Rapid response of identified resident endoneurial macrophages to nerve injury

Macrophages play a central role in the pathogenesis of peripheral neuropathy but the role of resident endoneurial macrophages is undefined because no discriminating markers exist to distinguish them from infiltrating hematogenous macrophages. We identified and characterized resident endoneurial macrophages during Wallerian degeneration in radiation bone marrow chimeric rats created by transplanting wild-type Lewis rat bone marrow into irradiated TK-tsa transgenic Lewis rats. In such animals, resident cells carry the transgene, whereas hematogenous cells do not. As early as 2 days after sciatic nerve crush and before the influx of hematogenous macrophages, resident transgene-positive endoneurial macrophages underwent morphological and immunophenotypic signs of activation. At the same time, resident macrophages phagocytosing myelin were found, and proliferation was detected by bromodeoxyuridine incorporation. Continuous bromodeoxyuridine feeding revealed that resident endoneurial macrophages sequentially retracted their processes, proliferated, and expressed the ED1 antigen, rendering them morphologically indistinguishable from hematogenous macrophages. Resident endoneurial macrophages thus play an early and active role in the cellular events after nerve lesion before hematogenous macrophages enter the nerve. They may thus be critically involved in the pathogenesis of peripheral neuropathy particularly at early stages of the disease and may act as sensors of pathology much like their central nervous system counterparts, the microglial cells.

Differential cyclin D1 requirements of proliferating Schwann cells during development and after injury

Neurons regulate Schwann cell proliferation, but little is known about the molecular basis of this interaction. We have examined the possibility that cyclin D1 is a key regulator of the cell cycle in Schwann cells. Myelinating Schwann cells express cyclin D1 in the perinuclear region, but after axons are severed, cyclin D1 is strongly upregulated in parallel with Schwann cell proliferation and translocates into Schwann cell nuclei. During development, cyclin D1 expression is confined to the perinuclear region of proliferating Schwann cells and the analysis of cyclin D1-null mice indicates that cyclin D1 is not required for this type of Schwann cell proliferation. As in the adult, injury to immature peripheral nerves leads to translocation of cyclin D1 to Schwann cell nuclei and injury-induced proliferation is impaired in both immature and mature cyclin D1-deficient Schwann cells. Thus, our data indicate that the molecular mechanisms regulating proliferation of Schwann cells during development or activated by axonal damage are fundamentally different.

Chronic blockade of melanocortin receptors alleviates allodynia in rats with neuropathic pain

We investigated the involvement of the spinal cord melanocortin (MC) system in neuropathic pain. Because we recently demonstrated that MC receptor ligands acutely alter nociception in an animal model of neuropathic pain, in this study we tested whether chronic administration was also effective. We hypothesized that chronic blockade of the spinal MC system might decrease sensory abnormalities associated with this condition. The effects of the MC receptor antagonist SHU9119 (0.5 microg/d) and agonist MTII (0.1 microg/d) were evaluated in rats with a chronic constriction injury of the sciatic nerve. Drugs were continuously infused into the cisterna magna. Antinociceptive effects were measured with tests involving temperature (10 degrees C or 47.5 degrees C) or mechanical (von Frey) stimulation. The administration of MTII increased mechanical allodynia, whereas SHU9119 produced a profound cold and mechanical antiallodynia, altering responses to control levels. The antiallodynic effects of SHU9119 were very similar to those produced by the alpha(2)-adrenergic agonist tizanidine (50 microg/d). The effects of SHU9119 and MTII are most likely mediated through the MC4 receptor, because this is the only MC-receptor subtype present in the spinal cord. We conclude that the chronic administration of MC4-receptor antagonists might provide a promising tool in the treatment of neuropathic pain.

IMPLICATIONS

In this study we demonstrated that continuous intrathecal infusion of the melanocortin-receptor antagonist SHU9119 reduces cold and mechanical allodynia in rats with a chronic constriction injury of the sciatic nerve, a lesion producing neuropathic pain.

