Effects of alpha-lipoic acid on impaired gastric fundus innervation in diabetic rats

Pathogenesis of diabetic gastrointestinal dysfunction

Diabetic gastrointestinal dysfunction is a common complication in patients with diabetes mellitus. Most of the symptoms are related to impaired gastrointestinal function. The pathogenesis and etiology of diabetic gastroenteropathy are complex, involving the parasympathetic and sympathetic nervous systems, enteric neurons, smooth muscle cells, the network of interstitial cells of Cajal, cholinergic receptors and neuronal nitric oxide synthase. This article reviews the pathogenesis of diabetic gastrointestinal dysfunction.

Effect of combination therapy consisting of enalapril, α-lipoic acid, and menhaden oil on diabetic neuropathy in a high fat/low dose streptozotocin treated rat

We have previously demonstrated that treating diabetic rats with enalapril, an angiotensin converting enzyme (ACE) inhibitor, α-lipoic acid, an antioxidant, or menhaden oil, a natural source of omega-3 fatty acids can partially improve diabetic peripheral neuropathy. In this study we sought to determine the efficacy of combining these three treatments on vascular and neural complications in a high fat fed low dose streptozotocin treated rat, a model of type 2 diabetes. Rats were fed a high fat diet for 8 weeks followed by a 30 mg/kg dose of streptozotocin. Eight weeks after the onset of hyperglycemia diabetic rats were treated with a combination of enalapril, α-lipoic acid and menhaden oil. Diabetic rats not receiving treatment were continued on the high fat diet. Glucose clearance was impaired in diabetic rats and significantly improved with treatment. Diabetes caused steatosis, elevated serum lipid levels, slowing of motor and sensory nerve conduction, thermal hypoalgesia, reduction in intraepidermal nerve fiber profiles, decrease in cornea sub-basal nerve fiber length and corneal sensitivity and impairment in vascular relaxation to acetylcholine and calcitonin gene-related peptide in epineurial arterioles of the sciatic nerve. Treating diabetic rats with the combination of enalapril, α-lipoic acid and menhaden oil reversed all these deficits to near control levels except for motor nerve conduction velocity which was also significantly improved compared to diabetic rats but remained significantly decreased compared to control rats. These studies suggest that a combination therapeutic approach may be most effective for treating vascular and neural complications of type 2 diabetes.

Autonomic neuropathy in experimental models of diabetes mellitus

Autonomic neuropathy complicates diabetes by increasing patient morbidity and mortality. Surprisingly, considering its importance, development and exploitation of animal models has lagged behind the wealth of information collected for somatic symmetrical sensory neuropathy. Nonetheless, animal studies have resulted in a variety of insights into the pathogenesis, neuropathology, and pathophysiology of diabetic autonomic neuropathy (DAN) with significant and, in some cases, remarkable correspondence between rodent models and human disease. Particularly in the study of alimentary dysfunction, findings in intrinsic intramural ganglia, interstitial cells of Cajal and the extrinsic parasympathetic and sympathetic ganglia serving the bowel vie for recognition as the chief mechanism. A body of work focused on neuropathologic findings in experimental animals and human subjects has demonstrated that axonal and dendritic pathology in sympathetic ganglia with relative neuron preservation represents one of the neuropathologic hallmarks of DAN but it is unlikely to represent the entire story. There is a surprising selectivity of the diabetic process for subpopulations of neurons and nerve terminals within intramural, parasympathetic, and sympathetic ganglia and innervation of end organs, afflicting some while sparing others, and differing between vascular and other targets within individual end organs. Rather than resulting from a simple deficit in one limb of an effector pathway, autonomic dysfunction may proceed from the inability to integrate portions of several complex pathways. The selectivity of the diabetic process appears to confound a simple global explanation (e.g., ischemia) of DAN. Although the search for a single unifying pathogenetic hypothesis continues, it is possible that autonomic neuropathy will have multiple pathogenetic mechanisms whose interplay may require therapies consisting of a cocktail of drugs. The role of multiple neurotrophic substances, antioxidants (general or pathway specific), inhibitors of formation of advanced glycosylation end products and drugs affecting the polyol pathway may be complex and therapeutic elements may have both salutary and untoward effects. This review has attempted to present the background and current findings and hypotheses, focusing on autonomic elements including and beyond the typical parasympathetic and sympathetic nervous systems to include visceral sensory and enteric nervous systems.

Effects of interleukin-6 treatment on neurovascular function, nerve perfusion and vascular endothelium in diabetic rats

AIM

Interleukin-6 (IL-6), a member of the neuropoietic cytokine family, participates in neural development and has neurotrophic activity. Recent research has also indicated actions to improve vasa nervorum function in diabetes. Both these facets are potentially relevant for treatment of diabetic neuropathy. The aim of this study was to determine whether IL-6 treatment corrected changes in neurovascular function in streptozotocin-induced diabetic rats.

