PARP inhibition or gene deficiency counteracts intraepidermal nerve fiber loss and neuropathic pain in advanced diabetic neuropathy

NAD+ Precursors Reverse Experimental Diabetic Neuropathy in Mice

Abnormal NAD+ signaling has been implicated in axonal degeneration in diabetic peripheral neuropathy (DPN). We hypothesized that supplementing NAD+ precursors could alleviate DPN symptoms through increasing the NAD+ levels and activating the sirtuin-1 (SIRT1) protein. To test this, we exposed cultured Dorsal Root Ganglion neurons (DRGs) to Nicotinamide Riboside (NR) or Nicotinamide Mononucleotide (NMN), which increased the levels of NAD+, the SIRT1 protein, and the deacetylation activity that is associated with increased neurite growth. A SIRT1 inhibitor blocked the neurite growth induced via NR or NMN. We then induced neuropathy in C57BL6 mice with streptozotocin (STZ) or a high fat diet (HFD) and administered NR or NMN for two months. Both the STZ and HFD mice developed neuropathy, which was reversed through the NR or NMN administration: sensory function improved, nerve conduction velocities normalized, and intraepidermal nerve fibers were restored. The NAD+ levels and SIRT1 activity were reduced in the DRGs from diabetic mice but were preserved with the NR or NMN treatment. We also tested the effect of NR or NMN administration in mice that overexpress the SIRT1 protein in neurons (nSIRT1 OE) and found no additional benefit from the addition of the drug. These findings suggest that supplementing with NAD+ precursors or activating SIRT1 may be a promising treatment for DPN.

Diabetic peripheral neuropathy: pathogenetic mechanisms and treatment

Diabetic peripheral neuropathy (DPN) refers to the development of peripheral nerve dysfunction in patients with diabetes when other causes are excluded. Diabetic distal symmetric polyneuropathy (DSPN) is the most representative form of DPN. As one of the most common complications of diabetes, its prevalence increases with the duration of diabetes. 10-15% of newly diagnosed T2DM patients have DSPN, and the prevalence can exceed 50% in patients with diabetes for more than 10 years. Bilateral limb pain, numbness, and paresthesia are the most common clinical manifestations in patients with DPN, and in severe cases, foot ulcers can occur, even leading to amputation. The etiology and pathogenesis of diabetic neuropathy are not yet completely clarified, but hyperglycemia, disorders of lipid metabolism, and abnormalities in insulin signaling pathways are currently considered to be the initiating factors for a range of pathophysiological changes in DPN. In the presence of abnormal metabolic factors, the normal structure and function of the entire peripheral nervous system are disrupted, including myelinated and unmyelinated nerve axons, perikaryon, neurovascular, and glial cells. In addition, abnormalities in the insulin signaling pathway will inhibit neural axon repair and promote apoptosis of damaged cells. Here, we will discuss recent advances in the study of DPN mechanisms, including oxidative stress pathways, mechanisms of microvascular damage, mechanisms of damage to insulin receptor signaling pathways, and other potential mechanisms associated with neuroinflammation, mitochondrial dysfunction, and cellular oxidative damage. Identifying the contributions from each pathway to neuropathy and the associations between them may help us to further explore more targeted screening and treatment interventions.

Novel drugs affecting diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) poses a significant threat, affecting half of the global diabetic population and leading to severe complications, including pain, impaired mobility, and potential amputation. The delayed manifestation of diabetic neuropathy (DN) makes early diagnosis challenging, contributing to its debilitating impact on individuals with diabetes mellitus (DM). This review examines the multifaceted nature of DPN, focusing on the intricate interplay between oxidative stress, metabolic pathways, and the resulting neuronal damage. It delves into the challenges of diagnosing DN, emphasizing the critical role played by hyperglycemia in triggering these cascading effects. Furthermore, the study explores the limitations of current neuropathic pain drugs, prompting an investigation into a myriad of pharmaceutical agents tested in both human and animal trials over the past decade. The methodology scrutinizes these agents for their potential to provide symptomatic relief for DPN. The investigation reveals promising results from various pharmaceutical agents tested for DPN relief, showcasing their efficacy in ameliorating symptoms. However, a notable gap persists in addressing the underlying problem of DPN. The results underscore the complexity of DPN and the challenges in developing therapies that go beyond symptomatic relief. Despite advancements in treating DPN symptoms, there remains a scarcity of options addressing the underlying problem. This review consolidates the state-of-the-art drugs designed to combat DPN, highlighting their efficacy in alleviating symptoms. Additionally, it emphasizes the need for a deeper understanding of the diverse processes and pathways involved in DPN pathogenesis.

Poly (ADP-ribose) polymerases as PET imaging targets for central nervous system diseases

Poly (ADP-ribose) polymerases (PARPs) constitute of 17 members that are associated with divergent cellular processes and play a crucial role in DNA repair, chromatin organization, genome integrity, apoptosis, and inflammation. Multiple lines of evidence have shown that activated PARP1 is associated with intense DNA damage and irritating inflammatory responses, which are in turn related to etiologies of various neurological disorders. PARP1/2 as plausible therapeutic targets have attracted considerable interests, and multitudes of PARP1/2 inhibitors have emerged for treating cancer, metabolic, inflammatory, and neurological disorders. Furthermore, PARP1/2 as imaging targets have been shown to detect, delineate, and predict therapeutic responses in many diseases by locating and quantifying the expression levels of PARP1/2. PARP1/2-directed noninvasive positron emission tomography (PET) has potential in diagnosing and prognosing neurological diseases. However, quantitative PARP PET imaging in the central nervous system (CNS) has evaded us due to the challenges of developing blood-brain barrier (BBB) penetrable PARP radioligands. Here, we review PARP1/2's relevance in CNS diseases, summarize the recent progress on PARP PET and discuss the possibilities of developing novel PARP radiotracers for CNS diseases.

NAD+ Precursors Repair Mitochondrial Function in Diabetes and Prevent Experimental Diabetic Neuropathy

Axon degeneration in diabetic peripheral neuropathy (DPN) is associated with impaired NAD+ metabolism. We tested whether the administration of NAD+ precursors, nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR), prevents DPN in models of Type 1 and Type 2 diabetes. NMN was administered to streptozotocin (STZ)-induced diabetic rats and STZ-induced diabetic mice by intraperitoneal injection at 50 or 100 mg/kg on alternate days for 2 months. mice The were fed with a high fat diet (HFD) for 2 months with or without added NR at 150 or 300 mg/kg for 2 months. The administration of NMN to STZ-induced diabetic rats or mice or dietary addition of NR to HFD-fed mice improved sensory function, normalized sciatic and tail nerve conduction velocities, and prevented loss of intraepidermal nerve fibers in skin samples from the hind-paw. In adult dorsal root ganglion (DRG) neurons isolated from HFD-fed mice, there was a decrease in NAD+ levels and mitochondrial maximum reserve capacity. These impairments were normalized in isolated DRG neurons from NR-treated mice. The results indicate that the correction of NAD+ depletion in DRG may be sufficient to prevent DPN but does not significantly affect glucose tolerance, insulin levels, or insulin resistance.

An Insight into Potential Pharmacotherapeutic Agents for Painful Diabetic Neuropathy

Diabetes is the 4th most common disease affecting the world's population. It is accompanied by many complications that deteriorate the quality of life. Painful diabetic neuropathy (PDN) is one of the debilitating consequences of diabetes that effects one-third of diabetic patients. Unfortunately, there is no internationally recommended drug that directly hinders the pathological mechanisms that result in painful diabetic neuropathy. Clinical studies have shown that anticonvulsant and antidepressant therapies have proven fruitful in management of pain associated with PDN. Currently, the FDA approved medications for painful diabetic neuropathies include duloxetine, pregabalin, tapentadol extended release, and capsaicin (for foot PDN only). The FDA has also approved the use of spinal cord stimulation system for the treatment of diabetic neuropathy pain. The drugs recommended by other regulatory bodies include gabapentin, amitriptyline, dextromethorphan, tramadol, venlafaxine, sodium valproate, and 5 % lidocaine patch. These drugs are only partially effective and have adverse effects associated with their use. Treating painful symptoms in diabetic patient can be frustrating not only for the patients but also for health care workers, so additional clinical trials for novel and conventional treatments are required to devise more effective treatment for PDN with minimal side effects. This review gives an insight on the pathways involved in the pathogenesis of PDN and the potential pharmacotherapeutic agents. This will be followed by an overview on the FDA-approved drugs for PDN and commercially available topical analgesic and their effects on painful diabetic neuropathies.