Induction of type 3 iodothyronine deiodinase by nerve injury in the rat peripheral nervous system

Thyroid hormones are essential for the development and repair of the peripheral nervous system. The type 2 deiodinase, which is responsible for the activation of T(4) into T(3), is induced in injured sciatic nerve. To obtain information on the type 3 deiodinase (D3) responsible for the degradation of thyroid hormones, we looked for its expression (mRNA and activity) in the sciatic nerve after injury. D3 was undetectable in the intact sciatic nerve of adult rats, but was rapidly and highly increased in the distal and proximal segments after nerve lesion. After cryolesion, D3 up-regulation disappeared after 3 d in the proximal segment, whereas it was sustained for 10 d in the distal segment, then declined to reach basal levels after 28 d, when functional recovery was completed. After a transsection preventing the nerve regeneration, up-regulation of D3 persisted up to 28 d at high levels in the distal segment. D3 was expressed in peripheral connective sheaths and in the internal endoneural compartment. D3 mRNA was inducible by 12-O-tetradecanoylphorbol-13-acetate in cultured fibroblasts or Schwann cells. In conclusion, induction of D3 in the peripheral nervous system after injury may play an important role during the regeneration process by adjusting intracellular T(3) levels.

Nerve growth factor signaling of p75 induces differentiation and ceramide-mediated apoptosis in Schwann cells cultured from degenerating nerves

In peripheral nerve regeneration or remyelination, immature Schwann cells expressing p75(NTR) play cardinal roles in the support and regeneration of axons (Griffin JW, Hoffman PN. Peripheral Neuropathy 361-376, 1993). Only one of four to six Schwann cells participate in remyelination of damaged or regenerating axons. The rest of the cells, or supernumerary Schwann cells, show severe atrophy and gradually decrease in number, reestablishing a 1:1 axon-Schwann cell relationship (Said G, Duckett S. Acta Neuropathol (Berl) 53:173-179, 1981). Recent reports demonstrated that severely atrophied supernumerary Schwann cells are eliminated by apoptosis during axonal regeneration or remyelination (Hirata H, Hibasami H. Apoptosis 3:353-360, 1998; Berciano MT, Calle E. Acta Neuropathol (Berl) 95:269-279, 1998). The mechanism to induce selective death of supernumerary Schwann cells without causing any damage to axon-associated Schwann cells or axons remains to be determined. In this article, we report that p75(NTR), the low-affinity receptor for all members of neurotrophins, signals both cell differentiation and apoptosis through intracellular ceramide elevation. The final response is dependent on the intracellular ceramide level and Schwann cells modulate their response by changing expression level of p75(NTR). This effect was selective for nerve growth factor (NGF). Taken together, the present study suggests that NGF contributes both to phenotypic regulation and to elimination of the dedifferentiated Schwann cells, while supporting survival or regeneration of certain types of axons during peripheral nerve repair or regeneration.

Wallerian degeneration of injured axons and synapses is delayed by a Ube4b/Nmnat chimeric gene

Axons and their synapses distal to an injury undergo rapid Wallerian degeneration, but axons in the C57BL/WldS mouse are protected. The degenerative and protective mechanisms are unknown. We identified the protective gene, which encodes an N-terminal fragment of ubiquitination factor E4B (Ube4b) fused to nicotinamide mononucleotide adenylyltransferase (Nmnat), and showed that it confers a dose-dependent block of Wallerian degeneration. Transected distal axons survived for two weeks, and neuromuscular junctions were also protected. Surprisingly, the Wld protein was located predominantly in the nucleus, indicating an indirect protective mechanism. Nmnat enzyme activity, but not NAD+ content, was increased fourfold in WldS tissues. Thus, axon protection is likely to be mediated by altered ubiquitination or pyridine nucleotide metabolism.

