Guanethidine adrenergic neuropathy: an animal model of selective autonomic neuropathy

The Influence of Indomethacin and Guanethidine on Experimental Streptozotocin Diabetic Neuropathy

In diabetic animals, reduced endoneurial perfusion and oxygen content have been linked to neuropathic abnormalities and might be amenable to pharmacological manipulation. In streptozotocin-induced diabetic rats, we studied the influence of guanethidine adrenergic sympathectomy, indomethacin treatment and a combined strategy on: serial in vivo motor and sensory conduction, resistance to ischemic conduction failure, in vitro myelinated and unmyelinated conduction, endoneurial perfusion and endoneurial oxygen tension. Unlike previous work diabetic animals had normal endoneurial perfusion but lower endoneurial oxygen tensions after six months of hyperglycemia. Guanethidine worsened sensory conduction despite lower microvascular resistance and an improvement in endoneurial oxygen tension. In contrast, indomethacin improved motor and sensory conduction but not oxygen tension. These studies do not support a linkage between conduction deficits and early endoneurial microangiopathy in experimental diabetes. Indomethacin, or related agents may offer a new therapeutic approach toward diabetic neuropathy through a mechanism independent of the endoneurial microvasculature.

Chemical sympathectomy alters food intake and thermogenic responses to catecholamines in rats

It has been suggested that the sympathetic nervous system contributes to the short-term control of feeding. The adrenergic innervation of some splanchnic organs seems to be especially involved in such processes, since catecholamines reduce feeding only when injected intraperitoneally or intraportally. In this work, the effects of neonatal sympathetic denervation with guanethidine (Gnt) upon food intake were assessed in adult rats. Gnt-treated male rats had lower body weight gain. The hypophagic response to intraperitoneal (ip) norepinephrine was 70% higher in Gnt-treated animals as compared to controls (P < 0.05); that of epinephrine (E) by 33% (P < 0.05) and that of isoproterenol was not significantly modified. As in normal rats, the hypophagic effect was much stronger after ip than after intramuscular (im) administration (P < 0.05). On the other hand, resting oxygen consumption (VO2) was consistently lower in denervated animals. Ip E administration did not modify VO2, while im E caused increased motor activity and VO2 (P < 0.05). In contrast to control rats, the respiratory exchange ratio in ad libitum fed Gnt rats did not decrease after Ip E administration, suggesting a lack of effect upon lipid mobilization. The lower rate of body weight gain induced by neonatal Gnt sympathectomy might be due to lower daily food intake possibly related, in part, to the sensitization of the alpha-adrenergic porto-hepatic response to endogenous catecholamines. Compared with controls, Gnt-treated rats also showed a limited thermogenic capacity not related to feeding, and a greater degree of carbohydrate oxidation, possibly due to a defect in E-induced lipolysis, which is beta-adrenergic.

Distal tibial mononeuropathy in diabetic and nondiabetic rats reared on wire cages: an experimental entrapment neuropathy

Using electrophysiological recordings, we studied a distal tibial mononeuropathy that involves the hind foot of rats reared in cages with wire grid flooring. In an initial set of experiments, serial sciatic-tibial motor conduction recordings were made in smaller or larger rats reared in cages with wire grid or sawdust flooring. Electrophysiological features of the neuropathy were loss in the amplitude of the distal tibial nerve M potential recorded over hind limb foot muscles, temporal dispersion of the potential, often into multiple peaks, and a prolonged distal latency of the response. The changes in M amplitude were more apparent in larger rats with a greater body weight. In a second series of experiments we studied sciatic-tibial conduction over 16 weeks in nondiabetic rats and rats rendered diabetic with streptozotocin raised and wire grid or plastic flooring. Tibial mononeuropathy developed in both wire grid-reared groups, but there was evidence that it appeared earlier in diabetic rats. Electrophysiological changes of distal mononeuropathy also obscured the expected slowing of sciatic-tibial motor conduction velocity from diabetics. Tibial mononeuropathy in rats reared on wire grid flooring may be a useful animal model of human entrapment neuropathy but its presence can confound studies of experimental neuropathy. Rats used in studies of experimental neuropathy should be housed in plastic cages with sawdust or shavings flooring.

