Central-peripheral distal axonopathy in the spontaneously diabetic BB-rat: ultrastructural and morphometric findings

Proliferative and nonproliferative lesions of the rat and mouse central and peripheral nervous systems

Harmonization of diagnostic nomenclature used in the pathology analysis of tissues from rodent toxicity studies will enhance the comparability and consistency of data sets from different laboratories worldwide. The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of four major societies of toxicologic pathology to develop a globally recognized nomenclature for proliferative and nonproliferative lesions in rodents. This article recommends standardized terms for classifying changes observed in tissues of the mouse and rat central (CNS) and peripheral (PNS) nervous systems. Sources of material include academic, government, and industrial histopathology databases from around the world. Covered lesions include frequent, spontaneous, and aging-related changes as well as principal toxicant-induced findings. Common artifacts that might be confused with genuine lesions are also illustrated. The neural nomenclature presented in this document is also available electronically on the Internet at the goRENI website (http://www.goreni.org/).

Encephalopathies: the emerging diabetic complications

Diabetic encephalopathies are now accepted complications of diabetes. They appear to differ in type 1 and type 2 diabetes as to underlying mechanisms and the nature of resulting cognitive deficits. The increased incidence of Alzheimer's disease in type 2 diabetes is associated with insulin resistance, hyperinsulinemia and hyperglycemia, and commonly accompanying attributes such as hypercholesterolemia, hypertension and obesity. The relevance of these disorders as to the emergence of dementia and Alzheimer's disease is discussed based on epidemiological studies. The pathobiology of accumulation of β-amyloid and tau the hallmarks of Alzheimer's disease are discussed based on experimental data. Type 1 diabetic encephalopathy is likely to increase as a result of the global increase in the incidence of type 1 diabetes and its occurrence in increasingly younger patients. Alzheimer-like changes and dementia are not prominently increased in type 1 diabetes. Instead, the type 1 diabetic encephalopathy involves learning abilities, intelligence development and memory retrieval resulting in impaired school and professional performances. The major underlying component here appears to be insulin deficiency with downstream effects on the expression of neurotrophic factors, neurotransmitters, oxidative and apoptotic stressors resulting in defects in neuronal integrity, connectivity and loss commonly occurring in the still developing brain. Recent experimental data emphasize the role of impaired central insulin action and provide information as to potential therapies. Therefore, the underlying mechanisms resulting in diabetic encephalopathies are complex and appear to differ between the two types of diabetes. Major headway has been made in our understanding of their pathobiology; however, many questions remain to be clarified. In view of the increasing incidence of both type 1 and type 2 diabetes, intensified investigations are called for to expand our understanding of these complications and to find therapeutic means by which these disastrous consequences can be prevented and modified.

Sequential abnormalities in type 1 diabetic encephalopathy and the effects of C-Peptide

Diabetic encephalopathy is a recently recognized complication in type 1 diabetes. In this review, we summarize a series of experimental results obtained longitudinally in the spontaneously type 1 diabetic BB/Wor-rat, and bringing out the beneficial effects of C-peptide replacement. It is increasingly clear that lack of insulin and C-peptide, and perturbations of their signaling cascades in type 1 diabetes are detrimental to the regulation of neurotrophic factors and their receptors. Other consequences of such deficits and perturbations are innate inflammatory responses with effects on synaptogenesis, neurite degeneration, and early behavioral abnormalities. Replacement of C-peptide, which does not effect hyperglycemia, has beneficial effects on a variety of pro-apoptotic stressors, oxidative stressors, and finally on apoptosis. Eventually, this cascade of events leads to neuronal loss and decreased densities of white matter myelinating cells, with more profound deficits in behavioral and cognitive function. Such changes are likely to underlie gray and white matter atrophy in type 1 diabetes, and are significantly prevented by full C-peptide replacement. Present data demonstrate that C-peptide replacement has beneficial effects on numerous sequential and partly interrelated pathogenetic mechanisms, resulting in prevention of neuronal and oligodendroglial cell loss, with significant prevention of neurobehavioral and cognitive functions.

