Vascular Impairment of Epineurial Arterioles of the Sciatic Nerve: Implications for Diabetic Peripheral Neuropathy

Role of TRPA1 in Painful Cold Hypersensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that plays a pivotal role in pain generation after exposure to irritant chemicals and is involved in the sensation of a wide variety of pathological pain. TRPA1 was first reported to be sensitive to noxious cold, but its intrinsic cold sensitivity still remains under debate. To address this issue, we focused on cold hypersensitivity induced by oxaliplatin, a platinum-based chemotherapeutic drug, as a peculiar adverse symptom of acute peripheral neuropathy. We and other groups have shown that oxaliplatin enhances TRPA1 sensitivity to its chemical agonists and reactive oxygen species (ROS). Our in vitro and animal model studies revealed that oxaliplatin, or its metabolite oxalate, inhibits hydroxylation of a proline residue within the N-terminus of human TRPA1 (hTRPA1) via inhibition of prolyl hydroxylase domain-containing protein (PHD), which induces TRPA1 sensitization to ROS. Although hTRPA1 is insensitive to cold, PHD inhibition endows hTRPA1 with cold sensitivity through sensing the small amount of ROS produced after exposure to cold. Hence, we propose that PHD inhibition can unveil the cold sensitivity of hTRPA1 by converting ROS signaling into cold sensitivity. Furthermore, in this review, we summarize the role of TRPA1 in painful cold hypersensitivity during peripheral vascular impairment.

New perspectives in diabetic neuropathy

Diabetes prevalence continues to climb with the aging population. Type 2 diabetes (T2D), which constitutes most cases, is metabolically acquired. Diabetic peripheral neuropathy (DPN), the most common microvascular complication, is length-dependent damage to peripheral nerves. DPN pathogenesis is complex, but, at its core, it can be viewed as a state of impaired metabolism and bioenergetics failure operating against the backdrop of long peripheral nerve axons supported by glia. This unique peripheral nerve anatomy and the injury consequent to T2D underpins the distal-to-proximal symptomatology of DPN. Earlier work focused on the impact of hyperglycemia on nerve damage and bioenergetics failure, but recent evidence additionally implicates contributions from obesity and dyslipidemia. This review will cover peripheral nerve anatomy, bioenergetics, and glia-axon interactions, building the framework for understanding how hyperglycemia and dyslipidemia induce bioenergetics failure in DPN. DPN and painful DPN still lack disease-modifying therapies, and research on novel mechanism-based approaches is also covered.

Diabetic Polyneuropathy: New Strategies to Target Sensory Neurons in Dorsal Root Ganglia

Diabetic polyneuropathy (DPN) is the most common type of diabetic neuropathy, rendering a slowly progressive, symmetrical, and length-dependent dying-back axonopathy with preferential sensory involvement. Although the pathogenesis of DPN is complex, this review emphasizes the concept that hyperglycemia and metabolic stressors directly target sensory neurons in the dorsal root ganglia (DRG), leading to distal axonal degeneration. In this context, we discuss the role for DRG-targeting gene delivery, specifically oligonucleotide therapeutics for DPN. Molecules including insulin, GLP-1, PTEN, HSP27, RAGE, CWC22, and DUSP1 that impact neurotrophic signal transduction (for example, phosphatidylinositol-3 kinase/phosphorylated protein kinase B [PI3/pAkt] signaling) and other cellular networks may promote regeneration. Regenerative strategies may be essential in maintaining axon integrity during ongoing degeneration in diabetes mellitus (DM). We discuss specific new findings that relate to sensory neuron function in DM associated with abnormal dynamics of nuclear bodies such as Cajal bodies and nuclear speckles in which mRNA transcription and post-transcriptional processing occur. Manipulating noncoding RNAs such as microRNA and long-noncoding RNA (specifically MALAT1) that regulate gene expression through post-transcriptional modification are interesting avenues to consider in supporting neurons during DM. Finally, we present therapeutic possibilities around the use of a novel DNA/RNA heteroduplex oligonucleotide that provides more efficient gene knockdown in DRG than the single-stranded antisense oligonucleotide.

