Acceleration of axonal outgrowth in motor axons from mature and old F344 rats after a conditioning lesion

Comparative study of pronator teres branch transfer and brachialis motor branch transfer to the anterior interosseous nerve to treat lower brachial plexus injury in rats

Distal nerve transfer is used to treat lower brachial plexus palsy, but outcome series on these transfer procedures following lower plexus injuries are sparse. The objective of this study is to compare treatment outcomes after nerve transfer using the brachialis motor branch (BMB) versus that using the pronator teres motor branch (PTMB). One hundred twenty adult rats with C8T1 nerve root avulsion were randomly divided into three groups (40 each): A: BMB transfer to the anterior interosseous nerve (AIN), B: PTMB transfer to the AIN, and C: no repair. Electrophysiological examination result, muscle tension test result, muscle weight and muscle fiber cross-sectional area of the flexor digitorum profundus and flexor pollicis longus, and number of myelinated nerve fibers in the AIN were compared among the groups to evaluate the treatment outcome. Nerve regeneration and muscle recovery in group B was better than those in group A at 4 and 8 weeks postoperatively (P < 0.05). There was no significant difference in the myelinated nerve fibers in groups A and B at 12 and 16 weeks postoperatively. The rats in group B showed greater and more significant improvement in other measured values than those in group A (P < 0.05). In conclusion, the PTMB seems a better donor nerve than the BMB for distal nerve transfer to treat lower brachial plexus injury according to the electrophysiological and histological examination in this rat study.

Regenerative medicine for the treatment of spinal cord injury: more than just promises?

Spinal cord injury triggers a complex set of events that lead to tissue healing without the restoration of normal function due to the poor regenerative capacity of the spinal cord. Nevertheless, current knowledge about the intrinsic regenerative ability of central nervous system axons, when in a supportive environment, has made the prospect of treating spinal cord injury a reality. Among the range of strategies under investigation, cell-based therapies offer the most promising results, due to the multifactorial roles that these cells can fulfil. However, the best cell source is still a matter of debate, as are clinical issues that include the optimal cell dose as well as the timing and route of administration. In this context, the role of biomaterials is gaining importance. These can not only act as vehicles for the administered cells but also, in the case of chronic lesions, can be used to fill the permanent cyst, thus creating a more favourable and conducive environment for axonal regeneration in addition to serving as local delivery systems of therapeutic agents to improve the regenerative milieu. Some of the candidate molecules for the future are discussed in view of the knowledge derived from studying the mechanisms that facilitate the intrinsic regenerative capacity of central nervous system neurons. The future challenge for the multidisciplinary teams working in the field is to translate the knowledge acquired in basic research into effective combinatorial therapies to be applied in the clinic.

Diagnosis and Prognosis of Iatrogenic Injury of the Recurrent Laryngeal Nerve

Objectives:

Following perioperative injury to a macroscopically intact recurrent laryngeal nerve (RLN), there are two possible intraneural injury types: 1) axonal injury, including disruption of axons, and 2) conduction block, only affecting the Schwann cells and the nodes of Ranvier. In this study, it was hypothesized that the functional outcome after RLN injury may depend on the type of nerve injury.

Methods:

Fifteen patients with acute postoperative unilateral RLN paralysis were prospectively studied. Electrophysiological examination (laryngeal electromyography) was used to differentiate between the two types of nerve injury. Vocal fold motions were monitored by repeated laryngoscopy during the study period (up to 6 months). Three of the patients with axonal injury were treated with the regeneration-promoting agent nimodipine.

Results:

The patients with conduction block all recovered normal vocal fold motion, whereas patients with axonal injury within the nerve had a significantly worse outcome. The 3 patients who were treated with nimodipine all recovered normal or near-normal vocal fold mobility despite the more severe axonal injury.

Conclusions:

In contrast to previous reports, our results show that laryngeal electromyography is a reliable tool for diagnosing the type of injury within the injured RLN, making it possible to predict the functional outcome in these patients. On the basis of the results, a future randomized study on nimodipine treatment for RLN axonal injury is suggested.

Preserved Regeneration and Functional Recovery of the Injured Recurrent Laryngeal Nerve after Secondary Surgical Repair in Adult Rats

Objectives:

Transection of the recurrent laryngeal nerve (RLN) is accompanied by poor functional recovery, despite primary repair, because of regeneration difficulties. Nimodipine can promote regeneration, but it is not yet clear whether preoperative treatment is necessary. It is also not clear whether surgical repair following RLN injury may be performed in a second procedure, with preserved regeneration. This study investigated the time window for secondary surgical repair of the transected RLN and the need for preoperative administration of nimodipine.

Methods:

In adult rats, the left RLN was transected and repaired at time intervals up to 3 weeks after transection, in combination with nimodipine treatment starting either before or after the operation. Regeneration and neuromuscular recovery were assessed by electrophysiology, retrograde tracing, and immunohistochemistry.

Results:

Similar (whether 0, 2, or 7 days) regenerative results were obtained when the RLN was repaired up to 1 week after injury, given nimodipine administration, whereas fewer motor neurons managed to regenerate after nerve repair at 3 weeks after the initial transection. No beneficial effect was detected from preoperative nimodipine administration.

