Quantitative assessment of respiratory function following contusion injury of the cervical spinal cord

Ampakines increase diaphragm activation following mid-cervical contusion injury in rats

Ampakines are positive allosteric modulators of AMPA receptors. We hypothesized that low-dose ampakine treatment increases diaphragm electromyogram (EMG) activity after mid-cervical contusion injury in rats. Adult male and female Sprague Dawley rats were implanted with in-dwelling bilateral diaphragm EMG electrodes. Rats received a 150 kDyn C4 unilateral contusion (C4Ct). At 4- and 14-days following C4Ct, rats were given an intravenous bolus of ampakine CX717 (5 mg/kg, n = 10) or vehicle (2-hydroxypropyl-beta-cyclodextrin; HPCD; n = 10). Diaphragm EMG was recorded while breathing was assessed using whole-body plethysmography. At 4-days, ampakine administration caused an immediate and sustained increase in bilateral peak inspiratory diaphragm EMG bursting and ventilation. The vehicle had no impact on EMG bursting. CX717 treated rats were able to increase EMG activity during a respiratory challenge to a greater extent vs. vehicle treated. Rats showed a considerable degree of spontaneous recovery of EMG bursting by 14 days, and the impact of CX717 delivery was blunted as compared to 4-days. Direct recordings from the phrenic nerve at 21-24 days following C4Ct confirmed that ampakines stimulated bilateral phrenic neural output in injured rats. We conclude that low-dose intravenous treatment with a low-impact ampakine can enhance diaphragm activation shortly following mid-cervical contusion injury, when deficits in diaphragm activation are prominent.

A novel mouse model of central cord syndrome

Patients with potential spinal stenosis are susceptible to central cord syndrome induced by blunt trauma. Suitable animal models are helpful for studying the pathogenesis and treatment of such injuries. In this study, we established a mouse model of acute blunt traumatic spinal cord injury by compressing the C6 spinal cord with 5 and 10 g/mm² compression weights to simulate cervical central cord syndrome. Behavioral testing confirmed that this model exhibited the characteristics of central cord syndrome because motor function in the front paws was impaired, whereas basic motor and sensory functions of the lower extremities were retained. Hematoxylin-eosin staining showed that the diseased region of the spinal cord in this mouse model was restricted to the gray matter of the central cord, whereas the white matter was rarely affected. Magnetic resonance imaging showed a hypointense signal in the lesion after mild and severe injury. In addition, immunofluorescence staining showed that the degree of nerve tract injury in the spinal cord white matter was mild, and that there was a chronic inflammation reaction. These findings suggest that this mouse model of central cord syndrome can be used as a model for preclinical research, and that gray matter is most vulnerable to injury in central cord syndrome, leading to impaired motor function.

Deglutition and the Regulation of the Swallow Motor Pattern

Despite centuries of investigation, questions and controversies remain regarding the fundamental genesis and motor pattern of swallow. Two significant topics include inspiratory muscle activity during swallow (Schluckatmung, i.e., "swallow-breath") and anatomical boundaries of the swallow pattern generator. We discuss the long history of reports regarding the presence or absence of Schluckatmung and the possible advantages of and neural basis for such activity, leading to current theories and novel experimental directions.

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.

Evidence That the Central Nervous System Can Induce a Modification at the Neuromuscular Junction That Contributes to the Maintenance of a Behavioral Response

Neurons within the spinal cord are sensitive to environmental relations and can bring about a behavioral modification without input from the brain. For example, rats that have undergone a thoracic (T2) transection can learn to maintain a hind leg in a flexed position to minimize exposure to a noxious electrical stimulation (shock). Inactivating neurons within the spinal cord with lidocaine, or cutting communication between the spinal cord and the periphery (sciatic transection), eliminates the capacity to learn, which implies that it depends on spinal neurons. Here we show that these manipulations have no effect on the maintenance of the learned response, which implicates a peripheral process. EMG showed that learning augments the muscular response evoked by motoneuron output and that this effect survives a sciatic transection. Quantitative fluorescent imaging revealed that training brings about an increase in the area and intensity of ACh receptor labeling at the neuromuscular junction (NMJ). It is hypothesized that efferent motoneuron output, in conjunction with electrical stimulation of the tibialis anterior muscle, strengthens the connection at the NMJ in a Hebbian manner. Supporting this, paired stimulation of the efferent nerve and tibialis anterior generated an increase in flexion duration and augmented the evoked electrical response without input from the spinal cord. Evidence is presented that glutamatergic signaling contributes to plasticity at the NMJ. Labeling for vesicular glutamate transporter is evident at the motor endplate. Intramuscular application of an NMDAR antagonist blocked the acquisition/maintenance of the learned response and the strengthening of the evoked electrical response.SIGNIFICANCE STATEMENT The neuromuscular junction (NMJ) is designed to faithfully elicit a muscular contraction in response to neural input. From this perspective, encoding environmental relations (learning) and the maintenance of a behavioral modification over time (memory) are assumed to reflect only modifications upstream from the NMJ, within the CNS. The current results challenge this view. Rats were trained to maintain a hind leg in a flexed position to avoid noxious stimulation. As expected, treatments that inhibit activity within the CNS, or disrupt peripheral communication, prevented learning. These manipulations did not affect the maintenance of the acquired response. The results imply that a peripheral modification at the NMJ contributes to the maintenance of the learned response.

Bilateral cervical contusion spinal cord injury: A mouse model to evaluate sensorimotor function

Spinal cord injury is a severe condition, resulting in specific neurological symptoms depending on the level of damage. Approximately 60% of spinal cord injuries affect the cervical spinal cord, resulting in complete or incomplete tetraplegia and higher mortality rates than injuries of the thoracic or lumbar region. Although cervical spinal cord injuries frequently occur in humans, there are few clinically relevant models of cervical spinal cord injury. Animal models are critical for examining the cellular and molecular manifestations of human cervical spinal cord injury, which is not feasible in the clinical setting, and to develop therapeutic strategies. There is a limited number of studies using cervical, bilateral contusion SCI and providing a behavioral assessment of motor and sensory functions, which is partly due to the high mortality rate and severe impairment observed in severe cervical SCI models. The goal of this study was to develop a mouse model of cervical contusion injury with moderate severity, resulting in an apparent deficit in front and hindlimb function but still allowing for self-care of the animals. In particular, we aimed to characterize a mouse cervical injury model to be able to use genetic models and a wide range of viral techniques to carry out highly mechanistic studies into the cellular and molecular mechanisms of cervical spinal cord injury. After inducing a bilateral, cervical contusion injury at level C5, we followed the recovery of injured and sham-uninjured animals for eight weeks post-surgery. Hindlimb and forelimb motor functions were significantly impaired immediately after injury, and all mice demonstrated partial improvement over time that remained well below that of uninjured control mice. Mice also displayed a significant loss in their sensory function throughout the testing period. This loss of sensory and motor function manifested as a reduced ability to perform skilled motor tasks in all of the injured mice. Here, we describe a new mouse model of moderate bilateral cervical spinal cord injury that does not lead to mortality and provides a comprehensive assessment of histological and behavioral assessments. This model will be useful in enhancing our mechanistic understanding of cervical spinal cord injury and in the development of treatments targeted at promoting neuroprotection, neuroplasticity, and functional recovery after cervical SCI.