Contrast-enhanced ultrasonography for evaluation of the blood perfusion of sciatic nerves in healthy dogs

Blood supply to the peripheral nerves is essential for fulfilling their structural and functional requirements. This prospective, experimental, exploratory study aimed to assess the feasibility of contrast-enhanced ultrasonography (CEUS) for evaluating blood perfusion of the sciatic nerve in normal dogs. Contrast-enhanced ultrasonography examinations were performed on the bilateral sciatic nerves after bolus injection of Sonazoid™ (0.015 mL/kg) in 12 healthy Beagles for 150 s. Then, qualitative assessment of the wash-in timing, degree and enhancement patterns, and quantitative measurement of the peak intensity and time to peak intensity were performed from the sciatic nerve. The results were compared to those obtained from the adductor muscle around the nerve and caudal gluteal artery. After contrast agent injection, the sciatic nerve was enhanced at approximately 13-14 s, immediately after wash-in of the caudal gluteal artery. The peak intensity of the sciatic nerve was significantly lower than that of the caudal gluteal artery and higher than that of the adductor muscle. The time to peak intensity was significantly slower than that of the caudal gluteal artery; but was not significantly different from that of the adductor muscle. There were no significant differences in the peak intensity and time to peak intensity between the left and right sciatic nerves. These results demonstrate the feasibility of CEUS to assess blood perfusion of the sciatic nerve in healthy dogs qualitatively and quantitatively. This result from healthy dogs could serve as a reference for further studies that evaluate the sciatic nerve under pathological conditions.

Lipids, Lipid-Lowering Therapy, and Neuropathy: A Narrative Review

Statins, or 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, are the mainstay of treatment for hypercholesterolemia as they effectively reduce LDL-C levels and risk of atherosclerotic cardiovascular disease. Apart from hyperglycemia, dyslipidemia and HDL dysfunction are known risk factors for neuropathy in people with obesity and diabetes. Although there are case reports of statin-induced neuropathy, ad hoc analyses of clinical trials and observational studies have shown that statins may improve peripheral neuropathy. However, large randomized controlled trials and meta-analyses of cardiovascular outcome trials with statins and other lipid-lowering drugs have not reported on neuropathy outcomes. Because neuropathy was not a prespecified outcome in major cardiovascular trials, one cannot conclude whether statins or other lipid-lowering therapies increase or decrease the risk of neuropathy. The aim of this review was to assess if statins have beneficial or detrimental effects on neuropathy and whether there is a need for large well-powered interventional studies using objective neuropathy end points.

Is Nerve Electrophysiology a Robust Primary Endpoint in Clinical Trials of Treatments for Diabetic Peripheral Neuropathy?

There is currently no FDA-approved disease-modifying therapy for diabetic peripheral neuropathy (DPN). Nerve conduction velocity (NCV) is an established primary endpoint of disease-modifying therapies in DPN and clinical trials have been powered with an assumed decline of 0.5 m/s/year. This paper sought to establish the time-dependent change in NCV associated with a placebo, compared to that observed in the active intervention group. A literature search identified twenty-one double-blind, randomised controlled trials in DPN of ≥1 year duration conducted between 1971 and 2021. We evaluated changes in neurophysiology, with a focus on peroneal motor and sural sensory NCV and amplitude in the placebo and treatment groups. There was significant variability in the change and direction of change (reduction/increase) in NCV in the placebo arm, as well as variability influenced by the anatomical site of neurophysiological measurement within a given clinical trial. A critical re-evaluation of efficacy trials should consider placebo-adjusted effects and present the placebo-subtracted change in NCV rather than assume a universal annual decline of 0.5 m/s/year. Importantly, endpoints such as corneal confocal microscopy (CCM) have demonstrated early nerve repair, whilst symptoms and NCV have not changed, and should thus be considered as a viable alternative.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Aldose Reductase and the Polyol Pathway in Schwann Cells: Old and New Problems

Aldose reductase (AR) is a member of the reduced nicotinamide adenosine dinucleotide phosphate (NADPH)-dependent aldo-keto reductase superfamily. It is also the rate-limiting enzyme of the polyol pathway, catalyzing the conversion of glucose to sorbitol, which is subsequently converted to fructose by sorbitol dehydrogenase. AR is highly expressed by Schwann cells in the peripheral nervous system (PNS). The excess glucose flux through AR of the polyol pathway under hyperglycemic conditions has been suggested to play a critical role in the development and progression of diabetic peripheral neuropathy (DPN). Despite the intensive basic and clinical studies over the past four decades, the significance of AR over-activation as the pathogenic mechanism of DPN remains to be elucidated. Moreover, the expected efficacy of some AR inhibitors in patients with DPN has been unsatisfactory, which prompted us to further investigate and review the understanding of the physiological and pathological roles of AR in the PNS. Particularly, the investigation of AR and the polyol pathway using immortalized Schwann cells established from normal and AR-deficient mice could shed light on the causal relationship between the metabolic abnormalities of Schwann cells and discordance of axon-Schwann cell interplay in DPN, and led to the development of better therapeutic strategies against DPN.

