Effect of antioxidant treatment of streptozotocin-induced diabetic rats on endoneurial blood flow, motor nerve conduction velocity, and vascular reactivity of epineurial arterioles of the sciatic nerve

Current and Future Strategies for the Management of Diabetic Neuropathy

Diabetic neuropathy is common, related to increased morbidity and mortality, and has no effective treatment at present. Interventions based on putative pathways thought to contribute to damage and repair of nerve fibres have yielded little success to date. Pain is a potentially debilitating manifestation of diabetic neuropathy and has many potential sites of origin and, hence, modulation. Its cause is unclear and it does not respond well to traditional pain therapies, proposed to mediate their benefits via multiple peripheral and central mechanisms. A better understanding of the mechanisms leading to nerve fibre degeneration and regeneration as well as pain has recently resulted in the development of a more targeted approach to the treatment of diabetic neuropathy. Thus, specific NMDA receptor antagonists and more specific neuronal serotonin and norepinephrine (noradrenaline) uptake inhibitors offer promise in the treatment of painful diabetic neuropathy. A number of treatments which include the aldose reductase inhibitors and neurotrophins have failed to reach the clinical arena. However, the antioxidant alpha-lipoic acid, as well as compounds which correct vascular dysfunction and hence neuropathy, such as ACE inhibitors and protein kinase C-beta inhibitors, have demonstrated more success.

Effect of treatment of diabetic rats with dehydroepiandrosterone on vascular and neural function

Nutritional supplementation with dehydroepiandrosterone (DHEA) may be a candidate for treating diabetes-induced vascular and neural dysfunction. DHEA is a naturally occurring adrenal androgen that has antioxidant properties and is reportedly reduced in diabetes. Using a prevention protocol, we found that dietary supplementation of streptozotocin-induced diabetic rats with 0.1, 0.25, or 0.5% DHEA caused a concentration-dependent prevention in the development of motor nerve conduction velocity and endoneurial blood flow impairment, which are decreased in diabetes. At 0.25%, DHEA significantly prevented the diabetes-induced increase in serum thiobarbituric acid-reactive substances and sciatic nerve conjugated diene levels. This treatment also reduced the production of superoxide by epineurial arterioles of the sciatic nerve. DHEA treatment (0.25%) significantly improved vascular relaxation mediated by acetylcholine in epineurial vessels of diabetic rats. Sciatic nerve Na+-K+-ATPase activity and myoinositol content was also improved by DHEA treatment, whereas sorbitol and fructose content remained elevated. These studies suggest that DHEA, by preventing oxidative stress and perhaps improving sciatic nerve Na+-K+-ATPase activity, may improve vascular and neural dysfunction in diabetes.

Lipoic acid in the treatment of smell dysfunction following viral infection of the upper respiratory tract

OBJECTIVES/HYPOTHESIS

The study aimed to investigate the potential therapeutic effects of alpha-lipoic acid in olfactory loss following infections of the upper respiratory tract. Possible mechanisms of actions include the release of nerve growth factor and antioxidative effects, both of which may be helpful in the regeneration of olfactory receptor neurons.

STUDY DESIGN

Unblinded, prospective clinical trial.

METHODS

A total of 23 patients participated (13 women, 10 men; mean age 57 y, age range 22-79 y; mean duration of olfactory loss, 14 mo; range, 4 to 33 mo); 19 of them were hyposmic and 4 had functional anosmia. Alpha-lipoic acid was used orally at a dose of 600 mg/day; it was prescribed for an average period of 4.5 months. Olfactory function was assessed using olfactory tests for phenyl ethyl alcohol odor threshold, odor discrimination, and odor identification.

RESULTS

Seven patients (30%) showed no change in olfactory function. Two patients (9%) exhibited a moderate decrease in olfactory function; in contrast, six patients (26%) showed moderate and eight patients (35%) remarkable increase in olfactory function. Two of the 4 patients with functional anosmia reached hyposmia; 5 of 19 hyposmic patients became normosmic. Overall, this resulted in a significant improvement in olfactory function following treatment (P =.002). At the end of treatment parosmias were less frequent (22%) than at the beginning of therapy (48%). Interestingly, recovery of olfactory function appeared to be more pronounced in younger patients than in patients above the age of 60 years (P =.018).

