Axonal regeneration from central respiratory neurons of the adult rat into peripheral nerve autografts: effects of graft location within the medulla

Respiratory plasticity following spinal cord injury: perspectives from mouse to man

The study of respiratory plasticity in animal models spans decades. At the bench, researchers use an array of techniques aimed at harnessing the power of plasticity within the central nervous system to restore respiration following spinal cord injury. This field of research is highly clinically relevant. People living with cervical spinal cord injury at or above the level of the phrenic motoneuron pool at spinal levels C3-C5 typically have significant impairments in breathing which may require assisted ventilation. Those who are ventilator dependent are at an increased risk of ventilator-associated co-morbidities and have a drastically reduced life expectancy. Pre-clinical research examining respiratory plasticity in animal models has laid the groundwork for clinical trials. Despite how widely researched this injury is in animal models, relatively few treatments have broken through the preclinical barrier. The three goals of this present review are to define plasticity as it pertains to respiratory function post-spinal cord injury, discuss plasticity models of spinal cord injury used in research, and explore the shift from preclinical to clinical research. By investigating current targets of respiratory plasticity research, we hope to illuminate preclinical work that can influence future clinical investigations and the advancement of treatments for spinal cord injury.

Thermogelling chitosan lactate hydrogel improves functional recovery after a C2 spinal cord hemisection in rat

The present study was designed to provide an appropriate micro-environment for regenerating axotomized neurons and proliferating/migrating cells. Because of its intrinsic permissive properties, biocompatibility and biodegradability, we chose to evaluate the therapeutic effectiveness of a chitosan-based biopolymer. The biomaterial toxicity was measured through in vitro test based on fibroblast cell survival on thermogelling chitosan lactate hydrogel substrate and then polymer was implanted into a C2 hemisection of the rat spinal cord. Animals were randomized into three experimental groups (Control, Lesion and Lesion + Hydrogel) and functional tests (ladder walking and forelimb grip strength tests, respiratory assessment by whole-body plethysmography measurements) were used, once a week during 10 weeks, to evaluate post-traumatic recoveries. Then, electrophysiological examinations (reflexivity of the sub-lesional region, ventilatory adjustments to muscle fatigue known to elicit the muscle metaboreflex and phrenic nerve recordings during normoxia and temporary hypoxia) were performed. In vitro results indicated that the chitosan matrix is a non-toxic biomaterial that allowed fibroblast survival. Furthermore, implanted animals showed improvements of their ladder walking scores from the 4th week post-implantation. Finally, electrophysiological recordings indicated that animals receiving the chitosan matrix exhibited recovery of the H-reflex rate sensitive depression, the ventilatory response to repetitive muscle stimulation and an increase of the phrenic nerve activity to asphyxia compared to lesioned and nonimplanted animals. This study indicates that hydrogel based on chitosan constitute a promising therapeutic approach to repair damaged spinal cord or may be used as an adjuvant with other treatments to enhance functional recovery after a central nervous system damage. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2004-2019, 2017.

Respiration following spinal cord injury: evidence for human neuroplasticity

Respiratory dysfunction is one of the most devastating consequences of cervical spinal cord injury (SCI) with impaired breathing being a leading cause of morbidity and mortality in this population. However, there is mounting experimental and clinical evidence for moderate spontaneous respiratory recovery, or "plasticity", after some spinal cord injuries. Pre-clinical models of respiratory dysfunction following SCI have demonstrated plasticity at neural and behavioral levels that result in progressive recovery of function. Temporal changes in respiration after human SCI have revealed some functional improvements suggesting plasticity paralleling that seen in experimental models-a concept that has been previously under-appreciated. While the extent of spontaneous recovery remains limited, it is possible that enhancing or facilitating neuroplastic mechanisms may have significant therapeutic potential. The next generation of treatment strategies for SCI and related respiratory dysfunction should aim to optimize these recovery processes of the injured spinal cord for lasting functional restoration.

Treatments to restore respiratory function after spinal cord injury and their implications for regeneration, plasticity and adaptation

Spinal cord injury (SCI) often leads to impaired breathing. In most cases, such severe respiratory complications lead to morbidity and death. However, in the last few years there has been extensive work examining ways to restore this vital function after experimental spinal cord injury. In addition to finding strategies to rescue breathing activity, many of these experiments have also yielded a great deal of information about the innate plasticity and capacity for adaptation in the respiratory system and its associated circuitry in the spinal cord. This review article will highlight experimental SCI resulting in compromised breathing, the various methods of restoring function after such injury, and some recent findings from our own laboratory. Additionally, it will discuss findings about motor and CNS respiratory plasticity and adaptation with potential clinical and translational implications.

