The effect of a conditioning lesion on the regeneration rate of peripheral nerve axons containing substance P

The regeneration rate of peripheral nerve axons containing substance P-like immunoreactivity (SPLI) was measured in rat sciatic nerve by radioimmunoassay of SPLI in nerve segments 2, 4 and 6 days after a test lesion made by briefly crushing the nerve at the hip. The regeneration rate of the fastest growing sensory axons was also measured in the same nerves using the pinch-reflex procedure. Three groups of animals were compared: group S, which received only the single test lesion, had regeneration rates of 3.57 +/- 0.26 (S.E.) mm/day for SPLI-containing axons and 3.53 +/- 0.14 mm/day for the fastest growing sensory axons. Group A/H, which received a conditioning lesion on the tibial nerve at the ankle 7 days prior to the test lesion at the hip, had a regeneration rate for SPLI-containing axons which was not significantly different from group S, of 3.35 +/- 0.17 mm/day. However, the regeneration rate for the sensory axons was significantly increased to 4.60 +/- 0.23 mm/day. Group H/H, which received both conditioning and test lesions at the hip, once again separated by 7 days, showed a significant increase in regeneration rate of SPLI-containing axons to 5.50 +/- 0.33 mm/day and a further increase over group A/H in the regeneration rate of sensory axons to 6.70 +/- 0.25 mm/day. We conclude that the small-diameter, unmyelinated axons containing SPLI in peripheral nerve normally regenerate at the same rate as the fastest growing sensory axons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Femoral nerve regeneration and its accuracy under different injury mechanisms

Surgical accuracy has greatly improved with the advent of microsurgical techniques. However, complete functional recovery after peripheral nerve injury has not been achieved to date. The mechanisms hindering accurate regeneration of damaged axons after peripheral nerve injury are in urgent need of exploration. The present study was designed to explore the mechanisms of peripheral nerve regeneration after different types of injury. Femoral nerves of rats were injured by crushing or freezing. At 2, 3, 6, and 12 weeks after injury, axons were retrogradely labeled using 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate(Dil) and True Blue, and motor and sensory axons that had regenerated at the site of injury were counted. The number and percentage of Dil-labeled neurons in the anterior horn of the spinal cord increased over time. No significant differences were found in the number of labeled neurons between the freeze and crush injury groups at any time point. Our results confirmed that the accuracy of peripheral nerve regeneration increased with time, after both crush and freeze injury, and indicated that axonal regeneration accuracy was still satisfactory after freezing, despite the prolonged damage.     

Expression and glycanation of the NG2 proteoglycan in developing, adult, and damaged peripheral nerve

We have investigated expression of the axon growth-inhibitory proteoglycan NG2 in peripheral nerve. In the adult, NG2 was present on endoneurial and perineurial fibroblasts, but not on Schwann cells. At birth, peripheral nerve NG2 was heavily glycanated, but was much less so in the adult. In vitro, sciatic nerve fibroblasts also produced heavily glycanated NG2. After peripheral nerve injury in rats and humans, an accumulation of NG2-positive cells was observed at the injury site. In the rat, there was an increase in NG2 glycanation for at least 2 weeks following injury. In mixed cultures of Schwann cells and peripheral nerve fibroblasts, the axons preferred to grow on the Schwann cells and seldom crossed onto the fibroblasts. Three-dimensional cultures of sciatic nerve fibroblasts were inhibitory to the growth of dorsal root ganglion axons. Inhibition of proteoglycan synthesis made the cells more permissive. NG2 may play a part in blocking axon regeneration through scar tissue in injured human peripheral nerve.     

Peripheral nerve regeneration is delayed in neuropilin 2-deficient mice

Peripheral nerve transection or crush induces expression of class 3 semaphorins by epineurial and perineurial cells at the injury site and of the neuropilins neuropilin-1 and neuropilin-2 by Schwann and perineurial cells in the nerve segment distal to the injury. Neuropilin-dependent class 3 semaphorin signaling guides axons during neural development, but the significance of this signaling system for regeneration of adult peripheral nerves is not known. To test the hypothesis that neuropilin-2 facilitates peripheral-nerve axonal regeneration, we crushed sciatic nerves of adult neuropilin-2-deficient and littermate control mice. Axonal regeneration through the crush site and into the distal nerve segment, repression by the regenerating axons of Schwann cell p75 neurotrophin receptor expression, remyelination of the regenerating axons, and recovery of normal gait were all significantly slower in the neuropilin-2-deficient mice than in the control mice. Thus, neuropilin-2 facilitates peripheral-nerve axonal regeneration.     

