Molecular alterations underlie nodal and paranodal degeneration in type 1 diabetic neuropathy and are prevented by C-peptide

Decreased glucagon in diabetic peripheral neuropathy patients with long duration type 2 diabetes

OBJECTIVE

The aim of this study was to investigate the association of fasting C-peptide and glucagon with diabetic peripheral neuropathy (DPN) in patients with type 2 diabetes (T2DM).

METHODS

A comprehensive evaluation was conducted on 797 patients with T2DM to assess the various risk factors affecting DPN. The subjects were categorized into short duration and long duration group according to the duration of diabetes with a threshold of 10 years. Logistic regression analysis was employed to examine the association between DPN and islet function, as well as other parameters. Receiver operating characteristic curve analysis was performed to evaluate the predictive capability of glucagon.

RESULTS

The fasting C-peptide levels were significantly lower in the DPN patients with short duration of diabetes, but lost significance in the long duration group. Conversely, a decreased level of glucagon was only observed in DPN patients with long duration of diabetes. For the group with long duration of diabetes, glucagon was the sole risk factor associated with DPN. The receiver operating characteristic curve analysis revealed that glucagon in the long duration group exhibited a moderate area under the curve of 0.706.

CONCLUSIONS

The serum glucagon levels in T2DM patients with DPN exhibited bidirectional changes based on the duration of diabetes. Decreased glucagon was associated with DPN in T2DM patients with long duration of diabetes.

The effects of voluntary exercise on histological and stereological changes of sciatic nerve, nitric oxide levels, and peripheral neuropathy caused by high-fat diet-induced type 2 diabetes in male rats

This research was conducted to investigate the possible beneficial impacts of voluntary exercise on sciatic tissue, nitric oxide levels, stereological changes, and peripheral neuropathy caused by "high-fat-diet (HFD)"-induced "type 2 diabetes mellitus (T2DM)" in male rats. Rats were put into four experimental groups at random: "healthy control (C), voluntary exercise (VE), diabetic (D), and diabetic rats treated by voluntary exercise (VED)"; each group contain eight animals. Animals in VE and VED groups performed "voluntary exercise (VE)" for ten weeks. Animals in D and VED groups became diabetic after receiving a HFD for four weeks and an intraperitoneal injection (IP) of "streptozotocin (STZ)" (35 mg/kg). In order to evaluate mechanical and thermal algesia, hot plate, tail withdrawal, and von Frey tests were carried out. At the end of this study, serum NOx levels were assessed, and histological and stereological analyses were conducted. Mechanical nociceptive thresholds indicated considerable reduction (p < 0.001) which was followed by a remarkable enhance (p < 0.001) in thermal nociceptive threshold of D group. Tissue changes were also seen in sciatic nerve of D group. Voluntary exercise modified thermal and mechanical sensitivity in diabetic rats. It also improved the damaged sciatic nerve in diabetic animals.

Diabetes Mellitus-Related Dysfunction of the Motor System

Although motor deficits in humans with diabetic neuropathy have been extensively researched, its effect on the motor system is thought to be lesser than that on the sensory system. Therefore, motor deficits are considered to be only due to sensory and muscle impairment. However, recent clinical and experimental studies have revealed that the brain and spinal cord, which are involved in the motor control of voluntary movement, are also affected by diabetes. This review focuses on the most important systems for voluntary motor control, mainly the cortico-muscular pathways, such as corticospinal tract and spinal motor neuron abnormalities. Specifically, axonal damage characterized by the proximodistal phenotype occurs in the corticospinal tract and motor neurons with long axons, and the transmission of motor commands from the brain to the muscles is impaired. These findings provide a new perspective to explain motor deficits in humans with diabetes. Finally, pharmacological and non-pharmacological treatment strategies for these disorders are presented.

Diabetic neuropathy in children and adolescents with type 1 diabetes mellitus: Diagnosis, pathogenesis, and associated genetic markers

Diabetic neuropathy (DN) is a common long-term complication of type 1 (T1D) and type 2 (T2D) diabetes mellitus, with significant morbidity and mortality. DN is defined as impaired function of the autonomic and/or peripheral nervous system, often subclinical, particularly in children and adolescents with T1D. Nerve conduction studies (NCS) and skin biopsies are considered gold-standard methods in the assessment of DN. Multiple environmental and genetic factors are involved in the pathogenesis of DN. Specifically, the role of metabolic control and glycemic variability is of paramount importance. A number of recently identified genes, including the AKR1B1, VEGF, MTHFR, APOE, and ACE genes, contribute significantly in the pathogenesis of DN. These genes may serve as biomarkers to predict future DN development or treatment response. In addition, they may serve as the basis for the development of new medications or gene therapy. In this review, the diagnostic evaluation, pathogenesis, and associated genetic markers of DN in children and adolescents with T1D are presented and discussed.

Methylglyoxal Disrupts Paranodal Axoglial Junctions via Calpain Activation

Nodes of Ranvier and associated paranodal and juxtaparanodal domains along myelinated axons are essential for normal function of the peripheral and central nervous systems. Disruption of these domains as well as increases in the reactive carbonyl species methylglyoxal are implicated as a pathophysiology common to a wide variety of neurological diseases. Here, using an ex vivo nerve exposure model, we show that increasing methylglyoxal produces paranodal disruption, evidenced by disorganized immunostaining of axoglial cell-adhesion proteins, in both sciatic and optic nerves from wild-type mice. Consistent with previous studies showing that increase of methylglyoxal can alter intracellular calcium homeostasis, we found upregulated activity of the calcium-activated protease calpain in sciatic nerves after methylglyoxal exposure. Methylglyoxal exposure altered clusters of proteins that are known as calpain substrates: ezrin in Schwann cell microvilli at the perinodal area and zonula occludens 1 in Schwann cell autotypic junctions at paranodes. Finally, treatment with the calpain inhibitor calpeptin ameliorated methylglyoxal-evoked ezrin loss and paranodal disruption in both sciatic and optic nerves. Our findings strongly suggest that elevated methylglyoxal levels and subsequent calpain activation contribute to the disruption of specialized axoglial domains along myelinated nerve fibers in neurological diseases.

Neurophysiological recovery 5 years after carpal tunnel release in patients with diabetes

INTRODUCTION

The long-term results of neurophysiological recovery after carpal tunnel release in patients with diabetes have not been studied.

