Radiculoneuropathy of ageing rats: a quantitative study

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

Excessive erythrocytosis in adult mice overexpressing erythropoietin leads to hepatic, renal, neuronal, and muscular degeneration

To investigate the consequences of inborn excessive erythrocytosis, we made use of our transgenic mouse line (tg6) that constitutively overexpresses erythropoietin (Epo) in a hypoxia-independent manner, thereby reaching hematocrit levels of up to 0.89. We detected expression of human Epo in the brain and, to a lesser extent, in the lung but not in the heart, kidney, or liver of tg6 mice. Although no acute cardiovascular complications are observed, tg6 animals have a reduced lifespan. Decreased swim performance was observed in 5-mo-old tg6 mice. At about 7 mo, several tg6 animals developed spastic contractions of the hindlimbs followed by paralysis. Morphological analysis by light and electron microscopy showed degenerative processes in liver and kidney characterized by increased vascular permeability, chronic progressive inflammation, hemosiderin deposition, and general vasodilatation. Moreover, most of the animals showed severe nerve fiber degeneration of the sciatic nerve, decreased number of neuromuscular junctions, and degeneration of skeletal muscle fibers. Most probably, the developing demyelinating neuropathy resulted in muscular degeneration demonstrated in the extensor digitorum longus muscle. Taken together, chronically increased Epo levels inducing excessive erythrocytosis leads to multiple organ degeneration and reduced life expectancy. This model allows investigation of the impact of excessive erythrocytosis in individuals suffering from polycythemia vera, chronic mountain sickness, or in subjects tempted to abuse Epo by means of gene doping.

Effects of pathway and neuronal aging on the specificity of motor axon regeneration

Youth is a strong predictor of functional recovery after peripheral nerve repair, while adulthood is commonly associated with poor outcome. Identification of the factors responsible for this difference could form the basis for strategies to improve regeneration in adults. Preferential reinnervation of motor pathways by motor axons (PMR) occurs strongly in young rats, but is often absent in older animals, and thus parallels the overall trend for superior results in young individuals. These experiments evaluate the individual contributions of peripheral nerve age and motoneuron age to the decline in regeneration specificity (PMR) which accompanies the aging process. The femoral nerves of young and old Lewis rats were removed as inverted "Y" grafts from the femoral trunk proximally to the terminal muscle and cutaneous branches distally. These grafts were transferred from (1) old to young, (2) young to old, (3) old to old, and (4) young to young bilaterally in 10 individuals per group. After 8 weeks of regeneration, reinnervation of cutaneous and muscle branches was assessed by dual labeling with HRP and Fluoro-Gold. Motor neuron regeneration was random in old to old (mean muscle branch (M) = 159, mean cutaneous branch (C) = 168), but PMR was seen when young pathways were used in old animals (M = 163, C = 116). PMR was vigorous when either type of graft was used in young animals (young graft, M = 218, C = 134; old graft, M = 204, C = 127). In this model, motoneuron age appears to be the primary determinant of specificity. However, the pathway also makes significant contributions, as shown by the ability of young pathways to generate specificity in old animals. Manipulation of graft Schwann cell behavior might therefore be an appropriate strategy to improve outcome in older individuals.

Age differences in nociception and pain behaviours in the rat

Much remains to be learned about the effects of ageing on pain. Studies of life-span changes in nociception and pain behaviours in the rat are equivocal making it difficult to draw firm conclusions. This paper reviews the available data and finds that age differences in nociception may be dependent on the pain test employed. Specifically, reflexive responses to nociceptive stimuli do not change with age while there may be no change or a linear decrease with age on more highly organized tests of nociception. Interestingly, age differences in pain behaviours on models of tissue injury and inflammation may not be linear. It is shown that important changes that begin at mid-life in neuroanatomy, neurochemistry and endogenous pain inhibition may be associated with alterations in pain sensitivity. Several testable hypotheses which might encourage future research in this domain are developed throughout this paper.

Peripheral neuropathy in B6C3F1 mice and SD rats induced by chronic intermittent insulin hypoglycemia

The effects of sustained insulin-induced hypoglycemia on peripheral nerves were examined in 9-10-week old female B6C3F1 mice and 9-10-week old female SD rats. Insulin was administered via osmotic minipumps at a dose of 81 IU/kg/day for 2 consecutive weeks. Mice and rats treated with this high insulin dose showed marked hypoglycemia, resulting in half the normal blood glucose level, hypothermia, impaired motor nerve conduction velocity, and an increased incidence of peripheral nerve lesions, consisting of nerve fiber degeneration characterized by irregular myelin sheaths and axonal atrophy.

