Changes in blood vessel permeability during degeneration and regeneration in peripheral nerves

Profiling sensory neuron microenvironment after peripheral and central axon injury reveals key pathways for neural repair

Sensory neurons with cell bodies in dorsal root ganglia (DRG) represent a useful model to study axon regeneration. Whereas regeneration and functional recovery occurs after peripheral nerve injury, spinal cord injury or dorsal root injury is not followed by regenerative outcomes. Regeneration of sensory axons in peripheral nerves is not entirely cell autonomous. Whether the DRG microenvironment influences the different regenerative capacities after injury to peripheral or central axons remains largely unknown. To answer this question, we performed a single-cell transcriptional profiling of mouse DRG in response to peripheral (sciatic nerve crush) and central axon injuries (dorsal root crush and spinal cord injury). Each cell type responded differently to the three types of injuries. All injuries increased the proportion of a cell type that shares features of both immune cells and glial cells. A distinct subset of satellite glial cells (SGC) appeared specifically in response to peripheral nerve injury. Activation of the PPARα signaling pathway in SGC, which promotes axon regeneration after peripheral nerve injury, failed to occur after central axon injuries. Treatment with the FDA-approved PPARα agonist fenofibrate increased axon regeneration after dorsal root injury. This study provides a map of the distinct DRG microenvironment responses to peripheral and central injuries at the single-cell level and highlights that manipulating non-neuronal cells could lead to avenues to promote functional recovery after CNS injuries or disease.

Circulatory dynamics of the cauda equina in lumbar canal stenosis using dynamic contrast-enhanced magnetic resonance imaging

BACKGROUND CONTEXT

There has been no study regarding the cauda equina circulation of patients with neurogenic intermittent claudication (NIC) in lumbar spinal canal stenosis (LSCS) using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

PURPOSE

The mechanism responsible for the onset of NIC was investigated using DCE-MRI to examine changes in cauda equina blood flow in patients with LSCS.

STUDY DESIGN

This was a retrospective longitudinal registry and magnetic resonance imaging study.

PATIENT SAMPLE

The subjects consisted of 23 patients who had LSCS associated with NIC (stenosis group). Ten asymptomatic volunteers who did not have NIC served as controls (control group). In the LSCS group, the cross-sectional area of the dural sac was <75 mm2 at the site of most severe stenosis. These patients were further divided into single and double stenosis subgroups.

OUTCOME MEASURES

The main measures we used were the signal intensity (S-I) ratio and the shape and size of the time intensity (T-I) curves. We compared these between the stenosis and control groups.

METHODS

At first, plain T1-weighted MR images were obtained and the lumbar dural sac cross-sectional area was measured using a digitizer. For DCE-MRI, sagittal T1-weighted images of the same slice were acquired continuously for 10 minutes after administration of gadolinium as an intravenous bolus to observe the distribution of contrast medium (gadolinium) in the cauda equina. To objectively evaluate changes in contrast enhancement of the cauda equina at the site of canal stenosis, regions of interest were established. The signal intensity (SI) ratio was calculated to compare the signal intensities before and after contrast enhancement, and time-intensity curves were prepared to investigate changes over time.

RESULTS

The static imaging findings and the changes of gadolinium uptake showed striking differences between the study and control patients. In the stenosis group, abnormal intrathecal enhancement showed around the site of stenosis on enhanced MR imaging. The SI ratio at the site of canal stenosis had a slower increase in the arterial phase when compared with that in the control group and remained high in the venous phase for up to 10 minutes. Finally, abnormal intrathecal enhancement was visible around the site of stenosis on enhanced MR imaging in all patients.

CONCLUSIONS

These clinical data indicate that cauda equina nerve roots in the LSCS patients are pathologic even when symptoms are not elicited in the supine position, suggesting that intraradicular venous congestion and edema themselves do not influence the existence of radicular symptoms.

The double-edged sword: effects of pregabalin on experimentally induced sciatic nerve transection and crush injury in rats

AIM

The aim of this study was to research the effects of pregabalin on experimentally induced peripheral nerve crush injuries in rats.

MATERIAL AND METHOD

Forty-two adult female Wistar albino rats were divided into seven groups: 1st group: healthy; 2nd group: axonotmesis control; 3rd group: anastomosis control; 4th group: axonotmesis+30 mg/kg of pregabalin; 5th group: axonotmesis+60 mg/kg of pregabalin; 6th group: anastomosis+30 mg/kg of pregabalin; 7th group: anastomosis+60 mg/kg of pregabalin. Evaluation of the sciatic functional index (SFI) was performed one day before and on days 7, 14, 21, and 28 following surgery. The right sciatic nerves of all animals were examined histopathologically and molecularly.

RESULTS

After 28 days post-injury, the histopathological regeneration in peripheral nerve injuries for pregabalin 30 mg/kg treated groups was significantly better than that of the control groups. Also the SFI increases and TGF-β gene expression up-regulation were significantly better in pregabalin 30 mg/kg treated groups.

CONCLUSION

The histopathological, functional and molecular data suggest that pregabalin 30 mg/kg treatment in axonotmesis and anostomosis groups improves nerve regeneration and increases SFI in peripheral nerve injuries by activating antiinflammatory cytokine TGF-β1.

Bilateral changes of cannabinoid receptor type 2 protein and mRNA in the dorsal root ganglia of a rat neuropathic pain model

Cannabinoid receptor type 2 (CB2R) plays a critical role in nociception. In contrast to cannabinoid receptor type 1 ligands, CB2R agonists do not produce undesirable central nervous system effects and thus promise to treat neuropathic pain that is often resistant to medical therapy. In the study presented here, we evaluated the bilateral distribution of the CB2R protein and messenger RNA (mRNA) in rat dorsal root ganglia (DRG) after unilateral peripheral nerve injury using immunohistochemistry, western blot, and in situ hybridization analysis. Unilateral chronic constriction injury (CCI) of the sciatic nerve induced neuropathic pain behavior and bilateral elevation of both CB2R protein and mRNA in lumbar L4-L5 as well as cervical C7-C8 DRG when compared with naive animals. CB2R protein and mRNA were increased not only in DRG neurons but also in satellite glial cells. The fact that changes appear bilaterally and (albeit at a lower level) even in the remote cervical DRG can be related to propagation of neuroinflammation alongside the neuraxis and to the neuroprotective effects of CB2R.

Plasminogen is a key proinflammatory regulator that accelerates the healing of acute and diabetic wounds

Despite decades of research on wound healing, effective biologic agents for the treatment of chronic wounds, especially diabetic wounds, are still lacking. In the present study, we report that the inert plasma protein plasminogen (plg) acts as a key regulatory molecule that potentiates wound healing in mice. Early in the healing process, plg bound to inflammatory cells is transported to the wound area, where the level of plg is increased locally, leading to the induction of cytokines and intracellular signaling events and to a potentiation of the early inflammatory response. Systemic administration of additional plg not only accelerates the healing of acute burn wounds in wild-type mice, but also improves the healing of chronic diabetic wounds in a mouse model of diabetes. Our results suggest that the administration of plg may be a novel therapeutic strategy to treat many different types of wounds, especially chronic wounds such as those caused by diabetes.