Changes in voltage-gated calcium channel alpha(1) gene expression in rat dorsal root ganglia following peripheral nerve injury

Although an increase in the excitability and ectopic spontaneous discharge (ESD) of primary sensory neurons can lead to abnormal burst activity, which is associated with neuropathic pain, the underlying molecular mechanisms are not fully understood. To investigate the relationship between these electrical abnormalities in injured neurons and voltage-gated calcium channel (VGCC) gene expression, reverse transcription-polymerase chain reaction (RT-PCR) was used to monitor the expression of the VGCC alpha(1) gene in the dorsal root ganglion (DRG) following chronic constriction injury (CCI) and axotomy of the rat sciatic nerve. Electrophoresis of the RT-PCR products showed the presence of multiple types of VGCC alpha(1) transcripts with various levels of basal expression in lumbar 4, 5, and 6 DRGs. CCI decreased alpha(1C), alpha(1D), alpha(1H), and alpha(1I) mRNA expression at 7 days in the ipsilateral DRG, to approximately 34-50% of the contralateral side. The same transcripts were repressed 7 days after sciatic axotomy and their reduction levels proved similar to those of CCI. Considering that changes of the intracellular calcium concentration modify the maintenance of ESD in injured DRG, these results suggest that the downregulation of alpha(1C), alpha(1D), alpha(1H) and alpha(1I) subunit gene expression in the rat DRG following peripheral nerve injury may contribute to the production of ESD associated with damaged nerves.

H reflex restitution and facilitation after different types of peripheral nerve injury and repair

This study addresses the restitution of monosynaptic H reflex after nerve injuries and their role in the recovery of walking. Adult rats were submitted to sciatic crush, complete section repaired by aligned or crossed fascicular suture, or an 8-mm resection repaired by autograft or tube repair. The sciatic nerve was stimulated proximal to the injury site and the M and H waves were recorded from gastrocnemius (GCm) and plantar (PLm) muscles at monthly intervals during 3 months postoperation. Walking track tests were also carried out and the sciatic functional index (SFI) calculated to assess gait recovery. The M and H waves reappeared in all the animals at the end of the follow-up. The H/M amplitude ratio increased during the first stages of regeneration and tended to decrease to control values as muscle reinnervation progressed. However, final values of the H/M ratio for the PLm remained significantly higher in all the groups except that with a nerve crush. The walking track pattern showed an appreciable recovery only after crush injury. Final SFI values correlated positively with the M wave amplitude and negatively with the H/M ratio. In conclusion, H reflex is facilitated after peripheral nerve injury and regeneration and tends to return to normal excitability with time. Changes in the H reflex circuitry and excitability correlated positively with the deficient recovery of walking pattern after severe nerve injury.

Decreases in endomorphin-2-like immunoreactivity concomitant with chronic pain after nerve injury

Nerve injury often leads to chronic, sometimes excruciating, pain. The mechanisms contributing to this syndrome include neurochemical plasticity in neurons involved in the earliest stages of pain transmission. Endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)) is an endogenous morphine-like substance that binds to the mu-opioid receptor with high affinity and selectivity. Endomorphin-2-like immunoreactivity (LI) is present in the superficial layers of the dorsal horn in the spinal cord and in primary afferents, suggesting a role for this peptide in pain transmission. To determine whether spinal endomorphin-2-LI is altered in an animal model of chronic pain, the left sciatic nerve of Swiss Webster and ICR mice was ligated in a modified Seltzer model of nerve injury. Changes in endomorphin-2-LI were assessed by immunocytochemistry at 2, 4 and 14 days after nerve injury. The side of the spinal cord ipsilateral to the nerve injury exhibited a dramatic decrease in endomorphin-2-LI relative to the contralateral side and to control animals. The change was restricted to the medial dorsal horn in the lumbar segments innervated by the sciatic nerve. Substance P-LI showed a small decrease, while calcitonin gene-related peptide-LI was unchanged. Both thermal hyperalgesia, as evidenced by significantly decreased paw withdrawal latencies, and decreased endomorphin-2-LI were observed within 2 days of injury and were most pronounced at 2 weeks after injury. The decrease in endomorphin-2-LI during the development of chronic pain is consistent with the loss of an inhibitory influence on pain transmission. These results provide the first evidence that reduction of an endogenous opioid in primary afferents is associated with injury-induced chronic pain.