METHODS

Thirty-five patients with diabetes and carpal tunnel syndrome (CTS) were matched with 31 patients without diabetes who had idiopathic CTS, and 27 and 30 patients, respectively, participated in this follow-up study. Nerve conduction results at 5 years were compared with previously published results at baseline and 1 year.

RESULTS

Significant neurophysiological improvement continued from 1 to 5 years after carpal tunnel release for patients with and without diabetes. However, wrist-palm sensory conduction velocity was still abnormal for 85% and 43% of patients with and without diabetes, respectively. Although diabetes had an impact on 4 of 10 measured neurophysiological parameters, the influence of peripheral neuropathy seemed insignificant.

DISCUSSION

After carpal tunnel release, significant long-term neurophysiological improvement is possible for patients with diabetes, and it is not influenced by the presence of peripheral neuropathy. Muscle Nerve 56: E59-E64, 2017.

C-peptide is independent associated with diabetic peripheral neuropathy: a community-based study

BACKGROUND

Because the relationship between C-peptide and diabetic peripheral neuropathy (DPN) is controversial, the aim of our study was to evaluate the relationship between C-peptide and DPN in community-based Chinese patients with type 2 diabetes.

METHODS

In total, 220 consecutive type 2 diabetic patients treated by our regional medical consortium were enrolled. DPN was assessed by clinical symptoms, signs, and electromyography.

RESULTS

Fasting C-peptide, 2-h postprandial C-peptide and ΔC-peptide (i.e., 2-h postprandial C-peptide minus the fasting C-peptide) serum concentrations in the non-DPN group were significantly higher than those in the clinical DPN group (all P ≤ 0.040) and the confirmed DPN group (all P < 0.002). The three C-peptide parameters were independently associated with DPN (all P < 0.05) after adjusting for age, sex, diabetes duration, smoking status, systolic pressure, body mass index, angiotensin-converting enzyme inhibitors/angiotensin receptor blocker use, fasting plasma glucose, HbA1c, triglyceride and estimated glomerular filtration rate. Compared with the ΔC-peptide quartile 1 (reference), patients in quartile 3 (odds ratio [OR], 0.110; 95% confidence interval [CI] 0.026-0.466; P = 0.003) and quartile 4 (OR, 0.012; 95% CI 0.026-0.559; P = 0.007) had a lower risk of DPN after adjusting for the confounders.

CONCLUSIONS

C-peptide was negatively associated with DPN in community-based Chinese type 2 diabetic patients in China.

Formation and disruption of functional domains in myelinated CNS axons

Communication in the central nervous system (CNS) occurs through initiation and propagation of action potentials at excitable domains along axons. Action potentials generated at the axon initial segment (AIS) are regenerated at nodes of Ranvier through the process of saltatory conduction. Proper formation and maintenance of the molecular structure at the AIS and nodes are required for sustaining conduction fidelity. In myelinated CNS axons, paranodal junctions between the axolemma and myelinating oligodendrocytes delineate nodes of Ranvier and regulate the distribution and localization of specialized functional elements, such as voltage-gated sodium channels and mitochondria. Disruption of excitable domains and altered distribution of functional elements in CNS axons is associated with demyelinating diseases such as multiple sclerosis, and is likely a mechanism common to other neurological disorders. This review will provide a brief overview of the molecular structure of the AIS and nodes of Ranvier, as well as the distribution of mitochondria in myelinated axons. In addition, this review highlights important structural and functional changes within myelinated CNS axons that are associated with neurological dysfunction.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

In vivo evidence of reduced nodal and paranodal conductances in type 1 diabetes

OBJECTIVES

Diabetic neuropathy is a debilitating complication of diabetes. Animal models of type 1 diabetes (T1DM) suggest that functional and structural changes, specifically axo-glial dysjunction, may contribute to neuropathy development. The present study sought to examine and characterise early sensory axonal function in T1DM patients in the absence of clinical neuropathy.

METHODS

Thirty patients with T1DM (15M:15F) without neuropathy underwent median nerve sensory and motor axonal excitability studies to examine axonal function. A verified mathematical model of human motor and sensory axons was used to elucidate the underlying causes of observed alterations.

RESULTS

Compared to controls (NC), T1DM patients demonstrated significant axonal excitability abnormalities in sensory and motor axons. These included marked reductions in sensory and motor subexcitability during the recovery cycle (T1DM 7.9 ± 0.4:10.4 ± 0.6%, NC 10.4 ± 0.7:15.4 ± 1.2%, P<0.01) and during hyperpolarizing threshold electrotonus at 10-20 ms (T1DM -75.5 ± 0.8:-69.7 ± 0.8%, NC -78.4 ± 1:-72.7 ± 0.9%, P<0.01). Mathematical modelling demonstrated that these changes were due to reduced nodal Na(+) currents, nodal/paranodal K(+) conductances and Na(+)/K(+) pump dysfunction, consistent with axo-glial dysjunction as outlined in animal models of T1DM.

CONCLUSIONS

The study provided support for the occurrence of early changes in nodal and paranodal conductances in patients with T1DM.

SIGNIFICANCE

These data indicate that axonal excitability techniques may detect early changes in diabetic patients, providing a window of opportunity for prophylactic intervention in T1DM.

C-peptide replacement therapy as an emerging strategy for preventing diabetic vasculopathy

Lack of C-peptide, along with insulin, is the main feature of Type 1 diabetes mellitus (DM) and is also observed in progressive β-cell loss in later stage of Type 2 DM. Therapeutic approaches to hyperglycaemic control have been ineffective in preventing diabetic vasculopathy, and alternative therapeutic strategies are necessary to target both hyperglycaemia and diabetic complications. End-stage organ failure in DM seems to develop primarily due to vascular dysfunction and damage, leading to two types of organ-specific diseases, such as micro- and macrovascular complications. Numerous studies in diabetic patients and animals demonstrate that C-peptide treatment alone or in combination with insulin has physiological functions and might be beneficial in preventing diabetic complications. Current evidence suggests that C-peptide replacement therapy might prevent and ameliorate diabetic vasculopathy and organ-specific complications through conservation of vascular function, as well as prevention of endothelial cell death, microvascular permeability, vascular inflammation, and neointima formation. In this review, we describe recent advances on the beneficial role of C-peptide replacement therapy for preventing diabetic complications, such as retinopathy, nephropathy, neuropathy, impaired wound healing, and inflammation, and further discuss potential beneficial effects of combined C-peptide and insulin supplement therapy to control hyperglycaemia and to prevent organ-specific complications.

