Enhancement of the conditioning lesion effect in rat sciatic motor axons after superimposition of conditioning and test lesions

Conditioning electrical stimulation promotes functional nerve regeneration

Peripheral nerve regeneration following injury is often incomplete, resulting in significant personal and socioeconomic costs. Although a conditioning crush lesion prior to surgical nerve transection and repair greatly promotes nerve regeneration and functional recovery, feasibility and ethical considerations have hindered its clinical applicability. In a recent proof of principle study, we demonstrated that conditioning electrical stimulation (CES) had effects on early nerve regeneration, similar to that seen in conditioning crush lesions (CCL). To convincingly determine its clinical utility, establishing the effects of CES on target reinnervation and functional outcomes is of utmost importance. In this study, we found that CES improved nerve regeneration and reinnervation well beyond that of CCL. Specifically, compared to CCL, CES resulted in greater intraepidermal skin and NMJ reinnervation, and greater physiological and functional recovery including mechanosensation, compound muscle action potential on nerve conduction studies, normalization of gait pattern, and motor performance on the horizontal ladder test. These findings have direct clinical relevance as CES could be delivered at the bedside before scheduled nerve surgery.

An In Vitro Model for Conditioning Lesion Effect

Axons of a peripheral nerve grow faster after an axotomy if it attains a prior injury a few days earlier. This is called conditioning lesion effect (CLE) and very much valued since it may provide new insights into neuron biology and axonal regeneration. There are established in vivo experimental paradigms to study CLE, however, there is a need to have an in vitro conditioning technique where CLE occurs in a maximally controlled environment. Mouse primary sensory neurons were isolated from lumbar 4-5 dorsal root ganglia and incubated at 37 °C on a silicon-coated watch glass that prevents cell attachment. After this conditioning period they were transferred to laminin coated culture dishes. Similar cultures were set up with freshly isolated neurons from control animals and from the animals that received a sciatic nerve cut 3 days earlier. All preparations were placed on a live cell imaging microscopy providing physiological conditions and photographed for 48 h. Axonal regeneration and neuronal survival was assessed. During the conditioning incubation period neurons remained in suspended aggregates and did not grow axons. The regeneration rate of the in vitro conditioned neurons was much higher than the in vivo conditioned and control preparations during the first day of normal incubation. However, higher regeneration rates were compromised by progressive substantial neuronal death in both types of conditioned cultures but not in the control preparations. By using neutralizing antibodies, we demonstrated that activity of endogenous leukemia inhibitory factor is essential for induction of CLE in this model.

A conditioning lesion enhances sympathetic neurite outgrowth

Axonal regeneration can be influenced by a conditioning lesion (an axonal injury made prior to a second test lesion). Previously, sympathetic neurons in vivo were shown to respond to a conditioning lesion with decreased neurite outgrowth, in contrast to the enhanced outgrowth observed in all other peripheral neurons examined. The present experiments tested the effects of a conditioning lesion on neurite outgrowth in vitro from the superior cervical ganglion (SCG) and the impact of several factors on that response. Ganglia axotomized 1 week earlier and then explanted in Matrigel or collagen gel responded with a significant increase in neurite extension compared to sham-operated ganglia. A distal axotomy produced by unilateral removal of the salivary glands (sialectomy) caused an increase in neurite outgrowth similar to that of a proximal axotomy. These conditioning lesions induced both an increase in the rate of elongation, and, in the case of the proximally axotomized SCG, a shorter initial delay of outgrowth. The enhanced outgrowth following sialectomy was specific to the nerve containing the majority of axons projecting to the salivary glands, suggesting that the conditioning lesion effect is restricted to previously injured neurons. Deletion of the gene for leukemia inhibitory factor (LIF), a gene induced by axotomy, did not abolish the conditioning lesion effect in SCG explants or dissociated cell cultures. In conclusion, sympathetic neurons are capable of responding to a conditioning lesion with increased neurite outgrowth. The hypothesis that the neuronal cell body response to axotomy plays an important role in the conditioning lesion response is discussed.