SP6616 as a Kv2.1 inhibitor efficiently ameliorates peripheral neuropathy in diabetic mice

BACKGROUND

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes severely afflicting the patients, while there is yet no effective medication against this disease. As Kv2.1 channel functions potently in regulating neurological disorders, the present work was to investigate the regulation of Kv2.1 channel against DPN-like pathology of DPN model mice by using selective Kv2.1 inhibitor SP6616 (ethyl 5-(3-ethoxy-4-methoxyphenyl)-2-(4-hydroxy-3-methoxybenzylidene)-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate) as a probe.

METHODS

STZ-induced type 1 diabetic mice with DPN (STZ mice) were defined at 12 weeks of age (4 weeks after STZ injection) through behavioral tests, and db/db (BKS Cg-m+/+Leprdb/J) type 2 diabetic mice with DPN (db/db mice) were at 18 weeks of age. SP6616 was administered daily via intraperitoneal injection for 4 weeks. The mechanisms underlying the amelioration of SP6616 on DPN-like pathology were investigated by RT-PCR, western blot and immunohistochemistry technical approaches against diabetic mice, and verified against the STZ mice with Kv2.1 knockdown in dorsal root ganglion (DRG) tissue by injection of adeno associated virus AAV9-Kv2.1-RNAi. Amelioration of SP6616 on the pathological behaviors of diabetic mice was assessed against tactile allodynia, thermal sensitivity and motor nerve conduction velocity (MNCV).

FINDINGS

SP6616 treatment effectively ameliorated the threshold of mechanical stimuli, thermal sensitivity and MNCV of diabetic mice. Mechanism research results indicated that SP6616 suppressed Kv2.1 expression, increased the number of intraepidermal nerve fibers (IENFs), improved peripheral nerve structure and vascular function in DRG tissue. In addition, SP6616 improved mitochondrial dysfunction through Kv2.1/CaMKKβ/AMPK/PGC-1α pathway, repressed inflammatory response by inhibiting Kv2.1/NF-κB signaling and alleviated apoptosis of DRG neuron through Kv2.1-mediated regulation of Bcl-2 family proteins and Caspase-3 in diabetic mice.

INTERPRETATION

Our work has highly supported the beneficial of Kv2.1 inhibition in ameliorating DPN-like pathology and highlighted the potential of SP6616 in the treatment of DPN.

FUNDING

Please see funding sources.

Effectiveness of acupuncture combined mecobalamin in the treatment of elderly diabetic peripheral neuropathy: A protocol of systematic review and meta-analysis

BACKGROUND

Although previous studies have reported the effectiveness of acupuncture combined mecobalamin (AM) in the treatment of elderly diabetic peripheral neuropathy (EDPN), no systematic study has assessed its effectiveness and safety. Thus, this study will evaluate the effectiveness and safety of AM for the treatment of patients with EDPN.

METHODS

Bibliographic electronic databases will be searched as follows: Cochrane Library, PUBMED, EMBASE, CINAHL, PsycINFO, WANGFANG, and China National Knowledge Infrastructure. All of them will be searched from each database initial to March 1, 2020 without language restrictions. All study selection, information extracted, and study quality evaluation will be performed by 2 independent authors. Any disagreements between 2 authors will be resolved by a third author via discussion. RevMan 5.3 software will be used for data pooling and meta-analysis performance if it is possible.

RESULTS

This study will provide synthesis of current evidence of AM for patients with EDPN through primary outcome of glycemic profile, and secondary of neuropathic pain intensity, plantar tactile sensitivity, sensory nerve conduction velocity and motor nerve conduction velocity, health-related quality of life, and adverse events.

CONCLUSION

This study will provide helpful reference for the efficacy and safety of AM for the treatment of patients with EDPN to the clinicians and further studies.Study registration number: INPLASY202040094.

