Comparative study of peripheral neuropathy and nerve regeneration in NOD and ICR diabetic mice

The non-obese diabetic (NOD) mouse was suggested as an adequate model for diabetic autonomic neuropathy. We evaluated sensory-motor neuropathy and nerve regeneration following sciatic nerve crush in NOD males rendered diabetic by multiple low doses of streptozotocin, in comparison with similarly treated Institute for Cancer Research (ICR) mice, a widely used model for type I diabetes. Neurophysiological values for both strains showed a decline in motor and sensory nerve conduction velocity at 7 and 8 weeks after induction of diabetes in the intact hindlimb. However, amplitudes of compound muscle and sensory action potentials (CMAPs and CNAPs) were significantly reduced in NOD but not in ICR diabetic mice. Morphometrical analysis showed myelinated fiber loss in highly hyperglycemic NOD mice, but no significant changes in fiber size. There was a reduction of intraepidermal nerve fibers, more pronounced in NOD than in ICR diabetic mice. Interestingly, aldose reductase and poly(ADP-ribose) polymerase (PARP) activities were increased already at 1 week of hyperglycemia, persisting until the end of the experiment in both strains. Muscle and nerve reinnervation was delayed in diabetic mice following sciatic nerve crush, being more marked in NOD mice. Thus, diabetes of mid-duration induces more severe peripheral neuropathy and slower nerve regeneration in NOD than in ICR mice.     

Myelinated afferents sprout into lamina II of L3–5 dorsal horn following chronic constriction nerve injury in rats

In order to investigate the consequences of chronic constriction injury (CCI) to nerve, we explored the relationship between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal horn. Following sciatic CCI neuropathy, mechanical allodynia developed in the corresponding footpad within two weeks and persisted throughout the experimental period which extended for an additional two weeks. The neuropathy of the sciatic injury includes extensive Wallerian-like degeneration of myelinated fibers but relative sparing of unmyelinated fibers. We observed that there was no significant change in the dorsal horn termination of unmyelinated C fibers in lamina II of the dorsal horn, using nerve injections of wheat germ agglutin-horseradish peroxidase for transganglionic axonal tracing of these fibers from the nerve injury site, and no evidence of sprouting into adjacent lamina. In contrast, myelinated afferent fibers were observed to be sprouting into lamina II of the dorsal horn, as indicated by cholera toxin β-subunit-horseradish peroxidase retrograde axonal tracings. This region of the dorsal horn is associated with nociceptive-specific neurons that are not generally associated with myelinated fiber input from mechanical and proprioceptive receptors. As previously suggested in nerve transection and crush injuries, and now demonstrated in CCI neuropathy, these morphological changes may have significance in the pathogenesis of chronic mechanical allodynia.     

Proinsulin C-peptide replacement in type 1 diabetic BB/Wor-rats prevents deficits in nerve fiber regeneration

We recently reported that early gene responses and expression of cytoskeletal proteins are perturbed in regenerating nerve in type 1 insulinopenic diabetes but not in type 2 hyperinsulinemic diabetes. We hypothesized that these differences were due to impaired insulin action in the former type of diabetes. To test this hypothesis, type 1 diabetic BB/Wor-rats were replaced with proinsulin C-peptide, which enhances insulin signaling without lowering blood glucose. Following sciatic nerve crush injury, early gene responses such as insulin-like growth factor, c-fos, and nerve growth factor were examined longitudinally in sciatic nerve. Neurotrophic factors, their receptors, and beta-tubulin and neurofilament expression were examined in dorsal root ganglia. C-peptide replacement significantly normalized early gene responses in injured sciatic nerve and partially corrected the expression of endogenous neurotrophic factors and their receptors, as well as neuroskeletal protein in dorsal root ganglia. These effects translated into normalization of axonal radial growth and significantly improved axonal elongation of regenerating fibers in C-peptide-replaced BB/Wor-rats. The findings in C-peptide replaced type 1 diabetic rats were similar to those previously reported in hyperinsulinemic and iso-hyperglycemic type 2 BB/Z-rats. We conclude that impaired insulin action may be more important than hyperglycemia in suppressing nerve fiber regeneration in type 1 diabetic neuropathy.     