Bilateral changes of IL-10 protein in lumbar and cervical dorsal root ganglia following proximal and distal chronic constriction injury of peripheral nerve

Interleukin-10 prevents transition of a physiological inflammatory reaction to a pathological state that may result in neuropathic pain. We studied bilateral changes of IL-10 protein levels in L4-L5 and C7-C8 dorsal root ganglia (DRG) after a chronic constriction injury (CCI) of either L4-L5 spinal nerves (pCCI) or the sciatic nerve (dCCI). Rats undergoing pCCI or dCCI were left to survive for 1, 3, 7 or 14 d, sham-operated rats for 3 or 14 d. After the survival time, C7-C8 and L4-L5 DRG were removed bilaterally from naïve, operated, and sham-operated rats and IL-10 protein was detected by immunohistochemical staining and measured using ELISA analysis. Unilateral pCCI and dCCI induced a transient bilateral elevation in IL-10 protein level not only in the homonymous lumbar DRG but also in the heteronymous cervical DRG nonassociated with the spinal segments of constricted nerve. Sham operations also induced bilateral elevation of IL-10 protein in both homonymous and heteronymous DRG. Our experiments revealed that the more proximal is a nerve injury the more rapid is the initial increase and slower the subsequent decrease of IL-10 protein level in DRG. Changes of IL-10 protein in DRG nonassociated with damaged nerve could be related to a general neuroinflammatory reaction of the nervous system to injury and thereby promote potential of the DRG neurons for regenerating their axons following a conditioning lesion.

Wallerian degeneration and peripheral nerve conditions for both axonal regeneration and neuropathic pain induction

Wallerian degeneration is a cascade of stereotypical events in reaction to injury of nerve fibres. These events consist of cellular and molecular alterations, including macrophage invasion, activation of Schwann cells, as well as neurotrophin and cytokine upregulation. This review focuses on cellular and molecular changes distal to various types of peripheral nerve injury which simultaneously contribute to axonal regeneration and neuropathic pain induction. In addition to the stereotypical events of Wallerian degeneration, various types of nerve damage provide different conditions for both axonal regeneration and neuropathic pain induction. Wallerian degeneration of injured peripheral nerve is associated with an inflammatory response including rapid upregulation of the immune signal molecules like cytokines, chemokines and transcription factors with both beneficial and detrimental effects on nerve regeneration or neuropathic pain induction. A better understanding of the molecular interactions between the immune system and peripheral nerve injury would open the possibility for targeting these inflammatory mediators in therapeutic interventions. Understanding the pleiotropic effects of cytokines/chemokines, however, requires investigating their highly specific pathways and precise points of action.

Bilateral changes of TNF-alpha and IL-10 protein in the lumbar and cervical dorsal root ganglia following a unilateral chronic constriction injury of the sciatic nerve

Background

There is a growing body of evidence that unilateral nerve injury induces bilateral response, the mechanism of which is not exactly known. Because cytokines act as crucial signaling molecules for response of peripheral nerves to injury, they may be induced to mediate the reaction in remote structures.

Methods

We studied levels of tumor necrosis factor α (TNF-α) and interleukin 10 (IL-10) proteins using ELISA in the ipsilateral and contralateral lumbar (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) from naïve rats, rats operated on to create unilateral chronic constriction injury (CCI) of the sciatic nerve, and sham-operated rats. Withdrawal thresholds for mechanical allodynia and thermal hyperalgesia were measured in the ipsilateral and contralateral hind and forepaws.

Results

The ipsilateral hind paws of all rats operated upon for CCI displayed decreased withdrawal thresholds for mechanical allodynia and thermal hyperalgesia, while no significant behavioral changes were found in the contralateral hind paws and both forepaws. Significantly lower baseline levels of TNF-α and IL-10 protein were measured by ELISA in the lumbar than cervical DRG of naïve rats. Bilateral elevation of TNF-α was induced in both the lumbar and cervical DRG by unilateral CCI of the sciatic nerve for 7 and 14 days, while the level of IL-10 protein was increased bilaterally in the lumbar DRG 1 and 3 days after operation. IL-10 levels declined bilaterally even below baseline level in both cervical and lumbar DRG 7 days from CCI and normalized after 14 days. In contrast to no significant changes in TNF-α, level of IL-10 protein was significantly increased in the ipsilateral lumbar DRG after 3 days and bilaterally in the lumbar DRG after 14 days from sham operation.

Conclusions

The results of our experiments show a bilateral elevation of TNF-α and IL-10 not only in the homonymous DRG but also in the heteronymous DRG unassociated with the injured nerve. This suggests that bilaterally increased levels of TNF-α and IL-10 in DRG following unilateral CCI are linked with general neuroinflammatory reaction of the nervous system to injury rather than only to development and maintenance of neuropathic pain.

Spatio-temporal changes of SDF1 and its CXCR4 receptor in the dorsal root ganglia following unilateral sciatic nerve injury as a model of neuropathic pain

There is a growing evidence that chemokines and their receptors play a role in inducing and maintaining neuropathic pain. In the present study, unilateral chronic constriction injury (CCI) of rat sciatic nerve under aseptic conditions was used to investigate changes for stromal derived factor-1 (SDF1) and its CXCR4 receptor in lumbal (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) from both sides of naïve, CCI-operated and sham-operated rats. All CCI-operated rats displayed mechanical allodynia and thermal hyperalgesia in hind paws ipsilateral to CCI, but forepaws exhibited only temporal changes of sensitivity not correlated with alterations in SDF1 and CXCR4 proteins. Naïve DRG displayed immunofluorescence for SDF1 (SDF1-IF) in the satellite glial cells (SGC) and CXCR4-IF in the neuronal bodies with highest intensity in small- and medium-sized neurons. Immunofluorescence staining and Western blot analysis confirmed that unilateral CCI induced bilateral alterations of SDF1 and CXCR4 proteins in both L4-L5 and C7-C8 DRG. Only lumbal DRG were invaded by ED-1+ macrophages exhibiting SDF1-IF while elevation of CXCR4-IF was found in DRG neurons and SGC but not in ED-1+ macrophages. No attenuation of mechanical allodynia, but reversed thermal hyperalgesia, in ipsi- and contralateral hind paws was found in CCI-operated rats after i.p. administration of CXCR4 antagonist (AMD3100). These results indicate that SDF1/CXCR4 changes are not limited to DRG associated with injured nerve but that they also spread to DRG non-associated with such nerve. Functional involvement of these alterations in DRG non-associated with injured nerve in neuropathic pain remains to be elucidated.

Experimental models of peripheral neuropathic pain based on traumatic nerve injuries - an anatomical perspective

Peripheral neuropathic pain (PNP) frequently occurs as a consequence of nerve injury and may differ depending upon the type of insult and the individual patient. Progress in our knowledge of PNP induction mechanisms depends upon the utilization of appropriate experimental models in rodents based on various types of peripheral nerve lesions. In this review, we draw attention to current knowledge on basic cellular and molecular events in various experimental models used to induce the PNP symptoms. Spontaneous ectopic activity of axotomized and non-axotomized primary sensory neurons, the bodies of which are located in the dorsal root ganglion (DRG), seems to be a key mechanism of PNP induction. The primary sensory neurons are directly affected by nerve injury or indirectly by activated satellite glial cells and adjoining immune cells that release a variety of molecules changing the microenvironment of the neurons. Recently, it has become clear that molecules produced during Wallerian degeneration play an important role not only in axon-promoting conditions distal to nerve injury but also in initiation of neuropathic pain. The molecules, transported by the blood, influence afferent neurons and their axons not only in DRG associated, but also those not directly associated with the injured nerve (i.e., in the contralateral DRG or at different spinal segments). Generally, all experimental PNP models based on a partial injury of peripheral nerve segments contain mechanisms initiated by signal molecules of Wallerian degeneration.

Macrophage depletion alters the blood-nerve barrier without affecting Schwann cell function after neural injury

Previous work has shown that, during the early phases of chronic nerve compression (CNC) injury, axonal pathology is absent while Schwann cells undergo a dramatic process of cellular turnover with marked proliferation. It is known that macrophages may release Schwann cell mitogens, so we sought to explore the role of macrophages in CNC injury by selectively depleting the population of hematogenously derived macrophages in nerves undergoing CNC injury by injecting clodronate liposomes at days 1, 3, and 6 postinjury and evaluating both the integrity of the blood-nerve barrier (BNB) and Schwann cell function. Integrity of the BNB was evaluated by intravenously injecting Evans blue albumin (EBA), and Schwann cell number was determined via stereologic techniques. The BNB was clearly altered by 2 weeks postinjury and continued to disintegrate at later time points. Macrophage depletion attenuated this response at all observed time points. Quantification of Schwann cell nuclei in CNC nerves showed no differences between compressed sections of macrophage-depleted and nondepleted animals. Although macrophages are largely responsible for the increased vascular permeability associated with CNC injury, it is likely that the Schwann cell response to CNC injury is not influenced by macrophage-derived mitogenic signals but rather must be mediated via alternative mechanisms.

