Two- and three-dimensional computer graphic evaluation of the subacute spinal cord injury

Schwann Cell Transplantation Subdues the Pro-Inflammatory Innate Immune Cell Response after Spinal Cord Injury

The transplantation of Schwann cells (SCs) has been shown to provide tissue preservation and support axon growth and remyelination as well as improve functional recovery across a diverse range of experimental spinal cord injury (SCI) paradigms. The autologous use of SCs has progressed to Phase 1 SCI clinical trials in humans where their use has been shown to be both feasible and safe. The contribution of immune modulation to the protective and reparative actions of SCs within the injured spinal cord remains largely unknown. In the current investigation, the ability of SC transplants to alter the innate immune response after contusive SCI in the rat was examined. SCs were intraspinally transplanted into the lesion site at 1 week following a thoracic (T8) contusive SCI. Multicolor flow cytometry and immunohistochemical analysis of specific phenotypic markers of pro- and anti-inflammatory microglia and macrophages as well as cytokines at 1 week after SC transplantation was employed. The introduction of SCs significantly attenuated the numbers of cluster of differentiation molecule 11B (CD11b)⁺, cluster of differentiation molecule 68 (CD68)⁺, and ionized calcium-binding adapter molecule 1 (Iba1)⁺ immune cells within the lesion implant site, particularly those immunoreactive for the pro-inflammatory marker, inducible nitric oxide synthase (iNOS). Whereas numbers of anti-inflammatory CD68⁺ Arginase-1 (Arg1⁺) iNOS⁻ cells were not altered by SC transplantation, CD68⁺ cells of an intermediate, Arg1⁺ iNOS⁺ phenotype were increased by the introduction of SCs into the injured spinal cord. The morphology of Iba1⁺ immune cells was also markedly altered in the SC implant, being elongated and in alignment with SCs and in-growing axons versus their amoeboid form after SCI alone. Examination of pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), and anti-inflammatory cytokines, interleukin-4 (IL-4) and interleukin-10 (IL-10), by multicolor flow cytometry analysis showed that their production in CD11b⁺ cells was unaltered by SC transplantation at 1 week post-transplantation. The ability of SCs to subdue the pro-inflammatory iNOS⁺ microglia and macrophage phenotype after intraspinal transplantation may provide an important contribution to the neuroprotective effects of SCs within the sub-acute SCI setting.

Strict perpendicular orientation of neural crest-derived neurons in vitro is dependent on an extracellular gradient of voltage

We report extraordinary perpendicular orientations of neurons dependent on the presence of an external direct current (DC) voltage gradient. We chose chick dorsal root and postganglionic sympathetic neurons to evaluate. These were cultured in observation chambers in which the cells were separated from electrode products or substrate effects and maintained at 35°C. Both types of neurons showed a rapid restructuring of their anatomy. Typically, neurites that were not perpendicular to the voltage gradient were quickly resorbed into the cell body within a few minutes. Over 3-6 hr, significant new neurite growth occurred and was patterned perpendicular to the DC electrical field (Ef). This preferred asymmetry was dependent on the Ef, as was the initial retrograde degeneration of fibers. At 400-500 mV/mm, over 90% of the cells in culture assumed this orientation. Removal of the DC Ef led to a loss of the preferred orientation, with further random growth within the chambers. This is the first report of such responses in dorsal root ganglion neurons. We also used sympathetic neurons as a meaningful comparison to analyze whether there were any qualitative or quantitative differences between these two cell types of neural crest origin. We discuss the means by which these orientations were achieved.

Theta rotation and serial registration of light microscopical images using a novel camera rotating device

An electromechanical video camera coupler was developed to rotate a light microscope field of view (FOV) in real time without the need to physically rotate the stage or specimen. The device, referred to as the Camera Thetarotator, rotated microscopical views 240 degrees to assist microscopists to orient specimens within the FOV prior to image capture. The Camera Thetarotator eliminated the effort and artifacts created when rotating photomicrographs using conventional graphics software. The Camera Thetarotator could also be used to semimanually register a dataset of histological sections for three-dimensional (3D) reconstruction by superimposing the transparent, real-time FOV to the previously captured section in the series. When compared to Fourier-based software registration, alignment of serial sections using the Camera Thetarotator was more exact, resulting in more accurate 3D reconstructions with no computer-generated null space. When software-based registration was performed after prealigning sections with the Camera Thetarotator, registration was further enhanced. The Camera Thetarotator expanded microscopical viewing and digital photomicrography and provided a novel, accurate registration method for 3D reconstruction. The Camera Thetarotator would also be useful for performing automated microscopical functions necessary for telemicroscopy, high-throughput image acquisition and analysis, and other light microscopy applications.

