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

Monocarboxylate transporter 1 in Schwann cells contributes to maintenance of sensory nerve myelination during aging

Schwann cell (SC)-specific monocarboxylate transporter 1 (MCT1) knockout mice were generated by mating MCT1 ^f/f mice with myelin protein zero (P0)-Cre mice. P0-Cre^+/- , MCT1 ^f/f mice have no detectable early developmental defects, but develop hypomyelination and reduced conduction velocity in sensory, but not motor, peripheral nerves during maturation and aging. Furthermore, reduced mechanical sensitivity is evident in aged P0-Cre^+/- , MCT1 ^f/f mice. MCT1 deletion in SCs impairs both their glycolytic and mitochondrial functions, leading to altered lipid metabolism of triacylglycerides, diacylglycerides, and sphingomyelin, decreased expression of myelin-associated glycoprotein, and increased expression of c-Jun and p75-neurotrophin receptor, suggesting a regression of SCs to a less mature developmental state. Taken together, our results define the contribution of SC MCT1 to both SC metabolism and peripheral nerve maturation and aging.

The Impact of Aging on Vasa Nervorum, Nerve Blood Flow and Vasopressin Responsiveness

Objective:

Aging impacts microvessels in a number of tissue beds. Vasopressin acts as a vasoconstrictor in most blood vessels but may also cause vasodilation. We evaluated the role of aging and vasopressin in the regulation of nerve blood flow (NBF) in rat peripheral nerve.

Methods:

We undertook a dose-response study to examine the impact of aging on resting NBF and its vasoreactivity to vasopressin. Nerve blood flow was measured using microelectrode hydrogen polarography. Argininevasopressin was administered both intra-arterially and topically.

Results:

In young adult rats (two months old) topical epineurial application of arginine-vasopressin produced a concentration-dependent reduction of NBF (ED50= 3.8 X 10-5 mol/L). Intra-arterial arginine-vasopressin also reduced NBF. Nerve blood flow was lower in aged rats (12 months old) and less responsive to topically applied vasopressin. The aging group had significantly higher concentrations of vasopressin in plasma than did the younger group.

Conclusions:

The results suggest that vasopressin constricts vessels in peripheral nerve and that there is an age related decline in the vasoconstrictive response to vasopressin. There may be a reduction in receptor sensitivity in vascular smooth muscle cells in peripheral nerve with increasing age.

Effect of ageing on tactile transduction processes

With advancing age, a decline in the main sensory modalities including touch sensation and perception is well reported to occur. This review mainly outlines the peripheral components of touch perception highlighting ageing influences on morphological and functional features of cutaneous mechanical transducers and mechanosensitive ion channels, sensory innervation, neurotransmitters and even vascular system required to ensure efferent function of the afferent nerve fibres in the skin. This, in conjunction with effect of ageing on the skin per se and central nervous system, could explain the tactile deficit seen among the ageing population. We also discuss appropriate tools and experimental models available to study the age-related tactile decline.

Effects of α-MSH on ischemia/reperfusion injury in the rat sciatic nerve

Background:

Ischemia/reperfusion (I/R) causes the production of toxic free radicals and leads to pathological changes in nerve tissue. We investigated the effect of alpha-melanocyte stimulating hormone (α-MSH) in a rat model for sciatic nerve I/R and discuss the possible cytoprotective and antioxidant mechanism of α-MSH against ischemic fiber degeneration.

Methods:

Experiments were performed using 42 adult male Wistar rats. Rats were divided into six experimental groups: control group, ischemia group, I/R groups, and α-MSH treated groups. Ischemia was produced by clamping of the femoral vessels. Immediately after ischemia that lasted 3 h, 75 μg/kg of α-MSH was administered subcutaneously before reperfusion and the tissue malondialdehyde (MDA) level was evaluated as an indicator of lipid peroxidation in groups with different reperfusion periods.

Results:

The reperfusion injury did not begin in the first hour of reperfusion after 3 h of ischemia, and MDA levels increased on the first day of reperfusion. During the first day, blood MDA levels were decreased in the α-MSH group compared to the control group. The tissue from animals pre-treated with α-MSH showed fewer morphological alterations. Myelin breakdown was significantly diminished after treatment with α-MSH, and the ultrastructural features of axons showed remarkable improvement. Two-way analysis of variance was used for comparing three or more groups. When a significant difference existed, the post-hoc multiple-comparison test was applied to demonstrate the differences.

