Dose-dependence of endoneurial fluid sodium and chloride accumulation in galactose intoxication

Is diabetes the risk factor for poor neurological recovery after cervical spine surgery? A review of the literature

The poor prognosis of cervical spine surgery is mainly manifested as poor neurological recovery and the presence of new upper extremity dysfunction that promotes significant psychological and physiological burdens on patients. Many factors influence the prognosis of cervical spine surgery, including the age of patients, the time and mode of surgery, and the surgical technique used. However, in clinical studies, it has been observed that patients with diabetes have a higher probability of poor prognosis after surgery. Therefore, we review the pathophysiology of diabetic neuropathies and discuss its impact on cervical nerve system function, especially in cervical nerve roots and upper limb peripheral nerve conduction.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Homeostatic regulation of the endoneurial microenvironment during development, aging and in response to trauma, disease and toxic insult

The endoneurial microenvironment, delimited by the endothelium of endoneurial vessels and a multi-layered ensheathing perineurium, is a specialized milieu intérieur within which axons, associated Schwann cells and other resident cells of peripheral nerves function. The endothelium and perineurium restricts as well as regulates exchange of material between the endoneurial microenvironment and the surrounding extracellular space and thus is more appropriately described as a blood-nerve interface (BNI) rather than a blood-nerve barrier (BNB). Input to and output from the endoneurial microenvironment occurs via blood-nerve exchange and convective endoneurial fluid flow driven by a proximo-distal hydrostatic pressure gradient. The independent regulation of the endothelial and perineurial components of the BNI during development, aging and in response to trauma is consistent with homeostatic regulation of the endoneurial microenvironment. Pathophysiological alterations of the endoneurium in experimental allergic neuritis (EAN), and diabetic and lead neuropathy are considered to be perturbations of endoneurial homeostasis. The interactions of Schwann cells, axons, macrophages, and mast cells via cell-cell and cell-matrix signaling regulate the permeability of this interface. A greater knowledge of the dynamic nature of tight junctions and the factors that induce and/or modulate these key elements of the BNI will increase our understanding of peripheral nerve disorders as well as stimulate the development of therapeutic strategies to treat these disorders.

The effects of repetitive compression on nerve conduction and blood flow in the rabbit sciatic nerve

The objective of this study was to clarify the effect of repetitive compression on nerve physiology in an experimental rabbit model. We defined 80 mmHg as a compression force which caused temporary disturbance of nerve conduction and blood flow with a brief compression. The following compressions were applied for 30 minutes to rabbit sciatic nerves: continuous compression, low frequency release compression (1 second of release time every 30 seconds) and high frequency release compression (1 second of release time every 10 seconds). Compound nerve action potentials and nerve blood flow were evaluated from the start of compression until 30 minutes after release. Endoneurial microvascular permeability was evaluated with Evans Blue albumin. The repetitive compression groups showed delay in recovery of compound nerve action potentials and blood flow after release, with endoneurial oedema. These findings suggest that repetitive compression may increase the risk of breakdown of the blood nerve barrier.

Expression of aldose reductase and sorbitol dehydrogenase genes in Schwann cells isolated from rat: effects of high glucose and osmotic stress

To investigate the polyol pathway activity in Schwann cells, we determined the mRNA levels of aldose reductase (AR) and sorbitol dehydrogenase (SDH) in cultured cells under hyperglycemic or hyperosmotic conditions using competitive RT-PCR technique. The expressions of AR and SDH mRNAs in Schwann cells were unaltered by high (30 mM) glucose content in the medium. On the other hand, osmotic stress elicited significant increases in AR mRNA without any effect on SDH mRNA expression. The levels of AR mRNA determined by this RT-PCR system were significantly correlated with AR activity, as well as the levels of sorbitol accumulated in Schwann cells cultured under hyperosmotic conditions. These findings suggest that in contrast to the induction of AR expression by osmotic stress, high glucose per se does not up-regulate expression of the enzymes constituting the polyol pathway in Schwann cells. The RT-PCR system developed in this study may be a useful tool in ascertaining the relative contributions of AR and SDH to the metabolic derangements leading to diabetic complications.

Effects of seven days of galactose feeding and aldose reductase inhibition on mast cells and vessel morphometry in rat sciatic nerve

The association between mast cells and vessel morphometry in sciatic nerve was examined after seven days in animals fed a diet of 40% D-galactose and compared to control rats and to galactose-fed animals treated with the aldose reductase inhibitor, Tolrestat. Electron microscopy revealed an increase in the total number of mast cells and the number of degranulated mast cells in galactose-fed animals (7.8 +/- 2.9; 2.6 +/- 2.9; mean +/- SD) compared to controls (4.6 +/- 2.1; degranulated mast cells were not seen in any control nerves) and Tolrestat-treated, galactose-fed animals (4.4 +/- 2.5; 0.1 +/- 0.4). Although no significant differences were noted in the numbers of vessels between the three groups, an index of vasoconstriction was significantly increased in the galactose-fed animals (0.115 +/- 0.048; mean +/- SD) compared to controls (0.068 +/- 0.011) and Tolrestat-treated, galactose-fed animals (0.075 +/- 0.20). These data suggest that mast cell degranulation is associated with the vascular constriction induced by seven days of galactose intoxication and that both may be prevented by inhibiting aldose reductase.