Spinal cord injury-induced up-regulation of AHNAK, expressed in cells delineating cystic cavities, and associated with neoangiogenesis

To investigate the molecular basis for the poor regenerative capacity of the mammalian central nervous system (CNS) after injury, we searched for genes whose expression was affected by an adult rat spinal cord hemi-section. Differential screening of a rat spinal cord expression library was performed using polyclonal antibodies raised against lesioned spinal cord tissue. A striking overexpression was found for ahnak, encoding a 700-kDa protein, in normal CNS present only in the blood-brain barrier (BBB) forming vascular endothelial cells. Indeed, very early after spinal cord injury (SCI), high levels of membrane-associated AHNAK are observed on non-neuronal cells invading the lesion site. With time, AHNAK distribution spreads rostrally and caudally concomitant with the process of tissue inflammation and axon degeneration, delineating the interior surface of cystic cavities, mainly in front of barrier-forming astrocytes. Strong overexpression is also observed on vascular endothelial cells reacting to BBB breakdown. Based on our detailed analysis of its spatiotemporal and cellular expression, and its previously described function in BBB, we suggest that AHNAK expression is associated with cell types displaying tissue-protective barrier properties. Our study may thus contribute to the elucidation of the precise molecular and cellular events that eventually render traumatic spinal cord tissue non-permissive for regeneration.

Inhibition of tumour necrosis factor-alpha by antisense targeting produces immunophenotypical and morphological changes in injury-activated microglia and macrophages

Microglia respond in a stereotypical pattern to a diverse array of pathological states. These changes are coupled to morphological and immunophenotypical alterations and the release of a variety of reactive species, trophic factors and cytokines that modify both microglia and their cellular environment. We examined whether a microglial-produced cytokine, tumour necrosis factor-alpha (TNF-alpha), was involved in the maintenance of microglial activation after spinal cord injury by selective inhibition using TNF-alpha antisense deoxyoligonucleotides (ASOs). Microglia and macrophages harvested from 3 d post-contused rat spinal cord were large and rounded (86.3 +/- 9.6%). They were GSA-IB4-positive (GSA-IB4(+)) (Griffonia simplicifolia lectin, microglia specific; 94.8 +/- 5.1%), strongly OX-42 positive (raised against a type 3 complement/integrin receptor, CD11b; 78.9 +/- 9.1%), ED-1 positive (a lysosomal marker shown to correlate well with immune cell activation; 97.2 +/- 2.6%) and IIA positive (antibody recognizes major histocompatibility complex II; 57.2 +/- 5.6%), indicative of fully activated cells, for up to 48 h after plating. These cells also secreted significant amounts of TNF-alpha (up to 436 pg/microg total protein, 16 h). Fluoroscein isothiocyanate-labelled TNF-alpha ASOs (5, 50 and 200 nm) added to the culture medium were taken up very efficiently into the cells (> 90% cells) and significantly reduced TNF-alpha production by up to 92% (26.5 pg/microg total protein, 16 h, 200 nm TNF-alpha ASOs). Furthermore, few of the treated cells at this time were round (5.4 +/- 2.7%), having become predominantly spindle shaped (74.9 +/- 6.3%) or stellate (21.4 +/- 2.7%); immunophenotypically, although all of them remained GSA-IB4 positive (91.6 +/- 6.2%), many were weakly OX-42 positive and few expressed either ED-1 (12.9 +/- 2.5%) or IIA (19.8 +/- 7.4%). Thus, the secretion of TNF-alpha early in spinal cord injury may be involved in autoactivating microglia/macrophages. However, at the peak of microglial activation after injury, the activation state of microglia/macrophages is not stable and this process may still be reversible by blocking TNF-alpha.