Sulindac in established experimental diabetes: a follow-up study

BACKGROUND

In two previous studies we have demonstrated prevention of electrophysiological abnormalities of nerve in experimental STZ (streptozotocin)-induced diabetes (ED) of rats using nonsteroidal anti-inflammatory agents: indomethacin and sulindac. Sulindac might benefit ED because it inhibits both cyclo-oxygenase and aldose reductase.

METHODS

In this work, we examined whether 1 month of sulindac treatment reversed or improved established biochemical and electrophysiological abnormalities in experimental diabetes of 3 months duration. Sulindac-treated diabetic rats (6.0 mg/kg 5/7 days weekly by gavage) were compared to untreated diabetics, nondiabetic controls and sulindac treated control rats.

RESULTS

Diabetic rats developed slowing of conduction velocity in caudal sensory, sural sensory, caudal motor and sciatic tibial motor fibers. Sulindac improved caudal motor and, to a lesser extent sural sensory conduction but not caudal sensory or sciatic tibial motor conduction. Sulindac did not alter sciatic sugars or polyols.

CONCLUSIONS

Sulindac provided modest improvement in some indices of experimental neuropathy in this reversal study, but there was less efficacy than in the preventative study. Reversal paradigms should be examined in all experimental therapies for diabetic neuropathy.

Autonomic involvement in Guillain-Barré syndrome: a review

Autonomic neuropathy is an important and common complication of Guillain-Barré syndrome (GBS). Manifestations be present in cardiovascular, sudomotor, gastrointestinal and other systems involving both sympathetic and parasympathetic fibers. Some apparently selective acute autonomic neuropathies may be subvarieties of GBS. Experimental work in animal models, pathological studies of GBS patients, and autonomic function studies have provided some help in the understanding of this complication. In managing GBS patients with autonomic dysfunction there are important practical considerations that can improve their care. In this article we review the literature on autonomic neuropathy in GBS and propose a management scheme to accommodate it in the overall treatment of the neuropathy.

Neonatal guanethidine treatment alters endoneurial but not dorsal root ganglion perfusion in the rat

In previous work, we suggested that there were differences in vasoregulation between dorsal root ganglia (DRG) and the endoneurium of peripheral nerve trunks. To investigate sympathetic control of both microvessel beds, we compared local perfusion in the sciatic nerve endoneurium and lumbar DRG of adult Sprague-Dawley rats treated from neonatal day 5 with guanethidine monosulfate to induce adrenergic sympathectomy. Control rats were injected with normal saline. Local blood flow and microvascular resistance were measured using microelectrodes sensitive to the clearance of hydrogen. Guanethidine-sympathectomized rats had higher sciatic endoneurial blood flow and lower endoneurial microvascular resistance than saline-injected controls. In contrast, DRG blood flow was not increased by sympathectomy and was comparable to control values despite the hypotension induced by sympathectomy. The results suggest that sympathetic control of local blood flow and may be less apparent in DRG than endoneurium and that local autoregulation may protect DRG from hypotension.

Potential therapeutic approaches to the treatment or prevention of diabetic neuropathy: evidence from experimental studies

Recent investigations using experimental models of diabetes mellitus have emphasized the importance of impaired blood flow for the development of nerve dysfunction. Other observations suggest that this may also be the case for patients. A number of studies have revealed that several types of vasodilators can prevent or successfully treat early conduction abnormalities in diabetic rodents. These include alpha 1-adrenoreceptor antagonists, calcium channel blockers, agents that inhibit the renin-angiotensin system, and vasomodulator prostanoids. Other treatments applied to animal models, such as omega-6 essential fatty acids, aldose reductase inhibitors, aminoguanidine which prevents the formation of advanced glycation end-products, and anti-oxidants all appear to have vascular-related effects that lead to improvements in nerve conduction. These findings suggest that endothelial dysfunction and oxidative stress could be important factors in the aetiology of diabetic neuropathy. Studies have also focused on deficits in axon growth and regeneration, their relation to impaired neuronal synthesis and transport of growth-related chemicals, and neuronotrophic abnormalities. Taken together, the data give rise to the notion that an optimal therapeutic strategy could consist of improving the microenvironment of damaged nerve fibres by manipulating nerve blood flow while concurrently encouraging repair with trophic agents.