Functional aspects of the regeneration of unmyelinated axons in the rat saphenous nerve

Electrophysiological experiments have been carried out to investigate aspects of unmyelinated axon regeneration in a transected cutaneous nerve. Some comparisons with regeneration of myelinated axons in the same nerve have also been made.

By 3 months after injury approximately 80% of the unmyelinated axons that had survived in the proximal stump had regenerated into the distal stump. About the same proportion of myelinated axons had regrown into the distal stump by this time. With both groups of axons there was no marked increase in the amount of regeneration across the injury site with longer recovery times. Conduction velocities in the regenerated unmyelinated axons tended to be slower across the injury site than proximally; the proximal conduction velocities did not differ from those in control nerves. The unmyelinated axons seemed to take longer to resupply the skin than did the myelinated ones, but in both cases the extent of skin innervation had reached about 60% of control values by 6 months after the injury.     

Long-term survival and axonal regeneration of retinal ganglion cells after optic nerve transection and a peripheral nerve graft

To investigate the effect of autologous peripheral nerve grafting on retinal ganglion cell survival and axonal regeneration after an injury, the optic nerve of adult Sprague-Dawley rats was transected and grafted with an autologous peripheral nerve from the peroneal branch of the left sciatic nerve. The numbers of both surviving and axon-regenerating retinal ganglion cells were determined at different times after surgery. The majority of retinal ganglion cells were rapidly lost within 3 weeks, followed by a slow and protracted phase of cell loss until the end of the 6-month study. FluoroGold-labelled axon-regenerating retinal ganglion cells were first detected by 2 weeks, followed by a period of high axonal regeneration that peaked at 8 weeks and accounted for over 35% of the total surviving retinal ganglion cells. However, retinal ganglion cells with regenerated axons eventually died. Our data thus indicate that axonal regeneration in the autologous peripheral nerve graft is insufficient to sustain the long-term survival of axotomized retinal ganglion cells.     

Spatial-Temporal progress of peripheral nerve regeneration within a silicone chamber: Parameters for a bioassay

The spatial-temporal progress of peripheral nerve regeneration across a 10-mmgap within a silicone chamber was examined with the light and electron microscope at 2-mm intervals. A coaxial, fibrin matrix was observed at 1 week with a proximal-distal narrowing that extended beyond the midpoint of the chamber. At 2 weeks, Schwann cells, fibroblasts, and endothelial cells had migrated into the matrix from both nerve stumps. There was a delay of 7–14 days after nerve transection and chamber implantation before regenerating axons appeared in the chamber. At 2 weeks, nonmyelinated axons were seen only in the proximal 1–5 mm of the chamber in association with Schwann cells. Axons reached the distal stump by 3 weeks and a proximal-distal gradient of myelination was observed. These observations define the parameters of a morphologic assay for regeneration in this chamber model which can be used to investigate cellular and molecular mechanisms underlying the success of peripheral nerve regeneration.     

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.     

Flunarizine enhances survival and regeneration of sensory and motor neurons after peripheral nerve injury

The diphenylpiperazine, flunarizine, partially prevents apoptosis after trophic factor deprivation in neural crest-derived neurons. Flunarizine protects dorsal root ganglion neurons (DRG) after nerve growth factor (NGF) withdrawal in vitro and after peripheral nerve injury in newborn rats in vivo. We have further studied the mechanisms of neuronal protection by flunarizine. Oligosomal DNA fragmentation, a hallmark of apoptosis, was significantly decreased by treatment of DRG neurons with flunarizine after NGF deprivation. We examined the effect on survival of the timing of administration of flunarizine to DRG neurons both in vitro and in vivo. Flunarizine effectively rescued dissociated DRG neurons if administered up to six hours after NGF withdrawal. In vivo, flunarizine prevented DRG neuronal death after sciatic axotomy in newborn rats if given soon after injury. Long-term experiments were done to test the ability of flunarizine to protect neurons and enhance regeneration after sciatic nerve injury. Newborn rats were subjected to peripheral nerve injury and administered flunarizine for four weeks; no further treatment was given for an additional 12 weeks. The group treated with flunarizine demonstrated a significantly increased number of DRG and spinal motor neurons that had regenerated axons into the distal sciatic nerve as determined by retrograde labeling with HRR Myelinated axons in the sural nerve in the group treated with flunarizine increased by nearly two-fold compared to control animals. Thus, flunarizine was able to enhance survival and promote long-term regeneration of sensory and motor spinal neurons after peripheral nerve injury.     

