Corinthian currant-supplemented diet restored serum polar phenolic compounds and decreased interleukin-1b (IL-1b) (p < 0.05) both in control and diabetic animals. Increased caecal lactobacilli counts (p < 0.05) and maintenance of enterococci levels within normal range were observed in the intestinal fluid of the DCCD group (p < 0.05 compared to DCD). Higher acetic acid levels were detected in the feces of diabetic rats that received the currant diet compared to the animals that received the control diet (p < 0.05). Corinthian currant could serve as a beneficial dietary component in the condition of T1D based on the results coming from the animal model of the STZ-induced T1D rat.

Sphaeranthus indicus Linn ameliorates streptozotocin-induced experimental diabetic neuropathy by targeting oxidative stress-mediated alterations

Background

Diabetes-induced neuropathic pain is manifested as a lowering of nerve transmission rate, increased discomfort, sensual loss, and axonal degradation, and is the most prevalent secondary consequence of diabetes. Diabetes is a devitalizing disease affecting people from diverse groups in both developing and industrialized countries. The inflammation pathway and oxidative stress both contribute considerably to diabetic peripheral neuropathy via the activation of inflammatory cytokines. Hyperglycemia-mediated neural oxidative stress and damage activates a number of metabolic pathways, causing diabetic neuropathy. The current study investigated the neuroprotective potential of methanolic extract of Sphaeranthus indicus Linn (MESI) in ameliorating diabetic neuropathic pain induced by administration of streptozotocin in rats.

Results

Four weeks after intraperitoneal treatment of streptozotocin (STZ), there was a significant decrease in mechano-tactile allodynia and mechanical and thermal hyperalgesia. Furthermore, STZ-induced oxidative stress increases the extent of neural lipid peroxidation (LPO), as evidenced by increased MDA levels, decreases the activities of endogenous antioxidants such as superoxide dismutase (SOD) and glutathione (GSH), and alters sciatic neural histoarchitecture. Chronic administration of methanolic extract of Sphaeranthus indicus Linn (MESI) for 4 weeks significantly and dose-dependently attenuated the decrease in levels of nociceptive thresholds, endogenous antioxidants (SOD and GSH), and increase in LPO. Furthermore, MESI significantly restored sciatic neural histoarchitecture.

Conclusion

The amelioration of streptozotocin-induced diabetic neuropathy by methanolic extract of Sphaeranthus indicus Linn (MESI) could be attributed to its antinociceptive, antioxidant, and neuroprotective properties.

The Development of Mechanical Allodynia in Diabetic Rats Revealed by Single-Cell RNA-Seq

Mechanical allodynia (MA) is the main reason that patients with diabetic peripheral neuropathy (DPN) seek medical advice. It severely debilitates the quality of life. Investigating hyperglycemia-induced changes in neural transcription could provide fundamental insights into the complex pathogenesis of painful DPN (PDPN). Gene expression profiles of physiological dorsal root ganglia (DRG) have been studied. However, the transcriptomic changes in DRG neurons in PDPN remain largely unexplored. In this study, by single-cell RNA sequencing on dissociated rat DRG, we identified five physiological neuron types and a novel neuron type MAAC (Fxyd7⁺/Atp1b1⁺) in PDPN. The novel neuron type originated from peptidergic neuron cluster and was characterized by highly expressing genes related to neurofilament and cytoskeleton. Based on the inferred gene regulatory networks, we found that activated transcription factors Hobx7 and Larp1 in MAAC could enhance Atp1b1 expression. Moreover, we constructed the cellular communication network of MAAC and revealed its receptor-ligand pairs for transmitting signals with other cells. Our molecular investigation at single-cell resolution advances the understanding of the dynamic peripheral neuron changes and underlying molecular mechanisms during the development of PDPN.

Attenuation of Some Inflammatory Markers by Endurance Training in the Spinal Cord of Rats with Diabetic Neuropathic Pain

Nervous inflammation is an important component of the pathogenesis of neurodegenerative diseases including chronic diabetic neuropathic pain. In order to obtain a decrease in the progression of diabetic neuronal damage, it may be necessary to examine therapeutic options that involve antioxidants and anti-inflammatory agents. The aim of this study was to investigate the attenuation of inflammatory factors with endurance training in the spinal cord of rats with neuropathic pain. Thirty-two 8-week-old male Wistar rats (with a weight range of 204 ± 11.3 g) were randomly divided into 4 groups (n = 8), including (1) diabetic neuropathy (50 mg/kg streptozotocin intraperitoneal injection), (2) diabetic neuropathy training (30 minutes of endurance training at 15 meters per minute, 5 days a week for 6 weeks), (3) healthy training, and (4) healthy control. After confirmation of diabetic neuropathy by behavioral tests, training protocol and supplementation were performed. The NLRP3, P38 MAPK, TNF-α, and IL-1β gene expressions were measured by a real-time technique in the spinal cord tissue. One-way analysis of variance and Tukey's post hoc test were used for statistical analysis. Endurance training reduced the sensitivity of the nervous system to thermal hyperalgesia and mechanical allodynia; also, compared to the diabetic neuropathy group, the gene expressions of NLTP3, P38 MAPK, TNF-α, and IL-1β were significantly reduced by endurance training (P < 0.05). Endurance training modulates NLRP3, P38 MAPK, and TNF-α, IL-1β gene expressions and improves the sensitivity of nociceptors to pain factors. Accordingly, it is recommended to use endurance training to reduce neuropathic pain for diabetics.