Node of Ranvier disruption as a cause of neurological diseases

Dysfunction and/or disruption of nodes of Ranvier are now recognized as key contributors to the pathophysiology of various neurological diseases. One reason is that the excitable nodal axolemma contains a high density of Nav (voltage-gated Na⁺ channels) that are required for the rapid and efficient saltatory conduction of action potentials. Nodal physiology is disturbed by altered function, localization, and expression of voltage-gated ion channels clustered at nodes and juxtaparanodes, and by disrupted axon–glial interactions at paranodes. This paper reviews recent discoveries in molecular/cellular neuroscience, genetics, immunology, and neurology that highlight the critical roles of nodes of Ranvier in health and disease.

Coenzyme Q10 prevents peripheral neuropathy and attenuates neuron loss in the db-/db- mouse, a type 2 diabetes model

Diabetic peripheral neuropathy (DPN) is the most common complication in both type 1 and type 2 diabetes. Here we studied some phenotypic features of a well-established animal model of type 2 diabetes, the leptin receptor-deficient db(-)/db(-) mouse, and also the effect of long-term (6 mo) treatment with coenzyme Q10 (CoQ10), an endogenous antioxidant. Diabetic mice at 8 mo of age exhibited loss of sensation, hypoalgesia (an increase in mechanical threshold), and decreases in mechanical hyperalgesia, cold allodynia, and sciatic nerve conduction velocity. All these changes were virtually completely absent after the 6-mo, daily CoQ10 treatment in db(-)/db(-) mice when started at 7 wk of age. There was a 33% neuronal loss in the lumbar 5 dorsal root ganglia (DRGs) of the db(-)/db(-) mouse versus controls at 8 mo of age, which was significantly attenuated by CoQ10. There was no difference in neuron number in 5/6-wk-old mice between diabetic and control mice. We observed a strong down-regulation of phospholipase C (PLC) β3 in the DRGs of diabetic mice at 8 mo of age, a key molecule in pain signaling, and this effect was also blocked by the 6-mo CoQ10 treatment. Many of the phenotypic, neurochemical regulations encountered in lumbar DRGs in standard models of peripheral nerve injury were not observed in diabetic mice at 8 mo of age. These results suggest that reactive oxygen species and reduced PLCβ3 expression may contribute to the sensory deficits in the late-stage diabetic db(-)/db(-) mouse, and that early long-term administration of the antioxidant CoQ10 may represent a promising therapeutic strategy for type 2 diabetes neuropathy.

Hyperpolarization of the membrane potential in cardiomyocyte tissue slices by the synchronization modulation electric field

Our previous studies have shown that a specially designed, so-called synchronization modulation electric field can entrain active transporter Na/K pumps in the cell membrane. This approach was previously developed in a study of single cells using a voltage clamp to monitor the pump currents. We are now expanding our study from isolated single cells to aggregated cells in a 3-dimensional cell matrix, through the use of a tissue slice from the rat heart. The slice is about 150 μm in thickness, meaning the slices contain many cell layers, resulting in a simplified 3-dimensional system. A fluorescent probe was used to identify the membrane potential and the ionic concentration gradients across the cell membrane. In spite of intrinsic cell-to-cell interactions and the difficulty in stimulating cell aggregation in the tissue slice, the oscillating electric field increased the intracellular fluorescent intensity, indicating elevation of the cell ionic concentration and hyperpolarization of the cell membrane. Blockage of these changes by ouabain confirmed that the results are directly related to Na/K pumps. These results along with the backward modulation indicate that the synchronization modulation electric field can influence the Na/K pumps in tissue cells of a 3-dimensional matrix and therefore hyperpolarize the cell membrane.

Prevalence of other diabetes-associated complications and comorbidities and its impact on health care charges among patients with diabetic neuropathy

INTRODUCTION

Diabetic neuropathy (DN) is a common complication associated with diabetes. This study assesses the prevalence of other diabetes-related complications or comorbidities among DN patients and its marginal contribution to health care charges.

METHODS

Using administrative claims database, we studied commercially insured patients below 65 years old with at least one claim of DN anytime from July 2004 through June 2005 (Year 1). Using propensity scoring, a 10:1 ratio of demographically matched controls with diabetes but no DN was constructed. Both DN patients and controls had 12 months of continuous enrollment in Year 1 and Year 2 (July 2005-June 2006). We compared the Year 1 prevalence of other diabetes-associated complications or comorbidities between DN patients and diabetic controls. Controlling for comorbidities, we used multivariate regressions to examine the incremental impact of DN or any other diabetes-related complication or comorbidity on Year 2 health care charges.

RESULTS

A higher percentage of DN patients had at least one other diabetes-related complication or comorbidity than diabetic controls. Individuals with DN had a higher prevalence of each individual other diabetes-related complication or comorbidity. Controlling for comorbidities, the presence of any other diabetes-related complication or comorbidity was statistically associated with higher outpatient pharmacy and total charges for both DN patients and controls. Total and outpatient pharmacy charges were also significantly higher for DN patients than for controls, among those with or without any other diabetes-related complications or comorbidities.

CONCLUSIONS

DN can occur in the absence of other diabetes-related complications or comorbidities. The presence of DN and any other diabetes-related complications or comorbidities significantly increases health care charges.

The beneficial effects of C-Peptide on diabetic polyneuropathy

Diabetic polyneuropathy (DPN) is a common complication in diabetes. At present, there is no adequate treatment, and DPN is often debilitating for patients. It is a heterogeneous disorder and differs in type 1 and type 2 diabetes. An important underlying factor in type 1 DPN is insulin deficiency. Proinsulin C-peptide is a critical element in the cascade of events. In this review, we describe the physiological role of C-peptide and how it provides an insulin-like signaling function. Such effects translate into beneficial outcomes in early metabolic perturbations of neural Na+/K+-ATPase and nitric oxide (NO) with subsequent preventive effects on early nerve dysfunction. Further corrective consequences resulting from this signaling cascade have beneficial effects on gene regulation of early gene responses, neurotrophic factors, their receptors, and the insulin receptor itself. This may lead to preventive and corrective results to nerve fiber degeneration and loss, as well as, promotion of nerve fiber regeneration with respect to sensory somatic fibers and small nociceptive nerve fibers. A characteristic abnormality of type 1 DPN is nodal and paranodal degeneration with severe consequences for myelinated fiber function. This review deals in detail with the underlying insulin-deficiency-related molecular changes and their correction by C-peptide. Based on these observations, it is evident that continuous maintenance of insulin-like actions by C-peptide is needed in peripheral nerve to minimize the sequences of metabolic and molecular abnormalities, thereby ameliorating neuropathic complications. There is now ample evidence demonstrating that C-peptide replacement in type 1 diabetes promotes insulin action and signaling activities in a more enhanced, prolonged, and continuous fashion than does insulin alone. It is therefore necessary to replace C-peptide to physiological levels in diabetic patients. This will have substantial beneficial effects on type 1 DPN.

