Prevention of sensory disorders in diabetic Sprague-Dawley rats by aldose reductase inhibition or treatment with ciliary neurotrophic factor

Nociceptive neurons differentially express fast and slow T-type Ca²⁺ currents in different types of diabetic neuropathy

T-type Ca²⁺ channels are known as important participants of nociception and their remodeling contributes to diabetes-induced alterations of pain sensation. In this work we have established that about 30% of rat nonpeptidergic thermal C-type nociceptive (NTCN) neurons of segments L4-L6 express a slow T-type Ca²⁺ current (T-current) while a fast T-current is expressed in the other 70% of these neurons. Streptozotocin-induced diabetes in young rats resulted in thermal hyperalgesia, hypoalgesia, or normalgesia 5-6 weeks after the induction. Our results show that NTCN neurons obtained from hyperalgesic animals do not express the slow T-current. Meanwhile, the fraction of neurons expressing the slow T-current did not significantly change in the hypo- and normalgesic diabetic groups. Moreover, the peak current density of fast T-current was significantly increased only in the neurons of hyperalgesic group. In contrast, the peak current density of slow T-current was significantly decreased in the hypo- and normalgesic groups. Experimental diabetes also resulted in a depolarizing shift of steady-state inactivation of fast T-current in the hyperalgesic group and slow T-current in the hypo- and normalgesic groups. We suggest that the observed changes may contribute to expression of different types of peripheral diabetic neuropathy occurring during the development of diabetes mellitus.

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.

Animal models of diabetes-induced neuropathic pain

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

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.

Specific functioning of Cav3.2 T-type calcium and TRPV1 channels under different types of STZ-diabetic neuropathy

Streptozotocin (STZ)-induced type 1 diabetes in rats leads to the development of peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia at early stages (4th week) followed by hypoalgesia after 8weeks of diabetes development. Here we found that 6-7 week STZ-diabetic rats developed either thermal hyper- (18%), hypo- (25%) or normalgesic (57%) types of PDN. These developmentally similar diabetic rats were studied in order to analyze mechanisms potentially underlying different thermal nociception. The proportion of IB4-positive capsaicin-sensitive small DRG neurons, strongly involved in thermal nociception, was not altered under different types of PDN implying differential changes at cellular and molecular level. We further focused on properties of T-type calcium and TRPV1 channels, which are known to be involved in Ca(2+) signaling and pathological nociception. Indeed, TRPV1-mediated signaling in these neurons was downregulated under hypo- and normalgesia and upregulated under hyperalgesia. A complex interplay between diabetes-induced changes in functional expression of Cav3.2 T-type calcium channels and depolarizing shift of their steady-state inactivation resulted in upregulation of these channels under hyper- and normalgesia and their downregulation under hypoalgesia. As a result, T-type window current was increased by several times under hyperalgesia partially underlying the increased resting [Ca(2+)]i observed in the hyperalgesic rats. At the same time Cav3.2-dependent Ca(2+) signaling was upregulated in all types of PDN. These findings indicate that alterations in functioning of Cav3.2 T-type and TRPV1 channels, specific for each type of PDN, may underlie the variety of pain syndromes induced by type 1 diabetes.

SNEDDS curcumin formulation leads to enhanced protection from pain and functional deficits associated with diabetic neuropathy: an insight into its mechanism for neuroprotection

Curcumin has shown to be effective against various diabetes related complications. However major limitation with curcumin is its low bioavailability. In this study we formulated and characterized self nano emulsifying drug delivery system (SNEDDS) curcumin formulation to enhance its bioavailability and then evaluated its efficacy in experimental diabetic neuropathy. Bioavailability studies were performed in male Sprague Dawley rats. Further to evaluate the efficacy of formulation in diabetic neuropathy various parameters like nerve function and sensorimotor perception were assessed along with study of inflammatory proteins (NF-κB, IKK-β, COX-2, iNOS, TNF-α and IL-6). Nanotechnology based formulation resulted in prolonged plasma exposure and bioavailability. SNEDDS curcumin provided better results against functional, behavioural and biochemical deficits in experimental diabetic neuropathy, when compared with naive curcumin. Further western blot analysis confirmed the greater neuroprotective action of SNEDDS curcumin. SNEDDS curcumin formulation due to higher bioavailability was found to afford enhanced protection in diabetic neuropathy.

FROM THE CLINICAL EDITOR

In this study the authors formulated and characterized a self-emulsifying drug delivery system for formulation to enhance curcumin bioavailability in experimental diabetic neuropathy. Enhanced efficacy was demonstrated in a rat model.

Hypertension-induced peripheral neuropathy and the combined effects of hypertension and diabetes on nerve structure and function in rats

Diabetic neuropathy includes damage to neurons, Schwann cells and blood vessels. Rodent models of diabetes do not adequately replicate all pathological features of diabetic neuropathy, particularly Schwann cell damage. We, therefore, tested the hypothesis that combining hypertension, a risk factor for neuropathy in diabetic patients, with insulin-deficient diabetes produces a more pertinent model of peripheral neuropathy. Behavioral, physiological and structural indices of neuropathy were measured for up to 6 months in spontaneously hypertensive and age-matched normotensive rats with or without concurrent streptozotocin-induced diabetes. Hypertensive rats developed nerve ischemia, thermal hyperalgesia, nerve conduction slowing and axonal atrophy. Thinly myelinated fibers with supernumerary Schwann cells indicative of cycles of demyelination and remyelination were also identified along with reduced nerve levels of myelin basic protein. Similar disorders were noted in streptozotocin-diabetic rats, except that thinly myelinated fibers were not observed and expression of myelin basic protein was normal. Superimposing diabetes on hypertension compounded disorders of nerve blood flow, conduction slowing and axonal atrophy and increased the incidence of thinly myelinated fibers. Rats with combined insulinopenia, hyperglycemia and hypertension provide a model for diabetic neuropathy that offers an opportunity to study mechanisms of Schwann cell pathology and suggests that hypertension may contribute to the etiology of diabetic neuropathy.