Decreased endoneurial fluid electrolytes in normal rat sciatic nerve after aldose reductase inhibition

The role of the enzyme aldose reductase in nerve homeostasis was examined by treating rats with an aldose reductase inhibitor. Female Sprague-Dawley rats were treated with Ponalrestat (25 mg/kg/day) or with excipient alone for 4 to 12 weeks before examining electrophysiologic function, endoneurial fluid electrolyte concentrations, nerve polyol levels, water content and (Na+,K+)-ATPase activity. Sorbitol, the product of glucose metabolism by aldose reductase, was detected in all nerves from control animals, whereas it was below detection limits in 7 of 11 nerves from Ponalrestat-treated rats. Ponalrestat treatment reduced endoneurial fluid sodium and chloride concentrations by 25% and 37%, respectively (both P < 0.001). No differences in nerve water content, conduction velocity, or ATPase activities were detected. These data, and previous studies demonstrating that increased flux through aldose reductase causes the accumulation of endoneurial electrolytes, suggest a role for this enzyme in modulation of the endoneurial microenvironment. However, short-term inhibition of aldose reductase does not appear to affect nerve function. Thus, our findings do not elicit concerns regarding the use of aldose reductase inhibitors in the treatment of clinical diabetic neuropathy.

Dose-dependent alterations in nerve polyols and (Na+,K+)-ATPase activity in galactose intoxication

Nerve polyol content and (Na+,K+)-adenosine triphosphatase (ATPase) activity of nerve homogenates were studied in a colony of rats fed diets containing either 0%, 10%, 20%, or 40% galactose for 4 months. Nerve water and dulcitol content exhibited dose-dependent increases, whereas nerve myo-inositol content declined with increasing dietary galactose. Homogenate (Na+,K+)-ATPase activity increased with increasing galactose consumption of up to 20% dietary intake and thereafter remained consistently elevated at twice the activity of 0% galactose-fed values. Nerves of rats fed 40% galactose were also examined at the light microscope level and showed evidence of both edema and myelin splitting. These data demonstrate that increased nerve water content, dulcitol accumulation, and myo-inositol depletion parallel the previously reported dose-related increase of endoneurial fluid sodium and chloride in nerves of galactose-fed rats and suggest that elevated nerve homogenate (Na+,K+)-ATPase activity may be related to one or more of these consequences of exaggerated polyol pathway flux.

Fine-structural localization of aldose reductase and ouabain-sensitive, K(+)-dependent p-nitro-phenylphosphatase in rat peripheral nerve

Aldose reductase was visualized by light and electron microscopy using a goat anti-rat antibody with immunoperoxidase and immunogold, respectively. Ouabain-sensitive, K(+)-dependent, p-nitro-phenylphosphatase, a component of (Na+, K+)-ATPase, was localized at the electron microscopic level by enzyme histochemistry using p-nitro-phenylphosphate as substrate. In peripheral nerve, spinal ganglia and roots, the Schwann cell of myelinated fibers was the principal site of aldose reductase localization. Immunostaining was intense in the paranodal region and the Schmidt-Lanterman clefts as well as in cytoplasm of the terminal expansions of paranodal myelin lamellae and the nodal microvilli. Schwann cell cytoplasm of unmyelinated fibers were faintly labelled. Endoneurial vessel endothelia, pericytes and perineurium failed to bind appreciable amounts of aldose reductase antibody. However, mast cell granules bound antibody strongly. In contrast, p-nitro-phenylphosphatase reaction product was detected in the nodal axolemma, terminal loops of Schwann cell cytoplasm and the innermost layer of perineurial cells. In endothelial cells, reaction product was localized on either the luminal or abluminal, or on both luminal and abluminal plasmalemma. Endothelial vesicular profiles were often loaded with reaction product. Occasional staining of myelin and axonal organelles was noted. Mast cells lacked reaction product.

Role of the blood-nerve barrier in experimental nerve edema

Nerve edema is a common response to the nerve injury seen in many peripheral neuropathies and is an important component of Wallerian degeneration. However, independent pathologic effects of nerve edema that aggravate or induce nerve injury extend the role of edema beyond that of an epiphenomenon of injury. New insights into the mechanism and impact of nerve edema come largely from animal models. In the following review, we discuss the cause and consequences of nerve edema with particular reference to endoneurial fluid pressure and its relevance to the nerve microenvironment. Experimental models of nerve edema include conditions with increased vascular permeability such as lead poisoning, experimental allergic neuritis, and murine globoid leukodystrophy. Increased perineurial permeability induced by local anesthetics and neurolytic drugs can also induce nerve edema sufficient to increase endoneurial fluid pressure. Both perineurial and vascular permeability are increased after damage induced by crush, freeze, or laser injury. One of the most important forms of nerve edema is induced by external compression; the significance of this change is that edema has local compressive effects that persist after the external pressure has been relaxed. Nerve edema and increased endoneurial fluid pressure also occur in conditions in which vascular permeability appears to be unchanged such as experimental diabetic neuropathy and in hexachlorophene intoxication. In both of these conditions, reduced nerve blood flow has been demonstrated in rats and is viewed as a consequence of increased endoneurial fluid pressure. Whatever its mechanism, endoneurial edema has important structural and functional consequences for nerve fibers. A clear understanding of the underlying pathology of the nerve microenvironment may provide useful insights into treatment of clinical neuropathies.