Effect of aminoguanidine on the impaired nitric oxide-mediated neurotransmission in anococcygeus muscle from diabetic rats

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 diabetes and elevated glucose on nitrergic relaxations in the isolated duodenum of the rat

Nitrergic relaxations of the isolated duodenum, induced by streptozotocin, were investigated in the experimental 8-week diabetes rat model. The effects of elevated glucose were also examined in the incubated duodenal muscles (in Krebs-Henseleit solution containing 44 mM glucose for 6 h) taken from nondiabetic rats. The relaxations induced by electrical field stimulation (EFS) and nicotine were significantly reduced in diabetic rats compared with control rats. Incubating of duodenal tissues in medium containing elevated glucose revealed significantly impaired relaxations to EFS and nicotine compared to responses obtained after normal glucose incubation. However, the relaxant responses to sodium nitroprusside and papaverine were similar in all groups. Incubating in hyperosmolar solutions containing sucrose, the relaxant responses were not affected. In conclusion, impairment of NO-mediated relaxations in diabetes may be related to hyperglycemia. The alterations caused by elevated glucose are not due to a hyperosmotic effect because the same concentration of sucrose had no effect on the relaxations.

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.

Neuropathology and pathogenesis of diabetic autonomic neuropathy

Autonomic neuropathy is a significant complication of diabetes resulting in increased patient morbidity and mortality. A number of studies, which have shown correspondence between neuropathologic findings in experimental animals and human subjects, have demonstrated that axonal and dendritic pathology in sympathetic ganglia in the absence of significant neuron loss represents a neuropathologic hallmark of diabetic autonomic neuropathy. A recurring theme in sympathetic ganglia, as well as in the pot-ganglionic autonomic innervation of various end organs, is the involvement of distal portions of axons and nerve terminals by degenerative or dystrophic changes. In both animals and humans, there is a surprising selectivity of the diabetic process for subpopulations of autonomic ganglia, nerve terminals within sympathetic ganglia and end organs, from end organ to end organ, and between vascular and other targets within individual end organs. Although the involvement or autonomic axons in somatic nerves may reflect an ischemic pathogenesis, the selectivity of the diabetic process confounds simple global explanations of diabetic autonomic neuropathy as the result of diminished blood flow with resultant tissue hypoxia. A single unifying pathogenetic hypothesis has not yet emerged from clinical and experimental animal studies, and it is likely that diabetic autonomic neuropathy will be shown to have multiple causative mechanisms, which will interact to result in the variety of presentations of autonomic injury in diabetes. Some of these mechanisms will be shared with aging changes in the autonomic nervous system. The role of various neurotrophic substances and the polyol pathway in the pathogenesis and treatment of diabetic neuropathy likely represents a two-edged sword with both salutary and exacerbating effects. The basic neurobiologic process underlying the diabetes-induced development of neuroaxonal dystrophy, synaptic dysplasia, defective axonal regeneration, and alterations in neurotrophic substance may be mechanistically related.

Diabetes causes an early reduction in autonomic ganglion blood flow in rats

Impaired blood flow to peripheral nerve trunks makes a major contribution to the neuropathic complications of diabetes mellitus. Comparatively little attention has been paid to perfusion abnormalities for the cell bodies of origin of the autonomic and sensory nerves, although they are severely affected in diabetic neuropathy. The aim was to examine the time course of changes in superior cervical ganglion (SCG) perfusion in streptozotocin-induced diabetic rats. Ganglion blood flow, measured by hydrogen clearance microelectrode polarography, was approximately 70 ml min(-1) 100 g(-1). One week of diabetes caused a 46% perfusion deficit, which was maintained (54%) over 24 weeks. Thus, an early, profound, and long-lived reduction in ganglion perfusion may deleteriously affect neural cell body function and could contribute to autonomic neuropathy.

Insulin reversal of impaired nitrergic transmission in the anococcygeus muscle from diabetic rats

The aim of this study was to investigate whether insulin could reverse the impairment in nitrergic neurotransmission in the anococcygeus muscle from streptozotocin-induced diabetic rats. Relaxations to nitrergic nerve stimulation and sodium nitroprusside were significantly reduced in precontracted muscles from 8- and 4-week diabetic rats compared to the corresponding control rats. Treatment of diabetic rats with Lente insulin (1-12 units/day s.c.) for the final 4 weeks of the 8-week diabetes duration reversed the reductions, but treatment of diabetic rats with insulin for the last week only did not. The impairment of relaxations was also not altered by in vitro exposure of muscles from 8-week diabetic rats to soluble insulin (0.02 units/ml, 2 h). The findings suggest that in vivo insulin treatment can reverse the existing diabetes-induced impairment of nitrergic transmission in rat anococcygeus muscle. However, short-term treatment with insulin, either in vivo or in vitro, does not reverse the impairment.