Role of A3 adenosine receptor in diabetic neuropathy

Neuropathy is the most common diabetic complication. Although the A1 and A2A adenosine receptors are important pharmacological targets in alleviating diabetic neuropathy, the role of the A3 adenosine receptor remains unknown. Because the A3 adenosine receptor regulates pain induced by chronic constriction injury or chemotherapy, its stimulation might also attenuate diabetic neuropathy. This study examines the effects of systemic treatment with the A3 adenosine receptor agonist 1‐deoxy‐1‐[6‐[[(3‐iodophenyl)methyl]amino]‐9H‐purin‐9‐yl]‐N‐methyl‐β‐d‐ribofuranuronamide (IB‐MECA) on diabetic neuropathy and explores the putative mechanisms underlying its pharmacological effects. We show that IB‐MECA alleviated mechanical hyperalgesia and thermal hypoalgesia in mice 2 weeks but not 4 weeks after streptozocin (STZ) treatment. Furthermore, IB‐MECA prevented the reduction in sciatic motor nerve conduction velocity and sensory nerve conduction velocity in diabetic mice 2 weeks but not 4 weeks after STZ treatment. Similarly, IB‐MECA inhibited the activation of nuclear factor‐κB and decreased the generation of tumor necrosis factor‐α in the spinal cord of mice 2 weeks but not 4 weeks after STZ treatment. These phenomena were associated with reduction of A3 adenosine receptor expression in the spinal cord after long‐term diabetes. Our results suggest that the A3 adenosine receptor plays a critical role in regulating diabetic neuropathy and that reduction in A3 adenosine receptor expression/function might contribute to the progression of diabetic neuropathy. © 2016 Wiley Periodicals, Inc.     

Impaired slow axonal transport in diabetic peripheral nerve is independent of RAGE

Diabetic peripheral nerve dysfunction is a common complication occurring in 30–50% of long‐term diabetic patients. The pathogenesis of this dysfunction remains unclear but growing evidence suggests that it might be attributed, in part, to alteration in axonal transport. Our previous studies demonstrated that RAGE (Receptor for Advanced Glycation Endproducts) contributes to the pathogenesis of diabetic peripheral neuropathy and impairs nerve regeneration consequent to sciatic nerve crush, particularly in diabetes. We hypothesize that RAGE plays a role in axonal transport impairment via the interaction of its cytoplasmic domain with mammalian Diaphanous 1 (mDia1) – actin interacting molecule. Studies showed that mDia1–RAGE interaction is necessary for RAGE‐ligand‐dependent cellular migration, AKT phosphorylation, macrophage inflammatory response and smooth muscle migration. Here, we studied RAGE, mDia1 and markers of axonal transport rates in the peripheral nerves of wild‐type C57BL/6 and RAGE null control and streptozotocin‐injected diabetic mice at 1, 3 and 6 h after sciatic nerve crush. The results show that in both control and diabetic nerves, the amount of RAGE accumulated at the proximal and distal side of the crush area is similar, indicating that the recycling rate for RAGE is very high and that it is evenly transported from and towards the neuronal cell body. Furthermore, we show that slow axonal transport of proteins such as Neurofilament is affected by diabetes in a RAGE‐independent manner. Finally, our study demonstrates that mDia1 axonal transport is impaired in diabetes, suggesting that diabetes‐related changes affecting actin binding proteins occur early in the course of the disease.     