Role of vasoconstrictor tone in arterial pressure lability after chronic sympathectomy and sinoaortic denervation in rats

In both chronically sympathectomized (SNX) and sinoaortic denervated (SAD) rats, removal of vasoconstrictor influences decreases mean arterial pressure (MAP) and its variability in parallel. This study examined if this decrease in arterial pressure lability is solely a result of decreasing vascular tone. In conscious 14-week-old male sympathectomized (guanethidine at 1-13 weeks of age) and sinoaortic denervated (2 weeks before study) rats, arterial pressure was recorded beat-to-beat during 30-min consecutive periods; control; ganglionic blockade in sinoaortic denervated rats and angiotensin converting enzyme inhibition plus vasopressin antagonism in sympathectomized rats; and restoration of the initial arterial pressure with continuous infusions of phenylephrine and angiotensin II. Sympathectomized and, even more, sinoaortic denervated rats had increased pressure variability. Neural or humoral blockade markedly reduced arterial pressure and its liability in both groups of rats and subsequent restoration of the arterial pressure with vasoconstrictor infusions returned lability to levels either slightly above (sympathectomy) or below (sinoaortic denervation) control values. In basal conditions, an increased frequency of occurrence of depressor episodes was evidenced in sympathectomized rats whereas a variable ratio of pressor to depressor events was observed in sinoaortic denervated rats. During vasoconstrictor infusions, blood pressure lability was mainly due to depressor events in both groups of rats. It is concluded that the background vascular tone provided by endogenous pressor systems is necessary for the expression of the depressor component of blood pressure lability in sinoaortic denervated and in sympathectomized rats. The study also suggests that in sympathectomized rats, humoral influences act to limit rather than enhance blood pressure lability, whereas in sinoaortic denervated rats, the sympathetic nervous system may directly generate part of the lability.

Effect of chemical sympathectomy on scintigraphic gastric and small bowel transit in the rat

Our aims were to measure gastric and small bowel transit of radiolabeled chow or liquids in rats and to assess the effect of pharmacologically induced chronic sympathectomy of at least 5 weeks duration. Three series of experiments were performed. In series I, four rats in each group underwent i.p. chloral hydrate anesthesia, tracheostomy with intubation and mechanical ventilation: they received by gavage 1.0 g rat chow dissolved in 1.5 ml saline labeled with 1.0 mCi 99mTc-DTPA In series II, four rats in each group were anesthetized with only i.p. chloral hydrate and received 1.5 ml labeled saline. Rats were placed on a gamma camera for dynamic acquisition of 1-min images for 7 h in series I, and 4 h in series II. Activity was quantitated in stomach and cecum; gastric emptying was analyzed by the power exponential model. In series III, four rats in each group were studied for 1 h on gamma camera and sacrificed after 5 h. At the end of each study, the stomach, small bowel, cecum and colon were removed and radioactivity counted in each organ in a dose calibrator. Quantitation of radioactivity by gamma camera was highly accurate: labeled chow in the stomach at 7 h was identical by external camera and counting of the excised organ (linear regression slope = 1.01, r = 0.97, P less than 0.0001). Similar precision was observed with labeled liquids (slope = 0.93, r = 0.93, P = 0.001). Chow emptied slower than liquid in both groups (P = 0.048). Ileocecal movement of isotope occurred by bolus transfers.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin converting enzyme inhibition prevents development of muscle and nerve dysfunction and stimulates angiogenesis in streptozotocin-diabetic rats