Comparison of age-related peripheral nerve changes in mice housed in either plastic cages with sawdust-covered solid flooring or wire-mesh-floor cages

A comparative histologic survey was conducted on the dorsal root, sciatic, tibial and medial plantar nerves of 90- and 110-week-old B6C3F1 female mice reared in either solid-floor cages covered in sawdust or wire-mesh-floor cages. Age-related peripheral nerve lesions, characterized by axonal degeneration and remyelination, were present in all nerves surveyed, and were especially prominent in the sciatic and medial plantar nerves at 110 weeks of age but, there were no differences associated with the type of cage floor in clinical signs, grasping power of the fore- and hind-limbs, motor nerve conduction velocity or histopathologic findings at these ages.

Age differences in the response to the formalin test in rats

We report the results of a study designed to assess age differences in the response to the formalin test, a model of tissue injury and inflammation, while controlling for differences in weight and motoric abilities in three groups of adult male Long-Evans rats: young (3 months old), middle-aged (18 months old), and old (24 months old). The first part of the study assessed initial differences in responsivity and found that the middle-aged group showed the greatest response, whereas the young and old groups did not differ from each other. In the second part of the study, the young and middle-aged animals were followed for a 4-month period. The formalin test was repeated at 2-month intervals. These results indicate that there may be an age-associated change in the sensitivity to tonic pain and that this sensitivity may peak at mid-life.

An immunohistochemical study of palmar and plantar digital nerves

We present a qualitative and semi-quantitative description of palmar and plantar digital nerves obtained from cadavers prepared for routine class dissection. The fingertips of the three most functionally important and sensitive digits, namely thumb, index and middle fingers, received more myelinated axons than those of the ring and little fingers. In both palmar and plantar nerves the numbers of myelin basic protein (MBP)-positive myelinated axons decreased proximodistally along each digit and also from thumb to little finger and hallux to little toe respectively. Our results suggest that there are up to ten times more myelinated axons in palmar digital nerves than in plantar nerves.

Aging is associated with divergent effects on Nf-L and GFAP transcription in rat brain

We studied the effects of advancing age on the expression of several proteins important in the structure and function of the nervous system. Brains of young (3 month), middle-aged (13 month), and old (29 month) male Fischer 344 rats were examined. Run-on transcription and Northern blot hybridizations were used to determine gene-specific transcription rates and mRNA levels, respectively. With advancing age, there was a decrement in the transcription rate and mRNA levels for neurofilament-light subunit (Nf-L), but an increment in the transcription rate and mRNA levels for glial fibrillary acidic protein (GFAP). Proteolipid protein (PLP) mRNA levels were attenuated between 3 and 13 months of age, whereas amyloid precursor protein (APP) mRNA levels were attenuated in the middle-aged but not the old animals. Transcription rates for alpha-actin and fos, and mRNA levels for alpha-actin, were unaffected. These observations indicate divergent transcriptional regulation of several genes, notably Nf-L and GFAP, in the aging mammalian forebrain.

A neuropathological study of paraparetic rats injected with HTLV-I-producing T cells

In order to clarify the pathogenesis of HTLV-I-associated myelopathy or tropical spastic paraparesis (HAM/TSP), we injected HTLV-I-producing rabbit or human T cells intravenously into WKA and F344 rats. Infection was confirmed from increase in the anti-HTLV-I antibody titer and from the presence of HTLV-I proviral DNA. Only WKA rats developed hindlimb paraparesis 78-124 weeks after the injection. Neuropathological examination of 5 rats showed degeneration of the anterolateral and posterior funiculi as well as the peripheral nerves, and this degeneration was characterized by prominent vacuolation and macrophage infiltration. The myelopathy and neuropathy were grossly similar to those in human HAM/TSP. Although pathological changes of the spinal cord were very mild in 2 paretic rats, and similar lesions were found in the spinal cords and peripheral nerves of 2 control WKA rats, the myelopathy, radiculoneuropathy, or both in the paretic rats showed greater severity than in the controls. The contribution of the aging process to the lesions of the spinal cord and peripheral nerve is discussed. It appears possible that HTLV-I may accelerate the aging process and give rise to paraparesis. The precise role of HTLV-I in the pathogenesis of rat paraparesis remains to be elucidated taking the role of the aging process of the spinal cord and peripheral nerve into account.

