Experimental rat models of types 1 and 2 diabetes differ in sympathetic neuroaxonal dystrophy.

Dysfunction of the autonomic nervous system is a recognized complication of diabetes, ranging in severity from relatively minor sweating and pupillomotor abnormality to debilitating interference with cardiovascular, genitourinary, and alimentary dysfunction. Neuroaxonal dystrophy (NAD), a distinctive distal axonopathy involving terminal axons and synapses, represents the neuropathologic hallmark of diabetic sympathetic autonomic neuropathy in man and several insulinopenic experimental rodent models. Although the pathogenesis of diabetic sympathetic NAD is unknown, recent studies have suggested that loss of the neurotrophic effects of insulin and/or insulin-like growth factor-I (IGF-I) on sympathetic neurons rather than hyperglycemia per se, may be critical to its development. Therefore, in our current investigation we have compared the sympathetic neuropathology developing after 8 months of diabetes in the streptozotocin (STZ)-induced diabetic rat and BB/ Wor rat, both models of hypoinsulinemic type 1 diabetes, with the BBZDR/Wor rat, a hyperglycemic and hyperinsulinemic type 2 diabetes model. Both STZ- and BB/Wor-diabetic rats reproducibly developed NAD in nerve terminals in the prevertebral superior mesenteric sympathetic ganglia (SMG) and ileal mesenteric nerves. The BBZDR/Wor-diabetic rat, in comparison, failed to develop superior mesenteric ganglionic NAD in excess of that of age-matched controls. Similarly, NAD which developed in axons of ileal mesenteric nerves of BBZDR/Wor rats was substantially less frequent than in BB/Wor- and STZ-rats. These data, considered in the light of the results of previous experiments, argue that hyperglycemia alone is not sufficient to produce sympathetic ganglionic NAD, but rather that it may be the diabetes-induced superimposed loss of trophic support, likely of IGF-I, insulin, or C-peptide, that ultimately causes NAD.

Sympathetic neuroaxonal dystrophy in the aged rat pineal gland.

Dysfunction of circadian melatonin production by the pineal gland in aged humans and rats is thought to reflect the functional loss of its sympathetic innervation. Our ultrastructural neuropathologic studies of the sympathetic innervation of the pineal gland of aged (24 months old) Fischer-344 and Sprague-Dawley rats showed loss of nerve terminals as well as the development of neuroaxonal dystrophy (NAD), an ultrastructurally distinctive distal axonopathy, far in excess of that in young control rats. Immunolocalization of tyrosine hydroxylase confirmed the age-related loss of normal noradrenergic innervation and development of NAD. NAD was more frequent in aged female rats compared to males and was particularly severe in aged female Sprague-Dawley rats compared to Fischer-344 rats. Pineal NGF content was significantly increased or unchanged in female and male aged Fischer-344 rats, respectively, compared to young controls. The rat pineal is a sensitive experimental model for the quantitative ultrastructural examination of age-related neuropathological changes in nerve terminals of postganglionic noradrenergic sympathetic axons, changes which may reflect similar changes in the diffusely distributed sympathetic innervation of other targeted endorgans.

A potent sorbitol dehydrogenase inhibitor exacerbates sympathetic autonomic neuropathy in rats with streptozotocin-induced diabetes.

We have developed an animal model of diabetic sympathetic autonomic neuropathy which is characterized by neuroaxonal dystrophy (NAD), an ultrastructurally distinctive axonopathy, in chronic streptozotocin (STZ)-diabetic rats. Diabetes-induced alterations in the sorbitol pathway occur in sympathetic ganglia and therapeutic agents which inhibit aldose reductase or sorbitol dehydrogenase improve or exacerbate, respectively, diabetes-induced NAD. The sorbitol dehydrogenase inhibitor SDI-711 (CP-470711, Pfizer) is approximately 50-fold more potent than the structurally related compound SDI-158 (CP 166,572) used in our earlier studies. Treatment with SDI-711 (5 mg/kg/day) for 3 months increased ganglionic sorbitol (26-40 fold) and decreased fructose content (20-75%) in control and diabetic rats compared to untreated animals. SDI-711 treatment of diabetic rats produced a 2.5- and 4-5-fold increase in NAD in the SMG and ileal mesenteric nerves, respectively, in comparison to untreated diabetics. Although SDI-711 treatment of non-diabetic control rat ganglia increased ganglionic sorbitol 40-fold (a value 8-fold higher than untreated diabetics), the frequency of NAD remained at control levels. Levels of ganglionic sorbitol pathway intermediates in STZ-treated rats (a model of type 1 diabetes) and Zucker Diabetic Fatty rats (ZDF, a genetic model of type 2 diabetes) were comparable, although STZ-diabetic rats develop NAD and ZDF-diabetic rats do not. SDI failed to increase diabetes-related ganglionic NGF above levels seen in untreated diabetics. Initiation of Sorbinil treatment for the last 4 months of a 9 month course of diabetes, substantially reversed the frequency of established NAD in the diabetic rat SMG without affecting the metabolic severity of diabetes. These findings indicate that sorbitol pathway-linked metabolic alterations play an important role in the development of NAD, but sorbitol pathway activity, not absolute levels of sorbitol or fructose per se, may be most critical to its pathogenesis.