Spinal cord injury and diaphragm neuromotor control

Introduction: Neuromotor control of diaphragm muscle and the recovery of diaphragm activity following spinal cord injury have been narrowly focused on ventilation. By contrast, the understanding of neuromotor control for non-ventilatory expulsive/straining maneuvers (including coughing, defecation, and parturition) is relatively impoverished. This variety of behaviors are achieved via the recruitment of the diverse array of motor units that comprise the diaphragm muscle.Areas covered: The neuromotor control of ventilatory and non-ventilatory behaviors in health and in the context of spinal cord injury is explored. Particular attention is played to the neuroplasticity of phrenic motor neurons in various models of cervical spinal cord injury.Expert opinion: There is a remarkable paucity in our understanding of neuromotor control of maneuvers in spinal cord injury patients. Dysfunction of these expulsive/straining maneuvers reduces patient quality of life and contributes to severe morbidity and mortality. As spinal cord injury patient life expectancies continue to climb steadily, a nexus of spinal cord injury and age-associated comorbidities are likely to occur. While current research remains concerned only with the minutiae of ventilation, the major functional deficits of this clinical cohort will persist intractably. We posit some future research directions to avoid this scenario.

Mid-cervical interneuron networks following high cervical spinal cord injury

Spinal interneuron (IN) networks can facilitate respiratory motor recovery after spinal cord injury (SCI). We hypothesized that excitatory synaptic connectivity between INs located immediately caudal to unilateral cervical SCI would be most prevalent in a contra- to ipsilateral direction. Adult rats were studied following chronic C2 spinal cord hemisection (C2Hx) injury. Rats were anesthetized and ventilated and a multi-electrode array was used to simultaneously record INs on both sides of the C4-5 spinal cord. The temporal firing relationship between IN pairs was evaluated using cross-correlation with directionality of synaptic connections inferred based on electrode location. During baseline recordings, the majority of detectable excitatory IN connections occurred in a contra- to- ipsilateral direction. However, acute respiratory stimulation with hypoxia abolished this directionality, while simultaneously increasing the detectable inhibitory connections within the ipsilateral cord. We conclude that propriospinal networks caudal to SCI can display a contralateral-to-ipsilateral directionality of synaptic connections and that these connections are modulated by acute exposure to hypoxia.

Plasticity Induced Recovery of Breathing Occurs at Chronic Stages after Cervical Contusion

Severe midcervical contusion injury causes profound deficits throughout the respiratory motor system that last from acute to chronic time points post-injury. We use chondroitinase ABC (ChABC) to digest chondroitin sulphate proteoglycans within the extracellular matrix (ECM) surrounding the respiratory system at both acute and chronic time points post-injury to explore whether augmentation of plasticity can recover normal motor function. We demonstrate that, regardless of time post-injury or treatment application, the lesion cavity remains consistent, showing little regeneration or neuroprotection within our model. Through electromyography (EMG) recordings of multiple inspiratory muscles, however, we show that application of the enzyme at chronic time points post-injury initiates the recovery of normal breathing in previously paralyzed respiratory muscles. This reduced the need for compensatory activity throughout the motor system. Application of ChABC at acute time points recovered only modest amounts of respiratory function. To further understand this effect, we assessed the anatomical mechanism of this recovery. Increased EMG activity in previously paralyzed muscles was brought about by activation of spared bulbospinal pathways through the site of injury and/or sprouting of spared serotonergic fibers from the contralateral side of the cord. Accordingly, we demonstrate that alterations to the ECM and augmentation of plasticity at chronic time points post-cervical contusion can cause functional recovery of the respiratory motor system and reveal mechanistic evidence of the pathways that govern this effect.

Diaphragm muscle function following midcervical contusion injury in rats

Midcervical spinal cord contusion injury results in tissue damage, disruption of spinal pathways, and motor neuron loss. Unilateral C4 contusion results in loss of 40%-50% of phrenic motor neurons ipsilateral to the injury (~25% of the total phrenic motor neuron pool). Over time after unilateral C4 contusion injury, diaphragm muscle (DIAm) electromyogram activity increases both contralateral and ipsilateral to the side of injury in rats, suggesting compensation because of increased activation of the surviving motor neurons. However, the impact of contusion injury on DIAm force generation is less clear. Transdiaphragmatic pressure (Pdi) was measured across motor behaviors over time after unilateral C4 contusion injury in adult male Sprague-Dawley rats. Maximum Pdi (Pdi_max) was elicited by bilateral phrenic nerve stimulation at 7 days postinjury. We hypothesized that Pdi_max is reduced following unilateral C4 contusion injury, whereas ventilatory behaviors of the DIAm are unimpaired. In support of our hypothesis, Pdi_max was reduced by ~25% after unilateral C4 contusion, consistent with the extent of phrenic motor neuron loss following contusion injury. One day after contusion injury, the Pdi amplitude during airway occlusion was reduced from ~30 to ~20 cmH₂O, but this reduction was completely reversed by 7 days postinjury. Ventilatory behaviors (~10 cmH₂O), DIAm-specific force, and muscle fiber cross-sectional area did not differ between the laminectomy and contusion groups. These results indicate that the large reserve capacity for DIAm force generation allows for higher-force motor behaviors to be accomplished despite motor neuron loss, likely reflecting changes in motor unit recruitment. NEW & NOTEWORTHY Respiratory muscles such as the diaphragm generate the pressures necessary to accomplish a variety of motor behaviors ranging from ventilation to near-maximal expulsive behaviors. However, the impact of contusion injury on diaphragm pressure generation across behaviors is not clear. The present study shows that contusion injury impairs maximal pressure generation while preserving the ability of the diaphragm to accomplish lower-force motor behaviors, likely reflecting changes in diaphragm motor unit recruitment.

Mid-cervical spinal cord contusion causes robust deficits in respiratory parameters and pattern variability

Mid-cervical spinal cord contusion disrupts both the pathways and motoneurons vital to the activity of inspiratory muscles. The present study was designed to determine if a rat contusion model could result in a measurable deficit to both ventilatory and respiratory motor function under “normal” breathing conditions at acute to chronic stages post trauma. Through whole body plethysmography and electromyography we assessed respiratory output from three days to twelve weeks after a cervical level 3 (C3) contusion. Contused animals showed significant deficits in both tidal and minute volumes which were sustained from acute to chronic time points. We also examined the degree to which the contusion injury impacted ventilatory pattern variability through assessment of Mutual Information and Sample Entropy. Mid-cervical contusion significantly and robustly decreased the variability of ventilatory patterns. The enduring deficit to the respiratory motor system caused by contusion was further confirmed through electromyography recordings in multiple respiratory muscles. When isolated via a lesion, these contused pathways were insufficient to maintain respiratory activity at all time points post injury. Collectively these data illustrate that, counter to the prevailing literature, a profound and lasting ventilatory and respiratory motor deficit may be modelled and measured through multiple physiological assessments at all time points after cervical contusion injury.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

The crossed phrenic phenomenon

The cervical spine is the most common site of traumatic vertebral column injuries. Respiratory insufficiency constitutes a significant proportion of the morbidity burden and is the most common cause of mortality in these patients. In seeking to enhance our capacity to treat specifically the respiratory dysfunction following spinal cord injury, investigators have studied the "crossed phrenic phenomenon", wherein contraction of a hemidiaphragm paralyzed by a complete hemisection of the ipsilateral cervical spinal cord above the phrenic nucleus can be induced by respiratory stressors and recovers spontaneously over time. Strengthening of latent contralateral projections to the phrenic nucleus and sprouting of new descending axons have been proposed as mechanisms contributing to the observed recovery. We have recently demonstrated recovery of spontaneous crossed phrenic activity occurring over minutes to hours in C₁-hemisected unanesthetized decerebrate rats. The specific neurochemical and molecular pathways underlying crossed phrenic activity following injury require further clarification. A thorough understanding of these is necessary in order to develop targeted therapies for respiratory neurorehabilitation following spinal trauma. Animal studies provide preliminary evidence for the utility of neuropharmacological manipulation of serotonergic and adenosinergic pathways, nerve grafts, olfactory ensheathing cells, intraspinal microstimulation and a possible role for dorsal rhizotomy in recovering phrenic activity following spinal cord injury.