Dynamic changes of neuroskeletal proteins in DRGs underlie impaired axonal maturation and progressive axonal degeneration in type 1 diabetes

We investigated mechanisms underlying progressive axonal dysfunction and structural deficits in type 1 BB/Wor-rats from 1 week to 10 month diabetes duration. Motor and sensory conduction velocities were decreased after 4 and 6 weeks of diabetes and declined further over the remaining 9 months. Myelinated sural nerve fibers showed progressive deficits in fiber numbers and sizes. Structural deficits in unmyelinated axonal size were evident at 2 month and deficits in number were present at 4 mo. These changes were preceded by decreased availability of insulin, C-peptide and IGF-1 and decreased expression of neurofilaments and β-III-tubulin. Upregulation of phosphorylating stress kinases like Cdk5, p-GSK-3β, and p42/44 resulted in increased phosphorylation of neurofilaments. Increasing activity of p-GSK-3β correlated with increasing phosphorylation of NFH, whereas decreasing Cdk5 correlated with diminishing phosphorylation of NFM. The data suggest that impaired neurotrophic support results in sequentially impaired synthesis and postranslational modifications of neuroskeletal proteins, resulting in progressive deficits in axonal function, maturation and size.

Zinc-activated C-peptide resistance to the type 2 diabetic erythrocyte is associated with hyperglycemia-induced phosphatidylserine externalization and reversed by metformin

Insulin resistance can broadly be defined as the diminished ability of cells to respond to the action of insulin in transporting glucose from the bloodstream into cells and tissues. Here, we report that erythrocytes (ERYs) obtained from type 2 diabetic rats display an apparent resistance to Zn(2+)-activated C-peptide. Thus, the aims of this study were to demonstrate that Zn(2+)-activated C-peptide exerts potentially beneficial effects on healthy ERYs and that these same effects on type 2 diabetic ERYs are enhanced in the presence of metformin. Incubation of ERYs (obtained from type 2 diabetic BBZDR/Wor-rats) with Zn(2+)-activated C-peptide followed by chemiluminescence measurements of ATP resulted in a 31.2 +/- 4.0% increase in ATP release from these ERYs compared to a 78.4 +/- 4.9% increase from control ERYs. Glucose accumulation in diabetic ERYs, measured by scintillation counting of (14)C-labeled glucose, increased by 35.8 +/- 1.3% in the presence of the Zn(2+)-activated C-peptide, a value significantly lower than results obtained from control ERYs (64.3 +/- 5.1%). When Zn(2+)-activated C-peptide was exogenously added to diabetic ERYs, immunoassays revealed a 32.5 +/- 8.2% increase in C-peptide absorbance compared to a 64.4 +/- 10.3% increase in control ERYs. Phosphatidylserine (PS) externalization and metformin sensitization of Zn(2+)-activated C-peptide were examined spectrofluorometrically by measuring the binding of FITC-labeled annexin to PS. The incubation of diabetic ERYs with metformin prior to the addition of Zn(2+)-activated C-peptide resulted in values that were statistically equivalent to those of controls. Summarily, data obtained here demonstrate an apparent resistance to Zn(2+)-activated C-peptide by the ERY that is corrected by metformin.

[Pathophysiology and treatment for diabetic neuropathy]

Diabetic neuropathy (DN) is the most frequent among peripheral neuropathies. Since its pathophysiology is so complicated, neither classification nor therapeutic management of DN has been established. Sensory/autonomic polyneuropathy (DP) is the main type of DN. Since diabetic patients occasionally have one or more subtypes of DN and/or other polyneuropathy including treatable neuropathy like CIDP, the treatment for DP has to be conducted after excluding the possibility of other conditions. Glycemic control is most essential to prevent the development of DP. However, it is practically difficult to keep HbA1c under 6.5% so that drinking and smoking better be restricted and blood pressure be properly maintained to retard the progression of DP. Aldose reductase inhibitor is only one commercially available drug for DP and its efficacy must be evaluated by nerve function tests along with subjective symptoms. More vigorous therapeutic procedure is expected by obtaining not only more potential drugs based on pathogenic mechanisms but also the technique targeting of DNA/siRNA of given peptides at dorsal root ganglion neurons.

Inflammation in Diabetic Encephalopathy is Prevented by C-Peptide

Encephalopathy is an increasingly recognized complication of type 1 diabetes. The underlying mechanisms are not well understood, although insulin deficiency has been implicated. The spontaneously diabetic BB/Wor-rat develops neuro-behavioral deficits and neuronal cell death in hippocampus and frontal cortex, which can be prevented by insulinomimetic C-peptide. Here we examined whether contributing factors such as activation of innate immune mediators are responsive to C-peptide replacement. Seven-month diabetic BB/Wor-rats and those treated with full C-peptide replacement were compared to age-matched control rats. Hippocampi of diabetic rats showed upregulation of RAGE and NF-kappaB, the former being localized to proliferating astrocytes. These changes were associated with increased expression of TNF-alpha, IL-1beta, IL-2 and IL-6 in hippocampi of diabetic rats. Full C-peptide replacement, which did not induce hyperglycemia, resulted in significant prevention of upregulation of RAGE expression, activation of NF-kappaB and activation of pro-inflammatory factors. In conclusion, impaired insulin activity is associated with upregulation of RAGE and pro-inflammatory factors, and these are likely to contribute to previously described oxidative and apoptotic neuronal cell death. Replacement of insulinomimetic C-peptide significantly prevents this cascade of events.

[C-peptide residual secretion makes difference on type 1 diabetes management?]

Type 1 diabetes is a chronic disease characterized by progressive destruction of the pancreatic beta cells, what leads to insulin deficiency and hyperglycemia. However, a significant secretory function may persist for long periods in a few patients, what is clinically evident through the detection of serum C peptide. This phenomenon might reduce the risk of chronic complications, severe hypoglycemias and allow easier metabolic control. It is possible that these advantages are caused, at least partially, by C peptide itself, acting directly in its target tissues.

Diabetic polyneuropathy: an update

PURPOSE OF REVIEW

To examine current issues in the diagnosis and treatment of diabetic polyneuropathy.