The emergence of insulin resistance following a chronic high-fat diet regimen coincides with an increase in the reinforcing effects of nicotine in a sex-dependent manner

The present study assessed the sex-dependent effects of insulin resistance on the reinforcing effects of nicotine. Female and male rats received a chronic high-fat diet (HFD) or regular diet (RD) for 8 weeks. A subset of rats then received vehicle or a dose of streptozotocin (STZ; 25 mg/kg) that induces insulin resistance. To assess insulin resistance, glucose levels were measured 15, 30, 60, 120, and 180 min after an insulin injection (0.75 U/kg). Nine days later, the rats were given extended access to intravenous self-administration (IVSA) of nicotine (0.015, 0.03, 0.06 mg/kg) in an operant box where they consumed their respective diet ad libitum and performed responses for water deliveries. Each nicotine dose was delivered for 4 days with 3 intermittent days of abstinence in their home cage. The day after the last IVSA session, physical signs were compared following administration of mecamylamine (3.0 mg/kg) to precipitate nicotine withdrawal. The results revealed that there were no changes in insulin resistance or nicotine intake in HFD alone rats regardless of sex. Insulin resistance was observed in HFD-fed rats that received STZ, and the magnitude of this effect was greater in males versus females. Our major finding was that nicotine intake was greater among HFD + STZ female rats as compared to males. Lastly, the physical signs of withdrawal were similar across all groups. Our results suggest that females diagnosed with disorders that disrupt insulin signaling, such as diabetes may be at risk of greater vulnerability to nicotine use due to enhanced reinforcing effects of this drug.

Use of human bone marrow mesenchymal stem cells immortalized by the expression of telomerase in wound healing in diabetic rats

Diabetes mellitus is associated with neural and micro- and macrovascular complications. Therapeutic options for these complications are limited and the delivery of mesenchymal stem cells into lesions have been reported to improve the healing process. In this work, the effects of the administration of a lineage of human bone marrow mesenchymal stem cells immortalized by the expression of telomerase (hBMSC-TERT) as a potential therapeutic tool for wound healing in diabetic rats were investigated. This is the first description of the use of these cells in diabetic wounds. Dorsal cutaneous lesions were made in streptozotocin-induced diabetic rats and hBMSC-TERT were subcutaneously administered around the lesions. The healing process was evaluated macroscopically, histologically, and by birefringence analysis. Diabetic wounded rats infused with hBMSC-TERT (DM-TERT group) and the non-diabetic wounded rats not infused with hBMSC-TERT (CW group) had very similar patterns of fibroblastic response and collagen proliferation indicating improvement of wound healing. The result obtained by birefringence analysis was in accordance with that obtained by the histological analysis. The results indicated that local administration of hBMSC-TERT in diabetic wounds improved the wound healing process and may become a therapeutic option for wounds in individuals with diabetes.

Neuroskeletal Effects of Chronic Bioelectric Nerve Stimulation in Health and Diabetes

Background/Aims

Bioelectric nerve stimulation (eStim) is an emerging clinical paradigm that can promote nerve regeneration after trauma, including within the context of diabetes. However, its ability to prevent the onset of diabetic peripheral neuropathy (DPN) has not yet been evaluated. Beyond the nerve itself, DPN has emerged as a potential contributor to sarcopenia and bone disease; thus, we hypothesized that eStim could serve as a strategy to simultaneously promote neural and musculoskeletal health in diabetes.

Methods

To address this question, an eStim paradigm pre-optimized to promote nerve regeneration was applied to the sciatic nerve, which directly innervates the tibia and lower limb, for 8 weeks in control and streptozotocin-induced type 1 diabetic (T1D) rats. Metabolic, gait, nerve and bone assessments were used to evaluate the progression of diabetes and the effect of sciatic nerve eStim on neuropathy and musculoskeletal disease, while also considering the effects of cuff placement and chronic eStim in otherwise healthy animals.

Results

Rats with T1D exhibited increased mechanical allodynia in the hindpaw, reduced muscle mass, decreased cortical and cancellous bone volume fraction (BVF), reduced cortical bone tissue mineral density (TMD), and decreased bone marrow adiposity. Type 1 diabetes also had an independent effect on gait. Placement of the cuff electrode alone resulted in altered gait patterns and unilateral reductions in tibia length, cortical BVF, and bone marrow adiposity. Alterations in gait patterns were restored by eStim and tibial lengthening was favored unilaterally; however, eStim did not prevent T1D-induced changes in muscle, bone, marrow adiposity or mechanical sensitivity. Beyond this, chronic eStim resulted in an independent, bilateral reduction in cortical TMD.

Conclusion

Overall, these results provide new insight into the pathogenesis of diabetic neuroskeletal disease and its regulation by eStim. Though eStim did not prevent neural or musculoskeletal complications in T1D, our results demonstrate that clinical applications of peripheral neuromodulation ought to consider the impact of device placement and eStim on long-term skeletal health in both healthy individuals and those with metabolic disease. This includes monitoring for compounded bone loss to prevent unintended consequences including decreased bone mineral density and increased fracture risk.

Neuroprotective validation of pectin in T2DM-induced allodynia and hyperalgesia in diabetic peripheral neuropathic pain

AIM

To validate neuroprotective effect of pectin against neuropathic pain in diabetic rodents.

MATERIAL AND METHOD

Pectin was isolated and characterised from different sources to validate its neuroprotective effect against T2DM associated neuropathic pain. The antioxidant activity of pectins was done by the DPPH method. Type-2 diabetes mellitus (T2DM) was induced in Wistar albino rats by high-fat diet and high-fat emulsion feeding for 2 weeks followed by a single i.p. of Sterptozotocin in 3rd week. The animals were grouped as positive control and Citrus sinensis (L.) Osbeck peel pectin (CSL-OP) as test group and treated for the next 4 weeks. Body weight and blood glucose were measured up to 8 weeks; however, behavioural assessment was done at the end of 5th to 8th week.

RESULT

CSL-OP restored the reduced body weight and elevated blood glucose with increased pain threshold and improved walking performance.

CONCLUSION

CSL-OP prevented progression of early diabetic neuropathy with anti-oxidant activity.