Lack of effects of transforming growth factor-alpha gene knockout on peripheral nerve regeneration may result from compensatory mechanisms

Transforming growth factor-alpha (TGF-alpha), previously identified as a major member of the epidermal growth factor (EGF) family of growth factors, plays a role in proliferation, differentiation, and survival of neuronal and glial precursors and is implicated in development of the nervous system. However, its roles in nerve injury-induced responses remain obscure. The current study examined roles of endogenous TGF-alpha in peripheral nerve regeneration using sciatic nerve injury models with TGF-alpha knockout mice. Three weeks after a sciatic nerve crush, no significant differences were found between TGF-alpha wild-type and mutant mice in the number of retrogradely labeled L5 dorsal root ganglion (DRG) sensory neurons and L5 spinal cord motor neurons and in the morphology of myelinated regenerating nerve fibers, indicating that TGF-alpha is not essential for sensory and motor nerve regeneration. To assess a possible functional redundancy among TGF-alpha-related ligands in response to a nerve injury, mRNA expression of the EGF family was analyzed by RT-PCR in L4/L5 DRG pools and distal degenerating sciatic nerve segments after sciatic nerve ligation. Prior to and 1 day after ligation, there was a higher level of EGF-R mRNA in DRGs and in nerve in TGF-alpha null mice compared to wild types, and there was an induction of ligand amphiregulin mRNA in DRGs in mutant mice in place of the TGF-alpha upregulation present in wild types. These results indicate that TGF-alpha gene knockout does not affect peripheral nerve regeneration, probably due to a functional redundancy within the EGF family through a compensatory expression mechanism at both the receptor and ligand levels in TGF-alpha knockout mice.

Systemic tizanidine hydrochloride (Zanaflex) relieves thermal hyperalgesia in rats with an experimental mononeuropathy

We sought to determine whether tizanidine, an alpha2-agonist, relieved thermal hyperalgesia in rats with surgically induced neuropathic pain. We used a Sprague-Dawley rat model in which a chronic constriction of the sciatic nerve caused the rats to develop postural changes, mechanical allodynia, and thermal hyperalgesia. Thermal hyperalgesia was verified through paw withdrawal latency (PWL). PWL was tested before surgery, after surgery, and after injections with tizanidine (0.5, 1.0, or 2.0 mg/kg) or normal saline. Ambulatory and total movements were evaluated by placing the rats in activity cages. Thermal hyperalgesia was induced in all rats after surgery. Tizanidine, but not saline, caused a significant improvement in PWL (P < 0.05), with complete reversal of thermal hyperalgesia at all doses on postoperative Day 6. Rats who received tizanidine 2 mg/kg maintained complete reversal of thermal hyperalgesia through postoperative Day 9. Some sedation was observed with tizanidine 2 mg/kg, but not with smaller doses. We conclude that tizanidine effectively reversed thermal hyperalgesia in a rat model.

IMPLICATIONS

This study was conducted to determine whether tizanidine could attenuate the thermal hyperalgesia that occurs in rats with surgically induced chronic constriction of the sciatic nerve. Tizanidine was effective in reducing sensitivity to heat, as measured by paw withdrawal latency, and did not cause sedation at smaller doses.

Effect of weight bearing on recovery from nerve injury in skeletal muscle

We examined the effect of weight bearing (WB) on muscle recovery after nerve injury. Rats were housed in individual cages for 2 wk under WB or hindlimb suspension (HS) after being subjected to sciatic nerve compression for 1 wk. Sham operated on rats served as controls (sham group). We used 31P- and 19F-nuclear magnetic resonance spectroscopy combined with histochemical, physiological, and biochemical techniques to assess the outcome in the three groups. Creatine kinase-BB (CK-BB) mRNA levels expression, CK activity, and type I fiber density in the WB group were elevated compared with those in the HS group. In addition, sciatic functional index, tetanic tension, energy state, and local circulation dynamics of the WB group were greater than those of the HS group. These results suggested that WB plays an important role in muscle regeneration, inhibits the reduction of CK activity, and facilitates the activation of neural recovery, energy state, and local circulation dynamics.

C-terminal flanking peptide of neuropeptide Y in DRG following nerve compression

The C-terminal flanking peptide of neuropeptide Y (CPON) was studied in dorsal root ganglia (DRG) by immunocytochemistry after different recovery periods (3, 6,14 and 28 days) following tourniquet compression of the rat hindlimb (sciatic nerve; 150 or 300 mmHg; 2 h). Compression induced a transient increase in the number of CPON-positive DRG-neurons (the contralateral uninjured side was devoid of CPON-positive cells). The compression-induced increase in CPON was less than that observed in separate rats subjected to sciatic nerve transection. The results show that compression induces regenerative changes in peripheral neurons and that such an injury of the nerve trunk is not limited to the site of the compression but results in the activation of the entire neuron.