Shedding light on restoring respiratory function after spinal cord injury

Loss of respiratory function is one of the leading causes of death following spinal cord injury. Because of this, much work has been done in studying ways to restore respiratory function following spinal cord injury (SCI) – including pharmacological and regeneration strategies. With the emergence of new and powerful tools from molecular neuroscience, new therapeutically relevant alternatives to these approaches have become available, including expression of light sensitive proteins called channelrhodopsins. In this article we briefly review the history of various attempts to restore breathing after C2 hemisection, and focus on our recent work using the activation of light sensitive channels to restore respiratory function after experimental SCI. We also discuss how such light-induced activity can help shed light on the inner workings of the central nervous system respiratory circuitry that controls diaphragmatic function.

Phrenic rehabilitation and diaphragm recovery after cervical injury and transplantation of olfactory ensheathing cells

Functional respiratory recovery was evaluated by recording diaphragm and phrenic nerve activity several months after cervical cord hemisection followed by olfactory ensheathing cell (OEC) transplantation. The intact side was taken as a control in each rat. Sham-transplanted rats did not recover respiratory activity from the ipsilateral lesioned side. By contrast, ipsilateral phrenic and diaphragmatic activities recovered in transplanted rats amounted to 80.7% and 73% of their controls, respectively. After contralateral acute C1 section eliminating any contralateral influence from crossed compensatory pathways, the ipsilateral phrenic activity remained at 57.5% of the control, indicating that the phrenic recovery originated from the ipsilateral side. Supralesional stimulation in these rats elicited sublesional ipsilateral postsynaptic phrenic responses showing that transplantation helped ipsilateral fibers to again transmit nervous messages to the phrenic target, leading to substantial functional recovery. The origin of mechanisms involved in respiratory recovery (regeneration, resurrection, sprouting, sparing, demasking of latent pathways) is discussed.

Functional reconnections established by central respiratory neurons regenerating axons into a nerve graft bridging the respiratory centers to the cervical spinal cord

The present work investigated, in adult rats, the long-term functional properties and terminal reconnections of central respiratory neurons regenerating axons within a peripheral nerve autograft bridging two separated central structures. A nerve graft was first inserted into the left medulla oblongata, in which the respiratory centers are located. Three months later, a C3 left hemisection was performed, and the distal tip of the graft was implanted into the C4 left spinal cord at the level of the phrenic nucleus, a natural central inspiratory target. Six to eight months after medullary implantation, the animals (n = 12) were electrophysiologically investigated to test 1) the phrenic target reinnervation by analyzing the phrenic responses elicited by bridge electrical stimulation and 2) the bridge innervation by unitary recordings of the spontaneous activity of regenerated axons within the nerve bridge. In the control group (n = 6), the medullary site of implantation corresponded to the dorsolateral medulla, a region known to be an unsuitable site for inducing respiratory axonal regrowth after nerve grafting. Stimulation of the nerve bridge never elicited phrenic nerve response, and no respiratory units were found within the nerve bridge. In the experimental group (n = 6), the proximal tip of the nerve bridge was implanted within the ventrolateral medulla at the level of the respiratory centers. Electrical stimulation of the nerve bridge induced phrenic nerve responses that reflected a postsynaptic activation of the phrenic target. Subsequent unitary recordings from teased fibers within the bridge revealed the presence of regenerated inspiratory fibers exhibiting discharge patterns typical of medullary inspiratory neurons, which normally make synaptic contacts with the inspiratory phrenic target. These results indicate that, when provided with an appropriate denervated target, central respiratory neurons with regenerated axons along a nerve bridge can remain functional for a long period and can make precise and specific functional reconnections with central homotypic target neurons.