RECENT FINDINGS

Diabetic neuropathies are common and rising in prevalence with the global burden of type 2 diabetes. Polyneuropathy is also emerging as a complication of impaired glucose tolerance, without frank diabetes. Ideas about the pathogenesis of diabetic polyneuropathy have evolved, and some new pathogenic mechanisms are being considered. These include roles for direct insulin signaling on neurons and axons that transduce growth signals, actions of the cleaved C-peptide product of the insulin prohormone, abnormal signaling by advanced glycation endproducts on neuronal and glial receptors and activation of poly (ADP-ribose) polymerase in microvessels and neurons. Manipulation of these pathways may offer new therapeutic approaches. Ideas about neuronal targets in the treatment of neuropathic pain have also advanced emphasizing abnormal sodium and calcium channel signaling.

SUMMARY

Consideration of diabetic polyneuropathy as a unique neurodegenerative condition has generated interest in new pathways involved in its development. A new round of clinical trials that address its pathogenesis may be welcome, as recent attempts have been largely disappointing. In the interim, several forms of therapy for neuropathic pain are available.

Diabetic neuropathy: mechanisms to management

Neuropathy is the most common and debilitating complication of diabetes and results in pain, decreased motility, and amputation. Diabetic neuropathy encompasses a variety of forms whose impact ranges from discomfort to death. Hyperglycemia induces oxidative stress in diabetic neurons and results in activation of multiple biochemical pathways. These activated pathways are a major source of damage and are potential therapeutic targets in diabetic neuropathy. Though therapies are available to alleviate the symptoms of diabetic neuropathy, few options are available to eliminate the root causes. The immense physical, psychological, and economic cost of diabetic neuropathy underscore the need for causally targeted therapies. This review covers the pathology, epidemiology, biochemical pathways, and prevention of diabetic neuropathy, as well as discusses current symptomatic and causal therapies and novel approaches to identify therapeutic targets.

The effects of C-peptide on type 1 diabetic polyneuropathies and encephalopathy in the BB/Wor-rat

Diabetic polyneuropathy (DPN) occurs more frequently in type 1 diabetes resulting in a more severe DPN. The differences in DPN between the two types of diabetes are due to differences in the availability of insulin and C-peptide. Insulin and C-peptide provide gene regulatory effects on neurotrophic factors with effects on axonal cytoskeletal proteins and nerve fiber integrity. A significant abnormality in type 1 DPN is nodal degeneration. In the type 1 BB/Wor-rat, C-peptide replacement corrects metabolic abnormalities ameliorating the acute nerve conduction defect. It corrects abnormalities of neurotrophic factors and the expression of neuroskeletal proteins with improvements of axonal size and function. C-peptide corrects the expression of nodal adhesive molecules with prevention and repair of the functionally significant nodal degeneration. Cognitive dysfunction is a recognized complication of type 1 diabetes, and is associated with impaired neurotrophic support and apoptotic neuronal loss. C-peptide prevents hippocampal apoptosis and cognitive deficits. It is therefore clear that substitution of C-peptide in type 1 diabetes has a multitude of effects on DPN and cognitive dysfunction. Here the effects of C-peptide replenishment will be extensively described as they pertain to DPN and diabetic encephalopathy, underpinning its beneficial effects on neurological complications in type 1 diabetes.

Early changes in insulin receptor signaling and pain sensation in streptozotocin-induced diabetic neuropathy in rats

The objective of the present study was to evaluate the time course of changes in peripheral nerve insulin receptor (IR) signaling and compare observed findings with behavioral responses to noxious mechanical and thermal stimuli in streptozotocin (STZ)-diabetic rats over 12 weeks of diabetes. Diabetic rats developed mechanical hyperalgesia, as indicated by decreased paw withdrawal thresholds to mechanical stimuli that were detectable after 2 weeks of diabetes; they also developed thermal hypoalgesia, as indicated by increased tail flick latencies to thermal stimuli that were detectable at 1 week of diabetes. Western blot analysis revealed decreased phosphorylated: total IR protein ratio that was detectable as early as 2 weeks of diabetes, whereas phosphorylated:total Akt protein ratio was decreased at 2 weeks and increased at 12 weeks of diabetes with unchanged PI-3K protein levels. To our knowledge, the present study is the first to demonstrate that impaired peripheral nerve IR signaling, as indicated by decreased phosphorylated:total IR protein ratio, coincides with early mechanical hyperalgesia and thermal hypoalgesia in STZ-diabetic rats. This finding may improve understanding of how altered pain sensation develops rapidly in this model.

PERSPECTIVE

This study examined peripheral nerve IR signaling during the early course of altered nociception in STZ-diabetic rats. In diabetic rats, impaired peripheral nerve IR signaling is observed shortly after STZ injection, as is altered nociception. This finding suggests a possible role of impaired IR signaling in diabetic sensory neuropathy.

Acetyl-L-carnitine in diabetic polyneuropathy: experimental and clinical data

Diabetic polyneuropathy (DPN) is the most common late complication of diabetes mellitus. The underlying pathogenesis is multifaceted, with partly interrelated mechanisms that display a dynamic course. The mechanisms underlying DPN in type 1 and type 2 diabetes mellitus show overlaps or may differ. The differences are mainly due to insulin deficiency in type 1 diabetes which exacerbates the abnormalities caused by hyperglycaemia. Experimental DPN in rat models have identified early metabolic abnormalities with consequences for nerve conduction velocities and endoneurial blood flow. When corrected, the early functional deficits are usually normalised. On the other hand, if not corrected, they lead to abnormalities in lipid peroxidation and expression of neurotrophic factors which in turn result in axonal, nodal and paranodal degenerative changes with worsening of nerve function. As the structural changes progress, they become increasingly less amendable to metabolic interventions. In the past several years, experimental drugs--such as aldose reductase inhibitors, antioxidants and protein kinase C inhibitors--have undergone clinical trials, with disappointing outcomes. These drugs, targeting a single underlying pathogenetic factor, have in most cases been initiated at the advanced stage of DPN. In contrast, substitution of acetyl-L-carnitine (ALC) or C-peptide in type 1 DPN target a multitude of underlying mechanisms and are therefore more likely to be effective on a broader spectrum of the underlying pathogenesis. Clinical trials utilising ALC have shown beneficial effects on nerve conduction slowing, neuropathic pain, axonal degenerative changes and nerve fibre regeneration, despite relatively late initiation in the natural history of DPN. Owing to the good safety profile of ALC, early initiation of ALC therapy would be justified, with potentially greater benefits.