FeTMPyP a peroxynitrite decomposition catalyst ameliorated functional and behavioral deficits in chronic constriction injury induced neuropathic pain in rats

Neuropathic pain is a maladaptive pain phenotype that results from injury or damage to the somatosensory nervous system and is proposed to be linked to a cascade of events including excitotoxicity, oxidative stress, mitochondrial dysfunction, neuroinflammation and apoptosis. Oxidative/nitrosative stress is a critical link between neuroinflammation and neurodegeneration through poly (ADP) ribose polymerase (PARP) overactivation. Hence, the present study investigated the antioxidant and anti-inflammatory effects of peroxynitrite decomposition catalyst; FeTMPyP in chronic constriction injury (CCI) of sciatic nerve-induced neuropathy in rats. CCI of the sciatic nerve manifested significant deficits in behavioral, biochemical, functional parameters and was markedly reversed by administration of FeTMPyP. After 14 days of CCI induction, oxidative/nitrosative stress and inflammatory markers such as iNOS, NF-kB, TNF-α and IL-6 were elevated in sciatic nerves of CCI rats along with depleted levels of ATP and elevated levels of poly (ADP) ribose (PAR) in both sciatic nerves in ipsilateral (L4-L5) dorsal root ganglions (DRG's), suggesting over activation of PARP. Additionally, CCI resulted in aberrations in mitochondrial function as evident by decreased Mn-SOD levels and respiratory complex activities with increased mitochondrial fission protein DRP-1. These changes were reversed by treatment with FeTMPyP (1 & 3 mg/kg, p.o.). Findings of this study suggest that FeTMPyP, by virtue of its antioxidant properties, reduced both PARP over-activation and subsequent neuroinflammation resulted in protection against CCI-induced functional, behavioral and biochemical deficits.

PARP overactivation in neurological disorders

Poly (ADP-ribose) polymerases (PARPs) constitute a family of enzymes associated with divergent cellular processes that are not limited to DNA repair, chromatin organization, genome integrity, and apoptosis but also found to play a crucial role in inflammation. PARPs mediate poly (ADP-ribosylation) of DNA binding proteins that is often responsible for chromatin remodeling thereby ensure effective repairing of DNA stand breaks although during the conditions of severe genotoxic stress PARPs direct the cell fate towards apoptotic events. Recent discoveries have pushed PARPs into the spotlight as targets for treating cancer, metabolic, inflammatory and neurological disorders. Of note, PARP-1 is the most abundant isoform of PARPs (18 member super family) which executes more than 90% of PARPs functions. Since oxidative/nitrosative stress actuated PARP-1 is linked to vigorous DNA damage and wide spread provocative inflammatory response that underlie the aetiopathogenesis of different neurological disorders, possibility of developing PARP-1 inhibitors as plausible neurotherapeutic agents attracts considerable research interest. This review outlines the recent advances in PARP-1 biology and examines the capability of PARP-1 inhibitors as treatment modalities in intense and interminable diseases of neuronal origin.

Insulin within the Arcuate Nucleus Has Paradoxical Effects on Nociception in Healthy and Diabetic Rats

Introduction

Broad neural circuits originate from the hypothalamic arcuate nucleus and project to many parts of the brain which are related to pain perception. Insulin receptors are found in the arcuate nucleus. Since nociception may be affected in type 1 diabetes, the present study aimed to investigate the intra-arcuate nucleus insulin role in pain perception in streptozotocin (STZ)-induced diabetic and healthy rats.

Methods

Regular insulin was microinjected within the arcuate nucleus and the pain tolerance was measured using the hot plate and the tail-flick apparatus in diabetic rats.

Results

The results showed that the arcuate nucleus suppression with lidocaine could increase thermal nociception in non-diabetic animals. Also, insulin within the arcuate nucleus decreased the acute thermal pain perception in these animals. STZ-induced diabetes produced hypoalgesia which the latency of these tests, progressively increased over time after induction of diabetes. Also, in the same animal group, intra-arcuate injection of insulin reduced the latency of nociception.

Conclusion

Intra-arcuate insulin has paradoxical and controversial effects in healthy and diabetic rats' nociception. These effects seem to be due to the insulin effect on releasing proopiomelanocortin and its derivatives.

Lysophosphatidic Acid Receptor 1- and 3-Mediated Hyperalgesia and Hypoalgesia in Diabetic Neuropathic Pain Models in Mice

Lysophosphatidic acid (LPA) signaling is known to play key roles in the initiation and maintenance of various chronic pain models. Here we examined whether LPA signaling is also involved in diabetes-induced abnormal pain behaviors. The high-fat diet (HFD) showing elevation of blood glucose levels and body weight caused thermal, mechanical hyperalgesia, hypersensitivity to 2000 or 250 Hz electrical-stimulation and hyposensitivity to 5 Hz stimulation to the paw in wild-type (WT) mice. These HFD-induced abnormal pain behaviors and body weight increase, but not elevated glucose levels were abolished in LPA₁^−/− and LPA₃^−/− mice. Repeated daily intrathecal (i.t.) treatments with LPA_1/3 antagonist AM966 reversed these abnormal pain behaviors. Similar abnormal pain behaviors and their blockade by daily AM966 (i.t.) or twice daily Ki16425, another LPA_1/3 antagonist was also observed in db/db mice which show high glucose levels and body weight. Furthermore, streptozotocin-induced similar abnormal pain behaviors, but not elevated glucose levels or body weight loss were abolished in LPA₁^−/− and LPA₃^−/− mice. These results suggest that LPA₁ and LPA₃ play key roles in the development of both type I and type II diabetic neuropathic pain.

Vasomodulation of peripheral blood flow by focused ultrasound potentiates improvement of diabetic neuropathy

OBJECTIVE

Effective treatment methods for diabetic peripheral neuropathy are still lacking. Here, a focused ultrasound (FUS) technique was developed to improve blood flow in diabetic peripheral vessels and potentially treat diabetic peripheral neuropathy.

RESEARCH DESIGN AND METHODS

Male adult Sprague-Dawley rats at 4 weeks poststreptozotocin injections were adopted as models for diabetic neuropathic rats. For single FUS treatment, blood perfusion in the skin of the pad of the middle toe was measured before, during, and after the medial and lateral plantar arteries were treated by FUS. For multiple FUS treatments, blood perfusion measurements, von Frey and hot plate testing and nerve conduction velocity measurements were performed before ultrasonic treatment on the first day of each week, and the microvascular and neural fiber densities in the pad of the toe were measured on the first day of the last week.

RESULTS

The blood perfusion rate significantly increased for 7-10 min in the control and neuropathic rats after a single ultrasound exposure. Multiple ultrasound treatments compared with no treatments significantly increased blood perfusion at the second week and further enhanced perfusion at the third week in the neuropathic rats. Additionally, the paw withdrawal force and latency significantly increased from 34.33±4.55 g and 3.96±0.25 s at the first week to 39.10±5.02 g and 4.77±0.71 s at the second week and to 41.13±2.57 g and 5.24±0.86 s at the third week, respectively. The low nerve conduction velocity in the diabetic rats also improved after the ultrasound treatments. Additionally, ultrasound treatments halted the decrease in microvessel and neural fiber densities in the skin of the diabetic toes. Histologic analysis indicated no damage to the treated arteries or neighboring tissue.

CONCLUSIONS

FUS treatment can increase upstream arterial blood flow in diabetic feet, ameliorate the decrease in downstream microvessel perfusion and halt neuropathic progression.

Evaluation of ameliorative effect of sodium nitrate in experimental model of streptozotocin-induced diabetic neuropathy in male rats

OBJECTIVE

Diabetes induces sensory symptoms of neuropathy as positive (hyperalgesia), negative (hypoalgesia), or both.

METHODS

In the present study, fifty male Wistar rats were allocated to five groups: control, control+nitrate, diabetes, diabetes+insulin, and diabetes+nitrate. Thirty days after diabetes confirmation, insulin (2-4 U/day) was injected subcutaneously in diabetes+insulin group and nitrate (100 mg/l) was added into drinking water of the control+nitrate and diabetes+nitrate groups for a period of 2 months. In order to assess the mechanical and thermal algesia, tail immersion, hot plate, and von Frey tests were performed. The serum insulin levels were determined with insulin ELISA Kit. Serum level of NOx was determined by the Griess method.