Diabetes does not alter the activity and localisation of nitric oxide synthase in the rat anococcygeus muscle

Functional studies have revealed diabetes specifically impairs smooth muscle reactivity to nitric oxide in the rat anococcygeus muscle. The present study was conducted to examine whether concurrent prejunctional defects in nitrergic neurotransmission exist in anococcygeus muscles from diabetic rats. Nitric oxide synthase (NOS) activity was assessed by the conversion of 3H-L-arginine to 3H-L-citrulline in homogenates of anococcygeus muscles obtained from 8-week diabetic rats and control rats. NOS activity measured in all tissue samples was dependent on the presence of calcium (2 mM), NADPH (1 mM), tetrahydrobiopterin (100 microM) and flavin adenine dinucleotide (10 microM); however, removal of calmodulin (50 U/ml) did not reduce L-citrulline production. Both N(G)-nitro-L-arginine (100 microM) and N(G)-nitro-L-arginine methyl ester (100 microM) produced significant inhibition of enzyme activity. NOS activity measured in tissue samples from diabetic rats (369.6 +/- 75.9 fmol L-citrulline/mg protein) did not significantly differ from that measured in samples from control rats (423.9 +/- 110.6 fmol L-citrulline/mg protein). However, NOS activity measured after removal of the cofactor tetrahydrobiopterin, was significantly greater in samples from control rats than that from the diabetic group. NOS-immunoreactive and NADPH-diaphorase reactive nerve terminals were found to be sparsely distributed throughout longitudinal sections or whole mounts of anococcygeus muscles from both control and diabetic rats. Quantification of NADPH-diaphorase positive fibres intersecting transects of whole tissue mounts, revealed no significant difference in fibre number between the treatment groups. All NOS-immunoreactive fibres also showed vasoactive-intestinal-polypeptide immunoreactivity. In conclusion, the findings together provide no evidence to indicate that diabetes can induce prejunctional changes in NOS activity or localisation, concurrent with the reported postjunctional impairment in smooth muscle reactivity to nitric oxide, in the rat anococcygeus muscle.

Comparison of relaxations evoked by photoactivation of NO-containing compounds and nitrergic nerve stimulation in 5-hydroxytryptamine- and potassium-contracted rat gastric fundus

1. The aim of the present study was to further investigate our earlier proposal of liberation of nitric oxide (NO) by photoactivation of molecules containing NO or NO2, which in turn relaxes gastric smooth muscle, and to determine whether presynaptic- and/or postsynaptic NO-mediated relaxation is affected differently by the degree of membrane depolarization in rat gastric fundus smooth muscle. 2. During contraction of rat gastric fundus with 5-hydroxytryptamine (5-HT, 10 microM), low (K+, 25 mM) and high potassium (K+, 65.4 mM), relaxation responses to nitrergic nerve stimulation, photo-activation of caged NO compounds (streptozotocin [STZ], N omega-nitro-L-arginine-methylacetate [L-NAME], N omega-nitro-D-arginine-methylacetate [D-NAME]), and sodium nitroprusside (SNP) were compared. 3. Nitrergic nerve (presynaptic) stimulation and photoactivation (postsynaptic) of all caged NO compounds produced rapid, transient and reversible relaxation of 5-HT and low-K(+)-contracted tissues. However, when contractions were induced by high K+, the relaxation induced by nerve stimulation was abolished, whereas relaxations induced by photoactivated NO compounds were significantly (P < 0.01) reduced. 4. The relaxation induced by sodium nitroprusside (SNP), but not papaverine, was also diminished in high-K(+)-contracted tissues. The magnitude of photoactivated NO-induced relaxation was related to the amount of NO release, light intensity and concentration of compounds. 5. The evidence that photoactivated NO-induced relaxation is mediated by cGMP comes from the observation that zaprinast, but not forskolin, potentiated the relaxation. 6. It is concluded that rat gastric smooth muscle relaxes to photoactivation of NO or NO2-carrying molecules via NO, and it appears that degree of membrane depolarization may be a critical factor in dissociating the response to presynaptic- and postsynaptic NO-mediated relaxation in this muscle.