SREBP-1c expression in Schwann cells is affected by diabetes and nutritional status

Our previous work demonstrated that the sterol response element binding proteins (SREBP)-1 and SREBP-2, which are the key regulators of storage lipid and cholesterol metabolism respectively, are highly expressed in Schwann cells of adult peripheral nerves. In order to evaluate the role of Schwann cell SREBPs in myelination and functioning of peripheral nerves we have determined their expression during development, after fasting and refeeding, and in a rodent model of diabetes. Our results show that SREBP-1c and SREBP-2, unlike SREBP-1a, are the major forms of SREBPs present in peripheral nerves. The expression profile of SREBP-2 follows the expression of genes involved in cholesterol biosynthesis, while SREBP-1c is co-expressed with genes involved in storage lipid metabolism. In addition, the expression of SREBP-1c in the endoneurial compartment of peripheral nerves depends on nutritional status and is disturbed in type 1 diabetes. In line with this, insulin elevates the expression of SREBP-1c in primary cultured Schwann cells by activating the SREBP-1c promoter. Taken together, these findings reveal that SREBP-1c expression in Schwann cells responds to metabolic stimuli including insulin and that this response is affected in type 1 diabetes mellitus. This suggests that disturbed SREBP-1c regulated lipid metabolism may contribute to the pathophysiology of diabetic peripheral neuropathy.

Diabetes mellitus and the peripheral nervous system: manifestations and mechanisms

Diabetes targets the peripheral nervous system with several different patterns of damage and several mechanisms of disease. Diabetic polyneuropathy (DPN) is a common disorder involving a large proportion of diabetic patients, yet its pathophysiology is controversial. Mechanisms considered have included polyol flux, microangiopathy, oxidative stress, abnormal signaling from advanced glycation endproducts and growth factor deficiency. Although some clinical trials have demonstrated modest benefits in disease stabilization or pain therapy in DPN, robust therapy capable of reversing the disease is unavailable. In this review, general aspects of DPN and other diabetic neuropathies are examined, including a summary of recent therapeutic trials. A particular emphasis is placed on the evidence that the neurobiology of DPN reflects a unique yet common and disabling neurodegenerative disorder.

Alzheimer-like changes in rat models of spontaneous diabetes

OBJECTIVE

To examine whether changes characteristic of Alzheimer's disease occur in two rat models with spontaneous onset of type 1 and type 2 diabetes.

RESEARCH DESIGN AND METHODS

The frontal cortices of 8-month-diabetic rats were examined with respect to neuronal densities, neurite degeneration, expression, and/or immunolocalization of amyloid precursor protein (APP), beta-secretase, beta-amyloid, COOH-terminal fragment (CTF), insulin receptor, IGF-1 receptor, glycogen synthase kinase 3-beta (GSK-3beta), protein kinase B (Akt), phosphorylated tau (phospho-tau), synaptophysin, and phosphorylated neurofilaments (SMI-31).

RESULTS

Neuronal loss occurred in both models, significantly more so in type 2 diabetic BBZDR/Wor rats compared with type 1 diabetic BB/Wor rats and was associated with a ninefold increase of dystrophic neurites. APP, beta-secretase, beta-amyloid, and CTF were significantly increased in type 2 diabetic rats, as was phospho-tau. The insulin receptor expression was decreased in type 1 diabetes, whereas IGF-1 receptor was decreased in both models, as were Akt and GSK-3beta expression.

CONCLUSIONS

The data show that beta-amyloid and phospho-tau accumulation occur in experimental diabetes and that this is associated with neurite degeneration and neuronal loss. The changes were more severe in the type 2 diabetic model and appear to be associated with insulin resistance and possibly hypercholesterolemia. The two models will provide useful tools to unravel further mechanistic associations between diabetes and Alzheimer's disease.

Diabetic neuropathy differs in type 1 and type 2 diabetes

In this article we describe differences in early metabolic abnormalities between type 1 and type 2 diabetic polyneuropathy (DPN), and how these differences lead to milder initial functional defects in type 2 diabetes, despite the same hyperglycemic exposures. This early reversible metabolic phase is progressively overshadowed by structural degenerative changes eventually resulting in nerve fiber loss. In comparison, the late structural phase of DPN affects type 1 diabetes more severely. Progressive axonal atrophy and loss is hence expressed to a larger extent in type 1 diabetes. In addition, type 1 DPN is characterized by paranodal degenerative changes not seen in type 2 DPN. These differences can be related to the differences in insulin action and signal transduction affecting the expression of neurotrophic factors and their receptors in type 1 diabetes. Downstream effects on neuroskeletal and adhesive proteins, their posttranslational modifications, and nociceptive peptides underlie the more severe resultant pathology in type 1 DPN. These differences in underlying mechanisms should be seriously considered in the future design of interventional paradigms to combat these common conditions.

C-peptide improves neuropathy in type 1 diabetic BB/Wor-rats

BACKGROUND

The spontaneously diabetic BB/Wor-rat is a close model of human type 1 diabetes and develops diabetic polyneuropathy (DPN) similar to that seen in type 1 patients. Here we examine the therapeutic effects of C-peptide, delivered as continuous infusion or once daily subcutaneous injections on established DPN.

METHODS

Diabetic rats were treated from four to seven months duration of diabetes with full continuous replacement dose of rat C-peptide via (a) osmopumps (OS), (b) full replacement dose (HSC) or (c) one-third of full replacement dose (LSC) by once daily injections.

RESULTS

Diabetic rats treated with OS showed improvements in motor nerve conduction velocity (p < 0.001), sural nerve myelinated fibre number (p < 0.005), size (p < 0.05), axonal area (p < 0.001), regeneration (p < 0.001) and overall neuropathy score (p < 0.001). The progressive decline in sensory nerve conduction velocity was fully prevented. The frequencies of Wallerian degeneration were decreased (p < 0.005). HSC-treated rats showed prevention of further progression of DPN (p < 0.001), whereas LSC-treated rats showed a milder progression of DPN (p < 0.001) compared to untreated rats as assessed by neuropathy score.

CONCLUSION

We conclude that (1) C-peptide is effective in the treatment of established DPN, (2) its effect is dose-dependent and (3) replacement by continuous infusion is the most effective administration of C-peptide.