Effect of diet‐induced obesity or type 1 or type 2 diabetes on corneal nerves and peripheral neuropathy in C57Bl/6J mice

We determined the impact diet‐induced obesity (DIO) and types 1 and 2 diabetes have on peripheral neuropathy with emphasis on corneal nerve structural changes in C57Bl/6J mice. Endpoints examined included nerve conduction velocity, response to thermal and mechanical stimuli and innervation of the skin and cornea. DIO mice and to a greater extent type 2 diabetic mice were insulin resistant. DIO and both types 1 and 2 diabetic mice developed motor and sensory nerve conduction deficits. In the cornea of DIO and type 2 diabetic mice there was a decrease in sub‐epithelial corneal nerves, innervation of the corneal epithelium, and corneal sensitivity. Type 1 diabetic mice did not present with any significant changes in corneal nerve structure until after 20 weeks of hyperglycemia. DIO and type 2 diabetic mice developed corneal structural damage more rapidly than type 1 diabetic mice although hemoglobin A₁C values were significantly higher in type 1 diabetic mice. This suggests that DIO with or without hyperglycemia contributes to development and progression of peripheral neuropathy and nerve structural damage in the cornea.     

Insulin-like growth factor-I prevents development of a vincristine neuropathy in mice

Vincristine is a commonly used antitumor agent whose major dose-limiting side-effect is a mixed sensorimotor neuropathy. To assess whether insulin-like growth factor-I (IGF-I), a neurotrophic agent that supports the survival of motoneurons and enhances regeneration of motor and sensory neurons, could prevent the peripheral neuropathy produced by vincristine, mice were treated with both vincristine (1.7 mg/kg, i.p., 2×/week) and/or IGF-I (0.3 or 1 mg/kg, s.c. daily) for 10 weeks. In mice treated with vincristine alone, there was evidence of a mixed sensorimotor neuropathy as indicated by changes in behavior, nerve conduction and histology. Caudal nerve conduction velocity was significantly slower in mice treated with vincristine alone as compared with vehicle-treated mice. Vincristine treatment alone also significantly increased hot-plate latencies and reduced gait support and stride length, but not toe spread distances. The effects of vincristine were accompanied by degeneration of sciatic nerve fibers and demyelination, indicating a peripheral neuropathy. IGF-I (1 mg/kg, s.c.) administered to vincristine-treated mice prevented the neurotoxic effects of vincristine as measured by nerve conduction, gait, response to noxious stimuli and nerve histology. At a lower dose of 0.3 mg/kg administered s.c., IGF-I partially ameliorated the neuropathy induced by vincristine as this dose only prevented the change in nerve conduction and hot-plate latencies. IGF-I administered alone had no effect on any of these parameters. These results suggest that IGF-I prevents both motor and sensory components of vincristine neuropathy and may be useful clinically in preventing the neuropathy induced by vincristine treatment.     

Nitrosative stress and peripheral diabetic neuropathy in leptin-deficient (ob/ob) mice