Endogenous galanin is required for the full expression of central sensitization following peripheral nerve injury

The neuropeptide galanin is known to be involved in nociceptive sensory processing in the spinal cord. We have attempted to better characterise the function of endogenous galanin in nociceptive signalling by examining a mouse strain carrying a loss of function mutation in the galanin gene (gal-/-). Galanin expression is significantly up-regulated following damage to a peripheral nerve. To address what effect this up-regulation has on spinal cord excitability we have examined wild type (gal+/+) and gal-/- mice 3 days after complete transection of the sciatic nerve using an electrophysiological paradigm, the flexor withdrawal reflex. We demonstrate that the up-regulation of galanin has no direct effect on basal spinal excitability after nerve injury. However, galanin is shown to be a crucial neuromodulator involved in the development of the central sensitization as both windup and the facilitation of spinal reflexes following conditioning stimulation are significantly impaired in gal-/- mice following peripheral nerve injury.

The geometric design of micromachined silicon sieve electrodes influences functional nerve regeneration

A neural interface could be used to control a limb prosthesis. Such an interface can be created by facilitating axonal regeneration through a sieve electrode and then register nerve signals intended to control the prosthesis. A key question is how to design the electrodes to ensure the best possible regeneration. Our previous studies have indicated that regeneration can be achieved using electrodes with square-shaped, 100 x 100 microm, via holes (holes that axons will regenerate through). Other reports have indicated a suitable range of these holes between 40 and 65 microm. In the present study we used silicon sieve electrodes with via holes of either 30 or 90 microm. The transparency, i.e. the percentage of the total via hole area, of these electrodes was either 20 or 30%. The electrodes were inserted into a silicone chamber which was used to bridge a gap in a rat sciatic nerve. After 12 weeks of nerve regeneration electrodes with a hole size of 30 microm and a 30% transparency had the most favourable result as judged by the regained gastrocnemius muscle force and the formation of reactive tissue inside the chamber. The sieve electrode transparency is crucial for ensuring regeneration.

Confocal imaging of Schwann-cell migration along muscle-vein combined grafts used to bridge nerve defects in the rat

Schwann cells guide axonal regrowth during peripheral nerve repair. In a case of a nerve lesion with substance loss, a graft conduit is necessary to enable axons to reach the distal nerve stump. If a non-nervous autograft is used, the question arises as to the presence and origin of Schwann cells along the grafted tube. We addressed this issue using a tubulization technique based on the use of an autologous vein filled with fresh skeletal muscle for the repair of sciatic nerve defects in the rat. We showed that both ends of the graft were early and progressively colonized by a number of glial fibrillar acid protein-immunopositive and S-100 immunonegative cells, an immunocytochemical pattern typical of immature Schwann cells. These cells, which were located in the interstice between grafted skeletal muscle fibers, are mainly organized into long chains oriented along the main axis of the graft and progressively colonize all the graft. Schwann cells coming from the distal nerve end are suitable for being responsible for guiding regeneration of nerve fibers along the graft toward the correct periphery (tissue specificity).

[Morphological changes and quantitative DNA analysis of Schwann cells in peripheral nerves after high voltage electrical injury]

OBJECTIVE

To explore the characteristics and the mechanism of the injury of Schwann cells and nerve fibres of the peripheral nerves inflicted by high voltage electricity.

METHODS

Rabbits injured by high voltage electricity were employed as the model. Thirty rabbits were randomly divided into control (9) and experimental (21) groups. The dynamic quantitative DNA analysis and the change of the morphology and structure of the sciatic nerve were observed with LM and EM on 0, 3, 7, 10, 14 and 21 postburn days (PBDs), respectively.

RESULTS

There was continuous degeneration of the axon and medullary sheath of sciatic nerve tissue and its neogenesis was inhibited. The DNA synthesis in Schwann cell initiated on 3 postburn day (PBD). The distribution of the current was heterogeneous on the cross section of peripheral nerve fibres.