Ciliary neurotrophic factor activates NF-κB to enhance mitochondrial bioenergetics and prevent neuropathy in sensory neurons of streptozotocin-induced diabetic rodents

Diabetes causes mitochondrial dysfunction in sensory neurons that may contribute to peripheral neuropathy. Ciliary neurotrophic factor (CNTF) promotes sensory neuron survival and axon regeneration and prevents axonal dwindling, nerve conduction deficits and thermal hypoalgesia in diabetic rats. In this study, we tested the hypothesis that CNTF protects sensory neuron function during diabetes through normalization of impaired mitochondrial bioenergetics. In addition, we investigated whether the NF-κB signal transduction pathway was mobilized by CNTF. Neurite outgrowth of sensory neurons derived from streptozotocin (STZ)-induced diabetic rats was reduced compared to neurons from control rats and exposure to CNTF for 24 h enhanced neurite outgrowth. CNTF also activated NF-κB, as assessed by Western blotting for the NF-κB p50 subunit and reporter assays for NF-κB promoter activity. Conversely, blockade of NF-κB signaling using SN50 peptide inhibited CNTF-mediated neurite outgrowth. Studies in mice with STZ-induced diabetes demonstrated that systemic therapy with CNTF prevented functional indices of peripheral neuropathy along with deficiencies in dorsal root ganglion (DRG) NF-κB p50 expression and DNA binding activity. DRG neurons derived from STZ-diabetic mice also exhibited deficiencies in maximal oxygen consumption rate and associated spare respiratory capacity that were corrected by exposure to CNTF for 24 h in an NF-κB-dependent manner. We propose that the ability of CNTF to enhance axon regeneration and protect peripheral nerve from structural and functional indices of diabetic peripheral neuropathy is associated with targeting of mitochondrial function, in part via NF-κB activation, and improvement of cellular bioenergetics.

Transplantation of bone marrow-derived mononuclear cells improves mechanical hyperalgesia, cold allodynia and nerve function in diabetic neuropathy

Relief from painful diabetic neuropathy is an important clinical issue. We have previously shown that the transplantation of cultured endothelial progenitor cells or mesenchymal stem cells ameliorated diabetic neuropathy in rats. In this study, we investigated whether transplantation of freshly isolated bone marrow-derived mononuclear cells (BM-MNCs) alleviates neuropathic pain in the early stage of streptozotocin-induced diabetic rats. Two weeks after STZ injection, BM-MNCs or vehicle saline were injected into the unilateral hind limb muscles. Mechanical hyperalgesia and cold allodynia in SD rats were measured as the number of foot withdrawals to von Frey hair stimulation and acetone application, respectively. Two weeks after the BM-MNC transplantation, sciatic motor nerve conduction velocity (MNCV), sensory nerve conduction velocity (SNCV), sciatic nerve blood flow (SNBF), mRNA expressions and histology were assessed. The BM-MNC transplantation significantly ameliorated mechanical hyperalgesia and cold allodynia in the BM-MNC-injected side. Furthermore, the slowed MNCV/SNCV and decreased SNBF in diabetic rats were improved in the BM-MNC-injected side. BM-MNC transplantation improved the decreased mRNA expression of NT-3 and number of microvessels in the hind limb muscles. There was no distinct effect of BM-MNC transplantation on the intraepidermal nerve fiber density. These results suggest that autologous transplantation of BM-MNCs could be a novel strategy for the treatment of painful diabetic neuropathy.

Animal models and biomarkers of neuropathy in diabetic rodents

Diabetic neuropathy (DN) is a multifactor complication of diabetes. It is a late finding in type 1 diabetes, but can be an early finding in type 2 diabetes. The cause of DN is still unclear and, like other complications of diabetes, it may be the result of various pathological conditions. Animal models and biomarkers of DN have been extensively used in neuropathic research. The most useful model of DN should exhibit the key feature present in human pathology. Diabetic rodents show behavioral, functional, structural and molecular biomarkers and they are widely used as models to investigate the etiology of DN as well as to screen the efficacy of the potential therapeutic interventions. We have reviewed the different animal models and biomarkers of neuropathy in diabetic rodents of either type 1 or type 2 diabetes.

Impaired sensory nerve function and axon morphology in mice with diabetic neuropathy

Diabetes is the most prevalent metabolic disorder in the United States, and between 50% and 70% of diabetic patients suffer from diabetes-induced neuropathy. Yet our current knowledge of the functional changes in sensory nerves and their distal terminals caused by diabetes is limited. Here, we set out to investigate the functional and morphological consequences of diabetes on specific subtypes of cutaneous sensory nerves in mice. Diabetes was induced in C57Bl/6 mice by a single intraperitoneal injection of streptozotocin. After 6-8 wk, mice were characterized for behavioral sensitivity to mechanical and heat stimuli followed by analysis of sensory function using teased nerve fiber recordings and histological assessment of nerve fiber morphology. Diabetes produced severe functional impairment of C-fibers and rapidly adapting Aβ-fibers, leading to behavioral hyposensitivity to both mechanical and heat stimuli. Electron microscopy images showed that diabetic nerves have axoplasm with more concentrated organelles and frequent axon-myelin separations compared with control nerves. These changes were restricted to the distal nerve segments nearing their innervation territory. Furthermore, the relative proportion of Aβ-fibers was reduced in diabetic skin-nerve preparations compared with nondiabetic control mice. These data identify significant deficits in sensory nerve terminal function that are associated with distal fiber loss, morphological damage, and behavioral hyposensitivity in diabetic C57Bl/6 mice. These findings suggest that diabetes damages sensory nerves, leading to functional deficits in sensory signaling that underlie the loss of tactile acuity and pain sensation associated with insensate diabetic neuropathy.

Emerging drugs for diabetic neuropathy

IMPORTANCE OF THE FIELD

Diabetic neuropathy (DN) is a very common and disabling diabetes-related complication. DN is associated with significant morbidity and mortality. Diabetic peripheral neuropathy (DPN) can be painful in the earlier stages of the disease before becoming painless. Most of the currently available therapies are symptomatic (focusing on pain relief) rather than disease-modifying. With the exception of good glycemic control, there is currently no effective treatment to slow the progression of or reverse DPN.

AREAS COVERED IN THIS REVIEW

In this article, we review the epidemiology, pathogenesis, currently available and future treatments for DPN, and the potential development issues/challenges related to such new therapies. Literature search was performed using PubMed, Medline and Pharmaprojects from 1950 onwards. Search terms include a combination of terms such as diabetic neuropathy, pathogenesis, pathophysiology, mechanisms, treatment, therapy, oxidative/nitrosative stress, anti-oxidants, serotonin, nitrotyrosine, protein kinase C, aldose reductase, sodium channels, taurine, lipoic acid and poly (ADP-ribose) polymerase.

WHAT THE READER WILL GAIN

The reader will gain an overview of the epidemiology, clinical features and risk factors of DN. In addition, the reader will have a better understanding of the mechanisms that underpin the development of DPN and their relationships to the current and future therapies. The reader will also develop an insight into the limitations of the current approach to DPN treatment and the potential avenues for future research.

