Insulin-ameliorated peripheral motor neuropathy in spontaneously diabetic WBN/Kob rats

Diabetic peripheral neuropathy: pathogenetic mechanisms and treatment

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

Superimposition of hypertension on diabetic peripheral neuropathy affects small unmyelinated sensory nerves in the skin and myelinated tibial and sural nerves in rats with alloxan-induced type 1 diabetes

Diabetic peripheral neuropathy (DPN) is a major complication of diabetes mellitus, and hypertension is considered to be a risk factor for DPN in patients with type 1 diabetes (T1DM). However, the morphological effects of hypertension on DPN are unclear. In this study, we investigated the effect of hypertension on DPN by investigating the changes in unmyelinated and myelinated nerve fibers in hypertensive rats with alloxan (AL)-induced T1DM. Thirteen-week-old WBN/Kob rats with AL-induced diabetes were allocated to receive tap water only (AL group), tap water containing 0.5% saline (0.5AN group), or tap water containing 0.75% saline (0.75AN group) for 15 weeks. Hyperglycemia was maintained for 15 weeks, and the animals were euthanized at 28 weeks. By 23 weeks of age, the systolic blood pressure was significantly higher in the 0.75AN and 0.5AN groups than in the AL group and was unchanged in all groups at 28 weeks. The number of intraepidermal sensory unmyelinated nerve fibers was significantly smaller in the 0.75AN and 0.5AN groups than in the AL group. The axonal size in the myelinated tibial and sural nerve fibers was significantly smaller in the 0.75AN group than in the AL group. Furthermore, luminal narrowing and endothelial hypertrophy were observed in the endoneurial tibial nerve vessels in the 0.75AN group. These findings suggest that superimposing hypertension on hyperglycemia may accelerate a reduction in the number of small unmyelinated sensory nerve fibers in the skin and induce mild axonal atrophy in myelinated tibial and sural nerve fibers in rats with AL-induced T1DM.

Reduce Muscle Fibrosis through Exercise via NRG1/ErbB2 Modification in Diabetic Rats

Diabetic myopathy refers to the manifestations in the skeletal muscle as a result of altered glucose homeostasis which reflects as fibrosis. Since physical exercise has been indicated a protective strategy for improving glucose metabolism in skeletal muscle, we tested a hypothesis under which the endurance exercise training could reverse the produced skeletal muscle fibrosis by diabetes. Eight-week-old male Wistar rats were randomly assigned into four groups including healthy control (HC), healthy trained (HT), diabetic control (DC), and diabetic trained (DT) groups. Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ; 45 mg/kg). Rats in the HT and DT groups carried out an exercise program on a motorized treadmill for five days a week over six weeks. Skeletal muscle levels of NRG1and ErbB2 were measured by the Western blot method. Exercise training decreased blood glucose levels in the DT group. Induction of diabetes increased skeletal muscle fibrosis in both the fast extensor digitorum longus (EDL) and slow soleus muscles, while endurance training modified it in diabetic trained rats. Moreover, muscle NRG1and ErbB2 levels were increased in diabetic rats, while training modified muscle NRG1and ErbB2 levels in diabetic trained rats. Our study provides novel evidence that endurance training could modify skeletal muscle fibrosis through NRG1/ErbB2 modification in STZ-induced diabetic rats.

Hyperglycemia Suppresses Age-Related Increases in Corneal Peripheral Sensory Nerves in Wistar Bon Kobori (WBN/Kob) Rats

Purpose

Nerve fiber density in the cornea is an alternative marker for diabetic peripheral neuropathy combined with intraepidermal nerve fiber density (IENFD). Recent studies investigated corneal nerves using rodent models of diabetes. Male Wistar Bon Kobori (WBN/Kob) rats spontaneously develop long-lasting diabetes and human-like diabetic peripheral neuropathy with vascular lesions. This study investigated corneal nerve fiber density and IENFD in diabetic male WBN/Kob rats as morphological markers of diabetic peripheral neuropathy.