CONCLUSION

The obvious features of peripheral nerve injured by high voltage electricity included delayed initiation of the DNA synthesis of Schwann cell and the secondary necrosis and inhibition of neogenesis process of Schwann cell.

Complications of hip arthroscopy

Complications associated with hip arthroscopy occur between 1.6% and 5%. Fortunately, with the greater understanding of the causes and advancements in techniques and equipment, the incidence is declining. Most of the complications were transient neuropraxias and fluid extravasations resulting in no permanent damage. Severe scuffing of two femoral heads and one case of avascular necrosis were considered serious and permanent, thereby resulting in a 0.5% rate in our series for significant complications.

The spatiotemporal relationship among Schwann cells, axons and postsynaptic acetylcholine receptor regions during muscle reinnervation in aged rats

To morphologically define the aging-related features during muscle reinnervation the spatiotemporal relationships among the major components of the neuromuscular junctions (NMJs) were investigated. A total of 64 rats, 30 adults (4 months old) and 34 aged adults (24 months old), were used. Between 1 and 12 weeks after sciatic nerve-crushing injury, cryosections of skeletal muscle were single or double labeled for S100, a marker of Schwann cells (SCs), for protein gene product 9.5, a neuronal marker, and for alpha-bungarotoxin (alpha-BT), a marker of the acetylcholine receptor site (AChR site), and then observed by confocal laser microscopy. The most obvious age changes were noted: (1) the regenerating SCs and axons were delayed in their arrival at the NMJ, (2) the dimensions of terminal SCs and AChR sites displayed a drastic and long-lasting drop (for terminal SCs, during 1-8 weeks; for AChR sites, during 1-12 weeks); (3) the degree of spatial overlap between AChR sites and terminal SCs was markedly low until 8 weeks post-crush; (4) damage and poor formation in the SCs, terminal axons and AChR sites, together with poor process extension from the terminal SC or terminal axon, were pronounced; (5) persistent aberrant changes, such as multiple innervation and terminal axon sprouting, together with poorly formed collateral innervation, nerve bundles, and NMJs, more frequently occurred in the later reinnervation period. Thus, with aging, regeneration is impaired during the period in which regenerating SC strands and axons extend into NMJs and the subsequent establishment of nerve-muscle contact is in progress. A complex set of morphological abnormalities between or among the TSCs, terminal axons, and AChR sites may be important in slowing of regeneration and reinnervation in aged motor endplates.

Interrelations of myogenic response, progressive atrophy of muscle fibers, and cell death in denervated skeletal muscle