Diaphragm electromyographic activity following unilateral midcervical contusion injury in rats

Contusion-type injuries to the spinal cord are characterized by tissue loss and disruption of spinal pathways. Midcervical spinal cord injuries impair the function of respiratory muscles and may contribute to significant respiratory complications. This study systematically assessed the impact of a 100-kDy unilateral C4 contusion injury on diaphragm muscle activity across a range of motor behaviors in rats. Chronic diaphragm electromyography (EMG) was recorded before injury and at 1 and 7 days postinjury (DPI). Histological analyses assessed the extent of perineuronal net formation, white-matter sparing, and phrenic motoneuron loss. At 7 DPI, ∼45% of phrenic motoneurons were lost ipsilaterally. Relative diaphragm root mean square (RMS) EMG activity increased bilaterally across a range of motor behaviors by 7 DPI. The increase in diaphragm RMS EMG activity was associated with an increase in neural drive (RMS value at 75 ms after the onset of diaphragm activity) and was more pronounced during higher force, nonventilatory motor behaviors. Animals in the contusion group displayed a transient decrease in respiratory rate and an increase in burst duration at 1 DPI. By 7 days, following midcervical contusion, there was significant perineuronal net formation and white-matter loss that spanned 1 mm around the injury epicenter. Taken together, these findings are consistent with increased recruitment of remaining motor units, including more fatigable, high-threshold motor units, during higher force, nonventilatory behaviors. Changes in diaphragm EMG activity following midcervical contusion injury reflect complex adaptations in neuromotor control that may increase the risk of motor-unit fatigue and compromise the ability to sustain higher force diaphragm efforts.

NEW & NOTEWORTHY

The present study shows that unilateral contusion injury at C4 results in substantial loss of phrenic motoneurons but increased diaphragm muscle activity across a range of ventilatory and higher force, nonventilatory behaviors. Measures of neural drive indicate increased descending input to phrenic motoneurons that was more pronounced during higher force, nonventilatory behaviors. These findings reveal novel, complex adaptations in neuromotor control following injury, suggestive of increased recruitment of more fatigable, high-threshold motor units.

Spontaneous Functional Recovery in a Paralyzed Hemidiaphragm Following Upper Cervical Spinal Cord Injury in Adult Rats

Previous studies have shown that latent respiratory pathways can be activated by as phyxia or systemic theophylline administration to restore function to a hemidiaphragm paralyzed by C2 spinal cord hemisection in adult female rats. Based on this premise, electrophysiologic recording techniques were employed in the present investigation to first determine qualitatively whether latent respiratory pathways are activated spon taneously following prolonged post hemisection periods (4-16 weeks) without any therapeutic intervention. Our second objective in a separate group of hemisected an imals was to quantitate any documented functional recovery under the following stan dardized recording conditions: bilateral vagotomy, paralysis with pancuronium bro mide, artificial ventilation, and constant PCO2(maintained at 25 mmHg).

The Impact of Midcervical Contusion Injury on Diaphragm Muscle Function

Midcervical contusion injuries disrupt descending ipsilateral excitatory bulbospinal projections to phrenic motoneurons, compromising ventilation. We hypothesized that a unilateral contusion injury at C3 versus C5 would differentially impact phrenic activity reflecting more prominent disruption of ipsilateral descending excitatory drive to more caudal segments of the phrenic motor pool with more cranial injuries. Phrenic motoneuron counts and evidence of diaphragm muscle denervation at individual neuromuscular junctions (NMJ) were evaluated at 14 days post-injury after unilateral contusion injury (100 kDynes). Whole body plethysmography and chronic diaphragm EMG were measured before the injury and at 3, 7, and 14 days post-injury. Contusion injuries at either level resulted in a similarly sized cavity. C3 contusion resulted in loss of 39 ± 13% of ipsilateral phrenic motoneurons compared with 13 ± 21% after C5 contusion (p = 0.003). Cervical contusion injuries resulted in diaphragm muscle denervation (C3 contusion: 17 ± 4%; C5 contusion: 7 ± 4%; p = 0.047). The pattern of denervation revealed segmental innervation of the diaphragm muscle, with greater denervation ventrally after C3 contusion and dorsally after C5 contusion. Overall, diaphragm root mean square electromyography activity did not change ipsilaterally after C3 or C5 contusion, but increased contralaterally (∼ 11%) after C3 contusion only on the first day post-injury (p = 0.026). Similarly, there were no significant changes in breathing parameters during eupnea or exposure to hypoxia (10% O2) - hypercapnia (5% CO2) at any time post-injury. Unilateral midcervical contusions minimally impair ventilatory behaviors despite phrenic motoneuron loss and diaphragm muscle denervation.

Intravenous multipotent adult progenitor cell treatment decreases inflammation leading to functional recovery following spinal cord injury

Following spinal cord injury (SCI), immune-mediated secondary processes exacerbate the extent of permanent neurological deficits. We investigated the capacity of adult bone marrow-derived stem cells, which exhibit immunomodulatory properties, to alter inflammation and promote recovery following SCI. In vitro, we show that human multipotent adult progenitor cells (MAPCs) have the ability to modulate macrophage activation, and prior exposure to MAPC secreted factors can reduce macrophage-mediated axonal dieback of dystrophic axons. Using a contusion model of SCI, we found that intravenous delivery of MAPCs one day, but not immediately, after SCI significantly improves urinary and locomotor recovery, which was associated with marked spinal cord tissue sparing. Intravenous MAPCs altered the immune response in the spinal cord and periphery, however biodistribution studies revealed that no MAPCs were found in the cord and instead preferentially homed to the spleen. Our results demonstrate that MAPCs exert their primary effects in the periphery and provide strong support for the use of these cells in acute human contusive SCI.