C-Peptide replacement therapy and sensory nerve function in type 1 diabetic neuropathy

OBJECTIVE

C-peptide replacement in animals results in amelioration of diabetes-induced functional and structural abnormalities in peripheral nerves. The present study was undertaken to examine whether C-peptide administration to patients with type 1 diabetes and peripheral neuropathy improves sensory nerve function.

RESEARCH DESIGN AND METHODS

This was an exploratory, double-blinded, randomized, and placebo-controlled study with three study groups that was carried out at five centers in Sweden. C-peptide was given as a replacement dose (1.5 mg/day, divided into four subcutaneous doses) or a dose three times higher (4.5 mg/day) during 6 months. Neurological examination and neurophysiological measurements were performed before and after 6 months of treatment with C-peptide or placebo.

RESULTS

The age of the 139 patients who completed the protocol was 44.2 +/- 0.6 (mean +/- SE) years and their duration of diabetes was 30.6 +/- 0.8 years. Clinical neurological impairment (NIA) (score >7 points) of the lower extremities was present in 86% of the patients at baseline. Sensory nerve conduction velocity (SCV) was 2.6 +/- 0.08 SD below body height-corrected normal values at baseline and improved similarly within the two C-peptide groups (P < 0.007). The number of patients responding with a SCV peak potential improvement >1.0 m/s was greater in C-peptide-treated patients than in those receiving placebo (P < 0.03). In the least severely affected patients (SCV < 2.5 SD below normal at baseline, n = 70) SCV improved by 1.0 m/s (P < 0.014 vs. placebo). NIA score and vibration perception both improved within the C-peptide-treated groups (P < 0.011 and P < 0.002). A1C levels (7.6 +/- 0.1% at baseline) decreased slightly but similarly in C-peptide-and placebo-treated patients during the study.

CONCLUSIONS

C-peptide treatment for 6 months improves sensory nerve function in early-stage type 1 diabetic neuropathy.

C-Peptide reverses nociceptive neuropathy in type 1 diabetes

We examined the therapeutic effects of C-peptide on established nociceptive neuropathy in type 1 diabetic BB/Wor rats. Nociceptive nerve function, unmyelinated sural nerve fiber and dorsal root ganglion (DRG) cell morphometry, nociceptive peptide content, and the expression of neurotrophic factors and their receptors were investigated. C-peptide was administered either as a continuous subcutaneous replacement dose via osmopumps or a replacement dose given once daily by subcutaneous injection. Diabetic rats were treated from 4 to 7 months of diabetes and were compared with control and untreated diabetic rats of 4- and 7-month duration. Osmopump delivery but not subcutaneous injection improved hyperalgesia and restored the diabetes-induced reduction of unmyelinated fiber number (P < 0.01) and mean axonal size (P < 0.05) in the sural nerve. High-affinity nerve growth factor (NGF) receptor (NGFR-TrkA) expression in DRGs was significantly reduced at 4 months (P < 0.01). Insulin receptor and IGF-I receptor (IGF-IR) expressions in DRGs and NGF content in sciatic nerve were significantly decreased in 7-month diabetic rats (P < 0.01, 0.05, and 0.005, respectively). Osmopump delivery prevented the decline of NGFR-TrkA, insulin receptor (P < 0.05), and IGF-IR (P < 0.005) expressions in DRGs and improved NGF content (P < 0.05) in sciatic nerve. However, subcutaneous injection had only marginal effects on morphometric and molecular changes in diabetic rats. We conclude that C-peptide exerts beneficial therapeutic effects on diabetic nociceptive neuropathy and that optimal effects require maintenance of physiological C-peptide concentrations for a major proportion of the day.

Degeneration of the Golgi and neuronal loss in dorsal root ganglia in diabetic BioBreeding/Worcester rats

AIMS/HYPOTHESIS

The aim of this study was to evaluate the nature and extent of neuronal loss in dorsal root ganglia (DRG) in diabetic polyneuropathy.

MATERIALS AND METHODS

We examined 10-month diabetic BioBreeding/Worcester (BB/Wor) rats with respect to DRG ultrastructure and morphometry, sural nerve morphometry, pro- and anti-apoptotic proteins, the expression of neurotrophic factors and their receptors, and sensory nerve functions.

RESULTS

In diabetic rats, DRG neurons decreased to 73% of normal, owing to loss of substance P and calcitonin gene-related peptide-positive neurons. Levels of pro-apoptotic active caspase-3, Bax and low-affinity nerve growth factor (NGF) were increased in DRG. The concentration of anti-apoptotic heat shock protein (HSP) 70 in DRG was decreased, whereas concentrations of Bcl-xl and HSP27 were unaltered. Levels of poly(ADP-ribose) polymerase (PARP) and cleaved PARP were unaltered. Levels of NGF in sciatic nerve and concentrations of the high-affinity NGF receptor, insulin receptor and IGF-I receptor in DRG were significantly decreased. Sensory nerve conduction velocity decreased to 78% of normal. Hyperalgesia increased up to 6 months. Myelinated and unmyelinated fibre numbers of the sural nerve were significantly decreased in diabetic rats. DRG examinations revealed no evidence of apoptosis, mitochondrial changes or abnormalities of the endoplasmic reticulum. Instead, neurons demonstrated progressive vacuolar degenerative changes of the Golgi apparatus, with fragmentation and formation of large cytoplasmic vacuoles. These data show that sustained apoptotic stress is present in DRG of chronically diabetic BB/Wor rats, but fails to proceed to apoptotic cell death.

CONCLUSIONS/INTERPRETATION

Progressive DRG neuronal loss, particularly of small neurons, occurs in the type 1 diabetic BB/Wor rat. This is associated with neurotrophic withdrawal and progressive degeneration of the Golgi apparatus.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

The preventive and therapeutic effects of GCPII (NAALADase) inhibition on painful and sensory diabetic neuropathy

Excitotoxic glutamate release occurs in several neurological disorders. One source is derived from the hydrolysis of the neuropeptide N-acetyl aspartyl glutamate (NAAG) by glutamate carboxypeptidase II (GCPII, also known as NAALADase). Drugs that attenuate glutamate transmission have been shown to relieve neuropathic pain, however side effects have limited their clinical use. It appears that GCPII is exclusively recruited to provide a glutamate source in hyperglutamatergic, excitotoxic conditions and therefore would be devoid of such side effects. Here we report on the therapeutic effects of an orally bio-available GCP II inhibitor on established painful and sensory neuropathy in the spontaneously diabetic BB/Wor rat. It significantly improved hyperalgesia, nerve conduction velocity and underlying myelinated fiber atrophy. The data suggest that GCP II inhibition may provide a meaningful and effective approach to the treatment of painful diabetic neuropathy.