Little is known concerning the time-course and structural dynamics of reactivation of compensatory myogenesis in denervated muscle, its initiating cellular mechanisms, and the relationship between this process and the progression of postdenervation atrophy. The purpose of this study was to investigate the interrelations between temporal and spatial patterns of the myogenic response in denervated muscle and progressive atrophy of muscle fibers. Another objective was to study whether reactivation of myogenesis correlates with destabilization of the differentiated state and death of denervated muscle cells. It has remained unclear whether muscle fiber atrophy was the primary factor activating the myogenic response, what levels of cellular atrophy were associated with its activation, and whether the initiation and intensity of myogenesis depended on the local and individual heterogeneity of atrophic changes among fibers. For this reason, our objective was also to identify the levels of atrophic and degenerative changes in denervated muscle fibers that are correlated with activation of the myogenic response. We found that the reactivation of myogenesis in the tibialis anterior and extensor digitorum longus muscles of the rat starts between days 10-21 following nerve transection, before atrophy has attained advanced level, long before dead cells are found in the tissue. Formation of new muscle fibers reaches its maximum between 2 and 4 months following denervation and gradually decreases with progressive postdenervation atrophy. The myogenic response is biphasic and includes two distinct processes. The first process resembles the formation of secondary and tertiary generations of myotubes during normal muscle development and dominates during the first 2 months of denervation. During this period, activated satellite cells form new myotubes on live differentiated muscle fibers. Most of the daughter myotubes in 1- and 2-month denervated muscle develop on the surface of fast type parent muscle fibers, and some of the newly formed muscle fibers express slow myosin. Some fast type parent fibers are weakly or, more rarely, moderately immunopositive for embryonic isomyosin. This indicates that reactivation of myogenesis may also depend on the fiber type. The level of atrophy, destabilization of the differentiated myofiber phenotype, and degenerative changes of individual fibers in denervated muscle are very heterogeneous. The myogenic response of the first type is associated predominantly with fibers of average and higher than average levels of atrophy. Muscle cells that undergo a lesser degree of atrophy also form daughter fibers, although with a lower incidence. We did not find any correlation between the size of newly formed fibers and the level of atrophy of parent fibers. The topographical distribution of new myotubes both in the peripheral and central areas of the mid-belly equatorial sections at the early stages following nerve transection indicates that myogenesis of the first type represents a systemic reaction of muscle to the loss of neural control. These data indicate that activation of the myogenic response does not depend on cell death and degenerative processes per se. The second type of myogenesis is a typical regenerative reaction that occurs mainly within the spaces surrounded by the basal laminae of dead muscle fibers. Myocytes of different sizes are susceptible to degeneration and death, which indicates that cell death in denervated muscle does not correlate with levels of muscle cell atrophy. The regenerative process frequently results in development of abnormal muscle cells that branch or form small clusters. Replacement of lost fibers becomes activated between 2 and 4 months following nerve transection, i.e., mainly at advanced stages of postdenervation atrophy, when cell death becomes a contributing factor of the atrophic process. In long-term denervated muscle, the first and second types of myogenesisoccur concurrently, and the topographical distribution of the myogenic response becomes more heterogeneous than during the first weeks following denervation. Thus, our data demonstrate differential temporal and spatial expression of two patterns of myogenesis in denervated muscle that appear to be controlled by different regulatory mechanisms during the postdenervation period. (c) 2001 Wiley-Liss, Inc.

Crosstalk between p38, Hsp25 and Akt in spinal motor neurons after sciatic nerve injury

The p38 stress-activated protein kinase pathway is involved in regulation of phosphorylation of Hsp25, which in turn regulates actin filament dynamic in non-neuronal cells. We report that p38, Hsp25 and Akt signaling pathways were specifically activated in spinal motor neurons after sciatic nerve axotomy. The activation of the p38 kinase was required for induction of Hsp25 expression. Furthermore, Hsp25 formed a complex with Akt, a member of PI-3 kinase pathway that prevents neuronal cell death. Together, our observations implicate Hsp25 as a central player in a complex system of signaling that may both promote regeneration of nerve fibers and prevent neuronal cell death in the injured spinal cord.

Experimental study on slow-speed elongation injury of the peripheral nerve: electrophysiological and histological changes

Slow-speed elongation of the sciatic nerves was induced in 41 rabbits by lengthening the osteotomized femur by 2.1 mm/day with an external fixator. Time courses were monitored. About 2 weeks after elongation began, the amplitude of the compound nerve action potentials decreased, and delayed latency began. Histological observations showed that the wavy structure of the nerve fibers was lost during the stage at which reduction in the amplitude of action potentials occurred. In some nerve fibers, narrowing of the axons was observed. Further elongation caused delay in the percent latency. Histologically, collagen fibers were noted in the nerve fascicles, as well as narrowing in all the nerve fibers. When elongation ceased at the stage at which a reduction in the amplitude of action potential was noted, recovery from nerve damage followed. When elongation did not cease until the stage at which latency was delayed, however, recovery was not apparent, either histologically or electrophysiologically. The critical level of percent amplitude for recovery was at 82.4% of the original level (the critical rate for bone lengthening was 20.5%).

Temporal and spatial distribution of Fos protein in the lumbar spinal dorsal horn neurons in the rat with chronic constriction injury to the sciatic nerve