Improvement of peripheral vascular impairment by a phosphodiesterase type 5 inhibitor tadalafil prevents oxaliplatin-induced peripheral neuropathy in mice

Oxaliplatin, a platinum-based chemotherapeutic drug, frequently induces peripheral neuropathy. Accumulating evidences suggest a possible relationship between peripheral vascular impairment and peripheral neuropathy. In this study, we investigated the effects of vasodilators on cumulative peripheral neuropathy induced by repeated injections of oxaliplatin (10 mg/kg) once a week for 8 weeks in mice. Single injections of vasodilators, including a phosphodiesterase type 5 inhibitor tadalafil acutely alleviated oxaliplatin-induced cold hypersensitivity, while tadalafil had no effect on the mechanical hypersensitivity. By contrast, long-term administration of tadalafil (0.1% in chow diets) during the oxaliplatin injection period reduced the oxaliplatin-induced decreases in skin temperature and blood flow without affecting platinum concentrations in blood, sciatic nerves, and dorsal root ganglion. The long-term administration significantly suppressed cold, mechanical, and electrical current hypersensitivities as well as thermal hypoesthesia. Furthermore, it prevented the decreases in sensory nerve conductance velocity and the number of endoneurial microvessels, and axon degeneration in the sciatic nerves. In vitro studies confirmed that tadalafil does not interfere with the cytotoxicity of oxaliplatin against human cancer cell lines. Altogether, these results suggest that improvement of peripheral vascular impairment by tadalafil could alleviate and prevent oxaliplatin-induced peripheral neuropathy.

Hypertension Contributes to Neuropathy in Patients With Type 1 Diabetes

BACKGROUND

Diabetic peripheral neuropathy (DPN) can lead to foot ulceration and amputation. There are currently no disease-modifying therapies for DPN. The aim of this study was to determine if hypertension contributes to DPN in patients with type 1 diabetes mellitus (T1DM).

METHODS

Subjects with T1DM (n = 70) and controls (n = 78) underwent a comprehensive assessment of DPN.

RESULTS

Hypertension was present in 40 of 70 T1DM subjects and 20 of 78 controls. Hypertension was associated with abnormal nerve conduction parameters (P = 0.03 to <0.001), increased vibration perception threshold (P = 0.01) and reduced corneal nerve fiber density and length (P = 0.02) in subjects with T1DM. However, after adjusting for confounding factors only tibial compound motor action potential and nerve conduction velocity were associated with hypertension (P = 0.03) and systolic blood pressure (P < 0.01 to <0.0001). Hypertension had no effect on neuropathy in subjects without diabetes.

CONCLUSIONS

This study shows that hypertension is associated with impaired nerve conduction in T1DM. It supports previous small trials showing that angiotensin-converting enzyme inhibitors improve nerve conduction and advocates the need for larger clinical trials with blood pressure lowering agents in DPN.

Zebrafish is a central model to dissect the peripheral neuropathy

The peripheral nervous system (PNS) is composed with all nerves extended from the brain and spinal cord, which are the central nervous system to other organs of the body. Dysfunctional peripheral motion resulting from the regressive neuronal axons in the defected PNS leads to several peripheral neuropathies including both inherited and non-inherited disorders. Because of poor understanding of cellular and molecular mechanisms involved in the peripheral neuropathy, there is currently non-targeted treatment of the disorder. Basic researches have paid attention to dissect roles of causative genes, identified from the inherited peripheral neuropathies, in PNS development. However, recent studies focusing on investigation of therapeutic targets have suggested that successful regeneration of the impaired peripheral nerves may be most effective treatment. The regeneration studies have been limited in the rodents system due to some of practical and physiological disadvantages until zebrafish model has emerged as an ideal system. Hence, this review aims to provide a comprehensive overview of the advantages of zebrafish as a model for the peripheral neuropathy researches and to suggest the disease genes-involved potential mechanisms targeting the PNS regeneration that may be demonstrated in zebrafish.