CONCLUSIONS

The results indicate that alpha-lipoic acid may be helpful in patients with olfactory loss after upper respiratory tract infection. However, to judge the true potential of this treatment, the outcome of double-blind, placebo-controlled studies in large groups of patients must be awaited, especially when considering the relatively high rate of spontaneous recovery in olfactory loss after upper respiratory tract infection.

NO- and non-NO-, non-prostanoid-dependent vasodilatation in rat sciatic nerve during maturation and developing experimental diabetic neuropathy

This study examined NO- and non-NO-, non-prostanoid-dependent pathways of agonist-induced vasodilatation in streptozotocin (STZ)-induced diabetic rats and their age-matched controls at 1-2, 8-10 and 18-20 weeks after induction of diabetes. Using laser Doppler flowmetry, vasodilatory responses to acetylcholine (ACh; 0.1 mM) and morpholino-sydnonimine (SIN-1) were determined in the presence of Ringer solution, during inhibition of NO synthase (NOS) and cyclo-oxygenase (COX) with N(omega)-nitro-L-arginine (L-NNA; 1 mM) + indomethacin (10(-5) M), and during inhibition of K(+) channels, NOS and COX with tetraethylammonium (TEA; 10 mM) + L-NNA + indomethacin. Basal NOS activity and nerve conduction velocity were also determined. In age-matched controls, SIN-1-induced vasodilatation in the presence of TEA + L-NNA + indomethacin, basal NOS activity and the initial vasodilatory response to ACh during NOS and COX inhibition all decreased with maturation. In STZ-induced diabetics, SIN-1-induced vasodilatation in the presence of TEA + L-NNA + indomethacin was impaired immediately after induction of diabetes, but not at 18-20 weeks. NOS activity in STZ-induced diabetics displayed a transient 2-fold increase at 8-10 weeks, decreasing to age-matched control levels at 18-20 weeks. At 18-20 weeks of STZ-induced diabetes, ACh-induced vasodilatation during NOS and COX inhibition was prolonged due to increased K(+) channel activity and experimental diabetic sensory neuropathy (EDN) had developed. Thus, in sciatic nerve microcirculation of STZ-induced diabetic rats: (1) diabetic impairment of vasodilatation in response to exogenous NO was transient; (2) non-NO-, non-prostanoid-dependent vasodilatation and K(+) channel activity were augmented in STZ-induced diabetes; and (3) alterations in NO bioactivity were not related to the development of EDN.

Transition metals and polyol pathway in the development of diabetic neuropathy in rats

BACKGROUND

The transition metal-catalyzed reaction is a major source of oxygen free radicals, which play an important role in vascular dysfunction leading to ischemia in diabetic tissues. The inhibition of polyol pathway hyperactivity has been reported to ameliorate neurovascular abnormalities in diabetic rats and has been proposed to improve the oxygen free radical scavenging capacity. The present study was conducted to compare the effect of a transition metal chelating agent, trientine (TRI), on diabetic neuropathy with that of an aldose reductase inhibitor, NZ-314 (NZ).

METHODS

Diabetic rats were divided into three groups: (1). untreated, (2). TRI-treated, and (3). NZ-treated. TRI (20 mg/kg) or NZ (100 mg/kg) was administered by gavage or chow containing NZ, respectively, for 8 weeks. Motor nerve conduction velocity (MNCV), coefficient of variation of the R - R interval on electrocardiogram (CVr-r), sciatic nerve blood flow (SNBF), platelet aggregation activities, and serum concentrations of malondialdehyde were measured.

RESULTS

Untreated diabetic rats showed delayed MNCV, decreased CV(R-R), and reduced SNBF compared to normal rats. TRI or NZ completely prevented these deficits. Platelet hyperaggregation activities in diabetic rats were prevented by NZ, but not by TRI. Increased concentrations of malondialdehyde in diabetic rats were partially but significantly ameliorated by either TRI or NZ.