The ubiquitin E3 ligase Nedd4-2 relieves mechanical allodynia through the ubiquitination of TRPA1 channel in db/db mice

Neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2) is a member of the E3 ubiquitin ligase family that is highly expressed in sensory neurons and involved in pain modulation via downregulation of ion channels in excitable membranes. Ubiquitination involving Nedd4-2 is regulated by adenosine monophosphate-activated protein kinase (AMPK), which is impaired in the dorsal root ganglion (DRG) neurons of db/db mice. AMPK negatively regulates the expression of transient receptor potential ankyrin 1 (TRPA1), a recognised pain sensor expressed on the membrane of DRG neurons, consequently relieving mechanical allodynia in db/db mice. Herein, we studied the involvement of Nedd4-2 in painful diabetic neuropathy and observed that Nedd4-2 negatively regulated diabetic mechanical allodynia. Nedd4-2 was co-expressed with TRPA1 in mouse DRG neurons. Nedd4-2 was involved in TRPA1 ubiquitination, this ubiquitination, as well as Nedd4-2-TRPA1 interaction, was decreased in db/db mice. Moreover, Nedd4-2 levels were decreased in db/db mice, while an abnormal intracellular distribution was observed in short-term high glucose-cultured DRG neurons. AMPK activators not only restored Nedd4-2 distribution but also increased Nedd4-2 expression. These findings demonstrate that Nedd4-2 is a potent regulator of TRPA1 and that the abnormal expression of Nedd4-2 in DRG neurons contributes to diabetic neuropathic pain.

Antihypernociceptive, Anxiolytic, and Antidepressant Properties of Aqueous and Ethanol Extracts of Dissotis thollonii Cogn. (Melastomataceae) in Mice

Diabetic neuropathy, which affects 7 to 9% of the world's population and that is usually accompanied by anxiety and depression, is chronic pain that results from impaired function of the central or peripheral nervous system. This study aimed at evaluating the antihypernociceptive, antiallodynic, anxiolytic, and antidepressant effects of Dissotis thollonii extracts. Diabetic neuropathy was induced by intraperitoneal injection of streptozotocin (200 mg/kg) in mice. The aqueous and ethanol extracts (250 and 500 mg/kg) were administered orally. Hyperalgesia (thermal and chemical), allodynia (mechanical and thermal), anxiety (high plus labyrinth, light-dark box, and social interaction), and depression (open field test, suspension test tail, and forced swimming test) were evaluated, and then the levels of some cytokines and growth factors were determined. The aqueous and ethanol extracts of Dissotis thollonii demonstrated significant antihypernociceptive (inhibition of hyperalgesia and allodynia), anxiolytic, and antidepressant activities in mice made diabetic by STZ. The extracts also significantly inhibited (p < 0.001) the levels of TNF-α, IL-1β, and IL-6 in the blood as well as the levels of TNF-α, IL-1β, IL-6, IGF, and NGF in the sciatic nerve. This study shows that the extracts of Dissotis thollonii have antihypernociceptive and neuroprotective effects which could be linked to the inhibition of proinflammatory cytokines and growth factors in the blood and the sciatic nerve.

Reaching task performance is associated to neuromuscular junction adaptations in rats with induced diabetes mellitus

Upper limb performance is affected by diabetes mellitus (DM). Neuromuscular junction (NMJ) is a key structure to understand the relationship between performance and morphology in DM. The aim of the study was to analyze NMJ plasticity due to DM in an animal model and its relationship with the function of forelimbs in rats. Twelve Wistar rats were divided into control (C) and DM groups. Animals were trained to perform a grasping task, following procedures of habituation, shaping, and reaching task. DM was induced using streptozotocin. Forelimb neuromuscular performance for dexterity was evaluated one day before DM induction and five weeks following induction. After that, biceps, triceps, and finger flexors and extensors were removed. Connective tissue and muscle fiber cross-sectional area (CSA) were measured. NMJ was assessed by its morphometric characteristics (area, perimeter, and maximum diameter), using ImageJ software. Motor performance analyses were made using single pellet retrieval task performance test. Student’s t-test was used for comparisons between groups. A significant decrease in all NMJ morphometric parameters was observed in the DM group compared with the C group. Results showed that DM generated NMJ retraction in muscles involved in a reaching task. These alterations are related to signs of muscular atrophy and to poor reaching task performance. In conclusion, induced DM caused NMJ retraction and muscular atrophy in muscles involved in reaching task performance. Induced DM caused significantly lower motor performance, especially in the final moments of evaluation, when DM compromised the tropism of the muscular tissue.

Effects of mesenchymal stromal cells on motor function and collagen in the skeletal muscles of rats with type I diabetes

The present study aimed to evaluate the effects of mesenchymal stromal cell (MSC) transplantation on motor function and collagen organization in the muscles of rats with type 1 diabetes mellitus. Male Wistar rats were randomly assigned to three groups: control (C), diabetic (DM) and diabetic treated with MSCs (DM-MSCs). Diabetes was induced by streptozotocin (50 µg/kg). Bone marrow cells were isolated from the tibia and femur. After 10 weeks of DM induction, the DM-MSC rats received four i.p. injections of MSCs (1 × 106). Ten weeks after MSC transplantation, motor performance was evaluated by the rotarod test and the anterior tibial (TA) muscles were collected for morphometric and quantification of collagen birefringence by polarizing microscopy analysis. Motor performance of the DM group was significantly reduced when compared to the C group and increased significantly in the DM + MSC group. The TA muscle mass was significantly reduced in the DM and DM + MSC groups compared to the C group. The connective tissue increased in the DM group compared to the C group and decreased in the DM + MSC group. The percentage collagen birefringence decreased significantly in the DM group when compared to the C group and increased in the DM + MSC group. Motor performance was positively correlated with collagen birefringence and negatively correlated with percentage of connective tissue. The results indicate that MSC transplantation improves both motor function and the collagen macromolecular organization in type 1 DM.

Insulin modulates the strong reinforcing effects of nicotine and changes in insulin biomarkers in a rodent model of diabetes

This study examined whether the strong reinforcing effects of nicotine and changes in insulin biomarkers observed in diabetic rats are modulated via insulin. A model of diabetes was employed involving administration of streptozotocin (STZ), which produces hypoinsulinemia in rats. The present study included vehicle- or STZ-treated rats that received sham surgery or insulin pellets. Two weeks later, the rats were given extended access to intravenous self-administration (IVSA) of saline or nicotine. Concomitant changes in food intake, water responses, and body weight were assessed during 12 days of IVSA. After the last session, plasma levels of insulin, leptin, amylin, and glucagon-like peptide-1 (GLP-1) were assessed using Luminex^® technology. In a separate cohort, phosphorylated insulin receptor substrate-2 (pIRS-2) and insulin growth factor-1 receptor β (IGF-1Rβ) were assessed in the nucleus accumbens (NAc) and ventral tegmental area (VTA) of vehicle- or STZ-treated rats that received sham surgery or an insulin pellet. STZ-treated rats displayed an increase in glucose levels, a decrease in body weight, and an increase in nicotine, food, and water intake relative to controls. STZ-treated rats also displayed a decrease in plasma insulin and leptin levels and an increase in amylin and GLP-1 levels relative to controls. Importantly, all of the STZ-induced changes in behavior and insulin biomarkers were prevented by insulin supplementation. STZ-treated rats also displayed a decrease in pIRS-2 and IGF-1Rβ in the NAc (but not VTA), an effect that was also prevented by insulin. These data suggest that insulin systems in the NAc modulate the strong reinforcing effects of nicotine in male diabetic rats.