BD™ PuraMatrix™ peptide hydrogel seeded with Schwann cells for peripheral nerve regeneration

This study investigated the effects of a membrane conduit filled with a synthetic matrix BD™ PuraMatrix™ peptide (BD) hydrogel and cultured Schwann cells on regeneration after peripheral nerve injury in adult rats.After sciatic axotomy, a 10 mm gap between the nerve stumps was bridged using ultrafiltration membrane conduits filled with BD hydrogel or BD hydrogel containing Schwann cells. In control experiments, the nerve defect was bridged using either membrane conduits with alginate/fibronectin hydrogel or autologous nerve graft. Axonal regeneration within the conduit was assessed at 3 weeks and regeneration of spinal motoneurons and recovery of muscle weight evaluated at 16 weeks postoperatively.Schwann cells survived in the BD hydrogel both in culture and after transplantation into the nerve defect. Regenerating axons grew significantly longer distances within the conduits filled with BD hydrogel when compared with the alginate/fibronectin hydrogel and alginate/fibronectin with Schwann cells. Addition of Schwann cells to the BD hydrogel considerably increased regeneration distance with axons crossing the injury gap and entering into the distal nerve stump. The conduits with BD hydrogel showed a linear alignment of nerve fibers and Schwann cells.The number of regenerating motoneurons and recovery of the weight of the gastrocnemius muscle was inferior in BD hydrogel and alginate/fibronectin groups compared with nerve grafting. Addition of Schwann cells did not improve regeneration of motoneurons or muscle recovery.The present results suggest that BD hydrogel with Schwann cells could be used within biosynthetic conduits to increase the rate of axonal regeneration across a nerve defect.     

The numbers of unmyelinated and myelinated axons in normal and regenerated rat saphenous nerves

Counts have been made of the numbers of unmyelinated and myelinated axons in the proximal and distal stumps of regenerated rat saphenous nerves and from equivalent sites in normal nerves. In the proximal part of normal nerves there were averages of 1 045 myelinated axons and 4 160 unmyelinated ones. Regenerated nerves contained the same number of myelinated axons in their proximal stumps but there was a 40% reduction in the unmyelinated axon count. In the distal stumps of these nerves the myelinated axon count had increased by an average of 620; this comes about because some regenerated myelinated axons support more than one process in the distal stump. In contrast, the number of unmyelinated axons was reduced further, from a mean of 2 476 in the proximal stump to one of 2 219.

The sizes of Schwann cell units in the normal and regenerated nerves were also noted. Schwann cell units in the proximal and distal stumps of the regenerated nerves were smaller than those in the normal ones.

These changes associated with unmyelinated axons in regenerated nerves are likely to contribute to the sensory, vasomotor and sudomotor abnormalities that sometimes occur after peripheral nerve injury and regeneration.     

Netrin-1 and peripheral nerve regeneration in the adult rat

Axonal guidance during development of the nervous system is thought to be highly regulated through interactions of axons with attractive, repulsive, and trophic cues. Similar mechanisms regulate axonal regeneration after injury. The netrins have been shown to influence the guidance of several classes of developing axons. Although netrins have been implicated as axonal guidance cues in the developing peripheral nervous system, there has been no direct evidence of netrin-1 expression in either developing or adult peripheral nerve. The present study utilized competitive PCR and immunohistochemistry to demonstrate the localization of netrin-1 within adult rat sciatic nerve. The expression of netrin-1 mRNA and protein was compared for normal or regenerated sciatic nerve 2 weeks following either a crush or a transection and repair injury. The PCR data show that netrin-1 mRNA is normally expressed at low levels in peripheral nerve, and similar low levels are found 2 weeks following a crush injury. However, 2 weeks following nerve transection and repair there is approximately a 40-fold increase in netrin-1 mRNA levels. Immunohistochemistry data show that Schwann cells are the major source of netrin-1 protein in peripheral nerve. Our results suggest that netrin-1 mRNA levels are profoundly affected during peripheral nerve injury and regeneration. The localization of netrin-1 to Schwann cells suggests that this protein is strategically situated to influence axon regeneration in adult peripheral nerve.     