Regeneration of acutely and chronically injured descending respiratory pathways within post-traumatic nerve grafts

Central respiratory neurons, which are acutely axotomized by peripheral nerve grafts implanted at the level of the descending respiratory pathways within the C2 spinal cord, can regenerate their axons within the grafts and still transmit normal physiological messages [Decherchi et al., 1996. Exp. Neurol. 137, 1-14]. The present work investigated the extent to which mature central neurons, acutely or chronically axotomized by a spinal lesion, still maintain the potential to regenerate an axon following post-traumatic nerve grafting within supra-lesional spinal structures and remained functional. This study is an extension of earlier work employing the more chronic lesions, that investigated whether respiratory neurons chronically axotomized by a spinal cord injury can retain the ability to regenerate their axonal process within a post-traumatic peripheral nerve graft. Here implantation was performed into the supra-lesional ventrolateral part of the ipsilateral C2 spinal cord (at the level of the descending respiratory pathways) previously hemisected at the C3 level. In the present study, these post-traumatic peripheral nerve grafts were performed either acutely (group I, n=15, 2.5 h post-injury: acute conditions) or chronically (group II, n=17, 3 weeks; group III, n=6, 3 months: chronic conditions) after the injury.Electrophysiological recording of teased filaments (n=2362) within the post-traumatic peripheral nerve grafts revealed the presence of regenerated nerve fibers with spontaneous unitary impulse traffic (graft units, n=954) in all animals. These graft units were respiratory (n=247) and non-respiratory (n=707). Respiratory discharges originated from central respiratory neurons which remained functional with preserved afferent connections. Except for the group III, post-traumatic C2 peripheral nerve grafts of the groups I and II contained a significantly higher occurrence rate (13.2+/-2% and 11.6+/-1.9%) of respiratory units than C2 spinal peripheral nerve grafts (5.9+/-1.6%) realized without previous CNS injury. The main conclusion of our study is that for a prolonged period of 3 weeks following a spinal cord injury, central respiratory neurons have the potential to remain functional and to regenerate their axonal process within post-traumatic peripheral nerve grafts inserted rostrally to the spinal damage. This indicates that supra-lesional post-traumatic nerve grafts may constitute an efficient delayed strategy for inducing axonal regrowth of chronically axotomized adult central neurons. This suggests that surgical intervention which is not always possible immediately after a spinal cord injury may be satisfactorily carried out after an appropriate delay.

Regrowth of acute and chronic injured spinal pathways within supra-lesional post-traumatic nerve grafts

The present work investigates the extent to which mature central neurons acutely or chronically axotomized by a spinal lesion still maintained the potential to regenerate an axon following post-traumatic nerve grafting within supra-lesional spinal structures. In adult rats, a C3 cervical hemisection (injury) was made and an autologous segment of the peroneal nerve was implanted 2mm rostrally into the ventrolateral part of the ipsilateral C2 spinal cord. Nerve graft implantations were carried out acutely at the time of injury (group I, acute conditions) or chronically, three weeks post-injury (group II, chronic conditions). Central neurons axotomized by the spinal lesion were labeled by True Blue injected at the lesion site at the time of trauma. Central neurons regenerating axons within the nerve grafts were labeled with either horseradish peroxidase (only in group I, n=4) or Nuclear Yellow (group I, n=3 and group II, n=6) applied two to four months post-grafting to the distal cut end of the nerve grafts. Neurons with dual staining (True Blue/Nuclear Yellow) represented central regenerating neurons which were previously axotomized by the spinal lesion and which had retained the capacity for axonal regeneration for a delayed period after injury. In group I (acute injury conditions), all types of labeled cells were found to be scattered with a clear bimodal distribution within the spinal cord and the brainstem. No labeled cells were found within the motor cortex. There was no statistically significant difference between horseradish peroxidase and all cells containing Nuclear Yellow (Nuclear Yellow and True Blue/Nuclear Yellow). In group II (chronic injury conditions), Nuclear Yellow- and True Blue/Nuclear Yellow-labeled cells had a similar dual distribution to that of group I, but were found to be significantly less represented (P=0.019). These differences are discussed in terms of capacity for cell survival and axonal regrowth after acute and chronic injury. The main conclusion is based on the evidence of dual staining of central neurons in both groups, which demonstrates that brainstem and spinal neurons involved in acute and chronic axotomy after spinal C3 lesion can survive the trauma and still maintain the capacity to regenerate lesioned axons within nerve grafts inserted rostrally (C2 spinal cord) to the primary site of injury. Although exhibited to a lesser extent in chronic than in acute conditions, this capacity was found to occur for as long as three weeks post-injury. These results indicate that supra-lesional post-traumatic nerve grafts may constitute an efficient delayed strategy for inducing axonal regrowth of chronically axotomized adult central neurons. We suggest that surgical intervention, which is not always possible immediately after a spinal cord injury, may be satisfactorily carried out after an appropriate delay.