Axonal transport of the cellular prion protein is increased during axon regeneration

The cellular prion protein, PrPc, is a glycosylphosphatidylinositol-anchored cell surface glycoprotein and a protease-resistant conformer of the protein may be the infectious agent in transmissible spongiform encephalopathies. PrPc is localized on growing axons in vitro and along fibre bundles that contain elongating axons in developing and adult brain. To determine whether the growth state of axons influenced the expression and axonal transport of PrPc, we examined changes in the protein following post-traumatic regeneration in the hamster sciatic nerve. Our results show (1) that PrPc in nerve is significantly increased during nerve regeneration; (2) that this increase involves an increase in axonally transported PrPc; and (3) that the PrPc preferentially targeted for the newly formed portions of the regenerating axons consists of higher molecular weight glycoforms. These results raise the possibility that PrPc may play a role in the growth of axons in vivo, perhaps as an adhesion molecule interacting with the extracellular environment through specialized glycosylation.

Conditioning lesions enhance axonal regeneration of descending brain neurons in spinal-cord-transected larval lamprey

In larval lamprey, with increasing recovery times after a transection of the rostral spinal cord, there is a gradual recovery of locomotor behavior, and descending brain neurons regenerate their axons for progressively greater distances below the transection site. In the present study, spinal cord "conditioning lesions" (i.e., transections) were performed in the spinal cord at 30% body length (BL; normalized distance from the head) or 50% BL. After various "lesion delay times" (D), a more proximal spinal cord "test lesion" (i.e., transection) was performed at 10% BL, and then, after various recovery times (R), horseradish peroxidase was applied to the spinal cord at 20% BL to determine the extent of axonal regeneration of descending brain neurons. Conditioning lesions at 30% BL, lesion delay times of 2 weeks, and recovery times of 4 weeks (D-R = 2-4 group) resulted in a significant enhancement of axonal regeneration for the total numbers of descending brain neurons as well as neurons in certain brain cell groups compared to control animals without conditioning lesions. Experiments with hemiconditioning lesions, which reduce interanimal variability, confirmed that conditioning lesions do significantly enhance axonal regeneration and indicate that axotomy rather than diffusible factors released at the injury site is primarily involved in this enhancement. Results from the present study suggest that conditioning lesions "prime" descending brain neurons via cell body responses and enhance subsequent axonal regeneration, probably by reducing the initial delay and/or increasing the initial rate of axonal outgrowth.

Valproic acid enhances axonal regeneration and recovery of motor function after sciatic nerve axotomy in adult rats

It has recently been demonstrated that valproic acid (VPA) robustly promotes neurite outgrowth, activates the extracellular signal regulated kinase pathway, and increases growth cone-associated protein 43 and bcl-2 levels in cultured human neuroblastoma SH-SY5Y cells. We hypothesized that VPA could also enhance peripheral nerve regeneration in adult animals. To test this hypothesis, we examined the effects of VPA (300 mg/kg daily for 16 weeks) on sciatic axonal regeneration following single or conditional axotomies in rats. The results showed that in VPA-treated rats there was a significant increase in the total numbers of regenerated myelinated nerve fibers and reinnervated muscle fibers in comparison with those rats not treated with VPA. As measured by sciatic function index and toe spread index, the motor function of the reinnervated hind limbs of rats receiving single axotomy without VPA treatment significantly improved at week 8 and reached plateau levels at about week 11, whereas the motor function of the reinnervated hind limbs of rats receiving single axotomy plus VPA and rats receiving conditional axotomy with or without VPA treatment significantly improved at week 4 and reached plateau levels at about week 8; there was no significant difference of the motor function among the three later groups. The results demonstrated that VPA is able to enhance sciatic nerve regeneration and recovery of motor function in adult rats, suggesting the potential clinical application of VPA for the treatment of peripheral nerve injury in humans.

Neuroprotective effects of gonadal steroids on regenerating peripheral motoneurons

In this review, the neuroprotective actions of testosterone on three different populations of injured rat peripheral motoneurons, i.e. facial (FMN), spinal (SMN) and pudendal (PMN), will be discussed. We have extrapolated concepts from the neuroendocrine field regarding the trophic effects of gonadal steroids on target neural tissue to the nerve regeneration field. Exogenous administration of testosterone immediately after nerve injury impacts positively on functional recovery through actions mediated by the androgen receptor. The mechanism by which steroidal enhancement of the regenerative properties of injured motoneurons occurs may involve pre-existing androgen receptors, heat shock proteins, and modulation of the cellular stress response.