Cytokine responses during chronic denervation

Background

The aim of the present study was to examine inflammatory responses during Wallerian degeneration in rat peripheral nerve when the regrowth of axons was prevented by suturing.

Methods

Transected rat sciatic nerve was sutured and ligated to prevent reinnervation. The samples were collected from the left sciatic nerve distally and proximally from the point of transection. The endoneurium was separated from the surrounding epi- and perineurium to examine the expression of cytokines in both of these compartments. Macrophage invasion into endoneurium was investigated and Schwann cell proliferation was followed as well as the expression of cytokines IL-1β, IL-10, IFN-γ and TNF-α mRNA. The samples were collected from 1 day up to 5 weeks after the primary operation.

Results

At days 1 to 3 after injury in the epi-/perineurium of the proximal and distal stump, a marked expression of the pro-inflammatory cytokines TNF-α and IL-1β and of the anti-inflammatory cytokine IL-10 was observed. Concurrently, numerous macrophages started to gather into the epineurium of both proximal and distal stumps. At day 7 the number of macrophages decreased in the perineurium and increased markedly in the endoneurium of both stumps. At this time point marked expression of TNF-α and IFN-γ mRNA was observed in the endo- and epi-/perineurium of the proximal stump. At day 14 a marked increase in the expression of IL-1β could be noted in the proximal stump epi-/perineurium and in the distal stump endoneurium. At that time point many macrophages were observed in the longitudinally sectioned epineurium of the proximal 2 area as well as in the cross-section slides from the distal stump. At day 35 TNF-α, IL-1β and IL-10 mRNA appeared abundantly in the proximal epi-/perineurium together with macrophages.

Conclusion

The present studies show that even during chronic denervation there is a cyclic expression pattern for the studied cytokines. Contrary to the previous findings on reinnervating nerves the studied cytokines show increased expression up to 35 days. The high expressions of pro-inflammatory and anti-inflammatory cytokines in the proximal epi-/perineurial area at day 35 may be involved in the formation of fibrosis due to irreversible nerve injury and thus may have relevance to the formation of traumatic neuroma.

Localization and changes of intraneural inflammatory cytokines and inducible-nitric oxide induced by mechanical compression

STUDY DESIGN

Investigation of intraneural inflammation induced by mechanical compression.

OBJECTIVES

In order to investigate the mechanism of neuropathy, this study used a median nerve compression model in dogs. Immunohistochemistry was used to examine the localization and changes of inflammatory cytokines and nitric oxide (NO).

SUMMARY OF BACKGROUND DATA

The manifestation of pain at sites of inflammation has a close relationship with the release of mediators from macrophages such as interleulin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), as well as with NO. However, the mediators involved in inflammation of nerve due to mechanical compression remain almost unknown.

METHODS

In this study, the median nerve of dogs was compressed with a clip for three weeks to observe the changes caused by compression. Immunohistochemistry was done by the avidin-biotin-peroxidase complex method to observe the changes of T cells (CD45) and macrophages (Mac-1) after compression. Antibodies against IL-1beta, TNF-alpha, and inducible nitric oxide synthesis (i-NOS) were used to examine the localization and changes of these mediators caused by nerve compression.

RESULTS

In control animals, resident T cells were detected, but there were no macrophages. IL-1beta was positive in the Schwann cells and vascular endothelial cells. However, no cells showed TNF-alpha or i-NOS positively. After nerve compression, numerous T cells and macrophages appeared among the demyelinized nerve fibers. The macrophages were positive for IL-1beta, TNF-alpha and i-NOS.

CONCLUSION

Inflammatory cytokines and NO may be involved in intraneural inflammatory changes arising from mechanical compression. Such mediators may be of importance in the manifestation of neuropathy.

Effect of acute nerve root compression on endoneurial fluid pressure and blood flow in rat dorsal root ganglia

The objective of the current study was to test the hypothesis that crush injury to nerve root increases endoneurial fluid pressure (EFP) and decreases blood flow in the associated dorsal root ganglion (DRG). A total of 21 adult, female Sprague-Dawley rats had their left L5 nerve root and DRG exposed. The L5 nerve root was clamped for 2 s with a vascular suture clip just proximal to the DRG (compression group). Sham-operated animals without compression were used for control (control group). EFP was recorded with a servo-null micropipette system using a glass micropipette with tip diameter of 4 mum before and after 3 h of treatment. After the final measurement of EFP, DRG was excised and processed for histology. Blood flow in the DRG was continuously monitored by laser Doppler flow meter for 3 h. Three hours after treatment, EFP was 4.7+/-2.7 cm H(2)O in the compression group and 2.2+/-1.2 cm H(2)O in the control group (P<0.05). Edema was the principal pathologic findings seen consistently in the DRG from animals in the compression group. Blood flow in the compression group was reduced 10 min after compression. This reduction was statistically significant compared with that of the control (P<0.01). An acute compression to the nerve root increased endoneurial edema, increased EFP in the associated DRG, and reduced DRG blood flow. This combination of increased EFP and decreased blood flow in the DRG may result in neuronal ischemia and sensory dysfunction. These acute pathophysiologic changes may thus have a role in the pathogenesis of low back pain and sciatica due to disc herniation and spinal canal stenosis.

The recovery of blood-nerve barrier in crush nerve injury--a quantitative analysis utilizing immunohistochemistry

The purpose of this study is to reveal whether the application of immunohistochemical examinations to the peripheral nervous system (PNS) can be a reliable method for the quantitative analysis of the blood-nerve barrier (BNB) and the relationship between restoration of BNB and nerve regeneration. Sciatic nerves in rats were examined after nerve crush. Immunohistochemical staining with anti-rat endothelial cell antigen-1 (anti-RECA-1) that recognizes endothelial cells and anti-endothelial barrier antigen (anti-EBA) for the detection of barrier-type endothelial cells were used. Neurofilament for staining axons was also performed. A quantitative analysis of the BNB was assessed using the ratio of EBA positive cells and RECA-1 positive cells. The ratio of EBA/RECA-1 decreased significantly 3 days postoperatively and reached its lowest level at day 7 in the segment 5 mm proximal and the entire distal stump. The ratio gradually recovered from the proximal and the regeneration of axons started a week earlier than BNB. The ratio of EBA/RECA-1 applied to the PNS can be a reliable method for the quantitative analysis of BNB. In crush injuries, the breakdown of BNB occurred simultaneously in the segment 5 mm proximal and the entire distal stump; restoration began from the proximal to distal and followed a week later to nerve regeneration.

Alleviated tension at the repair site enhances functional regeneration: the effect of full range of motion mobilization on the regeneration of peripheral nerves--histologic, electrophysiologic, and functional results in a rat model

BACKGROUND

In the clinical management of combined tendon and nerve injuries, competing treatment strategies are well known. The effect of mobilization on the functional regeneration of peripheral nerves remains controversial. This study sought to determine the effect of full range of motion mobilization on nerve repair by using tubular segmental nerve splinting to block movement, and thereby variable tension, at the nerve repair site.