Neurofilaments in diabetic neuropathy

This review discusses the role of abnormal neurofilament (NF) expression, processing, and structure as an etiological factor in diabetic neuropathy. Diabetic sensory and autonomic neuropathy in humans is associated with a spectrum of structural changes in peripheral nerve that includes axonal degeneration, paranodal demyelination, and loss of myelinated fibers-- the latter is probably the result of a dying-back of distal axons. NF filaments are composed of three subunit proteins, NFL, NFM, and NFH, and are major constituents of the axonal cylinder. It is clear that any abnormality in synthesis, delivery, or processing of these critical proteins could lead to severe impairments in axon structure and function. This article describes mechanisms of synthesis, phosphorylation, and delivery of NF and discusses how these processes may be abnormal in diabetics. The pathological alterations in the ganglion and preipheral nerve that occur in sensory and autonomic neuropath will be outlined and related to possible abnormal processing of NF. A major focus is the role or aberrant NF phosphorylation and its possible involvement in the imparied delivery of NF to the distal axon. Identification of stress-activated protein kinases (SAPKs) as NF kinases is discussed in detail and it is proposed that hyperglycemia-induced activation of SAPKs may be a primary etiological event in diabetic neuropathy.

Effect of IGF-I and neurotrophin-3 on gracile neuroaxonal dystrophy in diabetic and aging rats

Neuroaxonal dystrophy (NAD), a distinctive axonopathy characterized by dramatic swelling of preterminal axons and nerve terminals by the accumulation of a variety of subcellular organelles, develops in the central projections of sensory neurons to medullary gracile nuclei in aged animals and man, and in a number of diseases and experimental conditions. Although its pathogenesis is unknown, proposed mechanisms include abnormalities of axonal regeneration, collateral sprouting and synaptic plasticity which may reflect alteration in neurotrophic support. In the current study, we have demonstrated quantitatively that aging causes the expected marked increase in the frequency of gracile NAD; however, substantial numbers of dystrophic axons develop between 6 and 10 months of age, earlier than expected. Although diabetes has been reported to increase the frequency of NAD in the central processes of sensory neurons in the gracile fasciculus of genetically diabetic BB rats, we have found that 8-10 months of streptozotocin-induced diabetes results in fewer dystrophic axons in the gracile nucleus than in age-matched controls. Administration of neurotrophin-3 (NT-3) and insulin-like growth factor-I (IGF-I), which have been shown to affect synaptic plasticity (implicated in the pathogenesis of NAD), for the last two months before sacrifice did not affect the frequency of gracile NAD in controls or diabetics. The sensory terminals in the gracile nuclei provide a simple, well-characterized experimental system in which questions of pathogenesis and prevention of neuroaxonal dystrophy can be addressed.

Changes in sensory neuropeptides in dorsal root ganglion and spinal cord of spontaneously diabetic BB rats. A quantitative immunohistochemical study

This study examined the expression of the sensory neuropeptides, calcitonin gene-related peptide (CGRP) and substance P (SP), in the lumbar 4 and 5 dorsal root ganglion (DRG) and spinal cord of spontaneously diabetic BB rats and non-diabetic controls using quantitative immunohistochemical analysis. In both animal groups immunoreactivities for CGRP and SP were widely distributed within the neurons of DRG and in nerve fibres of the dorsal spinal cord. Image analysis of each neuropeptide subpopulation in the DRG showed that in diabetic rats the cell diameter of immunostained CGRP neurons was significantly decreased compared with controls, while no difference could be found for SP-immunoreactive (IR) neurons. The decrease in the CGRP-IR cell diameter appeared to occur mainly in medium to large neurons (30-50 microns diameter; 2.2% controls, < 1% diabetes), this change being parallel to an increased frequency of small-size neurons (< 20 microns diameter) in diabetic rats (62% controls, 69% diabetes; P < 0.05). However, there was no statistical difference in the total number of cells immunostained for either CGRP or SP between control and diabetic rats. The ratio of CGRP or SP neurons compared to total cells in the ganglion was similar in control and diabetic groups. No difference could be observed for peptide immunoreactivity in the dorsal and ventral horns of either control or diabetic animals. The observed changes of perikaryal size in diabetic rats might relate to the reduced axonal calibre and conduction velocity observed in these animals, and indicate that subpopulations of sensory neurons are affected differently by diabetes.

