Functional and structural abnormalities in the nerves of type I diabetic baboons: aminoguanidine treatment does not improve nerve function

Protective effect of pregabalin on the brain tissue of diabetic rats

Purpose

Diabetes mellitus (DM) is a metabolic disorder characterized by insulin deficiency or insulin resistance. Pregabalin (PGB) is an antiepileptic drug with proven efficacy in the treatment of epilepsy, generalized anxiety disorder, and neuropathic pain. In this study, we aimed to investigate the protective effects of PGB in brain tissue of rats with streptozotocin (STZ)-induced experimental diabetes.

Materials and methods

Twenty-eight Wistar albino male rats were randomly divided into four groups with seven rats each: (I) Control group, (II) PGB (50 mg/kg PBG), (III) DM, and (IV) DM + PGB (50 mg/kg/day PGB per orally for 8 weeks). Diabetes was induced with an intraperitoneal (i.p.) STZ injection (Sigma Chemical Co Louis Missour, USA) at a dose of 180 mg/kg. STZ was dissolved in 0.1 M phosphate-citrate tampon (pH 4.5). Paraffin sections were examined using histological and immunohistochemical analyses. To detect oxidative damage biochemically, malondialdehyde (MDA), the end product of lipid peroxidation; superoxide dismutase (SOD), catalase (CAT), glutathione (GSH) and glutathione peroxidase (GPx) which are antioxidant enzymes, levels were studied. In addition, bax, caspase-3 enzyme activities and TUNEL assay were studied to evaluate the apoptosis status.

Results

In the DM group, MDA concentrations were significantly higher and GPx and SOD activities were significantly lower compared to the control group. MDA concentrations were significantly lower and SOD activity was significantly higher in the DM + PGB group than in the DM group. The GPx activity in the DM group decreased significantly compared to the control group. In immunohistochemical examinations (Bax, Caspase-3 and TUNEL), the apoptosis rate was significantly lower in the in DM + PGB group than in the DM group.

Conclusion

Pregabalin may prevent harmful effects of oxidative damage by decreasing the MDA levels and increasing the SOD levels. In addition, it was thought that PGB may have antiapoptotic properties due to decreased bax and caspase-3 immunoreactivity and TUNEL positivity in PGB groups. Based on these findings, we think that PGB may be effective in reducing the risk of brain damage associated with DM.

Protective and therapeutic effects of swimming exercise training on diabetic peripheral neuropathy of streptozotocin-induced diabetic rats

BACKGROUND

Diabetic peripheral neuropathy (DPN) is a typical complication of diabetes. No definitive treatment and prevention of DPN has been established, and very few data on the role of exercise training on DPN have been reported.

AIM OF THE STUDY

The protective and therapeutic effects of aerobic physical activity on the development of DPN in Type 1 were investigated.

METHODS

Rats were assigned to 5 groups: C (control), E (exercise), D (diabetic), DEx (exercise after diabetic), ExD (diabetic after exercise); C containing 10 animals and E, D, DEx, ExD containing 15 animals. Diabetes was induced with streptozotocin (STZ) (45 mg/kg, ip). Development of diabetes was confirmed by measuring blood glucose levels 2 days after STZ treatment. Body weights of all the animals were evaluated weekly throughout the experiment. Motor dysfunction defined by a significant increase in compound muscle action potential (CMAP) latency was recorded. The amplitude of CMAP which mainly reflects axonal dysfunction was also measured using standard techniques. Sciatic nerve morphometry and blood glucose levels were analyzed in all the groups.

RESULTS

Blood glucose level significantly increased 2 days after STZ injection. All diabetic rats showed decreased body weight compared to control rats. An increase in motor latency of CMAP and a decrease in amplitude of CMAP, indicative of neuropathy, were seen in STZ rats. On the completion of the study (the 56th day post-STZ), histological examination revealed significant myelin loss (thinner myelin) in sciatic nerves of STZ rats. Treatment with swimming exercise had no effect on glycemic control but restored body weight, CMAP amplitude, CMAP latency or motor dysfunction in the diabetic animals.

CONCLUSIONS

This study suggests that swimming exercise training has protective and therapeutic effects on DPN of STZ-induced diabetic rats.

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.