Glycemia Lowering Effect of an Aqueous Extract of Hedychium coronarium Leaves in Diabetic Rodent Models

Hedychium coronarium has a long history of use worldwide as a food and in folk medicine. In this study, we aimed to investigate the effect of an aqueous extract of H. coronarium leaves (HC) on type 2 diabetes mellitus (T2DM). Two types of animal models were used in this study: Streptozotocin (STZ)-induced T2DM (Wistar rats; N = 8) and C57BKSdb/db mice (N = 5). After treatment with HC for 28 days, glucose tolerance improved in both of the diabetic animal models. As significant effects were shown after 14 days of treatment in the STZ-induced T2DM model, we carried out the experiments with it. After 28 days of treatment with HC, the levels of cholesterol, triglyceride, high-density lipoprotein, and low-density lipoprotein were significantly improved in the STZ-induced T2DM model. The lesions degree of islet β-cells was decreased after the HC treatment. Although the insulin level increased moderately, the aldosterone level was significantly decreased in the HC-treated groups, suggesting that aldosterone might play an important role in this effect. In summary, HC is a natural product and it is worth exploring its effect on T2DM.

miR-146a mediates thymosin β4 induced neurovascular remodeling of diabetic peripheral neuropathy in type-II diabetic mice

Diabetes induces neurovascular dysfunction leading to peripheral neuropathy. MicroRNAs (miRNAs) affect many biological processes and the development of diabetic peripheral neuropathy. In the present study, we investigated whether thymosin-β4 (Tβ4) ameliorates diabetic peripheral neuropathy and whether miR-146a mediates the effect of Tβ4 on improved neurovascular function. Male Type II diabetic BKS. Cg-m+/+Lepr^db/J (db/db) mice at age 20 weeks were treated with Tβ4 for 8 consecutive weeks, and db/db mice treated with saline were used as a control group. Compared to non-diabetic mice, diabetic mice exhibited substantially reduced miR-146a expression, and increased IL-1R-associated kinase-1 (IRAK1), tumor necrosis factor (TNFR)-associated factor 6 (TRAF6) levels and nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB) activity in sciatic nerve tissues. Treatment of diabetic mice with Tβ4 significantly elevated miR-146a levels and overcame the effect of diabetes on these proteins. Tβ4 treatment substantially improved motor and sensory conduction velocity of the sciatic nerve, which was associated with improvements in sensory function. Tβ4 treatment significantly increased intraepidermal nerve fiber density and augmented local blood flow and the density of fluorescein isothiocyanate (FITC)-dextran perfused vessels in the sciatic nerve tissue. In vitro, treatment of dorsal root ganglion (DRG) neurons and mouse dermal endothelial cells (MDEs) with Tβ4 significantly increased axonal outgrowth and capillary-like tube formation, whereas blocking miR-146a attenuated Tβ4-induced axonal outgrowth and capillary tube formation, respectively. Our data indicate that miR-146a may mediate Tβ4-induced neurovascular remodeling in diabetic mice, by suppressing pro-inflammatory signals.

Diabetic neuropathy and the sensory neuron: New aspects of pathogenesis and their treatment implications

Diabetic polyneuropathy (DPN) continues to be generally considered as a "microvascular" complication of diabetes mellitus alongside nephropathy and retinopathy. The microvascular hypothesis, however, might be tempered by the concept that diabetes directly targets dorsal root ganglion sensory neurons. This neuron-specific concept, supported by accumulating evidence, might account for important features of DPN, such as its early sensory neuron degeneration. Diabetic sensory neurons develop neuronal atrophy alongside a series of messenger ribonucleic acid (RNA) changes related to declines in structural proteins, increases in heat shock protein, increases in the receptor for advanced glycation end-products, declines in growth factor signaling and other changes. Insulin is recognized as a potent neurotrophic factor, and insulin ligation enhances neurite outgrowth through activation of the phosphoinositide 3-kinase-protein kinase B pathway within sensory neurons and attenuates phenotypic features of experimental DPN. Several interventions, including glucagon-like peptide-1 agonism, and phosphatase and tensin homolog inhibition to activate growth signals in sensory neurons, or heat shock protein overexpression, prevent or reverse neuropathic abnormalities in experimental DPN. Diabetic sensory neurons show a unique pattern of microRNA alterations, a key element of messenger RNA silencing. For example, let-7i is widely expressed in sensory neurons, supports their growth and is depleted in experimental DPN; its replenishment improves features of DPN models. Finally, impairment of pre-messenger RNA splicing in diabetic sensory neurons including abnormal nuclear RNA metabolism and structure with loss of survival motor neuron protein, a neuron survival molecule, and overexpression of CWC22, a splicing factor, offer further novel insights. The present review addresses these new aspects of DPN sensory neurodegeneration.