Conclusions:

Provided that nimodipine is administered, surgical reconstruction of the RLN can be performed within 1 week after the initial nerve trauma, with preserved neuromuscular function. Nimodipine may be administered at the time of RLN transection injury.

Chapter 26: Age-related differences in the reinnervation after peripheral nerve injury

Numerous and extensive functional, structural, and biochemical changes characterize intact aged peripheral nervous system. Functional recovery after peripheral nerve injury depends on survival of injured neurons and functional reinnervation of target tissue by regeneration of injured axons and collateral sprouting of uninjured (intact) adjacent axons. The rate of axonal regeneration becomes slower and its extent (density and number of regenerating axons) decreases in aged animals. Aging also impairs terminal sprouting of regenerated axons and collateral sprouting of intact adjacent axons, thus further limiting target reinnervation and its functional recovery. Decreased survival of aged noninjured and injured neurons, limited intrinsic growth potential of neuron, alteration in its responsiveness to stimulatory or inhibitory environmental factors, and changes in the peripheral neural pathways and target tissues are possible reasons for impaired reinnervation after peripheral nerve injury in old age. The review of present data suggests that this impairment is mostly due to the age-related changes in the peripheral neural pathways and target tissues, and not due to the limited intrinsic growth capacity of neurons or their reduced responsiveness to trophic factors. Age-related alterations in the soluble target derived neurotrophic factors, like nerve growth factor, and nonsoluble extracellular matrix components of neural pathways, like laminin, might be important in this respect.

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.

Sciatic nerve axotomy in aged rats: response of motoneurons and the effect of GM1 ganglioside treatment

The number, size, and staining intensity of choline acetyltransferase (ChAT)-immunopositive cells in the retrodorsal lateral nucleus (RDLN) of the spinal cord were studied in young (3-5 months old) and aged (22-24 months old) rats following left sciatic nerve distal transection (axotomy) and treatment with GM1 ganglioside. The cell size and the ChAT immunostaining density were decreased in the RDLN of non-manipulated as well as in the contralateral intact side of axotomized aged rats. Axotomy had no effect on the number of RDLN motoneurons in both aged and young rats. In the young rats, there was a decrease in the size of motoneurons 7 days post-axotomy and a partial spontaneous recovery occurred by 21 days. Axotomy did not reduce further the size of aged motoneurons, however. The ChAT staining intensity of the axotomized RDLN declined in both age groups after 7 days, and there was spontaneous near normal recovery by 21 days. In the aged rats, GM1 administration for 7 days corrected the cell size and ChAT immunoreactivity of the contralateral intact RDLN. With regard to axotomized RDLN neurons, 7 days of GM1 restored the cell size but not the ChAT immunostaining in young animals. The same treatment schedule, however, corrected both cell size and staining in aged rats. Administration of GM1 for 21 days had no further effect on the morphometric parameters of the axotomized motoneurons in aged rats, but slightly enhanced the recovery of ChAT immunostaining in young rats. Thus, it appears that GM1 facilitates the phenotypic recovery of RDLN motoneurons during aging and after axotomy.

Combination of gonadal steroid treatment and peripheral nerve grafting results in a peripheral motoneuron-like pattern of beta II-tubulin mRNA expression in axotomized hamster rubrospinal motoneurons

Rubrospinal motoneurons (RSMN) represent a population of androgen receptor-containing central motoneurons in rodents. In this study, the ability of testosterone propionate (TP), alone or in conjunction with a peripheral nerve graft (PNG), to alter the molecular program of injured RSMN was accomplished using betaII-tubulin cDNA probes and quantitative in situ hybridization (ISH). Initial fluoro-gold labeling experiments following a T1 hemisection established that, as in the rat, the hamster rubrospinal system is essentially crossed and that injured RSMN concentrate in the ventrolateral region of the red nucleus. In the second experimental series, adult gonadectomized male hamsters were subjected to a right T1 hemisection, with half of the operated animals immediately subcutaneously implanted with 1 10 mm TP Silastic capsule and the other half sham implanted. In a third experimental series, animals were subjected to T1 hemisection, followed by transplantation of a predegenerated autologous segment of peripheral nerve. Half of the animals in each group received TP implants at the time of spinal cord injury and PNG. Postoperative times were 2, 7, and 14 days (dpo). Quantitative ISH was performed using a betaII-tubulin-specific (33)P-labeled cDNA probe, emulsion autoradiography, and computerized image analysis for grain counting. Injury alone resulted in a short-lived increase in betaII-tubulin mRNA expression in the RSMN at 2 dpo, with a significant decline to well below control values at 7 and 14 dpo. TP treatment or PNG alone attenuated, but did not prevent, the down-regulation of betaII-tubulin mRNA. In contrast, the combination of TP with a PNG sustained the injury-induced increase in betaII-tubulin mRNA levels throughout the postoperative period of 2, 7, and 14 dpo. The synergistic effects of the two treatment strategies confirm the importance of targeting multiple aspects of the injury response for therapeutic intervention.