Advanced glycation endproducts (AGEs): pharmacological inhibition in diabetes

AGE inhibitors may act by various mechanisms at different steps of advanced glycation endproduct (AGE) formation (depending on oxidative stress and/or carbonyl stress) and AGE-mediated damage: trapping of reactive dicarbonyl species; antioxidant activity by transition metal chelation; other antioxidant activity including free radical scavenging; AGE cross-link breaking; AGE receptor (RAGE) blocking; RAGE signaling blocking; glycemia reduction by anti-diabetic therapy; aldose reductase inhibition; shunting of trioses-P towards the pentose-P pathway by transketolase activation. Most of the inhibitors have several sites of action. Practically one can distinguish drugs specifically developed as AGE inhibitors or AGE breakers; RAGE and receptor signaling blockers; other therapeutic compounds which were found subsequently to possess also AGE inhibitor activity, including dietary antioxidants. Encouraging results obtained in studies of various AGE inhibitors, conducted in vitro and in diabetic animals, are summarized in this review. However most of the clinical trials have been more or less disappointing, in part because of side effects; the long-term therapeutic interest of the most recently developed AGE inhibitors or breakers remains to be demonstrated in diabetes.

Effect of melatonin and vitamin E on diabetes-induced learning and memory impairment in rats

Previous studies indicate that diabetes mellitus might be accompanied by a certain erosion of brain function such as cognitive impairment. The aim of this study was to examine and compare the effects of melatonin and vitamin E on cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a spatial version of the Morris water maze test. The levels of lipid peroxidation and glutathione were detected in hippocampus and frontal cortex. The diabetic rats developed significant impairment in learning and memory behaviors as indicated by the deficits in water maze tests as compared to control rats. Furthermore, lipid peroxidation levels increased and glutathione concentration decreased in diabetic rats. Treatment with melatonin and vitamin E significantly ameliorated learning and memory performance. Furthermore, both antioxidants reversed lipid peroxidation and glutathione levels toward their control values. These results suggest that oxidative stress may contribute to learning and memory deficits in diabetes and further suggest that antioxidant melatonin and vitamin E can improve cognitive impairment in streptozotocin-induced diabetes.

Normative nerve conductions in the tail of rhesus macaques (Macaca mulatta)

BACKGROUND

The aim of this study was to test the feasibility of using the tail of Macaca mulatta for neurophysiological testing of the peripheral nervous system.

METHODS

Motor and sensory nerve conduction studies (NCS) of the tail were obtained by surface stimulation and recording. The technique utilized was novel. Unlike other NCS obtained from other peripheral nerves, this technique did not require any special neurophysiological expertise.

RESULTS

The latency of the motor and sensory response was 2.5 +/- 0.71 and 1.1 +/- 0.27 ms respectively. The amplitude of the motor and sensory response was 8.1 +/- 5.1 mV and 14.6 +/- 9.4 microV respectively. Similar to human beings, there was a statistically significant relationship between age and motor amplitude, motor latency and sensory latency.

CONCLUSIONS

Based on our results, a relatively simple, reproducible neurophysiological monitoring technique of the peripheral nervous system is possible.

Inhibitors of Advanced Glycation End Product Formation and Neurovascular Dysfunction in Experimental Diabetes

Advanced glycation and lipoxidation end products (AGEs/ALEs) have been implicated in the pathogenesis of the major microvascular complications of diabetes mellitus: nephropathy, neuropathy, and retinopathy. This article reviews the evidence regarding the peripheral nerve and its vascular supply. Most investigations done to assess the role of AGEs/ALEs in animal models of diabetic neuropathy have used aminoguanidine as a prototypic inhibitor. Preventive or intervention experiments have shown treatment benefits for motor and sensory nerve conduction velocity, autonomic nitrergic neurotransmission, nerve morphometry, and nerve blood flow. The latter depends on improvements in nitric oxide-mediated endothelium-dependent vasodilation and is responsible for conduction velocity improvements. A mechanistic interpretation of aminoguanidine's action in terms of AGE/ALE inhibition is made problematic by the relative lack of specificity. However, other unrelated compounds, such as pyridoxamine and pyridoxamine analogues, have recently been shown to have beneficial effects similar to aminoguanidine, as well as to improve pain-related measures of thermal hyperalgesia and tactile allodynia. These data also stress the importance of redox metal ion-catalyzed AGE/ALE formation. A further approach is to decrease substrate availability by reducing the elevated levels of hexose and triose phosphates found in diabetes. Benfotiamine is a transketolase activator that directs these substrates to the pentose phosphate pathway, thus reducing tissue AGEs. A similar spectrum of improvements in nerve and vascular function were noted when using benfotiamine in diabetic rats. Taken together, the data provide strong support for an important role for AGEs/ALEs in the etiology of diabetic neuropathy.