Vascular and Neural Complications in Type 2 Diabetic Rats: Improvement by Sacubitril/Valsartan Greater Than Valsartan Alone

Previously, we had shown that a vasopeptidase inhibitor drug containing ACE and neprilysin inhibitors was an effective treatment for diabetic vascular and neural complications. However, side effects prevented further development. This led to the development of sacubitril/valsartan, a drug containing angiotensin II receptor blocker and neprilysin inhibitor that we hypothesized would be an effective treatment for diabetic peripheral neuropathy. Using early and late intervention protocols (4 and 12 weeks posthyperglycemia, respectively), type 2 diabetic rats were treated with valsartan or sacubitril/valsartan for 12 weeks followed by an extensive evaluation of vascular and neural end points. The results demonstrated efficacy of sacubitril/valsartan in improving vascular and neural function was superior to valsartan alone. In the early intervention protocol, sacubitril/valsartan treatment was found to slow progression of these deficits and, with late intervention treatment, was found to stimulate restoration of vascular reactivity, motor and sensory nerve conduction velocities, and sensitivity/regeneration of sensory nerves of the skin and cornea in a rat model of type 2 diabetes. These preclinical studies suggest that sacubitril/valsartan may be an effective treatment for diabetic peripheral neuropathy, but additional studies will be needed to investigate these effects further.

TRPA1 sensitization during diabetic vascular impairment contributes to cold hypersensitivity in a mouse model of painful diabetic peripheral neuropathy

Background

Diabetic peripheral neuropathy is a common long-term complication of diabetes. Accumulating evidence suggests that vascular impairment plays important roles in the pathogenesis of diabetic peripheral neuropathy, while the mechanism remains unclear. We recently reported that transient receptor potential ankyrin 1 (TRPA1) is sensitized by hypoxia, which can contribute to cold hypersensitivity. In this study, we investigated the involvement of TRPA1 and vascular impairment in painful diabetic peripheral neuropathy using streptozotocin-induced diabetic model mice.

Results

Streptozotocin-induced diabetic model mice showed mechanical and cold hypersensitivity with a peak at two weeks after the streptozotocin administration, which were likely to be paralleled with the decrease in the skin blood flow of the hindpaw. Streptozotocin-induced cold hypersensitivity was significantly inhibited by an antagonist HC-030031 (100 mg/kg) or deficiency for TRPA1, whereas mechanical hypersensitivity was unaltered. Consistent with these results, the nocifensive behaviors evoked by an intraplantar injection of the TRPA1 agonist allyl isothiocyanate (AITC) were enhanced two weeks after the streptozotocin administration. Both streptozotocin-induced cold hypersensitivity and the enhanced AITC-evoked nocifensive behaviors were significantly inhibited by a vasodilator, tadalafil (10 mg/kg), with recovery of the decreased skin blood flow. Similarly, in a mouse model of hindlimb ischemia induced by the ligation of the external iliac artery, AITC-evoked nocifensive behaviors were significantly enhanced three and seven days after the ischemic operation, whereas mechanical hypersensitivity was unaltered in TRPA1-knockout mice. However, no difference was observed between wild-type and TRPA1-knockout mice in the hyposensitivity for current or mechanical stimulation or the deceased density of intraepidermal nerve fibers eight weeks after the streptozotocin administration.

Conclusion

These results suggest that TRPA1 sensitization during diabetic vascular impairment causes cold, but not mechanical, hypersensitivity in the early painful phase of diabetic peripheral neuropathy. However, TRPA1 may play little or no role in the progression of diabetic peripheral neuropathy.

Fasudil combined with methylcobalamin or lipoic acid can improve the nerve conduction velocity in patients with diabetic peripheral neuropathy: A meta-analysis

BACKGROUND

Fasudil (F) plus methylcobalamin (M) or lipoic acid (L) treatment has been suggested as a therapeutic approach for diabetic peripheral neuropathy (DPN) in numerous studies. However, the effect of the combined use still remains dubious.

OBJECTIVE

The aim of this report was to evaluate the efficacy of F plus M or L (F + M or F + L) for the treatment of DPN compared with that of M or L monotherapy, respectively, in order to provide the basis and reference for clinical rational drug use.

METHODS

Randomized controlled trials (RCTs) of F for DPN published up to September 2017 were searched. Relative risk (RR), mean difference (MD), and 95% confidence interval (CI) were calculated and heterogeneity was assessed with the I test. Sensitivity analyses were also performed. The outcomes measured were as follows: the clinical efficacy, median motor nerve conduction velocities (NCVs) (MNCVs), median sensory NCV (SNCV), peroneal MNCV, peroneal SNCV, and adverse effects.