Structure, innervation, and age-associated changes of mouse forearm muscles

In spite of a decline in muscle strength with age, the cause of the overall decrease in motor performance in aged mammals, including rodents, is incompletely understood. To add clarity, the gross organization, innervation, histochemical fiber types, and age-associated changes are described for mouse forearm muscles used in a variety of motor functions. The anterior (flexor) and posterior (extensor) forearm compartments have the same arrangement of muscles and gross pattern of innervation as the rat. Two primary histochemical fiber types, fast/oxidative/glycolytic (FOG) and fast/glycolytic (FG), with characteristic histochemical staining patterns were observed in all forearm muscles. Additionally, there was a small population of slow/oxidative (SO) fibers confined to the deep region of a single muscle, the flexor carpi ulnaris (FCU). Between 18 and 26 months the FCU muscle displayed fibers with morphological features distinct from earlier ages. Fibers displayed a greater variation in size, a loss of their uniform polygonal shape, and a dramatic increase in clumps of subsarcolemmal mitochondria, lysosomes, and lipofuscin granules. Many of the fibers had a distinctly atrophic, angular shape consistent with recent denervation. Morphometric analyses of the FCU's source of innervation, the ulnar nerve and one of its ventral roots (C8), were consistent with the denervation-like changes in the muscle fibers. Although, there was no net loss of myelinated axons between 4 and 26 months of age, there was a significant increase in the density of degenerating cells in both the ulnar nerve and ventral root C8.

Nimodipine effects on cerebral microvessels and sciatic nerve in aging rats

At the ultrastructural level different anomalies of the cerebral microvasculature were encountered in the brains of aged rats. These aberrations can either be attributed to degeneration processes or to the perivascular deposition of, e.g., collagen fibrils and other, unidentified, proteinous debris. We previously reported that chronic treatment with the calcium antagonist nimodipine from 24-30 months especially reduced the incidence of aging-related microvascular deposits in the frontoparietal motor cortex of rats. The same drug treatment did not interfere with the degeneration of pericytes. The reduction of the microvascular depositions was, however, not consistent throughout different cortical layers. We now demonstrate that an earlier onset (16-30 months) of the drug application yields a prominent and consistent reduction of microvascular deposits for all cortical layers studied. The earlier onset of the drug treatment again did not influence the quantity of pericyte degeneration. The effect of long-term nimodipine treatment (16-30 months) was also examined in the sciatic nerve. Compared to young animals the sciatic nerve of aged control rats (30 months) showed a variety of alterations of myelinated fiber (MF) morphometry. Nimodipine treatment from 16-30 months did not significantly change these morphometric aging-related changes. Approximately 6% of the MF in aged rats display morphological myelin irregularities. After nimodipine application the frequency of these alterations was reduced, which was, however, only significant for partial demyelination known as myelin ballooning. These results indicate a consistent influence of nimodipine on cerebral microvessels, while there is only a moderate effect on the morphology of sciatic myelinated fibers during the aging process.

Myelinated fiber regeneration after crush injury is retarded in sciatic nerves of aging mice

To compare nerve regeneration in young adult and aging mice, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two weeks later, both groups of mice were perfused with an aldehyde solution, and, after additional fixation, the sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. The same level was sectioned in unoperated contralateral nerves; these nerves served as controls. Electron micrographs and the Bioquant Image Analysis System IV were used to measure areas of posterior tibial fascicles and count the number of myelinated axons, the number of unmyelinated axons, and their frequency in Schwann cell units. In aging mice, the total number of regenerating myelinated axons was significantly reduced, but totals of regenerating unmyelinated axons in aging and young adults did not differ significantly. In aging mice, the frequency of Schwann cells that contained a single unmyelinated axon was greater, suggesting that before myelination began, Schwann cell ensheathment of axons also was slowed. After axotomy by a crush injury, the area of the posterior tibial fascicle was less than that in young adults and the distal disintegration of myelin sheath remnants also appeared to be retarded. The results indicate that responses of neurons, axons, and Schwann cells could be important in slowing the regeneration of myelinated fibers found in sciatic nerves from aging mice.

Spontaneous lesions in subchronic neurotoxicity testing of rats

Male and female Fischer 344 rats, 30 weeks of age, were examined for neuropathologic changes after a 13-week inhalation neurotoxicologic study. Tissues were preserved by whole-body perfusion with 1.5% glutaraldehyde/4% formaldehyde solution. An extensive set of neural tissues was embedded in paraffin, sectioned, and stained with hematoxylin and eosin, luxol fast blue/periodic acid-Schiff/hematoxylin, Sevier-Munger silver, and cresyl echt violet. Lesions in the central and peripheral nervous system were comparable between sexes and between control and treated animals. Bilateral swollen axons were present in the medial aspect of the nucleus gracilis adjacent to the area postrema. Occasional swollen axons also were observed in the dorsal and ventral funiculi of the spinal cord. Degeneration of individual nerve fibers was present in the trapezoid body, vestibular nerve root, trigeminal nerve, cerebellar peduncles, and the funiculi of the spinal cord. Individual nerve fiber degeneration also was present in the spinal nerve roots, sciatic and tibial nerves. Nerve fiber degeneration was characterized by myelin disruption and degeneration, vacuoles and axonal fragmentation. Similar spontaneous neuropathology may be encountered in rats from other subchronic neurotoxicologic studies and must be differentiated from treatment-related toxicity.