Assessment of hindlimb gait as a powerful indicator of axonal loss in a murine model of progressive CNS demyelination

Identifying the role of axonal injury in the development of permanent, irreversible neurologic disability is important to the study of central nervous system (CNS) demyelinating diseases. Our understanding of neurologic dysfunction in demyelinating diseases and the ability to assess therapeutic interventions depends on the development of objective functional assays that can non-invasively measure axonal loss. In this study, we demonstrate in a murine model of progressive CNS demyelination that assessment of the hindlimb width of stride provides a powerful indicator of axonal loss and can dissociate between deficits induced by demyelination versus axonal loss.

A role of migratory Schwann cells in a conditioning effect of peripheral nerve regeneration

The common peroneal nerve in mice was conditioned by axotomy around the head of the fibula. At various intervals from 1 day to 2, 3, 5, 15, and 25 days, a test lesion was made by axotomy 15 mm proximal to the conditioning lesion site. The proximal stump of the transected nerve was sandwiched between two sheets of thin plastic film and remained in vivo for various intervals from 3 h to 6, 9, 12, 24, 48, 72 and 96 h. The regenerating axons were visualized on the film with silver nitrate impregnation. Schwann cells were visualized migrating onto the film using immunohistochemistry with anti-S-100. To determine the effects of migratory Schwann cells on axonal outgrowth, a film model was established on one limb. After the nerve stump was removed from the film, the treated film was transferred to a new lesion on the contralateral limb and 2 days later the film was harvested for histological examination. Conditioned by a prior axotomy more than 3 days earlier, regenerating axons sprouted within less than 1 h after the test lesion was established and grew naked at five times higher rate: The growth rate was similar to that observed during regeneration in the presence of migratory Schwann cells (ordinary type). After a short interval, the axons, which had been ensheathed by migratory Schwann cells (reactive type), continued growing at a significantly (P < 0.01) higher rate. The reactive type of cells had fewer numbers of branches and higher activity in promoting axonal outgrowth than the ordinary type. Thus, both ordinary and reactive types of cells played key roles in initiating and maintaining a conditioning effect, respectively.

Androgenic regulation of the central glia response following nerve damage

Current research on the effects of gonadal steroids on the brain and spinal cord indicates that these agents have profound trophic effects on many aspects of neuronal functioning, including cell survival, growth and metabolism, elaboration of processes, synaptogenesis, and neurotransmission (Jones et al., 1985; Luine, 1985; Nordeen et al., 1985; Matsumoto et al., 1988a,b; Gould et al., 1990). Since many of the aspects of normal neuronal functioning altered by gonadal steroids are affected by injury to the nervous system, we initiated a series of experiments designed to exploit the trophic capabilities of steroids as therapeutic agents in neuronal injury and repair (Kujawa et al., 1989, 1991; Kujawa and Jones, 1990). Three steroid-sensitive model systems were used for these studies: the hamster facial motoneuron, the rat sciatic motoneuron, and the hamster rubrospinal motoneuron. The results of our initial series of experiments suggest that androgens, and possibly estrogens, act either directly or indirectly on the injured motoneuron and enhance elements of the neuronal reparative response that are critical to successful recovery of function. Recently, we discovered that gonadal steroids may also modulate the central glia response to nerve damage. In this review, a summary of our data identifying a therapeutic role for androgens in enhancing the reparative response of motoneurons to injury is presented. This is followed by a discussion of the effects of androgens on the glial response to injury.