CNS axonal regeneration with peripheral nerve grafts cryopreserved by vitrification: cytological and functional aspects

To test cool-warm protocols for storing peripheral nerves, 4-cm-long-nerve segments were removed from the hindleg of adult rats and cryopreserved using a vitrification solution (or cryoprotective mixture) containing a mixture of polyalcohols (2,3-butanediol, 1,2-propanediol, polyethylene glycol, and Belzer U.W. medium). Schwann cell viability and morphology were studied with regard to the effect of (i) cryoprotective mixture concentration (100, 50, and 30% diluted in human serum albumin at 4%), (ii) duration of exposure (10, 15, or 30 min in a single step) of nerves to the cryoprotective mixture, (iii) cooling rate (F1/F2, F3, and F4: 3, 12, and 231 degrees C/min, respectively), and (iv) type of replacement of cryoprotectant (T1, one step; or T2, perfusion) after warming. Nerves exposed 10 min to cryoprotective mixture 50% (2,3-butanediol, 1.926 mol.liter-1; 1,2-propanediol, 3.063 mol.liter-1; polyethylene glycol, 0.084 mol.liter-1; and Belzer U.W., 22.4 mosm-1) and cooled-warmed with the F2/F3/F4-T2 protocols contained live and correctly cryopreserved Schwann cells. The capacity of these cryopreserved nerve segments (n = 6) to be subsequently repopulated by regenerating axons from central neurons was compared to that of fresh nerves when used as peripheral nerve autografts implanted within the spinal cord at the level of the descending respiratory pathways. All cryopreserved nerve grafts were successfully reinnervated by regenerated central axons. Unitary spontaneous action potentials propagated along these axons were assessed by recording the discharge of tested nervous filaments (T) from the grafts in artificially ventilated and paralyzed animals. Out of 535 T, 32 (6 +/- 1.2%) presented spontaneous unitary activity with respiratory (R, n = 2) and nonrespiratory (NR, n = 30) pattern of discharge. The T mean number, the occurrence rate referenced to the total number of T (R/T, NR/T, and R + NR/T) and the mean number of spontaneous units (R, NR, R + NR) were compared to those of fresh spinal peripheral nerve grafts. Except for T, cryopreserved peripheral nerve grafts contained statistically significantly (P < 0.05) less spontaneous R and NR unitary activity, which represented, respectively, 6.2 +/- 6.2 and 26.8 +/- 5.7% of that found in the control group. These data indicate that nerves cryopreserved with the protocols described above contain viable Schwann cells which constitute a suitable support to induce regeneration of central fibers. The effectiveness of nerve cryopreservation by vitrification is discussed with regard to Schwann cell viability following cool-warm protocols and to subsequent reinnervation of the cryopreserved peripheral nerve grafts.

The effect of damage of the brachium of the superior colliculus in neonatal and adult hamsters and the use of peripheral nerve to restore retinocollicular projections

Using horseradish peroxidase (HRP) tracing technique, we were able to confirm the critical age in hamsters as reported previously (SO et al., 1981). Thus, following transection of the retinal fibers at the brachium of the superior colliculus (BSC) on postnatal-day 4 (P4) or later, no retinocollicular projections were observed in the adult stage. However, the retinal fibers were observed to reinnervate the superior colliculus (SC) if the BSC was cut on P3 or earlier. Physiological recording showed a close to normal retinocollicular map following a BSC damage on P0. Although retinal fibers did not reinnervate the SC following a BSC cut on or after P4, they could be observed to grow along a membrane over the damaged site. Bridging the site of BSC damage in adult hamsters using a segment of peripheral nerve (PN), retinal fibers labelled with WGA-HRP were observed to reinnervate the SC along the PN graft and visual evoked responses could be recorded in the SC showing the PN graft is effective in restoring damaged central visual pathways in adult mammals.