Tubulization Increases Axonal Outgrowth of Rat Sciatic Nerve after Crush Injury

It is important to develop methods which increase nerve regeneration since restoration of function following injury to peripheral nerves often requires outgrowth of the injured axons over long distances. In this study, axonal outgrowth after bilateral crush injury to the sciatic nerve of the rat was measured. One group with large-diameter nonpermeable silicone tubes and one group with large-diameter permeable silicone tubes applied around the crush site on one side had regeneration following nerve injury compared to controls on the other side. The length of regeneration of the regenerating axons were then measured 4, 5, and 6 days following the crush injury using the “pinch reflex test.” The presence of axons at the pinch level was confirmed by immunocytochemical staining for neurofilaments. The length of regeneration for rats with nonpermeable tubes was significantly greater than that of the contralateral control side and was so at all times (p < 0.05). The effect was present but not that pronounced where permeable tubes were used. We conclude that the outgrowth of regenerating sensory axons after sciatic nerve crush injury in the rat can be increased by enclosing the regeneration site in a silicone tube. The observed effect may be due to local mechanisms such as macrophage invasion or prevention of rapid wash-out of fluid from the crush zone.     

End-to-end and end-to-side neurorrhaphy between thick donor nerves and thin recipient nerves:an axon regeneration study in a rat model

During nerve reconstruction,nerves of different thicknesses are often sutured together using end-to-side neurorrhaphy and end-to-end neurorrhaphy techniques.In this study,the effect of the type of neurorrhaphy on the number and diameter of regenerated axon fibers was studied in a rat facial nerve repair model.An inflow-type end-to-side and end-to-end neurorrhaphy model with nerve stumps of different thicknesses(2:1 diameter ratio) was created in the facial nerve of 14 adult male Sprague-Dawley rats.After 6 and 12 weeks,nerve regeneration was evaluated in the rats using the following outcomes:total number of myelinated axons,average minor axis diameter of the myelinated axons in the central and peripheral sections,and axon regeneration rate.End-to-end neurorrhaphy resulted in a significantly greater number of regenerated myelinated axons and rate of regeneration after 6 weeks than end-to-side neurorrhaphy;however,no such differences were observed at 12 weeks.While the regenerated axons were thicker at 12 weeks than at 6 weeks,no significant differences in axon fiber thickness were detected between end-to-end and end-toside neurorrhaphy.Thus,end-to-end neurorrhaphy resulted in greater numbers of regenerated axons and increased axon regeneration rate during the early postoperative period.As rapid reinnervation is one of the most important factors influencing the restoration of target muscle function,we conclude that end-to-end neurorrhaphy is desirable when suturing thick nerves to thin nerves.     

Inhibitor of DNA binding 2 accelerates nerve regeneration after sciatic nerve injury in mice

Inhibitor of DNA binding 2 (Id2) can promote axonal regeneration after injury of the central nervous system. However, whether Id2 can promote axonal regeneration and functional recovery after peripheral nerve injury is currently unknown. In this study, we established a mouse model of bilateral sciatic nerve crush injury. Two weeks before injury, AAV9-Id2-3×Flag-GFP was injected stereotaxically into the bilateral ventral horn of lumbar spinal cord. Our results showed that Id2 was successfully delivered into spinal cord motor neurons projecting to the sciatic nerve, and the number of regenerated motor axons in the sciatic nerve distal to the crush site was increased at 2 weeks after injury, arriving at the tibial nerve and reinnervating a few endplates in the gastrocnemius muscle. By 1 month after injury, extensive neuromuscular reinnervation occurred. In addition, the amplitude of compound muscle action potentials of the gastrocnemius muscle was markedly recovered, and their latency was shortened. These findings suggest that Id2 can accelerate axonal regeneration, promote neuromuscular reinnervation, and enhance functional improvement following sciatic nerve injury. Therefore, elevating the level of Id2 in adult neurons may present a promising strategy for peripheral nerve repair following injury. The study was approved by the Experimental Animal Ethics Committee of Jinan University (approval No. 20160302003) on March 2, 2016.