Subcutaneous tri-block copolymer produces recovery from spinal cord injury

We have studied the ability of nonionic detergents and hydrophilic polymers to seal permeabilized membranes of damaged cells, rescuing them from progressive dissolution, degeneration, and death. We report that a single subcutaneous injection of the tri-block copolymer, Poloxamer 188 (P188) 6 hr after a severe compression of the adult guinea pig spinal cord is able to: (1). preserve the anatomic integrity of the cord; (2). produce a rapid recovery of nerve impulse conduction through the lesion; and (3). produce a behavioral recovery of a spinal cord dependent long tract spinal cord reflex. These observations stood out against a control group in blinded evaluation. Conduction through the lesion was monitored by stimulating the tibial nerve of the hind limb, and measuring the arrival of evoked potentials at the contralateral sensory cortex of the brain (somatosensory evoked potentials; SSEP). Behavioral recovery was determined by a return of sensitivity of formerly areflexic receptive fields of the cutaneous trunchi muscle (CTM) reflex. This contraction of back skin in response to tactile stimulation is totally dependent on the integrity of an identified bilateral column of ascending long tract axons. A statistically significant recovery of both SSEP conduction through the lesion and the CTM reflex occurred in P188-treated animals compared to vehicle-treated controls. Quantitative 3D computer reconstruction of the lesioned vertebral segment of spinal cord revealed a statistically significant sparing of spinal cord parenchyma and a significant reduction in cavitation of the spinal cord compared to control animals We determined that the proportion of P188-treated animals that recovered evoked potentials were nearly identical to that produced by a subcutaneous injection of polyethylene glycol (PEG). In contrast, P188 was not as effective as PEG in producing a recovery of CTM functioning. We discuss the likely differences in the mechanisms of action of these two polymers, and the possibilities inherent in a combined treatment.

Double labeling serial sections to enhance three-dimensional imaging of injured spinal cord

A method of double labeling a set of serial histological sections was performed to produce multiple three-dimensional (3D) reconstructions from the same segment of injured spinal cord. Alternate groups of consecutive histological sections were stained with Luxol fast blue with cresyl violet and Mallory's trichrome in order to reconstruct two different 3D images that reveal different pathological features of the same 1-month-old compression spinal cord injury. Three-dimensional visualization of the two reconstructions was accomplished using an isocontouring algorithm that automatically extracts surfaces of features of interest based on pixel intensity. The two 3D reconstructions demonstrated the sparing of myelinated nerve fibers and the composition of neuroglia through the chronic lesion of an adult guinea pig. The 3D images provided a comprehensive and explicit view of a chronically injured spinal cord that is not possible by the inspection of two-dimensional (2D) histological sections or from magnetic resonance imaging. Using every histological section, we believe this double labeling 3D reconstruction technique provides a more enhanced and accurate visualization of the entire spinal cord lesion than has been possible before. Furthermore, we contend that this double labeling technique can further elucidate the histopathological events of secondary injury at different time points post-injury by using different combinations of complementary histological makers.

Exposure to pulsed magnetic fields enhances motor recovery in cats after spinal cord injury

STUDY DESIGN

Animal model study of eight healthy commercial cats was conducted.

OBJECTIVE

To determine whether pulsed electromagnetic field (PMF) stimulation results in improvement of function after contusive spinal cord injury in cats.

SUMMARY OF BACKGROUND DATA

PMF stimulation has been shown to enhance nerve growth, regeneration, and functional recovery of peripheral nerves. Little research has been performed examining the effects of PMF stimulation on the central nervous system and no studies of PMF effects on in vivo spinal cord injury (SCI) models have been reported.

MATERIALS AND METHODS

PMF stimulation was noninvasively applied for up to 12 weeks to the midthoracic spine of cats with acute contusive spinal cord injury. The injury was produced using a weight-drop apparatus. Motor functions were evaluated with the modified Tarlov assessment scale. Morphologic analyses of the injury sites and somatosensory-evoked potential measurements were conducted to compare results between PMF-stimulated and control groups.

RESULTS

There was a significant difference in locomotor recovery between the PMF-stimulated and control groups. Although not statistically significant, PMF-stimulated spinal cords demonstrated greater sparing of peripheral white matter and smaller lesion volumes compared to controls. Somatosensory-evoked potential measurements indicated that the PMF-stimulated group had better recovery of preinjury waveforms than the control group; however, this observation also was not statistically significant because of the small sample size.

CONCLUSIONS

This preliminary study indicates that pulsed magnetic fields may have beneficial effects on motor function recovery and lesion volume size after acute spinal cord injury.