METHODS

After 1 month of diabetes, rats were given IL-6 for 1 month. The rats were subjected to sensory testing and measurements of nerve conduction velocities and nerve blood flow by hydrogen clearance microelectrode polarography. Further groups were used to study responses of the isolated gastric fundus and renal artery. Results were statistically analysed using ANOVA and post hoc tests.

RESULTS

Diabetic rats showed mechanical hyperalgesia, thermal hyperalgesia, and tactile allodynia. The former was unaffected by IL-6 treatment, whereas the latter two measures were corrected. Immunohistochemical staining of dorsal root ganglia for IL-6 did not reveal any changes with diabetes or treatment. The results showed that 22 and 17.4% slowing of sciatic motor and saphenous sensory nerve conduction velocities, respectively, with diabetes were improved by IL-6. Sciatic endoneurial perfusion was halved by diabetes and corrected by IL-6. A 40.6% diabetic deficit in maximal non-adrenergic, non-cholinergic relaxation of gastric fundus to nerve stimulation was unaffected by IL-6. Renal artery endothelium-dependent relaxation was halved by diabetes, the endothelium-derived hyperpolarizing factor (EDHF) component being severely attenuated. IL-6 did not affect nitric oxide-mediated vasorelaxation, but markedly improved EDHF responses.

CONCLUSIONS

IL-6 improved aspects of small and large nerve fibre and vascular endothelium dysfunction in diabetic rats. The functional benefits related to increased nerve blood flow via an EDHF mechanism, and IL-6 could have therapeutic potential in diabetic neuropathy and vasculopathy, which should be further evaluated.

Gastroparesis and functional dyspepsia: excerpts from the AGA/ANMS meeting

BACKGROUND

Despite the relatively high prevalence of gastroparesis and functional dyspepsia, the aetiology and pathophysiology of these disorders remain incompletely understood. Similarly, the diagnostic and treatment options for these two disorders are relatively limited despite recent advances in our understanding of both disorders.

PURPOSE

This manuscript reviews the advances in the understanding of the epidemiology, pathophysiology, diagnosis, and treatment of gastroparesis and functional dyspepsia as discussed at a recent conference sponsored by the American Gastroenterological Association (AGA) and the American Neurogastroenterology and Motility Society (ANMS). Particular focus is placed on discussing unmet needs and areas for future research.

Mechanisms of disease: the pathological basis of gastroparesis--a review of experimental and clinical studies

The pathogenesis of gastroparesis is complicated and poorly understood. This lack of understanding remains a major impediment to the development of effective therapies for this condition. Most of the scientific information available on the pathogenesis of gastroparesis has been derived from experimental studies of diabetes in animals. These studies suggest that the disease process can affect nerves (particularly those producing nitric oxide, but also the vagus nerve), interstitial cells of Cajal and smooth muscle. By contrast, human data are sparse, outdated and generally inadequate for the validation of data obtained from experimental models. The available data do, however, suggest that multiple cellular targets are involved. In practice, though, symptoms seldom correlate with objective measures of gastric function and there is still a lot to learn about the pathophysiology of gastroparesis. Future studies should focus on understanding the molecular pathways that lead to gastric dysfunction, in animal models and in humans, and pave the way for the development of rational therapies.

Effects of poly(ADP-ribose) polymerase inhibition on dysfunction of non-adrenergic non-cholinergic neurotransmission in gastric fundus in diabetic rats

Diabetes mellitus compromises nitric oxide (NO)-mediated endothelium-dependent relaxation of blood vessels, which has been linked to the excessive generation of reactive oxygen species. There are also deleterious effect on nitrergic innervation, contributing to autonomic neuropathy symptoms such as impotence and gastroporesis. Poly(ADP-ribose) polymerase (PARP) is a nuclear protein stimulated by DNA damage, caused, for example, by oxidative stress. Activation has been linked to impaired endothelial nitric oxide synthase (eNOS)-mediated vasodilation in experimental diabetes. There is no information on the potential role of PARP in nitrergic nerve dysfunction, therefore, the aim was to examine the effects of PARP inhibition, using 3-aminobenzamide (3-AB) on neurally mediated gastric fundus relaxation in streptozotocin-induced diabetic rats. Eight weeks of diabetes caused a 42.5% deficit in maximum relaxation of in vitro gastric fundus strips to electrical stimulation of the non-adrenergic non-cholinergic innervation. This was largely prevented or corrected (4 weeks of treatment following 4 weeks of untreated diabetes) by 3-AB. Diabetes also markedly attenuated the maintenance of relaxation responses to prolonged stimulation, and this was partially corrected by 3-AB treatment. Experiments in the presence of the NOS inhibitor, N(G)-nitro-L-arginine, and/or blockade of the co-transmitter, vasoactive intestinal polypeptide, by alpha-chymotrypsin, showed that the beneficial effects of 3-AB were primarily due to improved nitrergic neurotransmission. Thus, PARP plays an important role in defective nitrergic neurotransmission in experimental diabetes, which may have therapeutic implications for treatment of aspects of diabetic autonomic neuropathy.