Nitrosative stress contributes to nerve conduction slowing, thermal hypoalgesia, and impaired nitrergic innervation in animal models of Type 1 diabetes. The role for reactive nitrogen species in Type 2 diabetes-associated neuropathy remains unexplored. This study evaluated the role for nitrosative stress in functional and structural neuropathic changes in ob/ob mice, a model of Type 2 diabetes with mild hyperglycemia and obesity. Two structurally diverse peroxynitrite decomposition catalysts, Fe(III) tetrakis-2-(N-triethylene glycol monomethyl ether)-pyridyl porphyrin (FP15) and Fe(III) tetra-mesitylporphyrin octasulfonate (FeTMPS), were administered to control and 8-week-old ob/ob mice for 3 weeks at the doses of 5 mg kg(-1) day(-1) (FP15) and 5 and 10 mg kg(-1) day(-1) (FeTMPS). The 11-week-old ob/ob mice developed motor nerve conduction velocity (MNCV) and hind-limb digital sensory nerve conduction velocity (SNCV) deficits, thermal hypoalgesia, tactile allodynia, and a remarkable ( approximately 78%) loss of intraepidermal nerve fibers. They also had increased nitrotyrosine and poly(ADP-ribose) immunofluorescence in the sciatic nerve, spinal cord, and dorsal root ganglion neurons. Treatment with two structurally diverse peroxynitrite decomposition catalysts was associated with restoration of normal MNCV and SNCV, and alleviation of thermal hypoalgesia. Tactile response thresholds increased in response to peroxynitrite decomposition catalyst treatment, but still remained approximately 2.7- to 3.2-fold lower compared with non-diabetic controls. Intraepidermal nerve fiber loss was not alleviated by either FP15 or FeTMPS. Nitrotyrosine and poly(ADP-ribose) immunofluorescence in sciatic nerve, spinal cord, and dorsal root ganglia of peroxynitrite decomposition catalyst-treated ob/ob mice were essentially normal. In conclusion, nitrosative stress plays an important role in functional abnormalities associated with large motor, large sensory, and small sensory fiber neuropathy, but not in small sensory nerve fiber degeneration, in this animal model. Peroxynitrite decomposition catalysts alleviate Type 2 diabetes-associated sensory nerve dysfunction, likely by mechanism(s) not involving arrest of degenerative changes or enhanced regeneration of small sensory nerve fibers.     

Mice lacking basic fibroblast growth factor showed faster sensory recovery

Neurotrophic factors have been shown to stimulate and support peripheral nerve repair. One of these factors is basic fibroblast growth factor (FGF-2), which is up-regulated after peripheral nerve injury and influences early sciatic nerve regeneration by regulating Schwann cell proliferation. Our previous study on FGF-2 deficient mice indicated that FGF-2 is important for axonal maturation and remyelination one week after sciatic nerve crush (Jungnickel, J., Claus, P., Gransalke, K., Timmer, M. and Grothe, C., 2004. Targeted disruption of the FGF-2 gene affects the response to peripheral nerve injury. Mol. Cell. Neurosci. 25, 444-452). However, the functional impact of these effects on sensory and motor fibers was not clear. After performing pinch test, walking track analysis and rotarod, we found faster recovery of mechanosensory but not of motor function in mutant mice. To elucidate the role of FGF-2 on structural recovery, we analyzed FGF-2 deficient mice and wild-type littermates 2 and 4 weeks after sciatic nerve crush. Two weeks after peripheral nerve injury, regenerating fibers of mutant mice showed both significantly increased axon and myelin size, but no difference in the number of myelinated and unmyelinated fibers. Molecular analysis indicated that the expression level of myelin protein zero was significantly enhanced in lesioned nerves in the absence of FGF-2. These results suggest that loss of FGF-2 could positively influence restoration of mechanosensory function by accelerating structural recovery transiently.     

Diabetic neuropathy: Electrophysiological and morphological study of peripheral nerve degeneration and regeneration in transgenic mice that express IFNβ in β cells

Diabetic neuropathy is one of the most frequent complications in diabetes but there are no treatments beyond glucose control, due in part to the lack of an appropriate animal model to assess an effective therapy. This study was undertaken to characterize the degenerative and regenerative responses of peripheral nerves after induced sciatic nerve damage in transgenic rat insulin I promoter / human interferon beta (RIP/IFNβ) mice made diabetic with a low dose of streptozotocin (STZ) as an animal model of diabetic complications. In vivo, histological and immunohistological studies of cutaneous and sciatic nerves were performed after left sciatic crush. Functional tests, cutaneous innervation, and sciatic nerve evaluation showed pronounced neurological reduction in all groups 2 weeks after crush. All animals showed a gradual recovery but this was markedly slower in diabetic animals in comparison with normoglycemic animals. The delay in regeneration in diabetic RIP/IFNβ mice resulted in an increase in active Schwann cells and regenerating neurites 8 weeks after surgery. These findings indicate that diabetic‐RIP/IFNβ animals mimic human diabetic neuropathy. Moreover, when these animals are submitted to nerve crush they have substantial deficits in nerve regrowth, similar to that observed in diabetic patients. When wildtype animals were treated with the same dose of STZ, no differences were observed with respect to nontreated animals, indicating that low doses of STZ and the transgene are not implicated in development of the degenerative and regenerative events observed in our study. All these findings indicate that RIP/IFNβ transgenic mice are a good model for diabetic neuropathy. Muscle Nerve, 2010     