METHODS

In 96 rats, the right sciatic nerve was transected midthigh and coapted immediately microsurgically. The groups used in the study were as follows: group N, epineural nerve repair; group T, segmental tubular nerve splinting with fixed in situ tension at the nerve suture site,allowing segmental movement only; group TN, segmental tubular nerve splinting with alleviated in situ tension at the nerve suture site, allowing segmental movement only; and group TM, segmental tubular nerve splinting without fixed in situ tension at the nerve suture site, allowing movement of the nerve suture site. Full range of motion of the lower limbs was ensured by passive motion of hind limbs once a week after functional testing. Blinded histologic, immunohistochemical, and electrophysiologic assessment and 12 postoperative weekly function tests were carried out.

RESULTS

Functional and electrophysiologic results were significantly better in group TN, by segmental tubular nerve splinting with alleviated in situ tension at the nerve repair site, mainly because of less scar formation and enhanced endoneural angiogenesis at the nerve suture segment.

CONCLUSION

Full range of motion mobilization may impede functional nerve recovery by significant endoneural collagenization and decreased angiogenesis at the nerve suture segment. Complete alleviation of in situ (pathophysiologic) tension at the nerve suture site seems to improve functional peripheral nerve regeneration.

Pathology of lumbar nerve root compression. Part 1: Intraradicular inflammatory changes induced by mechanical compression

STUDY DESIGN

This study is to investigate the intraradicular inflammation induced by mechanical compression using in vivo model.

OBJECTIVES

The relationship between the intraradicular edema and nerve fiber degeneration induced by mechanical compression was determined in the nerve root.

SUMMARY OF BACKGROUND DATA

Recently some studies reported that mechanical compression increased microvascular permeability of the endoneurial capillaries and resulted in an intraradicular inflammation. These changes may be an important factor of the pathogenesis of radiculopathy. However, the natural courses of the intraradicular inflammation after mechanical compression are still poorly understood.

METHODS

In dogs, laminectomy was performed at L7 and the seventh nerve root was exposed to compression at 7.5 gram force (gf) clipping power. The animals were evaluated at 1 and 3 weeks after clipping. After the appropriate period of nerve root compression, Evans blue albumin (EBA) was injected intravenously. The nerve root sections were divided into two groups. The sections were used to investigate the status of the blood-nerve barrier function under the fluorescence microscope. The other sections were used for light and transmission electron microscopic study.

RESULTS

After 1 and 3 weeks, intraradicular edema was observed not only at the site of compression but also in the peripheral zone of a compressed anterior root and in the central zone of a compressed posterior root. The evidence of active Wallerian degeneration was also seen in the area of intraradicular edema. In addition, the nerve roots showing Wallerian degeneration were infiltrated by inflammatory cells, such as macrophages and mast cells.

CONCLUSIONS

Inflammatory reaction, such as Wallerian degeneration, breakdown of blood-nerve barrier and appearance of macrophage, may be deeply involved in radiculitis arising from mechanical compression, and these factors seem to be important in the manifestation of radiculopathy.

The effect of hyperbaric oxygen treatment on early regeneration of sensory axons after nerve crush in the rat

Abstract The effect of hyperbaric oxygen treatment (HBO) on sensory axon regeneration was examined in the rat. The sciatic nerve was crushed in both legs. In addition, the distal stump of the sural nerve on one side was made acellular and its blood perfusion was compromised by freezing and thawing. Two experimental groups received hyperbaric exposures (2.5 ATA) to either compressed air (pO2 = 0.5 ATA) or 100% oxygen (pO2 = 2.5 ATA) 90 minutes per day for 6 days. Sensory axon regeneration in the sural nerve was thereafter assessed by the nerve pinch test and immunohistochemical reaction to neurofilament. HBO treatment increased the distances reached by the fastest regenerating sensory axons by about 15% in the distal nerve segments with preserved and with compromised blood perfusion. There was no significant difference between the rats treated with different oxygen tensions. The total number of regenerated axons in the distal sural nerve segments after a simple crush injury was not affected, whereas in the nerve segments with compromised blood perfusion treated by the higher pO2, the axon number was about 30% lower than that in the control group. It is concluded that the beneficial effect of HBO on sensory axon regeneration is not dose-dependent between 0.5 and 2.5 ATA pO2. Although the exposure to 2.5 ATA of pO2 moderately enhanced early regeneration of the fastest sensory axons, it decreased the number of regenerating axons in the injured nerves with compromised blood perfusion of the distal nerve stump.

Evidence that Very Slow Wallerian Degeneration in C57BL/Ola Mice is an Intrinsic Property of the Peripheral Nerve

We have described a mutant mouse, C57BL/Ola, in which Wallerian degeneration following peripheral nerve transection is very slow. Our previous results suggested that recruited monocytes play a role in rapid Wallerian degeneration. The nature of the mutation in C57BL/Ola mice is not known and we have investigated whether the defect is intrinsic to the nerve or due to a defect in the circulating monocytes. We have made chimaeric mice in which bone marrow from histocompatible mice, with rapidly degenerating nerves and normal monocyte recruitment, was used to reconstitute irradiated C57BL/Ola mice and vice-versa. A substantial degree of donor repopulation of the hosts was confirmed by measures of the levels of glucose-phosphate isomerase alloenzymes in blood and tissue samples from the two different strains. The rate of degeneration of the transected sciatic nerve was found to be host-dependent, providing evidence that the mutation affects cell populations intrinsic to the nerve and not the circulating monocytes. We provide additional evidence that the peripheral nerves of C57BL/Ola mice are different from those of other mice as they degenerate at a slower rate in vitro.

Transferrin Receptor Expression and Iron Uptake in the Injured and Regenerating Rat Sciatic Nerve

Iron-saturated transferrin is a ubiquitous growth factor that plays a critical role in cellular iron uptake, growth and proliferation. Here we have studied the expression and distribution of transferrin receptors and iron uptake following injury of the rat sciatic nerve. Axotomy led to a massive but transient increase (days 2 - 9, maximum day 4) in [125I]transferrin binding at the site of the injury and in the distal, denervated part of the crushed or resected sciatic nerve, shortly preceding the time course of cellular proliferation (Friede and Johnstone, Acta Neuropathol, 7, 218 - 231, 1967; Jurecka et al., Acta Neuropathol, 32, 299 - 312, 1975). An additional, transient increase in specific binding was observed during reinnervation after reconnection of the resected sciatic nerve. Immunocytochemistry using the Ox-26 monoclonal antibody revealed strong and simultaneous expression of the transferrin receptor protein on two different cell types: on a subpopulation of blood-borne macrophages invading the injured peripheral nerve and on Schwann cells reacting to denervation and reinnervation. In addition, studies using intravenously injected radioactive iron (59Fe3+) showed a massive increase in endoneural iron uptake confined to the lesion site and to the distal part of the axotomised sciatic nerve, parallel to the time course of reactive transferrin receptor expression. Since iron is an essential cofactor of a number of key enzymes needed in energy metabolism and DNA synthesis, these data suggest that the induction of transferrin receptor expression may play an important role in the regulation of cellular growth and proliferation during peripheral nerve regeneration.

Development of the blood-nerve barrier in neonatal rats

The blood-nerve barrier (BNB) is constituted by the perineurium and the endothelium of endoneurial microvessels. We investigated the age at which the vascular component of BNB function is established in the rat and the ultrastructural modifications accompanying changes in permeability. BNB permeability was assessed with injections of Evans blue albumin (EBA) and horseradish peroxidase (HRP) in rats of different ages. Sciatic nerve sections were studied using fluorescence and electron microscopy. Nerves from animals injected with EBA indicated that the BNB is not functional before 13 days of life but that its function is established by 16 days. These results were confirmed by electron microscope examination of nerve sections from animals injected with HRP, which showed clefts between the endothelial cells of endoneurial vessels in young rats. In rats over 18 days, these clefts were occluded by tight junctions, which prevented HRP from leaving the vessel lumen and conferred BNB function. Systematic morphometric analysis of nerves from different age groups allowed the establishment of baseline normal histologic neural development with age.