RESULTS

Both thermal and mechanical nociceptive thresholds showed a significant decrease (p<0.05) which was followed by a significant increase (p<0.01) in the thermal nociceptive threshold in the diabetes group. Chronic nitrate or insulin treatment led to a significant decrease (p<0.01) in blood glucose levels, as well as a significant (p<0.05) increase in the body weight and serum NOx. Moreover, nitrate treatment significantly increased serum insulin levels (p<0.001) compared to the other groups.

CONCLUSION

Chronic nitrate treatment modified the thermal and mechanical sensitivities in diabetic animals.

Cardiovascular Autonomic Neuropathy and Distal Symmetric Sensorimotor Polyneuropathy: These Two Diabetic Microvascular Complications do not Invariably Co-Exist

Background:

Cardiovascular autonomic neuropathy (CAN) and distal symmetrical sensorimotor polyneuropathy (DSPN) are serious microvascular complications of diabetes mellitus (DM). Their simultaneous development remains disputable. The aim of the present study was to examine the correlation between CAN and the presence/severity of DSPN in DM.

Methods:

Subjects with type 1 (group A: n=51; mean age 40.4 years) and type 2 DM (group B: n=153; mean age 64.6 years) were studied. Evaluation of DSPN was based on neuropathy disability score. Assessment of CAN was based on the battery of 4 standardized cardiovascular autonomic function tests.

Results:

In group A, patients with moderate/severe DSPN exhibited a 12-fold higher likelihood of CAN in univariate analysis (p=0.035). However, significance was lost after adjustment for gender, age, DM duration, and haemoglobin A1c. In group A, likelihood for CAN did not correlate with the presence of mild DSPN in univariate and multivariate analysis. In group B, likelihood of CAN was similar in patients with mild and in those with moderate/severe DSPN compared with patients without DSPN in univariate and multivariate analysis. In between group comparison CAN was similarly distributed in the 2 groups (p for interaction=0.367), in patients with no, mild and moderate/severe DSPN.

Conclusion:

CAN does not always co-exist with degrees of DSPN, ranging from mild to moderate/ severe and is similarly distributed in T1DM and T2DM patients with mild and moderate/severe DSPN and in patients without DSPN.

Triglyceride-lowering effect of the aldose reductase inhibitor cemtirestat-another factor that may contribute to attenuation of symptoms of peripheral neuropathy in STZ-diabetic rats

Hyperglycemia is considered a key risk factor for development of diabetic complications including neuropathy. There is strong scientific evidence showing a primary role of aldose reductase, the first enzyme of the polyol pathway, in the cascade of metabolic imbalances responsible for the detrimental effects of hyperglycemia. Aldose reductase is thus considered a significant drug target. We investigated the effects of cemtirestat, a novel aldose reductase inhibitor, in the streptozotocin-induced rat model of uncontrolled type 1 diabetes in a 4-month experiment. Markedly increased sorbitol levels were recorded in the erythrocytes and the sciatic nerve of diabetic animals. Osmotic fragility of red blood cells was increased in diabetic animals. Indices of thermal hypoalgesia were significantly increased in diabetic rats. Tactile allodynia, recorded in diabetic animals in the early stages, turned to mechanical hypoalgesia by the end of the experiment. Treatment of diabetic animals with cemtirestat (i) reduced plasma triglycerides and TBAR levels; (ii) did not affect the values of HbA1c and body weights; (iii) reversed erythrocyte sorbitol accumulation to near control values, while sorbitol in the sciatic nerve was not affected; (iv) ameliorated indices of the erythrocyte osmotic fragility; and (v) attenuated the symptoms of peripheral neuropathy more significantly in the middle of the experiment than at the end of the treatment. Taking into account the lipid metabolism as an interesting molecular target for prevention or treatment of diabetic peripheral neuropathy, the triglyceride-lowering effect of cemtirestat should be considered in future studies. The most feasible mechanisms of triglyceride-lowering action of cemtirestat were suggested.

Schwann cells expressing nociceptive channel TRPA1 orchestrate ethanol-evoked neuropathic pain in mice

Excessive alcohol consumption is associated with spontaneous burning pain, hyperalgesia, and allodynia. Although acetaldehyde has been implicated in the painful alcoholic neuropathy, the mechanism by which the ethanol metabolite causes pain symptoms is unknown. Acute ethanol ingestion caused delayed mechanical allodynia in mice. Inhibition of alcohol dehydrogenase (ADH) or deletion of transient receptor potential ankyrin 1 (TRPA1), a sensor for oxidative and carbonyl stress, prevented allodynia. Acetaldehyde generated by ADH in both liver and Schwann cells surrounding nociceptors was required for TRPA1-induced mechanical allodynia. Plp1-Cre Trpa1fl/fl mice with a tamoxifen-inducible specific deletion of TRPA1 in Schwann cells revealed that channel activation by acetaldehyde in these cells initiates a NADPH oxidase-1-dependent (NOX1-dependent) production of hydrogen peroxide (H2O2) and 4-hydroxynonenal (4-HNE), which sustains allodynia by paracrine targeting of nociceptor TRPA1. Chronic ethanol ingestion caused prolonged mechanical allodynia and loss of intraepidermal small nerve fibers in WT mice. While Trpa1-/- or Plp1-Cre Trpa1fl/fl mice did not develop mechanical allodynia, they did not show any protection from the small-fiber neuropathy. Human Schwann cells express ADH/TRPA1/NOX1 and recapitulate the proalgesic functions of mouse Schwann cells. TRPA1 antagonists might attenuate some symptoms of alcohol-related pain.

Overexpression of Sirtuin 1 protein in neurons prevents and reverses experimental diabetic neuropathy

In diabetic neuropathy, there is activation of axonal and sensory neuronal degeneration pathways leading to distal axonopathy. The nicotinamide-adenine dinucleotide (NAD+)-dependent deacetylase enzyme, Sirtuin 1 (SIRT1), can prevent activation of these pathways and promote axonal regeneration. In this study, we tested whether increased expression of SIRT1 protein in sensory neurons prevents and reverses experimental diabetic neuropathy induced by a high fat diet (HFD). We generated a transgenic mouse that is inducible and overexpresses SIRT1 protein in neurons (nSIRT1OE Tg). Higher levels of SIRT1 protein were localized to cortical and hippocampal neuronal nuclei in the brain and in nuclei and cytoplasm of small to medium sized neurons in dorsal root ganglia. Wild-type and nSIRT1OE Tg mice were fed with either control diet (6.2% fat) or a HFD (36% fat) for 2 months. HFD-fed wild-type mice developed neuropathy as determined by abnormal motor and sensory nerve conduction velocity, mechanical allodynia, and loss of intraepidermal nerve fibres. In contrast, nSIRT1OE prevented a HFD-induced neuropathy despite the animals remaining hyperglycaemic. To test if nSIRT1OE would reverse HFD-induced neuropathy, nSIRT1OE was activated after mice developed peripheral neuropathy on a HFD. Two months after nSIRT1OE, we observed reversal of neuropathy and an increase in intraepidermal nerve fibre. Cultured adult dorsal root ganglion neurons from nSIRT1OE mice, maintained at high (30 mM) total glucose, showed higher basal and maximal respiratory capacity when compared to adult dorsal root ganglion neurons from wild-type mice. In dorsal root ganglion protein extracts from nSIRT1OE mice, the NAD+-consuming enzyme PARP1 was deactivated and the major deacetylated protein was identified to be an E3 protein ligase, NEDD4-1, a protein required for axonal growth, regeneration and proteostasis in neurodegenerative diseases. Our results indicate that nSIRT1OE prevents and reverses neuropathy. Increased mitochondrial respiratory capacity and NEDD4 activation was associated with increased axonal growth driven by neuronal overexpression of SIRT1. Therapies that regulate NAD+ and thereby target sirtuins may be beneficial in human diabetic sensory polyneuropathy.

Therapeutic efficacy of arginine-rich exenatide on diabetic neuropathy in rats

Diabetes mellitus is characterized by metabolic dysregulation associated with a number of health complications. More than 50% of patients with diabetes mellitus suffer from diabetic polyneuropathy, which involves the presence of peripheral nerve dysfunction symptoms. The aim of this study was to evaluate the potential of a new synthetic arginine-rich exendin-4 (Peptide D) in the treatment of complications caused by diabetes, including peripheral neuropathy, in rats. Diabetes was induced by administering streptozotocin (STZ). Three groups of diabetic rats were treated with Peptide D (0.1, 1, and 10 μg/kg). One group of diabetic rats was treated with Byetta® (1 μg/kg) for 80 days. Neuropathic pain development was assessed by tactile allodynia. STZ-treated rats showed an increased level of tactile allodynia unlike naïve animals. A histological study revealed that the diameter of the sciatic nerve fibers in STZ-treated rats was smaller than that of the naïve animals. An IHC study demonstrated decreased expression of myelin basic protein (MBP) in the sciatic nerve of diabetic rats compared to that in the naïve animals. Peptide D reduced the severity of tactile allodynia. This effect was more pronounced in the Peptide D treated groups than in the group treated with Byetta®. Peptide D and Byetta® treatment resulted in increased MBP expression in the sciatic nerve and increased diameter of myelinated nerve fibers. These findings suggest that poly-arginine peptides are promising agents for the treatment of peripheral polyneuropathies.