Baicalein alleviates diabetic peripheral neuropathy through inhibition of oxidative-nitrosative stress and p38 MAPK activation

With the consideration of the multifactorial etiology of diabetic peripheral neuropathy, an ideal drug or drug combination should target at least several key pathogenetic mechanisms. The flavonoid baicalein (5,6,7-trihydroxyflavone) has been reported to counteract sorbitol accumulation, activation of 12/15-lipoxygenase, oxidative–nitrosative stress, inflammation, and impaired signaling in models of chronic disease. This study evaluated baicalein on diabetic peripheral neuropathy. Control and streptozotocin-diabetic C57Bl6/J mice were maintained with or without baicalein treatment (30 mg kg^{− 1} d^{− 1}, i.p., for 4 weeks after 12 weeks without treatment). Neuropathy was evaluated by sciatic motor and hind-limb digital sensory nerve conduction velocities, thermal algesia (Hargreaves test), tactile response threshold (flexible von Frey filament test), and intraepidermal nerve fiber density (fluorescent immunohistochemistry with confocal microscopy). Sciatic nerve and spinal cord 12/15-lipoxygenase and total and phosphorylated p38 mitogen-activated protein kinase expression and nitrated protein levels were evaluated by Western blot analysis, 12(S)hydroxyeicosatetraenoic acid concentration (a measure of 12/15-lipoxygenase activity) by ELISA, and glucose and sorbitol pathway intermediate concentrations by enzymatic spectrofluorometric assays. Baicalein did not affect diabetic hyperglycemia, and alleviated nerve conduction deficit and small sensory nerve fiber dysfunction, but not intraepidermal nerve fiber loss. It counteracted diabetes-associated p38 mitogen-activated protein kinase phosphorylation, oxidative–nitrosative stress, and 12/15-lipoxygenase overexpression and activation, but not glucose or sorbitol pathway intermediate accumulation. In conclusion, baicalein targets several mechanisms implicated in diabetic peripheral neuropathy. The findings provide rationale for studying hydroxyflavones with an improved pharmacological profile as potential treatments for diabetic neuropathy and other diabetic complications.     

Diabetic Schwann cells suffer from nerve growth factor and neurotrophin‐3 underproduction and poor associability with axons

Schwann cells (SCs) are integral to peripheral nerve biology, contributing to saltatory conduction along axons, nerve and axon development, and axonal regeneration. SCs also provide a microenvironment favoring neural regeneration partially due to production of several neurotrophic factors. Dysfunction of SCs may also play an important role in the pathogenesis of peripheral nerve diseases such as diabetic peripheral neuropathy where hyperglycemia is often considered pathogenic. In order to study the impact of diabetes mellitus (DM) upon the regenerative capacity of adult SCs, we investigated the differential production of the neurotrophic factors nerve growth factor (NGF) and neurotrophin‐3 (NT3) by SCs harvested from the sciatic nerves of murine models of type 1 DM (streptozotocin treated C57BL/6J mice) and type 2 DM (LepR^?/? or db/db mice) or non‐diabetic cohorts. In vitro, SCs from diabetic and control mice were maintained under similar hyperglycemic and euglycemic conditions respectively. Mature SCs from diabetic mice produced lower levels of NGF and NT3 under hyperglycemic conditions when compared to SCs in euglycemia. In addition, SCs from both DM and non‐DM mice appear to be incapable of insulin production, but responded to exogenous insulin with greater proliferation and heightened myelination potentiation. Moreover, SCs from diabetic animals showed poorer association with co‐cultured axons. Hyperglycemia had significant impact upon SCs, potentially contributing to the pathogenesis of diabetic peripheral neuropathy. GLIA 2013;61:1990–1999     