Fine structure of presynaptic axonal terminals in sympathetic autonomic ganglia of aging and diabetic human subjects

The neuropathologic changes that may underlie autonomic nervous system dysfunction in nondiabetic elderly human subjects or as a complication of diabetes have been systematically examined in sympathetic ganglia of a series of autopsied human subjects. As in animal models of aging and diabetes, enormously swollen terminal axons were found closely apposed to the perikarya of principal sympathetic neurons in prevertebral superior mesenteric sympathetic ganglia of aged and diabetic human subjects. Dystrophic axons consisted of two stereotyped forms: the first was composed of large numbers of misaligned aggregates of neurofilaments surrounded by variable numbers of small dense core vesicles; the second was characterized by large numbers of mitochondria, vacuoles, and dense and multivesicular bodies. The fine structural characteristics of neuroaxonal dystrophy, its predilection for prevertebral rather than paravertebral sympathetic ganglia, and the tendency for multiple dystrophic axons to cluster preferentially around selected neurons were identical in aged and diabetic human ganglia and were similar to changes seen in animal models of aging and diabetes. Neither diabetic nor aging ganglia demonstrated evidence of neuronal degeneration. Such structural changes may represent a degenerative influence of diabetes and aging on the normal remodeling of nerve terminals in autonomic ganglia, i.e., the continually ongoing process of turnover and replacement of axonal terminals. Similarity of lesions in human diabetes and aging suggests the possibility of pathogenetic mechanisms that are common to diabetes and the aging process. The substantial parallels between humans and animal models provide support for the validity of testing some proposed pathogenetic mechanisms directly in animal models.

Impaired visual evoked potential and primary axonopathy of the optic nerve in the diabetic BB/W-rat

The spontaneously diabetic BB/W-rat has emerged as an important model system for somatic and autonomic diabetic polyneuropathy. In this study we examined visual evoked potentials and the presence of morphometric and structural changes in the optic nerve and the retinal ganglion cells and their afferent axons contained in the retinal nerve fibre layer. A six-month duration of diabetes mellitus was associated with significant increases in the latencies of the visual evoked potentials. The latency of the first positive potential showed a 44% increase, and that of the first negative potential was prolonged by 41%. No significant changes were demonstrated at any of the amplitudes. In the optic nerve mean myelinated fibre size was significantly reduced to 82% of control values, which was accounted for by a significant reduction in axonal size. Axo-glial dysjunction, a prominent structural defect of diabetic somato-sensory neuropathy in both man and diabetic rodents, was non-significantly increased in the optic nerve. In diabetic animals retinal ganglion cells displayed dystrophic changes. No such changes were observed in age- and sex-matched control animals. Proximal axons of the retinal nerve fibre layer showed an increase in dystrophic axons in diabetic BB/W-rats. Morphometric analysis of optic nerve capillaries revealed no abnormalities except for basement membrane thickening. The present data suggest that the diabetic BB/W-rat develops a central sensory neuropathy, characterized functionally by prolonged latencies of the visual evoked potentials and structurally by an axonopathy of optic nerve fibres.

Gracile nucleus of streptozotocin-induced diabetic rats

This study reports ultrastructural changes in the gracile nucleus of male Wistar rats after streptozotocin-induced diabetes. During the acute phase (3-7 days) degenerating electron-dense dendrites and axon terminals were dispersed in the neuropil. Degenerating dendrites were characterized by an electron-dense cytoplasm, swollen mitochondria, dilated endoplasmic reticulum and scattered ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals may form part of a synaptic glomerulus with a central electron-dense dendrite, or they may form the central element of a synaptic glomerulus. These degenerating profiles were absent in the gracile nucleus of the 3 and 7 days insulin-treated post-streptozotocin rats. Macrophages were present in the neuropil and were in the process of engulfing neuronal elements. During the medium phase (1-6 months), most of the degenerating dendrites and axon terminals had been engulfed or removed by macrophages. During the late phase (9-12 months) a second wave of degeneration occurred in the gracile nucleus, similar to the acute phase. During the medium and late phases, dystrophic axonal profiles were also significantly increased in the rats after streptozotocin treatment. It is concluded that the ultrastructural changes observed in the gracile nucleus in the present study were the result of streptozotocin-induced diabetes rather than a toxic effect of streptozotocin, even in the acute phase.