TAKE HOME MESSAGE

DN is a very common and disabling complication that currently has no effective treatments other than diabetes control. The pathogenesis of DPN is complex and multi-factorial. Several disease-modifying and symptomatic treatments are currently under development. Oxidative and nitrosative stress have been identified as key pathogenic factors in the development of DPN and new treatments target these pathways and/or their downstream consequences. Gene therapy and growth factors have also emerged as potential new therapies that target particular cellular compartments as opposed to being delivered systemically. The recognition of the difficulty in reversing established DN has focused efforts on slowing its progression.

Diet-induced obesity in Sprague-Dawley rats causes microvascular and neural dysfunction

BACKGROUND

The objective of this study was to determine the effect of diet-induced obesity (DIO) on microvascular and neural function.

METHODS

Rats were fed a standard or high fat diet for up to 32 weeks. The following measurements were carried out: vasodilation in epineurial arterioles using videomicroscopy, endoneurial blood flow using hydrogen clearance, nerve conduction velocity using electrical stimulation, size-frequency distribution of myelinated fibres of the sciatic nerve, intraepidermal nerve fibre density using confocal microscopy and thermal nociception using the Hargreaves method.

RESULTS

Rats fed a high fat diet for 32 weeks developed sensory neuropathy, as indicated by slowing of sensory nerve conduction velocity and thermal hypoalgesia. Motor nerve conduction velocity and endoneurial blood flow were not impaired. Mean axonal diameter of myelinated fibres of the sciatic nerve was unchanged in high fat-fed rats compared with that in control. Intraepidermal nerve fibre density was significantly reduced in high fat-fed rats. Vascular relaxation to acetylcholine and calcitonin gene-related peptide was decreased and expression of neutral endopeptidase (NEP) increased in epineurial arterioles of rats fed a high fat diet. In contrast, insulin-mediated vascular relaxation was increased in epineurial arterioles. NEP activity was significantly increased in the skin of the hindpaw. Markers of oxidative stress were increased in the aorta and serum of high fat-fed rats but not in epineurial arterioles.

CONCLUSION

Chronic obesity causes microvascular and neural dysfunction. This is associated with increased expression of NEP but not oxidative stress in epineurial arterioles. NEP degrades vasoactive peptides, which may explain the decrease in microvascular function.

Mitochondrial respiratory chain dysfunction in dorsal root ganglia of streptozotocin-induced diabetic rats and its correction by insulin treatment

OBJECTIVE

Impairments in mitochondrial physiology may play a role in diabetic sensory neuropathy. We tested the hypothesis that mitochondrial dysfunction in sensory neurons is due to abnormal mitochondrial respiratory function.

RESEARCH DESIGN AND METHODS

Rates of oxygen consumption were measured in mitochondria from dorsal root ganglia (DRG) of 12- to- 22-week streptozotocin (STZ)-induced diabetic rats, diabetic rats treated with insulin, and age-matched controls. Activities and expression of components of mitochondrial complexes and reactive oxygen species (ROS) were analyzed.

RESULTS

Rates of coupled respiration with pyruvate + malate (P + M) and with ascorbate + TMPD (Asc + TMPD) in DRG were unchanged after 12 weeks of diabetes. By 22 weeks of diabetes, respiration with P + M was significantly decreased by 31-44% and with Asc + TMPD by 29-39% compared with control. Attenuated mitochondrial respiratory activity of STZ-diabetic rats was significantly improved by insulin that did not correct other indices of diabetes. Activities of mitochondrial complexes I and IV and the Krebs cycle enzyme, citrate synthase, were decreased in mitochondria from DRG of 22-week STZ-diabetic rats compared with control. ROS levels in perikarya of DRG neurons were not altered by diabetes, but ROS generation from mitochondria treated with antimycin A was diminished compared with control. Reduced mitochondrial respiratory function was associated with downregulation of expression of mitochondrial proteins.

CONCLUSIONS

Mitochondrial dysfunction in sensory neurons from type 1 diabetic rats is associated with impaired rates of respiratory activity and occurs without a significant rise in perikaryal ROS.

Diabetic thermal hyperalgesia: role of TRPV1 and CB1 receptors of periaqueductal gray

Hyperalgesia is one of the debilitating complications of diabetes. This condition might be caused by defects in central or peripheral processing of pain signals. In the present study we aim to see if diabetic hyperalgesia is related to changes in Transient Receptor Potential Vanilloid 1 (TRPV1) or Cannabinoid CB1 receptors of periaqueductal gray (PAG). Activation of glutamatergic projecting neurons in midbrain ventrolateral periaqueductal gray (VL-PAG) induces antinociception. Agonists of TRPV1 in VL-PAG increase firing of these glutamatergic neurons. CB1 receptor agonists also cause antinociception by decreasing Gamma Aminobutyric Acid (GABA) release in PAG and disinhibiting these glutamatergic neurons. In the present study antinociceptive effect of intra VL-PAG microinjections of CB1 and TRPV1 agonists [WIN55,212-2 (WIN) and capsaicin respectively] were compared in diabetic vs. non-diabetic rats, meanwhile mRNA expression of these receptors in PAG of diabetic and non-diabetic rats were evaluated by real time polymerase chain reaction (real time PCR) assay. Our results showed an attenuation of capsaicin antinociceptive effect (P<0.05) and TRPV1 receptor expression (P=0.023) but an increase in WIN antinociceptive effect (P<0.05) and CB1 receptor expression (P<0.001) in PAG of diabetic vs. non-diabetic rats. It is concluded that down-regulation of TRPV1 receptors in PAG is responsible for reduced antinociceptive effect of TRPV1 agonist. This finding may be an underlying cause of diabetic hyperalgesia. Up-regulation of CB1 receptors might be a compensatory mechanism but the precise elucidation of the effects of CB1 changes on disinhibition needs further studies.

URB597, an inhibitor of fatty acid amide hydrolase, reduces hyperalgesia in diabetic rats

Diabetic rats display increased pain responses after injection of formalin into the paw or thermal stimulation of the tail, suggesting the presence of hyperalgesia. In this study, we investigated the efficacy of URB597 (0.1, 0.3, and 0.5 mg/kg, i.p.), an inhibitor of endocannabinoids metabolism, on 2 models of experimental hyperalgesia in streptozotocin (STZ)-induced diabetic rats. Animals were divided into control, URB597-treated control (0.1, 0.3, and 0.5 mg/kg), diabetic, and URB597-treated diabetic (0.1, 0.3, and 0.5 mg/kg) groups. Formalin and tail-flick tests were performed 4 and 8 weeks after the onset of hyperglycemia, respectively. Diabetes caused significant hyperalgesia during these tests. URB597 (0.3 and 0.5 mg/kg) reversed chemical and thermal hyperalgesia in diabetic rats. Administration of URB597 at a dose of 0.1 mg/kg did not alter pain-related behaviors in control and diabetic groups compared with those of the respective control groups. URB597 treatment did not affect body weight or plasma glucose level of treated animals compared with nontreated animals. This study shows that increasing endocannabinoid neurotransmission with URB597 displays efficacy in chemical and thermal models of diabetic hyperalgesia. It also suggests that URB597 is a promising tool for treatment of painful diabetic neuropathy.