Insulin-like Growth Factors Reverse or Arrest Diabetic Neuropathy: Effects on Hyperalgesia and Impaired Nerve Regeneration in Rats

Diabetic neuropathy is a debilitating disorder whose causation is poorly understood. A new theory proposes that neuropathy may arise as a consequence of loss of neurotrophic insulin-like growth factor (IGF) activity due to diabetes, superimposed on a slow continual loss due to aging. The prediction that IGF-I and IGF-II gene expression are reduced in diabetic nerves was recently tested and validated. Here we tested the prediction that IGF administration can prevent or reverse diabetic sensory neuropathy. Subcutaneous infusion of IGF-I or IGF-II, but not vehicle, halted (P < 0.01) the progression of hyperalgesia in streptozotocin-diabetic rats. Moreover, impaired sensory nerve regeneration was partially reversed within 2 weeks after treatment of diabetic rats with IGFs (P < 0.01). Impaired regeneration could also be prevented by daily subcutaneous IGF injections. The low replacement doses of IGFs were effective despite unabated hyperglycemia and weight loss. These results show that IGF replacement therapy can reverse or prevent diabetic sensory neuropathy independently of hyperglycemia or weight loss.     

Placental growth factor-2 gene transfer by electroporation restores diabetic sensory neuropathy in mice

Placental growth factor-2 (PlGF-2) exhibits neurotrophic activity in dorsal root ganglion (DRG) neurons through the neuropilin-1 (NP-1) receptor in vitro. To examine the potential utility of PlGF-2 therapy for treating diabetic neuropathy, we performed intramuscular PlGF-2 gene transfer by electroporation, and examined its effects on sensory neuropathy in diabetic mice. PlGF-2 was overexpressed in the tibial anterior (TA) muscles of streptozotocin-induced diabetic mice with hypoalgesia using a PlGF-2 plasmid injection with electroporation. The nociceptive threshold was measured using a paw-pressure test. In addition, we overexpressed PlGF-1, an isoform of PlGF that does not bind NP-1. The sciatic nerve and skin were examined 3 weeks after PlGF-2 electro-gene transfer. The overexpression and secretion of PlGF-2 in TA muscles were confirmed by an increase in PlGF levels in TA muscles and plasma, and strongly PlGF positive myofibers in TA muscles. Two weeks after electro-gene transfer into the bilateral TA muscles, the previously elevated nociceptive threshold was found to be significantly decreased in all treated mice. PlGF-1 gene transfer by electroporation did not significantly decrease the nociceptive threshold in diabetic mice. No increase in the number of endoneurial vessels in the sciatic nerve was found in the PlGF-2 plasmid-electroporated mice. A reduction of area of immunoreactivity in epidermal nerves in diabetic mice was restored by PlGF-2 gene transfer. These findings suggest that PlGF-2 electro-gene therapy can significantly ameliorate sensory deficits (i.e. hypoalgesia) in diabetic mice through NP-1 in DRG and peripheral nerves.     

Effect of glycemic control on corneal nerves and peripheral neuropathy in streptozotocin‐induced diabetic C57Bl/6J mice

We sought to determine the impact that duration of hyperglycemia and control has on corneal nerve fiber density in relation to standard diabetic neuropathy endpoints. Control and streptozotocin‐diabetic C57Bl/6J mice were analyzed after 4, 8, 12, and 20 weeks. For the 20‐week time point, five groups of mice were compared: control, untreated diabetic, and diabetic treated with insulin designated as having either poor glycemic control, good glycemic control, or poor glycemic control switched to good glycemic control. Hyperglycemia was regulated by use of insulin‐releasing pellets. Loss of corneal nerves in the sub‐epithelial nerve plexus or corneal epithelium progressed slowly in diabetic mice requiring 20 weeks to reach statistical significance. In comparison, slowing of motor and sensory nerve conduction velocity developed rapidly with significant difference compared with control mice observed after 4 and 8 weeks of hyperglycemia, respectively. In diabetic mice with good glycemic control, average blood glucose levels over the 20‐week experimental period were lowered from 589 ± 2 to 251 ± 9 mg/dl. All diabetic neuropathy endpoints examined were improved in diabetic mice with good glycemic control compared with untreated diabetic mice. However, good control of blood glucose was not totally sufficient in preventing diabetic neuropathy.     