Conclusions:

The results confirm that pre-treatment with α-MSH after ischemia protected the peripheral nerves against I/R injury.

Impact of diabetic polyneuropathy and cardiovascular autonomic neuropathy on the excretion of urinary 8-epi-PGF2αand its metabolites (2, 3-dinor and 2, 3-dinor-5, 6-dihydro)

The objective of this study was to establish if diabetes in the presence of polyneuropathy (PN) and/or cardiovascular autonomic neuropathy (CAN) is associated with alterations in the amounts of 8-epi-PGF2alpha (IP) and its metabolites including 2, 3-dinor-8-epi-PGF2alpha (dinor-IP) and 2, 3-dinor-5, 6 dihydro-8-epi-PGF2alpha (dinor-dihydro-IP) in urine. Mass spectrometric separation showed that excretion of IP was similar in the PN + /CAN- and PN+/CAN+ groups but higher than in the PN-/CAN- group (n = 103, 22 and 60, respectively; P < 0.05). By contrast, excretion of dinor-IP or dinor-dihydro-IP were similar in the PN-/CAN- and PN+/CAN- groups but higher than in PN+/CAN+ group. Correlations were obtained between IP and dinor-IP or dinor-dihydro-IP (r = 0.30; P < 0.001 and r = 0.31; P < 0.001, respectively). A significant association was also observed between dinor-IP and dinor-dihydro-IP (r = 0.48; P < 0.001). In conclusion, these biomarkers should prove useful in studies evaluating the impact of therapeutic drugs or antioxidant interventions aimed at delaying the onset of diabetic complications.

Ischemia–reperfusion injury of peripheral nerve in experimental diabetic neuropathy

The pathogenesis of human diabetic neuropathy likely involves the interplay of hyperglycemia, ischemia, and oxidative stress. Mild-moderate ischemia-reperfusion to streptozotocin (STZ)-induced diabetes results in florid fiber degeneration in diabetic but not in normal nerves. Uncertainty exists as to the influence of duration of diabetes on this susceptibility. We therefore studied diabetic tibial and sciatic nerves using a rat ischemia-reperfusion (IR) model after 1 month and 4 months of diabetes utilizing electrophysiological, behavioral, and neuropathological methods. Electrophysiological abnormalities were present in 1-month diabetic rats (D) and persisted over 4 months. Behavioral scores were decreased markedly at 4 months (p<0.05). Endoneurial edema and ischemia fiber degeneration (IFD) were observed at both the 1-month (p<0.01 and p<0.001) and 4-month (p<0.001) durations in diabetic nerves, whereas only mild or no damage was observed in age-matched control nerves. These findings demonstrate that STZ-induced diabetes exacerbates the morphological and electrophysiological pathology in peripheral nerve to IR injury both in the early timepoint of 1 month and late timepoint of 4 months, although there was a gradation of injury, which is more severe at the later timepoint. Reperfusion exaggerated morphological pathology in 1-month STZ-induced diabetic peripheral nerve.

Effect of trapidil in ischemia/reperfusion injury of peripheral nerves

OBJECTIVE

Ischemia plays an important role in the development of pathological changes in nerve tissue, and restoration of blood flow results in injury (ischemia/reperfusion [I/R] injury) mediated by toxic oxygen free radicals. Trapidil is currently used as a coronary artery vasodilating agent and is also used for the prevention of ischemic symptoms of cerebral vasospasm. The purpose of this study was to determine the effects of trapidil on I/R injury and the ischemic tolerance of rat peripheral nerves.

METHODS

Preischemia or prereperfusion administration of trapidil (8 mg/kg) was evaluated in the rat sciatic nerve I/R injury model. Nerve tissue samples from the I/R injury site were assayed for malondialdehyde (MDA), nitrites, and nitrates, as markers of I/R injury, and pathological changes were evaluated by electron microscopy.

