Diabetic neuropathies: pathogenetic mechanisms and therapeutic perspectives

Effects of ONO-2235, an aldose reductase inhibitor, on muscarinic receptors and contractile response of the urinary bladder in rats with streptozotocin-induced diabetes

This study was conducted to evaluate effects of the aldose reductase inhibitor ONO-2235 on the contractile response to acetylcholine of the urinary bladder dome of streptozotocin-induced diabetes mellitus (DM) rats and simultaneously observe the changes in the function and number of muscarinic receptors and the sorbitol content of the bladder. The contractile response to acetylcholine increased 51% in the DM rat bladder dome compared to the normal rats; however, this was attenuated to a 10% increase by administration of 100 mg/kg ONO-2235 for 2 weeks. Treatment with ONO-2235 significantly decreased the specific [3H]quinuclidinyl benzilate binding in DM rats. However there was no significant dose-dependency among the ONO-2235-treated groups. The sorbitol levels of the sciatic nerve and the bladder were higher in the DM rats compared to the control rats; ONO-2235 decreased the level, although it did not completely reverse them to the control level. These results suggest that an aldose reductase inhibitor attenuates the increase of the muscarinic receptor number and normalizes the enhanced contractile response to acetylcholine caused by hyperglycemia and diuresis, probably through suppression of the polyol-pathway in the DM rat bladder dome.

Diabetes-induced alterations in the properties of muscarinic cholinergic receptors in rat vas deferens

Muscarinic cholinergic receptors were identified and characterized by radioligand receptor binding assay using [3H]quinuclidinyl benzilate (QNB) in rat vas deferens membrane particulates of three experimental groups: 1) 8-week diabetic, 2) 8-week diabetic insulin-treated and 3) age-matched control. Diabetes was induced by the intravenous injection of 65 mg./kg. streptozotocin (STZ). The density of muscarinic receptors (Bmax values), as determined by saturation experiments with [3H]QNB, was demonstrated to be higher in the vas deferens of diabetic rats than in the vas deferens of control and diabetic insulin-treated rats. The equilibrium dissociation constants (KD values), however, were similar in all three groups. Muscarinic cholinergic antagonists competed with [3H]QNB binding sites in the vas deferens membrane particulates with the following rank order of Ki values: atropine < methoctramine < or = 4-DAMP < AF-DX 116 < HHSiD < pirenzepine = pfHHSiD. The pharmacological profile of muscarinic receptors was similar in all three groups. Additional pharmacological studies showed a similar rank order of Ki values for vas deferens, bladder dome and heart, but this rank order was significantly different in cerebral cortex and prostate. This is consistent with the predominance of the M2 muscarinic cholinergic receptor subtype in the rat vas deferens. It is concluded that STZ-induced diabetes causes an upregulation of muscarinic cholinergic receptor density in the rat vas deferens that can be prevented by the administration of insulin.

Effect of insulin and dietary myoinositol on muscarinic receptor alterations in diabetic rat bladder

Previous studies from our laboratory demonstrated that there is an up-regulation of muscarinic receptor density in the bladder dome of the 8-wks diabetic rat compared to control. To determine whether the changes observed in receptor density can be corrected by insulin or dietary myoinositol, five groups of rats were maintained for eight weeks: control (C), diabetic (D), diabetic insulin-treated (DI), diabetic myoinositol-treated (DMI), and control myoinositol-treated (CMI). Diabetes was induced by i.v. injection of 65 mg./kg. of streptozotocin. D and DMI animals were smaller, had higher serum glucose and lower serum insulin levels, higher water intakes and urine outputs, and larger bladder domes than the other groups. The density of the muscarinic receptors measured by radioligand receptor binding assays using [3H]quinuclidinyl benzilate were (in fmol./mg. protein): C, 88 +/- 13; D, 176 +/- 19; DI, 94 +/- 5; DMI, 158 +/- 8; CMI, 112 +/- 10. These data indicate that insulin, but not myoinositol treatment normalized diabetes induced abnormalities observed in the general features of streptozotocin-injected rats and prevented the diabetic-induced upregulation of bladder dome muscarinic receptors.

The field-stimulated vas deferens of the streptozotocin-diabetic mouse: effects of prazosin, alpha, beta-methylene ATP, and variation of stimulation parameters

The question of diabetic neuropathy was studied in the field-stimulated isolated vas deferens of the mouse. The animals were treated with either buffer or streptozotocin (170 mg/kg i.v.) 2 or 4 weeks, respectively, beforehand. Stimulus-response relationships were tested by variation of frequency (VF) at constant pulse width and by variation of pulse width (VP) at constant frequency. The adrenergic twitch component was eliminated by prazosin (1 microM) and the purinergic component by alpha, beta-methylene ATP (MeATP, 10 microM). The diabetes did not alter the muscular contractility (tested with KCl) and left the twitch-inhibiting effects of prazosin and MeATP unchanged, thereby revealing no difference in susceptibility between noradrenergic and purinergic mechanisms. However, in diabetic vasa, the maximal effectiveness of stimulation was decreased with VF but not VP, whereas the sensitivity of intramural neurons (50% effective frequency or pulse width, respectively) was unchanged with VF and reduced with VP. This may suggest that the diabetic neuron release less transmitter (VF), which can be compensated for by the activation of less sensitive neurons (VP). Actually, the uptake of 3H-noradrenaline into the (4 weeks-) diabetic vas was normal but the stimulation-induced fractional release of tritium was decreased by 26%. It is concluded that a sympathetic neuropathy occurred in the vas deferens of the streptozotocin-diabetic mouse.