Distribution of TGF-beta, the TGF-beta type I receptor and the R-II receptor in peripheral nerves and mechanoreceptors; observations on changes after traumatic injury

The mechanisms governing the regeneration of denervated peripheral mechanoreceptors are similar to those of peripheral nerves. The ability to regenerate depends partly on changes of the Schwann cell phenotype. The transforming growth factor beta (TGF-beta) family have been implicated in induction of Schwann cell proliferation, production of extracellular matrix and neurotrophin synthesis as well as synthesis or repression of cell adhesion molecules. Hence, they may prove to be of importance for regenerative mechanisms in peripheral mechanoreceptors. The distribution of TGF-beta, the receptors I and II and intra-cellular second messengers, Smad 2/3 and 4 was assessed in sensory neurones, peripheral nerves and mechanoreceptors by immuno-histochemistry, immuno-electron microscopy and in situ hybridisation. TGF-beta2 mRNA and TGF-beta2-like immunoreactivity (IR) were expressed in injured small and medium sized rat sensory neurones of dorsal root ganglia. TGF-beta and receptor II mRNA and immunoreactivities (IR) were present in satellite cells. Intact and injured sensory neurones expressed receptor I mRNA and Smad 2 mRNA. TGF-beta2 mRNA was found in transected nerve stumps and in sensory mechanoreceptors. TGF-beta1, 2 and Smad 4 were also observed in inner core lamellar cells of intact and denervated cat Pacinian corpuscles. Lamellar cells of intact and denervated Meissner corpuscles were TGF-beta immunoreactive. Merkel cells were receptors I and II immunoreactive. In conclusion, cutaneous and subcutaneous mechanoreceptors differ with regard to the expression of TGF-beta isoforms and receptors.

Effects of hyperbaric oxygen treatment on axonal outgrowth in sciatic nerve grafts in rats

We studied the effect of hyperbaric oxygen treatment on axonal outgrowth in grafts of sciatic nerves in 40 rats. The sciatic nerve was transsected and a 10 mm long segment from the opposite side was immediately sutured in as a nerve graft. Postoperatively 17 animals were treated with 100% oxygen at 3.2 atmospheres absolute pressure for 45 minutes and the treatment was repeated at four and eight hours postoperatively and then every eight hours until evaluation. At seven days the axonal outgrowth was evaluated by immunohistochemical staining of neurofilaments in the nerve grafts. The axonal outgrowth was significantly longer in animals treated with hyperbaric oxygen. We conclude that hyperbaric oxygen can improve nerve regeneration in sciatic nerve grafts in rats.

VEGF enhances intraneural angiogenesis and improves nerve regeneration after axotomy

Whilst there is an increased understanding of the cell biology of nerve regeneration, it remains unclear whether there is a direct interrelationship between vascularisation and efficacy of nerve regeneration within a nerve conduit. To establish this is important as in clinical surgery peripheral nerve conduit grafting has been widely investigated as a possible alternative to the use of nerve autografts. The aim of this study was to assess whether vascular endothelial growth factor (VEGF), a highly specific endothelial cell mitogen, can enhance vascularisation and, indirectly, axonal regeneration within a silicone nerve regeneration chamber. Chambers containing VEGF (500-700 ng/ml) in a laminin-based gel (Matrigel) were inserted into 1 cm rat sciatic nerve defects and nerve regeneration examined in relation to angiogenesis between 5 and 180 d. Longitudinal sections were stained with antibodies against endothelial cells (RECA-1), axons (neurofilament) and Schwann cells (S-100) to follow the progression of vascular and neural elements. Computerised image analysis demonstrated that the addition of VEGF significantly increased blood vessel penetration within the chamber from d 5, and by d 10 this correlated with an increase of axonal regeneration and Schwann cell migration. The pattern of increased nerve regeneration due to VEGF administration was maintained up to 180 d, when myelinated axon counts were increased by 78 % compared with plain Matrigel control. Furthermore the dose-response of blood vessel regeneration to VEGF was clearly reflected in the increase of axonal regrowth and Schwann cell proliferation, indicating the close relationship between regenerating nerves and blood vessels within the chamber. Target organ reinnervation was enhanced by VEGF at 180 d as measured through the recovery of gastrocnemius muscle weights and footpad axonal terminal density, the latter showing a significant increase over controls (P < 0.05). The results demonstrate an overall relationship between increased vascularisation and enhanced nerve regeneration within an acellular conduit, and highlight the interdependence of the 2 processes.

Favorable effect of VEGF gene transfer on ischemic peripheral neuropathy

Ischemic peripheral neuropathy is a frequent, irreversible complication of lower extremity vascular insufficiency. We investigated whether ischemic peripheral neuropathy could be prevented and/or reversed by gene transfer of an endothelial cell mitogen designed to promote therapeutic angiogenesis. Intramuscular gene transfer of naked DNA encoding vascular endothelial growth factor (VEGF) simultaneously with induction of hindlimb ischemia in rabbits abrogated the substantial decrease in motor and sensory nerve parameters, and nerve function recovered promptly. When gene transfer was administered 10 days after induction of ischemia, nerve function was restored earlier and/or recovered faster than in untreated rabbits. These findings are due in part to enhanced hindlimb perfusion. In addition, however, the demonstration of functional VEGF receptor expression by Schwann cells indicates a direct effect of VEGF on neural integrity as well. These findings thus constitute a new paradigm for the treatment of ischemic peripheral neuropathy.

Peripheral nerve injury induces endoneurial expression of IFN-gamma, IL-10 and TNF-alpha mRNA

Axotomy of a peripheral nerve leads to interruption of axon continuity with Wallerian degeneration in the distal segment and regenerative events in the proximal remaining neuron. Local inflammation is a consequence of trauma in general and signal molecules regulating inflammation, such as cytokines, participate in the outcome of nerve trauma. We studied a broad set of potent immunoregulatory cytokines after transection of rat sciatic nerve. The endoneurium of the transected rat sciatic nerve was taken from both proximal and distal stumps. The pooled endoneurium of 6 rats was studied using reverse transcription polymerase chain reaction (RT-PCR) after 14 h; 1, 3, 5, 7 days; 2 and 4 weeks after transection. A new observation was that TNF-alpha mRNA showed phasic expression pattern; three distinct peaks were seen, immediately (14 h), after 5 days and in the distal part also after 2 weeks. This phenomenon may be related to the breakdown of the blood-nerve barrier and to the recruitment of circulating macrophages. We further noticed that IFN-gamma mRNA was expressed between 5 days and 2 weeks. This suggests that T-cells may also take part in the regenerative processes. Furthermore, we observed that IL-10 mRNA is expressed continuously during Wallerian degeneration. The continuous expression of IL-10 mRNA may attenuate the production of inflammatory cytokines by macrophages and other cells.

Modulation of macrophage and microglial responses to axonal injury in the peripheral and central nervous systems

OBJECTIVE

After axonal injury, macrophages rapidly infiltrate and become activated in the mammalian peripheral nervous system (PNS) but not the central nervous system (CNS). We used the dorsal root pathway to study factors that modulate the response of macrophages to degenerating axons in both the PNS and the CNS.

METHODS

Lewis rats underwent transection of dorsal roots (Group 1), stab within the spinal cord (Group II), crush at the dorsal root entry zone (Group III), transection of dorsal roots combined with a CNS lesion (Group IV), or systemic administration of a known activator of macrophages, lipopolysaccharide, alone (Group V) or combined with transection of dorsal roots (Group VI). ED-1 antibody stained for macrophages and activated microglia at 7, 14, and 42 days postinjury.