Role of mitochondria in diabetic peripheral neuropathy: Influencing the NAD+-dependent SIRT1-PGC-1α-TFAM pathway

Survival of human peripheral nervous system neurons and associated distal axons is highly dependent on energy. Diabetes invokes a maladaptation in glucose and lipid energy metabolism in adult sensory neurons, axons and Schwann cells. Mitochondrial (Mt) dysfunction has been implicated as an etiological factor in failure of energy homeostasis that results in a low intrinsic aerobic capacity within the neuron. Over time, this energy failure can lead to neuronal and axonal degeneration and results in increased oxidative injury in the neuron and axon. One of the key pathways that is impaired in diabetic peripheral neuropathy (DPN) is the energy sensing pathway comprising the nicotinamide-adenine dinucleotide (NAD⁺)-dependent Sirtuin 1 (SIRT1)/peroxisome proliferator-activated receptor-γ coactivator α (PGC-1α)/Mt transcription factor A (TFAM or mtTFA) signaling pathway. Knockout of PGC-1α exacerbates DPN, whereas overexpression of human TFAM is protective. LY379268, a selective metabolomic glutamate receptor 2/3 (mGluR2/3) receptor agonist, also upregulates the SIRT1/PGC-1α/TFAM signaling pathway and prevents DPN through glutamate recycling in Schwann/satellite glial (SG) cells and by improving dorsal root ganglion (DRG) neuronal Mt function. Furthermore, administration of nicotinamide riboside (NR), a precursor of NAD⁺, prevents and reverses DPN, in part by increasing NAD⁺ levels and SIRT1 activity. In summary, we review the role of NAD⁺, mitochondria and the SIRT1-PGC-1α-TFAM pathway both from the perspective of pathogenesis and therapy in DPN.

Co-treatment With BGP-15 Exacerbates 5-Fluorouracil-Induced Gastrointestinal Dysfunction

Gastrointestinal (GI) side-effects of chemotherapy present a constant impediment to efficient and tolerable treatment of cancer. GI symptoms often lead to dose reduction, delays and cessation of treatment. Chemotherapy-induced nausea, bloating, vomiting, constipation, and/or diarrhea can persist up to 10 years post-treatment. We have previously reported that long-term 5-fluorouracil (5-FU) administration results in enteric neuronal loss, acute inflammation and intestinal dysfunction. In this study, we investigated whether the cytoprotectant, BGP-15, has a neuroprotective effect during 5-FU treatment. Balb/c mice received tri-weekly intraperitoneal 5-FU (23 mg/kg/d) administration with and without BGP-15 (15 mg/kg/d) for up to 14 days. GI transit was analyzed via in vivo serial X-ray imaging prior to and following 3, 7, and 14 days of treatment. On day 14, colons were collected for assessment of ex vivo colonic motility, neuronal mitochondrial superoxide, and cytochrome c levels as well as immunohistochemical analysis of myenteric neurons. BGP-15 did not inhibit 5-FU-induced neuronal loss, but significantly increased the number and proportion of choline acetyltransferase (ChAT)-immunoreactive (IR) and neuronal nitric oxide synthase (nNOS)-IR neurons in the myenteric plexus. BGP-15 co-administration significantly increased mitochondrial superoxide production, mitochondrial depolarization and cytochrome c release in myenteric plexus and exacerbated 5-FU-induced colonic inflammation. BGP-15 exacerbated 5-FU-induced colonic dysmotility by reducing the number and proportion of colonic migrating motor complexes and increasing the number and proportion of fragmented contractions and increased fecal water content indicative of diarrhea. Taken together, BGP-15 co-treatment aggravates 5-FU-induced GI side-effects, in contrast with our previous findings that BGP-15 alleviates GI side-effects of oxaliplatin.

Poly (ADP-Ribose) Polymerase Inhibition for Chemotherapy-Induced Peripheral Neuropathy: A Meta-Analysis of Placebo-Controlled Trials

Background: Chemotherapy-induced peripheral neuropathy is characterized by pain, numbness, and tingling in the hands and feet and by diminished quality of life. Multiple previous studies, mostly preclinical, suggest that poly (ADP-ribose) polymerase (PARP) inhibitors may help with these symptoms. Objective: To assess the relationship between PARP inhibition and prevention/palliation of peripheral neuropathy in a clinical setting. Design: Meta-analysis of placebo-controlled clinical trials with PARP inhibitors. Setting/Subjects: We conducted 9 literature searches that included PubMed and other sources to compile fully published placebo-controlled clinical trials that tested PARP inhibitors and that reported on peripheral neuropathy. Measurements: The relative risks for neuropathy of all grades based on PARP inhibition were calculated for each trial. Each trial was weighted by its respective sample size. A forest plot was constructed. Results: Five trials, inclusive of 843 patients, met this study's eligibility criteria. Four included a concomitant PARP inhibitor (either olaparib or veliparib) and paclitaxel, a neuropathy-causing chemotherapy agent; the remaining trial evaluated long-term monotherapy with olaparib. The pooled overall relative risk for the development of neuropathy with PARP inhibition was 1.06 (95% confidence interval: 1-1.4). Conclusions: PARP inhibition does not appear to reduce the risk of chemotherapy-induced peripheral neuropathy. Whether PARP inhibitors may palliate (rather than prevent) neuropathy remains an area in need of further investigation.

PARP inhibition in platinum-based chemotherapy: Chemopotentiation and neuroprotection

Cisplatin, carboplatin and oxaliplatin represent the backbone of platinum therapy for several malignancies including head and neck, lung, colorectal, ovarian, breast, and genitourinary cancer. However, the efficacy of platinum-based drugs is often compromised by a plethora of severe toxicities including sensory and enteric neuropathy. Acute and chronic neurotoxicity following platinum chemotherapy is a major constraint, contributing to dose-reductions, treatment delays, and cessation of treatment. Identifying drugs that effectively prevent these toxic complications is imperative to improve the efficacy of anti-cancer treatment and patient quality of life. Oxidative stress and mitochondrial dysfunction have been highlighted as key players in the pathophysiology of platinum chemotherapy-induced neuropathy. Inhibition of poly(ADP-ribose) polymerase (PARP), a nuclear enzyme activated upon DNA damage, has demonstrated substantial sensory and enteric neuroprotective capacity when administered in combination with platinum chemotherapeutics. Furthermore, administration of PARP inhibitors alongside platinum chemotherapy has been found to significantly improve progression-free survival in patients with breast and ovarian cancer when compared to those receiving chemotherapy alone. This review summarises the current knowledge surrounding mitochondrial damage and oxidative stress in platinum chemotherapy-induced neuropathy and highlights a potential role for PARP in chemopotentiation and neuroprotection.

Can Poly (ADP-Ribose) Polymerase Inhibitors Palliate Paclitaxel-Induced Peripheral Neuropathy in Patients With Cancer?

Background:

Paclitaxel-treated patients can suffer from years of peripheral neuropathy with pain, numbness, and tingling. Promising preclinical data with poly (ADP-ribose) polymerase (PARP) inhibitors led us to explore this class of agents to palliate this neuropathy.

Methods:

We relied on a completed trial that tested the antineoplastic effects of veliparib (NCT01012817). Data from patients who had been enrolled on NCT01012817, who previously received paclitaxel, and who had completed a validated pain assessment instrument were evaluated for improvement in their pain scores.

Results:

All 34 eligible patients were women, and all had a metastatic gynecological malignancy. On a 10-point scale (higher numbers indicative of worse pain), the average baseline score was 3.6 (range: 0-7). Seven patients (21%; 95% confidence interval: 9%-38%) manifested a drop in pain score (1 score lower than baseline followed by at least one consecutive value also below baseline). Of note, no patients initiated other therapy for neuropathy while on NCT01012817.