Polyethylene glycol rapidly restores physiological functions in damaged sciatic nerves of guinea pigs

OBJECTIVE

We have studied the ability of the hydrophilic polymer polyethylene glycol (PEG) to anatomically and physiologically reconnect damaged axons of the adult guinea pig spinal cord. Here we have extended this approach to test whether completely severed guinea pig sciatic nerves in isolation could be fused and whether PEG was able to repair severe standardized crush injuries to sciatic nerves in vivo.

METHODS

The fusion test was performed with isolated sciatic nerves maintained in a double-sucrose gap recording chamber. For in vivo experiments, the sciatic nerve was surgically exposed in the hind leg of deeply anesthetized adult guinea pigs and was crushed proximal to its insertion in the gastrocnemius muscle. PEG was injected just beneath the epineurium with a 29-gauge needle, allowed to remain in the damaged axon region for 2 minutes, and removed. Sham-treated guinea pigs received an injection of water or Krebs' solution. Three indices of recovery were simultaneously monitored in response to electrical stimulation of the proximal nerve, i.e., 1) recovery of compound muscle action potentials (in millivolts), 2) contraction force of the muscle (in dynes), and 3) displacement of the muscle (in millimeters).

RESULTS

When isolated sciatic nerves were severed within the double-sucrose gap chamber, compound action potential propagation through the transection plane was eliminated. After abutment of the two segments and 2-minute PEG application to this site, variable compound action potential recovery was measured in all four cases. The crush injuries to the sciatic nerve in vivo eliminated the three functional responses to sciatic nerve stimulation in all animals. Within the first 30 minutes after treatment, only 1 of 12 control animals exhibited spontaneous recovery in any of these measures, compared with six of eight PEG-treated animals. By 45 minutes, two more sham-treated animals and one more PEG-treated animal had recovered at least one functional response. This difference in proportions between PEG-treated and sham-treated animals was statistically significant (P < or =0.02).

CONCLUSION

We conclude that these preliminary data suggest that PEG application may be a way to interfere with the steady dissolution of peripheral nerve fibers after mechanical damage and to even functionally fuse or reconnect severed proximal and distal segments.

Three-dimensional morphometry of spinal cord injury following polyethylene glycol treatment

We are developing a novel means of restoring function after severe acute spinal cord injury. This involves a brief application of polyethylene glycol (PEG) to the site of injury. In the companion paper, we have shown that a delayed application of PEG can produce strikingly significant physiological and behavioral recovery in 90–100 % of spinal-cord-injured guinea pigs. In the present paper, we used three-dimensional computer reconstructions of PEG-treated and sham-treated spinal cords to determine whether the pathological character of a 1-month-old injury is ameliorated by application of PEG. Using a novel isocontouring algorithm, we show that immediate PEG treatment and treatment delayed by up to 7 h post-injury statistically increased the volume of intact spinal parenchyma and reduced the amount of cystic cavitation. Furthermore, in PEG-treated animals, the lesion was more focal and less diffuse throughout the damaged segment of the spinal cord, so that control cords showed a significantly extended lesion surface area. This three-dimensional computer evaluation showed that the functional recovery produced by topical application of a hydrophilic polymer is accompanied by a reduction in spinal cord damage.

Behavioral recovery from spinal cord injury following delayed application of polyethylene glycol

Topical application of the hydrophilic polymer polyethylene glycol (PEG) to isolated adult guinea pig spinal cord injuries has been shown to lead to the recovery of both the anatomical integrity of the tissue and the conduction of nerve impulses through the lesion. Furthermore, a brief (2 min) application of the fusogen (Mr 1800, 50 % w/v aqueous solution) to the exposed spinal cord injury in vivo can also cause rapid recovery of nerve impulse conduction through the lesion in association with functional recovery. Behavioral recovery was demonstrated using a long-tract, spinal-cord-dependent behavior in rodents known as the cutaneus trunci muscle (CTM) reflex. This reflex is observed as a contraction of the skin of the back in response to tactile stimulation. Here, we confirm and extend these preliminary observations. A severe compression/contusion injury to the exposed thoracic spinal cord of the guinea pig was performed between thoracic vertebrae 10 and 11. Approximately 7 h later, a topical application of PEG was made to the injury (dura removed) for 2 min in 15 experimental animals, and levels of recovery were compared with those of 13 vehicle-treated control animals. In PEG-treated animals, 93 % recovered variable levels of CTM functioning and all recovered some level of conduction through the lesion, as measured by evoked potential techniques. The recovered reflex was relatively normal compared with the quantitative characteristics of the reflex prior to injury with respect to the direction, distance and velocity of skin contraction. Only 23 % of the control population showed any spontaneous CTM recovery (P=0.0003) and none recovered conduction through the lesion during the 1 month period of observation (P=0.0001). These results suggest that repair of nerve membranes by polymeric sealing can provide a novel means for the rapid restoration of function following spinal cord injury.