Functional recovery after cervical spinal cord injury: Role of neurotrophin and glutamatergic signaling in phrenic motoneurons

Cervical spinal cord injury (SCI) interrupts descending neural drive to phrenic motoneurons causing diaphragm muscle (DIAm) paralysis. Recent studies using a well-established model of SCI, unilateral spinal hemisection of the C2 segment of the cervical spinal cord (SH), provide novel information regarding the molecular and cellular mechanisms of functional recovery after SCI. Over time post-SH, gradual recovery of rhythmic ipsilateral DIAm activity occurs. Recovery of ipsilateral DIAm electromyogram (EMG) activity following SH is enhanced by increasing brain-derived neurotrophic factor (BDNF) in the region of the phrenic motoneuron pool. Delivery of exogenous BDNF either via intrathecal infusion or via mesenchymal stem cells engineered to release BDNF similarly enhance recovery. Conversely, recovery after SH is blunted by quenching endogenous BDNF with the fusion-protein TrkB-Fc in the region of the phrenic motoneuron pool or by selective inhibition of TrkB kinase activity using a chemical-genetic approach in TrkB(F616A) mice. Furthermore, the importance of BDNF signaling via TrkB receptors at phrenic motoneurons is highlighted by the blunting of recovery by siRNA-mediated downregulation of TrkB receptor expression in phrenic motoneurons and by the enhancement of recovery evident following virally-induced increases in TrkB expression specifically in phrenic motoneurons. BDNF/TrkB signaling regulates synaptic plasticity in various neuronal systems, including glutamatergic pathways. Glutamatergic neurotransmission constitutes the main inspiratory-related, excitatory drive to motoneurons, and following SH, spontaneous neuroplasticity is associated with increased expression of ionotropic N-methyl-d-aspartate (NMDA) receptors in phrenic motoneurons. Evidence for the role of BDNF/TrkB and glutamatergic signaling in recovery of DIAm activity following cervical SCI is reviewed.

Acute theophylline exposure modulates breathing activity through a cervical contusion

Cervical spinal contusion injuries are the most common form of spinal cord injury (>50%) observed in humans. These injuries can result in the impaired ability to breathe. In this study we examine the role of theophylline in the rescue of breathing behavior after a cervical spinal contusion. Previous research in the C2 hemisection model has shown that acute administration of theophylline can rescue phrenic nerve activity and diaphragmatic EMG on the side ipsilateral to injury. However, this effect is dependent on intact and uninjured pathways. In this study we utilized a cervical contusion injury model that more closely mimics the human condition. This injury model can determine the effectiveness of therapeutic interventions, in this case theophylline, on the isolated contused pathways of the spinal cord. Three weeks after a 150 kD C3/4 unilateral contusion subjects received a 15 mg/kg dose of theophylline prior to a contralateral C2 hemisection. Subjects that received theophylline were able to effectively utilize damaged pathways to breathe for up to 2 min, while subjects treated with saline were unable to support ventilation. Through these experiments, we demonstrate that theophylline can make injured pathways that mediate breathing more effective and therefore, suggest a potential therapeutic role in the critical time points immediately after injury.

Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation

Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.

Early phrenic motor neuron loss and transient respiratory abnormalities after unilateral cervical spinal cord contusion

Contusion-type cervical spinal cord injury (SCI) is one of the most common forms of SCI observed in patients. In particular, injuries targeting the C3-C5 region affect the pool of phrenic motor neurons (PhMNs) that innervates the diaphragm, resulting in significant and often chronic respiratory dysfunction. Using a previously described rat model of unilateral midcervical C4 contusion with the Infinite Horizon Impactor, we have characterized the early time course of PhMN degeneration and consequent respiratory deficits following injury, as this knowledge is important for designing relevant treatment strategies targeting protection and plasticity of PhMN circuitry. PhMN loss (48% of the ipsilateral pool) occurred almost entirely during the first 24 h post-injury, resulting in persistent phrenic nerve axonal degeneration and denervation at the diaphragm neuromuscular junction (NMJ). Reduced diaphragm compound muscle action potential amplitudes following phrenic nerve stimulation were observed as early as the first day post-injury (30% of pre-injury maximum amplitude), with slow functional improvement over time that was associated with partial reinnervation at the diaphragm NMJ. Consistent with ipsilateral diaphragmatic compromise, the injury resulted in rapid, yet only transient, changes in overall ventilatory parameters measured via whole-body plethysmography, including increased respiratory rate, decreased tidal volume, and decreased peak inspiratory flow. Despite significant ipsilateral PhMN loss, the respiratory system has the capacity to quickly compensate for partially impaired hemidiaphragm function, suggesting that C4 hemicontusion in rats is a model of SCI that manifests subacute respiratory abnormalities. Collectively, these findings demonstrate significant and persistent diaphragm compromise in a clinically relevant model of midcervical contusion SCI; however, the therapeutic window for PhMN protection is restricted to early time points post-injury. On the contrary, preventing loss of innervation by PhMNs and/or inducing plasticity in spared PhMN axons at the diaphragm NMJ are relevant long-term targets.

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.

The role of the crossed phrenic pathway after cervical contusion injury and a new model to evaluate therapeutic interventions

More than 50% of all spinal cord injury (SCI) cases are at the cervical level and usually result in the impaired ability to breathe. This is caused by damage to descending bulbospinal inspiratory tracts and the phrenic motor neurons which innervate the diaphragm. Most investigations have utilized a lateral C2 hemisection model of cervical SCI to study the resulting respiratory motor deficits and potential therapies. However, recent studies have emerged which incorporate experimental contusion injuries at the cervical level of the spinal cord to more closely reflect the type of trauma encountered in humans. Nonetheless, a common deficit observed in these contused animals is the inability to increase diaphragm motor activity in the face of respiratory challenge. In this report we tested the hypothesis that, following cervical contusion, all remaining tracts to the phrenic nucleus are active, including the crossed phrenic pathway (CPP). Additionally, we investigated the potential function these spared tracts might possess after injury. We find that, following a lateral C3/4 contusion injury, not all remaining pathways are actively exciting downstream phrenic motor neurons. However, removing some of these pathways through contralateral hemisection results in a cessation of all activity ipsilateral to the contusion. This suggests an important modulatory role for these pathways. Additionally, we conclude that this dual injury, hemi-contusion and post contra-hemisection, is a more effective and relevant model of cervical SCI as it results in a more direct compromise of diaphragmatic motor activity. This model can thus be used to test potential therapies with greater accuracy and clinical relevance than cervical contusion models currently allow.

Inactivity-induced respiratory plasticity: protecting the drive to breathe in disorders that reduce respiratory neural activity

Multiple forms of plasticity are activated following reduced respiratory neural activity. For example, in ventilated rats, a central neural apnea elicits a rebound increase in phrenic and hypoglossal burst amplitude upon resumption of respiratory neural activity, forms of plasticity called inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF), respectively. Here, we provide a conceptual framework for plasticity following reduced respiratory neural activity to guide future investigations. We review mechanisms giving rise to iPMF and iHMF, present new data suggesting that inactivity-induced plasticity is observed in inspiratory intercostals (iIMF) and point out gaps in our knowledge. We then survey conditions relevant to human health characterized by reduced respiratory neural activity and discuss evidence that inactivity-induced plasticity is elicited during these conditions. Understanding the physiological impact and circumstances in which inactivity-induced respiratory plasticity is elicited may yield novel insights into the treatment of disorders characterized by reductions in respiratory neural activity.

Postinjury administration of 17β-estradiol induces protection in the gray and white matter with associated functional recovery after cervical spinal cord injury in male rats

The majority of spinal cord injuries (SCIs) in the clinic occur at the lower cervical levels, resulting in both white and gray matter disruption. In contrast, most experimental models of SCI in rodents induce damage in the thoracic cord, resulting primarily in white matter disruption. To address this disparity, experimental cervical SCI models have been developed. Thus, we used a recently characterized model of cervical hemicontusion SCI in adult male rats to assess the potential therapeutic effect of post-SCI administration of 17β-estradiol. Rats received a hemicontusion at the level of the fifth cervical vertebra (C5) followed by administration of 17β-estradiol via a slow release pellet (0.5 or 5.0 mg/pellet) beginning at 30 minutes post-SCI. Behavioral evaluation of skilled and unskilled forelimb function and locomotor function were conducted for 7 weeks after SCI. Upon conclusion of the behavioral assessments, spinal cords were collected and histochemistry and stereology were conducted to evaluate the effect of treatment on the lesion characteristics. We found that post-SCI administration of 17β-estradiol decreased neuronal loss in the ventral horn, decreased reactive astrogliosis, decreased the immune response, and increased white mater sparing at the lesion epicenter. Additionally, post-SCI administration of 17β-estradiol improved skilled forelimb function and locomotor function. Taken together, these data suggest that post-SCI administration of 17β-estradiol protected both the gray and white matter in cervical SCI. Moreover, this treatment improved function on skilled motor tasks that involve both gray and white matter components, suggesting that this is likely a highly clinically relevant protective strategy.