Spinal cord repair: strategies to promote axon regeneration

Neurons in the central nervous system have a remarkable capacity to regenerate their transected axons when provided with an appropriate growth environment. Advances in our understanding of axon regeneration have allowed the development of different experimental strategies to stimulate axon regeneration in animal models of spinal cord injury. Growth inhibitory proteins block axon regeneration in the CNS, and many of these proteins have been identified. Various methods that are now used to stimulate regeneration in the injured spinal cord are directed at overcoming the growth inhibitory environment of the CNS. Three general approaches tested in vivo stimulate regeneration in the spinal cord. First, antibodies that bind inhibitory proteins in myelin allow axon regeneration in the CNS. Second, methods that modulate neuronal intracellular signaling allow axons to grow directly on the inhibitory substrate of the CNS. Third, transplantation of cells to the lesioned spinal cord promotes repair. In this paper we review current advances in each of these research domains.

Regeneration of cerebral-buccal interneurons and recovery of ingestion buccal motor programs in Aplysia after CNS lesions

In the sea slug Aplysia, rhythmic biting is eliminated after bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion (dpl). The ability of cerebral-buccal interneuron-2 (CBI-2) to elicit ingestion buccal motor programs (iBMPs; i.e., fictive rhythmic ingestion) and to regenerate synaptic connections with target buccal neurons was assessed with intracellular recordings and dye injections. Isolated central ganglia were obtained from control animals and from lesioned animals at selected times after bilateral CBC crushes. Within 3 wk postlesion, transected CBI-2 axons sprouted at least 10 fine neurites confined to the core of the CBC that projected across the crush site toward the buccal ganglia. When fired with depolarizing current steps, CBI-2 was not observed to elicit iBMPs in preparations until 14 dpl. Thereafter a progressive enhancement in CBI-2's ability to elicit iBMPs was observed with time postlesion. By 40 dpl, CBI-2-elicited iBMPs were indistinguishable from those of controls. CBI-2 regenerated monosynaptic connections with appropriate buccal premotor- and motorneurons by 14 dpl, and the strength of these connections increased with time postlesion. Dramatic frequency facilitation was exhibited by the regenerating CBI-2 buccal synapses; for instance, at early postlesion times, no observable excitatory postsynaptic potentials (EPSPs) were obtained with 1- Hz stimulation of CBI-2, while at 7 Hz, a dramatic increase in EPSP amplitude was obtained with successive spikes. The present study shows that the time course of axonal and synaptic regeneration by command-like interneuron CBI-2 is correlated with the recovery of ingestion buccal motor programs elicited by CBI-2. These results parallel our previous findings of functional neural regeneration in the feeding system and suggest that functional neural regeneration is at least in part mediated by regeneration of specific synaptic pathways.

A 25-year perspective of peripheral nerve surgery: evolving neuroscientific concepts and clinical significance

In spite of an enormous amount of new experimental laboratory data based on evolving neuroscientific concepts during the last 25 years peripheral nerve injuries still belong to the most challenging and difficult surgical reconstructive problems. Our understanding of biological mechanisms regulating posttraumatic nerve regeneration has increased substantially with respect to the role of neurotrophic and neurite-outgrowth promoting substances, but new molecular biological knowledge has so far gained very limited clinical applications. Techniques for clinical approximation of severed nerve ends have reached an optimal technical refinement and new concepts are needed to further increase the results from nerve repair. For bridging gaps in nerve continuity little has changed during the last 25 years. However, evolving principles for immunosuppression may open new perspectives regarding the use of nerve allografts, and various types of tissue engineering combined by bioartificial conduits may also be important. Posttraumatic functional reorganizations occurring in brain cortex are key phenomena explaining much of the inferior functional outcome following nerve repair, and increased knowledge regarding factors involved in brain plasticity may help to further improve the results. Implantation of microchips in the nervous system may provide a new interface between biology and technology and developing gene technology may introduce new possibilities in the manipulation of nerve degeneration and regeneration.

Lumbar motor neuron size and number is affected by age in male F344 rats

Motor neurons in the spinal cord of old rats appear similar in size but less numerous compared with those in mature rats; they also contain a large amount of lipofuscin, the lipid peroxidation by-product whose function is largely unknown. The object of this study was to morphometrically characterize motor neurons found in the L4/L5 lumbar spinal cord of mature (6-month) and old (22-month) rats. Paraformaldehyde-fixed, lumbar spinal cords from six rats at each age were embedded in paraffin, sectioned at 6 microm and stained with 0.1% toluidine blue. The nucleolar diameter and area from a minimum of 34 motor neurons per spinal cord were measured. Motor neuron number was calculated using Abercrombie's (Abercrombie, 1946) formula after correcting for tissue shrinkage. Motor neuron number was decreased with age while the neuronal area increased with age. Nucleolar diameter also increased in old rats. Frequency distributions of motor neuron area revealed unimodal distributions of motor neurons rats of both ages. We suggest that larger nucleolar diameter reflects more metabolically active neurons in old rats while larger neuron area is a reflection of the presence of lipofuscin in old motor neurons.