IκB kinase 2 inhibition corrects defective nitrergic erectile mechanisms in diabetic mouse corpus cavernosum

OBJECTIVES

Oxidative or glyco-oxidative stress-induced activation of the transcription factor, nuclear factor (NF)-kappaB, is associated with the neurovascular complications of diabetes mellitus. Antioxidant treatment has beneficial effects in diabetic patients; however, delineating a possible role for NF-kappaB deactivation against direct antioxidant effects has been difficult. NF-kappaB is negatively regulated by the inhibitor of kappaB (IkappaB) complex that, in turn, is activated by specific kinases. Thus, the aim was to investigate the effects of the IkappaB kinase 2 inhibitor, AS602868, on corpus cavernosum function in diabetic mice.

METHODS

Diabetes was induced by streptozotocin; the duration was 6 weeks. Intervention AS602868 treatment (100 mg/kg/day) was given for 2 weeks after 4 weeks of untreated diabetes. Corpora cavernosum were isolated in organ baths for measurement of agonist-evoked or electrical stimulation-evoked smooth muscle tensions.

RESULTS

The maximal nitrergic nerve-mediated relaxation of phenylephrine-precontracted cavernosum was reduced approximately 30% by diabetes (P <0.001). AS602868 treatment completely reversed the deficit (P <0.001). Maximal nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was attenuated approximately 32% by diabetes (P <0.05). This was completely restored by IkappaB kinase 2 inhibition (P <0.01). Furthermore, AS602868 treatment also completely corrected (P <0.01) an approximate 20% diabetic deficit (P <0.001) in maximal endothelium-independent relaxation to the nitric oxide donor, sodium nitroprusside.

CONCLUSIONS

Inhibition of IkappaB kinase 2 can correct nitric oxide-dependent indexes of diabetic erectile dysfunction. This suggests that NF-kappaB activation is important in the development of diabetic cavernosum nitrergic neuropathy and vasculopathy.

Correction of nitrergic neurovascular dysfunction in diabetic mouse corpus cavernosum by p38 mitogen-activated protein kinase inhibition

Increased p38 mitogen-activated protein kinase (MAPK) in response to stress stimuli, including hyperglycemia, contributes to diabetic somatic neuropathy. However, effects on autonomic nerve and vascular function have not been determined. The aim of this study was to investigate the effects of the p38 MAPK inhibitor, LY2161793, on penile neurovascular function in streptozotocin-induced diabetic mice. Diabetes duration was 6 weeks and intervention LY2161793 treatment was given for the final 2 weeks. In vitro measurements on phenylephrine-precontracted corpus cavernosum revealed a 32% reduction in maximum nitrergic nerve-mediated relaxation with diabetes that was 74% corrected by LY2161793 treatment. Maximum nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was 42% attenuated by diabetes and 88% restored by LY2161793. Moreover, treatment partially corrected a diabetic deficit in endothelium-independent relaxation to a nitric oxide donor. Thus, p38 MAPK inhibition corrects nitric oxide-dependent indices of diabetic erectile autonomic neuropathy and vasculopathy, a therapeutic approach potentially worthy of consideration for clinical trials.

Effect of long-term administration of alpha-lipoic acid on retinal capillary cell death and the development of retinopathy in diabetic rats

Oxidative stress is increased in the retina in diabetes, and it is considered to play an important role in the development of retinopathy. alpha-Lipoic acid, a thiol antioxidant, has been shown to have beneficial effects on polyneuropathy and on the parameters of oxidative stress in various tissues, including nerve, kidney, and retina. The purpose of this study was to examine the effect of alpha-lipoic acid on retinal capillary cell apoptosis and the development of pathology in diabetes. Retina was used from streptozotocin-induced diabetic rats receiving diets supplemented with or without alpha-lipoic acid (400 mg/kg) for 11 months of diabetes. Capillary cell apoptosis (by terminal transferase-mediated dUTP nick-end labeling) and formation of acellular capillaries were investigated in the trypsin-digested retinal microvessels. The effect of alpha-lipoic acid administration on retinal 8-hydroxy-2'deoxyguanosine (8-OHdG) and nitrotyrosine levels was determined by enzyme-linked immunosorbent assay. alpha-Lipoic acid administration for the entire duration of diabetes inhibited capillary cell apoptosis and the number of acellular capillaries in the retina, despite similar severity of hyperglycemia in the two diabetic groups (with and without alpha-lipoic acid). Retinal 8-OHdG and nitrotyrosine levels were increased by over twofold and 70%, respectively, in diabetes, and alpha-lipoic acid administration inhibited these increases. Our results demonstrate that the long-term administration of alpha-lipoic acid has beneficial effects on the development of diabetic retinopathy via inhibition of accumulation of oxidatively modified DNA and nitrotyrosine in the retina. alpha-Lipoic acid supplementation represents an achievable adjunct therapy to help prevent vision loss in diabetic patients.