Degeneration of phrenic motor neurons induces long-term diaphragm deficits following mid-cervical spinal contusion in mice

A primary cause of morbidity and mortality following cervical spinal cord injury (SCI) is respiratory compromise, regardless of the level of trauma. In particular, SCI at mid-cervical regions targets degeneration of both descending bulbospinal respiratory axons and cell bodies of phrenic motor neurons, resulting in deficits in the function of the diaphragm, the primary muscle of inspiration. Contusion-type trauma to the cervical spinal cord is one of the most common forms of human SCI; however, few studies have evaluated mid-cervical contusion in animal models or characterized consequent histopathological and functional effects of degeneration of phrenic motor neuron-diaphragm circuitry. We have generated a mouse model of cervical contusion SCI that unilaterally targets both C4 and C5 levels, the location of the phrenic motor neuron pool, and have examined histological and functional outcomes for up to 6 weeks post-injury. We report that phrenic motor neuron loss in cervical spinal cord, phrenic nerve axonal degeneration, and denervation at diaphragm neuromuscular junctions (NMJ) resulted in compromised ipsilateral diaphragm function, as demonstrated by persistent reduction in diaphragm compound muscle action potential amplitudes following phrenic nerve stimulation and abnormalities in spontaneous diaphragm electromyography (EMG) recordings. This injury paradigm is reproducible, does not require ventilatory assistance, and provides proof-of-principle that generation of unilateral cervical contusion is a feasible strategy for modeling diaphragmatic/respiratory deficits in mice. This study and its accompanying analyses pave the way for using transgenic mouse technology to explore the function of specific genes in the pathophysiology of phrenic motor neuron degeneration and respiratory dysfunction following cervical SCI.

Phrenic motor neuron degeneration compromises phrenic axonal circuitry and diaphragm activity in a unilateral cervical contusion model of spinal cord injury

Respiratory dysfunction is the leading cause of morbidity and mortality following traumatic spinal cord injury (SCI). Injuries targeting mid-cervical spinal cord regions affect the phrenic motor neuron pool that innervates the diaphragm, the primary respiratory muscle of inspiration. Contusion-type injury in the cervical spinal cord is one of the most common forms of human SCI; however, few studies have evaluated mid-cervical contusion in animal models or characterized consequent histopathological and functional effects of degeneration of phrenic motor neuron-diaphragm circuitry. In an attempt to target the phrenic motor neuron pool, two unilateral contusion injury paradigms were tested, a single injury at level C4 and a double injury both at levels C3 and C4, and animals were followed for up to 6 weeks post-injury. Both unilateral cervical injury paradigms are reproducible with no mortality or need for breathing assistance, and are accompanied by phrenic motor neuron loss, phrenic nerve axon degeneration, diaphragm atrophy, denervation and subsequent partial reinnervation at the diaphragm neuromuscular junction, changes in spontaneous diaphragm EMG recordings, and reduction in phrenic nerve compound muscle action potential amplitude. These findings demonstrate significant and chronically persistent respiratory compromise following mid-cervical SCI due to phrenic motor neuron degeneration. These injury paradigms and accompanying analyses provide important tools both for understanding mechanisms of phrenic motor neuron and diaphragm pathology following SCI and for evaluating therapeutic strategies in clinically relevant cervical SCI models.

Respiratory function following bilateral mid-cervical contusion injury in the adult rat

The consequences of spinal cord injury (SCI) are often viewed as the result of white matter damage. However, injuries occurring at any spinal level, especially in cervical and lumbar enlargement regions, also entail segmental neuronal loss. Yet, the contributions of gray matter injury and plasticity to functional outcomes are poorly understood. The present study addressed this issue by investigating changes in respiratory function following bilateral C(3)/C(4) contusion injuries at the level of the phrenic motoneuron (PhMN) pool which in the adult rat extends from C(3) to C(5/6) and provides innervation to the diaphragm. Despite extensive white and gray matter pathology associated with two magnitudes of injury severity, ventilation was relatively unaffected during both quiet breathing and respiratory challenge (hypercapnia). On the other hand, bilateral diaphragm EMG recordings revealed that the ability to increase diaphragm activity during respiratory challenge was substantially, and chronically, impaired. This deficit has not been seen following predominantly white matter lesions at higher cervical levels. Thus, the impact of gray matter damage relative to PhMNs and/or interneurons becomes evident during conditions associated with increased respiratory drive. Unaltered ventilatory behavior, despite significant deficits in diaphragm function, suggests compensatory neuroplasticity involving recruitment of other spinal respiratory networks which may entail remodeling of connections. Transynaptic tracing, using pseudorabies virus (PRV), revealed changes in PhMN-related interneuronal labeling rostral to the site of injury, thus offering insight into the potential anatomical reorganization and spinal plasticity following cervical contusion.

Breathing patterns after mid-cervical spinal contusion in rats

Respiratory failure is the leading cause of death after cervical spinal injury. We hypothesized that incomplete cervical spinal injuries would alter respiratory pattern and initiate plasticity in the neural control of breathing. Further, we hypothesized that the severity of cervical spinal contusion would correlate with changes in breathing pattern. Fourteen days after C4-C5 contusions, respiratory frequency and tidal volume were measured in unanesthetized Sprague Dawley rats in a whole body plethysmograph. Phrenic motor output was monitored in the same rats which were anesthetized, vagotomized, paralyzed and ventilated to eliminate and/or control sensory feedback that could alter breathing patterns. The extent of spinal injury was approximated histologically by measurements of the injury-induced cyst area in transverse sections; cysts ranged from 2 to 28% of spinal cross-sectional area, and had a unilateral bias. In unanesthetized rats, the severity of spinal injury correlated negatively with tidal volume (R(2)=0.85; p<0.001) and positively with breathing frequency (R(2)=0.65; p<0.05). Thus, the severity of C4-C5 spinal contusion dictates post-injury breathing pattern. In anesthetized rats, phrenic burst amplitude was decreased on the side of injury, and burst frequency correlated negatively with contusion size (R(2)=0.51; p<0.05). A strong correlation between unanesthetized breathing pattern and the pattern of phrenic bursts in anesthetized, vagotomized and ventilated rats suggests that changes in respiratory motor output after spinal injury reflect, at least in part, intrinsic neural mechanisms of CNS plasticity initiated by injury.