Contributions of pathway and neuron to preferential motor reinnervation

Motor axons regenerating after transection of mixed nerve preferentially reinnervate distal muscle branches, a process termed preferential motor reinnervation (PMR). Motor axon collaterals appear to enter both cutaneous and muscle Schwann cell tubes on a random basis. Double-labeling studies suggest that PMR is generated by pruning collaterals from cutaneous pathways while maintaining those in motor pathways (the "pruning hypothesis"). If all collaterals projecting to muscle are saved, then stimulation of regenerative sprouting should increase specificity by increasing the number of motoneurons with at least one collateral in a muscle pathway. In the current experiments, collateral sprouting is stimulated by crushing the nerve proximal to the repair site before suture, a maneuver that also conditions the neuron and predegenerates the distal pathway. Control experiments are performed to separate these effects from those of collateral generation. Experiments were performed on the rat femoral nerve and evaluated by exposing its terminal cutaneous and muscle branches to HRP or Fluoro-Gold. Crush proximal to the repair site increased motor axon collaterals at least fivefold and significantly increased the percentage of correctly projecting motoneurons, consistent with the pruning hypothesis. Conditioning the nerve with distal crushes before repair had no effect on specificity. A graft model was used to separate the effects of collateral generation and distal stump predegeneration. Previous crush of the proximal femoral nerve significantly increased the specificity of fresh graft reinnervation. Stimulation of regenerative collateral sprouting thus increased PMR, confirming the pruning hypothesis. However, this effect was overshadowed by the dramatic specificity with which predegenerated grafts were reinnervated by fresh uncrushed proximal axons. These unexpected effects of predegeneration on specificity could involve a variety of possible mechanisms and warrant further study because of their mechanistic and clinical implications.

Androgenic enhancement of motor neuron regeneration

In conclusion, the available evidence to date suggests that many of the aspects of neuronal functioning affected by gonadal steroids under steady state conditions are also significantly affected by steroids under stress conditions such as axon disconnection. This argues toward a therapeutic usefulness of gonadal steroids in activating and/or accelerating the reparative response of neurons to injury, a concept that will be exciting to test in future clinical studies.

Testosterone regulation of the regenerative properties of injured rat sciatic motor neurons

We have previously demonstrated that systemic administration of testosterone differentially regulates the regenerative properties of injured hamster facial motor neurons, which are androgen receptor-containing cranial motor neurons. In this investigation, the hypothesis that testosterone alters the regenerative properties of rat sciatic motor neurons, which are androgen receptor-containing spinal motor neurons, was tested using fast axonal transport of radioactively labeled proteins to assess sciatic nerve regeneration. Adult castrated male rats were subjected to crush axotomy of the sciatic nerve at the level of the gemelli tendons (mid-thigh). One-half of the axotomized animals received subcutaneous implants of testosterone propionate (TP), with the remainder of the animals sham implanted with blank capsules. The outgrowth distances of the leading axons were measured at 5, 6, 7, and 11 days postoperative. Linear regression analysis was accomplished, with the slope of the line representing the regeneration rate and the x-intercept the initial delay of sprout formation. Systemic administration of testosterone resulted in a 13% increase in the rate of regeneration, relative to the control, -TP group. Outgrowth distances were significantly increased in the +TP group only in the later stages of regeneration. However, TP did not shorten the delay in sprout formation in regenerating sciatic motor neurons, but instead produced a small prolongation in the delay time. This pattern of hormonal regulation of the regenerative properties of spinal motoneurons is similar to that previously found in cranial motoneurons. The prolongation of the initial delay may have been a factor in the lack of significant outgrowth distances during the early stages of regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Pretreatment of rats with pulsed electromagnetic fields enhances regeneration of the sciatic nerve

Regeneration of the sciatic nerve was studied in rats pretreated in a pulsed electromagnetic field (PEMF). The rats were exposed between a pair of Helmholtz coils at a pulse repetition rate of 2 pps at a field density of 60 or 300 microT. The PEMF treatment was then discontinued. After an interval of recovery, regeneration of the sciatic nerve was initiated by a crush lesion. Regeneration of sensory fibers was measured by the "pinch test" after an additional 3-6 days. A variety of PEMF pretreatments including 4 h/day for 1-4 days or exposure for 15 min/day during 2 days resulted in an increased regeneration distance, measured 3 days after the crush lesion. This effect could be demonstrated even after a 14-day recovery period. In contrast, pretreatment for 4 h/day for 2 days at 60 microT did not affect the regeneration distance. The results showed that PEMF pretreatment conditioned the rat sciatic nerve in a manner similar to that which occurs after a crush lesion, which indicates that PEMF affects the neuronal cell body. However, the mechanism of this effect remains obscure.

Gonadal steroids as promoting factors in axonal regeneration

In this article, some of the trophic actions of gonadal steroids on receptor-concentrating neurons within the mammalian brain and spinal cord will be discussed. This will be followed by a summary of our recent data identifying a new role for gonadal steroids as therapeutic agents in neuronal injury and repair.