Chinese Library Classification No. R456; R745; R364.3+3     

Biological conduit small gap sleeve bridging method for peripheral nerve injury: regeneration law of nerve fibers in the conduit

The clinical effects of 2-mm small gap sleeve bridging of the biological conduit to repair peripheral nerve injury are better than in the traditional epineurium suture, so it is possible to replace the epineurium suture in the treatment of peripheral nerve injury. This study sought to identify the regeneration law of nerve fibers in the biological conduit. A nerve regeneration chamber was constructed in models of sciatic nerve injury using 2-mm small gap sleeve bridging of a biodegradable biological conduit. The results showed that the biological conduit had good histocompatibility. Tissue and cell apoptosis in the conduit apparently lessened, and regenerating nerve fibers were common. The degeneration regeneration law of Schwann cells and axons in the conduit was quite different from that in traditional epineurium suture. During the prime period for nerve fiber regeneration(2–8 weeks), the number of Schwann cells and nerve fibers was higher in both proximal and distal ends, and the effects of the small gap sleeve bridging method were better than those of the traditional epineurium suture. The above results provide an objective and reliable theoretical basis for the clinical application of the biological conduit small gap sleeve bridging method to repair peripheral nerve injury.     

Regeneration of axons after nerve transection repair is enhanced by degradation of chondroitin sulfate proteoglycan

Our past work indicates that growth-inhibiting chondroitin sulfate proteoglycan (CSPG) is abundant in the peripheral nerve sheaths and interstitium. In this study we tested if degradation of CSPG by chondroitinase enhances axonal regeneration through the site of injury after (a) nerve crush and (b) nerve transection and coaptation. Adult rats received the same injury bilaterally to the sciatic nerves and then chondroitinase ABC was injected near the injury site on one side, and the contralateral nerve was injected with vehicle alone. Nerves were examined 2 days after injury in the nerve crush model and 4 days after injury in the nerve transection model. Chondroitinase-dependent neoepitope immunolabeling showed that CSPG was thoroughly degraded around the injury site in the chondroitinase-treated nerves. Axonal regeneration through the injury site and into the distal nerve was assessed by GAP-43 immunolabeling. Axonal regeneration after crush injury was similar in chondroitinase-treated and control nerves. In contrast, axonal regrowth through the coaptation of transected nerves was markedly accelerated and the ingress of axons into the distal segment was increased severalfold in nerves injected with chondroitinase. On the basis of these results we concluded that growth inhibition by CSPG contributes critically to the poor regenerative growth of axons in nerve transection repair. In addition, degradation of CSPG by injection of chondroitinase ABC at the site of nerve repair increased the ingress of axonal sprouts into basal laminae of the distal nerve segment, presumably by enabling more latitude in growth at the interface of coapted nerve. This suggests that chondroitinase application may be used clinically to improve the outcome of primary peripheral nerve repair.     

Remnant neuromuscular junctions in denervated muscles contribute to functional recovery in delayed peripheral nerve repair

Schwann cell proliferation in peripheral nerve injury(PNI)enhances axonal regeneration compared to central nerve injury.However,even in PNI,long-term nerve damage without repair induces degeneration of neuromuscular junctions(NMJs),and muscle atrophy results in irreversible dysfunction.The peripheral regeneration of motor axons depends on the duration of skeletal muscle denervation.To overcome this difficulty in nerve regeneration,detailed mechanisms should be determined for not only Schwann cells but also NMJ degeneration after PNI and regeneration after nerve repair.Here,we examined motor axon denervation in the tibialis anterior muscle after peroneal nerve transection in thyl-YFP mice and regeneration with nerve reconstruction using allografts.The number of NMJs in the tibialis anterior muscle was maintained up to 4 weeks and then decreased at 6 weeks after injury.In contrast,the number of Schwann cells showed a stepwise decline and then reached a plateau at 6 weeks after injury.For regeneration,we reconstructed the degenerated nerve with an allograft at 4 and 6 weeks after injury,and evaluated functional and histological outcomes for 10 to 12 weeks after grafting.A higher number of pretzel-shaped NMJs in the tibialis anterior muscle and better functional recovery were observed in mice with a 4-week delay in surgery than in those with a 6-week delay.Nerve repair within 4 weeks after PNI is necessary for successful recovery in mice.Prevention of synaptic acetylcholine receptor degeneration may play a key role in peripheral nerve regeneration.All animal experiments were approved by the Institutional Animal Care and Use Committee of Tokyo Medical and Dental University on 5 July 2017,30 March 2018,and 15 May 2019(A2017-311C,A2018-297A,and A2019-248A),respectively.     