Central respiratory neuronal activity after axonal regeneration within blind-ended peripheral nerve grafts: time course of recovery and loss of functional neurons

Autologous segments of peroneal nerve were implanted into the medulla of adult rats to induce axonal regeneration of central neurons axotomised during the grafting procedure. Grafts were inserted in the midline--where respiratory axons decussate--or laterally, either in the nucleus tractus solitarius or in the nucleus ambiguus, close to respiratory cell bodies. The distal part of each graft was left unconnected (blind-ended graft). Between 2 and 30 months post-implantation, unit recordings from single fibres were made from small strands teased from the grafts to investigate activity of neurons regenerating axons. Spontaneous respiratory and non-respiratory activity was present only in grafts examined between 2 and 6 months post-implantation. Respiratory units had discharge patterns identical to those of normal inspiratory or expiratory neurons; their responses to lung inflation and asphyxia were also similar to those of central respiratory neurons. No spontaneous activity was present in the grafts examined 7-30 months post-implantation. Moreover, asphyxia, which normally enhances the activity of central respiratory neurons, failed to elicit activity. These results were similar in all grafts, regardless of the site of implantation. The presence of spontaneous activity only between 2 and 6 months post-implantation indicates that once axonal growth of respiratory neurons is stopped within blind-ended grafts, those neurons still exhibited normal functional properties for 3 months. The absence of activity 6 months after grafting suggests that loss of functional regenerating respiratory neurons does not occur progressively and follows an "all or nothing" rule within blind-ended grafts.

Forelimb motor performance following cervical spinal cord contusion injury in the rat

The purpose of this study was to examine the degree, persistence, and nature of forelimb behavioral deficits following cervical spinal cord contusion injury in the rat. Forelimb reaching and pellet retrieval, forehead adhesive sticker removal, and vibrissae-induced forelimb placing were examined for 16 weeks following a weight-drop injury (10.0 g-2.5 cm) at the C4-C5 spinal level. Nine of 13 rats studied were unable to perform the pellet retrieval task due to pronounced forelimb extension hypometria. However, these animals did carry out the forehead sticker removal and vibrissae-induced placing tasks. Therefore, the loss of reaching ability related to pellet retrieval was not due to generalized paralysis. This interpretation was further supported by evaluation of the rostrocaudal extent of relative motoneuron loss from 1-mm divisions through the lesion zone. The extent of motoneuron pathology ranged from 2 to 6 mm but was largely confined to the C4-C5 spinal segments. Morphometric assessments of axonal sparing revealed that pellet retrieval performance during the last month of observation was significantly correlated with fiber sparing in the dorsal columns and ventral white matter, whereas no significant correlation could be demonstrated with regard to dorsolateral white matter. While there were no conspicuous differences in qualitative assessments of damage to interneuron pools (i.e., laminae V to VII) between the nonreaching and retrieval-recovered rats, the possibility of combined white and gray matter pathology contributing to this deficit still exists. These initial findings thus demonstrate that the weight-drop contusion injury model can be adopted to studies of cervical spinal cord trauma in the rat. Such lesions yield permanent deficits in forelimb function lending to future studies of possible therapeutic interventions. Furthermore, performance deficits observed at 1 week postinjury in the placing and forehead sticker removal tasks can be predictive of any potential for long-range spontaneous recovery in pellet retrieval ability.

Central respiratory neurons of the adult rat regrow axons preferentially into peripheral nerve autografts implanted within ventral rather than within dorsal parts of the medulla oblongata

A great number of severed central nervous system (CNS) neurons of the adult rat have the capillary to regrow axons into peripheral nerve autografts. In the present experiment, autologous segments of the peroneal nerve were inserted perpendicularly to the dorsal surface of the medulla oblongata more or less laterally within either the ventral respiratory group or the so-called dorsal respiratory group (ventrolateral grafts, n = 5; dorsolateral grafts, n = 5). From 2 to 4.5 months after the graft implantation, spontaneous unitary activities (n = 197) were recorded within all the grafted nerves: they were found to arise from both central respiratory (R, n = 60) and non-respiratory (NR, n = 137) neurons which were giving off regenerated axons along the nerve grafts. The graft reinnervation by respiratory axons was found to be significantly more abundant within the medullary ventrolateral grafts than within the dorsolateral ones. The low rate of axonal regeneration from respiratory neurons observed within the dorsolateral grafts provides further evidence that the number of the respiratory neurons in the dorsal respiratory group, if present at all, is much smaller than that of the ventral respiratory group in the rat.