Advanced glycation endproducts--role in pathology of diabetic complications

Diabetes mellitus is a common endocrine disorder characterised by hyperglycaemia and predisposes to chronic complications affecting the eyes, blood vessels, nerves and kidneys. Hyperglycaemia has an important role in the pathogenesis of diabetic complications by increasing protein glycation and the gradual build-up of advanced glycation endproducts (AGEs) in body tissues. These AGE form on intra- and extracellular proteins, lipids, nucleic acids and possess complex structures that generate protein fluorescence and cross-linking. Protein glycation and AGE are accompanied by increased free radical activity that contributes towards the biomolecular damage in diabetes. There is considerable interest in receptors for AGEs (RAGE) found on many cell types, particularly those affected in diabetes. Recent studies suggest that interaction of AGEs with RAGE alter intracellular signalling, gene expression, release of pro-inflammatory molecules and free radicals that contribute towards the pathology of diabetic complications. This review introduces the chemistry of glycation and AGEs and examines the mechanisms by which they mediate their toxicity. The role of AGEs in the pathogenesis of retinopathy, cataract, atherosclerosis, neuropathy, nephropathy, diabetic embryopathy and impaired wound healing are considered. There is considerable interest in anti-glycation compounds because of their therapeutic potential. The mechanisms and sites of action of selected inhibitors, together with their potential in preventing diabetic complications are discussed.

Use of aminoguanidine (Pimagedine) to prevent the formation of advanced glycation endproducts

Aminoguanidine (AG) is a prototype therapeutic agent for the prevention of formation of advanced glycation endproducts. It reacts rapidly with alpha,beta-dicarbonyl compounds such as methylglyoxal, glyoxal, and 3-deoxyglucosone to prevent the formation of advanced glycation endproducts (AGEs). The adducts formed are substituted 3-amino-1,2,4-triazine derivatives. Inhibition of disease mechanisms, particularly vascular complications in experimental diabetes, by AG has provided evidence that accumulation of AGEs is a risk factor for disease progression. AG has other pharmacological activities, inhibition of nitric oxide synthase and semicarbazide-sensitive amine oxidase (SSAO), at pharmacological concentrations achieved in vivo for which controls are required in anti-glycation studies. AG is a highly reactive nucleophilic reagent that reacts with many biological molecules (pyridoxal phosphate, pyruvate, glucose, malondialdehyde, and others). Use of high concentrations of AG in vitro brings these reactions and related effects into play. It is unadvisable to use concentrations of AG in excess of 500 microM if selective prevention of AGE formation is desired. The peak plasma concentration of AG in clinical therapy was ca. 50 microM. Clinical trial of AG to prevent progression of diabetic nephropathy was terminated early due to safety concerns and apparent lack of efficacy. Pharmacological scavenging of alpha-oxoaldehydes or stimulation of host alpha-oxoaldehyde detoxification remains a worthy therapeutic strategy to prevent diabetic complications and other AGE-related disorders.

Altered expression of NCAM in hippocampus and cortex may underlie memory and learning deficits in rats with streptozotocin-induced diabetes mellitus

Neurological and structural changes are paralleled by cognitive deficits in diabetes mellitus. The present study was designed to evaluate the expression of neural cell adhesion molecules (NCAM) in the hippocampus, cortex and cerebellum and to examine cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a passive avoidance test and a spatial version of the Morris water maze test. NCAM expression was detected in the hippocampus, cortex and cerebellum by an immunoblotting method. The diabetic rats developed significant impairment in learning and memory behaviours as indicated by deficits in passive avoidance and water maze tests as compared to control rats. Expression of NCAM 180 and 120 kDa were found to be higher in hippocampus and cortex of diabetic rat brains compared to those of control, whereas expression of NCAM 140 kDa decreased in these brain regions. Our findings suggest that streptozotocin-induced diabetes impairs cognitive functions and causes an imbalance in expression of NCAM in those brain regions involved in learning and memory. Altered expression of NCAM in hippocampus may be an important cause of learning and memory deficits that occur in diabetes mellitus.

Glycation in diabetic neuropathy: characteristics, consequences, causes, and therapeutic options