Diabetes-induced Neuropathic Mechanical Hyperalgesia Depends on P2X4 Receptor Activation in Dorsal Root Ganglia

Peripheral diabetic neuropathy (PDN) manifests in 50-60% of type I and II diabetic patients and is the major cause of limb amputation. Adequate therapy for PDN is a current challenge. There are evidences that the activation of the P2X4 receptor (P2X4R) expressed on microglial cells of the central nervous system takes part in the development of neuropathic pain. However, there is an open question: Is P2X4R activation on dorsal root ganglia (DRG) involved in the development of neuropathic pain? To answer this question, this study verified the involvement of P2X4R expressed in DRG cells on diabetes-induced neuropathic mechanical hyperalgesia in rats. We found that intrathecal or ganglionar (L5-DRG) administration of a novel P2X4R antagonist (PSB-15417) or intrathecal administration of oligodeoxynucleotides (ODN)-antisense against the P2X4R reversed diabetes-induced neuropathic mechanical hyperalgesia. The DRG of the diabetic neuropathic rats showed an increase in P2X4R expression, and the DRG immunofluorescence suggested that P2X4R is expressed mainly in satellite glial cells (SGC). Finally, our study showed a functional expression of P2X4R in SGCs of the rat's DRG, because the P2X4R agonist BzATP elicits an increase in intracellular calcium concentration in SGCs, which was reduced by PSB-15417. These findings indicate that P2X4R activation in DRG is essential to diabetes-induced neuropathic mechanical hyperalgesia. Therefore, this purinergic receptor in DRG could be an interesting therapeutic target for quaternary P2X4R antagonists that do not cross the hematoencephalic barrier, for the control of neuropathic pain, preserving central nervous system functions.

Metformin attenuates increase of synaptic number in the rat spinal dorsal horn with painful diabetic neuropathy induced by type 2 diabetes: a stereological study

In our previous study, we have shown that number of synapses in the L5 segment of spinal dorsal horn increased significantly in a rat model of painful diabetic neuropathy (PDN) induced by high-dose of streptozotocin (an animal model of type 1 diabetes). The aims of this study were: (1) to determine whether high fat diet/low dose streptozotocin-diabetes, a rat model for type 2 diabetes, related PDN was also associated with this synaptic plasticity, (2) to reveal the range of this synaptic plasticity change occurred (in the whole length of spinal dorsal horn or only in the L5 lumbar segment of spinal dorsal horn) and (3) to discover whether treatment with metformin had effect on this synaptic plasticity. Male adult Sprague-Dawley rats were randomly allocated into the control group (n = 7), the PDN group (n = 6) and the PDN treated with metformin (PDN + M) group (n = 7), respectively. 28 days after medication, synaptic and neuronal numbers in the whole length of spinal dorsal horn or in 1 mm length of the L5 segment of spinal dorsal horn were estimated by the optical disector (a stereological technique). Compared to the control group and the PDN + M group, number of synapses in the L5 segment of spinal dorsal horn increased significantly in the PDN group (P < 0.05). There was no significant change between the control group and the PDN + M group in terms of the parameters in the L5 segment of the spinal dorsal horn (P > 0.05). Parameters of the whole length of spinal dorsal horn showed no significant changes (P > 0.05). Our results suggest that high fat diet/low dose streptozotocin diabetes related PDN is also associated with a numerical increase of synapses in the L5 segment of spinal dorsal horn but not in the whole length of spinal dorsal horn. Furthermore, the analgesic effect of metformin against PDN is related to its inhibition of numerical increase of synaptic number in the rat spinal dorsal horn.

Treatment with NADPH oxidase inhibitor apocynin alleviates diabetic neuropathic pain in rats

Increased reactive oxygen species by the activation of NADPH oxidase (NOX) contributes to the development of diabetic complications. Apocynin, a NOX inhibitor, increases sciatic nerve conductance and blood flow in diabetic rats. We investigated potential protective effect of apocynin in rat diabetic neuropathy and its precise mechanism of action at molecular level. Rat models of streptozotocin-induced diabetes were treated with apocynin (30 and 100 mg/kg per day, intragastrically) for 4 weeks. Mechanical hyperalgesia and allodynia were determined weekly using analgesimeter and dynamic plantar aesthesiometer. Western blot analysis and histochemistry/immunohistochemistry were performed in the lumbar spinal cord and sciatic nerve respectively. Streptozotocin injection reduced pain threshold in analgesimeter, but not in aesthesiometer. Apocynin treatment increased pain threshold dose-dependently. Western blot analysis showed an increase in catalase and NOX-p47phox protein expression in the spinal cord. However, protein expressions of neuronal and inducible nitric oxide synthase (nNOS, iNOS), superoxide dismutase, glutathion peroxidase, nitrotyrosine, tumor necrosis factor-α, interleukin-6, interleukin-1β, aldose reductase, cyclooxygenase-2 or MAC-1 (marker for increased microgliosis) in the spinal cord remained unchanged. Western blot analysis results also demonstrated that apocynin decreased NOX-p47phox expression at both doses and catalase expression at 100 mg/kg per day. Histochemistry of diabetic sciatic nerve revealed marked degeneration. nNOS and iNOS immunoreactivities were increased, while S-100 immunoreactivity (Schwann cell marker) was decreased in sciatic nerve. Apocynin treatment reversed these changes dose-dependently. In conclusion, decreased pain threshold of diabetic rats was accompanied by increased NOX and catalase expression in the spinal cord and increased degeneration in the sciatic nerve characterized by increased NOS expression and Schwann cell loss. Apocynin treatment attenuates neuropathic pain by decelerating the increased oxidative stress-mediated pathogenesis in diabetic rats.