Testosterone effects on ribosomal RNA levels in injured peripheral motor neurons: a preliminary report

We have previously demonstrated that administration of testosterone to hamsters during the early phases of axonal regeneration following facial nerve injury accelerates both the rate of regeneration of the fastest growing population of axons and the return of functional movement. We hypothesized from those studies that testosterone primes the neuronal cell body in such a way as to accelerate the "switch" from a normal to a reparative state. That hypothesis was tested in this study using ribosomal DNA (rDNA) probes in conjunction with in situ hybridization to map the molecular response of the polymerase I system to axotomy, with and without hormone exposure. Adult male hamsters were subjected to right facial nerve severance, with the left side serving as an internal control. Half the animals were administered testosterone propionate via subcutaneous implants. In situ hybridization using a genomic rDNA probe complementary to the 28S rRNA species was accomplished, and levels of rRNA in injured facial neurons assessed both qualitatively and quantitatively. Our initial findings indicate that testosterone markedly upregulates rRNA levels after injury, and support the hypothesis of an acceleration in the metabolic switch to a reparative state. This leads us, in turn, to suggest that this effect of testosterone on the ribosomal system is causally related to the increase in axonal regeneration rate and return of functional movement previously documented in this system.

Conditioning nerve crush accelerates cytoskeletal protein transport in sprouts that form after a subsequent crush

To examine the relationship between axonal outgrowth and the delivery of cytoskeletal proteins to the growing axon tip, outgrowth was accelerated by using a conditioning nerve crush. Because slow component b (SCb) of axonal transport is the most rapid vehicle for carrying cytoskeletal proteins to the axon tip, the rate of SCb was measured in conditioned vs. sham-conditioned sprouts. In young Sprague-Dawley rats, the conditioning crush was made to sciatic nerve branches at the knee; 14 days later, the test crush was made where the L4 and L5 spinal nerves join to form the sciatic nerve in the flank. Newly synthesized proteins were labeled in motor neurons by injecting 35S-methionine into the lumbar spinal cord 7 days before the test crush. The wave of pulse-labeled SCb proteins reached the crush by the time it was made and subsequently entered sprouts. The nerve was removed and sectioned for SDS-PAGE and fluorography 4-12 days after the crush. Tubulins, neurofilament proteins, and representative "cytomatrix" proteins (actin, calmodulin, and putative microtubule-associated proteins) were removed from gels for liquid scintillation counting. Labeled SCb proteins entered sprouts without first accumulating in parent axon stumps, presumably because sprouts begin to grow within hours after axotomy. The peak of SCb moved 11% faster in conditioned than in sham-conditioned sprouts: 3.0 vs. 2.7 mm/d (p less than 0.05). To confirm that sprouts elongate more rapidly when a test crush is preceded by a conditioning crush, outgrowth distances were measured in a separate group of rats by labeling fast axonal transport with 3H-proline 24 hours before nerve retrieval.(ABSTRACT TRUNCATED AT 250 WORDS)

The initial period of peripheral nerve regeneration and the importance of the local environment for the conditioning lesion effect

The aim of this study was to investigate the early period of neurite outgrowth in the regenerating rat sciatic nerve and to determine if the non-neuronal cells were important for the conditioning lesion effect. Regeneration distance was evaluated with the pinch-reflex test 6 h to 5 days after a test crush lesion. The regeneration velocity accelerated during approximately 3 days, whereupon outgrowth continued with a constant velocity. In unconditioned nerves the initial delay was 2.8 h and the constant rate of regeneration was 3.2 mm/day. In nerves with a distal conditioning lesion the initial delay was 2.4 h and the rate of regeneration increased by 52%. When the test crush was applied at the same place as the conditioning crush the initial delay was 1.9 h and the rate of regeneration increased by 61%. The conditioning lesion effect was not influenced by the distance between the cell body and the conditioning crush lesion. Furthermore, the conditioning lesion effect could not be expressed if conditioned axons grew into a freeze injured nerve section. Incorporation of [3H]thymidine increased in the regenerating nerve segment. The increase occurred earlier if this segment had been subjected to a conditioning crush lesion. The results of these experiments showed that peripheral neurites start to regenerate within a few hours after an injury, suggesting that growth cone formation is independent of the cell body reaction. A conditioning crush lesion increases the regeneration velocity and its acceleration, and the conditioning lesion effect cannot be expressed in the absence of living Schwann and other non-neuronal cells.