Conclusion:

The PARP inhibitors merit further study for chemotherapy-induced peripheral neuropathy. For patients suffering from peripheral neuropathy, these putative palliative effects might prompt earlier consideration of a PARP inhibitor as part of cancer treatment.

Neuropathic Pain: Delving into the Oxidative Origin and the Possible Implication of Transient Receptor Potential Channels

Currently, neuropathic pain is an underestimated socioeconomic health problem affecting millions of people worldwide, which incidence may increase in the next years due to chronification of several diseases, such as cancer and diabetes. Growing evidence links neuropathic pain present in several disorders [i.e., spinal cord injury (SCI), cancer, diabetes and alcoholism] to central sensitization, as a global result of mitochondrial dysfunction induced by oxidative and nitrosative stress. Additionally, inflammatory signals and the overload in intracellular calcium ion could be also implicated in this complex network that has not yet been elucidated. Recently, calcium channels namely transient receptor potential (TRP) superfamily, including members of the subfamilies A (TRAP1), M (TRPM2 and 7), and V (TRPV1 and 4), have demonstrated to play a role in the nociception mediated by sensory neurons. Therefore, as neuropathic pain could be a consequence of the imbalance between reactive oxygen species and endogen antioxidants, antioxidant supplementation may be a treatment option. This kind of therapy would exert its beneficial action through antioxidant and immunoregulatory functions, optimizing mitochondrial function and even increasing the biogenesis of this vital organelle; on balance, antioxidant supplementation would improve the patient's quality of life. This review seeks to deepen on current knowledge about neuropathic pain, summarizing clinical conditions and probable causes, the relationship existing between oxidative stress, mitochondrial dysfunction and TRP channels activation, and scientific evidence related to antioxidant supplementation.

Role of poly(ADP-ribose) polymerase activation in the pathogenesis of periodontitis in diabetes

AIM

The aetiology of progressive periodontitis in diabetes has not yet been elucidated. We previously demonstrated that nitrosative stress is increased in diabetic rats with periodontitis. Nitrosative stress induces poly(ADP-ribose) polymerase (PARP) activation. Here, we demonstrated the involvement of PARP activation in diabetic periodontitis and detailed the therapeutic effects of PARP inhibitor.

MATERIALS AND METHODS

Experimental periodontitis was induced by placing a nylon thread ligature. Half of the normal and diabetic rats received the PARP inhibitor, 1,5-isoquinolinediol, for 2 weeks. Gingival PARP activation was detected by immunostaining for poly(ADP-ribose). Periodontitis was evaluated by gingival inflammatory cell infiltration, inflammatory gene expressions and micro-CT analyses.

RESULTS

Although both periodontitis and the presence of diabetes increased PARP activation in the gingiva, diabetic rats with periodontitis had the highest activation of PARP. Diabetic rats with periodontitis also showed significant increases in monocyte/macrophage invasion into the gingiva, inflammatory gene expressions, nitrotyrosine-positive cells in the gingiva and alveolar bone loss, all of which were suppressed by treatment with the PARP inhibitor.

CONCLUSIONS

These results indicate the involvement of PARP activation in the pathogenesis and aggravation of periodontal disease in diabetes and suggest the therapeutic potential of PARP inhibition for treating periodontal disease, especially in patients with diabetes.

Association of myelinated primary afferents impairment with mechanical allodynia in diabetic peripheral neuropathy: an experimental study in rats

To investigate the mechanisms underlying the efficacy of surgical treatment for painful diabetic peripheral neuropathy. Rats were initially divided into 3 groups (I, control rats, II, streptozotocin-induced diabetic rats, III, streptozotocin-induced diabetic rats with latex tube encircling the sciatic nerve without compression). When mechanical allodynia (MA) became stable in the third week, one third of group III rats were sacrificed and the remainder were further divided into subgroups depending on whether the latex tube was removed. Except for some rats in group III, all rats were sacrificed in the fifth week. Morphometric analysis of nerve fibers was performed. Expression level of GABA_B receptor protein in spinal dorsal horn was determined. Changes of GABA_B receptor within areas of primary afferents central terminal were identified. Chronic nerve compression caused by the interaction of diabetic nerves swelling and the encircling latex tube increased the incidence of MA in diabetic rats, and nerve decompression could ameliorate MA. In diabetic rats with MA, demyelination of myelinated fibers was noted and reduction of GABA_B receptor was mainly detected in the area of myelinated afferent central terminals. MA in DPN should be partially attributed to compression impairment of myelinated afferents, supporting the rationale for surgical decompression.

Injury-induced gp130 cytokine signaling in peripheral ganglia is reduced in diabetes mellitus

Neuropathy is a major diabetic complication. While the mechanism of this neuropathy is not well understood, it is believed to result in part from deficient nerve regeneration. Work from our laboratory established that gp130 family of cytokines are induced in animals after axonal injury and are involved in the induction of regeneration-associated genes (RAGs) and in the conditioning lesion response. Here, we examine whether a reduction of cytokine signaling occurs in diabetes. Streptozotocin (STZ) was used to destroy pancreatic β cells, leading to chronic hyperglycemia. Mice were injected with either low doses of STZ (5×60mg/kg) or a single high dose (1×200mg/kg) and examined after three or one month, respectively. Both low and high dose STZ treatment resulted in sustained hyperglycemia and functional deficits associated with the presence of both sensory and autonomic neuropathy. Diabetic mice displayed significantly reduced intraepidermal nerve fiber density and sudomotor function. Furthermore, low and high dose diabetic mice showed significantly reduced tactile touch sensation measured with Von Frey monofilaments. To look at the regenerative and injury-induced responses in diabetic mice, neurons in both superior cervical ganglia (SCG) and the 4th and 5th lumbar dorsal root ganglia (DRG) were unilaterally axotomized. Both high and low dose diabetic mice displayed significantly less axonal regeneration in the sciatic nerve, when measured in vivo, 48h after crush injury. Significantly reduced induction of two gp130 cytokines, leukemia inhibitory factor and interleukin-6, occurred in diabetic animals in SCG 6h after injury compared to controls. Injury-induced expression of interleukin-6 was also found to be significantly reduced in the DRG at 6h after injury in low and high dose diabetic mice. These effects were accompanied by reduced phosphorylation of signal transducer and activator of transcription 3 (STAT3), a downstream effector of the gp130 signaling pathway. We also found decreased induction of several gp130-dependent RAGs, including galanin and vasoactive intestinal peptide. Together, these data suggest a novel mechanism for the decreased response of diabetic sympathetic and sensory neurons to injury.

Gene Expression Profiling Identifies Downregulation of the Neurotrophin-MAPK Signaling Pathway in Female Diabetic Peripheral Neuropathy Patients

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes mellitus (DM). It is not diagnosed or managed properly in the majority of patients because its pathogenesis remains controversial. In this study, human whole genome microarrays identified 2898 and 4493 differentially expressed genes (DEGs) in DM and DPN patients, respectively. A further KEGG pathway analysis indicated that DPN and DM share four pathways, including apoptosis, B cell receptor signaling pathway, endocytosis, and Toll-like receptor signaling pathway. The DEGs identified through comparison of DPN and DM were significantly enriched in MAPK signaling pathway, NOD-like receptor signaling pathway, and neurotrophin signaling pathway, while the "neurotrophin-MAPK signaling pathway" was notably downregulated. Seven DEGs from the neurotrophin-MAPK signaling pathway were validated in additional 78 samples, and the results confirmed the initial microarray findings. These findings demonstrated that downregulation of the neurotrophin-MAPK signaling pathway may be the major mechanism of DPN pathogenesis, thus providing a potential approach for DPN treatment.

Targeting the Diabetic Chaperome to Improve Peripheral Neuropathy

The chaperome constitutes a broad family of molecular chaperones and co-chaperones that facilitate the folding, refolding, and degradation of the proteome. Heat shock protein 90 (Hsp90) promotes the folding of numerous oncoproteins to aid survival of malignant phenotypes, and small molecule inhibitors of the Hsp90 chaperone complex offer a viable approach to treat certain cancers. One therapeutic attribute of this approach is the selectivity of these molecules to target high affinity oncogenic Hsp90 complexes present in tumor cells, which are absent in nontransformed cells. This selectivity has given rise to the idea that disease may contribute to forming a stress chaperome that is functionally distinct in its ability to interact with small molecule Hsp90 modulators. Consistent with this premise, modulating Hsp90 improves clinically relevant endpoints of diabetic peripheral neuropathy but has little impact in nondiabetic nerve. The concept of targeting the "diabetic chaperome" to treat diabetes and its complications is discussed.