Protective effects of methanolic extract of Juglans regia L. leaf on streptozotocin-induced diabetic peripheral neuropathy in rats

Background

Oxidative stress has a pivotal role in the pathogenesis and development of diabetic peripheral neuropathy (DPN), the most common and debilitating complications of diabetes mellitus. There is accumulating evidence that Juglans regia L. (GRL) leaf extract, a rich source of phenolic components, has hypoglycemic and antioxidative properties. This study aimed to determine the protective effects of Juglans regia L. leaf extract against streptozotocin-induced diabetic neuropathy in rat.

Methods

The DPN rat model was generated by intraperitoneal injection of a single 55 mg/kg dose of streptozotocin (STZ). A subset of the STZ-induced diabetic rats intragastically administered with GRL leaf extract (200 mg/kg/day) before or after the onset of neuropathy, whereas other diabetic rats received only isotonic saline as the same volume of GRL leaf extract. To evaluate the effects of GRL leaf extract on the diabetic neuropathy various parameters, including histopathology and immunohistochemistry of apoptotic and inflammatory factors were assessed along with nociceptive and biochemical assessments.

Results

Degeneration of the sciatic nerves which was detected in the STZ-diabetic rats attenuated after GRL leaf extract administration. Greater caspase-3, COX-2, and iNOS expression could be detected in the STZ-diabetic rats, which were significantly attenuated after GRL leaf extract administration. Also, attenuation of lipid peroxidation and nociceptive response along with improved antioxidant status in the sciatic nerve of diabetic rats were detected after GRL leaf extract administration. In other word, GRL leaf extract ameliorated the behavioral and structural indices of diabetic neuropathy even after the onset of neuropathy, in addition to blood sugar reduction.

Conclusion

Our results suggest that GRL leaf extract exert preventive and curative effects against STZ-induced diabetic neuropathy in rats which might be due to its antioxidant, anti-inflammatory, and antiapoptotic properties.

Graphical abstract

Protection against neuropathy

Endomorphin-2 Inhibition of Substance P Signaling within Lamina I of the Spinal Cord Is Impaired in Diabetic Neuropathic Pain Rats

Opiate analgesia in the spinal cord is impaired in diabetic neuropathic pain (DNP), but until now the reason is unknown. We hypothesized that it resulted from a decreased inhibition of substance P (SP) signaling within the dorsal horn of the spinal cord. To investigate this possibility, we evaluated the effects of endomorphin-2 (EM2), an endogenous ligand of the μ-opioid receptor (MOR), on SP release within lamina I of the spinal dorsal horn (SDH) in rats with DNP. We established the DNP rat model and compared the analgesic efficacy of EM2 between inflammation pain and DNP rat models. Behavioral results suggested that the analgesic efficacy of EM2 was compromised in the condition of painful diabetic neuropathy. Then, we measured presynaptic SP release induced by different stimulating modalities via neurokinin-1 receptor (NK1R) internalization. Although there was no significant change in basal and evoked SP release between control and DNP rats, EM2 failed to inhibit SP release by noxious mechanical and thermal stimuli in DNP but not in control and inflammation pain model. We also observed that EM2 decreased the number of FOS-positive neurons within lamina I of the SDH but did not change the amount of FOS/NK1R double-labeled neurons. Finally, we identified a remarkable decrease in MORs within the primary afferent fibers and dorsal root ganglion (DRG) neurons by Western blot (WB) and immunohistochemistry (IHC). Taken together, these data suggest that reduced presynaptic MOR expression might account for the loss of the inhibitory effect of EM2 on SP signaling, which might be one of the neurobiological foundations for decreased opioid efficacy in the treatment of DNP.

A Comparison of Surgical Invasions for Spinal Nerve Ligation with or without Paraspinal Muscle Removal in a Rat Neuropathic Pain Model

L5 spinal nerve ligation (SNL) in rats is one of the most popular models for studying neuropathic pain because of its high reproducibility. During the surgery, a part of the L5 paraspinal muscle is usually removed, which produces extra trauma and may potentially affect the physiological processes involved in neuropathic pain. To reduce the surgical trauma, the paraspinal muscle retraction was developed for exposure of the spinal nerve. The current study was aimed at comparing the surgical invasions between the L5 SNL models with paraspinal muscle removal or retraction. The results showed that both methods induced similar neuropathic pain behavior. However, the paraspinal muscle retraction group exhibited an average of 2.7 mg less blood loss than the muscle removal group. This group also showed a significantly lower increase in serum myoglobin and creatine phosphokinase levels on postoperative days 1 and 2, as well as a lower increase in interleukin-1β and interleukin-6 levels on postoperative day 1. The paraspinal muscle maintained normal morphological features following paraspinal muscle retraction. Our results indicate that the SNL rat model with paraspinal muscle retraction is a reliable physiological model that is reproducible, readily available, and less invasive than the model with muscle removal.