The effect of a conditioning lesion on sudomotor axon regeneration

The effects of a conditioning lesion on the rate of sudomotor axon regeneration were judged by the recovery of sweat gland (SG) secretion after cholinergic stimulation. Three groups of mice were given a conditioning lesion by crushing the sciatic nerve at mid-thigh 4, 7, and 14 days before a test lesion. A 4th group received a conditioning crush of the tibial nerve at the ankle 7 days before the test lesion. Control mice had a single test lesion. SG reinnervation in control mice began 19 days after the test lesion, and was functionally complete by 41 days. In groups with the conditioning lesion 4, 7, and 14 days before the test operation, the first reactive SGs reappeared at 16, 15, and 16 days respectively after the test lesion, and maximal recovery occurred by 33, 32, and 39 days. In mice with the distal conditioning lesion, reinnervation began at 19 days and was maximal by 36 days. In summary, a nerve conditioning lesion placed from 4 to 14 days prior to and at the same site as a test lesion significantly accelerated the growth rate of the fastest regenerating unmyelinated sudomotor axons and reduced the time until most SGs were reinnervated. A more distally placed test lesion reduced the interval for recovery.

Stimulation of rat sciatic nerve regeneration with pulsed electromagnetic fields

The effects of pulsed electromagnetic fields (PEMF) on rat sciatic nerve regeneration after a crush lesion were determined. The rats were placed between a pair of Helmholtz coils and exposed to PEMF of frequency 2 Hz and magnetic flux density of 0.3 mT. A 4 h/day treatment for 3-6 days increased the rate of nerve regeneration by 22%. This stimulatory effect was independent of the orientation of the coils. Exposure times of 1 h/day-10 h/day were equally effective in stimulating nerve regeneration. Rats exposed to PEMF for 4 h/day for 7 days before crush, followed by 3 days after crush without PEMF, also showed significantly increased regeneration. This pre-exposure 'conditioning' effect suggests that PEMF influences regeneration indirectly.

Neurofilament elongation into regenerating facial nerve axons

Immunocytochemistry was used to show that neurofilaments advance into regenerating facial nerve axons at 2.5 mm/day, which is less than the rate of axonal elongation (4.3 mm/day), measured from the transport of radiolabeled protein into the axons. Thus, the distal region of the newly-regenerated axons is deficient in neurofilaments, and this was confirmed by electron microscopy. These neurofilament-free regenerating axons could also be detected by immunocytochemistry using antibody to protein B50 (GAP43), a component of growth-cones. Immunoblots of nerve segments, incubated with monoclonal antibodies against the three neurofilament proteins, showed that all three proteins were present in the neurofilaments elongating into the regenerating axons, and confirmed the more distal extensions of B50 immunoreactivity. These results show that neurofilament immunocytochemistry underestimates the extent of axonal regeneration, and it is suggested that this technique should be employed with caution in regeneration studies. When the facial nerve received a conditioning lesion 7 days prior to a test lesion, axonal regeneration rate increased to 6.0 mm/day, and there was a proportional increase in neurofilament elongation rate to 4.4 mm/day. This occurred in spite of the reduction in cell body neurofilament protein synthesis induced by the lesions. It is concluded that the rate of neurofilament extension into regenerating axons is not governed by cell body synthesis but by local interactions with other cytoskeletal materials which support the increased regeneration rate of conditioned axons.

Herpes simplex virus latent infection: reactivation and elimination of latency after neurectomy

Section of the sciatic nerve during the period of herpes simplex virus (HSV) latent infection was performed to evaluate residual latency in mouse dorsal root ganglion. In control mice without sciatic neurectomy, latency was present in 90-100%, while in those which underwent a neurectomy procedure, latent infection was surprisingly decreased to 28-50%. To investigate the hypothesis that the decrease of latency resulted from HSV reactivation and replication (with subsequent neuron destruction), groups of mice were treated with acyclovir to inhibit HSV reactivation, after having undergone a neurectomy procedure. Acyclovir treatment largely prevented the neurectomy-related elimination of latency and supported the hypothesized mechanism.