RESULTS

At early time points, Group I demonstrated ED-1 cells in the PNS but not the CNS portion of the degenerating dorsal roots. Group II revealed ED-1 cells near the stab lesion. Group III demonstrated ED-1 cells adjacent to the dorsal root entry zone crush site. Group IV revealed ED-1 cells along both the PNS and the CNS portions of the degenerating dorsal roots when the CNS lesion was placed near the transected roots. Group V demonstrated few ED-1 cells in the PNS and the CNS, whereas Group VI revealed a marked ED-1 cellular response along both the PNS and the CNS portions of the transected dorsal roots.

CONCLUSION

Local CNS trauma and systemic administration of lipopolysaccharide can "prime" macrophages/microglia, resulting in an enhanced response to degenerating axons in the CNS. Such priming might prove useful in promoting axonal regeneration.

Quantitative analysis of edema in the dorsal nerve roots induced by acute mechanical compression

STUDY DESIGN

Edema in the dorsal nerve roots caused by acute compression was assessed quantitatively in the lumbar spine of the adult dog.

OBJECTIVE

To establish quantitative evaluation of edema in the dorsal nerve roots and to observe changes after acute compression with time.

SUMMARY OF BACKGROUND DATA

Mechanical compression induces an increase in microvascular permeability of the endoneurial capillaries and results in intraneural edema. However, there are no quantitative studies on edema in the nerve roots.

METHODS

The seventh lumbar nerve root was compressed with a 60-g force clip for 10 minutes. The nerve roots were removed immediately and at 24 hours, 1 week, and 3 weeks after compression. Nerve roots from the control and the sham groups were also obtained. Before removing the nerve roots, Evans blue albumin was injected intravenously. Changes in edema were examined using fluorescence microscopy. Evans blue albumin emits a bright red fluorescence. The relative red fluorescent area was calculated using computer image analysis, and the data were used to indicate the degree of edema.

RESULTS

In the compressed segment, edema was most pronounced just after decompression and reduced in nerves removed at 24 hours. In nerves removed at 1 week, edema was pronounced but was reduced at 3 weeks. In the segments closest to the spinal cord, edema was seen after 1 week and was significant after 3 weeks. In the segments closest to the dorsal root ganglion, edema was not detected at any time.

CONCLUSION

In the dorsal nerve roots the degree and the area of edema changed with time elapsed after acute compression. The degree of edema 24 hours after decompression was one third the degree immediately after decompression. These results show that edema induced by mechanical compression can recover after decompression.

Inter-relationships between angiogenesis and nerve regeneration: a histochemical study

Whilst increases in capillary number and permeability occurring during nerve regeneration suggest an interaction between regenerating axons and blood vessels, clinical attempts to improve nerve regeneration by augmenting nerve graft vascularisation have produced conflicting results and the nature of their relationship remains obscure. A better understanding of the process might be exploited in the development of a synthetic alternative to the autologous nerve graft and bring an improvement in the clinical results of nerve repair. To clarify this relationship the growth of axons and blood vessels through mats of orientated fibronectin grafted in rat sciatic nerves was assessed morphologically. Fibronectin, which supports axonal regeneration, is initially acellular, ensuring all vascular and neural elements within the graft are newly formed. To follow the progression of the elements, grafts were harvested between 3 and 30 days and stained with antibodies against endothelial cells (RECA-1), Schwann cells (S-100) and axons (a polyclonal or monoclonal panaxonal marker). Dual fluorescence staining combined with double exposure photography allowed the simultaneous visualisation of these elements and the demonstration of their true relative positions. Graft vascularisation came initially from the adjacent muscle bed. A neovascularisation front preceded axonal regeneration, although vessel and axonal orientation appeared similar. Schwann cells and axons extended together, never exceeding the area of vascularisation and appeared most numerous in well vascularised areas containing longitudinally orientated vessels. These results suggest that provision of a well vascularised, longitudinally orientated conduit may enhance nerve regeneration.

Vascular permeability in the peripheral autonomic and somatic nervous systems: controversial aspects and comparisons with the blood-brain barrier

Endothelium, choroidal epithelium, and arachnoid exclude plasma proteins from most parts of the mammalian central nervous system (CNS). Nerve roots, in contrast, have permeable capillaries and permeable pia-arachnoid sheaths. Diffusion of plasma proteins into the cerebrospinal fluid is probably prevented by slow bulk flow along a pressure gradient from the subarachnoid space into the veins of the roots. In nerves, the perineurium prevents diffusion of proteins from the epineurium into the endoneurium. Capillaries within fascicles are permeable to macromolecules, though less so than the microvessels of roots and ganglia. Endoneurial vascular permeability is lowest in rats and mice, but even in these species albumin is normally present in the extracellular spaces around the nerve fibers. The so-called blood-nerve barrier is not equivalent to the blood-brain barrier. Capillaries in sensory and sympathetic ganglia are fully permeable to macromolecules, and extravasated protein is in contact with neuronal cell bodies and neurites. An impenetrable perineurium surrounds each ganglion, but serves no obvious purpose when the vessels inside are as permeable as those outside. The enteric nervous system lacks a perineurium, and the neurons in its avascular ganglia and tracts are exposed to extracellular fluid formed by permeable vessels in adjacent tissues of the gut. The reasons for excluding macromolecules from some parts of the nervous system are obscure. Carrier-mediated transport, which maintains a constant supply of ions, glucose, and other metabolites to cells in the CNS, would be impossible if larger molecules could diffuse freely. Presumably the metabolic needs of ganglia are adequately met by exchange vessels similar to those of nonnervous tissues. Most of the CNS is protected from exogenous toxic substances that bind to plasma proteins. Peripheral neurons and glial cells are damaged by some such substances because of the lack of blood-tissue barriers.

Spinal axons in central nervous system scar tissue are closely related to laminin-immunoreactive astrocytes

Although transected central nervous system axons fail to regrow after injuries in adult mammals, they send sprouts into the scar tissue that forms at the lesion. We have investigated the relation between scar cells, laminin-like immunoreactivity and cut spinal axons in two previously characterized spinal cord lesion types. Labeling with antisera to glial fibrillary acidic protein and laminin demonstrated that the scar tissue formed after lesions in the rat and cat dorsal and ventral funiculi showed prominent gliosis and strong laminin-like immunoreactivity four days to one year postlesion. Axonal sprouts in the scar, visualized with antibodies to neurofilament (RT97) or by tracing using fluorescein-conjugated dextran, were ensheathed by a thin layer of strongly laminin-immunoreactive tissue. Immunoelectron microscopy demonstrated that axons in the scar were ensheathed predominantly by astrocytes, and that the surface of the cells outlining the axons in the scar showed strong laminin-like immunoreactivity. Adhesive and neurite orienting properties in the scar tissue were assessed in an in vitro system where PC12 cells were cultured on spinal cord slices from dorsal funiculus-lesioned rats. Very few cells adhered to the spinal cord section except for the part where the scar tissue had formed, where numerous cells were attached. The PC12 cells that had adhered to the scar tissue were mainly seen in parts of the scar that showed laminin-like immunoreactivity and their neurites predominantly followed tissue showing laminin-like immunoreactivity. The close association between axonal sprouts and laminin-like immunoreactivity indicates a role for laminin in axonal growth and/or guidance in the injured spinal cord.

The vascular response to nerve transection: neovascularization in the silicone nerve regeneration chamber

The rat sciatic nerve regeneration chamber was used to study the spatial and temporal response of the endoneurial vasculature during regeneration. Proximal and distal stumps of a transected rat sciatic nerve were placed in opposite ends of a silicone tube and allowed to regenerate for periods of 2, 3, 4 or 52 weeks after the surgery. Serial, transverse sections of nerve were studied at each time-point to quantitate the number of vessels, capillary density and the vessel luminal perimeter per nerve area. The results indicate that the vascular growth relative to the existing tissue in the chamber increases to a peak beyond normal levels and later decreases to values associated with control tissue. While this growth occurred from both the proximal and distal stumps, it appeared predominantly as a traveling wave in the proximal-distal direction preceding the major thrust of neuritic outgrowth from the proximal stump. Morphologic measurements of angiogenesis were paralleled in other animals by measurements of nerve blood flow using laser Doppler flowmetry at corresponding time-points. These data differ somewhat from previous reports of angiogenesis following nerve crush injury and are useful in formulating a general mathematical model of regeneration in the peripheral nervous system.