Glycation is the nonenzymatic reaction of glucose, alpha-oxoaldehydes, and other saccharide derivatives with proteins, nucleotides, and lipids. Early glycation adducts (fructosamines) and advanced glycation adducts (AGEs) are formed. "Glycoxidation" is a term used for glycation processes involving oxidation. Sural, peroneal, and saphenous nerves of human diabetic subjects contained AGEs in the perineurium, endothelial cells, and pericytes of endoneurial microvessels and in myelinated and unmyelinated fibres localized to irregular aggregates in the cytoplasm and interstitial collagen and basement membranes. Pentosidine content was increased in cytoskeletal and myelin protein extracts of the sural nerve of human subjects and cytoskeletal proteins of the sciatic nerve of streptozotocin-induced diabetic rats. AGEs in the sciatic nerve of diabetic rats were decreased by islet transplantation. Improved glycemic control of diabetic patients may be expected to decrease protein glycation in the nerve. Protein glycation may decrease cytoskeletal assembly, induce protein aggregation, and provide ligands for cells surface receptors. The receptor for advanced glycation and products (RAGE) was expressed in peripheral neurons. It is probable that high intracellular glucose concentration is an important trigger for increased glycation, leading to increased formation of methylglyoxal, glyoxal, and 3-deoxyglucosone that glycate proteins to form AGEs intracellularly and extracellularly. Oxidative stress enhances these processes and is, in turn, enhanced by AGE/RAGE interactions. An established therapeutic strategy to prevent glycation is the use of alpha-oxoaldehyde scavengers. Available therapeutic options for trial are high-dose nicotinamide and thiamine therapies to prevent methylglyoxal formation. Future possible therapeutic strategies are RAGE antagonists and inducers of the enzymatic antiglycation defense. More research is required to understand the role of glycation in the development of diabetic neuropathy.

Role of the Schwann cell in diabetic neuropathy

The relationships among Schwann cells, axons, and the perineurial barrier emphasize the key role Schwann cells play in normal functions of the nerve. Schwann cells are responsible for action potential velocity through insulation of axons, maintenance of axonal caliber, and correct localization of Na+ channels; immunological and funcitonal integrity of the nerve through the perineurial blood-nerve-barrier; and effective nerve regeneration. In diabetic neuropathy, many of these facets of nerve function are defective. Hypoxia, hyerglycemia, and increased oxidative stress contribute directly and indirectly to Schwann cell dysfunction. The results include impaired paranodal barrier function, damaged myelin, reduced antioxidative capacity, and decreased neurotrophic support for axons. This chapter discusses the role of the Schwann cell in the normal or regenerating nerve nad in the altered metabolic conditons of diabetes.

The baboon as a nonhuman primate model for the study of the genetics of obesity

OBJECTIVE

At present, rodents represent the most common animal model for research in obesity and its comorbidities (e.g., type 2 diabetes and coronary heart disease), however, there are several physiological and developmental differences between rodents and humans reflective of their relatively ancient evolutionary divergence (approximately 65 to 75 million years ago). Therefore, we are currently developing the baboon as a nonhuman primate model for the study of the genetics of obesity.

RESEARCH METHODS AND PROCEDURES

At present, we are collecting extensive phenotypic data in a large pedigreed colony (N > 2000) of baboons housed at the Southwest Foundation for Biomedical Research in San Antonio, Texas. The long-term goal of this project is to identify genes influencing adiposity-related phenotypes and to test hypotheses regarding their pleiotropic effects on other phenotypes related to increased risk for a variety of common diseases (e.g., coronary heart disease and type 2 diabetes).

RESULTS

To date we have obtained various adipose-specific endocrine measures, adipose tissue biopsies, and estimates of body composition on a substantial portion of our pedigreed colony. The pattern of adipose tissue accumulation follows closely that seen in humans, and we have detected significant additive genetic heritabilities for these obesity-related phenotypes.

DISCUSSION

Given the physiological and developmental similarities between humans and baboons, along with the ability to collect data under well-controlled situations and the extensive pedigree data available in our colony, the baboon offers an extremely valuable nonhuman primate model for the study of obesity and its comorbidities.

Future treatments for diabetic neuropathy: clues from experimental neuropathy

Neuropathy remains a major complication of diabetes and there is no approved treatment that prevents its progression or alleviates the associated symptoms. Animal models of diabetic neuropathy are hampered by their short life span, which precludes the development of overt structural pathology, and they are best viewed as exhibiting early metabolic, neurochemical, and functional indices of nerve disorders that may predict progression to overt diabetic neuropathy. In this context, diabetic animals have use in both establishing potential etiologic mechanisms and for screening novel therapeutic agents. Treatment strategies are evolving in concert with a developing understanding of how hyperglycemia causes nerve dysfunction and recent or ongoing clinical trials are investigating this rational approach to drug design. It is only by the successful demonstration of clinical efficacy of a compound developed by this approach that the use of animal models of diabetic neuropathy can be validated.

The effects of aminoguanidine on renal changes in a baboon model of Type 1 diabetes

BACKGROUND

The efficacy of aminoguanidine (AG) on primary prevention of diabetic nephropathy was investigated in a nonhuman primate model of Type 1 diabetes over a period of 4 years.