Characterization of a graded cervical hemicontusion spinal cord injury model in adult male rats

Most experimental models of spinal cord injury (SCI) in rodents induce damage in the thoracic cord and subsequently examine hindlimb function as an indicator of recovery. In these models, functional recovery is most attributable to white-matter preservation and is less influenced by grey-matter sparing. In contrast, most clinical cases of SCI occur at the lower cervical levels, a region in which both grey-matter and white-matter sparing contribute to functional motor recovery. Thus experimental cervical SCI models are beginning to be developed and used to assess protective and pharmacological interventions following SCI. The objective of this study was to characterize a model of graded cervical hemicontusion SCI with regard to several histological and behavioral outcome measures, including novel forelimb behavioral tasks. Using a commercially available rodent spinal cord impactor, adult male rats received hemicontusion SCI at vertebral level C5 at 100, 200, or 300 kdyn force, to produce mild, moderate, or severe injury severities. Tests of skilled and unskilled forelimb and locomotor function were employed to assess functional recovery, and spinal cord tissue was collected to assess lesion severity. Deficits in skilled and unskilled forelimb function and locomotion relating to injury severity were observed, as well as decreases in neuronal numbers, white-matter area, and white-matter gliosis. Significant correlations were observed between behavioral and histological data. Taken together, these data suggest that the forelimb functional and locomotor assessments employed here are sensitive enough to measure functional changes, and that this hemicontusion model can be used to evaluate potential protective and regenerative therapeutic strategies.

Involvement of ERK2 in traumatic spinal cord injury

Activation of extracellular signal-regulated protein kinase 1/2 (ERK1/2) are implicated in the pathophysiology of spinal cord injury (SCI). However, the specific functions of individual ERK isoforms in neurodegeneration are largely unknown. We investigated the hypothesis that ERK2 activation may contribute to pathological and functional deficits following SCI and that ERK2 knockdown using RNA interference may provide a novel therapeutic strategy for SCI. Lentiviral ERK2 shRNA and siRNA were utilized to knockdown ERK2 expression in the spinal cord following SCI. Pre-injury intrathecal administration of ERK2 siRNA significantly reduced excitotoxic injury-induced activation of ERK2 (p < 0.001) and caspase 3 (p < 0.01) in spinal cord. Intraspinal administration of lentiviral ERK2 shRNA significantly reduced ERK2 expression in the spinal cord (p < 0.05), but did not alter ERK1 expression. Administration of the lentiviral ERK2 shRNA vector 1 week prior to severe spinal cord contusion injury resulted in a significant improvement in locomotor function (p < 0.05), total tissue sparing (p < 0.05), white matter sparing (p < 0.05), and gray matter sparing (p < 0.05) 6 weeks following severe contusive SCI. Our results suggest that ERK2 signaling is a novel target associated with the deleterious consequences of spinal injury.

Descending bulbospinal pathways and recovery of respiratory motor function following spinal cord injury

The rodent respiratory system is a relevant model for study of the intrinsic post-lesion mechanisms of neuronal plasticity and resulting recovery after high cervical spinal cord injury. An unilateral cervical injury (hemisection, lateral section or contusion) interrupts unilaterally bulbospinal respiratory pathways to phrenic motor neurons innervating the diaphragm and leads to important respiratory defects on the injured side. However, the ipsilateral phrenic nerve exhibits a spontaneous and progressive recovery with post-lesion time. Shortly after a lateral injury, this partial recovery depends on the activation of contralateral pathways that cross the spinal midline caudal to the injury. Activation of these crossed phrenic pathways after the injury depends on the integrity of phrenic sensory afferents. These pathways are located principally in the lateral part of the spinal cord and involve 30% of the medullary respiratory neurons. By contrast, in chronic post-lesion conditions, the medial part of the spinal cord becomes sufficient to trigger substantial ipsilateral respiratory drive. Thus, after unilateral cervical spinal cord injury, respiratory reactivation is associated with a time-dependent anatomo-functional reorganization of the bulbospinal respiratory descending pathways, which represents an adaptative strategy for functional compensation.

Bilateral cervical contusion spinal cord injury in rats

There is increasing motivation to develop clinically relevant experimental models for cervical SCI in rodents and techniques to assess deficits in forelimb function. Here we describe a bilateral cervical contusion model in rats. Female Sprague-Dawley rats received mild or moderate cervical contusion injuries (using the Infinite Horizons device) at C5, C6, or C7/8. Forelimb motor function was assessed using a grip strength meter (GSM); sensory function was assessed by the von Frey hair test; the integrity of the corticospinal tract (CST) was assessed by biotinylated dextran amine (BDA) tract tracing. Mild contusions caused primarily dorsal column (DC) and gray matter (GM) damage while moderate contusions produced additional damage to lateral and ventral tissue. Forelimb and hindlimb function was severely impaired immediately post-injury, but all rats regained the ability to use their hindlimbs for locomotion. Gripping ability was abolished immediately after injury but recovered partially, depending upon the spinal level and severity of the injury. Rats exhibited a loss of sensation in both fore- and hindlimbs that partially recovered, and did not exhibit allodynia. Tract tracing revealed that the main contingent of CST axons in the DC was completely interrupted in all but one animal whereas the dorsolateral CST (dlCST) was partially spared, and dlCST axons gave rise to axons that arborized in the GM caudal to the injury. Our data demonstrate that rats can survive significant bilateral cervical contusion injuries at or below C5 and that forepaw gripping function recovers after mild injuries even when the main component of CST axons in the dorsal column is completely interrupted.

The crossed phrenic phenomenon and recovery of function following spinal cord injury

This review will focus on neural plasticity and recovery of respiratory function after spinal cord injury and feature the "crossed phrenic phenomenon" (CPP) as a model for demonstrating such plasticity and recovery. A very brief summary of the earlier literature on the CPP will be followed by a more detailed review of the more recent studies. Two aspects of plasticity associated with the CPP that have been introduced in the literature recently have been spontaneous recovery of ipsilateral hemidiaphragmatic function following chronic spinal cord injury and drug-induced persistent recovery of the ipsilateral hemidiaphragm lasting long after animals have been weaned from drug treatment. The underlying mechanisms for this plasticity and resultant recovery will be discussed in this review. Moreover, two new models involving the CPP have been introduced: a mouse model which now provides for an opportunity to study CPP plasticity at a molecular level using a genetic approach and light-stimulated induction of the CPP accomplished by transfecting mammalian cells with channelrhodopsin. Both models provide an opportunity to sort out the intracellular signaling cascades that may be involved in motor recovery in the respiratory system after spinal cord injury. Finally, the review will examine developmental plasticity of the CPP and discuss how the expression of the CPP changes in neonatal rats as they mature to adults. Understanding the underlying mechanisms behind the spontaneous expression of the crossed phrenic pathway either in the developing animal or after chronic spinal cord injury in the adult animal may provide clues to initiating respiratory recovery sooner to alleviate human suffering and eventually eliminate the leading cause of death in human cases of spinal cord injury.