CONCLUSIONS

These observations suggest that increased free radical formation through the transition metal-catalyzed reaction plays an important role in the development of diabetic neuropathy and that the preventive effect of an aldose reductase inhibitor on diabetic neuropathy may also be mediated by decreasing oxygen free radicals.

Nitrosative stress, uric Acid, and peripheral nerve function in early type 1 diabetes

The present study was performed to determine whether nitric oxide overproduction is associated with deterioration in peripheral nerve function in type 1 diabetes. We measured peripheral nerve function and biochemical indicators of nitrosative stress annually for 3 years in 37 patients with type 1 diabetes. Plasma nitrite and nitrate (collectively NO(x)) were 34.0 +/- 4.9 micro mol/l in the control subjects and 52.4 +/- 5.1, 50.0 +/- 5.1, and 49.0 +/- 5.2 in the diabetic patients at the first, second, and third evaluations, respectively (P < 0.01). Nitrotyrosine (NTY) was 13.3 +/- 2.0 micro mol/l in the control subjects and 26.8 +/- 4.4, 26.1 +/- 4.3, and 32.7 +/- 4.3 in the diabetic patients (P < 0.01). Uric acid was suppressed by 20% in the diabetic patients (P < 0.001). Composite motor nerve conduction velocity for the median, ulnar, and peroneal nerves was decreased in patients with high versus low NTY (mean Z score -0.522 +/- 0.25 versus 0.273 +/- 0.22; P < 0.025). Patients with high NO(x) had decreased sweating, and those with suppressed uric acid had decreased autonomic function. In conclusion, nitrosative stress in early diabetes is associated with suppressed uric acid and deterioration in peripheral nerve function.

Are diabetic neuropathy, retinopathy and nephropathy caused by hyperglycemic exclusion of dehydroascorbate uptake by glucose transporters?

Vitamin C exists in two major forms. The charged form, ascorbic acid (AA), is taken up into cells via sodium-dependent facilitated transport. The uncharged form, dehydroascorbate (DHA), enters cells via glucose transporters (GLUT) and is then converted back to AA within these cells. Cell types such as certain endothelial and epithelial cells as well as neurons that are particularly prone to damage during diabetes tend to be those that appear to be dependent on GLUT transport of DHA rather than sodium-dependent AA uptake. We hypothesize that diabetic neuropathies, nephropathies and retinopathies develop in part by exclusion of DHA uptake by GLUT transporters when blood glucose levels rise above normal. AA plays a central role in the antioxidant defense system. Exclusion of DHA from cells by hyperglycemia would deprive the cells of the central antioxidant, worsening the hyperglycemia-induced oxidative stress level. Moreover, AA participates in many cellular oxidation-reduction reactions including hydroxylation of polypeptide lysine and proline residues and dopamine that are required for collagen production and metabolism and storage of catecholamines in neurons. Increase in the oxidative stress level and metabolic perturbations can be expected in any tissue or cell type that relies exclusively or mainly on GLUT for co-transport of glucose and DHA including neurons, epithelial cells, and vascular tissues. On the other hand, since DHA represents a significant proportion of total serum ascorbate, by increasing total plasma ascorbate concentrations during hyperglycemia, it should be possible to correct the increase in the oxidative stress level and metabolic perturbations, thereby sparing diabetic patients many of their complications.

Dietary glutathione protects rats from diabetic nephropathy and neuropathy

Recently, much attention has focused on the role of oxidative stress in the various forms of tissue damage in patients with diabetes. The aim of this study was to examine the involvement of oxidative stress in the progression of kidney dysfunction and neuropathy in diabetes and to evaluate the potential usefulness of glutathione (GSH) in diabetes. We examined the effect that treatment of streptozotocin (STZ)-induced diabetic rats with GSH has on the renal and neural functions. Diabetic rats were treated with 1 g/100 g GSH as a dietary supplement. GSH significantly suppressed the diabetes-induced increase in urinary 8-hydroxy-2'-deoxyguanosine, one of the markers of oxidative stress. It also prevented the diabetes-induced increases in albumin and creatinine in urine. The diabetes-induced increase in the tail flick reaction time to thermal stimuli also was normalized by treatment with dietary GSH. In conclusion, GSH treatment can beneficially affect STZ-induced diabetic rats, with preservation of in vivo renal and neural function. This suggests a potential usefulness of dietary GSH treatment to reduce diabetic complications.