RESULTS

I/R resulted in an increase in MDA levels, which remained elevated for 2 weeks in control nerves. Rats that received trapidil before ischemia exhibited decreased MDA levels, and rats that received trapidil after the standard 3 hours of ischemia demonstrated increased tolerance to reperfusion, as reflected in significantly decreased MDA levels. Nitrite and nitrate levels in trapidil-treated rats were significantly higher than those in control animals. Histological evaluations of the sciatic nerve segments demonstrated that preischemia and postischemia trapidil treatments had a sparing effect against the myelin damage and axonal edema that are consistently noted in untreated ischemic reperfused nerves.

CONCLUSION

The results confirm that pretreatment with trapidil before the ischemic insult or before reperfusion provides marked protection against I/R injury in peripheral nerves.

Aging differentially modifies sensitivity of nerve blood flow to vasocontractile agents (endothelin-1, noradrenaline and angiotensin II) in sciatic nerve

This study examined the influence of the vaso-constricting agents (noradrenaline, endothelin-1 and angiotensin II) in Sprague-Dawley rats aged 2, 6 and 24 months by evaluating epineurial arteriolar vasoreactivity in response to superfused teat agents. Nerve blood flow (NBF) was measured using microelectrode H2 polarography. In 24-month-old rats, NBF was decreased and vascular resistance (VR) was increased compared with 2- and 6-month-old rats. All of the constricting agents reduced NBF in the 2-, 6- and 24-month groups, however, the effects of the constricting agents reduced significantly with age. These results suggest that during aging, there is a decline of vasoconstrictive responses to noradrenaline, endothelin-1 and angiotensin II in peripheral nerve and that these changes may be due to altered function of receptors.

Comparison of the effects of ascorbyl gamma-linolenic acid and gamma-linolenic acid in the correction of neurovascular deficits in diabetic rats

Essential fatty acid metabolism is impaired by diabetes mellitus and gamma-linolenic acid rich treatments such as evening primrose oil correct deficits in nerve conduction and endoneurial blood flow in diabetic rats. Other mechanistically unrelated treatments, such as antioxidants and aldose reductase inhibitors have a similar effect and there may be positive interactions with multiple treatments. Our aim was to compare the efficacy of a novel essential fatty acid derivative, ascorbyl gamma-linolenic acid, with that of gamma-linolenic acid in correcting diabetic neurovascular deficits. Eight weeks of diabetes caused 20.4 and 48.2% reductions in sciatic motor conduction velocity and nutritive endoneurial blood flow, respectively. Treatment was given for the last 2 weeks with gamma-linolenic acid (100 mg.kg-1.day-1) either in pure form or as ascorbyl gamma-linolenic acid, an equivalent dose of ascorbate (21 mg.kg-1.day-1) or jointly with ascorbate and gamma-linolenic acid. Conduction velocity was corrected by 39.8, 87.4, 13.2 and 66.8% with gamma-linolenic acid, ascorbyl gamma-linolenic acid, ascorbate and gamma-linolenic acid plus ascorbate, respectively. Corresponding ameliorations of the nutritive blood flow deficit were 44.0, 87.4, 87.4, 13.2 and 65.7%. For the gamma-linolenic acid plus ascorbate combinatin, and especially for ascorbyl gamma-linolenic acid, the magnitude of correction for conduction velocity and blood flow was greater than expected for simple addition of ascorbate and gamma-linolenic acid, indicating a synergistic interaction. Thus, with an efficacy 40 times that of evening primrose oil in rats, ascorbyl gamma-linolenic acid may be a suitable candidate for clinical trials of diabetic neuropathy.

Preservation of the cervical vagus nerve in aged rats: morphometric and enzyme histochemical evidence

The vagus nerve trunk, sampled at a mid-cervical level, has been analysed quantitatively by light and electron microscopy principally with respect to the numbers and sizes of the myelinated and unmyelinated axon populations in Wistar rats of 4, 24 and 30 months. No significant differences in total myelinated axon numbers were seen over the age range in counts made on light microscope montages of the entire cross-section of the nerve. The overall histological organisation and appearance of the nerve trunk did not change with age but age-associated alterations in the ultrastructure of some myelinated fibres and their Schwann cells was seen. Unmyelinated axons and their associated Schwann cells rarely showed age-associated changes. The numbers of myelinated and unmyelinated axons per mm2 determined from electron micrographs were, however, slightly decreased but the ratio of myelinated to unmyelinated axons was approximately 1:4 at all ages. Measurements of myelinated fibres showed a small but significant increase in size between young and old animals. There was an increase in the thickness of the myelin sheath, a decrease in myelinated axon diameter and in the the g ratio. The diameter of unmyelinated axons decreased with age and the number of unmyelinated axons per Schwann cell unit increased. The numbers, diameters and thickness of the walls of the vagal microvessels remained unchanged. Quantitation by microdensitometry of the activity of NADH tetrazolium reductase and succinate dehydrogenase in longitudinal sections of the vagus indicated an increase in the activity of these two metabolic markers whilst an increase in the activity of cytochrome oxidase indicated that neuronal activity in the vagus was unimpaired in old age. It is concluded that the structure of the rat vagus nerve, and in particular of its predominantly unmyelinated axon population, is not significantly affected in old age.