Modeling spinal cord contusion, dislocation, and distraction: characterization of vertebral clamps, injury severities, and node of Ranvier deformations

Spinal cord contusion and transection models are widely used for studying spinal cord injury (SCI). Clinically, however, other biomechanical injury mechanisms such as vertebral dislocation and distraction frequently occur, but these injuries are difficult to produce in animals. We mechanically characterize a vertebral clamping strategy that enables the modeling of vertebral dislocation and distraction injuries--in addition to the standard contusion paradigm--in the rat cervical spine. These vertebral clamps have a stiffness of 83.6+/-18.9 N/mm and clamping strength 64.7+/-10.2N which allows injuries to be modeled at high-speed (approximately 100 cm/s). Logistic regression indicated that a moderate-to-severe injury, with an acute mortality rate of 10%, occurs at 2.6 mm of C4/5 dorso-ventral dislocation and 4.1 mm of rostro-caudal distraction between C4 and C5. Injuries produced by dislocation and distraction exhibited features of axonal damage that were absent in contusion injuries. We conducted morphometric analysis at the nodes of Ranvier using immunohistochemistry for potassium channels (Kv1.2) in the juxtaparanodal region. Following distraction injuries, elongated nodes of Ranvier were observed up to 4mm rostral to the lesion. In contrast, contusion injuries produced distortions in nodal geometry which were restricted to the vicinity of the lesion. The greatest deformations in node of Ranvier geometry occurred at the dislocation epicenter. Given the importance of white matter damage in SCI pathology, the distinctiveness of these injury patterns demonstrate that the dislocation and distraction injury models complement existing contusion models. Together, these three animal models span a broader clinical spectrum for more reliably gauging the potential human efficacy of therapeutic strategies.

Respiratory neuroplasticity and cervical spinal cord injury: translational perspectives

Paralysis of the diaphragm is a severe consequence of cervical spinal cord injury. This condition can be experimentally modeled by lateralized, high cervical lesions that interrupt descending inspiratory drive to the corresponding phrenic nucleus. Although partial recovery of ipsilateral diaphragm function occurs over time, recent findings show persisting chronic deficits in ventilation and phrenic motoneuron activity. Some evidence suggests, however, that spontaneous recovery can be enhanced by modulating neural pathways to phrenic motoneurons via synaptic circuitries which appear more complex than previously envisioned. The present review highlights these and other recent experimental multidisciplinary findings pertaining to respiratory neuroplasticity in the rat. Translational considerations are also emphasized, with specific attention directed at the clinical and interpretational strengths of different lesion models and outcome measures.

Are rodents an appropriate pre-clinical model for treating spinal cord injury? Examples from the respiratory system

Because most studies of the effects of spinal cord injury (SCI) and resulting repair and treatments use rodent models, it is important to determine if these models are relevant to humans. In this review, we focus on alterations in respiratory function as a result of SCI. Several injury paradigms have been used in the rat to examine restoration of post-lesion respiratory function and potential benefits from repair strategies designed for humans. Unlike the corticospinal locomotor system, respiratory neural organization is well preserved between rodents and humans, and resembles the general organization of motor pathways in primates. These similarities justify the use of the rodent respiratory system as a model to analyze SCI and putative repair strategies.

Effect of spinal cord injury on the neural regulation of respiratory function

Injury at any level of the spinal cord can impair respiratory motor function. Indeed, complications associated with respiratory function are the number one cause of mortality in humans following spinal cord injury (SCI) at any level of the cord. This review is aimed at describing the effect of SCI on respiratory function while highlighting the recent advances made by basic science research regarding the neural regulation of respiratory function following injury. Models of SCI that include upper cervical hemisection and contusion injury have been utilized to examine the underlying neural mechanisms of respiratory control following injury. The approaches used to induce motor recovery in the respiratory system are similar to other studies that examine recovery of locomotor function after SCI. These include strategies to initiate regeneration of damaged axons, to reinnervate paralyzed muscles with peripheral nerve grafts, to use spared neural pathways to induce motor function, and finally, to initiate mechanisms of neural plasticity within the spinal cord to increase motoneuron firing. The ultimate goals of this research are to restore motor function to previously paralyzed respiratory muscles and improve ventilation in patients with SCI.

Effect of spinal cord injury on the respiratory system: basic research and current clinical treatment options

Spinal cord injury (SCI) often leads to an impairment of the respiratory system. The more rostral the level of injury, the more likely the injury will affect ventilation. In fact, respiratory insufficiency is the number one cause of mortality and morbidity after SCI. This review highlights the progress that has been made in basic and clinical research, while noting the gaps in our knowledge. Basic research has focused on a hemisection injury model to examine methods aimed at improving respiratory function after SCI, but contusion injury models have also been used. Increasing synaptic plasticity, strengthening spared axonal pathways, and the disinhibition of phrenic motor neurons all result in the activation of a latent respiratory motor pathway that restores function to a previously paralyzed hemidiaphragm in animal models. Human clinical studies have revealed that respiratory function is negatively impacted by SCI. Respiratory muscle training regimens may improve inspiratory function after SCI, but more thorough and carefully designed studies are needed to adequately address this issue. Phrenic nerve and diaphragm pacing are options available to wean patients from standard mechanical ventilation. The techniques aimed at improving respiratory function in humans with SCI have both pros and cons, but having more options available to the clinician allows for more individualized treatment, resulting in better patient care. Despite significant progress in both basic and clinical research, there is still a significant gap in our understanding of the effect of SCI on the respiratory system.

Loss and spontaneous recovery of forelimb evoked potentials in both the adult rat cuneate nucleus and somatosensory cortex following contusive cervical spinal cord injury

Varying degrees of neurologic function spontaneously recovers in humans and animals during the days and months after spinal cord injury (SCI). For example, abolished upper limb somatosensory potentials (SSEPs) and cutaneous sensations can recover in persons post-contusive cervical SCI. To maximize recovery and the development/evaluation of repair strategies, a better understanding of the anatomical locations and physiological processes underlying spontaneous recovery after SCI is needed. As an initial step, the present study examined whether recovery of upper limb SSEPs after contusive cervical SCI was due to the integrity of some spared dorsal column primary afferents that terminate within the cuneate nucleus and not one of several alternate routes. C5-6 contusions were performed on male adult rats. Electrophysiological techniques were used in the same rat to determine forelimb evoked neuronal responses in both cortex (SSEPs) and the cuneate nucleus (terminal extracellular recordings). SSEPs were not evoked 2 days post-SCI but were found at 7 days and beyond, with an observed change in latencies between 7 and 14 days (suggestive of spared axon remyelination). Forelimb evoked activity in the cuneate nucleus at 15 but not 3 days post-injury occurred despite dorsal column damage throughout the cervical injury (as seen histologically). Neuroanatomical tracing (using 1% unconjugated cholera toxin B subunit) confirmed that upper limb primary afferent terminals remained within the cuneate nuclei. Taken together, these results indicate that neural transmission between dorsal column primary afferents and cuneate nuclei neurons is likely involved in the recovery of upper limb SSEPs after contusive cervical SCI.

Effect of spinal cord injury on the respiratory system: basic research and current clinical treatment options

Spinal cord injury (SCI) often leads to an impairment of the respiratory system. The more rostral the level of injury, the more likely the injury will affect ventilation. In fact, respiratory insufficiency is the number one cause of mortality and morbidity after SCI. This review highlights the progress that has been made in basic and clinical research, while noting the gaps in our knowledge. Basic research has focused on a hemisection injury model to examine methods aimed at improving respiratory function after SCI, but contusion injury models have also been used. Increasing synaptic plasticity, strengthening spared axonal pathways, and the disinhibition of phrenic motor neurons all result in the activation of a latent respiratory motor pathway that restores function to a previously paralyzed hemidiaphragm in animal models. Human clinical studies have revealed that respiratory function is negatively impacted by SCI. Respiratory muscle training regimens may improve inspiratory function after SCI, but more thorough and carefully designed studies are needed to adequately address this issue. Phrenic nerve and diaphragm pacing are options available to wean patients from standard mechanical ventilation. The techniques aimed at improving respiratory function in humans with SCI have both pros and cons, but having more options available to the clinician allows for more individualized treatment, resulting in better patient care. Despite significant progress in both basic and clinical research, there is still a significant gap in our understanding of the effect of SCI on the respiratory system.