N-acetylcysteine inhibits hyperglycemia-induced oxidative stress and apoptosis markers in diabetic neuropathy

Several studies have indicated the involvement of oxidative stress in the development of diabetic neuropathy. In the present study, we have targeted oxidative stress mediated nerve damage in diabetic neuropathy using N-acetyl-l-cysteine (NAC), a potent antioxidant. After 8 weeks, streptozotocin-induced diabetic rats developed neuropathy which was evident from decreased tail-flick latency (thermal hyperalgesia). This was accompanied by decreased motor coordination as assessed by performance on rota-rod treadmill. Na(+) K(+) ATPase, a biochemical marker of development of diabetic neuropathy, was significantly inhibited in sciatic nerve of diabetic animals. NAC treatment at a daily dose between 1.4 and 1.5 g/kg body weight to diabetic animals for 7 weeks in drinking water ameliorated hyperalgesia, improved motor coordination and reversed reduction in Na(+) K(+) ATPase activity. There was an increase in lipid peroxidation in sciatic nerve of diabetic animals along with decrease in phospholipid levels, while NAC treatment attenuated lipid peroxidation and restored phospholipids to control levels. This was associated with decrease in glutathione and protein thiols. The activities of antioxidant enzymes; superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase and glutathione-S-transferase were reduced in sciatic nerve of diabetic animals. Cytochrome c release and active caspase 3 were markedly increased in nerve from diabetic animals suggesting activation of apoptotic pathway. NAC treatment significantly ameliorated decrease in antioxidant defense and prevented cytochrome c release and caspase 3 activation. Electron microscopy revealed demyelination, Wallerian degeneration and onion-bulb formation in sciatic nerve of diabetic rats. NAC on the other hand was able to reverse structural deficits observed in sciatic nerve of diabetic rats. Our results clearly demonstrate protective effect of NAC is mediated through attenuation of oxidative stress and apoptosis, and suggest therapeutic potential of NAC in attenuation of diabetic neuropathy.

Diabetic painful and insensate neuropathy: pathogenesis and potential treatments

Advanced peripheral diabetic neuropathy (PDN) is associated with elevated vibration and thermal perception thresholds that progress to sensory loss and degeneration of all fiber types in peripheral nerve. A considerable proportion of diabetic patients also describe abnormal sensations such as paresthesias, allodynia, hyperalgesia, and spontaneous pain. One or several manifestations of abnormal sensation and pain are described in all the diabetic rat and mouse models studied so far (i.e., streptozotocin-diabetic rats and mice, type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor rats, Zucker diabetic fatty rats, and nonobese diabetic, Akita, leptin- and leptin-receptor-deficient, and high-fat diet-fed mice). Such manifestations are 1) thermal hyperalgesia, an equivalent of a clinical phenomenon described in early PDN; 2) thermal hypoalgesia, typically present in advanced PDN; 3) mechanical hyperalgesia, an equivalent of pain on pressure in early PDN; 4) mechanical hypoalgesia, an equivalent to the loss of sensitivity to mechanical noxious stimuli in advanced PDN; 5) tactile allodynia, a painful perception of a light touch; and 5) formalin-induced hyperalgesia. Rats with short-term diabetes develop painful neuropathy, whereas those with longer-term diabetes and diabetic mice typically display manifestations of both painful and insensate neuropathy, or insensate neuropathy only. Animal studies using pharmacological and genetic approaches revealed important roles of increased aldose reductase, protein kinase C, and poly(ADP-ribose) polymerase activities, advanced glycation end-products and their receptors, oxidative-nitrosative stress, growth factor imbalances, and C-peptide deficiency in both painful and insensate neuropathy. This review describes recent achievements in studying the pathogenesis of diabetic neuropathic pain and sensory disorders in diabetic animal models and developing potential pathogenetic treatments.

Intranasal insulin ameliorates experimental diabetic neuropathy

OBJECTIVE

We hypothesized that intranasal insulin (I-I) delivery targets the nervous system while avoiding potential adverse systemic effects when compared with subcutaneous insulin (S-I) for experimental streptozotocin-induced diabetic peripheral neuropathy (DPN).

RESEARCH DESIGN AND METHODS

I-I or S-I at 0.87 IU daily or placebo were delivered in separate cohorts of diabetic and nondiabetic CD1 mice during 8 months of diabetes. Radiolabeled insulin detection was used to compare delivery and biodistribution for I-I and S-I. Biweekly behavioral testing and monthly electrophysiological and quantitative studies assessed progression of DPN. At and before end point, morphometric analysis of DRG, peripheral nerve, distal epidermal innervation, and specific molecular markers were evaluated.

RESULTS

Radiolabeled I-I resulted in more rapid and concentrated delivery to the spinal cord and DRG with less systemic insulin exposure. When compared with S-I or intranasal placebo, I-I reduced overall mouse mortality and sensory loss while improving neuropathic pain and electrophysiological/morphological abnormalities in diabetic mice. I-I restored mRNA and protein levels of phosphoinositide 3-kinase/Akt, cyclic AMP response element-binding protein, and glycogen synthase kinase 3beta to near normal levels within diabetic DRGs.

CONCLUSIONS

I-I slows the progression of experimental DPN in streptozotocin mice, avoids adverse effects associated with S-I treatment, and prolongs lifespan when compared with S-I. I-I may be a promising approach for the treatment of DPN.

CoMFA and CoMSIA analysis of 2,4-thiazolidinediones derivatives as aldose reductase inhibitors

Diabetes remains a life-threatening disease. The clinical profile of diabetic subjects is often worsened by the presence of several long-term complications, for example neuropathy, nephropathy, retinopathy, and cataract. Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were performed on a series of 2,4-thiazolidinediones derivatives as aldose reductase (ALR2) inhibitors. Molecular ligand superimposition on a template structure was finished by the database alignment method. The 3D-QSAR models resulted from 44 molecules gave q (2) values of 0.773 and 0.817, r (2) values of 0.981 and 0.979 for CoMFA and CoMSIA, respectively. The contour maps from the models indicated that a large volume group next to the R-substituent will increase the ALR2 inhibitory activity. In fact, adding a -CH(2)COOH substituent at the R-position would generate a new compound with higher predicted activity.