Differential effect of chronic vitamin E deficiency on the development of neuroaxonal dystrophy in rat gracile/cuneate nuclei and prevertebral sympathetic ganglia

Chronic vitamin E deficiency results in the premature and exaggerated development of neuroaxonal dystrophy (NAD) in primary sensory axon terminals in rat medullary gracile/cuneate nuclei, sites in which NAD develops normally with age. In the current study we determined if chronic Vitamin E deprivation had a similar effect on the development of NAD in the celiac/superior mesenteric sympathetic ganglia (C/SMG), another site with age-dependent NAD. The frequency of NAD failed to increase in the SMG of the same vitamin-E deficient animals in which a marked increase in severity of NAD was found in the gracile nucleus. These findings indicate that different populations of neurons are selectively involved in vitamin E deficiency and that the distribution of axonopathy in the E-deficient C/SMG does not duplicate the pattern of experimental diabetes and aging.

Neuroaxonal dystrophy in distal symmetric sensory polyneuropathy of the diabetic BB-rat

We and others have previously described neuroaxonal dystrophic changes as one of the hallmarks of structural diabetic autonomic polyneuropathy involving sympathetic nerves. In the present study, a systemic search for similar changes was undertaken in the mainly sensory symmetric polyneuropathy of the spontaneously diabetic BB-rat. Changes identical to those described in sympathetic nerves in this model were found in sensory ganglion cells, in their proximal extramedullary axons, and in proximal and distal myelinated axons of the spinal dorsal columns. The dystrophic substructures consisted of tubulovesicles, tubular rings, layered membranes, electron-dense membranous bodies, and neurofilamentous changes. Neuroaxonal dystrophic abnormalities increased with increasing duration of diabetes, and exhibited a topographic distribution along the sensory neuroaxonal axis, suggesting metabolic abnormalities as well as abnormalities in the turn-around mechanism of fast axonal transport in the pathogenesis of dystrophic changes in diabetic nerves.

Diabetic and hypoglycemic neuropathy--a comparison in the BB rat

Functional and structural neuropathy was examined in hyperglycemic (diabetic) BB rats maintained on small maintenance doses of insulin, hyperglycemic BB rats receiving no insulin, and BB rats in whom hypoglycemia was induced by the administration of excessive insulin doses. The data were compared with those of non-diabetic age- and sex-matched BB rats. Functional deficits and structural abnormalities were comparable in diabetic rats with and without insulin supplementation, suggesting that the generally necessary insulin dosing in this model does not per se account for the neuropathy. Hypoglycemic neuropathy was characterized by slowing of nerve conduction velocity, marked loss of anterior horn motoneurons and Wallerian degeneration, as well as loss of large myelinated fibers, suggesting a neuropathy involving predominantly motoneurons. Diabetic neuropathy was not associated with nerve cell loss but showed marked axonal atrophy involving predominantly sensory fibers. Thus, diabetic and hypoglycemic neuropathies are two distinguishable entities under strict experimental conditions, but may overlap in human diabetic subjects in whom tight insulin control is desirable.

Microcomputer collection and analysis of RR-interval data in the BB-rat

An analog to digital converter and microcomputer system for the collection of real-time RR-interval data in the BB-rat is described. Calculation of the statistic R is discussed and a commented program listing in Microsoft basic, for performing this transformation, is included as an appendix.

Can the BB-rat help to unravel diabetic neuropathy?

Diabetic neuropathy is probably the most common and one of the most disabling complications of diabetes mellitus. Although several possible pathogenetic mechanisms for this complication have been suggested, they cannot easily be tested in man. Therefore animal models with induced or spontaneous onset of diabetes mellitus have been used. The spontaneously diabetic BB-rat may provide a valuable model, since it displays both metabolic, functional and structural abnormalities of peripheral nerve similar to those present in humans. Based on systematic studies of these abnormalities in peripheral nerve in this model, inter-relationships between diabetic dysmetabolism, dysfunction and structural changes are starting to emerge. These findings are reviewed and a scheme of proven and proposed correlations which now can be tested in this animal model is presented.