Painful neuropathy: Mechanisms

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

Morin Mitigates Chronic Constriction Injury (CCI)-Induced Peripheral Neuropathy by Inhibiting Oxidative Stress Induced PARP Over-Activation and Neuroinflammation

Neuropathic pain is initiated or caused due to the primary lesion or dysfunction in the nervous system and is proposed to be linked to a cascade of events including excitotoxicity, oxidative stress, neuroinflammation and apoptosis. Oxidative/nitrosative stress aggravates the neuroinflammation and neurodegeneration through poly (ADP) ribose polymerase (PARP) overactivation. Hence, the present study investigated the antioxidant and anti-inflammatory effects of the phytoconstituent; morin in chronic constriction injury (CCI) induced neuropathy. Neuropathic pain was induced by chronic constriction of the left sciatic nerve in rats, and the effect of morin (15 and 30 mg/kg, p.o.) was evaluated by measuring behavioural and biochemical changes. Mechanical, chemical and thermal stimuli confirmed the CCI-induced neuropathic pain and treatment with morin significantly improved these behavioural deficits and improved the sciatic functional index by the 14th day after CCI induction. After 14 days of CCI induction, oxidative/nitrosative stress and inflammatory markers were elevated in rat lumbar spinal cord. Oxidative stress induced PARP overactivation resulted in depleted levels of ATP and elevated levels of poly (ADP) ribose (PAR). Treatment with morin reduced the levels of nitrites, restored glutathione levels and abrogated the oxidant induced DNA damage. It also mitigated the increased levels of TNF-α and IL-6. Protein expression studies confirmed the PARP inhibition and anti-inflammatory activity of morin. Findings of this study suggest that morin, by virtue of its antioxidant properties, limited PARP overactivation and neuroinflammation and protected against CCI induced functional, behavioural and biochemical deficits.

Pioglitazone Inhibits the Development of Hyperalgesia and Sensitization of Spinal Nociresponsive Neurons in Type 2 Diabetes

Thiazolidinedione drugs (TZDs) such as pioglitazone are approved by the U.S. Food and Drug Administration for the treatment of insulin resistance in type 2 diabetes. However, whether TZDs reduce painful diabetic neuropathy (PDN) remains unknown. Therefore, we tested the hypothesis that chronic administration of pioglitazone would reduce PDN in Zucker Diabetic Fatty (ZDF(fa/fa) [ZDF]) rats. Compared with Zucker Lean (ZL(fa/+)) controls, ZDF rats developed: (1) increased blood glucose, hemoglobin A1c, methylglyoxal, and insulin levels; (2) mechanical and thermal hyperalgesia in the hind paw; (3) increased avoidance of noxious mechanical probes in a mechanical conflict avoidance behavioral assay, to our knowledge, the first report of a measure of affective-motivational pain-like behavior in ZDF rats; and (4) exaggerated lumbar dorsal horn immunohistochemical expression of pressure-evoked phosphorylated extracellular signal-regulated kinase. Seven weeks of pioglitazone (30 mg/kg/d in food) reduced blood glucose, hemoglobin A1c, hyperalgesia, and phosphorylated extracellular signal-regulated kinase expression in ZDF. To our knowledge, this is the first report to reveal hyperalgesia and spinal sensitization in the same ZDF animals, both evoked by a noxious mechanical stimulus that reflects pressure pain frequently associated with clinical PDN. Because pioglitazone provides the combined benefit of reducing hyperglycemia, hyperalgesia, and central sensitization, we suggest that TZDs represent an attractive pharmacotherapy in patients with type 2 diabetes-associated pain.

PERSPECTIVE

To our knowledge, this is the first preclinical report to show that: (1) ZDF rats exhibit hyperalgesia and affective-motivational pain concurrent with central sensitization; and (2) pioglitazone reduces hyperalgesia and spinal sensitization to noxious mechanical stimulation within the same subjects. Further studies are needed to determine the anti-PDN effect of TZDs in humans.

Impairments of the primary afferent nerves in a rat model of diabetic visceral hyposensitivity

Background

Diabetic neuropathy in visceral organs such as the gastrointestinal (GI) tract is still poorly understood, despite that GI symptoms are among the most common diabetic complications. The present study was designed to explore the changes in visceral sensitivity and the underlying functional and morphological deficits of the sensory nerves in short-term diabetic rats. Here, we compared the colorectal distension (CRD)-induced visceromotor response (VMR, an index of visceral pain) in vivo, the mechanosensitivity of colonic afferents ex vivo as well as the expression of protein gene product (PGP) 9.5 and calcitonin gene-related peptide (CGRP) in colon between diabetic (3–6 weeks after streptozotocin injection) and control (age-matched vehicle injection) rats.

Results

VMR was markedly decreased in the diabetic compared to the control rats. There was a significant decrease in multiunit pelvic afferent nerve responses to ramp distension of the ex vivo colon and single unit analysis indicated that an impaired mechanosensitivity of low-threshold and wide dynamic range fibers may underlie the afferent hyposensitivity in the diabetic colon. Fewer PGP 9.5- or CGRP-immunoreactive fibers and lower protein level of PGP 9.5 were found in the colon of diabetic rats.

Conclusions

These observations revealed the distinctive feature of colonic neuropathy in short-term diabetic rats that is characterized by a diminished sensory innervation and a blunted mechanosensitivity of the remnant sensory nerves.

Jinmaitong decreases sciatic nerve DNA oxidative damage and apoptosis in a streptozotocin-induced diabetic rat model

Diabetic peripheral neuropathy (DPN) is a common chronic complication of diabetes. Jinmaitong (JMT), a Traditional Chinese Medicine, improves certain symptoms of DPN, such as limb pain and numbness. The aim of the present study was to investigate the effects of JMT on DNA oxidative damage and apoptosis in the sciatic nerve of diabetic rats. The rats were divided into a normal and a diabetic group. Diabetes was induced using streptozotocin (60 mg/kg). The diabetic model (DM) rats received vitamin C (0.05 g/kg/day) or JMT [low-dosage (L), 0.44 g/kg/day; medium-dosage (M), 0.88 g/kg/day or high-dosage (H), 1.75 g/kg/day]. After 16 weeks, the mechanical pain threshold of the rats was evaluated. The expression of 8-hydroxy-deoxyguanosine (8-OHdG), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase p22^phox, B-cell lymphoma 2 (Bcl-2), caspase 3 and cleaved-poly(ADP-ribose) polymerase 1 (PARP-1) in the sciatic nerve tissues was measured using the reverse transcription-quantitative polymerase chain reaction, immunohistochemistry and western blotting. JMT had no effect on body weight and fasting blood glucose levels. Following treatment, the rats in the JMT groups had an improved pain threshold compared with the DM controls (JMT-L, 52.9±6.5 g; JMT-M, 74.7±9.3 g; and JMT-H, 61.7±2.0 g vs. DM control, 35.32±12.06 g; all P<0.01), while the threshold in the JMT-M rats was similar to that of normal controls (P>0.05). 8-OHdG and NADPH oxidase p22^phox expression was significantly decreased in the three JMT groups compared with that in the DM controls (all P<0.05). Following JMT treatment, Bcl-2 levels were increased, while caspase 3 and cleaved-PARP-1 levels were decreased compared with those in the DM controls (all P<0.01). In conclusion, JMT may reduce DNA oxidative damage to the sciatic nerve in diabetic rats, as well as regulate genes involved in peripheral neuronal cell apoptosis, suggesting that JMT could be used to prevent or treat DPN in diabetic patients.

Upregulation of T-type Ca2+ channels in long-term diabetes determines increased excitability of a specific type of capsaicin-insensitive DRG neurons

Background

Previous studies have shown that increased excitability of capsaicin-sensitive DRG neurons and thermal hyperalgesia in rats with short-term (2–4 weeks) streptozotocin-induced diabetes is mediated by upregulation of T-type Ca²⁺ current. In longer–term diabetes (after the 8th week) thermal hyperalgesia is changed to hypoalgesia that is accompanied by downregulation of T-type current in capsaicin-sensitive small-sized nociceptors. At the same time pain symptoms of diabetic neuropathy other than thermal persist in STZ-diabetic animals and patients during progression of diabetes into later stages suggesting that other types of DRG neurons may be sensitized and contribute to pain. In this study, we examined functional expression of T-type Ca²⁺ channels in capsaicin-insensitive DRG neurons and excitability of these neurons in longer-term diabetic rats and in thermally hypoalgesic diabetic rats.