Gene therapy for the treatment of diabetic neuropathy

Neuropathy is a common, untreatable complication of type 1 and type 2 diabetes. In animal models peptide neurotrophic factors can be used to protect against the development of neuropathy, but the combination of short half-life and off-target effects of these potent pleiotropic peptides has limited translation to human therapy. Gene transfer is a promising strategy that may circumvent these limitations. In this article, we review the basic methods of gene transfer and the -preclinical data in rodent models that support the use of this approach in the treatment of diabetic neuropathy. The path to clinical applications and potential pitfalls in developing gene therapy for the treatment of diabetic neuropathy are considered.

Allodynia and hyperalgesia in diabetic rats are mediated by GABA and depletion of spinal potassium-chloride co-transporters

Diabetic rats show behavioral indices of painful neuropathy that may model the human condition. Hyperalgesia during the formalin test in diabetic rats is accompanied by the apparently paradoxical decrease in spinal release of excitatory neurotransmitters and increase in the inhibitory neurotransmitter GABA. Decreased expression of the potassium-chloride co-transporter, KCC2, in the spinal cord promotes excitatory properties of GABA. We therefore measured spinal KCC2 expression and explored the role of the GABA(A) receptor in rats with painful diabetic neuropathy. KCC2 protein levels were significantly reduced in the spinal cord of diabetic rats, while levels of NKCC1 and the GABA(A) receptor were unchanged. Spinal delivery of the GABA(A) receptor antagonist bicuculline reduced formalin-evoked flinching in diabetic rats and also dose-dependently alleviated tactile allodynia. GABA(A) receptor-mediated rate-dependent depression of the spinal H reflex was absent in the spinal cord of diabetic rats. Control rats treated with the KCC2 blocker DIOA, mimicked diabetes by showing increased formalin-evoked flinching and diminished rate- dependent depression. The ability of bicuculline to alleviate allodynia and formalin-evoked hyperalgesia in diabetic rats is consistent with a reversal of the properties of GABA predicted by reduced spinal KCC2 and suggests that reduced KCC2 expression and increased GABA release contribute to spinally mediated hyperalgesia in diabetes.

Diabetes and the peripheral nerve

Diabetes-induced damage to peripheral nerve culminates in development of peripheral diabetic neuropathy (PDN), one of the most devastating complications of diabetes mellitus and a leading cause of foot amputation. The pathogenesis of PDN occurs as a consequence of complex interactions among multiple hyperglycemia-initiated mechanisms, impaired insulin signaling, inflammation, hypertension, and disturbances of fatty acid and lipid metabolism. This review describes experimental new findings in animal and cell culture models as well as clinical data suggesting the importance of 1) previously established hyperglycemia-initiated mechanisms such as increased aldose reductase activity, non-enzymatic glycation/glycooxidation, activation of protein kinase C, 2) oxidative-nitrosative stress and poly(ADP-ribose) polymerase activation; 3) mitogen-activated protein kinase and cyclooxygenase-2 activation, impaired Ca(++) homeostasis and signaling, and several other mechanisms, in PDN.

Transplantation of bone marrow-derived mesenchymal stem cells improves diabetic polyneuropathy in rats

OBJECTIVE

Mesenchymal stem cells (MSCs) have been reported to secrete various cytokines that exhibit angiogenic and neurosupportive effects. This study was conducted to investigate the effects of MSC transplantation on diabetic polyneuropathy (DPN) in rats.

RESEARCH DESIGN AND METHODS

MSCs were isolated from bone marrow of adult rats and transplanted into hind limb skeletal muscles of rats with an 8-week duration of streptozotocin (STZ)-induced diabetes or age-matched normal rats by unilateral intramuscular injection. Four weeks after transplantation, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) productions in transplanted sites, current perception threshold, nerve conduction velocity (NCV), sciatic nerve blood flow (SNBF), capillary number-to-muscle fiber ratio in soleus muscles, and sural nerve morphometry were evaluated.

RESULTS

VEGF and bFGF mRNA expression were significantly increased in MSC-injected thigh muscles of STZ-induced diabetic rats. Furthermore, colocalization of MSCs with VEGF and bFGF in the transplanted sites was confirmed. STZ-induced diabetic rats showed hypoalgesia, delayed NCV, decreased SNBF, and decreased capillary number-to-muscle fiber ratio in soleus muscles, which were all ameliorated by MSC transplantation. Sural nerve morphometry showed decreased axonal circularity in STZ-induced diabetic rats, which was normalized by MSC transplantation.

CONCLUSIONS

These results suggest that MSC transplantation could have therapeutic effects on DPN through paracrine actions of growth factors secreted by MSCs.

Sulfasalazine blocks the development of tactile allodynia in diabetic rats

OBJECTIVE

Diabetic neuropathy is manifested either by loss of nociception (painless syndrome) or by mechanical hyperalgesia and tactile allodynia (pain in response to nonpainful stimuli). While therapies with vasodilators or neurotrophins reverse some functional and metabolic abnormalities in diabetic nerves, they only partially ameliorate neuropathic pain. The reported link between nociception and targets of the anti-inflammatory drug sulfasalazine prompted us to investigate its effect on neuropathic pain in diabetes.

RESEARCH DESIGN AND METHODS

We examined the effects of sulfasalazine, salicylates, and the poly(ADP-ribose) polymerase-1 inhibitor PJ34 on altered nociception in streptozotocin-induced diabetic rats. We also evaluated the levels of sulfasalazine targets in sciatic nerves and dorsal root ganglia (DRG) of treated animals. Finally, we analyzed the development of tactile allodynia in diabetic mice lacking expression of the sulfasalazine target nuclear factor-kappaB (NF-kappaB) p50.

RESULTS

Sulfasalazine completely blocked the development of tactile allodynia in diabetic rats, whereas relatively minor effects were observed with other salicylates and PJ34. Along with the behavioral findings, sciatic nerves and DRG from sulfasalazine-treated diabetic rats displayed a decrease in NF-kappaB p50 expression compared with untreated diabetic animals. Importantly, the absence of tactile allodynia in diabetic NF-kappaB p50(-/-) mice supported a role for NF-kappaB in diabetic neuropathy. Sulfasalazine treatment also increased inosine levels in sciatic nerves of diabetic rats.

CONCLUSIONS

The complete inhibition of tactile allodynia in experimental diabetes by sulfasalazine may stem from its ability to regulate both NF-kappaB and inosine. Sulfasalazine might be useful in the treatment of nociceptive alterations in diabetic patients.

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.