The effects of the angiotensin converting enzyme inhibitor lisinopril on slow and fast twitch muscle contractile properties, nerve conduction and hypoxic resistance, and muscle and nerve capillary density were examined in streptozotocin-diabetic rats. Prolongation of soleus contraction and relaxation were partially prevented by treatment (p less than 0.01). A 22% deficit in fast twitch extensor digitorum longus tetanic tension production was also ameliorated (p less than 0.01). Sciatic motor and sensory conduction velocity, 25% and 12% reduced by diabetes respectively, were 75% normalized by lisinopril (p less than 0.01). There was a 47% increase in resistance to hypoxic conduction block with diabetes (p less than 0.01). Lisinopril treatment resulted in normal hypoxic resistance. Capillarization of nerve and muscle was little affected by diabetes; however, there was a 17% increase in capillary density in sciatic nerve, and a 40% increase in extensor digitorum longus muscle with lisinopril (p less than 0.01). For soleus, a smaller treatment-induced increase in capillary density led to an elevated capillary/muscle fibre ratio (p less than 0.01). These results suggest that lisinopril promoted angiogenesis. It was concluded that the beneficial effect of preventive lisinopril treatment is likely to depend upon a reduction of peripheral vascular resistance and improvement of tissue blood flow, which implicates relative hypoxia as an important factor in the development of myopathy and neuropathy in experimental diabetes.

Influence of perivascular peptides on endoneurial blood flow and microvascular resistance in the sciatic nerve of the rat

1. A variety of vasoactive peptides has been identified in the axon terminals innervating vasa nervorum but their function is unknown. In mesenteric arterioles, substance P (SP) and calcitonin gene-related peptide (CGRP) have been postulated to have a role in tonic vasodilatation. 2. We explored the effect of epineurial capsaicin, SP, CGRP, spantide (SP antagonist), and hCGRP (8-37) (CGRP antagonist) on blood flow (EBF) and microvascular resistance (EMR) in the endoneurial compartment of the rat sciatic nerve, as measured by hydrogen clearance. 3. Epineurial capsaicin induced a prompt, intense and prolonged increase in EBF and lowering of EMR as compared to epineurial application of the carrier alone in a separate animal group. The hyperaemic response was also confirmed by studying serial clearance curves in individual animals. 4. Multifibre sciatic-tibial motor conduction was not changed by epineurial capsaicin. 5. When co-administered with capsaicin, hCGRP (8-37) completely blocked the hyperaemic response and increased EMR above the pooled control range. Spantide also blocked the capsaicin response. 6. When administered alone, both epineurial hCGRP (8-37) and spantide lowered EBF below and increased EMR above the control measurements in the same animals. 7. At 10(-5) M epineurial CGRP, but not SP lowered EMR. Vasodilatation from intra-arterial administration of CGRP was much greater and was more prolonged compared with that induced by SP. hCGRP (8-37), but not spantide reduced the intra-arterial response to CGRP. 8. The findings suggest that epineurial peptidergic terminals mediate a vasodilatory response (particularly through CGRP) that increases blood flow in the 'downstream' endoneurial compartment. Physiological peptide release (blocked by SP and CGRP receptor antagonism) may be important in maintaining tonic vasodilatation.

The effect of aging on endoneurial blood flow, hyperemic response and oxygen-free radicals in rat sciatic nerve

Electrophysiologic abnormalities and reduced energy metabolic rate develop in aging nerves. We investigated the effects of aging on nerve blood flow (NBF) in Fischer rats aged 2, 12, 24 and 30 months. NBF regressed negatively with increasing age and this decline was associated with an increase in nerve vascular resistance. Twenty minutes of nerve stimulation resulted in an increase in blood flow by about 50% in adult animals and did not decline with increasing age. As indices of oxygen free radical activity, we measured conjugated dienes, hydroperoxides, and norepinephrine from 2 to 30 months. There was a gradual decline with increasing age of all indices. We conclude that NGF declines with aging due to reduced microvascular caliber. These vessels retain their hyperemic response and oxygen free radical activity is less with increasing age.