Efficacy of limb cooling on the salvage of peripheral nerve from ischemic fiber degeneration

Since peripheral nerve has a large ischemic safety factor, hypothermia, by reducing metabolic demands, is potentially an efficacious technique to rescue nerve from ischemic fiber degeneration (IFD). We therefore evaluated the influence of temperature on the severity of IFD resulting from a standard ischemic stress. Ischemia to the left sciatic nerve in the rat was produced by embolization of 2 x 10(6) microspheres (14 microns) into its supplying arteries. The limb was embolized at three temperatures, 37 degrees C, 32 degrees C, and 28 degrees C and was maintained at each temperature for an additional 4 h. End points, evaluated 7 days after embolization, for the embolized limb were: (1) behavioral scores, 0-11 in increasing limb function; (2) compound nerve action potential of sciatic-tibial nerve; (3) sciatic nerve blood flow (NBF, in mL/100 g/min); and (4) histologic grade, expressed as percentage of fibers undergoing IFD (0 = < 5%; 1 = 5-25%; 2 = 26-50%; 3 = 51-75%; 4 = > 75%). NBF was reduced in all groups, varying with temperature, and all indices of nerve structure and function were significantly improved with hypothermia. We conclude that hypothermia, easily achievable in a limb nerve, is highly efficacious in the rescue of nerve from IFD. These findings are of clinical relevance.

Experimental ischemic neuropathy: salvage with hyperbaric oxygenation

Hyperbaric oxygenation is effective in augmenting the delivery of oxygen to tissue, but also causes oxidative stress. As part of our focus on improving peripheral nerve salvage from ischemic fiber degeneration, we evaluated whether hyperbaric oxygenation rescues peripheral nerve, rendered ischemic by microembolization, from ischemic fiber degeneration. The supplying arteries of rat sciatic nerve were embolized with microspheres of 14 microns diameter at moderate (2 x 10(6)) and high (5.6 x 10(6)) doses. Rats were randomized to receive hyperbaric oxygenation treatment (2.5 atm 100% oxygen for 2 hours/day for 7 days beginning within 30 minutes of ischemia), or room air. End points for the embolized limb were (1) behavioral scores (0-11 in increasing levels of limb function), (2) nerve action potential of sciatic-tibial nerve, (3) nerve blood flow, and (4) histological grade as percentage of fibers undergoing ischemic fiber degeneration (0 = < 5%; 1 = 5-25%; 2 = 26-50%; 3 = 51-75%; 4 = > 76%). Nerve blood flow and nerve action potential were uniformly absent and more than 90% of fibers had degenerated in both control and treatment groups receiving high doses. Control and treatment groups receiving moderate doses were well matched by level of ischemia (8.5 +/- 0.3 [N = 18] vs 7.7 +/- 0.4 ml/100 gm/min [N = 18], p > 0.05) but were significantly different by behavior score (5.6 +/- 0.7 vs 9.2 +/- 0.5 [N = 19], p < 0.001), nerve action potential (1.4 +/- 1.0 vs 3.9 +/- 0.5 [N = 6], p < 0.05), and histology (2.4 +/- 0.4 [N = 5] vs 0.8 +/- 0.5 [N = 4], p < 0.05). On single teased fiber evaluation, the predominant abnormality was E (axonal degeneration). We conclude that hyperbaric oxygenation will effectively rescue fibers from ischemic fiber degeneration, providing the ischemia is not extreme.