RESULTS

Thirteen RCTs with 1148 participants were included. Clinical efficacy of F + M combination therapy was significantly better than M monotherapy (8 trials; RR 1.26, 95% CI 1.17-1.35, P < .00001, I = 0%), the efficacy of F + L combination therapy was also obviously better than L monotherapy (4 trials; RR 1.27, 95% CI 1.16-1.39, P < .00001, I = 0%). Compared with monotherapy, the pooled effects of combination therapy on NCV were (MD 6.69, 95% CI 4.74-8.64, P < .00001, I = 92%) for median MNCV, (MD 6.71, 95% CI 1.77-11.65, P = .008, I = 99%) for median SNCV, (MD 4.18, 95% CI 2.37-5.99, P < .00001, I = 94%) for peroneal MNCV, (MD 5.89, 95% CI 3.57-8.20, P < .00001, I = 95%) for peroneal SNCV. Furthermore, there were no serious adverse events associated with drug intervention.

CONCLUSION

Combination therapy with F plus M or L was superior to M or L monotherapy for improvement of neuropathic symptoms and NCVs in DPN patients, respectively. Moreover, no serious adverse events occur in combination therapy.

Effect of Dietary Content of Menhaden Oil with or without Salsalate on Neuropathic Endpoints in High-Fat-Fed/Low-Dose Streptozotocin-Treated Sprague Dawley Rats

In this study, we wanted to extend our investigation of the efficacy of fish oil with or without salsalate on vascular and neural complications using a type 2 diabetic rat model. Four weeks after the onset of hyperglycemia, diabetic rats were treated via the diet with 3 different amounts of menhaden oil with or without salsalate for 12 weeks. Afterwards, vascular reactivity of epineurial arterioles and neuropathy-related endpoints were examined. The addition of salsalate to high-fat diets enriched with 10% or 25% kcal of menhaden oil protected vascular reactivity to acetylcholine and calcium gene-related peptide, motor and sensory nerve conduction velocity, thermal nociception, intraepidermal nerve fiber density, and cornea sensitivity to a greater extent than 10% or 25% menhaden oil alone. Vascular and neural function was maximally protected with diet containing 45% kcal as menhaden oil, and adding salsalate did not provide any additional benefit. Salsalate alone in the high-fat diet of diabetic rats provided minimal protection/improvement of vascular and neural dysfunction. These studies imply that dietary salsalate in combination with lower amounts of menhaden oil can provide greater benefit toward diabetes-induced vascular and neural impairment than menhaden oil alone.

Tadalafil Promotes the Recovery of Peripheral Neuropathy in Type II Diabetic Mice

We previously demonstrated that treatment of diabetic peripheral neuropathy with the short (4 hours) half-life phosphodiesterase 5 (PDE5) inhibitor, sildenafil, improved functional outcome in diabetic db/db mice. To further examine the effect of PDE5 inhibition on diabetic peripheral neuropathy, we investigated the effect of another potent PDE5 inhibitor, tadalafil, on diabetic peripheral neuropathy. Tadalafil is pharmacokinetically distinct from sildenafil and has a longer half-life (17+hours) than sildenafil. Diabetic mice (BKS.Cg-m+/+Lepr^db/J, db/db) at age 20 weeks were treated with tadalafil every 48 hours for 8 consecutive weeks. Compared with diabetic mice treated with saline, tadalafil treatment significantly improved motor and sensory conduction velocities in the sciatic nerve and peripheral thermal sensitivity. Tadalafil treatment also markedly increased local blood flow and the density of FITC-dextran perfused vessels in the sciatic nerve concomitantly with increased intraepidermal nerve fiber density. Moreover, tadalafil reversed the diabetes-induced reductions of axon diameter and myelin thickness and reversed the diabetes-induced increased g-ratio in the sciatic nerve. Furthermore, tadalafil enhanced diabetes-reduced nerve growth factor (NGF) and platelet-derived growth factor-C (PDGF-C) protein levels in diabetic sciatic nerve tissue. The present study demonstrates that tadalafil increases regional blood flow in the sciatic nerve tissue, which may contribute to the improvement of peripheral nerve function and the amelioration of diabetic peripheral neuropathy.