Results

Here we have demonstrated that in STZ-diabetes T-type current was upregulated in capsaicin-insensitive low-pH-sensitive small-sized nociceptive DRG neurons of longer-term diabetic rats and thermally hypoalgesic diabetic rats. This upregulation was not accompanied by significant changes in biophysical properties of T-type channels suggesting that a density of functionally active channels was increased. Sensitivity of T-type current to amiloride (1 mM) and low concentration of Ni²⁺ (50 μM) implicates prevalence of Ca_v3.2 subtype of T-type channels in the capsaicin-insensitive low-pH-sensitive neurons of both naïve and diabetic rats. The upregulation of T-type channels resulted in the increased neuronal excitability of these nociceptive neurons revealed by a lower threshold for action potential initiation, prominent afterdepolarizing potentials and burst firing. Sodium current was not significantly changed in these neurons during long-term diabetes and could not contribute to the diabetes-induced increase of neuronal excitability.

Conclusions

Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Ca_v3.2 subtype of T-type channels. This upregulation results in the increased excitability of these neurons and may contribute to nonthermal nociception at a later-stage diabetes.

Antioxidant strategies in the management of diabetic neuropathy

Chronic hyperglycaemia (an abnormally high glucose concentration in the blood) resulting from defects in insulin secretion/action, or both, is the major hallmark of diabetes in which it is known to be involved in the progression of the condition to different complications that include diabetic neuropathy. Diabetic neuropathy (diabetes-induced nerve damage) is the most common diabetic complication and can be devastating because it can lead to disability. There is an increasing body of evidence associating diabetic neuropathy with oxidative stress. Oxidative stress results from the production of oxygen free radicals in the body in excess of its ability to eliminate them by antioxidant activity. Antioxidants have different mechanisms and sites of actions by which they exert their biochemical effects and ameliorate nerve dysfunction in diabetes by acting directly against oxidative damage. This review will examine different strategies for managing diabetic neuropathy which rely on exogenous antioxidants.

Biology of diabetic neuropathy

More than half of all patients with diabetes develop neuropathic disorders affecting the distal sensory and/or motor nerves, or autonomic or cranial nerve functions. Glycemic control can decrease the incidence of neuropathy but is not adequate alone to prevent or treat the disease. This chapter introduces diabetic neuropathy with a morphological description of the disease then describes our current understanding of metabolic and molecular mechanisms that contribute to neurovascular dysfunctions. Key mechanisms include glucose and lipid imbalances and insulin resistance that are interconnected via oxidative stress, inflammation, and altered gene expression. These complex interactions should be considered for the development of new treatment strategies against the onset or progression of neuropathy. Advances in understanding the combined metabolic stressors and the novel study of epigenetics suggest new therapeutic targets to combat this morbid and intractable disease affecting millions of patients with type 1 or type 2 diabetes.

The co-occurrence of myocardial dysfunction and peripheral insensate neuropathy in a streptozotocin-induced rat model of diabetes

BACKGROUND

Cardiomyopathy and distal symmetrical polyneuropathy (DSPN), including sensory and autonomic dysfunction, often co-occur in diabetic mellitus (DM) patients. However, the temporal relationship and progression between these two complications has not been investigated. Using a streptozotocin DM animal model that develops insensate neuropathy, our aim was to examine in parallel the development of DSPN and DM-associated changes in cardiac structure and function as well as potential mechanisms, such as autonomic dysfunction, evaluated by changes in urinary and myocardial norepinephrine content and myocardial neuronal markers.

METHODS

Sensory neuropathy was measured by behavioral tests using Von Frey filaments and Hargreaves methods. Echocardiography was used to evaluate myocardial structure and function. Autonomic function was evaluated by measuring urinary and myocardial norepinephrine (NE) levels by enzyme-linked immunosorbent assay and high-performance liquid chromatography/mass spectrometry. Quantitative immunohistochemistry was used to measure the myocardial neuronal markers, calcitonin gene-related peptide (CGRP) and general neuronal protein gene product 9.5 (PGP 9.5).

RESULTS

The DM group developed tactile and thermal insensate neuropathy 4-5 weeks after DM onset. Cardiovascular changes were found between 4 and 12 weeks after DM onset and included bradycardia, diastolic and systolic dysfunction and cardiac dilation. There was a 2.5-fold reduction in myocardial NE levels and a 5-fold increase in urinary NE levels in the DM group. Finally, there was a 2.3-fold increase in myocardial CGRP levels in the DM group and no change in PGP9.5 levels.

CONCLUSIONS

Cardiovascular structural and functional changes developed early in the course of DM and in combination with insensate neuropathy. In parallel, signs of cardiac autonomic dysfunction were also found and included decreased myocardial NE levels and altered CGRP levels. These results may indicate the need for early cardiovascular evaluation in DM patients with insensate neuropathy.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

PARP inhibitors attenuate chemotherapy-induced painful neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major toxicity of chemotherapy treatment for which no therapy is approved. Poly(ADP-ribose) polymerase (PARP)1/2 are nuclear enzymes activated upon DNA damage, and PARP1/2 inhibition provides resistance against DNA damage. A role for PARP inhibition in sensory neurotransmission has also been established. PARP inhibitors attenuate pain-like behaviors and neuropathy-associated decreased peripheral nerve function in diabetic models. The hypothesis tested was that PARP inhibition protects against painful neuropathy. The objective of this study was to investigate whether the novel, selective PARP1/2 inhibitors (ABT-888 and related analogues) would attenuate development of mechanical allodynia in vincristine-treated rats. PARP inhibitors were dosed for 2 days, and then co-administered with vincristine for 12 days. Mechanical allodynia was observed in rats treated with vincristine. PARP1/2 inhibition significantly attenuated development of mechanical allodynia and reduced poly ADP-ribose (PAR) activation in rat skin. The data presented here show that PARP inhibition attenuates vincristine-induced mechanical allodynia in rats, and supports that PARP inhibition may represent a novel therapeutic approach for CIPN.

Association of reduced glyoxalase 1 activity and painful peripheral diabetic neuropathy in type 1 and 2 diabetes mellitus patients

AIMS

The present study was undertaken to investigate the relationship between glyoxalase 1 (Glo1) enzyme activity and painful diabetic neuropathy (DN) in patients with diabetes mellitus.

METHODS

Glo1 activity and biochemical markers were determined in blood samples from 108 patients with type 1 diabetes, 109 patients with type 2 diabetes, and 132 individuals without diabetes as a control. Painful and painless peripheral DN was assessed and multivariate regression analysis was used to determine independent association of Glo1 activity with occurrence of painful DN.

RESULTS

In patients with type 1 and type 2 diabetes mellitus and painful DN compared to patients with painless DN, Glo1 activity was significantly reduced by 12 and 14%, respectively. The increase in Glo1 activity was significantly associated with reduced occurrence of painful DN after adjusting for confounders by multivariate analysis.

CONCLUSIONS

Our results demonstrate for the first time that Glo1 activity is lower in patients with both types of diabetes mellitus who were diagnosed with painful DN. These data support the hypothesis that Glo1 activity modulates the phenotype of DN and warrant further investigation into the role of Glo1 in DN.

Endoplasmic reticulum stress plays a key role in the pathogenesis of diabetic peripheral neuropathy

Endoplasmic reticulum stress resulting from abnormal folding of newly synthesized proteins impairs metabolism, transcriptional regulation, and gene expression, and it is a key mechanism of cell injury. Endoplasmic reticulum stress plays an important role in cardiovascular and neurodegenerative diseases, cancer, and diabetes. We evaluated the role for this phenomenon in diabetic peripheral neuropathy. Endoplasmic reticulum stress manifest in upregulation of multiple components of unfolded protein response was identified in neural tissues (sciatic nerve, spinal cord) of streptozotocin diabetic rats and mice. A chemical chaperone, trimethylamine oxide, administered for 12 weeks after induction of diabetes (110 mg·kg⁻¹·d⁻¹, a prevention paradigm) attenuated endoplasmic reticulum stress, peripheral nerve dysfunction, intraepidermal nerve fiber loss, and sciatic nerve and spinal cord oxidative-nitrative stress in streptozotocin diabetic rats. Similar effects on diabetes-induced endoplasmic reticulum stress and peripheral nerve dysfunction were observed with a structurally unrelated chemical chaperone, 4-phenylbutyric acid (100 mg·kg⁻¹·d⁻¹, intraperitoneal). CCAAT/enhancer-binding protein homologous protein (CHOP)(-/-) mice made diabetic with streptozotocin displayed less severe sciatic nerve oxidative-nitrative stress and peripheral neuropathy than the wild-type (C57Bl6/J) mice. Neither chemical chaperones nor CHOP gene deficiency reduced diabetic hyperglycemia. Our findings reveal an important role of endoplasmic reticulum stress in the development of diabetic peripheral neuropathy and identify a potential new therapeutic target.