An examination of autonomic nervous function in genetically diabetic mice

1 This study was designed to determine whether the autonomic innervation of the heart and vas deferens in genetically diabetic mice exhibited dysfunction similar to those seen in chemically diabetic animals and diabetic patients. 2 Diabetic mutant mice (outcrossed from the C57 BL/KS db/db strain) were compared with their non-diabetic litter-mates at age 20 to 22 weeks. Right and left atria and vasa deferentia were removed from freshly killed animals and subjected to nerve stimulation and treatment with noradrenaline (NA) or acetylcholine (ACh) in organ baths. 3 Right atria from diabetic animals were less responsive to noradrenergic nerve stimulation than control preparations but there was no such difference between the noradrenergic responses of left atria from the two groups of mice. Both atria were hypersensitive to exogenous NA. 4 Atria from diabetic mice responded to cholinergic nerve stimulation and exogenous ACh in a fashion similar to those of non-diabetic mice. Likewise in the responses of vasa deferentia to nerve stimulation were similar in the two groups. These findings are indicative of some autonomic nervous dysfunction characteristic, to an extent, of diabetes mellitus.

Effects of streptozotocin diabetes on the noradrenergic innervation of the rat heart: a longitudinal histofluorescence and neurochemical study

The effects of the age of induction and total duration of streptozotocin diabetes on the sympathetic noradrenergic innervation of the rat heart was examined with glyoxylic acid induced histofluorescence to demonstrate the distribution of noradrenergic fibers within the heart, and with high performance liquid chromatography with electrochemical detection to measure tissue levels of the neurotransmitter norepinephrine. Diabetes was induced in male Sprague-Dawley rats at 1, 2, and 4 months of age. Within each of these groups, diabetic rats survived for periods of 1, 2, and 4 months. Additional groups of diabetic rats survived to a chronological age of 8 months. Norepinephrine levels in the hearts of diabetic rats were increased over those of control rats in all groups at 1 month duration of diabetes. Ventricles were generally affected to a greater extent than atria. At 2 months duration of diabetes, ventricular levels remained elevated while atrial norepinephrine levels were at or below control levels. At 4 months duration of diabetes, and in all groups at 8 months of age, the norepinephrine levels were at or below control levels, except in the ventricles of rats induced at 4 months of age, which remained elevated. Histofluorescence studies demonstrated the presence of axon bundles and varicose noradrenergic profiles in the diabetic rat hearts, distributed in a pattern similar to that seen in controls. However, at 1 month duration of diabetes in all groups, the density of noradrenergic varicosities in diabetic rat hearts appeared increased with abundant branched profiles. These results are surprising, since studies on genetic models of diabetes have suggested decreased norepinephrine levels in the heart. The present study suggests that during the early phases of streptozotocin induced diabetes, noradrenergic nerves are still intact and may be susceptible to pharmacologic manipulation. The later fall of norepinephrine levels back to or below control levels may indicate actual neuronal damage, suggesting that early intervention may be necessary to protect these nerves from degeneration. This issue is potentially important in view of the reported toxic effects of high NE levels on the heart, and the high incidence of death from myocardial infarct in diabetic humans with autonomic neuropathy.

Axonal transport of noradrenaline and noradrenergic transmission in rats with streptozotocin-induced diabetes

The accumulation of noradrenaline in constricted sciatic nerves was measured in 6 month diabetic rats (streptozotocin 35 mg/kg) and 4 day diabetic rats (streptozotocin 70 mg/kg) together with two groups of age-matched control animals. There was no alteration in the amount of noradrenaline accumulated in the nerves of the diabetic animals when compared with the controls. The vasa deferentia of the long-term diabetic animals showed an impaired response to stimulation of their noradrenergic nerves and a hypersensitivity to exogenous noradrenaline. These vasa were not wasted and showed a normal contractility in response to potassium chloride. Vasa deferentia from the short-term diabetic rats showed no abnormalities of function. Vasa deferentia from all groups of rats were also examined at the ultrastructural level. Specimens from all the chronically diabetic animals contained many abnormal nerve terminals. These lesions were not seen in vasa from the short-term diabetic rats. Taken together these findings indicate that rats with chronic streptozotocin-induced diabetes exhibit pathological changes in the noradrenergic nerves supplying the vas deferens. These animals do not, however, show an impairment of the axonal transport of noradrenaline.