Nodal persistent Na+ currents in human diabetic nerves estimated by the technique of latent addition

OBJECTIVE

To investigate the effects of hyperglycemia on persistent Na+ currents in human diabetic nerves, eliminating the factors of passive membrane properties as a factor. Previous studies show that strength-duration time constant of a nerve is shortened under hyperglycemia, suggesting reduced axonal persistent Na+ currents. However, the time constant is also affected by changes in passive membrane properties. Latent addition using computerized threshold tracking is a new method that can separately evaluate Na+ currents and passive membrane properties.

METHODS

Latent addition was used to estimate nodal Na+ currents in median motor axons of 83 diabetic patients. Brief hyperpolarizing conditioning current pulses were delivered, and threshold changes at the conditioning-test interval of 0.2 ms were measured as an indicator of nodal persistent Na+ currents. Seventeen patients were examined before and after insulin treatment.

RESULTS

There was an inverse linear relationship between hemoglobin A1c levels and threshold changes at 0.2 ms (P=0.02); the higher hemoglobin A1c levels were associated with smaller threshold changes. After insulin treatment, there was a significant improvement in nerve conduction velocities associated with greater threshold changes at 0.2 ms (P=0.03), suggesting an increase in persistent Na+ currents. The fast component of latent addition, an indicator of passive membrane properties, was not affected by the state of glycemic control.

CONCLUSIONS

Hyperglycemia could suppress nodal persistent Na+ currents, presumably because of reduced trans-axonal Na+ gradient or impaired Na+ channels, and this can be rapidly restored by glycemic control.

SIGNIFICANCE

Reduced nodal Na+ currents may partly contribute to the pathophysiology of human diabetic neuropathy.

Apoptotic stress is counterbalanced by survival elements preventing programmed cell death of dorsal root ganglions in subacute type 1 diabetic BB/Wor rats

Several groups have reported apoptosis of dorsal root ganglion (DRG) cells as a prominent feature of diabetic polyneuropathy (DPN), although this has been controversial. Here, we examined subacute (4-month) type 1 diabetic BB/Wor rats with respect to sensory nerve functions, DRG and sural nerve morphometry, pro- and antiapoptotic proteins, and the expression of neurotrophic factors and their receptors. Sensory nerve conduction velocity was reduced by 13% and was accompanied by significant hyperalgesia. The numbers of DRG neurons including substance P-and calcitonin gene-related peptide-positive neurons were not altered, although they showed significant atrophy. Sural nerve morphometry showed decreased numbers of myelinated and unmyelinated fibers. Active caspase-3 and Bax expressions were increased, whereas antiapoptotic Bcl-xl and heat shock protein (HSP) 27 expressions in DRGs were increased. Nerve growth factor (NGF) contents in sciatic nerves and the expression of NGF receptor TrkA in DRGs were decreased. Immunohistochemistry showed increased numbers of active caspase-3-, HSP70-, and HSP27-positive neurons. Examinations of DRGs revealed no structural evidence of apoptosis but rather progressive hydropic degenerative changes. We conclude that apoptotic stress is induced in DRGs but is counterbalanced by survival elements in subacute type 1 diabetic BB/Wor rats and that distal nerve fiber loss reflects a dying-back phenomenon caused by impaired neurotrophic support.

Unmyelinated fiber sensory neuropathy differs in type 1 and type 2 diabetes

BACKGROUND

Neuropathic pain is common in diabetic patients. Degeneration of sensory C-fibers in peripheral nerve plays a prominent role in the generation of neuropathic pain. We examined degenerative changes of C-fibers in two rat models with type 1 and type 2 diabetes.

METHODS

Type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor-rats of 8 months duration with equal exposure to hyperglycemia were examined. Thermal hyperalgesia was monitored using an infrared thermal probe. C-fiber size, number, frequencies of denervated Schwann cells, regenerating C-fibers, type 2 axon/Schwann cell relationship and collagen pockets in the sural nerve were examined morphometrically. Neurotrophic receptor expression was examined by Western blotting. Neurotrophins and neuropeptides were examined by ELISA.

RESULTS

Type 1 rats showed increased thermal hyperalgesia followed by a decrease. Hyperalgesia in type 2 rats showed a slower progression. These findings were associated with a 50% (p < 0.001) loss of C-fibers, increased frequencies of denervated Schwann cells (p < 0.001), regenerating fibers (p < 0.001), collagen pockets (p < 0.001) and type 2 axon/Schwann cell relationship (p < 0.001) in type 1, but not in type 2 rats. Expression of insulin receptor, IGF-1R, TrkA and C was decreased in BB/Wor rats, whereas BBZDR/Wor rats showed milder or no deficits. NGF and NT-3 in sciatic nerve and substance P and calcitonin gene-related peptide in dorsal root ganglia were decreased in type 1, but not in type 2 rats.

CONCLUSION

The more severe molecular, functional and morphometric abnormalities of nociceptive C-fibers in type 1 insulinopenic rats compared to type 2 hyperinsulinemic rats suggest that impaired insulin action may play a more important pathogenetic role than hyperglycemia per se.

The effect of C-peptide on cognitive dysfunction and hippocampal apoptosis in type 1 diabetic rats

Primary diabetic encephalopathy is a recently recognized late complication of diabetes resulting in a progressive decline in cognitive faculties. In the spontaneously type 1 diabetic BB/Wor rat, we recently demonstrated that cognitive impairment was associated with hippocampal apoptotic neuronal loss. Here, we demonstrate that replacement of proinsulin C-peptide in this insulinopenic model significantly prevented spatial learning and memory deficits and hippocampal neuronal loss. C-peptide replacement prevented oxidative stress-, endoplasmic reticulum-, nerve growth factor receptor p75-, and poly(ADP-ribose) polymerase-related apoptotic activities. It partially ameliorated apoptotic stresses mediated via impaired insulin and IGF activities. These findings were associated with the prevention of increased expression of Bax and active caspase 3 and the frequency of caspase 3-positive neurons. The results show that several partially interrelated apoptotic mechanisms are involved in primary encephalopathy and suggest that impaired insulinomimetic action by C-peptide plays a prominent role in cognitive dysfunction and hippocampal apoptosis in type 1 diabetes. Although these abnormalities were not fully prevented by C-peptide replacement, the findings suggest that this regime will substantially prevent cognitive decline in the type 1 diabetic population.