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

Evidence that poly(ADP-ribose) polymerase (PARP) activation plays an important role in diabetic complications is emerging. This study evaluated the role of PARP in rat and mouse models of advanced diabetic neuropathy. The orally active PARP inhibitor 10-(4-methylpiperazin-1-ylmethyl)-2H-7-oxa-1,2-diaza-benzo[de]anthracen-3-one (GPI-15427; formulated as a mesilate salt, 30 mg kg(-1) day(-1) in the drinking water for 10 weeks after the first 2 weeks without treatment) at least partially prevented PARP activation in peripheral nerve and DRG neurons, as well as thermal hypoalgesia, mechanical hyperalgesia, tactile allodynia, exaggerated response to formalin, and, most importantly, intraepidermal nerve fiber degeneration in streptozotocin-diabetic rats. These findings are consistent with the lack of small sensory nerve fiber dysfunction in diabetic PARP -/- mice. Furthermore, whereas diabetic PARP +/+ mice displayed approximately 46% intraepidermal nerve fiber loss, diabetic PARP -/- mice retained completely normal intraepidermal nerve fiber density. In conclusion, PARP activation is an important contributor to intraepidermal nerve fiber degeneration and functional changes associated with advanced Type 1 diabetic neuropathy. The results support a rationale for the development of potent and low-toxicity PARP inhibitors and PARP inhibitor-containing combination therapies.

Influence of TRPV1 on diabetes-induced alterations in thermal pain sensitivity

A common complication associated with diabetes is painful or painless diabetic peripheral neuropathy (DPN). The mechanisms and determinants responsible for these peripheral neuropathies are poorly understood. Using both streptozotocin (STZ)-induced and transgene-mediated murine models of type 1 diabetes (T1D), we demonstrate that Transient Receptor Potential Vanilloid 1 (TRPV1) expression varies with the neuropathic phenotype. We have found that both STZ- and transgene-mediated T1D are associated with two distinct phases of thermal pain sensitivity that parallel changes in TRPV1 as determined by paw withdrawal latency (PWL). An early phase of hyperalgesia and a late phase of hypoalgesia are evident. TRPV1-mediated whole cell currents are larger and smaller in dorsal root ganglion (DRG) neurons collected from hyperalgesic and hypoalgesic mice. Resiniferatoxin (RTX) binding, a measure of TRPV1 expression is increased and decreased in DRG and paw skin of hyperalgesic and hypoalgesic mice, respectively. Immunohistochemical labeling of spinal cord lamina I and II, dorsal root ganglion (DRG), and paw skin from hyperalgesic and hypoalgesic mice reveal increased and decreased TRPV1 expression, respectively. A role for TRPV1 in thermal DPN is further suggested by the failure of STZ treatment to influence thermal nociception in TRPV1 deficient mice. These findings demonstrate that altered TRPV1 expression and function contribute to diabetes-induced changes in thermal perception.

Defects in tongue papillae and taste sensation indicate a problem with neurotrophic support in various neurological diseases

Neurotrophic support of developing neurons by neurotrophins is of critical importance in the development of fungiform papillae and taste buds. A number of neurological disorders show a decrease or increase in fungiform papillae or taste sensation. These can be grouped into disorders with reduced papillae (Machado-Joseph disease, Stüve-Wiedemann syndrome, familial dysautonomia, dystonia musculorum, and Behçet's disease) and those with taste defects only (Alzheimer's disease, Huntington's disease, hereditary sensory and autonomic neuropathy type IV, and diabetes mellitus). In addition, Parkinson's disease results in increased taste sensation. Here, we hypothesize that the main problem in these disorders is either not enough or too much neurotrophic support. Proneurotrophic drugs such as some antidepressants and aldose reductase inhibitors may prove useful in the treatment of these sensory and central nervous system disorders. Similarly, antineurotrophic drugs may also be useful in Parkinson's disease. Here we show that the protein involved in familial dysautonomia, IKAP, localizes to keratin filaments in HeLa cells, suggesting a role for the keratin cytoskeleton in neurotrophic support.

Pathogenesis of pain in peripheral diabetic neuropathy

Recent advances in understanding the pain associated with diabetic neuropathy are likely to provide significant mechanistic insights and offer better therapies. In clinical research, new tools for measuring neuropathic pain and validation of histologic and other biomarkers will provide the foundation for research advances, and new clinical trial designs will allow better discrimination of beneficial treatments and may reveal underlying pathogenic mechanisms. Ongoing refinement of relevant animal models and assays to more accurately reflect the clinical condition will improve evaluation of novel pharmacologic approaches while dissecting peripheral versus central effects of diabetes on pain pathways will provide a more complete picture of the pathophysiologic mechanisms. Such multidisciplinary work may soon allow physicians to offer improved therapeutic options to patients suffering this distressing condition.

Role of nitrosative stress in early neuropathy and vascular dysfunction in streptozotocin-diabetic rats

Evidence for important roles of the highly reactive oxidant peroxynitrite in diabetic complications is emerging. We evaluated the role of peroxynitrite in early peripheral neuropathy and vascular dysfunction in STZ-diabetic rats. In the first dose-finding study, control and STZ-diabetic rats were maintained with or without the potent peroxynitrite decomposition catalyst Fe(III)tetrakis-2-(N-triethylene glycol monomethyl ether) pyridyl porphyrin (FP15) at 3, 5, or 10 mg.kg(-1).day(-1) in the drinking water for 4 wk after an initial 2 wk without treatment for assessment of early neuropathy. In the second study with similar experimental design, control and STZ-diabetic rats were maintained with or without FP15, 5 mg.kg(-1).day(-1), for vascular studies. Rats with 6-wk duration of diabetes developed motor and sensory nerve conduction velocity deficits, mechanical hyperalgesia, and tactile allodynia in the absence of small sensory nerve fiber degeneration. They also had increased nitrotyrosine and poly(ADP-ribose) immunofluorescence in the sciatic nerve and dorsal root ganglia. All these variables were dose-dependently corrected by FP15, with minimal differences between the 5 and 10 mg.kg(-1).day(-1) doses. FP15, 5 mg.kg(-1).day(-1), also corrected endoneurial nutritive blood flow and nitrotyrosine, but not superoxide, fluorescence in aorta and epineurial arterioles. Diabetes-induced decreases in acetylcholine-mediated relaxation by epineurial arterioles and coronary and mesenteric arteries, as well as bradykinin-induced relaxation by coronary and mesenteric arteries, were alleviated by FP15 treatment. The findings reveal the important role of nitrosative stress in early neuropathy and vasculopathy and provide the rationale for further studies of peroxynitrite decomposition catalysts in long-term diabetic models.

C-fos expression at the spinal dorsal horn of streptozotocin-induced diabetic rats

BACKGROUND

Pain during diabetic neuropathy is associated with peripheral nerve damage but recent evidences suggest the occurrence of central effects. We used the activation of the c-fos protooncogene to study the activity of spinal dorsal horn neurons in streptozotocin (STZ)-induced diabetic rats in the absence of stimulation or in response to innocuous or noxious stimuli.