Nerve blood flow in early experimental diabetes in rats: relation to conduction deficits

A reduction in nerve blood flow in chronic experimental diabetes has been linked to impaired conduction. Recently, there have been reports that this is preceded by a period of functional hyperemia. The present investigation explored early changes in sciatic nerve endoneurial blood flow and function in streptozocin-treated rats with durations of diabetes from 1 wk to 4 mo. Blood flow was monitored by microelectrode polarography and hydrogen clearance in thiobutabarbital (Inactin)-anesthetized animals. It was reduced by 41% as early as 1 wk after diabetes induction. There was no evidence of an early functional hyperemia, flow remaining 44% depressed up to 4 mo. In another investigation, similar reductions in blood flow were acutely induced in normal rats rendered hyperglycemic by glucose infusion. In diabetic animals, conduction velocity in sciatic branches supplying gastrocnemius and tibialis anterior muscles was correlated with blood flow. The link was further tested using a group of 2-mo diabetic rats treated with guanethidine. Treatment caused a functional adrenergic sympathectomy, and blood flow increased to within the normal range. Conduction velocity, depressed by 26% with diabetes, was normalized by treatment. These observations support the hypothesis that hyperglycemia-induced blood flow reductions and resultant endoneurial hypoxia are important factors underlying nerve conduction deficits early in the development of diabetic neuropathy.

Effects of sympathectomy in a model of causalgiform pain produced by partial sciatic nerve injury in rats

In a previous report we presented a novel behavioral model of neuropathic pain disorders, produced in rat by a unilateral ligation of about half of the sciatic nerve. The model is characterized by rapid onset of behaviors suggesting spontaneous pain and disordered responses to non-noxious and noxious stimuli. These include reduced withdrawal thresholds to repetitive touch in the partially deafferented skin ('touched-evoked hyperesthesia'), touch-evoked allodynia, reduced withdrawal thresholds to noxious thermal stimuli and exaggerated responses to noxious heat and mechanical stimuli ('thermal hyperalgesia'). Some of these disorders are seen at mirror image sites on the hind limb opposite the lesion. These disorder start within hours after partial nerve injury, last many months and are very similar to causalgia in humans following partial nerve injury. Since sympathetic efferent activity is known to aggravate causalgia in humans and sympathectomy is known to relieve it, we studied the effect of changing sympathetic outflow in the rat model. Reversible sympathectomy was carried out using guanethidine injected intraperitoneally in 3 experiments, each at a different time in relation to the partial nerve injury. We found that: (1) sympathectomy performed several months postoperatively alleviated the sensory disorders bilaterally; (2) sympathectomy prior to nerve injury partially prevented the appearance of thermal hyperalgesia but did not affect hyperesthesia to repetitive touch; and (3) sympathectomy at the time of nerve injury aggravated the sensory disorders during the first few days. As maintenance and production of the sensory disorders in this animal model depended on sympathetic nervous outflow, we conclude that the rats were suffering from a syndrome analogous to sympathetically maintained causalgia in man.

Stimulation-induced peripheral nerve hyperemia: mediation by fibers innervating vasa nervorum?

We studied the influence of graded nerve trunk stimulation on endoneurial blood flow (NBF), oxygen tension and pH in the rat sciatic nerve. The purpose of the investigation was to explore changes in the in vivo microenvironment of axons which result from their metabolic activation. Using endoneurial microelectrodes, we made serial measurements before, during and after varying stimulation protocols in curarized animals. NBF was increased by stimulation and the effect was directly related to the frequency, length and intensity of the stimulation train. At an intensity designed to supramaximally recruit myelinated fibers, high frequency (50 and 100 Hz) and prolonged duration (5-15 min) stimulation increased NBF. The effect was partly blocked by infiltration of lidocaine at the stimulating site. High intensity stimulation, designed to also recruit unmyelinated fibers, was particularly effective in enhancing NBF and lowering microvascular resistance. Continuous measurements of endoneurial oxygen and pH tension failed to identify associated declines with stimulation irrespective of the protocol. Both oxygen tension and pH tended to rise with stimulation. These studies confirm the presence of stimulation-induced endoneurial hyperemia but provide no evidence that local hypoxia or acidosis are mechanisms. Stimulation recruitment of vasodilatory axons innervating vasa nervorum may be an alternative explanation.