Analysis of the Zucker Diabetic Fatty (ZDF) type 2 diabetic rat model suggests a neurotrophic role for insulin/IGF-I in diabetic autonomic neuropathy.

Dysfunction of the autonomic nervous system is a recognized complication of diabetes. Neuroaxonal dystrophy (NAD), a distinctive axonopathy involving distal axons and synapses, represents the neuropathologic hallmark of diabetic sympathetic autonomic neuropathy in human and several insulinopenic experimental rodent models. Recent studies have suggested that loss of the neurotrophic effects of insulin and/or IGF-I on sympathetic neurons and not hyperglycemia per se, may underlie the development of sympathetic NAD. The streptozotocin (STZ)-diabetic and BB/W rat, the most commonly used experimental rodent models, develop marked hyperglycemia and concomitant deficiency in both circulating insulin and IGF-I. These animals reproducibly develop NAD in nerve terminals in the prevertebral sympathetic ganglia and the distal portions of noradrenergic ileal mesenteric nerves. The Zucker Diabetic Fatty (ZDF) rat, an animal model of type 2 diabetes, also develops severe hyperglycemia comparable to that in the STZ- and BB/W-diabetic rat models, although in the presence of hyperinsulinemia. In our study, ZDF rats maintained for 6 to 7 months in a severely diabetic state, as assessed by plasma glucose and glycated hemoglobin levels, maintained significant hyperinsulinemia and normal levels of plasma IGF-I at sacrifice. NAD did not develop in diabetic ZDF rat sympathetic ganglia and ileal mesenteric nerves as assessed by quantitative ultrastructural techniques, which is in dramatic contrast to neuropathologic findings in comparably hyperglycemic 6-month STZ-diabetic insulinopenic rats. These data combined with our previous results argue very strongly that hyperglycemia is not the critical and sufficient element in the pathogenesis of diabetes-induced NAD, rather that it is the loss of trophic support, most likely of IGF-I or insulin, that causes NAD.

Non-obese diabetic mice rapidly develop dramatic sympathetic neuritic dystrophy: a new experimental model of diabetic autonomic neuropathy.

To address the pathogenesis of diabetic autonomic neuropathy, we have examined the sympathetic nervous system in non-obese diabetic (NOD) and streptozotocin (STZ)-induced diabetic mice, two models of type 1 diabetes, and the db/db mouse, a model of type 2 diabetes. After only 3 to 5 weeks of diabetes, NOD mice developed markedly swollen axons and dendrites ("neuritic dystrophy") in the prevertebral superior mesenteric and celiac ganglia (SMG-CG), similar to the pathology described in diabetic STZ- and BBW-rat and man. Comparable changes failed to develop in the superior cervical ganglia of the NOD mouse or in the SMG-CG of non-diabetic NOD siblings. STZ-induced diabetic mice develop identical changes, although at a much slower pace and to a lesser degree than NOD mice. NOD-SCID mice, which are genetically identical to NOD mice except for the absence of T and B cells, do not develop diabetes or neuropathology comparable to diabetic NOD mice. However, STZ-treated NOD-SCID mice develop severe neuritic dystrophy, evidence against an exclusively autoimmune pathogenesis for autonomic neuropathy in this model. Chronically diabetic type 2 db/db mice fail to develop neuritic dystrophy, suggesting that hyperglycemia alone may not be the critical and sufficient element. The NOD mouse appears to be a valuable model of diabetic sympathetic autonomic neuropathy with unambiguous, rapidly developing neuropathology which corresponds closely to the characteristic pathology of other rodent models and man.