Pathophysiology of glial growth factor receptors

Activation and proliferation of glial cells are common events in the pathology of the nervous system. Although we are only beginning to understand the molecular signals leading to glial activation in vivo, there is increasing evidence that growth factors and their receptors may play an important part. In this paper we summarize the data on the pathophysiology of glial growth factor receptors and their ligands in the central and peripheral nervous systems.

Molecular characterization of anionic sites on the luminal front of endoneurial capillaries in sciatic nerve

The distribution of anionic microdomains has been described in cerebral vessels and more recently in capillaries of peripheral nerve. Evidence is accumulating that these sites play a role in the barrier function of vascular endothelia in the PNS and CNS. The chemical nature of anionic sites has been at least partly determined for cerebral vessels but not in peripheral nerve. This study reports our preliminary investigations to determine the nature of endothelial anionic sites in sciatic nerve. The effects of digestion of ultra-thin sections of nerve with a battery of proteolytic and glycolytic enzymes (papain, trypsin, proteinase K, hyaluronidase, heparinase, heparitinase and neuraminidase) on the distribution of anionic sites was determined using the label, cationic colloidal gold. Papain, a proteolytic enzyme of broad specificity, succeeded in removing the majority of cationic colloidal gold-binding sites on the luminal surface of vascular endothelia. In contrast trypsin and proteinase K were less effective, reflecting their narrower specificity. Hyaluronidase, heparinase and heparitinase did not significantly affect cationic colloidal gold-labelling. However, a considerable reduction in cationic colloidal gold-binding occurred following neuraminidase digestion. These results suggest that, as in cerebral vessels, sialic acid-containing glycoproteins are largely responsible for the negatively charged domains on the luminal membrane of endothelial cells in peripheral nerve.

The vascular response to nerve crush: relationship to Wallerian degeneration and regeneration

The response of the endoneurial vasculature in rat sciatic nerve following crush injury was investigated by morphometric analysis of serial nerve transverse sections at the site of injury and in distal segments at 1, 2, 3, 6, and 9 weeks after injury. Quantitative analysis included determination of the number of vessels, vessel radius vessel perimeter, and transfascicular area. The vascular response to crush injury consisted of two phases: an early phase, which peaked at 1 week after crush, consisted of an increase in vessel size but not vessel number. The second phase, which peaked at 6 weeks after crush, consisted of an increase in the number of vessels and in their density. This two-phase response was also evident as a dual peak in the total endoneurial vessel perimeter, a measure of vascular surface area, when this variable was plotted against time. The first phase of the vascular response was temporally related to the recruitment of macrophages and the clearance of degenerating axonal and myelin tissue during the early phase of Wallerian degeneration. The second phase involved an increase in the number of blood vessels and was associated with cellular proliferation, neurite elongation, and myelination during the subsequent period of nerve regeneration.

Growth of ascending spinal axons in CNS scar tissue

The aim of the present study was to test the capacity of spinal cord scar tissue to assist and sustain axon regrowth. In adult rats and cats the dorsal funiculus (DF) was cut at mid-thoracic or lumbar level, and a superficial incision in the DF rostral to the lesion was made in order to extend the penetrating lesion. Axonal tracing in rats 50-100 days postinjury with anterogradely transported wheatgerm agglutinin-conjugated horseradish peroxidase or rhodamine-conjugated dextran demonstrated that nerve fibers had entered the scar tissue. Axon ingrowth in the scar was further indicated by axonal immunoreactivity to the growth-associated protein GAP-43. The scar tissue showed low-affinity neurotropin receptor-like immunoreactivity in association with blood vessels and in the interstitium. The integrity of the blood-brain barrier in the extended dorsal funiculus lesion was disrupted for at least 11 months postinjury, assessed by i.v. injections of free HRP or Evans blue. The present study shows that penetrating injury in the dorsal funiculus produces a CNS environment permissive for axonal sprouting and that PNS influence is not necessary for spinal tract regrowth. A possible relationship between the absence of an intact BBB and injury-induced axonal sprouting is discussed.

Peripheral nerve regeneration: role of growth factors and their receptors

Growth factors play a central role in the regulation of normal and injury-induced regenerative cell growth. The purpose of this article is to summarize the available data on the expression of different growth factors and their receptors in the injured peripheral nervous system and to discuss their possible role in promoting peripheral nerve regeneration.

Blood-nerve barrier: ultrastructural and endothelial surface charge alterations following nerve crush

Nerve crush results in an enhanced vascular permeability of the endoneurial vessels distal to the lesion. Vascular permeability at the blood-nerve barrier (BNB) to serum proteins is influenced by many factors, including anionic surface charge, endothelial vesicular transcytosis and the presence or absence of fenestrated vessels. Using mice and rats, the present ultrastructural investigation examined the effect of nerve crush (axonotmesis) on: (1) the distribution of endothelial anionic sites and (2) the appearance of fenestrations in endoneurial vessels after 4 and 14 day intervals as demonstrated with cationic probes. Transient anionic fenestrations developed in a minority of mouse endoneurial vessels in 4-day crushed nerves, but were not found in 14-day crushed nerves of mice nor in crushed nerves of rats. The known increase in the permeability of endoneurial vessels in rats and mice was not associated with reduced luminal labelling with cationic ferritin at physiological pH. At pH 2.0 the labelling of glycocalyx moieties (such as sialic acid) with cationic colloidal gold was disrupted in some epi- and endoneurial vessels of 4-day rats, but in a greater proportion after 14 days. The enhanced permeability of the BNB during degeneration and regeneration is related to the formation of anionic fenestrations in endoneurial vessels of mice and to the reduced and uneven distribution of endothelial glycocalyx moieties that are anionic at pH 2.0 in rats.

Effects of alcohol feeding on synthesis and secretion of apolipoproteins by regenerating rat sciatic nerve

The amounts of apolipoprotein (apo) E and A1 released into the culture medium were examined in the regenerating nerves distal to a crush site following chronic alcohol feeding. Cultured minced segments of regenerating nerves taken from rats fed an alcohol-containing liquid diet for 5 weeks released only 50% of apoE but nearly 200% of apoA1 when compared with rats pair-fed with a control diet. The extent of decrease in medium apoE corresponded to the decrease of apoE mRNA in the nerve. Thus, chronic alcohol ingestion affects apoE synthesis of regenerating nerves by changing its mRNA level. On the other hand, apoA1 mRNA remained undetectable in regenerating and intact nerves whether the rats were fed alcohol or not. Furthermore, the amount of apoA1 released by the regenerating nerve into the culture medium was not significantly larger than that present in the nerve tissue prior to incubation. Therefore, it is most likely that apoA1 released by the injured nerve originated from the bloodstream and the increase in apoA1 content seen in the crushed nerve of alcohol-fed rats is due to an enhanced permeability of the nerve-blood barrier. Since the burst of apolipoproteins in the injured nerve is likely to play a role in nerve regeneration, the perturbation of apolipoprotein contents in regenerating nerves by chronic alcohol consumption may contribute to the pathogenesis of alcoholic neuropathy.

Schwann cell endocytosis: a role in nerve regeneration?