Nerve ischaemia in diabetic rats: time-course of development, effect of insulin treatment plus comparison of streptozotocin and BB models

This study sought to determine the time-course of development of reduced nerve laser Doppler flux in experimental diabetes and the effect on this anomaly of insulin treatment. In addition, we aimed to compare nerve laser Doppler flux in streptozotocin- and genetically-diabetic BB rat models. Sciatic nerve laser Doppler flux in diabetic rats was variable during the 2 days following streptozotocin injection; from day 4, when the measurement was 80% of control, fluxes fell steadily and formed a plateau at 40% of control values after 4 weeks of diabetes. In a second study, using rats with 4-week streptozotocin-diabetes, sciatic nerve laser Doppler flux was reduced to 44% of the value measured in control rats. Treatment of a parallel group of diabetic rats with insulin, by sustained release implants, prevented this ischaemia, so that nerve laser Doppler flux was 91% of controls. Nerve Doppler flux in BB rats with 6-week genetic diabetes was 57% of a control (non-diabetic) BB group. There were no differences in mean arterial pressures between control and diabetic rats in any of the studies. Heart rates of control and insulin-treated diabetic animals were higher than those of the untreated diabetic group; in the other studies heart rates of diabetic animals were numerically lower than controls, but not significantly so. These observations suggest that sciatic nerves of rats with short-term diabetes, whether induced with streptozotocin or of genetic origin, are markedly ischaemic and that this ischaemia in streptozotocin-diabetes is evident within a week of diabetes onset, forms a plateau after 4 weeks and is maintained for at least 2 months.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative analysis of the cervical sympathetic trunk in young adult and aged rats

The cervical sympathetic trunk of rats aged 4 and 24 months was examined quantitatively with particular reference to the predominating population of unmyelinated axons by using electron microscope photomontages of transverse sections of the entire nerve trunk. No obvious neuropathological or degenerative changes were observed in animals of either age. At 24 months the mean diameter of unmyelinated axons, the mean number of axons/100 microns2 and the mean number of axons associated with individual Schwann cell units was significantly greater than in the young adult animals. With regard to the small population of myelinated axons in the nerve trunk, the thickness of the myelin sheath did not change with age. The numbers of endoneurial blood vessels in the trunk was not significantly different. This analysis of the cervical sympathetic trunk is consistent with other morphological, histochemical and biochemical evidence which suggests that elements of the cervical sympathetic system are well preserved in aged rats.

Age-related changes in maturation of regenerated motor innervation

The dependence of spontaneous transmitter release by regenerated nerve endings on age was studied in rat extensor digitorum longus muscle after sciatic nerve crush during the first year of life. Intracellular recordings were carried out at different times after denervation in muscles of 1.5-, 3- or 12-month-old animals. The frequency of miniature endplate potentials was detected. In order to determine the percentage of multiple innervation, nerve-evoked endplate potentials were recorded. The time course of the percentage of polyinnervated muscle cells was similar in the three groups of animals, but mepp frequency increased more slowly with increasing age. Moreover, by extrapolating the intercept of linear regression for mepp frequency vs days from denervation, a conventional initial time of reinnervation may be computed: the results are consistent with a decrease in the nerve regeneration rate during the first year of life.

Hyperemia of injured peripheral nerve: sensitivity to CGRP antagonism

Intrinsic mechanisms of vasodilatation may prevent injury-related ischemia in peripheral nerve endoneurium. We examined local perfusion up to 10 days following local crush, partial injury or simple exposure of the rat sciatic nerve. By employing epineurial hCGRP(8-37), a receptor antagonist of CGRP, and serial hydrogen clearance measurements, we estimated the component of post-injury blood flow related to local CGRP action. Injury-related ischemia was not observed at any of the time points studied at or proximal to injury. Instead, endoneurial blood flow (EBF) increased at 24 h proximal to crush or partial injury, and at 48 h within the crush zone when compared to sham operated controls or to a pooled reference range of EBF. Composite blood flow (F) was also elevated at 48 h and 5 days within the crush zone suggesting hyperemia involving the epineurial plexus, perineurial vessels and AV shunts. hCGRP(8-37) constricted vasa nervorum at most time points but its effect on EBF was maximum and exceeded controls within the crush zone at 48 h. The findings indicate that certain types of nerve injury, including focal crush, are associated with hyperemia, not ischemia. CGRP vasodilatation may account for part of this response, implying a local peptidergic afferent fiber response to nerve trunk injury.