Oxidative stress participates in the breakdown of neuronal phenotype in experimental diabetic neuropathy.

AIMS/HYPOTHESIS: This study compared the effects of streptozotocin-induced diabetes in rats with those of two pro-oxidant interventions; a diet deficient in vitamin E and treatment with primaquine. METHODS: Measurements were made by the classic motor and sensory conduction velocity deficits and by indicators of the breakdown of small fibre phenotype i.e., sciatic nerve content of nerve growth factor and the neuropeptides, substance P and neuropeptide Y. RESULTS: As with diabetes, the pro-oxidant interventions decreased conduction velocities (though the effect of vitamin E deficiency was not significant), the sciatic nerve content of nerve growth factor and the neuropeptides (all percentages refer to the mean value for the appropriate control groups). In diabetes, nerve growth factor was depleted to 50% in the control rats (p < 0.05); oxidative stress depleted nerve growth factor to 64% (primaquine; p < 0.05) and 81% (vitamin E deficient; not significant) of controls. Substance P was depleted to 51% in the control rats (p < 0.01) with depletions to 74% and 72% (both p < 0.01) by oxidative stress; equivalent depletions for neuropeptide Y were 38% controls in diabetes (p < 0.001) and 67% (primaquine; p < 0.001) and 74% (vitamin E deficient; p < 0.05) for oxidative stress. CONCLUSION/INTERPRETATION: The relative magnitudes of these changes suggest an effect in diabetes of oxidative stress, coupled with some other cellular event(s). This is supported by the effects of a diester of gamma-linolenic acid and alpha-lipoic acid, which completely prevented the effects on the pro-oxidant interventions on conduction velocity, nerve growth factor and neuropeptide contents, but was only partially preventative in diabetes.

Effect of streptozotocin-induced diabetes on NGF, P75(NTR) and TrkA content of prevertebral and paravertebral rat sympathetic ganglia.

Diabetic autonomic neuropathy results in significant morbidity and mortality. Both diabetic humans and experimental animals show neuroaxonal dystrophy of autonomic nerve terminals, particularly in the prevertebral superior mesenteric ganglia (SMG) and celiac ganglia (CG) which innervate the hyperplastic/hypertrophic diabetic small intestine. Previously, investigators suggested that disturbances in ganglionic nerve growth factor (NGF) content or transport might play a pathogenetic role in diabetic autonomic pathology. To test this hypothesis, we measured NGF content and NGF receptor expression, p75(NTR) (low affinity neurotrophin receptor) and trkA (high affinity NGF receptor), in control and diabetic rat SMG, CG and superior cervical ganglia (SCG). Surprisingly, rather than a decrease, we observed an approximate doubling of NGF content in the diabetic SMG and CG, a result which reflects increased NGF content in the hyperplastic diabetic alimentary tract. No change in NGF content was detected in the diabetic SCG which is relatively spared in experimental diabetic autonomic neuropathy. NGF receptor expression was not consistently altered in any of the autonomic ganglia. These observations suggest that increased NGF content in sympathetic ganglia innervating the diabetic alimentary tract coupled with intact receptor expression may produce aberrant axonal sprouting and neuroaxonal dystrophy.

Aberrant neurofilament phosphorylation in sensory neurons of rats with diabetic neuropathy.

Aberrant neurofilament phosphorylation occurs in many neurodegenerative diseases, and in this study, two animal models of type 1 diabetes--the spontaneously diabetic BB rat and the streptozocin-induced diabetic rat--have been used to determine whether such a phenomenon is involved in the etiology of the symmetrical sensory polyneuropathy commonly associated with diabetes. There was a two- to threefold (P < 0.05) elevation of neurofilament phosphorylation in lumbar dorsal root ganglia (DRG) of diabetic rats that was localized to perikarya of medium to large neurons using immunocytochemistry. Additionally, diabetes enhanced neurofilament M phosphorylation by 2.5-fold (P < 0.001) in sural nerve of BB rats. Neurofilaments are substrates of the mitogen-activated protein kinase (MAPK) family, which includes c-jun NH2-terminal kinase (JNK) or stress-activated protein kinase (SAPK1) and extracellular signal-regulated kinases (ERKs) 1 and 2. Diabetes induced a significant three- to fourfold (P < 0.05) increase in phosphorylation of a 54-kDa isoform of JNK in DRG and sural nerve, and this correlated with elevated c-Jun and neurofilament phosphorylation. In diabetes, ERK phosphorylation was also increased in the DRG, but not in sural nerve. Immunocytochemistry showed that JNK was present in sensory neuron perikarya and axons. Motoneuron perikarya and peroneal nerve of diabetic rats showed no evidence of increased neurofilament phosphorylation and failed to exhibit phosphorylation of JNK. It is hypothesized that in sensory neurons of diabetic rats, aberrant phosphorylation of neurofilament may contribute to the distal sensory axonopathy observed in diabetes.

A lipoic acid-gamma linolenic acid conjugate is effective against multiple indices of experimental diabetic neuropathy.

Untreated streptozotocin-diabetic (7 weeks duration) rats showed reductions (all p < 0.01; percentages in brackets) in motor and sensory nerve conduction velocity (MNCV; 14%, SNCV; 17%) and in sciatic nerve contents of nerve growth factor (NGF; 57%), substance P (SP; 53%) and neuropeptide Y (NPY; 39%). Treatment with a gamma-linolenic acid-alpha-lipoic acid conjugate (GLA-LA; 35 mg x day(-1) x rat(-1)) attenuated (p < 0.05) these reductions to MNCV (8%), SNCV (5%), NGF (19%), SP (23%), NPY (20%), such that the values in GLA-LA-treated diabetic rats did not differ significantly from those of control non-diabetic animals. Treatment with alpha-lipoic acid alone at 100 mg/kg i.p. was without effect on these variables except for NGF (33% reduction, p < 0.05) and treatment with the antioxidant, butylated hydroxytoluene (1.5% dietary supplement) did not affect any deficits. These data show that GLA-LA is effective in improving both electrophysiological and neurochemical correlates of experimental diabetic neuropathy.

Does neuropathy develop in animal models?

Defects of the peripheral nervous system are common in patients with diabetes mellitus. At least 50% of diabetic patients will develop a form of diabetic neuropathy within 25 years after diagnosis. Currently the cornerstone of treatment lies with the maintenance of euglycaemia using insulin, which has inherent problems of its own. In addition, the signs and symptoms of diabetic neuropathy are often intractable. Therefore, the development of effective treatments for diabetic neuropathy is urgently needed. Thus, animal models have been developed to investigate the pathogenesis of diabetic neuropathy and evaluate potential therapeutic agents. However, no model is perfect and no one would suggest that diabetic rats can replicate the human condition fully. In this review the appropriateness of established animal models of diabetic neuropathy is discussed with reference to the pathology and pathophysiology of the human case with the hope of addresssing some of the questions surrounding this general issue.