METHODS

Adolescent male baboons (Papio hamadryas) were assigned to four groups: control, diabetic, and control and diabetic treated with AG. Diabetes was induced with streptozocin (60 mg/kg) and treated with insulin to maintain a mean HbA1c level of about 9%. AG was given subcutaneously (10 mg/kg) each day. All animals had annual renal biopsies and 24-h urine collections for measurements of glomerular basement membrane (GBM) thickness, fractional mesangium volume (FMV), albumin excretion rate (AER), and creatinine clearance. Glomerular filtration rate (GFR) and renal plasma flow (RPF) were also determined.

RESULT

The diabetic animals had increased GBM after 2 years of diabetes, but there was no increase in FMV over the study period. AG prevented the thickening of GBM at the 3- and 4-year time points. AG and diabetes synergistically increased the GFR. All diabetic animals developed increased albuminuria during the study although lower than the conventionally accepted microalbuminuria range. AG was not able to prevent this and, in fact, led to the nondiabetic animals also developing albuminuria.

CONCLUSION

This is the first study to investigate the early use of AG in ameliorating renal damage in a primate model of Type 1 diabetes. The structural and functional changes in the kidney of these animals resemble those seen in the early stages of the human disease. AG was able to significantly reduce the thickening of GBM due to diabetes. This may suggest a potential role for this in primary prevention of diabetic nephropathy in the future.

Cardiovascular structure and function in baboons with Type 1 diabetes -- a transvenous ultrasound study

Diabetes mellitus is an important risk factor for both macrovascular and cardiac disease in humans. The availability of a novel intravenous ultrasound probe allows detailed interrogation of a large proportion of the vasculature, as well as the heart, during the same examination. Six male baboons (Papio hamadryas) with Type 1 diabetes and known microangiopathy, and six control animals were studied. Vascular structure in the major large arteries and cardiac function were studied using transvenous ultrasound introduced via the right femoral vein and positioned under fluoroscopy. All arteries were examined for atherosclerotic plaque and for presence of increased intima-media thickness (IMT). Left ventricular function was assessed at rest and following infusions of dobutamine (positive inotrope) and esmolol (negative inotrope). The procedure was performed safely and successfully in all cases. No atherosclerotic plaque was seen in either diabetic or normal baboons. There was no difference in the aortic IMT (0.38+/-0.04 vs. 0.37+/-0.05 mm, normal vs. diabetic, P=NS) or in doppler flow in the renal or iliac arteries between diabetic and normal baboons. Left ventricular wall thickness and systolic tissue velocity were similar in the two groups at rest (6.9+/-2.5 vs. 6.2+/-1.4 cm/s, normal vs. diabetic, P=NS), after dobutamine (15.5+/-2.2 vs. 12.7+/-3.9 cm/s, normal vs. diabetic, P=NS), and after esmolol (4.3+/-1.0 vs. 5.6+/-1.0 cm/s, normal vs. diabetic, P=NS). In a high primate model of diabetes with microangiopathy, the presence of hyperglycemia for 7 years per se does not produce abnormalities of macrovascular or cardiac structure.

Nitric oxide-mediated corpus cavernosal smooth muscle relaxation is impaired in ageing and diabetes

OBJECTIVE

To examine nitric-oxide (NO)-mediated relaxation in cavernosal smooth muscle in a rat model of diabetes, as previous experiments showed that HbA1c (an isoform of glycosylated haemoglobin and a marker of long-term diabetic control) impaired NO-mediated relaxation of normal corpus cavernosal tissue through the generation of superoxide anions.

MATERIALS AND METHODS

Eight weeks after the induction of diabetes, male Wistar rats were killed and cavernosal tissue obtained. Strips were contracted with 1 micromol/L noradrenaline before applying acetylcholine or electrical field stimulation (EFS) or sodium nitroprusside (SNP). Relaxation responses were repeated in the presence of L-arginine (100 micromol/L), indomethacin (10 micromol/L) or superoxide dismutase (SOD, 120 IU/mL). Young and age-matched control animals were examined in the same way.

RESULTS

Eight weeks of uncontrolled diabetes caused a significant impairment in mean relaxation responses to acetylcholine (P < 0.05) and to EFS (P < 0.05), but not to SNP, compared with young and age-matched controls, respectively. L-arginine, indomethacin and SOD had no significant effect on this impairment. Ageing caused a lesser but significant impairment in EFS-mediated cavernosal smooth muscle relaxation (P < 0.05).

CONCLUSION

Diabetes impairs endothelial and neuronal NO-mediated cavernosal smooth muscle relaxation in rats in vitro. This effect is not mediated by an alteration in the intracellular action of NO, the availability of NO, superoxide anion inactivation of NO or the generation of constrictor prostanoids. It is possible that cholesterol or advanced glycation end products are responsible for the effect of diabetes on penile smooth function.