Changes in endoneurial blood flow, motor nerve conduction velocity and vascular relaxation of epineurial arterioles of the sciatic nerve in ZDF-obese diabetic rats

BACKGROUND

We have previously reported that in streptozotocin-induced diabetic rats, reduction in endoneurial blood flow (EBF) and impairment of acetylcholine-mediated vascular relaxation of arterioles that provide circulation to the sciatic nerve precedes slowing of motor nerve conduction velocity (MNCV). However, in animal models of type 2 diabetes it is unknown whether slowing of MNCV is accompanied by vascular dysfunction.

METHODS

Using ZDF-lean and ZDF-obese diabetic rats we examined whether diabetes-induced slowing in MNCV was associated with a reduction in EBF and impaired vascular relaxation in epineurial arterioles of the sciatic nerve. We measured MNCV in the sciatic nerve using a non-invasive procedure, and sciatic nerve nutritive blood flow using microelectrode polarography. In vitro videomicroscopy was employed to quantify arteriolar diameter responses to acetylcholine in arterioles overlying the sciatic nerve.

RESULTS

MNCV and EBF in hyperglycemic (4-week duration) ZDF-obese diabetic rats were significantly decreased by 30% and 63%, respectively, compared to age-matched ZDF-lean rats. Acetylcholine elicited a dose-dependent dilation of epineurial vessels from ZDF-lean and ZDF-obese diabetic rats, although acetylcholine-induced dilation was significantly reduced in ZDF-obese diabetic rats. Determination of markers of oxidative stress provided ambiguous results. Superoxide levels were increased in epineurial vessels from ZDF-obese diabetic rats. Lens glutathione levels were decreased and serum thiobarbituric acid reactive substances increased in ZDF-obese diabetic rats but sciatic nerve conjugated diene and glutathione levels were not significantly different compared to ZDF-lean rats.

CONCLUSIONS

Diabetes causes a reduction in EBF and impairment in vascular relaxation in epineurial vessels in ZDF-obese diabetic rats. This impaired vascular response is associated with neural dysfunction.

Effect of M40403 treatment of diabetic rats on endoneurial blood flow, motor nerve conduction velocity and vascular function of epineurial arterioles of the sciatic nerve

1. To further explore the effect of antioxidants in preventing diabetes-induced vascular and neural dysfunction we treated streptozotocin-induced diabetic rats daily with subcutaneous injections of 10 mg kg(-1) of M40403 (n=11) and compared the results obtained from 17 control rats and 14 untreated diabetic rats. M40403 is a manganese(II) complex with a bis(cyclo-hexylpyridine)-substituted macrocyclic ligand that was designed to be a selective functional mimetic of superoxide dismutase. Thus, M40403 provides a useful tool to evaluate the roles of superoxide in disease states. 2. Treatment with M40403 significantly improved diabetes-induced decrease in endoneurial blood flow, acetylcholine-mediated vascular relaxation in arterioles that provide circulation to the region of the sciatic nerve, and motor nerve conduction velocity (P<0.05). M40403 treatment also reduced the appearance of superoxide in the aorta and epineurial vessels and peroxynitrite in epineurial vessels. Treating diabetic rats with M40403 reduced the diabetes-induced increase in thiobarbituric acid reactive substances in serum but did not prevent the decrease in lens glutathione level. Treating diabetic rats with M40403 did not improve sciatic nerve Na(+)/K(+) ATPase activity or the sorbitol, fructose or myo-inositol content of the sciatic nerve. 3. These studies provide additional evidence that diabetes-induced oxidative stress and the generation of superoxide and perhaps peroxynitrite may be partially responsible for the development of diabetic vascular and neural complications.