Guanethidine-induced adrenergic sympathectomy augments endoneurial perfusion and lowers endoneurial microvascular resistance

Chronic administration of guanethidine sulfate in the rat induces a selective adrenergic neuropathy. We studied the effects of guanethidine-induced adrenergic sympathectomy on rat sciatic nerve blood flow (NBF), microvascular resistance (MR), vessel caliber and norepinephrine (NE) content. A control group of animals was studied following chronic administration of mammalian Ringer's solution. NBF and MR were measured with an endoneurial microelectrode, using the technique of hydrogen clearance (HC). Following HC, the sciatic nerve was perfused with India Ink, removed, frozen and sectioned. Measurements were made of endoneurial microvessel numbers, diameter, circumference and area. The contralateral sciatic nerve was removed for measurements of NE content. In guanethidine-treated animals we observed elevated NBF, reduced MR and dilated microvessels. Numbers of microvessels and fascicular areas were similar to controls. NE content was markedly reduced following sympathectomy. These studies suggest that NBF, unlike cerebral blood flow (CBF), is regulated by its adrenergic input. Removal of adrenergic innervation of the vasa nervorum appears to result in a loss of tonic vasoconstrictive action.

Role of sympathoadrenomedullary system in cardiovascular response to stress in rats

Sympathetic nerve and/or adrenal medulla contributions to stress-induced cardiovascular responses were investigated by factoring out their influence using adrenal demedullation (DMED) and/or chemical sympathectomy with guanethidine (GUAN). Rats divided into 4 groups [sham-operated/saline (SHAM/SAL), SHAM/GUAN, DMED/SAL and DMED/GUAN] were injected i.p. over 4 weeks with either saline or GUAN (25 mg/kg/day). At the end of this treatment period, blood pressure (BP) and heart rate (HR) were monitored via carotid catheter prior to and during restraint in conscious rats. Treatments did not alter basal BP or HR when compared to controls. Restraint increased HR (delta 72 bpm) and systolic, diastolic and mean BP (delta approximately 20 mm Hg) in control animals. Restraint-induced HR change was significantly greater in DMED/SAL animals (delta 88 bpm), but less in SHAM/GUAN animals (delta 40 bpm) than in controls. DMED/GUAN was not different from SHAM/GUAN alone in altering HR response to stress, supporting the greater influence of sympathetic nerves over adrenal medulla in controlling HR. Chronic GUAN abolished normal pressor responses to restraint stress. DMED increased diastolic blood pressure response to stress. However, in DMED/GUAN rats, not only did stress fail to increase blood pressure but rather stress produced hypotension (delta - 34 mm Hg MAP), demonstrating the role of adrenal medulla in maintaining BP during stress. Differential effects of the various treatments on diastolic and systolic pressure suggest that the treatments had effects on peripheral vasculature. These results demonstrate that sympathetic nerves and adrenal medulla have important influences on cardiovascular function during stress and that in the absence of either, the other system may partially compensate.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic sympathectomy ablates unmyelinated fibers in the rat 'preganglionic' cervical sympathetic trunk

Classical anatomical depictions of the cervical sympathetic trunk label it as a cholinergic preganglionic structure. We studied the cervical sympathetic trunk of the rat following daily injection for 5 weeks of guanethidine monosulphate, a regimen known to selectively destroy adrenergic neurons outside of the blood-brain barrier leaving cholinergic systems and preganglionic structures intact. The drug-treated animals were compared with a group of physiologic saline-injected animals. In the drug-treated animals, there was an approximately 40% reduction in the numbers of unmyelinated fibers per unit area compared to controls. The finding of swollen and degenerative appearing unmyelinated fibers at 7 days of drug treatment confirmed that the fiber loss resulted from active axonal degeneration. The pattern of unmyelinated fiber loss was expressed as a reduction of fibers per Schwann cell-basement membrane profile with an appearance of 'empty profiles', and a conversion of large profiles (with large numbers of fibers per profile) to smaller size categories. There were no differences in axon diameters, fascicular areas, and numbers of microvessels between the groups. Microvessels were dilated in the drug-treated animals. These findings suggest that a large component of the cervical sympathetic chain in the rat consists of postganglionic adrenergic fibers which appear to intermingle with preganglionic cholinergic axons coursing through the chain.