Axonal regrowth in the amyelinated optic nerve of the myelin-deficient rat: ultrastructural observations and effects of ganglioside administration

It has been postulated that myelin degradation products may inhibit regrowth of mammalian central axons and that central nervous system (CNS) myelin and oligodendrocytes may constitute a "nonpermissive substrate" for axonal growth. To address these issues, we utilized an X-linked rat mutant, myelin-deficient or md. In the optic nerve of this mutant, 40 days and more postnatally, normal myelin is absent and oligodendrocytes are few (Dentinger et al. Brain Res. 344:255-266, 1985). Twenty-eight days before sacrifice, we operated on four groups of 50-day-old md rats and age-matched normal littermates according to the following protocols: 1) unilateral intraorbital optic nerve crush; 2) beginning within 1 hour of nerve crush, daily intraperitoneal injection of GM1 ganglioside (20 mg/kg) dissolved in phosphate-buffered saline (PBS); 3) daily intraperitoneal injection of PBS alone, also begun within 1 hour of nerve crush; 4) severance of the optic nerve immediately behind the papilla 16 or 21 days after the primary crush lesions. Additionally, normal and md rats were killed 4 and 14 days after unilateral optic nerve injury. Nerves of unoperated md rats and their normal littermates were also processed. In the operated animals that did not receive GM1, ultrastructural analysis 4, 14, and 28 days after lesioning revealed that md optic nerves contained significantly greater numbers of regenerating axons, including growth cones and varicosities, than nerves of normal rats. Notably, 28 days postoperatively, (group 1), regenerating axons were still abundant in md nerve, whereas, in nerves of normally myelinated littermates, axonal numbers were diminished markedly. Regenerating optic axons of both md and normally myelinated rats were oriented by linear astrocytic arrays and often were enclosed by astrocytic cytoplasm. In normal littermates, GM1 administration (group 2) induced a significant increase in the number of axons within the operative lesion. Paradoxically, GM1 inhibited the ordinarily robust regeneration of md axons. PBS-injected md and normal rats (group 3) showed no significant differences from noninjected, operated animals. Severance of the nerve at the papilla (group 4) 7-12 days before sacrifice confirmed the origination of axonal regrowth by retinal ganglion cells. The data provide in vivo support for a role of myelin breakdown products or the secretory products of oligodendroglia in the inhibition of regenerative axonal sprouting within mammalian CNS.     

Recovery of C-Fiber-Induced Extravasation Following Peripheral Nerve Injury in the Rat

Peripheral nerve injury leads to substantial alterations in injured sensory neurons. These include cell death, phenotypic modifications, and regeneration. Primary sensory neurons have recently been shown not to die until a time beyond 4 months following a nerve crush or ligation and this loss is, moreover, limited to cells with unmyelinated axons, the C-fibers. The late loss of C-fibers may be due to a lack of target reinnervation during the regenerative phase. In order to investigate this, we have used a particular peripheral function, unique to C-fibers, as a measure of peripheral reinnervation: an increase in capillary permeability on antidromic activation of C-fibers, i.e., neurogenic extravasation. This was investigated in rats that had received a nerve crush injury 1 to 50 weeks earlier. Some recovery of the capacity of C-fibers to generate extravasation was detected at 8–10 weeks, which increased further at 12–14 weeks, and then plateaued at this level with no further recovery at 30 or 50 weeks. In intact and damaged sciatic nerves, Aβ-fibers never induced extravasation. These findings are compatible with the hypothesis that those C-fibers which make it back to their peripheral targets do not subsequently die and those that do not, may die.