The temporal and spatial expression pattern of Fos protein in spinal dorsal horn neurons was examined by immunohistochemistry in rats with chronic constriction injury (CCI) to the sciatic nerve. In normal animals, a few Fos-immunoreactive (-IR) neurons were detected in the dorsal horn of the lumbar spinal cord. Following induction of CCI, a very large number of Fos-IR neurons appeared in the spinal dorsal horn, but a significant number of Fos-IR neurons were also observed in the contralateral dorsal horn where primary afferents of the injured sciatic nerve rarely project. Sham-operated animals also had a significant number of Fos-IR neurons in the dorsal horn bilaterally. The number of Fos-IR neurons reached its maximal level 1 day following placement of the ligatures (PO 1d). The ratio of the number of Fos-IR neurons in the ipsilateral dorsal horn to the contralateral dorsal horn, however, had its peak level 3 days following CCI (3.1-fold increase compared to the contralateral dorsal horn). The number of Fos-IR neurons in the dorsal horn gradually decreased, but increased again around PO 15d. On PO 30d, the number of Fos-IR neurons decreased and became comparable to that in normal animals. The present results indicate that the induction of Fos-IR neurons in the dorsal horn caused by CCI is biphasic and reaches its maximal level on PO 3d, near the time of hyperalgesia onset.

Glucocorticoid inhibition of neuropathic limb edema and cutaneous neurogenic extravasation

Sciatic nerve section in rats evokes chronic limb edema, pain behavior, and hindpaw hyperalgesia, a syndrome resembling the complex regional pain syndrome type II (CRPS II or causalgia) in man. Glucocorticoids such as methylprednisolone (MP) have been used as analgesic and anti-edematous agents in patients suffering from CRPS, and interestingly these therapeutic effects appear to persist in some patients after stopping the medication. Similar to the CRPS clinical response to glucocorticoids, we now demonstrate that chronic hindpaw edema in the sciatic transection CRPS model is reversed by a continuous infusion of MP (3 mg/kg/day over 21 days), and this anti-edematous effect persists for at least 1 week after discontinuing MP. Furthermore, there is a chronic increase in spontaneous protein extravasation in the hindpaw skin of rats after sciatic transection, similar to the increased protein extravasation observed in the edematous hands of CRPS patients. A 2-week infusion of MP (3 mg/kg/day) reduced spontaneous protein extravasation in the hindpaw skin by 80%. We postulated that increased spontaneous neurogenic extravasation resulted in development of limb edema in both the animal model and the CRPS patient, and that the anti-edematous effects of MP are due to an inhibition of spontaneous extravasation. Additional experiments examined the inhibitory effects of MP infusion on electrically-evoked neurogenic extravasation in the hindpaw skin of normal rats. MP inhibition was dose- and time-dependent, with an ED(50) of 1.2 mg/kg/day for a 14-day continuous infusion of MP, and a maximum inhibitory effect requiring 17 days of MP infusion (3 mg/kg/day). MP (3 mg/kg/day for 14 days) also blocked both capsaicin- and SP-evoked neurogenic extravasation, indicating a post-junctional inhibitory effect. Our interpretation is that increased spontaneous neurogenic extravasation in this CRPS model contributed to the development and maintenance of hindpaw edema, and that chronic MP administration dose- and time-dependently blocked neurogenic extravasation at a post-junctional level, thus reversing spontaneous extravasation and limb edema in this model.

Peptide accumulations in proximal endbulbs of transected axons

Axons proximal to a transection develop into enlarged, but presumed 'passive' endbulb structures. In previous studies, we observed that proximal stumps of transected sciatic nerves accumulate discrete and striking deposits of calcitonin gene-related peptide (CGRP) that have apparent direct and local actions on nearby microvessels. In this work, we provide evidence that CGRP, in the company of several additional peptides, are deposited through 'arrested' anterograde transport into axon endbulbs that develop after transection. In proximal stump tips of rat sciatic nerves transected 48 h earlier, CGRP accumulation colocalized with a label for neurofilament that was accentuated at axon tips, but was prevented by a concurrent more proximal sciatic section. Similarly, interruption of CGRP deposition eliminated its apparent actions on local microvessels following injury. CGRP accumulation was also observed in sural nerve proximal stump tips, indicating its presence in sensory axons despite the known declines in the sensory neuronal synthesis of CGRP that occur following axotomy. Peptide accumulation was not unique to CGRP, with a similar pattern of anterograde accumulation observed for substance P (SP), neuropeptide Y (NPY) and galanin. Deposited peptides and perhaps other axonal constituents in the milieu of a peripheral nerve injury may be associated with important local physiological actions in the regenerative microenvironment.