Inhibition of AAK1 Kinase as a Novel Therapeutic Approach to Treat Neuropathic Pain

To identify novel targets for neuropathic pain, 3097 mouse knockout lines were tested in acute and persistent pain behavior assays. One of the lines from this screen, which contained a null allele of the adapter protein-2 associated kinase 1 (AAK1) gene, had a normal response in acute pain assays (hot plate, phase I formalin), but a markedly reduced response to persistent pain in phase II formalin. AAK1 knockout mice also failed to develop tactile allodynia following the Chung procedure of spinal nerve ligation (SNL). Based on these findings, potent, small-molecule inhibitors of AAK1 were identified. Studies in mice showed that one such inhibitor, LP-935509, caused a reduced pain response in phase II formalin and reversed fully established pain behavior following the SNL procedure. Further studies showed that the inhibitor also reduced evoked pain responses in the rat chronic constriction injury (CCI) model and the rat streptozotocin model of diabetic peripheral neuropathy. Using a nonbrain-penetrant AAK1 inhibitor and local administration of an AAK1 inhibitor, the relevant pool of AAK1 for antineuropathic action was found to be in the spinal cord. Consistent with these results, AAK1 inhibitors dose-dependently reduced the increased spontaneous neural activity in the spinal cord caused by CCI and blocked the development of windup induced by repeated electrical stimulation of the paw. The mechanism of AAK1 antinociception was further investigated with inhibitors of α2 adrenergic and opioid receptors. These studies showed that α2 adrenergic receptor inhibitors, but not opioid receptor inhibitors, not only prevented AAK1 inhibitor antineuropathic action in behavioral assays, but also blocked the AAK1 inhibitor-induced reduction in spinal neural activity in the rat CCI model. Hence, AAK1 inhibitors are a novel therapeutic approach to neuropathic pain with activity in animal models that is mechanistically linked (behaviorally and electrophysiologically) to α2 adrenergic signaling, a pathway known to be antinociceptive in humans.

Virus-Mediated Knockdown of Nav1.3 in Dorsal Root Ganglia of STZ-Induced Diabetic Rats Alleviates Tactile Allodynia

Diabetic neuropathic pain affects a substantial number of people and represents a major public health problem. Available clinical treatments for diabetic neuropathic pain remain only partially effective and many of these treatments carry the burden of side effects or the risk of dependence. The misexpression of sodium channels within nociceptive neurons contributes to abnormal electrical activity associated with neuropathic pain. Voltage-gated sodium channel Nav1.3 produces tetrodotoxin-sensitive sodium currents with rapid repriming kinetics and has been shown to contribute to neuronal hyperexcitability and ectopic firing in injured neurons. Suppression of Nav1.3 activity can attenuate neuropathic pain induced by peripheral nerve injury. Previous studies have shown that expression of Nav1.3 is upregulated in dorsal root ganglion (DRG) neurons of diabetic rats that exhibit neuropathic pain. Here, we hypothesized that viral-mediated knockdown of Nav1.3 in painful diabetic neuropathy would reduce neuropathic pain. We used a validated recombinant adeno-associated virus (AAV)-shRNA-Nav1.3 vector to knockdown expression of Nav1.3, via a clinically applicable intrathecal injection method. Three weeks following vector administration, we observed a significant rate of transduction in DRGs of diabetic rats that concomitantly reduced neuronal excitability of dorsal horn neurons and reduced behavioral evidence of tactile allodynia. Taken together, these findings offer a novel gene therapy approach for addressing chronic diabetic neuropathic pain.

Dendritic spine dysgenesis in neuropathic pain

Neuropathic pain is a significant unmet medical need in patients with variety of injury or disease insults to the nervous system. Neuropathic pain often presents as a painful sensation described as electrical, burning, or tingling. Currently available treatments have limited effectiveness and narrow therapeutic windows for safety. More powerful analgesics, e.g., opioids, carry a high risk for chemical dependence. Thus, a major challenge for pain research is the elucidation of the mechanisms that underlie neuropathic pain and developing targeted strategies to alleviate pathological pain. The mechanistic link between dendritic spine structure and circuit function could explain why neuropathic pain is difficult to treat, since nociceptive processing pathways are adversely "hard-wired" through the reorganization of dendritic spines. Several studies in animal models of neuropathic pain have begun to reveal the functional contribution of dendritic spine dysgenesis in neuropathic pain. Previous reports have demonstrated three primary changes in dendritic spine structure on nociceptive dorsal horn neurons following injury or disease, which accompany chronic intractable pain: (I) increased density of dendritic spines, particularly mature mushroom-spine spines, (II) redistribution of spines toward dendritic branch locations close to the cell body, and (III) enlargement of the spine head diameter, which generally presents as a mushroom-shaped spine. Given the important functional implications of spine distribution, density, and shape for synaptic and neuronal function, the study of dendritic spine abnormality may provide a new perspective for investigating pain, and the identification of specific molecular players that regulate spine morphology may guide the development of more effective and long-lasting therapies.

Enhanced nicotine self-administration and suppressed dopaminergic systems in a rat model of diabetes

Patients with diabetes display a heightened propensity to use tobacco; however, it is unclear whether they experience enhanced rewarding effects of nicotine. Thus, this study examined the reinforcing effects of nicotine in a rodent model of diabetes involving administration of streptozotocin (STZ), a drug that is toxic to pancreatic insulin-producing cells. The first study compared STZ- and vehicle-treated rats that had 23-hour access to intravenous self-administration (IVSA) of nicotine or saline and concomitant access to food and water. In order to examine the contribution of dopamine to our behavioral effects, dopamine transporter (DAT), D1 and D2 receptor levels were compared in the nucleus accumbens (NAc) following 10 days of nicotine or saline IVSA. Dopamine levels in the NAc were also compared following nicotine administration. Lastly, nicotine metabolism and dose-dependent effects of nicotine IVSA were assessed. The results revealed that STZ-treated rats displayed enhanced nicotine intake and a robust increase in food and water intake relative to controls. Protein analysis revealed an increase in DAT and a decrease in D1 receptor levels in the NAc of STZ- versus vehicle-treated rats regardless of IVSA condition. STZ-treated rats also displayed suppressed NAc dopamine levels during baseline and in response to nicotine. STZ treatment did not alter our assessment of nicotine metabolism. Furthermore, STZ treatment increased nicotine IVSA in a dose-dependent manner. Our findings suggest that STZ-treatment increased the rewarding effects of nicotine. This suggests that strong reinforcing effects of nicotine may contribute to greater tobacco use in patients with diabetes.

Assessment of diabetic peripheral neuropathy in streptozotocin-induced diabetic rats with magnetic resonance imaging

OBJECTIVE

To determine the role of magnetic resonance (MR) imaging and quantitative T2 value measurements in the assessment of diabetic peripheral neuropathy (DPN).