12/15-Lipoxygenase inhibition counteracts MAPK phosphorylation in mouse and cell culture models of diabetic peripheral neuropathy

BACKGROUND

Increased mitogen-activated protein kinase (MAPK) phosphorylation has been detected in peripheral nerve of human subjects and animal models with diabetes as well as high-glucose exposed human Schwann cells, and have been implicated in diabetic peripheral neuropathy. In our recent studies, leukocytetype 12/15-lipoxygenase inhibition or gene deficiency alleviated large and small nerve fiber dysfunction, but not intraepidermal nerve fiber loss in streptozotocin-diabetic mice.

METHODS

To address a mechanism we evaluated the potential for pharmacological 12/15-lipoxygenase inhibition to counteract excessive MAPK phosphorylation in mouse and cell culture models of diabetic neuropathy. C57Bl6/J mice were made diabetic with streptozotocin and maintained with or without the 12/15-lipoxygenase inhibitor cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC). Human Schwann cells were cultured in 5.5 mM or 30 mM glucose with or without CDC.

RESULTS

12(S) HETE concentrations (ELISA), as well as 12/15-lipoxygenase expression and p38 MAPK, ERK, and SAPK/JNK phosphorylation (all by Western blot analysis) were increased in the peripheral nerve and spinal cord of diabetic mice as well as in high glucose-exposed human Schwann cells. CDC counteracted diabetes-induced increase in 12(S)HETE concentrations (a measure of 12/15-lipoxygenase activity), but not 12/15-lipoxygenase overexpression, in sciatic nerve and spinal cord. The inhibitor blunted excessive p38 MAPK and ERK, but not SAPK/ JNK, phosphorylation in sciatic nerve and high glucose exposed human Schwann cells, but did not affect MAPK, ERK, and SAPK/JNK phosphorylation in spinal cord.

CONCLUSION

12/15-lipoxygenase inhibition counteracts diabetes related MAPK phosphorylation in mouse and cell culture models of diabetic neuropathy and implies that 12/15-lipoxygenase inhibitors may be an effective treatment for diabetic peripheral neuropathy.

Marrow-derived cells regulate the development of early diabetic retinopathy and tactile allodynia in mice

The hypothesis that marrow-derived cells, and specifically proinflammatory proteins in those cells, play a critical role in the development of diabetes-induced retinopathy and tactile allodynia was investigated. Abnormalities characteristic of the early stages of retinopathy and allodynia were measured in chimeric mice lacking inducible nitric oxide synthase (iNOS) or poly(ADP-ribosyl) polymerase (PARP1) in only their marrow-derived cells. Diabetes-induced capillary degeneration, proinflammatory changes, and superoxide production in the retina and allodynia were inhibited in diabetic animals in which iNOS or PARP1 was deleted from bone marrow cells only. Of the various marrow cells, neutrophils (and monocytes) play a major role in retinopathy development, because retinal capillary degeneration likewise was significantly inhibited in diabetic mice lacking the receptor for granulocyte colony-stimulating factor in their marrow-derived cells. Immunodepletion of neutrophils or monocytes inhibited the endothelial death otherwise observed when coculturing leukocytes from wild-type diabetic animals with retinal endothelium. iNOS and PARP1 are known to play a role in inflammatory processes, and we conclude that proinflammatory processes within marrow-derived cells play a central role in the development of diabetes complications in the retina and nerve.

Diabetic peripheral neuropathy: should a chaperone accompany our therapeutic approach?

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes that is associated with axonal atrophy, demyelination, blunted regenerative potential, and loss of peripheral nerve fibers. The development and progression of DPN is due in large part to hyperglycemia but is also affected by insulin deficiency and dyslipidemia. Although numerous biochemical mechanisms contribute to DPN, increased oxidative/nitrosative stress and mitochondrial dysfunction seem intimately associated with nerve dysfunction and diminished regenerative capacity. Despite advances in understanding the etiology of DPN, few approved therapies exist for the pharmacological management of painful or insensate DPN. Therefore, identifying novel therapeutic strategies remains paramount. Because DPN does not develop with either temporal or biochemical uniformity, its therapeutic management may benefit from a multifaceted approach that inhibits pathogenic mechanisms, manages inflammation, and increases cytoprotective responses. Finally, exercise has long been recognized as a part of the therapeutic management of diabetes, and exercise can delay and/or prevent the development of painful DPN. This review presents an overview of existing therapies that target both causal and symptomatic features of DPN and discusses the role of up-regulating cytoprotective pathways via modulating molecular chaperones. Overall, it may be unrealistic to expect that a single pharmacologic entity will suffice to ameliorate the multiple symptoms of human DPN. Thus, combinatorial therapies that target causal mechanisms and enhance endogenous reparative capacity may enhance nerve function and improve regeneration in DPN if they converge to decrease oxidative stress, improve mitochondrial bioenergetics, and increase response to trophic factors.

Glucagon-like peptide 1, insulin, sensory neurons, and diabetic neuropathy

Like insulin, glucagon-like peptide 1 (GLP-1) may have direct trophic actions on the nervous system, but its potential role in supporting diabetic sensory neurons is uncertain. We identified wide expression of GLP-1 receptors on dorsal root ganglia sensory neurons of diabetic and nondiabetic mice. Exendin-4, a GLP-1 agonist, increased neurite outgrowth of adult sensory neurons in vitro. To determine the effects ofexendin-4 in comparison with continuous low- or high-dose insulin in vivo, we evaluated parallel cohorts of type 1 (streptozotocin-induced) and type 2 (db/db) mice of 2 months' diabetes duration with established neuropathy during an additional month of treatment. High-dose insulin alone reversed hyperglycemia in type 1 diabetic mice, partly reversed thermal sensory loss, improved epidermal innervation but failed to reverse electrophysiological abnormalities. Exendin-4 improved both sensory electrophysiology and behavioral sensory loss. Low-dose insulin was ineffective. In type 2 diabetes, hyperglycemia was uncorrected, and neither insulin nor exendin-4 reversed sensory electrophysiology, sensory behavior, or loss of epidermal axons. However, exendin-4 alone improved motor electrophysiology. Receptor for advanced glycosylated end products and nuclear factor-κB neuronal expression were not significantly altered by diabetes or treatment. Taken together, these results suggest that although GLP-1 agonists and insulin alone are insufficient to reverse all features of diabetic neuropathy, in combination, they might benefit some aspects of established diabetic neuropathy.

Triglyceride, nonesterified fatty acids, and prediabetic neuropathy: role for oxidative-nitrosative stress

Peripheral neuropathy develops in human subjects with prediabetes and metabolic syndrome before overt hyperglycemia. The contributions of impaired glucose tolerance and insulin signaling, hypertriglyceridemia and/or increased nonesterified fatty acids (NEFA), and hypercholesterolemia to this condition remain unknown. Niacin and its derivatives alleviate dyslipidemia with a minor effect on glucose homeostasis. This study evaluated the roles of impaired glucose tolerance versus dyslipidemia in prediabetic neuropathy using Zucker fatty (fa/fa) rats and the niacin derivative acipimox, as well as the interplay of hypertriglyceridemia, increased NEFA, and oxidative-nitrosative stress. Sixteen-week-old Zucker fatty rats with impaired glucose tolerance, obesity, hyperinsulinemia, hypertriglyceridemia, hypercholesterolemia, and increased NEFA displayed sensory nerve conduction velocity deficit, thermal and mechanical hypoalgesia, and tactile allodynia. Acipimox (100 mg kg(-1) day(-1), 4 weeks) reduced serum insulin, NEFA, and triglyceride concentrations without affecting glucose tolerance and hypercholesterolemia. It alleviated sensory nerve conduction velocity deficit and changes in behavioral measures of sensory function and corrected oxidative-nitrosative stress, but not impaired insulin signaling, in peripheral nerve. Elevated NEFA increased total and mitochondrial superoxide production and NAD(P)H oxidase activity in cultured human Schwann cells. In conclusion, hypertriglyceridemia and/or increased NEFA concentrations cause prediabetic neuropathy through oxidative-nitrosative stress. Lipid-lowering agents and antioxidants may find a use in the management of this condition.