Rapid recovery from spinal cord injury after subcutaneously administered polyethylene glycol

Arguably a seminal event in most trauma and disease is the breakdown of the cell membrane. In most cells, this is first observed as a collapse of the axolemmas barrier properties allowing a derangement of ions to occur, leading to a progressive dissolution of the cell or its process. We have shown that an artificial sealing of mechanically damaged membranes by topical application of hydrophilic polymers such as polyethylene glycol (PEG) immediately restores variable levels of nerve impulse conduction through the lesion. This was documented by a rapid recovery of somatosensory evoked potential (SSEP) conduction, and by recovery of the cutaneous trunchi muscle (CTM) reflex in PEG-treated animals. The CTM reflex is a sensorimotor behavior dependent on an intact (and identified) white matter tract within the ventrolateral funiculus of the spinal cord, and is thus an excellent index of white matter integrity. We show that PEG can be safely introduced into the bloodstream by several routes of administration. Using a fluorescein decorated PEG, we demonstrate that the polymer specifically targets the hemorrhagic contusion of the adult guinea pig spinal cord when administered through the vasculature, but not intact regions of the spinal cord. A single subcutaneous injection (30% weight by weight in sterile saline) made 6 hr after a standardized spinal cord contusion in adult guinea pigs was sufficient to produce a rapid recovery of SSEP propagation through the lesion in only PEG-treated animals, accompanied by a statistically significant recovery of the CTM reflex. These data suggest that parenterally administered PEG may be a novel treatment for not only spinal injury, but head injury and stroke as well.

Degeneration and sprouting of identified descending supraspinal axons after contusive spinal cord injury in the rat

Contusive spinal cord injury (SCI) results in the formation of a chronic lesion cavity surrounded by a rim of spared fibers. Tissue bridges containing axons extend from the spared rim into the cavity dividing it into chambers. Whether descending axons can grow into these trabeculae or whether fibers within the trabeculae are spared fibers remains unclear. The purposes of the present study were (1) to describe the initial axonal response to contusion injury in an identified axonal population, (2) to determine whether and when sprouts grow in the face of the expanding contusion cavity, and (3) in the long term, to see whether any of these sprouts might contribute to the axonal bundles that have been seen within the chronic contusion lesion cavity. The design of the experiment also allowed us to further characterize the development of the lesion cavity after injury. The corticospinal tract (CST) underwent extensive dieback after contusive SCI, with retraction bulbs present from 1 day to 8 months postinjury. CST sprouting occurred between 3 weeks and 3 months, with penetration of CST axons into the lesion matrix occurring over an even longer time course. Collateralization and penetration of reticulospinal fibers were observed at 3 months and were more extensive at later time points. This suggests that these two descending systems show a delayed regenerative response and do extend axons into the lesion cavity and that the endogenous repair can continue for a very long time after SCI.

Advances in three-dimensional reconstruction of the experimental spinal cord injury

Three-dimensional (3D) computer reconstruction is an ideal tool for evaluating the centralized pathology of mammalian spinal cord injury (SCI) where multiple anatomical features are embedded within each other. Here, we evaluate three different reconstruction algorithms to three-dimensionally visualize SCIs. We also show for the first time, that determination of the volume and surface area of pathological features is possible using the reconstructed 3D images themselves. We compare these measurements to those calculated by older morphometric approaches. Finally, we demonstrate dynamic navigation into a 3D spinal cord reconstruction.