Methods

Male WBN/Kob rats exhibit abnormal glucose tolerance and diabetes at approximately 30 weeks of age, which progresses until approximately 90 weeks of age. Male WBN/Kob rats aged 36 and 90 weeks were therefore used for histological investigations and compared with age-matched nondiabetic female rats.

Results

Terminal epithelial nerve density and subbasal nerve plexus density in the central cornea were significantly greater in nondiabetic female rats aged 90 weeks when compared with nondiabetic female rats aged 36 weeks. However, terminal epithelial nerve density and subbasal nerve plexus density did not increase with age in diabetic male WBN/Kob rats, instead lowering by up to 40%, relative to measurements in nondiabetic female rats aged 90 weeks. However, this difference was not statistically significant. IENFD was significantly lower in diabetic male rats aged 90 weeks than in male rats aged 36 weeks, but did not differ between diabetic male rats and nondiabetic female rats aged 90 weeks.

Conclusions

In WBN/Kob rats, hyperglycemia suppresses an age-related increase in peripheral sensory corneal nerve density; therefore, corneal sensory nerves may be important morphological markers of diabetic peripheral sensory neuropathy.

Effect of combined dyslipidemia and hyperglycemia on diabetic peripheral neuropathy in alloxan-induced diabetic WBN/Kob rats

Clinical and experimental research have suggested that dyslipidemia aggravates diabetic peripheral neuropathy (DPN). However, whether dyslipidemia is a risk factor for DPN remains unclear. To investigate the effect of dyslipidemia on DPN, morphological features of peripheral nerves were analyzed in diabetic rats treated with a high-fat diet (HFD). Male rats were divided into four groups: nondiabetic rats (N), alloxan-induced diabetic rats (AL), diabetic rats treated with an HFD (AH), and nondiabetic rats treated with an HFD (HF). Combined hyperglycemia and dyslipidemia (AH group) induced a significant increase in plasma triglyceride and cholesterol levels. In addition, the combined effects contributed to a reduction in myelin size and a reduction in myelin thickness as indicated on sensory sural nerve histograms. There was also a reduction in the size of motor nerve axons when compared with the effects of hyperglycemia or dyslipidemia alone. However, the sensory nerve conduction velocity in the AH group was slightly but not significantly lower than those in the HF and AL groups. These results suggest that combined hyperglycemia and dyslipidemia induced mild peripheral motor and sensory nerve lesions, without significantly affecting sensory nerve conduction velocity.

The relationship between blood pressure and sciatic nerve blood flow velocity in rats with insulin-treated experimental diabetes

Peripheral nerve blood flow (NBF) does not autoregulate but, instead, responds passively to changes in mean arterial pressure (MAP). How this relationship is impacted by insulin-treated experimental diabetes (ITED) is unknown. We tested the hypothesis that ITED will reduce NBF across a range of MAP in Sprague Dawley rats. Following 10 weeks of control or ITED conditions, conscious MAP (tail-cuff) was measured, and under anaesthesia, the MAP (carotid artery catheter, pressure transducer) and NBF (Doppler ultrasound, 40 MHz) responses to sodium nitroprusside (60 µg/kg) and phenylephrine (12 µg/kg) infusion were recorded (regression equations for MAP vs NBF were created for each rodent). Thereafter, motor nerve conduction velocity (MNCV) and nerve vascularization (haematoxylin and eosin stain) were determined. Conscious MAP was higher and MNCV was lower in the ITED group (p < 0.01). In response to drug infusions, the ΔMAP and ΔNBF were similar between groups (p ≥ 0.18). Estimated conscious NBF (based on substituting conscious MAP values into each individual regression equation) was greater in the ITED group (p < 0.01). Sciatic nerve vascularization was similar between groups (p ≥ 0.50). In contrast to the hypothesis, NBF was not reduced across a range of MAP. In spite of increased estimated conscious NBF values, MNCV was reduced in rats with ITED.