METHODS

Sequential MR imaging, T2 measurement, and quantitative sensory testing of sciatic nerves were performed in streptozotocin-induced diabetic rats (n = 6) and normal control rats (n = 6) over a 7-week follow-up period. Histological assessment was obtained from 48 diabetic rats and 48 control rats once weekly for 7 weeks (n = 6 for each group at each time point). Nerve signal abnormalities were observed, and the T2 values, mechanical withdrawal threshold (MWT), and histological changes were measured and compared between diabetic and control animals.

RESULTS

Sciatic nerves in the diabetic rats showed a gradual increase in T2 values beginning at 2 weeks after the induction (P = 0.014), while a decrease in MWT started at 3 weeks after the induction (P = 0.001). Nerve T2 values had a similar time course to sensory functional deficit in diabetic rats. Histologically, sciatic nerves of diabetic rats demonstrated obvious endoneural oedema from 2 to 3 weeks after the induction, followed by progressive axonal degeneration, Schwann cell proliferation, and coexistent disarranged nerve regeneration.

CONCLUSION

Nerve T2 measurement is potentially useful in detecting and monitoring diabetic neuropathy.

KEY POINTS

• Sciatic nerves in diabetic rats showed a gradual increase in T2 values • Nerve T2 values were negatively correlated with sensory function impairment • Longitudinal T2 values can be used to monitor the disease progress • Nerve degeneration contributed mainly to progressive prolongation of nerve T2 values.

Spatio-temporal expression and functional involvement of transient receptor potential vanilloid 1 in diabetic mechanical allodynia in rats

Diabetic neuropathic pain (DNP) is one of the most common clinical manifestations of diabetes mellitus (DM), which is characterized by prominent mechanical allodynia (DMA). However, the molecular mechanism underlying it has not fully been elucidated. In this study, we examined the spatio-temporal expression of a major nociceptive channel protein transient receptor potential vanilloid 1 (TRPV1) and analyzed its functional involvement by intrathecal (i.t.) application of TRPV1 antagonists in streptozocin (STZ)-induced DMA rat models. Western blot and immunofluorescent staining results showed that TRPV1 protein level was significantly increased in the soma of the dorsal root ganglion (DRG) neurons on 14 days after STZ treatment (DMA 14 d), whereas those in spinal cord and skin (mainly from the central and peripheral processes of DRG neurons) had already been enhanced on DMA 7 d to peak on DMA 14 d. qRT-PCR experiments confirmed that TRPV1 mRNA level was significantly up-regulated in the DRG on DMA 7 d, indicating a preceding translation of TRPV1 protein in the soma but preferential distribution of this protein to the processes under the DMA conditions. Cell counting assay based on double immunostaining suggested that increased TRPV1-immunoreactive neurons were likely to be small-sized and CGRP-ergic. Finally, single or multiple intrathecal applications of non-specific or specific TRPV1 antagonists, ruthenium red and capsazepine, at varying doses, effectively alleviated DMA, although the effect of the former was more prominent and long-lasting. These results collectively indicate that TRPV1 expression dynamically changes during the development of DMA and this protein may play important roles in mechanical nociception in DRG neurons, presumably through facilitating the release of CGRP.

Berberine ameliorates cold and mechanical allodynia in a rat model of diabetic neuropathy

This study evaluated the antiallodynic properties of berberine on cold and mechanical allodynia after streptozotocin (STZ)-induced diabetes using a rat model. Diabetic neuropathy was induced in rats by intraperitoneal injection of STZ. To measure cold and mechanical allodynia, a 4°C plate and von Frey filament were used, respectively. Cold and mechanical allodynia induced by diabetes were significantly decreased by single and repeated intraperitoneal treatment of amitriptyline at 10 mg/kg, and berberine at 10 and 20 mg/kg. The hepatic malondialdehyde, superoxide dismutase, catalase, and glutathione peroxidase activities were significantly increased in diabetic rats as compared with those in intact rats; however, in amitriptyline- and berberine-treated rats, they were significantly decreased as compared to the STZ control. The overall effects of berberine 20 mg/kg on cold and mechanical allodynia were quite similar to those of amitriptyline 10 mg/kg, and berberine exhibited similar antioxidant effects as the same dosage of amitriptyline. In conclusion, berberine (10 and 20 mg/kg) was observed to have antiallodynic effects against diabetes, which are presumed to be associated with antioxidative effects. It can be considered that the anti-inflammatory or antidepressant capacity of berberine could contribute to the antiallonynic effects shown in this study.

Streptozotocin-induced diabetic hyperalgesia in rats is associated with upregulation of Toll-like receptor 4 expression

Neuropathic pain is one of the common complications of diabetes mellitus, and current treatments often do not meet medical needs. Toll-like receptor 4 (TLR4) has been implicated as a potential therapeutic target in neuropathic and other pain models. In this study, we investigated whether TLR4 expression in spinal cord would be altered in streptozotocin-induced diabetic rat model, which had persistent mechanical and thermal hypersensitivity. The results showed that the mRNA expression of TLR4 was upregulated in streptozotocin-treated animals. Furthermore, TLR4 expression was associated with both paw-pressure withdrawal threshold toward mechanical stimulus and paw withdrawal latency toward thermal stimulus. The protein levels of TNF-α and IL-1β, two downstream proinflammatory cytokines of TLR4 signaling pathway, were also significantly raised and correlated with mechanical/thermal hypersensitivity in diabetic rats. Together, these data have demonstrated that TLR4 and its signaling pathway are associated with neuropathic pain in a diabetic model. It may imply that TLR4 could be a novel target for treating diabetic neuropathy.