The macrophage in acute neural injury: changes in cell numbers over time and levels of cytokine production in mammalian central and peripheral nervous systems

We evaluated the timing and density of ED-1-positive macrophage accumulation (ED 1 is the primary antibody for the macrophage) and measured cytokine production by macrophages in standardized compression injuries to the spinal cord and sciatic nerves of individual rats 3, 5, 10 and 21 days post-injury. The actual site of mechanical damage to the nervous tissue, and a more distant site where Wallerian degeneration had occurred, were evaluated in both the peripheral nervous system (PNS) and the central nervous system (CNS) at these time points. The initial accumulation of activated macrophages was similar at both the central and peripheral sites of damage. Subsequently, macrophage densities at all locations studied were statistically significantly higher in the spinal cord than in the sciatic nerve at every time point but one. The peak concentrations of three cytokines, tumor necrosis factor α (TNF α), interleukin-1 (IL-1) and interleukin-6 (IL-6), appeared earlier and were statistically significantly higher in injured spinal cord than in injured sciatic nerve. We discuss the meaning of these data relative to the known differences in the reparative responses of the PNS and CNS to injury.

Immediate recovery from spinal cord injury through molecular repair of nerve membranes with polyethylene glycol

A brief application of the hydrophilic polymer polyethylene glycol (PEG) swiftly repairs nerve membrane damage associated with severe spinal cord injury in adult guinea pigs. A 2 min application of PEG to a standardized compression injury to the cord immediately reversed the loss of nerve impulse conduction through the injury in all treated animals while nerve impulse conduction remained absent in all sham-treated guinea pigs. Physiological recovery was associated with a significant recovery of a quantifiable spinal cord dependent behavior in only PEG-treated animals. The application of PEG could be delayed for approximately 8 h without adversely affecting physiological and behavioral recovery which continued to improve for up to 1 month after PEG treatment.

Electrically mediated regeneration and guidance of adult mammalian spinal axons into polymeric channels

An extracellular electric field has been shown to influence the regeneration of nerve fibers within the adult mammalian spinal cord. However, in these studies, few axons were labeled by local application of intracellular markers relative to the number of axons transected. This has limited an evaluation of the robustness of the response, and the direction of growth of regenerating axons that might be influenced by the orientation of the applied voltage gradient. In this study, a hollow silicone rubber tube (c. 6 mm x 1 mm outside diameter) containing a cathodal (negative) electrode was inserted longitudinally into the dorsal half of the adult guinea-pig spinal cord. The electric field ( approximately 100 microV/mm) was imposed within the damaged spinal cord with an implanted d.c. stimulator for about three weeks. Based on previous studies, this orientation of the electric field would be expected to both initiate axonal regeneration and guide growing axons to, and into, the silicone guidance channel. In experimental animals (n = 20), a robust regeneration of axons into the tube was observed in more than half the cases. These axons were traced from surrounding white and gray matter by anterograde and retrograde labeling using a tetramethylrhodamine-conjugated dextran as an intracellular marker. Control animals (n = 16) received tubes with inactive electrodes. It was rare to find any axons within control guidance channels, since adult mammalian central nervous system axons do not regenerate. This report provides evidence for not only the facilitated regeneration of adult mammalian central axons, but also their guidance, by an imposed electric field.

Acute repair of crushed guinea pig spinal cord by polyethylene glycol

Acute repair of crushed guinea pig spinal cord by polyethylene glycol. We have studied the responses of adult guinea pig spinal cord white matter to a standardized compression within a sucrose gap recording chamber. This injury eliminated compound action potential (CAP) conduction through the lesion, followed by little or no recovery of conduction by 1 h postinjury. We tested the ability of polyethylene glycol (PEG) to repair the injured axons and restore physiological function. Local application of PEG (1,800 MW, 50% by weight in water) for approximately 2 min restored CAP conduction through the injury as early as 1 min post PEG application. The recovery of the CAP </=1 h was significantly greater in treated compared with control spinal cords (controls = 3.6% of the preinjury amplitude; PEG treated = 19%; P < 0.0001, unpaired Student's t-test). Stimulus-response analysis indicated that the susceptibility for recovery was similar for all calibers of axons after PEG application. The enhanced recovery of conduction after PEG treatment was associated with an early alteration in conduction properties relative to control spinal cords. This included increased refractoriness and sensitivity to potassium channel blockade using 4-aminopyridine (4-AP). Normally 4-AP enhanced the amplitude of the recovering CAPs by approximately 40% in control spinal cords; however this effect was nearly doubled to approximately 72% in PEG treated spinal cords. Because severe clinical injuries to the spinal cord (and some peripheral nerves) are both resistant to medical treatment and usually produced by compression, we discuss the possible clinical benefits of PEG application.