Myelinated afferents sprout into lamina II of L3-5 dorsal horn following chronic constriction nerve injury in rats

In order to investigate the consequences of chronic constriction injury (CCI) to nerve, we explored the relationship between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal horn. Following sciatic CCI neuropathy, mechanical allodynia developed in the corresponding footpad within two weeks and persisted throughout the experimental period which extended for an additional two weeks. The neuropathy of the sciatic injury includes extensive Wallerian-like degeneration of myelinated fibers but relative sparing of unmyelinated fibers. We observed that there was no significant change in the dorsal horn termination of unmyelinated C fibers in lamina II of the dorsal horn, using nerve injections of wheat germ agglutin-horseradish peroxidase for transganglionic axonal tracing of these fibers from the nerve injury site, and no evidence of sprouting into adjacent lamina. In contrast, myelinated afferent fibers were observed to be sprouting into lamina II of the dorsal horn, as indicated by cholera toxin beta-subunit-horseradish peroxidase retrograde axonal tracings. This region of the dorsal horn is associated with nociceptive-specific neurons that are not generally associated with myelinated fiber input from mechanical and proprioceptive receptors. As previously suggested in nerve transection and crush injuries, and now demonstrated in CCI neuropathy, these morphological changes may have significance in the pathogenesis of chronic mechanical allodynia.     

Metformin exacerbates and simvastatin attenuates myelin damage in high fat diet‐fed C57BL/6 J mice

Diabetic neuropathy is one of the most deleterious complications of diabetes mellitus in humans. High fat diet (HFD)‐fed C57BL/6 J mice are a widely used animal model for type 2 diabetes mellitus and metabolic syndrome. We investigated the effects of metformin and simvastatin on the ultrastructural characteristics of sciatic nerve fibers in these mice. Metformin treatment increased the number of structural defects of the myelin sheet surrounding these fibers in already affected nerves of HFD fed mice, and simvastatin treatment reduced these numbers to the levels seen in control mice. These results warrant further research on the effects of metformin and statins in patients developing diabetic neuropathy and advise caution when deciding about optimal treatment modalities in these patients.     

Cerebrolysin improves sciatic nerve dysfunction in a mouse model of diabetic peripheral neuropathy

To examine the effects of Cerebrolysin on the treatment of diabetic peripheral neuropathy, we first established a mouse model of type 2 diabetes mellitus by administering a high-glucose, high-fat diet and a single intraperitoneal injection of streptozotocin. Mice defined as diabetic in this model were then treated with 1.80, 5.39 or 8.98 mL/kg of Cerebrolysin via intraperitoneal injections for 10 consecutive days. Our results demonstrated that the number, diameter and area of myelinated nerve fibers increased in the sciatic nerves of these mice after administration of Cerebrolysin. The results of several behavioral tests showed that Cerebrolysin dose-dependently increased the slope angle in the inclined plane test (indicating an improved ability to maintain body position), prolonged tail-flick latency and foot-licking time (indicating enhanced sensitivity to thermal and chemical pain, respectively, and reduced pain thresholds), and increased an index of sciatic nerve function in diabetic mice compared with those behavioral results in untreated diabetic mice. Taken together, the anatomical and functional results suggest that Cerebrolysin ameliorated peripheral neuropathy in a mouse model of type 2 diabetes mellitus.