Maladaptive dendritic spine remodeling contributes to diabetic neuropathic pain

Diabetic neuropathic pain imposes a huge burden on individuals and society, and represents a major public health problem. Despite aggressive efforts, diabetic neuropathic pain is generally refractory to available clinical treatments. A structure-function link between maladaptive dendritic spine plasticity and pain has been demonstrated previously in CNS and PNS injury models of neuropathic pain. Here, we reasoned that if dendritic spine remodeling contributes to diabetic neuropathic pain, then (1) the presence of malformed spines should coincide with the development of pain, and (2) disrupting maladaptive spine structure should reduce chronic pain. To determine whether dendritic spine remodeling contributes to neuropathic pain in streptozotocin (STZ)-induced diabetic rats, we analyzed dendritic spine morphology and electrophysiological and behavioral signs of neuropathic pain. Our results show changes in dendritic spine shape, distribution, and shape on wide-dynamic-range (WDR) neurons within lamina IV-V of the dorsal horn in diabetes. These diabetes-induced changes were accompanied by WDR neuron hyperexcitability and decreased pain thresholds at 4 weeks. Treatment with NSC23766 (N(6)-[2-[[4-(diethylamino)-1-methylbutyl]amino]-6-methyl-4-pyrimidinyl]-2-methyl-4,6-quinolinediamine trihydrochloride), a Rac1-specific inhibitor known to interfere with spine plasticity, decreased the presence of malformed spines in diabetes, attenuated neuronal hyperresponsiveness to peripheral stimuli, reduced spontaneous firing activity from WDR neurons, and improved nociceptive mechanical pain thresholds. At 1 week after STZ injection, animals with hyperglycemia with no evidence of pain had few or no changes in spine morphology. These results demonstrate that diabetes-induced maladaptive dendritic spine remodeling has a mechanistic role in neuropathic pain. Molecular pathways that control spine morphogenesis and plasticity may be promising future targets for treatment.

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.

Predictive validity of behavioural animal models for chronic pain

Rodent models of chronic pain may elucidate pathophysiological mechanisms and identify potential drug targets, but whether they predict clinical efficacy of novel compounds is controversial. Several potential analgesics have failed in clinical trials, in spite of strong animal modelling support for efficacy, but there are also examples of successful modelling. Significant differences in how methods are implemented and results are reported means that a literature-based comparison between preclinical data and clinical trials will not reveal whether a particular model is generally predictive. Limited reports on negative outcomes prevents reliable estimate of specificity of any model. Animal models tend to be validated with standard analgesics and may be biased towards tractable pain mechanisms. But preclinical publications rarely contain drug exposure data, and drugs are usually given in high doses and as a single administration, which may lead to drug distribution and exposure deviating significantly from clinical conditions. The greatest challenge for predictive modelling is, however, the heterogeneity of the target patient populations, in terms of both symptoms and pharmacology, probably reflecting differences in pathophysiology. In well-controlled clinical trials, a majority of patients shows less than 50% reduction in pain. A model that responds well to current analgesics should therefore predict efficacy only in a subset of patients within a diagnostic group. It follows that successful translation requires several models for each indication, reflecting critical pathophysiological processes, combined with data linking exposure levels with effect on target.

Antinociceptive effects of combination of tramadol and acetaminophen on painful diabetic neuropathy in streptozotocin-induced diabetic rats

OBJECTIVE

The purpose of this study was to establish the streptozotocin (STZ)-induced diabetic model with rats and investigate the antinociceptive effect of combination of Tramadol (TR) and Acetaminophen (NAPA) on the animal model for the first time.

METHODS

Diabetic model was induced by a single injection of STZ (60 mg/kg, intraperitoneal). Nociceptive thresholds were measured by means of electronic von Frey test, hot-plate test, and tail-flick test. On the 28th day of diabetes induction, diabetic rats with significant hyperalgesia were randomly divided into three groups: TR, NAPA, and TR-NAPA combination group. Each group was randomly divided into four subgroups. Three geometric series of drugs were given to each group respectively. Antinociceptive effects of the drugs were assessed at 15, 30, 60, 120, and 180 minutes after drug administration. 50% Maximum antinociceptive effect of each drug was determined by probit analysis, whereas interaction between TR and NAPA was evaluated by isobolographic analysis.

RESULTS

Hyperalgesia, along with hyperglycemia, developed 4 days after STZ injection and persisted at all tested time points until 28 days. TR, NAPA, and TR-NAPA combination administration all produced dose-dependent antinociceptive effects. Isobolographic analysis showed a significant deviation of TR/NAPA 50% maximum antinociceptive effect (in tail-flick test, but not in von Frey test) from the additive line.

CONCLUSIONS

Combination of the two drugs produces an additive antinociceptive effect in tail-flick test, whereas probable additive antinociceptive effect in von Frey test in painful diabetic neuropathy rats.

Key role for spinal dorsal horn microglial kinin B1 receptor in early diabetic pain neuropathy

Background

The pro-nociceptive kinin B₁ receptor (B₁R) is upregulated on sensory C-fibres, astrocytes and microglia in the spinal cord of streptozotocin (STZ)-diabetic rat. This study aims at defining the role of microglial kinin B₁R in diabetic pain neuropathy.

Methods

Sprague-Dawley rats were made diabetic with STZ (65 mg/kg, i.p.), and 4 days later, two specific inhibitors of microglial cells (fluorocitrate, 1 nmol, i.t.; minocycline, 10 mg/kg, i.p.) were administered to assess the impact on thermal hyperalgesia, allodynia and mRNA expression (qRT-PCR) of B₁R and pro-inflammatory markers. Spinal B₁R binding sites ((¹²⁵I)-HPP-desArg¹⁰-Hoe 140) were also measured by quantitative autoradiography. Inhibition of microglia was confirmed by confocal microscopy with the specific marker Iba-1. Effects of intrathecal and/or systemic administration of B₁R agonist (des-Arg⁹-BK) and antagonists (SSR240612 and R-715) were measured on neuropathic pain manifestations.

Results

STZ-diabetic rats displayed significant tactile and cold allodynia compared with control rats. Intrathecal or peripheral blockade of B₁R or inhibition of microglia reversed time-dependently tactile and cold allodynia in diabetic rats without affecting basal values in control rats. Microglia inhibition also abolished thermal hyperalgesia and the enhanced allodynia induced by intrathecal des-Arg⁹-BK without affecting hyperglycemia in STZ rats. The enhanced mRNA expression (B₁R, IL-1β, TNF-α, TRPV1) and Iba-1 immunoreactivity in the STZ spinal cord were normalized by fluorocitrate or minocycline, yet B₁R binding sites were reduced by 38%.

Conclusion

The upregulation of kinin B₁R in spinal dorsal horn microglia by pro-inflammatory cytokines is proposed as a crucial mechanism in early pain neuropathy in STZ-diabetic rats.