METHODS

Four weeks after saline or STZ (50 mg/kg) injection, rats were anaesthetized and either not further manipulated or submitted to innocuous (gentle touch), noxious mechanical (pinching) or noxious thermal (radiant heat) stimulation of the hindlimb skin. In each experimental situation, the numbers of Fos-immunoreactive (Fos-IR) neurons occurring in the superficial (laminae I-II) or deep (laminae III-V) dorsal horn were compared.

RESULTS

In the absence of stimulation, STZ-injected rats presented significantly higher numbers of Fos-IR neurons than controls, both in the superficial and deep dorsal horn (DDH). In comparison with the respective baseline levels, innocuous stimulation did not induce a significant increase in the numbers of Fos-IR neurons in controls or STZ-rats. Noxious mechanical and noxious thermal stimuli increased the numbers of Fos-IR neurons, both in control and STZ-rats, but in a more pronounced manner when diabetic rats were subjected to noxious mechanical stimulation.

CONCLUSIONS

The present study demonstrates that the responses of spinal cord neurons are strongly affected during diabetes. The higher baseline neuronal activity probably underlies the spontaneous pain detected during diabetes since the spinal dorsal horn is the major relay station in the ascending transmission of nociceptive input to the brain.

High-fat diet induced neuropathy of pre-diabetes and obesity: effects of "healthy" diet and aldose reductase inhibition

OBJECTIVE

Subjects with dietary obesity and pre-diabetes have an increased risk for developing both nerve conduction slowing and small sensory fiber neuropathy. Animal models of this type of neuropathy have not been described. This study evaluated neuropathic changes and their amenability to dietary and pharmacological interventions in mice fed a high-fat diet (HFD), a model of pre-diabetes and alimentary obesity.

RESEARCH DESIGN AND METHODS

Female C57BL6/J mice were fed normal diets or HFDs for 16 weeks.

RESULTS

HFD-fed mice developed obesity, increased plasma FFA and insulin concentrations, and impaired glucose tolerance. They also had motor and sensory nerve conduction deficits, tactile allodynia, and thermal hypoalgesia in the absence of intraepidermal nerve fiber loss or axonal atrophy. Despite the absence of overt hyperglycemia, the mice displayed augmented sorbitol pathway activity in the peripheral nerve, as well as 4-hydroxynonenal adduct nitrotyrosine and poly(ADP-ribose) accumulation and 12/15-lipoxygenase overexpression in peripheral nerve and dorsal root ganglion neurons. A 6-week feeding with normal chow after 16 weeks on HFD alleviated tactile allodynia and essentially corrected thermal hypoalgesia and sensory nerve conduction deficit without affecting motor nerve conduction slowing. Normal chow containing the aldose reductase inhibitor fidarestat (16 mg x kg(-1) x day (-1)) corrected all functional changes of HFD-induced neuropathy.

CONCLUSIONS

Similar to human subjects with pre-diabetes and obesity, HFD-fed mice develop peripheral nerve functional, but not structural, abnormalities and, therefore, are a suitable model for evaluating dietary and pharmacological approaches to halt progression and reverse diabetic neuropathy at the earliest stage of the disease.

A peroxynitrite decomposition catalyst counteracts sensory neuropathy in streptozotocin-diabetic mice

Whereas an important role of free radicals and oxidants in peripheral diabetic neuropathy is well established, the contribution of nitrosative stress and, in particular, of the highly reactive oxidant peroxynitrite, has not been properly explored. Our previous findings implicate peroxynitrite in diabetes-associated motor and sensory nerve conduction deficits and peripheral nerve energy deficiency and poly(ADP-ribose) polymerase activation associated with Type 1 diabetes. In this study the role of nitrosative stress in diabetic sensory neuropathy is evaluated. The peroxynitrite decomposition catalyst Fe(III) tetrakis-2-(N-triethylene glycol monomethyl ether)pyridyl porphyrin (FP15) was administered to control and streptozotocin (STZ)-diabetic mice at the dose of 5 mg kg(-1) day(-1) (FP15), for 3 weeks after initial 3 weeks without treatment. Mice with 6-week duration of diabetes developed clearly manifest thermal hypoalgesia (paw withdrawal, tail-flick, and hot plate tests), mechanical hypoalgesia (tail pressure Randall-Sellito test), tactile allodynia (flexible von Frey filament test), and approximately 38% loss of intraepidermal nerve fibers. They also had increased nitrotyrosine and poly(ADP-ribose) immunofluorescence in the sciatic nerve, grey matter of spinal cord, and dorsal root ganglion neurons. FP15 treatment was associated with alleviation of thermal and mechanical hypoalgesia. Tactile response threshold tended to increase in response to peroxynitrite decomposition catalyst treatment, but still remained approximately 59% lower compared with non-diabetic controls. Intraepidermal nerve fiber density was 25% higher in FP15-treated than in untreated diabetic rats, but the difference between two groups did not achieve statistical significance (p=0.054). Nitrotyrosine and poly(ADP-ribose) immunofluorescence in sciatic nerve, spinal cord, and dorsal root ganglion neurons of peroxynitrite decomposition catalyst-treated diabetic mice were markedly reduced. In conclusion, nitrosative stress plays an important role in sensory neuropathy associated with Type 1 diabetes. The findings provide rationale for further studies of peroxynitrite decomposition catalysts in a long-term diabetic model.

The kinin B1 receptor antagonist SSR240612 reverses tactile and cold allodynia in an experimental rat model of insulin resistance

BACKGROUND AND PURPOSE

Diabetes causes sensory polyneuropathy with associated pain in the form of tactile allodynia and thermal hyperalgesia which are often intractable and resistant to current therapy. This study tested the beneficial effects of the non-peptide and orally active kinin B(1) receptor antagonist SSR240612 against tactile and cold allodynia in a rat model of insulin resistance.

EXPERIMENTAL APPROACH

Rats were fed with 10% D-glucose for 12 weeks and effects of orally administered SSR240612 (0.3-30 mg kg(-1)) were determined on the development of tactile and cold allodynia. Possible interference of SSR240612 with vascular oxidative stress and pancreatic function was also addressed.

KEY RESULTS

Glucose-fed rats exhibited tactile and cold allodynia, increases in systolic blood pressure and higher plasma levels of insulin and glucose, at 12 weeks. SSR240612 blocked tactile and cold allodynia at 3 h (ID(50)=5.5 and 7.1 mg kg(-1), respectively) in glucose-fed rats but had no effect in control rats. The antagonist (10 mg kg(-1)) had no effect on plasma glucose and insulin, insulin resistance (HOMA index) and aortic superoxide anion production in glucose-fed rats.