High Cervical Lateral Spinal Cord Injury Results in Long-Term Ipsilateral Hemidiaphragm Paralysis

Although axon regeneration is limited in the central nervous system, partial lesions of the spinal cord induce neuroplasticity processes that can lead to spontaneous functional improvement. To determine whether such compensatory mechanisms occur in the respiratory system, we analyzed the incidence of partial injury of the cervical spinal cord on diaphragm activity in adult rats. We show that a section of the lateral area of the C2 cervical spinal cord induces complete phrenic nerve inactivation and ipsilateral hemidiaphragm paralysis, whereas medial or dorsolateral sections had only a moderate effect on respiratory activity. In the case of lateral hemisection, activity of the ipsilateral phrenic nerve was partially restored after a lapse of 3 months. No spontaneous diaphragm recovery was observed, however, even after a lapse of several months in the case of hemisection or lateral section. Ipsilateral hemidiaphragm activity could however be restored after transection of the contralateral phrenic nerve, by activation of the "crossed phrenic phenomenon" (involving activation of previously latent respiratory contralateral pathways crossing the midline). These data suggest that the respiratory system develops important long-term plasticity processes at the level of phrenic motoneuron innervation. However, they do not by themselves allow substantial diaphragm recovery, underscoring the continued need for developing repair strategies. These studies also validates the use of the respiratory system as a model to evaluate the functional incidence of repair strategies not only after hemisection but also after more limited sectioning restricted to the lateral side of the cervical cord.

Behavioral and histological characterization of unilateral cervical spinal cord contusion injury in rats

Most experimental studies of spinal cord injury (SCI) in rats damage the thoracic cord, with the consequent functional loss being due to interruption of long tracts connecting the caudal spinal cord to the rostral nervous system. Less work has been done evaluating injury to the cervical cord, even though it is the most common level of human SCI. In addition to the long tracts, the cervical spinal cord contains the sensory and motor neurons responsible for upper extremity function. The purpose of this study was to further develop a rat model of cervical spinal cord contusion injury using a modified NYU/MASCIS weight drop device. Mild (6.25 mm) and moderate (12.5 mm) C5 unilateral injuries were produced. Behavioral recovery was examined using a grooming test, a paw preference test, a walkway test (The Catwalk), and a horizontal ladder test. Histological outcome measures included sparing at the lesion epicenter, sparing throughout the extent of the lesion, quantification of myelin loss rostral and caudal to the lesion, and motor neuron counts. Compared to controls, animals receiving SCI exhibited injury severity-specific deficits in forelimb, locomotor, and hindlimb function persisting for 6-weeks post-SCI. Histological analysis revealed ipsilateral containment of the injury, and differentiation between groups on all measures except motor neuron counts. This model has many advantages: (1) minimal animal care requirements post-SCI, (2) within subject controls, (3) functional loss involves primarily the ipsilateral forelimb, and (4) it is a behavioral and histological model for both gray and white matter damage caused by contusive SCI.

A new model of upper cervical spinal contusion inducing a persistent unilateral diaphragmatic deficit in the adult rat

Research on spinal cord trauma requires models reflecting the contusion mechanisms encountered in clinical situation. The aim of this study was to develop in the adult rat a reproducible model of upper cervical spinal cord contusion inducing persistent unilateral diaphragm deficit. After dura and pia matter removal, weight drop and compression were targeted at the ventro-lateral funiculi which contain the bulbospinal descending respiratory pathways that command the phrenic motoneurons innervating the diaphragm. At 7 days post-injury, the left diaphragm activity recorded in contused rats (27.4 +/- 5.1% of the contralateral activity) was significantly lower than in the sham group (97.6 +/- 1.2%). This respiratory deficit still persisted 1 month later. Histology showed a reproducible left C2-lateralized lesion that involved both white and gray matter including the ventro-lateral funiculi. This C2 contusion model provides a basis for testing both regenerative and neuroprotective strategies aimed at improving functional respiratory recovery after spinal cord trauma.

Respiratory abnormalities resulting from midcervical spinal cord injury and their reversal by serotonin 1A agonists in conscious rats

Respiratory dysfunction after cervical spinal cord injury (SCI) has not been examined experimentally using conscious animals, although clinical SCI most frequently occurs in midcervical segments. Here, we report a C5 hemicontusion SCI model in rats with abnormalities that emulate human post-SCI pathophysiology, including spontaneous recovery processes. Post-C5 SCI rats demonstrated deficits in minute ventilation (Ve) responses to a 7% CO2 challenge that correlated significantly with lesion severities (no injury or 12.5, 25, or 50 mm x 10 g weight drop; New York University impactor; p < 0.001) and ipsilateral motor neuron loss (p = 0.016). Importantly, C5 SCI resulted in at least 4 weeks of respiratory abnormalities that ultimately recovered afterward. Because serotonin is involved in respiration-related neuroplasticity, we investigated the impact of activating 5-HT1A receptors on post-C5 SCI respiratory dysfunction. Treatment with the 5-HT1A agonist 8-hydroxy-2-(di-n-propylmino)tetralin (8-OH DPAT) (250 microg/kg, i.p.) restored hypercapnic Ve at 2 and 4 weeks after injury (i.e., approximately 39.2% increase vs post-SCI baseline; p < or = 0.033). Improvements in hypercapnic Ve response after single administration of 8-OH DPAT were dose dependent and lasted for approximately 4 h(p < or = 0.038 and p < or = 0.024, respectively). Treatment with another 5-HT1A receptor agonist, buspirone (1.5 mg/kg, i.p.), replicated the results, whereas pretreatment with a 5-HT1A-specific antagonist, 4-iodo-N-[2-[4(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (3 mg/kg, i.p.) given 20 min before 8-OH DPAT negated the effect of 8-OH DPAT. These results imply a potential clinical use of 5-HT1A agonists for post-SCI respiratory disorders.

Histopathological and Behavioral Characterization of a Novel Cervical Spinal Cord Displacement Contusion Injury in the Rat

Cervical contusive trauma accounts for the majority, of human spinal cord injury (SCI), yet experimental use of cervical contusion injury models has been limited. Considering that (1) the different ways of injuring the spinal cord (compression, contusion, and transection) induce very different processes of tissue damage and (2) the architecture of the spinal cord is not uniform, it is important to use a model that is more clinically applicable to human SCI. Therefore, in the current study we have developed a rat model of contusive, cervical SCI using the Electromagnetic Spinal Cord Injury Device (ESCID) developed at Ohio State University (OSU) to induce injury by spinal cord displacement. We used the device to perform mild, moderate and severe injuries (0.80, 0.95, and 1.1 mm displacements, respectively) with a single, brief displacement of <20 msec upon the exposed dorsal surface of the C5 cervical spinal cord of female (180-200 g) Fischer rats. Characterization of the model involved the analysis of the temporal histopathological progression of the injury over 9 weeks using histochemical stains to analyze white and gray mater integrity and immunohistochemistry to examine cellular changes and physiological responses within the injured spinal cord. Accompanying the histological analysis was a comprehensive determination of the behavioral functionality of the animals using a battery of motor tests. Characterization of this novel model is presented to enable and encourage its future use in the design and experimental testing of therapeutic strategies that may be used for human SCI.