Schwann cell plasma membrane vesicles have been shown to increase in numerical density after nerve injury but their function is unclear. In this study, ultrastructural tracers were micro-injected in vivo into crushed rat sciatic nerves after various time intervals to ascertain whether plasma membrane vesicles of Schwann cells are involved in the uptake and utilization of molecules from the endoneurium during axonal regeneration and remyelination. Horseradish peroxidase (HRP), a tracer of fluid-phase endocytosis, was taken up by macrophages and fibroblasts but remained external to Schwann cells throughout the study. After 14-16 days of crush injury, HRP was present within vessel lumina and in cytoplasmic vesicles of pericytes and vascular endothelia. Low-density lipoprotein-gold, which is primarily internalized by receptor-mediated endocytosis, and bovine serum albumin-gold, proposed as a tracer for fluid-phase endocytosis, were internalized by macrophages and fibroblasts but were not taken up by Schwann cells. Although Schwann cells formed pits in the plasma membrane and vesicles were evident in the cytoplasm, none of the tracers used were internalized by Schwann cells. It is suggested that Schwann cell plasmalemmal and cytoplasmic vesicles have a cellular role unrelated to endocytosis or alternatively the Schwann cell basal lamina may function as a diffusion barrier to the tracers employed.

A breakdown of the blood-brain barrier is associated with optic nerve regeneration in the frog

We have examined the integrity of the blood-brain barrier during optic nerve regeneration in the frog Litoria (Hyla) moorei using rhodamine B-labeled bovine serum albumin (RBA). A transient localized breakdown of the blood-brain barrier was observed between 1 and 5 weeks after extracranial optic nerve crush. The zone of breakdown progressed along the experimental optic nerve, ascended the opposite optic tract, and swept rostro-caudally across the tectum contralateral to the crushed nerve. By 7 weeks, the blood-brain barrier was once again intact along the length of the optic pathway. In a concurrent series of frogs, regenerating optic axons were visualized by anterograde transport of horseradish peroxidase (HRP). At each stage examined, the region reached by the front of regenerating axons corresponded to that in which the blood-brain barrier had been shown to break down. In contrast to the results after nerve crush, the blood-brain barrier remained intact along the length of the optic pathway following optic nerve ligation to prevent regeneration. We conclude that the breakdown of the blood-brain barrier which occurs during optic nerve regeneration in the frog is triggered by the regenerating axons.

Perineurial permeability to sodium during Wallerian degeneration in rat sciatic nerve

In rat sciatic nerves, the effect of Wallerian degeneration on the rate of transperineurial passage of sodium between the endoneurium and the epineurial extracellular space was investigated. In nerves transected and ligated at the sciatic notch, an in situ technique was used to measure the permeability coefficient-surface area product (PS) of the mid-thigh portion of the perineurium to 22Na. Sampling times ranged from one day to sixteen weeks after the lesion. Additionally, endoneurial water content (an indicator of nerve edema) was also measured in transected, degenerating nerves at the same sampling times. Endoneurial water content increased significantly by the fourth day after transection, peaked at four weeks, and then remained elevated through 16 weeks of post-lesion measurement. The PS of the perineurium to 22Na on the 4th day after transection was significantly greater than that of control animals. This increase then declined to normal levels through the 2nd week, and finally increased to values that were 3-fold to 4-fold of control values for the remainder of the observation period. The earlier, short lasting increase in perineurial PS is probably associated with the inflammatory response to nerve section, and proliferation of perineurial layers and cells. The later increase in perineurial permeability is proposed to play a role in the dissipation of endoneurial hydrostatic pressure and clearance of myelin debris from the endoneurium. In view of the complex changes in perineurial permeability described herein, it would seem inappropriate to consider these phenomena merely as passive breakdowns of the barrier properties of the perineurium.

Nerve cables formed in silicone chambers reconstitute a perineurial but not a vascular endoneurial permeability barrier

The passage of molecules into the endoneurial environment of the axons of normal peripheral nerve is regulated by two permeability barriers, the perineurial-nerve barrier and the endoneurial blood-nerve barrier. These barriers exist because of the presence of tight junctions between adjacent perineurial cells and adjacent endothelial cells. In the present study we investigated whether permeability barriers form in nerve cables, which develop inside silicone chambers. The sciatic nerves of adult rats were cut, and the proximal and distal ends sutured into opposite ends of silicone chambers that were filled with dialyzed plasma. The presence of barriers was determined with the tracer horseradish peroxidase (HRP), which was injected intravenously and detected histochemically in tissues by light and electron microscopy. At four weeks, a regenerated nerve cable extended across the 10 mm length of each chamber. However, no permeability barriers were present since the reaction product for HRP was visible throughout the cable. At twenty-six weeks, all the axons in cables were gathered into minifascicles. Each minifascicle of axons was surrounded by perineurial cells. Blood vessels were excluded from the minifascicles by the perineurial cells and the vessels were permeable to HRP, thus indicating that their endothelial cells had not formed tight junctions. Despite the leakage of HRP from the excluded vessels, the tracer did not reach the axons because the perineurial cells encircling the minifascicles developed tight junctions. In some animals, the chambers were removed at four weeks to determine whether the chamber influenced barrier development. This manipulation had no effect since cables, with or without chambers, exhibited similar findings at twenty-six weeks. Our results indicate that nerve cables regenerate a perineurial but not an endoneurial permeability barrier. We conclude that axons in long-term cables are protected by only a perineurial permeability barrier.

Extravasation of staphylococcal alpha-toxin in normal and injured CNS regions lacking blood-brain barrier function: observations after ventral root replantation

Staphylococcus aureus plays an important role as a bacterial pathogen after traumatic injury. The majority of isolated strains produces alpha-toxin, a 33-kDa protein, with membrane-damaging and lethal effects. The central nervous system (CNS) has been considered as the possible target for the lethal action of this toxin. A transfer of alpha-toxin across an intact blood-brain barrier (BBB) is however unlikely. The aim of the present study was to determine if alpha-toxin is accumulated in CNS regions which lack the BBB function. The distribution of alpha-toxin after intravascular injections, in normal mice and rats as well as in rats subjected to ventral root replantation, was assessed using immunogold technique. The results show that, although alpha-toxin does not cross the BBB, alpha-toxin-like immunoreactivity could be detected in the area postrema and at the optic nerve-retinal junction. Extravasation of alpha-toxin was also shown to occur in the spinal cord even 22 months after ventral root replantation. This finding suggests that axon regeneration after ventral root replantation takes place in a macromolecular environment which is totally different from the normal CNS. The implications of vascular spread of alpha-toxin to regions devoid of BBB function are discussed in relation to the bacterial infections which might complicate severe spinal injuries.

Blood-nerve barrier in the frog during wallerian degeneration: are axons necessary for maintenance of barrier function?

Blood-nerve barrier tissues (endoneurial blood vessels and perineurium) of the frog's sciatic nerve were studied during chronic Wallerian degeneration to determine whether barrier function depends on the presence of intact axons. Sciatic nerves of adult frogs were transected in the abdominal cavity; the ends were tied to prevent regeneration and the distal nerve stumps were examined. Vascular permeabilities to horseradish peroxidase and to [14C]sucrose increased to day 14, returned toward normal levels by 6 weeks, and continued at near normal levels to 9 months. Perineurial permeabilities to the tracers increased by day 10 and remained elevated at 9 months. Proliferation of perineurial, endothelial, and mast cells occurred between 3 days and 6 weeks, resulting in an increased vascular space (measured with [3H]dextran) and number of vascular profiles. The perineurium increased in thickness and the mast cells increased in number. This study indicates that during Wallerian degeneration of the frog's sciatic nerve there is 1) a transitory increase in vascular permeability distal to the lesion, that is related to changes within the endoneurium; 2) an irreversible increase in permeability of the perineurium, which begins later than that seen in the endoneurial blood vessels; and 3) proliferation of non-neuronal components in the absence of regenerating neuronal elements. The results indicate that maintenance of vascular integrity does not require the presence of axons in the frog's peripheral nerve, whereas perineurial integrity and barrier function are affected irreversibly by Wallerian degeneration.