The aldose reductase inhibitor sorbinil does not prevent the impairment in nitric oxide-mediated neurotransmission in anococcygeus muscle from diabetic rats

This study investigated whether increased polyol pathway activity could contribute to alterations in nitrergic neurotransmission in anococcygeus muscles from 8-week diabetic rats. In the presence of guanethidine (10-30 microM) and clonidine (0.01-0.05 microM), relaxations obtained to nitrergic nerve stimulation (0.5-5 Hz, 10-s train), to sodium nitroprusside (5-500 nM) and to nitric oxide (0.1-3 microM) were significantly reduced in muscles from diabetic rats compared to responses from control rats. Treatment of diabetic rats with the aldose reductase inhibitor sorbinil (42 mg/kg per day via feed for 8 weeks) did not affect impaired reactivity to nitrergic nerve stimulation, sodium nitroprusside or nitric oxide. The results suggest increased polyol pathway activity does not contribute to the alterations in nitrergic neurotransmission in anococcygeus muscles from diabetic rats.

Effect of diabetes on relaxations to non-adrenergic, non-cholinergic nerve stimulation in longitudinal muscle of the rat gastric fundus

1. The effect of 8-week streptozotocin-induced diabetes has been examined on relaxations to non-adrenergic, non-cholinergic (NANC) nerve stimulation in longitudinal strips of rat gastric fundus. 2. In the presence of noradrenergic and cholinergic blockade and raised tissue tone, electrical field stimulation (0.5-4 Hz, 30 s trains) induced frequency-dependent relaxations that were significantly smaller in gastric fundus strips from diabetic rats than in strips from control rats. 3. NG-nitro-L-arginine methyl ester (NAME, 100 microM) significantly reduced NANC relaxations in muscle strips from both control and diabetic rats, but the reduction was greater in muscle strips from diabetic rats than in those from control rats at frequencies of 2 and 4 Hz. alpha-Chymotrypsin (1 u ml-1) slightly reduced relaxations to nerve stimulation in muscle strips from both control and diabetic rats. 4. The duration of NANC nerve relaxations (1-4 Hz, 30 s trains) was smaller in muscle strips from diabetic rats than in those from control rats. The duration of NANC relaxations was reduced by alpha-chymotrypsin (1 u ml-1) in muscle strips from control rats but not in muscle strips from diabetic rats. 5. Relaxations to both nitric oxide (NO; 1-30 microM) and vasoactive intestinal polypeptide (VIP; 0.1-30 microM) were concentration-dependent and did not differ between muscle strips from control and diabetic rats. 6. The results suggest that streptozotocin-induced diabetes impairs relaxations to NANC nerve stimulation in the rat gastric fundus, which are largely mediated by NO and to a lesser extent by VIP. The impairment appears to occur at the prejunctional level, as smooth muscle reactivity to NO and VIP is not altered.

Effect of diabetes and elevated glucose on nitric oxide-mediated neurotransmission in rat anococcygeus muscle

1. Nitric oxide (NO)-mediated neurotransmission is impaired in anococcygeus muscle from 8-week streptozotocin-induced diabetic rats. This study investigated the effects of insulin treatment, and the duration of diabetes on this impairment. In addition, the effect of in vitro exposure to elevated glucose has been investigated on NO-mediated relaxations, in muscles from untreated rats. 2. Relaxant responses to field stimulation (0.5-5 Hz, 10s train), sodium nitroprusside (SNP; 5 and 10 nM) and NO (1 and 3 microM) were significantly impaired in anococcygeus muscles from 8-week diabetic rats, compared to responses from control rats. Insulin treatment (5 u Lente day-1, s.c.) of diabetic rats prevented the development of this impairment. 3. Consistent with findings in 8-week diabetic rats, relaxation induced by field stimulation, SNP and NO were attenuated in tissues from 2-week and 4-week diabetic rats compared to corresponding control responses, whereas relaxations to papaverine (3 and 10 microM) were not reduced. In contrast, diabetes of 3-days duration did not affect relaxations to field stimulation, SNP or NO. 4. Incubation of anococcygeus muscles from untreated rats in medium containing elevated glucose (44.1 mM) for 6 h, significantly impaired relaxations to field stimulation compared to responses obtained after normal glucose (11.1 mM) incubation. Relaxations to SNP and to NO were not affected by 6 h exposure to elevated glucose. Similarly, incubation in hyperosmolar solutions containing mannose or sucrose for 6 h, impaired relaxations to field stimulation, but not to SNP or NO. 5. The results indicate that the diabetes-induced impairment of NO-mediated neurotransmission in the rat anococcygeus muscle develops between 3 days and 2 weeks after the induction of diabetes with streptozotocin. Prevention of the impairment by insulin treatment suggests that it is specific for the diabetic state. In addition, the impairment may be related to hyperglycaemia and the consequent rise in osmolarity, since in vitro exposure to high glucose as well as to other hyperosmolar media impaired NO-mediated relaxations to field stimulation.