CONCLUSIONS AND IMPLICATIONS

We provide the first evidence that the B(1) receptors are involved in allodynia in an experimental rat model of insulin resistance. Allodynia was alleviated by SSR240612 most likely through a direct inhibition of B(1) receptors affecting spinal cord and/or sensory nerve excitation. Thus, orally active non-peptide B(1) receptor antagonists should have clinical therapeutic potential in the treatment of sensory polyneuropathy.

Novel sites of aldose reductase immunolocalization in normal and streptozotocin-diabetic rats

Glucose metabolism by aldose reductase (AR) is implicated in the pathogenesis of many diabetic complications, including neuropathy. We have re-evaluated the distribution of AR in the sciatic nerve and dorsal root ganglion (DRG) of normal rats, expanded these observations to describe the location of AR in the spinal cord and footpad skin, and investigated whether diabetes alters the distribution of AR. In sciatic nerve, AR was restricted to cytoplasm of myelinated Schwann cells and endothelial cells of epineurial, but not endoneurial, blood vessels. AR immunoreactivity (IR) was present in satellite cells in the DRG. In skin, AR-IR was detected in vascular endothelial cells, Schwann cells of myelinated fibers, and axons of perivascular sympathetic nerves. AR was localized selectively to oligodendrocytes of the white matter of spinal cord. The distribution of AR-IR in sciatic nerve, DRG, skin, and spinal cord was not altered by up to 12 weeks of streptozotocin-induced diabetes. Identification of perineuronal satellite cells, oligodendrocytes, and perivascular sympathetic nerves as AR-expressing cells reveals them as cellular sites with the potential to contribute to diabetic neuropathy.

Early diabetic neuropathy: Triggers and mechanisms

Peripheral neuropathy, and specifically distal peripheral neuropathy (DPN), is one of the most frequent and troublesome complications of diabetes mellitus. It is the major reason for morbidity and mortality among diabetic patients. It is also frequently associated with debilitating pain. Unfortunately, our knowledge of the natural history and pathogenesis of this disease remains limited. For a long time hyperglycemia was viewed as a major, if not the sole factor, responsible for all symptomatic presentations of DPN. Multiple clinical observations and animal studies supported this view. The control of blood glucose as an obligatory step of therapy to delay or reverse DPN is no longer an arguable issue. However, while supporting evidence for the glycemic hypothesis has accumulated, multiple controversies accumulated as well. It is obvious now that DPN cannot be fully understood without considering factors besides hyperglycemia. Some symptoms of DPN may develop with little, if any, correlation with the glycemic status of a patient. It is also clear that identification of these putative non-glycemic mechanisms of DPN is of utmost importance for our understanding of failures with existing treatments and for the development of new approaches for diagnosis and therapy of DPN. In this work we will review the strengths and weaknesses of the glycemic hypothesis, focusing on clinical and animal data and on the pathogenesis of early stages and triggers of DPN other than hyperglycemia.

PROTECTIVE EFFECTS OF BENDAZAC LYSINE ON DIABETIC PERIPHERAL NEUROPATHY IN STREPTOZOTOCIN-INDUCED DIABETIC RATS

1. Diabetic neuropathy is a many faceted complication of both type I and II diabetes. The aim of the present study was to investigate the effects of bendazac lysine (BDL), an anticataract drug, on experimental diabetic peripheral neuropathy (DPN) in rats. 2. Diabetes was induced in rats by intraperitoneal injection of 75 mg/kg streptozotocin (STZ) dissolved in 0.1 mol/L citrate buffer (pH 4.4). Bendazac lysine was administered to rats at doses of 50, 100 and 200 mg/kg twice a day for 12 weeks. 3. Diabetic rats without treatment showed hypopraxia, polydipsia, polyuria, slow weight gain, cataract, increased tail-flick threshold temperature, decreased motor nerve conduction velocity (nd induced pathological morphological changes of myelinated nerve fibres. All these symptoms were ameliorated in diabetic rats treated with BDL. Bendazac lysine ameliorated the blood glucose concentration, glycosylated haemoglobin levels and insulin levels in the plasma of diabetic rats, reduced aldose reductase activity in erythrocytes and advanced glycation end-products in both nerves and serum and increase the activity of glutathione peroxidase in the nerves and Na(+)/K(+)-ATPase in the nerves and erythrocytes. 4. Bendazac lysine exerts its protective effects against the progression of diabetic peripheral neuropathy in STZ-diabetic rats through multiple mechanisms and is a potential drug for the prevention of deterioration in DPN.

The leptin-deficient (ob/ob) mouse: a new animal model of peripheral neuropathy of type 2 diabetes and obesity

Whereas functional, metabolic, neurotrophic, and morphological abnormalities of peripheral diabetic neuropathy (PDN) have been extensively explored in streptozotocin-induced diabetic rats and mice (models of type 1 diabetes), insufficient information is available on manifestations and pathogenetic mechanisms of PDN in type 2 diabetic models. The latter could constitute a problem for clinical trial design because the vast majority of subjects with diabetes have type 2 (non-insulin dependent) diabetes. This study was aimed at characterization of PDN in leptin-deficient (ob/ob) mice, a model of type 2 diabetes with relatively mild hyperglycemia and obesity. ob/ob mice ( approximately 11 weeks old) clearly developed manifest sciatic motor nerve conduction velocity (MNCV) and hind-limb digital sensory nerve conduction velocity (SNCV) deficits, thermal hypoalgesia, tactile allodynia, and a remarkable ( approximately 78%) loss of intraepidermal nerve fibers. They also had increased sorbitol pathway activity in the sciatic nerve and increased nitrotyrosine and poly(ADP-ribose) immunofluorescence in the sciatic nerve, spinal cord, and dorsal root ganglion (DRG). Aldose reductase inhibition with fidarestat (16 mg . kg(-1) . d(-1)), administered to ob/ob mice for 6 weeks starting from 5 weeks of age, was associated with preservation of normal MNCV and SNCV and alleviation of thermal hypoalgesia and intraepidermal nerve fiber loss but not tactile allodynia. Sciatic nerve nitrotyrosine immunofluorescence and the number of poly(ADP-ribose)-positive nuclei in sciatic nerve, spinal cord, and DRGs of fidarestat-treated ob/ob mice did not differ from those in nondiabetic controls. In conclusion, the leptin-deficient ob/ob mouse is a new animal model that develops both large motor and sensory fiber and small sensory fiber PDN and responds to pathogenetic treatment. The results support the role for increased aldose reductase activity in functional and structural changes of PDN in type 2 diabetes.